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Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
B. U. ONSHORE
DESIGN CRITERIA
GAS DEHYDRATION
PRG.PR.GAS.0001
Rev. 0
January 2010
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 2 (90)
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
I N D E X
1. SCOPE AND PURPOSE 5
2. REFERENCE DOCUMENTS 5
3. DEFINITIONS 5 3.1 Specific Definitions 5 3.2 Symbols and Abbreviations 8
4. ACTIVITIES DESCRIPTION 12 4.1 Glycol Gas Dehydration 13
4.1.1 General 13 4.1.2 System Description and Process Flow Diagram 16 4.1.2.1 Gas Stream 17 4.1.2.2 Glycol Stream 17 4.1.2.3 Enhanced Regeneration Systems 19 4.1.3 Design Variables 22 4.1.3.1 Inlet Gas Flow rate 22 4.1.3.2 Inlet Gas Temperature 22 4.1.3.3 Inlet Gas Pressure 25 4.1.3.4 Lean TEG temperature 25 4.1.3.5 Lean TEG Concentration 25 4.1.3.6 Glycol Circulation Rate 26 4.1.3.7 Number of Stage in the Contactor 27 4.1.4 Equipment Design 27 4.1.4.1 Inlet Gas Cooler 27 4.1.4.2 Inlet gas Separator or Scrubber 27 4.1.4.3 Contactor 28 4.1.4.4 Glycol Flash Vessel 35 4.1.4.5 Regenerator and Reboiler 36 4.1.4.6 Surge Drum 44 4.1.4.7 Heat Exchangers 45 4.1.4.8 Condensed Overheads Separator 46 4.1.4.9 Filters 47
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 3 (90)
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
4.1.4.10 Glycol Circulation Pumps 49 4.1.4.11 Glycol StorageTank 49 4.1.4.12 Glycol Sump Vessel 50 4.1.4.13 Enhanced Regeneration Equipment Design 51 4.1.5 Materials 55 4.1.6 Operation and Maintenance 56 4.1.6.1 Oxidation 56 4.1.6.2 Ph Control 56 4.1.6.3 Salt Contamination 57 4.1.6.4 Hydrocarbon Contamination 57 4.1.6.5 Sludge Accumulation 57 4.1.6.6 Foaming 57 4.1.6.7 Acid gas solubilities and stripping 58 4.1.6.8 Mercury in Feed gas 58
4.2 Molecular Sieves Dehydration 59 4.2.1 General 59 4.2.2 System Description and Process Flow Diagram 65 4.2.2.1 Adsorption Mechanism 67 4.2.3 Design Variables 68 4.2.3.1 Gas Composition 68 4.2.3.2 Flow Rate 68 4.2.3.3 Pressure 68 4.2.3.4 Temperature 69 4.2.3.5 Further variables affecting the efficiency of
regeneration 69 4.2.3.6 Utilities Availability and Local Conditions 69 4.2.4 Equipment Design 70 4.2.4.1 Scrubber 70 4.2.4.2 Gas Dehydrator 71 4.2.4.3 Regeneration Calculations 76 4.2.4.4 Special Bed Configurations 79 4.2.5 Operation and Maintenance 80 4.2.5.1 Operation Records 80 4.2.5.2 Good Operational Techniques 80 4.2.5.3 Depressurization / Repressurization 80
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 4 (90)
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
4.2.5.4 Analyzers 81 4.2.5.5 Insulation 81 4.2.5.6 Effects of contaminants on molecular sieves 81 4.2.6 Process Control and Safeguarding 82
5. FLOW CHART 83
6. APPENDIX 84 6.1 Appendix I – Water removal at different TEG
concentration and TEG circulation rates 84 6.2 Appendix II – Reboiler Heat Duty 87 6.3 Appendix III – Molecular Sieve Equipment Sizing –
Example 88
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 5 (90)
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
1. SCOPE AND PURPOSE
The purpose of this document is to provide the design guidelines for natural gas dehydration processes.
For equipment mechanical details the relevant SAIPEM or Project criteria shall be considered.
The application of this Design Criteria shall be subjected to accurate review of the results by a qualified and
competent designer to avoid unnecessary over sizing or design not adequate to the scope.
Moreover, margins indicated in the following paragraphs shall be considered as the minimum to be applied
in case no indications are present in the Project documents or in other SAIPEM standards.
2. REFERENCE DOCUMENTS
- PRG.PR.VES.0001 Guide to selecting and process sizing vessels
- PRG.PR.VES.0011 Guide to process sizing fractionating columns
- PRG.PR.GEN.0003 Guide to selecting standards for process equipment internals
- PRG.PR.HEB.0001 Guide to process selection and sizing of heat exchangers
- PRG.PR.MAC.0001 Guide to selecting and process sizing pumps
- PRG.PR.TUB.0001 Process piping sizing guide
- PRG.PR.HEB.0002 Guide to process sizing furnaces
- PRG.PR.MAC.0002 Guide to the process design of compressors
- GPSA Engineering Data Book
- Natural Gas Conditioning Though Adsorption Technology (J. M. Campbell, W. P. Cummings)
3. DEFINITIONS
3.1 Specific Definitions
Absorber See contactor Absorption Process The attraction and retention of vapours (water) by liquids
(glycol) / solids from a gas stream. Actual Trays The number of trays installed in a column or the equivalent
number of actual trays for a packed column. Bubble Cap Tray Horizontal plate holding bubble caps and downcomers in the
contactor. Bubble Caps Slotted metal caps attached over elevated nozzles (risers) on
the bubble cap trays. The slots cause the gas to break upinto small bubbles for intimate contact with the glycol.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 6 (90)
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
Condensate Light hydrocarbon liquids. Contactor (or Absorber) A vertical pressure column where gas and glycol are
intermingled counter-currently to remove water vapour fromthe gas. The contactor usually contains bubble cap trays,valve trays or structured packing.
Dehydration Removal of water vapour from a gas or a liquid. Design Pressure The pressure used in the design of a vessel for the purpose
of determining the minimum permissible wall thickness orphysical characteristics of the different parts of the vessel.
Dewpoint The temperature at which vapour begins to condense into a
liquid at a particular system pressure. A natural gas streamexhibits both hydrocarbon and water dewpoints.
Dewpoint Depression The difference in water dewpoint temperature between the
gas entering and leaving the contactor. Downcomer The vertical conduit between trays which allows liquid to pass
from tray to tray. Flood The condition wherein excess liquid hold-up occurs and
normal counterflow action is prevented in the glycolcontactor, regeneration still or stripping column. It is a designlimit which when reached in operation causes an excessiveloss of liquid from the top of the column.
Free Water Liquid water which is not dissolved in any other substance. Gas/Glycol Heat Exchanger A heat exchanger employed to cool the lean glycol by the gas
leaving the contactor before the glycol enters the contactor. Glycol A hygroscopic liquid. Mono-ethylene Glycol (MEG) and Di-
ethylene Glycol (DEG) are commonly used in hydrateinhibition service and Tri-ethylene Glycol (TEG) is mostcommon in gas dehydration service.
Lean Glycol (or Dry Glycol) Glycol which has been regenerated and has a low water
content. Rich Glycol (or Wet Glycol) Glycol which has absorbed water and thus has a high water
content. Glycol Flash Separator A three phase separator which is used in the rich glycol
stream to remove entrained gas and hydrocarbon liquids. Glycol/Glycol Exchanger A heat exchanger employed to recover heat from the
outgoing hot lean glycol from the reboiler and for pre-heatingthe incoming cool rich glycol from the contactor.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 7 (90)
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
Heat Duty The rate of heat absorption by the process. Heat Flux The average heat transfer rate to the fluid. Inlet Gas Separator (Scrubber)
A separator which removes free liquids from the inlet gasstream. The separator may be included or not in thecontactor.
Packing Material installed in the contactor, still column or stripping
column that provides a large surface area for merging liquidand vapour to facilitate mass transfer during absorption,distillation or stripping. Random packing consists of shaped pieces (e.g. rings,saddles) that have been dumped, not stacked, in the column.Structured packing is essentially a series of parallel formedmetal sheets.
pH Measure of the acidity of a liquid on a scale of 0 to 14 with 7
being neutral. 0 to 7 is acidic and 7 to 14 is alkaline. Reboiler A heat exchanger for boiling water out of the glycol. Regeneration System A unit including reboiler, still column and other related
facilities to regenerate (or re-concentrate) rich glycol to leanglycol.
Reflux Condensed liquid which flows back a column to maximise
separation efficiency. Saturated Gas (with respect to water)
A gas stream which contains the maximum amount of watervapour at a given temperature and pressure withoutcondensing the water.
Sparging Tube Internal pipe in the reboiler used to distribute stripping gas. Standard (pressure and temperature)
Unit of gas volume at reference conditions of 1 bar and 15°C.Abbreviated: m3(st).
Still Reflux Column Vertically mounted distillation (fractionation) column on top of
the reboiler. Stripping Column A packed column where glycol from the reboiler flows
downward to the surge drum while gas flows upwardstripping the water from glycol.
Stripping Gas Gas that is contacted with glycol to help remove water from
the glycol. Surge Drum Reservoir for regenerated glycol which is included or not in
the reboiler.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 8 (90)
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
Theoretical Tray One in which the vapour and liquid leaving the stage are in
equilibrium. The number of actual trays is equal to thenumber of theoretical trays divided by the overall trayefficiency.
Transfer Unit The dimensionless distance within which the solute
molecules transfer to the gas phase. A transfer unit can becalculated for a theoretical stage.
Tray Efficiency The ratio between the number of theoretical and actual trays. Valve Tray Horizontal plate holding valves and downcomers in the
contactor. A valve consists of a liftable metal plate whichcovers a hole in the tray, providing a variable area for gasflow.
3.2 Symbols and Abbreviations
A Area [m2] BTEX Aromatic components: benzene, toluene, ethylbenzene and xylene - Cp Heat Capacity [kJ/(kg K)] Css Saturation correction factor for sieve CT Temperature correction factor D Inside diameter of column or vessel [m] DEG Diethylene glicol - D Nozzle inside diameter, or diameter (with subscript) [m] EG Ethylene glicol - F Packing factor [m-1] FF Fraction of flood - G Acceleration due to gravity [9.81 m/s2] G Gas mass flowrate/unit area [kg/(s.m2)] H Height [m] H Henthalpy [kJ/kg] HC Height of channel or riser height of distributor [m]
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 9 (90)
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
HETP Height equivalent to a theoretical plate [m] ILL Interface level of condensate and glycol [m] L Length of vessel between tangent lines [m] Ls Length of racket bed saturation zone [m] HLL High level pre-alarm [m] LLL Low level pre-alarm [m] LCV Level control valve - HHLL High level trip [m] LLLL Low level trip [m]
•
m Mass flowrate [kg/s]
MEA Methylethanol amine - MMSCF Milion of standard cubic feet - MTD Mean temperature difference [°C] MTZ Mass Transfer Zone - N Number of bubble cap - NLL Normal level of liquid [m] NPSH Net positive suction head [m] OVHD Overhead - P Pressure [Pa] PG Propilene glicol - PSV Safety relief valve - PVT Pressare-Volume-Temperature - Q Volumetric flow rate or Heat [m3/s] or [kJ] Re Reynold's number - T Temperature [°C] or [K]
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 10 (90)
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
t Thickness [mm] TCV Temperature control valve - TEG Triethylene glicol - TREG Tetraethylene glicol - TS Tray spacing in contactor [m] TTL Top tangent line on vessel - U Overall heat transfer coefficient [W/(m2K)] UCR Unit circulation rate, volumetric flowrate of lean glycol per mass
flowrate of water removed [l/kg]
V Velocità [m/s] W Width [mm] Wr Water removed per cycle [kg]
Greek Symbols
Δ Separation between plates in separator, difference in parameter values (as in Δρ)
[m]
ρ Density [kg/m3] λ Gas load factor:
[m/s]
μ Dynamic viscosity [Pa.s] or [mPa.s] ψ Ratio of ρ of water to ρ of glycol - ϕ Flow parameter:
-
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 11 (90)
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
Subscripts
B Bed G Gas (as in vg) Hl Heat loss I Initial In Inlet (as in vin, Yin) L Liquid M Mixture (as in vm) Min Minimum Max Maximum Out Outlet Rg Regeneration Si Sieve St Steel Tr Total regeneration W Water
Superscripts
* Density correction (e.g. in Q*max)
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 12 (90)
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
4. ACTIVITIES DESCRIPTION
Gas Dehydration is the process of reducing the water content in the gas down to an acceptable level for the
downstream systems. At the wellhead, reservoir fluids almost invariably contain water and, except for a few
shallow wells, produced natural gas is saturated with water. The major reasons for dehydrating gas are:
• natural gas can combine with liquid or free water to form solid hydrates that can plug valves,
fittings, equipment or even pipelines;
• if not separated from the produced water, natural gas is corrosive, especially when CO2 and/or H2S
are also present;
• water can condense in the pipeline causing slug and possible erosion and corrosion;
• water vapour increases the volume and decreases the heating value of the gas;
• sales gas contracts and/or pipeline specifications have a maximum water content (usually 110 kg
H2O / million (st)m3 i.e. 7 lb H2O per MMSCF) or a specified dew point value;
• freezing in cryogenic and refrigerated absorption plants even in small quantities.
Natural gas is commercially dehydrated in one of the following methods:
1. Absorption Glycol Dehydration
2. Adsorption Molecular Sieve or Silica Gel
3. Condensation Refrigeration with Glycol Injection
Glycol dehydration (absorption) is the most common dehydration process used to meet pipeline sales
specification and field requirements (gas lift, fuel, etc.). Adsorption process are used to obtain very low
water contents (0.1 ppm vol or less) required in low temperature processing such as deep NGL extraction
and LNG plants. Condensation is commonly used as a dehydration process when the gas is employed in
moderate levels of refrigeration or in pipeline transportation. An inhibitor such as ethylene glycol (EG) or
methanol is used to prevent hydrate formation.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 13 (90)
4.1 Glycol Gas Dehydration
4.1.1 General
Glycol dehydration plants have many applications for natural gas dehydration. They are used to reach a
water dew point in the range of -30 ÷ -40°C. They are preferred for the following reasons:
• low installation costs ;
• operating costs similar to other dehydration systems;
• glycol dehydration is continuous process rather than batch;
• low pressure drop across the plant (0.35÷0.7 bar);
• easiness to treat gas at low pressure;
• possibility to receive gas at high feed temperature (till 66°C);
• low costs for obtaining small reductions in the gas dew point.
The principle of glycol dehydration is contacting a gas stream with a hygroscopic liquid which has a greater
affinity for the water vapour than does the gas.
After contacting the gas, the water-rich glycol is regenerated by heating at approximately atmospheric
pressure to a temperature high enough to drive off virtually all the absorbed water. The regenerated glycol
is then cooled and recirculated back.
All the following glycol types possess suitable properties to be used in a dehydration plant:
• Triethylene glycol (TEG) is the most commonly used dehydration liquid and it is the assumed glycol
type in this process description because it is the most cost-effective choice. TEG is the most easily
regenerated to a concentration of 98÷99.5% (using stripping gas) in an atmospheric stripper
because of its high boiling point. This permits higher dew-point depression of natural gas in the
range of 27÷66°C. Moreover, TEG has an initial theoretical decomposition temperature of 206.7°C,
higher than other glycol types and it is not too viscous above 21°C.
• Diethylene glycol (DEG) is sometimes used when hydrate inhibition is required upstream of
dehydration or due to the greater solubility of salt in DEG. Moreover, DEG is somewhat cheaper to
buy and sometimes is used for this reason. Compared to TEG, DEG has a larger carry-over loss,
offers lower dew point depression, and regeneration to high concentrations is more difficult due to
lower degradation temperature and consequent lower regeneration temperature.
• Tetraethylene glycol (TREG) is more viscous and more expensive than the other glycols. The only
real advantage is its lower vapour pressure which reduces absorber vapour loss. For this reason, it
should only be considered for rare cases where glycol dehydration will be employed on a gas
whose temperature exceeds about 50°C, such as when extreme ambient conditions prevent
cooling to a lower temperature.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 14 (90)
• Propylene glycol (PG) has been employed in some units and it is the least toxic glycol. It has a
lower affinity for aromatics, but PG has a much higher vapour pressure than TEG, and a much
lower flash point.
Physical properties of glycols are the following:
Glicol MEG DEG TEG TREG
Formula HOC2H4OH HO(C2H4O)2H HO(C2H4O)3H HO(C2H4O)4H
Molecular mass 62.07 106.12 150.17 194.32
Boiling point 101.3 kPa (°C) 197 245 287 327
Freezing point (°C) -13 -8 -7.2 -6.2
Density 20°C (kg/m3) 1113 1116 1123 1246
Viscosity 20°C (mPa.s) 20.9 35.7 47.9 60.0
Degradation temperature (°C) 165 164 206 238
Flash Point (°C) 111 124 165 202
Auto Ignition temperature (°C) 410 229 370 358
Toxicity YES YES YES YES
Table 1 – Physical Properties of MEG, DEG, TEG and TREG
Glycols as a class are of a low order of toxicity (with the exception of oral toxicity). They do not vaporise
readily at normal temperatures and, therefore, do not constitute a hazard from inhalation. They are also not
active skin irritants.
Glycols will burn and should be handled as if they were hydrocarbons. As they are highly miscible with
water, alcohol type foam concentrate shall be used for extinguishing of a glycol fire. For extinguishing of
small fires some dry powder types can be effective.
TEG has been applied downstream of production facilities that use MEG or DEG as a hydrate inhibitor
without apparently leading to contamination problems. Methanol used as a hydrate inhibitor in the feed gas
to a glycol dehydration unit will be absorbed by the glycol, and according to the GPSA Engineering Data
Book it can cause the following problems:
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 15 (90)
1. methanol will add additional reboiler heat duty and still vapour load and therefore increase
glycol losses;
2. aqueous methanol causes corrosion of carbon steel. Corrosion can thus occur in the still
and reboiler vapour space;
3. high methanol injection rates and consequent slug carry-over can cause flooding.
Glycol entrainment may lead to the following downstream problems:
• coalescing and partial condensation in pipelines resulting in localised corrosion;
• in cryogenic plants, particularly at temperatures below -25°C, freezing of TEG and plugging of
equipment;
• reduced performance of downstream adsorption plant, e.g. molecular sieves or silica gel.
Any entrained glycol should be removed upstream of cryogenic plant in high efficiency gas/liquid separators
to prevent possible plugging.
A range of lean TEG concentrations can be achieved with the basic regeneration flow schemes and various
enhancements summarised in
Table 2 and further described in the following paragraphs. It should be noted that the corresponding
dewpoint depressions are approximate and achievable figures are affected by actual process conditions.
Regeneration Process Basic Cold Finger Vacuum Stripping Gas Azeotropic
Stripping Lean TEG
concentration (wt%) 98.75 99.5 99.9 99.96 99.99
Dewpoint Depression (°C)
45 60 65 70 100
Table 2 – Lean TEG Concentrations with Regeneration Systems
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 16 (90)
4.1.2 System Description and Process Flow Diagram
The basic flow scheme without enhancements, such as stripping gas, is described below with reference to
Fig. 1, and follows the two main streams, gas and glycol. It is typical and many variations are possible.
Flash Gas
Fig. 1 – Simplified Process Flow Diagram for TEG Dehydration System
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 17 (90)
4.1.2.1 Gas Stream
Where feed temperatures are high, especially relative to ambient conditions, an inlet cooler may be used.
Feed gas is scrubbed of free liquids (water and condensate) and solid particles before entering the glycol
contactor. The separator may stand alone or form an integral part of the glycol contactor column. The latter
solution is usually preferred for small gas flowrate.
The saturated feed gas is introduced in the bottom of the contactor and rises up through the column where
it contacts lean glycol which is injected in the top of the column. The contacting devices may be trays or
packing. Dry gas leaves the column via a de-entrainment device, e.g. a demister mat, to remove entrained
droplets.
Flash gas from glycol flash vessel and off gas from still column overhead could have a high content of H2S,
BTEX and volatile hydrocarbon and are usually routed to low pressure flare.
4.1.2.2 Glycol Stream
Lean glycol from surge drum flows through the rich/lean glycol heat exchanger to cool the lean glycol
stream before entering the glycol circulation pumps. In some arrangements there will be two rich/lean glycol
heat exchangers in series.
Although there will be some pressure drop through the heat exchanger, due to the temperature reduction
the glycol should not flash at this point. If there is insufficient NPSH for the glycol pumps in the location
shown, they may be located between the surge drum and the lean/rich glycol heat exchanger. In this
location the pumps will operate at a higher temperature. Sufficient NPSH can be created for the main glycol
pumps by installing glycol booster pumps, if necessary.
The lean glycol then flows to the final cooler, which is often an air-cooled heat exchanger but could also be
a glycol-gas heat exchanger. In some cases (generally for low glycol circulation rate) lean glycol cooling
coils are installed in the top section of contactor thus avoiding the installation of the gas/glycol heat
exchanger.
From the glycol final cooler, the lean glycol enters the top of the contactor. On its way down the column the
glycol absorbs water and the rich glycol collects at the bottom of the contactor.
The rich glycol passes from the contactor via a level control valve to a coil in the top of the regeneration still
column, thereby providing reflux cooling in the still column.
Rich glycol is heated to about 60°C or 70°C in the rich/lean glycol heat exchanger before it enters the glycol
flash vessel. In this 3-phase separator, dissolved and entrained gas is removed from the glycol and liquid
hydrocarbon condensate, if present, is separated from the glycol. These hydrocarbon components would
flash in the regenerator and lead to an increased still column vapour load, a higher reboiler duty
requirement, greater glycol losses and a loss of recoverable product. These components would also lead to
coking of the reboiler heating elements, fouling and foaming.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 18 (90) While this arrangement is typical (heating reduces glycol viscosity and makes gas and condensate
separation easier), in some units the glycol flash vessel is located immediately downstream of the
contactor, operating at a lower temperature.
From the flash vessel the rich glycol flows through a full flow particle filter and an activated carbon filter
often in slipstream service, to remove solids and dissolved hydrocarbons and degradation products,
respectively, in order to prevent foaming and sludge build-up in the regenerator.
The rich glycol is further heated in a second rich/lean glycol heat exchanger and then flows to the
regenerator still column between two sections of packing. This is a typical arrangement, in some systems
the glycol flows directly to the regenerator without passing through a second glycol/glycol heat exchanger.
Heat is provided at the bottom of the regenerator evaporating water from the glycol. The reboiler may be
directly fired or indirectly heated by electricity, hot oil or steam. Typical operating temperatures are up to
204°C, i.e. 2°C lower than degradation temperature (see Fig. 2). Water and volatile species present are
evaporated from the rich glycol, the reflux is provided to reduce glycol losses. Because of the wide
difference in volatility only a small reflux is needed to effect water/glycol separation. Off gas is normally
cooled and sent to a three phase separator where condensate and oily water are recovered and sent to
treatment/disposal. The regeneration units are designed to operate at prevailing atmospheric pressure and
at temperature lower than the initial thermal decomposition temperatures of the glycols shown below:
GLYCOL DECOMPOSITION
TEMPERATURE
LEAN GLYCOL
CONC., wt%
SUGGESTED
REGENERATION
TEMPERATURE
EG 165°C 96.0 164°C
DEG 164°C 97.1 163°C
TEG 206°C 98.7 204°C
TREG 238°C - 236°C
Table 3 – Glycol Decomposition Temperatures
These are the temperatures at which measurable decomposition begins to occur in the presence of air. In
the units containing no air (oxygen), it has been found that the reboiler can be operated very close to the
above temperatures without noticeable decomposition. The composition of the lean glycol is set by the
bubble point composition at the regenerator pressure. Maximum concentration achievable in an
atmospheric regenerator operating at decomposition temperature is also shown in the above Table 3.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 19 (90)
Fig. 2 – TEG concentration vs. reboiler temperature
If the lean glycol concentration required at the absorber (to meet the dew point specification) is higher than
the maximum concentrations above, then some enhanced methods (see par. 4.1.2.3) of increasing the
glycol concentration at the regenerator shall be incorporated in the unit. Generally all of these methods
involve lowering the partial pressure of the glycol solution either by pulling a vacuum on the regenerator, or
by introducing stripping gas into the regenerator.
4.1.2.3 Enhanced Regeneration Systems
A. COLDFINGER
The "Coldfinger" method is a patented process which has been used in a number of locations, mostly in the
USA, to give enhanced glycol regeneration. It consists of a heat exchanger tube bundle with a liquid
collection through which is inserted into the vapour space of the surge drum. The heat exchanger tubes are
fed with either rich glycol before flowing to the still column reflux, or with cooling water, which gives a lower
temperature. This "cold finger" leads to condensation of some vapour which is richer in water than the
regenerated TEG in the liquid space of the surge drum. The condensed vapours collected are continuously
recycled back to the still column feed. The H2O partial pressure in the vapour space is thus lowered and the
lean glycol concentration increased. Lean TEG concentration of 99.5÷99.9 wt% have been achieved in
Coldfinger units without the use of stripping gas, although a small amount of gas is introduced into the
surge tank for pressure balancing. A simplified scheme of this regeneration process is shown in Fig. 3.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 20 (90) B. AZEOTROPIC STRIPING (DRIZO)
The azeotropic striping "DRIZO", shown in Fig. 3, is a patented process to give enhanced regeneration of
glycol. It utilises a circulating solvent, such as heptane or octane, to remove water by azeotropic stripping.
The regenerator vapours are condensed in a 3-phase separator and condensed solvent is returned to the
regenerator via a pump and solvent heaters. Some solvent is lost in the remaining vapour stream to vent.
Depending on the composition of the gas being dried, sufficient heavy ends can be absorbed by the glycols
in the contactor to more than compensate for these losses. Thus, an initial charge may only be needed for
start-up and excess hydrocarbon liquid can be recovered as a product.
This unit has the advantage of providing very high stripping gas rates with little or no venting of
hydrocarbons. Glycol concentrations in excess of 99.99% have been achieved with the DRIZO process. It
has an added advantage of condensing and recovering aromatic hydrocarbons from the still column
overhead. In fact, these unit often operate with a stripping solvent which is not iso-octane but a mixture of
aromatic, naphthenic and paraffin hydrocarbons in the C5-C8 range.
C. STRIPPING GAS
Glycol concentration can be increased by injecting a small quantity of stripping gas via a sparge pipe in the
surge drum or via a packed stripping column with a counter-current glycol flow, as shown in Fig. 3. The
latter alternative is preferred since it allows either a glycol concentration of 99.96 % instead of 99.9 % to be
achieved or alternatively a reduction in the stripping gas rate. If introduced directly to the reboiler, it is
common to use a distributor pipe along the bottom of the reboiler. Any inert gas is suitable as stripping gas:
it may be drawn from the fuel gas system, from the gas being dehydrated or from exhaust gas from a gas-
powered glycol pump if used. The use of stripping gas, however, is not recommended due to the increase
of hydrocarbon emissions when vent gas from still column is routed to flare.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 21 (90)
Rich TEG
Vent gas to flare or recycle
Still Column
Direct Fired
Reboiler
Surge Tank
Lean TEG
Stripping Gas
A) Stripping Gas
Rich TEG
Vent gas to flare or recycle
Still Column
Direct Fired
Reboiler
Surge Tank
Lean TEG
Rich TEG
Still Column
Direct Fired
Reboiler
Surge Tank
Lean TEG
Cooling MediumWater/Rich
TEG mixture to Still Column
B) Coldfinger ®
C) Drizo ®
Solvent Condenser
Solvent Vaporizer
Excess Solvent
Water
Vent
Drizo® Separator
Stripping Solvent
Fig. 3 – TEG Regeneration Alternatives (Stripping Gas, Coldfinger® and Drizo®)
D. VACUUM
The vacuum process utilises a low partial pressure over the glycol solution to achieve a higher glycol
concentration. This is achieved by drawing a vacuum on the stripping column. These units are not common
due to their high operating costs, control complexity and problems with glycol degradation due to air
infiltration.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 22 (90)
4.1.3 Design Variables
A properly designed and operated TEG unit will dehydrate natural gas with only minor difficulties and
require modest maintenance.
High glycol losses, excessive TEG recirculation rates, improperly operating pumps, needless energy
consumption, frequent plant shutdowns and excessive equipment replacement can lead to very high
operating costs. The overall design of a glycol system requires optimisation of many variables that interact
with each other.
This paragraph explains the effect of process variables on TEG unit optimisation.
4.1.3.1 Inlet Gas Flow rate
The load (kg/hr H2O to be removed) varies directly with the feed gas flow rate. Normally the trays (bubble
cap usually and valve type sometimes) are operating in a severe spray regime, i.e. very little liquid glycol
compared to the gas flow rate. Increases in feed gas flow rate can exacerbate this delicate condition of
“blowing flood” and can be very detrimental to contactor performance. Random or structural packing is not
susceptible to this “blowing flood” because the liquid flows as wetted film on the packing surface. In
addition, an increase in gas flowrate can lead to excessive glycol losses and overload of regenerator; of
course the capability of the other units (i.e pumps and reboilers) must be considered. The lower flow limit is
determined by the “overdesign” and the turndown ratio (depending on the type of the internals used).
4.1.3.2 Inlet Gas Temperature
Inlet gas temperature is a significant variable. Higher gas temperature requires more glycol circulation,
since:
- there is an exponential increase in the water vapour content of the saturated gas (see Fig. 4);
- a higher inlet dew point requires a higher dew point depression for a given outlet dew point;
- a higher unit circulation rate (UCR) is required due to an increase in the equilibrium value of water
dew point for a given lean glycol concentration.
Higher temperature also reduces the gas density leading to a higher volumetric gas flow and load factor.
The operating inlet gas temperature shall be in the range 25÷50°C.
Hydrates may form if the temperature of the water saturated gas is less than about 20°C, therefore it is
desirable to have the inlet gas temperature at least 5°C above the hydrate formation temperature. It has to
be noted that at very low inlet gas temperature (i.e. < 15°C) the increase of glycol viscosity will reduce the
efficiency of absorption and will increase the foaming tendency.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 23 (90) Gas cooling upstream of the inlet scrubber is generally recommended, especially for temperatures above
50°C, since it is usually more economic than providing increased glycol circulation and regeneration
capacity. Reducing the glycol circulation rate is also preferred to reduce losses.
Above 50°C the inlet gas contains too much water, and the drying ability of the glycol is reduced. Moreover,
at higher temperature glycol vaporisation losses become appreciable. In addition, if the inlet gas
temperature is much higher than ambient temperature, heavier hydrocarbons could condensate on the wall
of the contactor.
Glycol into contactor needs to be slightly warmer (3 ÷ 6°C) than the inlet gas to avoid hydrocarbon
condensation.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 24 (90)
Fig. 4 – Equilibrium Water Dew Point Vs Temperature
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 25 (90)
4.1.3.3 Inlet Gas Pressure
The pressure effect is quite small. Operation is theoretically possible up to pressures of 150 to 200 bar.
However, in practice operations at pressures above 135 bar, i.e. outside 900# ANSI pressure rating, are
not recommended. The HETP increases with pressure and glycol vapour loss may become excessive at
gas densities greater than about 100 kg/m3.
At pressure below about 35 bar, the gas contains significantly more water vapour and the effect of pressure
is quite appreciable.
4.1.3.4 Lean TEG temperature
While TEG can theoretically dehydrate natural gas at operating temperatures from 10°C to 55°C the
preferred temperature range is 30 ÷ 55°C. The equilibrium water dew point decreases with decreasing
temperature, but cooling below 20°C the glycol becomes too viscous. This reduces tray efficiency,
promotes foaming, and increases glycol losses. Below 10°C the drop in dehydration efficiency is very
pronounced. The inlet glycol temperature should be 3÷6°C higher than the inlet gas temperature. If the
glycol enters cooler than the gas, the resulting chilling condenses hydrocarbons, which, in turn, promote
foaming. If the glycol enters more than 8°C above the effluent gas temperature, TEG vaporization losses
are increased unnecessarily.
4.1.3.5 Lean TEG Concentration
The lean glycol concentration has the greatest effect on the dew point depression highly affecting
equilibrium water dew point of dry gas. For standard dehydration systems, the factors affecting
concentration are the reboiler operating temperature and the quantity of stripping gas used, as discussed in
par. 4.1.2.3. The drying ability of TEG increases rapidly with concentration as illustrated in Fig. 5.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 26 (90)
Fig. 5 – Equilibrium Water Dew Point Vs TEG Concentration
4.1.3.6 Glycol Circulation Rate
Total glycol circulation rate is a function of the total amount of water to be removed from the gas and the
Unit Circulation Rate (UCR). As indicated in 4.1.3.2, a higher inlet gas temperature increases the water
vapour content of the saturated gas, the dew point depression required, and the UCR, all of which
compound total circulation requirement.
Increasing the circulation rate, the dew point depression will increase by providing a higher mean difference
between the operating and equilibrium lines. However, for UCR above 40 litres/kg, the improvement is
usually small and the reboiler duty becomes excessive. Increasing the number of trays (or packing height)
or the glycol concentration are generally more effective means to increase the dew point depression,
especially when the percentage of the inlet water to be removed is high. A lower circulation rate will also
reduce the sensible heat requirement of the system. A minimum design UCR of 18 litres/kg is
recommended. Typically UCR varies between 25 and 40 litres of glycol per kg of water removed.
Figures in Appendix I illustrate the effect of different TEG circulation rates on water removal, at different
TEG concentration. Curves have been determined at fixed temperature and number of equilibrium stages.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 27 (90)
4.1.3.7 Number of Stage in the Contactor
At a fixed value of lean TEG concentration, it can be selected a contactor with several stages and a low
circulation rate, or one with few stages and a high circulation rate.
At fixed values of lean TEG concentration and circulation rate the water concentration in the treated gas
has an asymptotic trend when the number of theoretical trays is 5 or higher.
In conclusion, the number of trays in the Contactor is economically less important than other design
variables (i.e. lean TEG concentration and circulation rate) in fact, due to the large difference between
boiling point of glycol and water, few stages are required for dehydration; hence more efforts shall be made
to optimise other process variables above mentioned.
4.1.4 Equipment Design
This section recommends the minimum requirements for the design, material selection and fabrication of
glycol-type gas dehydration systems. The components of a basic system are described in a detailed
manner. Supplementary design details for enhanced regeneration methods (see 4.1.2.3) are present.
The equipments described in this section shall be equipped with piping, instruments, valve actuators, level
shutdown devices and other accessories to make a complete and functional system. It shall be understood
that sample connections, vents, low point drains and minor material items required to make the system
functional are part of the assembly.
4.1.4.1 Inlet Gas Cooler
Whenever possible, the feed gas should be cooled by air or water ahead of the inlet separator, since this is
generally a cheaper form of dehydration. The inlet cooler shall operate above the hydrate formation
temperature, taking into account minimum ambient temperature scenario with all fans switch off.
4.1.4.2 Inlet gas Separator or Scrubber
An inlet gas separator with a minimum liquid removal efficiency of 99% of liquid droplets > 10 µm shall be
provided upstream of the dehydration unit, even if it is downstream of a production separator. The need of
removing entrainment from the gas stream before the contactor is emphasized by the fact that most of all
gas dehydration problems are caused by inadequate scrubbing of the inlet gas. Five of the more common
contaminants that impair the performance of the glycol systems are:
1. Entrained or Free Water. This water increases the glycol recirculation, reboiler duty, and
fuel costs. If the unit becomes overloaded, glycol can be carried over from the contactor
and/or the still column.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 28 (90)
2. Oils or Hydrocarbons. Dissolved oils (aromatic or asphaltic) reduce the drying capacity of
the glycol and, with water, cause foaming. Undissolved oils coke on the heat transfer
surfaces in the reboiler and increase the viscosity of the glycol.
3. Entrained Brine. These salts dissolve in the glycol, are corrosive to steels (especially
stainless) and can deposit on the fire tubes in the reboiler, causing hot spots and fire-tube
burnout.
4. Down-Hole Additives. They are for example corrosion inhibitors, acidizing and fracturing
fluids. These materials cause foaming, corrosion, and hot spots if they deposit on the fire
tubes.
5. Solids. They are for instance sand and corrosion products. They promote foaming, erode
valves and pumps and eventually plug trays and packing.
Heat tracing may be used to eliminate condensing liquids between the separator and the contactor.
The separator can also be arranged inside the contactor column below the contacting section. The
combination of the contactor and separator in one column offers savings in total weight, space and costs.
Liquids that might otherwise condense in the piping between the separator and contactor are also avoided.
The separator internals may either be of the demister type or a high efficiency type (High efficiency inlet
device-mist mat-multicyclone separator). In the latter case the required diameter for separation will be
considerably smaller than that required for contacting.
4.1.4.3 Contactor
As shown in Fig. 6, the absorber consists of an optional integral scrubber at the bottom, a mass transfer or
drying section in the middle, and a mist extractor at the top. In smaller units, the lean glycol is cooled in a
coil located in the contactor just below the mist extractor, while in larger units, a separate, external heat
exchanger is used (see 4.1.4.7). Wet natural gas enters the integral scrubber tangentially, and then passes
through a wire-mesh mist extractor that removes most of the remaining entrained liquid droplets.
In the drying section, the gas flows upward and is contacted intimately by the descending glycol solution.
This counter-current contact usually employs 4 to 12 bubble cap or valve trays or packed bed.
Liquid glycol carryover should not exceed 16 kg/MMSm3 (1 lb/MMscf), therefore a mist extractor should be
provided at the top of the absorber.
Various configurations for the contactor are possible; the main variables are the choice of contacting
internals, and whether or not the inlet gas separator is arranged in the column.
The following Fig. 7 and Fig. 8 show the possible different configurations.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 29 (90)
Fig. 6 – TEG Contactor
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 30 (90)
Dry gas outlet
Glycol outlet
Demister mat
Manhole
Glycol inlet
Bubble cap trays (5 to 8 typical)
Wet gas inlet
Manhole
Vortex breaker
LAHH
HC skimmer
0.15 D (min 0.15 m)0.10 m
2 TS
TS (0.6 m typical)
TS
TS
TS
0.5 d (min 0.3 m)
d + 0.02 m0.05 D (min 0.15 m)
Inlet gas distributor
Glycol inlet
Bubble cap trays (5 to 8 typical)
TS (0.6 m typical)
TS
TS
TS
0.10 m
2 TS
0.15 D (min 0.15 m)
Glycol outlet
0.05 D (min 0.15 m)
d + 0.02 m
d (min 0.3 m)0.1 m0.2 D (min 0.3 m)
hc (0.8 m typical)
min 0.4 m
HC skimmer
Glycol outlet
Gas riserRiser cap
Chimney tray
Demister mat
Wet gas inlet
Manhole
Vortex breaker
LAHH
Inlet gas distributor
Demister mat
Dry gas outlet
Manhole
Fig. 7 – Glicol Contacor with Bubble Cap Trays (left side) and with Bubble Cap Trays and inelt Separator (right side)
0.05 D (min 0.15 m)
d + 0.02 mWet gas inlet
LAHH
Inlet gas distributor
Demister matManhole (0.6 m typical)
Glycol inlet
Liquid distributor
Structured packing
Gas riserRiser cap
Chimney trayDrain pipe
HC skimmer
Manhole
Vortex breaker
0.4 m typically
0.10 m0.6 m (or 0.3 m for a full dia. top flange)
0.15 D (min 0.15 m)
Packed height 2.5 – 4.0 m
typically
0.2 D (min 0.3 m)
hc (0.25 m typical)
min 0.4 m
Glycol outlet
Dry gas outlet
Demister matManhole (0.6 m typical)
Glycol inlet
Liquid distributor 0.4 m typically
0.10 m0.6 m (or 0.3 m for a full dia. top flange)
0.15 D (min 0.15 m)
Dry gas outlet
Structured packing
0.05 D (min 0.15 m)d + 0.02 mWet gas inlet
LAHH
Inlet gas distributor
HC skimmer
Manhole
0.2 D (min 0.3 m)
Drain pipe
Vortex breaker
Liquid outlet
Packed height 2.5 – 4.0 m
typically
min 0.4 m
hc (0.25 m typical)
0.3 m
0.51 m
0.10 md (min 0.3m)
Glycol outlet
Demister mat
Multicycle trays
Chimney trayGas riser
Riser cap
Fig. 8 – Glicol Contacor with Structured Packing (left side) and with Structured Packing and Inlet Separator (right side)
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 31 (90)
Manufacturers shall be required to provide fully dimensioned and detailed drawings of the internals,
including layout and location of gas risers, liquid inlet and, where applicable, drip pipes. A skim line is
required from the bottom section of the contactor to prevent accumulation of condensate. This should be
located such that condensate can be skimmed with glycol at the normal operating level.
A check valve should be installed as close as possible to the injection point of the contactor to prevent a
reversed flow and gas entering the glycol injection line in the event of a pump failure or line rupture.
For design details of the internals refer to PRG.PR.VES.0011 and to PRG.PR.GEN.0003.
• Chimney Tray A chimney tray is required immediately below the contacting section for all configurations except for trays
with an external separator. The chimney collects the wet glycol and, for columns with an integral separator,
it provides a liquid volume for level control. For columns with structured packing this tray also acts as a gas
distributor.
• Contacting Internals The options for the contacting internals include bubble cap trays, valve trays, random packing, structured
packing and multicyclone trays. Since glycols tend to foam, the trays should be spaced at least 450 mm
and preferably 600÷760 mm apart. The use of structured packing is recommended in view of its high
specific gas capacity and low glycol entrainment characteristic, further reduction in column diameter can be
achieved by using multicyclone trays.
A. Bubble Cap Trays Prior to the late eighties, most glycol contactors used bubble cap trays. They have proved effective and
reliable and have good gas and liquid turndown ratios; the latter being limited primarily by the bypassing of
gas. There is industry experience with columns up to 4.2 m in diameter with bubble cap trays.
The gas turndown for bubble caps is superior to that for valve trays; turndown requirements should be
stated in the specification.
They are suitable for viscous liquid also. For gas flow rates below about 40 % of design, the tray efficiency
may decrease. Blinding off several of the bubble caps has been proven as a cheap and effective way of
overcoming problems at high turndown ratios.
For design purposes most manufacturers use tray efficiencies in the range of 25 to 33 %, although actual
efficiency may differ.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 32 (90) Valve Trays Valve trays have about 10 to 15 % greater capacity than bubble cap trays for a given contactor diameter
and, at design gas flow rates, possibly a little higher efficiency. However, due to their design, valve trays
are more prone to weeping, i.e. liquid seepage through the valves. This is not significant at relatively high
liquid rates, but with the low liquid rates normally encountered in glycol contactors, weeping causes the
column to be inefficient unless the glycol rate is maintained at a high level.
B. Random Packing Random (or dumped) packings, consisting of ceramic saddles or pall rings, are not as well suited to glycol
contactors as structured packings or trays due to the high gas-liquid ratios. They were previously used in
preference to bubble cap trays, in small diameter columns less than 450 mm diameter due to easier
installation and lower cost. For further information about turndown, please refer to Supplier data.
C. Structured Packing At low liquid loads such as encountered in glycol contactors, structured packings are superior to random
packing because of their higher specific area and better mass transfer efficiency.
The gas handling capacity of structured packings is about 150 % to 190 % greater than that of bubble cap
trays, which allows smaller diameters and thus cheaper contactors.
A gas turndown ratio of 10 to 1 can be achieved with structured packing, and the glycol losses due to carry-
over of liquid from the contactor are extremely low. The latter can be explained by the liquid film formed on
packing, which is not as easily entrained as the liquid from a droplet bed on a tray.
Structured packings can also be attractive for revamping existing columns in order to increase the gas
handling capacity, to reduce the glycol carry-over from the contactor, or to improve the dew point
suppression capability.
The recommended minimum liquid superficial velocity based on contactor cross-sectional area for
structured packing is 0.5 mm/s to ensure complete wetting of the packing surface area.
Structured packings and their liquid distributors are more sensitive to liquid turndown (please refer to
Supplier data).
Packing, packing supports, hold-down grids and liquid distributor should be designed / verified by a single
entity.
D. Multicyclone Trays Contacting multicyclone trays are a recent development that have been successfully field tested. Vanes are
used to impart a rotation to the gas which forces the liquid droplets to the wall of the tube where they are
collected.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 33 (90) The gas handling capacity of multicyclone trays is of the order of four times that of bubble cap trays. At gas
flows as low as about 30 % of design, contacting occurs with co-current upward flow of glycol and gas in
the multicyclones. A gas turndown to 20 % is achievable in combination with a higher unit circulation rate.
At the lowest gas flows contacting occurs with counter-current (downward) glycol flow. At maximum
capacity glycol losses are extremely low, similar to structured packing.
Multicyclone trays are more expensive than structured packing for the same capacity. However, for typical
operating conditions and capacities of about 5 million m3(st)/day or more, the advantage consists of a
substantial cost savings for the column shell.
• Column Diameter The column diameter is governed by the gas load in the contacting section and is not affected by the glycol
circulation rate.
The design should be based on the operation mode under the severest conditions with the highest value of
the volumetric load factor Q*, defined by:
gl
ggQQ
ρρρ−
=* ; [m3/s] Eq. 1
where ρ
l is the density of the rich glycol leaving the contactor [kg/m3]
ρg is the density of the gas entering into contactor [kg/m3]
Qg is the inlet gas volumetric flow rate [m3/s]
Having identified the most severe loading from the highest value of Q*, it is then necessary to add a margin
to give the value on which the design shall be based. This value, Q*max, should include margins for
inaccuracies in basic data, for operational flexibility.
The column inside diameter may be calculated from:
max
*max4
πλQD = ; [m] Eq. 2
in which λmax is the maximum allowable gas load factor.
A λmax of 0.055 m/s should be used for bubble cap trays at 0.6 m spacing. For smaller tray spacings (TS)
the capacity decreases as: 33.0
max 60.0055.0 ⎟
⎠⎞
⎜⎝⎛=
TSλ ; [m/s] Eq. 3
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
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Sheet 34 (90) With greater tray spacings only a marginal improvement is achieved.
λmax is higher for structured packings and, depending on the type selected, the column diameter required is
a factor of 1.4 to 1.25 smaller than that required for bubble cap trays.
This is a simplified calculation method and final diameter shall be confirmed by trays / structured packing
Manufacturer.
A- TRAYS NUMBER
The number of actual trays required is determined by the number of theoretical trays divided by the tray
efficiency. The height of the contacting section follows from the actual number of trays and the tray spacing.
An adequate space shall be taken into account for demister, distributor, chimney tray,…
B- PACKING HEIGHT
For structured packing the packing height is determined by the number of theoretical trays required for the
process duty and the height of transfer unit given by Vendor, Supplier or Literature. Structured packing
consists of prefabricated elements. The calculated packing height should be rounded up to a multiple of this
height and the total packing height should not exceed 6100 mm or 10 times the column diameter whichever
is smaller without redistribution of glycol in the tower.
• Liquid Distributor A liquid distributor is required when packing or multicyclone are used. It consists of a number of liquid drip
pipes and vapour risers evenly distributed over the column cross-sectional area.
• Pressure Drop The pressure drop over a glycol contactor is the sum of pressure drops over the internals:
- inlet and outlet nozzles;
- demister;
- separation cyclones (if any);
- chimney tray;
- contacting section;
- liquid distributor.
Contactor pressure drop shall take into account the overall system and should not exceed 0.5 bar. The
following equations can be used for the calculation:
Pressure drop over chimney tray (accurate figures should be obtained from tray Manufacturers):
32 10*2 −=Δ ggvP ρ ; [kPa] Eq. 4
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
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Sheet 35 (90)
Pressure drop over bubble cap trays:
ΔP=0.01 bar/tray is typical. Accurate figures should be obtained from tray manufacturers.
For pressure drop over structured packing refer to Manufacturers data.
Pressure drop over the liquid distributor: 32 10*2 −=Δ ggvP ρ ; [kPa] Eq. 5
Actual gas velocity should be calculated through chimney tray and liquid distributor.
4.1.4.4 Glycol Flash Vessel
The glycol flash vessel is used to remove gaseous hydrocarbons that have been absorbed or entrained
with the glycol. It also provides to separate any liquid hydrocarbons from the glycol to prevent them from
entering the reboiler and causing fouling, foaming and flooding.
Both sulphur compounds and carbon dioxide are very soluble in water/glycol mixtures and react to some
degree with glycols. Degassing in the flash vessel before the still column reduces their concentration and
reduces high temperature corrosion. Degassing is more efficient if the rich glycol is first pre-heated in the
top of the still column and in the glycol/glycol exchanger. Pre-heating, to about 60 ÷ 70°C, also reduces
glycol viscosity. However, increased temperatures increase the solubility of liquid hydrocarbons in the
glycol. The pressure in the flash separator shall be sufficient to permit the exist glycol stream to flow
through all downstream equipment, i.e. the heat exchangers and the filters.
The general requirement for glycol vessel pressure is a maximum pressure of 15 % of the contactor
operating pressure. Thus, for 70 bar contactor pressure the flash vessel should operate at a pressure lower
than 10 bar. Flash gas may be routed to flare or to other Plant destination. In addition a blanket gas
connection should be provided to ensure sufficient operating pressure in the glycol flash vessel
It may be appropriate to design the piping and equipment upstream of the flash vessel for the design
pressure of the contactor.
The glycol flash vessel rich glycol LCV should be installed as close as possible to the still feed nozzle to
minimise vaporisation.
The vessel should normally be horizontal and sized for a residence time of 20 minutes. Since liquid
capacity is limiting and gas rates are low, it should usually be designed to operate between about 60% to
80 % full.
In order to separate liquid hydrocarbons from glycol, a bucket or trough and weir design is recommended
due to the simplicity of the controls for the small density difference between the collected hydrocarbons and
the glycol (for details refer to PRG.PR.VES.0001.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
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Sheet 36 (90)
4.1.4.5 Regenerator and Reboiler
The glycol regenerator includes a reboiler to supply heat for the glycol regeneration system and it is sized
for the total heat demand. This is governed primarily by the total glycol circulation rate and the quantity of
water removed, and secondly by the efficiency of heat exchange and system losses. Heat is required for
vaporising the water to regenerate the glycol, for sensible heat lost to the treated gas stream and for
system heat losses.
Data on typical onshore TEG regenerators are given in the following table.
Rating [kW]
23 51 110 150 220 290 370 440 590
Glyol rate [m3/hr]
0.15 0.32 0.68 0.95 1.4 1.9 2.4 2.8 3.8
Reboiler dia & length
[m]
0.4x3.0 0.5x2.7 0.6x3.0 0.76x3.0 0.86x4.0 0.86x4.9 0.92x5.8 1.07x6.1 1.14x6.6
Storage dia & length
[m]
Integral Integral 0.5x3.0 0.6x3.0 0.6x4.0 0.6x4.9 0.76x5.8 0.76x6.1 0.91x6.6
Firebox dia & length
[m]
0.1x1.2 0.15x1.5 0.2x2.7 0.25x2.6 0.3x3.4 0.3x4.3 0.3x5.2 0.4x5.4 0.5x5.8
Still dia & length
[m]
0.15x2.0 0.2x2.1 0.25x2.4 0.3x2.4 0.4x3.0 0.45x3.0 0.45x3.7 0.5x3.7 0.6x3.7
Skid size [m]
1.2x3.7 1.5x3.6 1.7x3.7 1.7x3.7 1.7x4.6 1.7x5.8 2.0x6.7 2.1x7.0 2.3x7.6
Shipping weight [tonnes]
64 107 164 220 310 410 540 640 850
Operating weight [tonnes]
83 144 251 350 490 640 880 1020 1370
Glycol volume [m3]
0.17 0.33 0.80 1.2 1.7 2.1 3.1 3.9 4.7
Table 4 – Data on Typical Onshore TEG Regenerators
• Reboiler The reboiler should have a spill-over column or a weir to prevent the heating coils from becoming exposed.
The spill-over column may be packed to serve as a stripping gas contact column. A separate stripping
column is, however, recommended if stripping gas is required. The reboiler bundle should be easily
removable.
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Sheet 37 (90) HEAT SOURCES
Alternative heat sources for the glycol regenerator are:
- direct gas fired;
- hot oil/steam;
- electricity;
- turbine or engine exhaust;
- furnace flue gas.
Waste heat recovery, e.g. from engine or turbine exhaust with a heating fluid system, is most common in
offshore oil applications and should in all cases be considered as the preferred option for environmental
reasons. Where this is not possible or reasonably practicable, electrical heating is preferred to direct fired
heating for safety.
a) Direct gas fired
Direct fired heaters are most common in onshore applications due to the lower cost. The source of
permanent ignition shall always be installed in a non-hazardous area and fired heaters should be located
as far as practicable away from a hazardous area.
The minimum distance between a fired heater and a process equipment should be 30 m. They are not as
inherently safe as waste heat recovery (except when hot oil is used) or electric heating units and need
additional detection/protection measures. In common with all detection/protection systems, they require
maintenance and adherence to procedures to ensure they work satisfactorily.
b) Hot oil/steam
In some location, such as offshore platform, indirect heating with oil or steam is required by fire code and
prudent practice. A steam condensate trap and strainer with block valves and a bypass should be provided
to drain the steam condensate. The reboiler should be sized as a heat exchanger using the U values.
c) Electricity
A minimum of three heating coils should be used. Over-temperature protection of the heater elements
should be provided by means of at least two thermocouple elements clamped or welded to the heater
sheath, located in an area of highest anticipated sheath temperature. Solid state proportional current
control on part of the load is preferred to on-off control.
d) Turbine or engine exhaust
The use of exhaust gases form gas turbines and engines as the heating medium achieves substantial
energy savings.
e) Furnace flue gas
For onshore locations, all flash gas and condensate may be incinerated in a furnace and the flue gas used
as a reboiler heat source. This may be the most favourable alternative when it is necessary to dispose of all
waste hydrocarbon streams.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
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Sheet 38 (90)
It is generally environmentally favourable to maximise the lean glycol concentration by using high reboiler
temperatures since this will minimise the circulation rate or the volume of stripping gas required.
HEAT DUTY
The following maximum flux rates should not be exceeded at normal operating conditions:
Electric 12.5 kW/m2
Direct fired 19 kW/m2
Steam 24 kW/m2
Hot oil 24 kW/m2
For an example refer to Appendix II.
The reboiler should typically be sized in compliance with PRG.PR.HEB.0001.
• Still Column The column shall be located on top of the reboiler with a flanged connection.
A direct acting PSV set at the design pressure of the regenerator should be installed on the vapour line
between the still column and overhead condenser. The vapour line should be able to withstand full vacuum.
Trays, structured packing and random packing may be used in still columns. Random packing is
recommended in case of the small column size.
With the larger surface areas recommended on the lean/rich glycol heat exchangers, some still feed
vaporisation will occur. Feed piping to the still column should be designed for this two-phase flow to prevent
surging which could upset the still operation, possibly causing flooding and glycol carry-over.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
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Sheet 39 (90) DESIGN METHODS
Column design for random packings, such as pall rings, is typically based on curves in Fig. 9.
Fig. 9 – Pressure Drop and Flooding Correlation
These curves give the generalised pressure drop correlation for packed column design.
Packed columns are usually designed for 60 to 85 % of the flood point. However, for glycol units, care
should be exercised in determining the appropriate mass flow rates, ml and mg. Due to the unsteady flows
encountered in glycol units together with potential for overloads at start-up, still columns are usually
designed at the lower end of the range, or about 60 % of the flood point. Columns designed in this manner
are consistent with the sizes recommended by Manufacturers for onshore applications, see
Table 5.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
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Sheet 40 (90) Since regenerators usually operate below the design rate, this conservative design will result in a low
velocity and a reduction of column efficiency. The wide difference between the boiling points of glycol and
water makes separation easy even with a relatively short column, with a minimum of reflux and with
reduced packing efficiencies.
PACKING TYPES
Table 5 gives recommended pall ring sizes and associated data for a range of still column diameters.
Column Diameter
[mm]
Recommended pall ring size
[mm]
Packing Factor
[m-1]
Density
[kg/m3]
HETP
[m]
<250 16 230 593 0.30 to 0.45
<300 25 157 480 0.38 to 0.53
<750 38 92 415 0.45 to 0.75
>750 51 66 385 0.6 to 0.9
Table 5 – Pall Ring Data for Still Column
COLUMN DIAMETER
Fig. 9 can be used to determine the allowable mass flowrate per unit area, G.
The following equation can be used to determine the required diameter of packing: 5.04
⎥⎦
⎤⎢⎣
⎡=
GFFm
D g
π; [m] Eq. 6
where: mg = gas mass flow rate [kg/s]
G = allowable flow rate per unit area [kg/(s.m2)]
D = internal diameter of packed column [m]
FF = fraction of flood. Take as 0.6.
The size of the still column is pre-determined by most manufacturers of packaged systems to match the
glycol reconcentration capacity of the reboiler. A still column cross-sectional area of typically 0.55 x 10-3 m2
should be supplied for every kW of reboiler heat duty.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
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Sheet 41 (90) COLUMN HEIGHT
The large difference between the boiling points of water (100°C) and TEG (288°C) allows a sharp
separation to be accomplished in the still with a relatively short column and with a minimum reflux.
Fig. 10 shows a typical McCabe-Thiele diagram for the reboiler and still column. Due to the wide difference
between the equilibrium line and the operating line, three theoretical trays are sufficient to effect the
separation.
Fig. 10 - McCabe - Thiele diagram for glycol reboiler and still column
Since the reboiler and condenser are each one theoretical stage, only one stage of packing is required.
The height of packing for a theoretical tray (HETP) ranges from 0.3 to 0.9 m for operation in normal loading
ranges, although it can be much higher with low loading or poor liquid distribution. A total of 1.8 m of
packing is recommended, 0.6 m below the feed point and 1.2 m above. Although this is effectively two
stages, the cost of the additional packing is small and provides a safety margin. A stage can be needed
above or below the feed point, depending on extremes of rich glycol water content and feed temperature.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
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Sheet 42 (90) Poor packing performance is also expected above the feed point because of low liquid loading and the
extra packing height helps to minimise glycol losses.
Random packing requires a simple liquid distributor which should be supplied by the same Manufacturer as
that of the packing.
• Reflux Reflux is provided at the top of the still column to effect rectification of the vapours and to minimise glycol
losses. This reflux is normally supplied by using the rich glycol stream circulating through a condensing coil
inserted in the top of the column. Cooling water, an air-cooled finned section at the top of the column, or an
external water source (such as de-mineralised water or steam condensate) could alternatively be used.
The rich glycol stream is the preferred method for cost and simplicity of operation and energy efficiency. By
not using the rich glycol stream as the condenser coolant the sensible heat duty of the reboiler is increased.
The advantage of increased reflux ratios is to reduce the number of theoretical trays required for a given
level of separation. Fig. 10 also shows the effect of reflux on the still column height. The slope L/V of the
operating line is termed the "internal reflux ratio", where "L" and "V" are the liquid and vapour phase rates,
respectively. The fraction or percentage of the vapour which is reflux, "R", is termed the "external reflux
ratio". L/V and R are related by the equation:
RR
VL
+=
1 Eq. 7
The figure shows that increasing the external reflux ratio from 15 to 50 %, the number of theoretical stages
required decreases only slightly. Only minimal amounts of reflux are therefore required for glycol still
columns.
Fig. 11 shows the minimum reflux rate required to control glycol losses as a function of feed temperature.
The slope increases with the feed temperature due to the increasing amount of glycol in the overhead
vapours. For a still column with a large number of theoretical trays, the reflux rate shown would be
adequate.
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PRG.PR.GAS.0001
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Sheet 43 (90)
Fig. 11 – Minimum Reflux Required to Control TEG Losses
When the maximum still feed temperature is 165°C and lower, the recommended design external reflux
ratio is 15 %. As a example, for a 35°C inlet gas temperature and a UCR of 25 litres/kg, the temperature of
the combined stream after the reflux condenser will be about 40°C for 15 % reflux and 50°C for 50 % reflux.
The temperature of the vapours leaving the reflux condenser will be between 93 and 100°C depending on
the amount of stripping gas and vaporised hydrocarbons in the vapours. The presence of gas lowers the
partial pressure of the water vapour to less than atmospheric pressure thereby lowering the boiling point of
the water. Vapour temperatures above 100°C indicate that the vapours contain glycol and that additional
reflux should be provided to reduce the glycol losses.
Since the difference between the condensing and feed temperatures is large, reflux may be effectively
controlled by controlling the temperature at a point halfway between the feed and the top of the packing
above the feed point (see Fig. 1). By adjusting the bypass to the reflux coil, the amount of reflux can be
easily and accurately controlled; 10 % of the total glycol flow to the coils is nominally sufficient, with 20 %
recommended for design. If the temperature at the control point is between 110 and 125°C, then the
minimum amount of reflux is assured.
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PRG.PR.GAS.0001
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Sheet 44 (90)
• Reboiler The following method may be used to size indirectly heated reboiler heat exchange bundles.
1) Calculate the MTD assuming a constant reboiler temperature, normally between
190°C and 204°C for calculating both lower and higher temperature differences between the
heating medium and the glycol.
2) Use a U-value of 340 W/m2.K for hot oil or 425 W/m2.K for steam to calculate the required
surface area. The outside film coefficient is controlling.
3) Table 6 indicates the surface area per length of bundle with 19 mm tubes.
Diameter
[mm]
Surface Area per
length of bundle
[m2/m]
300 4.8
350 6.0
400 7.7
450 10.3
500 13.2
600 19.1
Table 6 – Surface Areas – Steam or Hot Oil Heated Bundles
For hot oil tubes, the tube side velocity shall be a minimum of 1.2 m/s.
4) The shell diameter should be approximately 350 to 450 mm larger than the tube bundle to
provide adequate vapour disengagement area.
5) The reboiler should be designed for a minimum internal pressure of 0.1 barg of or full of water,
whichever is greater.
For details refer also to PRG.PR.HEB.0001
4.1.4.6 Surge Drum
The surge drum should be sized to provide the following:
- handle start up, normal operation and shut down fluctuations due to drain-down of TEG from the
contactor.
- a retention time not less than 20 minutes between low and high levels, based on the design circulation
rate;
- a reasonable length of time between glycol additions;
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Sheet 45 (90) - sufficient volume to accept the glycol drained from the reboiler to allow repair or inspection of the heating
coil.
In case no stripping gas is provided, the surge drum could be conveniently vented to the reboiler but a
small amount (typically 0.4 m3/h) of nitrogen or dry fuel gas purge is needed to prevent water vapour from
the reboiler flowing through the vent line and being absorbed by the glycol in the surge drum. If it is not
located directly below the reboiler, it should be provided with a separate purge gas supply.
Provisions should be made for:
- make-up of the glycol inventory;
- batch addition of chemicals to the glycol surge drums, e.g. for pH control, corrosion inhibition, etc.
In addition, the glycol surge drum elevation shall always supply adequate liquid head to the glycol
circulation pump.
4.1.4.7 Heat Exchangers
For general design details refer to PRG.PR.HEB.0001.
Heat exchangers in glycol dehydration systems have low overall heat transfer coefficient (U) values
because glycol is a poor heat transfer fluid. Fouling from glycol contaminants and degradation products
may also be severe. Table 7 gives the range of values for the exchangers discussed herein.
Recommended
Fouling Factors [m2K/W] Type of
Exchanger
Range of Uvalues
[W/m2K] Shell Tube Overall U [W/m2K]
Glycol-Glycol (double pipe type)
Finned Bare
45 to 57 110 to 140
0.0004 0.0004
0.0004 0.0004
50 125
Reflux Coil
110 to 230
0.0002
0.0004
170
Reboiler
Steam Hot Oil
400 to 510340 to 400
0.0004 0.0004
0.0002 0.0002
425 340
(double pipe type) Gas-Glycol
Finned Bare
51 to 57 170 to 200
0.0004 0.0004
0.0002 0.0002
50 180
Table 7 – Typical Values for Heat Exchangers TEG Service
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Sheet 46 (90)
• Glycol-Glycol Exchangers The recommended design criterion for lean-rich glycol heat exchangers is to cool the lean glycol to a
maximum temperature of 90°C. As shown in Fig. 1, two heat exchangers may be used, with the rich glycol
being heated to about 70°C before the glycol flash vessel, then heated to between 150°C and 165°C before
feeding the still column. The choice of temperatures then becomes an economic decision based on the
availability and cost of process heat and the cost of larger glycol/glycol exchangers. The duty of the
glycol/glycol exchanger upstream of the glycol flash vessel will be affected by solution gas, but this will be
typically less than 4 % assuming 50 m3(st)/m3 solution gas and a UCR of 25 litres/kg. The lean glycol
should flow through the shell side of the heat exchangers where the pressure drop is small.
Double pipe or hair pin type, with bare or finned tubes, or plate-type heat exchangers may be used. The
latter are increasingly used, especially for offshore or large systems, because they are more compact,
lighter, cheaper, less susceptible to fouling and easier to clean. A new type of exchanger named
Compablock® are now available in the market and its application needs to be deeply analyzed with
Supplier due to sensitivity to fouling. Some incidents of gasket leakage in plate-type units have been traced
to carry-over of hydrocarbon condensate (due to poor system design) and flashing in the heat exchanger. It
should be recognised that gasket leakage also occurs due to high pressure shock loads seen during gas
blow-by.
• Lean Glycol Cooler A lean glycol cooler is required so that the lean glycol entering the top of the contactor is cooled to 3 ÷ 6°C
higher than gas temperature entering the contactor. The lean glycol may be cooled by either an air-cooled
heat exchanger, a water-cooled heat exchanger or a lean glycol-gas heat exchanger. Although air-cooled
glycol coolers risk under-cooling the lean glycol they are recommended since control of the glycol
temperature is then independent of the gas flow. This provides greater operational flexibility, e.g. during
start-up or gas turndown, and avoids the risk of excessive contactor temperatures.
Failure to cool the glycol makes the top tray act as a heat exchanger. The temperature of the glycol on this
tray is higher, which increases the partial pressure of the water vapour and thus reduces drying efficiency.
4.1.4.8 Condensed Overheads Separator
As shown in Fig. 1, a condenser and three-phase separator should be installed in the vapour line from the
regenerator still column. The vapour and contaminated water are waste streams, hydrocarbon emissions or
flaring/incineration are reduced by the amount of produced condensate. The 3-phase separator can be
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
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Sheet 47 (90) similar in design to the glycol flash vessel. Similarly to the overhead condenser, pressure losses should be
minimised. Further processing of the outlet streams should also be considered with regard to
environmental issues.
4.1.4.9 Filters
The solid content in the glycol should be kept below an acceptable value to prevent pump wear, plugging of
heat exchangers, foaming, fouling of contactor trays and still packing, cell corrosion (where the solids settle
on metal surfaces), and hot spots on fire tubes.
Filters should be located in a common area in order to limit areas subject to spillage/pollution. They should
be designed for rapid, simple and safe filter element replacement. Space around the filters should ensure
accessibility for easy maintenance and the handling of fouled filter elements. Filter elements shorter than
0.5 m are suggested to avoid crack formation by expansion. The high viscosity of glycol at low
temperatures should be taken into account in the design of the glycol filters. The design should be such
that no gas can be trapped in the filter cover.
A quick-closure lid with pressure safety device should be used. The lid should be easily removable by
means of a rotating davit or a hinge with counterweight.
Block, vent and drain valves should be provided for each filter. Vent and drain connections should be large
enough to allow water flushing before opening for cleaning and maintenance.
The following steps should be taken if the feed gas contains mercury:
- replaceable filter elements should be non-metallic to reduce the problem of disposal in case they become
contaminated;
- special provisions should be made for storage and disposal of used filter elements;
- the possibility of mercury accumulation should be borne in mind for all connections to the filter. Drainage
points are preferred to be vertical. In addition, since mercury migrates through gaskets, suitable drip pans
or pits should be constructed under the filters with a low point in which the mercury can be collected and
sucked up with a vacuum truck.
A) PARTICLE FILTERS
Sock or pleated paper cartridge type glycol filters should be used to remove 95 % of solid impurities > 5 μm
in size before they cause fouling, foaming or plugging. Two full flow filters with change-over valves and no
by-pass lines should be installed in parallel to permit continuous operation during change-out. The filters
are installed in the rich glycol line downstream of the glycol-flash vessel. In addition a single full flow filter
may be installed downstream of the glycol pump to intercept particles that may originate in the reboiler and
eliminate the need for a specific filter upstream of any liquid distributor in the contactor.
Frequent filter changes may be needed during start-up or when neutralisers are added to the system to
control the glycol pH. 25 μm cartridges can be used for initial start-up, then changed to 5 μm cartridges.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
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Sheet 48 (90) The filters should be depth type filter elements, since these have a longer service life and greater efficiency
than barrier filters. If the glycol is contaminated with condensate, this will dissolve the impregnating agent,
which reduces the life of the surface filters. The filters should have a holding capacity of at least 20 kg, but
preferably 50 kg. Alternatively, two filters in series may be specified, the first a cleanable filter down to 50
μm followed by a 5 μm filter. The coarse filter will prolong the life of the fine filter, possibly improving
lifecycle cost.
The maximum pressure drop across the filter should be limited to 170 kPa. The filter elements should be
designed for a pressure difference of at least 2 bar. The flow through the filter should not exceed 44
litre/min/m2 based on the external area of the filter elements. Each filter shall be sized for 110% of the
design flow, in compliance with the Glycol Circulation Pumps overdesign margin.
B) ACTIVATED CARBON FILTERS
An activated carbon filter can effectively remove most foam promoting compounds such as well-treating
chemicals, compressor oils and other troublesome organic impurities in the glycol. It should be installed
downstream of the particle filters in the rich glycol line. Sidestream filtration with not less than 10 % - 20%
flow can be used for higher glycol flow rates. For low glycol flowrate it could be also foreseen a full flow
carbon filter. This flowrate should be verifiable during normal system operation. Sidestream filtration may
be achieved with a three-way control valve equipped with a mechanical stop for the design flow rate.
Only one carbon filter with block valves and a bypass should be installed, since it will not harm the glycol
system to take the filter out of service to change the elements. Carbon filters are usually sized to give a
flowrate of 2.5 to 5.0 m3/h per m2 of filter cross sectional area.
The method for determining the need to change the activated carbon filters differs from that of particle
filters. Due to the method of adsorption in the carbon elements, the differential pressure will remain
constant even though the filters are no longer active. The elements should thus be changed when the
clarity of the glycol deteriorates or when the glycol analysis indicates a high hydrocarbon content.
If the feed gas contains mercury, it may also be necessary to install a sulphur-impregnated activated
carbon filter in the vapour outlet line downstream of the condensed overheads separator.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
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Sheet 49 (90)
4.1.4.10 Glycol Circulation Pumps
The glycol recirculation pumps, which represent the only moving part in the entire unit, return the low
pressure lean glycol to the high pressure contactor.
Depending on Plant operating philosophy, Glycol pumps could be classified as essential duty and
continuous operation.
Glycol circulation pumps are usually reciprocating type. The pumps shall be sized to provide a minimum of
10% excess capacity because the circulation rate is a key parameter to ensure the unit performances. Two
glycol pumps should be provided in parallel: one running and one standby.
A start-up strainer should be installed in the glycol line upstream from each pump. The strainer should be
easily removable for cleaning.
For other details of pump design refer to PRG.PR.MAC.0001.
4.1.4.11 Glycol StorageTank
Glycol storage tank is provided for the make-up of glycol losses in the circuit. It consists of an atmospheric
tank whose pressure is maintained by means of a inert gas (e.g. nitrogen) blanketing system that prevents
glycol from oxidation due to possible air infiltration. Off gas is routed to safe location or flare.
Make-up glycol is filtered and then pumped into glycol surge drum by using reciprocating or rotative pumps.
Such operation is discontinuous and locally controlled by operator. A typical design of glycol storage
system is shown in Fig. 12.
N PC
At safe location
Truck unloading connection
to TEG surge drum
Fig. 12 – TEG storage process flow diagram
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
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Sheet 50 (90)
4.1.4.12 Glycol Sump Vessel
A vessel is provided in order to collect the drains of glycol unit for reprocessing or disposal. Drainages are
usually gravity flow lines taking from the bottom of the equipment, so glycol sump is located underground.
Operating pressure of glycol sump is maintained at about atmospheric value by means of a pressure
control valve and a inert gas blanketing system, e.g. nitrogen, that prevents glycol from oxidation. Off gas is
routed to flare or safe location. Such system could be used also to pressurize and therefore empty the
vessel. Alternatively, sump can be emptied by using self-priming or submerged pumps. When glycol is
recovered from sump and then reprocessed, it shall be filtered before recycling into the circuit; otherwise,
contaminated glycol is loaded into a truck and sent for disposal.
Pressure relief valve installed on glycol sump, shall be sized in order to protect itself against, as minimum,
the following overpressure scenarios: pressure control valve to flare or safe location blocked outlet,
pressure control valve inlet blow-by and inlet blow-by from high pressure upstream equipment, due to
failure during drainage operation. A typical design of glycol sump is shown in Fig. 13.
N PC
grade grade
TEG sewer underground
header
Vacuum tanker connection
TEG make-up
flare
M
Fig. 13 - TEG sump process flow diagram
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
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Sheet 51 (90)
4.1.4.13 Enhanced Regeneration Equipment Design
As introduced in par. 4.1.2.3, a more detailed description of enhanced regeneration equipment design will
be given in this paragraph.
• COLDFINGER Equipment Application of the Coldfinger process requires an additional pump, with a spare, to recycle glycol from the
Coldfinger collection in the surge drum to the still column feed (Fig. 3). Changes to the following equipment
are also required:
- still column;
- reboiler;
- surge drum.
The loading of the still column and the duty of the reboiler are increased by the recycle stream from the
surge drum. The surge drum needs to be well insulated to maintain a temperature close to that of the
reboiler. It must also accommodate the Coldfinger heat-exchanger bundle and liquid collection trough,
which occupies space that is not available for liquid surge.
• Gas Stripping Equipment Stripping gas cannot be condensed from the still overheads and it also reduces the amount of
hydrocarbons recovered in the condensed overheads separator. "End-of-pipe" facilities, e.g. incinerators,
shall have sufficient capacity for the stripping gas in addition to vapours from the reboiler and glycol flash
vessel.
Normally fuel gas is used as stripping gas. To prevent accumulation and freezing of liquid, there should be
no low points in the stripping gas line to the surge drum. There should be adequate facilities incorporated in
the stripping gas line upstream of the surge drum to prevent liquid from the reboiler entering the stripping
gas line.
A heater may be required upstream of the stripping column to ensure that the stripping gas is dry. The need
and duty of this heater depend on the gas composition and the ambient temperature.
A) GAS RATE
Using counter-current stripping in a packed column, the quantity of stripping gas required to achieve a
given concentration can be reduced by a factor of five or more compared with injecting gas into the reboiler.
Fig. 14 shows the effect of counter-current stripping on glycol concentration.
A 10% overdesign margin should be kept on the minimum amount of stripping gas because the gas flow
rate is a key parameter for the unit performances. A maximum design rate of 40 m3(st)/m3 should not be
exceeded, to prevent the possibility of column flooding.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
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Sheet 52 (90) A measure device should be used for measuring the flow of stripping gas.
Fig. 14 – Glycol Concentration vs. Stripping Gas Rate
B) STRIPPING COLUMN
The stripping column allows the stripping gas to make intimate contact with the lean glycol and it is installed
between the reboiler and the surge drum. The stripping gas thus strips the water remaining in the lean
glycol and is not diluted by the water previously removed in the reboiler.
The size of the column can be calculated as for other packed columns and in compliance with
PRG.PR.VES.0011. However, since the design conditions are always the same, the cross-sectional area of
the stripping column can be found from Fig. 15.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
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Sheet 53 (90)
Fig. 15 – Stripping Column Size
The height of the stripping column is governed by the same variables as the height of other columns; i.e.
the taller the packing, the more theoretical trays available.
Due to the low gas rates involved and the nature of stripping/absorbing, a greater packing height per
theoretical tray is required than in the still column: the values in stripping columns are typically 0.6 to 0.9 m
per theoretical tray.
• Azeotropic Stripping (DRIZO) Equipment The DRIZO process (Fig. 3) requires additional equipment and modifications to the standard ones, as listed
below:
- still column;
- stripping column;
- still overheads condenser and separator;
- glycol flash vessel overheads condenser and separator (optional extra);
- solvent pumps (extra);
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
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Sheet 54 (90) - solvent heater (extra).
One simplification is the elimination of activated carbon filters in case BTEX components are present in the
feed gas due to the high affinity between the solvent and the BTEX. BTEX components would flood the
carbon bed making them ineffective in removing degradation products.
The DRIZO process may lead to an approximate doubling of the gas stream in the reboiler and a higher
concentration of BTEXs in the waste water stream.
The still column is similar to the traditional one but allowance is required for the solvent loading. A demister
mat may be installed above the reflux condenser to reduce entrainment losses. This causes an increase in
backpressure, but can probably be accepted in view of the higher glycol concentration.
The stripping column is similar the traditional one but takes account of the different properties and flow
rates.
The Still Overheads Condenser and Separator should be similar in design to those already described. The
quantity of condensed hydrocarbon will, however, be greater due to the recovery of condensate from the
glycol flash vessel and condensation of additional BTEX components due to absorption by the solvent.
Glycol Flash Overheads Condenser and Solvent Recovery Drum may be placed downstream of the glycol
flash vessel vapour outlet to recover C5+ components which reduces solvent losses. The condensed liquid
stream flows to the still column and is added to the solvent extraction process.
Two full-flow solvent pumps are required to overcome the backpressure of the solvent heaters and
stripping column.
A heat exchanger using lean glycol leaving the surge drum heats the solvent downstream of the solvent
pump. This is followed by an electric heater upstream of the stripping column to superheat the solvent to
above 150°C.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
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Sheet 55 (90)
4.1.5 Materials
All materials indicated below are the minimum requirement and are to be evaluated / confirmed during
detailed design considering project peculiarity involving also material specialists.
The contactor shell should be made of carbon steel with a minimum corrosion allowance of 3 mm. A 3 mm
thick AISI 316L stainless steel cladding should be applied below the bottom tray when bubble caps are
used or below the chimney tray when structured packing or multicyclones are used. All internals should be
AISI 316L stainless steel.
Material for the regeneration system where rich glycol is present should be carbon steel with AISI 316L
cladding or solid stainless steel AISI 316L depending on economical evaluation.
Material for regeneration system where lean glycol is present is normally carbon steel with 3 mm corrosion
allowance.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
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Sheet 56 (90)
4.1.6 Operation and Maintenance
This section describes some of the most important operating and maintenance aspects of glycol
dehydration process including generalised start-up and shutdown procedures.
4.1.6.1 Oxidation
Degradation is a natural occurrence and it is accelerated in the presence of sulphur compounds. Oxygen
may enter a system with the inlet gas, through storage tanks and sumps without inert-gas blankets, through
the pump packing glands or via spilled or leaked glycol recovered from the drain base plates. Glycols will
oxidise readily in the presence of oxygen and form corrosive organic acids. Recovered glycol, which is old
or dirty or has been exposed to oxygen, should not be returned to the glycol systems. Oxidation inhibitors
may be used to prevent corrosion if oxygen contamination cannot be avoided. Degradation products
contribute to foaming but they also are major sources of corrosion problems; the answer is effective
filtration of circulating glycol.
4.1.6.2 Ph Control
Solutions in a glycol unit that is not downstream of an amine unit become acidic and corrosive, especially
when the inlet gas contains H2S or CO2. Glycols are very reactive with sulphur compounds. The resultant
materials tend to polymerize and form a product which is very corrosive. Also the pH becomes lower.
The metallic corrosion rate increases rapidly with a decrease in the glycol pH. Organic acids resulting from
the oxidation of glycol and thermal decomposition products or acid gases absorbed from the gas stream
are the most troublesome corrosive compounds. The glycol pH should be regularly checked and kept
between 6.5 and 8.0 by neutralising the acidic compounds with ethanolamines, soda ash or other alkaline
chemicals.
About 100 g of methylethanolamine (MEA) per m3 of solution is usually sufficient to raise the pH from 6.5 to
a safe level of 7.3. Maintaining the pH around 7.3 may, however, require excessive addition of neutraliser.
It is generally better to allow the pH to swing between the control limits. When the glycol pH is extremely
low, the amount of neutraliser required can be determined by laboratory analysis. The pH should not raised
above 8 to 8.5 because the glycol solution will tend to foam and emulsify more easily. An overdose of
neutraliser will also precipitate a blank sludge that will be suspended in the glycol solution. There is a risk of
over-dosing when using MEA, leading to a pH of greater than 10 causing cracking due to stress corrosion.
Frequent filter changes are also required when pH buffers are being added.
If CO2 is present in the gas stream, sodium acetate or another base should be used instead of MEA to
avoid reaction of CO2 with MEA.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
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Sheet 57 (90)
4.1.6.3 Salt Contamination
Salt over carry-over, either as slugs or fine mist, should be prevented by the use of an efficient separator
upstream of the glycol contactor.
Salt deposits on the reboiler fire tube cause hot spots, fire-tube plugging and hence failures.
Salt deposits accelerate equipment corrosion, reduce heat transfer in the reboiler tubes and affect specific
gravity reading when a hydrometer is used to determine glycol-water concentrations. Troublesome salts
are not removed with normal regeneration and accumulate in the glycol circuit.
When it is not possible to remove all the salt water from the gas stream from the feed gas to the Contactor,
the removal of salt from the glycol is necessary. Several methods are available for salt removal from glycol
such as vacuum distillation, electrodialysis or ion-exchange resin bed.
4.1.6.4 Hydrocarbon Contamination
Liquid hydrocarbons, resulting from carry-over with the inlet gas or from condensation in the contactor,
increase glycol foaming, degradation and losses. Carry-over can be prevented by maintaining proper levels
in the inlet separator and by keeping mist mats clean. If hydrocarbons enter into the contactor, they should
be removed in the glycol flash vessel or in the activated carbon filter.
4.1.6.5 Sludge Accumulation
Dust, sand, pipeline scale, reservoir fines, and corrosion products such as iron sulphide and rust are picked
up by the glycol if not removed by the inlet separator. These solids, together with tarry hydrocarbons,
eventually settle out and form an abrasive, sticky, black gum which can erode the glycol pump and other
equipment, plug the contactor trays and the packing in the stripper, and deposit on the fire tubes.
4.1.6.6 Foaming
Foaming may be mechanical or chemical. Mechanical foaming is caused by excessively-high gas flow rates
in the Contactor, while contaminants such as solid particles, salts, corrosion inhibitors, and liquid
hydrocarbon cause chemical foaming. Foaming increases glycol losses and reduces the capacity of the
system. Entrained glycol will carry over the top of the contactor with the dry gas if a stable foam builds up
on the trays. Foaming also causes poor contact between the gas and the glycol solution, which in turn
leads to a poorer dew point depression.
Foaming is best detected by monitoring the pressure drop across the contactor. Erratic readings followed
by a rapid increase in pressure are symptoms of foaming. Chemical foaming can be detected by bubbling
air through a sample of glycol for five minutes and observing the resulting foam height and stability.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
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Sheet 58 (90) The most satisfactory cure for foaming problems is proper care of the glycol solution. The most important
measures to achieve this are effective gas cleaning ahead of the glycol system, a proper filtration of the
circulating solution and by raising the temperature of the gas and the glycol above the dew point of the
condensed hydrocarbon. The use of anti-foaming agents (AF) will not solve the underlying problem. The
use of AF should be regarded as a temporary control until the foam promoters can be determined and
eliminated.
The success of the AF is usually dependent upon when and how it is added. Some AFs, when added after
the foam is generated, act as good inhibitors, but when added before foam generation act as good foam
stabilisers, which makes the problem worse. Most AFs are deactivated within a few hours under high
temperature and pressure conditions and their effectiveness can be dissipated by the heat of the glycol
solution. AFs should generally be added continuously, a drop a time, for best results. The use of a chemical
feed pump will help meter the AF accurately and give better dispersion into the glycol solution. Water-
soluble AFs are sometimes made more effective by diluting them before addition into the system. AFs with
limited solubility should be added via the pump suction to ensure good dispersion in the glycol solution. If
foaming is not a serious problem, the AF may be added in slugs of about 100g when needed. The addition
of too much AF is usually worse than no AF at all. Excessive amounts sharply increase the foaming
problem.
4.1.6.7 Acid gas solubilities and stripping
CO2 and/or H2S, when present in significant quantities, have the following effects:
- they increase the saturation water content of natural gas.
- they readily dissolve in glycol, reducing its pH and promoting corrosion.
- CO2 and H2S react with MEA, an acid neutraliser.
For high CO2 and/or H2S contents, it may be advantageous to employ a stripper upstream of the glycol
flash vessel. This approach can reduce corrosion, the flare SO2 level and the size of the glycol reboiler and
still column.
4.1.6.8 Mercury in Feed gas
The presence of mercury in the feed gas will lead to contamination of the entire glycol system.
Used glycol and filters should be stored in a safe, confined area and disposed of in an environmentally
acceptable manner. There are also consequences for the design of the filters themselves.
The overhead condenser and separator reduces the mercury content of the residual vapour but may not
eliminate the need for an activated carbon filter impregnated with sulphur for this stream.
There is also the possibility of mercury induced stress cracking in small bore pipelines, hence precautions
should be taken in the material selection, especially for instrument connections.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
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Sheet 59 (90)
4.2 Molecular Sieves Dehydration
4.2.1 General
The use of solid desiccant is tipically limited to applications such as high H2S content gases, very low water
dewpoint requirements, simultaneous control of water and hydrocarbon dewpoints, and special cases such
as oxygen containing gases. In processes where cryogenic temperatures are encountered, solid dessicant
dehydration is usually preferred over conventional methanol injection to prevent hydrate and ice formation.
Solid desiccant units are generally more expensive to buy and to operate than glycol units.
Commercial desiccants are summarized in three categories:
• Gels – Alumina or silica gels manufactured and conditioned to have an affinity for water.
• Alumina – A manufactured or natural occurring form of aluminium oxide that is activated by
heating.
• Molecular Sieves – Manufactured or naturally occurring alumino-silicates exhibiting a degree of
selectivity, based on crystalline structure, in the adsorption of natural gas constituents.
In particular Silica Gel is a generic name for a gel manufactured from sulphuric acid and sodium silicate. It
is essentially pure silicon dioxide, SiO2. It is used for gas and liquid dehydration and hydrocarbon recovery
from natural gas. When used for dehydration, silica gel will give outlet dewpoints of approximately -50°C .
Alumina is a hydrated form of alumina oxide (Al2O3). It is used for gas and liquid dehydration and will give
outlet dewpoints of about -68°C. Less heat is required to regenerate alumina and silica gel than for
molecular sieve and the regeneration temperature is lower. Particular attention should be taken when
drying sour gas because Al2O3 will catalyze the H2S + CO2 reaction and form so much COS + H2O that the
process would be unable to dry the gas. Alumina can be a prolific producer of COS.
Molecular Sieves are a class of aluminosilicates. They produce the lowest water dewpoints, and can be
used to simultaneously sweet and dry gases and liquids. Their equilibrium water capacity is much less
dependent on adsorption temperature and relative humidity. They are usually more expensive and the
common crystalline forms used in commercial adsorption are synthetically manufactured materials.
For comparison Fig. 16 presents the important properties of commercial solid dessicants. Fig. 17 shows
static equilibrium capacity vs. relative humidity for various new dessicants.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
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Sheet 60 (90)
Dessicant Shape Bulk density [kg/m3] Particle size Heat capacity
[kJ/kg°k]
Approx, minimum moisture Content of
Effluent Gas [mm/kg]
Activated Alumina Alcoa F200 Beads 770 7x14 Tyler mesh or 3.2 mm / 4.8mm /
6.4mm
1.00 -68°C dew point
Activated Alumina UOP A-201 Beads 735 3-6 mesh or 5-8 mesh
0.92 5-10 ppmv
Mole Sieve Grace – Davison 4A Beads 675-720 4-8 mesh or 8-12 mesh
0.96 0.1 ppmv (-101°C)
Molecular Sieve UOP 4A-DG Extrudate 640-705 3.2 / 1.6 mm pellets 1.00 0.1 ppmv Molecular Sieve Zeochem 4A Beads 720-735 4-8 mesh or 8-12
mesh 1.00 0.1 ppmv
Silica Gel Sorbead® -R Beads 785 5x8 mesh 1.05 -51°C dew point Silica Gel Sorbead® -H Beads 720 5x8 mesh 1.05 -51°C dew point Silica Gel Sorbead® -WS Beads 720 5x8 mesh 1.00 -51°C dew point
Fig. 16 – Typical Dessicant Properties
Fig. 17 – Static Equilibrium Capacity vs. Relative Humidity for Selected Solid Desiccants
The Molecular Sieves activated crystalline material is porous. The pore openings in a given structure are all
exactly the same size and are determined by the molecular structure of the crystal and the size of
molecules present in the crystal. The pores are formed by driving off water of crystallization that is present
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
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Sheet 61 (90) during the synthesis process. The exactness of the pore size and distribution has given rise to the name
molecular sieves, which is used almost universally to describe these materials.
Molecular Sieves have the large surface area typical of any solid adsorbent. In addition, however, they
have highly localized polar charges and high adsorptive selectivity for polar and un-saturated compounds.
These localized charges are the reason for the very strong adsorption of polar or polarisable compounds
on molecular sieves. This also results in much higher adsorptive capacities for these materials by
molecular sieves than by other adsorbents, particularly in the lower concentration ranges.
Molecular Sieves in the gas industry vary in pore size from 3-10 Å, and are generally found as the following
types:
• 3A – Potassium variation of sodium aluminosilicate, used for water removal.
• 4A – Sodium aluminosilicate, used for removal of water, CO2, incidental H2S removal.
• 5A – Calcium variation of sodium aluminosilicate, used for water, H2S and light mercaptans
removal
• 13X – Sodium aluminosilicate with X crystal structure, used for water and mercaptan removal.
Table 8 shows that in most industrial applications molecular sieves are used for the removal of water,
hydrogen sulphide, organic sulphides, and carbon dioxide (either singly or in combination) from fluids.
Data included in the table shall be considered as a preliminary indication and Molecular Sieves type and life
time shall be confirmed by a specialized Vendor.
Application Molecular sieves
Industry Type Phase Type Estimated life [years]
Petroleum Super Drying Kerosene/Jet fuel Liquid 4A 4 Alkylation feed Liquid 4A 3 Reformer recycle gas Gas 4A 4 Hydrocracker gas Gas 4A 4 Purifying Sweetening LPG Liquid 13X 2-3 Sweetening recycle gas Gas 4A/5A 2-3 Separation n-Paraffin Gas/Liquid 5A 4 Petrochemical Chemical Super Drying Olefin Cracked gas Gas 3A 2 Finished product Liquid 3A 2 Propylene Liquid 3A 2 Alcohols Liquid 3A 4 Feed to low temperature plants Liquid/Gas 4A 4 Purifying CO2 from Xylenes Liquid 5A 3
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
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Sheet 62 (90)
Cyclohexane purification Liquid 13X 3 CO2 from Ethylene Gas 5A 2 Natural Gas Super Drying Pipeline Gas Gas 4A 4 Feed to LNG plants Gas 4A 4 Feed to helium plants Gas 4A 4 Purifying Sweetening natural gas Gas 4A 2-3 Sweetening plus CO2 Gas 4A 2-3 Air Separation Purifying Water and CO2 from air Gas 13X 4 CO2 removal Gas 13X 4 Refrigeration Super Drying Liquid/Gas 4A Not
regenerable General Super Drying Instrument air Gas 4A 4 Waveguide driers Gas 4A 4 Inert gases and rare gases Gas 4A 4 Purifying Ammonia and water
from furnace atmosphere gas Gas 5A 3-4
CO2 from inert gas Gas 5A/13X 3-4
Table 8 – Typical Application of Molecular Sieves
The static equilibrium adsorption capacity for type 4A, type 5A, and type 13X molecular sieves for H2O are
shown in Fig. 18, Fig. 19 and Fig. 20 and Make no allowances for co-adsorption.
Fig. 18 – Adsorption Equilibrium Loading of H2O on 4A Pellets
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Sheet 63 (90)
Fig. 19 – Equilibrium Adsorption Loading of H2O on 5A
Fig. 20 – Equilibrium Adsorption Loading of H2O on 13X
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Sheet 64 (90)
As long as the component molecule is small enough to fit through the pore opening, the adsorption of
components on sieve is:
• directly proportional to MW;
• inversely proportional to vapour pressure;
• directly proportional to dipole moment (i.e. polarity).
Adsorption in order of retention is:
1. water (strongest retention)
2. methanol (coadsorbes with water)
3. heavier mercaptans
4. light mercaptans
5. H2S
6. CO2
7. COS (weakest retention)
In general, published isotherms showing loading on sieve are only for that specific component in a carrier
gas and can be significantly different in presence of other contaminants.
In terms of heat of adsorption it can be helpful to refer to the following data: the heat of adsorption of water
on sieve of about 4180 kJ/kg or almost twice its atmospheric latent heat.
CS2 cannot be removed with sieve because its polarity is too low. COS can be removed by some sieves but
it is not strongly adsorbed and would be displaced with RSH’s. CO2 is strongly adsorbed and the presence
of CO2 will decrease COS loadings. Unless the CO2 content is low (< 0.1-0.5%) COS removal will require
large beds, infact COS is non-polar and very weakly adsorbed.
Some specific molecular sieves are used for sour gas drying. They can provide good dryness of natural
gas with minimum COS formation.
Methanol is very polar but not as polar as water. However the presence of methanol will decrease the water
loading. This is true for all sieves except 3A because the pore size is too small. Since methanol is not as
thermally stable as water, there may be methanol degradation during regeneration on certain sieves.
The typical parameter shown in this document should be confirmed with Molecular Sieve Vendor.
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Sheet 65 (90)
4.2.2 System Description and Process Flow Diagram
A schematic flow diagram of one typical molecular sieve process is shown in the following Fig. 21.
Regeneration gas to flare
Sour gasinlet
Sweet gas out
Regeneration gasRegeneration
heater
Tower 1adsorpt.
Tower 2regener.
Fig. 21 – Schematic Flow Diagram for a Simple Molecular Sieve Adsorptive Process
The continuous process requires two (or more) towers with one (or more) on-line removing water while the
other is being regenerated. Generally a bed is designed to be on-line in adsorption for 8 to 24 hours. Gas
flow during adsorption is typically downflow: this allows higher gas velocities (thus smaller diameter towers)
since bed fluidization is avoided.
When the bed is taken off-line, the water is removed by heating to 230°C÷320°C, depending on the
desiccant used and the performance specification (i.e., 315°C for molecular sieve, with alumina gel and
activated alumina falling in between). The regeneration gas used to heat the bed is usually a slipstream of
dry process gas and it can be sent to flare or reprocessed after it has been cooled and the free water
removed. Any heat source can be used including waste heat from engines and turbines. This is an
important design consideration since heat is often a major operating cost.
Regeneration gas is upflow during the heating period since the flow rate is usually much less than during
dehydration, so velocity is not a problem. In this way, any residual water left on the desiccant will be at the
top of the bed and will not affect the effluent dewpoint when adsorption is resumed. In addition, upflow
heating helps to strip the contaminants from the top of the bed, where they are accumulated, extending
desiccant life. The regeneration cycle continues until H2O has been driven off. Before starting another
dehydration cycle, the hot regenerated bed shall be cooled to 15-20°C higher than the feed gas
temperature, in order to avoid liquids condensation on the bed itself. Regeneration gas flow during the
cooling period may be upflow if the gas is completely free of water, which saves two switching valves per
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Sheet 66 (90) tower. If the cooling gas contains water, cooling flow should be downflow to avoid preloading of the
desiccant at the bottom of the bed with water.
A more sophisticated process has been developed in order to overcome the difficulties encountered in
flaring a portion of sweetened gas stream which has been used for regeneration. This Closed Cycle
process is shown in Fig. 22.
TC
1 2 3 4
Heater
Cycle Control
TC
Cooler
Separator
Sour Vent
Flashed SolventContactor
Recycle
Wet Gas Inlet
Separator
Filter
Flash Tank
Dry Gas Out
Water Hydrocarbon
Fig. 22 – Closed Cycle Process
Wet gas enters into the unit through a separator and a filter which will remove all liquids and entrained
solids. The wet gas then flows downward through two molecular sieve treating beds and leaves the plant as
dried gas. A portion of the dry gas stream is removed and flows downward through a third bed which has
been regenerated but is still hot. The dried gas removes heat from the bed and flows through a gas-to-gas
exchanger before going through the regeneration heater. After being heated, this gas flows upward through
the bed (bed 4) on regeneration cycle, heating it and removing the adsorbed water. The gas from the bed
then flows through heat exchanger with the dried gas to the tower and then through a cooler. Any
condensed liquids are removed in a separator before the regeneration stream flows into a contactor where
approximately 90% of the H2S, if present, is removed by circulating solvent (i.e. amine). This process
rejects from the gas stream only the acid gas constituents and burns only the amount of gas required to
provide regeneration heat.
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Sheet 67 (90)
In some cases a portion of the wet gas, spilled upstream the adsorption bed, is heated and sent to the bed
in regeneration mode.
4.2.2.1 Adsorption Mechanism
The mechanisms of adsorption in a molecular sieve bed are relatively complex. In most processes there is
a combination of “sieving” action and physical adsorption. In simplest terms, a molecule must pass through
the pore opening in the sieve before it can be adsorbed on the active centres inside the crystal structure.
In most gas-purification cases water vapour is also present in the impure gas and is removed by the
molecular-sieve adsorbent along with the other impurities. Since water is adsorbed more strongly than any
of the other components, it concentrates initially at the inlet portion of the bed where it displaces the other
impurities which had previously been adsorbed. These desorbed impurities are then re-adsorbed further
down the column, and this impurity adsorption zone moves through the bed in advance of the water
adsorption zone. Fig. 23 shows the four major adsorption zones in a sieve bed designed to remove both
water and sulphur compounds. In zone 1, equilibrium has been established between the water and the
sieve bed. Zone 2 is the water-sulphur exchange zone in which the water is displacing sulphur compounds
on the sieve surface. Zone 3 is the sulphur equilibrium section where the sulphur compounds are desorbed
to the equilibrium capacity of the bed. Zone 4 is the mass transfer section where the sulphur compounds
are transferred from the gas phase to the adsorbed phase. In a bed in which the sulphur zones are allowed
to develop fully, the sweet gas leaving will be at a very, very low water content.
As indicated in Fig. 23, a much smaller bed would be required, for the same adsorption period, if
dehydration alone were desired. In that case a small amount of mercaptan sulphur would be removed with
the water, but, at the water breakthrough point, the sulphur content of the product gas would be the same
as that of the feed. In a similar manner, the presence of any more strongly adsorbed component will affect
capacity and breakthrough point for a given impurity.
Zone 1Water
Equilibrium Section
Zone 2Water- Sulfur
Exchange
Zone 3Sulfur Equilibrium
Section
Zone 4Sulfur
Mass-Transfer Section
Sour, Wet Gas Inlet
Sweet, Dry Gas Outlet
Lenght of Adsorption Bed
ConcentrationH2O
H2S
Fig. 23 – Schematic Representation of the Adsorption Process
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Sheet 68 (90) Normally before dehydration system a sweetening facility is foreseen. Therefore usually only Zone 1 and
Zone 2 (Water Mass Transfer Zone) are to be taken into account.
4.2.3 Design Variables
A review of the performance of adsorption plants, particularly those encountering some operational
difficulty, has shown that many are processing gas stream of different composition or under different
conditions then were used as a basis for the original design. The greatest problem seems to arise from
inadequate specification, although over-specification also occurs. Here below are outlined those factors
that should be specified in adsorption plants to obtain optimum performance. Obviously many of these
factors are specific for hydrocarbon recovery units, while others apply to straight dehydrators as well.
As a matter of practicability, the adsorber should be designed to operate under the most common
conditions expected, with allowance for the normal variations.
4.2.3.1 Gas Composition
The designer needs a complete gas analysis that includes at least the percentages of methane through
heptanes plus fraction, as well as the molecular weight and specific gravity of that heavy fraction. Also the
composition of the gas “contaminants” such as hydrogen sulphide, carbon dioxide, nitrogen, and other non-
hydrocarbon components is needed. If the feed gas is a high pressure gas condensate it is also desirable
to outline the PVT (pressure-volume-temperature) properties of the gas, particularly the dew point.
The probable water dew point of the gas as it enters the unit should be specified. It is not to be assumed
that the gas is saturated. This, of course, will lead to the highest bed loading, but relative saturation affects
the dynamic performance of a unit.
4.2.3.2 Flow Rate
An adsorption plant can accommodate relatively large velocity changes so long as the frequency is not too
pronounced. Situations are sometimes encountered, particularly with longer cycle dehydration units,
wherein the flow rate falls to a point that efficient regeneration and cooling becomes a problem.
As for composition, it is desirable to specify the nominal flow rate, with estimates of the normal variations
(max and min) and their frequency.
4.2.3.3 Pressure
In most cases a hydrocarbon recovery unit operates most effectively in the pressure range 35-55 barg.
Operating pressure depends on the circuit in which the unit is operating.
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Sheet 69 (90) When applying specified pressure loss parameters to equipment design it is good practice to allocate up to
70% of the available loss to the column design and the reminder to the interconnecting pipework manifolds,
valves, and ancillary equipment items.
4.2.3.4 Temperature
The proper specification of temperature requires a careful appraisal of the operating conditions if the gas
being processed comes more or less directly from producing wells. Both adsorbent capacity and water
content are fairly sensitive to increases in temperature above 38°C. Gas should never enter a unit above
the temperature to which it could be reduced by means of available cooling media. Cooling is the cheapest
way to dehydrate where gas temperatures are above these ranges. Positive cooling ahead of the tower
also allows positive control of inlet temperatures.
4.2.3.5 Further variables affecting the efficiency of regeneration
The regeneration heating sequence can be broken down into the following component heat requirements:
• applying heat to the molecular sieve its supports, and any protective media employed;
• Heating the ironware, for example, the vessel, pipework and valves;
• Allowing for heat losses;
• Heating and vapourising the adsorbate;
• In the case of liquid-phase treaters it may be necessary to provide heat to remove liquid remaining
in the adsorbers after draining.
The heating step can be sub-divided into two distinct steps, each with particular temperature range.
An initial low temperature range during which desorption of hydrocarbons and bulk of the impurities takes
place and the second stage during which the final design regeneration temperature is attained.
4.2.3.6 Utilities Availability and Local Conditions
The utilities available and their reliability are significant parameters in optimum design. The following should
be outlined for the designer:
• availability of electricity with specification of voltage, amperage, phase, KWH and reliability;
• availability of water with specification of quality and quantity;
• design wet and dry bulb of air;
• amount of supervision and maintenance normally available and / or preferred;
• altitude;
• availability of “sweet” natural gas suitable for use as fuel.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
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Sheet 70 (90)
4.2.4 Equipment Design
This section recommends the minimum requirements for the design, material selection and fabrication of
molecular sieves gas dehydration systems.
The equipment described in this section should be equipped with piping, instruments, valve actuators, level
shutdown devices and other accessories to make a complete and functional system in line with the rest of
the Plant. It should be understood that sample connections, vents, low point drains and minor material
items required to make the system functional are part of the assembly.
4.2.4.1 Scrubber
The presence of contaminants in the feed gas strongly reduces the adsorption capability of solid desiccant.
Therefore, an adequate scrubber at the inlet of the dehydration bed is an important element to ensure as
long life as possible of the bed.
Some Vendors provide special adsorbent materials at the inlet of the bed, which prevent contamination of
liquids (e.g. H2O and gasoline). These adsorbents can represent up to 15% of the bed, however they
should be considered as a complement and not as a substitute of the inlet scrubber.
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Sheet 71 (90)
4.2.4.2 Gas Dehydrator
A detailed representation of adsorber tower is shown in Fig. 24.
2:1 SEMI ELLIPSOIDAL HEAD
M1
N1
J3
M2
INLET COLLECTOR
FLOATING MESH SCREEN
150 DEEP HOLD DOWN LAYER, 12 DIA. CERAMIC BALLS
MOLECULAR SIEVE BED.
J2
3 DIA. CERAMIC BALLS, 75 DEEP
6 DIA. CERAMIC BALLS, 75 DEEP
12 DIA. CERAMIC BALLS, 75 DEEP
MESH RETAINER SCREEN AND BED SUPPORT GRATING / BEAMS2:1 SEMI ELLIPSOIDAL HEAD
OUTLET DISTRIBUTOR -STRAINER
N3
J1
N4
N2
J4
N5
Fig. 24 – Adsorber tower
The inlet and outlet distributors should be designed so that the process and regeneration gases do not
impinge directly on the bed, in order to ensure as uniform as possible flow through the bed and to avoid
channelling. The top and bottom of the bed should contain two or three layers of inert support balls in
graduated sizes. The support should be designed to carry the weight of the desiccant and the inert balls,
plus 1 ÷ 2 bar pressure differential. The anular space between the bed support and the vessel wall should
be filled with high temperature packing to avoid bypassing.
The first step is to determine the bed diameter, which depends on the superficial velocity. Too large
diameter will require a high regeneration gas rate to prevent channelling. Too small diameter will cause too
high pressure drop and will damage the sieve.
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Sheet 72 (90) As a general rule, the diameter of the tower should be such that the adsorption gas velocities are within the
following ranges:
Adsorption Pressure Superficial Gas Velocity 69 bar 7.6 to 12.2 m/min
34.5 bar 10.7 to 16.8 m/min
The pressure drop is determined by a modified Ergun equation, which relates pressure drop to superficial
velocity as follows:
2VCVBLP ρμ +=
Δ Eq. 8
Constants for the previous equation are detailed in the following table:
Particle Type B C
3.2 mm bead (4x8 mesh) 0.0693 3.75 x 10-7
3.2 mm extrudate 0.0893 5.23 x 10-7
1.6 mm bead (8x12 mesh) 0.1881 5.74 x 10-7
1.6 mm extrudate 0.2945 8.86 x 10-7
Table 9 – Constants for Pressure Drop Equation
Fig. 25 was derived from the previous equation by assuming a gas composition and temperature setting the
maximum allowable pressure drop through the bed should be about 35 kPa. A design pressure drop higher
than 55 kPa is not recommended as the desiccant is fragile and can be crushed by the total bed weight and
pressure drop forces. Remember to check the pressure drop after the bed height has been determined.
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Sheet 73 (90)
Fig. 25 – Allowable Velocity for Mole Sieve Dehydrator
Once the allowable superficial velocity is estimated, calculate the bed minimum diameter (i.e. D minimum), and
select the diameter (i.e. D selected): 5.0
maxmin
4⎟⎟⎠
⎞⎜⎜⎝
⎛=
VqD imum π
; [m] Eq. 9
ρ
•
=mq [m3/s] Eq. 10
Where should be 110% of the maximum operating case since the bed diameter is a key parameter for
the unit performances.
•
m
Obtain the corresponding superficial velocity, Vadjusted as follows: 2
minmax ⎟⎟
⎠
⎞⎜⎜⎝
⎛=
selected
imumadjusted D
DVV [m/s] Eq. 11
An alternative and more exact method is to calculate the maximum superficial velocity by solving Eq. 8
algebraically, which gives Eq. 12 below. The second term inside the square root of Eq. 12 may be deleted
since its value is insignificant compared to the first term, which gives Eq. 13.
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Sheet 74 (90)
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎠⎞
⎜⎝⎛
−
⎪⎪
⎭
⎪⎪
⎬
⎫
⎪⎪
⎩
⎪⎪
⎨
⎧
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎠⎞
⎜⎝⎛
+⎟⎠⎞
⎜⎝⎛ Δ
=22
21
2
maxmax
ρμ
ρμ
ρCB
CB
CLP
V [m/s] Eq. 12
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎠⎞
⎜⎝⎛
−
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡⎟⎠⎞
⎜⎝⎛ Δ
=2
21
maxmax
ρμ
ρCB
CLP
V [m/s] Eq. 13
The value of (ΔP/L)max in this equation depends on the sieve type, size and shape, but a typical value for
design is 7.5 kPa/m.
Next step is to choose an adsorption period and calculate the mass of desiccant required. Eight to twelve
hour adsorption periods are common. Periods of greater than 12 hours may be justified especially if the
feed gas is not water saturated. Long adsorption periods mean less regenerations and longer sieve life, but
larger beds and additional capital investment.
Both water capacity and the rate at which solid desiccant adsorb water decline as the material ages. The
object of the design is to install enough desiccant such that after three to five years, the water mass
transfer zone (Zone 2, Fig. 25) will be at the bottom of the bed at the end of the adsorption period.
In the saturation zone, molecular sieve is expected to hold approximately 13 kg of water per 100 kg of
sieve. New sieve will have an equilibrium capacity near 20% (20 kg of water per 100 kg of sieve); 13%
represents the approximate capacity of a 3-5 year old sieve. This capacity needs to be adjusted when the
gas is not water saturated or the temperature is above 24°C. See Fig. 26 and Fig. 27 to find the correction
factors for molecular sieve.
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PRG.PR.GAS.0001
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Sheet 75 (90)
Fig. 26 – Mole Sieve Capacity Correction for Unsaturated Inlet Gas
Fig. 27 – Mole Sieve Capacity Correction for Temperature
To determine the mass of desiccant required in the saturation zone (SS), calculate the amount of water to
be removed during the cycle and divide by the effective capacity.
TSS
rs CC
WS13.0
= Eq. 14
( )densitybulkDSL S
S ⋅= 2
4π
Eq. 15
where
SS: amount molecular sieves required in saturation zone, [kg];
LS: length of packed bed saturation zone, [m];
Wr: water removed per cycle, [kg];
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Sheet 76 (90) Css: saturation correction factor for sieve;
CT: temperature correction factor.
Molecular sieve bulk density is 675-735 kg/m3 for spherical particles and 640-705 kg/m3 for extruded
cylinders.
Even though the mass transfer zone (MTZ) will contain some water (approximately 50% of the equilibrium
capacity), the saturation zone is estimated assuming it will contain all the water to be removed.
The length of the mass transfer zone can be estimated as follows:
( )ZV
L adjustedMTZ ⋅⎟⎟
⎠
⎞⎜⎜⎝
⎛=
3.0
640 Eq. 16
Where: Z = 0.52 m for 3 mm sieve
0.26 m for 1.5 mm sieve
The total bed height is the summation of the saturation zone and the mass transfer zone heights. It should
be not less than the vessel inside diameter, or 1.8 m, whichever is greater.
Now the total bed pressure drop is checked. The ΔP/L for the selected diameter, Dselected, is adjusted using
the following equation and approximation:
( )2
max
/46.7 ⎟⎟⎠
⎞⎜⎜⎝
⎛≅⎟
⎠⎞
⎜⎝⎛ Δ
VV
mkPaLP adjusted
adjusted
Eq. 17
The result is multiplied times the total bed height (Ls + LMTZ) to get the total design pressure drop, which
should be 45-55 kPa max. This is important, because the operating pressure drop can increase to as much
as double the design value over three years. If the pressure drop exceeds 55 kPa, the bed diameter should
be increased and the sieve amount and vessel dimensions recalculated.
To estimate the total cylindrical length of a tower, add an adequate spacing to the bed height, which
provides the space for an inlet distributor and for bed support and hold-down balls under and on top of the
sieve bed.
4.2.4.3 Regeneration Calculations
The first step is to calculate the total heat required to desorb the water, to heat the desiccant and vessel. A
10% heat loss is assumed.
( bedonwaterofkgkgkJQw ⋅⋅⋅⋅⎟⎟
⎠
⎞⎜⎜⎝
⎛= 4200 ) Eq. 18
where Qw: desorption of water heat duty, [kJ];
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Sheet 77 (90)
( ) ( irgsi TTKkg
kJsieveofkgQ −⎟⎟⎠
⎞⎜⎜⎝
⎛⋅
⋅⋅=0.1 ) Eq. 19
where Qsi: duty required to heat molecular sieve to regeneration temperature, [kJ];
( ) ( irgst TTKkg
kJsteelofkgQ −⎟⎟⎠
⎞⎜⎜⎝
⎛⋅
⋅⋅=5.0 )
)
Eq. 20
where Qst: duty required to heat vessel and piping to regeneration temperature, [kJ];
( ) ( )( 10.0stsiwhl QQQlossheatQ ++=⋅= Eq. 21
where Qhl: regeneration heat loss, [kJ];
The temperature, Trg, is the temperature at which the bed and vessel must be heated, based on the vessel
being externally insulated (i.e. no internal insulation which is usually the case). This is about 28°C below
the temperature of the hot regeneration gas entering the tower.
It is recommended to evaluate the thickness of the vessel according to design requirement. As short-cut
formula, the weight of the vessel steel can estimated from the equations below. Eq. 22 is the ASME Section
VIII equation in terms of the vessel inside diameter. It is based on a maximum tensile stress of 130 MPa.
The design pressure is usually set at 110% of the maximum operating pressure. The value of 3.2 mm in
Eq. 23 is the corrosion allowance in inches. The term 0.75 Dbed is to account for the weight of the tower
heads. The value of 0.91 mm provides the space for the inlet distributor and support and hold down balls.
design
designbed
PkPaxPD
t2.1)2600001300002(
1000−=
= ; [mm] Eq. 22
bedbedMTZS DDLLtkgsteelofMass )91.075.0)(2.3(8.29)( ++++=⋅⋅ Eq. 23
For determining the regeneration gas rate, the total regeneration heat load should be calculated from the
following equation:
( )( )hlstsiwtr QQQQQ +++= 2 Eq. 24
The 2 factor is based on operating plants feedback and corrects for the change in temperature difference
(in – out) across the bed with time during the regeneration period. It assumes that 50% of the heat in the
gas transfers to the bed, vessel steel and heat loss to atmosphere; and the balance leaves with the hot
gas.
The regeneration gas flow rate [mrg] and the heat capacity [Cp] are calculated from the Eq. 25 and Eq. 26,
with the enthalpies obtained from the enthalpy vs temperature plots for various pressures.
Moreover the regeneration furnace should be designed for the 110% of the calculated regeneration flow
rate.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 78 (90)
⋅
⋅−=
))(( timeheatingTTCQm
bhotp
trrg ; [kg/time] Eq. 25
( )( )bhot
ihotp TT
HHC−−
= ; [kJ/kg°C] Eq. 26
The temperature Thot is 28°C above the temperature Trg at which the bed must be heated. The temperature
Tb is the bed temperature at the beginning of the regeneration, which is the same as the dehydration-plant
feed temperature.
The heating time is usually 50% to 60% of the total regeneration time which must include a cooling period.
Fig. 28 shows a typical temperature profile for a regeneration period (heating and cooling). For 8 hour
adsorption periods, the regeneration normally consists of 4 ½ hours of heating, 3 hours of cooling and ½
hour for stand-by and switching. For longer periods the heating time can be extended as long as a
minimum pressure drop of 0.23 kPa/m is maintained to ensure even flow distribution across the bed.
Fig. 28 – Inlet and Outlet Temperatures During Typical Solid Desiccant Bed Regeneration Period
The superficial velocity of the regeneration gas is calculated with the following equation:
24DqV
π= [m/s] Eq. 27
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
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Sheet 79 (90) The calculated superficial velocity can not be less than the value that corresponds with a minimum bed
pressure drop of 0.23 kPa/m. If the calculated velocity is less than this, the regeneration gas rate should be
ecreased by
ultiplying it times V/Vmin.
tion of the above formulas for equipment sizing.
d only one of the group is
eens to provide the
lower side. This two foot space and the
e dryer. This saves
on vessel size and reactivation costs. Also in a very large system where multiple beds must be used in
parallel it may be possible to save a complete unit, and bypass that amount of gas.
increased by multiplying it by the ratio Vmin/V, and the period of regeneration should be d
m
Appendix III shows an example of the applica
4.2.4.4 Special Bed Configurations
1. Multiple Beds – Dividing the total bed area between two or more towers can result in fabrication saving
because smaller heads and lighter shell plate may be used, even though piping will be somewhat more
expensive. In addition, it is sometimes possible to reduce total desiccant and regeneration
requirements, thus affecting further savings. When the low L/D results from relatively small amounts of
water in the inlet gas, normal cycle times of 8 to 12 hours frequently result in very shallow beds of large
diameter. Multiple beds of a height higher than the minimum calculated allow each tower to operate
several times as long as a normal cycle. Thus, instead of two large towers, one on dehydration and one
on regeneration, several small dryers share the load during dehydration an
removed periodically for regeneration. Under some circumstances the total desiccant requirements for
a system of multiple beds is less than for more conventional double towers.
2. Radial Flow Dehydrators – Occasionally dehydrators are designed for radial gas flow. The gas enters a
distribution tube which runs full length through the central axis of a vertical cylindrical shell. It exits
through an annular space between the desiccant and the shell itself. This configuration provides a large
facial area in a shell of reasonable cost. There is an added cost of support scr
annular space outside the desiccant. The system also provides very short contact time, so dew points
as low as those obtained with more conventional designs are not usually possible.
3. Horizontal Beds – Supporting a bed with large area and relatively shallow depth in a cylindrical vessel
resting on its side is another alternative. In such an arrangement the desiccant depth can be about
60% of the diameter of the vessel. Thus a ten foot diameter shell would hold a six foot deep bed
supported parallel with its axis and about two feet above its
equal space above the desiccant, act as space for gas distribution. Various mechanical headers and
baffles may also be incorporated to improve gas distribution.
4. Partial Bypass – Another system may be used where the requirements are exactly opposite – only
normal drying is required. Because solid desiccants, by their nature, tend to produce very dry gas, a
portion of a wet stream may sometimes be bypassed and blended downstream of th
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
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Sheet 80 (90)
4.2.5 Operation and Maintenance
Although the adsorption plant is basically simple and easy to operate, there are certain details that should
be appreciated if both satisfactory performance and adsorbent life are to be assured. This involves
measurement of temperature and flow, as well as sampling connections, at key points. Ideally, the location
and type of such facilities should be indicated. The Vendor will not normally furnish complete testing
facilities unless they are specified.
4.2.5.1 Operation Records
Adequate record maintenance and periodic tests are key to good operation. Such records should include a
diary of plant problems as well as the usual information. Routine records should show daily gas volume
processed, inlet pressure and temperature, pressure drop across the unit, daily liquid recovery,
regeneration temperatures and regeneration gas flow rate. Recording of such data on a routine basis is
advisable.
4.2.5.2 Good Operational Techniques
In addition to keeping good records there are several operating practises that lead to optimum plant
performance.
1. Protect the towers from the entrance of all liquids. This implies a good scrubber with a good mist
extractor element. A filter coalescer can also be foreseen.
2. Minimize the regeneration and cooling gas necessary to perform their functions within the prefixed
time.
3. Never allow a rapid pressure release or increase on the towers. Severe breakage can result,
particularly in the second case.
4. Make an effort to maintain as constant an inlet gas flow as possible. Minimizing rapid fluctuation in inlet
analysis is also desirable if feasible.
5. Always design and operate for maximum cycle length, consistent with the purpose of the unit in order to
increase the life of the bed.
4.2.5.3 Depressurization / Repressurization
The regeneration cycle frequently includes depressurizing / repressuring to match the regeneration gas
pressure and / or to maximize the regeneration gas volume to meet the velocity criterion. In these
applications, the rate of depressuring or repressuring should not exceed 350 kPa per minute.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
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Sheet 81 (90)
4.2.5.4 Analyzers
Moisture analyzers for very low water contents require care to prevent damage to the probes. When
inserted into the beds, sample probes and temperature probes should be installed to reach the centre of
the gas phase.
A water monitor, to indicate bed switching time is necessary part of this system. It may be an automatic
system with probe 75-100 cm up in the bed at the exit end, or it may be only an indicating system in the
feed stream which will display the water content for an operator, so that he can compensate by manually
changing the cycle time.
4.2.5.5 Insulation
Solid desiccant towers are insulated externally or internally. Internal refractory requires careful installation
and curing, usually before the desiccant is installed. It saves energy but the greatest benefit is it can
dramatically reduce the required heating and cooling times. This is often an important benefit for systems
where regeneration times are limited. The primary disadvantage is the potential for wet gas bypassing the
desiccant through cracks and defects in the insulation during the adsorption cycle.
4.2.5.6 Effects of contaminants on molecular sieves
Molecular sieves performance is strongly affected by the presence of contaminants. The nature of silica gel
molecular sieve and physical structure, which contributes to its efficiency as an adsorbent, make it subject
to chemical attack by many compounds.
- Oil, glycols, diisopropynalomine, diglycolamine and associates: this includes all the high boiling
viscous that comes along, from compressor lube systems, from the wells, from upstream
absorption plants. It enters a bed in the form of droplets or aerosol and it will be adsorbed by the
binder, or in the macropores of the molecular sieves. Such contaminant is not completely removed
during regeneration, and it eventually cracks, polymerises or otherwise goes through any number
of reactions which result in coke formation. When this occurs on some types of thermally stable
sieve, the heavy hydrocarbons can be removed by an actual burn-off under a controlled oxygen
atmosphere. Some sieves, including most of the available 3A grades do not have sufficient thermal
stability to permit this type of treatment.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 82 (90)
- Ammonia: the ammonia formed from decomposition of amine derivates during regeneration, will
chemically attack the silica gel in the most commonly used molecular sieve binder systems and
weaken the physical structure of the particles.
- Potassium carbonate solutions: another upstream liquid process which involves the use of hot
liquid for CO2 removal. As in the previous cases for amine and glycols, the carbonate solution will
have a blockage and blinding effect on the sieve; if present in a secondary phase. Even mild
vaporized, entry will result in cation exchange on the standard natural gas dehydration molecular
sieve 4A. The soda will be replaced by potassium, altering the 4A into 3A material. This will impact
on overall water capacity as well as thermal stability of the altered 4A if regeneration exceeds
270°C.
- Oxygen: if there is any oxygen in the system, or in the regeneration gas, it will react with H2S and
some other sulphur compounds on the surface of the sieves and deposit elemental sulphur. Since
oxygen can enter a system by a number of routes, it is a good idea to request an oxygen
determination during any routine stream analysis.
- Salt: usually enters a desiccant bed dissolved in entrained water. Unfortunately it does not leave
when the water is vapourised and removed from bed during regeneration. Thus the solid
accumulates and blocks pores, macropores and in extreme cases, all the voids between the
molecular sieve beads.
- Heavy metals, arsenic and other dense elements: large molecular particles such as mercury, lead
and arsenic will not be adsorbed into classic synthetic A and X type crystal molecular sieve
because of the pore openings. These contaminants are not considered to be harmful to the
molecular sieves performance, but may alter its toxicity rating on disposal.
4.2.6 Process Control and Safeguarding
The switching valves may be controlled on a time basis, outlet regeneration gas temperature or outlet gas
analysis. The latter is best (and most expensive) but requires a time or temperature override to ensure that
a properly regenerated and cooled tower is ready when switching occurs.
A detailed logic sequence shall be developed containing as minimum a description for the Manual and
Automatic Mode selection and operation of the switching valve control, the system start-up procedure, the
Step Permissive and the Step description, the Process Abnormal Conditions, the Sequence Extension and
the Stop Procedure.
The following figure shows a typical control configuration.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 83 (90)
LC
AIH2O
AIH2O
AIH2O
Wet Gas
SourWater
Dry GasRegeneration
Dry Gas
FC
Regeneration Wet Gas
Filter
Adsorp. Adsorp. Regen.
TC
to regenerationgas heater
Fig. 29 – Typical configuration of switching valve control
5. FLOW CHART
NA
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
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Sheet 84 (90) 6. APPENDIX
6.1 Appendix I – Water removal at different TEG concentration and TEG circulation rates
Fig. 30 – Water removal Vs TEG Circulation Rates (N = 1)
Win/out: water content in the inlet/outlet gas
Fig. 31 – Water removal Vs TEG Circulation Rates (N = 1.5)
Win/out: water content in the inlet/outlet gas
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 85 (90)
Fig. 32 – Water removal Vs TEG Circulation Rates (N = 2)
Win/out: water content in the inlet/outlet gas
Fig. 33 – Water removal Vs TEG Circulation Rates (N = 2.5)
Win/out: water content in the inlet/outlet gas
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 86 (90)
Fig. 34 – Water removal Vs TEG Circulation Rates (N = 3)
Win/out: water content in the inlet/outlet gas
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 87 (90) 6.2 Appendix II – Reboiler Heat Duty
Table 10 gives typical reboiler heat duties. The heat duty may be calculated from:
Q = (122.6 + 2461/UCR) kJ/litre
UCR
[l/kg]
Reboiler heat duty
[kJ/l TEG circulated]
10 369
15 287
20 246
25 221
30 205
40 184
50 172
60 164
Table 10 – Approximate Reboiler Heat Duty per Litre of TEG
The above duties are only typical because the sensible heat demanded from the reboiler depends on the
temperature of the rich glycol entering the still column. This temperature is dependent on the design of the
glycol-glycol heat exchangers. For the reboiler heat duties stated, the following assumptions were used:
- sufficient glycol/glycol heat exchanger surface is provided to reduce the temperature of the dry
glycol entering the surge drum to 93°C;
- a gas temperature of 50°C;
- a stripping gas rate of 35 m3 (st) per m3 of glycol at 38°C;
- a power gas rate of 50 m3 (st) (heated to 100°C) per m3 of glycol for pumps.
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
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Sheet 88 (90) 6.3 Appendix III – Molecular Sieve Equipment Sizing – Example
Data
Design Pressure = 2790 kPa
Adsorption Pressure = 2060 kPa
Adsorption Temperature = 17.5°C
Regeneration Temperature = 300°C
Cycle Period = 24 h (Heating Time = 6.45 h)
Water Removed per Cycle (kg) = 160 kg/h * 24 h = 3840 kg
No. of Beds in Adsorption per Cycle = 2
Gas Flowrate (m) = 132378 k/h
Gas Density (ρ) = 32.01 kg/m3
Gas Viscosity (μ) = 0.01 cP
Gas Heat Capacity (Cp) = 2.05 kJ/kg°C
mkPa
LP 5.7
max
=⎟⎠⎞
⎜⎝⎛ Δ
Particle Type = 3A Beads, 1.6 mm (B=0.1881, C=5.74 10-7, Z=0.26 m, ref. Table 9)
Molecular Sieve Capacity Correction Factors: CSS = 0.96, CT = 1
Bulk Density = 700 kg/m3
Sieve Supposed Weight = 20330 kg
Adsorber
hm
hmV 588
201.3201.0
1074.51881.0
01.321074.55.7 72
1
7max =
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡⎟⎠⎞
⎜⎝⎛⋅⎟
⎠⎞
⎜⎝⎛
⋅−⎥⎦⎤
⎢⎣⎡
⋅⋅=⎟
⎠⎞
⎜⎝⎛ −
− (Eq. 13)
hm
hmq
33
5.413501.32
132378==⎟⎟
⎠
⎞⎜⎜⎝
⎛ (Eq. 10)
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 89 (90)
( ) mmD 3588
5.41354 5.0
min =⎟⎠⎞
⎜⎝⎛
⋅⋅
=π
(Eq. 9)
( ) mmDsel 1.3=
hm
hmVadj 569
1.33588 =⎟
⎠⎞
⎜⎝⎛=⎟
⎠⎞
⎜⎝⎛
(Eq. 11)
( ) ( )m
kPam
kPaLP 01.756901.321074.556901.01881.0 27 =⋅⋅⋅+⋅⋅=⎟
⎠⎞
⎜⎝⎛Δ − (Eq. 8)
( ) kgkgSs 3080096.0113.0
3840=
⋅⋅= (Eq. 14)
( ) mmLs 9.27001.3
2308004
2 =⋅⋅
⋅=
π (Eq. 15)
( ) mmLMTZ 25.026.0640569 3.0
=⋅⎟⎠⎞
⎜⎝⎛= (Eq. 16)
( ) mmLTOT 15.325.09.2 =+=
( ) kPakPakPaPTOT 451.2215.301.7 <=⋅=Δ
Regeneration
( ) kJkJQW610*128.1638404200 =⋅= (Eq. 18)
( ) ( ) kJkJQsi610174.55.17272120330 ⋅=−⋅⋅= (Eq. 19)
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.
PRG.PR.GAS.0001
Rev. 0 Date Jan 2010
Sheet 90 (90)
( ) mmmmt 7.3327902.1260000
27901.31000=
⋅−⋅⋅
= (Eq. 22)
( ) ( )( ) kgkgSteelofMass 217651.391.01.375.025.09.22.37.338.29 =+⋅+++=⋅⋅ (Eq. 23)
( ) ( ) kJkJQst61077.25.172725.021765 ⋅=−⋅⋅= (Eq. 20)
( ) ( ) kJkJQhl6666 10407.21077.210174.510128.161.0 ⋅=⋅+⋅+⋅⋅= (Eq. 21)
( ) ( ) kJkJQtr76666 10296.510407.21077.210174.510128.162 ⋅=⋅+⋅+⋅+⋅⋅= (Eq. 24)
( ) hkg
hkgmrg 12899
45.65.17328050.210296.5 7
=⋅−⋅
⋅=⎟
⎠⎞
⎜⎝⎛
(Eq. 25)
Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.