-
Technology using alkanolamines,or amines, for the removal
ofhydrogen sulphide and carbondioxide from natural gases has
beenaround for decades. Since the 1960sand 1970s, several amines
have comeinto general use, but there is littleinformation available
on whichamine is best suited to a particularservice. Many
inefficient amine gas-sweetening units can be optimisedby simply
changing their aminesolutions.
The basic flow scheme for an amine-sweetening unit is shown in
Figure 1. Inthe design of the process, the primaryconcern is that
the sweetened gasshould meet the required purityspecifications with
respect to H2S andCO2. The secondary objective is to selectthe
amine, which optimises equipmentsize and minimises plant
operatingcosts. The following points should alsobe addressed in the
selection of theproper amine for the design orevaluation of an
existing plant: Can the amine circulation rate bereduced by
selecting an amine that maybe used at a higher concentrationand/or
at a higher acid gas loading? Could the equipment be designedmore
efficiently using an amine thatrequires a lower circulation rate
and/orhas lower heats of reaction with H2S andCO2? Could H2S be
selectively absorbedfrom the sour gas while CO2 is rejected?Can the
selective absorption of H2S andCO2 from the sour gas be optimised
bythe use of a suitable amine blend? Could corrosion and solvent
lossproblems be improved with an amine ormixture of amines more
resistant todegradation?
Between 5070% of the initialinvestment for an
amine-sweeteningunit is directly associated with themagnitude of
the solvent circulationrate, and another 1020% of the
initialinvestment depends on the regenera-tion energy requirement
(Astarita et al,1983). Approximately 70% of gas-
sweetening plant operating costs,excluding labour expenses, are
due tothe energy required for solventregeneration. Appropriate
amineselection can significantly reduce theregeneration energy
requirement andsolution circulation rate. Therefore, theselection
of amines best suited to theprocess conditions can have a
dramaticimpact on the overall costs associatedwith a sweetening
unit.
Several alternative flow schemes foramine-sweetening plants were
discussedin detail by Polasek, Donnelly andBullin (1983) and
therefore will not bediscussed here.
General considerations foramine selection The general criteria
for amine selectionin sweetening plants have changedover the years.
Until the 1970s,monoethanolamine (MEA) was the onlyamine considered
for any sweeteningapplication. In the 1970s, as exemplifiedin
papers by Beck (1975) and Butwelland Perry (1975), a major switch
fromMEA to diethanolamine (DEA) occurred.In the past ten years,
MDEA, DGA andmixed amines have steadily gainedpopularity too.
In order to become accepted on anindustry-wide basis, different
operating
Predicting amine blendperformance
A process simulator is able to predict the performance of an
Iranian gas-sweeteningplant. Various mixtures of diethanolamine
(DEA) and Methyl diethanolamine(MDEA) are used to investigate the
potential for an increase in plant capacity
Hamid Reza Khakdaman and Ali Taghi Zoghi National Iranian Oil
Company Majid Abedinzadegan Abdi Memorial University of
Newfoundland
PETROCHEMICALS & GAS PROCESSING
PTQ Q4 2005w w w. e p t q . c o m
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Alkanolamine type MEA DEA DGA MDEASolution strength, wt% 1520
2535 5070 2050Acid gas loading, mole/mole 0.300.35 0.300.35
0.300.35 UnlimitedAbility for selective absorption of H2S No Under
limited No Under most
conditions conditions
Typical operating conditions and data for amines
Table 1
Figure 1 Process flow diagram for a common sweetening plant
Acid gas
Stm.
Cond.
Refluxpump
Reboiler
Sourgas
Sweetgas
Stripper
Absorber
StripperO/H
condenser
Boosterpump
StoragetankLean amine
cooler
Crossexchanger
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conditions should be tested and provenwith a particular amine.
Each alka-nolamine solution has an acceptedrange of process
conditions andparameters associated with it. Typicaloperating
conditions for commonalkanolamines are summarised inTable 1.
Diethanolamine (DEA)DEA is the most commonly used amine,within
the 2535 wt% range. The totalacid gas loading for DEA is limited
to0.300.35 mole/mole for carbon steel asthe equipments construction
material.DEA can be safely loaded up toequilibrium level (~1
mole/mole) whenstainless steel is used. The degradationproducts of
DEA are relatively lesscorrosive than those of MEA. Exposureto
oxygen forms corrosive acids, andCOS and CS2 may, to some extent,
reactirreversibly with DEA. DEA is notreclaimable under regenerator
condi-tions; it decomposes below its boilingpoint at atmospheric
pressure. Vacuumreclaimers, however, have been success-fully used
to reclaim DEA solutions(Meisen et al, 1996).
Since DEA is a secondaryalkanolamine, it has a reduced
affinityfor reaction with H2S and CO2.Therefore, it may not be able
to producepipeline-quality gas for some low-pressure gas streams.
In general, as thegas pressure is lowered, the strippingsteam must
be increased or a split flowdesign used. In some cases, even
thesemeasures will not suffice and anothersolvent must be used.
Under some conditions, such as lowpressures and a liquid
residence time onthe tray of about two seconds, DEA isselective
toward H2S and will permit asignificant fraction of the CO2 to
remainin the sales gas.
Methyldiethanolamine(MDEA)An accepted set of operating
conditionshas not been firmly established forMDEA, as compared to
the previouslymentioned amines. This is due to theflexibility and
versatility of MDEA, andthe resulting wide range of
applications.MDEA is a tertiary amine andcommonly used in the 2050
wt%range. Solutions with lower amine
concentration are typically employed inlow-pressure,
high-selectivity applica-tions such as the selective removal ofH2S
in the Shell Claus off-gas treating(SCOT) units.
Due to the considerably reducedcorrosion problems, acid gas
loadings ashigh as 0.70.8 mole/mole areconsidered practical in
carbon steel-made equipment. Higher loadings mayalso be possible
with a few problems.
MDEA exposure to oxygen formscorrosive acids, which, if not
removedfrom the system, can result in the build-up of iron sulphide
in the system.MDEA has several distinct advantagesover primary and
secondary amines,which include lower vapour pressure,lower heats of
reaction, higherresistance to degradation, fewercorrosion problems
and selectivitytoward H2S in the presence of CO2.
Most of these advantages have alsobeen reported by Blanc et al
(1982).Depending on the application, some ofthem have special
significance; forexample, due to its lower heat ofreaction, MDEA
can be employed inpressure swing plants for bulk CO2removal. Here,
the rich amine is merelyflashed at or near atmospheric pressureand
little or no heat is added forstripping.
The overwhelming advantage thatMDEA currently possesses over the
otheramines is that it is readily selectivetoward H2S in the
presence of CO2. Athigh CO2/H2S ratios, a major portion ofthe CO2
can be slipped through theabsorber and into the sales gas
whileremoving most of the H2S. Theenhanced selectivity of MDEA for
H2S isattributed to the inability of tertiaryamines to form
carbamates with CO2.MDEA does not have a hydrogenattached to the
nitrogen and cannotreact directly with CO2 to formcarbamate. The
CO2 reaction can onlyoccur after the CO2 dissolves in water toform
a bicarbonate ion, which thenundergoes an acid-base reaction
withthe amine:
(1)
At least six mechanisms for the CO2-MDEA reaction have been
proposed byCornelissen (1982), Barth et al (1981)
and Danckwerts (1979). MDEA can,however, react with H2S by the
sameproton transfer mechanism of primaryand secondary amines (Jou
et al, 1982):
(2)
Selective absorption of H2S can beenhanced by optimising
absorber designto obtain a liquid tray residence time of1.53.0
seconds and by increasing thetemperature in the absorber. Both
ofthese conditions favour H2S absorptionwith CO2 rejection.
Mixed aminesAmine blends are generally mixtures ofMDEA and DEA
or MEA and are used toenhance CO2 removal by MDEA, asdescribed by
Polasek, Bullin and Iglesias-Silva (1992). Such mixtures are
referredto as MDEA-based amines, with DEA orMEA as the secondary
amines. Thesecondary amines generally compriseless than 20% of the
total amine contenton a molar basis. At lower concentra-tions of
MEA and DEA, the overallamine concentration can be as high as55 wt%
without the implementation ofexotic metal equipment.
MDEA-based mixtures are normallyused to increase the CO2 pickup
in caseswhere the MDEA is allowing too muchCO2 to slip overhead in
the absorber.Spiking the MDEA with MEA or DEA toachieve the desired
CO2 pickup is oftenpreferable to a complete amine switch-out to a
DEA or MEA system, becausethe MDEA regenerator reboiler may
beundersized for a purely formulated DEAor MEA system. Operating
problemsassociated with mixed amines influenceamine mixture
concentration and itsmaintenance.
However, finding an optimumconcentration for mixed
amines(DEA+MDEA) strongly depends on theH2S and CO2 content of the
sour gas,operating pressures and sale gasspecifications. For
natural gas-sweetening purposes, mixed amines aretypically mixtures
of MDEA and DEA orMEA, which enhance CO2 removalwhile retaining
desirable characteristicsof MDEA, such as reduced corrosionproblems
and low heats of reaction
Case study A typical Iranian gas plant is selected forthis
study. The gas-sweetening facilityhas five identical amine trains
for H2Sand CO2 removal. The plant manage-ment decided to consider
one of theunits for substituting DEA with amixture of DEA and MDEA.
Each trainwas composed of two absorbers and twostripper columns,
which operated inparallel in the unit. The HYSYS plantsimulator was
used to simulate the
PTQ Q4 2005
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PETROCHEMICALS & GAS PROCESSING
Amine MEA DEA DGA MDEASolution strength 1520 2535 4060 3050Acid
gas loading, mole/mole 0.30.4 0.30.4 0.30.4 UnlimitedHr for H2S,
Btu/lb 550 511 674 522Hr for CO2, Btu/lb 825 653 850 600
Heat of reaction for different types of amines
Table 2
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process. The absorber feed gascomposition is shown in Table 3,
andoperating conditions are summarised inTable 4.
Current plant operating conditionswere initially simulated to
obtain theconfidence that the simulation wasperformed effectively.
The simulationproduced a very good agreementbetween the
HYSYS-generated resultsand the actual operating data. Theresults
are listed in Table 5.
The process was subsequentlysimulated using various mixtures of
DEAand MDEA with the followingconstraints: Solution circulation
rate wasconsidered constant at 935m3/hr H2S content in sweet gas
should bekept less than 2ppm CO2 content in sweet gas should bekept
less than 1% Duty of each reboiler was consideredconstant at
1.32e+8kJ/hr (125MMBTU/hr) Condenser temperature equals 52C.
DEA and MDEA concentrations in thesolution were changed from
1030 andfrom 539 wt% respectively. The aminemixtures, which met a
targeted value forthe following parameters, were selectedas the
alternative solvent for optimummixture concentration: Amine
system
Acid gas composition in thesweetened gas (absorber overhead)
Figure 2 shows how the plantcapacity can be increased for
variousamine blends. The throughput can beraised from the base
value ofapproximately 14 600kmole/hr to theindicated gas flow rate
shown in Figure
4 for various amineblend compositions,and will be
furtherdiscussed. It shouldbe noted that thereboiler duty andother
parameterspreviously indicatedwere fixed and thatonly the
gasthroughputs were
changed. Since the maximum MDEAconcentration in an
industrialapplication is limited to below 50%, thetotal composition
was kept below thispercentage. It can be concluded that a49% amine
blend with 2030% DEAcontent will be an optimisedcomposition. A
lower-end (closer to20%) concentration for DEA will berecommended
due to the need forsolution corrosion and viscosity control.As can
be seen, by blending DEA andMDEA mixtures, for the
indicatedcomposition, the plant capacity can beincreased to 17
00020 000 kmole anrise of approximately 1637%.
In order to check if the plant canhandle higher gas flow rates,
otherpieces of equipment and plant parts,
PTQ Q4 2005
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PETROCHEMICALS & GAS PROCESSING
Total amine 35%Total amine 40%
Total amine 45%Total amine 49%
17 500
18 000
18 500
19 000
19 500
20 000
20 500
17 000
So
ur g
as f
low
rate
, km
ole
/hr
10 15 20
250 MMBtu/hr
DEA, %25 30 355
Figure 2 Unit revamp at fixed reboiler duty
Parameter Operating data Simulation resultsRich amine loading
0.450.50 0.49Lean amine loading 0.0270.031 0.028H2S in sweet gas,
ppm 1.52.5 2.0CO2 in sweet gas, mol% 0.01 0.01Absorber col.
top/bottom temperature, C 55.0/77.0 61.5/86.2Stripper col.
top/bottom temperature, C 52.0/120.4 1
1 The condenser temperature was set to 52C and the reboiler duty
of 125MMBTU/hr caused thereboiler temperature of 120.4C.
Comparison between simulation and actual operating data
Parameter Typical valueAmine circulation rate, m3/hr 935
Absorber col. top/bottom pressure, bara 1058/1063Absorber col.
top/bottom temperature, C 55.0/77.0Stripper col. top/bottom
pressure, bara 22.0/27.9Stripper col. top/bottom temperature, C
52.0/120.4 No. of actual tray (absorber) 20No. of actual tray
(stripper) 24
Gas-sweetening operating conditions
Component Flow rate, Mole %kmole/hr
H2O 4.28 0.03N2 75.94 0.52CO2 936.3 6.41H2S 562.36 3.85COS 0.26
17ppmC1 12 909.22 88.35C2 81.8 0.58C3 13.16 0.09i-C4 2.92 0.02n-C4
4.38 0.03i-C5 2.92 0.02n-C5 2.92 0.1C6
+ 14.62 DEA 0Total 14 611.16 100Pressure, bara 1063Temperature,
C 21
Absorbers feed gas composition(design basis)
Table 3
Table 4
Table 5
o Rich amine loading: ( ), H2S(v/v)=
, CO2 (v/v)=
o Lean amine loading: ( ) , H2S(v/v), CO2 (v/v)
DEAmolMDEAmolSHmolCOmol 22
+
+
FlowVoleA
FlowVolSH
.min
.2
FlowVoleA
FlowVolCO
.min
.2
DEAmolMDEAmolSHmolCOmol 22
+
+
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including heat exchangers, pumps, pipesizes and towers, should
be examined. Itwas observed that the diameter of theexisting
absorption tower columnscould handle an increase in gasthroughput
of around 22%. Using thisfigure, the performance of the plant
wasthen evaluated using various amineblends. The lean loading
increases withDEA content, as a higher heating rate isrequired to
release the acid gas from theamine. The rich amine loadings
remainrelatively constant for a particularamine blend, but since a
solvent blendwith a higher concentration of totalamine can
naturally absorb more acidgas a reduction of acid gas loading
willbe expected when the total aminecontent increases.
Figure 3 and the previouslymentioned Figure 4 show a variation
inthe acid gas (CO2 and H2S) content ofthe sweetened gas for the
enhancedcapacity scenario a 22% gas flow rateincrease when the
amine flow rate and
reboiler duties were kept constant. It isevident that unless the
total amineconcentration was increased beyond the35 wt% mark, the
acid gas specificationscould not be met. Beyond this totalamine
composition, the acid gascontent of the sweetened gas remainsnearly
constant for varying DEAcontent.
MDEA capabilities Due to its lower corrosion tendency andheat of
reactions with acid gasescompared to other amines, MDEA is
afavourable option. Using the HYSYSplant simulation, different
mixtures ofDEA and MDEA were investigated. Sincemixed amines have a
higher capacity foracid gas removal at constant aminecirculation
rates compared to DEAsolvents, the gas-processing capacity
ofgas-sweetening units can be increased.These results show that the
gas flow ratecapacity for a typical unit can easily beincreased by
up to 20%.
References1 Beck J E, Diethanolamine an energy
conserver, Laurence Reid Gas ConditioningConference, Norman,
Oklahoma, 1975.
2 Donnelly S T, Henderson D R, A newapproach to amine selection,
Proceedingsof the AIChE Spring National Meeting NewYork, 1982.
3 Holmes J W, Spears M L, Bullin J A,Sweetening LPGs with
amines, ChemicalEngineering Progress, 80, 5, 47, 1984.
4 Jou F Y, Lal D, Mather A E, Otto F P,Solubility of H2S and CO2
in aqueousmethyldiethanolamine solutions, Ind. Eng.Chem. Proc. Des.
Dev., 21, 539, 1982.
5 Kohl A and Riesenfeld F, Gas purification,Gulf Publishing
Company, Houston, Texas,1979.
6 Meisen A, Abedinzadegan Abdi M, Abry R,Millard M, Degraded
amine solutions:nature, problems, distillative
reclamation,Proceedings of the 45th Annual LaurenceReid Gas
Conditioning Conference,Norman, Oklahoma, 168190, 1996.
7 Polasek J C, Bullin J A, Selective absorptionusing amines,
Proceedings of the 61stAnnual Gas Processors Convention, Tulsa,USA,
1992.
8 Polasek J C, Bullin J A, Iglesias-Silva G A,Using mixed amine
solutions for gassweetening, presented at the 71st AnnualGas
Processors Association 2528 February,Norman, Oklahoma, 2001.
Hamid Reza Khakdaman is head of themodelling and simulation
department atthe das division of the Research Institute ofPetroleum
Industry (RIPI) of the NationalIranian Oil Company (NIOC),
Tehran,Iran. He is involved in natural gassweetening and
gas-to-liquids processes asa research engineer. He holds a BSc
fromthe Iranian University of Science andTechnology (IUST) and MSc
from TehranUniversity, both in chemical engineering.Email:
[email protected] Taghi Zoghi is a senior projectengineer at
NIOC-RIPI, Tehran, Iran wherehe is involved in gas-processing
projects.He holds a BSc from Tehran University andMSc from IUST,
both in chemicalengineering. Email: [email protected]
Abedinzadegan Abdi is the formerdirector of the gas division at
NIOC-RIPI,Tehran, Iran and is currently a facultymember at the
Memorial University ofNewfoundland and process engineeringlead at
the centre for marine CNG in StJohns, NL, Canada. Email:
[email protected]
PTQ Q4 2005
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PETROCHEMICALS & GAS PROCESSING
0.02
0.00
0.04
0.06
0.08
0.10
0.12
0.14C
O2,
%
10 15
Total amine 35%Total amine 40%Total amine 45%Total amine 49%
20DEA, %
25 30 355
Figure 3 CO2 concentration in sweet gas for different amine
mixtures (DEA+MDEA)
2.0
0.0
4.0
6.0
8.0
10.0
12.0
14.0
H2S
, pp
m
10 15
Total amine 35%Total amine 40%Total amine 45%Total amine 49%
20DEA, %
25 30 355
Figure 4 H2S concentration in sweet gas for different amine
mixtures (DEA+MDEA)
Since mixed amines have ahigher capacity for acid gasremoval at
constant aminecirculation rates compared to DEA solvents, the
gas-processing capacity ofgas-sweetening units can beincreased
