ETCE 2001

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1 Copyright 2001 by ASME

Proceedings of ETCE 2001Engineering Technology Conference on Energy

February 57, 2001, Houston, TX

ETCE2001-17085

INNOVATIVE APPROACH TO THE MERCURY CONTROL

DURING NATURAL GAS PROCESSING

Zdravko SpiricEnvironmental Protection Manager

INA-Naftaplin, Subiceva 29, 10000 Zagreb, CroatiaT: 385-1-459 22 36, F: 385-1-459 21 73, E: [email protected]

ABSTRACT

Natural gas, being produced from gas fields around the

globe, along with a large number of other harmful substances

(CO2, H2S, RSH, COS, etc.), often contains mercury.

Mercurys potentially harmful effect on humans and on the

ecological system as a whole as well as the risk regarding

mercurys corrosive effects to the very sophisticated and

expensive process material, equipment and catalysts is making

its removal an imperative.

The frequently applied procedure to remove mercuryas an impurity and environment pollutant from process streams

utilizes adsorption on a fixed bed consisting of sulfur

impregnated activated carbon.

This paper deals with data, results and more than seven

years of practical field experience obtained by research of

mercury removal unit efficiency during production and

enhancement of natural gas at Molve, Croatia. Paper details the

operating implications of handling and processing natural gas

containing mercury, showing the results of the innovative

approach in the process control, resulting in safety reliability

and process efficiency improvements by plant modification due

to change of gas flow direction.

KEYWORDS: natural gas, mercury removal, sulfurimpregnated activated carbon, process safety,

environmental protection

INTRODUCTION

Chemical and petroleum processes and pipelines pose

very special risks to public safety and the environment

Whether the cause of the risk/damage is a catalyst poisoning

corrosion damage, sudden spill or the gradual contamination of

a site through underground leakage, major financial losses are a

constant threat. Environmental laws focused on occupationa

and public safety, along with "right to know" laws, have

become increasingly more stringent relative to these issues

Rising public pressure and regulation prescribe minimum

design, construction, operations, control and maintenancerequirements for such facilities. Due to stringent environmenta

laws, which make activities more difficult and more

expensive, all industry sectors, especially petrochemica

operators have to consider removal of hazardous pollutants.

Even in small amounts, mercury and its compounds

have an extremely harmful effect on human health [1]. Mercury

also represents a very important and complex problem in

hydrocarbon exploration & production conditions (detection

protection, and removal). Moreover, mercury corrosion attack

endangers dramatically process plants and facilities. Numerous

reported cases all over the world (USA, Algiers, Indonesia

describe huge failures resulting with great damage andenvironmental catastrophes as a consequence of uncontrolled

presence of mercury in gas. Therefore, prevention of mercury

entering into eco-technology-system is absolutely critical. The

problem of mercury content in the natural gas has to be

approached and solved in an accomplished and safe way, taking

into consideration three seemingly contradictory and ye

compatible and equally important criteria:

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1. Technological

2. Ecological

3. Economical

Upon investigating the mercury removal technologies

available, Croatian scientists concluded that according to the

best world experience, sulfur impregnated activated carbon was

the best suited for achieving the mercury removal objectives at

Molve natural gas processing plant [2].

MERCURY CORROSION

The implication of the effects of mercury in natural

gas was not reported until 1973, when a catastrophic failure of

aluminium heat exchangers occurred at the Skikda liquefied

natural gas plant in Algeria[3]. Investigations determined thatmercury corrosion caused the failure and that the mercury

likely came from an accidental source, such as test instruments

used in plant and field start-up. After the Skikda failure, a study

of the Groningen field in Holland revealed similar corrosion in

the gas-gathering system. CO2 was initially thought to be the

cause [4], but later investigations [5] pinpointed mercury, with

concentrations ranging from 0,001 to as high as 180 g /m3.

Phannenstiel et al. [6] state that most if not all of the

mercury in natural gas is in the elemental form and that no

natural gas processing plant problems are suspected to have

been caused by organic or inorganic mercury compounds, and

that elemental mercury is the probable cause of mercury

corrosion problems. Even more, trace quantities of H2S, very

often present in the natural gas, are the catalyst for the reaction

of mercury with iron oxide from the pipe (vessel/reactor).

Although the concentration of mercury in a given natural gasmay be considered extremely low, Audeh [7]observes that its

effect is cumulative as it amalgamates. Elemental mercury

forms an amalgam with the surface layer of the metal it

contacts.

To date, the most serious problems reported by the

industry owing to mercury corrosion have been the result of

mercury forming an alloy with aluminium (amalgam), which is

much weaker than the metal itself and is often referred to as an

embrittlement. To initiate aluminium corrosion, the tightly

adhering aluminum oxide layer on the surface of the aluminium

must be removed. The mercury/aluminium amalgam process

removes this oxide layer. Saunders et al [8] observed thatbrazed aluminium plate-fin heat exchangers are the

predominant choice for cryogenic service. Aluminium is used

due to its brazeability, excellent mechanical properties at cold

temperatures, and superior heat transfer characteristics. They

further state that mercury can damage the aluminium used in

these exchangers and must be completely removed to no

detectable levels in upstream equipment.

MERCURY REMOVAL

Separation processes represents a practical and cost-

effective approach for eliminating pollution substances from

petroleum and chemical processing applications. Mercury is

very often removed as an impurity and environment pollutan

from process fluid gas streams by adsorption on a fixed

activated carbon bed [9].The adsorbent is primarily designedto extract elemental mercury, i.e. mercury in its vapor state

Physical forces of adsorption are not always sufficient for tota

adsorption of a particular component, e.g. mercury. In this case

the large inner surface may serve as a carrier of the active

component and/or chemical compound, i.e. to take over and

distribute an optimum quantity of impregnating agent. The

impregnation process increases the activated carbon capacity

significantly, in order to produce a special type of sulfur

impregnated activated carbon with a great adsorption capacity

for mercury vapors [10]. The mercury removal process is based

on principle of adsorption and of chemical reaction

(chemisorption) of mercury present in natural gas using

impregnated elemental sulphur in a micro-porous adsorbent

This reaction results in a stable and insoluble compound, the

mercury sulfide.

Several factors are influencing efficiency of mercury

removal from natural gas: composition of gas as well as

concentration of mercury vapor, presence of higher

hydrocarbons, water and other impurities, temperature

pressure, gas flow rate, activated carbon characteristics, contac

time, etc. [11,12]. It is extremely hard to design the

breakthrough curve, or to accurately predict the effective

capacity and expected bed life. To solve all problems a multi

discipline approach to a mercury removal problem is necessary.

The purpose of this research was to define mercury

removal efficiency, mercury loading and distribution profile in

the activated carbon bed and variation in concentration of

elemental sulfur through the bed in the real process conditions

as well as to optimize plant performance and gas flow

directions trying to prolong bed life and improve process

efficiency.

MERCURY ADSORBER

Mercury removal unit (MRU) consists of an adsorber

Fig. 1b. [13], located in the process plant, treating waterwashed natural gas. Beside hydrocarbons (more than 70%

methane), natural gas contains CO2, H2S, and near saturated

levels of water vapor. MRU contains 64 m3 commercia

granular palletized sulfur impregnated activated carbon and is

designed to remove mercury from an average inle

concentration of 1000 g/m3 to less than 5 g/m3 for a period

of 3 years.

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MERCURY MEASUREMENTS

The levels of mercury in natural gas were detected and

quantified at the selected sampling points (inlet to and outlet

from the adsorber bed), applying the mercury analyzer based on

fluorescence detection technique (PS Analytical Sir Galahad)

[14].

Sulfur content in mercury saturated activated carbon,

as total sulfur, was detected by using the instrument with X-ray

fluorescence spectroscopy method.

As to problems related to preservation and storage of

field samples containing mercury [15], it is recommended to

conduct the detection and analysis of activated carbon mercury

content on site, immediately upon unloading, since there is no

need for special preparation prior to sample treatment by means

of suitable field analytical devices. The samples are subject to

significant mercury evaporation at ambient temperature and

pressure. Therefore, in order to complete the investigation, we

have used the standard gravimetric procedure (Escha), that

covers adequately the required concentrations, resulting with

reliable and reproducible data under given circumstances [16].

RESULTS AND DISCUSSION

The mercury removal efficiency results obtained

during this test period clearly indicate good performance of the

sulfur impregnated activated carbon bed [17].

However, after two years of operation, analyses

performed on activated carbon samples obtained from the

mercury removal bed indicated (unexpectedly) high mercuryloading, with mercury loading through very long mass transfer

zone (MTZ), requiring a change-out of the activated carbon

load. Following the carefully prepared unloading and

replacement schedule, the level of the activated carbon

withdrawn from the adsorber vessel has been investigated. The

spent activated carbon was replaced by fresh load, Fig. 2. Based

on sample analysis data, i.e. on results of saturation

investigation, along with determination of fill-up weight and

sulfur content in replaced activated carbon, it was desired and

expected to establish the behavior/progress of the mass transfer

front wave and efficiency of the adsorption process.

The results of determination and investigation of spentactivated carbon did not reveal expected efficiency as was

suggested by literature data. Nevertheless, the obtained results

indicated almost linear regularity of activated carbon mercury

load distribution across the depth of adsorber bed, Fig. 1.a.

A reduced concentration of impregnated sulfur (8 %)

is evidenced at the top of the bed layer, when compared to

original carbon. Namely, in the deeper shifts of adsorber bed,

sulfur content rises to values typical for the fresh carbon (15

%). Regarding the above assumption, we were able to find out

and establish a non-uniformity of (total) sulfur content across

the adsorber bed, Fig. 1.c.

It was found that mercury removal efficiency is

strongly related to the MRU inlet stream temperature. This

finding provides some fundamental data for research into

sulphur impregnated activated carbon mercury remova

inefficiency caused by the loss of active species.

Considering that elevated temperatures promote the

chemical reaction with sulfur, forming mercury sulfide, and the

possibility that mercury occurs in its vapor state increases, it is

desirable to obtain the mercury removal at the highest possible

temperature. Moreover, natural gas is saturated with steam

and, condensation of water within carbon pores should be

prevented. This problem can also be solved by application o

the corresponding temperature regime.

When problems with condensation in the activated

carbon bed, as well as reduction of sulfur content in activatedcarbon became evident, a process improvement was introduced

related to change of gas flow direction, Fig. 3. Gas was sen

towards the heat exchanger E-3201A/B, and then to the

adsorber V-3102 and not, as originally designed, first to the

adsorber and then to the heat exchanger.

Following the process enhancement, an efficient and safe

operation of investigated system has been established and

confirmed [18], since the most adequate removal prevented

introduction of mercury into process and transportation system.

CONCLUSIONS

Improving plant safety/reliability, economic and

environmental efficiency is a major goal of all petroleum

companies. In recent years, there have been significant

advances in safety standards in the industry, and the

development and implementation of environmental, healthy and

safety (EHS) management systems have become the norm. A

key component of EHS effective management is the

CONTROL - recording and analysis of EHS performance

measures. The objectives of advanced EHS management and

effective process control are to reduce risk: to establish safe and

environmentally friendly production, to control a process units

product qualities closer to specification and to maximize itsthroughput against operating constraints.

This research is dealing with the process of mercury

removal from natural gas, based on principle of adsorption and

of chemisorption of mercury by means of sulphur impregnated

activated carbon. In spite of remarkable advances in contro

systems, due to a large number of determining parameters

variability of process variables and to their interaction

(composition and properties of the adsorbent, process stream

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flow diagram) an accurate and safe control of separation

process is not achievable. It is extremely hard to accurately

predict the effective capacity, the breakthrough curve, or the

expected adsorber bed life. The numerous restrictions make it

also impossible to elaborate a reliable automatic control system

for removal of mercury from natural gas. The problem lies in

selection and application of suitable mathematical expression

and/or equation (process modeling techniques) that would

allow the numeric evaluation and description of the correlation,

i.e. of the function of interdependency of all systems key

properties and of the process performances. Therefore, to avoid

safety/reliability problems in the process, the complex

procedure and the key parameters of the system under research

have to be closely and continuously monitored and analyzed

following the program defined and scheduled ahead.With sucha monitoring of process conditions and MRU performances,

failures can be predicted and sometimes avoided by making

operating changes.

By defining mercury distribution profile in the

activated carbon bed and variation of elemental sulfur contentthroughout the bed, we determined the behavior/progress of the

mass transfer front wave, i.e. mass transfer zone, as well as the

dynamics and efficiency of the adsorption process. The process

modification step, that improves mercury removal efficiency

from natural gas in real process conditions, was suggested. The

MRU was re-designed to increase process safety and to meet

the stringent purity requirements, less than 1,0 microgram

Hg/m3 natural gas.

This investigation also revealed that described

methodology enables an appropriate characterization of

analyzed system and insures reliable results for mercury

content in natural gas, in saturated activated carbon as well asin process plant environment. One of the main advantages of

this approach is that this techniques offers a direct, very

accurate and reliable introspection into dynamics and efficiency

of mercury removal from natural gas by means of activated

carbon, exposing also the problems and restrictions that

accompany this procedure.

Finally, all test results clearly indicate that our

innovative approach in mercury control program has been, and

continues to be fully effective in the process equipment

protection and at minimizing contribution of mercury in the

working and living environment.

LITERATURE

1. Agency for Toxic Substances and Disease Registry

(ATSDR)., Toxicological profile for mercury. Atlanta,

GA: U.S. Department of Health and Human Services,

Public Health Service, 1994.

2. Spiric Z. and Vadunec J.: Protection of Process Plantand Environment against Mercury during Natural Gas

Production, 12-th International Congress of Chemical

and Process Engineering - CHISA 96, Prague 1996.

3. Kinney, G.T.: Skikda LNG Plant Solving Troubles,

Oil&Gas J. Sept, 15, 1975

4. Leeper J.E.: Processing/ (A study of) Mercury

Corrosion in (Mixes-Refrigerant) Liquefied Natural

Gas Plants, Q. Can. Gas Process. Assoc. Meet.

(Calgary 9/10/80) Energy Process., Vol.73, N.3, Jan.-

Feb. 1981. 46-51,

5. Situmorang M.S. and Muchlis M.: Mercury Problems

in the Arun LNG Plant. 8th Int. Gas Union-Int. Inst.

Refrig.-Inst.Gas Tecnol., Jt. Int. LNG Congress, LosAngeles, paper 1 II-6., 1986,

6. Phannenstiel L.L., McKinley C. and Sorensen J.S.:

Mercury in Natural gas, Paper PAP76-T-12 presented

at the American gas Assn. Operation section

Transmission Conference, Las Vegas, May 3-5, 1976.

7. Audeh C.A.. Hoffman B.E. and Kirker G.W.: Process

for the production of natural gas condensate having a

reduced amount of mercury from a mercury-

containing natural gas well stream, Patent: United

States; US 5209913 A, Application: US 343693

890427, PP.: 7 pp., 1996.

8. Saunders J.B., Pahade R.F. and Delnicki W.V.:

Cryogenic Nitrogen rejection, Proc. ASME Annual

Energy- Sources Technology Conf. Hydrocarbon

Process Symposium, Dallas, pp. 43-49, Feb.15-18,

1987.

9. Ruthven DM (1984) Principles of Adsorption and

Adsorption Processes, Wiley, New York

10. Hutchins RA (1979) Activated Carbon Systems for

Separations of Liquids, in Scweitzer PA (Ed.),

Handbook of Separation Techniques for Chemical

Engineers, McGraw-Hill, New York pp. 1-415

11. Biscan D.A. and McNamara J.D.: Mercury Detection

and Removal: Field Experience in the USA and

Abroad, Proceedings 14-th Int. Conf. LNG., pp. 8-17,

April 7-14, 1980.

12. Bourke M.J. and Mazzoni A.F.: The roles of activatedcarbon in gas conditioning, Proc. Laurance Reid Gas

Cond. Conf., PP.: 137-58, 1989.

13. Spiric Z (1996) Mercury Removal from natural gas

with sulfur impregnated activated carbon - our

experience, 4-th International Conference on Mercury

as a Global Pollutant, Hamburg

14. Spiric Z. and Stockwell P.B., Ambient Air Mercury

measurements during Natural Gas Production,

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American Environmental Laboratory Vol. 10, No. 5.,

16-20 (1998)

15. Bloom NS (1992) Considerations in sampling for and

analysis of mercury at uncharacterized spill sites, in

Charlton DS and Harju JA, eds., Workshop on

Mercury Contamination at Natural Gas Industry Sites:

Chicago, GRI-92/0214

16. Spiric Z. and Hraste M, (1998) Mercury saturation

profile across the sulphur impregnated activated

carbon bed, In Ebinghaus R, Turner RR, Lacerda D,

Vasiliev O. and Salomons W. (eds), Mercury

Contaminated Sites: Characterization, Risk

Assessment and Remediation, 409-417, Springer

Environmental Science, Springer Verlag Heidelberg,17. Spiric Z, Dragas M, Vadunec J, Mashyanov NR and

Ozerova N. (1999) Investigation of mercury content in

Podravina gas fields and environment, 6th International

Petroleum Environmental Conference, November 16-

19, Houston, USA18. Horvat M, Jeran Z, Spiric Z, Jacimovic R and

Miklavcic V.: Mercury and other elements in lichensat INA-Naftaplin gas treatment plant, Molve, Croatia,

Journal of Environmental Monitoring, 2000, volume 2,

issue 2, 139-144.

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Documents

ETCE 2001