001 CET 1112 012 Dynamic Simulation Sweetening Process Natural Gas

Canadian Journal on Chemical Engineering & Technology Vol. 2 No. 9, December 2011

156

DYNAMIC SIMULATION OF SWEETENING

PROCESS OF NATURAL GAS T. Abbas, M. Ghauri, Z. Rashid, M. Shahid

Abstract — Natural gas is incomparable to other

energy sources in economics amenity and in environmental

interests. Carbon dioxide (CO2), hydrogen sulphide (H2S)

and other impurities are found in natural gas stream that

concern the quality and performance of gas. In order to

reduce their concentration less than 2 % to meet the

contractual specification, amine sweetening process is

used. In this paper, simulation study and modeling of

equipment for gas sweetening plant with di-ethanolamine

(DEA) has been investigated on HYSYS in “DYNAMIC

STATE”. The aim of this study is to design, optimize the

sweetening process and study of absorber design in both

steady and dynamic state. Simulation results show that in

dynamic mode the absorption of CO2 is constant

throughout the process.

Key Words — Natural Gas, Amine Process,

Simulation, Modeling, CO2, H2S.

I. INTRODUCTION

Discovery and utilization of coal has tremendous

effects in industrial development on this planet earth.

That is why the nineteenth century was known as a

century of coal and with the help of this fossil fuel

industrial revolution in Europe took place as this

provided the electricity to the entire Europe. As a

matter of fact, the twentieth century was called the

century of oil, the major energy source to support the

expansion of global economy. Natural gas took over

the position of coal as the number two energy source

behind oil in the late twentieth century. This boom of

global expansion is because of numerous factors,

including development of new markets, substitution

of coal as fuel for providing space and industrial

utilities requirement, use of natural gas in making

petrochemicals and fertilizers, and strong demand for

low-sulfur fuels [1,2].

Natural gas mainly consists of large quantity of

methane along with heavier hydrocarbons such as

higher alkanes and alkenes; moreover in the raw state

it often contains a considerable amount of non

hydrocarbons, such as nitrogen and the acid gases

(carbon dioxide and hydrogen sulfide).A natural gas

stream is approximately two mole percent (mol%)

sour. It means for every 100 kgmoles of gas 2

kgmoles of hydrogen sulfide (H2S) are present in it

[3]. As a matter of fact, H2S and CO2 are corrosive in

aqueous solution. Furthermore, they are very toxic in

nature and have very low heating value. Due to these

facts, sales gas is required to be sweetened to contain

no more than a quarter grain H2S per 100 standard

cubic feet (4 parts per million) and to have a heating

value of no less than 920 to 980 Btu/SCF, depending

on the contract [4].

There are quite a few treating processes available for

removal of acid gases from natural gas, including

Chemical solvents, Physical solvents, Adsorption

Processes Hybrid solvents and Physical separation

[5].

In the past few years, amine solvents for the removal

of acid gases have received increased attentions. In

this process the acidic components react with an

alkanol-amine absorption liquid via an exothermic,

reversible reaction in a gas/liquid contactor. In a

following process step the acidic components are

removed from the solvent in a regenerator, usually at

low pressure and/or high temperature [6]. Mono-

ethanol amine (MEA), di-ethanol amine (DEA), di-

isopropanol amine (DIPA) and N-methyl di-ethanol

amine (MDEA) are widely accepted alkanol amines

for industrial operations [7].

In the present paper, the use of amine di-

ethanolamine (DEA) has been investigated for a

variety of cases using a process simulation program

HYSYS in dynamic state.

II. AMINE PROCESS

The low operating cost and flexibility of tailoring

solvent composition to suit gas compositions make

this process one of the most widely used [8].


157

Sour gas is firstly fed into a unit, scrubber, for the

removal of entrained liquid phase, water and liquid

hydrocarbons. The gas then enters the bottom of

absorption tower most of the sweet gas exits at the

top of tower.The regenerated amine usually called

lean amine enters at the top of this tower and the two

streams are contacted counter-currently. In this

tower, CO2 and H2S are absorbed with the chemical

reaction into the amine phase. The exit amine

solution, loaded with CO2 and H2S is called rich

amine. This stream is flashed, filtered and then fed to

the top of a stripper to recover the amine and acid

gases are stripped and exit at the top of the tower.

The refluxed water helps in steam stripping the rich

amine solution. The regenerated amine is recycled

back to the top of the absorption tower as shown in

figure 1.

Table.1 Comparison of Amine solvents [10]

Solvent MEA DEA DIPA

Ch.Formula. RNH2 R2NH R`NH

Mol.wt 61 105 133

Amine type Primary Secondary Secondary

Vap.Pressue 100oF(mm

Hg) 1.05 0.058 0.01

Freezing Point oF. 15 20 16

Loading (mol Acid-

gas/mol amine)

0.35 0.5 0.7

H2S/CO2 selectivity 1 1 2

Solvent conc.(wt%) 15-20 20-35 30-40

Acid gas (mol/m) 0.3-0.4 0.5-0.6 0.3-0.4

Circulation

(gal/mole acid gas)

100-

165

60-125 --------

Steam rate(lb/gal) 1-1.2 0.9-1.1 ------

Reboiler temperature oF 240 245 255

Heat of reaction(Btu/lb

acid gas, H2S)

620 630 0

Among the amines shown in the above table 1, the

most common, less expensive, easy to install and

operate absorbent is Di-ethanolamine (DEA). It is a

comprehensive substitute of MEA and becoming the

most widely used gas sweetening solvent. It is a

secondary amine with lower reactivity and less

corrosive than MEA. Moreover, it reacts with COS

and CS2 and the product can be regenerated. It is less

volatile hydrocarbon and possesses low heat of

reaction than MEA. It means DEA is easy in

processing and easy to regenerate [8].

The basic reactions with CO2 and H2S are [8,9]

2�� + �� ⇔ �� 1

H2S reacts with MEA, DEA, or MDEA to form

hydrogen sulfide and protonated amine.

�� + �� ⇔ 2�� 2

2�� + �� ⟺ �� 3

Based on above reactions, 1.7 lbs of DEA can be

used to react with the same amount of acid gas as 1.0

lb of MEA. Because of its lower corrosive nature,

higher strength upto 35% (wt) of DEA can be used.

Loading up to 0.65 mol of acid gas per mole of DEA

can cause fewer operational problems than MEA

because the elimination of the degradation products

and the absence of a reclaimer. DEA is weaker than

MEA so that Corrosion is less in a process where

DEA is used instead of MEA. Foaming is reduced

due to absence of degradation and corrosion products

[8].

III. Simulation and Modeling

The simulation model is developed on Aspentech

HYSYS 7.1.The type of fluid package selected is

Amine Package.DEA used as an aqueous absorbent

to absorb H2S and CO2 from sour gas streams. Before

entering the contractor, the sour gas is passed through

an inlet separator where entrained droplets of liquid

are removed from the gas stream. Specification of

sour gas is shown in table 2.

Table 2. Inlet sour gas and amine specifications

Parameters Values

Inlet gas flow rate 1245 kgmole/hr

Inlet liquid flow rate 1886 kgmole/hr

Inlet gas temperature 86 oF

Inlet liquid Temperature 99 oF

Amine concentration 28%

Gas in pressure 998 psia

L. Amine in pressure 995 psia

H2S in sour gas 2.5 mole%

CO2 in sour gas 4.13 mole %


158

Figure 1. Process Flow Sheet.

The whole process is modeled and simulated in

dynamic state as shown in figure (1) in order to get

the best absorption results, to avoid foaming, to meet

the sweet gas specification which may be result of

poor contact of sour gas and solvent. In order to

avoid all these problems controller loops are used

that after regular interval of time check the

composition of streams and take the action to get

exact required sweet gas specification all the times.

IV. Results and Discussion

The aim of the study is to investigate the effect of

using diethanolamine (DEA)) on the natural gas

treatment process using the process simulation

program HYSYS in dynamic state.

Temperature profile

It can be observed that in figure (2, steady state) and

figure (3, dynamic state) the temperature continues to

increase. This may be explained as follows. As the

liquid flows down the tower, it continues to absorb

acid gas. This absorption is accompanied by a heat of

reaction, which causes the temperature of the liquid

to continue to rise. The temperature drop at the

bottom of the tower results from the cold gas entering

the bottom and contacting the hot liquid flowing

downwards. The cold gas absorbs heat from the hot

liquid causing its temperature to decrease. This

results in a temperature profile at bottom of the

tower.

No. of Stages

0 5 10 15 20 25

Te

mp

era

ture

[o

C ]

30

35

40

45

50

55

60

65

70

Figure 2. Temperature Profile Vs No. of Stages (steady state).


159

No. of Stages

0 5 10 15 20 25

Tem

pera

ture

[oC]

30

40

50

60

70

80

Figure 3. Temperature Profile Vs No. of Stages (dynamic state)

Absorption of CO2 and H2S

The absorption of CO2 and H2S on different stages of

absorber with 20 trays can be observed in figure (4).

The sweet gas has composition of CO2 and H2S in

mole fraction are 0.0000004 and 0.0000001

respectively with net molar flow in MMSCFD for

both liquid and vapor phases in absorber as shown in

figure(5). The whole process is simulated in dynamic

mode, sweet gas with constant composition of CO2

and H2S is obtained, the strip chart covering the

complete simulation results of CO2 and H2S is shown

in figure (6)

No.of Stages

0 5 10 15 20 25

Mo

le F

ract

ion

Ab

sorb

ed

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

No.of Stages vs CO2 Absorbed

No.of Stages vs H2S Aborbed

Figure 4. Mole Fraction of CO2 and H2S absorbed


160

No. of Stages

0 5 10 15 20 25

Net M

ola

r F

low

[M

MS

CF

D]

22

24

26

28

30

32

34

36

38

40

42

No.of Stages vs Vapor Molar Flow

No.of Stages vs Liquid Molar Flow

Figure 5. Molar Flow in absorber

Strip Chart of Simulation Results

Strip chart of simulation model shows the constant

result of sweetening process. The integrator runs

from 0 to 20 minutes, CO2 concentration in sweet gas

is regularly observed. In figure (6) it can be seen that

CO2 concentration is 0.000000 (mole fraction) with

same flow rate of DEA to absorber throughout the

process.

Figure 6: Strip chart of Simulation Results


Conclusion

The principle investigation was directed to study the

effect of using diethanolamine (DEA), as solvent on

the gas treatment process using the software HYSYS

In all cases, DEA was the best result in the absorption

processes of CO2 and H2Sfrom the natural g

dynamic state The simulation result show that, by

using of the 1886(kg mole /h) DEA with T=99

P=995 (psia) were the best result in the absorption

processes of CO2 (0.0000004, mole fraction) and H

(0.0000001, mole fraction) without any fluctu

throughout the dynamic process.

REFERENCES

[1] Vadud.A.S. “Natural gas desulfurization.Slovenska Technicka

Univerzita V Bratislave”, 2009.

[2] Ikoku,C.U, Natural Gas Production Engineering.

John Weley & Sons, Krieger Publishing Company.,

13. ISBN: 0-89464-639-7.

[3] Abdel-Al. H.K., Aggour. M., and Fahim M.A, Petroleum and

gas Field Processing , Marcel Dekker, Inc., 2003, pp 269

ISBN: 0-8247-0962-4.

BIOGRAPHIES

Engr. Tauqeer Abbas is Research Associate in

the department of “Environmental and Energy

Engineering” at COMSATS Institute of

Information Technology Vehari

Born in 1988-Pakistan. He holds is B.Sc

degree in the field of Chemical Engineering

from COMSATS Institute of Information

Technology, Lahore, Pakistan. His area of

interest is Energy Conservation, Modeling and Simulation of

Separation Processes. He is an active member of Proces

Engineering Group of Biomass Conversion Research Center, CIIT,

Lahore, Pakistan. He is quite energetic and young researcher,

always ready to accept challenges. His email id is

[email protected]

Dr. Moinuddin Ghauri is Associate Professor

in Chemical Engineering Department at

COMSATS Institute of Information

Technology Lahore-Campus

in 1963-Pakistan. He holds Ph.D. in the field

of Chemical Engineering, Energy and

Environment Conservation from the

University of Sheffield, UK. Energy and

Environment is his focus area. He has exposure of working with

leading National, Multinational and European instituti

author of twenty technical teaching-learning resources for

Environmental Control Technology and modules for Chemical

Technology at technical education Pakistan. He is creative by

nature and loves creative and challenging assignments.

INSERT


161

The principle investigation was directed to study the

effect of using diethanolamine (DEA), as solvent on

process using the software HYSYS.

In all cases, DEA was the best result in the absorption

Sfrom the natural gas in

The simulation result show that, by

using of the 1886(kg mole /h) DEA with T=99 oF and

P=995 (psia) were the best result in the absorption

(0.0000004, mole fraction) and H2S

(0.0000001, mole fraction) without any fluctuation

Slovenska Technicka

[2] Ikoku,C.U, Natural Gas Production Engineering.,New York:

Krieger Publishing Company., 1984. pp 12-

Al. H.K., Aggour. M., and Fahim M.A, Petroleum and

, 2003, pp 269-271.

[4] Mackenzie. D. H., Prambil .F.C., Daniels. C.A., and Bullin

J.A, “ Design and operation of selective sweetening Plant using

MDEA”, Energy Progress., pp 31-36. 1987.

[5] Sohbi.B., Meakaff.M., Emtir.M., and Elgarni M, “The using of

mixing amines in an industrial gas sweetening plant”,

Academy of Science, Engineering and Technology

,2007.

[6] Huttenhuis.P.J.G., Agrawal.N.J., Hogendoorn.J.A., and

Versteeg. G.F, “Gas solubility of H2S and CO

solutions of N-methyl diethanol amine”, Journal of Petroleum

Science and Engineering., vol.55, pp.122-

DOI:10.1016/j.petrol.2006.04.01.

[7] Zare Aliabad.H, Mirazei S,“Removal of CO2 and H2S using

Aqueous Alkanolamine solusions”, World Academy of Science,

Engineering and Technology.,vol.49, pp.1

[8] Abdel-Al.H.K., Aggour.M., and Fahim.M.A., Petroleum and

gas Field Processing ” Marcel Dekker, Inc., 2003.pp. 281

ISBN: 0-8247-0962-4.

[9]Danckwerts.P.V, “The reaction of CO2 with ethanolamines”,

Chem.Eng.Sci.vol.,34,pp.443-446. 1979.

2509(79)85087-3.

[10] Abdel-Al. H.K., Aggour. M., and Fahim M.A., Petroleum and

gas Field Processing , Marcel Dekker, Inc.

ISBN: 0-8247-0962-4.

Engr. Tauqeer Abbas is Research Associate in

the department of “Environmental and Energy

Engineering” at COMSATS Institute of

Information Technology Vehari-Pakistan.

He holds is B.Sc

degree in the field of Chemical Engineering

from COMSATS Institute of Information

Technology, Lahore, Pakistan. His area of

interest is Energy Conservation, Modeling and Simulation of

Separation Processes. He is an active member of Process System

Engineering Group of Biomass Conversion Research Center, CIIT,

Lahore, Pakistan. He is quite energetic and young researcher,

His email id is

Dr. Moinuddin Ghauri is Associate Professor

Chemical Engineering Department at

COMSATS Institute of Information

Campus, Pakistan. Born

He holds Ph.D. in the field

of Chemical Engineering, Energy and

Environment Conservation from the

University of Sheffield, UK. Energy and

is his focus area. He has exposure of working with

leading National, Multinational and European institutions. He is

learning resources for

Environmental Control Technology and modules for Chemical

. He is creative by

nature and loves creative and challenging assignments.

Engr. Zeeshan Rashid is Lecturer in Chemical

Engineering Department at COMSATS

Institute of Information Technology Lahore

Campus, Pakistan. Born in 1985

did his M.S from University of Teesside, U.K

in Process Manufacturi

area of research is Modeling and

and process design. He worked as Process Technologist in Aker

Solution Pvt Limited, UK.

Dr. Muhammad Shahid is Assistant Professor

in the department of “Environmental

Energy Engineering” at COMSATS Institute

of Information Technology Vehari

He is Ph.D. from INP

France in the field of “Soil & Environmental

Sciences”. His area of interest includes

Environmental Toxicology, Soil Science and

Green Energy. His doctoral thesis has been awarded “

Escande” award by INP-ENSAT. He has several publications in

the field of Soil & Environmental Sciences in high impact factor

journals.

INSERT

[4] Mackenzie. D. H., Prambil .F.C., Daniels. C.A., and Bullin

Design and operation of selective sweetening Plant using

36. 1987.

[5] Sohbi.B., Meakaff.M., Emtir.M., and Elgarni M, “The using of

mixing amines in an industrial gas sweetening plant”, World

and Technology.,vol. 31,pp 1-2.

] Huttenhuis.P.J.G., Agrawal.N.J., Hogendoorn.J.A., and

S and CO2 in aqueous

Journal of Petroleum

-134. 2007.

] Zare Aliabad.H, Mirazei S,“Removal of CO2 and H2S using

World Academy of Science,

.,vol.49, pp.1-2. 2009.

Fahim.M.A., Petroleum and

gas Field Processing ” Marcel Dekker, Inc., 2003.pp. 281-282.

“The reaction of CO2 with ethanolamines”,

446. 1979. DOI:10.1016/0009-

Al. H.K., Aggour. M., and Fahim M.A., Petroleum and

Marcel Dekker, Inc., 2003. pp 280-281.

Engr. Zeeshan Rashid is Lecturer in Chemical

Engineering Department at COMSATS

Institute of Information Technology Lahore-

Born in 1985- Pakistan. He

did his M.S from University of Teesside, U.K

Process Manufacturing Engineering. His

Modeling and Simulation

He worked as Process Technologist in Aker

Dr. Muhammad Shahid is Assistant Professor

in the department of “Environmental and

Energy Engineering” at COMSATS Institute

of Information Technology Vehari-Pakistan.

He is Ph.D. from INP-ENSAT, Toulouse-

France in the field of “Soil & Environmental

Sciences”. His area of interest includes

Environmental Toxicology, Soil Science and

een Energy. His doctoral thesis has been awarded “Léoplod

ENSAT. He has several publications in

the field of Soil & Environmental Sciences in high impact factor

001 CET 1112 012 Dynamic Simulation Sweetening Process Natural Gas

Documents

Transcript of 001 CET 1112 012 Dynamic Simulation Sweetening Process Natural Gas