Presentation

MODELING CARBON DIOXIDE GAS ABSORBER UNIT FOR SWEETENING OF NATURAL GAS

USING METHYL DIETHANOL AMINE

BY

OKOROMA, JUSTICE(B.TECH, CHEMICAL/PETROCHEMICAL ENGINEERING)

RIVERS STATE UNIVERSITY OF SCIENCE & TECHNOLOGY.2012

1

Highlights of Presentation

1. INTRODUCTION

2. LITERATURE REVIEW

3. MODEL DEVELOPMENTS

4. RESULTS AND DISCUSSION

5. CONCLUSION AND RECOMMENDATIONS

6. REFERENCES

2

INTRODUCTION Carbon dioxide (CO2) is considered to be the largest contributor to

global warming problem. CO2 removal from industrial streams is needed to control greenhouse gas emissions for environment protection.

The presence of tiny amounts of CO2 can act as poison in catalytic processes such as ethylene polymerization. CO2 also results to severe corrosion damage on the production facilities and pipelines, and icing in natural gas liquids (NGL) production and transportation.

Thus, the removal of CO2 from gas streams is a very important operation for natural gas processing, oil refining and petrochemical processes.

AIM:The aim of this project is to develop and simulate a mathematical model of an absorber for the removal of carbon dioxide (CO2) from natural gas using Methyl Diethanol Amine (MDEA).

3

Introduction (continued)

OBJECTIVES: Develop a mathematical model that will predict the variation of CO2

concentration, variation of gas (CO2) and MDEA temperatures across the absorber.

Solve the resulting model equations numerically by developing algorithm and program using MATLAB; validate models results using plant data.

Perform a simulation of the absorber using the models developed to determine the effect of the process variables on the absorber, thus predict the performance of the absorber.

SCOPE: Methyl Dimethyl Amine (MDEA) is used as the absorbent. Packed column is considered as the gas absorber.

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LITERATURE REVIEW The most useful concept of the process of absorption is given by the

two-film theory due to Whitman (Richardson et al, 2002).

The separated pure CO2 is further compressed and injected into an aquifer, accounting for an annual amount of 2 million tons of CO2 (Jimmy Xiao et al, 2000).

The Pitzer model has recently been applied for the physical and chemical modeling of aqueous systems of CO2 and alkanolamines (Li and Mather, 1997).

Kaewschian et al. (2001) used a similar approach based upon the electrolyte-UNIQUAC model (Sander et al., 1986) to predict the solubility of CO2 and H2S in aqueous solution of MEA and MDEA.

The first attempt to treat absorption equilibria in a thermodynamically rigorous manner was made by Edwards et al (1975).

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Literature Review (continued)

Process Description: A simplified flow diagram of a typical CO2 removal unit using MDEA is

shown below. Sweet Gas

Lean Amine solvent

Absorber

Rich Amine Solvent

Sour Gas

Regenerator

Lean Amine

Carbon dixode

Schematic of the CO2 Absorber and MDEA Regenerator (Richardson et al, 2002).

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MODEL DEVELOPMENT

Model Assumptions: Only CO2 and MDEA are transferred across the interface Steady state conditions are applied.

To illustrate the mass and heat transfers in differential section of the Absorber, let’s consider an elemental packed height or differential section of the absorber, with its associated inflow and outflow streams, shown here.

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Model Development (continued)

Models of Mass transfer rates: For CO2:

NCO2.a .dz = KGa (yCO2 - yCO2,e) dz = - G .dyCO2

𝑑𝑦𝐶𝑂2𝑑𝑧 = − 𝐾𝐺𝑎(𝑦𝐶𝑂2 − 𝑦𝐶𝑂2,𝑒)𝐺 (3.1) For MDEA:

𝑁𝑀𝐷𝐸𝐴 𝑎∙𝑑𝑧 = 𝐾𝐿,𝑀𝐷𝐸𝐴𝑎൫𝑋𝑀𝐷𝐸𝐴 − 𝑋𝑀𝐷𝐸𝐴,𝑒൯𝑑𝑧 = −𝐺∙𝑑𝑋𝑀𝐷𝐸𝐴 𝑑𝑋𝑀𝐷𝐸𝐴𝑑𝑧 = − 𝐾𝐿,𝑀𝐷𝐸𝐴𝑎(𝑋𝑀𝐷𝐸𝐴 − 𝑋𝑀𝐷𝐸𝐴,𝑒)𝐺 (3.2)

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Model Development (continued)

Energy (Enthalpy) Balance:

ቌ𝐻𝑒𝑎𝑡 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑟𝑎𝑡𝑒 𝑓𝑟𝑜𝑚 𝑔𝑎𝑠𝑡𝑜 𝑙𝑖𝑞𝑢𝑖𝑑 𝑝ℎ𝑎𝑠𝑒ቍ = ൬𝐼𝑛𝑝𝑢𝑡 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑒𝑛𝑡ℎ𝑎𝑙𝑝𝑦൰ − ൬𝑂𝑢𝑡𝑝𝑢𝑡 𝑟𝑎𝑡𝑒𝑜𝑓 𝑒𝑛𝑡ℎ𝑎𝑙𝑝𝑦൰

± ቌ𝐻𝑒𝑎𝑡 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑐𝑎𝑢𝑠𝑒𝑑 𝑏𝑦𝑚𝑎𝑠𝑠 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟ቍ ሺ3.3ሻ Model for Gas Phase:

Model for Liquid Phase:

𝑑𝑡𝐿𝑑𝑧 = ℎ𝐺𝑎ሺ𝑡𝐺 − 𝑡𝐿ሻ𝐿𝐶𝐿 + ሺ𝐶𝑝𝐶𝑂2ሺ𝑡𝐺 − 𝑡𝑜ሻ+ 𝐻𝑂𝑆ሻൣ �−𝐾𝐺𝑎(𝑦𝐶𝑂2 − 𝑦𝐶𝑂2,𝑒൧𝐿𝐶𝐿

+ ሺ𝐶𝑝𝑀𝐷𝐸𝐴ሺ𝑡𝐺− 𝑡𝑜ሻ+ 𝐻𝑉ሻൣ �−𝐾𝐿,𝑀𝐷𝐸𝐴𝑎൫𝑋𝑀𝐷𝐸𝐴 − 𝑋𝑀𝐷𝐸𝐴,𝑒൯൧𝐿𝐶𝐿 (3.11)

𝑑𝑡𝐺𝑑𝑧 = −ℎ𝐺𝑎(𝑡𝐺− 𝑡𝐿)𝐺ȋ𝐶𝑝𝐵+𝐶𝑝𝐶𝑂2𝑦𝐶𝑂2+𝐶𝑝𝑀𝐷𝐸𝐴𝑋𝑀𝐷𝐸𝐴Ȍ (3.8)

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Process Parameters and Operating conditions:

Property Inlet Outlet Gas Temperature (K) 313.67 325.11 Liquid (MDEA) Temperature (K) 313.40 316.33 CO2 mole fraction (mol/mol%) 0.0167 0.00000

Table 3.1: Main properties of Absorber column feedstock and products (NAOC OB/OB Gas Plant data)

Table 3.2: Mass and Heat Transfer coefficients; parameters of gas and liquid phases and Absorption column operation conditions

Parameter Symbol Value Unit Reference Height of column 𝑧 20 m Plant data Specific interfacial surface area

𝑎 416 m2/m3 Plant data

Gas – film transfer coefficient in terms of mole fraction

𝐾𝐺 0.000096 kmol/m2s Karl A. H., 2003

Liquid – film transfer coefficient in terms of mole fraction

𝐾𝐿,𝑀𝐷𝐸𝐴 0.000051 kmol/m2s Karl A. H., 2003

Molar gas flux or gas phase molar velocity

𝐺 0.0148 kmol/m2s Tontiwachwuthikul, et al (1992)

Equilibrium (interface) mole fraction of CO2 in gas phase

𝑦𝐶𝑂2,𝑒 0.191 unitless Plant data

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Process Parameters and Operating conditions (cont’d)

Parameter Symbol Value Unit Reference Equilibrium (interface) mole fraction of MDEA in gas phase

𝑋𝑀𝐷𝐸𝐴,𝑒 0.325 unitless Plant data

Heat transfer coefficient of gas phase

ℎ𝐺 0.01 KJ/m2sK Karl A. H., 2003

Enthalpy of gas phase 𝐻𝐺 19000 KJ/kmolK Karl A. H., 2003 Enthalpy of liquid phase 𝐻𝐿 26000 KJ/kmolK Karl A. H., 2003 Specific heat of CO2 𝐶𝑝𝐶𝑂2 37.13 KJ/kmolK Karl A. H., 2003 Specific heat of MDEA 𝐶𝑝𝑀𝐷𝐸𝐴

49.982 KJ/kmolK Tontiwachwuthikul, et al

(1992) Specific heat of gas 𝐶𝑝𝐵 37.13 KJ/kmolK Tontiwachwuthikul, et al

(1992) Specific heat of liquid 𝐶𝐿 49.982 KJ/kmolK Karl A. H., 2003 Heat of reaction – include heat of solution for CO2

𝐻𝑂𝑆 3.9 x 105 KJ/kmolK Tontiwachwuthikul, et al (1992)

Heat of vapourization of MDEA

𝐻𝑉 2.6 x 104 KJ/kmolK Tontiwachwuthikul, et al (1992)

Reference temperature 𝑡𝑜 298 K Karl A. H., 2003 Molar liquid flux 𝐿 0.0095 kmol/m2s Tontiwachwuthikul, et al

(1992)

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RESULTS AND DISCUSSIONS1. Variation of CO2 concentration through absorption column height :

0 5 10 15 20 250

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

Absorption column height, Z (m)

CO2

Conc

entr

ation

, yCO

2 (m

ol/m

ol%

)

12

Results and Discussions (continued)

2. Variation of temperature of liquid (Methyl diethanol Amine -MDEA) across the absorption column height :

0 5 10 15 20 25309

310

311

312

313

314

315

316

317

Absorption column height, Z (m)

Liqu

id T

empe

ratu

re, K

(oC)

13

Results and Discussions (continued)

3. Variation of temperature of gas through absorption column height :

0 5 10 15 20 25308

310

312

314

316

318

320

322

324

326

Absorption column height, Z (m)

Gas

Tem

pera

ture

, Tg

(oC)

14

Results and Discussions (continued)

4. Variation of CO2 concentration through absorption column height at different MDEA weight %:

0 5 10 15 20 250

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

MDEA = 55%

MDEA = 50%

MDEA = 45%

MDEA = 38.22%

Absorption column height, Z (m)

CO2

Conc

entr

ation

, yCO

2 (m

ol/m

ol%

)

15

Results and Discussions (continued)

5. Variation of CO2 concentration through absorption column height at different MDEA weight % (3D Surface Plot for visualization):

16

02

46

810

1214

1618

20

00.0020.0040.0060.0080.01

0.0120.0140.0160.018

MDEA = 55%

MDEA = 50%

MDEA = 45%

MDEA = 38.22%

Absorption column height, Z (m)

CO2

Conc

entr

ation

, yCO

2 (m

ol/m

ol%

)

Results and Discussions (continued)

6. Effect of MDEA concentration on the outlet CO2 concentration :

35 40 45 50 55 600

0.0005

0.001

0.0015

0.002

0.0025

MDEA Concentration (wt %)

Out

let C

O2

con

cent

ratio

n, y

A (m

ol/m

ol%

)

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Results and Discussions (continued)

7. Variation of CO2 concentration through absorption column height at different Gas Flow Rate :

0 5 10 15 20 250

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

G = 80,000 std.m3/hr

G = 120,000 std.m3/hr

G = 160,000 std.m3/hr

G = 200,000 std.m3/hr

Absorption column height, Z (m)

CO2

Conc

entr

ation

, yCO

2 (m

ol/m

ol%

)

18

Results and Discussions (continued)

8. Variation of CO2 concentration through absorption column height at different Gas Flow Rate (3D Surface Plot) :

02

46

810

1214

1618

20

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

G = 80,000 std.m3/hr

G = 160,000 std.m3/hr

Absorption column height, Z (m)

CO2

Conc

entr

ation

, yCO

2 (m

ol/m

ol%

)

19

Results and Discussions (continued)

9. Variation of CO2 concentration through absorption column height at different Gas Flow Rate :

0 5 10 15 20 250

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

MDEA = 55%

MDEA = 50%

MDEA = 45%

MDEA = 38.22%

Absorption column height, z(m)

CO2

conc

entr

ation

, yA

(mol

/mol

%)

20

CONCLUSION AND RECOMMENDATIONSTable 4.1: Comparison between Plant and Predicted Process Parameters

Process Parameter Model Prediction

Plant Data % Deviation

CO2 concentration 0.00000 0.00000 0.00 Gas Outlet temperature (K) 324.32 325.11 0.24 Liquid MDEA Outlet temperature (K)

312.00 313.40 0.44

CONCLUSION: Good agreements were found between the plant data and the model

predictions, since the values derived from model predictions showed little or no deviations from the plant data.

MDEA weight % of 45 is preferable for optimal absorption. It is also economical too as additional MDEA concentration would have a cost implication on the absorption facility operation.

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CONCLUSION AND RECOMMENDATIONS

RECOMMENDATIONS: Future research work should focus on the regeneration section of the

absorption plant.

More plant data should be obtained from companies like Total E&P and SPDC that operate CO2 Absorption to allow a comparative study to be made.

Future research works should also take into cognizance the CO2

dependence on residence time of the absorption process. The effect of a higher or lower time interval spent by natural gas and MDEA in the absorber would provide further breakthrough on efficient or optimal plant operations.

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REFERENCES• Edwards, T.J., Newman, J. and Prausnitz, J.M. (1975), Thermodynamics of

Aqueous Solutions Containing Volatile Weak Electrolytes, AIChE J., 21:248259.

• Jimmy Xiao, Chih-Wei Li, Meng-Hui Li, (2000) Kinetics of absorption of carbon dioxide into aqueous solutions of 2-amino-2-methyl-1-

propanol+monoethanolamine, Chemical Engineering Science, Vol. 55, pp. 161-175.

• Karl A. H. (2003) Modelling and experimental study of CO2 absorption in a membrane contactor, Thesis – NUST, pg.102.

• Kaewsichan,.L. ,Al-Bofersen, O., Yesavage, V.F. . Sami Selim, M. (2001), Predictions of the solubility of acid gases in monoethanolamine (MEA) and methyldiethanolamine (MDEA) solutions using the electrolyte- Uniquac model, Fluid Phase Equilibria, 183, 159-171.

• Li, Y. G., and Mather, A. E. (1997) Correlation and prediction of the solubility of CO2 and H2S in aqueous solutions of methyldiethanolamine. Industrial and Engineering Chemistry Research, 36, 2760-2765.

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REFERENCES

• NAOC OB/OB Gas Plant Data (2006) Agip KCO Training Special Project, OJT Technical Workbook – Obiafu/Obrikom Gas Plant.

• Richardson, J.F , Harker J. H., Backhurst J. R. (2002) Coulson & Richardson’s Chemical Engineering: Particle Technology and

Separation Process, Vol.2, 5th ed., Elsevier, New Delhi, India.

• Sander G., Carey, T., Rochelle, G., (1986) A model of acid gasabsorption/stripping using methyldiethanolamiene with added acid. Gas separation and purification, 5, 95-109.

• Tontiwachwuthikul, P.; Meisen, A.; Lim, J. (1992) CO2 absorption by NaOH, Monoethanolamine and 2-Amino-2-Methyl-1-Propanol in a packed column, Chemical Engineering science, 47 (2), 381– 390.

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THANKS FOR LISTENING&

GOD BLESS

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