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Aspen Plus
Rate-Based Model of the CO2 Capture Process by Methanol using Aspen Plus
Copyright (c) 2008 by Aspen Technology, Inc. All rights reserved.
Aspen Plus, the aspen leaf logo and Plantelligence and Enterprise Optimization are trademarks or registered trademarks of Aspen Technology, Inc., Cambridge, MA.
All other brand and product names are trademarks or registered trademarks of their respective companies.
This document is intended as a guide to using AspenTech's software. This documentation contains AspenTech proprietary and confidential information and may not be disclosed, used, or copied without the prior consent of AspenTech or as set forth in the applicable license agreement. Users are solely responsible for the proper use of the software and the application of the results obtained.
Although AspenTech has tested the software and reviewed the documentation, the sole warranty for the software may be found in the applicable license agreement between AspenTech and the user. ASPENTECH MAKES NO WARRANTY OR REPRESENTATION, EITHER EXPRESSED OR IMPLIED, WITH RESPECT TO THIS DOCUMENTATION, ITS QUALITY, PERFORMANCE, MERCHANTABILITY, OR FITNESS FOR A PARTICULAR PURPOSE.
Aspen Technology, Inc. 200 Wheeler Road Burlington, MA 01803-5501 USA Phone: (1) (781) 221-4300 Toll Free: (1) (888) 996-7100 URL: http://www.aspentech.com
Contents 1
Contents
Introduction............................................................................................................2
1 Components .........................................................................................................3
2 Process Description..............................................................................................4
3 Physical Properties...............................................................................................6
4 Simulation Approaches.......................................................................................15
5 Simulation Results .............................................................................................17
6 Conclusions ........................................................................................................18
References ............................................................................................................19
2 Introduction
Introduction
This document describes an Aspen Plus rate-based model of the CO2 capture process by methanol (MEOH) from a gas mixture of H2, CO2, CO, N2, CH4, H2S and COS from gasification of Western Kentucky coal char[1]. The operation data from a pilot scale absorber[1] are used to specify the feed conditions and unit operation block specifications in the model. Thermophysical property models have been validated against DIPPR correlations[2] for component vapor pressure and liquid density, and literature data for vapor-liquid equilibrium from Semenova (1979)[3] and Leo(1992)[4]. Transport property models have been validated against literature data for viscosity[5-9], thermal conductivity[10-13] , surface tension[7, 14-18], and diffusivity[19].
The model includes the following key features:
• PC-SAFT equation of state model for vapor pressure, liquid density and phase equilibrium
• Transport property models
• Rate-based model for absorber with ceramic Intalox saddles packing
1 Components 3
1 Components
The following components represent the chemical species present in the process:
Table 1. Components Used in the Model
ID Type Name Formula
MEOH CONV METHANOL CH4O
CO2 CONV CARBON-DIOXIDE CO2
H2S CONV HYDROGEN-SULFIDE H2S
CO CONV CARBON-MONOXIDE CO
N2 CONV NITROGEN N2
COS CONV CARBONYL-SULFIDE COS
H2 CONV HYDROGEN H2
CH4 CONV METHANE CH4
4 2 Process Description
2 Process Description
The flowsheet for the pilot plant[1] for CO2 capture by MEOH includes an absorber, a flash tank, a stripper and so on. However, only the absorber data are reported.
The sour gas enters the bottom of the absorber, contacts with lean MEOH solvent from the top counter-currently and leaves at the top as sweet gas, while the solvent flows out of the absorber at the bottom as the rich solvent with absorbed CO2 and some other gas components.
Table 2 presents the absorber’s typical operation data.
2 Process Description 5
Table 2. Data of the Absorber from the Pilot Plant [1]
Absorber
Diameter 0.127 m
Nominal Packing Height* 2.2 m
Packing Type ceramic Intalox saddles
Packing Size 6.25 mm(0.25 in)
Sour Gas
Flow rate 2.17 lbmol/hr
CO2 in Sour Gas 0.2801(mole fraction)
H2S in Sour Gas 0.00807(mole fraction)
Sweet Gas
CO2 in Sweet Gas 0.0095 (mole fraction)
H2S in Sweet Gas 0.00037 (mole fraction)
Lean MEOH
Flow rate 8.29lbmol/hr
Temperature -34.7F
Pressure 400psia**
* The column was found to be too high for the experiments and no absorption was detected above certain height of the packing [1]. Liquid and gas samples were taken at the height of 1.5m from the bottom as liquid feed and gas product. Therefore, effective packing height (1.5m), is used instead of the real height (2.2m) in this simulation model. This effective height was also used in the literature model [1].
** Because pressure unit is not reported explicitly [1], it is assumed to be psia based on the pressure data in Table II and Figure 12 of [1]
6 3 Physical Properties
3 Physical Properties
The PC-SAFT equation of state model is used to calculate vapor pressure, liquid density and phase equilibrium. The PC-SAFT pure component parameters for CO2, H2S, CO and COS have been regressed against vapor pressure and liquid density data generated from DIPPR correlations[2] for each component. For all other components, the PC-SAFT pure component parameters are taken from the work by Gross and Sadowski (2001, 2002)[20,21]. The binary parameters between CO2 and MEOH and H2S and MEOH have been regressed against vapor-liquid equilibrium data from Semenova (1979)[3] and Leu (1992)[4].
DIPPR correlation models[2] are used to calculate MEOH viscosity, thermal conductivity and surface tension, respectively; the predictions are in excellent agreement with literature data[5-18] as showed in Figures 13-15.
Wilke-Chang model[22] is used for calculating the gas diffusivity in MEOH. The model quality has been justified by CO2 diffusivity data from Littel (1991)[19] as showed in Figure 16.
Figures 1-16 show property predictions together with literature data.
3 Physical Properties 7
MEOH vapor pressure
0.00001
0.0001
0.001
0.01
0.1
1
10
100
150 250 350 450 550
Temperature, K
Vapo
r pre
ssur
e, b
ar
DataPC-SAFT
Figure 1. MEOH vapor pressure. PC-SAFT is used to fit data generated from DIPPR correlation[2] for methanol.
MEOH liquid density
300
400
500
600
700
800
900
1000
150 250 350 450 550
Temperature, K
Liqu
id d
ensi
ty, k
g/m
3
Data
PC-SAFT
Figure 2. MEOH liquid density. PC-SAFT is used to fit data generated from DIPPR correlation[2] for methanol.
8 3 Physical Properties
CO2 vapor pressure
0
10
20
30
40
50
60
70
200 220 240 260 280 300 320
Temperature, K
Vapo
r pre
ssur
e, b
ar Data
PC-SAFT
Figure 3. CO2 vapor pressure. PC-SAFT is used to fit data generated from DIPPR correlation[2] for CO2.
CO2 liquid density
500
600
700
800
900
1000
1100
1200
1300
200 220 240 260 280 300 320
Temperature, K
Liqu
id d
ensi
ty, k
g/m
3
Data
PC-SAFT
Figure 4. CO2 liquid density. PC-SAFT is used to fit data generated from DIPPR correlation[2] for CO2
3 Physical Properties 9
H2S vapor pressure
01020304050607080
180 230 280 330 380
Temperature, K
Vapo
r pre
ssur
e, b
ar Data
PC-SAFT
Figure 5. H2S vapor pressure. PC-SAFT is used to fit data generated from DIPPR correlation[2] for H2S.
H2S liquid density
300
400
500
600
700
800
900
1000
1100
180 230 280 330 380
Temperature, K
Liqu
id d
ensi
ty, k
g/m
3
Data
PC-SAFT
Figure 6. H2S liquid density. PC-SAFT is used to fit data generated from DIPPR correlation[2] for H2S.
10 3 Physical Properties
CO vapor pressure
05
10152025303540
70 90 110 130
Temperature, K
Vapo
r pre
ssur
e, b
arData
PC-SAFT
Figure 7. CO vapor pressure. PC-SAFT is used to fit data generated from DIPPR correlation[2] for CO
CO liquid density
400450500550600650700750800850
70 90 110 130
Temperature, K
Liqu
id d
ensi
ty, k
g/m
3
Data
PC-SAFT
Figure 8. CO liquid density. PC-SAFT is used to fit data generated from DIPPR correlation[2] for CO.
3 Physical Properties 11
COS vapor pressure
0
10
20
30
40
50
60
130 180 230 280 330 380
Temperature, K
Vapo
r pre
ssur
e, b
arData
PC-SAFT
Figure 9. COS vapor pressure. PC-SAFT is used to fit data generated from DIPPR correlation[2] for COS.
COS liquid density
600
700
800
900
1000
1100
1200
1300
1400
130 180 230 280 330 380
Temperature, K
Liqu
id d
ensi
ty, k
g/m
3
Data
PC-SAFT
Figure 10. COS liquid density. PC-SAFT is used to fit data generated from DIPPR correlation[2] for COS.
12 3 Physical Properties
Figure 11. Vapor-liquid equilibria of CO2-MEOH at three temperatures. Comparison of experimental data[3] to calculation results of PC-SAFT with adjustable binary parameter.
Figure 12. Vapor-liquid equilibria of H2S-MEOH at three temperatures. Comparison of experimental data[4] to calculation results of PC-SAFT with adjustable binary parameter.
3 Physical Properties 13
MEOH liquid viscosity
0.0001
0.001
0.01
0.1
150 200 250 300 350
Temperature, K
Visc
osity
, Pa.
s
Data
DIPPR
Figure 13. MEOH liquid viscosity. Comparison of literature data[5-9] to calculation results of DIPPR correlation model[2].
MEOH liquid thermal conductivity
0.15
0.2
0.25
200 250 300 350 400
Temperature, K
Ther
mal
con
duct
ivity
, W/m
-K
Data
DIPPR
Figure 14. MEOH liquid thermal conductivity. Comparison of literature data[10-13] to calculation results of DIPPR correlation model[2].
14 3 Physical Properties
MEOH surface tension
0.0001
0.001
0.01
0.1
250 300 350 400 450 500 550
Temperature, K
Surf
ace
tens
ion,
N/M
Data
DIPPR
Figure 15. MEOH liquid surface tension. Comparison of literature data[7,14-18] to calculation results of DIPPR correlation model[2].
Diffusivity of CO2 in MEOH
0
2
4
6
8
250 275 300 325 350
Temperature, K
Diff
usiv
ity (m
2/s)
*E9
Data
Wilke-Chang
Figure 16. CO2 diffusivity in MEOH. Comparison of experimental data[19] to calculation results of Wilke-Chang model[22].
4 Simulation Approaches 15
4 Simulation Approaches
Run 35I of the pilot absorber [1] is used in this work.
Simulation Flowsheet – The pilot absorber has been modeled with the following simulation flowsheet in Aspen Plus as shown in Figure 17.
LEANIN
GASIN
GASOUT
RICHOUT
ABSORBER
Figure 17. Rate-Based MEOH Flowsheet in Aspen Plus
16 4 Simulation Approaches
Unit Operations - The unit operation in this model has been represented by an Aspen Plus Block as outlined in Table 3.
Table 3. Aspen Plus Unit Operation Blocks Used in the Rate-Based MEOH Model
Unit Operation Aspen Plus Block Comments / Specifications
Absorber RadFrac 1. Calculation type: Rate-Based
2. Number of Stages: 10
3. Top Pressure: 400psia
4. Packing: 6.25mm(0.25in) ceramic Intalox saddles
5. Packing Height: 1.5m*
6. Mass transfer coefficient method: Onda (1968)
7. Interfacial area method: Onda (1968)
8. Interfacial area factor: 1
9. Film resistance option: Film for liquid and vapor
10. Flow model: Mixed
* The column was found to be too high for the experiments and no absorption was detected above certain height of the packing[1]. Liquid and gas samples were taken at the height of 1.5m from the bottom as liquid feed and gas product. Therefore, effective packing height(1.5m), is used instead of the real height(2.2m) in this simulation model. This effective height was also used in the literature model [1].
Streams - Feeds to the Rate-Based MEOH model are gas stream GASIN containing H2, CO2, CO, N2, CH4, H2S and COS and liquid solvent stream LEANIN containing pure MEOH solvent. Feed conditions are summarized in Table 4.
Table 4. Feed specification
Stream ID GASIN LEANIN
Substream: MIXED
Temperature: F 53.9 -34.7
Pressure:psia 400 400
Mole-flow: lbmol/hr
MEOH 0 8.29
CO2 0.608109 0.0
H2S 0.01752 0.0
CO 0.438551 0.0
N2 0.340854 0.0
COS 0.000977 0.0
H2 0.720569 0.0
CH4 0.043421 0.0
5 Simulation Results 17
5 Simulation Results
The simulation was performed using Aspen Plus version 2006.5. Key simulation results are presented in Table 5 and Figure 18. To illustrate the effectiveness of the rate-based approach, simulation results for the absorber using the equilibrium stage calculation type are also shown in Figure 18.
Table 5. Key Simulation Results
Measurement Rate-Based MEOH model
CO2 mole fraction in GASOUT 0.95% 1.235%
H2S mole fraction in GASOUT 0.037% 0.0008%
Temperature of RICHOUT, F 0.7 2.65
0
0.15
0.30.45
0.6
0.75
0.9
1.051.2
1.35
1.5
-45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15
Temperature, F
Pack
ing
Hei
ght,
m
Literature DataASPEN RateSepASPEN Equilibrium Stages
Figure 18. Absorber Liquid Temperature Profile
18 6 Conclusions
6 Conclusions
The Rate-Based MEOH model provides a rate-based rigorous simulation of the process. Key features of this rigorous simulation include the PC-SAFT equation of state model for vapor pressure, liquid density and phase equilibrium, rigorous transport property modeling, rate-based multi-stage simulation with Aspen Rate-Based Distillation which incorporates heat and mass transfer correlations accounting for columns specifics and hydraulics.
The model is meant to be used as a guide for modeling the CO2 capture process with MEOH. Users may use it as a starting point for more sophisticated models for process development, debottlenecking, plant and equipment design, among others.
References 19
References
[1] Kelly, R.M.; Rousseau, R.W.; Ferrell, K.F., “Design of Packed, Adiabatic Absorber: Physical Absorption of Acid Gases in Methanol,” Ind. Eng. Chem. Process. Des. Dev., 23, 102-109 (1984).
[2] DIPPR® 801 database, BYU-Thermophysical Properties Laboratory (2007).
[3] Semenova, A.I.; Emelyanova, E.A.; Tsimmerman, S.S.; Tsiklis, D.S., “The Phase Equilibrium in the System Methanol - Carbon Dioxide,” Zh. Fiz. Khim., 53, 2502-2505 (1979).
[4] Leu, A.D.; Carroll, J.J.; Robinson, D.B., “The Equilibrium Phase Properties of the Methanol – Hydrogen Sulfide Binary System,” Fluid Phase Equilib., 72, 163-172 (1992).
[5] Komarenko, V.G.; Manzhelii, V.G.; Radtsig, A.V., "Viscosity and Density of Normal Monobasic Alcohols at Low Temperatures, " Ukr. Fiz. Zh., 12, 4, 681 (1967).
[6] Bretsznajder, S., "Prediction of Transport and Other Physical Properties of Fluids, " International Series of Monographs in Chemical Engineering, Pergamon Press, Oxford, 2 (1971).
[7] Selected Values of Properties of Chemical Compounds, Data Project, Thermodynamics Research Center, Texas A&M University, College Station, Texas (1980-extant); loose-leaf data sheets.
[8] Rauf, M.A.; Stewart, G.H.; Farhataziz, "Viscosities and Densities of Binary Mixtures of 1-Alkanols from 15 to 55 C, " J. Chem. Eng. Data, 28, 324 (1983).
[9] Stephan, K.; Lucas, K., "Viscosity of Dense Fluids, " New York: Plenum Press (1979).
[10] Raal, J.D., Rijsdijk, R.L., "Measurement of Alcohol Thermal Conductivities Using a Relative Strain-Compensated Hot-Wire Method, " J. Chem. Eng. Data, 26, 351 (1981).
[11] Takizawa, S.; Murata, H.; Nagashima, A., "Measurement of the Thermal Conductivity of Liquids by Transient Hot-Wire Method, " Bull. Jsme., 21, 152, 273 (1978).
[12] Rastorguev, Yu. L.; Ganiev, Yu. A., "Thermal Conductivity of Aqueous Solutions of Organic Liquids, " Russ. J. Phys. Chem., 40, 7, 869 (1966).
20 References
[13] Mukhamedzyanov, I.Kh.; Mukhamedzyanov, G.Kh.; Usmanov, A.G., "Thermal Conductivity of Liquid Saturated Monobasic Alcohols at Pressures Below 2500 Bars, " Proc. of Kazan Chem. Tech. Inst. of S.W. Kirov, 44, 57 (1971).
[14] Kaye, G.W.C.; Laby, T.H., "Tables of Physical and Chemical Constants, 14th ed., " Longman Group, Limited, London (1973).
[15] Vargaftik, N.B., "Tables on the Thermophysical Properties of Liquids and Gases, 2nd ed., " Halsted Press, New York (1975).
[16] Jasper, J.J., "The Surface Tension of Pure Liquid Compounds, " J Phys Chem Ref Data, 1, 4, 841-1009 (1972).
[17] Riddick, J.A.; Bunger, W.B., "Organic Solvents: Physical Properties and Methods of Purification, 3rd ed., " Wiley Interscience, New York (1970).
[18] Won, Y.S.; Chung, D.K.; Mills, A.F., "Density, Viscosity, Surface Tension, and Carbon Dioxide Solubility and Diffusitivity of Methanol, Ethanol, Acqueous Propanol, and Acqueous Ethylene Glycol at 25 C, " J. Chem. Eng., 26, 2, 140 (1981).
[19] Littel, R.J.; Versteeg,G.F.; van Swaaij,W.P.M., “Physical absorption into nonaqueous solutions in a stirred cell reactor,” Chem. Eng. Sci., 46, 3308-3313 (1991).
[20] Gross, J.; Sadowski, G., “Perturbed-Chain SAFT: An Equation of State Based on a Perturbation Theory for Chain Molecules,” Ind. Eng. Chem. Res., 40, 1244-1260 (2001).
[21] Gross, J.; Sadowski, G., “Application of the Perturbed-Chain SAFT Equation of State to Associating Systems”, Ind. Eng. Chem. Res., 41, 5510-5515 (2002).
[22] Reid, R.C.; Prausnitz, J.M.; Poling, B.E., “The Properties of Gases and Liquids,” 4th ed.; McGraw-Hill: New York (1987).