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Aspen Plus
Aspen Plus Model of theCO2Capture Process byN-methyl 2-pyrrolidone
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Copyright (c) 2008 by Aspen Technology, Inc. All rights reserved.
Aspen Plus, the aspen leaf logo and Plantelligence and Enterprise Optimization are trademarks or registeredtrademarks of Aspen Technology, Inc., Burlington, MA.
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Revision History 1
Revision HistoryVersion Description
V7.0 First version
V7.1 Re-verified simulation results using Aspen Plus V7.1
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2 Contents
ContentsIntroduction............................................................................................................31 Components .........................................................................................................42 Process Description..............................................................................................53 Physical Properties...............................................................................................74 Simulation Approaches.......................................................................................165 Simulation Results .............................................................................................196 Conclusions........................................................................................................21References ............................................................................................................22
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Introduction 3
Introduction
This document describes an Aspen Plus model of the CO2capture process by
the physical solvent N-methyl 2-pyrrolidone(NMP) from a gas mixture of CO,CO2, H2, H2O, N2, Ar, CH4, NH3, and H2S from gasification of Illinois No. 6
bituminous coal[1]. Due to lack of design data for NMP, the operation datafrom an engineering evaluation design case using DEPG as solvent by Energy
Systems Division, Argonne National Laboratory (1994)[1]are used to specifythe feed conditions and unit operation block specifications in the process
model. Since only the equilibrium stage results for the DEPG design case are
available in the literature and the Aspen Plus DEPG model uses equilibriumstage simulation, the process model developed here is also based on the
equilibrium stage distillation model instead of the more rigorous rate-based.
In addition to the gases present in the design case, many other gascomponents such as COS, CH3SH and so on are also included in this model for
potential needs by model users. Pure and/or binary parameters have beendetermined and included in the model for these compounds.
NMP data for vapor pressure[2], liquid density[2], viscosity[3-5], thermal
conductivity[6]
and surface tension[4,7]
are used to determine parameters inthermophysical property and transport property models used in this work. For
all other components, thermophysical property models have been validated
against DIPPR correlations[2], which are available in Aspen Plus, for
component vapor pressure and liquid density. Vapor-liquid equilibrium data
from Xu et al. (1992)[8]between propylene carbonate and selected
components and solubility ratios[9,10]of gases in propylene carbonate and in
NMP are used to estimate vapor-liquid data between NMP and gascomponents and then to adjust binary parameters in thermophysical property
models. The designed packing information from the literature[1]is alsoincluded in the process model, which allows rigorous rate-based simulation to
be performed.
The model includes the following key features: PC-SAFT equation of state model for vapor pressure, liquid density, heat
capacity, and phase equilibrium
Transport property models Equilibrium distillation model for absorber with designed packing
information from the literature[1]
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4 1 Components
1 Components
The following components represent the chemical species present in the
process.
Table 1. Components Used in the Model
ID Type Name Formula
NMP Conventional N-METHYL-2-PYRROLIDONE C5H9NO-D2
CO2 Conventional CARBON-DIOXIDE CO2
H2S Conventional HYDROGEN-SULFIDE H2S
CO Conventional CARBON-MONOXIDE CO
H2O Conventional WATER H2O
CS2 Conventional CARBON-DISULFIDE CS2
NH3 Conventional AMMONIA H3N
N2 Conventional NITROGEN N2
COS Conventional CARBONYL-SULFIDE COS
O2 Conventional OXYGEN O2
SO2 Conventional SULFUR-DIOXIDE O2SSO3 Conventional SULFUR-TRIOXIDE O3S
CH3SH Conventional METHYL-MERCAPTAN CH4S
C2H5SH Conventional ETHYL-MERCAPTAN C2H6S-1
CH3SCH3 Conventional DIMETHYL-SULFIDE C2H6S-2
HCN Conventional HYDROGEN-CYANIDE CHN
H2 Conventional HYDROGEN H2
BENZENE Conventional BENZENE C6H6
CH4 Conventional METHANE CH4
C2H6 Conventional ETHANE C2H6
C2H4 Conventional ETHYLENE C2H4
C3H8 Conventional PROPANE C3H8
IC4H10 Conventional ISOBUTANE C4H10-2NC4H10 Conventional N-BUTANE C4H10-1
C2H2 Conventional ACETYLENE C2H2
C6H14 Conventional N-HEXANE C6H14-1
C7H16 Conventional N-HEPTANE C7H16-1
NO2 Conventional NITROGEN-DIOXIDE NO2
NO Conventional NITRIC-OXIDE NO
AR Conventional ARGON AR
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2 Process Description 5
2 Process Description
In this NMP model, we use the operation data taken from a CO2capture
design case by DEPG reported by Energy Systems Division, Argonne NationalLaboratory (ANL)[1]. The reported flowsheet includes an absorber for CO2
absorption by DEPG at elevated pressure, flash tanks to release CO2andregenerate solvent at several different pressure levels, and compressors and
turbines to change pressures of streams. However, the process modelpresented in this work focuses only on the absorber and the other unit
operations are not included.
The sour gas enters the bottom of the absorber, contacts with lean NMP
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 CO2and some other gas components.
Two pressure levels for absorption were evaluated in the ANL report: 250psia
and 1000psia. For each pressure case study, the gas feeds into the absorber
is the same, but solvent flow rates and number of equilibrium stages used aredifferent. Typically, to achieve a certain CO2recovery, the high pressure case
used less solvent and fewer stages. Table 2 represents some operation data.In this NMP model, we used the operation data of the low pressure case.
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6 2 Process Description
Table 2. Data of the Absorber
Low Pressure Case High Pressure Case
Absorber
Number of Stages 12 10Diameter, ft 17 11
Packing Height, ft 3 3
Packing Type Pall ring Pall ring
Packing Size, mm 50 50
Sour Gas
Flow rate, lbmol/hr 17614.58 17614.58
CO2in Sour Gas, mole fraction 0.2461 0.2461
Lean DEPG
Flow rate, lbmol/hr 23000 6900
Temperature, F 30 30Pressure, psia 250 1000
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3 Physical Properties 7
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 componentparameters for gases have been regressed against vapor pressure and liquid
density generated from DIPPR correlations[2]for each component or takenfrom the work by Gross and Sadowski (2001, 2002)[11,12]. The PC-SAFT pure
parameters for NMP have also been regressed to fit vapor pressure and liquiddensity data from DIPPR correlations[2].
No vapor-liquid equilibrium data for the gases in NMP were found to regress
the PC-SAFT binary parameters. However, Xu et al. (1992)[8]reported Henrysconstants for CO2, H2S and SO2with propylene carbonate and according to
reference [9], CO2solubility in propylene carbonate and in NMP are very
similar in both the volume-solvent basis and the mole-solvent basis. So CO2
Henrys constant with propylene carbonate were used as a starting point to
regress binary parameters between CO2and NMP. Then CO2solubility in NMPat 25C and 1atm was calculated using the binary parameters. Comparison of
the calculated CO2solubility and the literature data[9]supplies direction to
adjust the Henrys constant data. Several iterations were made to get suitable
Henrys constant data for CO2with NMP, which can give suitable binaryparameters between CO2and NMP, allowing accurate estimation of CO2
solubility in NMP at 25C and 1atm. A diagram of the process is shown in
Figure 1.
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8 3 Physical Properties
Estimate CO2Henrysconstantwith NMP
Regress kijbetween CO2and NMP
Estimate CO2solubility inNMP at 25C and 1atm
MatchCO2solubility Data
[9]in NMP?
CO2 Henrys constant data[8]
with PC
Output CO2Henrysconstant and kijin NMP
Yes
No
Figure 1. Diagram of estimation process of PC-SAFT binary parameter for CO2and NMP.
Once Henrys constant for CO2with NMP were figured out, solubility ratios[10]
of the other gases to CO2were used to determine their Henrys constants with
NMP, with the assumption that solubility ratios are equivalent to Henrys
constant ratios. Then these estimated Henrys constant served to regressbinary parameters between these gas components and NMP.
DIPPR model parameters for NMP are regressed to fit data for viscosity[3-5],thermal conductivity[6]and surface tension[4,7].
Figures 2-16 show property predictions together with literature data.
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3 Physical Properties 9
NMP vapor pressure
1.00E-06
1.00E-04
1.00E-02
1.00E+00
1.00E+02
0 200 400 600 800
Temperature, K
VaporPresure,ba
rData
PC-SAF
Figure 2. NMP vapor pressure. PC-SAFT is used to fit data from DIPPRcorrelation[2]for NMP.
NMP liquid density
600
800
1000
1200
100 200 300 400 500 600
Temperature, K
Liquiddensity,kg/m3
Data
PC-SAF
Figure 3. NMP liquid density. PC-SAFT is used to fit data from DIPPRcorrelation[2]for NMP.
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10 3 Physical Properties
CO2 vapor pressure
0
10
20
30
40
50
60
70
200 220 240 260 280 300 320
Temperature, K
Vaporpressure,ba
r Data
PC-SAFT
Figure 4. CO2vapor pressure. PC-SAFT is used to fit data generated fromDIPPR 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
Liquiddensity,kg/m3
Data
PC-SAFT
Figure 5. CO2liquid density. PC-SAFT is used to fit data generated fromDIPPR correlation[2]for CO2.
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3 Physical Properties 11
H2S vapor pressure
0
10
20
30
40
50
60
70
80
180 230 280 330 380
Temperature, K
Vaporpressure,ba
rData
PC-SAFT
Figure 6. H2S vapor pressure. PC-SAFT is used to fit data generated fromDIPPR correlation[2]for H2S.
H2S liquid density
300
400
500
600
700
800
900
1000
1100
180 230 280 330 380
Temperature, K
Liquiddensity,kg/m3
Data
PC-SAFT
Figure 7. H2S liquid density. PC-SAFT is used to fit data generated fromDIPPR correlation[2]for H2S.
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12 3 Physical Properties
CO vapor pressure
0
5
10
15
20
25
30
35
40
70 90 110 130
Temperature, K
Vaporpressure,ba
r
Data
PC-SAFT
Figure 8. CO vapor pressure. PC-SAFT is used to fit data generated fromDIPPR correlation[2]for CO.
CO liquid density
400
450
500
550
600
650
700
750
800
850
70 90 110 130
Temperature, K
Liquiddensity,kg/m3
Data
PC-SAFT
Figure 9. CO liquid density. PC-SAFT is used to fit data generated from DIPPRcorrelation[2]for CO.
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3 Physical Properties 13
NH3 vapor pressure
0
10
20
30
40
50
60
70
80
90
200 250 300 350 400
Temperature, K
Vaporpressure,ba
r
Data
PC-SAFT
Figure 10. NH3vapor pressure. PC-SAFT is used to fit data generated fromDIPPR correlation[2]for NH3.
NH3 liquid density
400
450
500
550
600
650
700
750
200 250 300 350 400
Temperature, K
Liquiddensity,kg/m3
Data
PC-SAFT
Figure 11. NH3liquid density. PC-SAFT is used to fit data generated fromDIPPR correlation[2]for NH3.
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14 3 Physical Properties
VLE for CO2-NMP
0
0.005
0.01
0.015
290 300 310 320 330 340 350
Temperature, K
Pressure,MPa
Data
PC-SAFT
Figure 12. Vapor-liquid equilibria of CO2-NMP. Comparison of estimated datato calculation results of PC-SAFT with adjustable binary parameter.
VLE for H2S-NMP
0
0.005
290 300 310 320 330 340 350
Temperature, K
Pressure,M
Pa Data
PC-SAFT
Figure 13. Vapor-liquid equilibria of H2S-NMP. Comparison of estimated datato calculation results of PC-SAFT with adjustable binary parameter.
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3 Physical Properties 15
Surface tension of NMP
0
0.01
0.02
0.03
0.04
0.05
0.06
200 300 400 500 600 700
Temperature (K)
Surfacetension(N/m)
DIPPR
Data
Figure 14. NMP liquid surface tension. DIPPR correlation model[4] is used tofit data[4,7].
Viscosity of NMP
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0.0045
0.005
200 300 400 500 600
Temperature (K)
Viscosity(Pa.s)
DIPPR
Data
Figure 15. NMP liquid viscosity. DIPPR correlation model[4] is used to fitdata[3-5].
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16 3 Physical Properties
Thermal conductivity of NMP
0.09
0.1
0.11
0.12
0.13
0.14
0.15
200 300 400 500
Temperature (K)
Thermalconductivity(W
/m.K)
DIPPRData
Figure 16. NMP liquid thermal conductivity. DIPPR correlation model[4] isused to fit data[6].
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4 Simulation Approaches 17
4 Simulation Approaches
As stated in the previous sections, this NMP model uses operation data of a
DEPG design case from [1], the low pressure case. Feed conditions, absorberconfigurations and operation conditions of the DEPG low pressure case were
used in this model as a base case and then solvent flow rate is adjusted toreach the same CO2capture amount as DEPG does.
The absorber is modeled with the Equilibrium calculation type instead of the
more rigorous rate-based calculation type because the design cases from [1]were based on equilibrium stage calculations. This allows us to make
meaningful comparison between our model and the DEPG model, which alsouses the Equilibrium calculation type because only equilibrium results are
available for comparison in [1]. However, we included packing design
information from the literature in the model so that the rate-based calculationtype can be used. In addition, as shown above, transport properties, which
are crucial for rate-based calculations, have also been validated. Therefore,
this model is ready for rate-based calculations, in which correlations and scalefactors of interfacial area, mass transfer coefficient, heat transfer coefficient,
liquid holdup and so on can be selected and adjusted. You can also select thefilm resistance types and flow models to be used.
Simulation Flowsheet The absorber has been modeled with the following
simulation flowsheet in Aspen Plus, shown below.
LEANIN
GASIN
GASOUT
RICHOUT
ABSORBER
Figure 17. NMP Process Flowsheet in Aspen Plus
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18 4 Simulation Approaches
Unit Operations Major unit operations in this model have beenrepresented by Aspen Plus Blocks as outlined in Table 3.
Table 3. Aspen Plus Unit Operation Blocks Used in theNMP Model
Unit Operation Aspen Plus Block Comments / Specifications
ABSORBER RadFrac The absorber for the low pressure case with the followingsettings:
1. Calculation type: Equilibrium stage
2. Number of stages: 12
3. Top Pressure: 250psia
4. Column diameter: 17ft
5. Packing Type: Pall ring
6. Packing Size: 50mm(2in)
7. Packing Height per stage: 3ft
Streams The gas feeds of the NMP model is GASIN, containing CO, CO2, H2,H2O, N2, Ar, CH4, NH3, and H2S.
The solvent liquid feeds is LEANIN, containing NMP and a small amount of CO2
and H2O.
Feed conditions are summarized in Table 4.
Table 4. Feed specification
Stream ID GASIN LEANIN
Substream: MIXED
Temperature: F 68.13 30
Pressure:psia 248 250Mole-flow: lbmol/hr
NMP 0 23000
CO 77.37 0.0
CO2 4335.99 395.00
H2 5611.86 0.0
H2O 61.91 2.25
N2 7306.65 0.0
AR 88.6 0.0
CH4 128.77 0.0
NH3 2.99 0.0
H2S 0.4 0.0
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5 Simulation Results 19
5 Simulation Results
The simulation was performed using Aspen Plus V7.1 with the absorber
calculation type set to Equilibrium. Key simulation results are presented inFigure 18 and 19, together with the simulation results of the DEPG model
using the low pressure case operation data.
As shown by Figures 18 and 19, with the same flow rate (23000lbmol/hr) andtemperature (30F) for the fed solvent to the absorber, DEPG (Squares in
Figures 18 and 19) has a much higher remove capacity than NMP (Solid linesin Figures 18 and 19). To achieve a similar CO2removal to what DEPG does,
NMP flow rate should be increased to about 50700lbmol/hr (Dashed Lines inFigures 18 and 19), which is about 2.2 times of DEPG flowrate.
According to Table 1 in reference [9], at 25C, CO2solubility is
0.485ft3/gallon DEPG and 0.477ft3/gallon propylene carbonate. At 25C,specific gravity is 1030kg/m3for DEPG, whose molecular weight is 280, and
1027kg/m3for propylene carbonate, whose molecular weight is 99. If
transformed to a mole-solvent base, CO2solubility in DEPG is about 2.9 timesof the solubility in propylene carbonate at 25C
1
2
3
4
5
6
7
8
9
10
11
12
0 5 10 15 20 25 30 35 40 45 50 55 60 65
T e m p e r a t u r e F
Sta
umb
DEPG 23000 30F
NMP 23000 30F
NMP 50700 30F
Figure 18. Absorber Temperature Profile
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20 5 Simulation Results
1
2
3
4
5
6
7
8
9
10
11
12
0 0.05 0.1 0.15 0.2 0.25 0.3
CO Mole Fraction
Saumb
DEPG 23000 30F
NMP 23000 30F
NMP 50700 30F
Figure 19. Absorber Vapor Phase CO2Composition Profile
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6 Conclusions 21
6 Conclusions
The NMP model provides an equilibrium stage simulation of the process and
validated property models which allow rigorous rate-based simulation. Keyfeatures of this model include the PC-SAFT equation of state model for vapor
pressure, liquid density and phase equilibrium, rigorous transport propertymodeling, equilibrium stage simulation with RadFrac and packing information
from the literature[1].
The model is meant to be used as a guide for modeling the CO2captureprocess with NMP. Users may use it as a starting point for more sophisticated
models for process development, debottlenecking, plant and equipmentdesign, among others.
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References
[1] R.D. Doctor, J.C. Molburg, P.R. Thimmapuram, G.F. Berry, C.D. Livengood,
Gasification Combined Cycle: Carbon Dioxide Recovery, Transport, andDisposal, Energy System Divison, Argonne National Laboratory (1994)
[2] DIPPR801 database, BYU-Thermophysical Properties Laboratory (2007)
[3] V.A. Granzhan, O.G. Kirillova, "Physico-Chemical Analysis of the Systemn-Methyl-alpha-Pyrrolidone-Methanol," J. Appl. Chem. USSR, 43, 1898 (1970).
[4] M-Pyrol Handbook, GAF Corporation, New York (1972)
[5] J.A. Riddick, W.B. Bunger, "Organic Solvents: Physical Properties and
Methods of Purification, 3rd ed., " Wiley Interscience, New York (1970)
[6] A. Missenard, "Conductivite Thermique des Solides, Liquides, Gaz et de
Leurs Melanges, " Editions Eyrolles, Paris, 5 (1965); Also see Missenard, A.,Comptes Rendus, 260, 5521 (1965)
[7] S. Sugden, "The Variation of Surface Tension with Temperature and Some
Related Functions," J. Chem. Soc. (London, Transactions), 125, 32 (1924)
[8] Y. Xu, R.P. Schutte, L.G. Helper, Solubilities of Carbon Dioxide, HydrogenSulfide and Sulfur Dioxide in Physical Solvents, Can. J. Chem. Eng., 70, 569-
573 (1992)
[9] G. Ranke, V.H. Mohr, The Rectisol Wash: New Developments in Acid GasRemoval from Synthesis Gas, from Acid and Sour Gas Treating Processes,
Stephen A. Newman, ed., Gulf Publishing Company, Houston, 80-111(1985)
[10] R. Epps, Processing of Landfill Gas for Commercial Applications: the
SELEXOL Solvent Process, Union Carbide Chemicals & Plastics TechnologyCorporation, June, 1992. (Prepared for Presentation at ECO WORLD 92, June
15, 1992, Washington D. C.)
[11] J. Gross, G. Sadowski, Perturbed-Chain SAFT: An Equation of State
Based on a Perturbation Theory for Chain Molecules, Ind. Eng. Chem. Res.,40, 1244-1260 (2001)
[12] J. Gross, G. Sadowski, Application of the Perturbed-Chain SAFTEquation of State to Associating Systems, Ind. Eng. Chem. Res., 41, 5510-
5515 (2002)