Aspen Plus NMP Model

8/13/2019 Aspen Plus NMP Model

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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.

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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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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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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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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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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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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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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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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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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)

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