Aspen Plus: Aspen Plus Model of The CO Capture Process by Depg [PDF]

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Aspen Plus

Aspen Plus Model of the CO2 Capture Process by DEPG

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Revision History Version

Description

V7.0

First version

V7.1

Re-verified simulation results using Aspen Plus V7.1

V7.2

Add formic acid and its PC-SAFT parameters

V7.3

Re-verified simulation results using Aspen Plus V7.3

V7.3.2

Re-verified simulation results using Aspen Plus V7.3.2

V8.2

Update the model to V8.2

V8.4

Update the model to V8.4

V8.6

Update the model to V8.6

Revision History

1

Contents Introduction ............................................................................................................3 1 Components .........................................................................................................4 2 Process Description..............................................................................................5 3 Physical Properties...............................................................................................6 4 Simulation Approaches.......................................................................................15 5 Simulation Results .............................................................................................18 6 Conclusions ........................................................................................................20 References ............................................................................................................21

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Contents

Introduction

This document describes an Aspen Plus model of the CO2 capture process by the physical solvent DEPG from a gas mixture of CO, CO2, H2, H2O, N2, Ar, CH4, NH3, and H2S from gasification of Illinois No. 6 bituminous coal[1]. The operation data from an engineering evaluation design case by Energy Systems Division, Argonne National Laboratory (1994)[1] are used to specify the feed conditions and unit operation block specifications in the process model. Since only the equilibrium stage results are available in the literature, the process model developed here is based on the equilibrium stage distillation model instead of the more rigorous rate-based model. DEPG[2] is a mixture of the dimethyl ethers of polyethylene glycol with formula CH3O(C2H4O)nCH3 where n ranges from 2 to 9. However, DEPG in this model is represented by an Aspen Plus databank component, also called DEPG (dimethyl ether of polyethylene glycol), with the average molecular weight of 280 - corresponding to n = 5.3. DEPG data from Coastal Chemical[3] for vapor pressure, liquid density, heat capacity, viscosity, and thermal conductivity are used to determine parameters in thermophysical property and transport property models used in this work. For all other components, thermophysical property models have been validated against DIPPR correlations[4] , which are available in Aspen Plus, for component vapor pressure and liquid density. Vapor-liquid equilibrium data from Xu et al. (1992)[5] between DEPG and selected components are used to adjust binary parameters in thermophysical property models. The designed packing information from the literature[1] is also included in the process model, which allows rigorous rate-based simulation to be performed. The model includes the following key features:

Introduction



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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1 Components

The following components represent the chemical species present in the process. As already stated, DEPG in real processes is a mixture of the dimethyl ethers of polyethylene glycol with formula CH3O(C2H4O)nCH3 where n ranges from 2 to 9 [2] and in this model an average molecular weight of 280 corresponding to n = 5.3 is used to represent the DEPG solvent by an Aspen Plus databank component DEPG.

Table 1. Components Used in the Model

4

ID

Type

Name

Formula

DEPG

CONV

DIMETHYL-ETHER-POLYETHYLENE-GLYCOL

DEPG

CO

CONV

CARBON-MONOXIDE

CO

CO2

CONV

CARBON-DIOXIDE

CO2

H2

CONV

HYDROGEN

H2

H2O

CONV

WATER

H2O

N2

CONV

NITROGEN

N2

AR

CONV

ARGON

AR

CH4

CONV

METHANE

CH4

NH3

CONV

AMMONIA

H3N

H2S

CONV

HYDROGEN-SULFIDE

H2S

HCN

CONV

HYDROGEN-CYANIDE

CHN

COS

CONV

CARBONYL-SULFIDE

COS

CH2O2

CONV

FORMIC-ACID

CH2O2

1 Components

2 Process Description

The flowsheet for CO2 capture by DEPG in the report by Energy Systems Division, Argonne National Laboratory (ANL) [1] includes an absorber for CO2 absorption by DEPG at elevated pressure, flash tanks to release CO2 and regenerate solvent at several different pressure levels, and compressors and turbines to change pressures of streams. However, the process model presented 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 DEPG 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. 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 are different. Typically, to achieve a certain CO2 recovery, the high pressure case used less solvent and fewer stages. Table 2 represents some operation data:

Table 2. Data of the Absorber Low Pressure Case

High Pressure Case

Number of Stages

12

10

Diameter, ft

17

11

Packing Height, ft

3

3

Packing Type

Pall ring

Pall ring

Packing Size, mm

50

50

Flow rate, lbmol/hr

17614.58

17614.58

CO2 in Sour Gas, mole fraction

0.2461

0.2461

Flow rate, lbmol/hr

23000

6900

Temperature, F

30

30

Pressure, psia

250

1000

Absorber

Sour Gas

Lean DEPG

2 Process Description

5

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 CO, CO2, NH3, H2S have been regressed against vapor pressure and liquid density generated from DIPPR correlations[4] for each component. The PC-SAFT pure parameters for DEPG have been regressed to fit vapor pressure and liquid density data from Coastal Chemical[3]. For all other components, the PC-SAFT pure parameters are taken from the work by Gross and Sadowski (2001, 2002)[6,7]. The binary parameters between CO2 and DEPG and H2S and DEPG have been regressed against vapor-liquid equilibrium data form Xu et al. (1992)[5]. Based on solubility ratio of H2 to H2S in DEPG at 25°C[8,9] and experimental vapor-liquid equilibrium data for H2S in DEPG, we also estimated vapor-liquid equilibrium data for H2 in DEPG and used these estimated data for regression of binary parameters between H2 and DEPG. In the same way, we obtained binary parameters between the other gas components and DEPG[8,9], except for Ar because of the missing solubility ratio of Ar. DIPPR model parameters for DEPG are regressed to fit data from Coastal Chemical[3] for viscosity and thermal conductivity. The Aspen ideal gas heat capacity model parameters for DEPG are also regressed to fit liquid heat capacity data from Coastal Chemical[3]. Finally, the dipole moment from the DIPPR database[4] for pentaethylene glycol dimethyl ether is used for DEPG. Figures 1-15 show property predictions together with literature data.

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3 Physical Properties

DEPG vapor pressure

Vapor pressure, bar

0.1 Data

0.01

PC-SAFT

0.001 0.0001 0.00001 0.000001 0.0000001 250

300

350

400

450

Temperature, K Figure 1. DEPG vapor pressure. PC-SAFT is used to fit data from Coastal Chemical[3].

DEPG liquid density

Liquid density, kg/m3

1200 1150

Data

1100

PC-SAFT

1050 1000 950 900 850 800 250

300

350

400

450

Temperature, K Figure 2. DEPG liquid density. PC-SAFT is used to fit data from Coastal Chemical[3].

3 Physical Properties

7

CO2 vapor pressure

Vapor pressure, bar

70 60 50

Data PC-SAFT

40 30 20 10 0 200

220

240

260

280

300

320

Temperature, K Figure 3. CO2 vapor pressure. PC-SAFT is used to fit data generated from the DIPPR correlation[4] for CO2.

CO2 liquid density

Liquid density, kg/m3

1300 1200 1100 1000 900 Data

800

PC-SAFT

700 600 500 200

220

240

260

280

300

320

Temperature, K Figure 4. CO2 liquid density. PC-SAFT is used to fit data generated from the DIPPR correlation[4] for CO2.

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3 Physical Properties

H2S vapor pressure

Vapor pressure, bar

80 70 60 50

Data PC-SAFT

40 30 20 10 0 180

230

280

330

380

Temperature, K Figure 5. H2S vapor pressure. PC-SAFT is used to fit data generated from the DIPPR correlation[4] for H2S.

H2S liquid density

Liquid density, kg/m3

1100 1000 900 800 700 600

Data PC-SAFT

500 400 300 180

230

280

330

380

Temperature, K Figure 6. H2S liquid density. PC-SAFT is used to fit data generated from the DIPPR correlation[4] for H2S.

3 Physical Properties

9

CO vapor pressure

Vapor pressure, bar

40 35 30 Data

25

PC-SAFT

20 15 10 5 0 70

90

110

130

Temperature, K Figure 7. CO vapor pressure. PC-SAFT is used to fit data generated from the DIPPR correlation[4] for CO.

CO liquid density

Liquid density, kg/m3

850 800 750 700 650 600

Data

550

PC-SAFT

500 450 400 70

90

110

130

Temperature, K Figure 8. CO liquid density. PC-SAFT is used to fit data generated from the DIPPR correlation[4] for CO.

10

3 Physical Properties

Vapor pressure, bar

NH3 vapor pressure 90 80 70 60 50 40 30 20 10 0 200

Data PC-SAFT

250

300

350

400

Temperature, K Figure 9. NH3 vapor pressure. PC-SAFT is used to fit data generated from the DIPPR correlation[4] for NH3.

NH3 liquid density

Liquid density, kg/m3

750 700 650 600 550

Data

500

PC-SAFT

450 400 200

250

300

350

400

Temperature, K Figure 10. NH3 liquid density. PC-SAFT is used to fit data generated from the DIPPR correlation[4] for NH3.

3 Physical Properties

11

VLE for CO2-DEPG 1

Pressure, psia

0.9

Data PC-SAFT

0.8

0.7 0.6

0.5 50

70

90

110

130

150

Temperature, F

Figure 11. Vapor-liquid equilibria of CO2-DEPG. Comparison of experimental data[5] to calculation results of PC-SAFT with adjustable binary parameters.

VLE for H2S-DEPG 0.15

Pressure, psia

0.13

Data PC-SAFT

0.11

0.09 0.07

0.05 50

70

90

110

130

150

Temperature, F

Figure 12. Vapor-liquid equilibria of H2S-DEPG. Comparison of experimental data[5] to calculation results of PC-SAFT with adjustable binary parameters.

12

3 Physical Properties

Heat capacity, J/kmol-K

DEPG liquid heat capacity

750000 Data DIPPR 650000

550000 250

300

350

400

450

Temperature, K

Figure 13. DEPG liquid heat capacity. The Aspen ideal gas heat capacity model is used to fit data from Coastal Chemical[3].

DEPG liquid viscosity 0.1 Data

Viscosity, Pa.s

DIPPR 0.01

0.001

0.0001 200

250

300

350

400

450

Temperature, K

Figure 14. DEPG liquid viscosity. The DIPPR correlation model[4] is used to fit data from Coastal Chemical[3].

3 Physical Properties

13

DEPG liquid thermal conductivity

Thermal conductivity, W/m-K

0.21 Data DIPPR

0.19

0.17

0.15

0.13 200

250

300

350

400

450

Temperature, K

Figure 15. DEPG liquid thermal conductivity. The DIPPR correlation model[4] is used to fit data from Coastal Chemical[3].

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3 Physical Properties

4 Simulation Approaches

The high pressure case and the low pressure case are included in the process model as two separate absorber columns. The absorbers are 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 literature. However, we included designed packing information from the literature in the model so that the rate-based calculation type 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 scale factors of interfacial area, mass transfer coefficient, heat transfer coefficient, liquid holdup and so on can be selected and adjusted. You can also select the film resistance types and flow models to be used. Simulation Flowsheet – The absorbers for the two cases have been modeled with the simulation flowsheet in Aspen Plus shown in Figure 16, in which ABSORB-H is the absorber for the high pressure case and ABSORB-L is the absorber for the low pressure case.

GASOUT-L

LEAN-L

GASOUT-H

LEAN-H ABSORB-L

GASIN-L

ABSORB-H GASIN-H

RICH-L

RICH-H

Figure 16. DEPG Process Flowsheet in Aspen Plus

4 Simulation Approaches

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Unit Operations – Major unit operations in this model have been represented by Aspen Plus blocks as outlined in Table 3.

Table 3. Aspen Plus Unit Operation Blocks Used in the DEPG Model Unit Operation

Aspen Plus Block

Comments / Specifications

ABSORB-H

RadFrac

The absorber for the high pressure case with the following settings: 1. Calculation type: Equilibrium stage 2. Number of stages: 10 3. Top Pressure: 1000psia 4. Column diameter: 11ft 5. Packing Type: Pall ring 6. Packing Size: 50mm(2in) 7. Packing Height per stage: 3ft

ABSORB-L

RadFrac

The absorber for the low pressure case with the following settings: 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

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4 Simulation Approaches

Streams – The gas feeds of the DEPG model are GASIN-H for the high pressure absorber ABSOR-H and GASIN-L for the low pressure absorber ABSORB-L, both containing CO, CO2, H2, H2O, N2, Ar, CH4, NH3, and H2S. The solvent liquid feeds are LEAN-H for the high pressure absorber ABSORB-H and LEAN-L for the low pressure absorber ABSORB-L, both containing DEPG and a small amount of CO2 and H2O. Feed conditions are summarized in Table 4.

Table 4. Feed specification Stream ID

GASIN-H

LEAN-H

GASIN-L

LEAN-L

Temperature: F

68.17

30

68.13

30

Pressure:psia

998

1000

248

250

Substream: MIXED

Mole-flow: lbmol/hr DEPG

0

6900

0

23000

CO

77.37

0.0

77.37

0.0

CO2

4335.99

115.55

4335.99

395.00

H2

5611.86

0.0

5611.86

0.0

H2O

61.91

0.07

61.91

2.25

N2

7306.65

0.0

7306.65

0.0

AR

88.6

0.0

88.6

0.0

CH4

128.77

0.0

128.77

0.0

NH3

2.99

0.0

2.99

0.0

H2S

0.4

0.0

0.4

0.0

HCN

0.0

0.0

0.0

0.0

COS

0.0

0.0

0.0

0.0

CH2O2

0.0

0.0

0.0

0.0

4 Simulation Approaches

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5 Simulation Results

The simulation was performed using Aspen Plus with the absorbers' calculation type set to Equilibrium. Key simulation results are presented in Table 5 and 6 and Figure 17 and 18, together with available design data from the report of the Energy Systems Division, Argonne National Laboratory[1] . A problem was found in the literature that their calculation was based on improper solubility ratios of the gas components in DEPG solvent, in which N2:H2 is 0, while UOP reported a ratio of 1.5[2] and this model reports a ratio of about 3.8. As a result, this model gives less CO2 absorption than that reported in the literature. In addition, the temperature of the rich solvent from the bottom of the absorbers is also lower in our simulation.

Table 5. Key Simulation Results for the High Pressure Case Literature

This model

CO2 mole fraction in GASOUT-H

0.01619

0.050

Temperature of RICH-H, F

83.82

60.4

Table 6. Key Simulation Results for the Low Pressure Case

18

Literature

This model

CO2 mole fraction in GASOUT-L

0.01629

0.036

Temperature of RICH-L, F

46.68

42.1

5 Simulation Results

1 2

Stage Number

3 4 5 6 7 8 ABSORB-H 9 10 0

5

10 15 20 25 30 35 40 45 50 55 60 65 Temperature, F

Figure 17. Absorber Temperature Profile for the High Pressure Case

1 2 3 Stage Number

4 5 6 7 8 9 10

ABSORB-L

11 12 0

5

10 15 20 25 30 35 40 45 50 55 60 65 Temperature, F

Figure 18. Absorber Temperature Profile for the Low Pressure Case

5 Simulation Results

19

6 Conclusions

The DEPG model provides an equilibrium stage simulation of the process and validated transport property models which allow rigorous rate-based simulation. Key features of this model include the PC-SAFT equation of state model for vapor pressure, liquid density, and phase equilibrium; rigorous transport property modeling; 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 CO2 capture process with DEPG. Users may use it as a starting point for more sophisticated models for process development, debottlenecking, plant and equipment design, among others.

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6 Conclusions

References

[1] R.D. Doctor, J.C. Molburg, P.R. Thimmapuram, G.F. Berry, C.D. Livengood, “Gasification Combined Cycle: Carbon Dioxide Recovery, Transport, and Disposal”, Energy System Divison, Argonne National Laboratory (1994) [2] D.J. Kubek, E. Polla, F.P. Wilcher, “Purification and Recovery Options for Gasification,” Gasification Technologies Conference, San Francisco (1996) [3] Coastal AGR Solvent Bulletin, Coastal Chemical Co., L.L. C [4] DIPPR® 801 database, BYU-Thermophysical Properties Laboratory (2007). [5] Y. Xu, R.P. Schutte, L.G. Helper, “Solubilities of Carbon Dioxide, Hydrogen Sulfide and Sulfur Dioxide in Physical Solvents,” Can. J. Chem. Eng., 70, 569573 (1992) [6] 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) [7] J. Gross, G. Sadowski, “Application of the Perturbed-Chain SAFT Equation of State to Associating Systems”, Ind. Eng. Chem. Res., 41, 5510-5515 (2002) [8] G. Ranke, V. H. Mohr, “The Rectisol Wash: New Developments in Acid Gas Removal from Synthesis Gas,” from Acid and Sour Gas Treating Processes, Stephen A. Newman, ed., Gulf Publishing Company, Houston, 80-111 (1985) [9] R. Epps, “Processing of Landfill Gas for Commercial Applications: the SELEXOL Solvent Process,” Union Carbide Chemicals & Plastics Technology Corporation, June, 1992. (Prepared for Presentation at ECO WORLD ’92, June 15, 1992, Washington D. C.)

References

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