Regeneration with Rich Bypass of Aqueous Piperazine and

Article pubs.acs.org/IECR

Regeneration with Rich Bypass of Aqueous Piperazine and Monoethanolamine for CO2 Capture Yu-Jeng Lin, Tarun Madan, and Gary T. Rochelle* Texas Carbon Management Program, McKetta Department of Chemical Engineering, The University of Texas at Austin, 200 E Dean Keeton St, C0400 Austin, Texas 78712-1589, United States ABSTRACT: Amine scrubbing is the most mature CO2 capture technology for fossil fuel power plants, but the energy use for CO2 regeneration and compression will be 20 to 25% of the power plant output. The objective of this work is to develop alternative stripper configurations that reduce the energy use of CO2 capture. The advanced stripper configurations were modeled and optimized using Aspen Plus. Total equivalent work was used as an indicator of overall energy performance accounting for reboiler duty, compression work, and pump work. The rich exchanger bypass recovers stripping steam heat by using an exchanger. To get better energy performance, this strategy was applied to advanced configurations including a reboilerbased stripper, an interheated stripper, and a flash stripper. Both 9 m monoethanolamine (MEA) and 8 m piperazine (PZ) were investigated. The best energy performance was obtained from the stripper with a warm rich bypass and a rich exchanger bypass, which provides 10% less equivalent work for PZ and 6% less for MEA compared to the simple stripper. A flash stripper with a warm rich bypass and rich exchanger bypass uses 9% less energy with PZ and 5% less with MEA. With the warm rich bypass and rich exchanger bypass, MEA can provide 8% less equivalent work at 135 °C with acceptable thermal degradation. simple stripper.6−12 Rochelle and co-workers6−9 evaluated a multipressure configuration that operates the stripper at different pressure levels. The improvement comes from the recovery of stripping steam heat at a higher pressure. Van Wagener and Rochelle13 emphasized the importance of increasing process reversibility by introducing more complex configurations including the multistage flash and the interheated stripper. Van Wagener and Rochelle showed that the interheated stripper with 8 m piperazine (PZ) offers the best energy savings. Madan11 demonstrated that the multifeed flash stripper can effectively reduce the energy requirement by bypassing a portion of rich solvent into the stripper at different temperature levels. Loss of steam heat from the stripper is one of the reasons that the amine scrubbing process is inefficient.14 When the stripper is operated at 120−150 °C, water is vaporized and emitted with CO2 from the top of the stripper. The water vapor in the stripper overhead can be reduced using a cold rich bypass that recovers the heat either in the stripper or in a heat exchanger.11,15,16 The rich exchanger bypass proposed in this work applies a heat exchanger to recover steam heat from CO2 rich vapor product by bypassing a portion of the cold rich solvent. The objective of this work is to investigate the improvement brought by the rich exchanger bypass. Both the reboiler-based stripper and the flash stripper were modified with the rich exchanger bypass. The stripper modeling and optimization of novel configurations have been done using Aspen Plus. The relative amount of water vaporized depends on the solvent

1. INTRODUCTION The most mature technology for postcombustion CO2 capture is amine scrubbing.1 A typical absorption/stripping CO2 capture process is shown in Figure 1. Desulfurized flue gas

Figure 1. Simple stripper.

from coal combustion with 12% CO2 is contacted with the aqueous amine in the absorber where 90% of the CO2 is removed. The rich solvent from the bottom of the absorber is sent to the stripper and heated for CO2 regeneration. The stripped CO2 is then compressed to 150 bar for further storage and sequestration. For postcombustion CO2 capture, steam usage for CO2 regeneration in the stripper and CO2 compression work are the main contributions to the energy use. CO2 capture will reduce electricity output by 20−25% at a typical coal-fired power plant.2−5 To reduce the energy requirement, alternative stripper configurations have been proposed to improve capture efficiency. Several previous studies have shown that alternative stripper configurations have better energy performance compared to the © 2014 American Chemical Society

Received: Revised: Accepted: Published: 4067

November 5, 2013 January 31, 2014 February 18, 2014 February 18, 2014 dx.doi.org/10.1021/ie403750s | Ind. Eng. Chem. Res. 2014, 53, 4067−4074

Industrial & Engineering Chemistry Research

Article

effective turbine efficiency, η, is set at 90% based on the work of Van Wagener.10

properties such as the heat of desorption and acceptable operating temperature. In order to show the improvement with different solvent properties, two common solvents, piperazine (PZ) and monoethanolamine (MEA), were investigated. The total equivalent work is used to evaluate the overall energy requirement of the process rather than relying only on reboiler duty.

⎛ kJ ⎞ Weq ⎜ ⎟ = WHeat +Wpump +Wcomp ⎝ mol CO2 ⎠

(1)

⎛ kJ ⎞ Wcomp⎜ ⎟ ⎝ mol CO2 ⎠

2. SIMULATION METHODS This work simulates only the solvent regeneration process, including the main cross exchanger, stripper column, and overhead condenser as shown in Figure 1. The temperature and CO2 loading of the rich solvent are assumed to be constant. The lean loading is optimized to minimize the use of equivalent work by the regeneration system and may not minimize the overall cost of the process including the capital cost of the absorber and cross exchanger. 2.1. Simulation Model. Simulation results were obtained from Aspen Plus version 7.3. The Electrolyte Non-Random Two-Liquid (e-NRTL)17,18 property method is used to describe the CO2−amine−H2O chemistry accounting for the nonideality in the aqueous electrolyte system. For the gas− liquid contacting separator unit, Aspen Plus RateSep provides a rigorous rate-based model for heat and mass transfer using a nonequilibrium approach, applying two-film theory. The application of the rate-based model to the amine scrubbing process has offered accurate prediction against pilot plant data.19 8 m (mol/kg H2O) PZ and 9 m (mol/kg H2O) MEA were chosen as solvents. 7 m MEA (30 wt %) is the standard solvent for amine scrubbing in industry. 9 m MEA provides greater CO2 capacity than 7 m MEA, which should give less energy use. PZ is an advanced solvent with a greater reaction rate and CO2 capacity than MEA. It can also be used up to 150 °C without significant thermal degradation.20,21 8 m PZ can be used at 0.26 to 0.42 mol CO2/mol alkalinity without solids precipitation down to 20 °C.20 The thermodynamic models used for MEA and PZ in this work were “Phoenix” and “Independence,” respectively.22,23 These models have been regressed in Aspen Plus with experimental data including amine volatility, heat capacity, CO2 solubility, and amine pKa over a range of amine concentration and CO2 loading. 2.2. Total Equivalent Work. The energy requirement for the CO2 capture process is evaluated as total equivalent work. As eq 1 shows, total equivalent work consists of pump work, compression work, and heat work and is normalized by the moles of CO2 removed. The pump work includes the head at 72% efficiency to move the rich solvent from the absorber to the pressure of the stripper. Compression work to 150 bar was calculated by eq 2 from stripper pressure. This equation was developed by Van Wagener10 with Aspen Plus, assuming intercooling (and water condensation) at 40 °C between stages at a compression ratio of 2.0 or less, with a polytropic efficiency of 72%, and no pressure drop in the intercooling. The equivalent work accounts for reboiler heat duty with Carnot cycle efficiency and turbine efficiency as shown in eq 3. In the Carnot efficiency, the driving force for the steam exchanger is taken as 5 °C from the steam condensing temperature, and the heat sink is assumed to be 40 °C. The

⎧ ⎞ ⎛ ⎪ 4.572 ln⎜ 150 ⎟ − 4.096 Pin ≤ 4.56bar ⎪ ⎝ Pin ⎠ =⎨ ⎛ 150 ⎞ ⎪ ⎟ − 2.181 Pin > 4.56bar ⎪ 4.023 ln⎜ ⎝ Pin ⎠ ⎩

(2)

⎛ T + ΔT − Tsink ⎞ ⎛ kJ ⎞ WHeat⎜ ⎟Q reb ⎟ = ηturbine⎜ reb Treb + ΔT ⎝ mol CO2 ⎠ ⎝ ⎠

(3)

2.3. Process Specifications. Process specifications used in the simulations are shown in Table 1. Because the absorber was Table 1. Process Simulation Specifications solvent process modeling tool thermodynamic model packing reboiler T (°C) reboiler LMTD (°C) rich loading (mol CO2/mol alkalinity) rich solvent T (°C) main exchanger LMTD (°C) rich exchanger LMTD (°C) interheated exchanger LMTD (°C)

8 m PZ

9 m MEA

Aspen Plus v7.3 Independence Phoenix 2 m Mellapak 250X 150 120 5 0.40 0.50 46 5 20 5

not simulated, typical rich solvent conditions including rich loading and temperature were fixed as constants. The total packing height in the stripper is fixed at 2 m except for the configurations with two packing sections where 1 m is used for each section. Rich solvent was set at 46 °C with CO2 loading (mol/mol alkalinity) of 0.5 for MEA and 0.4 for PZ according to typical results with an intercooled absorber.22 It is usually more energy-efficient when the stripper is operated at high temperature due to higher CO2 partial pressure. However, a high temperature could result in significant thermal degradation of the amine in the reboiler. A reasonable compromise of 150 °C for PZ and 120 °C for MEA was used.20,24 Three different heat exchangers were used in this work, the main exchanger, the interheated exchanger, and the rich exchanger. For the main exchanger and interheated exchanger, the log mean temperature difference (LMTD) was specified as 5 °C. For the rich exchanger, where heat transfer happens between vapor and liquid, the LMTD was set at 20 °C. Because phase change happens in some of the exchangers, LMTD calculations have considered the temperature profile along the heat exchanger instead of using inlet/outlet temperature only.

3. REBOILED STRIPPER CONFIGURATIONS Reboiled stripper configurations will be described in this section, flash-based stripper configurations in the next section. 4068

dx.doi.org/10.1021/ie403750s | Ind. Eng. Chem. Res. 2014, 53, 4067−4074


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3.1. Simple Stripper. The simple stripper shown in Figure 1 is the base case. The cold rich solvent is heated by the hot lean solvent in the main exchanger and then sent to the top of the stripper. The reboiler provides the sensible heat, the heat of CO2 desorption, and the vaporization heat of water. The hot lean solvent from the reboiler is returned to the absorber through the main exchanger. The hot CO2 rich vapor from the top of the stripper is cooled to 40 °C in the overhead condenser with a loss of the latent heat of the excess water vapor. 3.2. Simple Stripper with Cold Rich Bypass. Figure 2 shows the flow sheet of the simple stripper with the cold rich

usually contains 30−50% H2O. By bypassing part of the cold rich solvent to the rich exchanger, the latent heat of steam in the CO2 rich vapor can be partially recovered. After being heated, the bypass rich solvent is mixed with the hot rich solvent and fed to the top of the stripper. Instead of condensing steam in the stripper using the cold rich bypass, the rich exchanger bypass provides an alternative to recover stripping steam heat. Since condensed water is not directly recycled back to the stripper, there is more flexibility in managing the water balance of the system. 3.4. Interheated Stripper with Rich Exchanger Bypass. Figure 4 shows the flow sheet of the interheated stripper with

Figure 2. Simple stripper with the cold rich bypass. Figure 4. Interheated reboiled stripper with the rich exchanger bypass.

bypass. Part of the cold rich solvent is bypassed and sent to the top of the stripper without being heated by the main exchanger. The stripper serves as a direct contact exchanger that condenses steam out of the overhead CO2 product. Van Wagener10 has shown that the cold rich bypass can effectively mitigate flashing at the top of the stripper caused by high temperature and reduces reboiler heat duty. Madan11 has extended this concept by feeding cold, warm, and hot rich bypasses into appropriate stages in the stripper. With multiple feeds of rich solvent at different temperature levels, stripping steam can be recovered more reversibly. However, there is a diminishing return of energy savings with an increasing number of bypasses, and more stripper packing is required to fully realize the benefits. 3.3. Simple Stripper with Rich Exchanger Bypass. Figure 3 shows the flow sheet of the simple stripper with the rich exchanger bypass. In this configuration, an exchanger is used to recover stripping steam heat from the CO2 vapor that

the rich exchanger bypass. The interheated stripper was proposed by Leites et al.25 and has been studied by Rochelle and co-workers.9−11 Solution is taken from the middle of the stripper to be heated by the hot lean solvent in the interheated exchanger and sent back to the next stage of the stripper. The hot rich solvent temperature decreases because only part of the heat from the hot lean solvent is transferred by the main heat exchanger. A lower rich solvent temperature can reduce the flashing at the top of the stripper so that less steam comes out with the CO2. Similar to the cold rich bypass, energy savings come from condensing steam in the stripper, but the temperature at the top of the stripper is warmer. In this work, the interheated stripper configuration was modified with the rich exchanger bypass to recover the rest of the steam heat from the CO2 vapor. 3.5. Reboiled Stripper with Rich Exchanger Bypass and Warm Rich Bypass. Figure 5 shows the flow sheet of the stripper with the rich exchanger bypass and warm rich bypass. This configuration includes both concepts of recovering stripping steam heat from the stripper and the cross exchanger. The warm rich bypass recovers the steam heat in the stripper, and the cold rich bypass recovers the rest in the rich heat exchanger. The warm rich bypass is drawn from the main heat exchanger and fed to the top of the stripper. The temperature of the warm rich solvent is chosen as its bubble point. The remaining heat in the CO2 is recovered with bypassing cold rich solvent in the rich exchanger. Applying the warm rich bypass makes the heat transfer driving force between rich solvent and hot CO2 vapor smaller in both the stripper and the rich heat exchanger when steam is condensed. The combination is expected to work more efficiently than with only the rich exchanger bypass or cold rich bypass. As shown by Madan,11 the stripper can be operated

Figure 3. Simple stripper with the rich exchanger bypass. 4069



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Figure 5. Reboiled stripper with the warm rich bypass and rich exchanger bypass. Figure 7. Flash stripper with the cold rich bypass.

somewhat more reversibly by feeding more bypass flows at appropriate temperature levels into corresponding locations of the stripper.

4. FLASH STRIPPER CONFIGURATIONS In the flash stripper, the reboiler is replaced by a convective steam heater. This heats the rich solvent, and then CO2 and lean solvent are produced by flash. Since there is less solvent hold-up and residence time at elevated temperatures, the convective steam heater will minimize thermal degradation of the solvent. The capital cost of a convective steam heater should be less than a reboiler. However, it is expected that the flash stripper is less efficient than the reboiler-based stripper due to a lack of counter-current contact between vapor and liquid. Flash stripper modeling starts with the single-stage flash as Figure 6 shows. Multistage flash that regenerates CO2 at

Figure 8. Flash stripper with the rich exchanger bypass.

Figure 9. Flash stripper with the warm rich bypass and rich exchanger bypass.

Figure 6. Single-stage flash.

5. RESULTS AND DISCUSSION Equivalent work is the metric of the overall energy performance. Optimum equivalent work can be obtained by varying lean loading, the cold rich bypass rate, and the warm rich bypass rate. The results with optimum lean loading and mass split fraction are shown in Tables 2 and 3. Figures 11−14 show the equivalent work with lean loading from 0.26 to 0.34 mol CO2/mol alkalinity for PZ and 0.30−0.44 for MEA. As lean loading varies, there are tradeoffs between stripping steam heat, sensible heat, compression work, and pump work. The heat of desorption does not change significantly when the lean loading is less than 0.5.21 Higher lean loading leads to higher CO2

different pressure levels can improve energy performance over the single-stage flash, but diminishing returns of energy savings were observed with increasing stages. The rich exchanger bypass and cold rich bypass were also applied to the flash stripper as shown in Figures 7 and 8. Figure 9 shows the flash stripper with the warm bypass and rich exchanger bypass. After being heated by the steam heater to 150 °C for PZ and 120 °C for MEA, the rich solvent is sent to the bottom of the flash stripper. CO2 vapor flashes in the column and contacts with the warm rich bypass. A portion of the cold rich solvent recovers the stripping steam heat from the CO2 vapor in the rich exchanger. 4070



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Table 2. Optimum Results for 8 m PZ energy (kJ/mol CO2) Ldg (mol/mol)

P (bar)

simple stripper (SS) SS + cold rich BPS SS + rich ex BPS interheated stripper interheated stripper + rich ex BPS stripper + warm rich BPS + rich ex BPS

0.30 0.30 0.30 0.30 0.28 0.28

8.5 8.5 8.5 8.5 7.4 7.4

single stage flash (SSF) SSF + cold rich BPS SSF + rich ex BPS flash + warm rich BPS + rich ex BPS

0.32 0.30 0.30 0.30

10.2 8.4 8.5 8.2

cold rich BPS (%)

warm rich BPS (%)

Qreb

Wreb

Wcomp

WEQ

14

105.1 95.7 93.5 95.0 90.3 89.5

25.4 23.1 22.6 23.0 21.8 21.6

9.3 9.3 9.3 9.3 9.9 9.9

35.9 33.6 33.1 33.4 32.6 32.4

10

118.7 99.1 98.5 91.5

28.7 23.9 23.8 22.1

8.6 9.4 9.3 9.5

39.0 34.5 34.3 32.7

reboiled stripper 5 7 5 7 flash stripper 7 10 5

Table 3. Optimum Results for 9 m MEA energy (kJ/mol CO2) Ldg (mol/mol)

P (bar)

cold rich BPS (%)

warm rich BPS (%)

Qreb

Wreb

Wcomp

WEQ

14

145.6 137.1 137.1 133.9 130.8 130.2

28.0 26.3 26.3 25.7 25.1 25.0

13.3 13.3 13.3 13.8 13.8 13.8

41.8 40.2 40.2 39.9 39.3 39.2

14

165.5 142.2 146.6 132.0

31.8 27.3 28.2 25.4

12.7 13.4 12.7 14.0

45.3 41.2 41.7 39.7

reboiled stripper simple stripper (SS) SS + cold rich BPS SS + rich ex BPS interheated stripper interheated stripper + rich ex BPS stripper + warm rich BPS + rich ex BPS

0.40 0.40 0.40 0.38 0.38 0.38

3.3 3.3 3.3 3.0 3.0 3.0

single stage flash (SSF) SSF + cold rich BPS SSF + rich ex BPS flash + warm rich BPS + rich ex BPS

0.42 0.40 0.42 0.38

3.8 3.3 3.8 2.9

5 7 6 7 flash stripper 8 8 9

Figure 10. Water content in the CO2 vapor and stripper pressure of simple stripper with 8 m PZ and 9 m MEA.

Figure 11. Reboiled stripper results for 8 m PZ.

partial pressure and reduced compression work. The concentration of water vapor in the CO2 product also decreases, but sensible heat increases because of reduced solvent capacity. These configurations can be compared with those at optimum equivalent work. 5.1. Rich Exchanger Bypass and Cold Rich Bypass. The rich exchanger bypass and cold rich bypass both recover the stripping steam heat. When more heat from CO2 rich vapor is effectively recovered, less sensible heat is needed in the reboiler. The stripping steam that is available to be recovered from the simple stripper is shown in Figure 10. The water concentration in the CO2 product is 25−40% with 8 m PZ and 30−50% with 9 m MEA.

Figures 11 and 12 show the equivalent work of the reboiled strippers using PZ and MEA, respectively. Compared to the simple stripper, the rich exchanger bypass has 7.8% improvement of equivalent work for PZ and 3.8% for MEA. At lower loading, the improvement is more significant than at higher loading because more steam is available to be recovered at low lean loading. The rich exchanger bypass has better energy performance than the cold rich bypass with PZ, but they are similar to MEA, which implies the rich exchanger bypass is relatively inefficient when more water vapor is in the CO2 produced. The temperature approach in the rich exchanger and the amount of packing for the cold rich bypass can determine the performance of the heat recovery. Reasonable specifications 4071



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stripper reduced equivalent work by 9.2% for PZ and 6.0% for MEA. 5.3. Stripper with Warm Rich Bypass and Rich Exchanger Bypass. For the stripper with the warm rich bypass and rich exchanger bypass, both bypass flows are optimized. With both bypasses recovering heat from the CO2 product, this configuration provides the lowest equivalent work, 9.7% improvement for PZ and 6.2% for MEA. Figures 11 and 12 show that the optimum is broad; equivalent work does not vary much with lean loading in the region of the optimum. Therefore, this configuration allows the system to operate at lower lean loading and greater capacity with little loss of energy efficiency. This configuration has energy performance similar to the interheated stripper but is less complex to implement and should have a lower capital cost. 5.4. Flash Strippers. Figures 13 and 14 show the total equivalent work of flash strippers using PZ and MEA, respectively. The single-stage flash uses 9% more work than the simple stripper because a large amount of water vapor is lost with the CO2 product. This process can be improved with the cold rich bypass or rich exchanger bypass or both. When only one of the bypasses is used with the flash stripper, compared to the simple stripper, the energy use is reduced by 4.2% for PZ and 0.2 to 1.5% for MEA. The flash stripper with both the warm rich bypass and rich exchanger bypass uses 8.9% less work with PZ and 5.0% less with MEA. Because the contribution of stripping steam heat to the total equivalent work decreases, the optimum lean loading is shifted to a lower value. Compared to the reboiled stripper with the warm rich bypass and rich exchanger bypass, the flash stripper is less efficient at low lean loading but provides almost the same energy performance at optimum lean loading. 5.5. Variable Regeneration Temperature. This work also demonstrates the flexibility to use variable steam temperature with advanced stripper configurations. Since the existing power plants may have different steam pressure at the crossover between the low and intermediate pressure turbines, the process concept needs flexibility to adapt to a different steam temperature. Operating at a higher temperature is always more efficient because of higher CO2 partial pressure. It should also reduce the capital cost of the compressor. However, not only is the limit of the temperature restricted by acceptable thermal degradation but the available pressure of steam from the power plant should also be considered. Another two cases of the flash stripper with the warm rich bypass and rich exchanger bypass have been simulated at different temperature levels: 120 °C for PZ and 135 °C for MEA. The operating temperatures correspond to a range of steam pressure from 2.3 bar (125 °C) to 5.4 bar (155 °C) if a 5 °C temperature approach is used for heat transfer. Figures 15 and 16 show the comparison of equivalent work of the flash stripper with the warm rich bypass and rich exchanger bypass using PZ and MEA, respectively. Table 4 shows the optimum parameters. MEA operating at 135 °C shows better energy performance with equivalent work of 1.1 kJ/mol CO2 lower than at 120 °C. The flash stripper with less residence time has the potential to be operated at a higher temperature without dramatic thermal degradation. At a lower temperature, the ratio of the amount of stripping steam to CO2 removed becomes higher. Stripping steam heat required is the main factor causing the inefficiency of the lower temperature case at lower lean loading. However, as lean loading increases, the equivalent work of the two temperature

Figure 12. Reboiled stripper results for 9 m MEA.

Figure 13. Flash stripper results for 8 m PZ.

Figure 14. Flash stripper results for 9 m MEA.

of a 20 °C LMTD in the rich exchanger and 2 m of stripper packing were used in this work. Less equivalent work would be used with a tighter temperature approach or more packing to provide better heat transfer, but the capital cost also increases. 5.2. Interheated Stripper with Rich Exchanger Bypass. Compared to the simple stripper, the interheated stripper reduces equivalent work by 7.0% for PZ and 4.5% for MEA. Having the same function as the cold rich bypass, the interheated stripper provides rich solvent at a lower temperature that condenses water vapor in the stripper overhead. When combined with the rich exchanger bypass to recover the rest of the stripping steam heat from the CO2, the interheated 4072



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6. CONCLUSIONS The reboiled stripper with the warm rich bypass and rich exchanger bypass provides the best energy performance. It uses 9.7% less energy with 8 m PZ and 6.2% less with 9 m MEA than the simple stripper. The reboiled stripper is even better than the flash configuration at lower lean loading. The flash stripper with the warm rich bypass and rich exchanger bypass offers the potential for excellent energy performance. Because this configuration uses a convective steam heater rather than a reboiler, it should have significantly lower capital cost. Other potential advantages of the advanced flash stripper will be investigated. Because the convective steam heater can be designed with less solvent hold-up at the maximum temperature, the flash configuration can be used at an elevated temperature to match the condensing temperature of the available steam supply. This flexibility should reduce the capital cost of the compressor and provide additional energy savings, especially with MEA, which is otherwise limited by thermal degradation.

Figure 15. Variable temperature results of the flash stripper with the warm rich bypass and rich exchanger bypass for 8 m PZ.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare the following competing financial interest(s): One author of this publication consults for Southern Company and for Neumann Systems Group on the development of amine scrubbing technology. The terms of this arrangement have been reviewed and approved by the University of Texas at Austin in accordance with its policy on objectivity in research. The authors have financial interests in intellectual property owned by the University of Texas that includes ideas reported in this paper.

Figure 16. Variable temperature results of the flash stripper with the warm rich bypass and rich exchanger bypass for 9 m MEA.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Texas Carbon Management Program in the preparation of this work.

cases gets closer, and the crossover is near 0.34 lean loading for PZ and 0.42 for MEA. The higher temperature case becomes inefficient at higher lean loading because the heat duty at higher temperature is more valuable (as calculated using the Carnot cycle efficiency expression). At higher lean loading, the difference in the stripping steam heat required between two temperature cases is not significant so that the Carnot cycle efficiency term becomes a more important factor. In a higher temperature case, the shift of optimum lean loading toward a lower value can be observed because when the temperature increases, the contribution of stripping steam heat to total equivalent work becomes less compared to the contribution of sensible heat.

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REFERENCES

(1) Rochelle, G. T. Amine scrubbing for CO2 capture. Science 2009, 325, 1652−4. (2) Romeo, L. M.; Espatolero, S.; Bolea, I. Designing a supercritical steam cycle to integrate the energy requirements of CO2 amine scrubbing. Int. J. Greenh. Gas Control 2008, 2, 563−570. (3) Romeo, L. M.; Bolea, I.; Escosa, J. M. Integration of power plant and amine scrubbing to reduce CO2 capture costs. Appl. Therm. Eng. 2008, 28, 1039−1046.

Table 4. Optimum Results of Flash Stripper with Warm Rich Bypass and Rich Exchanger Bypass Using 9 m MEA and 8 m PZ with Variable Temperature energy (kJ/mol CO2) steam heater T (°C)

Ldg (mol/mol)

P (bar)

cold rich BPS (%)

warm rich BPS (%)

Qreb

Wreb

Wcomp

WEQ

12 10

102.9 91.5

16.5 22.1

13.7 9.5

33.9 32.7

14 10

132.0 122.2

25.4 22.2

14.0 11.3

39.7 38.6

8 m PZ 120 150

0.32 0.30

3.0 8.2

6 5

120 135

0.38 0.36

2.9 5.3

9 7

9 m MEA

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