Article pubs.acs.org/IECR
Integrated Bench-Scale Parametric Study on CO2 Capture Using a Carbonic Anhydrase Promoted K2CO3 Solvent with Low Temperature Vacuum Stripping Guojie Qi,† Kun Liu,† Reynolds A. Frimpong,† Alan House,§ Sonja Salmon,§ and Kunlei Liu*,†,‡ †
Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, Kentucky 40511, United States Department of Mechanical Engineering, University of Kentucky, Lexington, Kentucky 40506, United States § Novozymes North America Inc., 77 Perry Chapel Church Road, Franklinton, North Carolina 27525, United States ‡
S Supporting Information *
ABSTRACT: A bench-scale unit was fabricated and used to investigate use of carbonic anhydrase (CA) promoted K2CO3 solvent as an option for CO2 capture from coal-fired power plants. Bench-scale parametric tests were performed at various CA concentrations, solvent flow rates, and reboiler duties. The CO2 capture efficiency significantly increases, and regeneration energy requirement decreases, with increasing CA concentrations up to 2.5 g/L, with capture performance leveling off at higher enzyme doses (up to 4 g/L). Thus, at higher enzyme doses, the capture efficiency is equilibrium rather than kinetically controlled at the top of absorber, when using solvent regenerated via vacuum stripping at high (>35%) lean carbonate to bicarbonate (CTB) conversion levels, which limits the driving force for CO2 absorption. The CO2 capture efficiency also increases when reboiler duty was increased from 0.85 to 1.1 kW, although this also increases the regeneration energy penalty. In contrast, the effect of solvent flow rate on CO2 capture efficiency is less pronounced. Further improvements to the CO2 capture process using CA promoted K2CO3 solvent with low temperature vacuum stripping could be potentially advanced by lowering vacuum pressure, improving strategies for increasing rich CTB conversion (e.g., advanced packing column and optimized L/G ratio), and decreasing absorption temperature. However, the major challenge of using a K2CO3−KHCO3 solvent is the low absorption rate due to the slow CO2 hydration step resulting in poor absorption performance. Piperazine (PZ), amino acids, and boric acid, as mass transfer promoters, have been well studied and applied to accelerate the absorption rate.5,8,10−13 However, drawbacks of these promoters, such as toxicity, instability, and corrosiveness, inhibit their extensive use.9 The enzyme carbonic anhydrase (CA), as an efficient and eco-friendly biocatalyst, has been shown to improve the CO2 absorption rate in the K2CO3−KHCO3 solvent.14 CA is a zinc
1. INTRODUCTION Solvent based postcombustion CO2 capture is one of the preferred options for CO2 removal from coal-fired combustion power plants.1,2 A variety of aqueous alkanolamine solutions have been identified as capture solvents.3 Alkali salts based solvents are an attractive alternative being actively investigated. 4−7 Such solvents are less toxic with minimal degradation, nonvolatile with no aerosol emissions, and have heat of absorptions of CO2 (for example, 24 kJ/mol CO2 for carbonate to bicarbonate) much lower than those of amine based solvents.8 One of the examples of commonly used alkali salt based solvents for CO2 capture is a 20−30 wt % potassium carbonate−bicarbonate (K2CO3−KHCO3) aqueous solution, which converts between carbonate and bicarbonate during CO2 absorption and desorption.9 © 2016 American Chemical Society
Received: Revised: Accepted: Published: 12452
September 3, 2016 November 9, 2016 November 9, 2016 November 9, 2016 DOI: 10.1021/acs.iecr.6b03395 Ind. Eng. Chem. Res. 2016, 55, 12452−12459
Article
Industrial & Engineering Chemistry Research
Figure 1. Schematic of the integrated bench-scale system.
increased 5-fold by adding free or immobilized enzymes to 20 wt % K2CO3 solvent.23 Akermin Inc. has demonstrated that CA coated M500X structured packing localized in the absorber can achieve 10-fold absorption enhancement in a lab-scale loop reactor, and 7-fold improvement in an integrated slip-stream field test, using 20 wt % K2CO3. This process was tested at the National Carbon Capture Center (USA), where >2800 h long testing was completed with 90% CO2 capture at ∼19.5 N m3/h flue gas and 80% CO2 capture at 30 N m3/h flue gas.21 However, a drawback of CA immobilization on structured packing is the logistical challenge and expense of replacing or recoating packing that eventually becomes inactivated. In addition, CA immobilization on a fixed surface can potentially limit the full CA activity exploitation, so further work with a free dissolved enzyme warrants investigation. Lu et al. proposed an integrated vacuum carbonate absorption process through process simulation, wherein rich CO2 solution could be regenerated at low temperature under vacuum pressure using low quality steam from the power plant. The simulation results demonstrated that the proposed process is economically attractive, suggesting a process that may be amenable to using a dissolved enzyme and avoiding the thermal stress of conventional high temperature stripping.19 The challenges of vacuum stripping should be also evaluated as its increasing of the CO2 compression cost and the scalability limitations to deep vacuum level ( 99.5%) from Armand Products Company, and deionized water. The carbon dioxide (purity > 99.9%) and calibrating gas (14 vol %, CO2, nitrogen balance gas) were purchased from Purity Plus. 2.3. Integrated Bench-Scale System. Figure 1 shows the schematic of the integrated bench-scale absorption/regeneration system at the University of Kentucky (UKy) Center for Applied Energy Research, which consists of a 7.6 cm ID clear PVC scrubber with 2 m of packing, a 7.6 cm ID stainless steel stripper with 2 m packing, and a condenser for solvent recovery in the stripper exhaust. The packing inside both absorber and stripper are 6 × 6 mm ceramic Raschig rings. The vacuum system includes a vacuum pump and a vacuum regulator to maintain the desired vacuum condition in the stripper. A heater and a chiller were used to provide separately sensible heat for solvent heating and cooling (a lean/rich heat exchanger was not employed). A hot oil system connected to the reboiler provided necessary heat for solvent regeneration. The lean and rich solvent pumps were selected and installed to minimize cavitation under vacuum stripping conditions. Two in-line flow meters were installed to monitor the volumetric solvent flow rates both entering and exiting the stripper. Feed gas was supplied by two mass flow controllers (MFC) deployed to control the CO2 and N2 flow rates. CO2 and N2 were mixed to simulate flue gas composition. The simulated flue gas mixture was sent through a water saturator and then was fed to the bottom of the absorber. A Horiba CO2 Analyzer was used to measure the online CO2 concentration. Carbon content and alkalinity of the lean and the rich solution samples were measured using methods described previously.27
φCO =
VNin2
(1)
where φCO2, CO2 capture efficiency, %; ninCO2, gas inlet CO2 mole flow rate, mol/s; nout CO2, gas outlet CO2 mole flow rate, mol/s. The inlet CO2 flow rate, ninCO2 could be calculated by the CO2 MFC flow rate. The outlet CO2 flow rate was calculated by eq 2:
process variable
value
liquid flow rate (mL/min) lean solvent inlet temperature (°C) rich solvent inlet temperature (°C) K2CO3 concentration (wt %) CTB conversion of lean solvent (%) enzyme concentration (g/L) gas flow rate (L/min) absorber gas inlet temperature (°C) gas CO2 concentration (vol %) stripper top pressure (kPa) hot oil flow rate (L/min) a reboiler inlet oil temperature (°C) reboiler duty (kW) b reboiler bulk solvent temperature (°C) estimated heat loss (kW)
300, 500 40 65−70 23.5 34%−43% 0, 1, 2.5, 4 30 40 14.8 35 10 90, 95 0.7−1.2 75−80 0.09
a
Reboiler inlet oil temperature was maintained by both hot oil heater and immersion heater. bRich solvent inlet temperature was set ∼10 °C lower than the reboiler bulk temperature to simulate a typical heat exchanger approach temperature (10 °C). 12454
DOI: 10.1021/acs.iecr.6b03395 Ind. Eng. Chem. Res. 2016, 55, 12452−12459
Article
Industrial & Engineering Chemistry Research
Figure 2. Effect of CA concentration on (a) CO2 capture efficiency and (b) regeneration energy requirement with 500 mL/min solvent flow rate at 75.5 ± 0.5 °C reboiler bulk temperature with 0.8 kW reboiler duty.
and flow rate. Data in repeated tests with the same parametric set points was used to obtain experimental error. Liquid samples collected during the steady-state operation were used to determine the carbon content (CCO2) and alkalinity (AlkK) for CTB conversion calculations. The CTB conversion was calculated according to eq 5: conversion =
corresponding energy requirements of 322, 300, and 284 kJ/ mol CO2, respectively. To analyze further the effect of CA concentration on the driving force limitation for CO2 absorption, Figure 3 presents
2CCO2 − AlkK AlkK
(5)
3. RESULTS AND DISCUSSION 3.1. Effect of CA Concentration on CO2 Capture. The effect of CA concentration on CO2 capture efficiency and regeneration energy was measured at various CA concentrations with 500 mL/min cyclic solvent flow rate, 0.8 kW reboiler duty, and 75.5 ± 0.5 °C reboiler bulk solvent temperature (reboiler oil inlet temperature 90 °C). Figure 2a shows that the CO2 capture efficiency increases 3.9 to 4.6 times when dosing CA (1 to 4 g/L) in the 23.5 wt % K2CO3 solvent compared to the no enzyme case (18% capture efficiency). When the CA concentration increases from 1 to 2.5 g/L, the CO2 capture efficiency increased from 72% to 84%, but no further increase is observed at 4 g/L CA, as shown by the plateau in CO2 capture efficiency in Figure 2a at the higher enzyme concentrations. The flocculation and impurities inhibition of CA at high concentration (>2.5 g/L) might contribute to the formation of the CO2 capture efficiency plateau.17 The more than 35% lean solution CTB conversion in these cases might also limit the increase of CO2 capture efficiency and the driving force for CO2 mass transfer in the absorber, which is further discussed below. Accordingly, the regeneration energy requirement displayed in Figure 2b shows that the enzyme promoted cases can reduce the energy consumption for CO2 release in the stripper by 4-fold compared to the blank 23.5 wt % K2CO3 solvent (1330 kJ/ mol CO2, calculated using experimental data from UKy’s bench-scale unit). The reduction in energy requirement in the presence of CA can be attributed to the higher solvent cyclic capacity and increased rich CTB conversion, which reduced the sensible heat and amount of water evaporation in the stripper. In the presence of CA, the energy requirements for the enzyme doses tested were similar, decreasing slightly with increasing CA concentration across the range 1 to 2.5 to 4 g/L, with
Figure 3. Driving force analysis in absorber using equilibrium CO2 partial pressures at different CTB conversions in 23.5 wt % K2CO3 solvent (PoCO2 is the operational CO2 partial pressure).
the equilibrium CO2 partial pressure versus CTB conversion in 23.5 wt % K2CO3 at various temperatures. Although the local CO2 concentration in the absorber and stripper is temperature and water-vapor dependent, in order to simplify the calculation, the absorber inlet (bottom) CO2 partial pressure was assumed to be 15 kPa (purple dashed line). The absorber outlet (top) CO2 partial pressure was assumed to be 1.5 kPa (purple dotted line) by assuming the 1.5 vol % outlet CO2 concentration to achieve the 90% CO2 capture target of the U.S. Department of Energy.28 The intersecting point (lower blue open circle) between the equilibrium CO2 partial pressure line at 40 °C and the 1.5 kPa absorber outlet CO2 partial pressure line is the maximum allowable CTB conversion for lean solution entering the top of absorber to achieve 90% CO2 capture, and is around 35%. However, all the lean CTB conversions in the parametric tests were above 35% under the low stripper operating temperature, as highlighted in the CTB conversion working range (solid blue line) shown in Figure 3. Hence, the level of CO2 absorption was equilibrium controlled at the top of 12455
DOI: 10.1021/acs.iecr.6b03395 Ind. Eng. Chem. Res. 2016, 55, 12452−12459
Article
Industrial & Engineering Chemistry Research
Figure 4. Effect of solvent flow rate on (a) CO2 capture efficiency and (b) regeneration energy requirement at various solvent flow rates and 75.5 ± 0.5 °C reboiler bulk temperature with 0.8 kW reboiler duty.
than at 300 mL/min at various CA concentrations. Note that the regeneration energy was calculated by only considering reboiler heat supply, vacuum duty, and heat loss, as shown in eq 3, and did not include the sensible heat from the rich solvent heater and lean solvent chiller as shown in Figure 1. Thus, the data shown in Figure 4b only indicates a limited impact of solvent flow rate on reboiler performance, although the trend of lower energy requirement with lower solvent flow rate is as expected. Table 2 summarizes the lean and rich CTB conversions, cyclic capacity, and regeneration energy requirements corre-
absorber due to higher than targeted lean solution CTB conversion, meaning less CO2 was released in the stripper than targeted. An indicator that the CO2 absorption was equilibrium limited is that, even in the presence of CO2, increasing the CA concentration beyond 2.5 g/L gave no further improvement in CO2 capture efficiency. Rich CTB conversions, as shown in Figure 3, were below 60% for parametric tests at varied CA concentration under the selected inlet CO2 concentration (15 vol % on dry basis). The high lean and low rich solution CTB conversions result in the low cyclic capacity and high regeneration energy as shown in Figure 2b, compared to the conventional MEA based CO2 capture process, with regeneration energy reported at 163 kJ/mol CO2.29 It is worth noting that the vacuum stripping process (0.35 bar) requires more CO2 compression work than the conventional MEA process (2 bar), which is not included in the energy calculation in this work. Lower CTB conversion and lower temperature of the solvent returning from the stripper to the absorber would be beneficial to increase the CO2 mass transfer driving force for absorption. As shown in Figure 3, the maximum rich CTB conversion at the absorber bottom and the maximum lean CTB conversion to reach 90% CO2 capture at the absorber top shift left with increasing temperature. When the temperature decreases from 40 to 30 and 20 °C, the maximum theoretical rich CTB conversions (upper circles) are increased from 74% to 79% and 84%, and the maximum lean CTB conversions (lower circles) to reach 90% CO2 capture are around 41% and 48%. However, the solubility of K2CO3/KHCO3 solution, and the impact on kinetics of CA promoted K2CO3, should also be considered for operation at the lower absorber temperatures. Also, the extra cooling duty to reach the low temperature would need to be considered as this could contribute to an increase in the overall cost. 3.2. Effect of Solvent Flow Rate on CO2 Capture. To determine the effect of solvent flow rate on CO2 capture, the parametric tests were carried out at around 0.8 kW reboiler duty (corresponding to a reboiler bulk solvent temperature of 75.5 ± 0.5 °C) with solvent flow rates at 300 and 500 mL/min and different CA concentrations. Figure 4a shows that the CO2 capture efficiency difference is small when the solvent flow rate increases from 300 to 500 mL/min at CA concentrations from 1 to 4 g/L. Figure 4b reveals that the regeneration energy requirement at 500 mL/min solvent flow rate is slightly higher
Table 2. CTB Conversions, Capture Efficiency, and Energy Requirement Summary in the Solvent Flow Rate Parametric Tests CA concentration, g/L solvent flow rate, mL/min CO2 capture efficiency, % lean CTB conversion, % rich CTB conversion, % cyclic capacity, mol CO2/mol solvent reboiler duty, kW energy requirement, kJ/mol CO2
1 300
1 500
2.5 300
2.5 500
4 300
4 500
69% 43% 63% 0.20
72% 43% 57% 0.14
81% 37% 57% 0.20
84% 40% 58% 0.18
84% 37% 57% 0.20
83% 36% 54% 0.18
0.72 301
0.79 322
0.75 273
0.84 300
0.79 275
0.80 284
sponding to the solvent flow rate parametric tests presented in Figure 4. As shown in Table 2, the lean CTB conversion is around 36% to 43%, and the rich CTB conversion is around 54% to 63%. The high lean CTB conversion limits the driving force for CO2 capture in the absorber, generally leading to a lower solvent cyclic capacity and higher regeneration energy penalty, even at various solvent flow rates. As shown in Table 2 and Figure 4, the highest cyclic capacity (0.2 mol CO2/mol solvent) and CO2 capture efficiency (84%) were achieved with the lower flow rate (300 mL/min), overall resulting in the lowest energy requirement (275 kJ/mol CO2) across the conditions tested. The performance of aqueous 30 wt % MEA was also determined in the bench-scale system to compare with the CA promoted K2CO3 solvent and give a relative reference for the CO2 capture efficiency and energy requirement. As shown in Table S1, in the bench-scale system, the CO2 capture efficiency with MEA (94%) is higher, and the energy 12456
DOI: 10.1021/acs.iecr.6b03395 Ind. Eng. Chem. Res. 2016, 55, 12452−12459
Article
Industrial & Engineering Chemistry Research
3.3. Effect of Reboiler Duty on CO2 Capture. The effect of reboiler duty on CO2 capture was determined in 23.5 wt % K2CO3 solvent with and without CA with the goal of lowering the carbonate content and increasing the capture efficiency. The reboiler duty was increased from 0.85 to 1.1 kW (corresponding to maintained reboiler temperatures of 75.5 and 80 °C, respectively). Figure 6a shows that the CO2 capture efficiency slightly increases from 83.7% to 88.8% with reboiler duty increasing from 0.85 to 1.1 kW and with 2.5 g/L CA in the solvent. Figure 6b shows that the regeneration energy requirement increases with reboiler duty in both the presence and absence of CA. In the case of no enzyme present, the hydration reaction is so slow that the capture efficiency is kinetically limited. Therefore, the effect of increasing reboiler duty on CO2 capture via decreasing the CTB conversion of the lean solvent exiting the stripper in the tests without CA is minimal as shown in Figure 6. In the parametric tests that varied the reboiler duty, the reboiler bulk solvent temperature was increased from 75.5 to 80 °C. No higher reboiler duties were investigated to minimize CA thermal stress caused by contact with the hot oil tube skin temperature in the reboiler. 3.4. Potential Paths for Improvement. Figure 7 presents several potential paths to overcome the absorption and desorption limitations as shown in the parametric tests with low temperature vacuum stripping. The equilibrium CO2 partial pressures versus CTB conversions at 40 and 80 °C, theoretically represent the typical CO2 absorption/regeneration loop in bench-scale parametric tests (loop a-b-c-d-a, solid lines). The 90% CO2 capture line (purple dashed line) and the 15% CO2 partial pressure line (red dashed line) are defined the same as in Figure 3. Higher rich CTB conversion is beneficial to increase the solvent cyclic capacity. As shown in Figure 7, the absorption/regeneration loop (a-b′-c′-d-a) displays the maximum rich CTB conversion (b′) that can be reached in the absorber, which is much higher than in the parametric tests. Use of taller absorber columns and advanced packing with optimized L/G ratio have the potential to expand the CTB conversion working range. Lower vacuum pressure is another option to achieve leaner CTB conversions. The absorption/ regeneration loop (a″-b-c-d″-a″) with 0.2 bar vacuum pressure could potentially reduce maximum lean CTB conversion to 28% (d″) and achieve more than 90% CO2 capture (a″). However, a lower vacuum pressure consumes more work and
requirement with MEA (262 kJ/mol CO2) is lower than the CA promoted K2CO3 condition mentioned above. To understand further the stripping limitation in the varied solvent flow rate cases, Figure 5 presents the equilibrium and
Figure 5. Driving force analysis in the stripper using equilibrium and operational CO2 partial pressures versus CTB conversion in 23.5 wt % K2CO3.
operational CO2 partial pressures versus CTB conversion in 23.5 wt % K2CO3 at 75 °C. To simplify the calculation, the operational CO2 partial pressure line (red dashed and dotted line) is calculated from the vacuum stripping pressure (0.35 bar) and the equilibrium CO2 mole fraction at various CTB conversions. As illustrated in Figure 5, the driving force for solvent regeneration is relatively small (
