Article pubs.acs.org/IECR
Antisolvent Precipitation of Potassium Bicarbonate from KHCO3 + H2O + Ethanol/2-Propanol Systems in the CO2 Capture Process Gyo Hee Kim,† Tae-Sung Jung,‡ Won Hi Hong,† Jong-Nam Kim,*,‡ and Jong-Duk Kim*,† †
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Guseong-dong 373-1, Yusong-gu, Daejeon, Republic of Korea ‡ High Efficiency and Clean Energy Research Division, Korea Institute of Energy Research, 71-2 Jang-dong, Yusong-gu, Daejeon, Republic of Korea S Supporting Information *
ABSTRACT: For the energy-saving hot carbonate process, which contains a crystallization step, the antisolvent precipitation of potassium bicarbonate was investigated by adding ethanol and 2-propanol in a simulated effluent solution of CO2 absorption column. The feasibility of using the antisolvent was verified with the equilibrium data of ternary systems and a KHCO3 recovery. The ternary systems of KHCO3 + H2O + ethanol/2-propanol were equilibrated with varying amounts of alcohols at a preset temperature, and the equilibrium concentrations were determined using the cloud point method. In the ethanol system, a homogeneous and a solid−liquid phase were observed, whereas in the 2-propanol system, a liquid−liquid phase as well as the homogeneous and solid−liquid phases were observed. The equilibrium data were correlated with a local concentration parameter in the range of the antisolvent. From the correlation, the amount of antisolvent that has an effect equivalent to a cooling-only method and the optimum concentration of the antisolvent were evaluated. An addition of 17 wt % of ethanol and 57 wt % of 2propanol had an effect equivalent to cooling the solution from 333 K to 303 K. The optimum concentrations of the ethanol system and the 2-propanol system were found to range from 32.93 wt % to 60.07 wt % and from 6.24 wt % to 17.88 wt %, respectively, and the bicarbonate recovery yield was almost doubled when the optimum concentration of the antisolvent was applied. Considering the antisolvent concentration that has an effect equivalent to cooling crystallization and the optimum concentration of the antisolvent, ethanol was found to be a more feasible antisolvent than 2-propanol.
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INTRODUCTION The reduction of CO2 emissions into the atmosphere has been widely studied in an effort to solve the problem of global warming. Among the practical processes, the absorption of CO2 with a solvent is known to be a suitable means of capturing CO2 from flue gas on a large scale. The main challenge associated with the conventional amine-based absorption process is that it requires a high energy,1 which increases the operation cost in practice. Thus, it is necessary to design a lowenergy process. According to previous studies, the energy consumed during the regeneration of the solvent in the amine-based CO2 capture process exceeds 60% and accounts for the largest share, while the compression energy was the second-largest, in terms of energy consumption.1,2 For this reason, a process which reduces these energies was proposed recently.3−8 A hot carbonate process containing a high-pressure stripping step is one of the alternatives to the amine-based CO2 capture process, as the stripping and compression energy levels are lower than those associated with conventional absorption.3,8 During this process, because of the high-pressure stripping step, the generation of bicarbonate slurry is unescapable; hence, an essential bicarbonate crystallization step is introduced. In an earlier hot carbonate process containing a high-pressure stripping step, a cooling crystallization method was used for the crystallization step. For an economical crystallization step, reducing the energy of the crystallizer deserves consideration, and two means of © 2015 American Chemical Society
economical crystallization step were considered in this study. One of the method is to reduce temperature gap between the CO2 absorber and the crystallizer. During a hot carbonate process containing a cooling crystallization step, the crystallization step is introduced after the absorption step, which takes place at a high temperature exceeding 60 °C. This temperature gap between the CO2 absorber and the crystallizer can determine the economics of the process. Another means of economical crystallization step is to increase the bicarbonate precipitation yield. Crystallization is generally available through the supersaturation of the solute by cooling or by adding a nonsolvent. For the crystallization process to reduce the temperature gap between the CO2 absorber and the crystallizer, the antisolvent crystallization method is favorable, because it mainly relies on the difference between the concentrations in the solution and not a decrease in temperature. The antisolvent crystallization method also has the advantage of increasing the bicarbonate precipitation yield considerably. Before studying antisolvent crystallization as used to improve the hot carbonate process, it is necessary to select an antisolvent and a primary solvent. Ethanol and 2-propanol were selected as antisolvents, because they are miscible with Received: Revised: Accepted: Published: 8287
May 6, 2015 July 20, 2015 August 12, 2015 August 12, 2015 DOI: 10.1021/acs.iecr.5b01696 Ind. Eng. Chem. Res. 2015, 54, 8287−8294
Article
Industrial & Engineering Chemistry Research
similar to that in another study.10 These values are listed in Table 1.
water and immiscible with potassium bicarbonate, a salt. As the primary solvent, a simulated effluent solution of an alkaline CO2 absorption column was considered. In the crystallizer, where the antisolvent was mixed with the effluent solution of the alkaline CO2 absorption column, there appear many complicated components, but the data for the entire component system are very limited. Furthermore, the data in a three-component system consisting of salt, water, and antisolvents are even more limited. Therefore, the components in the crystallizer may be simplified to a ternary systems of KHCO3 + H2O + ethanol or 2-propanol. In this work, the equilibria data in ternary systems of KHCO3 + H2O + ethanol or 2-propanol and the recovery of potassium bicarbonate salt at various temperatures are reported. Based on the reported data, the amount of antisolvent that has an effect equivalent to a cooling solution and the optimum concentration of the antisolvent are proposed. With these suggestions, the feasibility of using an antisolvent in an improved hot carbonate process is verified.
Table 1. Solubility of KHCO3 in Water at 303, 313, and 333 K Solubility of KHCO3 (g KHCO3/100 g Water) literature this work
T = 303 K
T = 313 K
T = 333 K
39.1 40.3
45.4 46.8
60.0 60.3
In the ethanol-containing system, only a homogeneous phase and a liquid−solid phase were observed at all temperatures from 303 K to 333 K (Figure 1). The area of the homogeneous phase broadened as the temperature increased, because of the increased solubility of KHCO3 in H2O.
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EXPERIMENTAL SECTION Materials. The potassium bicarbonate was purchased from Daejung (>99.5 wt %). Ethanol (absolute) and 2-propanol (99.7 wt %) were obtained from Merck and Junsei, respectively. Deionized water was used to prepare the solution. All chemicals were used without further purification. Experimental Procedure. The solid−liquid and liquid− liquid equilibrium lines were determined by a cloud point method, as proposed in the literature.9 Known ratios of the antisolvent and potassium bicarbonate were prepared in an equilibrium cell and then were titrated with water until the solution became clear. The final concentration of the mixture was determined by the weight of each component, as measured by an analytical balance with a precision of 0.001 g. The solution was placed in a 50 mL cell and stirred with a magnetic stirrer. The cell was placed in a water bath to maintain a constant temperature during the experiment. For the KHCO3 salt recovery test, samples were prepared by mixing known masses of KHCO3, H2O, and ethanol/2propanol in an equilibrium cell. The prepared solution was stirred with a magnetic stirrer until equilibrium was reached. As an equilibrium cell, 50 mL falcon tubes were used. The equilibrium cell was also placed in a water bath to maintain a constant temperature. After reaching equilibrium, the samples were placed in an oven for a sufficient time until the solid was adequately precipitated. After precipitating, the solid was collected by filtration and dried in an oven at least 4 h. The mass of the filtered solid was measured with a Sartorius analytical balance with a precision of 0.001 g. Both experiments, the experimental temperatures were 303, 313, 323, and 333 K, and the temperature remained constant during each experiment.
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Figure 1. Ternary phase diagram of KHCO3 + H2O + ethanol at 303, 313, and 323 K (L+S, liquid−solid phase; H, homogeneous phase).
In the 2-propanol system, there were distinct differences in the phases compared to those of the ethanol systems; the liquid−liquid and liquid−liquid−solid phases were observed in the 2-propanol system (Figure 2). Equilibrium data containing solid−liquid and binodal data for the 2-propanol-containing systems are presented in the Supporting Information (Tables S2 and S3, respectively). The distinct differences in the two antisolvent system arose because of the better affinity of ethanol than 2-propanol in water. The degrees of affinity with water of ethanol and 2propanol were identified with the polarity, as there are no differences in the solubility of ethanol and 2-propanol in water. They are both miscible with water at all concentrations. With regard to the polarity, a small difference between the substance and the water indicates the higher affinity between them. The polarity of water is 9.0, that of ethanol is 5.2, and that of 2propanol is 4.3.11 Correlation for Solid−Liquid Equilibrium Data to Predict the Amounts of Salt. To predict the amount of precipitated salt, correlations of the solid−liquid equilibrium data and the solid−liquid−liquid equilibrium data were required. For the correlation of the solid−liquid equilibrium, the following empirical equation12,13 was used.
RESULTS AND DISCUSSION
Ternary Phase Diagram of KHCO3 + H2O + Antisolvent. To examine the effect of the antisolvent on the KHCO3 solution, ternary phase diagrams were obtained. Solid−liquid equilibrium data for the ethanol-containing systems are presented in the Supporting Information (Table S1). The solubility of KHCO3 in water, which is the equilibrium concentration when the antisolvent concentration is zero, was
ln w1 = a0 + a1w3 + a 2w32 + a3w33 8288
(1)
DOI: 10.1021/acs.iecr.5b01696 Ind. Eng. Chem. Res. 2015, 54, 8287−8294
Article
Industrial & Engineering Chemistry Research
Figure 2. Ternary phase diagram of KHCO3 (1) + H2O (2) + 2-propanol (3) at 303, 313, 323, and 333 K (L+L+S, liquid−liquid−solid phase; L+S, liquid−solid phase; L+L, liquid−liquid phase; H, homogeneous phase).
with the first and second terms. The parameters of eq 1 for the 2-propanol system are shown in Table 3.
Here, w1 and w3 are the mass fractions of the KHCO3 and antisolvent, respectively, and a0, a1, a2, and a3 are fitting parameters. The parameters for the ethanol system are listed in Table 2. The reliability levels of the data in the ethanol system were >99.9%.
Table 3. Fitting Parameters of eq 1 for the 2-Propanol System a0 a1
Table 2. Fitting Parameters of eq 1 for the Ethanol System a0 a1 a2 a3
T = 303 K
T = 313 K
T = 323 K
T = 333 K
−1.2416 −3.5104 −0.3053 −2.0357
−1.1384 −3.4410 1.4151 −3.9467
−1.0326 −2.9107 0.2286 −2.7960
−0.9904 −2.5638 −0.6883 −1.9525
T = 303 K
T = 313 K
T = 323 K
T = 333 K
−1.2500 −3.4904
−1.1473 −3.5073
−1.0566 −2.9081
−0.9752 −2.1304
To apply eq 1 in the 2-propanol system, the concentrations of KHCO3 (w1) were higher than 0.0288, 0.0198, 0.0130, and 0.0089 at 303, 313, 323, and 333 K, respectively. For the solid−liquid−liquid equilibrium, the solid equilibrium line is appeared as a straight line in the ternary diagram. Therefore, for the correlation of the solid−liquid−liquid equilibrium in the 2-propanol-containing system, the following linear equation was used:
The parameters in Table 2 can be used when liquid−liquid phase separation is not observed and when the temperatures are 303, 313, 323, and 333 K. For 2-propanol systems, solid−liquid and solid−liquid− liquid equilibria were observed. For the solid−liquid equilibrium, eq 1 was used, and only the a0 and a1 parameters were obtained because the solid−liquid data were well correlated
w3 = b0 + b1w1
(2)
The fitting parameters from eq 2 are listed in Table 4. 8289
DOI: 10.1021/acs.iecr.5b01696 Ind. Eng. Chem. Res. 2015, 54, 8287−8294
Article
Industrial & Engineering Chemistry Research
With the parameters in the ternary systems containing ethanol or 2-propanol, the data pertaining to potassium bicarbonate recovery and the optimum concentration of the antisolvent discussed in the following sections are predicted. Precipitation Ability of the Antisolvent: Recovery of Potassium Bicarbonate Salt. To investigate the ability of the antisolvent with regard to salt formation, the recovery of KHCO3 was determined according to the amount of added antisolvent. A bicarbonate recovery experiment was conducted with an aqueous KHCO3 saturated solution at 333 K. The percent of recovery was calculated with the following equation:
Table 4. Fitting Parameters for eq 2 in the 2-PropanolContaining System b0 b1
T = 303 K
T = 313 K
T = 323 K
T = 333 K
0.5825 −2.5484
0.6869 −2.5956
0.7306 −2.3103
0.7611 −2.1120
The concentration ranges used with eq 2 differed from those of the operating temperature, as listed in Table 5. Table 5. Concentration Range for eq 2 temperature, T (K)
w1 (KHCO3)
w3 (2-propanol)
303 313 323 333
0.0288−0.1896 0.0198−0.2273 0.0130−0.2992 0.0089−0.2677
0.1264−0.5451 0.0975−0.6073 0.0532−0.7140 0.0474−0.7440
KHCO3 recovery (%) amount of KHCO3 precipitated = amount of KHCO3 in saturated KHCO3 solution at 333 K (3)
Using eqs 1 and 2, the amount of precipitated KHCO3 could be predicted. The amount of salt was calculated from the mass balance with the concentration in the initial solution and the concentrations of the liquid and solid on the equilibrium line (eqs 1 and 2), as well as the precipitated solid/total liquid ratio (S/L ratio). The S/L ratio was calculated from the lever-arm rule.
Equations 1 and 2 describe the behavior of the phase diagram in the 2-propanol system well; moreover, the reliability levels of the data in the 2-propanol system were >98.8%. The parameters in Tables 3 and 4 can be used when the temperatures are 303, 313, 323, and 333 K.
Figure 3. Potassium bicarbonate recovery, according to the amount of ethanol added. (Legend: (■) experimental data in the solid−liquid region, () prediction by eq 1.) 8290
DOI: 10.1021/acs.iecr.5b01696 Ind. Eng. Chem. Res. 2015, 54, 8287−8294
Article
Industrial & Engineering Chemistry Research
Figure 4. Potassium bicarbonate recovery, according to the amount of 2-propanol added. (Legend: (■) experimental data in the solid−liquid region, (□) experimental data in the solid−liquid−liquid region, () prediction by eq 1, (···) prediction by eq 2.)
As shown in Table 6, ethanol was more capable of precipitating KHCO3 than 2-propanol. In terms of the KHCO3 recovery level, an addition of
