In order to solve the global warming problem, reducing $CO_2$ emission into the atmosphere has been required, and reducing the $CO_2$ emission from the power plant, the main emission source studied worldwide. However, when the $CO_2$ separation process is applied to the power plant, the cost of power generation is sig-nificantly increased. Therefore, in this thesis, methods to lower the cost of $CO_2$ separation was studied.
The emitted gas condition contains $CO_2$ was differed from the types of power plant. In the coal-fired power plant, the gas is mainly consist of $CO_2$ and $N_2$ and emitted at the atmospheric pressure. Meanwhile, in the integrated gasification combined cycle (IGCC), the gas was mainly composed of $CO_2$ and $H_2$, and emit-ted at the high pressure, over 30bar. The methods of $CO_2$ separation from the former gas and the latter gas can be classified to post-combustion, and pre-combustion. According to the $CO_2$ separation methods, the approach to reduce the cost of $CO_2$ separation should be differently established. Therefore, in this study, an anti-solvent and a novel physical solvent were considered for the cost reduction of $CO_2$ separation processes for pre-combustion and post-combustion methods, respectively.
In part I, for the post-combustion $CO_2$ capture process, an antisolvent process was investigated. Etha-nol, methanol, propylene glycol, and 2-propanol were selected as an antisolvent to determine the precipita-tion ability of anti-solvents in a simulated effluent solvent from a $CO_2$ absorption column. The effect of the molecular structure of the antisolvent on the precipitation yield was investigated. The precipitation yields were affected by the chain length, the number of hydroxyl groups, and the number of carbon branches, as substituted by the methyl group of the antisolvent. The optimum length and optimum number of hydroxyl groups and carbon branches were studied in relation to the hydrophilicity and octanol-water partition coeffi-cient.
To compare the performance levels of anti-solvents in potassium carbonate solutions, solid-liquid equilibrium on the systems of $KHCO_3+H_2O+anti-solvent$ were measured. From the area of solid phase in ter-nary diagram, the effect of antisolvent on precipitation could be roughly evaluated with the area of solid phase region. As a result, ethanol showed the best $KHCO_3$ precipitation ability, 2-propanol showed the sec-ond, methanol showed the third, and propylene glycol was the worse precipitation ability in saturated $KHCO_3$ solution. Therefore ethanol and 2-propanol were selected for the further study. To specify the effect of anti-solvent on the Hot-carbonate solution, the precipitation ability and antisolvent recovery characteristics were investigated. The precipitation ability was measured in terms of $KHCO_3$ recovery in ternary systems of the $KHCO_3+H_2O+antisolvent$. 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 to maximize the solid/liquid ratio were evaluated. Considering the optimum concentration of the antisolvent and $KHCO_3$ recovery at the opti-mum concentration, ethanol was found to be the more feasible antisolvent than 2-propanol. For the operat-ing temperature, 303K was the most effective to maximize precipitation yield.
For the antisolvent recovery characteristics, liquid-liquid phase separation ability of ethanol was measured. The liquid-liquid phase separation was investigated by adding ethanol in 40wt% $K_2CO_3$ solution, so a ternary systems of $K_2CO_3 + H_2O$ + ethanol was measured. The equilibrium data were correlated with a four types of local concentration parameter. From this correlations, the weight ratio of organic to aqueous phase and the concentration of three components in organic phase and aqueous phase were predicted. From the result of the prediction, the optimum amount of ethanol over the initial solution was determined, and it was 10 ~ 40wt%. In this optimum amount of ethanol, the ratio of organic phase over the total solution was 0.12~0.5, and the concentration of ethanol in the organic phase was about 80wt%, and these are reasonable values for the operation of practical process.
To verify the effect of an antisolvent on the simulated effluent solution from a $CO_2$ absorption col-umn, a quaternary system of $K_2CO_3+KHCO_3+H_2O+ethanol$ was investigated. The equilibrium data in the quaternary system was correlated with an empirical equation, and mass balance and energy consumption in a simplified practical process was roughly evaluated. The flow rate of $CO_2$ lean solution in cooling precipita-tion process was 17% reduced than that in no-precipitation process. Also, the energy consumption in cooling precipitation process decreased 35.92%, compared to no-precipitation process. For the evaluations on the energy consumption in the two precipitation process, only the sensible heat was compared, and the sensible heat was reduced about 4% in anti-solvent precipitation process compared to the cooling precipitation pro-cess. Considering the evaluations on mass balance and energy consumption, the anti-solvent precipitation process in the Hot-carbonate process was feasible and can be an economic process.
In part II, for the pre-combustion $CO_2$ capture process, Novel physical solvent was investigated and compared with polyethylene glycol dimethyl ether (DMPEG). Dimethyl carbonate, diethyl carbonate, and triacetin were selected as novel physical solvent candidate, with the results of the preliminary $CO_2$ solubility and binding energy calculations between $CO_2$ and the novel physical solvent. The novel physical solvent im-proved the $CO_2$ solubility in terms of the mass unit, viscosity, and mass transfer. However, triacetin was the only feasible physical solvent for DMPEG among the three candidates, because diethyl carbonate and dime-thyl carbonate may increase the vapor pressure of the blended solvent significantly.
To evaluate the blending solvents, the $CO_2$ solubility in the selected physical solvents were measured. The equilibrium data was correlated with a perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state with the regression program Aspen Plus. After the correlation, the cost evaluation was con-ducted.
For the novel solvent, triacetin and mixture of triacetin and DMPEG are considered, and they are compared with DMPEG, the solvent in the Selexol process with commercial simulation program, Aspen plus. For the economic evaluation, utility cost, equipment cost, and solvent purchase cost in the candidate pro-cesses were evaluated and compared with that of Selexol process. As a result, the process using triacetin only was the most economical process among the three candidates, and the process reduced 20% of cost com-pared to the reference process, Selexol process. Particular, the cost on the solvent purchase was reduced 28%, due to the lower price of triacetin than DMPEG, and lower flowrate in process with triacetin than that with DMPEG.
As a result of the mass balance and economic evaluation, triacetin can be the alternative solvent for the pre-combustion $CO_2$ capture.