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Title:Absorption based processes applying phase transition for energy efficient postcombustion CO2 capture
Author(s):Ye, Qing
Director of Research:Wang, Xinlei
Doctoral Committee Chair(s):Wang, Xinlei
Doctoral Committee Member(s):Lu, Yongqi; Kenis, Paul; Tumbleson, Mike; Singh, Vijay
Department / Program:Engineering Administration
Discipline:Agricultural & Biological Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):Phase transitional solvent
CO2 absorption
Abstract:The conventional CO2 absorption process, using an aqueous 30 wt% monoethanolamine (MEA) solvent for postcombustion CO2 capture, is energy intensive and thus costly. The novel phase transitional CO2 absorption process has been regarded as a promising alternative to the benchmark MEA process. The solvent in a phase transitional process turns into two phases after CO2 absorption; one phase contains the majority of the absorbed CO2 while the other is CO2 lean. This key feature of a phase transitional process enables the use of only the CO2 rich phase for CO2 stripping and solvent regeneration, which can reduce the energy requirement compared with the MEA process. We investigated two scenarios of phase transitional processes (i.e., liquid-solid and liquid-liquid) to overcome the energy disadvantages associated with the MEA process. For the liquid-solid CO2 absorption process, an aqueous solution of potassium carbonate was applied and the CO2 absorption product, i.e., potassium bicarbonate (KHCO3), was subjected to a crystallization process due to its limited aqueous solubility at low temperatures (<40℃). The KHCO3 rich slurry separated from the mother solution was sent to the stripper for CO2 desorption at elevated temperatures (>140℃). The kinetics of KHCO3 crystallization and the solubility of KHCO3 both were investigated under typical operating conditions. The resultant data were fit to a size dependent crystal growth model, and applied to perform the crystallizer design analysis for the proposed carbonate based CO2 absorption process. The liquid-solid process had drawbacks related to issues incurred by the slurry operation. Therefore, additional research efforts were focused on studying the liquid-liquid phase transition which constitutes the major part of this dissertation. The first step was to select the proper liquid-liquid phase transitional (i.e., biphasic) solvents. A feasible biphasic solvent that is subject to a liquid-liquid phase separation during CO2 absorption can be a blend of compounds “A” and “B”. Compound “A”, containing primary or secondary amino groups, acts as an absorption accelerator. Compound “B”, containing tertiary amino groups, serves as a promoter for solvent regeneration. A screening study was conducted using a large pool of candidate compounds “A” and “B”. The aqueous blends of “A” and “B” were investigated with respect to their performance of CO2 absorption (i.e., rate and capacity), CO2 desorption (i.e., CO2 desorption pressure) and phase separation behavior. The qualitative relationship between the solvent structure and performance was evaluated. A polyamine based compound “A” was indispensable for liquid-liquid phase separation. The compound “A” with 4 to 6 carbon atoms and up to 3 nitrogen atoms favored the performance of its solvent blend. A further screening study was conducted using two aqueous biphasic solvents, with diethylenetriamine (DETA) as the common absorption accelerator. The first solvent blended N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) as the regeneration promoter, and the second blended bis[2-(N,N-dimethylamino)ethyl]ether. Similar screening criteria as for the prior study were adopted. The formula of a solvent blend affected its CO2 loading capacity and phase separation behavior. The blend of DETA and PMDETA was superior to MEA with respect to its larger capacity and higher desorption pressure of CO2; thus, it was selected for further study. Two formulae of the blend of DETA and PMDETA, i.e., 2 m DETA + 3 m PMDETA, and 1.5 m DETA + 3.5 m PMDETA, were adopted to study the mechanisms of liquid-liquid phase separation associated with reactions of CO2 absorption and desorption. Quantitative speciation analyses in the dual phases were conducted with 1H, 13C and two dimensional (2D) nuclear magnetic resonance (NMR) spectroscopic techniques, focusing on the evolvement of species upon different CO2 loadings. CO2 absorption into the biphasic solvent blend of DETA and PMDETA underwent a two stage process. In the early stage, CO2 reacted with DETA, and monocarbamate and dicarbamate species (including their protonated species) were formed. In the latter stage, further CO2 absorption resulted in the protonation of PMDETA via proton exchange reactions and production of HCO3-/CO32- (i.e., PMDETA catalyzed CO2 hydration). The volumetric fraction of the CO2 rich phase increased gradually in the early stage and abruptly in the latter stage. CO2 desorption from the CO2 rich phase, separated from the CO2 laden biphasic solvent, was a reverse process of CO2 absorption and coupled with a dual phase transition. The biphasic solvent blend of DETA and PMDETA was characterized further with respect to its thermodynamic and kinetic behaviors. A comprehensive thermodynamic model was developed to predict the liquid-liquid phase separation and vapor-liquid-liquid equilibrium in the aqueous biphasic solvents comprised of DETA and PMDETA for CO2 capture. The unknown reaction equilibrium constants and binary interaction parameters were retrieved by data regression. The predicted partial pressures of CO2 over the biphasic solvent or the separated CO2 rich phase agreed with the experimental data. Detailed speciation in either liquid phase evolving with changing CO2 loadings and temperatures was predicted based on the model, and the results were consistent with the prior NMR data. Moreover, the phase separation behavior with respect to the volumetric fraction of each liquid phase and the partitioning of DETA or PMDETA between the dual phases was predicted with accuracy. The heat of absorption reactions for the mother biphasic solvent was predicted to be smaller than the heat of desorption reactions for the separated CO2 rich phase. The thermodynamic modeling approach developed in this study could be applied to other biphasic solvent systems. The kinetics of CO2 absorption into neat DETA solutions, neat PMDETA solutions and their blends were studied. The intrinsic rate constants of reactions between CO2 and DETA, and between CO2 and PMDETA (PMDETA catalyzed CO2 hydration), were retrieved from the data measured for the monophasic solutions of neat DETA and neat PMDETA, respectively, based on an established mass transfer and kinetic model. Reactions of forming dicarbamate species from monocarbamate of DETA were as fast as the reactions of forming monocarbamate from molecular DETA. The obtained rate constants and the concentrations of species estimated by the prior thermodynamic model were used together to predict the kinetics of CO2 absorption into the solvent blend of DETA and PMDETA with and without the presence of dual liquid phases. A modified two film mass transfer model was applied to account for the scenario when dual phases were present. The prediction results were consistent with the experimental data, indicating that the absorption of CO2 into two liquid phases could be represented as the simultaneous absorption of CO2 into each of the two monophasic liquid solvents.
Issue Date:2018-04-18
Type:Text
URI:http://hdl.handle.net/2142/101328
Rights Information:Copyright 2018 Qing Ye
Date Available in IDEALS:2018-09-04
2020-09-05
Date Deposited:2018-05


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