369212 Adsorbent Screening for Post-Combustion Carbon Capture Based on Optimum VSA Process Performance: A Simple Method Based on Equilibrium Isotherm Properties
New adsorbents are being synthesized in large numbers claiming suitability for post-combustion carbon capture and concentration (CCC). However, no effective method has yet been developed to evaluate their performance in an actual CCC process. Proper screening of an adsorbent for CCC must either be based on extensive process experimentation or process simulation and optimization using realistic inputs for adsorption equilibrium and rate kinetics of fluid transport into the adsorbent pores. Experimental evaluation of adsorbents synthesized is too resource and time intensive to be practically feasible for routine screening. On the other hand, process simulation and optimization is computationally demanding and generally not available to all material researchers.
Since most of the adsorbents for post-combustion capture are equilibrium controlled, their equilibrium isotherm properties strongly influence the process performance. A proper relationship between easily quantifiable isotherm properties on process performance has not been established yet. Performance indicators such as breakthrough times, high loading capacity, etc., for CO2 have not been proven to be directly correlated to process level performance of the adsorbent.
In the current work, a predictive correlation has been developed for estimating minimum energy penalty for post-combustion CCC yielding 95% purity and 90% recovery for an adsorbent as a function of some easily quantifiable CO2 and N2 equilibrium isotherm properties using a 4-step VSA process with light product pressurization (LPP). The 4-step VSA process with LPP has been reported in the literature to be the best cycle for post-combustion capture from dry gas using 13X zeolite. A closer physical insight of this process gives further assurance that its effectiveness should in general apply to adsorbents with favorable CO2 isotherms. The process simulation is based on a non-isothermal, non-isobaric model, which uses extended dual site Langmuir isotherm to represent binary equilibrium and assumes macropore molecular diffusion controlled transport kinetics. The equilibrium isotherm model has been shown in the literature to reliably capture experimental results for a wide variety of systems, while the transport model is justified for an equilibrium controlled process. Thus, the chosen cycle and features of the simulation model are most appropriate for developing a versatile adsorbent screening tool for CCC application.
A parametric study has been conducted using design of experiments for estimating the effects of the following five CO2 equilibrium isotherm properties: binary mixture selectivity over N2, Henry’s constant, CO2 loading, non-linearity and local slope at CO2 partial pressure in the feed gas on the minimum energy penalty of the VSA process. Each combination of the aforementioned CO2 isotherm properties, representing a possible adsorbent, has been optimized for the operating process variables using genetic algorithm based optimization. The significance of the independent variables and their interactions were tested using the analysis of variance (ANOVA) with 95% confidence limits (α= 0.05). The correlation has been validated by comparing with the minimum energy predictions from full process optimization for several known and promising hypothetical MOF adsorbents reported in the literature.
It has been found that the optimized process performance of any adsorbent is most sensitive to binary mixture selectivity for CO2 over N2. An important non-intuitive observation is that for the same binary selectivity, higher CO2 loading means increased N2 loading, which increases energy penalty for the system. The meta-model provides the much needed easy-to-use tool for material researchers for evaluating new adsorbents.