(115c) Integrated Adsorbent-Process Optimization for Carbon Capture and Concentration Using Vacuum Swing Adsorption Cycles

Authors: 
Khurana, M., National University of Singapore
Farooq, S., National University of Singapore

A large number of adsorbents are being regularly reported in the literature claiming their suitability for carbon capture and concentration (CCC) from flue gas. One thing common in all these adsorbents is they all show higher selectivity of carbon dioxide over nitrogen. Adsorbents that have been tested for the transport mechanism of the diffusing gases in the pores show macropore molecular diffusion as the controlling step. As such, it is possible to simulate a CCC process for each adsorbent using only equilibrium isotherm information, a suitable mixture equilibrium model based on single component parameters and molecular diffusivity as inputs, and determine the optimum operating conditions as decision variables under which the process will give minimum energy or maximum productivity while meeting the purity-recovery constraints imposed by the regulatory agencies. We have developed such a simulation and optimization platform for CCC using Vacuum Swing Adsorption (VSA). This platform provides a reliable tool to evaluate adsorbents for their suitability for CCC. It has been applied to many adsorbents reported in the literature.  

Here we propose a novel approach for integrated adsorbent and process design. The scope of our aforementioned simulation and optimization platform is expanded to include adsorbent isotherm characteristics, in addition to the process operating conditions, as decision variables. By including adsorbent isotherm characteristics as decision variables, we are now able to ask what is the minimum energy or maximum productivity possible in a VSA process for CCC and what are the shapes of the corresponding isotherms? We have answered these questions for two VSA cycles, namely a 4-step process with light product pressurization (LPP) and a 6-step process with light and heavy reflux in addition to LPP. We have established the minimum energy and maximum productivity bounds for these two cycles as functions of the evacuation pressure. While the 4-step VSA cycle enjoys lower energy penalty advantage, it requires a very low evacuation pressure and fails to achieve the purity-recovery constraints if the evacuation pressure is increased beyond 0.04 bar. In contrast, the 6-step cycle enjoys a wider operating window for the low evacuation pressure and higher productivity, but these come at the expense of increased energy penalty.

The integrated adsorbent-process optimization adopted here directly gives the shape of the isotherm necessary to achieve a certain target for a chosen VSA cycle along with the necessary operating conditions.  It may be viewed as adsorbent design by process inversion. The results obtained set benchmarks for the development of new adsorbent and VSA cycles for CCC.