(361d) Simulation of a Reactive Separation Application for CO2 Removal

Authors: 
Hwang, H. T., University of Southern California
Harale, A., University of Southern California
Tsotsis, T. T., University of Southern California
Sahimi, M., University of Southern California
Liu, P. K. T., Media and Process Technology Inc


For long-duration human presence on Mars or the Moon one requires highly reliable and efficient systems which provide basic life support provisions, such as food, water and air for the crews. A key function of the space life-support systems is effective removal of metabolic CO2 from the environment in the living quarters, and in the cabin during space flights. Without continuous removal, the amount of CO2 in a closed environment would increase to unacceptable levels.

Several methods of CO2 removal have been studied for use in the space habitats and suits. Adsorption of CO2 by LiOH, for example, is the current standard approach. For long-term flights the method presents challenges, however, since LiOH is not easily regenerable. Other similar systems based on CO2 chemisorption by silver oxide and solid phase amines also face the same challenge. Molecular sieves, such as zeolites, can trap CO2 in their pores by physical adsorption instead, and are more amenable to regeneration, e.g., via pressure swing adsorption (PSA), at the cost, nevertheless, of a greater process complexity.

A promising approach is the use of the methanation (Sabatier) reaction, in which CO2 is catalytically activated in the presence of hydrogen, and is hydrogenated to methane with simultaneous production of water. The simplest life support systems are "open-loop" and provide all the required resources, from water and oxygen to food, from either storage or re-supply. For such systems the amount of provisions needed increases proportionally as the mission duration and crew size increase. More complex systems are "closed-loop", at least to a certain extent, and though they also require an initial supply of resources, they subsequently process the waste products generated, such as CO2, urine, and wastewater, to recover additional useful resources.

In this study, we investigate the application of a reactive separation technology, in which the catalytic and separation steps are coupled in situ through the use of CO2 permselective membranes. We also present an overview of our current experimental and modeling efforts in this area aiming to establish the feasibility of the proposed reactive separation application for "closed-loop" life support systems.

Key words: Membrane Reactor, Carbon dioxide, Methanation, Sabatier reaction, Modeling, Air revitalization system, Closed-loop system