Two-Stage Psa System For CO2 Removal And Concentration During Closed-Loop Human Space Exploration Missions (2016)

A novel two-stage pressure swing adsorption (PSA) system has been developed to remove metabolic CO2 removal from the spacecraft cabin air of the International Space Station (ISS). This PSA system enriches and recovers the CO2 to make it suitable for use in a Sabatier reactor (CO2 reduction). This two-stage PSA process utilizes Stage 1 to concentrate metabolic CO2 from about 0.2667 vol% to about 40 to 60 vol% and Stage 2 to further enrich the CO2 product from Stage 1 up to > 97 vol% CO2, while recovering at least 95% of it, which corresponds to removing 4.0 kg/day of CO2. Each stage of this PSA system utilizes a combination of equalization, cocurrent depressurization, heavy and light reflux cycle steps to facilitate significant heavy component enrichment and recovery (i.e., CO2) from a dilute feed stream. The first generation of this two-stage PSA process utilizes beaded commercial adsorbent, i.e., 13X zeolite, in both stages. These two PSA systems were designed via simulation using the Dynamic Adsorption Process Simulator (DAPS). The DAPS results were validated using an experimental multi-bed PSA system. DAPS was then used to scale up Stage 1 and scale down stage 2 to size the full scale two-stage PSA system that might someday be used on the ISS. The modeling results from Stage 1 revealed that longer heavy and light reflux step times played an important role in concentrating the CO2 in the heavy product and modeling results from Stage 2 showed that a heavy reflux step was essential to achieving the desired performance. Implications from these modeling and experimental results began to hint at the possibility of significantly concentrating CO2 from ambient air up to around 10 to 15 vol% at relatively high recovery using a simple PSA cycle. The Environmental Control and Life Support System (ECLSS) for the Space Station performs several functions such as O2 and N2 supply and control, CO2 removal and reduction, potable water supply, comfortable cabin temperature and humidity levels and total cabin pressure, and .adequate nutrition. An atmosphere revitalization system (ARS) includes oxygen and nitrogen supply and control, carbon dioxide removal and reduction, trace contaminant removal. For a closed loop system for future long term duration spaceflights, CO2 removal system has an important role on not only cabin atmosphere quality but also water recovery, via CO2 reduction, and O2 recovery, via water electrolysis. Detailed information about CO2 capture from flue gas, CO2 capture from atmospheric air, International Space Station (ISS), Environmental Control and Life Support System (ECLSS) and its subsystems such as Humidity and Temperature Control, Atmosphere Control and Supply, Waste Management, Food Management, Fire Detection and Suppression, and Atmosphere Revitalization: Oxygen Generation, Nitrogen Supply, Trace Contaminant Removal and Monitoring, mostly Carbon Dioxide Removal subsystem, Carbon Dioxide Reduction, Water Recovery and Management and lastly about Pressure Swing Adsorption (PSA) were given in Chapter 1. Chapter 2 is about model description of a FORTRAN based in house dynamic adsorption process simulator (DAPS) that simulations of all PSA cycles in this thesis were carried out. In Chapter 3, simulations of a 3-bed 9-step pressure swing adsorption (PSA) cycles were carried out to study the enrichment and recovery trace amount of CO2 from a CO2-air mixture using 13X zeolite using dynamic adsorption process simulator DAPS. Extensive parametric studies were investigated in order to determine how process performances are affected by process parameters such as HR/LR step time (thus cycle time), light reflux ratio, co-current and counter-current depressurization pressures, and light reflux pressure. In Chapter 4, initial simulations using the full scale flow rates in search of the bed size, light reflux ratio, cycle time and vacuum pressure that lead to the desired performance with a 3-bed 8-step pressure swing adsorption (PSA) cycles (0.4% CO2 from a CO2-N2 mixture) were carried out by using 13X zeolite as an adsorbent using dynamic adsorption process simulator DAPS. The initial DAPS results were then used to determine the 3-bed experimental conditions. Experimental runs have been done by using a 4-bed PSA apparatus. Model validation was carried out via running simulations with no adjustable parameters against experimental results. In Chapter 5, a novel two-stage pressure swing adsorption (PSA) system has been developed to remove metabolic CO2 removal from the spacecraft cabin air of the International Space Station (ISS). These two PSA systems were designed via simulation using the Dynamic Adsorption Process Simulator (DAPS). The DAPS results were validated using an experimental multi-bed PSA system. DAPS was then used to scale up Stage-1 and scale down Stage-2 to size the full scale two-stage PSA system. Overall this study showed that target process performance for NASA has been achieved with two-stage PSA system. Two-stage PSA process performances are 98.10% CO2 purity with 97.46% CO2 recovery, (4.13kg CO2 removal / day) from 0.2667 vol. % CO2 concentration in feed stream.