Lila M. Mulloth

High pressure excess isotherms for adsorption of oxygen and nitrogen in zeolites.

High-pressure oxygen is an integral part of fuel cell systems, many NASA in situ resource utilization concepts, and life support systems for extravehicular activity. Due to the limited information available for system designs over wide ranges of temperature and pressure, volumetric methods are applied to measure adsorption isotherms of O(2) and N(2) on NaX and NaY zeolites covering temperatures from 105 to 448 K and pressures up to 150 bar. Experimental data measured using two apparatuses with distinctly different designs show good agreement for overlapping temperatures. Excess adsorption isotherms are modeled using a traditional isotherm model for absolute adsorption with a correction for the gas capacity of the adsorption space. Comparing two models with temperature-dependent coefficients, a virial isotherm model provides a better description than a Toth isotherm model, even with the same number of parameters. With more virial coefficients, such as a cubic form in loading and quadratic form in reciprocal temperature, the virial model can describe all data accurately over wide ranges of temperature and pressure.

Evaluation of Commercial Off-the-Shelf Sorbents and Catalysts for Control of Ammonia and Carbon Monoxide

Designers of future space vehicles envision simplifying the Atmosphere Revitalization (AR) system by combining the functions of trace contaminant (TC) control and carbon dioxide removal into one swing-bed system. Flow rates and bed sizes of the TC and CO2 systems have historically been very different. There is uncertainty about the ability of trace contaminant sorbents to adsorb adequately in high-flow or short bed length configurations, and to desorb adequately during short vacuum exposures. There is also concern about ambient ammonia levels in the absence of a condensing heat exchanger. In addition, new materials and formulations have become commercially available, formulations never evaluated by NASA for purposes of trace contaminant control. The optimal air revitalization system for future missions may incorporate a swing-bed system for carbon dioxide (CO2) and partial trace contaminant control, with a reduced-size, low-power, targeted trace contaminant system supplying the remaining contaminant removal capability. This paper describes the results of a comparative experimental investigation into materials for trace contaminant control that might be part of such a system. Ammonia sorbents and low temperature carbon monoxide (CO) oxidation catalysts are the foci. The data will be useful to designers of AR systems for future flexible path missions. This is a continuation of work presented in a prior year, with extended test results.

Development Status of a Low-Power CO2 Removal and Compression System for Closed-Loop Air Revitalization

The ‘low-power CO2 removal (LPCOR) system’ is an advanced air revitalization system that is under development at NASA Ames Research Center. The LPCOR utilizes the fundamental design features of the ‘four bed molecular sieve’ (4BMS) CO2 removal technology of the International Space Station (ISS). It will reduce the cabin air CO2 concentration by 60% with a 50% power savings compared to the current ISS standard. In addition, it will recover pure, compressed CO2 for oxygen recovery. LPCOR improves the power efficiency by replacing the desiccant beds of the 4BMS with a membrane dryer and a state-of the art structured adsorbent device that require 25% of the thermal energy required by the 4BMS desiccant beds. The CO2 removal and recovery functions are performed in a two-stage adsorption compressor. CO2 is removed from the cabin air and partially compressed in the first stage. The second stage performs further compression and delivers the compressed CO2 to a reduction unit such as a Sabatier reactor for oxygen recovery. This paper describes the development status of the LPCOR system, including the breadboard experiments to determine the performance parameters of the full-scale LPCOR components for an optimized process, characterization tests and long-term performance testing of individual components. Also discussed in this paper are the flow distribution challenges encountered in a low pressure-drop system such as the residual water adsorber, configured as an engineered structured sorbent, and the efforts to mitigate the flow-related issues.

A Solid-State Compressor for Integration of CO 2 Removal and Reduction Assemblies

Integration of CO2 removal and reduction assemblies in a spacecraft air revitalization system requires an interface with the functionality of a vacuum pump/compressor and a buffer tank. The compressor must meet the vacuum needs of the CO2 removal unit and the pressure needs of the CO2 reduction device, and must also store sufficient CO2 to accommodate the differences in cycle times of the two processes. In this presentation, we describe the design and operation of an adsorption-based device sized for use on the International Space Station. The adsorption compressor functions at a power level approximately ten times lower than a comparable mechanical compression/buffer tank system. The unit is also smaller, lighter, and quieter than its mechanical counterpart.

Integrated System Design for Air Revitalization in Next Generation Crewed Spacecraft

Human space initiatives that involve long-duration space voyages and interplanetary missions are possible only with the technologies that enable integration of the air, water, and solid waste treatment systems that minimizes the loss of consumables. However, the capabilities of NASAs existing environmental control and life support (ECLS) system designs consist of many independent systems that are energy extensive. This paper discusses the concept of an integrated system of CO 2 removal and trace contaminant control units that utilizes novel gas separation and purification techniques and optimized thermal and mechanical design for future spacecraft. The integration process will enhance the overall life and economics of the existing systems by eliminating multiple mechanical devices with moving parts.

The Concept and Experimental Investigation of CO2 and Steam Co-electrolysis for Resource Utilization in Space Exploration

CO 2 acquisition and utilization technologies will have a vital role in designing sustainable and affordable life support and in situ fuel production architectures for human and robotic exploration of the Moon and Mars. For long-term human exploration to be practical, reliable technologies have to be implemented to capture the metabolic CO 2 from the cabin air and chemically reduce it to recover oxygen. Technologies that enable the in situ capture and conversion of atmospheric CO 2 to fuel are essential for a viable human mission to Mars. This paper describes the concept and mathematical analysis of a closed-loop life support system based on combined electrolysis of CO 2 and steam (co-electrolysis). Products of the coelectrolysis process include oxygen and syngas (CO and H 2 ) that are suitable for life support and synthetic fuel production, respectively. The model was developed based on the performance of a co-electrolysis system developed at Idaho National Laboratory (INL). Individual and combined process models of the co-electrolysis and Sabatier, Bosch, Boudouard, and hydrogenation reactions are discussed and their performance analyses in terms of oxygen production and CO 2 utilization are presented.

Air-Cooled Design of a Temperature-Swing Adsorption Compressor for Closed-Loop Air Revitalization Systems

This paper describes the mechanical design, thermal model development, and compression test results of a solid-state compressor prototype. This compressor utilizes the principle of temperature-swing adsorption compression and has no rapidly moving parts. Temperature-swing adsorption compressors (TSAC) have multiple applications in space exploration where acquisition, separation, purification, transportation or compression of gases are involved. NASA Ames Research Center (ARC) has developed a TSAC for potential application in a closed-loop air revitalization system (ARS) for a future spacecraft or crew exploration vehicle. Since the ARS of International Space Station (ISS) naturally serves as the baseline for future systems, we chose the design guidelines for this TSAC based on the ISS requirements. ARC has designed and successfully tested similar compressors that use a liquid cooling medium in the past. Considering the availability, handling, and safety factors of air compared to a liquid coolant in a spacecraft environment, the design of the current TSAC was developed based on air-cooling.

Integrated Testing of a 4-Bed Molecular Sieve and a Temperature-Swing Adsorption Compressor for Closed-Loop Air Revitalization

Accumulation and subsequent compression of carbon dioxide that is removed from space cabin are two important processes involved in a closed-loop air revitalization scheme of the International Space Station (ISS). The 4-Bed Molecular Sieve (4BMS) of ISS currently operates in an open loop mode without a compressor. This paper reports the integrated 4BMS and liquid-cooled TSAC testing conducted during the period of March 3 to April 18, 2003. The TSAC prototype was developed at NASA Ames Research Center (ARC). The 4BMS was modified to a functionally flight-like condition at NASA Marshall Space Flight Center (MSFC). Testing was conducted at MSFC. The paper provides details of the TSAC operation at various CO2 loadings and corresponding performance of CDRA.

Development of a Low-Power CO2 Removal and Compression System for Closed-Loop Air Revitalization in Future Spacecraft

The current CO2 removal technology of NASA is very energy intensive and contains many non-optimized subsystems. This paper discusses the design and prototype development of a two-stage CO2 removal and compression system that will utilize much less power than NASA s current CO2 removal technology. This integrated system contains a Nafion membrane followed by a residual water adsorber that performs the function of the desiccant beds in the four-bed molecular sieve (4BMS) system of the International Space Station (ISS). The membrane and the water adsorber are followed by a two-stage CO2 removal and compression subsystem that satisfies the operations of the CO2 adsorbent beds of the 4BMS aid the interface compressor for the Sabatier reactor connection. The two-stage compressor will utilize the principles of temperature-swing adsorption (TSA) compression technology for CO2 removal and compression. The similarities in operation and cycle times of the CO2 removal (first stage) and compression (second stage) operations will allow thermal coupling of the processes to maximize the efficiency of the system. In addition to the low-power advantage, this processor will maintain a lower CO2 concentration in the cabin than that can be achieved by the existing CO2 removal systems. The compact, consolidated, configuration of membrane gas dryer and CO2 separator and compressor will allow continuous recycling of humid air in the cabin and supply of compressed CO2 to the reduction unit for oxygen recovery. The device has potential application to the International Space Station and future, long duration, transit, and planetary missions.

Development and Testing of a Temperature-swing Adsorption Compressor for Carbon Dioxide in Closed-loop Air Revitalization Systems

The air revitalization system of the International Space Station (ISS) operates in an open loop mode and relies on the resupply of oxygen and other consumables from earth for the life support of astronauts. A compressor is required for delivering the carbon dioxide from a removal assembly to a reduction unit to recover oxygen and thereby dosing the air-loop. We have developed a temperature-swing adsorption compressor (TSAC) that is energy efficient, quiet, and has no rapidly moving parts for performing these tasks. The TSAC is a solid-state compressor that has the capability to remove CO2 from a low- pressure source, and subsequently store, compress, and deliver at a higher pressure as required by a processor. The TSAC is an ideal interface device for CO2 removal and reduction units in the air revitalization loop of a spacecraft for oxygen recovery. This paper discusses the design and testing of a TSAC for carbon dioxide that has application in the ISS and future spacecraft for closing the air revitalization loop.