Title: Development of a Microchannel In Situ Propellant Production System

An in situ propellant production (ISPP) plant on future Mars robotic missions can produce oxygen (O2) and methane (CH4) that can be used for propellant for the return voyage. By producing propellants from Mars atmospheric carbon dioxide (CO2) and hydrogen (H2) brought from Earth, the initial mass launched in low Earth orbit can be reduced by 20% to 45%, as compared to carrying all of the propellant for a round-trip mission to the Mars surface from Earth. Pacific Northwest National Laboratory used microchannel architecture to develop a Mars-based In Situ Propellant Production (ISPP) system. This three year research and development effort focused on process intensification and system miniaturization of three primary subsystems: a thermochemical compressor, catalytic reactors, and components for separating gas phases from liquid phases. These systems were designed based on a robotic direct return mission scenario, but can be scaled up to human flight missions by simply numbering up the microchannel devices. The thermochemical compression was developed both using absorption and adsorption. A multichannel adsorption system was designed to meet the full-scale CO2 collection requirements using temperature swing adsorption. Each stage is designed to achieve a 10x compression of CO2. A compression ratio to collect Martian atmospheric CO2 at ~0.8 kPa and compress it to at least 100 kPa can be achieved with two adsorption stages in series. A compressor stage incorporates eight thermally coupled adsorption cells at various stages in the adsorption/desorption cycle to maximize the recuperation of thermal energy and provide a nearly continuous flow of CO2 to the downstream reactors. The thermochemically compressed CO2 is then mixed with hydrogen gas and fed to two reactors: a Sabatier Reaction unit and a Reverse Water/Gas Shift unit. The microchannel architecture allows better heat control than is possible in an adiabatic system, resulting in significantly higher conversion. The reactors can also have reduced mass over conventional hardware. Over 60% conversion was achieved using a two stage RWGS reactor in which water was removed between stages. Sabatier conversions of greater than 85% were achieved in a single stage system. Since the RWGS is endothermic and the Sabatier is exothermic, by combining the two reactions, heat generated from the Sabatier can be used to fuel the RWGS reaction. A combined Sabatier/RWGS reactor was successfully tested. Both the Sabatier and RWGS reactions generate water. The water will be collected and electrolyzed to produce oxygen and recycle the hydrogen. A microchannel phase separator is also under development to separate liquid water from vapor and other gases in these product streams. This phase separator relies on surface forces, not gravitational effects, to separate the water and is therefore suited to space applications. The specific energy of this device reached values of 1200 to 8000 W/K for water mole fractions of 20 to 70%. The phase separator technology was scaled up in a system to removed water from a cathode effluent of a 5 kW PEM fuel cell. In this case a three channel device can remove 43 mL/min of water in 95 SCFM of air. This exceeded the design requirements of the device. A system model of the microchannel ISPP plant was generated to predict the size, weight and performance for the individual components and use it to optimize the overall system. The microchannel technologies developed for CO2 collection, reaction, and phase separation can be used not only for an ISPP system, but also life support, EVA, and lunar applications. The use of microchannel technologies reduces both mass and volume of the system as well as improving the system efficiency.