New Technology for Gas Absorption

The research focused on whether liquids (particularly liquid amines) could be an acceptable substitute to solid sorbents for the capture of metabolically-generated CO2 in closed environments. There is a large body of research looking at liquids for terrestrial (i.e. power plant) applications, but closed environments and small scale capture plants have separate concerns. Initially, the research focused on whether electrospray (see Figure 1) could be an effective gas-liquid contactor by creating sub-micron droplets of liquid amines. It was expected that the extraordinarily high surface area to volume ratio would yield smaller devices, saving weight and volume, but would require greater care in liquid selection, particularly a low conductivity, low vapor pressure liquid. In the quest to find a lower vapor pressure amine, I discovered a combination of common solvents (aminoethylethanolamine and triethylene glycol) that absorb both water and CO2 and have naturally low volatility. Finally, the research investigated the suitability for liquid absorbers in closed environments. The conclusions of the electrospray research are that while nanodroplets have extraordinarily high surface area to volume ratios, which would ordinarily be favorable, the short residence time of the droplets (<1 millisecond) does not provide enough time for the reaction to proceed to completion. The bulk of the reaction actually occurs in the condensed phase where the droplets collect, but is not practical for large absorption needs given the complexity and area required to create a large array of nozzles for electrospraying. However, the method has been demonstrated as a means for validating mass transfer characteristics and reaction rates of designer liquids in small quantities (such as ionic or designer organic liquids). Figure 2 shows how the extent of reaction (i.e. the amount of CO2 absorbed per volume of liquid) changes as a factor of several variables. The extent of reaction is dependent on the CO2 partial pressure [P], the diffusivity [D] of CO2 in the liquid (a function of viscosity), the reaction rate [k], the radius of the liquid puddle [Rpuddle], the liquid flow rate [Q] and the solubility of CO2 in the liquid as given by the Henry’s law constant [H]. Together, these quantities form a dimensionless group. Each of the series represent some variation on one or more of these variables, yet all seem to collapse along one curve (certainly much more so than by any other combination of the variables). The collapse of data with respect to a dimensionless group is a key finding for chemical engineering research and allows extension of the research to other liquid systems. Finally, I had the opportunity to look at short-term solutions for CO2 removal which would be easily scalable, work that was performed as part of my final visiting technologist experience at JSC, in collaboration with work for AES and Honda. In order to be more off-the-shelf, proposed microporous PTFE membranes which are chemically inert and would work in microgravity to permit passage of CO2 from a gas phase into the liquid phase. It appears that PTFE-based microporous membrane contactors can be favorable with aqueous amine mixtures. An AEEA/water mixture showed high fluxes with relatively low water loss. Such a contactor can absorb 100% of the metabolic CO2 waste with a 10% impact to condenser load while still maintaining competitive size. Additional work is required to determine ideal regeneration of aqueous mixtures in space