Liquid Systems for Carbon Dioxide Removal in Spacecraft Environments

• Main Author: Paragano, Matthew Vincent
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01.01.2017
• Subjects: Aerospace engineering; Chemical engineering; Systems science
• ISBN: 0355709295; 9780355709292

As humans strive to explore deeper into the solar system, the need for efficient, compact, and reliable life support systems for providing breathable air and drinkable water become critical to mission success. One element of providing breathable air is the removal of metabolic gaseous waste products, primarily consisting of carbon dioxide, from the cabin air. Recent work on human performance has suggested that carbon dioxide has effects on human performance at lower concentrations than previously anticipated and at concentrations lower than presently controlled to on the International Space Station. Such performance requirements represent a substantial challenge and provide the opportunity for an alternative solution to the zeolites presently in use. The present work examines the feasibility of using liquid a bsorbents to perform carbon dioxide absorption in enclosed microgravity environments. The use of liquid absorbents for carbon dioxide removal (or capture) is well studied in literature. Chief among the absorbents studied is monoethanolamine, an organic base which reacts to neutralize carbon dioxide. The reactivity of bases with amines increases the mass transfer rate, a particularly desirable feature for systems requiring compact architectures. To improve the diffusivity of carbon dioxide and reaction products, amines are typically dissolved in water, which has a low viscosity and solubilizes amines well. However, as analyzed in this work, aqueous sorbents are unattractive in enclosed environments because the water will evaporate into the cabin. As an alternative, an off-the-shelf, non-aqueous mixture of aminoethylethyleneamine and triethylene glycol was developed which shows moderate viscosity characteristics with low-volatility components and full miscibility. This work investigates for the first time the use of electrospray as a nanoscalegas -liquid contactor to improve the mass transfer while using a viscous liquid absorbent. Experimental investigation of this phenomenon concludes that electrospray shows a high overall mass transfer rate at all loadings than a stationary film. The proposed reason for the continued high mass transfer rate is the continuous refreshing of the gas-liquid interface with unreacted amine. Varying parameters for the concentrations of carbon dioxide and a mine, the liquid flow rate, and the driving electricfield show the data may be collapsed by an empirical dimensionless group which relates to the liquid pool at the bottom of the spray that represents a well-mixed interface. In addition, the influence of carbon dioxide reacting with the surface of the electrospray cone was studied experimentally with monoethanolamine, concluding that emitted current increases with carbon dioxide partial pressure due to production of ionic reaction products changing electrical conductivity. Finally, the mass transfer of water vapor and carbon dioxide through a microporous polytetrafluoroethylene membrane into aqueous and non-aqueous aminoethylethanola mine solutions. Aqueous solutions show water vapor losses, consistent with expectations, which would impose a condensation risk to cabin environments.