Title

Supported amine sorbents under temperature swing absorption for CO2 and moisture capture

Date of Completion

January 2007

Keywords

Engineering, Chemical

Degree

Ph.D.

Abstract

In 2004, NASA landed two rovers on the surface of Mars that continue today looking for traces of water, or even life. Eventually, humans will walk on Mars. To get there, however, scientists must overcome a multitude of obstacles, including capturing CO2 produced by respiration of the crew while in transit to Mars and then ultimately capturing CO2 on a global scale and converting to oxygen, as 95% of Mars' atmosphere is composed of the gas. This Ph.D. research, which focused on capturing CO2 and moisture in long-term enclosed environments using supported amine sorbents, is intended to be one step down that path toward reaching Mars. ^ The first portion of this research involved verifying that the supported amines were capable of cyclic CO2 capture under temperature swing absorption conditions. Three amines were tested, including triethylenetetramine (TETA), an acrylonitrile-modified tetraethylenepentamine (TEPAN), and an acrylonitrile-modified ethyleneamine (ME-100). TETA required desorption at relatively high temperatures of 80°C due to the presence of primary amines within the molecule, which have a strong affinity for CO2. TETA was found to be too volatile at 80°C, however, and evaporated from the support. TEPAN and ME-100 in contrast were found to have a very low volatility and were found to only begin evaporating at 150°C based on GC/MS measurements of the composition of the effluent gas, a temperature well above the suggested operating temperatures. Controlling parameters determining the cyclic CO2 capacity of these supported amine sorbents under temperature swing absorption conditions were identified from a set of seven variables that were considered important for TEPAN and ME-100. For TEPAN, CO2 partial pressure (12% variability to the system), absorption temperature (6%), desorption temperature (19%), and desorption pressure (61%) were found to be important to cyclic CO 2 capacity. For ME-100, desorption pressure was found to be the most important variable in determining cyclic CO2 capacity. ^ Once identified, the two most important controlling parameters, desorption temperature and desorption pressure, were examined in a more detailed study for ME-100. With an expanded range of both parameters, desorption temperature proved to be most influential in determining cyclic CO2 capacity in this system. Energy requirements were calculated after determining the most successful desorption temperature and pressure settings for the supported amine system and were found to compare favorably with the current zeolite system for CO2 capture used onboard the International Space Station. ^ Subsequent research focused on modeling the rates of reaction between CO2 and the supported amine TEPAN at 25°C. A mathematical model was developed based on the Caplow literature mechanism and the rate expression developed by Danckwerts. It was speculated that the amine was agglomerating by hydrogen bonding across adjacent TEPAN chains and forming a highly viscous gel-like by-product as the CO2-TEPAN reaction proceeded. The reaction kinetics were modeled using a general kinetic expression that did not agree with the commonly accepted Caplow mechanism, possibly due to the formation of the gel. From the model a rate expression of the form RCO2molcm3 ˙sec=8.8˙10 10&sqbl0;AM&sqbr0;3.8tot CO2 was derived. ^ It was also hypothesized that there was a relationship between the wettability of the support material and the CO2 capture capacity of the supported amine. A high hydrophilicity (or wettability) of the surface might lead to a larger spreading of the amine in the pores, which exposes more of the amine surface area to the gas stream. A completely hydrophobic polymer was surface modified to add oxygen-functional groups to the surface and to increase wettability of the support. TEPAN was loaded into the modified support and total CO 2 capacity was examined compared to the original loaded hydrophobic support. The capacity of CO2 doubled for the supported amine when the surface was modified with sulfuric acid and neutralized with potassium hydroxide. ^