Direct Air Capture

Abstract

Rising concentration of CO2 in the atmosphere has become a significant concern, paving the way for worldwide research on mitigation techniques like carbon capture and storage (CCS) and carbon capture and utilization (CCU). Zero Emission Fuel (ZEF), an aspiring startup based in the Netherlands, aims to develop a micro plant that produces methanol from just solar energy and air. Their process involves capturing CO2 and H2O directly from the air, splitting H2O to obtain H2 and producing methanol by reacting CO2 with H2. The focus of this research is on the absorption process of ZEF's direct air capture unit. Instead of capturing CO2 and H2O through a widely used batch direct air capture process, ZEF chooses to side with a novel continuous absorption and desorption process involving a flow of bulk polyamines without any conventional support structures. Polyethyleneimine (PEI-MW-600) and Tetraethylenepentamine (TEPA) are used as absorbents to achieve a target of capturing 825 grams of CO2 in 8 hours every day from a single direct air capture unit. Preliminary investigations show that the viscosity of PEI-600 is significantly higher than that of TEPA. An increase in CO2 concentration increases the viscosity of both polyamines significantly. In comparison, increasing H2O concentration leads to a maximum viscosity point in both polyamines and subsequent decrease in viscosity with a further increase in H2O concentration. Although, compared to CO2, H2O has a smaller effect on viscosity. Moreover, premixed samples of PEI-600 and TEPA with H2O show an increase in the CO2 absorption rate when pure CO2 is bubbled through the samples. In order to study and characterize the novel absorption process, an experimental setup facilitating a flow of absorbent is developed, and experiments are conducted following an experimental approach to calculate the mass transfer rates and average concentrations of CO2 and H2O. Fourier-transform infrared spectroscopy is used to estimate these concentrations. Experiments are performed on different initial concentrations of PEI and TEPA. TEPA is found to have a better absorption performance with an average CO2 absorption rate that is two times higher than PEI-600. Contrary to preliminary investigations done by bubbling CO2 into polyamines, absorbents with premixed water flowing down a channel were found to have lower CO2 absorption rates than pure polyamines. A multiple regression viscosity model, heat model, and a model of absorption column is developed using the data collected to estimate average viscosity, the heat of absorption of water, and characteristics of the absorption process. Diffusion of CO2 through the absorbent layers is found to be the primary limiting factor, while mass flow rates of absorbent and air are also found to influence the absorption process. Finally, a design of the absorption column is made to meet ZEF's requirement taking into account various performance characteristics studied during the thesis.