Characterization, Optimization and Design of the Sorbent System for a Continuous Direct Air Capture System

Abstract

With the influx of the industrial revolution of the past centuries, the global energy demand has grown proportional to the global economical growth. Consequently, an enormous rise in greenhouse gas emissions has been observed, where the major contribution to global warming comes from the rising concentration of CO2. Therefore, the concept of capturing CO2 directly from the air (DAC) has gained world wide attention as it can reduce the carbon footprint of men kind.
Zero Emission Fuels (ZEF), an inspiring start-up based in Delft, aims to develop a micro plant that utilizes the DAC concept to produce methanol as a fuel, using only the sun as its energy source. CO2 and H2O is captured directly from the atmosphere by the DAC unit which operates continuously with the help of an amine sorbent that flows through an absorption and stripping column. Previous research shows pure tetraethylenepentamine (TEPA) having great potential as a CO2 capturing sorbent. However, it has some limitations regarding slow absorption and desorption kinetics due to a high sorbent viscosity, that prevent ZEF from reaching their goal of capturing 825 grams of CO2 per DAC unit in
eight hours of operation. The focus of this research is on the sorbent selection process of ZEF’s DAC system. Research has shown that adding a diluent to TEPA could potentially improve the performance of the ZEF DAC unit. Therefore, a total of nine diluents, DEG, PEG-200, PEG-400, PEG-600, selexol-250, selexol-500, glycerol, 1,4-butanediol, sulfolane, mixed in different ratio’s with TEPA, have been put to the test through the developed framework of sorbent selection.
In the framework of sorbent selection all the sorbents are tested through four different experiments, keeping efficiency and costs in consideration. The Airfarm experiments are used to evaluate absorption capacity and viscosity of the sorbent at specific ambient conditions. The Vapor-Liquid-Equilibrium (VLE) experiments are utilized to evaluate the desorption characteristics of the sorbent from where the regeneration energy demand can be calculated with the help of a mathematical model. The re-pumping experiments are used to evaluate the absorption kinetics of the sorbent and lastly, the degradation experiments
are used to evaluate the sorbent life time performance. After the experimental procedure the sorbents can be judged on the key performance indicators regarding the ZEF DAC unit operated in a specific climate. It was concluded through the literature research and proved by an extensive experimental research
that selexol, glycerol and sulfolane did not improve the characteristics of the sorbent. However, adding DEG, PEG or 1,4-butanediol to TEPA did have a profound positive effect on the overall performance of the DAC unit. Since the diluents reduce the sorbent viscosity, and therefore, increase the absorption
and desorption kinetics. Based on the experimental results and the design specifications set by ZEF, PEG-200 proved the most promising out of all diluents in the mixing ratio TEPA:PEG-200 2:5. This sorbent has been tested for two different climates; the dry Sahara climate and the more humid Mediterranean climate to see how the sorbent would have to be changed for different climates. It was concluded, for a more humid climate, it requires less diluent for the sorbent to stay within the 2 푃푎 ⋅ 푠 viscosity limit. The experimental results were included in a full DAC model to design the DAC unit utilizing the optimized sorbent for the two different climates considering the design specifications set by ZEF. Following
a sensitivity analysis an optimized conceptual DAC design was obtained for both climates. Where it was concluded that the optimized sorbent resulted in reduction in energy requirement for regeneration of the sorbent, where 1554 푘푊ℎ/푡표푛 of CO2 is needed for the Sahara climate and 1636 푘푊ℎ/푡표푛 of CO2 for the Mediterranean climate. Finally, a cost analysis has been performed regarding the operational and capital costs of the newly designed conceptual DAC units for both climates.