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M. Sinha

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Direct Air Capture

Characterization and design of a novel absorption process

Master thesis - M. Sinha, Earl Goetheer, R. Delfos, J. van Kranendonk

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.

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Continuous Direct Air Capture, understanding mass transfer in reactive absorption of carbon dioxide

Experimenting and modelling a novel DAC absorption process

Master thesis - A. de Matteis, Earl Goetheer, R. Delfos, Jan Van Kranendonk, M. Sinha

Increase in global energy demand has brought to a massive rise in greenhouse gases emissions. In particular, the continuously increasing atmospheric concentration of carbon dioxide is a major concern among the scientific community. This paved the way for an intense research of CO2 emissions mitigating technologies. Those ones including carbon capture, storage and utilisation are seen as part of the solution for this global problem. In this framework, Zero Emission Fuels (ZEF), a startup company located in Delft (The Netherlands) set the ambitious goal of developing a micro process plant producing methanol from ambient air and sunlight only. Part of the process involves direct capture of carbon dioxide from the atmosphere (direct air capture) with the use of a pure tetraethylenepentamine (TEPA) absorbent. ZEF sets itself aside from the rest of the industry developing an absorber which collects CO2 with no support structure by continuously flowing TEPA inside open channels. Previous research shows how this approach brings limitations connected to slowly diffusing CO2 molecules in the absorbent liquid film in proximity of the gas-liquid interface. In this framework, circulation of TEPA particles from the gas-liquid interface into the bulk of the flowing absorbent is seen as a viable solution to improve the process. This thesis focuses on investigating this hypothesis by inducing mixing on the liquid side. That is done by building two experimental setups investigating both passive and active mixing. Experimental results show that active mixing can be used to improve the rate of CO2 absorption, while passive mixing does not bring significant advantages. Results from the passive mixing experiments are further investigated by modelling the fluid dynamics of the process through a Direct Numerical Simulation of the particular Stokes’ flow in the engineered absorption channel. The process is characterised with the definition of a theoretical framework describing mass transfer in the liquid side. Following an analogy with ice formation on top of a frozen lake, this theory, also known as the ”Ice-Sheet” theory, shines some light on the way this diffusion limitations are happening at a molecular level. In particular, a highly viscous, heavily loaded layer of sorbent on the gas-liquid interface is believed to be the cause for observed CO2 diffusion limitations. This theory is backed-up by defining its mathematical equivalent in the form of a mass transfer model. Comparing the results of the model with experimental results, a very good agreement is observed. That is believed to add credibility to the proposed theory. Moreover, that is also found to be in line with the latest knowledge available in literature about CO2 absorption in TEPA films. Finally, the experimental results and the developed models are used to engineer a new iteration of ZEF’s absorber on a cost reduction basis.

Dynamic operation of an Integrated Direct Air Capture and CO2 Compression System

Master thesis - K. Vidyarthi, Earl Goetheer, M. Sinha

CO2 concentration in the atmosphere has soared since the industrial era owing to the rapid and unabated increase in fossil fuel consumption globally. This has resulted in an increase in global average temperature by ≈ 1.1 C in this period; and has sparked the concern of climate disaster events across the globe. In order to tackle the issue of climate change, research into Carbon dioxide removal (CDR) technology such as Carbon Capture and Storage (CCS) and Carbon Capture and Utilization (CCU) has gained prominence. Zero Emission Fuels (ZEF) B.V is a technology startup based in Delft, working on the CCU pathway for CDR. ZEF is developing micro plants to capture CO2 and H2O from the atmosphere using Direct Air Capture (DAC) system. The captured CO2 is then compressed to 50bar using compressors while H2O is split electro-chemically to H2 at 50bar. This high-pressure CO2 and H2 are then used to produce methanol. The energy for the whole process is derived from the sun, making the entire process sustainable and emission-free. ZEF’s micro-plant will derive its raw materials i.e. CO2 and H2O, from the environment. Thus, variation in the environmental conditions, i.e., humidity, temperature, and solar radiation (external disturbances for the system), will affect the overall system output. Current work focused on the design of control scheme for the integrated DAC and CO2 compression system, which will be able to meet ZEF’s performance target. The effect of variation in solar radiation was not considered for this work. At first, the performance parameters for the various sub-systems of the integrated DAC+FM system were identified. With this information, the operating scenarios and the process constraints for the system were identified. Then, models for the sub-systems of the DAC and compressor system, i.e., Absorber, sump, desorber, flash tank, and compressor system were developed. Parts of the model, the desorber and flash tank, were validated using experimental setup developed by integrating the existing DAC prototype at ZEF with a representative compression system. The main target of the experimental setup was to develop control scheme to maintain the pressure of the flash tank to the required target levels. Through multiple iterations, the final layout and control system for the integrated setup was identified which was able to control the pressure of the flash tank at the target level. Finally, a control scheme for DAC production control was developed based on the steady-state output from the desorber model. Through this, the feed and power (heat) input to the desorber is varied to control the production of CO2 and H2O. The system model developed as part of this work can be further used as a tool by ZEF to integrate different sub-systems, test the performance of different sorbents and assess the viability of different control schemes for the integrated system