Removal of residual CO₂ from feed water to increase the lifetime of KOH electrolyte

Removal of residual CO₂ from feed water to increase the lifetime of KOH electrolyte

Nauta, Marieke (TU Delft Electrical Engineering, Mathematics and Computer Science)

de Jong, W. (mentor)

Delft University of Technology

2020-01-15

Zero Emission Fuels B.V. (ZEF) is a start-up who aims to design a sustainable methanol production micro-plant, using solely air and solar energy as an input. A subsystem that has yet to be designed just upstream of the alkaline (KOH) electrolyser, is the so-called degasser. The purpose of the degasser is to remove a residue of carbon dioxide (CO₂) present in the feed water for the KOH electrolyser. The presence of this CO₂ is caused by it being absorbed together with water, in the direct air capture (DAC) subsystem. It is important that the concentration of carbon dioxide in the feed water is minimized since it is known to react with KOH, decreasing the performance of the reactor. The aim of this thesis, is to develop a method to minimize the concentration of carbon dioxide in the feed water, which is feasible for the ZEF micro-plant. A literature study was performed based on which is concluded on the feasibility of three different degassing methods: membrane permeation, distillation and stripping. It was decided to further investigate the efficiency of a stripping device, by making a numerical model using Matlab and validating this model with experiments. For the base case model, a bubble column in the shape of a cylindrical pipe operating in the Taylor flow regime was used. The bubble column had an inner diameter of 6 mm. Operation at steady state, at a pressure of 50 bar and a temperature of 50 °C, resulted in a required length of 65 cm. It should be noted here, that the liquid flow rate was equal to that aimed by ZEF, but the gas flow rate was about three times as high. A sensitivity analysis was performed in which the temperature of the system was changed. As expected, the required length of the bubble column decreases with increasing temperature. A sensitivity analysis on the influence of recirculation of the gas phase, showed that the accumulation of carbon dioxide in the gas phase is of no limiting factor in the stripping process, so that with the gas flow rate of ZEF, recirculation can be an interesting option. Different setups were tested to validate the model with. However, since none of them showed a steady state behaviour, the model could not be validated with these experiments. The setup was changed slightly in order to perform batch experiments, so that the continuous system (based on theory) could be compared with the batch system (based on experiments). Results showed that both the batch and continuous mode are feasible for the ZEF micro-plant.

Taylor bubble flow

http://resolver.tudelft.nl/uuid:8fd20529-1596-467d-a732-717f9667c13f

2022-01-15

Student theses

master thesis

© 2020 Marieke Nauta