Characterisation of flow and bubble phenomena in a high pressure alkaline water electrolyser

Abstract

Zero Emission Fuels B.V (ZEF) is currently in the process of designing and prototyping a sustainable methanol micro-plant, which captures water vapor and carbon dioxide from the atmosphere and uses those components to eventually produce methanol. In order to produce methanol, hydrogen is required which is produced from the captured water by electrolysis in a zero gap design, high pressure alkaline water electrolyser. The pressure in the electrolysis setup is regulated at a desired operating pressure of 50 bar by a pressure control system, designed by ZEF.

Insight is required in how the flow of electrolyte and bubbles behaves in the electrolysis setup and the electrolyser cells, in order to implement design or operation condition changes to allow for an efficient operation in the eventual micro-plant.

This study focuses on the experimental characterization of the flow and bubble phenomena in the ZEF electrolyser design, during operation at a desired pressure of 50 bar, with the use of visualization by video footage inside the electrolyser cells. Camera equipment used for the visualization consists of a microscope camera and a GoPro camera. An ultrasound Doppler velocimetry device (UDV) was used to measure the electrolyte flow velocity and direction in different parts of the electrolysis setup, in order to substantiate findings made by the camera equipment.

Experiments were conducted with an electrolysis setup based on the current electrolyser design made by ZEF, with the addition of a transparent add-on to allow for visualization of the flow and bubbles inside an electrolyser cell on the oxygen side. First the operation of the electrolyser was characterized by means of IV curves and temperatures, followed by visualization experiments for varying operating pressures and current densities at different parts of the electrolyser cell.

One of the main observations made during the visualization experiments is the periodic occurrence of stagnation of electrolyte and gas flow out of the electrolyser. Stagnation of the flow occurs inside the electrolyser cells on the oxygen side, at the top channels leading the flow out of the electrolyser cells, to eventually a flash tank. Different multiphase flow regimes were observed during active flow through the top channels, which vary based on the operating current density, pressure and temperature.
Bubble behavior was observed at different positions on the electrode in the electrolyser cell and possible enhancement of gas crossover was detected in the electrolysis setup and investigated.

The effect of the pressure control system on the operation of the electrolyser was also investigated and showed to affect the electrolyte flow direction in the electrolysis system.
Models were made to validate and explain the findings from the visualization video footage and characterization of the electrolyser. A previous made model that characterizes the flow in the electrolysis system is used for comparison.

Due to the lack of a temperature regulation system, a finned tube heat exchanger was modelled to be implemented in the experimental setup as for the eventual electrolysis setup.

Based on the findings from the experiments, recommendations are proposed regarding the experimental and eventual design, and for further development of the high pressure alkaline water electrolyser and the electrolysis system.