Increasing renewable electricity production calls for innovative methods to store electricity as well as a desire for electrification of processes that currently rely on fossil fuels. Zero Emission Fuels is a company that is developing a system to convert carbon dioxide and water from the air into methanol, a liquid hydrocarbon fuel, using photovoltaic energy. The scale of the system is fit for a single solar PV panel. The desired methanol output is 25 grams per hour.
In this work, a new design for the methanol synthesis reactor of the system is developed, built and experimentally characterized. Knowledge from work by Basarkar and Gutierrez on a previous prototype is used as the starting point. The focus of the new design is placed on the heat integration network, the natural circulation effects, and tilting of the reactor. The heat exchanger makes use of heat pipes to transfer heat. Natural circulation is increased by increasing the channel dimensions of the system. This led to an increase in mass flow rate of over 3000%, making the mass flow rate the limiting factor for reactor performance. The mass flow rate is roughly 800% higher than assumed in the design phase; the resulting energy flows are too high for the heat exchanger to work effectively. Nevertheless, the heat exchanger heating duty relative to total heating is 230% higher than Basarkar. Tilting the reactor 20 degrees reduced the mass flow rate by 46% to 0.41 g/s, improving almost all aspects of reactor performance. Productivity increased by 58% to 15.7 g/h; 182% higher than Basarkar, though the space time yield is lower (4.1 vs 6.8 mmol/gcat/h). The energy efficiency of the system is close to Basarkar at 36.5% (vs. 37.5%). It is clear from experimental correlations that reducing mass flow rate will increase productivity, energy efficiency and heat exchanger performance.
By simulating the requirements for an autothermal reactor it is found that the catalyst bed dimensions should be increased in terms of diameter and length to increase to residence time in the catalyst bed. The results agree with the experimental conclusion that the mass flow rate should be reduced. Furthermore, it is recommended to heat the fluid by convection instead of conduction to ensure a more homogeneous temperature profile in the catalyst bed. Also, the heat exchanger should be expanded by adding more heat pipes and increasing the heat transfer surface area.