(255b) Methanol Synthesis in a Membrane Reactor

Methanol Synthesis in a Membrane Reactor

SAHAR SOLTANI, XINHAI YU, MUHAMMAD SAHIMI and THEODORE T. TSOTSIS

University of Southern CALIFORNIA

LOS ANGELES, CA 90089

Methanol synthesis has undergone continuous improvements for nearly a century, as it represents the starting raw material for the production of a variety of other chemicals and solvents, including formaldehyde, methyl tertiary butyl ether, and acetic acid and fuel additives. Methanol has a number of advantages as a fuel and a source of chemical products, such as being more easily transportable than methane and other gaseous fuels, having a high energy density, needing no desulphurization, and its reactions (e.g., steam reforming) proceeding at moderate temperatures.  Recent global energy shortages and more strict emission regulations have motivated research and development of new fuel cells, among which a direct methanol fuel cell is a prime candidate.

In the present work the CO/CO₂conversion into methanol in both a traditional reactor (TR) and a membrane reactor (MR) has been studied. The purpose of this study is to investigate the possibility of using a MR to increase the total carbon conversion into methanol relative to what a TR can convert.

MR with solvent purge, which is proposed in this work, incorporates the advantages of the existing reactive separation systems that result from the in-situ product removal. In our proposed process, we use a solvent as a sweep fluid. The membrane serves as an interface contactor for the selective permeation of methanol. The solvent is chosen in a way that the main product, methanol, has the highest solubility in it. Using TetraEthylene Glycol Dimethyl Ether as an inert, high boiling point agent, the alcohol will be selectively removed in-situ from the reactor using the membrane as an interface contactor between the methanol and the solvent. Due to the very low solubility of H₂, CO, and CO₂, they will remain in the reactor, since the solvent blocks the membrane’s pores.

Prior to the start of the membrane reactor experiments, the surface of a membrane was successfully modified in order to increase membrane hydrophobicity and, therefore, to achieve higher mass transfer rate that, in return, results in higher methanol production. The modified membrane can be operated as a membrane contactor under the proposed operating conditions without the loss of the inert solvent. The kinetics of the methanol synthesis from CO, CO₂ and H₂ on a commercial Cu/Al₂O₃/ZnO catalyst was investigated in an autoclave reactor. The effect of various CO/(CO₂+CO) ratios, stoichiometric number, pressure, temperature and flow rate was investigated in the kinetic experiments.

Based on the experimental kinetics and the membrane surface modification results, we developed a model of the MR to explore the effect of several factors on the total conversion of carbon. The rate equation derived from the kinetic experiments was used in the model. Membranes with various thicknesses were simulated, and their performance was compared. Our results indicate great potential for application of membrane reactor in methanol synthesis using CO, CO₂ and H_2.