(699c) Simulation of Membrane-Based Separation for a Methanol-OCM Synthesis Gas

Methanol can be produced from synthesis gas at the pressure of 30 bar and a temperature of 250 °C. Since the conversion of methane in the Oxidative Coupling of Methane (OCM) reaction could not reach 100%, it is a good alternate to produce methanol from this non-reacted gases. The OCM synthesis gas is produced from the non-reacted methane stream coming from the OCM reactor, and it is represented by hydrogen, carbon dioxide, carbon monoxide and water. Due to the thermodynamic equilibrium, the methanol production is limited and the yield of methanol is not high. Thus, to avoid the high condenser utility costs of the demethanizer column in the separation section, the membrane unit could replace this column to remove light gas out of the system, i.e. hydrogen. In this work, the membrane-based gas separation process of methanol-OCM synthesis gas is presented, as shown in the figure. In this process, the products stream, with 5 wt% methanol at 30 bar and 145 °C, is firstly cooled down and decompressed to 10 bar, and further separated in the flash. Partial methanol and water from the gas mixture are in the liquid phase, meanwhile the residue gas stream flows into the membrane unit. In the membrane process, partial hydrogen and water are removed from the gas system, while the methanol and other gases stay in the residue under high pressure. In order to reduce the methanol loss in the permeating gas phase, two stages of membrane units are used in this process. The collected liquid phase is fed to a distillation column to achieve the final separation of methanol and water, and the residue gases and the hydrogen in the permeate flow of membrane unit could be recycled to the methanol synthesis process. The membrane used for hydrogen removal is a kind of polysulfone (PSF) dense membrane with the separation factor 24, 24 and 2 for hydrogen/carbon monoxide, hydrogen/methane and hydrogen/carbon dioxide, respectively. Hydrogen shows a high permeability in this kind of polymer. However, in the sulfonated PSF membrane methanol exhibits a lower permeability. In addition, since the hydrogen is the main component with mole fraction of 33 mol% in contrast to methanol in the gas system, it is reasonable to use hydrogen selective membrane for removing hydrogen from the gas mixture. A simplified model for membrane-based gas permeation, which has been evaluated with experimental data in a mini-plant scale for CO₂ removal in OCM process, was used to simulate the methanol separation process in Aspen Plus^®. The result shows that 95 wt% methanol is recovered with the purity of 95.8 wt% in this separation process.