The earliest commercial methanol-synthesis plant was set-up by BASF in the 1920’s. A chrome-oxide / zinc-oxide catalyst was applied at temperatures ranging from 320-380oC, at a pressure of 300 bar. Development of copper-based catalyst active at lower temperatures enabled lower operating pressures in the range of 50 bar. It seemed natural to reduce the pressure as much as possible, in order to:
reduce thickness of steel piping and reactor, and
reduce syngas compression
However, a higher pressure favours the equilibrium reaction, and results in smaller piping and reactor sizes, and smaller recycles. A reduced temperature enables higher conversions, but also yields lower catalytic activity and larger reactors. Higher temperatures also negatively affect product distribution (by-products as CH4, dimethyl ether (DME), methylformate, higher alcohols and acetones) and catalyst lifetime due to e.g. catalyst sintering.
The main item in reducing the pressure is related to investment costs in the syngas production section: for conventional methanol processes at 70 bar, the investment costs for the syngas production section only is 50 % (natural gas) up to 80 % (coal) of the total plant costs (Olah et al., 2006). Spath and Dayton (2003) report data on the ICI process starting from natural gas: de-sulphurisation (2%), reforming / gas cooling (32 %), steam production (14%), compression (24%), methanol synthesis (22%) and distillation (6%).
For coal, oil or natural gas as a feed for syngas the best solution would be to operate the system at a high-pressure as that would yield reasonable return on capital. Here, the operating pressure of the syngas production itself is a limitation: syngas production is taking place at pressures of up to 40 bar for entrained flow gasification (Lurgi / Texaco / Winkler / Shell), up to 35 and 25 bar for moving bed gasification for the Lurgi gasifier, respectively the British Gas-Lurgi gasifier. In syngas production from steam reforming of natural gas, maximum pressures of up to 70 bar are common. The limit in the pressure of the methanol synthesis is therefore caused by the syngas production.