The synthesis loop

The synthesis reactor is the core of methanol synthesis, but in conventional methanol synthesis a series of other operations take place as well – heating, cooling, condensing, re-compressing. The design of a synthesis plant is almost the same for all systems operating with a gas recycle loop. A general layout is given in the figure below. For details on several process details the reader is referred to Supp (1990) or Olah et al. (2006).

The compressed syngas is mixed with recycle gas and the complete quantity, or some of it, is heated in a heat exchanger to the desired reactor inlet temperature, usually about 220-230 oC. In case of a cold quench operation, only some of the feed gas, about 2/3, is preheated, and the rest used to cool the product gas in between the individual catalyst layers.

The reactor gas leaves the reactor at about 250-270 oC. This is send to a separator, after heat exchange with reactor feed gas, air cooler, water coolers or any other cooling system. In the separator water and methanol are separated from the not-reacted gas, which is returned to the recycle compressor. The pressure in the reactor system generally is 50 - 100 bar (ICI, Lurgi), with recycle ratios of 5 to 7.

General (simplified) low-pressure methanol synthesis loops: (a) recycle compressor, (b) heat exchanger, (c) reactor, (d) cooler/condenser, (e) methanol separator, (f) start-up heater.

To prevent build up of inert gases, a small fraction of the gas is vented. Depending on the CO2 concentration in the feed inlet, the raw methanol contains up to 12% water, as well as a number of impurities varying with the selectivity of the catalyst and the CO concentration in the feed gas.

The pressure in the reactor system generally is 50 - 100 bar (ICI, Lurgi), with recycle ratios of 3 to 7. Largest plants have methanol reactors with an individual production capacity of 1,800 to 2,500 tpd. Mitsubishi Gas Chemicals (MGC) originally designed their system for 150 bar, but operated successfully at 100 bar or less. The process is offered at a pressure range in between 50 and 200 bar, and temperatures in between 235 and 270 oC. Haldor Topsoe provides a design for pressures up to 150 bar, and temperatures of 200 up to 310 oC. For Linde AG process pressures of 50 to 150 bar are stated but at lower temperatures of 240 to 270 oC. Also, a slurry process is developed, where reactions are taking place in a liquid fluid bed on an agitated catalyst The latter enables converting low SN-ratio syngas into methanol.