Methanol synthesis is generally performed by passing a synthesis gas comprising hydrogen, carbon oxides and any inert gases at an elevated temperature and pressure through one or more beds of a methanol synthesis catalyst, which is often a copper-containing composition. Methanol is generally recovered by cooling the product gas stream to below the dew point of the methanol and separating off the product as a liquid. The crude methanol is typically purified by distillation. The process is often operated in a loop: thus the remaining unreacted gas stream is usually recycled to the synthesis reactor as part of the synthesis gas via a circulator. Fresh synthesis gas, termed make-up gas, is added to the recycled unreacted gas to form the synthesis gas stream. A purge stream is often taken from the circulating gas stream to avoid the build up of inert gasses.
U.S. Pat. No. 5,631,302 describes a process in which methanol is catalytically produced from a synthesis gas containing hydrogen and carbon oxides on copper-containing catalysts under pressures in the range from 20 to 20 bars and at temperatures in the range from 200 to 350 DEG C. The synthesis gas is passed through a first synthesis reactor, which consists of a shaft reactor and contains a fixed bed of a copper-containing catalyst. The reaction in the shaft reactor is carried out adiabatically and without a recycling of synthesis gas. Together with recycle gas, the gas mixture which has not been reacted in the first synthesis reactor is passed through a second synthesis reactor, which contains a copper-containing catalyst, which is disposed in tubes and is indirectly cooled through boiling water.
U.S. Pat. No. 5,827,901 describes a process in which methanol is catalytically produced from a synthesis gas containing hydrogen and carbon oxides on copper-containing catalysts at pressures in the range 20 to 120 bar and temperatures in the range 130 DEG to 350 DEG C. The synthesis gas is first of all passed through a first synthesis reactor, in which the catalyst is provided in tubes surrounded by water as a coolant, which is boiling at an elevated pressure. From the first reactor a first mixture containing gases and methanol vapour is withdrawn and passed without cooling through a second synthesis reactor. In the second reactor the catalyst is cooled with synthesis gas to which a make up gas has been added.