Processes for producing methanol are known for example from EP 0 790 226 B1. The methanol is produced in a cyclic process in which a mixture of fresh and partly reacted synthesis gas first is supplied to a water-cooled reactor and then to a gas-cooled reactor, in each of which the synthesis gas is converted to methanol on a copper catalyst. The methanol produced in the process is separated from the synthesis gas to be recirculated, which then is countercurrently passed through the gas-cooled reactor as coolant and preheated to a temperature of 220 to 280° C., before it is introduced into the first synthesis reactor. In the case of unsteady plant conditions and in particular in the case of an improper start-up of the reactor by too early addition of fresh synthesis gas, this procedure can, however, lead to a condensation of the methanol product in the gas-cooled second reactor, since the reactor wall temperature at the outlet can approach the dew point of the product gas.
From U.S. Pat. No. 7,144,923 B2 a process for methanol synthesis is known, in which the synthesis gas first passes through an adiabatic reactor stage, in which a part of the synthesis gas is converted to methanol, and subsequently is supplied to a gas-cooled reactor for further conversion. The gas entering the gas-cooled reactor is guided cocurrent to the fresh gas, in order to achieve a better temperature control of the synthesis gas and a better heat recovery. When using an adiabatic first reactor, however, the CO content of the usable synthesis gas is restricted, since the control of the exotherm in the first reactor can create problems. In addition, it is required to perform cooling of the partly converted synthesis gas after each stage of the first reactor, which complicates the design of the reactor.