3.3 METHANOL SYNTHESIS PLANTS

Methanol or methyl alcohol CH3OH is obtained today by synthesis processes. It is mainly used for the following:

• as solvent in the chemical industry for preparing paints, lakes etc.
• as the starting product for preparing formic aldehyde (formalin)
• preparing antifreeze aqueous solutions (a solution made up of 40% methyl alcohol in water freezes around -40 °C)
• as fuel, especially blended with petrol (10 to 20%) in order to considerably increase the fuel grade (the octane rating of methyl alcohol is 120).

As regards the process in recent years there has been a tendency‘ to use low pressure processes while plant capacities have grown considerably.

Nowadays 2000 to 2500 MTPD plants are normally required and there are projections for 5000 MTPD plants

A typical low pressure process (ICI) makes it possible to obtain 99.5 to 99.99% methanol starting with natural gas, steam and carbon dioxide.

After the natural gas is desulphurized it is reformed with steam to obtain synthesis gas consisting of CO, CO2, H2 and CH4. The reforming pressures and temperatures depend on the quality of the initial materiai but rangebetween 10 and 20 bars, 800 and 900°C.

The heat developed by the synthesis gas reactions is mainly used to produce steam.

The gas is then cooled and sent to the compressors which compress it to 80-90 bars and send it to the reactor for conversion into methanol. On leaving the reactor the gas is cooled and the methanol separated from the gas that has not reacted. As usual there is a recycle impeller to recompress the gas and send it back into the reactor.

As regards the machines the methanol synthesis service is normally made up of 2 or 3 compression stages in 2 barrel type compressor casings.

These compressors that handle the synthesis gas are normally directly coupled to a steam turbine, fre- quently the type with a double exhaust.

The recycle compressor is sometimes driven sepa- rate from another steam turbine.

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