Production of Grade AA Methanol

The flowsheet in Fig. 4.2 shows a typical distillation system which may be used to describe the process reactions and main characteristics of the equipment used. In addition to dimethyl ether, methyl formiate, acetone and hydrocarbons up to about Cg, the low boilers in a broader sense include also that part of the gases dissolved in the raw methanol which is not expelled when this is flashed into an interim storage tank. This flashing operation is normally performed at pressures between 5-10 bar in order to keep methanol losses down, but frequently also in order to ensiure that the raw methanol can be transferred to the light ends column without using a pump. 

Whenever the occurence of amines has to be expected - for instance due to the presence of nitrous oxides or ammonia in the synthesis gas - a cation exchanger is normally provided in the predegassed raw methanol. Raw methanol is fed to the bottom half - usually between trays 8 and 10 counting from the top - of the prerun column which is equipped with a total of some 40 trays. Valve trays are normally used as they are characterized by a much better partial load behaviour than the cheaper sieve trays. The column is heated by a reboiler 

As is well known, the stripping factor in the distillation section of a column is that figure which indicates whether a certain component tends to migrate towards the column top or as a liquid into the column bottom. The stripping factor is defined by 

The second key component in this system is methyl formiate. Acetone occurs in the raw methanol only in the order of a few ppm, and dimethyl ether is much more volatile than methyl formiate and can therefore be separated more easily from the raw methanol. Essentially, only the prerun column is a pure stripping column, the heat supply to the colunrn bottoms and the number of trays being designed to ensure that methyl formiate is, for all practical purposes, removed completely from the bottom product, the so-called stabilized methanol. Refluxing the methanol condensed in the ovahead condensers is inevitable to keep overhead methanol losses to a minimum. 

Heating the prerun column with its approximately 40 trays requires some 0.86 GJ per ton of methanol or about 390kg of low-pressure steam. This results in a reflux ratio to the cold feed of about 1 3 at tiie column head. The low-pressure steam of at least 2 bar may be replaced also by waste heat, for instance from the raw gas, at temperatures above 120°C. 

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