Basic Methanol Distillation Systems

With the single column system it is not possible to produce pure methanol of Grade A or even AA under economically justifiable conditions, and it can therefore be used only in plants which are scheduled to turn out methanol for burner fuel or motor fuel applications. 

A two-column system always consists of a light ends column, which is sometimes also termed an extraction column, and a refining column. The former is used to expel the light ends overhead, or sometimes only the dimethyl ether, which can be conveniently marketed. If this is the case, the other low boilers are withdrawn from a side outlet above the inlet. Methanol, water and high boilers are withdrawn from the bottoms and fed to the refining column in which as a rule the pure methanol is obtained at the column top, the high boilers below the feed nozzle, and the water in the column bottoms. 

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Nitromethane shows the simplest residue curve map with one unstable curved separatrix dividing the triangle in two basic distillation regions. Methanol and acetonitrile give rise two binary azeotropic mixtures and three distillation regions that are bounded by two unstable curved separatrices. Water shows the most complicated residue curve maps, due to the presence of a ternary azeotrope and a miscibility gap with both the n-hexane and the ethyl acetate component. In all four cases, the heteroazeotrope (binary or ternary) has the lowest boiling temperature of the system. As it can be seen in Table 3, all entrainers except water provide the n-hexane-rich phase Zw as distillate product with a purity better than 0.91. Water is not a desirable entrainer because of the existence of ternary azeotrope whose n-hexane-rich phase has a water purity much lower (0.70). Considering in Table 3 the split... 

Hexagonal polyoxymethylene of polydispersity 1 and h h molecular weight can also be prepared in methanol instead of water. The equilibria are analogous to those in the aqueous system. Basic catalysts are also preferred in this system. The polymerization is best carried out at 110°C and a highly concentrated solution of 85% formaldehyde can be used. Under these conditions the polymerization must be carried out under pressure, and the methanol is distilled overhead while new formaldehyde is added. Small amounts of water in the system induced side reactions, mainly the formation of methyl formate. In one reported run 22% of polyoxymethylene of a molecular weight M of 46,000 was obtained after 90 h. The loss of formaldehyde by side reactions was less than 1%. 

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