Purification of Mixed Alcohols

As mentioned already in the previous chapter, an alcohol mixture which is to be used as octane booster for motor fuels must not contain more than approximately 0.5 wt. % of water. It must also be stable , i.e. storable. Alcohol/water mixes, which contain much higher quantities of water, must therefore be purified to remove the low boilers and distilled to reduce their water content. 

For their MAS product, SNAM suggest purification in several stages [4.2]. As shown in Fig. 4.6, a first distillation stage is used to separate methanol and all low boiling components from the high boilers. At this stage, methanol is withdrawn from the distillation column separately from the low boilers and un- 

The overhead product of the third column is an azeotropic mix of higher alcohols, cyclohexane and water which is fed to a second phase separator. The light phase in this separator, consisting essentiaUy of cyclohexane, is recycled to the second column whereas the aqueous phase is fed either to the first column or to the first phase separator. The diagram does not show the equipment to handle the cyclohexane and to remove the residual alcohol from the waste water. This plant arrangement yields a product containing less than 0.1 wt. % of water. 

As the raw product from LURGI s Octamix Synthesis does not contain more water than can be tolerated in a blending component to natural motor fuels, a marketable product can be obtained simply by eliminating the uncondensibles and those components with boiling points lower than methanol, insofar as they may cause trouble during transportation or storage. This is done in a low boilers column similar to that for pure methanol distillation [4.3]. 

If this water content in the bottoms product exceeds the maximum that can be tolerated for motor fuel components, it may be corrected by using a conventional molecular sieve. Such molecular sieves can reduce the water contents to as little as 200ppm. 

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