Still, continuous rectifying

Rectifyingx—When alcohol of special purity is required, it is further purified by special rectification. This is usually done by diluting the spirit with water to a strength of about 45 to 50 per cent., and redistilling it. Intermittent stills which fractionate the distillate (the impurities being in the first and last runnings), and continuous rectifying stills are used for the purpose. 

In larger establishments a continuous rectifying still is used in place of the intermittent still for the second operation. Figure 22 is a diagram of such a continuous rectifying still. The still consists essentially of a purifying column C and a concentrating... 

In other districts where the wines have a strong, earthy flavor somewhat more elaborate apparatus is used. The La Rochelle district uses the Alembic des lies which is a pot still with rectifying equipment. The Midi uses a continuous distilling column of the kind in favor in this country, excepting that it is equipped with a faucet or tap at each plate. This arrangement enables the operator to distil at higher or lower strengths at will. 

Those using both pot stills or vat stills and Coffey or other continuous rectifying stills. 

In the arrangement discussed, the feed is introduced continuously to the column and two product streams are obtained, one at the top much richer than the feed in the MVC and the second from the base of the column weaker in the MVC. For the separation of small quantities of mixtures, a batch still may be used. Here the column rises directly from a large drum which acts as the still and reboiler and holds the charge of feed. The trays in the column form a rectifying column and distillation is continued until it is no longer possible to obtain the desired product quality from the column. The concentration of the MVC steadily falls in the liquid remaining in the still so that enrichment to the desired level of the MVC is not possible. This problem is discussed in more detail in Section 11.6. 

Many processes are used throughout the world for distilling wine into brandy. Continuous distillation in plate columns, discontinuous or batch distillation using pot stills (with and without adjunct rectifying columns), and even distillation apparatuses involving both continuous and discontinuous aspects are used. 

Only a small amount of pot-still brandy is produced in California. It is used primarily for blending, in small percentages, with continuous still brandy after aging. Equipment used for this purpose consists of pot-rectifiers. In at least one case, the fractionating column is so arranged that it can serve either as the concentrating section for the pot or as a part of a continuous still unit. 

After approx. 48 h of fermentation, the fermented mash shows an alcohol content of approx. 6-10% by vol. When distilled in continuous stills with 3 or more colunms, the alcohol level in the distillate can be enriched up to 96.6% by vol. The purification of the raw distillate is normally carried out in continuous rectification equipment, for which the raw alcohol is usually diluted to approx. 15% by vol. before rectification in order to better separate fusel oil components. The first fractions of the rectified distillate - the so-called head fraction - contain significant quantities of acetaldehyde, methyl alcohol and low-boiling esters the middle part - heart fraction - represents so-called neutral spirits with an alcohol content of approx. 96.6% by vol. The tail fractions contain higher alcohols and higher esters etc. Since head and tail fractions contain organoleptic properties undesirable for neutral alcohol, they usually are employed as technical alcohol or have lately also been isolated and used as starting raw materials for the production of natural aroma components. 

In the United States alcohol is distilled and rectified from its wash by means of continuous stills. In smaller establishments all non-volatile materials and a substantial portion of the water are removed in a so-called beer still, Figure 20. On account of the partial rectification in the preheater the distillate from a 6% beer will frequendy run as high as 40 to 60 per cent alcohol. The crude alcohol is neutralized with some suitable alkali such as soda ash, and then purified and concentrated in an intermittent still, Figure 21. Assuming that the feed runs as high as 60 per cent alcohol, the feed is diluted so as to reduce the con-... 

The operation of a batch still and column can be analyzed using a McCabe-Thiele diagram, with the same operating-line equation that was used for the rectifying section of a continuous distillation ... 

The reader should be aware that the minimum reflux scenarios presented here are just one of three possible ways the minimum reflux limit can be obtained in distributed feed columns. The designs shown thus far all depicted minimum reflux when the vertex of the internal CS adjacent to the topmost rectifying section lies exactly on its profile, that is, a pinch occurs on the topmost rectifying CS. It is perfectly valid for the minimum reflux condition to be determined by the bottommost stripping profile, or indeed where the TTs of the internal CSs do not overlap one another. The latter case is shown in Figure 6.12 where the column reflux has been reduced and TTs cascade around one another, thereby limiting any further column reflux reduction. The general requirement for minimum reflux is however the same as for simple columns any reflux value below the minimum reflux value will lead to a discontinuous path of profiles, and minimum reflux is therefore the last reflux where a continuous path is still maintained. 

Continuous distillation is achieved by returning the liquid condensate to a point approximately midway up the fractionating column, where the liquid in the column is about the same composition as the returning liquid (reflux). The relatively pure vapour leaving the top of the column may be taken off continuously, leaving only the unwanted still bottoms in the boilers. The section of the column above the inlet is called the rectifying section, and that below is called the stripping section. 

---