The Composition of Plate Samples from Distilling Columns with Particular Reference to the Distribution of Higher Alcohols

Abstract

The compositions of plate or tray liquid samples from a 54-inch diameter commercial concentrating column and a 12-inch diameter pilot plant column, while producing grape brandy under different operating conditions, are reported. The following conclusions and observations are made: a) Fusel oil (higher alcohols) concentrate on plates nearest in apparent proof to about 135. The boiling point of ethyl alcohol-water at 135 proof is 180°F at atmospheric pressure, but the observed temperature of plates of highest fusel oil content are likely to be nearer to 185°F, owing to the effect of column pressure on temperature and the fact that a significant percentage of higher boiling alcohols in ethyl alcohol-water mixtures increases the apparent proof and also raises the boiling point toward that of the azeotropic mixture. (The azeotropic mixture of isoamyl alcohol and water boils at 95°C, 203°F). b) The highest attainable fusel oil on any plate in a concentrating column is less than about 0.5 percent when a concentrating column is used to produce a beverage brandy-type distillate at 170 or less proof, owing to the fact that fusel oil present in the feed passes over into the product. c) The fusel oil concentration on plates in the region of an apparent proof of 135 may reach 25 to 50 percent, sufficient to cause separation into two phases upon cooling, with or without further dilution with water. d) The percentage of isobutyl alcohol of the total higher alcohols, estimated by the Komarowsky color reaction, is greater in the upper plate levels, approaching 100 percent. On the lower plates and in the fusel region where two phases are present, the isobutyl alcohol content may be less than about 10 percent. e) Esters, principally ethyl acetate, reach the highest concentration at the top of a column, but a secondary maximum at lower plate levels is sometimes found. f) Aldehydes, principally acetaldehyde, also accumulate at the top plate level. Occasionally there is some analytical evidence for higher boiling aldehydes at lower plate levels of low proof. The significance of this in plant operation includes the following: a) No separation of fusel oil by the customary practice of dilution with water, passage through a decanter and return of the wash water to the column is possible when the distillate is beverage brandy received at 170 or less proof. A small reduction in fusel oil content of the product produced at about 170 proof is possible by removing a "low oils" cut one or two plates below that used for product removal, but with a significant decrease in the yield of the main product. b) The accumulation and removal of fusel oil from wine spirits is most complete if the highest attainable proof of distillation is used. This requires proper control of column operation such that the concentration of alcohol begins on the lowest plates of the concentrating column. c) The most neutral wine spirit is obtained as a side-stream from a plate numbering 5 to 10 below the top, so as to permit removal of low boiling aldehydes and esters in a heads cut. d) That portion of the fusel oil which does pass over into wine spirits (about 185 to 190 proof) is likely to consist of a high proportion of isobutyl alcohol and probably other alcohols present, more volatile than amyls, such as <i>n</i>-propyl alcohol.