Distiller magazine a publication of the American Distilling Institute, the Voice of Artisan Distilling; devoted to the craft spirits industry: vendors and distillers alike.
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fall 2020 | 155 and with a higher steam consumption since you are re-boiling liquid that has already been vaporized. e principle is used in gin distillation where you want reflux so you can more efficiently remove the essential oils from the botanicals. Several commercial spirits are made by the two-stage distillation process, nota- bly Scotch malt whisky, Cognac, some tequila that is not distilled continuously, select bourbon brands, cachaça and grappa, to name a few. Most other prod- ucts are made in continuous stills or the Adam's two-retort still that uses a hybrid system involving a stripping pot. e principle for pot still stripping is illustrated in Exhibit 1. CONTINUOUS STILL STRIPPNG Continuous still stripping involves a column still operating continuously with anywhere from 16–20 stripping trays spaced about 18" apart. e sur- face of the tray usually has punched holes that vary in diameter based on the substrate to be distilled and the operat- ing conditions in the still. e diameter of the column also varies depending on the required throughput on the column. (See table on p. 158.) e other variable is that some col- umns have downcomers, whereby the liquid from the tray flows over a weir to the tray below in a way that the liquid has to traverse the tray before descending. e downcomers have to be sized to accommodate the flow of liquid from tray to tray. Some larger columns employ multiple downcomers. ere have been many column–tray designs over the years; Exhibit 2 shows the most modern system. In the early days, the column shell was made of wood with copper trays. Now the shell and trays can be stainless steel, since copper is not needed in the column to remove mercaptans. e other feature of a continuous column is the three heat exchangers that condense the vapor leaving the column to create a reflux and produce a high wine or surge that will feed other columns for rectification and removal of congeners. e surge will normally be about 55% ABV if there are no recti- fying trays in the column. e exception to having heat exchangers is the Coffey still design, where the vapor from the stripping column goes directly to a rectifier to produce finished spirit. e Coffey still was developed in 1831 by Aeneas Coffey, but is outside this discussion. Some modern columns have dual flow trays where the sieve holes are bigger than normal and there are no downcomers. e vapor going up the column still supports a liquid layer on the tray but the holes are big enough to allow liquid to traverse down the tray. e disadvantage of these trays is that they must be operated at the designed flow rate because the turn-down Exhibit 1 Several commercial spirits are made by the two- stage distillation process. Notably Scotch Malt Whisky, Cognac, some Tequila that is not distilled continuously, select Bourbon brands, Cachaça, Grappa, to name a few.