Stripping is the process used to separate the alcohol from the water in grain fermentation’s “beer,” or in the case of molasses, “dead wash.” Different terms may be used for other substrates.
In the case of beer, it contains spent yeast, grains, (except in the case of malt whiskey) congeners and alcohol. A stripping still should only remove the congeners and alcohol; fractionation of the volatile compounds — of which there are many, each with different boiling points — is not the objective.
There are two different stripping methods used in the industry: batch, or pot, stripping and continuous, or column, stripping.
A pot distilling system consists of a boiling kettle heated with steam in various ways (not usually direct injection) or with an open fire under the pot, in the traditional method. The vapor containing the volatile components and some water is then condensed. In the case of pots heated by open fire, the condenser should be in a separate building to avoid contact with the fire. In the old days the pot was referred to as an alembic.
This no-reflux system can produce a low wine with about 35% alcohol. This will require a second distillation in a spirit still (also a pot) or hybrid still, with a column, to produce the desired spirit for consumption.
It is not practical to provide reflux — a process that enhances the volume of alcohol in the distillate — but some refluxing does occur when the vapor hits the inside of the vapor pipe that leads to the condenser. The lyne arm can also be sloped back to the pot to allow condensed liquid in the arm to reflux (flow) back into the pot. This re-boiling of liquid actually enhances the strength of the distillate, if only marginally.
Another method is to install a cooling jacket around the swan neck, which will condense some vapor before it gets to the lyne arm, thus causing internal reflux. This can produce a low wine with up to 40% alcohol, but the distillation will be slightly longer and with a higher steam consumption since you are re-boiling liquid that has already been vaporized.
The 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, notably Scotch malt whisky, Cognac, some tequila that is not distilled continuously, select bourbon brands, cachaça and grappa, to name a few. Most other products are made in continuous stills or the Adam’s two-retort still that uses a hybrid system involving a stripping pot.
The principle for pot still stripping is illustrated in Exhibit 1.
Continuous still stripping involves a column still operating continuously with anywhere from 16–20 stripping trays spaced about 18” apart. The surface of the tray usually has punched holes that vary in diameter based on the substrate to be distilled and the operating conditions in the still. The diameter of the column also varies depending on the required throughput on the column. (See table on p. 158.)
The other variable is that some columns 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. The downcomers have to be sized to accommodate the flow of liquid from tray to tray. Some larger columns employ multiple downcomers.
There 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.
The 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. The surge will normally be about 55% ABV if there are no rectifying trays in the column.
The 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. The 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. The 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. The disadvantage of these trays is that they must be operated at the designed flow rate because the turn-down ratio is not as large as with trays that have downcomers.
In some older beer/wash columns, designers used bubble caps on the trays or tunnel trays, but the problem, especially with a molasses wash, is extreme scaling. This still takes place on sieve trays, but they are much easier to clean. Molasses with a high calcium content can build up a scale that can be up to 2 cm thick in just two weeks, especially if high pressure steam is used to heat the column.
Scaling is not a big issue with grain beer, but some scaling can occur because of the hard water used in the cooking process. A dilute caustic cleaning is recommended on a regular basis.
The three heat exchangers mentioned above are (a) a beer preheater that uses the exiting vapor to preheat the incoming beer, (b) a rectifying condenser that condenses vapor leaving the beer preheater and(c) a vent condenser that is run at a warm temperature to allow incondensable gases, methanol and some volatile components to vent off into the atmosphere through a flame arrester vent.
The sizing of all three heat exchangers is crucial. The beer temperature leaving the preheater should be about the same temperature as the entry tray on the column; this determines the number of tubes in the preheater (the heating surface) in relation to the beer feed rate.Some claim that this reduces scaling in the column. Surfactants have been used to reduce scaling, but this is expensive and somewhat ineffectual.
The rectifying condenser sizing is based on the water temperature for cooling and the amount of vapor that should go to the vent condenser, which produces the heads cut and the vapor expelled to atmosphere.
The beer column produces, as above, a high wine with all the nonvolatile components that will be selectively removed in the following column or columns that redistill the high wine. The high wine contains most, if not all, the nonvolatiles and some of the volatiles that will travel to the other columns for further rectification. These will produce anything from a flavorful distillate to a neutral spirit required for the production of vodka, gin or some liqueurs.
The column still shown (Exhibit 2) has some rectifying trays to have the option of producing a viable product — e.g., bourbon at a maximum of 160 proof, a flavorful rum or a low-proof tequila. Bourbon can also be produced just using a stripping still with fewer trays and a thumper or doubler. This is a progression from the original method using the Irish system employing three pot stills.
The hot effluent leaving the column — dunder (rum), vinasse (Spanish) or whole stillage (grain mashes) — can be used to preheat water for cooking or boiler feed water. It should have no more than 0.25% ABV, which can be tested by condensing vapor from, say, the 10th tray, with an ebulliometer or by distillation and an alcohol hydrometer.
There is also a system used in heating a column that employs a thermo-compressor to recover steam from the effluent, but that is not the subject of this article.
Original beer columns used vertical, not the modern multi-pass horizontal design.
The table illustration below is a rough chart of capacities for beer columns with different diameters. The principle of column distillation is illustrated in Exhibit 2.
Please note that the foregoing is not a definitive guide to the design of the two stripping systems described. Many variations can be employed.
Also, with regard to the above engineering factors, it is necessary to consider tray design, condenser configurations and materials of construction. Sizing of condensers in relation to water temperature and availability is also a factor. Air-cooled condensers have also been used in countries where water is a limited resource. Pump motors in the vicinity of the still should be explosion proof.