Distillation is the purification of gases or liquids by taking advantage of their boiling point
differences. Ethanol and water have a fairly large difference in boiling point, but only up to a
certain concentration. At 1 atmosphere and about 95 volume % ethanol, the boiling point of
this mixture has a boiling point less than either of the pure components and is known to be a
minimum boiling azeotrope.
The earliest known distillation was between ethanol and water. For millennia, man has made spirits, or “strong drink”, from the fermentation of various sugars and starches. These early distillations were typically a single stage, or more simply a pot boiling and a condenser to capture the vapors. This single stage would yield an alcohol content of 40 volume % ethanol (or 80 proof), hence your typical whiskey or brandy. If done twice you can improve the ethanol content further to 70 volume % ethanol, which is your typical cognac before aging.
For fuels ethanol we need an alcohol that has no water. Therefore we distill fermentation products with many stages. However, nature does not allow us to get past the 95 volume % azeotrope by distillation no matter how many stages we might have. The rectifier in a fuels ethanol plant is the tower that enables us to produce the azeotropic ethanol product (sometimes referred to as the 190 proof product). This 190 proof product typically has the remaining water removed by molecular sieve.
A typical modern ethanol plant has three main towers in its distillation system. These include a beer mash tower, the rectifier and a side stripper (sometimes called the water column). The beer mash tower takes feed directly from the fermenters complete with all the solids, proteins, remaining starches and other assorted “cats and dogs.” This tower removes all the solids and other potentially fouling agents along with a majority of the water(1) . This tower will typically contain 22 trays. The rectifier takes the overhead vapor from the beer mash tower and concentrates the ethanol up to 190 proof which is the azeotrope. The rectifier also removes small amounts of some middle boilers such as propanol and heavier alcohols along with some aldehydes in a side stream a few trays from the bottom of the tower. These heavier alcohols are typically called fusels and need to be withdrawn or they can contaminate the ethanol or the water withdrawn from the system. The rectifier typically has 25 to 30 trays. The side stripper is basically a beer tower minus the solids. It takes the water from the bottom of the rectifier and, with about 16 trays, strips out any remaining ethanol. Figure 1 shows a schematic of a typical fuels ethanol distillation unit.
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