Batch distillation is a process in which two liquids with different boiling points are heated, vaporized and then cooled to obtain a new product with a concentration higher in one of the two liquids. During a batch distillation, no new liquids are added to the heating vessel, so over time, the product concentration will change as the mixture is boiled and the final product is removed. Pilot plant operations, pharmaceuticals and specialty chemical applications commonly use batch distillation to control quality of small amounts of product.
A batch distillation system might contain a heating vessel or still, a distillation column that can be as simple as an empty pipe or a complex column with trays or packing, a liquid condenser that uses water or air cooling and final product collection lines and storage. The still heats the liquid mixture, which then travels up through the riser or column to the condenser. Water typically is used to cool the condenser, and some or all of the liquid condensate is permitted to fall back through the column, a technique known as reflux. Batch systems that return all of the condensed liquid to the column are referred to as total reflux systems.
One advantage of using tray columns for batch distillation is that the product composition is slightly different at each tray position. Product collection lines can be installed at each tray position, and a final product can be obtained by taking distillate from one tray or a number of trays. As the composition of the still changes during the run, the collection points can be changed to maintain a desired product quality.
A common application of batch distillation is the production of whiskey and other spirits. The fermented grain and water mixture is placed in the still and allowed to boil. Vapor resulting from the boiling fermented grain has a higher alcohol content than the grain mixture, because the alcohol has a lower boiling point than water and vaporizes to a greater extent than the water. The rising vapor condenses to a liquid that is a mixture of alcohol and water, and additional batch distillations of this alcohol-water mixture will raise the alcohol content even higher. The distillate with the desired alcohol content is collected, stored and aged to meet the distiller’s requirements.
High-value pharmaceuticals benefit from batch distillation processes. Product quality control is critical for drug production, and batch distillation allows small quantities to be manufactured and checked against product specifications without risking large quantities of off-quality product. Batch systems also provide an easier method for customizing drug production, because small changes can be made to the batch chemicals, or the distillation system can be adjusted for temperature, reflux or operating pressure to vary final product specifications.