Mixtures of liquids can often be separated by a process of evaporation and recondensation; this will concentrate one of them in the end product. The principle on which this process depends is that different liquids evaporate at different temperatures. This approach to separating mixtures is called distillation. Not only is it very important in chemistry labs, but it also plays a key role in the manufacture of products like alcoholic spirits, crude oil products and distilled water.
Distillation columns may be either batch-process or continuous. The former distills one batch of mixture at a time -- fresh batches are only introduced once the distillation of the first batch is complete. A continuous column, by contrast, constantly feeds in fresh mixture, and the unit handles a continuous feed stream without any interruptions. This type is more common in industry, since it can distill more of the mixture more rapidly.
A distillation column has multiple components, each with its own part to play. The reboiler provides the heat needed to vaporize the liquids in the mixture, while the condenser cools the vapor to turn it back into liquid. The reflux drum stores the condensate from the condenser so that some of it can be recycled into the column. The column itself contains trays or plates, which assist in the separation. The plates are housed within a vertical case called a shell.
Distillation columns typically use a process similar to the following. First, feed enters the column about midway up the vertical shell; it's introduced via a feed tray and flows down the column. At the bottom, a pipe conveys the feed into the reboiler, where it is heated to high temperatures. The vapor re-enters and ascends the column; once it exits at the top, it is cooled by a condenser and flows into the reflux drum. The reflux drum returns some of this material to the column as reflux, while the distillate will be removed.
This process either separates some components from the others or produces a distillate enriched in a particular component. The feed, for example, might be 15 percent ethanol; the resulting distillate would have a much higher ethanol content. Variations on the same basic theme are possible; the fractional distillation columns at oil refineries, for example, use multiple trays at different heights in the column to draw off components of the vapor that condense at certain temperatures. This way, the distillation process can separate the crude oil into its various components.
- Newcastle University, School of Chemical Engineering and Advanced Materials: Distillation
- Elmhurst College: Distillation Oil Refining
- "Chemical Principles: The Quest for Insight"; Peter Atkins, et al.; 2008
- Photo Credit Jupiterimages/Photos.com/Getty Images
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