Distillation Fundamentals

In the chemicals processing industry, distillation is the most commonly employed separation technique.  Different chemicals have different boiling points.  After a simple flash, the vapor product contains more light compounds than does the liquid product.  A series of flash vessels can be employed to effect purifications of the light and heavy compounds.  Similarly, a series of equilibrium stages arranged vertically in a distillation column can effect simple and difficult separations.  Column operating pressure is an important variable as is column pressure drop.  A reboiler is required at the bottom of a column to boil the heavy liquid.  A condenser is required at the top of a column to condense the light liquid and create a reflux stream that flows down the column and cleans the upflowing vapor.  When the reflux flow rate is divided by the top (liquid) product rate, the resultant number is called the reflux ratio.  Generally, reflux requirements are a function of the relative volatilities of the light and heavy components.  Pumps, pipes, valves, meters, thermocouples and other instrumentation are all essential parts of a distillation unit.  Batch, azeotropic and extractive distillations are also employed industrially.