Distillation - Principles of Chemical Separations with Environmental Applications - page 86

4.2 Background

Distillation is the most widely used separation method in the chemical and petrochemical
industries. It is not usually considered initially for environmental applications but can serve
as a useful approach. It is, however, frequently utilized to recover the mass-separating agent
for recycle back into the system following absorption, stripping and extraction processes.
Some environmental applications of distillation are the separation of organic solvent/water
mixtures and water removal for volume reduction prior to disposal of hazardous waste
mixtures. In addition, the principles used to design and analyze distillation columns are
also applicable to other separation methods (extraction, absorption, stripping). So, the
knowledge gained from the subject matter in this chapter will be applied in the study of
these other methods.
Distillation is an equilibrium-limited separation which uses heat as an energy-separating
agent. It is applied when two or more relatively volatile liquids, that vaporize at different
temperatures, need to be separated or fractionated into almost pure product streams. Dis-
tillation separates components of a liquid mixture based on their different boiling points.
When the boiling points of the entering species are significantly different, distillation can
easily separate the feed into almost pure product streams of each component. However,
as the boiling points become closer, distillation requires a large number of equilibrium
stages to perform the separation.
Distillation is the baseline process for the chemical process industry, with 40,000
columns in operation in the US, handling 90-95% of all separations for product recovery
and purification. The capital invested in distillation systems in the US alone is at least
$8 billion [1].
Because it is prevalent throughout industry, there are numerous advantages to distillation
as a separation technology. The process flow sheets are relatively simple and no mass-
separating agent is required. The capital costs are low as is the risk associated with lesser
known technologies. There is an abundance of data describing vapor-liquid equilibrium
for many systems. Usually, distillation can be designed using only physical properties
and vapor-liquid equilibrium (VLE) data, so scale-up is often very reliable. The primary
disadvantage of distillation is its lack of energy efficiency. It is not a useful technique for
systems containing an azeotrope or those with close boiling points. An azeotrope occurs
when the vapor and liquid compositions of a system become identical such that, without
altering the system, no further separation is possible. Distillation also cannot be applied
to feed streams which are sensitive to thermal degradation or that polymerize at elevated
temperatures. Operating the column under vacuum, however, can reduce or eliminate these
problems.
The separation in a distillation process is governed by a difference in the composition of
a liquid and vapor phase. This difference is usually characterized by a difference in actual
vapor pressures, or volatilities, of the liquid-phase components. Vapor-liquid equilibrium
data for the mixture components are, therefore, an important element for design and