hmmmmm, chemical engineering

Distillation is a liquid-liquid separation technique based on differences in boiling points. Separation of a mixture of two components is called binary distillation, three components ternary distillation, and so on.

Several other fluid separation techniques exist, such as absorption, adsorption, and liquid-liquid extraction. However, distillation is by far the most common separation process. The importance of distillation as a unit operation in the chemical process industry (CPI) is enormous. For instance, consider the petrochemical industry: crude oil is catalytically cracked into lower molecular weight organic chemicals that in turn are distilled to form pure reactants for all sorts of products (gasoline, solvents, plastics. Industrial gases are generally obtained by cryogenic distillation; a low temperature distillation technique in which the process gas is cooled down below the boiling point of its components. Air is separated in this way to form oxygen, nitrogen, and several inert gases (many billion pounds per year production).

A typical schematic for a (continuous) distillation column is given in the figure. The feed enters the column at the feed tray on a chosen height in the column. The liquid trickles down past several separation stages, where it comes into contact with its vapor. The separation stages consist of contacting plates or bubble caps. Contact between liquid and vapor is essential, since vapor-liquid equilibrium (VLE) is the fundamental principle for separation. The liquid stream flows down through the bottom section of the column (called the stripping section). The product with the lowest boiling point is collected as bottoms, but part of the liquid is vaporized in a condenser, and re-enters the column. The condenser supplies the energy input to the system, required for separation. The vapor follows its way through the top section, called the Rectifying section, and enters a reflux drum. In the drum, the vapor is cooled down to liquid. Part of the product is returned to the column (called the reflux), and the remainder of the (high boiling point) product leaves the column as distillate.

             __|__     _|_
            |     |___|~~~| Reflux drum
            |     |   |___|
 Rectifying |_____|     |__________
  Section   |_____|      Distillate
      Feed  |_____|
 Stripping  |_____|
  Section   |     |_______
            |     |     _|_
            |_____|    |~~~| Condenser
               |       |___|

Batch distillation, such as it is often applied used for small scale operations, sensitive or expensive products, or noncontinuous feeds generally works on the same principle. In this case there is no feed stream, but a large heated bottom flask. Separation is usually carried out with a high surface area column packing, and distillate is collected in a similar way to continuous distillation. The bottoms product remains inside the bottom flask until bottoms or distillate meet the required product purity.

Distillation is based on the chemical equilibrium stage concept. The liquid to be separated is in contact with its corresponding vapor at any location inside the distillation column. At each point, the vapor stream and liquid stream are in equilibrium (VLE), i.e. the pressure, temperature and chemical potential of the liquid and vapor phase are equal. The equilibrium conditions are governed by the Gibbs Phase Rule.

Consider a mixture of ethanol (EtOH, alcohol) and water (w) at an equilibrium stage inside the distillation column. A vapor stream (V) flows upward, contacting a downward flowing liquid stream (L). The concentrations of the liquid phase are given by xw and xEtOH. The concentrations of the vapor phase are given by yw and yEtOH. At the given operating conditions (e.g. T= 85.3 C, P= 1 atm.), the mole fractions (concentrations) of the liquid phase are: xEtOH= 0.1238 (12.38%), xw= 1 - 0.1238 = 0.8762 (87.62%). The corresponding vapor phase compositions in equilibrium with the liquid phase are: yEtOH = 0.4704, yw = 0.5296 (Note that the mole fractions in each phase add up to 1, or 100%).

     L              yW=0.5296
     ↓              ↑
    ___________________ Equilibrium Stage

     ↓              ↑
     xEtOH=0.1238   V

As we can see, at this equilibrium stage, the ethanol concentration in the vapor phase is significantly higher than in the liquid phase. We have enriched the vapor stream with ethanol. The vapor stream flows up and interacts with more liquid at a higher equilibrium stage. The vapor stream leaving this stage will have an even higher ethanol concentration. The purity of the product (bottoms and distillate) can be improved by adding sufficient equilibrium stages for separation.

It is important to note that it is impossible to obtain a 100% purity of products. As the purity of the product goes up, the efficiency of the separation becomes less, em>e.g. it is more difficult to increase a concentration from 90% to 95% than it is from 50% to 55%. For 100% product purity, an infinite number of equilibrium stages is required. Another complication is the interaction of the liquid components called an azeotrope. For instance, an azeotrope is the reason why a binary mixture of ethanol and water normally can't be separated beyond 89% alcohol content.