As we discussed in the first chapter, distillation is based on the fact that the vapor from a boiling
mixture will be richer in the components that have lower boiling points.
Therefore when this vapor is cooled and condensed, the condensate will contain more of
the volatile components. At the same time, the original mixture will contain more of the less
volatile material. Distillation columns are designed to achieve this separation. Distillation
is the most common separation technique in the process industries. The major drawback is that
distillation consumes enormous amounts of energy, both in terms of cooling and heating.
As shown in Figure 1, a simple distillation apparatus consists essentially of three parts:
a pot with an outlet tube for the vapor to pass,
a condenser above the pot which cools the vapor, returning it back to a liquid stream,
and a heat source which causes the initial mixture to vaporize. In the simplest case,
the substance with the lowest boiling point vaporizes first and the temperature remains
constant until that substance has completely evaporated. The vapor passes into the
condenser where, when cooled, condenses to a liquid, which is collected in another vessel.
This overhead product is called the distillate.
The substance having a higher boiling point remains in the original pot as the residue.
Such a perfect separation is never achieved.
Some of the lower volatile material also vaporizes and ends up in the distillate,
and some of the higher volatile material remains in the original pot.
The above description is for a batch process. The original mixture is added to the distillation
pot as a single batch and the process proceeds to a desired endpoint. The contents of the pot
and the distillate are removed and another batch is processed.
The example on the previous page is for a batch process.
It is also possible to operate the single pot in a continuous manner, as shown in Figure 2.
This is known as flash distillation. The feedstream is fed continuously to the distillation
pot, and the residue and distillate are also removed in a continuous manner.
In either batch or flash distillation, only a single equilibrium separation is achieved.
What if we desire a distillate in a more pure form? This can be done by taking the distillate
from the first pot and passing it to a second pot. The vapor from this pot will be even more
concentrated in the more volatile component. We can condense this vapor and pass it on to
a third pot, and so on, until the distillate meets our purity specification.
Let's look at the sequence of steps on the equilibrium diagram for ethanol-water.
Suppose the initial ethanol-water mixture is 25% ethanol and 75% water.
The ethanol concentration of the vapor of this first stream is 56%, as we see on the
equilibrium curve. If we condense this vapor to a liquid and feed it to a second pot, the liquid
mixture is also 56% ethanol. What is the concentration of ethanol in the vapor in the
second pot? We can see this easily on the equilibrium curve by using the 45° line.
Remember that the 45° line is nothing more than the point where the vapor and liquid
concentrations are the same. If we draw a horizontal line from the equilibrium curve to the
45° line, this point now represents the liquid mixture in the second pot.
By drawing a vertical line up to the equilibrium line, we can determine what is the ethanol
concentration in the vapor in the second pot. The concentration is 68%.
What about the third pot? We draw the horizontal line to the 45° line and the vertical line
to the equilibrium line and find our answer: 75%.
This procedure can be repeated for any number of pots. Each one of these steps is called a
stage. It takes three stages to produce a product with an ethanol concentration of 75%
from a liquid mixture with an initial ethanol concentration of 25%.
Instead of using a sequence of pots we can essentially do the same thing in a distillation
column. A distillation column is nothing more than a series of pots stacked on top of one
another with continuous flow from one pot to another.
Each of these pots are also called a stage in the column. In practice, the separation is either
done with trays or packing instead of pots. We can think of a tray or a certain height of packing
as a stage in the process. The distillation column is a continuous process. It has a
feedstream containing the original liquid mixture. From the top of the column, the distillate
containing the more volatile components is removed. From the bottom of the column,
the residue containing the less volatile components is removed.