Distillation


an introduction
© Copyright 1997-2018 by M.T. Tham and R.C. Costello, P.E.
Introduction
Types of Columns
Basic Equipment and Operation
Reboilers
Distillation Principles
Vapour Liquid Equilibria
Distillation Column Design
Effects of the Number of Trays or Stages
Factors Affecting Operation
Crossword
Other Resources
Copyright Information
Costello Distillation
EFFECTS OF THE NUMBER OF TRAYS OR STAGES
Here we will expand on the design of columns by looking briefly at the effects of
arrored.gif (1174 bytes) the number of trays, and
arrored.gif (1174 bytes) the position of the feed tray, and
on the performances of distillation columns.
  Effects of the Number of Trays
 

It can be deduced from the previous section on distillation column design that the number of trays will influence the degree of separation. This is illustrated by the following example.

Consider as a base case, a 10 stage column. The feed is a binary mixture that has a composition of 0.5 mole fraction in terms of the more volatile component, and introduced at stage 5. The steady-state terminal compositions of about 0.65 at the top (stage 1) and 0.1 at the bottom (stage 10) are shown below:


Composition Profile: 10 stages, feed at stage 5

Suppose we decrease the number of stages to 8, and keep the feed at the middle stage, i.e. stage 4. The resulting composition profile is:


Composition Profile: 8 stages, feed at stage 4

We can see that the top composition has decreased while the bottom composition has increased. That is, the separation is poorer.

Now, if we increase the number of stages to 12, and again introduce the feed at mid-column, i.e. stage 6, the composition profile we get is:


Composition Profile: 12 stages, feed at stage 6

Again, the composition has changed. This time the distillate is much richer in the more volatile component, while the bottoms has less, indicating better separation.

Thus, increasing the number of stages will improve separation.

  Effect of Feed Tray Position
 

Here we look at how the position of the feed tray affects separation efficiency. Suppose we have a 20 stage column, again separating a binary mixture that has a composition of 0.5 mole fraction in terms of the more volatile component. The terminal compositions obtained when the feed is introduced at stages 5, 10 and 15 (at fixed reflux and reboil rates) are shown in the following plots.


Composition profile: 20 stages, feed at stage 5


Composition profile: 20 stages, feed at stage 10


Composition profile: 20 stages, feed at stage 15


[Click on green button to see animated display of how the composition
profiles change with feed stage position
]

As the feed stage is moved lower down the column, the top composition becomes less rich in the more volatile component while the bottoms contains more of the more volatile component. However, the changes in top composition is not as marked as the bottoms composition.

The preceding examples illustrate what can happen if the position of the feed tray is shifted for this particular system. They should not be used to generalise to other distillation systems, as the effects are not straightforward.


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