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#1 2016-07-16 11:53:10

Registered: 2016-07-12
Posts: 11

What is the purpose of plates in a distillation column?

A plate is a physical place in the column where the liquid/vapor can establish equilibrium, and the important point here is that the equilibrium point will be DIFFERENT for each plate. That's the whole point of having a column with plates (or packing).

Let's look at an example. If your distillation apparatus consisted of only the boiler and the condenser (equivalent to one plate system), and you run one distillation cycle, you could distill 10% ethanol to perhaps 40% ethanol, and that will happen a certain temperature. Why is this? Because it's a limitation of, and governed by, the liquid/vapor equilibrium data for the mixture.

Next, we could take that 40% mixture and run it again through your single plate apparatus, and lookng at the liquid/vapor equilibrium data for the mixture, this time, we could distill it to about 60% ethanol, and the most important point to understand is that this is all happening at a different temperature and different equilibrium point on the chart. The liquid concentrations are different than before, the vapor concentrations are different than before, and the temperature is diferent than before.

What colum plates do is they establish these needed equilibrium zones at the needed different conditions as the vapor moves up the column and the liquid moves back down the column. Hotter conditions at the bottom of the column vaporize ethanol and water, and cooler conditions at the top condense the water while the ethanol remains a vapor to pass to the condenser.

As the concentration of ethanol rises at each stage, the mixture approaches its azeotrope. A water/ethanol mixture reaches it's azeotrope at 95.6% ethanol. An azeotrope is a mixture of two or more liquids in ratios that its composition cannot be changed by simple atmospheric distillation. In other words, the vapor concentrations are exactly the same as the liquid concentrations that you started with. It means that condening the vapor leaves you with exactly the same concentration as the original liquid, and therefore boiling/distillation accomplishes nothing. This mixture cannot be seperated further by simple atmospheric distillation.


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