Question

Why is HETP important? Why not just use theoretical plates to measure the efficiency of a fractional distillation head?

Answer 1

Background:

Some descriptions:

Theoretical plates are hypothetical areas of the fractionating column (width or height calibrated using known liquids) in which liquid and vapor phases of the same substance co-exist in equilibrium. More the number of theoretical plates, the more the efficiency of the distillation process

Distillation head in a simple fractional distillation apparatus is where the fractionating column joins the delivery tube that goes into the receiving flask.

HETP refers to Height Equivalent to Theoretical Plate. This term is used when instead of separate plates a single packing is used in the fractionating column because of some disadvantages of theoretical plates discussed below.

In a fractional distillation apparatus, the fractionating column provides beads or glass plates which increase the surface area for less volatile components to condense. Each time this happens, the vapors become enriched in the more volatile component. Also, in industrial fractional distillation, reflux is provided from top to bottom of theoretical plates to further separate the less volatile component from more volatile vapors and thus increase the efficiency of the distillation head.

Why HETP? Why not theoretical plates?

The vapors leaving the theoretical plate are always enriched in volatile components as compared to the vapors entering it. Further, the concentrations vary as one moves from bottom to top of the fractionating column. This makes the calculation of efficiency of distillation head a tedious process as the proportion of vapors entering and leaving each theoretical plate are not the same.

To overcome the above problem, a single packing material (such as resin) is used and its HETP value calibrated using a known mixture of liquids. The efficiency of the distillation head can be more easily predicted before the actual process is carried out.