[rajmehta]

What is the effect of changing operating pressure of  distillation column on column diameter ?

[fledarmus]

Well, if your distilling column is made out of thin tygon or latex tubing, reducing the pressure will suck it all in so there is only a very narrow path through, and increasing the pressure will make it inflate and increase the diameter. If it is made out of a rigid material like glass or steel, changing the pressure won't change the diameter at all. 

But I don't think that is what you were intending to ask. To answer the question I think you were asking, try answering these questions -

1) what effect do changes in pressure have on the differences in boiling points between your desired compound and your impurity?

2) What does this do to the efficiency of your separation, all other things being equal?

3) What effect does changing the column diameter have on the efficiency of your separation?

[baem84]

What is the effect of changing operating pressure of  distillation column on column diameter ?

Pressure change in column will effect to temperature profiles. Separation of mixture much depending on temperatures which have been calculated & designed initially. You will facing composition changes either top or bottom product. Beside, at lower pressure will cause weeping on tray & subsequently loss of separation ability.

TQ