Process Equipment and Plant Design
The function of manholes provided in the shell of a distillation column is to

Give access to the individual trays for cleaning, maintenance and installation
All of these
Keep a check on the liquid gradient over the plate by direct visual observation
Guard against foaming & entrainment by dumping antifoaming agent through it

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Process Equipment and Plant Design
Smoker's equation for the calculation of number of equilibrium stages in a continuous binary distillation column is used, when the

Relative volatility is close to one (e.g., separation of close boiling isomers)
Number of equilibrium stages required is likely to be very small
Feed is not at its bubble point
Number of equilibrium stages in only stripping section is to be calculated

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