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Mass Transfer - Engineering

Q1:

A distillation column with N plates is being operated under normal conditions. At some point of time, the operation is shifted to total reflux condition (i.e., no product and residue are being withdrawn and feed to the column is stopped). At the new steady state,

A composition of vapors and that of liquid do not vary throughout the column.

B reboiler load and condenser load are minimum.

C the top and bottom compositions are unchanged with and without total reflux.

D the top and bottom compositions correspond to the maximum enrichment available.

ANS:D - the top and bottom compositions correspond to the maximum enrichment available.

At the new steady state with total reflux condition in a distillation column, the following statements hold true: The top and bottom compositions correspond to the maximum enrichment available. Explanation:

  • Total reflux condition means that all the vapor leaving the top of the column is condensed and returned to the column as liquid reflux. This results in the liquid continuously cascading down the column, undergoing multiple equilibrium stages.
  • Under total reflux, the composition of the liquid and vapor at each stage of the column reaches equilibrium with each other. As a result, the concentration of components in both the liquid and vapor phases becomes uniform throughout the column.
  • The top of the column, where the most volatile components are enriched, reaches its maximum possible enrichment under total reflux conditions. Similarly, the bottom of the column, where the least volatile components are enriched, also reaches its maximum possible enrichment.
  • The reboiler load (heat input to the bottom of the column) and the condenser load (heat removal from the top of the column) are at their minimum values under total reflux conditions since no product is being withdrawn from the column, and all the energy input into the system is used for maintaining reflux.
  • Therefore, the top and bottom compositions correspond to the maximum enrichment available under total reflux conditions, and they remain unchanged throughout the column.