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Mass Transfer - Engineering

Q1:

Inside the distillation column, the

A driving force for the vapour flow is the pressure drop.

B liquids are not always at their bubble points.

C pressure increases gradually from bottom to the top of the column.

D none of these

ANS:A - driving force for the vapour flow is the pressure drop.

The statement "driving force for the vapor flow is the pressure drop" is not entirely accurate in the context of distillation columns. In distillation, the driving force for vapor flow is primarily the difference in vapor pressure between the liquid and vapor phases, not the pressure drop along the column. Here's a more detailed explanation:

  1. Vapor Pressure Difference: In a distillation column, the liquid and vapor phases are in equilibrium at each stage. The vapor pressure of a component in the liquid phase depends on its temperature and composition. If the vapor pressure of a component in the liquid phase is higher than the partial pressure of that component in the vapor phase, it will tend to evaporate into the vapor phase. This difference in vapor pressures between the liquid and vapor phases creates a driving force for vaporization.
  2. Boiling Point Difference: Components in a liquid mixture have different boiling points. As the mixture is heated in the distillation column, components with lower boiling points vaporize first, rising to the top of the column. This process continues as the temperature increases along the column, with each stage having a slightly higher temperature than the stage below it. The difference in boiling points between the components creates a driving force for vapor flow.
  3. Pressure Drop: While there is indeed a pressure drop along the height of the distillation column due to factors such as gravity, frictional losses, and changes in vapor and liquid densities, this pressure drop does not directly drive the vapor flow. Instead, it affects the conditions under which vaporization occurs, such as boiling point elevation, but it does not serve as the primary driving force for vapor flow.
In summary, while there is a pressure drop along the distillation column, the primary driving force for vapor flow is the difference in vapor pressures between the liquid and vapor phases, which is influenced by factors such as temperature and composition gradients.