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Mass Transfer - Engineering

Q1:

To get high tray efficiency

A interfacial surface between liquid and gas phase should be large.

B time of contact between the two phases should be less.

C gas velocity should be very low.

D liquid entrainment should be severe.

ANS:A - interfacial surface between liquid and gas phase should be large.

To achieve high tray efficiency in a distillation column, the following conditions should generally be met:

  1. Interfacial surface between liquid and gas phase should be large: A large interfacial surface area allows for greater mass transfer between the liquid and vapor phases, facilitating efficient separation of components. This is often achieved through the use of structured packing or trays with effective designs that promote turbulence and intimate contact between the phases.
  2. Time of contact between the two phases should be sufficient: Contrary to the statement, the time of contact between the two phases should be sufficient to allow for adequate mass transfer. This ensures that equilibrium is reached between the phases, allowing for efficient transfer of components. Therefore, a shorter contact time may lead to reduced efficiency as it limits the opportunity for mass transfer.
  3. Gas velocity should be moderate: While excessively high gas velocities can lead to increased liquid entrainment and poor efficiency due to flooding, very low gas velocities may result in reduced turbulence and inadequate mixing between the phases. Therefore, maintaining a moderate gas velocity is preferable for promoting efficient mass transfer and avoiding flooding.
  4. Liquid entrainment should be minimized: Liquid entrainment occurs when liquid is carried over with the vapor phase to the tray above, reducing separation efficiency. Minimizing liquid entrainment is crucial for achieving high tray efficiency. This can be accomplished by optimizing tray design, operating conditions, and ensuring proper liquid distribution across the tray surface.
In summary, to achieve high tray efficiency, it is essential to maximize interfacial surface area, ensure sufficient contact time between the phases, maintain moderate gas velocities, and minimize liquid entrainment. These factors collectively contribute to efficient mass transfer and separation in the distillation column.