Introduction:
Distillation columns are commonly used in various industries for separating mixtures based on their boiling points. The efficiency of a distillation column depends on various factors, including the design of the trays or plates present in the column. Bubble caps are one type of tray design used in distillation columns. The number of bubble caps per tray plays a crucial role in determining the column's performance.

Explanation:
The number of bubble caps per tray primarily depends on the allowable gas and liquid velocities. Let's understand why this is the correct answer.

Gas Velocity:
Gas velocity refers to the speed at which the vapor flows through the column. It is important to maintain an optimal gas velocity to ensure proper vapor-liquid contact. If the gas velocity is too low, the vapor may not effectively interact with the liquid, leading to poor separation. On the other hand, if the gas velocity is too high, it can cause flooding, which hampers the separation efficiency. The number of bubble caps per tray is determined based on the allowable gas velocity to ensure proper vapor-liquid contact.

Liquid Velocity:
Liquid velocity refers to the speed at which the liquid flows through the column. Similar to gas velocity, maintaining an appropriate liquid velocity is crucial for efficient separation. If the liquid velocity is too low, it can result in stagnant liquid pools on the tray, reducing the contact between the vapor and liquid phases. Conversely, if the liquid velocity is too high, it can cause liquid entrainment, where the liquid is carried away with the vapor, again leading to poor separation. The number of bubble caps per tray is also influenced by the allowable liquid velocity.

Gas and Liquid Velocities:
Both gas and liquid velocities are interrelated and affect the column's performance. The design of the bubble caps and the tray layout should be selected to ensure that the gas and liquid velocities are within the allowable range. The number of bubble caps per tray is determined based on these allowable velocities to ensure efficient vapor-liquid contact and separation.

Feed Composition:
While the feed composition does impact the distillation process, it is not the primary factor influencing the number of bubble caps per tray. The feed composition affects the column's operating conditions, such as temperature and pressure profiles, reflux ratio, and tray efficiency. However, the number of bubble caps per tray is primarily determined by the allowable gas and liquid velocities.

Conclusion:
In distillation columns, the number of bubble caps per tray primarily depends on the allowable gas and liquid velocities. Optimal gas and liquid velocities ensure efficient vapor-liquid contact and separation. While feed composition does impact the overall separation process, it is not the primary factor influencing the number of bubble caps per tray.