ANS:B - number of plates increases.

When the reflux ratio decreases in a distillation column, the number of plates required to achieve the same separation increases. This is because the reflux ratio plays a critical role in the efficiency of the distillation process, particularly in terms of the degree of separation achieved per theoretical plate. Here's why the number of plates increases as the reflux ratio decreases:

1. Effect on Separation Efficiency: The reflux ratio is defined as the ratio of the amount of reflux (liquid returned to the column) to the amount of distillate (product) withdrawn from the column. A higher reflux ratio means that more liquid is being returned to the column, resulting in greater contact between the liquid and vapor phases on each tray. This increased contact enhances the separation efficiency of the column, allowing for better separation of components.
2. Role of Theoretical Plates: The theoretical plates in a distillation column are the stages or trays where vapor and liquid come into equilibrium, facilitating the transfer of components between the vapor and liquid phases. Each theoretical plate represents a stage of separation, with higher numbers of plates typically leading to better separation efficiency.
3. Impact of Reflux Ratio: When the reflux ratio decreases, less liquid is returned to the column, resulting in reduced liquid holdup on each tray. With less liquid available for contact with the rising vapor, the efficiency of each theoretical plate decreases. As a result, more theoretical plates are needed to achieve the same degree of separation between the components.
4. Increased Number of Plates: Therefore, as the reflux ratio decreases, the number of plates required to achieve the desired separation increases. This is necessary to compensate for the reduced separation efficiency per plate and ensure that the desired product purity is maintained.