ANS:B - use of increased tray spacing.

Increasing tray spacing refers to increasing the vertical distance between individual trays or plates within a distillation column. Each tray or plate in a distillation column provides a stage for the separation of components in a mixture. Here's how increasing tray spacing can affect the tower diameter:

1. Enhanced Mass Transfer: Increasing tray spacing allows for more vertical distance between trays, providing more space for the vapor and liquid phases to interact. This can enhance mass transfer between the phases, improving separation efficiency. With better mass transfer, the column can achieve the same separation with fewer trays, reducing the overall height of the column.
2. Reduced Pressure Drop: A larger tray spacing can reduce the pressure drop across the column because there are fewer trays in the same height of the column. This can be beneficial for reducing energy consumption and operating costs.
3. Decreased Tower Diameter: By improving separation efficiency and reducing the number of trays required, increasing tray spacing can lead to a smaller tower diameter for the same separation task. This is because the increased tray spacing allows for a more efficient use of column height, effectively reducing the number of theoretical stages required to achieve the desired separation.
4. Design Considerations: However, increasing tray spacing needs to be balanced with other design considerations such as maintaining adequate residence time for separation, avoiding flooding or weeping, and ensuring proper liquid and vapor distribution across the trays. Additionally, larger tray spacing may require modifications to tray designs or operating conditions to optimize performance.

Overall, increasing tray spacing can be a strategy to reduce the tower diameter in a distillation column while maintaining or improving separation efficiency, but it must be carefully evaluated in the context of specific process requirements and design constraints.