ANS:A - small sizes.

The use of packed towers for distillation is generally limited to: vacuum distillation. Packed towers are often preferred over tray/column distillation for vacuum distillation processes due to their lower pressure drop characteristics. In vacuum distillation, operating at reduced pressures helps lower the boiling points of the components being separated, allowing for distillation at lower temperatures and reducing the risk of thermal degradation or undesired reactions. Packed towers provide a large surface area for mass transfer between the vapor and liquid phases, facilitating efficient separation even at the lower vapor pressures encountered in vacuum distillation. Additionally, the open structure of packed beds allows for better vapor-liquid contact and mass transfer compared to trays, making them well-suited for vacuum distillation applications. While packed towers can be used for multicomponent distillation and may be suitable for high-pressure operations in some cases, they are particularly advantageous for vacuum distillation due to their favorable pressure drop characteristics and efficient mass transfer capabilities under vacuum conditions.