ANS:B - avoid channelling

Very tall packed towers are divided into series of beds primarily to avoid flooding. Flooding occurs when the liquid flow rate in the packed bed exceeds its capacity to efficiently handle the flow. This leads to the accumulation of liquid in the packing, which hinders the contact between the gas and liquid phases and reduces the efficiency of the mass transfer process. Dividing the tower into multiple beds allows for better liquid distribution and ensures that each bed operates within its optimal range of liquid flow rates, thereby reducing the likelihood of flooding. Additionally, it facilitates better control over the flow dynamics and enhances the overall performance of the packed tower. While it may indirectly help in reducing pressure drop, avoiding channeling, and reducing liquid hold-up, the primary purpose of dividing the tower into beds is to prevent flooding. Certainly! Let's break down the explanation further:

1. Flooding: Flooding is a phenomenon that occurs when the liquid flow rate through a packed tower becomes too high, overwhelming the tower's ability to effectively handle the flow. When flooding occurs, liquid accumulates in the packed bed, impeding the flow of gas and reducing the efficiency of mass transfer. This can lead to decreased separation efficiency and compromised tower performance.
2. Division into Beds: Dividing a very tall packed tower into a series of beds helps mitigate the risk of flooding. By dividing the tower into multiple beds, the liquid flow can be distributed more evenly across each bed, reducing the likelihood of any single bed becoming overwhelmed with liquid. Each bed operates within its optimal range of liquid flow rates, allowing for better control over the flow dynamics and preventing flooding.
3. Better Liquid Distribution: Dividing the tower into beds allows for better liquid distribution throughout the packed bed. This ensures that each section of the tower receives an adequate amount of liquid, preventing localized flooding and promoting efficient mass transfer between the gas and liquid phases.
4. Optimal Operation: Ensuring that each bed operates within its optimal range of liquid flow rates is crucial for preventing flooding. When the liquid flow rate exceeds the capacity of a single bed, flooding can occur, leading to reduced efficiency and performance. Dividing the tower into beds helps maintain the proper balance of liquid flow rates, thereby avoiding flooding and ensuring optimal tower operation.

In summary, dividing very tall packed towers into series of beds primarily helps avoid flooding by distributing liquid flow more evenly, ensuring optimal operation of each bed, and promoting efficient mass transfer. While it indirectly aids in reducing pressure drop, avoiding channeling, and reducing liquid hold-up, the primary goal is to prevent flooding and maintain tower performance.