Which of the following quantities need not be fixed before starting the design of a co-current absorber ?

ANS:B - Flow rate of entering liquid

The "flow rate of entering liquid" refers to the rate at which the liquid phase (solvent) is introduced into the absorber unit of a gas-liquid absorption system. In the context of an absorber, which is typically a packed column or tower, the liquid flows downward while the gas flows upward, facilitating the contact between the two phases for mass transfer to occur. Explanation:

1. Importance:
   • The flow rate of the entering liquid is a crucial parameter in the design and operation of an absorber. It directly affects the efficiency and effectiveness of the absorption process.
   • The rate at which the liquid is introduced determines the residence time of the liquid within the absorber and thus the contact time between the gas and liquid phases.
   • Properly controlling the flow rate of the entering liquid is essential for achieving the desired level of solute removal or absorption efficiency.
2. Contact Time:
   • The flow rate of the entering liquid influences the contact time between the gas and liquid phases. Higher flow rates result in shorter residence times and less contact between the phases, while lower flow rates allow for longer contact times.
   • Optimal contact time is necessary for effective mass transfer to occur, ensuring that sufficient solute is absorbed from the gas phase into the liquid phase.
3. Absorption Efficiency:
   • The flow rate of the entering liquid also affects the absorption efficiency of the system. Higher flow rates may lead to higher throughput but could result in reduced absorption efficiency due to insufficient contact time.
   • Conversely, lower flow rates may improve absorption efficiency but could reduce the overall capacity or throughput of the absorber.
4. Design Considerations:
   • The flow rate of the entering liquid is typically determined based on factors such as the required absorption capacity, desired separation efficiency, and the properties of the gas and liquid streams.
   • Design considerations also include hydraulic constraints, such as pressure drop limitations and flooding considerations, which may influence the selection of the optimal flow rate.

In summary, the flow rate of the entering liquid is a critical parameter in the design and operation of an absorber, as it directly impacts the contact time between the gas and liquid phases, absorption efficiency, and overall performance of the system. Adjusting this flow rate is essential for optimizing the absorber's performance to meet process requirements.