Co-current absorber design — Which quantity does NOT need to be fixed in advance before starting the preliminary design of a co-current gas absorber?

Introduction:
Absorber design begins with setting flows, compositions, and performance targets. Thermal effects can influence solvent temperature and equilibrium, but early sizing commonly assumes adiabatic operation or estimates heat effects later. This question checks which parameter need not be fixed before preliminary sizing of a co-current absorber.

Given Data / Assumptions:

• Co-current gas and liquid flow with specified inlet compositions.
• Preliminary design stage: determine size (height/area) to meet outlet specs.
• Heat effects are typically secondary and may be approximated as adiabatic initially.

Concept / Approach:

Essential inputs are gas and liquid flow rates and required removal (outlet specs). These determine operating line and driving force. Heat of absorption and sensible heat changes can be included later to refine temperature profiles. Therefore, a fixed value for heat gain/loss is not mandatory at the outset, although it becomes important in detailed design, especially for exothermic absorptions (e.g., SO2 into alkaline solutions, CO2 into amines).

Step-by-Step Solution:

Verification / Alternative check:

Standard design procedures (NTU/HTU or stage-based) start with mass balances and equilibrium; temperature corrections are incorporated after initial sizing, confirming that fixed heat gain/loss is not prerequisite.

Why Other Options Are Wrong:

B and C are essential throughputs. E (target spec) is fundamental to define the separation duty. D is incorrect because one quantity (heat gain/loss) truly need not be fixed initially.

Common Pitfalls:

Ignoring heat effects entirely; while not fixed initially, they should be revisited to avoid underestimating solvent rates or packing height for strongly exothermic systems.

Final Answer:

heat gain or loss