MECH6019 Chap.9 Absorber Design and Sulphur Control
Absorber Design and Sulphur Control
Gas absorption transfers a soluble pollutant from gas into liquid across an interface. Solubility is necessary, but useful equipment also needs interfacial area, a concentration driving force and time for transfer. Packed towers supply area while gas and liquid pass through void spaces.
Pressure drop, wetting, fouling and flooding determine whether the contact can be maintained at the selected rates.
Countercurrent flow sends gas upward and liquid downward. Fresh solvent meets the cleanest gas at the top, while increasingly rich liquid meets stronger gas toward the bottom. This arrangement helps preserve driving force along the column.
Crossflow or plate equipment creates different contact patterns, but every design still needs an overall pollutant balance and an equilibrium relationship.
Begin with solute entering and leaving the gas. Their difference is absorbed into liquid or consumed by reaction. On a dilute solute-free basis, the gas-side removal equals the liquid flow multiplied by its change in loading.
This balance sets the required carrying duty before a transfer coefficient or tower height is selected. If removed solute cannot be found downstream, the boundary is incomplete.
The operating line represents the material balance between gas and liquid compositions. The equilibrium relationship represents the limiting gas composition associated with a liquid loading. Their separation provides transfer driving force.
At minimum liquid rate a pinch is approached and the required contact becomes impractically large. Real operation uses more solvent while remaining below hydraulic and regeneration constraints.
Sulphur control adds reaction chemistry. Reduced sulphur compounds may be absorbed and regenerated, while sulphur dioxide can be scrubbed with alkaline reagent or limestone systems.
Reaction can strengthen absorption by consuming dissolved pollutant, but it introduces reagent demand and products. Oxidation state influences recovery, saleable-product possibilities and disposal requirements.
Use a staged solution: close the gas balance, calculate liquid loading, test equilibrium feasibility, estimate transfer requirement and check hydraulics.
Then add regeneration energy, reagent consumption, solids or wastewater and pressure drop. A gas-side removal percentage is only one part of the answer; the complete design shows where every mole of sulphur goes and whether all constraints meet at one operating point.
What this chapter covers
- 01
Gas solubility
- 02
Countercurrent packed towers
- 03
Operating line
- 04
Mass-transfer driving force
- 05
Sulphur oxidation state
- 06
Wet scrubbing
Worked example · free
Close a dilute absorber balance
- 1Find 2.0 kmol/h inlet solute.
- 1Find 0.2 kmol/h outlet.
- 1Subtract to obtain 1.8 kmol/h.
Key terms
- Absorption
- Transfer of a soluble gas constituent into liquid.
- Operating line
- Material-balance relation between gas and liquid compositions.
- Packing
- Tower material that provides gas-liquid contact area.
- Rich solvent
- Liquid leaving absorption with increased solute loading.
Absorber Design and Sulphur Control FAQ
Why use countercurrent contact?
Fresh solvent meets the cleanest gas while progressively richer liquid meets stronger gas, preserving a useful driving force along the tower. The arrangement does not remove equilibrium or hydraulic limits, so solvent rate, packing and approach to flooding must still be checked together.
What must the first balance show?
Pollutant entering, leaving and absorbed must close. The removed amount then sets liquid loading or reaction-product demand. Only after this overall balance is secure should equilibrium, transfer coefficients and tower height be introduced, because those relations cannot repair a missing solute stream.
Why track sulphur after capture?
Scrubbing moves or reacts sulphur rather than making it disappear. Regeneration, recovered product and disposal streams affect the complete environmental burden. Reagent consumption, liquid loading and reaction-product quantities should all reconcile with the gas-side removal balance.
Exam move
Draw gas and liquid arrows before writing equations. Close the overall solute balance first, then introduce equilibrium and mass-transfer relations. Label dirty gas, clean gas, fresh solvent and rich liquid before inserting compositions. Calculate solute entering and leaving the gas, and require the difference to appear in liquid loading or reaction products.
Use a dilute solute-free basis only when its approximation is stated. Next sketch equilibrium and operating relationships and point to their separation as the driving force. Explain why minimum liquid rate approaches a pinch and why an actual design uses more solvent. Add hydraulic limits: higher liquid or gas rate can increase pressure drop and approach flooding even while mass-transfer capacity appears better.
Compare packed and alternative contact equipment through area, flow pattern and operating constraints rather than labels alone. For sulphur control, map oxidation state, reagent demand and the fate of captured sulphur. Separate physical absorption, chemical consumption and regeneration because they may use different loading measures. Work one sensitivity check for warmer solvent, lower liquid rate and higher inlet loading.
In the final pass, follow a fixed order: gas balance, liquid loading, equilibrium feasibility, transfer requirement, hydraulics and residual handling. If any earlier step fails, do not continue to a tower dimension. This sequence makes the design auditable and keeps removal percentage connected to a real downstream stream.
