Abatement Scheme for Acetone in Dryer Off-Gas: 23-Chem-A5 Chemical Plant Design and Economics, December 2018, Q6

Question 6: Abatement Scheme for Acetone in Dryer Off-Gas (20 marks)

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Given / Find. A nitrogen dryer off-gas, saturated with water and acetone at 75–90 °C and near atmospheric pressure, must be treated so the discharged nitrogen contains <200 ppm acetone. The task is to propose a justified process scheme (an abatement and solvent-recovery train), not a numerical design.

Figure 6.1 — Proposed abatement/recovery train for the nitrogen dryer off-gas: chilled condensation (bulk removal) → water scrubber (deep removal) → activated-carbon adsorber (polish to <200 ppm), with condensate and scrub liquor sent to distillation for acetone recovery and the nitrogen recycled to the dryer.

Proposed scheme. A three-stage recovery-plus-polish train on the dryer off-gas, with the nitrogen recycled in a closed loop:

Chilled condensation (bulk removal). Cool the 75–90 °C saturated gas against chilled water/refrigerant. Most of the water and the bulk of the acetone condense out, sharply lowering the acetone loading before any polishing step. This recovers the majority of the solvent as a liquid at the lowest cost and shrinks every downstream unit.
Water scrubbing / absorption (deep removal). Contact the cooled gas counter-currently with water in a packed absorber. Acetone is completely miscible with water and highly soluble, so a modest water rate strips it to a few hundred ppm or below; this is the workhorse step that brings the gas near the 200 ppm limit.
Activated-carbon adsorption (final polish). Pass the scrubbed gas through an activated-carbon bed to adsorb the residual acetone and guarantee compliance below 200 ppm even during scrubber upsets. The bed is regenerated (steam or hot inert gas), and the desorbed acetone is returned to recovery.

Solvent recovery and nitrogen recycle. The condensate and the acetone-rich scrubber liquor are sent to a distillation column that recovers acetone overhead (reused in the process) and returns clean water to the scrubber. The polished nitrogen is recycled back to the dryer in a closed loop, with a small make-up and purge.

Reasons for selection. The scheme exploits acetone's key properties: it is fully water-miscible and highly soluble, so water absorption is cheap and effective; it is volatile, so condensation and steam-regenerable carbon both work well; and it is flammable (lower flammable limit ≈ 2.5 vol%), which is exactly why nitrogen—an inert—is used as the drying medium and why the nitrogen is recycled in a closed loop rather than vented: keeping oxygen out prevents a flammable acetone/air mixture and recovers the valuable, costly nitrogen. Staging the units from cheap bulk removal (condensation) through deep removal (scrubbing) to a guaranteed polish (carbon) minimises the size and cost of the expensive final stage while ensuring the <200 ppm limit is always met; recovering the acetone by distillation turns an emissions problem into a solvent-recovery credit.

Check: the exact split between the three stages depends on the acetone loading (fixed by the dryer solvent rate and the 75–90 °C saturation) and on the achievable chilled-condensation temperature; a full design would set the condenser temperature, the scrubber water rate and stages, and the carbon-bed cycle from a mass balance and the acetone–water vapour–liquid equilibrium. If solvent recovery were not economic, a thermal/catalytic oxidiser could replace stages 2–3, but it destroys (rather than recovers) the acetone and burns the nitrogen loop's inert blanket, so recovery is preferred here.

Concept & Theory

Topic. Volatile-organic-solvent emission control and recovery from an inert-gas dryer loop.

Key relations. Condensation removes solvent down to its saturation at the coolant temperature; absorption removes it per Henry's-law/VLE solubility (acetone–water is fully miscible); adsorption polishes to trace ppm; distillation recovers the solvent for reuse. Closed inert-gas loop keeps the vapour space out of the flammable envelope.

Why this works. Layering unit operations by cost and duty — cheap bulk condensation first, effective water absorption second, guaranteed carbon polish last — achieves a stringent ppm limit at minimum cost, while the water-miscibility and volatility of acetone make both absorption and regenerative recovery highly effective. Recycling the nitrogen both saves the inert and, by excluding oxygen, removes the fire/explosion hazard.

Common pitfall. Venting the nitrogen after treatment (wasting an expensive inert and admitting the risk of an acetone/air flammable mixture on the next cycle), or relying on a single unit — carbon adsorption alone would be swamped by the high inlet loading, and condensation alone cannot reach 200 ppm.

Source. Peters, Timmerhaus & West (equipment selection; drying); Sinnott & Towler, Ch. multi (absorption, adsorption, environmental control); Canadian practice per Environment and Climate Change Canada VOC guidance and provincial air permits.

Question 6 of 6: Abatement Scheme for Acetone in Dryer Off-Gas

Question 6: Abatement Scheme for Acetone in Dryer Off-Gas (20 marks)