02 · Recovery vs Thermal Oxidation
Recovery or destruction: Genosorb® vs thermal oxidation
Thermal oxidisers — regenerative (RTO), recuperative, catalytic and direct-fired — meet an emission limit by burning the VOC to CO₂ and water. Recovery meets the same limit and returns the solvent to production. Here is how they compare on the points plant engineers ask about.
| Parameter | Absorption — Genosorb® | Thermal oxidation (RTO / recuperative) |
|---|---|---|
| What happens to the solvent | Recovered as liquid and reused | Destroyed — burned to CO₂ and water |
| Efficiency | 95–99.9% recovery | 98–99.99% destruction (design) |
| Operating temperature | Ambient, 20–30 °C — the vent is cooled to 30 °C or below before it meets Genosorb®; no preheating | About 650–1,100 °C; catalytic units run several hundred degrees cooler |
| Fuel | No burner fuel; steam for regeneration | Burner fuel needed below roughly 3–10% of LEL |
| CO₂ from the solvent | None — about 1.3 kg CO₂-eq avoided per kg by replacing fresh solvent | 1.4–3.3 kg CO₂ per kg of solvent (Table 3), plus fuel CO₂ |
| Chlorinated or sulphur solvents | Recovered as solvent (MDC case: 99% at 950 kg/hr) | Form HCl, Cl₂ or SO₂; an acid-gas scrubber can double the cost |
| Combustion by-products | None — no combustion | NOx and CO from the burners |
| Inlet concentration | Operates below 20% of LEL | Kept below 25% of LEL (up to 50% with continuous LEL monitoring) |
| Variable flow | Designed for maximum flow; runs at lower flows without a problem | Handles variable flow poorly (recuperative and direct-fired) |
| What you get back | Solvent, worth its purchase price | Heat, usable for steam or hot water |
| Capital cost | Depends on flow rate and solvent concentration | Depends on flow; a scrubber adds cost on chlorinated streams |
| Counts towards a % recovery target | Yes | No — burned solvent is not recovered |
Oxidiser figures: US EPA Air Pollution Control Cost Manual and control-technology fact sheets. Recovery figures: CCDC project data.
How thermal oxidisers work
- Direct-fired / flare — simplest and cheapest to build; no heat recovery, so the highest fuel cost.
- Recuperative — a heat exchanger recovers 40–70% of the heat; going higher risks auto-ignition in the exchanger.
- Regenerative (RTO) — ceramic beds recover 95% or more of the heat; about 80% of recent oxidiser installations are RTOs.
- Catalytic — runs several hundred degrees cooler, but halogens, sulphur, phosphorus and heavy metals poison the catalyst.
Concentration decides the oxidiser’s fuel bill
An oxidiser has to hold its chamber at temperature. On a lean vent the burners supply that heat; only above a few percent of the lower explosive limit (LEL) does the solvent itself keep the chamber hot — and then the solvent is the fuel.
| Concentration | % of LEL | Thermal oxidiser | Genosorb® recovery |
|---|---|---|---|
| Below ~2 g/Nm³ | under ~3% | Burns gas continuously | Concentrate first with a rotary concentrator, then absorb |
| ~2–6.5 g/Nm³ | ~3–10% | Nears self-sustaining; the solvent pays the fuel bill and is lost | Recovered directly |
| ~6.5–13 g/Nm³ | ~10–20% | Surplus heat | Recovered directly; more solvent captured per Nm³ treated |
| Above ~13 g/Nm³ | over ~20% | Must be diluted below 25% of LEL or continuously monitored | No dilution. The stream is cooled to 30 °C or below and the heat is recovered |
Band edges are approximate, from the EPA’s 3–10% LEL self-sustaining range; real thresholds depend on the solvent, heat recovery and design. Very dilute streams: see low-concentration VOC recovery.
CO₂ released when common solvents are burned
| Solvent | Formula | kg CO₂ per kg burned | Also formed |
|---|---|---|---|
| Toluene | C₇H₈ | 3.34 | — |
| Hexane | C₆H₁₄ | 3.06 | — |
| Acetone | C₃H₆O | 2.27 | — |
| Isopropyl alcohol | C₃H₈O | 2.20 | — |
| Ethyl acetate | C₄H₈O₂ | 2.00 | — |
| Methanol | CH₄O | 1.37 | — |
| Methylene dichloride (MDC) | CH₂Cl₂ | 0.52 | 0.86 kg HCl — needs scrubbing |
Stoichiometric values from each molecular formula; the oxidiser’s fuel adds more CO₂.
When thermal oxidation is the better choice
Recovery is not always the answer. An oxidiser can be the better fit for:
- Very small or intermittent loads — when the recoverable solvent is worth too little to repay the equipment.
- Mixtures with no reuse value — many low-value components that cannot be separated or reused in the process.
- Odour and toxic traces — where the goal is destroying trace compounds rather than recovering solvent.
- Both together — recovery first for the valuable solvent, a small oxidiser to polish what remains. CCDC can supply this combination.
CPCB’s draft 95% recovery target
The draft guideline for pharmaceutical plants is a share of solvent recovered. Solvent burned in an oxidiser does not count towards it, however high the destruction efficiency, whereas Genosorb® can recover up to 99.9%. See VOC norms in India.