03 · Absorption vs Refrigeration vs Adsorption
Non-destructive, non-reactive systems recover and recycle the VOC as a liquid or in concentrated form — nothing is burned or chemically converted. Three principal technologies exist:
Adsorption
Gas adsorbed onto activated carbon (or zeolite / molecular sieve) beds. Near-100% removal possible. Ketones and mixed streams are problematic — high heats of adsorption risk fire. Cyclic operation requires at least two beds. High maintenance; large footprint.
Refrigeration & Cryogenic
Reduces gas temperature until VOC condenses out. As concentration falls, cryogenic temperatures may be needed. Best for high-concentration, intermittent streams — tank farms, loading stations, marine unloading. Requires only power; smallest footprint.
Physical Absorption — Genosorb®
Our technologyA high-boiling PEG ether absorbent dissolves VOC at ambient conditions, then releases it in a stripping column for condensation and reuse. Ambient temperature, low pressure, no carbon beds, no periodic replacement. Works across polar, non-polar VOCs and gases such as CO₂, H₂S and SO₂.
Table 1 — Comparison among non-destructive technologies
| Parameter | Refrigeration | Absorption ✦ | Adsorption |
|---|---|---|---|
| Process | Condensation | Separation and condensation | Separation and condensation |
| Operating temp. | Very low temp. | Ambient or below ambient | Ambient and above |
| Pressure | Very low | Low | High |
| Vacuum | Not required | Optional | Required |
| Operations | Automatic / continuous | Automatic continuous | Automatic batch process |
| Maintenance | Low | Low | High |
| Power cost | On demand only | Even at no load | Even at no load |
| Installation cost | Very low | Low | Large |
| Conc. measurement | Not required | Required (outlet) | Required (outlet) |
| Space | Smallest | Small | Large |
| Steam | Not required | Required | Required |
| Inert gas | Not required | Special circumstances | Required for safety |
| Carbon replacement | Not required | Not required | Periodic replacement |
✦ Genosorb® physical absorption — recommended for mixed-solvent, continuous-process streams.
Operating cost per kg recovered: two Indian cases
| Stream | Absorption | Refrigeration | Carbon adsorption |
|---|---|---|---|
| Acetone — 487 m³/hr at 123 g/m³ | 2.27 | 5.70 | 7.47 |
| Ethyl acetate / toluene / MEK / cellosolve — 9,500 m³/hr at 97 g/m³ | 7.30 | — | 10.6 |
From “VOC recovery from gaseous streams” by Dr D M Mohunta, CCDC (2004). Costs are in 2004 rupees and show how the technologies compare with each other, not today’s prices. Read the original study.
In both cases absorption had the lowest operating cost. More on the acetone case in acetone recovery unit, and on mixed printing solvents in flexible packaging and gravure printing.
Which technology fits which stream
- Absorption (Genosorb®) suits continuous streams from low to medium concentration, mixed solvents, ketones and wet gas — the absorbent also dries the gas.
- Refrigeration and condensation suit high-concentration, intermittent streams such as tank farms and tanker loading.
- Carbon adsorption can reach very low outlet levels, but ketones and mixed streams risk bed fires and the beds need regeneration and replacement.
- Thermal oxidation destroys the solvent, needs supplemental fuel on lean streams and adds CO₂ — nothing is recovered. See recovery vs thermal oxidation.
- Very dilute, very large streams are best concentrated first with a rotary concentrator, then recovered.