How pharmaceutical solvent incinerators work
Pharmaceutical plants generate spent solvents that cannot be recovered economically: mixtures of methanol, toluene, acetone, IPA, ethyl acetate, DMF and chlorinated solvents such as dichloromethane. These have a high calorific value, often 5,000 to 8,000 kcal/kg, so a solvent incinerator usually burns them with little or no support fuel and recovers the heat in a waste heat boiler raising steam for the site. The chamber is held at 1,100°C or more with two seconds residence time where halogenated solvent exceeds about 1% chlorine. Gas leaves the boiler at about 220 to 280°C, is quenched to saturation and scrubbed with caustic, and the ID fan pulls it through to the stack. A combustion air fan feeds the solvent burner.
Where the ID fan sits
- Solvent burner and chamber
- Waste heat boiler
- Quench
- Caustic scrubber
- Mist eliminator
- ID fan
- Stack
The fan sits after the caustic scrubber and mist eliminator, at the base of the stack, keeping the chamber, boiler and scrubber under suction. It works against the boiler, quench, packed column and demister, typically 300 to 500 mmWC. A few plants place the fan between the boiler and the quench to keep it dry, but then hot gas containing HCl is above the fan and the scrubber runs under pressure, which most operators avoid.
Gas at the ID fan
| Gas temperature, normal | 55 to 75°C, saturated after the scrubber |
| Maximum or upset | Hot gas on quench water failure; emergency vent and trips protect the fan |
| Dust at the fan | Negligible solids; scrubber droplets and sodium salt mist |
| Moisture and dew point | Saturated gas with free droplets |
| Corrosive species | HCl and free chlorine from chlorinated solvents; hypochlorite in carried-over scrubber liquor |
| Density at the design temperature | About 1.05 kg/m³ at 65°C, 88% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
ID Fan selection for pharmaceutical solvent incinerators
| Blade form | Backward curved, with smooth surfaces and drainage |
| Material of construction | FRP with a chemical-resistant barrier or rubber-lined mild steel; high-alloy stainless only where chlorine is absent |
| Drive and arrangement | Belt or direct drive with the motor outside the wet zone |
| Bearings and cooling | Pedestal bearings with sealed shaft entry |
| Flow control | Variable speed drive on chamber pressure |
| Also specified | Casing drain, inspection door and interlocks with the emergency vent and quench water |
What this ID fan has to cope with
Chlorine and hypochlorite attack
Burning dichloromethane and chloroform releases some free chlorine as well as HCl. In a caustic scrubber this forms hypochlorite, and chlorine with chloride is aggressive to SS 316. FRP with a suitable resin, or rubber-lined steel, is the usual answer, with sodium thiosulphate or sulphite dosing in the scrubber to reduce free chlorine.
Quench failure on a non-metallic fan
FRP and rubber linings tolerate wet gas but fail quickly in hot gas. Two independent quench water supplies, a temperature trip ahead of the fan and an emergency vent are standard protection.
Variable gas volume with solvent blend
Switching from a high calorific solvent to one with more water or chlorine changes the gas volume and composition. The fan is sized on the worst blend, and drive control on chamber pressure handles the swings.
Energy and control
Solvent incinerators usually run steadily, burning a blended feed from storage tanks. A variable speed drive on chamber pressure trims ID fan power as the feed rate changes. Because the waste heat boiler recovers most of the heat as steam, keeping excess air low raises steam output as well as reducing fan duty, and good boiler cleaning keeps the gas temperature at the quench within design.
ID fan and FD fan in pharmaceutical solvent incinerators
| ID fan | FD fan | |
|---|---|---|
| Position | After the caustic scrubber | At the burner windbox |
| What it handles | Saturated gas with HCl and chlorine | Ambient combustion air |
| Temperature at the fan | 55 to 75°C | Ambient, 25 to 45°C |
| Main design concern | Chlorine and chloride corrosion | Fast response to solvent changes |
| Control | Chamber pressure | Air-fuel ratio with oxygen trim |
See also the Forced Draft (FD) Fan for pharmaceutical solvent incinerators.
Questions buyers ask
Why is the ID fan on a solvent incinerator made of FRP and not stainless steel?
Chlorinated solvents produce HCl and some free chlorine. After a caustic scrubber the wet gas and droplets contain chlorides and hypochlorite, which pit and crack stainless steel. FRP with a chemical-resistant liner, or rubber-lined steel, resists this better at scrubber outlet temperatures.
Does a pharma solvent incinerator need a quench before the ID fan?
Yes, when the fan follows a wet scrubber. The quench cools the boiler outlet gas to saturation before the scrubber, and the fan then handles gas at about 55 to 75°C, which allows non-metallic materials.
How is the ID fan sized for a solvent incinerator with a waste heat boiler?
On the gas volume at the fan, which is the combustion gas from the worst solvent blend plus quench water vapour and leakage air, at saturated conditions. The system resistance includes the boiler, quench, scrubber and demister.
Information needed to quote an ID fan for pharmaceutical solvent incinerators
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Solvent blend. Range of chlorinated solvent content and calorific value of the feed.
- Scrubber chemistry. Scrubbing liquor, pH and any dechlorination dosing.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.