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 FD fan sits
- Inlet filter
- FD fan
- Air preheater
- Burner windbox
- Solvent burner
- Combustion chamber
The fan sits near the burner and delivers ambient air to the windbox, through an air preheater on some plants. It works against the burner swirl register and windbox, typically 300 to 600 mmWC. Because the site handles flammable solvents, the fan location and motor rating are set by the hazardous area classification, and the inlet is taken from outside any classified zone where possible.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | Up to 200 to 250°C where fitted, downstream of the fan |
| Dust | Clean air; filtered inlet on pharma sites |
| Pressure at the fan outlet | Typically 300 to 600 mmWC |
| Hazardous area | Fan and motor may sit in a classified zone near solvent storage |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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. |
FD Fan selection for pharmaceutical solvent incinerators
| Blade form | Backward curved for efficiency and a stable curve |
| Material of construction | Mild steel; non-sparking features where the area classification calls for them |
| Drive and arrangement | Direct mounted or direct coupled, with a flameproof motor where required |
| Bearings and cooling | Standard pedestal or motor bearings |
| Flow control | Variable speed drive on air-fuel ratio with oxygen trim |
| Also specified | Airflow measurement, air pressure switch interlock, and inlet filter |
What this FD fan has to cope with
Air shortage on high calorific solvent
A change from aqueous waste to a pure solvent can double the air demand in seconds. If the fan cannot respond, the chamber produces soot and CO. Measured airflow and a drive tuned to lead fuel on rising load prevent this.
Flame instability on mixed feeds
Solvent blends with water or halogens burn less easily. The fan must deliver steady pressure at low flow to keep the swirl stable when the burner turns down.
Ignition source near solvents
Fans close to solvent handling can be an ignition source. Fan and motor are selected for the area classification, and the air intake is taken from a safe location.
Energy and control
With a waste heat boiler, every percent of excess air carries heat to the stack and reduces steam output. A variable speed FD fan with oxygen trim holds excess air at the lowest stable level, saving fuel equivalent and fan power, and reducing gas volume through the boiler, scrubber and ID fan.
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 Induced Draft (ID) Fan for pharmaceutical solvent incinerators.
Questions buyers ask
What size combustion air fan does a solvent incinerator need?
It depends on the maximum solvent firing rate and its stoichiometric air, plus excess air. Stoichiometric air ranges from roughly 5 Nm³ per kilogram for methanol to about 10 Nm³ for toluene, so the blend matters. We size the fan on the maximum firing case with margin for blend changes.
Does the FD fan need a flameproof motor?
It depends on the hazardous area classification of the fan location. Many pharma solvent incinerators sit near solvent tanks, and the motor and fan are selected to suit the zone.
Why does the incinerator smoke when the solvent changes?
Usually because combustion air does not follow the change in solvent heat input quickly enough, so the burner runs short of air for a few seconds. Airflow measurement, oxygen trim and a fast drive, with air leading fuel on rising load, solve most cases.
Information needed to quote an FD fan for pharmaceutical solvent incinerators
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Firing range. Minimum and maximum solvent firing rate and calorific value.
- Area classification. Hazardous area zone at the fan location, if any.
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.