How vent gas incinerators for PVC and chlor-Alkali plants work
PVC and chlor-alkali complexes collect vent streams from EDC cracking, VCM recovery, oxychlorination and storage, and burn them in a vent gas incinerator rather than releasing chlorinated organics. The vents carry ethylene dichloride, vinyl chloride, other chlorinated hydrocarbons, hydrogen and nitrogen, often with wide swings in heating value. They are oxidised at typically 1,100 to 1,200°C with about two seconds of residence time, supported by natural gas or fuel oil. The flue gas carries hydrogen chloride and some free chlorine. It is quenched rapidly to below 100°C to limit dioxin re-formation, then passes an HCl absorber that recovers dilute hydrochloric acid and a caustic scrubber before the ID fan and stack. A combustion air fan supplies the burner and the oxidation air.
Where the ID fan sits
- Vent headers
- Burner and combustion chamber
- Quench
- HCl absorber
- Caustic scrubber
- Mist eliminator
- ID fan
- Stack
Placing the fan after the scrubbing train means it handles gas at 50 to 75°C instead of 1,100°C, and the acid has already been removed down to a few mg/Nm³. The fan works against the combined resistance of the quench, packed absorber, packed or tray scrubber, mist eliminator and ducting, typically 250 to 450 mmWC. Because the whole hot section runs under suction, any leak draws air in rather than letting chlorinated gas out, which is the main safety reason for an induced draft arrangement on this plant.
Gas at the ID fan
| Gas temperature, normal | 50 to 75°C, saturated, after the caustic scrubber |
| Maximum or upset | Up to 90 to 100°C if quench water or scrubber circulation fails, until the interlock trips the burner |
| Moisture and dew point | Saturated; free water and mist carry-over if the eliminator floods |
| Corrosive species | Residual HCl, traces of chlorine and hypochlorite mist from the caustic scrubber |
| Dust at the fan | Essentially none; occasional salt deposits from caustic carry-over |
| 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 vent gas incinerators for PVC and chlor-Alkali plants
| Blade form | Backward curved, in FRP or rubber-lined steel, sized for moderate tip speed |
| Material of construction | Chemical-grade FRP with a vinyl ester resin barrier, or rubber-lined mild steel; nickel alloy only where high temperature excursions must be tolerated. Plain SS 316 is not chosen, as wet chlorides pit it |
| Drive and arrangement | Belt or direct drive, arrangement 9 or 8, with the motor outside the gas stream |
| Bearings and cooling | Pedestal bearings outside the casing; no cooling needed at this temperature |
| Flow control | Variable speed drive on combustion chamber pressure |
| Also specified | Casing drain to the scrubber sump, shaft seal against gas escape, and a spare rotor for critical service |
What this ID fan has to cope with
Chloride attack on metal parts
Stainless steel that would serve on a sulphur-bearing duty pits and cracks in wet chloride gas, often at welds and the hub. An FRP or rubber-lined fan, with fasteners and shaft sleeve in a chloride-resistant alloy, avoids the rapid failures seen when a standard stainless fan is installed after the scrubber.
Liquid carry-over
A flooded mist eliminator or high scrubber circulation can send slugs of liquor into the fan, adding load, unbalance and salt deposits. A low-point drain on the casing, an inlet that falls toward the scrubber and an alarm on fan current help catch carry-over early.
Temperature excursion on quench failure
If the quench water stops, gas at several hundred degrees can reach the absorber and fan within seconds and destroy FRP or rubber. Interlocks trip the burner and vent feed on quench flow and temperature, and the fan material is chosen with a stated short-time temperature limit.
Gas escape at the shaft
The fan casing is the last enclosure before the stack, and the outlet side runs at slight positive pressure. A packed or lip seal with a purge connection keeps residual acid gas from leaking into the plant area. The seal housing is in a non-metallic or chloride-resistant material so that it does not become the first part to corrode and open a leak path.
Energy and control
Vent gas flow varies with plant operation, from a steady base of storage and process vents to large surges during start-up, purging or a unit trip. The ID fan must hold chamber suction across that range. A variable speed drive on chamber pressure lets the fan follow the vent load and saves energy at the low base flow where the plant spends most of its time, while keeping enough margin for the purge surge without opening a bypass.
ID fan and FD fan in vent gas incinerators for PVC and chlor-Alkali plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the quench and scrubbers, before the stack | At grade, feeding burner and oxidation air |
| What it handles | Saturated gas with HCl and chlorine traces | Clean ambient air |
| Temperature at the fan | 50 to 75°C, saturated | Ambient |
| Main design concern | Chloride corrosion and liquid carry-over | Purge capacity and air for rich vents |
| Control | Combustion chamber pressure | Chamber temperature and oxygen |
See also the Forced Draft (FD) Fan for vent gas incinerators for PVC and chlor-Alkali plants.
Questions buyers ask
What material should an ID fan be after an HCl scrubber?
For wet gas after an HCl absorber and caustic scrubber, FRP with a vinyl ester barrier or rubber-lined mild steel is usual. These resist both residual acid and hypochlorite. Standard stainless steel is a poor choice in wet chloride service because it pits and suffers stress corrosion cracking.
Why is a vent gas incinerator in a PVC plant run under suction?
The vents contain vinyl chloride and other chlorinated organics, and the flue gas contains hydrogen chloride. Running the incinerator and scrubbing train under suction from an ID fan means any leak draws air inward instead of releasing toxic or corrosive gas into the plant. It also lets the scrubbers work at a stable pressure.
How do I protect an FRP ID fan from high temperature?
Protect it at the source: interlock the burner and vent feed on quench water flow and quench outlet temperature, and alarm on fan inlet temperature. The fan is specified with a short-time temperature limit so the trip setting can be chosen safely below it. Check the trips regularly in service.
Information needed to quote an ID fan for vent gas incinerators for PVC and chlor-Alkali plants
- 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.
- Gas analysis. Expected HCl, chlorine and hypochlorite at the fan inlet, and the scrubber liquor chemistry.
- Upset cases. Maximum temperature and duration the fan must withstand if the quench or scrubber circulation fails.
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.