How pickling acid regeneration plants work
Steel strip and wire pickling lines use hydrochloric acid, and the spent liquor, mainly ferrous chloride, is regenerated rather than neutralised. In a spray roaster the liquor is sprayed into a tall reactor fired by tangential burners on natural gas, LPG or oil; the droplets roast at about 400 to 700°C to iron oxide powder and HCl gas. Fluidised bed roasters do the same in a hot bed of oxide granules. The roaster gas, leaving at about 350 to 450°C, passes a cyclone and a venturi preconcentrator, where it evaporates water from incoming liquor and cools to about 90 to 100°C. HCl is then absorbed in water in an absorption column to give regenerated acid, typically around 18%, and the remaining gas is cleaned in scrubbers before the exhaust fan and stack.
Where the ID fan sits
- Spray or fluid bed roaster
- Cyclone
- Venturi preconcentrator
- Absorption column
- Exhaust (ID) fan
- Scrubber and stack
The fan is placed after the absorption column, often between the absorber and a final scrubber or demister. Placing it here keeps the whole plant upstream under suction, so HCl cannot leak out of the roaster or columns. The gas at the fan is saturated, at about 70 to 90°C, with residual HCl and droplets. The suction covers the roaster, cyclone, venturi and absorber, typically 300 to 600 mmWC.
Gas at the ID fan
| Gas temperature, normal | 70 to 90°C, saturated, after the absorber |
| Maximum or upset | Up to about 100°C or more if absorber water fails |
| Dust at the fan | Very low; traces of iron oxide |
| Corrosive species | HCl and traces of chlorine, with condensed acid droplets |
| Moisture | Saturated with entrained droplets |
| Density at the design temperature | About 0.99 kg/m³ at 85°C, 83% 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 pickling acid regeneration plants
| Blade form | Backward curved or radial, depending on construction material and pressure |
| Material of construction | FRP, rubber-lined steel or other acid-resistant construction |
| Drive and arrangement | Belt or direct drive, arrangement 9 or 8, motor outside the gas |
| Bearings and cooling | Standard bearings outside the corrosive gas; shaft seal against acid leakage |
| Flow control | Variable speed drive on roaster pressure |
| Also specified | Casing drain to acid sump, gas-tight shaft seal and protective coating on the base |
What this ID fan has to cope with
Acid corrosion of the fan
Saturated gas with HCl and condensed acid attacks mild steel and most stainless steels quickly; chlorides cause pitting even on SS 316. The fan must be built in FRP, rubber-lined steel or a suitable alloy, with linings checked for blisters and cracks at each shutdown and the shaft entry sealed so acid does not reach the bearings.
Temperature limits of non-metallic fans
FRP and rubber linings have temperature limits, often not far above the normal gas temperature. If absorber water fails, gas at 100°C or more reaches the fan and the lining can soften, blister or delaminate. A temperature trip at the fan inlet, an emergency water supply to the absorber and a resin system chosen for the upset temperature protect the fan.
Roaster pressure control
The roaster must stay under slight suction so that HCl and oxide dust do not escape at the burners, the oxide discharge or the top. When liquor feed or firing changes, the gas volume changes with it. The fan must respond without large swings, using speed control with a well-tuned loop and a pressure measurement close to the roaster top.
Energy and control
Acid regeneration plants run continuously, with throughput following the pickling line and the volume of spent liquor in the storage tanks. A variable speed drive on roaster pressure reduces fan power when liquor feed is cut back. Keeping air ingress low at roaster seals, the oxide discharge and inspection doors reduces gas volume, lowers fan power and improves HCl absorption.
ID fan and FD fan in pickling acid regeneration plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the absorption column | Feeding the roaster burners |
| What it handles | Saturated gas with HCl traces | Ambient air from a clean intake |
| Temperature at the fan | 70 to 90°C, saturated | Ambient |
| Main design concern | Acid corrosion and lining temperature limits | Burner stability and clean intake |
| Control | Roaster pressure | Burner firing rate |
See also the Forced Draft (FD) Fan for pickling acid regeneration plants.
Questions buyers ask
What material is used for an acid regeneration plant exhaust fan?
The gas after the absorber is saturated and carries HCl and acid droplets, so the fan is usually FRP, rubber-lined steel or another acid-resistant construction. Mild steel and ordinary stainless steel corrode quickly in this gas. The choice depends on fan size, pressure, the upset temperature and the downstream scrubber arrangement.
Where is the ID fan placed in an HCl regeneration plant?
Usually after the absorption column, so that the roaster, cyclone, venturi preconcentrator and absorber all run under suction and HCl cannot leak out to the plant. A final scrubber or demister often follows the fan before the stack, which means the fan casing itself runs under slight positive pressure and must be gas tight.
What temperature does the acid regeneration exhaust fan handle?
About 70 to 90°C, saturated, after the absorber. Roaster gas leaves at 350 to 450°C, but the venturi preconcentrator and absorber cool it before the fan. The fan material, especially FRP or rubber lining, must tolerate short excursions to higher temperature if the absorber water supply fails.
Information needed to quote an ID fan for pickling acid regeneration 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.
- Roaster type. Spray roaster or fluidised bed, and the regeneration capacity.
- Gas composition. HCl and chlorine content at the fan inlet and the downstream scrubber.
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.