How ammonium sulphate dryers work
Ammonium sulphate is crystallised from ammonia and sulphuric acid, either in a saturator on coke oven gas or in an evaporative crystalliser on caprolactam by-product liquor. The slurry is centrifuged to about 1 to 3% moisture and dried in a fluid bed or rotary dryer. Drying air is heated to typically 120 to 180°C in a steam coil or an indirect oil or gas fired air heater and leaves at 70 to 90°C, carrying fine crystals. Product is cooled to below about 50°C in a fluid bed cooler to prevent caking in storage. Exhaust air passes cyclones and a wet scrubber, which return dissolved salt to the crystalliser, before the exhaust fan discharges to atmosphere.
Where the ID fan sits
- Fluid bed or drum dryer
- Cyclones
- Wet scrubber
- Mist eliminator
- ID fan
- Stack
The fan is located after the scrubber, where the air is close to saturation at 40 to 60°C and the dust load has been reduced to tens of mg/Nm³. It works against the resistance of the bed or drum, cyclones, venturi or packed scrubber and mist eliminator, typically 350 to 600 mmWC. Some older plants place the fan between the cyclones and the scrubber to keep it dry, but it then handles more abrasive crystals and the scrubber runs under pressure, so the downstream position is more common.
Gas at the ID fan
| Gas temperature, normal | 40 to 60°C, near saturation after the scrubber |
| Maximum or upset | 70 to 90°C if the scrubber is bypassed or circulation stops |
| Dust at the fan | Fine ammonium sulphate, typically 20 to 50 mg/Nm³, more with poor mist elimination |
| Moisture and dew point | Saturated, with liquor droplets that dry to hard salt on hot surfaces |
| Corrosive species | Ammonium sulphate solution, slightly acidic from free sulphuric acid in the product |
| Density at the design temperature | About 1.08 kg/m³ at 55°C, 90% 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 ammonium sulphate dryers
| Blade form | Backward curved or radial tipped, with smooth surfaces that shed salt |
| Material of construction | SS 316 casing and impeller, or FRP; mild steel corrodes rapidly in wet ammonium sulphate |
| Drive and arrangement | Belt or direct drive, arrangement 9 or 8, with bearings out of the wet zone |
| Bearings and cooling | Pedestal bearings with a slinger and shaft seal; no cooling needed |
| Flow control | Variable speed drive on dryer suction |
| Also specified | Water wash nozzles in the casing and inlet, a casing drain, and an inspection door |
What this ID fan has to cope with
Salt build-up on the impeller
Liquor droplets that pass the mist eliminator dry on the blades to a hard crust of ammonium sulphate. It grows unevenly, unbalances the rotor, and finally breaks off in pieces. Periodic water wash through fixed nozzles, a better eliminator and smooth blade surfaces without pockets keep the wheel clean.
Corrosion of mild steel
Wet ammonium sulphate with free acid attacks mild steel and galvanised parts within months, especially at the casing bottom where liquor collects. SS 316 or FRP for wetted parts and a free-draining casing are the standard response. Bearing pedestals and base frames outside the gas stream also need a good coating, since spilled liquor and salt dust attack them too.
Dust escaping from the dryer
If exhaust suction falls as the scrubber packing scales up, the dryer goes positive and fine crystals escape into the building. A fan with pressure margin for a fouled scrubber, and a drive controlling dryer suction, keeps the dryer contained. Scrubber pressure drop is trended so packing can be cleaned before suction is lost.
Energy and control
Ammonium sulphate dryers follow the crystalliser output, which changes with ammonia or caprolactam plant rate rather than with time of day. The exhaust fan usually runs for long periods well below its design flow, and the scrubber resistance rises between cleanings. A variable speed drive on dryer suction both follows the production rate and compensates for scrubber fouling, which a fixed-speed fan with a damper handles only by wasting power.
ID fan and FD fan in ammonium sulphate dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclones and wet scrubber | Before the air heater, feeding the bed plenum |
| What it handles | Humid air with salt droplets | Clean ambient air |
| Temperature at the fan | 40 to 60°C, near saturation | Ambient |
| Main design concern | Corrosion and salt build-up | Steady fluidising pressure |
| Control | Dryer suction | Air flow and bed pressure drop |
See also the Forced Draft (FD) Fan for ammonium sulphate dryers.
Questions buyers ask
What material is used for an ammonium sulphate dryer exhaust fan?
SS 316 is the usual choice for the casing and impeller on an ammonium sulphate dryer exhaust fan after a wet scrubber, with FRP as an alternative. Mild steel corrodes quickly in wet ammonium sulphate with free acid and is not recommended for wetted parts.
How do I stop salt build-up on a dryer exhaust fan?
First reduce the liquor reaching the fan with a well-maintained mist eliminator. Then fit water wash nozzles in the inlet and casing and run them at set intervals, so deposits are dissolved before they harden. Smooth blades without pockets shed deposits more easily. Wash water drains back to the scrubber.
Should the ID fan be before or after the scrubber?
After the scrubber is more common, because the fan then sees little abrasive dust and the scrubber runs under suction. The fan must then be built for wet, corrosive air. Placing it before the scrubber keeps it dry but exposes it to more abrasive crystals.
Information needed to quote an ID fan for ammonium sulphate dryers
- 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.
- Scrubber. Type of scrubber and mist eliminator, and their clean and fouled pressure drops.
- Product chemistry. Free acid content of the product and the scrubber liquor concentration.
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.