Numerous desorber types are available today. Some of the more common types are listed below.
* Indirect fired rotary
* Direct fired rotary
* Heated screw (hot oil, molten salt, electric)
* Infrared
* Microwave
Most indirect fired rotary systems use an inclined rotating metallic cylinder to heat the feed material. The heat transfer mechanism is usually conduction through the cylinder wall. In this type of system neither the flame nor the products of combustion can contact the feed solids or the offgas. Think of it as a rotating pipe inside a furnace with both ends sticking outside of the furnace. The cylinder for full-scale transportable systems is typically five to eight feet in diameter with heated lengths ranging from twenty to fifty feet. With a carbon steel shell, the maximum solids temperature is around 1,000 °F, while temperatures of 1,800 °F with special alloy cylinders are attainable. Total residence time in this type of desorber normally ranges from 30 to 120 minutes. Treatment capacities can range from 2 to 30 tons per hour for transportable units.
Direct-fired rotary desorbers have been used extensively over the years for petroleum contaminated soils and soils contaminated with Resource Conservation and Recovery Act hazardous wastes as defined by the United States Environmental Protection Agency. A 1992 paper on treating petroleum contaminated soils estimated that between 20 and 30 contractors have 40 to 60 rotary dryer systems available. Today, it is probably closer to 6 to 10 contractors with 15 to 20 portable systems commercially available. The majority of these systems utilize a secondary combustion chamber (afterburner) or catalytic oxidizer to thermally destroy the volatilized organics. A few of these systems also have a quench and scrubber after the oxidizer which allows them to treat soils containing chlorinated organics such as solvents and pesticides. The desorbing cylinder for full-scale transportable systems is typically four to ten feet in diameter with heated lengths ranging from twenty to fifty feet. The maximum practical solids temperature for these systems is around 750 to 900 °F depending on the material of construction of the cylinder. Total residence time in this type of desorber normally ranges from 3 to 15 minutes. Treatment capacities can range from 6 to over 100 tons per hour for transportable units.
Heated screw systems are also an indirect heated system. Typically they use a jacketed trough with a double auger that intermeshes. The augers themselves frequently contain passages for the heating medium to increase the heat transfer surface area. Some systems use electric resistance heaters instead of a heat transfer media and may employ a single auger in each housing. The augers can range from 12 to 36 inches in diameter for full-scale systems, with lengths up to 20 feet. The auger/trough assemblies can be connected in parallel and/or series to increase throughput. Full scale capabilities up to 4 tons per hour have been demonstrated. This type of system has been most successful treating refinery wastes.
In the early days, there was a continuous infrared system that is no longer in common use. In theory, microwaves would be an excellent technical choice since uniform and accurately controlled heating can be achieved with no heat transfer surface fouling problems. One can only guess that capital and/or energy costs have prevented the development of a microwave thermal desorber at the commercial scale.