Indirect fired thermal desorption systems are comprised of two general categories: Continuous feed and batch type. Although some designs utilize electrical resistance heating, most use a petroleum fired heat source. These systems are constructed in such a way that the flame and its products of combustion are unable to come in contact with the soil or the vaporized contaminants.
Continuous Feed Indirect Fired Thermal Desorbers
Most continuous feed systems utilize an externally heated rotating cylinder as the desorber, where heat is conducted through the cylinder wall and into the soil on the other side. Treatment capacities generally range from 4 to 25 tons per hour for transportable units. The second most common design consists of a jacketed screw conveyor where heat transfer fluids are pumped through the device and indirectly heat the conveyed material. These systems have been most successful in removing petroleum hydrocarbons from solids at capacities up to approximately 4 tons per hour.
Batch Type Indirect Fired Thermal Desorbers
There are two basic variations of batch processors: Mixing and static. One benefit of a batch unit is that the processing retention time can be infinitely adjusted as needed to meet the treatment criteria. Otherwise, these units typically produce lower treatment capacities compared to continuous feed systems.
Mixing type units generally consist of a sealed rotating drum enclosed within an insulated furnace jacket. Internals of these drums contain paddles that provide additional mixing during rotation. These desorbers also typically operate under a vacuum which increases the volatility of the compounds being vaporized. These systems can be applied to most any waste profile including mercury and radioactive material.
Static desorbers are usually of rectangular construction and heat the contaminated media via direct steam contact, or infrared heaters. Direct contact steam units provide a stripping, or washing effect that aids in treatment performance, but are limited to treatment temperatures of 212°F unless super heating is employed. These units are best suited for media contaminated with low boiling point compounds such as chlorinated solvents. Typical steam unit batch durations range from 1 to 3 hours. Infrared systems can achieve higher treatment temperatures and be applied to higher boiling point contaminants, but generally rely more on conduction rather than convective heat transfer. These units can require batch times of up to 24 or 48 hours in order to achieve treatment goals.