Catalytic pressureless oiling (PVD) or thermocatalytic low temperature conversion (LTC) is a technical depolymerization process. Artificial or natural polymers and long-chain hydrocarbons are converted into shorter-chain aliphatic hydrocarbons, comparable to synthetic light oil (diesel fuel), by adding a zeolitic catalyst at temperatures below 400 °C without excess pressure. The efficiency depends on the starting material and ranges from 20% (for biomass) to up to 80% (for energy-rich plastics and oils).
The main field of application is the conversion of waste into fuels as so-called synthetic diesel (i.e. neither biodiesel nor mineral diesel). The highly concentrated mineral catalyst causes a conversion of the input materials under technically controllable conditions at high temperature and air exclusion in hot oil.
In the reaction tank, the dry, highly crushed input material and the catalyst are mixed and heated up to 400°C. At these temperatures the input material loses its solid consistency and liquefies. Most plastics melt and emulsify in thermal oil. The hydrocarbons are cracked by the added catalyst.
From the hot oil mixture, the resulting short-chain hydrocarbons evaporate at boiling temperatures of less than 340°C. In a subsequent distillation column they are recovered from the steam as a mixture of aliphatic hydrocarbons (of the type C10-C22). This mixture has the properties of conventional diesel fuel. If the input mass contains sulphur compounds, these must be removed even further. Halogens, which are contained e.g. in PVC or other plastics, are bound by a neutralizer to form salts. The hydrocarbons not cracked in the reaction and the other substances present in the input material, such as metals, salts, carbon, lignin (in the case of wood), but also spent and decomposed catalyst, remain in the reaction vessel and are discharged by a screw conveyor. The mixture of spent catalyst and the other residues and oils is further processed and the catalyst is reprocessed.
In contrast to pyrolysis, the temperatures of less than 400°C do not cause the formation of highly toxic dioxins or furans from halogen-containing plastics, as these are only formed at higher temperatures. Due to the catalyst and the temperatures of more than 300°C all bacteria, viruses and prions should be destroyed during the reaction. Contaminated input materials should, however, be safely disinfected in advance.
Zeolites of the type Pentasil and Wassalith are used as catalysts. The required quantity of the catalyst is approx. 5 - 9 % of the input, depending on the input material. In the case of materials containing chlorine and fluorine, hydrated lime is used to neutralise the acids produced during the process. Catalyst and neutralizer are common, commercially available products which can be handled chemically without any problems.
