Plastics pyrolysis is based on thermal and sometimes catalytic breakdown of the polymer structure. The target product may be monomer (from PMMA, PTFE, PS), oil fractions ranging from petrol (C-5-C-10) and kerosene (C-10-C-15) to waxy fractions (greater than or equal to C-20), aromatics, synthesis raw materials formed by hydrogenation or synthesis gas (H-2, CO). The desired product determines the operating conditions required and hence the technology used. A survey is given of laboratory, pilot and industrial scale reactors, pyrolytic extruders, stirred tanks and single- or double-bed fluidized systems. Moreover, the use of plastics in oil refinery-based processes, such as fluid catalytic cracking, hydrocracking, viscosity breaking and delayed coking, and also (coal-derived) hydrogenation and gasification are briefly covered. The product distribution in a pyrolysis process has to be derived experimentally, with the operating temperature, heating rate, catalytic effects, residence time of feed material and reaction products, reaction pressure, possible supply of reactive gases (such as oxygen, hydrogen) and, of course, the chemical constitution and structure of the original plastics as major factors. Pyrolysis involves the breaking of chemical bonds and is normally endothermic. The required heat of reaction can be supplied indirectly through the reactor walls, by a circulating heat carrier, which may be the polymer content of the reactor or the sand in a dual fluid-bed system, or by partial oxidation. The reactor technology is generally determined by the temperature level required, heating method, necessary residence time and possibly the presence of a catalyst, required to rearrange linear structures to high-octane-number branched ones, to eliminate heteroatoms (Cl, N, O, S), etc. Some potential operating problems are addressed in an EU-funded project with acronym Cycleplast, a joint project of five teams from Central and Western Europe, the aims of which are briefly described. [References: 9]
展开▼
