Waste continued pyrolysis

In this section we discuss the process flow diagram for the pyrolysis of waste plastics. This is required to be a compact process that can be controlled in a stable and continuous way, because of movement of the high-viscosity material for each unit system in the process. This material can block the flow line and make difficult the continuous control of unit process. 

A pilot plant for the high temperature pyrolysis of polymers to recycle plastic waste to valuable products based on rotating cone reactor (RCR) technology. The RCR used in this pilot plant, the continuous RCR was an improved version of the bench-scale RCR previously used for the pyrolysis of biomass, PE and PP. 9 refs. 

Free carbon thereby is deposited on the reactive mass of silicon, covering it over and serving as a catalyst for further pyrolysis of methyl groups. Furthermore, the methane and hydrogen which appear in the exit gases impair the efficiency of the condensers and represent a waste of organic halide. For these reasons the formation of trichlorosilanes is to be avoided as uneconomical and detrimental to the continued production of dichlorosilanes. 

The pyrolysis of mixed waste thermoplastics in a pilot plant of 360 ton/year at the Korea Institute of Energy Research (KIER), as shown in Figure 5.6, has an oil yield of about 82% for continuous process control over two days. The distribution of oil product is 27% gasoline product and 73% heavy oil product. Also the yield, of gas product is 10-15% and consists of about 18.1% Ci, 15.2% C2, 30.3% C3, 21.9% C4 and 14.3% C5 components. Similar results were obtained by other researchers, as shown in Table 5.1. 

An oil of low flash point in the range 14-18°C, and of 41-43 MJ Kg gross calorific value has been obtained in batch pyrolysis [36] of automobile tyre waste. In a pilot plant with semi-continuous feeding [37] the liquid yield of tyre waste decreased seriously with increasing temperature, and it was always lower in an atmosphere containing oxygen that in nitrogen. 

A 20 t/day plant (6000 t/yr) is equipped with two pyrolysis vessels (with dimensions of 2800 mm ID and 2000 m height). The vessels are fed with molten plastics by four extruders each with a capacity of 250 kg/h. The plant runs continuously and can feed waste plastics and discharge the solid residue while the plant is running. The liquid fuels are fractionated in a fractionation tower. The plant produces a liquid fuel yield of up to 80% (by weight), depending on the nature of the feedstock. 

The 1973 petroleum crisis intensified research on coal liquefaction and conversion processes. The technology developed in this field was later harnessed in chemical recycling of plastics. Mastral et al. [32], for example, employed two different batch reaction systems (tubing bomb reactors and magnetically stirred autoclave) and a continuous reactor (swept fixed bed reactor). Chemical recycling techniques such as pyrolysis [28, 33-38] or coliquefaction with coal [39, 40] convert plastic wastes into hydrocarbons that are valuable industrial raw materials. 

In contrast to other recycling processes (mechanical recycling, vacuum pyrolysis), tluidized-bed pyrolysis has a number of advantages. Different kinds of plastics can be degradated into monomers in higher yields than with other methods and without needing to mill wastes into small particle sizes. The most important advantages are that monomers produced can be purified before repolymerization which allows production of a more valuable product and that the process allows continuous operation. 

The main advantage of rotary kiln pyrolyzers is that the rotation of the pyrolyzing chamber guarantees the permanent turning and mixing of the waste plastic, so that the mixture is continuously homogenized and blended with inert pyrolysis gas. A rotary kiln pyrolyzer is shown in Figure 27.3. 

The pyrolysis of mixed plastics containing PVC in supercritical water has also been demonstrated [96]. The temperature in the reactor increases from 200°C at the top to 1200°C at the bottom. HCl is generated in the first reaction zone. In the second zone, HCl continues to react with alkali metal and is removed, and residue and fuel gas which mainly consists of H2 and CHj are produced by reaction of plastic waste and supercritical water. In the third reaction zone, part of the residue produced was oxidized and CO and fuel gases were generated. 

Pyrolysis can be used for the thermal decomposition of waste materials that are predominantly organic in nature, e.g. scrap tyres, scrap cables, waste plastics, shredder wastes, and acid sludge. Rotary kilns are particularly suitable as universally applicable pyrolysis units for continuous operation. Highly aromatic pyrolysis oils for use as chemical raw materials are obtained at reactor temperatures of about 700 °G. Such pyrolysis oils could form the basis for the production of aromatics such as benzene, naphthalene, and their homologues, thermoplastic hydrocarbon resins and precursors of industrial carbon, when the proven processes for the refining of coal tar and crude benzene are applied. 

Silica particles were prepared by the pyrolysis-cum-water vapor treatment of waste green tires and used as a filler in fresh SBR rubber blends (Ivanov and Mihaylov, 2011). Pyrolysis was carried out by gradually heating tires between 300°C and 1000°C and continuously purging air, smoke gases, carbon dioxide, and nitrogen with the addition of vapor from 0% to 100%. The final product was... 

In addition to the thermogravimetry experiments, batch pyrolysis experiments continuously monitoring HCl formation were performed with PVC to determine the optimum temperature for HCl formation. Under conditions of maximum HCl formation, PVC was pyrolyzed with and without oxygen in a fluidized bed reactor and the formation of polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) measured. Avoiding the formation of these highly toxic compounds would be a critical element in any waste stream processing scheme. 

The design of a pyrolysis oven is special and depends on complete sealing, with no ingress of air and with the waste products continuously being removed with a purge of N2. This process forms a product, which when exposed to air, can absorb O2 followed by evolution of heat, which must be dissipated, failing which a fire may occur. 

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