Polymer mixed feedstock

The presence of halogenated polymers and fire retardants, of heavy metals, and the potential formation of dioxins are some of the problems addressed in various studies presented at the successive Symposia on Feedstock Recycling (ISFR). Dehalogenation is a major topic. Some important commercial polymers (PVC, PVDC, chlorinated PE) introduce the element chlorine in almost any mixed feedstock, including those that are... 

Structure of the material breaks down. It can be applied to pure or to mixed feedstocks, including mixtures of plastics and other organic compounds. In all cases with a mixed feedstock, and in many cases even with a pure polymer, the resultant product contains a wide variety of compounds, some liquid and some gas. Separating out pure streams for use as chemical raw materials is often not economically viable. The most common use for these materials is as fuel, after separation only into gas and liquid streams. 

Several types of bacteria are capable of forming PHB or of producing PHA/PHB copolymers from a mixed feedstock containing appropriate precursor molecules. The amount of hydroxyvalerate (HV) depends on the feed and also on the species of bacteria which is used. However, only the Pseudomonas species of bacteria can produce and store polymers from monomers containing 8 or more carbons. ... 

D.M. Bigg, Interrelation among feedstock form, product requirements, equipment type, and operating parameters in polymer mixing processes, Polymer-Plastics Technology and Engineering 23 (2) (1984) 133-168. 

The BP Chemicals polymer cracking process is based at Grangemouth in Scotland and uses mixed plastics as the raw material. The reactor uses a fluidised bed which operates at 500 °C in the absence of air, and under these conditions the plastics crack thermally to yield hydrocarbons. These vaporize and are carried away from the bed with the fluidising gas. Solid impurities such as metals from PVC stabilisers accumulate in the bed or are carried away in the hot gas to be captured by a cyclone further along in the plant. PVC decomposes to HCl and this is neutralized on a solid lime absorbent to yield CaCl2 which is disposed of in landfill. The purified gas is cooled to condense most of the hydrocarbon which can be employed as commercially useful distillate feedstock. The light hydrocarbons which are less easy to condense are compressed, reheated and recycled as fluidising gas. 

The cracked gases are cooled and fractionated to remove fuel oil and water (2-5) then compressed (6), processed for acid-gas removal (8) and dried (9). The C3 and lighter material is separated as an overhead product in the depropanizer (10) and acetylene is hydrogenated in the acetylene converter (11). The acetylene converter effluent is processed in the demethanizer system (12-14) to separate the fuel gas and hydrogen products. The demethanizer bottoms is sent to the deethanizer (15) from which the overhead flows to the C2-splitter (16), which produces the polymer-grade ethylene product and the ethane stream, which is recycled to the furnaces as a feedstock. The deethanizer bottoms flows to the C3-splitter (18) where the polymer-grade propylene is recovered as the overhead product. The C3-splitter bottoms product, propane, is typically recycled to the furnaces as a feedstock. The depropanizer bottoms product, C4S and heavier, flow to the debutanizer (19) for recovery of the mixed-C4 product and aromatic-rich pyrolysis gasoline. 

In terms of the materials used, there is a major difference in the complexity of the feedstocks of food extrusion compared to synthetic polymers when one examines any level below that of the bulk rheology of the mix. For example ... 

Thermal processes are mainly used for the feedstock recycling of addition polymers whereas, as stated in Chapter 2, condensation polymers are preferably depolymerized by reaction with certain chemical agents. The present chapter will deal with the thermal decomposition of polyethylene, polypropylene, polystyrene and polyvinyl chloride, which are the main components of the plastic waste stream (see Chapter 1). Nevertheless, the thermal degradation of some condensation polymers will also be mentioned, because they can appear mixed with polyolefins and other addition polymers in the plastic waste stream. Both the thermal decomposition of individual plastics and of plastic mixtures will be discussed. Likewise, the thermal coprocessing of plastic wastes with other materials (e.g. coal and biomass) will be considered in this chapter. Finally, the thermal degradation of rubber wastes will also be reviewed because in recent years much research effort has been devoted to the recovery of valuable products by the pyrolysis of used tyres. 

Feeedstock recycling, also known as thermolysis of rubbers and plastics. The method is used to recycle soiled, mixed rubbers and plastics into monomers, feedstock gasses, oils, and condensates, from which new polymers can be synthesized. It should be used only when and where the first method cannot be carried out. 

The method of feeding wet polymer to FD is very important. Granular products are relatively free flowing when wet polymers are fed with devices such as screw and rotary star feeders sticky polymers may be best handled with a table feeder. Lumpy or pasty polymer must be broken np or mixed with dry product recycle to produce a more uniform and free-flowing feedstock. 

For extrusion compounding, if the components have different densities (e.g. polymer versus fillers) or different shapes (pellets versus regrind flakes) then prefer to use the extruder for mixing. That is, operate the extruder with starved feeding conditions with the components metered separately into the extruder, see Section 9.11. On the other hand, for the blending of polymer feedstock for extrusion, if the components have very similar properties, then use tumble, rotating drum or ribbon blenders or rotor-stator blenders of the feed upstream of the extruder and use flood feeding of the mixture to the extruder. 

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