Chemical recycling techniques

Incineration with energy recovery is examined as a means for the disposal of plastics waste, and data are presented for the calorific values of a number of materials. Chemical recycling techniques are also briefly reviewed. 

The origins and composition of plastics wastes and factors affecting their recycling are discussed. Partieular attention is paid to chemical recycling and incineration, with and without energy recovery, and a number of developments in chemical recycling techniques are examined. 19 refs. 

The recycling of PET is an important environmental topic as well as a commercial opportunity due to its widespread use, abundance and availability in bottles, packaging and fibres. While mechanical recycling of PET is now well established, newer chemical recycling techniques rely on depolymerization routes which cleave the polymer chains into new monomer building blocks (see Chapter 16). 

Spanish initiatives in PVC recycling are examined with particular reference to mechanical recycling, and developments in chemical recycling techniques are also reviewed. Statistics are presented for sources of PVC waste and forms and applications of the recycled materials, and a map shows the geographical distribution of PVC recyclers in Spain. 

Among polyesters synthesized from 1,4-benzenedicarboxylic acid and aliphatic diols, poly (ethylene terephthalate) (PET) and poly (butylene terephthalate) (PBT) are the most frequently applied ones. Hydrolysis is evidently the easiest chemical recycling technique of polyesters, however they may be mixed with other waste plastics, thus it is useful to know the properties of their pyrolysis product. 

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. 

Both incineration and chemical recycling techniques produce i) gaseous products that could represent potential health hazards and, therefore, should be treated carefully, and ii) residual solid products, which need further disposal [1]. 

Sueoka, Y. et al.. Chemical recycling techniques for plastics, Mitsubishi Juko Giho, 36, 3,146,1999. 

The Vinyloop process was developed by Solvay as a response to a challenge from one of its customers, Ferrari Textiles Techniques (France), who produces architectural tarpaulin and canvas in PVC/polyester compound. This is a rather difficult formulation for recycling, since the PVC is mixed with a matrix. At the same time, Ferrari felt it was important that their material would be recyclable. The first Vinyloop installation is now operational. It is a form of mechanical rather than feedstock or chemical recycling, since the PVC matrix is not changed in the process. 

A pyrolysis technique was investigated as a method for the chemical recycling of glass fibre-reinforced unsaturated polyester SMC composites. The proeess yielded liquid products and gases and also a solid residue formed in the pyrolysis of glass fibres and fillers. The solid residue was used as a reinforeement/filler in unsaturated polyester BMC composites, and the influenee on mechanical properties was studied in comparison with BMC prepared entirely from virgin materials. 

Techniques for chemical recycling of waste saturated polyesters, mainly poly(ethylene terephthalate) and of waste polyurethanes to produce polyurethanes are reviewed. 29 refs. Articles from this journal can be... 

Processes for the mechanical and chemical recycling of PETP bottles and other containers are described, and waste separation techniques are also examined. 9 refs. ASSORIMAP PETCORE REPLASTIC REKO BV PURE TECH INTERNATIONAL INC. DOW CHEMICAL CO. 

Techniques for the chemical recycling of plastics into monomers and petrochemical feedstocks are described, including chemical and thermal depolymerisation, pyrolytic liquefaction, pyrolytic gasification and partial oxidation. BRITISH PETROLEUM CO.PLC... 

Pyrolysis is a tertiary or feedstock recycling technique capable of converting plastic waste into fuels, monomers, or other valuable materials by thermal and catalytic cracking processes. This method can be applied to transform both thermoplastics and thermosets in high-quality fuels and chemicals. Moreover it allows the treatment of mixed, unwashed plastic wastes. 

Heterogeneous process mainly involves chemolysis/solvolysis, and it uses chemical reagents or solvents for depolymerization of the polymer into monomers. It is a secondary recycling technique depending on the nature of polymers, a wide variety of solvents are used for the depolymerization process. Based on the nature of solvents, it is named as alcoholysis, methanolysis, glycolysis, etc. 

In addition to the academic literature on chemical recycling of aromatic polyesters, especially PET, there are many patents dealing with this topic. These cover four basic techniques, combinations of same, and some novel approaches. To make a survey of this area less unwieldy, the six apparent main players will be described first, then the others discussed by the category of chemistry used in the particular process. 

Both solids recycle techniques allow some combination of faster rise rate, imjHoved clarity of overflow, higher concentration of solids in the underflow, and reduced consumption of treating chemicals. 

Nylon recycling has increased substantially in the last several years. Most recycling efforts have focused on recovery of carpet. According to the U.S. Department of Energy, about 3.5 billion lb of waste carpet are discarded each year in the United States, with about 30% of them made from nylon 6. (For more on carpet recycling, see Sec. 12.4.15.) Recycling systems for condensation polymers, such as nylon and PET, can more effectively use chemical depolymerization techniques than can systems for addition polymers such as polyolefins and PVC. Most of the efforts directed at nylon recycling have taken this route. 

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