Chemolysis Methods

In recent years new processes for PET chemical recycling have been patented that cannot be classified in any of the previous sections, because they use more than a chemical agent to promote the polyester cleavage. Usually, these methods consist of two or more steps which combine different types of treatments glycolysis-hydrolysis, methanolysis-hydrolysis, glycolysis-methanolysis, etc. The major goal of these combined treatments is to benefit from the advantages of each individual process. 

Similarly, the combination of methanolysis and hydrolysis has also been proposed as an interesting alternative to obtain fibre grade TPA from PET 

Finally, a third possible combination of treatments (glycolysis-methanolysis) for PET recycling has been reported by Gamble et alP In this case, the first step 

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Feedstock recycling is examined as a method of plastics recovery. The range of techno logics currently employed are described, and include pyrolysis, hydrogenation, gasification, and chemolysis. Methods for the recycling of mixed plastics wastes are discussed, which include work by BP Chemicals, VEBA Oil, Shell Chemicals and Leunawerke. 

Different PET chemolysis methods have been developed aimed at the production of TPA, DMT or BHET, all of them being possible monomers for the reconstruction of fresh polyesters. The exact monomer formed by PET depolymerization depends on the type of chemical agent used to break down the polymeric chains. In certain processes, the final product of PET chemolysis is a mixture of polyols, useful in the formulation of other polymers such as unsaturated polyesters, polyurethanes and polyisocyanurates. This is an interesting case of chemical recycling because the breakdown of one polymer leads to the raw materials for the preparation of a quite different class of plastics. 

Depending on the depolymerization agent, polyester chemolysis methods have been classified as follows glycolysis, methanolysis, hydrolysis, ammono-lysis, aminolysis and combined processes. Figure 2.1 summarizes the different alternatives for PET chemolysis, as well as the type of products derived from each one. All these PET degradation alternatives are reviewed in the following sections. 

The most important chemolysis methods so far developed to reverse the polyurethane polymerization reaction shown in Scheme 2.2 are glycolysis and hydrolysis. These processes are reviewed next, together with other less widely investigated treatments. 

Other less well-established PET chemolysis methods include degradation by ammonolysis and aminolysis. Degradation of PET by combined treatments has evolved in recent years as an interesting alternative, benefiting from the advantages of each individual process. Thus, a number of combined processes have recently been developed glycolysis-hydrolysis, methanolysis-hydrolysis and glycolysis-methanolysis. 

Company Chemolysis method Capacity (tons/year) Application... 

Current methods of feedstock recovery are reviewed. Brief details are given of pyrolysis, hydrogenation, gasification, and chemolysis. Activities of some European companies are briefly discussed in the areas of recycling mixed plastics waste and closed-loop recycling. 

Conversion into basic chemicals by chemolysis or thermolysis is a secondary method. 

Chemolysis certain polymer families such as polyurethane are chemically depolymer-ized. This is theoretically the best recycling solution if the performances of the original material are to be recovered and if the recyclate is used in the same application. This is, technically and economically, a difficult method that is industrialized in few cases. 

As with most chemical transformations the waste generated are many folds greater than the amount of material produced. Our waste treatment philosophy is to eliminate, reduce, recycle, or treat in that order of preferences. Eliminate, reduce, and recycle are basic process alternatives that are routinely part of the process strategy and are the most cost effective choices. Waste treatment is usually the last alternative and the most expensive option and is necessary in order for a chemical company to maintain its obligations to the environment and community. However, it also offers the most and diverse methods that could be tailored to a specific waste problem. Examples of methods to treat waste are incineration, wet air oxidation, chemolysis, ozonolysis, chlorinolysis, UV treatment, activated carbon, biotreatment, and supercritical oxidation. 

The processes of feedstock recycling of plastic wastes considered in this chapter are based on contact of the polymer with a catalyst which promotes its cleavage. In fact, plastic degradation proceeds in most cases by a combination of catalytic and thermal effects which cannot be isolated. As was described in Chapter 3, the use of catalysts is also usual in chemolysis processes of plastic depolymerization. However, there are two main differences between catalytic cracking and chemolysis there is no chemical agent incorporated to react directly with the polymer in catalytic cracking methods, and the products derived from the polymer decomposition are not usually the starting monomers. 

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