Depolymerization feedstock

Various processes have been developed for hydrolyzing lignocellulose to its major constituents, i.e., to sugars and (partly) depolymerized lignin. The lignin is usually precipitated from the aqueous solution and either used as chemical feedstock or burned as process fuel. The aqueous sugar solution is then applied for fermentation to ethanol after neutralization and purification. 

Claims of perpetual motion create moments of mirth and consternation for those knowledgeable in the laws of thermodynamics. Yet, is it only hyperbole when a responsible journal such as the European Plastics News [1] proclaims that depolymerization of polyethylene terephthalate (PET) can be repeated indefinitely The second law of thermodynamics brings us back to reality. The depolymerization of PET does not operate at 100% yields, but does offer the opportunity for near-stoichiometric recovery of the monomers used to make the polyester. With high yields of potentially valuable monomers, the commercial potential for polyester depolymerization to regain feedstocks must be considered. 

Depolymerization processes have been proposed for poly(butylene terephtha-late) by the glycolysis of PBT with 1,4-butanediol and a titanium catalyst [65]. Methanolysis of poly(ethylene naphthalate) to dimethyl naphthalate and ethylene glycol has also been proposed [66, 67], but not implemented. The lack of commercial depolymerization of PEN is probably due not to technical limitations, but to insufficient supplies of PEN polymer feedstock to meet the minimum quantities needed for economical operations. 

Other than for simple glycolysis, a substantial capital investment must be made to conduct commercial depolymerization of PET to regain PET monomers for repolymerization of PET. As the capital costs rise at roughly the 0.6 power of the relative volume [68], larger facilities are more economically attractive than smaller facilities. Besides the availability of capital to build very large depolymerization facilities, the limiting criterion has been and is likely to continue to be the sure supply of adequate PET feedstock at acceptable prices. 

These benzaldehydes could then be directly used as a feedstock for various polymeric products or reduced to form phenolic benzylic alcohol derivatives (i.e., p-methylol groups). The p-methylol groups would thus be active sites, whereas in unmodified lignins, the C-l site is blocked and unreactive. In addition, the oxidative-cleavage step will hydrolyze a portion of the lignin interunit ether bonds, and thus increase the total fraction of free phenolic units to further enhance the reactivity. Other possible benefits are that the lignin would be extensively depolymerized and would form a more uniform feedstock material both conditions would give a product that is easier to handle. 

Pyrolysis treatments are interesting regarding the aforementioned plastic refuse makeup. Other successful treatments for feedstock recycling of condensation polymers (PET, ABS, etc.), that allows for the depolymerization and recovery of their constituent monomers (e.g. hydrolysis, alcoholysis, methanolysis, etc.), cannot be applied for polyolefin plastics recycling. In contrast, pyrolysis of polyolefins yields valuable hydrocarbon mixtures of... 

Neat polystyrene feedstocks will depolymerize in a pyrolysis process to give predominantly styrene monomer-a liquid fuel with good energy content. 

In summary, pyrolysis is a tertiary recycling process that is used to break down large polymer molecules. In this process, the polymer samples are heated in an inert atmosphere, which causes the carbon-carbon bonds to break along the polymer backbone. This depolymerization step results in monomers (short-chained compounds) being formed. Generally, three types of products are formed from pyrolysis reactions gas, oil and char. All three have the potential to be nsed as a fnel or chemical feedstock. Depending on the feed polymer and the reaction conditions, different products can be obtained. The pyrolysis oil can either be used directly or can be nsed as a raw material for the petroleum industry [1-5]. 

Serrano et al. [11] studied the use of a laboratory-scale screw kiln reactor to transform low-density polyethylene (LDPE) into petrochemical feedstock. In this process, pyrolysis was carried out at reaction temperatures of 400-550°C and screw speeds of 3-20 rpm (Figure 19.6). In this process the plastic feed is initially heated in a feed hopper until the feed is melted. The melted plastic is then fed into the screw conveyor where it is depolymerized into gas, liquid and solid. The hopper is equipped with a stirrer to mix the feed plastic. Nitrogen is also used to provide an inert medium for pyrolysis. 

Depolymerization of the biopolymer structure is an issue in almost all utiUzation concepts. The established chemical technology using cellulose as a base material is a significant exception. Recovery of proteins is proving to be a second major exception. For the most, pari the carbohydrate structure needs to be broken down into useful monomers like glucose or xylose. Over the years, the assessment of hydrolysis with catalysts, both mineral and biological, has been evaluated with a range of biomass feedstocks. 

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. 

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. 

Five main methods of feedstock recycling have been considered in this book, classified according to the degradation conditions and the products obtained chemical depolymerization, gasification, thermal treatments, catalytic cracking and reforming, and hydrogenation. 

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