Hydrolysis-glycolysis process

A variation of steam technology is a hydrolysis-glycolysis process at 190-200 °C. LiOH proved to be an excellent catalyst of these reactions and the degradation process is accelerated markedly to just a few minutes while the temperature may be decreased at 170-190 °C [12,34]. 

Therefore, since step-growth polymers are often prepared by reversible reactions, it is feasible to convert them back to their monomers or ohgomers/chemicals by solvolytic processes such as hydrolysis, glycolysis,... 

Table 16.4 provides the summary of economic costs and results. The presumption throughout is that each process can be operated with environmental and human safety to produce a fully satisfactory product of sufficient quality to command market prices. The processes which produce discrete products, i.e. methanolysis and hydrolysis, will have to meet commercial specifications. The mixed-species product processes, i.e. simple glycolysis, hybrid and glycolysis with color filtration, must be able to feed adjacent polymerization facilities to make satisfactory product. Because the simple glycolysis process has little purification capability,... 

The important question of comparative value, mentioned earlier in Section 6, now must be considered. The material output of each process per unit of feed is estimated and multiplied by the market price of the material to arrive at a value of product. The output of the methanolysis processes, DMT and EG, are shown as a methanolysis-type product value in Table 16.5. The stoichiometric ratios are adjusted with a presumed 99 % recovery of DMT and 93 % recovery of EG. The output of the hydrolysis processes, TPA and EG, have the stoichiometric ratios adjusted for a presumed 99 % recovery of TPA and 93 % recovery of EG. The glycolysis processes, including methanolysis/BHET hybrid, are valued at 99 % recovery of terephthalic acid, 95 % recovery of ethylene glycol, and a US 0.022/kg esterification credit for making BHET. The EG recovery is higher for glycolysis-type products because of less loss of useful moieties. The three... 

This process involves reversing chemical reaction where the organic polymer is converted back to more basic chemical building blocks. Among these processes, hydrolysis, glycolysis, methanolysis, and aminolysis are well known. This depolymerization process will also free the glass fiber reinforcement, which is an added value. 

Chemical recovery processes by PU polymer breakdown through hydrolysis, glycolysis and aminolysis processes are extremely important because by using chemical reactions, the PU wastes are chemically transformed into new products which can possibly be used in the fabrication process of new PU. PU wastes are important raw materials for new polyols destined to become rigid and flexible foams. 

For a better understanding of the PU foam wastes recovery by chemical processes. The model reactions for hydrolysis, glycolysis and aminolysis of urethane and urea groups will be presented in the next sections. 

In Sections 20.1-20.4 the main reactions involved in the chemical recovery of PU wastes i.e., hydrolysis, glycolysis, aminolysis and alkoxylation reactions were presented. Several important processes for chemical recovery of PU polymers will be presented in the next chapters. 

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. 

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. 

The principal solvolysis reactions for PET are methanolysis with dimethyl terephthalate and ethylene glycol as products, glycolysis with a mixture of polyols and BHET as products, and hydrolysis to form terephthalic acid and ethylene glycol. The preferred route is methanolysis because the DMT is easily purified by distillation for subsequent repolymerization. However, because PET bottles are copolyesters, the products of the methanolysis of postconsumer PET are often a mixture of glycols, alcohols, and phthalate derivatives. The separation and purification of the various products make methanolysis a cosdy process. In addition to the major product DMT, methanol, ethylene glycol, diethylene glycol, and 1,4-cyclohexane dimethanol have to be recovered to make the process economical.1... 

Hydrolysis, although a simple method in theory, yields terephthalic acid (TPA), which must be purified by several recrystallizations. The TPA must be specially pretreated to blend with ethylene glycol to form premixes and slurries of the right viscosities to be handled and conveyed in modern direct polyesterification plants. Hie product of the alkaline hydrolysis of PET includes TPA salts, which must be neutralized with a mineral acid in order to collect the TPA. That results in the formation of large amounts of inorganic salts for which commercial markets must be found in order to make the process economically feasible. There is also the possibility that the TPA will be contaminated with alkali metal ions. Hydrolysis of PET is also slow compared to methanolysis and glycolysis.1... 

Methanolysis products are separated and purified by distillation. BHET, the monomer obtained by PET glycolysis, is normally purified by melt filtration under pressure. One of the problems encountered in neutral hydrolysis of PET is that the terephthalic acid isolated contains most of the impurities initially present in the PET waste. Hence very elaborate purification processes are required to obtain terephthalic acid of commercial purity. 

The technical advantages of the chemical recycling of PETP bottles are discussed, and some developments in depolymerisation processes are examined. Particular attention is paid to glycolysis, hydrolysis and solvolysis processes respectively developed by TBl, Tredi and Eastman Chemical. 

In Figure 2.4, data for the equilibrium constants of esterification/hydrolysis and transesterification/glycolysis from different publications [21-24] are compared. In addition, the equilibrium constant data for the reaction TPA + 2EG BHET + 2W, as calculated by a Gibbs reactor model included in the commercial process simulator Chemcad, are also shown. The equilibrium constants for the respective reactions show the same tendency, although the correspondence is not as good as required for a reliable rigorous modelling of the esterification process. The thermodynamic data, as well as the dependency of the equilibrium constants on temperature, indicate that the esterification reactions of the model compounds are moderately endothermic. The transesterification process is a moderately exothermic reaction. 

The chemistry of the solid-state polycondensation process is the same as that of melt-phase poly condensation. Most important are the transesterification/glycolysis and esterification/hydrolysis reactions, particularly, if the polymer has a high water concentration. Due to the low content of hydroxyl end groups, only minor amounts of DEG are formed and the thermal degradation of polymer chains is insignificant at the low temperatures of the SSP process. 

The operating rates of each process were fixed at 22 700 annual tonnes. The yield of gross product to Class I or prime product was estimated by this author and is included in Table 16.4. Semi-washed PET flake will contain some extraneous material, by definition. All processes will lose some potential ethylene glycol due to decomposition. Hydrolysis processes are expected to lose isophthalates in the crystallization step. The methanolysis and glycolysis with filtration processes generally recover more materials for inclusion with product. Simple glycolysis recovers the most, but still loses some ethylene glycol. 

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