Mixed plastics hydrogenation

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. 

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. 

Polyolefin resins contain only carbon and hydrogen, and additives, such as some antioxidants and UV stabilizers. Moreover, the presence of hetero-elements, such as chlorine and bromine is undesirable, as these elements distribute over the three product phases-gas, liquid, and solids, reducing the market potential and value of each of these. Studying their elimination is a major consideration in developing processes for mixed plastics. 

In the Pemis plant, Texaco has adopted a downdraft reactor fed with oxygen and/or steam according to the waste feed inpnt. This technology has been developed for coal, heavy oils, petroleum coke, orimulsion, wastes and mixed plastics. End prodncts conld be energy (combined cycles), hydrogen, ammonia or methanol according the type of feed [75]. 

Thermal cracking often yields a low-value mixture of hydrocarbons with a very broad volatility range that can extend from hydrogen to coke. It is therefore important to determine the optimal pyrolysis conditions and/or the most advantageous catalyst to obtain marketable products (e.g. diesel fuel) from mixed plastic wastes. 

The reaction of various metal oxides with hydrogen chloride and the reverse reactions have been extensively smdied [47] the reaction behavior of hydrogen chloride with various bivalent and trivalent metal oxides has been reported. Sakata et al. reported the spontaneous degradation of municipal waste plastics at low temperature [48] and also the dechlorination of chlorine compounds from PVC mixed plastics-derived oil using solid sorbents [22], Courtemanche and Levendis [49] reported the control of HCl emission from the combustion of PVC by in-fumace injection of calcium-magnesium-based sorbents at gas temperatures of 850 and 1050°C. In the present study, the adsorption temperature 350°C was found to be optimum for the complete removal of hydrogen chloride at moderate concentrations (1820 ppm). 

The main difference between Veba process and other processes lies in that hydrogenation technology is used in this process, which improves the quality of products. At the same time, waste plastics are stirred and fully mixed by hydrogen. This whole apparatus is capable of disposing of 40000 tons of waste plastics per year, but is relatively complicated and expensive. 

There are all kinds of processes and reactors for pyrolysis of PVC-containing mixed plastics which can be basically divided into three classes thermal cracking, catalytic cracking and hydrogenation. The main products are gasoline, diesel oil, fuel gas and HCl. 

From mixed plastics, the formation of ethene, methane and hydrogen increase with temperature but the formation of propene and higher hydrocarbons decreases. Whether the isolation of olefins can compete economically with the oil cracking process remains to be seen, but the total hydrocarbon pyrolysate is similar to naphtha, the feedstock for petrochemicals production. It should be noted that because pyrolysis is primarily a non-oxidative process carried out under carefully controlled conditions, the possibility of dioxin formation is much reduced. In 1995 about 100 kT of mixed plastics wastes were converted into chemical feedstocks in Europe by pyrolysis, and this is expected to increase at the expense of incineration over the next ten years. 

Considering the preservation or saving of the material, the material recycling is the most effective procedure. The usable fractions are about 70 to 80 % [6]. Also raw-material recycling processes achieve an output of 70 up to 80 %, exceptional the pyrolysis of mixed plastics waste. Hydrogenation of a clean... 

This is very similar to pyrolysis, but in this process the mixed plastic waste (MPW) is heated with hydrogen. As the molecules are cracked (the process is often termed hydrocracking), they are saturated with the hydrogen molecules to produce a saturated liquid and gaseous hydrocarbons. The synthetic crude oil produced is of a very high quality. It is necessary to keep the pressure of the hydrogen sufficient to suppress repolymerisation or the generation of undesirable by-products. 

One of the most expensive and time-consuming aspects of polymer recycling is the separation process. It would therefore make economic sense if mixed plastics waste could be taken and recycled back to suitable feedstocks. The essential feature of feedstock recycling processes is the use of heat (thermolysis) to break bonds, similar to crude oil refining. The process may be carried out by heat alone (pyrolysis), in a hydrogen atmosphere, or in the presence of oxygen (gasification). In the latter case... 

The good miscibility of PBI-PVPy blends can be useful for property modifications in the plastics industry, particularly in the areas of processability, where intermediate Tg values are sufficient and economical, and with retention of good mechanical properties at elevated temperatures. Blending can be achieved by solution mixing, and hydrogen bonding promotes compatibility. 

The unit recycles up to 40,000 metric tons per year of mixed plastic containing up to 10% PVC. Up to 20% of its feed will be mUled plastic particles less than 8 mm in diameter. [The] unit operates at 150-300 bars and about 470°C in a hydrogen atmosphere, producing a syncrude containing 60% paraffins, 30% naphtha, 9% aromatics and 1% olefins. Chlorine... is converted to HCl in the reactor, then neutralized with calcium carbonate. 

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