Hydrocracking waste

The present state of technology is reviewed (mainly from German literature of 1993 -4) in the Add of three principal thermal methods used for plastics wastes, namely pyrolysis (high-temperature carbonisation, coking), hydrocracking and gasification. 36 refs. Articles from this journal can be requested for translation by subscribers to the Rapra produced International Polymer Science and Technology. 

MITI is currently undergoing research to develop technology for the recycling of non-flammable plastics such as those used in business machines and computers. Their National Institute for Resources and Environment plans to decompose, without the production of harmful substances, non-flammable polymers by means of liquid phase hydrocracking, and to recover from them light oils such as benzene, toluene and xylene. The key to the technology, it is claimed, lies in the development of a catalyst which will be able to combine hazardous substances such as bromine and chlorine contained in the waste plastics. 

Catalytic processes (finid catalytic cracking, catalytic hydrocracking, hydro-treating, isomerization, ether manufacture) also create some residuals in the form of spent catalysts and catalyst fines or particulates. The latter are sometimes separated from exiting gases by electrostatic precipitators or filters. These are collected and disposed of in landfills or may be recovered by off-site facilities. The potential for waste generation and hence leakage of emissions is discussed below for individual processes. 

Coprocessing of waste plastics with heavy petroleum fractions have considerable interest in feedstock recycling. In this study, we aimed to investigate the processing of municipal waste plastics (MWP) in presence of conventional and non-conventional catalysts in a refinery stream. For this purpose, the hydrocracking of MWP in vacuum gas oil (VGO) over metal loaded active carbon and conventional acidic catalysts (HSZM-5, DHC-8) was carried out to obtain liquid fuel. 2 refs. 

High-temperatnre pyrolysis and cracking of waste thermoplastic polymers, such as polyethylene, polypropylene and polystyrene is an environmentally acceptable method of recycling. These type of processes embrace both thermal pyrolysis and cracking, catalytic cracking and hydrocracking in the presence of hydrogen. Mainly polyethylene, polypropylene and polystyrene are used as the feedstock for pyrolysis since they have no heteroatom content and the liquid products are theoretically free of sulfur. 

W. Ding, J. Liang, and L. L. Anderson, Hydrocracking and hydroisomerization of high-density polyethylene and waste plastic over zeolite and silica-alumina supported Ni and Ni-Mo sulfides. Energy Fuel, 11, 1219-1224 (1997). 

Waste plastics potentially can also be processed in hydrocracking process as an additional feed stream in mixture with vacuum gas oil or crude oil residues. Careful plastic segregation is then necessary since inorganic additives and impurities of plastics can foul the hydrocracking catalyst. Noncatalytic high-temperature olefin pyrolysis (700-800°C) and coking are insensitive to fouling. 

The liquid product obtained from thermal cracking can be either catalytically cracked/ hydrocracked or co-processed with a refinery feed. Since the catalytic cracking of oil derived from MWP is more or less problematic, any cracking catalyst can be applied to oil derived from pyrolysis of plastics. But the yield and the quality of gasohne obtained from cracking step vary with the type of catalyst and the properties of the pyrolytic oil derivated from waste plastics. 

Activity of these catalysts depends on the balance between the hydrogenation and acidic functions. For example, it was found that HZSM-5 was effective for the hydrocracking of HDPE and plastic waste [24]. But the liquid product contained much less n-paraffins and a greater amounts of aromatics (34%) and naphthenes (21.7%) because of a lack of sufficient hydrogenation function. The reaction mechanism over HZSM-5 can be considered as follows ... 

Based on the above results, it can be mentioned that the catalyst having both hydrogenation and acidic functions can successfully convert heavy oil derived from plastic wastes (relatively clean) into environmentally acceptable transport fuels. However, for the heavy oils containing impurities, the dual functional hydrocracking catalysts still need to be improved. In the hydrocracking process over the acidic catalyst, nitrogen content in feed is limited because basic nitrogen compounds poison the acidic sites of the catalyst. 

In the case of waste polymer mixture, it is expected that impurities in the waste plastic mixture, as well as polymer type, have an effect on the hydrocracking process. In addition, the degradation of single polymers might be different from that of mixtures of polymers. 

As a result, MWP sorted from municipal solid wastes can be processed in a refinery, which has a dechlorination unit installed prior to the hydrocracking unit. Even though commercial catalysts showed satisfactory performance at high temperatures, neutral catalysts based on activated carbon can also be utilized for this purpose. 

R. Holighaus and Klaus Nieman, Hydrocracking of waste plastics, 1996, personal communication. 

There is available proven technology for hydrogen production from resid, natural gas, coal, biomass, and waste, as well as for hydrocracking of oil resid. In fact, as shown in Figure 11.26, as natural gas price increases in recent years, it makes more sense to make H2 from coal and/or resid via gasification followed by WGS reaction. 

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