Liquefaction plastics waste

Texaco gasification is based on a combination of two process steps, a liquefaction step and an entrained bed gasifier. In the liquefaction step the plastic waste is cracked under relatively mild thermal conditions. This depolymerisation results in a synthetic heavy oil and a gas fraction, which in part is condensable. The noncondensable fraction is used as a fuel in the process. The process is very comparable to the cracking of vacuum residues that originate from oil recycling processes. 

Polymers have inherently high hydrocarbon ratios, making liquefaction of waste plastics into liquid fuel feedstocks a potentially viable commercial process. The objective is to characterise the thermal degradation of polymers during hydrogenation. LDPE is studied due to its simple strueture. Isothermal and non-isothermal TGA were used to obtain degradation kinetics. Systems of homopolymer, polymer mixtures, and solvent-swollen polymer are studied. The significant variables for... 

Plastic Waste Management Institute Newsletter (Japan), Liquefaction Technology 15(3) 1 (1998). 

According to a US patent [46], cracking or liquefaction of waste plastics is realized in a sequential three- or four-screw extruder system with increasing process temperature. Similarly, the catalyst (if used) is discharged with coke and mineral residue. At the end of the process, the distillable hydrocarbon fraction is separated from solid residue and coke. 

There are different methods for carrying out the catalytic liquefaction of plastic wastes. 

Texaco has recently adapted its gasification process to plastics waste [76]. The project is to develop a capacity of 40-50 kt/yr in Pemis (Netherlands). This process consists in two parts a liquefaction step and an entrained bed gasifier. The plastic waste is depolymerized... 

Although direct liquefaction of waste plastic looked promising, problems associated with impurities (paper, aluminum, etc.) and chlorine derived from PVC caused operational difficulties. Consequently, it currently appears that the first step of any feedstock recycling process for waste plastics or tires should be pyrolysis, which allows much easier separation of solid impurities and chlorine. Research on pyrolysis of post-consumer plastic has been carried out by Kaminsky and co-workers [17, 18], Conrad Industries [19, 20], and Shah et al. [21]. Shah et al. [21] conducted pyrolysis experiments on relatively dirty post-consumer waste plastic obtained from the DSD. The pyrolysis oils were then subjected to hydroprocessing to convert them into high-quality transportation fuels (gasoline, kerosene, diesel). 

The research on the liquefaction of waste polymers and the coprocessing of waste plastics with vacuum residue or coal has been a subject of study by many groups at universities and environmental agencies. A significant amount of effort and funds are being diverted to develop process technology in order to convert mixed plastics waste from municipal... 

The 1973 petroleum crisis intensified research on coal liquefaction and conversion processes. The technology developed in this field was later harnessed in chemical recycling of plastics. Mastral et al. [32], for example, employed two different batch reaction systems (tubing bomb reactors and magnetically stirred autoclave) and a continuous reactor (swept fixed bed reactor). Chemical recycling techniques such as pyrolysis [28, 33-38] or coliquefaction with coal [39, 40] convert plastic wastes into hydrocarbons that are valuable industrial raw materials. 

The liquefaction of waste plastics is not an entirely new technique, but the challenge to operate commercial plants has been tried in Germany by pyrolysis and hydrogenation. However, the former, operated by BASF was stopped in 1996, and the latter by Veba Oel GmbH in 1999 [1]. It is well known that the reason is not a technical problem, but an economic one the cost is higher than those of competitive techniques, such as mechanical recycling or blast furnace application. 

Table 26.1 Brief history of liquefaction of waste plastics in Japan...

Co. Ltd (SPR) was founded to recycle the waste plastics collected in Sapporo city, where 13 500 t has been baled every year. This system has received excellent support by Sapporo city. However, only half of the baled plastics is used in the SPR liquefaction plant located just next to the baling plant due to the tender system controlled by JCPRA. Thus, the operating ratio of the SPR plant is not so high, and this also raises the treatment cost of waste plastics, as does the small scale. The remaining half is used for the iron and steel industry, far from Sapporo. The third liquefaction plant constructed at Mikasa city, Hokkaido stopped this March, since sufficient raw plastic waste could not be obtained. 

Figure 26.1 The Niigata liquefaction plant (Reproduced by permission of Plastic Waste Management Institute)...

K. R. Prakash and A. R. Tarrer, High temperature liquefaction of waste plastics. Fuel, 77(4), 293-299 (1998). 

Utilization of wood-biomass residues as well as waste polymers is the important direction of recent research activities. It is known that direct catalytic liquefaction of plant biomass can be used to produce liquid fuels and chemicals [1,2]. Co-pyrolysis and co-hydropyrolysis processes have the potential for the environmentally friendly transformation of lignocellulosic and plastic waste to valuable chemicals. 

In a further work,2 the authors studied the processing of plastic wastes in mixtures of tetralin and used automotive oil. Figure 6.2 shows the results obtained with different waste oil/tetralin ratios. ZSM-5 was again more active than the ferrihydrite-based catalyst, while the latter led to oil yields very close to those of the thermal degradation. In both thermal and catalytic experiments, the activity was strongly enhanced by an increase in the waste oil/tetralin ratio, which shows the latter is not the best solvent for the liquefaction of aliphatic plastics. 

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