Plastic cracking

The BP Chemicals polymer cracking process is based at Grangemouth in Scotland and uses mixed plastics as the raw material. The reactor uses a fluidised bed which operates at 500 °C in the absence of air, and under these conditions the plastics crack thermally to yield hydrocarbons. These vaporize and are carried away from the bed with the fluidising gas. Solid impurities such as metals from PVC stabilisers accumulate in the bed or are carried away in the hot gas to be captured by a cyclone further along in the plant. PVC decomposes to HCl and this is neutralized on a solid lime absorbent to yield CaCl2 which is disposed of in landfill. The purified gas is cooled to condense most of the hydrocarbon which can be employed as commercially useful distillate feedstock. The light hydrocarbons which are less easy to condense are compressed, reheated and recycled as fluidising gas. 

Resources, Conservation Recycling 23,No.3, 1998,p.l63-81 CATALYTIC PLASTICS CRACKING FOR RECOVERY OF GASOLINE-RANGE HYDROCARBONS FROM MUNICIPAL PLASTIC WASTES Buekens A G Huang H Brussels,Free University... 

I6D. M. Parks, Advances in Characterization of Elastic-Plastic Crack-Tip Fields, Topics in Fracture and Fatigue, A. S. Argon, Ed., Springer-Verlag, Berlin (1992). 

C. F. Shih and A. Needleman, Fully Plastic Crack Problems, Part I Solutions by a Penalty Method, J. Appl. Mech., 51[3], 48-56 (1984). 

The book also explores the application of various acidic catalysts, such as silica-alumina, zeolites (HY, HZSM-5, mordenite) or alkaline compounds such as zinc oxide. However, the main problem with catalytic cracking is that in the course of the cracking process all catalysts deactivate very quickly. Expensive zeolite catalysts increase the cost of waste plastics cracking process to the point where it becomes economically unacceptable since the catalyst becomes contained in coke residue and therefore cannot be recovered and regenerated. 

Table 3.1 Structural features of common zeolites used as catalysts in plastic cracking...

Chemical composition of waste plastic cracking products depends on shares of the individual polymers (PE, PP, PS) in the feed and process parameters. This fact decides the technological application of the final products. Important products of the cracking process, both petroleum fractions and waste plastics, are coke residues. Coke residue yield increases considerably, up to 10 wt%, in cracking of municipal and industrial waste plastics since they contain various inorganic impurities and additives. It can be applied as solid fuel, like brown coal. In the fluid cracking the solid residue is continuously removed from the process by combustion in a regenerator section. 

Sophisticated catalysts, such as ZSM-5 or HZSM-5 [22] and other zeolites are also suggested in numerous papers, e.g. KEY [23], HY and H-mordenite [24], Re-zeolite-based Engelhardt FCC commercial catalyst [25], and steamed commercial zeolite catalyst [26]. These investigations are mainly devoted to fundamental studies and the correlation between feed composition, catalyst properties, process parameters and efficiency connected with prodnct distribution. Iron supported on silica-alumina, mesoporous silica and active carbons serves as the next example of materials applied in the waste plastics cracking [27, 28]. On the other hand, according to some results [29] application of cracking catalysts such as Zn-13X, Fe-5A and CoMo-HY are ineffective in waste plastics cracking. 

It seems that fluid-bed cracking reactor (thermal or catalytic) is the best solution for industrial scale. However, regeneration and circulation of so-called equilibrium cracking catalyst is possible for relatively pure feeds, for instance crude oil derived from vacuum gas oils. Municipal waste plastics contain different mineral impurities, trace of products and additives that can quickly deactivate the catalyst. In many cases regeneration of catalyst can be impossible. Therefore in waste plastics cracking cheap, disposable catalysts should be preferably applied. Expensive and sophisticated zeolite and other molecular sieves or noble-metal-based catalysts will find presumably limited application in this kind of process. The other solution is thermal process, with inert fluidization agent and a coke removal section or multi-tube reactor with internal mixers for smaller plants. 

A. G. Buekens and H. Huang, Catalytic plastics cracking for recovery of gasoline-range hydrocarbons from municipal plastic wastes. Resources, Conservation and Recycling, 23, 163 (1998). 

Most commodity hydrocarbon plastics are suitable for pyrolysis. Generally the larger the substituent in the side chain, the easier the plastic can be degraded. The order of increasing side chain size is polyethylene<polyvinyl chlorideProblems with many pre-existing plastic cracking technologies include ... 

Table 28.1 Laboratory experiments on catalytic plastics cracking...

The introduction of catalysts can lower the activation energy for plastics cracking, which can greatly lower the reaction temperature needed. But temperature is still a very important factor that can greatly affect the pyrolysis process. 

Ying Huang, Hongxia Yan, and Qiuyu Zhang, Liquid fuel manufactured from waste plastics cracking, Plastics, 31(4), 36-40 (2002). 

Fig. 1 Evaluation of the elastic-plastic crack propagation energy rate, J, from incremental crack growth measurements.

Paris, P.C., Tada, H., Zahoor, A. and Ernst, H., (1979), The theory of instability of the tearing mode of elastic-plastic crack growth. In J.D. Landes, J.A. Begley and G.A. Clarke (Eds), Elastic-Plastic Fracture, ASTM STP 668. American Society for Testing Materials, Philadelphia, 5. 

The most common catalysts used in plastic cracking are acidic solids, mainly alumina, amorphous silica-alumina and zeolites. These materials are the catalysts typically used in the petroleum processing and petrochemical industries. They have very different textural and acid properties, which directly determine their catalytic activity and product selectivity. Thus, while the acidity of alumina is of Lewis type, both Brdnsted and Lewis acid sites may be present in amorphous silica-alumina and zeolites. This is an important factor because... 

A retarded mix must be adequately cured in order to prevent plastic cracking, which can arise if the concrete is allowed to dry out before sufficient strength has developed. 

---