Cracking and Reforming

The processes of feedstock recycling of plastic wastes considered in this chapter are based on contact of the polymer with a catalyst which promotes its cleavage. In fact, plastic degradation proceeds in most cases by a combination of catalytic and thermal effects which cannot be isolated. As was described in Chapter 3, the use of catalysts is also usual in chemolysis processes of plastic depolymerization. However, there are two main differences between catalytic cracking and chemolysis there is no chemical agent incorporated to react directly with the polymer in catalytic cracking methods, and the products derived from the polymer decomposition are not usually the starting monomers. 

Compared to the simple cleavage of the polymer by thermal effects, catalytic cracking has a number of advantages  

In the following sections of this chapter, the catalytic conversion of individual plastics (polyethylene, polypropylene and polystyrene) is first reviewed, followed by a description of the processes developed for the catalytic cracking of plastic and rubber mixtures. Finally, methods based on a combination of thermal and catalytic treatments are considered. However, taking into account that the key factor in the catalytic conversion of plastic wastes is the catalyst itself, we will first describe the main properties of the most widely used catalytic systems for the degradation of polymers. 

---

Natural gas and crude oils are the main sources for hydrocarbon intermediates or secondary raw materials for the production of petrochemicals. From natural gas, ethane and LPG are recovered for use as intermediates in the production of olefins and diolefms. Important chemicals such as methanol and ammonia are also based on methane via synthesis gas. On the other hand, refinery gases from different crude oil processing schemes are important sources for olefins and LPG. Crude oil distillates and residues are precursors for olefins and aromatics via cracking and reforming processes. This chapter reviews the properties of the different hydrocarbon intermediates—paraffins, olefins, diolefms, and aromatics. Petroleum fractions and residues as mixtures of different hydrocarbon classes and hydrocarbon derivatives are discussed separately at the end of the chapter. 

CH3)2. CH.CH2.CH3 mw 72.15, colorl liq, mp -159.9°, bp 27.85°, d 0.6201 g/cc at 20/4°, RI 1.35370. Sol in ethanol, ether, hydrocarbons and oils, insol in w. First prepd by Frankland in 1850 by treating iso-amyl iodide with Zn in w at 140° (Ref 2). It was isolated by Young from American petroleum (Ref 3). Present methods of prepn include fractional distn of petroleum and subsequent purification of the crude isopentane by rectification, as well as cracking and reforming of crude oil components and natural gasolines in oil refineries (Refs 4 7)... 

Chemical reactions enhanced by catalysts or enzymes are an integral part of the manufacturing processes for the majority of chemical products. The total market for catalysts and enzymes amounts to 11.5 billion (2005), of which catalysts account for about 80%. It consists of four main applications environment (e.g., automotive catalysts), 31% polymers (e.g., polyethylene and polypropylene), 24% petroleum processing (e.g., cracking and reforming), 23% and chemicals, 22%. Within the latter, particularly the catalysts and enzymes for chiral synthesis are noteworthy. Within catalysts, BINAPs [i.e., derivatives of 2,2 -bis(diphenylphosphino) -1, l -bis-l,l -binaphthyl) have made a great foray into chiral synthesis. Within enzymes, apart from bread-and-butter products, like lipases, nitrilases, acylases, lactamases, and esterases, there are products tailored for specific processes. These specialty enzymes improve the volumetric productivity 100-fold and more. Fine-chemical companies, which have an important captive use of enzymes, are offering them to third parties. Two examples are described here ... 

Oil refineries (hydrogen produced in cracking and reforming operations ... 

Fractionation of straight-run, cracked, and reforming distillates, or even fractionation of crude petroleum... 

Benzene is one of the major chemicals produced by the petroleum industry. More than 1.6 billion gallons are produced each year by cracking and reforming various petroleum fractions. Most of this is used in the production of styrene, which is then polymerized to polystyrene. Other arenes that are made in large amounts include toluene (830 million gallons), cumene, o-xylene, and p-xylene. At one time, benzene was an important solvent in the organic laboratory. Recently, however, its use has been phased out because of its potential adverse health effects. Long exposure to benzene has been shown to lead to bone marrow depression and leukemia. 

See Figure 13.36.) There has been a tremendous increase in the demand for a variety of petroleum products in the early twentieth century. This demand has forced the oil industry to develop new techniques to increase the yield from each barrel of oil. These techniques are called cracking and reforming. 

In this section, you learned that petrochemicals, an essential part of our society and technology, are obtained from petroleum. You discovered how petroleum is separated into its components by fractional distillation, cracking, and reforming. 

The introduction of catalytic converters has had a tremendous impact on the composition of gasoline. The catalysts used became poisoned by small amounts of impurities in particular the lead compounds present in high octane gasoline were detrimental. Processes which produce high octane number compounds were therefore stimulated. First, cracking and reforming increased in importance. More recently, the aromatics content is also expected to have to decrease and alternative processes are in use or under way, e.g. the production of MTBE (methyl tertiary-butyl ether). 

N and ppm levels of various metals such as vanadium and nickel). These elements are harmful to the environment, as upon combustion they produce SOx and NOx gases (responsible for acid rain), and to the chemical industry, as these molecules can poison the catalysts needed for the subsequent cracking and reforming operations (see Box 2, and Chapter 1, Section 1.2.1). 

Figure 15.6 Process flow for commercial pyrolysis plant (Thermofuel ) for converting waste plastics into diesel fuel. The plastic is heated to 375-425°C and the pyrolysis vapours are catalytically cracked and then selectively condensed. Note that the pyrolysis vessel is purged with nitrogen gas and that the hot pyrolytic vapours pass from the pyrolysis vessel to the catalytic reaction tower where they are cracked and reformed to give a high-purity diesel stream. (Reproduced by permission of Ozmotech Pty Ltd)...

Fig. 7.3. Development of Research Octane Number of two grades of gasoline in Germany, with the technologies responsible for the changes. The octane quality of the original unprocessed hydrocarbon (S.R. tops) was improved and augmented by material from cracking and reforming processes, from tetra-alkyl lead addition and by alcohols which were used at times of crisis because they could be produced from internal resources.

---