Recycling methods cracking

Although there are minor differences in the HCl—vinyl chloride recovery section from one vinyl chloride producer to another, in general, the quench column effluent is distilled to remove first HCl and then vinyl chloride (see Eig. 2). The vinyl chloride is usually further treated to produce specification product, recovered HCl is sent to the oxychlorination process, and unconverted EDC is purified for removal of light and heavy ends before it is recycled to the cracking furnace. The light and heavy ends are either further processed, disposed of by incineration or other methods, or completely recycled by catalytic oxidation with heat recovery followed by chlorine recovery as EDC (76). 

Figure 2.3.2 (Kraemer and deLasa 1988) shows this reactor. DeLasa suggested for Riser Simulator a Fluidized Recycle reactor that is essentially an upside down Berty reactor. Kraemer and DeLasa (1988) also described a method to simulate the riser of a fluid catalyst cracking unit in this reactor.

A less effective, but more economically viable method, would be to recycle all low-value hydrocarbon by-products to the cracker furnace. This particularly focuses on methane which within the confines of an operation is typically valued relative to the fuel oil price. However, this equally applies to ethane and propane which are generally recycled to the feedstock side of the cracking furnace. Depending on the relative value, it may be optimal for minimising carbon emissions in some operations to use ethane as a fuel rather than a feedstock. 

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. 

Furthermore, a study of the behaviour of a fluidized-bed reactor is very important as one of the options for a possible commercialization of such a pyrolysis recycle process is to co-feed plastic waste into the FCC cracker unit of an oil refinery. Studies of plastic being a fraction of the feed of a cracking process have been carried out by Ng et al. [7] and Arandes et al. [12, 27, 28]. They not only showed the applicability of the method, but discovered a synergetic effect on the cracking of the oil decreasing the amount of aromatics. 

Pyrolysis is a tertiary or feedstock recycling technique capable of converting plastic waste into fuels, monomers, or other valuable materials by thermal and catalytic cracking processes. This method can be applied to transform both thermoplastics and thermosets in high-quality fuels and chemicals. Moreover it allows the treatment of mixed, unwashed plastic wastes. 

Hydrogenation, usually in the presence of catalysts, is the final method of feedstock recycling considered. In the process, the polymers are cracked in a hydrogen... 

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. 

Five main methods of feedstock recycling have been considered in this book, classified according to the degradation conditions and the products obtained chemical depolymerization, gasification, thermal treatments, catalytic cracking and reforming, and hydrogenation. 

Catalytic cracking is the cracking of heavy hydrocarbons using catalyst. The polyolefins such as PP and PE are recycled through this method. In the laboratory scale setup, these reactions are carried out in a flow reactor. There are two modes of catalytic treatment, liquid phase contact and vapor phase contact. In first case, the catalyst is in contact with molten polymers and here the catalyst reacts mainly with oligomers. In vapor phase contact, the catalyst is in contact with thermally degraded polymer [27]. 

Reshape, or surface recycling, is used to transform a partially failed surface layer to a good as new layer without the addition of new hot mix asphalt or new aggregate material. Through this method, surface cracks are healed and surface profile is reshaped free of deformations. 

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