Commercial pyrolysis

Pyrolytic routes to hexafluorobenzene have also attracted attention but have not been commercialized. Pyrolysis of tribromofluoromethane [353-54-8] CBr F, at 630—640°C in a platinum tube gives hexafluorobenzene in 55% yield (251—253). The principal disadvantage of this process is the low weight yield of product 90% of the costly CBr F that is charged is lost as bromine. Of economic potential is the related copyrolysis of dichlorofluoromethane [754-34-0] and chlorofluoromethane [593-70-4] (254,255). 

The most common type of commercial pyrolysis equipment is the direct fired tubular heater in which the reacting material flows through several tubes connected in series. The tubes receive thermal energy by being immersed in an oil or gas furnace. The pyrolysis products are cooled rapidly after leaving the furnace and enter the separation train. Constraints on materials of construction limit the maximum temperature of the tubes to 1500 °F. Thus the effluent from the tubes should be restricted to temperatures of 1475 °F or less. You may presume that all reactor tubes and return bends are exposed to a thermal flux of 10,000 BTU/... 

Pressure effects are, in general, not significant in the range of commercial pyrolysis interests. For hydrocarbon partial pressures of 0.2 to 2.0 x 105 Pa, few differences are seen. When pressures are increased to the range of 50 to 100 x 105 Pa, however, the global rate constants sometimes double. 

TABLE V Typical Amounts of Steam for Commercial Pyrolysis... 

Overview of Commercial Pyrolysis Processes for Waste Plastics... 

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)...

The Beijing Roy Environment Technology Co., Ltd (also known as Royco) has developed a commercial pyrolysis process for turning waste plastics into oil known as the EZ-Oil Generator process. 

Thermal reactions of acetylene, butadiene, and benzene result in the production of coke, liquid products, and various gaseous products at temperatures varying from 4500 to 800°C. The relative ratios of these products and the conversions of the feed hydrocarbon were significantly affected in many cases by the materials of construction and by the past history of the tubular reactor used. Higher conversions of acetylene and benzene occurred in the Incoloy 800 reactor than in either the aluminized Incoloy 800 or the Vycor glass reactor. Butadiene conversions were similar in all reactors. The coke that formed on Incoloy 800 from acetylene catalyzed additional coke formation. Methods are suggested for decreasing the rates of coke production in commercial pyrolysis furnaces. 

Further research is required on combustion properties of various commercial pyrolysis oils in order to identify the reasons for emission behaviour, nozzle blockages and related phenomena. 

Efforts should be directed to facilitate good interaction between scientific policy formulators, research organizations, and organizations wanting to commercialize pyrolysis conversion processes. 

Coke and carbon oxides, both undesirable by-products, are always formed to some extent In commercial pyrolysis units. The carbon oxides, are produced when part of the coke reacts with steam that Is used as a diluent with the hydrocarbon feedstock. Most, If not all, of these undesired products are formed by surface reactions that reduce the yields of olefins and other desired products. Coke also acts to Increase heat transfer resistances through the tube walls, and most pyrolysis units must be periodically shut down for decoking of the tubes. During decoking, pure steam or steam to which a small amount of oxygen (or air) Is added Is fed to the reactor, and the coke Is oxidized to produce carbon oxides. 

Several gaseous components present during most commercial pyrolysis runs react with or at the surface. For example, hydrogen reduces the surface oxides (6), desulfurizes coke (7), and reacts with the coke Itself to produce methane (8). Cleaning coke from the surface may act to promote more coke fonnatlon, but reduction of surface oxides presumably often decreases the rate of coke formation. Carbon monoxide also Is a reducing agent for metal oxides and Is sometimes employed during the manufacture of steel. 

How to best apply the results of the present Investigation to commercial units Is not yet known. A small diameter reactor (such as operated In this Investigation at atmospheric pressure) has much larger surface-to-volume and surface-to-mass of gas ratios hence the relative Importance of surface reactions Is much greater In these reactors than In larger diameter conmerclal reactors operated at several atmospheres of pressure. Information currently being obtained will hopefully Indicate Improved techniques needed to predict the Importance of surface reactions In commercial pyrolysis units. Presumably, complicated mathematical models that can be solved with the aid of computers can be developed In the near future. 

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