By-products of Pyrolysis

Etliylene production involves liigh temperatures (1500°F) in tlie pyrolysis section and cryogenic temperatures in tlie purification section. The feedstocks, products, and by-products of pyrolysis are flaimnable and pose severe fire liazards. Benzene, wliich is produced in small amounts as a byproduct, is a known carcinogen. Table 21.7.1 summarizes some of the properties of etliane (feedstock) and tlie product gases. Figure 21.7.1 shows a simplified schematic diagram of the pyrolysis and waste heat recovery section on an etliylene plant. 

Pyrolysis produces three principal products - pyrolytic gas, oil, and char. Char is a fine particulate composed of carbon black, ash, and other inorganic materials, such as zinc oxide, carbonates, and silicates. Other by-products of pyrolysis may include steel (from steel-belted radial tires), rayon, cotton, or nylon fibers from tire cords, depending on the type of tire used. 

The by-products of pyrolysis (i.e. char and gases) can be further processed (e.g. combusted to provide the needed heat for the pyrolysis process) [14], Typically, the char... 

Carbonization. The molded part is carbonized by slow heating in a reducing or inert environment. A typical heating cycle is 1 - 5°C/min to 800°C and 5 -10°C/min from 800 -1000°C. The heating rate is a function of the rate of diffusion of the volatile by-products of pyrolysis. This diffusion rate must be slowto avoid disruption and rupture of the carbon structure the thicker the part, the slower the rate. For very thin parts, i.e., 10 m or less, only a few minutes may be needed for thicker parts, carbonization may take weeks. For that reason, wall thickness is usually limited to less than 4 mm. 

It is produced from dry - wood by thermal decomposition (400-500 °C) in kilns with controlled access of air and in retorts in the absence of air. Saw dust, fruit stones, nut shells and com cobs may be used also. By-products of pyrolysis are acetic acid, methanol, wood creosote and many others. 

Pyrolysis gasoline is a by-product of the steam cracking of hydrocarbon feeds in ethylene crackers (see Ethylene). Pyrolysis gasoline typically contains about 50—70 wt % aromatics, of which roughly 50% is benzene, 30% is toluene, and 20% is mixed xylenes (which includes EB). 

Hydrogen is also a significant by-product of other industrial processes, such as steam pyrolysis (68) of hydrocarbons to produce ethylene (qv), eg,... 

Liquid Fuels. Liquid fuels can be obtained as by-products of low temperature carbonization by pyrolysis, solvent refining, or extraction and gasification followed by catalytic conversion of either the coal or the products from the coal. A continuing iaterest ia Hquid fuels has produced activity ia each of these areas (44—46). However, because cmde oil prices have historically remained below the price at which synthetic fuels can be produced, commercialization awaits an economic reversal. 

Propjiene [115-07-17, CH2CH=CH2, is perhaps the oldest petrochemical feedstock and is one of the principal light olefins (1) (see Feedstocks). It is used widely as an alkylation (qv) or polymer—ga soline feedstock for octane improvement (see Gasoline and other motor fuels). In addition, large quantities of propylene are used ia plastics as polypropylene, and ia chemicals, eg, acrylonitrile (qv), propylene oxide (qv), 2-propanol, and cumene (qv) (see Olefin POLYMERS,polypropylene Propyl ALCOHOLS). Propylene is produced primarily as a by-product of petroleum (qv) refining and of ethylene (qv) production by steam pyrolysis. 

Chlorinated by-products of ethylene oxychlorination typically include 1,1,2-trichloroethane chloral [75-87-6] (trichloroacetaldehyde) trichloroethylene [7901-6]-, 1,1-dichloroethane cis- and /n j -l,2-dichloroethylenes [156-59-2 and 156-60-5]-, 1,1-dichloroethylene [75-35-4] (vinyhdene chloride) 2-chloroethanol [107-07-3]-, ethyl chloride vinyl chloride mono-, di-, tri-, and tetrachloromethanes (methyl chloride [74-87-3], methylene chloride [75-09-2], chloroform, and carbon tetrachloride [56-23-5])-, and higher boiling compounds. The production of these compounds should be minimized to lower raw material costs, lessen the task of EDC purification, prevent fouling in the pyrolysis reactor, and minimize by-product handling and disposal. Of particular concern is chloral, because it polymerizes in the presence of strong acids. Chloral must be removed to prevent the formation of soflds which can foul and clog operating lines and controls (78). 

Miscellaneous Fuels A variety of gases have veiy minor market shares. These include reformed gas, oil gases, producer gas, blue water gas, carbureted water gas, coal gas, and blast-furnace gas. The heating values of these gases range from 3.4 to 41 MJ/m (90 to 1100 Btii/ftq. They are produced by pyrolysis, the water gas reaction, or as by-products of pig-iron production. 

Background Converting coal to combustible gas has been practiced commercially since the early nineteenth century. The first gas-producing companies were chartered in 1812 in England and in 1816 in the United States to produce gas for illumination oy the heating or pyrolysis of coal. This method of producing gas is still in use the gas is a by-product of the carbonization of coal to manufacture coke for metallurgical purposes. 

Pyrolysis of scrap tires was studied by several mbber, oil, and carbon black industries [14]. Pyrolysis, also known as thermal cracking is a process in which polymer molecules are heated in partial or total absence of air, until they fragment into several smaller, dissimilar, random-sized molecules of alcohols, hydrocarbons, and others. The pyrolysis temperature used is in the range of 500°C-700°C. Moreover, maintenance of partial vacuum during pyrolysis in reactors lowered the economy of the process. Several patents were issued for the pyrolysis of worn out tires to yield cmde oil, monomers, and carbon black in economic ways [15-18]. The major drawback of chemical recycling is that the value of the output is normally low and the mixed oils, gases, and carbon black obtained by pyrolysis cannot compete with similar products from natural oil. Pyrolyzing plant produces toxic wastewater as a by-product of the operation [19]. 

Applications Applications of IC extend beyond the measurement of anions and cations that initially contributed to the success of the technique. Polar organic and inorganic species can also be measured. Ion chromatography can profitably be used for the analysis of ionic degradation products. For example, IC permits determination of the elemental composition of additives in polymers from the products of pyrolysis or oxidative thermal degradation. The lower detection limit for additives in polymers are 0.1% by PyGC... 

Cyclic gas generators converted coke, a by-product of high-temperature pyrolysis process, to a synthetic gas by alternatively exposing the coke to air to provide heat and to steam to produce a gas that burned with a blue flame. The coal gas was know as blue water gas (Probstein, R. F. and Hicks, R. E., Synthetic Fuels, McGraw-Hill, 1982, p. 7). 

Pyrolysis processes can be divided into two groups low temperature and high temperature. The products of pyrolysis processes differ and can be controlled by temperature and the rate of material heating. Table 4.1 provides a summary of variations of the product slate for biomass and coal feedstocks. 

This can certainly be extended to other metal sulphides, using other complexes of sulphur (and also selenium). However, the complex and anion of the metal salt need to be chosen so that all the by-products of the pyrolysis reaction are volatile, otherwise the film will be contaminated with the nonvolatile by-products. For example, using cadmium nitrate and thiourea, all the by-products are volatile ... 

Several zeolites were used both in original and calcined form. The volatile products of pyrolysis either analyzed directly by GC/MS or after passing over the respective zeolites. 

V (at oxygen consumption over 90%) indicates that Reactor III gave the highest selectivity (ca. 60% ) as to hydrogen peroxide formation, and the lowest yield of by-products from pyrolysis and combustion. Reactor... 

Thus, at today s naphtha prices and by-product markets, pyrolysis of light vacuum gas oil appears closely competitive with naphtha for ethylene production. 

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