Cracking, thermal yields

Thermal Cracking. Thermal chlorination of ethylene yields the two isomers of tetrachloroethane, 1,1,1,2 and 1,1,2,2. Introduction of these tetrachloroethane derivatives into a tubular-type furnace at temperatures of 425—455°C gives good yields of trichloroethylene (33). In the cracking of the tetrachloroethane stream, introduction of ferric chloride into the 460°C vapor-phase reaction zone improves the yield of trichloroethylene product. 

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. 

Furnace Black One of the three principal processes used for making carbon black the others being the Thermal Black and the Channel Black processes. In the Furnace Black process, aromatic fuel oils and residues are injected into a high velocity stream of combustion gases from the complete burning of an auxiliary fuel with an excess of air. Some of the feedstock is burned, but most of it is cracked to yield carbon black and hydrogen. The products are quenched with water. 

Thermal cracking often yields a low-value mixture of hydrocarbons with a very broad volatility range that can extend from hydrogen to coke. It is therefore important to determine the optimal pyrolysis conditions and/or the most advantageous catalyst to obtain marketable products (e.g. diesel fuel) from mixed plastic wastes. 

Countercurrent operation of shaft and rotary kiln furnaces leads to extensive heat transfer between feedstock and outgoing products. The sensible heat of the products remains within the system which enhances its thermal yield. Cocurrent operation shows none of these advantages yet it is a simple and efficient method to avoid tar problems. Indeed the tar is extensively cracked while moving through the hearth zone. Also the problem of wastewater is somewhat simplified. Crosscurrent operation is only seldom used. 

This fixed bed process itself was already representing a major improvement over the earlier thermal-cracking methods yielding more gasoline of higher octane rating and less low-value, heavy fuel oil by-product [146]. 

Improved catalyst/oil mixing achieved by good atomizing nozzles results in less thermal cracking and yields that are more selective to gasoline. The presence of the large quantities of diluent in the riser (the steam) reduces hydrocarbon partial pressure and hence reduces coke make (see Table 2). 

The carbonylation of acetaldehyde dimethyl acetal or the hydrocarbonyla-tion of methyl acetate by use of rhodium catalysts yields ethylidene diacetate, which can be cracked thermally to give vinyl acetate ... 

Bergius cerlairdy recognised the relationship between his work and the catalytic hydrogenation of heavy cmde oil fractions, relative to the newly introduced thermal cracking. Thermal cracking of cmde oil fractions was first rrsed in refineries aroimd 1911-1912 to increase the yield of gasoline. By about 1924 the... 

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