Products of thermal cracking

Because of the different chemistry of cracking processes their products have different compositions. The major product of thermal cracking is ethylene, with large amounts of Ci and C3 hydrocarbons, and C4— C15 terminal alkenes. Thermal cracking, consequently, is used mainly in olefin manufacture. 

Today, the majority of ethylene is produced by thermal cracking of hydrocarbon feedstocks ranging fi-om ethane to heavy vacuum gas oils. Over 60% of the world s propylene is produced as a by-product of thermal cracking, with the balance being supplied from refinery sources and others. Raw materials are mosdy natural gas condensate components (principally ethane and propane) in the US and Mideast and naphtha in Europe and Asia. Alkanes/olefins are broken apart at high temperatures, often in the presence of a zeolite catalyst, to produce a mixture of primarily aliphatic alkenes and lower molecular weight alkanes. The mixture is feedstock and temperature dependent and separated by fractional distillation. 

Production of maleic anhydride by oxidation of / -butane represents one of butane s largest markets. Butane and LPG are also used as feedstocks for ethylene production by thermal cracking. A relatively new use for butane of growing importance is isomerization to isobutane, followed by dehydrogenation to isobutylene for use in MTBE synthesis. Smaller chemical uses include production of acetic acid and by-products. Methyl ethyl ketone (MEK) is the principal by-product, though small amounts of formic, propionic, and butyric acid are also produced. / -Butane is also used as a solvent in Hquid—Hquid extraction of heavy oils in a deasphalting process. 

Butadiene, used for the production of elastomers, is produced by the catalytic thermal cracking of butane and as a by-product of other cracking reactions. 

The atmospheric bottom, also known as reduced oil, is then sent to the vacuum unit where it is further separated into vacuum gas oil and vacuum residues. Vacuum distillation improves the separation of gas oil distillates from the reduced oil at temperatures less than those at which thermal cracking would normally take place. The basic idea on which vacuum distillation operates is that, at low pressure, the boiling points of any material are reduced, allowing various hydrocarbon components in the reduced crude oil to vaporize or boil at a lower temperature. Vacuum distillation of the heavier product avoids thermal cracking and hence product loss and equipment fouling. 

Because of the tendency of thermal cracking to produce coke as a by-product of the process, most refiners have replaced thermal cracking units with catalytic cracking processes. However, in certain parts of the world, thermal cracking units still exist. 

The original source of feed stock for the production of aviation gasoline was the butylene-isobutane portion of products from thermal cracking. Later, the thermal cracking units were replaced by catalytic cracking units, and most of the feed stocks are now derived from this source. To a... 

Thermal stability refers to the ability of the oil components, both base oil and additive, to resist degradation or "cracking" due to the effects of heat. The products of thermal decomposition may be corrosive and form a deposit. Corrosion of engine parts (of non-ferrous metals) occurs during the winter months for the most part under low temperature driving conditions. Water vapor may condense in the crankcase, and rusting (of ferrous metals) may occur if the motor oil does not have adequate rust inhibiting properties (Anon, 1958). 

Although the conversion by thermal cracking was below 0 5 %, the influence on certain reaction products was significant More than 50 % of ClT C2 and of the linear olefins in C4 and C, were formed by thermal cracking The influence of thermal cracking on the other products was negligible ... 

Gravimetric Results of Catalytic Cracking. Experiments were conducted to assess the effects of temperature, cat-to-oil ratio, and feedstock composition. In addition to the effect of variables on product yields, it was also important to identify the relative influence of thermal reactions, since free-radical reactions may adversely affect product quality. A series of experiments was conducted in the temperature range of 412°-415°C because this is the temperature of maximum increase in production from thermal cracking and catalytic vs. thermal effects are more easily discernible at this temperature. 

Because the process is dominated by acidic catalysis, if the cracking processes are taken to the extreme, the cracking reactions (known as P-scission) result in propylene and branched olefins such as isobutene. These olefins dominate the light gas products. Ethylene is a very minor component and its presence may be due to a small amount of thermal cracking taking place. 

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