Cracking to Produce Ethylene

There are several objectives to produce ethylene by catalytic cracking, namely  

Attempts realise these objectives are based on catalysts able to handle heavy feedstock at relatively low temperatures (550°C versus the 850°C for steam cracking). 

However there are several major hurdles. The most common catalysts are based on acid catalysis with Bronsted or Lewis acid sites these sites promote the formation of propylene rather than ethylene as is witnessed by conventional FCC operations. Ethylene is promoted by free radical processes. Catalysis of free radical reactions is rare, but not unknown . One route is to take a conventional acid catalysis and to neutraUse the acid sites with alkaline metals (magnesium, calcium) or phosphorus or a mixture of such. This can generate a further problem, in that the catalyst promotes the formation of carbon (coke) and hydrogen which are thermodynamically favoured at the reaction temperatures. 

The higher ethylene yields observed in the DCC type processes has led developments towards the catalytic cracking of heavy oils to ethylene. A typical yield from cracking a gas oil (b.p. 229-340°C) with 45% paraffins, 35.7% naphthenes and aromatics 18.2% is illustrated in Table 10.5 . 

The higher ethylene yield is dehvered by a high temperature (660°C). This is high compared to normal FCC type operations, but considerably lower than the temperatures typical for steam cracking 

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Reactions of /l-Butane. The most important industrial reactions of / -butane are vapor-phase oxidation to form maleic anhydride (qv), thermal cracking to produce ethylene (qv), Hquid-phase oxidation to produce acetic acid (qv) and oxygenated by-products, and isomerization to form isobutane. 

Coal is poised to eventually replace oil as the primary industrial feedstock [862a], In the way that oil is cracked to produce ethylene, coal (or other organic material, such as natural gas) may be converted to synthesis gas. A large number of oxygenates are derivable from synthesis gas, some of which (such as ethanoic acid and methanol) are already in full commercial production. In conventional (ethylene-based) technology, dichlorine is normally... 

Dr Dry from Sasol proposed in 1982 to use slurry phase reactors to ultimately produce mainly diesel with naphtha as a significant co-product by using a scheme in which the reactor wax is hydrocracked (19, 20). It was further proposed that this naphtha is a good feedstock for thermal cracking to produce ethylene. Gulf Oil in 1985 proposed the use of a slurry reactor with a modem precipitated cobalt catalyst to produce mainly diesel as a final product (21). The advent of still more active cobalt catalysts has now resulted in the ability to consider gas velocities for the LTFT reactors that are in line with those used for the HTFT fluid bed reactors. 

Most of the petrochemical facihties that depend on a cheap and abundant supply of natural gas are also located in the Gulf Coast, where a surplus of offshore gas is available. These facihties use cracking of natural gas to produce ethylene as the starting point for the refining of their products. This... 

The pattern of commercial production of 1,3-butadiene parallels the overall development of the petrochemical industry. Since its discovery via pyrolysis of various organic materials, butadiene has been manufactured from acetylene as weU as ethanol, both via butanediols (1,3- and 1,4-) as intermediates (see Acetylene-DERIVED chemicals). On a global basis, the importance of these processes has decreased substantially because of the increasing production of butadiene from petroleum sources. China and India stiU convert ethanol to butadiene using the two-step process while Poland and the former USSR use a one-step process (229,230). In the past butadiene also was produced by the dehydrogenation of / -butane and oxydehydrogenation of / -butenes. However, butadiene is now primarily produced as a by-product in the steam cracking of hydrocarbon streams to produce ethylene. Except under market dislocation situations, butadiene is almost exclusively manufactured by this process in the United States, Western Europe, and Japan. 

In addition to conventional thermal cracking in tubular furnaces, other thermal methods and catalytic methods to produce ethylene have been developed. None of these are as yet commercialized. 

The newly formed free radical may terminate by abstraction of a hydrogen atom, or it may continue cracking to give ethylene and a free radical. Aromatic compounds with side chains are usually dealkylated. The produced free radicals further crack to yield more olefins. 

PVC production, on the other hand, is carried out by first high severity steam cracking of gas oil to produce ethylene. Vinyl chloride monomer (VCM) is then... 

In steam cracking the homogeneous pyrolysis of ethane to produce ethylene... 

Table III gives a range of the possible feedstocks that can be used to produce ethylene and the kinds and amounts of by-products that can be made from them. For our purposes we have selected a constant basis of 1 billion lbs/year ethylene production. The feedstocks illustrated in Table III include ethane, propane, n-butane, a full range naphtha, a light gas oil, and a heavy gas oil. The yields reflect high severity conditions with recycle cracking of ethane in all cases. For propane feed, propane recycle cracking has been included as well.

Steam cracker tar (SCT) is a by-product from the steam cracking of naphtha or gas oils to produce ethylene. The characteristics and yield of SCT is dependent on the feed characteristics, the plant design and severity of cracking,... 

Ethylene and propylene are produced primarily by the cracking of naphtha. They also are available from the fractionation of natural gas. Ethylidene norbornene is produced by reacting butadiene with cyclopentadiene. 1,4 Hexadiene is produced from butadiene and ethylene. Dicyclopentadiene is obtained as a by-product from the cracking of heavy feedstocks to produce ethylene. 

Hydrocarbon waxes produced in a fixed-bed reactor, which has operated since 1955, have found a variety of uses. Also, byproducts from the Sasol Lurgi coal gasifiers are recovered for chemical and solvent applications. These products include phenol, cresols, toluene, xylenes, ammonia, and sulfur. An addition to the spectrum of chemical products from Sasol is polypropylene. Also, ethane is being cracked to supplement ethylene production for sale to polyethylene producers. Additional work is in progress to evaluate the recovery of organic acids from aqueous waste streams. 

This chapter describes the basic process of ethane steam cracking operations to produce ethylene and the integration with downstream operations. The approach to economic analysis for various types of ethane cracking operations is described and the economic analysis for ethane cracking in a standardised approach is developed. The production of olefins from other feed stocks and the economics of production are developed in later chapters ... 

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