Basic Chemistry of LPG Cracking

The steam pyrolysis of LPG follows the same pathway of that for ethane, namely by a complex branching chain free radical mechanism. This can be divided into initiation, chain propagation and chain termination reactions. This gives rise to a large number of intermediates and products. As with ethane, products of higher carbon number than the feed are formed. 

Because the cracking process involves the rupture of carbon-carbon bonds, products of the same carbon number as the feedstock are low in concentration. Thus for propane, the major product of cracking is ethylene and methane rather than propylene. Normal butane gives more propylene, but the main end product is again ethylene. 

The branched nature of isobutane, however, makes propylene the major product. Table 8.1 gives the single pass yields for ethane, propane, normal and isobutane to illustrate these points. 

Because the formation of heavy liquid products is low, LPG is cracked in a very similar process to ethane cracking. Often LPG can be co-fed to the pyrolysis furnace with ethane and there is no need for an additional process plant. 

However, if the LPG is from a refinery operation or downstream petrochemical production, olefins are often present and these can lead to 

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