Feedstocks

Feeds to steam cracking units vary appreciably, from light hydrocarbon gases to petroleum residues. Due to the difference in the cracking rates of the various hydrocarbons, the reactor temperature and residence time vary. As mentioned before, long chain hydrocarbons crack more easily than shorter chain compounds and require lower cracking temperatures. For example, it was found that the temperature and residence time that gave 60% conversion for ethane yielded 90% conversion for propane.   Feedstock composition also determines operation parameters. The rates of cracking hydrocarbons differ according to structure. Paraffinic hydrocarbons are easier to crack than cycloparaffins, and aromatics tend to pass through unaffected. Isoparaffins such as isobutane and isopentane give high yields of propylene. This is expected, because cracking at a tertiary carbon is easier:

As feedstocks progress from ethane to heavier fractions with lower H/C ratios, the yield of ethylene decreases, and the feed per pound ethylene product ratio increases markedly. Table 1.1 shows yields from steam cracking of different feedstocks,45 and how the liquid by-products and BTX aromatics increase dramatically with heavier feeds.

Table 1.1: Ultimate yields from steam cracking various feedstocks