TABLE OF CONTENTS The most important olefins and diolefins used to manufacture petrochemicals are ethylene, propylene, butylenes, and butadiene. Butadiene, a conjugated diolefin, is normally coproduced with C 2-C 4 olefins from different cracking processes. Separation of these olefins from catalytic and thermal cracking gas streams could be achieved using physical and chemical separation methods. However, the petrochemical demand for olefins is much greater than the amounts these operations produce. Most olefins and butadienes are produced by steam cracking hydrocarbons.
Butadiene can be alternatively produced by other synthetic routes discussed with the synthesis of isoprene, the second major diolefin for rubber production.
The main route for producing light olefins, especially ethylene, is the steam cracking of hydrocarbons. The feedstocks for steam cracking units range from light paraffinic hydrocarbon gases to various petroleum fractions and residues. The properties of these feedstocks are discussed in Chapter 2.
The cracking reactions are principally bond breaking, and a substantial amount of energy is needed to drive the reaction toward olefin production.
The simplest paraffin (alkane) and the most widely used feedstock for producing ethylene is ethane. As mentioned earlier, ethane is obtained from natural gas liquids. Cracking ethane can be visualized as a free radical dehydrogenation reaction, where hydrogen is a coproduct:
The reaction is highly endothermic, so it is favored at higher temperatures and lower pressures. Superheated steam is used to reduce the partial pressure of the reacting hydrocarbons' (in this reaction, ethane). Superheated...
