Operation of an Ethane Steam Cracker
Ethane and steam are fed to a steam cracker at a user-set total pressure (in bars). The water/ethane molar ratio is also set by the user. This Demonstration estimates the equilibrium distribution of products (methane, ethylene, acetylene, carbon dioxide, carbon monoxide, oxygen, hydrogen, water, and ethane) for temperatures ranging from 550 K to 1100 K. The Gibbs free energy for the mixture is minimized subject to elemental mass balance constraints, which are easily obtained. It turns out that the equilibrium mole fractions of ethane, ethylene, acetylene, and oxygen are negligible for the selected range of and . Thus, only methane, hydrogen, carbon monoxide, and carbon dioxide need be plotted, in orange, yellow, green, and cyan, respectively.[more]
With only steam and ethane fed into the reactor, hydrogen is the main product. A small amount of methane is also formed. All the ethane is consumed by the reaction , but one does not get the full yield of hydrogen. The rest of the hydrogen fed into the reactor is converted into water by the reaction .
It is worth noting that no prior knowledge of the reactions (i.e., which products are formed) is required to perform the free-energy minimization. The ideal-gas contributions to the total Gibbs free energy are obtained from experimental data . We compute the residual Gibbs free energy using the Peng–Robinson equation of state . This determines the equilibrium composition for pressures much higher than 1 bar. It is clear that the ethane steam cracking produces no methane at high temperatures and low pressures. However, a significant amount of is obtained at high pressures (see Snapshot 3).[less]
 I. Barin and G. Platzki, Thermochemical Data of Pure Substances, 3rd ed., New York: VCH Publishers, Inc., 1995.
 S. I. Sandler, Chemical and Engineering Thermodynamics, 3rd ed., New York: John Wiley & Sons, 1999.