Ethane Pyrolysis in a Tubular Reactor
Olefin compounds such as ethylene can be produced by the pyrolysis of hydrocarbons at an elevated temperature. The olefin yield depends on the reaction temperature and the partial pressure of the hydrocarbon, which can be controlled by adding steam as a diluent. This Demonstration analyzes the isothermal pyrolysis of ethane to produce ethylene in a tubular reactor operating at steady state. The feed stream to the reactor contains ethane with steam as a diluent. The gas phase reaction is assumed to behave as an ideal gas in plug flow (no velocity gradients in the reactor). You can vary the reaction temperature, the inlet composition, and the inlet pressure. During the pyrolysis, reaction intermediates in the form of radicals are produced. The kinetics of the reaction can be simplified by assuming that the rates of production of the radical species are zero. This is known as the quasi-steady state assumption (QSSA). You can assess the validity of the QSSA by examining the rates of production profiles of the radical species along the length of the reactor (expressed in terms of reactor volume).
The kinetic steps for the pyrolysis reaction of ethane are
In the above steps is the methyl radical, is the ethyl radical, and is a hydrogen radical. The rate of reaction for each kinetic step is determined by mass action kinetics. Thus for the kinetic step, the rate of reaction is , where represents the reactant species at the reaction step, and is the stoichiometric coefficient of the reactant at the kinetic step. The species balance for the species is , where is the volumetric flow rate and is the volume of the reactor. The rate constants are assumed to have Arrhenius dependence on temperature: . The values of are taken from .
 J. B. Rawlings and J. G. Ekerdt, Chemical Reactor Analysis and Design Fundamentals, Madison, WI: Nob Hill Publishing, 2002.