Propane Pyrolysis in a Tubular Reactor with Cyclic Flow.
Griffiths, Robert Henry, Jr.
1980
Abstract
An experimental comparison of cyclic-steady-state flow versus steady-state flow for propane pyrolysis in a tubular reactor found no significant difference. The reactor was a 1/4 inch I.D., 3/8 inch O.D., 20 inch long mullite tube which passed through an electrically-heated furnace. The experimental program consisted of sets of experiments in which four cyclic flow runs over a frequency range of 0.25 to 4.0 hertz were performed between two steady flow runs. While the propane flow rate was maintained at 3.0 x 10('-5) gmol/sec, the diluent flow rate was cycled between zero and twice the average value by driving a solenoid valve with a square-wave signal. The remaining independent variables for the experimental program were type of diluent (nitrogen or methane), average diluent flow rate (3.0--4.5 x 10('-5) gmol/sec), and maximum wall temperature (726-864(DEGREES)C). The reactor pressure was maintained at atmospheric. Gas and liquid samples were analyzed with a gas chromatograph with a flame ionization detector and a linear temperature programmer. The primary constituents for analysis were methane, ethane, ethylene, propane, propylene, benzene, toluene, and the xylenes, although peaks were observed for C(,4)'s, C(,5)'s and hydrocarbons heavier than the xylenes. Propane conversions were maintained above 95% for five of the seven data sets, and were 37% and 74% for the other two sets. The material balances generally closed within 5%. A generalized computer program for the stochastic simulation of real (non-ideal) chemical reactors with periodic input streams was developed. The program performs the stochastic simulation by first dividing each inlet (or reactor port) stream into a user-specified number of equal molar cells (batch reactors). These cells enter one at a time and pass through a network of user-specified modules (reactor sections). Three types of modules are employed: INLT, CSTR, and PFR. The INLT modules store the cells for one cycle of operation for each inlet stream to the reactor. The CSTR modules model the operation of non-ideal Continuously Stirred Tank Reactor sections, while the PFR modules model the operation of non-ideal Plug Flow Reactor sections. Conceptually, the cells in CSTR modules are r and omly positioned while the cells in PFR modules are linearly positioned. Any pattern of flow between these modules is possible except for backflow to an INLT module. For example, streams may be split between several modules or may be recycled to a previous module. Mixing within the reactor modules occurs by coalescing two r and om cells into one perfectly mixed cell which immediately disperses into two cells. Mixing within each reactor module is controlled by a user-specified intensity of mixing which is expressed as the number of coalescences per entering cell. A User's Guide for the program is provided in the appendix along with a listing of the FORTRAN source code. Simulations performed with the generalized computer model indicated general agreement with the trends found in the experimental program. The reaction stoichiometry and kinetics were based on a molecular reaction scheme for propane pyrolysis derived by Van Damme, Narayanan, and Froment (1975) with the following modifications: (a) five reactions involving water and acetylene were omitted because in the experimental study water was not used as a diluent and acetylene was not detected in the gas samples, (b) one of the four propane decomposition reactions was omitted because of its minor contribution to the total propane decomposition, and (c) three of the remaining ten reactions were changed from first order to second order on the basis of reaction rate computations.Types
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