Kinetics of the nonisothermal pyrolysis of propane

Abstract

A method was devised to yield chemical reaction kinetic parameters from nonisothermal, nonisobaric flow experiments. The system studied was the pyrolysis of propane a t high temperatures (800" to 1,OOO"C.). A t these temperatures the rates of the various reactions are so high that a batch or even an isothermal flow experiment is impossible. To keep the conversions low so that the initial stages of decomposition could be studied, the feed gas was diluted with varying amounts of nitrogen. Residence times in the reactor were in the millisecond range. The reactor exit gas was analyzed by mass spectrometry. The method developed in this work is not limited to simple kinetic studies, but can be useful in complicated series and parallel reactions which often require nonisothermal conditions.As reaction temperatures are raised, it becomes increasingly difficult to measure kinetic rate constants experimentally. In this work, the pyrolysis of propane was studied at 800" to 1,OOO"C. where the reactions are extremely rapid. Furthermore, the products of reaction themselves decompose under these conditions, tending to mask the primary kinetics. Conversions, therefore, were kept low through the use of a steady state flow system, where residence times can be shorter than in batch systems; the concentration of the reaction products was minimized by diluting the feed gas with nitrogen to about 5% propane.The extent of this dilution was limited however, because the higher the concentration of diluent and the lower the conversion, the more difficult becomes the exit gas analysis.Since rate constants are strongly dependent upon temperature, it would be most convenient to conduct kinetic experiments isothermally. However, because of the low residence times required and the physical limitations on heat transfer rates, a 'nonisothermal experiment results. Some previous workers have chosen an equivalent, average temperature for all or a fraction of the reactor, but this procedure leads to only partially satisfactory results. An alternative is to measure the gas temperature profile throughout the reactor and to devise some method of treating the data to yield the desired rate constants. Such was the method used in this study.Once the rate constants are obtained as functions of temperature, the kinetic model can be programmed on an analog computer; it is then possible to test the consistency of the data and to predict product distributions, conversions, etc., for any arbitrary set of conditions. This is extremely valuable if similar studies are made for the other low hydrocarbons which are products of propane pyrolysis. Then the entire series could be studied simultaneously; that is, product distributions could be predicted not only for the simple case of low conversions but also for the more complicated cases of consecutive reactions where reaction products themselves react further.There is much ublished information in the literature in general and propane in particular. While much kinetic work has been done with propane at lower temperatu...

Novel microreactor with quench system for kinetic study of propane pyrolysis

“…It can be seen that the experimental data values are about 5-8% higher than those predicted by simulations of Froment et a!. (1979) and Kershenbaum and Martin (1967) or the results obtained by Layokun and Slater (1979). Since tubular reactors were used in these studies, the temperatures presented in the literature are probably suspect.…”

“…The reason for this can be explained by the difference in temperature a t which the studies were conducted. Several previous workers (Laidler et al, 1962;Martinet al, 1964;Kershenbaum, 1964) have noted that as the temperature of the hydrocarbon pyrolysis is increased, the activation energy for the reaction "appears" to decrease rather than remain constant. This effect can also be noted in this work.…”

“…The simulation involved the mass balances of seven molecular species and four radical species. A simple reaction scheme presented by Kershenbaum and Martin (1967) to predict the kinetics of nonisothermal pyrolysis of propane was also used. The simulation involved the primary decomposition reactions yielding the primary products.…”

Enhancement of wall-to-gas heat-transfer coefficients by homogeneous chemical reactions

“…The example which will be considered here in some detail is propane pyrolysis which can be represented by the following first-order, irreversible rate expression in the 800-1000 "C temperature range (Kershenbaum and Martin, 1967) r = 2.4 x 10" exp(-52100/R,T)CA Propane pyrolysis is known to produce equivalent amounts of ethylene, propylene, methane, and hydrogen at low conversions. Although the product ratio and consequently the heat of reaction changes with increasing conversions, the heat of reaction estimated at low conversions (24800 cal/gmol) is used as a first approximation in the calculations to be presented here.…”

“…Considerable attention has been given to the beneficial contribution of homogenous chemical reactions to increasing the interfacial mass-transfer rates in chemical processing systems (Danckwerts, 1970;Astarita;1967). The basis of these considerations is that the dimensions of the interphase region in which the major changes in concentration take place is primarily determined by the *Author to whom correspondence should b e addressed.…”