Thermal Cracking of Propane

All the reactions, except 4 and S, are considered to be elementary, so that their order equals the molecularity. Reactions 4 and S are more complex and first ordo- 

The experimental study of Froment et al. (loc. cit) was carried out in a tubular reactor with plug flow. Tlie data were obtained as follows total conversion of propane versus a measure of the residence time, lyfF cH.fe conversion of propane into propylene versus 1V(Kcjh,)o reactor volume reduced to 

It will be shown in Chapter 9 that a mass balance on propane over an isothermal differential volume element of a tubular reactor with plug flow may be written 

After integration over the total volume of an isothermal reactor, Eq. a yields the various ffow rates Fj at the exit of the reactor, for which Vr/(,Fcjh )o has a certain value, depending on the propane feed rate of the experiment. If Eq. a is integrated with the correct set of values of the rate coefficients ki...kg the experimental values of Fj should be matched. Conversely, from a comparison of experimental and calculated Pj the best set of values of the rate coefficients may be obtained. The fit of the experimental Fj by means of the calculated ones, Fj, can be expressed quantitatively by computing the sum of squares of deviations between experimental and calculated exit flow rates, for example. These may eventually be weighted to account for differences in degrees in accuracies between the various Fj so that the quantity to be minimized may be written, for n experiments  

Sundaram and Froment [loc. cit] systematized this estimation by applying nonlinear regression. 

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This chapter contains a discussion of two intermediate level problems in chemical reactor design that indicate how the principles developed in previous chapters are applied in making preliminary design calculations for industrial scale units. The problems considered are the thermal cracking of propane in a tubular reactor and the production of phthalic anhydride in a fixed bed catalytic reactor. Space limitations preclude detailed case studies of these problems. In such studies one would systematically vary all relevant process parameters to arrive at an optimum reactor design. However, sufficient detail is provided within the illustrative problems to indicate the basic principles involved and to make it easy to extend the analysis to studies of other process variables. The conditions employed in these problems are not necessarily those used in current industrial practice, since the data are based on literature values that date back some years. 

The thermal cracking of propane is practiced industrially for the primary purpose of making ethylene and propylene, but other reactions also occur. A scheme worked out by Sundaram Froment (Chem Eng Sci 32 601, 1977) consists of the nine reactions of the table. Equilibrium constants were deduced from thermodynamic data and the other constants by nonlinear regression from the extensive data on this topic in the literature and laboratory. 

FIGURE 19.2 Changes in the product distribution with temperature on the thermal cracking of propane. (Drawn from the data of Frolich and Wiezevich [13].)... 

Table / Molecular scheme for the thermal cracking of propane... 

Example 9.1-1 Derivation of a Kinetic Equation from Experiments in an Isothermal Tubular Reactor with Plug Flow. Thermal Cracking of Propane... 

The thermal cracking of propane was studied at atmospheric pressure and 800°C in a tubular reactor of the integral type. The experimental results are given in Table 1. 

Table Thermal cracking of propane. Conversion versus space time data...

Tdjle 2 Thermal cracking of propane. Rate versus conversion, k-vaiues from the integral and differential method of kinetic analysis... 

THERMAL CRACKING OF PROPANE. CONVERSION VERSUS SPACE TIME DATA... 

Detemmerman and Froment [1998] carried out a three dimensional coupled CFD simulation of furnaces and reactor tubes for the millisecond thermal cracking of propane into ethylene in the presence of steam, a process already discussed in Chapters 1 and 9. 

Van Damme,P.So, SoNarayanan and G.F.Froment. "Thermal Cracking of Propane and Propane-Propylene Mixtures, Pilot Plant versus Industrial Data". AIChE.J. v21 (1975) 1065-1073. 

Modeling of thermal cracking of propane was performed by Sundaram and Froment [46] on the basis of experimental work by Van Damme et al. [47]. The kinetics of mercury-photosensitized decomposition of propane was studied in a pair of papers by Papic and Laidler [14, 15] in the seventies. 

Van Damme, P.S., Narayanan, S. and Froment, G.F. (1975) Thermal cracking of propane and propane-propylene mixtures pilot plant versus industrial data , AIChE J., 21, 6, 1065-1073. 

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