Propane cracking parameters

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. 

Table 9. Relevant geometric parameters for the cracking transition states of ethane, propane and i-butane.

The possible influence of nonideal flow is to be estimated. For this approximate study assume that the temperature in the tubes is constant and that the reaction gases are completely mixed in the U-bend at the end of each tube section. The cracking reactions are approximately first order and kinetic data indicate that three tube sections would be required to obtain 96% conversion of the propane for plug flow in each section. The flow rate in such that the Reynolds number in the tubes in 10,000. For these conditions the axial dispersion parameter, Djudi, where d the tube... 

In a naphtha, 200-300 components may lead to these olefins, so that all the networks have to be developed and their ethylene production summed up. It is clear that a realistic kinetic analysis of the cracking of a complex hydrocarbon mixture into olefins is not a simple task. The rate parameters of the elementary steps can only be accessed by an investigation of the kinetics of a number of specific feed components, preferably with increasing complexity. Even for the paraffin family, cracking ethane and propane will not suffice because they do not generate all the steps encountered with the higher members. 

The results given in Section 9.3.2 for the thermal cracking of naphtha and of a mixture of ethane-propane were obtained with very detailed radical kinetic schemes for these processes [Willems and Froment, 1988a, b]. The present problem formulates ethane cracking in terms of a drastically simplified molecular model containing 7 reactions. This reaction scheme and the corresponding kinetic model was derived from the radical scheme developed by Sundaram and Froment [1977]. Table 1 gives the kinetic parameters of these reactions. It should be mentioned that the kinetic parameters for the reverse reactions (2) and (5) were obtained from equilibrium data. Table 2 is the matrix of stoichiometric coefficients ay defined by... 

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