Kinetic relationships of propane dehydrogenation

Abstract

THERMODYNAMICS of propane dehydrogenation indicate that a practical degree of conversion in this direction is only achieved at temperatures higher than 700°K. At this temperature the equilibrium constant of dehydrogenation K~=0.005, degree of conversion x=0.063 and propylene content in the equilibrium mixture reaches 5.94% by weight. However, at 700°K cracking is thermodynamically a more likely mode of propane conversion, K~ being 10.75 and x 0.96. To avoid the effect of secondary processes and accelerate the reaction in the required direction, it is quite correct to use active catalysts with a high selectivity to dehydrogenation. There are several papers which deal with propane dehydrogenation [1-8], however, only one [6] gives a theoretical analysis of data to determine the kinetic constants of the process. I t should be noted first of all that the authors of this paper [6] analysed average data on propylene yield by the Dodd and Watson empirical equation [9] proposed for butane dehydrogenation. Secondly, process conditions selected in former [6-7] and in our studies considerably differ. These were the reasons for this investigation. Further, catalytic dehydrogenation of propane is more efficient and has greater selectivity than pyrolytic dehydrogenation. There is therefore an urgent need for a sound approach to the evaluation and calculation of the rates of this process, in order to develop it industrially. The problem of calculating the rate of monomolecnlar irreversible continuous reaction in a particular form was solved by Balandin [10] and Frost [11]. The general method of solving the problem of continuously calculating the rates of chemical reactions and physico-chemical processes for stationary and non-stationary systems was proposed by one of us in 1948 [12-14]. According to this method the rate of heterogeneous reaction under conditions of ideal displacement will be