OP-II-3

PP-II-49Hinselwood kinetic equations for the main reactions pathways, namely:dehydrogenation, cracking and coke formation.Mathematical modelling of an industrial fluidized bed reactor for propaneisobutanedehydrogenation. A two-phase non-isothermal reactor model taking intoaccount axial gas and heat dispersion was combined with kinetics and used for theindustrial-scale reactor modelling.Previous thermodynamic calculations for dehydrogenation of a model C 3 –i-C 4paraffins mixture indicate that there is a complex mutual effect of the components onthe equilibrium yield of olefins [2]. Each component can act simultaneously: (i) as adiluent shifting the dehydrogenation equilibrium of the mixture components to thedesired olefin and (ii) as an additional source of hydrogen, which shifts thedehydrogenation equilibrium to the left. Experiments on dehydrogenation of modelC 1 –C 4 paraffin mixtures on Cr 2 O 3 /Al 2 O 3 catalysts in pilot fluidized and fixed bedsshow an increase of the total conversion of C 3 –C 4 paraffins to olefins and an increaseof the total process selectivity to olefins.As a result of the present modelling the using propane-isobutane mixture for theindustrial-scale fluidized bed olefins production at subatmospheric pressure wasshown to have the following benefits:• Selectivity to i-C 4 H 8 is high enough and grows slightly from 86-87 to 90 % onincreasing inlet C 3 H 8 fraction from 0 to 60 % weight.• Inlet concentrations of C 3 H 8 up 20 % weight lead to the apparent selectivity toC 3 H 6 exceeding 100 %. This effect arises due to i-C 4 cracking leading toadditional propene formation and kinetic reasons.• Reasonably high propene concentration of 35 % weight in C 3 H 8 -C 3 H 6 mixturecould be obtained at relatively low bed temperature 577-585 °C, allowing toobtain high olefins selectivity.• Coke yield rises slowly (from 0.7-1.0 % weight at 100 % i-C 4 at the inlet to 1.2-1.4 % weight at 45 % C 3 H 8 at the inlet) allowing safe industrial fluid bedreactor operation.References[1]. L.A. Isupova, Yu.Yu. Tanashev, I.V. Kharina, E.M. Moroz et al., Chemical Engineering Journal(2005), 107, 163-169.[2]. RF Patent No 2322290 / V.V. Molchanov, N.A. Pakhomov, L.A. Isupova, V.A. Balashov et al.(2008), Priority of 18.12.2006.423