Use of an axial dispersion model for kinetic description of hydrocracking

Abstract

A novel approach to the description of the kinetics of hydrocracking of vacuum gas oils (VGO) is developed, in which the progress of cracking with increasing space time is modeled by the consideration of two aspects: (i) the distribution of boiling point around the mid-boiling temperature T₅₀, and (ii) the decay kinetics of T₅₀. Using the data of Bennett and Bourne (1972) for the hydrocracking of Kuwait VGO it is shown that the distribution of boiling points around T₅₀, after appropriate scaling, can be adequately described by an axial - dispersion model. To gain an insight into the physico - chemical basis of the axial-dispersion parameter Pe, and the kinetics decay of T₅₀, a detailed kinetic model is also developed for hydrocracking, consisting of lumped species: paraffins, naphthenes, aromatics and sulphur compounds. The rate constants for the detailed reaction network for various cuts were determined from pilot plant data for Kuwait VGO reported by Bennett and Bourne (1972). With the aid of the detailed kinetic model, verified for its predictive capability by testing with two other feedstocks, it is shown that the axial-dispersion parameter Pe is almost solely governed by the paraffinicity of the feedstock and may be considered to reflect the "selectivity" of the catalyst. The decay order of T₅₀, n, and the corresponding decay rate constant, k₅₀, are also found to be primarily governed by the feed paraffinicity; the decay order n varies from nearly second-order for feedstocks with extremely low paraffin content to approximately first-order for highly paraffinic feeds. Together, these two parameters, n and k₅₀, portray the "activity" of the catalyst.