Alok K. Saxena

Use of an axial-dispersion model for kinetic description of hydrocracking

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.

Measurement and prediction of solubility of petroleum waxes in organic solvents

A microscopic procedure is suggested for the measurement of wax dissolution (solubility), wax appearance and gel formation temperatures of n-alkanes and petroleum waxes in organic solvents. All three characteristics can be measured in a single experiment and are better in repeatability and accuracy than those obtained from the corresponding ASTM procedures. The method has the advantage of no restrictions on the nature, type or colour of the sample (only a very small amount of which is needed) and the gel structure can be observed and a photomicrograph recorded if desired. The equations developed by earlier workers for predicting solubility of n-alkanes and their binary mixtures are modified by considering the contributions of all solid-solid transitions, and are used to calculate the dependence of solubility on the concentration of waxes in some organic solvents. If wax is assumed to be a single-component system (n-alkane components forming solid solutions), all the earlier as well as the presently modified equations show only order-of-magnitude agreement with the experimental solubility. Considering wax instead as a multicomponent system of n-alkanes, a new solubility equation is developed, giving calculated values in good agreement with the experimental values.

Crystallization behaviour of n-paraffins in Bombay-High middle-distillate wax/gel☆

Abstract The crystallization behaviour of n-paraffins in the waxes obtained from the middle distillate of Bombay-High crude oil has been studied using X-ray diffractometry and photomicrography. The effects of solvents and of a pour-point depressant additive on the lattice properties of the waxes have been investigated and the mechanisms of gel formation and additive action have been studied. The correlations between wax crystal structure and morphology have been discussed.

EFFECT OF COIL PITCH AND CROSS-SECTIONAL ELLIPTICITY ON RTD FOR DIFFUSION-FREE LAMINAR FLOW IN COILED TUBES

The effect of coil pitch and the cross-sectional ellipticity on residence time distribution (RTD) for diffusion-free laminar flow in helically coiled tubes has been studied. The numerically computed RTDs reveal that the increase in the coil pitch broadens the RTD while the increase in the cross-sectional ellipticity narrows the RTD. The condition under which the effect of coil pitch on RTD can be ignored has been reported.