Liquid phase hydrogenation of aromatic compounds on nickel catalyst

摘要

The major applications of aromatic hydrogenation (dearomatisation) are in the production of aromatic-free fuels and solvents. Health risks related to aromatic compounds, such as benzene and some polyaromatic compounds, have encouraged legislators to tighten the restrictions on aromatic content in end products. In diesel fuel, aromatic compounds have the further effect of lowering fuel quality, and they are reported to be responsible for undesired particle emissions in exhaust gases. Indeed, the major remaining concern in regard to exhaust gases is particle emissions, as fuels are already low in sulphur and the emissions of CO, SOx and NOx have been significantly reduced.The aim of the work was, on the basis of experimental data from the liquid phase to develop kinetic and deactivation models of the hydrogenation of aromatic compounds suitable for use in the design and optimisation of hydrogenation reactors operating in the liquid phase. To this end, the hydrogenation of toluene, tetralin, naphthalene and mixtures of these on a commercial nickel catalyst was studied in a continuously working three-phase reactor. These model compounds were chosen to represent monoaromatics (toluene), partly hydrogenated polyaromatics (tetralin) and polyaromatics (naphthalene).The solvent effect on toluene hydrogenation was studied in cyclohexane, n-heptane and isooctane. At low temperatures the hydrogenation rates were similar, but at higher temperature the rate in cyclohexane was significantly lower than the rate in n-heptane and isooctane. It was concluded that the difference in the rates at higher temperatures was primarily due to the different solubility of hydrogen. Thus, the matrix effects of all compounds need to be included in the models for reliable parameters and rate expressions to be achieved.Toluene and tetralin were assumed to form a π-complex with adsorbed hydrogen and surface nickel. Intermediates were presumed to retain their aromatic nature and to react further to corresponding cyclohexenes and thereafter to fully saturated products. The difference between the hydrogenation rates of naphthalene and monoaromatic compounds was explained in terms of adsorption strength and adsorption mode of aromatic compounds. Naphthalene, adsorbing more strongly than monoaromatic compounds, was proposed to react through π/σ-adsorption rather than π-adsorption.The kinetic models of toluene, tetralin and naphthalene were successfully applied to the hydrogenation of aromatic mixtures of these compounds. Naphthalene was observed to inhibit the hydrogenation of toluene and tetralin, but toluene and tetralin had no effect on the hydrogenation of naphthalene. The inhibition effect could be explained with the adsorption terms obtained during single component experiments, decreasing in the order naphthalenetetralin>toluene. The simulation of the data obtained in the hydrogenation of mixtures with the kinetic models of the single compounds showed that the inhibition effect can successfully be estimated from single compound experiments if well defined adsorption coefficients are available for all compounds.Severe catalyst deactivation was observed during the work. Coking (formation of hydrogen-deficient species) was assumed to be the cause of this deactivation since no sulphur or nitrogen impurities were detected. Besides increase in the cis-to-trans ratio, the catalyst deactivation suppressed the hydrogenation of tetralin to decalins relative to the hydrogenation of naphthalene to tetralin. This was explained by the π-adsorption of tetralin, which was proposed to require an ensemble of Ni-atoms, which further on, with deactivation, led to a more severe decrease in the hydrogenation rate of tetralin than in the hydrogenation rate of naphthalene.