(87e) Modeling and Simulation of Hydrotreating Reactors with the Effects of Vapor-Liquid Equilibrium

Hydrotreating (HDT) is a one of the most important processes used in refineries to improve the quality of distillate fractions by removing of sulfur, nitrogen, oxygen, and metals, and saturating olefins and aromatics. The HDT process is usually operated at temperatures of 350?400°C and pressures of 50?100 bar in a fixed bed. Under such conditions, the process typically involves gas, liquid, and solid phases (trickle bed) due to the presence of high-boiling-point fractions in the feedstocks.

In the trickle bed, hydrogen gas and liquid oil flow through a fixed bed of solid catalyst. There are complex couplings between the reactions, mass transfer, and heat transfer in this three-phase process. Although efforts to model trickle-bed reactors have achieved considerable progress in combining transport phenomena and heterogeneous reactions, variations in the flow rates and compositions of the individual phases due to the volatilization of liquid oil are often ignored because of the lack of vapor-liquid equilibrium (VLE) data for the HDT system.

It is generally assumed that the feed oil is non-volatile and the flow rates of both liquid and vapor phases therefore remain constant along the reactor. In fact, this assumption is not true for HDT reactors. Our previous work showed that large amounts of hydrocarbons are present in both phases, especially at higher temperatures and lower pressures. Accordingly, it is necessary to take VLE into account to ensure accurate modeling and simulation. Moreover, commercial HDT reactors are operated under non-isothermal conditions?temperature varies along the reactor. Since the vaporization of oil consumes significant energy, it is not possible to accurately predict reactor bed temperatures if this change of state is neglected.

In previous studies we conducted VLE flash experiments in a continuous-flow unit with hydrogen and various petroleum middle distillates under typical HDT conditions. The interaction coefficients between hydrogen and hydrocarbon boiling pseudo-components, required to perform VLE flash calculations, were estimated. Our previous work also demonstrated flow mapping of the desired operating regimes in a pilot plant trickle-bed reactor for hydrotreating two different middle distillate feeds. The present paper describes the development of a steady-state trickle-bed reactor model with VLE effects and gives predictions of commercial HDT reactor behavior. The detailed results are presented and discussed.