(23e) Accounting for VLE in Modeling and Simulation of a Multi-Bed Industrial Hydrotreater

Hydrotreating of middle distillates is an important step in petroleum refining for producing clean diesel fuels. With the ultra-low sulphur diesel (ULSD) specification already implemented in many developed countries including Canada and the USA, deep hydrotreating of middle distillates to reduce their sulphur content to 15 ppm or lower requires more severe hydrotreating conditions (higher temperature, pressure and hydrogen:oil ratio, low space velocity, etc.). Under these conditions vapour-liquid equilibrium (VLE) inside the hydrotreater greatly affects the fluid’s flow dynamics, mass and heat transfer as well as the hydrotreating reactions. Therefore, the requirement to implement such severe processing conditions to meet government regulations greatly affects the hydrotreating reactor performance. To accurately predict the performance and behaviours of a hydrotreater under such operation, VLE must be accounted for during reactor modelling and simulation.

The present paper discusses the modelling and simulation of a three-bed light cycle oil (LCO) hydrotreater with inter-stage quenching with consideration of VLE under typical commercial operating conditions. The hydrotreater was represented by a one-dimensional plug-flow adiabatic reactor model. During the simulation the mass and heat balance equations of the reactor model are solved with simultaneous VLE calculation by using an in-house developed flash program specifically designed and calibrated for hydrogen-petroleum systems. Simulations with and without VLE revealed significant differences in simulated reactor temperature profiles, hydrotreating conversions, fluid flow rates, and phase compositions. Temperature impact analysis indicated that the entire reactor performance depended heavily on the axial temperature distribution, which is established based on the catalyst bed layout and bed inlet temperatures. It was also observed that although the simulated hydrodesulphurization (HDS) and hydrodearomatization (HDA) conversions were improved at higher inlet temperatures, there was an increase in quenching gas requirement, vaporization rate, and formation of high-temperature zones towards the inlet of the reactor. In a similar manner, pressure increased hydrotreating performance but with extensive heat release due to increased HDA conversions. Despite the strong vaporization of LCO into the gas phase under the operating conditions investigated, it was verified that the plug-flow of liquid and full catalyst wetting criteria were fully satisfied for establishing the reactor model.