Canadian oil sand bitumen, equivalent to one-third of world's useful petroleum resources, is a promising alternative to counter declines in conventional crude oil sources. Upgrading, including hydrotreating, is necessary to comply with low sulphur specifications for fuels. Bitumen-derived feeds contain non-filterable ultrafines, typically <20 µm. These fines impose economic and processing setbacks in the downstream hydrotreaters where H2 and Heavy Gas Oil (HGO) flow co-currently downwards in a trickle bed in which the hydrotreating reactions remove objectionable materials. In the course of operation, plugging develops and leads to progressive increase in bed obstruction and pressure drop. The cumulative effect of tens of thousands of barrels of feed processed each day diverts the catalyst bed from its primary function to a huge filter. Fines accumulation causes the pressure drop to rise by restricting the flow. Eventually, the pressure drop becomes so high that the hydrotreaters are shutdown and the still chemically active catalyst is replaced. Fines deposition shortens hydrotreater cycle life, increases operational problems and maintenance work leading to poor overall energy efficiency (Gray et al., 2002). Therefore, research is needed to understand different features of this problem. In our group, the effect of gas flow was first studied on the extent of deposition by raising the gas superficial velocity from low to moderate values (Edouard et al., 2006). A decreasing trend of specific deposit (the mass of deposition per unit volume of the reactor) was observed. A correlation was established to relate the filtration rate and liquid holdup in the bed. Later on, the results of some experimental hydrodynamic observations were reported (Hamidipour et al., 2007). The complications to interpret the results of liquid circulation mode in comparison to the one-pass mode were discussed. An area was highlighted on the superficial velocity map of gas versus liquid to specify the conditions in which the trickle to pulse flow regime changeover is plausible under filtration conditions. The occurrence of plateau in pressure drop signal at the severe plugging conditions was attributed to the short circuits of fluids near the wall which consist of deposition islands and plug-free corridors.

In this communication, a cold-flow setup using kerosene-kaolin suspensions was studied with the goal to understand the deposition and release of fines in trickle bed reactors and in monolith beds. Iso-flow feed policies and cyclic operation (liquid, gas and alternating) were evaluated in terms of specific deposit, pressure drop and flow regime evolutions in the quest of the best approach for extending reactor operational life under filtration conditions. Electrical Capacitance Tomography (ECT), with the help of suitable reconstruction algorithms, was used as an aiding tool to track as a function of time the evolution of the plugging and release patterns inside the bed. The effects of bed length as well as gas and liquid superficial velocities was examined on the structure of deposition along the bed and recommendation are proposed regarding the most suitable feed policy to reduce deposition.

References:

-Gray M.R., Srinivasan N., Masliyah J.H. 2002. Pressure buildup in gas–liquid flow through packed beds due to deposition of fine particles. Can. J. Chem. Eng. 80, 346-354.

- D. Edouard, I. Iliuta, F. Larachi, 2006. Role of gas phase in the deposition dynamics of fine particles in trickle-bed reactors, Chem. Eng. Sci., 61, 3875.

-Hamidipour, M., Larachi, F., Ring, Z. 2007. Hydrodynamic observation of trickle beds under filtration conditions. Ind. Eng. Chem. Res. 46, 8336-8342.