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Ind. Eng. Chem. Res. 1999, 38, 4878-4888
Deposition of Fine Particles in Packed Beds at Hydrotreating Conditions: Role of Surface Chemistry Sanyi Wang,† Keng H. Chung,‡ Jacob H. Masliyah,† and Murray R. Gray*,† Department of Chemical & Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G6, and Syncrude Research Centre, Edmonton, AB, Canada T6N 1H4
Accumulation of fine particles by packed-bed catalytic reactors causes increased pressure drop, sometimes causing premature shut down. Deposition in a reactor, i.e., filtration, is enhanced when the particle suspension is unstable. A model system of 0.68 µm of kaolin in light gas oil was selected to study deposition at hydrotreating conditions. The surface of the kaolin was modified by adsorbing polymeric asphaltene molecules. Particle suspensions were circulated through a packed bed of either glass beads or catalyst in a batch reactor at 375 °C. The filtration mechanism switched from cake filtration to deep-bed filtration when kaolin was pretreated with asphaltene. Deposition of kaolin particles in the bed decreased with increasing asphaltene concentration. These results are consistent with steric stabilization of colloids by adsorbed polymers. Hydrotreating of the liquid phase enhanced desorption of the asphaltene polymer from particle surfaces and increased particle flocculation and deposition along the length of the reactor bed. Introduction Packed-bed reactors are widely used in the petroleum industry for hydrotreating various process streams from residual oil through to naphtha. Depending on the source of the oil, several types of particles may be present in hydrotreater feed streams, including clays, coke, and corrosion products. In fixed-bed hydrotreating of residue fractions, corrosion products, such as iron sulfide, are deposited in the reactor bed and cause severe plugging.1 Athabasca bitumen contains finely dispersed clay solids coated with humic material that closely resembles asphaltenes.2 Some of these particles are entrained into the gas oil fraction during distillation. Similarly, in coking processes such as delayed coking, coke particles can be entrained into coker distillates. These entrained fine solids subsequently enter downstream hydrotreaters. Filtration prior to the reactor bed can effectively remove particles larger than approximately 20 µm but not the smaller fine particles. A low concentration of fine solids, in the range of several parts per million in hydrotreater feed, can accumulate significantly over months of operating by depositing throughout the catalyst bed. The ensuing increase in pressure drop can result in premature reactor shutdown.1,3 The deposition of fine solids in packed-bed hydrotreaters is analogous to aqueous deep-bed or granular filtration, wherein the size of solid particles is much smaller than the flow channels through the packed bed. The catalyst pellets can be considered collectors of fine particles in the feed oil, but at a microscopic level the retention of fine particles will depend on particle* Author to whom correspondence should be addressed. Phone:(780)492-7965.Fax:(780)492-2881.E-mail:Murray.Gray@ UAlberta.CA. † University of Alberta. ‡ Syncrude Research Centre.
particle forces. In order for fine particles (
