The quantity of spent hydroprocessing catalysts discarded as solid wastes in the petroleum refining industries has increased remarkably in recent years due to a rapid growth in the hydroprocessing capacity to meet the rising demand for low sulfur fuels. Due to their toxic nature, spent hydroprocessing catalysts have been branded as hazardous wastes, and the refiners are experiencing pressure from environmental authorities to handle them safely. Several alternative methods such as reclamation of metals, rejuvenation and reuse, disposal in landfills and preparation of useful materials using spent catalysts as raw materials are available to deal with the spent catalyst problem. The technical feasibility as well as the environmental and economic aspects of these options are reviewed. In addition, details of two bench scale processes, one for rejuvenation of spent hydroprocessing catalysts, and the other for producing nonleachable synthetic aggregate materials that were developed in this laboratory, are presented in this paper.
Large quantities of catalysts are used in the refining industry for the purification and upgrading of various petroleum streams and residues1. The catalysts deactivate with time and the spent catalysts are usually discarded as solid wastes. The quantity of spent catalysts discharged from different processing units depends largely on the amount of fresh catalysts used, their life and the deposits formed on them during use in the reactors. In most refineries, a major portion of the spent catalyst wastes come from the hydroprocessing units. The volume of spent hydroprocessing catalysts discarded as solid wastes has increased significantly due to a steady increase in the processing of heavier feedstocks containing higher sulfur, nitrogen and metal contents, together with a rapid growth in the distillates hydroprocessing capacity to meet the increasing demand for low sulfur fuels2. In Kuwait alone around 7,000 tons of spent catalysts are generated every year as solid wastes from the hydrotreating and hydrocracking units.
Environmental laws concerning spent catalyst disposal have become increasingly more severe in recent years. Spent hydroprocessing catalysts have been classified as hazardous wastes by the Environmental Protection Agency in the USA3. The most important hazardous characteristic of spent hydroprocessing catalysts is their toxic nature. Chemicals such as V, Ni, Mo and Co present in the catalyst can be leached by water after disposal and pollute the environment4. Besides the formation of leachates, the spent hydroprocessing catalysts, when in contact with water, can liberate toxic gases. The formation of the dangerous HCN gas from the coke deposited on hydroprocessing catalysts that contain