Catalyst Stacking Technology as a Viable Solution to Ultralow Sulfur Diesel Production

Production of ultralow sulfur diesel (ULSD) is a tough challenge for refineries previously producing low sulfur diesel. Some common solutions to meet the stricter standards are revamping pre-existing units, building new facilities, purchasing high-activity catalysts, adapting alternative desulfurization techniques, and changing the feedstock; however, these methods usually entail high capital investments or are simply not viable. Another option, generally overlooked, is to place different catalysts in multibed configurations to produce a synergistic effect between them, i.e., catalyst stacking technology. This work aims to broaden the current knowledge and scope of this technology and suggest it as a viable low-investment solution for ULSD production. As we have described, the differences between the catalysts can be exploited to maximize the performance and minimize the operating costs of HDS units. Moreover, any new HDS catalyst performance could benefit from a fitting stacked-bed system, especially if it has high production costs. However, the synergistic effect between the catalysts is not fully understood yet, and designing the best stacking system (number, order, and proportions of the catalysts) is not straightforward. Many factors must be considered, such as the characteristics of the catalysts (active phase, support, and kinetic parameters), feedstock composition, operating constraints, and target end product. Currently, the selection of the optimal configuration is inadequately approached case by case and requires multiple catalytic evaluations on an industrially relevant scale because a reliable predictive method has not been established yet. Therefore, valuable time and resources could be saved by thoroughly studying the influences of the variables on the synergetic effects and developing new mathematical models. Furthermore, there are still many alternative applications of this technology that could solve current and future issues of the refining industry, like the reuse of spent catalysts and the coprocessing of bio-oils with conventional feedstocks.