There is an increasing awareness in the industry that refiners can substantially improve the profitability of their hydroprocessing reactors by installing the latest-generation reactor internals. However, the importance of taking steps to prevent foulants from reaching those trays and internals is often underappreciated. Fouling of catalyst beds causes unwanted pressure drop increases that limit the performance of many hydroprocessing units. The resultant issues can include short run lengths, unplanned downtime costs, unused catalyst activity, increased maintenance and lost revenue. The fouling challenge is complex and depends on the type of feed, the upstream processing and the unit operating conditions.