Fixed Bed Reactor Fouling

Fixed bed reactors are a workhorse in the chemical and petroleum refining industry. A reactor is often expected to run uninterrupted for multiple years between planned shutdowns. Chemical engineers are expected to skillfully monitor and manage the forecasted remaining catalyst life, as unplanned outages are significantly more costly than scheduled maintenance shutdowns.

Catalyst poisoning may occur when a hydrocarbon reactor receives feed material contaminated with non-hydrocarbon materials, such as metal ions. In addition to poisoning, the catalyst bed is subject to mechanical obstruction from solids in the feed. Catalyst extrudates are often very small to reduce mass transfer resistance, and a deep bed of fine catalyst extrudates is an efficient particulate capture system. Particulate fouling will be evidenced by increasing reactor differential pressure. Examples of undesired feed particles include carbonaceous materials, polymeric material, pipe scale, and inorganic suspended matter.

Either loss of catalyst activity or rising differential pressure may necessitate reactor throughput reduction or an expensive unplanned shutdown to replace the catalyst. In the event of a shutdown it is the job of the chemical engineer to adjust the catalyst loading plan to maximize the cycle life of the unit. Removing a portion of the catalyst and replacing it with grading material will increase tolerance to feed particulates but at the expense of fewer active catalyst sites and a higher deactivation rate. Increasing the amount of catalyst loaded either by dense-loading or by removing a portion of the grading layer volume may increase tolerance to poisons and reduce deactivation rates, but at the expense of reduced tolerance to feed particulate contamination.

In this engineer’s experience, there is a third reactor fouling mode that is unusual and unexpected by most: particulate plugging of the bottom bed of a multi-bed reactor while the upper bed stays clean and free of particulate contamination, despite both beds containing the same size, type, and density catalyst. This failure mode will be discussed along with potential root causes.