A process vessel includes a body defining an interior vessel surface and a fixed catalyst bed in the body. A retainer is provided on the fixed catalyst bed to retain the fixed catalyst bed in position. The retainer defines at least one trapping formation at or in close proximity to the interior vessel surface above the fixed bed catalyst to trap solid objects whirled around in an annular zone above the fixed catalyst bed by fluid passing through the vessel, which whirling solid objects can erode the interior vessel surface in the annular zone.