The excessive fouling you experienced in the radiant section, was it in the upper part or in the lower part? There are a few reasons for fouling in the upper part. One reason is resid incompatibility. Another reason for fouling in that area that has been reported is inorganic contamination of the feed. One of the contaminants has been determined to be an antifoam that is used in the production of the base crude oil.
Fouling in the lower portion is typically the conventional coke formation mechanism.