Method for inhibiting fouling of petrochemical processing equipment

A method of inhibiting formation of a foulant on process equipment through which a stream of hot crude oil or naphtha is fed and comprising adding to the stream an effective amount of a mixture consisting essentially of (1) a thiodipropionate and, (2) either a dialkyl acid phosphate ester or dialkyl acid phosphite ester, said phosphate ester being characterized by the general formula ##STR4## where:

• X is sulfur or oxygen,
• R.sub.1, R.sub.2, and R.sub.3 are each individually selected from the group consisting of hydrogen and addition complexes of hydrogen with an amine, alkyl, aryl, alkaryl, cycloalkyl, alkenyl, and aralkyl provided that in any given such phosphate ester at least one and not more than two of each R.sub.1, R.sub.2 and R.sub.3 are hydrogen or an addition complex of hydrogen with an amine;
• and said phosphite ester being characterized by the general formula ##STR5## where:
• X is sulfur or oxygen,
• R.sub.4, R.sub.5, and R.sub.6 are each independently selected from the group consisting of hydrogen and addition complexes of hydrogen with an amine, alkyl, aryl, alkaryl, cycloalkyl, alkenyl, and aralkyl, provided that in any given such phosphite ester at least one and not more than two of each R.sub.4, R.sub.5, and R.sub.6 are each hydrogen or an addition complex of hydrogen with an amine.