Reducing sulfur trioxide concentration in regeneration zone flue gas

In a catalytic cracking process comprising:

• contacting a hydrocarbonaceous feed with a cracking catalyst to produce cracked hydrocarbon vapors and deactivated catalyst containing carbonaceous deposits;
• separating the deactivated catalyst from the hydrocarbon vapors and conducting the deactivated catalyst to a regeneration vessel;
• at least partially removing the carbonaceous deposits from the deactivated catalyst in the regeneration vessel by means of an oxygen-containing gas introduced into the regeneration vessel, thereby forming a flue gas comprising oxygen, sulfur dioxide, sulfur trioxide, carbon monoxide and carbon dioxide;
• the improvement which comprises monitoring the sulfur trioxide and the oxygen concentration in the flue gas from the regeneration vessel;
• and
• adjusting the amount of the oxygen-containing gas in the regeneration vessel in relation to the concentration of the sulfur trioxide to maintain the concentration by volume of the sulfur trioxide in the flue gas such that the ratio SO.sub.3 /SO.sub.x in the flue gas is less than 5%, thereby preventing the appearance of a visible condensation plume in the flue gas.