Reduction of foaming in a slurry catalyst hydrocarbon conversion process

In a process for the conversion of a hydrocarbonaceous black oil comprising asphaltenes and indigenous trace metals wherein a slurry of said black oil and a finely divided, unsupported metal catalyst is subjected to hydrocarbon conversion conditions in the presence of hydrogen in a hydrocarbon conversion zone, thereby causing the formation within said conversion zone of foam which impairs the efficiency of the conversion process, the method of reducing or eliminating said foam formation in the conversion zone which comprises introducing to said zone containing the black oil-catalyst slurry-from about 0.001 to about 1 weight percent, based on the weight of said black oil, of a silicone antifoaming agent to mitigate foaming in the conversion zone.