Refoaming is a coke drum is usually due to excessive amounts of unconverted oil left in the coke drum at the end of the coking phase. Operating the coke drum at a higher drum outlet temperature should help reduce or eliminate the refoaming events. A target drum outlet temperature of 825 F (440 C) is high enough to convert enough of the feed that refoaming will not be a problem.
The small steam step is used to push the last of the feed into the coke drum up through the coke bed and into the bubbling liquid layer on the top of the bed. This steam also transfers heat from the coke below into the still reacting liquid to complete the coking reactions.
Superficial vapor velocity is one of the key variables in foam generation, so increasing the height of the foam front when the coke drum is depressurized to the blowdown system is expected. Depressurizing the coke drum slowly, over 10 minutes or so, should limit the increase in foam front depth due to the drum pressure decreasing.
A high VCM content on the coke is an indication of the coke drum being too cold during the coking phase. Targeting a coke VCM of between 9% and 11% by increasing heater outlet temperatures and steaming the coke bed to blowdown at the big steam rate for a reasonable time should help prevent the refoaming events.
I think that coke morphology has very little to do with refoaming. Some resids, those produced from paraffinic feeds, need to have more severe conditions to finish the coking reactions completely.