Reducing drum pressure will reduce the coke yield and generate more liquid products. The vapor velocity will increase due to the lower pressure so the likelihood of coke fines carry over will increase. Similarly, the pressure on the fractionator will decrease which will increase the likelihood of tray flooding. The suction pressure of the wet gas compressor will decrease which will reduce the capacity of the compressor on a total mass flow rate basis.
If you are currently producing sponge coke, lowering the drum pressure will increase the likelihood of producing shot coke. Shot coke can be more difficult to quench which will lead to hot spots in the coke drum. If you are currently producing shot coke, lowering drum pressure will increase the likelihood of producing loose shot. Shot coke in general and loose shot coke in particular require the initial water quench rate to be low and the water rate should remain low for as long as possible before increasing the water rate. The rate of increase should be kept low until the cone section of the coke drum is full of liquid water. The low water rates are done to prevent channeling of the water through the coke bed. If there is channeling, there will be hot spots in the coke bed.
Pressurized quenching by holding back pressure on the coke drum during the water quench phase will help prevent hot spots in the coke bed by increasing the likelihood that liquid water will contact all of the coke in the drum. If the coke bed is adequately quenched, there should not be any hot spots.
Minimizing unconverted oil left in the drum by operating the drum hot enough during the coking phase and by proper steaming during the little and big steam steps during the off-line drum operation will also minimize hot spots. Drum outlet temperatures prior to quench should target 825 F (440 C).