The same goes for industrial processes like coke drum systems in the refining industry. Coke drums are cleaned after each cycle, which is why they’re commonly used in pairs – one is in use while the other is being cleaned. The vapour overhead nozzle line, however, which runs to the fractionator from the coke drum to the next step in the process, typically only gets cleaned during a forced shutdown. If coke drum levels are too high or inaccurate, this line clogs up sooner, leading to more frequent shutdowns and lost revenue.
VEGA, a global manufacturer of process measurement technology, has the solution to minimising those shutdowns: a vapour compensation gauge paired with continuous level detectors. With VEGA’s coke drum solution, the vapour compensation gauge serves a dual purpose: it monitors the particulate matter over carry into the vapour overhead line, and it sends the percent change in vapour density above the rising level in the vessel to the continuous level system to increase the accuracy of the overall level measurement. Each of these functions allows operators to better determine the proper switch point from one coke drum to the next while minimising particulate matter reaching the vapour overhead nozzle, leading to fewer shutdowns for cleanouts.
Filling the coke drum
The coking process begins with resid being fed into a coke drum. The resid is extremely hot and violent in nature, and the coke drum process level is very uneven from one side of the drum to the other because of bubbling, foaming, and the overall nature of the fluid. In this step, the refinery is boiling oil to capture the released hydrocarbons in the vapour overhead nozzle to transport them to the coker fractionator. Ideally, the level inside the coke drum should be high enough to maximise throughput, but low enough to prevent any contaminants from getting into the vapour overhead nozzle.
As the resid fills the drum, it commonly foams up, similar to a carbonated beverage being poured too fast – only on a much larger scale. The foam inside the coke drum can rise a few feet rather quickly, so to combat this, antifoam agents made of silica and gas oil are added to the mix during this process. Excess antifoam can also contribute to a blockage in the overhead vapour line, and possibly travel farther downstream, prematurely poisoning catalyst beds. The VEGA continuous level coke drum system can help prevent this from happening.
Vapour compensation gauge and continuous level sensors save the day
Operators should watch two different measurements to get the most efficient fill each time they fill the coke drum: the overall level and the vapour compensation gauge. The overall level can show when any antifoam agents need to be added, and the vapour compensation gauge will begin to spike near the end of the cycle when particulate matter tends to carry over more due to the overall level in the drum.
Using a vapour compensation gauge with continuous level sensors provides refinery operators with the ability to optimise the switch out point for each drum cycle and know when to add antifoam agent. By slowly increasing the switch out level point over time and monitoring how often the refinery has to clean out the vapour overhead lines, they can optimise these two variables to fit their needs.
A recent study in the Coking.com newsletter looked at the value of cleaning the vapour overhead lines when the pressure drop increases 3 psig over the normal 2 to 5 psig drop. The increasing pressure is a result of matter buildup in the overhead lines. Based on the analysis, the value of regularly cleaning the vapour lines could be as much as $876 000 per year. The VEGA coke drum solution provides operators a means to minimise carryover and extend time between shutdowns to clean the vapour overhead lines.
VEGA currently recommends monitoring the continuous level output in combination with the vapour compensation gauge output to maximise throughput and minimise any particulate carryover. Watching these two variables will help in optimising the switch point for the coke drum. Now thanks to this most recent study, the savings from monitoring this pressure drop and cleaning the vapour overhead lines can be given an actual value in savings.
Authors: Jack Rodgers, Vice President Business Development, with contributions from Tai Piazza, Refining Industry Manager and Jason Meyers, Marketing Content Specialist
Read the article online at: https://www.hydrocarbonengineering.com/refining/13092018/how-a-vapor-compensation-gauge-on-coke-drum-systems-pays-for-itself/