We are trying to figure out how to improve the feed control to our new Hydrocracking and Hydrotreating Units, since one or two of the feeds come from the Coker Unit, we want to know how variable are the quality and flow of the HCGO, Naphtha and LCGO?.
We are aware it would be changing while coker cycles are taking place. We don’t have tanks to store LCGO and Naphtha as feed to the hydrotreater, so these streams go to the hydrotreater directly from the coker unit, also if there is something wrong with the hydrotreater, those streams could go to the HCGO tank that feeds the hydrocracker and if there is a sudden change in composition or flow, it could lead on a runaway on both plants.
The variation would depend on the fractionator internals and process control scheme. The control operation can make pro-active process adjustments prior to backwarm and during drum switching that can minimize the swing. Chimney trays at the various draws can be useful but only if your flows are large enough. Advanced control applications can be useful as well but you will have to invest a lot of time customizing it and tuning it before it will be truly useful.
Simplest option, sample a the maximum time between switches on a 4/6 drum unit. Then sample 20 min after drum preheat, again 10-20 min after switch, again 1 hour after switch. The make some process adjustments (pumparound, spray wash, draw rates, etc) during the next cycle and re-sample for improvements.