June 21, 2018 at 8:48 am #28654
Dear Coking members,
Feed incompatibility is normally a cause for fouling increase in the upper radiant section, due to asphaltene precipitation.
p-value is an analisys that is normally used to measure asphaltene precipitation tendency in other processes (like visbreaking, fueloil, etc).
Has anybody experience of the sucessful application of this analissy (or any other similar) to predict compatibility issues in Delayed coker feed? If so, what is the monimum p-value recommended?
Thanks for your help
June 21, 2018 at 3:00 pm #28659
P-value tests is not really applicable with resid. But I would suggest you use Colloidal Instability Index (CII) is the ticket for resid. You must first get SARA analysis on your feed and ideally on heater charge too.
What values constitute stability are very challenging because of the cracking across the heater. Traditionally CII > 1 is a problem in all cases but CII > 0.5 can be a problem at some sites in the furnace itself. So you will have to find your operating envelope.
June 26, 2018 at 11:10 pm #28670
The p-value test is used to measure the stability and compatibility of visbroken pitch, asphalt and fuel oils at moderate temperatures. This test does not predict what the stability of the blended resids will be as the temperature is increased. Part of the reason that resid incompatibility fouls the upper radiant section of the heater is that is the point of initial vaporization of the resid or bubble point. That is the lowest solvent power of the resid matrix and asphaltenes precipitate at that point.
As Evan stated, the CII is an index that can provide guidance on the stability and compatibility of the blended resids. The SARA analysis is not normally available for the resids so the CII cannot be calculated. Even if you can calculate it, there is uncertainty around the value of this index where the blend becomes unstable.
Process film temperature is dependent on the heat flux and the velocity of the process fluid. The variability of these factors between different heaters is part of the uncertainty around heater fouling.
I have used the Watson K factor and the Bureau of Mines correlation index (BMCI) as guidance around resid compatibility. They can provide some insight into the potential for precipitation of asphaltenes. These values are normally easy to generate, but they are probably less accurate than the CII.
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