During the RefComm Galveston 2015 conference, held the week of May 4th, the use of catalyst additives to enhance propylene production and trap metals was discussed in several presentations and classes. One question came up several times;
“How do these additives affect the overall activity of the catalyst blend”?
Apparently, many assume that using additives entails a tradeoff between the desired function of the additive and unit conversion because the additive will dilute the cracking catalyst. Is this really what happens on a commercial unit?
The answer, as with many questions involving the FCC unit, is “it depends”.
To understand the dynamics that control how additives affect a commercial unit, it is useful to review a paper that C. F. Guthrie and I prepared for the 1991 AIChE Symposium on Advanced FCC Technology while I was with Chevron Research and Technology Company. This paper explains that the coke selectivity of the additive and how the operator chooses to control the unit heat balance will determine if overall cracking activity decreases or remains constant as the catalyst inventory is diluted with additives. The paper involved an SOx reduction additive that had little cracking activity and contributed only a negligible amount to coke production, but the mechanisms and conclusions are applicable to all additives (such as ZSM-5 or metals traps) that do not produce significant amounts of coke.
For this study, blends of cracking catalyst and catalytically inert additive were tested in a pilot plant at constant riser outlet temperature and two solids circulation rates (circulation rate of the catalyst plus additive blend). Figure 1 shows that when solids circulation rate is held constant the expected dilution of catalyst activity is observed. The figure also shows that increasing the circulation rate successfully returns the conversion to its zero dilution value even with a high additive concentration.
Figure 2 shows the effect that diluting the catalyst has on coke production. Again, as expected, coke production decreases as the cracking catalyst is diluted. In the pilot plant, there is no direct relationship between the amount of coke produced and the regenerator temperature because external heaters are used to control reactor and regenerator temperatures. In a commercial unit, the coke yield has an integral relationship with the regenerator temperature and therefore the solids circulation rate. This data illustrates that diluting the catalyst inventory would not only reduce conversion, but coke yield as well. The coke yield reduction would result in less heat release during the regeneration phase which would force a series of secondary changes to the unit’s operating conditions. The effect of these secondary changes depends on how the operator chooses to adjust unit conditions.
The operator could choose make up for the lost heat of coke combustion by adding external heat to the system (such as increasing feed preheat) or by increasing the amount of heat released during coke burning (moving from partial to total combustion). In this scenario, the solids circulation rate will remain constant and the change in overall unit conversion will be directly proportional to the concentration of additive; similar to that observed in Figures 1 and 2. Alternatively, the operator could choose to keep heat inputs and combustion constant. In this scenario, the regenerator temperature will decrease. In the most common control scheme used on FCC units, the regenerated catalyst slide valve will automatically open to allow more catalyst to circulate to maintain constant riser outlet temperature. The solids circulation rate will increase until the coke production returns to the point where the unit heat balance is again satisfied. Figure 3 shows the results of heat balance calculations based on the pilot plant data. It shows that when the second scenario is chosen, the resulting increase in solids circulation rate returns the circulation rate of active catalyst back to nearly the initial point resulting an almost complete recovery of the lost conversion.
Thus, the operator need not be concerned about diluting the cracking catalyst with additives as long as:
- The additive has little coke producing activity.
- The unit is capable of circulating sufficient solids to compensate for the level of dilution.
- The reduction in regenerator temperature is not large enough to cause regeneration problems.