Controlling Contaminant Sodium Improves FCC Octane and Activity
Sodium or alkali earth metals are contaminants that produce detrimental effects on catalyst's performance in the FCC unit. Their presence on the catalyst can be attributed to a poor or inadequate crude desalting operation, use of caustic downstream of the crude unit, processing refinery "slops", purchasing sea water contaminated gas oil, using contaminated spray water or feed water, and leakage in steam generators. Regardless of the source, this additive sodium will change catalytic activity / selectivity which will be reflected in lower unit conversion and a loss of RONC and MONC gasoline octane. Severe problems will be encountered in both of these areas if the additive Na level exceeds 0.5 wt. % and regenerator temperature is greater than 1325° F.
Sodium and Catalyst Activity
The sodium, once deposited on the catalyst, deactivates the cracking function by neutralizing the acid sites directly and by forming a low temperature eutectic with the matrix. The resultant eutectic melting temperatures are in the regions normally encountered in the FCCU regenerator. Thus, melting of the matrix will result in pore collapse and zeolite "encapsulation". This effect is directly proportional to the severity (temperature) in the regenerator and the presence of other contaminants on the catalyst such as vanadium. The combination of acid site neutralization and "encapsulation" of the zeolite can lead to significant permanent loss in catalytic activity.
BASF has studied the effect of added sodium on catalytic activity in the laboratory(Ref. 1). This was done by impregnating a catalyst with additive sodium levels of 0.5, 1.0, 1.5 and 2.0 wt. % using aqueous NaCI and steamed at the following conditions:
The heat treated samples were evaluated at BASF's standard MAT conditions of 910° F. 5 C/O, 15 WHSV and using standard Mid-Continent gas oil. The results are illustrated in Figure 1. The added sodium significantly affects the activity as its level on the catalysts exceeds 0.5 wt. % and the heat treat temperatures increase past the equivalent regenerator temperature of 1325° F.
Sodium and Gasoline Octane
The effect of added sodium on gasoline octane was the subject of a 1982 BASF Patent(Ref. 2). The data in it reviewed pilot plant testing of a zero rare earth FCC catalyst inherently contained 0.20 wt. % Na20 that was impregnated to different sodium levels. These levels and the results of testing are shown in Figure 2.
This testing shows a reduction in both RONC and MONC as added sodium increases.
Commercial data supports our laboratory results. As shown in Figures 3 thru 5, there is an octane penalty as sodium increases in the circulating catalysts inventory. The mechanism for the observed octane penalty was discussed by Pine, et al(Ref. 3,4). An outline of which is as follows:
With the strongest acid sites preferentially poisoned, only the weaker sites are available for the cracking reactions. The weaker acid sites are more closely spaced than the stronger sites. As such, bimolecular hydrogen transfer reactions are more likely to occur:
Hydrogen is transferred from naphthenic LCO boiling range compounds to olefinic gasoline boiling range compounds to form higher paraffin content gasoline and higher aromatic content LCO. This results in lower gasoline octane and lower LCO cetane.
Additive sodium is a contaminant that permanently deactivates cracking catalyst through neutralization of acid sites and possible encapsulation of zeolite. Gasoline octanes are negatively affected as a result of the reduction in strongly acidic cracking sites which in turn promotes an increase in bimolecular hydrogen transfer reactions. Care should be exercised by the refiners to exclude or minimize sodium from the FCC unit.
1. Himpsl, F. L. , BASF internal publication (TSR #8191).
2. Brown, S.M. et al, U.S. Patent 4,325,813 ( 1982).
3. Pine, L.A., et al, J. Catal, 85, 466 (1984)
4. Pine, L.A., et al, J. Catal, 85, 466 ( I 984)