Improve Catalyst Resistance to Feedstock Poisons by Increasing Matrix Surface Area
For a variety of reasons, a refiner's economics may at times, dictate a need to process heavy oil / resid in his FCC unit. It may be because extra FCC capacity is available, a changing crude slate, or a decrease in heavy oil / resid demand due to lower resid pricing. Whenever resid processing is increased in an FCC unit, catalyst operating severity is increased. Both metals and basic nitrogen compounds, which are known to poison FCC catalysts, are concentrated in the heavier end of gas oils, especially in the residuum. These poisons, which are contained within large hydrocarbon molecules deposit on the catalyst matrix. Each type of poison affects the FCC catalyst differently. Vanadium, for example, causes permanent deactivation of the catalyst while nitrogen causes only temporary deactivation which is reversed by regeneration. Nickel causes little or no deactivation, however, it is deleterious to product selectivities. The catalyst matrix plays an important role in trapping these feed contaminants and inhibiting their adverse effects on both catalyst activity and selectivity.
In previous Catalyst Reports, data was presented which showed that increasing a catalyst's matrix surface area will result in improvements to product quality and bottoms upgrading. Commercial data showed that Gasoline Octane increases of 0.5 to 1.0 RON and Cetane Index improvements of 1.7 to 2.9 numbers were attained and attributed directly to increasing matrix surface area. Similarly it has been commercially demonstrated that the upgrading properties of catalyst with active matrices can reduce FCC unit bottoms by as much as 40%. This report will discuss the positive effect matrix can have on improving resistance to feed poisons and contaminants.
High Matrix Surface Area Catalysts Resist Vanadium Poisoning
Vanadium deposits on catalyst particles in the riser as organo-metallic compounds that are converted to vanadium oxides in the regenerator. In the regenerator environment, vanadium can migrate to zeolite crystals and form a low melting eutectic with the silica alumina of the zeolite. This leads to destruction of the crystal structure and loss in activity. The binding of vanadium to catalyst matrix inhibits the vanadium from reaching the zeolite and thus minimizes deactivation.
A plot of laboratory data on commercial catalysts which demonstrates this effect is shown in Figure1. Plotted against Matrix Surface area is the ratio of the measured MAT activity of a metals free "clean" catalyst divided by the activity of the same catalyst when subjected to 4500 ppm of vanadium contamination. In both cases, the catalysts were aged by steaming for 4 hours at 1450°F. The data shows the correlation of significant improvements in contaminated catalyst activity with increasing Matrix Surface area up to an upper limit of about 130 m2/gm.
Both the amount of matrix surface area in a catalyst and its composition contribute to a catalyst's ability to resist feedstock poisons. In general, with a few notable exceptions, a catalyst's matrix surface area and its alumina content tend to vary in the same direction. However, alumina content alone is not responsible for the improvement in an FCC catalyst's vanadium tolerance. Surface chemistry is important as well. In addition to alumina, other materials have been shown to be effective at trapping vanadium. These materials include calcium, magnesium and rare earth compounds which are clearly basic in nature. It is possible that acid-base chemistry as well as the formation of high melting species of vanadium are involved in the trapping of vanadium by the matrix. In Figure 2, the vanadium tolerance data shown is for three catalysts with nearly the same high alumina content and matrix chemical composition (all are REY catalysts). From this data it is apparent that the moderate and high matrix surface area catalysts have superior vanadium tolerance and the loss in conversion with increasing metals concentration is greatest for the low matrix surface area catalyst.
Commercial support for the vanadium tolerance effects of an active matrix is shown in Figure 3. In this resid cracking operation at Refinery D, a low matrix surface area catalyst replaced one with a moderate matrix surface area. The catalysts were similar in other respects, such as matrix alumina content and zeolite type. Over a period of weeks as the catalyst inventory changed, the equilibrium catalyst MAT activity gradually declined by five numbers. The activity decline closely followed the decline in catalyst matrix surface area. Unit operating conditions, fresh catalyst makeup, and equilibrium catalyst vanadium loading were essentially constant during the catalyst changeout period. The refinery then changed back to the previously used moderate surface area catalyst and the activity trend reversed itself .
When nickel deposits on FCC catalysts, it acts as a dehydrogenation catalyst which contributes to coke and gas production. As a general rule of thumb, the level of nickel concentration found on a catalyst will be one half the level of the vanadium present. The FCC catalyst matrix may act as a catalyst support for nickel, so that a high matrix surface area could increase the catalytic effects of nickel. However, the use of antimony passivators largely offsets the effects of nickel and greatly reduces the nickel effects relative to those of vanadium.
Ultimately, increasing the matrix surface area within a given family of catalysts can have the drawback of some increase in coke and gas make. The optimal situation for a refiner is to use a catalyst which maximizes matrix surface area up to the limits of his particular unit so that he may take the maximum possible advantage of the benefits derived from increasing matrix surface area, i.e., improved bottoms upgrading, octane and cetane improvement and feedstock poison resistance. BASF 's Dynamics Family of catalysts have been designed to be especially effective in doing this for a refiner.
Active Matrix Improves Nitrogen Tolerance
The heavy ends of gas oils and residue contain both basic and non-basic nitrogen compounds. The basic nitrogen compounds are the type mainly responsible for poisoning FCC catalysts because poisoning occurs by the reaction of the basic nitrogen species with acid sites on the catalyst. The neutralization of catalytically active acid sites results in the deactivation of the catalyst. Nitrogen poisons also adversely affect selectivities. In addition, nitrogen in the feed is associated with asphaltenes and other coke precursors which contribute to catalyst poisoning.
The acid sites on the matrix of an FCC catalyst act to intercept nitrogen poisons and provide ''sacrificial sites" which protect zeolite in the catalyst from being poisoned. Thus, the matrix can reduce the deactivation effects of nitrogen. Figure 4 illustrates the effectiveness of an active catalyst matrix at reducing deactivation by basic nitrogen in feed. In this laboratory test, the activity of a steamed catalyst run on a high nitrogen feed and the activity of the same steamed catalyst run on a low nitrogen mid continent feed were measured and the ratio of their activities is plotted versus matrix surface. A comparison of the qualities of the two feedstocks is given in Figure 5. The higher matrix surface area catalysts, which have high alumina contents are far more resistant to poisoning.
Figure 5a shows that an active matrix still plays its major selectivity role of helping to upgrade bottoms to LCO on the high nitrogen feed. Laboratory data also shows an advantage in other selectivities (gas, gasoline, and coke) for active matrix catalysts, as shown in Figure 5b for gasoline.
In conclusion, the matrix of an FCC catalyst significantly influences bottoms upgrading, liquid product qualities, and feed contaminant tolerance. Matrix cracking increases LCO yield while reducing bottoms by cracking large molecules. Both gasoline octane and LCO cetane are improved by matrix cracking because of low hydrogen transfer and selective cracking of paraffinic portions of the bottoms. Finally, the catalyst matrix traps vanadium and basic nitrogen contaminants in the feed to reduce catalyst deactivation.
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