Vanadium passivation

Occelli, M. L. and Stencel, J. M., "Cracking Metals-Contaminated Oils with Catalysts Containing Metal Scavengers. Part II. The Effect of Aluminuma Particles Addition on Vanadium Passivation." (In preparation)... 

On non-zeolitic particles in the absence of a vanadium passivator, vanadium (when present at the 0.4 wt% level) makes a greater contribution to contaminant coke and hydrogen yields than nickel at constant surface area and metals loading. Incorporation of a vanadium passivator into the catalyst matrix can greatly alter the selectivity effects of vanadium, and can essentially negate its effect on non-zeolitic particles as in the case of magnesium. 

Use a vanadium passivator when vanadium level exceeds 3000 ppm on catalyst... 

Models have been developed to predict cat cracker yields based on operating parameters and feedstock properties (34) These have aided in application and evaluation of metals passivation benefits. Miller and Pawloski (35) reported the use of mathematical models to calculate the benefits of vanadium passivation, and Teran (27) reported the need for FCCU hydrogen modeling and metals tracking to optimize passivation benefits. 

Sepiolite has been made exchangable by chemical treatments and Mg2+ at the border of the channels has been substituted by Al3+. In this way sepiolite with mild acidity, controlled mesopore, and improved stability has been obtained. This material is active for gasoil cracking, giving a good bottom conversion, and light cycle oil products without excessive gas and coke formation. Meanwhile, it is active for vanadium passivation. 

Vanadium pas ation has been a more difficult challenge to overcome. In reviewing the hterature it is quickly realized that a host of materials have been studied for vanadium passivation and some have been commercialized. However, not all materials perform equally. In fact, actual unit performance may vary dgnificantly from that predicted by testing methods commonly used in many laboratories. 

Metal-tolerant catalysts are formed by active matrices with macropores which can entrap large molecules such as the vanadium-containing compounds and, ideally, can produce metal agglomeration and a strong interaction with the matrix. However, most of these catalysts have to balance between metal tolerance and other desired catalyst characteristics such as yield and cost. Due to this, separate metal-trap additives have been used, and in the case of vanadium besides MgO-containing materials (von Ballmoos 1993) and a liquid vanadium passivator (Stonecipher 1997), anew rare-earth-based dual particle system has been introduced (Dougan 1994). 

Examples of liquid additives currently in use include bismuth and antimony based additives for passivation of nickel contaminants. A number of solid catalytic additives have been developed that are specific for certain functions. Approximately two-thirds of North American units utilize a noble metal promoter to reduce emissions of CO as well as provide beneficial yield effects. During the early to mid-1980 s, SOX removal additives came into use due to tighter environmental restrictions. A ZSM-5 based additive for octane enhancement and light olefin production was developed during the mid-1980 s and is used commercially. Additives have also been proposed as metal traps especially for vanadium passivation. These solid FCC additives have become an increasingly important tool by which refiners meet yield and environmental requirements. 

Catalysts. In industrial practice the composition of catalysts are usuaUy very complex. Tellurium is used in catalysts as a promoter or stmctural component (84). The catalysts are used to promote such diverse reactions as oxidation, ammoxidation, hydrogenation, dehydrogenation, halogenation, dehalogenation, and phenol condensation (85—87). Tellurium is added as a passivation promoter to nickel, iron, and vanadium catalysts. A cerium teUurium molybdate catalyst has successfliUy been used in a commercial operation for the ammoxidation of propylene to acrylonitrile (88). 

A number of indices relate metal activity to hydrogen and coke production. (These indices predate the use of metal passivation in the FCC process but are still reliable). The most commonly used index is 4 X Nickel + Vanadium. This indicates that nickel is four times as actiw as vanadium in producing hydrogen. Other indices [9] used are ... 

In general, vanadium concentrations above 2,000 ppm on the E-Cat can justify passivation. 

The corrosion behaviour of amorphous alloys has received particular attention since the extraordinarily high corrosion resistance of amorphous iron-chromium-metalloid alloys was reported. The majority of amorphous ferrous alloys contain large amounts of metalloids. The corrosion rate of amorphous iron-metalloid alloys decreases with the addition of most second metallic elements such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, copper, ruthenium, rhodium, palladium, iridium and platinum . The addition of chromium is particularly effective. For instance amorphous Fe-8Cr-13P-7C alloy passivates spontaneously even in 2 N HCl at ambient temperature ". (The number denoting the concentration of an alloy element in the amorphous alloy formulae is the atomic percent unless otherwise stated.)... 

Sepiolite passivates most of the nickel via formation of non interactive silicate-like materials. Heating at high temperatures induces migration of nickel to the interior and of vanadium to the exterior of the catalyst surface. Metal-surface interactions are observed also in Ni-loaded kaolin microspheres however, V on kaolin behaves like bulk V20g with respect to reduction, thus explaining this xlay s inability to passivate V-contaminants. 

The relative ease with which VpOr can be reduced to V(III) in aluminosilicates indicate the exiirence of weak metal-surface interactions and the inability of the surface to effectively passivate vanadium. Similarly, V on Kaolin (and metakaolin) exist mostly as the "free oxide and can (in part) be reduced to V(III) species. Therefore, DFCC systems containing metakaolin microspheres (or amorphous aluminosilicates (15)) should not be as effective as sepiolite in passivating metals TTke Ni and V. In fact, DCC mixtures loaded with 5000 ppm Ni-equivalents (that is 0.6% V + 0.38% Ni) are not metals resistant when metakaolin is used as a metals scavenger (1) ... 

DFCC mixtures containing 40% sepiolite and 60% GRZ-1 are equally effective in passivating high (10,000 wtppm) levels of vanadium impurities (1 ). In both cases, metakaolin microspheres do not... 

These results suggest that oxidation state is not solely responsible for catalyst deactivation but that other factors such as V location and mobility may play an important role. Basic alkaline earth oxide passivators such as MgO, admixed to the catalyst, interact strongly with vanadium during the regeneration period. Although the oxidation state of vanadium is essentially unaffected, MgO structurally modifies V as evidenced by a unique X-ray absorption spectrum. 

Understanding the interaction of vanadium with the catalyst is an important step in the development of technology to passivate... 

VANADIUM DEACTIVATION UNDER SIMULATED CONDITIONS. The degree of catalyst deactivation was measured by comparing the activity of the catalyst or catalyst/passivator blend containing 5000 ppm V to that of the corresponding sample with no V added after similar treatment conditions (Table II). For the USY catalyst (catalyst A), steaming with VgO at 1450 F resulted in a 6855 decline in activitjr ... 

The passivity of vanadium is referred to on p. 23, and the electrolytic decomposition of anhydrous fused vanadium salts on p. 17. 

Previously proposed mechanisms of the biosynthesis of certain chlorinated compounds have invoked electrophilic bromination of alkenes followed by passive chloride attack [62], Although this mechanism could explain the origin of adjacent brominated and chlorinated carbons, it does not readily account for compounds containing chlorine only. Thus, with the discovery of chloroperoxidase activity of the vanadium enzyme, the origin of specific chlorinated marine natural products can now be addressed. 

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