Vanadium mobility

Sodium and vanadium react to form sodium vanadates. These mixtures have a low melting point (<1,200°F or 649°C) and increase vanadium mobility. 

FCC catalyst development to reduce the effect of vanadium has been aimed at reduction of vanadium mobility the application of special ingredients in the catalyst which function as metal scavengers or metal catchers. In the past (2, 10) transport experiments were used to show that during steam-aging, intraparticle transfer of vanadium occurs and that migrating vanadium can be irreversibly sorbed by a metal trap such as sepiolite (2) in the form of a heat stable vanadate. 

A simple vanadium mobility test can be setup, in which the vanadium which migrates from vanadium-loaded catalyst towards non-vanadium loaded catalyst can be measured under catalyst aging (usually steaming) conditions (2). 

This approach minimizes the effects of catalyst attrition phenomena on the vanadium mobility determination. 

The following table (Table VI.), shows an example of vanadium mobility and its dependence on catalyst composition. 

Factors that influence vanadium mobility in commercial FCC units are ... 

Regenerator temperature it has been observed that the vanadium mobility can double with 40°C regenerator temperature (see figure 8). 

Figure 8 illustrates the effect of temperature on vanadium mobility (measured on equilibrium catalysts from three FCC units) and the large impact of certain unit variables. For these measurements the coarse equilibrium catalyst was tested in a mobility test using the method described above by substituting sieved coarse equilibrium catalyst for vanadium impregnated catalyst. 

The efficiency of a Vanadium trap additive is illustrated in Figure 5.8 (Si reflects the presence of the zeolite based catalyst while La and V are present on the same additive particle, the V-trap) and the catalytic effect demonstrated in Figure 5.9. The effectiveness of V-traps is particularly difficult to test in the laboratory, because the level of vanadium mobility in commercial units is difficult to simulate, and the competitive reaction to form sulfate is not taken into account by most laboratory testing. 

The rate of vanadium mobility in FCCU s is dependent on many operating factors. These might be whether the unit is operated in lull or partial combustion, which will effect the average oxidation state of the vanadium. Other factors will be freshness of the vanadium, presumably older vanadium has had more of an opportunity to react with the catalyst matrix and become immobile. Steam concentration in the regenerator, catalyst make-up rate, temperature, and two-stage regeneration will all effect the mobility of vanadium. 

The results of this work show that even though the vapor pressure of vanadium is low, the transfer velocity of vanadium vapor is high and the rate of mass transfer in a fluidized bed is high. A high rate of vanadium transport to traps and a low rate of vanadium transport by transpiration are consistent with the vapor phase transport model. The vapor pressure of the vanadic acid follows a second order Freundlieh isotherm, which reflects a coverage dependent heat of adsorption. The rate of vanadium transfer from catalyst to trap is only weakly dependent on the number density of the catalyst or trap particles. This lack of dependence suggests that inter-particle collisions are not the dominant mechanism for vanadium transfer. Vanadium mobility in FCCU s is a complex issue dependent on many operating variables. 

New Developments in FCC Catalyst Deactivation by Metals Metals Mobility and the Vanadium Mobility Index (VMI)... 

Vanadium, in particular, is a metal which has large effects on FCC activity and selectivities. Not only does vanadium migrate to and destroy the zeolite, but also under certain conditions, it has high dehydrogenation activity [2]. Recent work has shown that vanadium is most mobile in its fully oxidized state, the state in which it is most destructive towards the zeolite [3], and that there are many factors which affect vanadium mobility. Nickel, which also has dehydrogenation activity, is much less mobile than vanadium and does not attack and destroy the zeolite. Thus, from a refiners perspective, it would be valuable to know how mobile the vanadium is in their FCC unit. In this paper, we use commercial Beat and laboratory experiments on fresh catalyst to help answer the question of how mobile is vanadium and what are the factors that affect this mobility. 

A summary of commercial FCC operating conditions and Vanadium Mobility... 

R.F.Wormsbecher, W.-C. Cheng, G. Kim and R.H. Harding, Ch. 21, Vanadium Mobility in Fluid Catalytic Cracking, in Deactivation and Testing of Hydrocarbon-Processing... 

Dual particle or separate traps such as RV4+ must have attrition and fluidization properties similar to FCC catalyst. Their advantages are that they do not change the selectivity of the base catalyst and theoretically have a higher capacity for vanadium capture. Performance evaluation of dual particle traps is usually simpler. They can often be isolated from equilibrium catalyst and analyzed for vanadium capture. Confirmation of preferential pick up on integral traps tends to be a bit more qualitative. A disadvantage may be that they are more dependent on vanadium mobility than integral traps. 

Since the effectiveness of a separate particle vanadium trap such as RV4+ depends on the ability of the vanadium to migrate from the catalyst to the trap, a number of laboratory experiments and commercial evaluations were designed to measure vanadium mobility. Vanadium mobility can be discussed in terms of intraparticle mobility, interparticle mobility from the catalyst to the trap, and interparticle mobility from the trap to the catalyst (irreversibility). These three areas are discussed below[6]. 

Interparticle mobility is proven by electron microprobe scans of cyclic metal impregnated (CMI)[6] Residcat 767Z4+ which incorporates RV4+ technology. Since the catalyst and the RV4+ were simultaneously exposed to the metals during the CMI procedure, the rate of deposition of vanadium on the catalyst and trap surfaces should be similar. However, the catalyst particles, contain virtually no detectable vanadium. In contrast, the RV4+ particles containing the Active Trap Component are high in vanadium. This is another indication of particle to particle vanadium mobility[6]. ... 

At the cessation of treatment, vanadium mobilized rapidly from the liver and slowly from the bones. Other tissue levels decreased rapidly after oral exposure was discontinued. Thus, retention of vanadium was much longer in the bones (Edel et al. 1984 Parker and Sharma 1978). 

Martin HW and Kapian DI (1998) Temporal changes in cadmium, thallium, and vanadium mobility in soil and phytoavailability under field conditions. Water, Air, and Soil Pollution 101 399-410. 

Strong metal support interaction is commonly proposed in the literature for reduction of oxide [17]. This model is in agreement for an interface like Ni-O-V over nickel particle. Ease vanadium reduction can be proposed by SMSI and also a decrease of vanadium acidity. This effect lowers vanadium zeolite poising and also vanadium mobility. 

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