Controlling the metals content of FCC equilibrium catalyst
A method for calculating catalyst make-up provides a means to control the stability of FCC equilibrium catalyst in terms of metals content
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Fluid catalytic cracking (FCC) is one of the key process technologies in the refining and petrochemical industry, upgrading low value streams such as vacuum gas oils and residues to valuable fuel components such as high octane naphtha and basic petrochemical building blocks such as propylene and ethylene.
The activity and selectivity of FCC equilibrium catalyst (E-cat) are among the most important variables to be monitored in an FCC unit to maintain the desired yield of valuable products. On the other hand, the presence of metals such as nickel, vanadium and sodium in E-cat affects these catalytic properties, thus the metals content in E-cat is perhaps the most relevant variable to be controlled when managing the fresh catalyst make-up rate. The aim of this article is to provide a mathematical method to control E-cat metals content based on classical mass balance equations. In addition, two case studies are presented to illustrate the calculations.
Effects of metals on E-cat performance
Metals in FCC feed, in particular nickel, vanadium and sodium, are detrimental to the FCC unit’s performance. Nickel and vanadium are present in crude oil and concentrate in the high boiling point fractions,1 which are the typical feeds to the FCC units and include residual feed such as atmospheric and vacuum residues in the case of residue FCC units.
On the other hand, sodium contamination is usually caused by the presence of sodium chloride in FCC feeds, mainly due to upsets in the crude unit desalter,2 or excessive caustic injection, also in the crude unit.
Other metals such as iron, copper, calcium and magnesium are also present in FCC feeds, however nickel, vanadium and sodium are usually present in much higher concentrations. The main effects of these three contaminants are dehydrogenation, coke formation and catalyst deactivation.
Dehydrogenation and coke formation promoted by nickel and vanadium
Nickel at relatively low pressures, such as those in FCC units, promotes dehydrogenation reactions, taking hydrogen atoms from hydrocarbon molecules to form hydrogen gas (H2). In this process, as hydrocarbon molecules lose hydrogen they become more prone to forming olefins and aromatic compounds.
Furthermore, aromatic compounds in a dehydrogenating environment tend to condense to form heavy polynuclear aromatics (PNA) and subsequently coke (see Figure 1). These effects are magnified when processing highly aromatic feeds and residues.
Coke-make reactions increase delta coke and the regenerator temperature, therefore the catalyst to oil (C/O) ratio is reduced, and hence conversion as well.
Vanadium is also considered a dehydrogenation promoter, however its dehydrogenation activity is approximately 25% compared with nickel. For this reason, when the combined effect of nickel and vanadium is being studied, a dehydrogenation metals factor must be used (FDHM), defined here as Ni + V/4 (wtppm).
Figure 2 illustrates the typical combined effects of nickel and vanadium on E-cat, showing in this case the coke and dry gas formation factors as a function of the FDHM for two different catalysts in two different units. (Unit 1 processes VGO feed and Unit 2 processes residual feeds.)
In general, the dehydrogenation and coking effect of nickel and vanadium correlates very well with the E-cat FDHM, unlike simple Ni+V which is inaccurate since the Ni/V ratio is generally different for different kinds and sources of feeds.3
On the other hand, unstable olefins formed by dehydrogenation tend to polymerise, forming heavy molecules and lowering selectivity to propylene and butylenes. Furthermore, hydrogen by itself affects the operation of the wet gas compressor since its low molecular weight might decrease the wet gas average molecular weight to values below the compressor design value.
Catalyst deactivation promoted by vanadium and sodium
In addition to dehydrogenation, vanadium has a second effect. Unlike nickel, which deposits on the catalyst matrix, vanadium oxidises in the regenerator to V2O5 and then reacts with steam to form vanadic acid (H3VO4).2 Thereafter, H3VO4 migrates to the zeolite crystal structure, decreasing its melting point to typical regenerator temperature conditions, which slowly destroys the silica-alumina framework.4
In addition, it has been reported that vanadium promotes the transition of the Faujasite structure of Y-zeolite to a Mullite structure with no cracking activity due to the lack of acid sites.5 Consequently, catalyst activity is decreased due to the collapse of the zeolite crystal structure (see Figure 3).
Regarding sodium, its main negative effect occurs when it associates with vanadium. H3VO4 and vanadium oxides react with sodium to form sodium vanadates, mainly NaVO3, which forms eutectics with SiO2,6 causing not only the crystal structure of zeolite, but also the matrix, to collapse rapidly at typical regenerator temperatures due to localised sintering.
This combined effect impacts catalyst activity and conversion drastically. Figure 4 illustrates the typical combined effects of vanadium and sodium on E-cat, showing again the E-cat for two different catalysts in two different units.
In the most severe cases, when a high concentration of sodium vanadates is combined with a temperature excursion in the regenerator, a unit shutdown may be needed to remove sintered catalyst and replace it with fresh and/or equilibrium catalyst. On the other hand, any loss of catalyst activity will increase the occurrence of thermal cracking reactions, affecting conversion and selectivity, due to the increase of dry gas and coke make.
Metals balance in an FCC unit
The most effective way to control the metals content in E-cat is through catalyst formulation and appropriate catalyst make-up management. In this article, catalyst make-up management is discussed as an alternative to control metals content in E-cat, thereby controlling their detrimental effects in the FCC unit performance as well.
The first step in effective catalyst make-up management is conducting a metals balance around the unit. A typical FCC metals balance is shown in Figure 5, which describes the metals inlets (catalyst make-up and metals added by the feed itself) and outlets (catalyst withdrawals and losses).
Since some FCC units, especially those processing residue, also utilise E-cat with low metals content from other units as a catalyst make-up, in addition to fresh catalyst, the catalyst make-up is also considered as a potential metals source in the metals balance around a typical FCC unit.
This balance can be done on an individual component basis (for Ni, V, Na or any other metal) or considering grouping of species such as Ni+V/4 or Na+V, depending on the kind of monitoring and control to be performed on the E-cat.
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