Increase refinery profitability using Innovative FCC catalysts to process residue feedstock (1)
Use catalyst fundamentals to select the best catalysts. Globally, around 35% of FCC units’ process predominantly vacuum gasoil (VGO) feedstock and 65% process feedstock containing residue feed; in some regions, the tendency to process residue is even greater.
Vasileios Komvokis, Carl Keeley and Stephen Challis
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To improve the profitability of the FCC unit, increased flexibility to process residue feedstock is desired. This can be achieved through the use of advanced FCC catalysts, supported by value-added technical service. Part 1 of this paper will explain how residue feed quality impacts the FCC operation, and what key catalyst features are required to process residue feedstock. Part 2 will explore options for conducting residue catalyst selection studies and use case studies to demonstrate how innovative FCC catalysts, supported by advanced technical services and tools, can be used to increase refinery profitability. 1-5
Impact of H2 and coke on FCC operation
Additional production of H2 and coke from processing residue type feeds has a very significant impact on the FCC operation, as most FCC’s operate to coke and Gas handling limits.
Coke is any carbonaceous, high molecular weight non-volatile residue formed from cracking, and has low hydrogen-to-carbon ratio, i.e. has low hydrogen content.
There are four types of FCC coke.
• Catalytic coke is coke associated with acid site catalysed cracking. This coke usually increases linearly with the second order activity of the catalyst.
• Additive coke is related to feedstock properties, such as, basic nitrogen content, average molecular weight and Conradson Carbon. It is independent of catalyst activity.
• Contaminant coke is derived from feed contaminants that are dehydrogenation catalysts, e.g. Nickel (Ni), Vanadium (V), Iron (Fe) etc. that remove H2 from hydrocarbon molecules, thereby increasing the tendency to form coke.
• Cat-to-oil coke results from un-stripped hydrocarbons remaining on the catalyst when it leaves the spent catalyst stripper.
Excessive production of H2 and coke will constrain the amount of residue feedstock that can be processed due to wet gas compressor (WGC) and regenerator operating limits, respectively.
H2 has a low molecular weight and density and is difficult to compress in the WGC.
Increasing content of H2 in the WGC inlet reduces the hydraulic capacity of the WGC and also the downstream equipment. Thus, it is desirable to minimise the amount of H2 produced in the FCC.
The formation of coke on the catalyst results in FCC catalyst deactivation due to the blocking of active acid sites. Thus, the coke must be burned off the catalyst in the regenerator to restore catalyst activity. The coke burning requires O2 from air supplied from the main air blower (MAB) and the reaction generates a significant heat release.
The MAB capacity will limit the coke burning capability. The heat release is useful and necessary to close the FCC unit heat balance as feed needs to be heated to the reactor riser temperature, and cracking reactions in the riser are endothermic (heat absorbing).
However, any heat generated in excess of unit heat balance requirements can lead to higher regenerator temperatures, which can contribute to higher catalyst deactivation and consumption, and reduced equipment operating life. Thus, it is very important to control the production of coke, and this is particularly challenging when processing residue feedstock.
The additional H2 and coke associated with residue feedstock can be accommodated and controlled through equipment improvements, and/or by the use of advanced FCC catalysts supported by value-added technical service, or a combination of these.
Part 1 of this paper will explain how residue feed quality impacts the FCC operation, and what key catalyst features are required to process residue feedstock.1-4
FCC Catalyst features required to process residue feedstock
FCC catalysts with the following features are required to increase the flexibility to process residue feeds:
• Coke selective matrix for bottoms upgrading and metals tolerance;
• Optimised zeolite and matrix content, to maximise either conversion, or distillate yield;
• Moderate to high zeolite content to provide selective cracking;
• Low fresh catalyst sodium (Na) content to minimise Na-V zeolite deactivation.5
To maximise refinery profitability, FCC catalysts need to be tailored to meet the specific requirements of each refinery. The customised catalyst should have a carefully designed pore structure, matrix type and surface area, zeolite surface area, rare earth on zeolite (RE/Z), metals traps, additives etc.
These FCC catalyst fundamentals are summarised in Figure 1 and are discussed in the following sections.
Zeolite and matrix
Zeolite is an ordered tetrahedral structure of silica and alumina atoms. Several forms of zeolite exist; zeolite Y is the type used in FCC catalysts. It has a regular repeating pattern giving the characteristic measured as the unit cell size. The ratio of aluminato-silica in a zeolite gives it its cracking characteristics. Zeolites are deactivated by the loss of alumina from the structure in a hot atmosphere in the presence of steam (i.e. the conditions found in the FCC Regenerator). A controlled dealumination by the manufacturer can be used to give the catalyst good performance characteristics. These are known as ultra-stable Y (USY) or reduced unit cell size catalysts. Zeolites are responsible for most of the FCC cracking activity. Diffusion of oil through a zeolite crystal has a marked effect on product selectivities.
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