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Nov-2019

Improved distribution of spent catalyst

Application of a newly designed spent catalyst distributor improves spent catalyst distribution, coke combustion and bed temperatures in the FCC.

RAJ SINGH, PAUL MARCHANT and STEVE SHIMODA
TechnipFMC Process Technology
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Article Summary
In the fluid catalytic cracking (FCC) process, distribution of spent catalyst in the regenerator is important for effective catalyst regeneration. Uniform contact between combustion air and coke laden spent catalyst results in an even temperature profile throughout the bed and minimises hot spots, resulting in better coke removal and retention of catalyst activity.

This article highlights the development of TechnipFMC’s spent catalyst distributor design from simple ‘hockey stick’ to ‘compound angle wye bathtub’ distributor. This development has focused on improving spent catalyst distribution in the regenerator bed with the process benefit of uniform coke combustion and bed temperatures. Computational fluid dynamic (CFD) modelling was used to guide the development of TechnipFMC’s latest design and confirm improved catalyst distribution. Data from commercial FCC units showing a close to even temperature profile in the regenerator with our latest spent catalyst distributor confirms our design expectation.

TechnipFMC is known in the refining and petrochemical industry for its FCC technology, in particular its two stage regeneration R2R technology. The R2R FCC technology is designed to process residue feedstocks and is often referred to as RFCC. Key features (see Figure 1) are specifically engineered to achieve desired unit performance, operational flexibility and multiple turnaround reliability. A differentiating design feature of TechnipFMC’s R2R technology is catalyst regeneration, which is achieved in two stages in series, where the first stage is operating in partial combustion mode followed by complete combustion mode in the second stage. The first regenerator typically burns 60-80% of the coke on the catalyst and any hydrocarbons entrained from the stripper. The resulting low first stage regenerator temperature minimises hydrothermal deactivation of the catalyst. Partially regenerated catalyst is then transferred to the second stage regenerator through an internal lift riser, where it is completely regenerated.

The configuration rejects a portion of the heat of combustion as carbon monoxide (CO) enriched flue gas from the first stage regenerator, resulting in low first and second stage regenerator temperatures. A lower regenerator temperature increases catalyst-to-oil ratio, maximising unit conversion. Additionally, it also reduces catalyst deactivation, resulting in low catalyst make-up rate. Although the FCC process is mature and well established, TechnipFMC is continuously innovating and improving its licensed FCC technology features to extend the operation beyond current unit limitations and into new markets.

Catalyst regeneration
The performance of the regenerator depends on effective distribution and mixing of spent catalyst and combustion air. Even distribution of spent catalyst across the catalyst bed maintains effective combustion of coked catalyst and a consistent temperature throughout the catalyst bed, resulting in better coke burn and improved catalyst activity retention. Ensuring even distribution of combustion air is relatively easy, however the uniformity of spent catalyst into the regenerator bed depends on the distributor design. The progression of TechnipFMC’s spent catalyst distributor technology from a simple ‘hockey stick’ with open slots at the bottom for catalyst outflow to a ‘compound angle wye bathtub’ distributor is shown in Figure 2. The main driving force behind this development has been to improve the spent catalyst coverage in the regenerator, especially for large size regenerators.

FCC regenerator catalyst beds are essentially ‘back mixed’ beds where the inherent mixing of gas and catalyst is reasonably good. In reality, the bed geometry and in-flow/out-flow of catalyst results in better axial (vertical) mixing compared to radial mixing. The degree of radial mixing suffers with an increase in regenerator diameter. If spent catalyst is not well distributed across the vessel, then variation in temperatures in the catalyst bed is seen. A comparison of early designed commercial units indicates an increase in dense bed temperature variation with regenerator size (see Figure 3). These regenerators include either the ‘hockey stick’ or short ‘single arm bathtub distributor’, which are generally extended from the regenerator wall towards the vessel’s centre line, providing limited coverage and distribution of spent catalyst into the catalyst bed.

The limited distribution of spent catalyst across the regenerator can result in uneven coke burn-off from the catalyst, impacting unit performance through reduced catalyst activity, leading to increased catalyst addition as well as the potential for afterburn. In R2R units, the first stage regenerator operating in partial burn mode removes the majority of the hydrogen from the coke, reducing the potential for high temperature hydrothermal deactivation in the second stage. The impact of uneven coke burn-off in the first stage does not greatly influence catalyst activity in two stage regeneration. However, it can result in localised afterburn in the first stage and can impact the performance of the second stage regenerator.

In single stage full burn regenerators, where the temperatures are higher, the impact on the catalyst is more severe. The uneven coke burn-off may result in excessively coked particles flowing to the riser and poor catalytic performance, poor yields and more dry gas formation. Bed and dilute phase temperature variation and afterburn can impact the mechanical reliability of the internals and may often require a capacity reduction to control the dilute phase temperatures. These issues have driven the development of spent catalyst distributors for use in all types of regenerator designs, especially for large regenerators.
 
Compound angle wye bathtub distributor
TechnipFMC’s compound angle wye bathtub spent catalyst distributor has been developed to address catalyst maldistribution issues, which are often observed as non-uniform bed temperatures in the regenerator. This distributor is now offered as a standard design for the first stage regenerator in R2R technology as well as for single stage regenerators. The compound angle wye bathtub distributor design, which is an improvement to an original concept of slanted wye bathtub distributor design, was optimised using extensive computational fluid dynamics (CFD) modelling and has been validated through commercial experience. CFD modelling was conducted using Barracuda CPFD software, which is well suited for simulating gas solid flow hydrodynamics in fluidised beds, such as commercial FCC regenerators.

Compared to the original slanted wye bathtub, the optimised design is initially inclined at a steep angle to aid catalyst flow into and down the arms, followed by a shallower angle to reduce catalyst velocity and prevent catalyst from overflowing the end of the bathtub arms. At the crotch where catalyst flows into the distributor arms, a weir is positioned to prevent catalyst from overflowing and to direct it to flow down the two branches. The open slots in the upper section of the arms are eliminated to prevent premature distribution.

A comparison of CFD modelling results from the original slanted wye and the optimised compound angle wye bathtub is shown in Figure 4. The catalyst flow in the bathtub is presented as density distribution (see Figure 4a) and velocity profile (see Figure 4b) along the length of the arms. The modelling results of the original design show that a large proportion of catalyst from the standpipe accumulates and overflows at the crotch section and from the initial top section of the bathtub, with a relatively small amount of catalyst flowing through the slots. The plan view of density profile indicates that catalyst coverage is limited around the crotch section only. The catalyst velocity profile indicates that the flow in the initial section of the arms is more active compared to the latter half.
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