Nov-2023

Enhancing FCC reliability: The impact of cyclone technology developments

How optimising cyclone design and implementing advanced internal hardware enhance operational reliability, illustrated with three real-world examples.

Mohammad Umer Ansari, Todd Foshee and Robert A Ludolph
Shell Catalysts & Technologies

Article Summary

Cyclones are indispensable components in the fluid catalytic cracking (FCC) unit, crucial for separating solid catalyst particles from gases. This enables catalyst retention within the FCC unit, facilitating cracking reactions and subsequent regeneration, while also averting the transfer of contaminants to the main fractionator and reducing particulate emissions into the flue gas system. By maintaining optimal catalyst particle size distribution and fluidisation conditions, cyclones increase catalyst circulation capacity, collectively enhancing catalytic cracking efficiency and yielding valuable gasoline and olefinic products.

However, cyclones operate within an intensely harsh environment, and their reliability has been a consistently high pain point for FCC operators, a message supported by an industrial survey conducted in 2022. This supports the findings of an earlier Grace Davison survey, which found that catalyst losses from cyclones were the number one problem in FCC operation. Furthermore, after longer durations in service, cyclones frequently suffer from erosion, leading to hole formation and dipleg-related plugging events.

The severity of these problems is different for primary and secondary cyclones, which operate within different solid loading and particle velocity environments. Primary and secondary cyclones are usually installed in series, located within both reactor and regenerator vessels above the dense fluidised beds. This arrangement is shown in Figure 1.

In particular, the issues are exacerbated in secondary cyclones, which contain a very intense vortex and where higher particle velocities are endured. The scenario of reduced particle loading subsequently causing higher erosion may appear counter-intuitive at first. In a secondary cyclone, although the particle loading is reduced, the vortex pathway is greater, more energetic, and prone to instability. These factors combine to cause faster erosion of the cone and the top of the dipleg. This effect is illustrated in Figure 2.

Common issues
The impact this can have on an FCC plant operator is not trivial. Extensive erosion can lead to holes at the inlet and the bottom of cyclones, resulting in high catalyst losses and leading to reduced throughput, which consequentially has a big impact on margins. Reduced FCC throughputs create a bottleneck for the refinery, with subsequent loss in crude processing rate. Additionally, inefficient cyclone operation will exacerbate catalyst loss and create environmental compliance issues in the regenerator as the flue gas from the regenerator becomes progressively harder to treat. Additionally, operational and product quality issues emerge from the reactor as more catalyst escapes to the main fractionator, impacting column operation and bottoms products.

Catalyst loss of between 3-5 tonnes per day (TPD) is not uncommon and is linked to a substantial increase in operational costs. An unresolved issue could result in a step change in catalyst loss, with the potential for rates to rise by two to three times. Higher catalyst loss may lead to poor fluidisation that will limit catalyst circulation rate, resulting in reduced throughput or yield degradation. Higher catalyst losses further inflate operational costs because of the need for higher fresh catalyst additions. In the worst-case scenario, severe cyclone erosion could typically lead to a three-week shutdown. The cost of such an outage could amount to $25-30 million for a medium-sized unit.

Development of the solutions
Fortunately, technology exists to mitigate these undesirable outcomes, thereby enhancing the reliability and performance of FCC cyclone operation. During the 1980s, Shell found cyclone-related issues to be a major cause of unplanned shutdowns in its FCC units. Subsequently, the Shell FCC group at Shell Technology Centre Houston, USA, embarked on a development plan to improve cyclone technology. Following the implementation of the improved cyclone designs in the early 1990s, there was a 90% decline in cyclone-related shutdowns across all FCC units in Shell’s fleet, with the decline progressing almost exponentially over an eight-year period. This level of performance has been sustained into current-day operations, with Shell units experiencing a low level of unexpected shutdowns due to cyclone malfunction.

Case studies
The next section of this article will illustrate how the economic and operational challenges due to poor cyclone performance can be resolved by installing leading-edge technology. In many cases, this can have a positive outcome for the operation of downstream plant, such as the fractionator or the flue gas scrubber. The first case study focuses on primary cyclone design within a medium-sized regenerator unit, whereas case study two highlights the successful avoidance of coke plugging in a secondary cyclone within a small-scale FCC reactor. In the third case study, the replacement of regenerator cyclones in a medium-sized unit is detailed as part of a revamp aimed at increasing the unit’s capacity.

Case study 1: Regenerator cyclone replacement to improve operational reliability and reduce catalyst loss
The regenerator cyclones in a 60,000 barrels per day (BPD) unit located in Japan were reaching the end of their life, having been in operation for 20 years. The operational efficiency of the unit had declined because of elevated catalyst losses, leading to economic challenges. Additionally, there was deformation of the secondary cyclone to central plenum cross-over ducting, and stress cracks were discovered within the support lug to cyclone body joints, among other structural issues with the support system.

The cyclones operated as 12 pairs, and the old configuration within the vessel was changed from a mirror-image layout to a more radially symmetric design, as shown in Figure 3. A corrugated cross-over ducting was introduced to eliminate the observed structural cracking and distortion.

The Shell design team was able to take a holistic approach to the evaluation of the regenerator. By leveraging modelling tools, it was able to re-assess the design and loading of the whole system, such that cyclone operation was not considered in isolation. By considering the loading to the cyclones, the design team was able to calculate the flux required in the diplegs. In this way, and further to addressing the structural issues, the team also enhanced the performance of the diplegs.

Consequently, the primary cyclone dipleg diameter was reduced to improve the flux of catalyst and reduce dipleg stalling. When spent catalyst resides within a dipleg for too long, it will de-gas, which is the process by which gases or vapours trapped or adsorbed within the catalyst particles are removed. Excessive de-gassing leads to a loss of fluidity and subsequent compaction, exacerbating the likelihood of stalling events. The modified design also reduced dipleg submergence into the fluidised bed for more reliable operation.