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 effi- ciency of the unit had declined because of elevated catalyst losses, leading to economic challenges. Additionally, there was deformation of the secondary cyclone to central ple- num 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 configu- ration 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 model- ling 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 address- ing the structural issues, the team also enhanced the per- formance 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 stall- ing events. The modified design also reduced dipleg sub- mergence into the fluidised bed for more reliable operation. The benefits of this overhaul include a reduction in cat- alyst losses from 5-6 TPD to typical values of less than 1 TPD, which resulted in a substantial reduction in fresh catalyst additions to the process. Additionally, there have been no issues related to stalled diplegs and no other oper- ational or reliability issues in the last six years. A small unit with a capacity of 21,000 BPD faced signifi- cant issues related to coke plugging in its secondary reactor cyclones. Although the primary cyclones were functioning well, the secondary cyclones experienced frequent coking due to their lower catalyst load, particularly in areas of low gas velocity. This coking problem led to unplanned shut- downs in the past, causing disruption to operations. The coke deposits on the leeward side of the inlet scroll and out- let tube posed a serious risk, as they could break off during thermal cycles when differential expansion and contraction between the adhered coke and the refractory wall occurs. By falling into the entrance of the associated dipleg, the large coke deposits blocked catalyst flow, compromis- ing catalyst separation and subsequently causing loss of catalyst from the secondary cyclone. Consequently, this Case study 2: Reactor secondary cyclone replacement to address coke plugging
Primary cyclone
Secondary cyclone
Vortex
Longer, energetic, asymmetrical vortex
Solid ow path
Many more turns; more potential for erosion
Cyclone loading: High
Cyclone loading: Low
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 sce- nario, 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 undesir- able outcomes, thereby enhancing the reliability and per- formance 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 eco- nomic 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 fraction- ator or the flue gas scrubber. The first case study focuses on primary cyclone design within a medium-sized regen- erator unit, whereas case study two highlights the suc- cessful 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. Figure 2 Illustration of vortex characteristics within primary and secondary cyclones
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