【Weekly Topic】What are the advantages and disadvantages of coaxial catalytic cracking units?

Coaxial: It has good degassing performance, is simple to operate, is suitable for small FCC units, and has relatively low energy consumption. High and low in parallel: capable of processing low-quality raw materials, slow catalyst deactivation, high reagent-to-oil ratio; according to empirical data, the liquid yield ratio is also higher.

Parallel high and low: 1. The reactor is higher than the regenerator, and the regeneration pressure is 20-40 KPa higher than the reaction pressure； 2. Catalyst circulation between the two units is carried out using inclined tubes, and the catalyst flow rate is adjusted via a slide valve ； 3. The regenerated catalyst provides good degassing effects, resulting in a reduction in the flue gas carried into the reaction system by it ； 4. The pressure buildup in the regeneration line is high, preventing backflow of the catalyst. Coaxial: 1. The two units adopt a coaxial configuration with the settler at the top and the regenerator at the bottom; this arrangement offers advantages such as a simple structure, flexible and convenient operation and control, strong resistance to operational disturbances especially catalyst backflow, and low floor space requirements ； 2. The catalyst to be activated is transported via vertical pipes, and the level in the settler is controlled by a plug valve for the catalyst to be activated ； 3. The raw catalyst distribution technology is employed, with a specially designed raw catalyst distributor installed at the outlet of the raw catalyst sleeve, thereby ensuring a relatively uniform distribution of the raw catalyst in the upper part of the dense phase bed of the regenerator – this provides a fundamental guarantee for achieving efficient single-stage countercurrent regeneration ； 4. The startup riser is relatively long and has a high pressure reserve; its static pressure is generally greater than that of the startup inclined tube.

As a supplementary point, the disadvantages of the coaxial design are: the stripping section is prone to wear out, and coke clumps can easily get stuck in the plug valves

Additional note: A previous view was that the coaxial design saves space and reduces steel usage.

I’d like to share something: I’ve worked with both coaxial catalysis and parallel high-low catalysis in the past, and I have this experience. In coaxial catalysis, the differential pressure between the two reactors has a significant impact on its operation, as it is this differential pressure that maintains the circulation of the catalyst. Whenever issues such as moisture in the feed material, an increase in feed volume, or the shutdown of the compressor occur and cause the differential pressure to exceed acceptable levels, it becomes difficult to maintain normal operation; on several occasions this has led to incidents of yellow smoke being emitted from the chimneys. However, this problem does not exist in the case of parallel high and low catalyst configurations. Due to factors such as extremely high temperatures after cooling in the distillation tower, large pressure drops in the distillation tower, and bottlenecks in the compressors, the differential pressure remains very low; at times it even reached -10 KPa (for a short period), yet no problems occurred and operations could continue normally. Therefore, the operational flexibility and safety aspects are better than those of coaxial catalysis. Of course, the complexity of operation and the parameters that need to be adjusted are also greater than those in the coaxial type.

The subsequent maintenance of coaxial catalytic cracking units is inferior to that of parallel high and low configurations, as the key areas where the catalyst wears out – namely the stripping section and the raw catalyst riser – are located inside the regenerator, making it impossible to take measures in a timely manner. However, their flexibility and performance in the event of disturbances are better than those of parallel high and low configurations. It is recommended that attention be paid to the coaxial stripping section and the raw catalyst riser, with efforts made for renovation and innovation.

The two units are coaxially stacked, with the reaction settler on top and the regenerator below; the stripping section extends into the regenerator, which helps to reduce the overall height of the installation, save steel usage, minimize space requirements, and cut down on investment costs, as there is no reactor frame. A foldable lift pipe is used, which can enter from the side wall of the settler or from the top.

This post was last edited by jbcyf on 2012-3-28 at 15:36. Coaxial type: it has good degassing performance, is simple to operate, suitable for small FCC units, and has relatively low energy consumption. Parallel high and low: capable of processing low-quality raw materials, slow catalyst deactivation, high reagent-to-oil ratio; empirical data shows a higher liquid yield ratio

Firstly, in terms of investment, coaxial investment is small, while the parallel high and low investment approach involves larger amounts. In terms of operation, the differential pressure between the two components in a coaxial system has a significant impact on its functioning. Since coaxial catalysis relies on differential pressure to maintain catalyst circulation, any situation that causes the differential pressure to exceed acceptable levels – such as water content in the feed material, increased feed volume, or shutdown of the compressor – makes it difficult to maintain proper operation, and this can lead to incidents of yellow smoke being emitted from the chimney. This problem does not exist in the high-low parallel catalysis system, and its operational flexibility and safety are better than those of coaxial catalysis. The complexity of operation and the parameters that need to be adjusted are also greater than those in the coaxial type. Parallel types with high throughput have greater elasticity.