Oct-2011

FCC air/steam rings

Ring distributors are a simple yet effective means of improving air/steam distribution within the FCC unit

Clint Cooper
Shaw’s Energy & Chemicals Group

Article Summary

Two of the most critical stages of the fluid catalytic cracking (FCC) process are the combustion of coke in the regenerator and the stripping of entrained hydrocarbons from spent catalyst in the stripper. The success or failure of each of these stages — and of the FCC operation as a whole — is heavily dependent on the effectiveness of the selected air/steam distributor design. 

Purpose of FCC air/steam distributors
The primary purpose of an FCC air/steam distributor is to induce stable and uniform fluidisation across the entire cross-section of the catalyst bed. Maintaining a fluidised bed with a constant density is a central concept of the FCC unit, and it is critical to reliably maintain the unit’s pressure balance. An effective combustion air distributor encourages air to mix intimately with spent catalyst in the regenerator bed, resulting in minimal afterburn in the dilute phase and optimum regeneration with minimum damage to the catalyst. An effective stripping steam distributor maximises the recovery of valuable reactor products and reduces the thermal load and coke burning duty of the regenerator. Distributors must be reliable enough to survive harsh operating conditions and to operate for extended periods of time without plugging or failing. It is also crucial that the distributors do not cause catalyst attrition.

Comparison of FCC distributors
There are several types of FCC air/steam distributors in operation (see Figure 1). The plate grid is basically a solid plate towards the base of the vessel with holes that allow air to be distributed across the vessel. The plate grid can be designed flat, concave, convex or double-dished. While these are generally simple, cost-effective and easy to modify and clean, they do have several potential disadvantages that should be noted. With plate grids, the catalyst bed has a tendency to weep into the plenum. In order to avoid this weepage, and to provide better distribution, a relatively high pressure drop is required. Maintaining a high pressure drop can cause operational difficulty, particularly at turndown rates. This type of distributor needs to be sealed with the vessel wall and requires an expansion joint to allow for thermal expansion of the plate against the vessel wall. For this reason, plate grids typically require more maintenance than other distributors. Furthermore, the vessel volume below the plate grid is not an active bed, and a significant percentage of total vessel volume does not contribute to regeneration, especially in vessels with conical or hemispheral bottoms.

The pipe grid is based on a system of cantilevered arms, each of which contains rows of nozzles. This distributor is generally able to operate with lower pressure drops than the plate grid, resulting in improved turndown flexibility and reduced air blower discharge pressure; however, the pipe grid has a less forgiving mechanical design. Cyclic oscillation of the arms can cause fatigue-based failure, resulting in damage to the distributor. 

At its most basic level, a ring is simply a circular pipe with rows of nozzles that distribute air or steam into a vessel. 

Advantages of rings
Rings have several advantages over other types of distributors. They have robust designs, tailored specifically to the demands of the unit. Rings have been found to have longer life spans, with relatively little maintenance. Reliability is one of the most important attributes of any mechanical feature inside an FCC unit. Since they are circular, rings have the simplest geometry and most easily accommodate thermal expansion. They do not have cantilevered arms, such as the pipe grid, which can experience cyclic oscillations and fatigue. Rings can be placed concentrically around the centre of the vessel, allowing complete coverage of the catalyst bed’s cross-sectional area. The nozzles have a design that minimises both nozzle erosion and catalyst attrition. Rings are also a low pressure drop distributor, and they have excellent turn-up and turn-down capabilities. Finally, the jet penetration of ring nozzles promotes the horizontal mixing of air/steam and catalyst in addition to vertical mixing.

Figure 2 shows some of the common locations of rings within a resid fluidised catalytic cracker. Rings in the reactor/stripper section use steam as an aeration medium and are generally fabricated with carbon steel. Rings in the regenerator section use blower air as an aeration medium and are generally fabricated with stainless steel, which provides protection from higher temperatures.

Steam ring applications
The steam ring distributor has several potential applications in the reactor/stripper section of the FCC unit.
The main stripping steam ring, located in the stripper, is designed to displace entrained hydrocarbons from the spent catalyst. One or more of these rings may be necessary, based on the configuration and diameter of the stripper. Recent designs often split the ring into two half-rings. This design allows for greater flexibility in the operation of each half-ring, which is helpful in cases where catalyst flows preferentially down one side of the vessel. The normal flow rate of the main stripping steam ring is dependent on the catalyst circulation rate, quality of feed and the type of stripper internals.

The fluffing steam ring is used to maintain adequate fluidisation as catalyst leaves the stripper and enters the standpipe. This ring also helps to prevent “dead” catalyst build-up at the bottom of the stripper vessel. The normal operating flow rate of the fluffing ring is based on a desired superficial velocity in the stripper.

The dome steam ring is placed near the top of the reactor to prevent reactor vapours from condensing and causing coke to build up on the sides of the vessel. The normal operating flow rate for this ring is set by a desired “sweep” velocity down the cross-sectional area of the reactor. Drilled holes —rather than dual-diameter nozzles — are sufficient for this ring since it is not located in an active catalyst zone.

For Shaw designs, a small steam ring is placed at the bottom of the riser in the wye section. This ring provides the necessary steam to achieve the desired “reverse seal” bed density at the base of the riser.

Air ring applications
There are also several different applications for air rings within the regeneration section of the FCC unit. Combustion air rings distribute air into the regenerator, combusting the coke on the catalyst. The heat released from the combustion reactions is transferred by the catalyst into the feed riser, where it provides heat to crack the fresh feed.