Cyclone dipleg

In comparison, units that are designed with turbulent beds have a lower superficial velocity limit because of soflds entrainment and are unable to independently control the entrained soflds recycle. The soflds loading in the turbulent-bed regenerator configuration are equal to the reactor—regenerator circulation plus the entrained soflds via the cyclone diplegs. 

Selective rotation of the regenerator primary cyclone dipleg termination... 

ABB Lummus s RTD consists of a two-stage reactor cyclone system (see Figure 9-2). The riser cyclones (the first stage) are hard-piped to the riser. Attached to the end of each riser cyclone dipleg is a conventional trickle valve as shown in Figure 9-3. Each trickle valve has a small opening to prevent catalyst defluidization, which can be a problem, especially during start-ups. 

No trickle or flapper valves are used on the first stage. The riser cyclone diplegs terminate with a splash plate (Figure 9-4A). The upper reactor cyclone diplegs use conventional trickle valves. Sealing the upper reactor cyclone diplegs with about two feet of catalyst provides... 

Zenz, F. A., Size Cyclone Diplegs Better, Hydrocarbon Processing, pp. 125-128 (1975)... 

The pressure at the solids outlet of a gas cyclone is usually about 0.7 kPa (0.1 lbf/in2) lower than the pressure at the discharge of the leg. Total pressure to be balanced by the fluid leg in the cyclone dipleg is... 

Oversized cyclone diplegs (this is typically more common in the second stage cyclone)... 

In order to minimize vapor entrainment down the cyclone dipleg, ensuring the primary cyclone diplegs are sufficiently submerged in the dense bed and maintaining the dipleg valves on the secondary cyclone diplegs are essential. 

Knowlton, T., and Brevoord, E. PRSI Offers Valuable Insight into Cyclone Dipleg Efficiency. A/fcemar/e Catalysts Courier, Issue 65, 2006. 

Bed components in a dense-phase fluidized bed include the gas distributor, cyclone, dipleg, heat exchanger, expanded section, and baffles, as shown in Fig. 9.4. The basic function of each component in a fluidized bed is described in the following. 

Among the multitude of possible configurations based on the preceding concept, Fig. 35 shows two designs (a) a fast bed in the annular space, and the cyclone dipleg located in the center (b) a fast bed in the center with the annular space serving as the dipleg. 

There are two basic types of standpipe configurations, the overflow standpipe (Fig. 6A) and the underflow standpipe (Fig. 6B). The overflow standpipe is so named because the solids overflow from the top of the fluidized bed into the standpipe, and there is no bed of solids above the standpipe. In the underflow standpipe, the solids are introduced into the standpipe from the underside, or bottom, of the bed or hopper, and a bed of solids is present above the standpipe. With this definition, a cyclone dipleg is classified as an overflow standpipe because there is no bed of solids above the entrance to the dipleg. 

When a cyclone dipleg is operating with a dense phase at the bottom and a dilute phase at the top, the pressure drop through the dilute-phase section at the top is generally assumed to be zero when calculating the required dipleg length. This is not the case. 

Figure 8 Cyclone dipleg pressure drop profiles.

WHY DO CYCLONE DIPLEGS OFTEN OPERATE IN STREAMING FLOW ... 

Many chemical processes recirculate solids. Catalytic systems recirculate catalyst in a reaction/regeneration cycle. First the catalyst is used to supply heat or a reactant to the process it is then transferred to a separate vessel to regenerate the catalyst, and then it is returned to the reactor. Circulating fluidized bed combustors recirculate fuel and ash around a loop to burn the fuel completely. A system with a cyclone collecting entrained solids above a fluidized bed and returning the solids to the bed via the cyclone dipleg is also a recirculating solid system. All of these recirculation systems employ standpipes. 

One of the simplest types of automatic solids recirculation systems is the cyclone/dipleg system utilized to collect entrained solids and return them to the bed. This type of system is shown in Fig. 13 for a fluidized bed containing both primary and secondary internal cyclones. External cyclone return systems are also automatic recycle systems. All solids entering the primary and secondary cyclones are returned to the bed automatically via the primary and secondary diplegs. 

The pressure balance around the system for returning solids to the bed via the secondary cyclone dipleg is... 

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