Dipleg

A generic multipurpose fluidized bed is illustrated in Figure 2 (1). The soHds are contained in a vessel and gas is introduced into the system via a distributor, which is typically a drilled plate at the bottom of the vessel. A plenum chamber is provided below the distributor plate. The height of the soHds level above the distributor is called the bed height, and the vertical space above the bed height is called the freeboard. A splash zone may exist as a transition between the bed and freeboard. Cyclones, located either in the freeboard or external to the vessel, are used to remove soHds from the gas stream. Diplegs can return entrained soHds directly to the bed. 

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. 

Most FCC unit regenerators employ 6-16 sets of primary and secondary cyclones in series, depending on unit size. These cyclones are designed to recover catalyst particles that are larger than 20 p. The recovered catalyst particles are returned to the regenerator via the diplegs. The flue gas exits the cyclones through a plenum chamber. 

The penalty of higher efficiency is usually higher pressure drop. Pressure drop through cyclones is of some concern because of gas compression costs. A more critical reason is usually the effect on dipleg operability and length mentioned previously. 

Single-stage or primary dipleg mass flux = 100-125 Ib/ft /sec (500-600 kg/m2/sec)... 

Primary diplegs internally lined with I inch (2.5 cm) thick, erosion-resistant lining. Secondary diplegs lined internally with I inch refractory on the top 5 feet (1.5 m) from the dust-bowl. Externally, diplegs should be lined where the spent catalyst returns and other known turbulent or wear areas. 

Holes have formed in the diplegs because of high cyclone velocity or external impingement. 

Spalled coke or refractory is lodged in the diplegs. This can be caused by improper curing or inadequate refractory supports. 

Low catalyst level in the regenerator could uncover the diplegs and allow backflow. 

Trickle Valves ara either stuck closed or opened Trickle Valves are either warped or eroded Trickle Valves have fallen off Holes in the Diplegs Cracks in the Plenum Diplegs diameter either "too small or too large ... 

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. 

In the KBR system, as with the ABB Lummus design, the riser cyclones are hard-piped to the riser. The diplegs of both the riser cyclone and the upper reactor cyclone are often sealed with catalyst. This minimizes the carry-under of reactor vapors into the reactor housing and maximizes the collection efficiency of the riser cyclones. 

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... 

The catalyst must be fluidized to provide an effective seal for the diplegs. Fluidization is critical without it, the diplegs cannot discharge the catalyst and will plug, with possible massive carry-over to the main fractionator. To ensure this uniform fluidization, the system uses an additional steam distributor. 

The recovered catalyst enters the reactor via an external dipleg. Aside from external rough-cut cyclones, SWEC also offers riser-cyclones, referred to as LD (Linear Disengaging Device), intended to separate catalyst from reactor vapors quicker than conventional cyclones (see Figure 9-7A). 

Dipleg is the part of a cyclone separator that provides a barometric seal between the cyclone inlet and the cyclone solid outlet. 

Dipleg Diameter. In the foregoing examples, the minimum 4" I.D. (see Fig. 19) was sufficiently large to pass the collected solids. The basic relationship for sizing diplegs takes the dimensionless form ... 

Figure 26. Cyclone pressure balance (no restrictive valve on dipleg end).

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