Seal leg

Circulating fluidized beds (CFBs) are high velocity fluidized beds operating well above the terminal velocity of all the particles or clusters of particles. A very large cyclone and seal leg return system are needed to recycle sohds in order to maintain a bed inventory. There is a gradual transition from turbulent fluidization to a truly circulating, or fast-fluidized bed, as the gas velocity is increased (Fig. 6), and the exact transition point is rather arbitrary. The sohds are returned to the bed through a conduit called a standpipe. The return of the sohds can be controUed by either a mechanical or a nonmechanical valve. 

When the solid is one of the reactants, such as in ore roasting, the flow must be continuous and precise in order to maintain constant conditions in the reactor. Feeding of free-flowing granular solids into a fluidized bed is not difficult. Standard commercially available sohds-weighiug and -conveying equipment can be used to control the rate and dehver the solids to the feeder. Screw conveyors, dip pipes, seal legs, and injectors are used to introduce the solids into the reactor... 

Seal legs are frequently used in conjunction with solids-flow-control valves to equ ize pressures and to strip trapped or adsorbed gases from the sohds. The operation of a seal leg is shown schemati-caUy in Fig. 17-19. The sohds settle by gravity from the fluidized bed into the seal leg or standpipe. Seal and/or stripping gas is introduced near the bottom of the leg. This gas flows both upward and downward. Pressures indicated in the ihustratiou have no absolute value but are only relative. The legs are designed for either fluidized or settled solids. 

In some cases, it is possible to combine the functions of blowdown and disengaging drums in one vessel. However, PR devices discharging liquid hydrocarbons lighter than pentane should not be connected into the drum if there is a possibility that such liquids could accumulate and be released to the sewer through the seal leg. Also, the drum vent should be sized to prevent pressure buildup due to vaporization. In these applications, the design criteria for both services must be met and special attention should be paid to potential hazards and problems which may be introduced, such as ... 

A secondary seal loop is provided for water withdrawal during major blows when turbulence at the downstream overflow connection to the primary seal loop interferes with normal drainage. Extending the base of the flare stack 3 diameters below the sloped inlet line provides vapor disengaging for the secondary seal leg. The bottom of the stack and inlet line up to 1.5 m above the seal water level are gunite lined for corrosion protection. 

When vacuum can form in the system due to condensing/ cooling hot vapor entering, the seal drum liquid volume and possibly the seal drum diameter/length must be adjusted to maintain a seal when/if the seal fluid is drawn up into the inlet piping. A vacuum seal leg should be provided on the inlet 1.2 times the expected equivalent vacuum height in order to maintain a seal. 

The hot well is the sump where the barometric leg is sealed. It must be designed to give adequate cross-section below the seal leg and for upward and horizontal flow over a seal dam or weir. At sea level the hot well must be a minimum of 34.0 ft below the base of the barometric condenser. For safety to avoid air in-leakage, a value of 35-36 ft is used. For an altitude corresponding to a 26-in. Hg. barometer, the theoretical seal height is 29.5 ft actual practice still uses about 34 ft. 

Example 1 Length of Seal Leg The length of the seal leg can be estimated as shown. 

The amount of fast coke was determined from a correlation based on observation of operating kilns. As the reactor outlet temperature changes, the amount of oil on the catalyst changes. For the usual purge conditions existing in the seal leg connecting the reactor to the kiln,... 

Catalyst-circulation rate was originally measured by use of wattmeters or torquemeters connected to the bucket-elevator system (175, 185). The energy required to drive the elevator motors was calibrated against catalyst-circulation rate, as calculated from average bucket loading and number of buckets transported per hour. Later a radioactive-tracer method was developed that involves introducing a few radioactive particles into the catalyst inventory and determining the time required for each of these particles to pass from one end of the reactor seal leg to... 

It is important to maintain correct levels in the hoppers into which the elevators discharge. Too high a level causes spillage of catalyst, and too low a level results in insufficient pressure in the reactor seal leg. Hoppers in some units are equipped with a continuously rotating vertical shaft with paddles which ride on the surface of the catalyst the position of the upper end of the shaft is communicated to the control room by means of a pneumatic transmitter (185,191). 

Reactors of air-lift units are generally similar to previous concurrent-flow TCC design. Steam is used in the seal leg above the reactor. The reactors are operated at pressures from 5 to 15 p.s.i.g. (107,333). Spent catalyst is stripped with steam, in the purge zone in the bottom of the reactor, and then flows through external pipes to the regenerator. These pipes are equipped with shutoff safety valves which are normally wide open. No adjustment can be made at this point to control catalyst flow. 

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