Plenum chamber

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. 

Figure 13 shows two pipe distributors, one in a branched and one in a ring configuration. These distributors minimize weeping, have good turndown, may requite the lowest pressure drop, and avoid the need for a plenum chamber. They are also well suited to multiple-level fluid injection. The disadvantages of these distributors are that there are defluidized soHds beneath the distributor and the mechanical design is more complex. 

Fan Ring and Plenum Chambers The air must be distributed from the circular fan to the rec tangular face of the tube bundle. The air velocity at the fan is between 3.8 and 10.2 m/s (750 and 2000 ft/in). The plenum-chamber depth (from fan to tube bundle) is dependent upon the fan dispersion angle (Fig. 11-46), which should have a maximum value of 45°. 

FIG. 12-48 Doiible-tnick dryer. (A) Air-inlet duct. (B) Air-exhaiist duct with damper. (C) Adjnstahle-pitch fan 1 to 15 hp. (D) Fan motor. (E) Fin heaters. (F) Plenum chamber. (G) Adjustable air-blast nozzles. (H) Trucks and trays. (J) Turning vanes. 

Some units have been built and successbilly operated with simple slot-type distributors made of heat-resistant steel. This requires a heat-resistant plenum chamber but eliminates the frequently encountered problem of corrosion caused by condensation of acids and water vapor on the cold metal of the distributor. 

In some cases, it is impractical to use a plenum chamber under the constriction plate. This condition arises when a flammable or explosive mixture of gases is being introduced to the reactor. One solution is to pipe the gases to a multitude of individual gas inlets in the floor of the reactor. In this way it may be possible to maintain the gas velocities in the pipes above the flame velocity or to reduce the volume of gas in each pipe to the point at which an explosion can be safely contained. Another solution is to provide separate inlets for the different gases and depend on mixing in the fluidized bed. The inlets should be fairly close to one another, as lateral gas mixing in fluidized beds is poor. 

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 velocity of the air, flowing through the fan, can be as much as 3 to 4 times the velocity across the face of the tube bundle. Also, the air, coming from the circular shape of the fan, must be distributed across the square or rectangular shape of the bundle. The air plenum chamber is intended to make this velocity and shape transition, such that the distribution of air is uniform across the bundle. Common practice is to install the fan in a chamber, such that the distance from the first row of the tube bundle to the fan is about one-half the fan diameter... 

The plenum chamber design may be a simple box shape, formed by flat sides and bottom, or curved transition sections may be used to obtain a tapered smooth transition from the rectangular bundle to the circular fan. Either design may be used for forced-draft or induced-draft air cooled heat exchangers. 

Full use IS made of ceiling diffusers, which ensure that the cold air spreads out over a wide area before dropping. Perforated ceilings may be used, with the ceiling void being the plenum chamber. A typical downward system is shown in Fig. 9.28. 

Plenum chamber Any air compartment connected to one or more ducts or to a slot in an air distribution hood. 

System efficiency is influenced by the air plenum chamber and fan housing. Industrial axial flow fans in proper system design will have efficiencies of approximately 75% based on total pressure. Poor designs can run 40%. Speed reducers are about 75% mechanically efficient. 

The flue gas exits the cyclones to a plenum chamber in the top of the regenerator. The hot flue gas holds an appreciable amount of energy. Various heat recovery schemes are used to recover this energy. In some units, the flue gas is sent to a CO boiler where both the sensible and combustible heat are used to generate high-pressure steam. In other units, the flue gas is exchanged with boiler feed water to produce steam via the use of a shell/tube or box heat exchanger. 

Follow proper start-up procedures. Introduce feed to the riser only when the reactor system is adequately heated up. Local cold spots cause coke to build up in the reactor cyclones, the plenum chamber, or the vapor line. 

A schematic diagram of the experimental apparatus is shown in Fig. 1. A rotating fluidized bed composes of a plenum chamber and a porous cylindrical air distributor (ID400xD100mm) made of stainless sintered mesh with 20(xm openings [2-3]. The horizontal cylinder (air distributor) rotates around its axis of symmetry inside the plenum chamber. There is a stationary cylindrical filter (ID140xD100mm, 20(o.m openings) inside the air distributor to retain elutriated fine particle. A binary spray nozzle moimted on the metal filter sprays binder mist into the particle bed. A pulse air-jet nozzle is also placed inside the filter, which cleans up the filter surface in order to prevent clogging. 

The principles of fluidisation, discussed in Chapter 6, are applied in this type of dryer, shown typically in Figure 16.25. Heated air, or hot gas from a burner, is passed by way of a plenum chamber and a diffuser plate, fitted with suitable nozzles to prevent any back-flow of solids, into the fluidised bed of material, from which it passes to a dust separator. Wet material is fed continuously into the bed through a rotary valve, and this mixes immediately with the dry charge. Dry material overflows through a downcomer to an integral after-cooler. An alternative design of this type of dryer is one in which a thin bed is used. 

As the composition of the catalyst changed, the inability to predict the CO/CO2 ratio becomes more and more limiting on the use of the model. The explicit solution was originally applied to the catalyst containing chromia (Cr203) and with relatively poorer diffusivity. With this catalyst and with the operating conditions at that time, the assumption of a constant value of a in each zone was adequate. The chromia was introduced into the catalyst early in the use of the TCC units to oxidize CO and to reduce the concentration of CO in the bottom plenum chamber and in the stack. The presence of too much CO in these parts of the unit could lead... 

Fig. 31. Effect of adding cooling air (simulated) to the bottom plenum chamber on plenum temperature.

Figure 29.3 Experimental setup and instrumentation 1 — fuel 2 — oxidizer, N2, seed-particles 3 — plenum chamber 4 — flow straightener 5 — c/d nozzle (Dthroat = 19.0 mm, Dexu = 24.7 mm) 6 — cavity 7 — laser sheet 8 — Mie-scattering collection device 9 — CCD 10 — afterburning flame and 11 — microphone...

Notes 1) The structure, consisting of the plenum chamber filter, shall be of such design that it can be supported by means of four (4) steel columns, one (1) at each corner. 2) Plenun chamber shall be of sufficient strength bracing to prevent wall, floor or ceiling from pulsating or oscillating. 3) For ammonia/urea plants or other installations with ammonia in the atmosphere, no copper, alianinum or their alloys shall be used for parts exposed to the air. 4) Seals filter material shall be resistant to vapors present in the inlet air (ammonia, hydrocarbons, etc.). 5) Unit shall be completely assembled and shipped as one unit. 6) Instruments controls to be conpletely piped and wired. 7) Outlet connection screen (0.25 in.) (6 mm) wire mesh, with back up bars to withstand seven (7) psi/0.5 kg/cm Press, across the screen. 

GHz microwave generator feeds microwave energy into a rectangular wave guide and then through a quartz window into a plenum chamber, as shown in Figure 16. 

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