Gas distributor

To avoid such types of undesirable gas flows, it is necessary to adjust and control the pressure distribution within the reactor chamber or, more precisely, before the gas flows reach the substrates located in the chamber. 

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Fig. 2. Multipurpose fluidized bed where 1 represents the sheU 2, soHd particles 3, the blower 4, the gas distributor 5, the heat exchanger for fluidizing gas 6, internal heating or cooling 7, external heating or cooling 8, cyclones 9, the soHds feeder 10, soHds offtake 11, Hquid feed 12, the freeboard 13, the...

As bubbles rise through the bed, they coalesce into larger bubbles. The actual bubble size at any height above the distributor, in the bed is a function of the initial bubble size as it emerges from the gas distributor and the gas flow rate (16) ... 

Fig. 12. Cap-type gas distributors where the fluid is directed laterally.

Fig. 13. Examples of pipe gas distributors (a) simple sparger, (b) details of the pipe, (c) wagon wheel, and (d) multilevel distributor.

Quench Converter. The quench converter (Fig. 7a) was the basis for the initial ICl low pressure methanol flow sheet. A portion of the mixed synthesis and recycle gas bypasses the loop interchanger, which provides the quench fractions for the iatermediate catalyst beds. The remaining feed gas is heated to the inlet temperature of the first bed. Because the beds are adiabatic, the feed gas temperature increases as the exothermic synthesis reactions proceed. The injection of quench gas between the beds serves to cool the reacting mixture and add more reactants prior to entering the next catalyst bed. Quench converters typically contain three to six catalyst beds with a gas distributor in between each bed for injecting the quench gas. A variety of gas mixing and distribution devices are employed which characterize the proprietary converter designs. 

When a stationary vessel is employed for fluidization, all sohds being treated must be fluidized nontluidizable fractions fall to the bottom of the bed and may eventually block the gas distributor. The addition of mechanical vibration to a fluidized system offers the following advantages ... 

Gas Distrihutor The gas distributor has a considerable effect on proper operation of the flmdized bed. Basically there are two types (1) For use when the inlet gas contains solids and (2) for use when the inlet gas is clean. In the latter case, the distributor is designed to prevent Back flow of sohds during normal operation, and in many cases it is designed to prevent back flow during shutdown. In order to provide distribution, it is necessary to restrict the gas or gas and solids flow so that pressure drops across the restriction amount to from 0.5 kPa (2 in of water) to 20 kPa (3 Ibf/iu ). 

FK . 17-11 Miildple -pipe gas distributor. [Ftvm Stemerding, de Qroot, and Kuypers, Soc. Cbem. Ind. J. Symp. Fluidization Proc., 35 6, London (1963).]... 

Measurements in large fluidized beds of fine particles indicate that bubble coalescence often ceases within a short distance above the gas distributor plate. Indications from density measurements or single bubble velocities are that bubble velocity Ug and diameter often reach maximum stable values, which are invariant with height or fluidizing gas velocity. 

The pressure drop in the Y or J-bend section could be from improper fluidization or a flaw in the mechanical design. There are often fluffing gas distributors in the bottom of the Y or along the J-bend that are designed to promote uniform delivery of the cataly.st into the feed nozzles. Mechanical damage to these distributors or too little or too much fluffing gas affect the catalyst density, causing pressure head downstream of the slide valve. 

Some data on gas holdup are also reported by Stemerding (SI6). Hoogendoorn and Lips (H10) have reported gas-holdup data for counter-current bubble flow in the experimental system described in Section V,A,4. Gas holdup was not influenced by changes of liquid flow rate, but increased with nominal gas velocity in the range from 0.03 ft/sec to 0.3 ft/sec. The results are somewhat lower than those obtained by Weber, the difference being explained as due to the difference in gas distributor. Weber used a porous plate and Hoogendoorn and Lips a set of parallel nozzles. 

Verschoor (V5) studied the motion of swarms of gas bubbles formed at a porous glass gas distributor. Gas holdup was observed to increase approximately linearly with nominal gas velocity up to a critical point (corresponding to a nominal gas velocity of about 4 cm/sec), whereupon it decreased to a minimum and then increased again on further increase of the gas velocity. Higher holdup was observed for a water-glycerine mixture than for water. 

Houghton et al. (HI3) have reported data on the size, number, and size-distribution of bubbles. Distinction is made between bubble beds, in which bubble diameter and gas holdup tend to become constant as the gas velocity is increased (these observations being in agreement with those of other workers previously referred to), and foam beds, in which bubble diameter increases and bubble number per unit volume decreases for increasing gas velocity. Pore characteristics of the gas distributor affect the properties of foam beds, but not of bubble beds. Whether a bubble bed or a foam bed is formed depends on the properties of the liquid, in particular on the stability of bubbles at the liquid surface, foam beds being more likely to form in solutions than in pure liquids. 

Siemes and Weiss (SI4) investigated axial mixing of the liquid phase in a two-phase bubble-column with no net liquid flow. Column diameter was 42 mm and the height of the liquid layer 1400 mm at zero gas flow. Water and air were the fluid media. The experiments were carried out by the injection of a pulse of electrolyte solution at one position in the bed and measurement of the concentration as a function of time at another position. The mixing phenomenon was treated mathematically as a diffusion process. Diffusion coefficients increased markedly with increasing gas velocity, from about 2 cm2/sec at a superficial gas velocity of 1 cm/sec to from 30 to 70 cm2/sec at a velocity of 7 cm/sec. The diffusion coefficient also varied with bubble size, and thus, because of coalescence, with distance from the gas distributor. 

Westerterp et al. (W5) measured interfacial areas in mechanically agitated gas-liquid contactors. The existence of two regions was demonstrated At agitation rates below a certain minimum value, interfacial areas are unaffected by agitation and depend only on nominal gas velocity and the type of gas distributor, whereas at higher agitation rates, the interfacial areas are... 

The results provided in the literature for stress with biological particle systems, whereby gas distributors with small hole diameters, i.e. with smaller bubble sizes, have a more negative effect on cells (see e.g. [4, 30,31]), are frequently not comparable, as in these studies there was differing stress during bubble formation at the gas distributor due to different hole velocities. 

As a simplification, the term in Eq. (10) that accounts for the kinetic energy of the gas jets emerging from the gas distributor is based on the expression ( 9goVl/2, which is valid for incompressible flow. Experimental investigations show [27], that for relatively low gas velocities it is possible to represent the empirically determined loss coefficients q as accurately with this simplification as by the use of expressions for compressible flow. 

The effects of the different stresses mentioned in Sect. 4.1.3 cannot be determined individually by experimental studies, so that only collective conclusions are possible. Of practical interest are the effects of the gas velocity, the geometry of the gas distributor, and the filling height. 

The effects of these factors were observed by numerous investigations [27] or [44, 45], with various gas distributors uniformly arranged at the base of the bubble columns. Some results are shown as examples in Figs. 12 and 13. 

An increase in the hole diameter dL with otherwise constant velocities Vl in the gas distributor leads initially to a decrease in the stress within the dL range from 0.2 mm to 0.5 mm, and then again to an increase over the range from... 

Much higher shear forces than in stirred vessels can arise if the particles move into the gas-liquid boundary layer. For the roughly estimation of stress in bubble columns the Eq. (29) with the compression power, Eq. (10), can be used. The constant G is dependent on the particle system. The comparison of results of bubble columns with those from stirred vessel leads to G = > 1.35 for the floccular particle systems (see Sect. 6.3.6, Fig. 17) and for a water/kerosene emulsion (see Yoshida and Yamada [73]) to G =2.3. The value for the floe system was found mainly for hole gas distributors with hole diameters of dL = 0.2-2 mm, opening area AJA = dJ DY = (0.9... 80) 10 and filled heights of H = 0.4-2.1 m (see Fig. 15). 

The bubble size distribution is closely related to the hydrodynamics and mass transfer behavior. Therefore, the gas distributor should be properly designed to give a good performance of distributing gas bubbles. Lin et al. [21] studied the influence of different gas distributor, i.e., porous sinter-plate (case 1) and perforated plate (case 2) in an external-loop ALR. Figure 3 compares the bubble sizes in the two cases. The bubble sizes are much smaller in case 1 than in case 2, indicating a better distribution performance of the porous sinter-plate. Their results also show the radial profile of the gas holdup in case 1 is much flatter than that in case 2 at the superficial gas velocities in their work. 

Fig. 3. Radial profiles of the bubble size with Fig. 4. Influence of internals on the gas holdup different gas distributors (air-water system) [22]. (air-water-solid slurry system) [23].

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