Gas rates

Group D particles are large, on the order of 1 or more millimeters (1000 fim) in average particle size. In a fluidized bed, they behave similarly to Group B particles. Because of the high gas velocities required to fluidize Group D particles, it is often more economical to process these particles in spouted or in moving beds, where lower gas rates suffice. 

Fig. 21. Gassed power curves for constantand JF where mixer speeds = and gas rates = < Q2 < Qs-...

Schemes to control the outlet temperature of a process furnace by adjusting the fuel gas flow are shown in Figure 13. In the scheme without cascade control (Fig. 13a), if a disturbance has occurred in the fuel gas supply pressure, a disturbance occurs in the fuel gas flow rate, hence, in the energy transferred to the process fluid and eventually to the process fluid furnace outlet temperature. At that point, the outlet temperature controller senses the deviation from setpoint and adjusts the valve in the fuel gas line. In the meantime, other disturbances may have occurred in the fuel gas pressure, etc. In the cascade control strategy (Fig. 13b), when the fuel gas pressure is disturbed, it causes the fuel gas flow rate to be disturbed. The secondary controller, ie, the fuel gas flow controller, immediately senses the deviation and adjusts the valve in the fuel gas line to maintain the set fuel gas rate. If the fuel gas flow controller is well tuned, the furnace outlet temperature experiences only a small disturbance owing to a fuel gas supply pressure disturbance.

Fig. 23. Log—log plot of pressure drop per unit height of typical packing as a function of gas rate at two Hquid rates and for the unirrigated packing.

Gas-Solid Mixtures Carlson, Frazier, and Engdahl [Trans. Am. Soc. Mech. Eng., 70, 65-79 (1948)] describe the use of a flow nozzle and a square-edged orifice in series for the measurement of both the gas rate and the solids rate in the flow of a finely divided solid-in-gas mixture. The nozzle differential is sensitive to the flow of both phases, whereas the orifice differential is not influenced by the sohds flow. 

Farbar [Trans. Am. Soc. Mech. Eng., 75,943-951 (1953)] describes how a venturi meter can be used to measure solids flow rate in a gas-solids mixture when the gas rate is held constant. Separate calibration curves (solids flowversus differential) are required for each gas rate of interest. 

The left-hand side of Eq. (14-55) represents the efficiency of absorption of arw one component of the feed-gas mixture. If the solvent oil is denuded of solute so that Xo = 0, the left-hand side is equal to the fractional absorption of the component from the rich feed gas. When the number of theoretical plates N and the hquid and gas rates L i and G, f have been fixed, the uractional absorption of each component may be computed directly and the operating lines need not be placed by trial and error as in the graphic approach described earlier. 

Plate-Column Capacity The maximum allowable capacity of a plate for handling gas and liquid flow is of primaiy importance because it fixes the minimum possible diameter of the column. For a constant hquid rate, increasing the gas rate results eventually in excessive entrainment and flooding. At the flood point it is difficult to obtain net downward flow of hquid, and any liquid fed to the column is carried out with the overheaa gas. Furthermore, the column inven-toiy of hquid increases, pressure drop across the column becomes quite large, and control becomes difficult. Rational design caUs for operation at a safe margin below this maximum aUowable condition. 

Flooding may also be brought on by increasing the liquid rate while holding the gas rate constant. Excessive liquid flow can overtax the capacity of downcomers or other passages, with the ultimate result of increased liquid inventoiy, increased pressure drop, and the other characteristics of a flooded column. 

Ne = Bquid-pbase transfer units X = mG,n/L,n (stripping factor) m = slope of equibbrium curve G, = gas rate, moFs L,n = bquid rate, moFs... 

The effect of increasing gas rate is to increase /cg and decrease 0g, with the result that tends to be constant over a range of gas rates. 

Increased gas rate increases turbulence and the degree of back mixing of liquid. 

If the packing surface is discontinuous in nature, a phase inversion occurs, and gas oubbles through the liquid. The column is not unstable and can be brought back to gas-phase continuous operation by merely reducing the gas rate. Analogously to the flooding condition, the pressure drop rises rapidly as phase inversion occurs. 

Typical total holdup data for packings are shown in Figs. 14-58 and 14-59. It should be noted that over much of the preloading range gas rate has little effecl on holdup. 

The various models for predicting values of He and Hi are given in Sec. 5. The important parameters in the models include gas rate, liquid rate, gas and liquid properties (density, viscosity, siirrace tension, diffiisivity), packing type and size, and overall bed dimensions. 

The gas rate at which coalescence begins to reduce the effectiveness of dispersion appears to depend not only on the pore size and pore structure of tlie di.spersiiig medium but also on the li( iiid properties, li( iiid depth, agitation, and other features of the pin giiig environment coalescence is strongly dependent on the concentration of... 

Typical exponents on the effect of power and gas rate on tend to be arounaO.5 for each variable, 0.1. 

Finding F Either Eq. (22-45) or Eq. (22-46) can be used to find the surface excess indirectly from experimental measurements. To assure a close approach to operation as a single theoretical stage, coalescence in the rising foam should be minimized by maintaining a proper gas rate and a low foam height [Brunner and Lemhch, Ind. Eng. Chem. Fundam. 2, 297 (1963)]. These precautions apply particularly with Eq. (22-45). 

A very low gas rate in a column several feet tall with internal reflux can sometimes be used to effect difficult multicomponent separations in batch operation [Lemlich, Principles of Foam Fractionation, in Periy (ed.). Progress in Separation and Purification, vol. 1, Inter-science, New York, 1968, chap. 1]. 

The slowly rising foam in a tall column can Be employed as the sorbent for continuous chromatographic separations [Talman and Rubin, Sep. Sci., 11, 509 (1976)]. Low gas rates are also employed in short columns to produce the scumlike froth of batch-operated ion flotation, microflotation, and precipitate flotation. 

Downward flow of both fluids imposes no restriction on the gas rate, except that the pressure drop will be high. On the whole, the trickle bed is preferred to the flooded bed. 

Liquid Holdup The major factor influencing this property is the liquia flow rate, but the shape, size, and wetting characteristics of the particles and the gas rate and the initial distribution of liquid also enter in. One of the simpler correlations is that of Midoux et al. (J. 

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