Liquids rates

Minimum Wetting Rate The minimum liquid rate required for complete wetting of a vertical surface is about 0.03 to 0.3 kg/m s for water at room temperature. The minimum rate depends on the geom-etiy and nature of the vertical surface, liquid surface tension, and mass transfer between surrounding gas and the liquid. See Ponter, et al. Int. J. Heat Mass Tran.fer 10, 349-359 [1967] Trans. Inst. Chem. Eng. [London], 45, 345—352 [1967]), Stainthorp and Allen Trans. Inst. Chem. Eng. [London], 43, 85-91 [1967]) and Watanabe, et al. ]. Chem. Eng. [Japan], 8[1], 75 [1975]). 

Operating Lines The McCabe-Thiele method is based upon representation of the material-balance equations as operating lines on the y-x diagram. The lines are made straight (and the need for the energy balance obviated) by the assumption of constant molar overflow. The liqmd-phase flow rate is assumed to be constant from tray to tray in each sec tiou of the column between addition (feed) and withdrawal (produc t) points. If the liquid rate is constant, the vapor rate must also be constant. 

The liquid holdup on each of the Nt eqmlibrium trays is assumed to be perfectly mixed but will vary as liquid rates leaving the trays vary. Vapor holdup is assumed to be negligible everywhere. Tray molar vapor rates V vary with time but at any instant in time are eveiy-where equal. 

Liquid rates are very low and/or vapor rates are high, in which case structured packing may be particularly desirable. 

Selection of Solubility Data Solubility values determine the liquid rate necessaiy for complete or economic solute recoveiy and so are essential to design. Equihbrium data generally will be found in one of three forms (1) solubility data expressed either as solubility in weight or mole percent or as Heniy s-law coefficients, (2) pure-component vapor pressures, or (3) equilibrium distribution coefficients (iC values). Data for specific systems may be found in Sec. 2 additional references to sources of data are presented in this section. 

Calculation of Liquid-to-Gas Ratio The minimum possible liquid rate is readily calculated from the composition of the entering gas and the solubility of the solute in the exit liquor, saturation being assumed. It may be necessaiy to estimate the temperature of the exit liquid based on the heat of solution of the solute gas. Values of latent and specific heats and values of heats of solution (at infinite dilution) are given in Sec. 2. 

Table 14-2 illustrates the observed variations in values for different packing types and sizes for the COg-NaOH system at a 25 percent reactant-conversion level for two different liquid flow rates. The lower rate of 2.7 kg/(s-m ) or 2000 lb/(h-ft ) is equivalent to 4 (U.S. gal/min)/ft and is typical of the liquid rates employed in fume scrubbers. The higher rate of 13.6 kg/(s-m ) or 10,000 lb/(h-fU) is equivalent to 20 (U.S. gal/min)/ft and is more typical of absorption towers such as are used in CO9 removal systems. For example. We note also that two different gas velocities are represented in the table, corresponding to superficial velocities of 0.59 and 1.05 m/s (1.94 and 3.44 ft/s). 

TABLE 14-2 Typical Effects of Packing Type, Size, and Liquid Rate on K o in a Chemically Reacting System, KgO, kmol/(h m )... 

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. 

Downflow Flooding Columns can flood because of their inability to handle large quantities of liqmd. For crossbow plates this hmit on liquid rate Is evidenced by downcomer backup to the plate above. To avoid downflow flooding one must size the column downcomers such that excessive backup does not occur. 

Increased liquid rate favors plug flow. 

For total-reflux distillations carried out in packed columns, regions of loading and flooding are identified by their effects on mass-transfer efficiency, as shown in Fig. 14-47. Gas and liquid rate increase... 

FIG. 14-53 Pressure for metal Intalox saddles, sizes No, 25 (nominal 25 mm) and No, 50 (nominal 50 mm). Air-water system at atmospheric pressure, 760-mm (30-in) column, hed height, 3,05 m (10 ft), L = liquid rate, kg/(s-m ). To convert kilograms per second-square meter to pounds per hour-square foot, multiply hy 151,7 to convert pascals per meter to inches of water per foot, multiply hy 0,1225, (Coutiesy Notion Company, Akron, Ohio.)... 

FIG. 14-54 Pressure drop for Flexipac packing, sizes No, 1 and No.. 3, Air-water system at atmospheric pressure. Liquid rate in gallons per minute-square foot. To convert (feet per second) (younds per cubic foot) " to (meters per second) (kilograms per cubic meter) " , multiply by 1,2199 to convert gallons per minute-square foot to pounds per hour-square foot, multiply by 500 to convert inches of water per foot to millimeters of water per meter, multiply by 83,31 and to convert pounds per hour-square foot to kilograms per second-square meter, multiply by 0,001.356, Coutiesy Koch Engineering Co., Wichita, Kansas.)... 

For larger diameter columns, and for low liquid rates, the distributor must be almost exactly level (e.g., within 6 mm for a 3-m diameter) or all pour points will not function. On the other hand, the rises must be high enough to accommodate the backup caused by high liquid rates. The needed head can be estimated from the orifice equation, with a discharge coefficient of 0.5. In some cases the orinces discharge directly into tubes that extend to the packed bed (the Tubed drip-pan distributor ). 

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 coefficients are usually corrected to a hydroxide conversion of 25 percent at 24°C. For other conversions. Fig. 14-15 may be used. Reported values of Kog< for representative random packings are given in Table 14-8. The effect of liquid rate on the coefficient is shown in Figs. 14-70 and 14-71. 

The effect of chemical reaction in reducing the effect of variation of the liquid rate on the rate of absorption in the laminar-flow regime was illustrated by the evaluation of the rate of absorption of chlorine in ferrous chloride solutions in a wetted-waU column by Gilliland, Baddoiir, and White [Am. In.st. Chem. Eng. J., 4, 323 (1958)]. 

FIG. 20-88 Effect of gas velocity on maximiTm liquid rate for a sponted-bed seed coater. [Liu Litster, Powder Tech., 74, 259 (1993).] With laud permission from Elsevier Science SA, Lansanue, Switzerland. 

Liquid distribution Good only at high liquid rate Good Good Good... 

An apparent first-order specific rate increases with liquid rate as the fraction of wetted surface improves. Catalyst effectiveness of particles 3 to 5 mm (0.12 to 0.20 in) diameter has been found to be about 40 to 60 percent. 

L = Effective total molar liquid rate in top section L - Effective total molar liquid rate in bottom section M = Total equilibrium stages below tbe feed stage including reboiler... 

Table 6 show s maximum liquid loading rates per fr of column diameter. Minimum liquid rate runs 0.5 to 2gpm/ft ... 

L = Fractionator liquid rate, Ib/hr or packed column liquid rate, Ibs/fLsec... 

The larger value from Equations 1, 2, and 3 applies. Equation 2 applies only for liquid rates less than 0.50GPM/L , where L ,i is the weir length in inches. Equation 3 applies where the dowmcomers are unusually small relative to the required downcomer area. 

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