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Data Interpolation Interpolation of experimental HETP data is the most reliable means of obtaining design HETP values. This is hardly surprising in an area where our understanding of the theory is so poor that rules of thumb can do better than theoretical models. The author believes that it is best to derive HETP from experimental data, and to check it against a rule of thumb. Eckert [Chem. Eng. Progr. 59(5), 76 (1963)], Chen (Chem. Eng. p. 40, March 5, 1984), and Vital et al. [Hydroc. Proc. 63(12), 75 (1984)] tabulated experimental HETP data for various random packings. Kister (Distillation Design, McGraw-Hill, 1992) extended these tabulations and included published HETP data and a detailed procedure for interpolating such HETP data. A prerequisite to any interpolation of packing data is thorough familiarity with the factors that affect HETP. Overlooking any of the factors listed can easily lead to poor interpolation and grossly incorrect design. In particular, it is imperative to recognize that the quality of distribution in pilot towers is generally superior to the quality of distribution in commercial towers. Underwetting Laboratory- and pilot-scale distillation experiments with systems that exhibit large differences in surface tension along the column such as methanol-water showed a sharp drop in efficiency at the high-surface-tension end of the column [Ponter et al., Trans. Instn. Chem. Engineers [London], 45, T345 (1967)]. There appeared to be a critical methanol composition below which performance deteriorated rapidly. The poor performance at the lowmethanol-concentration end appeared independent of the type and size of packing. Visual observations with disk columns attributed these effects to underwetting. Underwetting is a packing surface phenomenon, which breaks up liquid film. The tendency of the liquid film to break (the degree of wetting) is expressed by the contact angle (Fig. 14-61). A contact angle of 0° indicates perfect wetting; an angle of 180° indicates no wetting. Mersmann and Deixler [Chem. Ing. Tech. 58(1), 19 (1986)] provide a preliminary chart for estimating contact angles. The contact angle depends on both the surface and the liquid and is a strong function of composition. In systems with large surface tension gradients, both contact angles and minimum wetting rates may vary rapidly with changes of composition or surface tension. Extensive studies by Ponter et al. [loc. cit.; also, Ponter and Au-Yeung, Chem. Ing. Tech., 56(9), 701 (1984)] showed that • Underwetting is most significant in aqueous-organic systems, and tends to occur at the high-surface-tension (aqueous) end of the composition range. Liquid viscosity may also have an effect. • Underwetting may be alleviated by changing the material and surface roughness of the packing. • In systems susceptible to underwetting, column efficiency can sometimes (but not always) be improved by the addition of small amounts of surfactants. Effect of Lambda Most packed-column efficiency testing has been at total reflux. Some tests for both random and structured packings [Billet, “<strong>Packed</strong> Towers Analysis and Design,” Ruhr University, Bochum, Germany, 1989; Meier, Hunkeler, and Stocker, IChemE Symp. Ser. 56, 3.3/1 (1979); Eckert and Walter, Hydroc. Proc. 43(2), 107 (1964)] suggest that efficiencies at finite reflux are similar to those at total reflux when lambda (λ=mG M/LM, which is the ratio of the slope of the equilibrium curve to the slope of the operating line) ranges between 0.5 and 2.0. This range is typical for most distillation systems. Koshy and Rukovena [Hydroc. Proc., 65(5), 64 (1986)], experimenting with methanol-water and water-DMF using #25 IMTP pack- FIG. 14-61 Contact angles. (a) Acute, good wetting. (b) Obtuse, poor wetting. EQUIPMENT FOR DISTILLATION AND GAS ABSORPTION: PACKED COLUMNS 14-67 ing in a pilot-scale column, observed a sharp efficiency drop when the group λ was greater than 2 or lower than 0.5. The efficiency loss escalated as λ deviated more from this range. Koshy and Rukovena recognized that surface tension gradients and underwetting may have influenced some of their findings, but argue that the lambda effect is the major cause for the efficiency differences observed in their tests. High-relative-volatility systems are those most likely to be affected by λ, because at low volatility, λ ranges from 0.5 to 2. Strigle (loc. cit.) quantified the lambda effect on HETP using the following equation: Actual HETP/standard HETP = 1 + 0.278[ABS(ln λ) 3 ] (14-160) For 0.5
14-68 EQUIPMENT FOR DISTILLATION, GAS ABSORPTION, PHASE DISPERSION, AND PHASE SEPARATION 11 of Kister’s Distillation Design (McGraw-Hill, New York, 1992) leads to a similar conclusion for structured packings. For water-rich systems, packing HETPs tend to be much higher than for nonaqueous systems due to their high lambda or surface underwetting, as discussed above. High hydrogen concentrations (>30 percent or so in the gas) have also led to low packing efficiencies (Kister et al., Proc. 4th Ethylene Producers Conference, AIChE, New Orleans, La., p. 283, 1992), possibly due to the fast-moving hydrogen molecule dragging heavier molecules with it as it diffuses from a liquid film into the vapor. Errors in VLE These affect packing HETP in the same way as they affect tray efficiency. The discussions and derivation earlier in this subsection apply equally to tray and packed towers. Comparison of Various Packing Efficiencies for Absorption and Stripping In past editions of this handbook, extensive data on absorption/stripping systems were given. Emphasis was given to the following systems: Ammonia-air-water Liquid and gas phases contributing; chemical reaction contributing Air-water Gas phase controlling Sulfur dioxide-air-water Liquid and gas phase controlling Carbon dioxide-air-water Liquid phase controlling The reader may refer to the data in the 5th edition. For the current work, emphasis will be given to one absorption system, carbon dioxide-air-caustic. Carbon Dioxide-Air-Caustic System The vendors of packings have adopted this system as a “standard” for comparing the performance of different packing types and sizes for absorption/stripping. For tests, air containing 1.0 mol % CO2 is passed countercurrently to a circulating stream of sodium hydroxide solution. The initial concentration of NaOH in water is 1.0 N (4.0 wt %), and as the circulating NaOH is converted to sodium carbonate it is necessary to make a mass-transfer correction because of reduced mass-transfer rate in the liquid phase. The procedure has been described by Eckert et al. [Ind. Eng. Chem., 59(2), 41 (1967); Chem. Eng. Progr., 54(1), 790 (1958)]. An overall coefficient is measured using gas-phase (CO2) concentrations: moles CO2 absorbed KOGae = �� time-bed volume-partial pressure CO2 driving force (14-161) The coefficients are usually corrected to a hydroxide conversion of 25 percent at 24°C. For other conversions, Fig. 14-14 may be used. Reported values of KOGa for representative random packings are given in Table 14-15. The effect of liquid rate on the coefficient is shown in Fig. 14-63. While the carbon dioxide/caustic test method has become accepted, one should use the results with caution. The chemical reaction masks TABLE 14-15 Overall Coefficients for Representative Packings CO2-air-caustic system Nominal size, mm Overall coefficient KOGa, kg⋅moles/(hr⋅m3⋅atm) Ceramic raschig rings 25 37.0 50 26.1 Ceramic Intalox saddles 25 45.1 50 30.1 Metal pall rings 25 49.6 50 34.9 Metal Intalox saddles (IMTP ® ) 25 54.8 50 39.1 NOTE: Basis for reported values: CO2 concentration in inlet gas, 1.0 vol %; 1N NaOH solution in water, 25 percent NaOH conversion; temperature = 24°C; atmospheric pressure: gas rate = 1.264 kg/(s⋅m2 ); liquid rate = 6.78 kg/(s⋅m2 ). SOURCE: Strigle, R. L., <strong>Packed</strong> Tower Design and Applications, 2d ed., Gulf Publ. Co., Houston, 1994. K Ga, lb-moles/hr, ft 3 , atm K Ga, lb-moles/hr, ft 3 , atm 10 5 3 2 1 .8 .6 .4 .3 .2 0.1 100 500 1000 2000 5000 2 Liquid rate, lbm/(hr•ft ) (a) 10 5 3 2 1 .8 .6 .4 .3 .2 0.1 100 1 ' ' p ' ' 1 2 a r ' ' the effect of physical absorption, and the relative values in the table may not hold for other cases, where much of the resistance to mass transfer is in the gas phase. Background on this combination of physical and chemical absorption may be found earlier in the present section, under “Absorption with Chemical Reaction.” l l n i r a a r s ' ' 2 s c c g i h p h s l a g i g l r r i r i n i n n g g g s 500 1000 2000 5000 2 Liquid rate, lbm/(hr•ft ) FIG. 14-63 Overall mass transfer coefficients for carbon dioxide absorbed from air by 1N caustic solution. (a) 1-in Pall rings and Raschig rings. (b) 2-in Pall rings and Raschig rings. Air rate = 0.61 kg/s⋅m 2 (450 lb/hr⋅ft 2 ). To convert from lb/hr⋅ft 2 to kg/s⋅m 2 , multiply by 0.00136. To convert from lb-moles/hr⋅ft 3 atm to kg-moles/s⋅m 3 atm, multiply by 0.0045. [Eckert et al., Chem. Eng. Progr., 54(1), 70 (1958).] (b) s s
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