Surface tension column packing

Rizzuti et al. [Chem. Eng. Sci, 36, 973 (1981)] examined the influence of solvent viscosity upon the effective interfacial area in packed columns and concluded that for the systems studied the effective interfacial area a was proportional to the kinematic viscosity raised to the 0.7 power. Thus, the hydrodynamic behavior of a packed absorber is strongly affected by viscosity effects. Surface-tension effects also are important, as expressed in the work of Onda et al. (see Table 5-28-D). 

The above rules of thumb apply to organic and hydrocarbon systems, whose surface tensions are relatively low (a < 25 mN/m). For higher surface tensions, the liquid does not adhere well to the packing surfaces (underwetting), causing higher HETPs. In a water-rich system (a = 70 mN/m or so) HETPs obtained from Eqs. (14-156), (14-158), and (14-159) need to be doubled. For intermediate surface tension systems (some amines and glycols, whose surface tension at column conditions is 40 to 50 mN/m), HETPs obtained from Eqs. (14-156), (14-158), and (14-159) need to be multiplied by 1.5. 

Earlier studies on the measurements of the gas holdup in a packed bubble-column were performed by Weber,39 Stemberding,33 and Voyer and Miller.38 Weber found that the gas holdup was unaffected by the liquid flow rate, a result similar to the one observed in an unpacked bubble-column. A decrease in the surface tension of the liquid was also found to increase the gas holdup. The correlations presented by him are summarized in Table 7-1. 

Sharma (S35), including the use of organic solvents (such as toluene, xylene, diethylene glycol, and polyethylene glycol) for the measurement of a and by the chemical method. In each case, the reaction between CO2 and selected amines is employed to determine a. For example, values of interfacial areas obtained in a reaction of CO2 with cyclohexylamine in xylene plus 10% isopropanol, in a 10-cm-i.d. column packed with 0.5 in. ceramic Raschig rings, are reported in Fig. 15. A comparison with the values for aqueous systems shows a 50% improvement attributable to the lower surface tension of xylene ([Pg.74]

Transport and interfacial properties are often neglected in favor of research and development efforts directed to phase equilibrium properties. Even less attention has been devoted to such properties for electrolytes and polymers. In industrial practice, the needs for transport and interfacial properties are numerous, i.e., detailed design of heat exchangers, and distillation column tray and packing sizing calculations. Both predictive and correlative models are needed for liquid viscosity, thermal conductivity, surface tension, diffusion coefficients, etc. 

The influence of column diameter and surface tension on the HTU in packed columns in the countercurrent distillation of binary mixtures was studied by Gomez and StrumUlo [8a]. They found the relation... 

Consider a fixed-bed column with downward cocurrent flow of liquid and gas phases. The column is packed with 0.3 cm diameter catalyst particles, with bed void fraction of 0.48, bed diameter of 5 cm, and bed length of 150 cm. Gas and liquid fluxes (both superficial) are 10 and 10 kg/m -h, respectively. Under reaction conditions the relevant gas properties are average molecular weight = 10, density 0.06g/cm, viscosity = 0.6cP, surface tension = lOdynes-cm, specific gravity = 0.9, and the molecular diffusivity of reactant = 10 ft /h. From these data estimate... 

The wetting of the packing depends on the nature of the packing surface and on the surface tension of the liquid. For the wetting to be complete, an efficient liquid distribution at the top is required, while the column to particle diameter should exceed 20 to 25 to avoid liquid bypassing along the wall. Henry and Gilbert [24] recommend that > 10. 

---