Spray-columns column height

Mass Transfer As mentioned earlier, spray columns rarely develop more than 1 theoretical stage due to the axial mixing in the column. Nevertheless, it is necesary to determine what column height will give this theoretical stage. It is recommended by Cavers in Lo et al. Handbook of Solvent Extraction p. 323 and p. 327, John Wiley Sons, New York, 1983 that the following equation be used to estimate the overall efficiency coefficient ... 

Fig. 9.1 Spray column for Hquid-liquid extraction. The water phase flows from top to bottom, the toluene as the lighter phase from bottom to top. If the column is always filled with water at the marked height, the toluene breaks into drops at the feeding point. When the drops have reached the water interface, they coalesce and form a continuous toluene phase. The phenol transfers from the water phase to the toluene phase.

Spray columns. These are columns fitted with rows of sprays located at different heights. Gas rises vertically, and liquid is sprayed downward at each of these rows. Mass transfer is usually poor because of low gas and liquid... 

The height of a spray diyer column, z, necessary to take a liquid to dryness is given by [7] (assmning that boundary layer mass transfer is the rate determining step)... 

Efficiencies of several kinds of small-scale extractors are shown in Fig. 19-28. Larger-diameter equipment may have less than one-half these efficiencies. Spray columns are inefficient and are used only when other kinds of equipment may become clogged. Packed columns as liquid-liquid reactors are operated at 20 percent of flooding. Their height equivalent to theoretical stage (HETS) range is from... 

Heat transfer, dehunrtidification, absorption, distillation, and extraction operations in general involve two-film resistances. Where there is direct contact between the two fluid streams as in packed or spray columns, the use of an overall value of H.T.U. greatly simplifies calculations... the overall H.T.U. and resulting column height do not vary from one of these systems to another as much as might be supposed. 

No appreciable effect of column height (5 ft 4 in. to 8 ft 8 in.) on the transfer coefficient was noticed. Sea water and fresh water gave similar results. Contrary to Garwin s experience, the heat-transfer coefficients were found to be larger when the top of the column was hot than when the top was cold. With GJGc = 2.58 and F=0.6, values of 8180 and 7030 Btu/ft /hr/°F for hot and cold top, respectively, were obtained with spray base as the dispersed phase. The maximum values of the volumetric heat-transfer coefficient were 11,500 and 8500 Btu/ft /hr/°F for the hot and cold top, respectively. These results probably reflect the natural convection currents in the column as well as with the very pronounced effect of fluidity of the oil on the volumetric heat-transfer coefficient (W12, Fig. 12). Since Garwin worked with benzene, it is likely that this fluidity effect would have been relatively less pronounced in his experiments. 

For condensation of methylene chloride in water, in cocurrent downflow 4-in. and 6-in. diam columns packed with i-in. Intalox saddles, the volumetric transfer coefficients reported (HlOa) were less than half those obtained with the sieve-plate column. The difference may be due partially to the different definition of the temperature driving-force applied for these two columns. (The log-mean AT was used for the packed bed, and a 2-in. transfer height was assumed.) The volumetric heat transfer coefficients increased with the 0.4-0.6 power of the liquid rate from 65,000 to 150,000 Btu/hr/ft /°F with the liquid rate increasing from 1 to 4 x 10" Ib/hr/ft. Contrary to the sieve-plate and spray-column studies, no effects of the vapor flow rate (from 1100 to 2500 Ib/hr/ft ) on the heat-transfer coefficient were noted in the packed bed study. 

The values of interfacial area and of overall mass transfer coefficient increase with decreasing distance S between the spray nozzle and gas inlet, whatever the nozzle type, column dimensions and flowrates. Indeed the spray provides a large interfacial area in the vicinity of the nozzle, where there is intensive circulation. Then a decreases quickly away from the nozzle, as a result of both coalescence of droplets and collection of liquid on the column walls. kQa and a are approximately proportional to for absorption and desorption processes, which shows that kQ is practically independent of the column height. Moreover Mehta and Sharma indicate that a is unaffected by ionic strength and viscosity but may decrease about 20 % when solids are generated by the reaction of gas with the liquid. Thus the following correlations pay be used for design (112)... 

In a spray column with internals (Fig. 6-29a) [6.34, 6.35] droplets are formed mainly at the feed location by means of annular sprinklers. Droplets now move under the influence of gravity and buoyancy forces through the continuous phase. The column throughput depends considerably on the phase density difference and the viscosity of the phases. Considerable axial mixing is disadvantageous. This increases with increasing column diameter/column height ratio. 

Figure 8.16 shows the relationship between the mass transfer of product (K a) and applied voltage for penicillin extraction in a 25 mm diameter X 0.65 m height spray column. The importance of electrostatic spraying in the improvement of overall mass transfer rate is clearly demonstrated (Weatherley, 1993). 

Rates of absorption of ammonia in water in countercunent spray towers liave been determined by Kowalke et al. (1925) and Pigford and Pyle (1951). Data obtained by Pigford and Pyle are shown in Figure 4-14 for a S2-in. high spray tower. In this case the data are given in terms of the number of transfer units because spray column performance is not proportional to height. The indicated number of transfer units for the system is almost directly proportional to the liquid rate because the area for mass transfer increases almost linearly with flow rate in spray units. 

Water resistance test methods include AATCC 127 (hydrostatic pressure test), AATCC 42 (impact penetration test), and AATCC 35 (rain test). In the hydrostatic pressure test, a sample is subjected to a column of increasing water pressure until leakage occurs. The impact penetration test requires water to be sprayed on the taut surface of a fabric sample from a height of two feet. The fabric is backed by a blotter of predeterrnined weight, which is reweighed after water penetration. The rain test is similar in principle to the impact penetration test. 

If these conditions of drop formation and rise are reproduced in a spray tower of 1.8 m in height and 0.04 m2 cross-sectional area, what is the transfer coefficient, Kwa, kmol/sm3 (kmol/m3), where a represents the interfacial area in m2/unit volume of the column The benzene phase enters at the flowrate of 38 cm3/s. 

---