Extractor-Sizing Calculations

The previous sections have pointed out that mathematical models of the processes must be proved by experiments, or adapted to experimental results with the aid of pilot extractors. For economic reasons, pilot extractors are usually much smaller than large-scale industrial apparatus. Pulsed pilot columns, for example, have a diameter between 30 and 250 mm, whereas industrial-size columns are up to 2500 mm and more in size. Thus, the question arises of whether or not the calculations or pilot experiments may be used for the dimensions of large-scale apparatus. This is a general problem for engineers. 

The most frequently used component separators are absorbers, strippers, fractona-tors, and extractors. According to Humphrey [74], fractionators are used in 90 to 95% of the separations in the US. The principles of conponent separators are covered extensively in several texts such as Treybal [29], King [30] and Henley and Seader [31, 65]. We will only consider short cut sizing methods. These methods are useful for preliminary design estimates and for first guesses for more exact calculations, requiring iterative calculation procedures. 

Table 6.33 Calculation Procedure for Sizing the Karr, Reciprocating-Plate Extractor ...

A spray-tower extractor operates at 30°C with hydrocarbon drops dispersed in water. The density of the hydrocarbon phase is 53 Ib/ft. If the average drop size is 2.0 mm, calculate the terminal velocity of the drops. If the slip velocity is assumed to be independent of the holdup and if the dispersed-phase volumetric flow rate is taken to be twice that of the continuous phase, at what flow rate of the dispersed phase... 

The different process units are sized according to general engineering rules [55] as well as by using simulations in the commercial process design package AspenPlus. It should be noted that the calculated sizes of the extractor and separator shown in Table 14.9 are relatively small as compared to the typical size of an industrial emulsion polymerization reactor (50 m ). 

In each case, the fluidized bed velocities, during extraction, pressurizing, and depressurizing, have to be determined taking into account dependence on the given particle size and on the solvent density. The maximum allowable fluid velocity, about 90% of the one from fluidization, is taken to calculate the extractor diameter. 

The mean relative velocity ur is usually referred to as slip velocity . It is dependent on the gas content (l-h°) and the falling velocity of a droplet in an infinitely extended continuous phase. The falling velocity of a droplet, acc. to Eq. (2-31), only takes into account the physical properties of the system and the droplet size, whereas the gas content (l-h°) also includes the mutual interaction of the individual droplets. There are numerous formulas for calculating the function (1—h°) in Eq. (2-34) for bubble columns, fluidised beds, liquid/liquid extractors, and for fluidisation and sedimentation [14, 38, 52]. These can be found in literature. However, there are no such methods available for systems, in which gas, as a continuous phase, flows through the packing counter-current to the downward moving liquid phase. 

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