Transfer unit experimental values

The dominant mechanism of purification for column crystallization of solid-solution systems is recrystallization. The rate of mass transfer resulting from recrystallization is related to the concentrations of the solid phase and free liquid which are in intimate contact. A model based on height-of-transfer-unit (HTU) concepts representing the composition profile in the purification section for the high-melting component of a binary solid-solution system has been reported by Powers et al. (in Zief and Wilcox, op. cit., p. 363) for total-reflux operation. Typical data for the purification of a solid-solution system, azobenzene-stilbene, are shown in Fig. 20-10. The column crystallizer was operated at total reflux. The solid line through the data was com-putecfby Powers et al. (op. cit., p. 364) by using an experimental HTU value of 3.3 cm. 

A systematic study of mass transfer in bubble columns by Mashelkar and Sharma (M8, M9, S23) is summarized in Fig. 23. Increasing the superficial gas velocity increases the gas holdup a, the volumetric mass-transfer coefficients, and the interfacial area per unit volume of dispersion, but not the true mass-transfer coefficients. Correlations proposed for ki, seem too specific to be extended to practical systems (H13, FI, A3). Sharma and Mashelkar (S21) found good agreement between their experimental values of and the values from Geddes stagnant sphere model equation (G3) ... 

Table 6 summarizes the absolute rate coefficients and rate parameters for representative types of organic structures. Rate coefficients for the reaction of the H-atom donor with its own R02 radical (k) and with f-Bu02- (k ) are reported at 30°C. Values of log A and E are reported for H-atom transfer to the f-Bu02 radical only. E was calculated from the value of k and the assigned value of log A with sufficient accuracy to recalculate k at 30°C. The probable error in E calculated in this way is about 1 kcalmole-1, or a factor of six in rate at 30°C. Where reliable experimental evidence indicates that some other value of log A is applicable, we have also listed the experimental value. However, in no case do the estimated and experimental values differ by more than one log unit and larger differences, which are often reported, should be viewed with considerable skepticism. 

Interpretation of Data. Operating data may be interpreted in terms of either mass-transfer coefficients, HTU s or H.E.T.S. s, depending upon which of the methods of Chap. 8 it is planned to use later in design. The determination of the values of H.E.T.S. from such data requires no particular explanation. In the case of the others, under ordinary circumstances the experimental data lead to over-all values of coefficients or transfer units, and these should be expressed in terms of the phase where the principal resistance to diffusion lies, as explained in Chap. 5. Over-all HTU s and Ka 8 can be converted, one into the other, through Eqs. (8.12) and (8.14). 

From the experimental data, we obtain yj, Xj, L, and G , and we need to solve the equation for each point. We are going to use gPROMS to model the column and determine the number of transfer units. We start with fitting the equilibrium line. We do in piecewise due to the complexity and the range of data. For the sake of the room, we present the parameter and variable definition as well as the values in Table 9.6. 

Farnworth also presented a theoretical model ofthe combined conductive and radiative heat flows through fibrous insulating materials and compared them with experimental values of the thermal resistances of several synthetic fibre battings and of a down and feather mixture (Table 4.9). No evidence of convective heat transfer is found, even in very low-density battings. The differences in resistance per unit thickness among the various materials may be attributed to their different absorption constants. 

According to this method, it is not necessaiy to investigate the kinetics of the chemical reactions in detail, nor is it necessary to determine the solubihties or the diffusivities of the various reactants in their unreacted forms. To use the method for scaling up, it is necessaiy independently to obtain data on the values of the interfacial area per unit volume a and the physical mass-transfer coefficient /c for the commercial packed tower. Once these data have been measured and tabulated, they can be used directly for scahng up the experimental laboratory data for any new chemic ly reac ting system. 

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