Transfer unit method

In order to determine the packed height it is necessary to obtain a value of the overall number of transfer units methods for doing this are available for binary systems in any standard text covering distillation (73) and, in a more complex way, for multicomponent systems (81). However, it is simpler to calculate the number of required theoretical stages and make the conversion ... 

Mass transfer in packed columns is a continuous, differential, process, so the transfer unit method should be used to determine the column height, as used in absorption see Section 11.14.2. However, it often convenient to treat them as staged processes and use the HETS for the packing employed. For random packings the HETS will, typically, range from 0.5 to 1.5 m, depending on the type and size of packing used. 

The design of most mass-transfer equipment requires evaluation of the number of theoretical stages or transfer units. Methods for carrying out these calculations for various types of mass-transfer operations are presented in many general chemical engineering books, such as those indicated in the Chemical Engineering Series list of books given at the front of this text. 

Use the HTU-NTU (height of transfer unit-number of transfer units) method to determine the height of packing in ... 

The logarithmic mean driving force used in Equations 3-14 and 3-17 is applicable only when both the operating line and the equilibrium curve are straight. However, the transfer unit method is not restricted to this situation. A method for determining the number of transfer units required when the equilibrium curve is not straight is illustrated in Chapter 6. 

The transfer unit method may be more convenient for use in calculating the packed depth required. The height of a transfer unit tends to vary less with flow rates than the overall liquid-phase mass transfer coefficient. Using this method, the packed depth is ... 

Example 6 Solvent Rate for Absorption Let us consider the absorption of acetone from air at atmospheric pressure into a stream of pure water fed to the top of a packed absorber at 25 C. The inlet gas at 35 C contains 2 percent by volume of acetone and is 70 percent saturated with water vapor (4 percent H2O by volume). The mole-fraction acetone in the exit gas is to be reduced to 1/400 of the inlet value, or 50 ppmv. For 100 kmol of feed-gas mixture, how many Idlomoles of fresh water should be fed to provide a positive-driving force throughout the pacldug How many transfer units will be needed according to the classical adiabatic method What is the estimated height of pacldug required if Hqq = 0.70 m ... 

Determine the height of an overall transfer unit Hog using methods given in Sec. 5, Table 5-28. For a multicomponent mixture, the key components are often used in determining Hog-... 

The transfer matrix method extends rather straightforwardly to more than one dimension, systems with multiple interactions, more than one adsorption site per unit cell, and more than one species, by enlarging the basis in which the transfer matrix is defined. 

Hughmark [181] has proposed empirical correlations for better fit of experimental data to transfer units and thus tray efficiency comparison with the AIChE method [2]. 

An alternate method to determining the number of transfer units is the graphical integration of dy/(y - y). The procedure is basically the same as for absorbers, that is ... 

Ploty, from y bottoms to y overhead versus l/(y - y). The position of y-feed can be noted on the graph, and the integration so arranged as to reveal the split betiveen rectifying and stripping transfer units. The total number by this method should check closely with the graphical step-wise method. 

The height of a transfer unit for this system is not available, therefore it may be roughly approximated by the method of additive Hq and Hl which is questionable at best, or the approximation of 2 ft for HTU may be used. The latter is just about as reliable as the former. 

Hanna, T, Graphical Method, Find Mass Transfer Units, Chem. Eng. April 6 (1959), p. 127. 

The methods for estimating the heights of transfer units, HTU, given in Section 11.14.3 can be used for distillation. The relationship between transfer units and the height of an equivalent theoretical plate, HETP is given by ... 

There is no entirely satisfactory method for predicting the height of a transfer unit. In practice the value for a particular packing will depend not only on the physical properties... 

Many correlations have been published for predicting the height of a transfer unit, and the mass-transfer coefficients several are reviewed in Volume 2, Chapter 12. The two methods given in this section have been found to be reliable for preliminary design work, and, in the absence of practical values, can be used for the final design with a suitable factor of safety. 

The effectiveness—NTU method is a procedure for evaluating the performance of heat exchangers, which has the advantage that it does not require the evaluation of the mean temperature differences. NTU stands for the Number of Transfer Units, and is analogous with the use of transfer units in mass transfer see Chapter 11. 

The probability per unit time for photodestruction of the donor ( pb,z>) is always the same, in the presence and absence of the acceptor. However, in the presence of the competing process of energy transfer the overall rate of photobleaching is less. Therefore, we can use the rate of photobleaching to measure the rate of energy transfer. This method uses measurements recorded in the second to minute range in order to measure rates in the nanosecond range. 

Gas-phase analytical methods, flash vacuum pyrolysis and, 21 139-140 Gas-phase decomposition, of Group III organometallics, 22 156 Gas-phase grown carbon fibers, 26 736-737 Gas-phase height of a transfer unit (HG), 1 51-52... 

In this analysis, it is assumed that the total number of moles of gas and liquid remain the same. This is true in absorption only when a small change in concentration takes place. With distillation, the total number of moles of gas and liquid does remain more nearly constant so that no difficulty then arises. In Chapter 11, the following relationships between individual and overall heights of transfer units are obtained and methods of obtaining the values of HG and HL are discussed ... 

As the potentialities of liquid extraction as a separation method were developed, the need for efficient, continuously operated, multistage equipment became apparent. It was natural therefore to turn to devices which had been so successful in other similar fluid-contacting operations, such as the bubble-tray tower and the packed tower of distillation. These devices have proved to be disappointing in liquid-extraction service, however for example, bubble-tray towers provide tray efficiencies in liquid-extraction operations of less than 5% (S7), and conventional packed towers show heights of transfer units of 10 to 20 ft. or more (T3). 

Example 9.11 employs this method for finding the number of transfer units as a function of liquid to gas ratio, both with finite and infinite values of km/kh. The computer programs for the solution of this example are short but highly desirable. Graphical methods have been widely used and are described for example by Foust et al. (1980). 

---