AIChE method

The AIChE method is the most detailed method for predicting plate efficiencies that is available in the open literature. It takes into account all the major factors that are known to affect plate efficiency this includes  

The mass transfer characteristics of the liquid and vapour phases. 

The method is well established, and in the absence of experimental values, or proprietary prediction methods, should be used when more than a rough estimate of efficiency is needed. 

The approach taken is semi-empirical. Point efficiencies are estimated making use of the two-film theory , and the Murphree efficiency estimated allowing for the degree of mixing likely to be obtained on real plates. 

The procedure and equations are given in this section without discussion of the theoretical basis of the method. The reader should refer to the AIChE manual, AIChE (1958) or to Smith (1963) who gives a comprehensive account of the method, and extends its use to sieve plates. A brief discussion of the method is given in Volume 2 to which reference can be made for the definition of any unfamiliar terms used in the equations. 

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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]. 

Chan and Fair (1984a) have published an alternative method for point efficiencies on sieve plates which they demonstrate gives closer predictions than the AIChE method. 

The relation between the plate efficiency and point efficiency with the Peclet number as a parameter is shown in Figure 11.16a and b. The application of the AIChE method is illustrated in Example 11.12. 

To use the AIChE method, and Van Winkle s correlation, estimates of the physical properties are required. It is unlikely that experimental values will be found in the literature for all systems that are of practical interest. The prediction methods given in Chapter 8, and in the references given in that chapter, can be used to estimate values. 

The AIChE method was developed from measurements on binary systems. The AIChE manual should be consulted for advice on its application to multicomponent systems. 

The AIChE method, and that of Van Winkle, predict the dry Murphree plate efficiency. In operation some liquid droplets will be entrained and carried up the column by the vapour flow, and this will reduce the actual, operating, efficiency. 

For the plate design in Example 11.11, estimate the plate efficiency for the plate on which the concentration of acetone is 5 mol per cent. Use the AIChE method. 

To use the AIChE method and Van Winkle s correlation, estimates of the physical properties are required. It is unlikely that experimental values will be found in the... 

AIChE Method. The AIChE correlation of the number of transfer units for the vapor phase on both bubble caps and sieve trays is... 

The original AIChE method includes a correlation for the clear liquid height it is preferable, however, to use the more recent correlation of Bennett et al. (1983)... 

AIChE method Chan and Fair method Zuiderweg method... 

The AIChE and Chan and Fair methods have given quite different estimates of the point efficiency for this system Zuiderweg s method falls in between. In fact, this example is based on experiments carried out by Plaka et al. (1989) and the column dimensions are for their column. The point efficiency determined by Plaka et al. in a number of experiments is around 75-80%. In this case we see that the method of Chan and Fair has given an acceptable estimate of the experimentally determined value, whereas the AIChE method severely underpredicts the point efficiency. The latter result is in accord with the findings of Plaka et al. ... 

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