Efficiency, tray AIChE method

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]. 

In spite of all the effort that has been expended on this topic, the prediction of mass transfer efficiency still is not on a satisfactory basis. The relatively elaborate method of the AIChE Bubble-Tray Manual (AIChE, New York, 1958) is based on the two-film theory but has not had a distinguished career. A number of simpler correlations have been proposed and have some value as general guidance. That literature has been surveyed recently by Vital, Grossel, and Olsen [Hyd. Proc., 55-56 (Oct. 1984) 147-153 (Nov. 1984) 75-78 (Dec. 1984)]. 

Some of the available methods for estimating tray efficiencies will be described. A useful summary of the AIChE bubble-cap tray method is in the book of King (1980, pp. 621-626). Some of the literature that has found fault with this method is cited by Vital et al. (1984). 

Theoretical Efficiency Prediction Theoretical tray efficiency prediction is based on the two-film theory and the sequence of steps in Fig. 14-41. Almost all methods evolved from the AIChE model (AIChE Research Committee, Bubble Tray Design Manual, New York, 1958). This model was developed over 5 years in the late 1950s in three universities. Since then, several aspects of the AIChE model have been criticized, corrected, and modified. Reviews are given by Lockett (Distillation Tray Fundamentals, Cambridge University Press, Cambridge, England, 1986) and Chan and Fair [Ind. Eng. Chem. Proc. 

Individual component efficiency method. Another simple procedure, it was recommended lor the AIChE tray efficiency correlation (125). It yields individual component efficiency but takes no account of diffu-sional interaction. It proceeds as follows (12) ... 

The best-established theoretical method for predicting is that of the AIChE (Bubble-Tray Design Manual, American Institute of Chemical Engineers, New York, 1958). It is based on the sequential prediction of point efficiency, Murphree efficiency, and overall column efficiency ... 

Considerable work on methods for pre-predicting fractionator tray efficiency continues to the present. Shortcut methods from the past differed rather widely. The AIChE 1958 calculation method is 90% accurate,but definitely cannot be termed shortcut. 

Lee and Dudukovic [18] described an NEQ model for homogeneous RD in tray columns. The Maxwell-Stefan equations are used to describe interphase transport, with the AIChE correlations used for the binary (Maxwell-Stefan) mass-transfer coefficients. Newton s method and homotopy continuation are used to solve the model equations. Close agreement between the predictions of EQ and NEQ models were found only when the tray efficiency could correctly be predicted for the EQ model. In a subsequent paper Lee and Dudukovic [19] presented a dynamic NEQ model of RD in tray columns. The DAE equations were solved by use of an implicit Euler method combined with homotopy continuation. Murphree efficiencies calculated from the results of an NEQ simulation of the production of ethyl acetate were not constant with time. 

Numerous attempts have been made to develop useful correlations for predicting fire efficiency of tray towers. The most comprehensive study was carried out under sponsorship of the American Insthute of Chemical Engineers (AIChE) and resulted in the publication of the AlChE bubble tray design manual. The AIChE prediction method, which is aimed primarily at bubble-cap trays, involves first estimating the number of gas-phase and liquid-phase transfer units, Nc and N, for the specific conditions on the tray, combining these to obtain the number of overall gas transfer units Nqc, and predicting the point efficiency by the relationship... 

This expression corresponds essentially to the limit of Pet —> 00 shown in Figure 8.3.6. A comprehensive background on stage efficiency for a plate in a distillation column, its relation to point efficiency, as well as a brief description of the AIChE tray-efHciency prediction method" has been provided by King (1980, pp. 621-622). 

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