Transfer units, prediction height

The use of Figure 11.40 will slightly overestimate the number of stages and a more accurate estimate would be made by graphical integration of equation 11.104 but this is not justified in view of the uncertainty in the prediction of the transfer unit height. Molecular weights SO2 = 64, H20 = 18, air = 29... 

Rate of Mass Transfer in Bubble Plates. The Murphree vapor efficiency, much like the height of a transfer unit in packed absorbers, characterizes the rate of mass transfer in the equipment. The value of the efficiency depends on a large number of parameters not normally known, and its prediction is therefore difficult and involved. Correlations have led to widely used empirical relationships, which can be used for rough estimates (109,110). The most fundamental approach for tray efficiency estimation, however, summarizing intensive research on this topic, may be found in reference 111. 

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. 

Predictions of the column height required for any given separation can be obtained by using either a staged approach or a transfer unit approach. The plug flow models for determining the height of a column are of limited value due to the effect of axial dispersion, which is caused by... 

The efficiency of a plate can be predicted with more certainty than the equivalent term for packing (height of a theoretical plate [HETP] or heights of transfer units [HTU]). 

To predict the overall mass-transfer coefficient or the height of a transfer unit, separate correlations are required for the gas phase and the liquid phase. Such correlations are generally based on experimental data for systems in which one... 

Under practical conditions, the effective height of a transfer unit, including the effects of bubbles and residence time distribution in the gas phase, is on the order of 1 m, which is very much larger than one commonly finds in fixed beds (see eq. (4.39)), despite the much larger surface area of the fluidized powder An accurate prediction of the mass transfer rates in fluidized beds is quite difficult, due to the large number of variables that determine the hydrodynamic behaviour of the bed. A brief description of these is given below ... 

Note that the relations above predict that the volumetric mass transfer coefficient is proportional to the superficid gas velocity. For a bubble column the number of transfer units is defined in analogy with eq. (4.38). It is found to be approximately proportional to the column height / ... 

The empirical equations for predicting the height of the gas and liquid film transfer units [9] ... 

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