Height of a transfer unit

Hq and aie called the overall gas-phase height of a transfer unit and the number of overall gas-phase transfer units, respectively. In the case of a straight equiUbrium line, is often neady concentration-independent as explained earher. In such cases, use of equation 47 is especially convenient... 

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. 

The heights of a transfer unit ia each phase thus contribute to the overall heights of a transfer unit. Data on values of HTU for various types of countercurrent equipment have been reviewed (1,10). In normal operating practice, the extraction factor is chosen to be not greatiy different from unity, within the range of 0.5—2. 

These models provide values of the height of a transfer unit for the Hquid-phase and the vapor-phase Hy. These values are combined to form the heieht of an overall transfer unit, H ... 

Likewise, the height of a transfer unit based on raffinate-phase compositions is the height of tower divided by the number of transfer units [Eq. (15-30)]. 

At high extraction factors the height of a transfer unit is mostly dependent on the resistance to the transfer of solute from the raffinate phase. 

FIG. 16-13 Effect of Re Sc group, distribution ratio, and diffiisivity ratio on height of a transfer unit. Dotted hnes for gas and sohd hnes for liquid-phase systems. 

HTU = Height of a transfer unit, ft Kga = Overall gas mass-transfer coefficient, lb moles/(hr) (fF) (atm)... 

HETP = height equivalent to a theoretical plate, ft HTU = height of a transfer unit, ft L = liquid mass velocity, Ib/hr-ft m = exponent a 1.0 n = exponent 0.44 Pr = Prandtl number, dimensionless Sc = Schmidt number dimensionless U, = linear velocity of gas based on total column cross-sectional area, ft/sec... 

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. 

The relationship between the overall height of a transfer unit and the individual film transfer units H, and HG, which are based on the concentration driving force across the liquid and gas films, 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. 

For packed column separations using Pall rings, the height of a transfer unit (HTU) may be estimated as follows ... 

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