Transfer units, prediction

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. 

Since the term (1 -i- k i)/k approaches unity for large /c -value, the number of plates is equal to one half the number of transfer units for a strongly retained component. For these conditions, when Np = N/2, Eq. (16-174) and Eq. (16-161) produce the same peak retention time, peak spreading, ana predict essentially the same peak profile. 

Calculate N,og and Table 1, from Predict the number of Transfer Units for Cooling Towers, by Kamal Adham Khodaparast, Chemical Engineering Progress, Vol. 88, No. 4, pp. 67-68 (1992). Reproduced by permission of the American Institute of Chemical Engineers. 1992 AlChE. ... 

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 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... 

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... 

Nonequilibrium methods attempt to get around the difficulty of predicting efficiencies by doing away with the equilibrium-stage concept. Instead, they apply a transport phenomena approach for predicting mass transfer rates. The mass transfer rates are calculated continuously along the column length and not in discrete equilibrium stages. This process is similar to the transfer unit concept (Sec. 10.3.1). 

As predicted from Eq. (14), the reciprocal of the number of transfer units n varies linearly with the reciprocal of qx, the column capacity per unit volume of adsorbent. The value of l/nmt obtained by extrapolating the straight line to I /c/x = 0 is 0.038 and agrees with the results of Table I performed... 

Figure 6. The driving-force (in eV) dependence of non-radiative transfer rates (in arbitrary units) predicted by the Marcus-Jortner equation for total reorganization energies of 0.1, 0.3, 0.5, 1.0, and 1.5 eV. Various ratios of As and Ay were used for the individual traces.

Packings for columns are available in a variety of shapes and sizes. Modern models for predicting their efficiency utilize the transfer unit concept. For the multicomponent case, NTU values are found from theoretical stages ... 

Example 13.18 is included to show the predicted mass transfer efficiency of an acetone-water column in which structured packing is used. Of particular interest are the results from Eqs. 13.255, 13.259, 13.261 and 13.262 are used to determine transfer units. The value of HETP is calculated using Eq. 13.172. Plug flow of the phases is usually assumed, although extreme ratios of the flowing phases can cause some back mixing. 

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]). 

---