Transfer unit, height number

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... 

Driving force Number of transfer units Height of a transfer unit ... 

Height of a Transfer Unit, HTU Number of Transfer Units, NTU ... 

Gj /k aPh.3.s the dimension of length or height and is thus designated the gas-phase height of one transfer unit, The integral is dimensionless and indicates how many of these transfer units it takes to make up the whole tower. Consequently, it is called the number of gas-phase transfer units, N. Equation 40 may therefore be written as... 

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. 

In order to determine the packed height it is necessary to obtain a value of the overall number of transfer units methods for doing this are available for binary systems in any standard text covering distillation (73) and, in a more complex way, for multicomponent systems (81). However, it is simpler to calculate the number of required theoretical stages and make the conversion ... 

Computation of Tower Height The required height of a gas-absorption or stripping tower depends on (1) the phase equilibria involved, (2) the specified degree of removal of the solute from the gas, and (3) the mass-transfer efficiency of the apparatus. These same considerations apply both to plate towers and to packed towers. Items 1 and 2 dictate the required number of theoretic stages (plate tower) or transfer units (packed tower). Item 3 is derived from the tray efficiency and spacing (plate tower) or from the height of one transfer unit (packed tower). Solute-removal specifications normally are derived from economic considerations. 

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)]. 

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... 

Hg, Hi = height of gas, liquid phase transfer unit, inches Re = Reynolds number of gas phase... 

Because the packed tower is a continuous contacting device as compared to the step-wise plate tower, performance capacity is expressed as the number of transfer units, N, the height of the transfer unit, H.T.U., and mass transfer coefficients K a and Kj a. Figure 9-68 identifies the key symbols and constant flow material balance. 

Nql = number of transfer units, based on overall liquid film coefficients Z = height of packing, ft... 

Determine the number of transfer units, and the packed tower height. 

The right-hand side of equation 10.158 is the height of the column divided by the HTU and this is known as the number of transfer units N. It is obtained by evaluating the integral on the left-hand side of the equation. 

Units in SI system Si Stanton number h/Cf,pu Dimensions depend ort order of reaction. Suffixes 0 Value in bulk of phase 1 Phase 1 2 Phase 2 A Component A B Component B AB Of A in B b Bottom of column equilibrium with bulk of other phase G Gas phase / Interface value. L Liquid phase u Overall value (for height and number of transfer units) value in bulk of phase i Top of column Dimensions in in M. N, 1. T. 

The height and number of overall transfer units are defined as H0g and Nog by ... 

The concentrations v, y refer to conditions on either side of the gas film, and hence Ng is the number of gas-film transfer units, and HG the height of a gas-film transfer unit. 

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