Height of a mass transfer unit

If the Zt- s of the previous equations are solved, they will be expressed in terms of the product of the reciprocal of the overbarred factors by the respective integrals. Thus, the tower height may be expressed as the product of two factors. Consider the first as the height of a mass transfer unit, H, and the second as the number of mass transfer units, N,. Therefore,... 

Define the concepts number of mass-transfer units (NTU), and height of a mass-transfer unit (HTU). 

C = the gas concentration in parts per million V = the air sample volume in cubic centimeters K = a constant for a given type of indicator tube and test gas H = a mass transfer proportionality factor having the dimensions of centimeters, and known as the height of a mass transfer unit. 

Other values often used instead of the mass transfer coefficient and die average driving force are the height of a mass transfer unit and the number of mass transfer units. To definite them, let us proceed fitan equations (194) and (198) and fi om the equation of the mass balance of the apparatus. From these equations it follows ... 

The number of die mass transfer units and the height of a mass transfer unit can be defined also for any eqiuUfarium equation. 

I.e., the height equivalent to one theoretical stage HETP is equal to the height of a mass transfer unit multiplied by the number of theoretical stages, or... 

According to equation (210) the height of the packing is a product of the number of mass transfer units and the height of one mass transfer unit. 

The height of a single plate in the unit is then defined by the total height of the mass transfer zone and the practical plate number. 

There are several methods that may be used to calculate the height of the overall transfer unit all are based on empirically determined packing constants [7-9]. One commonly used method involves determining the overall gas and liquid mass transfer coefficients k, k. A major difficulty in using this approach is that the values of k and /cl is frequently unavailable for specific pollutant-solvent systems of interest. For this purpose, the method used to calculate the height of the overall transfer unit is based on estimating the height of the gas and liquid film transfer units, Hl and Hq, respectively [8-13]. 

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. 

HTU (Height Equivalent to One Transfer Unit) Frequently the values of the individual coefficients of mass transfer are so strongly dependent on flow rates that the quantity obtained by dividing each coefficient by the flow rate of the phase to which it apphes is more nearly constant than the coefficient itself. The quantity obtained by this procedure is called the height equivalent to one transfer unit, since it expresses in terms of a single length dimension the height of apparatus required to accomplish a separation of standard difficulty. 

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. 

The HETP of a packed-tower section, valid for either distillation or dilute-gas absorption and stripping svstems in which constant molal overflow can be assumed and in which no chemical reactions occur, is related to the height of one overall gas-phase mass-transfer unit Hqc by the equation... 

Whenever these conditions on the ratio yjy apply, the design can be based upon the physical rate coefficient /cg or upon the height of one gas-phase mass-transfer unit He- The gas-phase mass-transtor hmited condition is approximately vahd, for instance, in the following systems absorption oi NH3 into water or acidic solutions, vaporization of water into air, absorption of H9O into concentrated sulfuric acid solutions, absorption of SO9 into alkali solutions, absorption of H9S from a dllute-... 

The contribution to the height of a transfer unit overall based on the raffinate-phase compositions is the sum of the contribution from the resistance to mass transfer in the raffinate phase plus the contribution from the resistance to mass transfer in the extract phase divided bythe extraction factor [Eq. (15-31)]. 

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... 

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