Transfer unit approach

RATE MEASURES, TRANSFER UNITS, APPROACH TO EQUILIBRIUM, AND BYPASSING... 

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... 

If a stream of air is intimately mixed with a quantity of water in an adiabatic system, the temperature of the air will drop and its humidity will increase. If the equilibration time or the number of transfer units approaches infinity, the air-water mixture will reach saturation. The acUabatic saturation temperature T is given by a heat balance between the initial unsaturated vapor-gas mixture and the final saturated mixture at thermal equilibrium ... 

Rate Measures, Transfer Units, Approach to Equilibrium,... 

Packing height can be evaluated with either the transfer-unit or the height equivalent theoretical plate (HETP) approach. Traditionally, the transfer-unit approach has been used with absorption and stripping systems and the HETP approach for fractionation services. The method used here is the HETP approach. The method is applicable to distillation and... 

The transfer unit approach permits consideration of varying vapor and liquid flows, varying properties, and differences in packing geometry. However, computation of transfer units, especially for multicomponent systems, is not straightforward. Instead, a hybrid approach can be used ... 

The transfer unit approach could also be used. For the same assumptions used for the Kremser equation (straight lines, no reaction, dilute concentrations) the Colburn equation, with a molar stripping factor = (mPc)IFi = S = 0.7, gives an estimate of the NTU required. For a molar stripping factor S = 0.7 the number of theoretical mass transfer units, NTU, corresponding to driving force ratio = ((yout - )/(yj - mXi )) are ... 

What is the relationship between the theoretical stage and the transfer unit approach ... 

First, we will consider the transfer unit approach because it relates directly to the mass transfer coefficient. Where the gas flow does not change significantly over a short packed depth ... 

I umber of Transfer Units. Eor relatively dilute systems the ratios involvingjgj, jy gj, and 1 — y approach unity so that the computation of from equation 50 and -Nq from equation 49 may be simplified to... 

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. 

Mass Transfer Relationships for calculating rates of mass transfer between gas and liquid in packed absorbers, strippers, and distillation columns may be found in Sec. 5 and are summarized in Table, 5-28. The two-resistance approach is used, with rates expressed as transfer units ... 

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. 

A cooling tower operates in the countercurrent mode as illustrated by Figure 5.13. Entering air has a 5% wet-bulb temperature of 65°F. Hot process water enters the tower at 118°F and cold water leaves at a 15° approach to the wet-bulb (i.e., at 80°F). The cross-sectional area of the tower is 676 ft2. Determine the number of transfer units (Ntu ) required to meet the process requirements. Air is supplied to the tower by a blower having a capacity of 250,000 cfm and the water loading is 1500 lb/(hr)(ft2). 

Approach to Equilibrium—Finite Contactor with No Bypassing When there is no bypassing, the measure that sets the approach is the ratio of change to driving force. This ratio is called the number of transfer units Nc. It is dimensionless. For heat-transfer applications, it can be envisioned as a conventional heat exchanger wliere a vaporizing liquid cools a gas ... 

For quenching operations, this means that even though residence time is cut as gas velocity goes up, the effective approach to equilibrium increases. Since the volume for a given length of pipe falls with (velocity)-1, the expected number of transfer units NG in a given length of pipe increases with (velocity)0 8... 

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). 

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