Number of gas transfer units

To calculate the height, one needs both the height of a gas transfer imit, Hqq, and the number of gas transfer units, Nqq. Since equilibrium data are not available, assume m (slope of equilibrium curve) approaches zero. This is not an mneasonable assumption for most solvents that preferentially absorb (or react with) the solute. For this condition ... 

The next step is to estimate the number of gas transfer units from equation (5-15). The operating curve equation for this system in terms of mole fractions was developed in Example 5.2 and is... 

When a computer simulation is used for a distillation column, the number of gas transfer units can be calculated for each theoretical stage in the rectihcation section with Equation... 

Mathematically expressed, NT = Pf, where NT is the number of particulate transfer units achieved and PT is the total energy expended within the collection device, including gas and liquid pressure drop and thermal and mechanical energy added in atomizers. NT is further defined as NT = In [1/(1 -1 )], where Tj is the overall fractional collection efficiency. This was intended as a universal principle, but the constants and y have been found to be functions of the chemical nature of the system and the design of the control device. Others have pointed out that the principle is applicable only when the primary collection mechanism is impaction and direct interception. Calvert (R-10, R-12) has found that plotting particle cut size versus pressure drop (or power expended) as in Fig. 14-129 is a more suitable way to develop a generalized energy-requirement curve for impaction... 

Using a special weighted averaging procedure, calculate the number of ternary transfer units for the gas phase. Repeat for the liquid phase. 

Combine the number of ternary transfer units in each phase to obtain the number of ternary overall gas-phase transfer units. 

Also, it follows from Eq. (13-34) that the number of overall transfer units may be expressed in terms of the number of gas-phase and liquid-phase units as follows... 

HEAT-TRANSFER UNITS. Some adiabatic dryers, especially rotary dryers, are conveniently rated in terms of the number of heat-transfer units they contain. Heat-transfer units are analogous to the mass-transfer units discussed in Chap. 22 one heat-transfer unit is the section or part of the equipment in which the temperature change in one phase equals the average driving force (temperature difference) in that section. Transfer units may be based on the temperature change in either phase but in dryers they are always based on the gas. The number of transfer units in the dryer is given by... 

Solutions for the heat-transfer problem, which arises in the use of packed beds as direct-contact recuperative heat exchangers, were presented by Furnas in 1930. The parameter Njj is the number of heat-transfer units. For heat transfer, the dimensionless time t is the heat capacity of the gas times the amount of gas that has passed through the bed divided by the total bed capacity. For Nu = co, the breakthrough curve of T /Tq vs. t would be a vertical line at t = 1.0, just as for mass transfer. The defining equations are... 

For dilute mixtures and when Henry s law applies, prove that the number of overall transfer units for cocurrent gas absorption in packed towers is given by... 

Another way of combining the terms for mass transfer and reaction in series is to use N, the number of reaction units, and N, the number of mass transfer units. The group kpi,L/ug is equivalent to kt for a homogeneous reaction and is called the number of reaction units. The group KL/ug is the number of mass transfer units and is equivalent to the NTU in mass transfer operations such as gas absorption. Using Eq. (9.19), these terms can be combined to give N, the overall number of units for mass transfer and reaction ... 

K number of heat transfer units based upon the gas e porosity... 

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