Stripping Kremser Equation

On an XY diagram for case C the operating line will go through points Xr, Ys and Xf, with a slope of R /S similar to Fig. 15-13. When using the Kremser equation for case C, one uses the pseudo feed concentration X from Eq. (15-21) and the stripping factor from Eq. (15-22). One uses the raffinate concentration X and inlet solvent concentration Y, without modification. 

The effluent gas of a fermentation process contains carbon dioxide and ethanol. It is required to recover 97.5% of the ethanol by absorption with water in a countercurrent absorber. Using the Kremser equation, determine the smallest number of stages required to meet the specification. Use trial calculations with number of stages ranging from 6 to 9. The A -valuc of ethanol at average column temperature and pressure may be assumed constant at 0.57. The stripping factor can be estimated based on the inlet vapor and liquid flows. 

An alternative method for the design of an absorption or stripping column is based on the modular group method (Kremser equation, 12.33), primarily for dilute mixtures. The required number of stages for a specified separation is first determined. Once this is known, the packing height can be estimated from the relation... 

Use the Kremser equation with a molar stripping factor or molar absorption factor of F/Ffy m = 1.4. A molar stripping or adsorption factor = 5 is approximately the same... 

Use Kremser equation for the design of dilute units with molar L/absorption factor for economic stripping of F m/F = 1.2 to 2 with starting value of 1.4 m = Henry s constant/total pressure. For packing, HETS = 1.83 m. 

The stages are determined by combining the McCabe-Thiele graphical construction, for a suitable region in the middle, with the Kremser equations for the low and/or high ends, where absorption and stripping factors are almost constant. 

To estimate the number of theoretical stages required, NTS, the Kremser-Sou-ders-Brown equations may be used. (The Kremser equation assumes that the operating and equilibrium lines are straight, the systems are nonreacting and that the concentrations are dilute (that is < 10%). These are reasonable assumptions for many absorptions.) The key parameter is the ratio of the product of the molar equilibrium constant m (as in y = mx ) with the molar gas flowrate divided by the molar liquid flowrate, = molar stripping factor = fnpG)/Pi = S. 

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 ... 

Assume 99% absorption Use the Kremser equation for the design of dilute units with molar stripping factor, S, = 0.7. Tray spacing = 0.6 m with valve or sieve trays. See also Section 4.2 for sucii details as downcomer sizing. 

Equation (12-12) is a special case of the Kremser equation. When this equation is applicable, absorption and stripping problems can be solved quite sinple and accurately without the need for a stage-by-stage calculation. 

Exanple 12-2. Stripping analysis with Kremser equation... 

Use the Kremser equation for dilute absorption and stripping problems... 

The Kremser equation was used for absorption, stripping, and extraction. When total flows, pressure, and tenperature are constant and the heat of contacting the phases is negligible, we can use the Kremser... 

Often the term A is called the absorption factor and S the stripping factor, where S = /A. These equations can be used with any consistent set of units such as mass flow and mass fraction or molar flow and mole fraction. Such series of equations are often called Kremser equations and are convenient to use. If A varies slightly from the inlet to the outlet, the geometric average of the two values can be used, with the value of m at the dilute end being used for both values of A. 

The Kremser equation has a different form when applied to a stripper. The form of the Kremser equation (Kremser, 1930) for a stripper in which the stripping gas or vapor having a mole fraction enters at the column... 

Steam introduced at the column bottom is used to strip a volatile impurity i from an aqueous solution introduced at the column top. Develop the following simplified form of the Kremser equation for the steam stripper ... 

This development has been phased completely in terms of concentrations in the vapor. Obviously, we could have phased the development in terms of concentrations in the liquid. Had we done so, we would derive a completely parallel set of equations containing a stripping factor, which is no more than the reciprocal of the absorption factor. When I try to use this second set of equations, I often confuse myself. As a result, I find it easier simply to recognize that labeling the phases is arbitrary, and just plug into the Kremser equation in the form given. 

When A is the only unknown, it may be found by trial solution of these equations, or the Kremser-Brown stripping chart may be used. Example 14.4 applies these results. 

This is of the same form as the Kremser-Brown equation for gas absorption and stripping and the Turner equation for leaching. The... 

The Kremser-Brown relationship [Equation (13.8)] can be modified for stripping ... 

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