Constant molal overflow column distillation

Most of the properties change somewhat from one end to the other of industrial columns for effecting separations, so that the mass transfer coefficients likewise vary. Perhaps the property that has the most effect is the mass rate of flow which appears in the Reynolds number. Certainly it changes when there is a substantial transfer of material between the two phases in absorption or stripping and even under conditions of constant molal overflow in distillation processes, the mass rate of flow changes because of differences of the molecular weights of the substances being separated. As a practical expedient, however, mass transfer coefficients are evaluated at mean conditions in a column. 

Estimates must be made of V/L at the top and bottom and the feed zone. In distillation problems, assumption of constant molal overflow in each zone probably is within the accuracy of the method. In stripping or absorption columns, first iteration evaluations of the amounts of stripping or absorption will provide improved estimates of V/L at the key points in the columns. 

The initial total flow rate and temperature profiles can make the difference between success and failure of a rigorous method. Usually for distillation columns, the condenser and reboiler temperatures are estimated and a calculation that assumes constant molal overflow Sec. 2.2.2) is used to initialize tbe internal vapor and liquid flow... 

Multicomponent rectification Consider a multicomponent mixture of m-species continuously fed into a distillation column (Acrivos and Amundson, 1955 Amundson, 1966 Ramkrishna and Amundson, 1985). Let and be the compositions of the ith species on the nth plate for the liquid phase and the vapor phase, respectively. Based on constant molal overflow of liquid with a downflow rate L and a vapor upward flow rate V, the steady-state mass balance for the ith species in the rectifying section above the nth plate leads to the equation... 

The actual variations in the V and L streams in a distillation column depend on the enthalpies of the vapor and liquid mixtures. The limitations imposed by assuming constant molal overflow can be removed by enthalpy balances used in conjunction with material balances and phase equilibria. The enthalpy data may be available from an enthalpy-concentration diagram, such as the one in Fig. 18.24. Since benzene-toluene solutions are ideal, this diagram was constructed using molar average heat capacities and heats of vaporization. Some... 

Consider what is required for a heavy component to be completely absent from the distillate. If jCb is zero and constant molal overflow is assumed, the material-balance equation for the upper part of the column [Eq. (18.14)] becomes... 

A distillation column with a partial reboiler and a total condenser is being used to separate a mixture of benzene, toluene, and cumene. The feed is 40 mol% benzene, 30 mol% toluene, and 30 mol% cumene and is input as a saturated vapor. We desire 95% recovery of the toluene in the distillate and 95% recovery of the cumene in the bottoms. The reflux is returned as a saturated liquid, and constant molal overflow (CMO) can be assumed. Pressure is 1 atm... 

On the other hand, calculation of diffusion in distillation columns tends to be easier if the molar average reference velocity v gf moi is used. In distillation, constant molal overflow is often valid or close to valid fSection 4.2T The resulting equimolar counterdiffusion results in = -Ng, and there is no convection in the reference frame with v,.pf mni = 0. If we choose the reference velocity as the molar average velocity, then Eq. ri5-16ei becomes... 

The situation is analogous to that of a distillation column assuming constant molal overflow. ... 

There are a number of additional modes of operating a distillation column. These include partial condenser open steam introduced at the column bottom without a reboiler enriching distillation column stripping distillation column operation with a side stream, etc. We will briefly describe each one of these in the context of equilibrium stages/ plates and constant molal overflow. 

We split the saturated liquid feed into two fractions having flow rates of and introduce one fraction at the top of a distillation column, vaporize the second fraction completely and introduce it at the bottom the column. Plot the q-lines for the two streams entering the column if jtij= 0.5. Develop, under conditions of constant molal overflow, the operating line for this distillation column. Plot the line for the situation where Wtfi = Wtfi. Obtain graphically the compositions of the overhead vapor product and the bottoms liquid product for two values of the number of ideal equilibrium plates in column, N =1 and N—5. 

Although simple distillation marks the first analysis of batch distillation process, the graphical analysis presented by [39] provided the basis for analyzing batch distillation operating modes. The difference between simple distillation and batch distillation operations is the relation between the distillate composition xd and the bottom composition xb due to the presence of reflux and column internals as shown in Figure 4.4. If one assumes constant molal overflow Lj = Lj-i =. . = Lo = L and Vj = Vj- =. . = Vi = V), then McCabe-Theile s method can be used. 

---