Constant Distillate Composition

A constant-composition distillate may be produced by progressively increasing the reflux rate. When it reaches the maximum allowable from a hydraulic or economic standpoint, the distillate is redirected to a new receiver, and the reflux rate is lowered to a new starting point. The procedure is repeated for subsequent cuts, some of which may be off specification. Here again, lower distillate rates generally result in better separation at the expense of longer distillation times. 

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Operation at constant reflux ratio is better than operation with constant distillate composition for high-yield batch separations. However, operation with constant distillate composition might be necessary if high product purity is required. In fact, it is not necessary to operate in one of these two special cases of constant reflux ratio or constant distillate composition. Given the appropriate control scheme, the reflux ratio can be varied through the batch... 

Figure 14.10 Integration of the equation for constant distillate composition allows the energy requirement to be determined.

This measure was based upon the ratio of the minimum necessary number of plates, A min (averaged over the reboiler composition) in a column to the actual number of plates in the given column, Nj. Christensen and Jorgensen assumed that the mixture has a constant relative volatility a and the column operates at total reflux using constant distillate composition (x o) strategy (section 3.3.2) and evaluated Nmin using the Fenske equation ... 

Step 5 (section 3.3.1) now simply requires to run the column (i.e. simulate the column operation using a model) at total reflux until the unit reaches the steady state (Holland and Liapis, 1983 Nad and Spiegel, 1987) or until the instant distillate composition reaches the product composition so that the product can be collected at constant distillate composition (Coward, 1967 Kerkhof and Vissers, 1978 Logsdon and Biegler, 1993). A variation of this total reflux operation concept can also be found in the literature and in practice. In this variation only a part of the condensed liquid is returned to the column and the rest is taken out as product (product period starts from step 3 of section 3.3.1) (Converse and Huber, 1965 Mayur and Jackson, 1971 Mujtaba, 1989 Mujtaba and Macchietto, 1992). Mujtaba and Macchietto (1988) and Mujtaba (1989) clearly explain whether and when an initial total reflux operation is required. 

Kerkhof and Vissers showed that for difficult separations an optimal reflux control policy yields up to 5% more distillate, corresponding to 20-40% higher profit, than either constant distillate composition or constant reflux ratio policies. 

Bogart [Trans. Am. Inst. Chem. Eng., 33, 139 (1937)] developed the following equation for constant distillate composition with the column holdup assumed to be negligible ... 

As an example, consider the same ethanol-water mixture used above to illustrate constant reflux but now with a constant distillate composition of Xd = 0.90. The following table is compiled ... 

The graphical procedure is applicable to any binary batch distillation process and is not limited to operations at constant reflux ratio or constant distillate composition. 

FIGURE 17.3 Graphical method for constant distillate composition. 

The distillation time is obtained by graphical or numerical integration of this equation. If the process is at constant distillate composition, the instantaneous and cumulative distillate compositions are identical ... 

The process is approximated as a number or relatively small steps, each of which corresponds to a defined time interval. In each step the shortcut distillation model is applied based on the initial reboiler charge for that step and the desired constant distillate composition. The resulting product is used as the reboiler charge for the next step. The computations are continued until the available number of stages cannot produce the required distillate composition. 

Batch distillation columns may be operated uader either of two reflux policies, constant reflux with varying distillate composition, and variable reflux, with constant distillate composition. These policies have the usual limitations of total reflux and minimum stages,... 

FIGURE 5.5-11 Batch distillailon with constant distillate composition aud varying reflux (three theoretical stages),... 

Calculate, by modified McCabe-Thiele methods, distillation time for binary batch rectification with constant distillate composition and speci-... 

Batch Distillation with Constant Distillate Composition... 

An alternative to the constant-reflux-ratio policy described above is to maintain a constant-molar-vapor rate, but continuously vary the reflux ratio to achieve a constant distillate composition that meets the specified purity. This policy requires a more complex control system, which may be justified only for large batch distillation systems. 

Calculations for the policy of constant distillate composition can also be made with the McCabe-Thiele diagram for binary mixtures. An overall material balance for the light-key component at any time, t, is given by rearrangement of equation (6-108) at constant xD for W as a function of... 

Shortcut methods for handling multicomponent batch distillation have been developed for the two cases of constant reflux and constant distillate composition (Diwekar and Mandhaven, 1991 Sundaram and Evans, 1993). Both methods avoid tedious stage-by-stage calculations of vapor and liquid compositions by employing the Fenske-Underwood-Gilliland (FUG) shortcut procedure for continuous distillation, described in Section 6.8, at succesive time steps. In essence, they treat batch distillation as a sequence of continuous, steady-state rectifications. As in the FUG method, no estimations of compositions or temperatures are made for intermediate stages. 

If a constant distillate composition is required, this can be achieved by increasing the reflux ratio as the system is depleted in the more volatile material. Calculations are again made with the McCabe-Thiele diagram as described by Bogart and illustrated by the following example. Other methods of operating batch columns are described by Ellerbe. ... 

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