Special distillations batch distillation

When evaluating a batch distillation, either through simulation or from scale-up of a pilot plant, one thing that needs special attention is hold-up in the column. A large hold-up in the column reduces the sharpness of separation and will require increased reflux or increased number of stages. If hold-up in the column is not accounted for, products might be off specification6. 

The one level optimal control formulation proposed by Mujtaba (1989) is found to be much faster than the classical two-level formulation to obtain optimal recycle policies in binary batch distillation. In addition, the one level formulation is also much more robust. The reason for the robustness is that for every function evaluation of the outer loop problem, the two-level method requires to reinitialise the reflux ratio profile for each new value of (Rl, xRI). This was done automatically in Mujtaba (1989) using the reflux ratio profile calculated at the previous function evaluation in the outer loop so that the inner loop problems (specially problem P2) could be solved in a small number of iterations. However, experience has shown that even after this re-initialisation of the reflux profile sometimes no solutions (even sub-optimal) were obtained. This is due to failure to converge within a maximum limit of function evaluations for the inner loop problems. On the other hand the one level formulation does not require such re-initialisation. The reflux profile was set only at the beginning and a solution was always found within the prescribed number of function evaluations. 

This section presents the dynamic optimisation problem formulation of Mujtaba (1989) and Mujtaba and Macchietto (1992) to obtain optimal recycle policies in multicomponent batch distillation. Some special cases were identified where the methods used for the binary case could be applied fairly easily to multicomponent mixtures. The previously mentioned measure q of the degree of difficulty of separation was used to identify those special cases. A new operational strategy regarding the order of off-cuts recycle in a multicomponent environment was discussed. The Benefits of recycling were correlated against the measure q. 

It is important to note that in using computer-aided models for batch distillation, the various assumptions of the model can have a significant impact on the accuracy of the results e.g., see the discussion of the effects of holdup above. Uncertainties in the physical and chemical parameters in the models can be addressed most effectively by a combination of sensitivity calculations using simulation tools, along with comparison to data. The mathematical treatment of stiffness in the model equations can also be very important, and there is often a substantial advantage in using simulation tools that take special account of this stiffness. (See the 7th edition of Perry s Chemical Engineers Handbook for a more detailed discussion of this aspect). 

Rgure 8-35. Batch distillation constant reflux ratio after McCabe-Thlele diagram. Revised/adapted and used by permission, Schweitzer, PA Handbookaf Separation Techniques for Chemical Engineers, McGraw-Hill Book Co. (1979) also reprinted by special permission, Chem. Eng. Jan. 23 (1961), p. 134., 1961 by McGraw-Hill, Inc., New York. 

All the vapour rising from the liquid must be condensed in the specially provided Liebig condenser and be collected as distillate. Subject to this condition, and in view of the rapid stirring effected by the rising vapour, it would seem safe to assume that the distillate really represents the vapour which is in equilibrium with the liquid at the time in question. The compositions of the liquid and vapour are of course continually changing as the batch distillation proceeds. 

With a view to the high purification of liquids by means of batch distillation Wilcox [140a] derived special equations for distillations which aim at achieving ver high distillate purities. 

In the special case of a high coluttm with many equilibrium stages, the liquid concentration Xg in the vessel is approximately equal to the point of intersection of the operating and the equilibrium lines. Thus the relationship between Xp, Xb and. Rp is given by (5.2-28) which is valid for minimum reflux ratio. After inserting this relationship (5.2-80) and (5.2-82) can be analytically solved for ideal systems. The results are shown in Fig. 5.2-41 for both modes of batch distillation and, additionally, for continuous distillation. 

Let us consider a mixture of three components A, B, and R. Of special importance are the so-called residue curves, which represent the liquid residue compositions with time in a simple batch distillation. Different curves are obtained for different starting compositions, and a collection of these curves for a given ternary system is called an RCM. A typical RCM is shown in Figure 14.4a. Some important features of these curves should be noted If the direction of the curve is assumed to be from the starting composition to the ending composition, then the arrow on each curve points from the lower-boiling component or azeotrope (if one is formed) to the higher. The presence of azeotropes can create boundaries... 

Although gradually diminishing In favor, batch distillation still is tin interesting process to control, and deserves more than casual attention. Like most batch processes, its control system requires special consideration, ultimately bearing only faint resemblance to that of its continuous counterpart. 

Dialkylphenols are also produced in specialized plants. These plants combine complex batch reactors with vacuum distillation trains or other recovery systems. Alkenes with carbon numbers between 4 and 9 react with phenol to make an unrefined alkylphenol mixture, which is fed into the recovery section where very high purity product is isolated. The product is stored, handled, and shipped just as are the monoalkylphenols. 

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... 

The batch rectifier shown schematically in Eig. 13-119 is by far the most common configuration of equipment. Several alternative special-purpose configurations have been studied and offer potential advantages in particular applications. Also see Doherty ancl Malone Conceptual Design of Distillation Systems, McGraw-Hill, 2001, pp. 407-409, 417- 19). 

A poor batch of wine can sometimes be improved by chemical means, and then marketed as a blend. If it is not possible to correct the off wine, it may be distilled for recovery of alcohol, which again may be used for wine fortification. The aqueous residue, now significantly lower in BOD, is then discarded. An off wine may also be converted to wine vinegar by acetifica-tion [37](Eq. 16.15). This process is sufficiently profitable that one California winery has specialized in producing wine vinegar, rather than wine. 

Brandies have the special feature that they are prepared by distillation of a wine, generally a grape wine, rather than a beer . The great variety in flavor and qualities of the various brandies originates from the different types of wines distilled, whether the distillation was batch or continuous, and the aging and blending details. 

Gin manufacture in the United States may be carried on along with the manufacture of whiskey and spirits. Its place in this unitized operation is shown in Fig. 26. A specialized plant for the manufacture of gin is shown in Fig. 34. The process is as follows Pure spirit from the charge tank is drawn as needed to the gin still. Sufficient good-quality water is added to dilute the alcohol to about 125% proof. The juniper berries and other flavors required for a batch are placed in the gin head. High pressure steam is run through a coil in the still to cause distillation. The heads and tails are discarded, and the middle run, after dilution with distilled water in the blending tank to 80 or 90% proof, is drawn off to bottles. 

Distillations of specially chemicals where contamination can be a problem. The betel) equipaient can be cleaned or sterilized between batch runs. 

This disadvantage can partially be compensated by special batch operating modes, of which the upwards mode is already used in some industrial processes. Here the distillation vessel containing the mixture to be separated is located at the top of the column. The individual components are collected one after the other in order of their volatilities, starting with the lowest boiling fraction, at the bottom of the column (Figure 2.3.2-15). 

The kinetics of a chemical reaction have a significant influence on the products that can be attained from a RD process. The attainable products of counterinfinite reflux ratio can be obtained as singular points of a reactive reboiler batch process (bottom product) or a reactive condenser batch process (distillate product). The compositions of both products are located on a unique singular point curve. This curve is independent of any special type of reaction kinetics. However, the locations of the top and bottom products on this curve depend on the structure of the rate equation and on the intensity of the reaction (Damkbhler number) in the considered reaction system. 

Distillation is applied at small scales on a batch or continuous basis to separate and purify specially chemicals. At the other extreme it is the basic method for separating commodity chemicals on a tonnr basis, a familiar example being the distillation of crude peUoleum to obtain a number of hydrocarbon... 

---