Feasible distillates

As mentioned previously, ternaiy mixtures can be represented by 125 different residue curve maps or distillation region diagrams. However, feasible distillation sequences using the first approach can be developed for breaking homogeneous binaiy azeotropes by the addition of a third component only for those more restricted systems that do not have a distillation boundaiy connected to the azeotrope and for which one of the original components is a node. For example, from... 

FIG. 13-64 Feasible distillation region diagrams for breaking homogeneous binary azeotrope A-B, a) Low-boiling entranoes. 

To a first approximation, the composition of the distillate and bottoms of a single-feed continuous distillation column lies on the same residue curve. Therefore, for systems having separatrices and multiple regions, distillation composition profiles are also constrained to lie in specific regions. The precise boundaries of these distillation regions are a function of reflux ratio, but they are closely approximated by the RCM separatrices. If a separatrix exists in a system, a corresponding distillation boundary also exists. Also, mass balance constraints require that the distillate composition, the bottoms composition, and the net feed composition plotted on an RCM for any feasible distillation be collinear and spaced in relation to distillate and bottoms flows according to the well-known lever rule. 

FIG. 13-83 Feasible distillation region diagrams and associated distillation system for breaking a homogeneous minimum-boiling binary azeotrope A-B. Component B boils at a higher temperature than does A. 

Fig. 6.9 Feasible products from column simu- bottoms and the open squares represent lations with different Nj, feed stage location, r, feasible distillates. The rectifying and stripping s, and D recorded in transformed variable cascade trajectories from Fig. 6.8 are also space. The filled squares represent feasible shown for comparison...

Fig. 6.11 Feasibility diagram showing the feasible distillates (unstable nodes) and feasible bottom products (stable nodes) from the rectifying and stripping cascade bifurcation diagrams, respectively...

The design equations would include, in addition to the usual heat and mass balances and vapor-liquid equilibria, equations for chemical equilibria and/or reaction kinetics. The occurrence of a chemical reaction can severely restrict the allowable ranges of temperatures and phase compositions by virtue of the additional equations for chemical equilibrium/kinetics. This effect can be quantitatively analyzed by constructing a residue curve map (RCM). It explicitly shows the shifting of distillation boundaries in the presence of reaction and defines the limits of feasible distillation column operation. We illustrate this (Venimadhavan et al., 1994) by considering the reaction... 

Figure 7J1 Regions of feasible distillate and bottoms product compositions (shaded) for a ternary mixture (a) system without azeotropes (b) system with two binary azeotropes (c) system with binary and ternary azeotropes (Stichlmair et al., 1989).

In the first part of this chapter, the important factors of an effective entrainer for separation of an azeotrope using extractive distillation have been demonstrated using IPA dehydration as an example. The isovolatility curve can be used to determine the feasible distillate produet in the extractive distillation column. The isovolatility and equivolatility curves can be used to determine which candidate entrainer is more effeetive in enhancing the relative volatility of the azeotropic mixture. The VLB information of the binary pairs between the candidate entrainer and either of the two original eomponents should also be plotted to make sure there is no problem for the separation in the leetilying section of the extractive distillation column and in the entrainer reeovery eolumn. 

In summary, the BVDM or residue curve plots can be used to conceptualize feasible distillation sequences. It is particularly useful for azeotropic systems involving three conponents, either when there are three components in the feed or when an entrainer is added to break a binary azeotrope. This method has only been introduced in this section, so care must be taken when applying this method without additional reading. Reference [171 is su ested for further reading. 

Despite the advances in the thermodynamics for predicting azeotropic mixture, feasible distillation boundaries, and sequence of cuts, the azeotropic batch distillation system is still incipient in terms of design, optimization, and optimal control. 

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