Azeotrope reactive

The situation becomes more complicated when the reaction is IdneticaUy controlled and does not come to complete-chemical equilibrium under the conditions of temperature, hquid holdup, and rate of vaporization in the column reactor. Venimadhavan et al. [AIChE J., 40, 1814 (1994)] and Rev [Jnd. Eng. Chem. Res., 33, 2174 (1994)] show that the existence and location of reactive azeotropes is a function of approach to equilibrium as well as the evaporation rate. 

Reaction yields, optimizing, 9 443 Reactive aluminas, 2 405, 408 Reactive azeotropes, 22 331-332 Reactive-chemical hazards, assessment of, 21 845-846... 

Section 4.2 is focused on phase equilibrium-controlled vapor-liquid systems with kinetically or equihbrium-controlled chemical reactions. The feasible products are kinetic azeotropes or reactive azeotropes, respectively. 

Fig. 4.2. Potential singular point surfaces (dashed-dotted curve) for an ideal ternary system with single reaction A + B C. (a) Ellipse-type system (b) hyperbola-type system. RA = reactive azeotrope solid curve = chemical equilibrium surface.

Figure 4.6 illustrates the PSPS and the chemical equilibrium surface. The PSPS has a hyperbola-type shape and passes through all pure component vertices and the stoichiometric pole n. It intersects the isobutene-MeOH edge and the MeOH-MTBE edge at two points, which are nonreactive binary azeotropes. From Fig. 4.6 one can also see that there exists no reactive azeotrope in this system. All the bifurcation branches and the pure component vertices, as discussed by Venimadhavan et al. [7], are located on the PSPS. 

Figure 4.9(a) and (b) illustrate the system behavior at a total pressure of 15 atm and 8 atm, respectively. As can be seen from the location of the PSPS, this system has similar features as the ideal system example 1 which has an elhpse-shaped PSPS (see Fig. 4.2(a)), as discussed above. Due to the boiling sequence of the reaction components, the PSPS is fully located outside the physically relevant composition space and, as a consequence, no reactive azeotrope can appear. It is worth noting that inside the phase-splitting region, the PSPS of the real heterogeneous system and the PSPS of the pseudohomogeneous system are different However, this does not affect the feasible top and bottom products of a fully reactive distillation column. 

The surfaces described by Eqs. (27a) and (27b) in the three-dimensional composition space intersect with each other and yield the PSPS as curves given in Fig. 4.10(a). The PSPS contain several branches, three of which pass through the pure components HOAc, IPOAc and water, and are located outside the composition space but are not depicted. The branch passing through the I PA-vertex locates four nonre-active azeotropes - that is, IPA-IPOAc, IPOAc-Water, IPA-IPOAc-Water, and IPA-Water. This branch also contains the reactive azeotrope. The PSPS is also displayed in the transformed composition space (Fig. 4.10(b)). 

This example considers distillation of a reacting ternary mixture in an open batch distillery with flowing sweep gas. From this example, one can see the determination of reactive azeotropes and reactive arheotropes . The considered hypothetical reaction is... 

A singular point of reactive membrane separation should be denoted as kinetic arheotrope because it is a process phenomenon rather than a thermodynamic phenomenon. The condition for arheotropy can be elegantly expressed in terms of new transformed variables, which are a generalized formulation of the transformed composition variables first introduced to analyze reactive azeotropes. 

Note that there are no reactive azeotropes between the components of the reaction mixture, although the volatile reactants lay between light and heavy products. The explanation may be found in the fact that both acid and alcohol have high boiling points that do not contribute significantly to the total vapor pressure, dominated by alcohol and water. 

Figure A.2 (right) emphasizes a particular position where phase equilibrium and stoichiometric lines are collinear. In other words the liquid composition remains unchanged because the resulting vapor, after condensation, is converted into the original composition. This point is a potential reactive azeotrope, but when the composition satisfies chemical equilibrium too it becomes a true reactive azeotrope. Some examples of residue curve maps are presented below. Ideal mixtures are used to illustrate the basic features, which may be applied to some important industrial applications.

Figure A.3 also illustrates graphically the formation of a reactive azeotrope as the point where a particular stoichiometric line becomes tangential to the nonreac-tive residue curve and intersects simultaneously the chemical equilibrium curve.

Figure A.4 (left), the equilibrium curve from the previous case becomes a surface of conical shape limited by the inert-free equilibrium curve and the edges AI and BI. It is to be noted that all the reactive residue curves start at the reactive azeotrope but with different paths towards the vertex I.

Another possibility is the representation in a two-dimensional diagram, as in Figure A.4 (right). The component C being chosen as the reference, the relation (A.3) gives the transformed co-ordinates XA = (xA + xc)/(l + xc) and X, = (xB + xc) / (1 + xc). The residue curves run from the reactive azeotrope to the vertex of component I. This situation is denoted by two degrees of freedom systems . 

Summing up, the influence of the kinetics of a chemical reaction on the vapor-liquid equilibrium is very complex. Physical distillation boundaries may disappear, while new kinetic stable and unstable nodes may appear. As result, the residue curve map with chemical reaction could look very different from the physical plots. As a consequence, evaluating the kinetic effects on residue curve maps is of great importance for conceptual design of reactive distillation systems. However, if the reaction rate is high enough such that the chemical equilibrium is reached quickly, the RCM simplifies considerably. But even in this case the analysis may be complicated by the occurrence of reactive azeotropes. 

The accuracy of the thermodynamic data has a significant effect on RCM computation. In the case of slow reactions both kinetics and phase equilibrium should be modelled accurately. If the reaction is fast enough the chemical reaction prevails. In many cases chemical equilibrium may be taken as the reference. Consequently, accurate knowledge of the chemical equilibrium constant is needed. When reactive azeotropes and/or phase splitting might occur accurate modelling of phase equilibrium is also needed. 

The mathematical solution of Eq. (A.15) is tedious. An elegant graphical solution has been proposed by Stichlmair and Fair [1]. The occurrence of a reactive azeotrope is expressed geometrically by the necessary condition that the tangent to the residue (distillation) curve be collinear with the stoichiometric line. Such points form the locus of potential reactive azeotropes. In order to become a true reactive azeotrope the intersection point must also belong to the chemical equilibrium... 

Figure A.8 Graphical identification of reactive azeotropes for the reaction A + B C.

This simple graphical illustration allows the formulation of a practical rule reactive azeotropes may occur for ideal mixtures having segregated volatilities (reactants either lighter or heavier with respect to products), but should not form in the case of mixed volatilities . 

A similar construction can be imagined when A and B form a minimum boiling azeotrope, as illustrated in Figure A.9. This time a second reactive azeotrope curve appears in the upper distillation region, between the AB-minimum azeotrope and the reactant A. On the other hand, when the volatility order is changed, as for example B becomes the heaviest and C the intermediate, no reactive azeotropes can be found since the residue curves and the equilibrium curves are aligned in the same direction. 

Reactive azeotropes are not easily visualized in conventional y-x coordinates but become apparent upon a transformation of coordinates which depends on the number of reactions, the order of each reaction (for example, A + B C or A + B C + D), and the presence of nonreacting components. The general vector-matrix form of... 

---