Azeotrope fixed point

This time, the distillation lines and residue curves follow each other fairly closely because the difficult separation means that the changes from stage to stage in a staged column become smaller and approach the continuous changes in a packed column. It is important to note that distillation lines and residue curves have the same properties at fixed points (when the distillation lines and residue curves converge to a pure product or an azeotrope). 

Fixed points boiling points of pure components and azeotropes. They can be nodes (stable and unstable) and saddles. 

In this chapter, we describe an algorithm for predicting feasible splits for continuous single-feed RD that is not limited by the number of reactions or components. The method described here uses minimal information to determine the feasibility of reactive columns phase equilibrium between the components in the mixture, a reaction rate model, and feed state specification. This is based on a bifurcation analysis of the fixed points for a co-current flash cascade model. Unstable nodes ( light species ) and stable nodes ( heavy species ) in the flash cascade model are candidate distillate and bottom products, respectively, from a RD column. Therefore, we focus our attention on those splits that are equivalent to the direct and indirect sharp splits in non-RD. One of the products in these sharp splits will be a pure component, an azeotrope, or a kinetic pinch point the other product will be in material balance with the first. 

As an example, consider the residue curve map for a ternary system with a minimumboiling binary azeotrope of heavy (H) and light (L) species, as shown in Figure 7.23. There are four fixed points one unstable node at the binary azeotrope (A), one stable node at the vertex for the heavy species (H), and two saddles at the vertices of the light (L) and intermediate (I) species. 

Azeotropic Systems. An azeotropic system is one wherein two or more components have a constanl boiling point at a particular composition. Such mixtures cannot be separated by conventional distillation methods. If rhe constant boiling point is a minimum, the system is said lo exhibit negomv azeotropy if it is a maximum, positive azeotropy. Consider a mixture of water and alcohol in the presence of the vapor. This system of two phases and two components is divarianl. Now choose some fixed pressure and study the composition of the system at equilibrium us a function of temperature. The experimental results arc shown schematically in Fig. 5. 

In contrast, certain mixtures of two (binary) or three (ternary) components form constant boiling mixtures that cannot be separated by distillation. In such cases, each component contributes a fixed amount and the boiling point of the mixture is characteristic of the components. Such a system is called an azeotrope. The boihng point of an azeotrope may be higher or lower than that of the individual components. Common binary azeotropes are listed in Table 4.7 and ternary azeotropes are listed in Table 4.8. 

When crystallization applies care should be paid to the fixed-composition points (eutectics) as with the azeotropes. In this case both ethylbenzene and o-xylene have to be removed before since they give eutectics with p-xylene. Although demanding, the distillation can be used. A direct sequence scheme is appropriate with ethylbenzene separated in the top of the first split and then o-xylene in bottoms of the second split. 

Fig.4 and Fig.5 show adsorption isotherms for single component systems, obtained from fixed bed experiments and molecular simulation, respectively. Adosorption equilibria were simulated well. Except EtOH system, quantitative order of amount adsorbed was good agreement with experimental data. As for BEN and TOL systems, the amount of adsorbed for simulations were lower than experimental data. So it is necessary to examined van der Waals parameter for benzene ring. Fig.6 and Fig.7 show adsorption equilibria For binary component systems, obtained from fixed>bed experiments and molecular simulation, respectively. These are examples, which show azeotropic adsorption. Especially, IPA-TCE, BEN-EtOH systems show two azeotropic points. Result of simulation shows only one azeotropic point. More investigative is necessary. 

There are 3/ic + 2 variables (,y and Kj for all species, temperature T, and pressure P) in these 3 c + 1 equations. If we fix pressure, the model is completely fixed. Solving, we will determine the bubble-point temperature for the azeotrope as well as its composition. We note, therefore, that an azeotropic composition for a mixture is. as we already knew, pressure-dependent. 

The separation process depends on the nature of the vapor-liquid equilibrium relationships of the system, which can be represented on a ternary diagram. Figure 10.3a shows a ternary diagram at some fixed system pressure. Components A and B are close boilers, and A forms an azeotrope with the entrainer E. The curves in the triangle represent liquid isotherms. A corresponding vapor isotherm (not shown) could be drawn to represent the vapor at equilibrium with each liquid curve with tie lines joining vapor and liquid compositions at equilibrium. The temperature of the isotherms reaches a minimum at point Z that corresponds to the composition of the azeotrope formed between A and E. 

In a system that does not separate into individual phases, an increase in bulk concentration, x, also corresponds to an increase in the surface concentration, x(s). Depending on the nature of the surfactant and solid surface, one may observe two types of x(s) = fix) dependencies (Fig. III-10). For a high surface activity of adsorbing component at low solution concentrations, x, one observes a steep rise in x(s) until surface saturation is reached (x(s) =1), as shown by curve 1 in Fig. Ill-10. At low surface activity of the adsorbing substance, x(s) =fix) may be an S-shaped (curve 2). The intersection point A corresponds to the identical compositions of the surface layer and the bulk solution, i.e., a kind of surface azeotrope is formed. 

Figure 4.19 shows, as one would expect, that the lowest boiling temperatures are near the low-boiling acetone vertex, while the higher boiling isotherms are near the high-boiling ethanol vertex. The azeotrope between methanol and acetone cause profiles to have some curvature. It is important to point out that the location of these isotherms are fixed for a certain pressure, and that neither Xa nor will shift them (refer to Section 3.6.4). 

If the mixture is sufficiently nonideal that an azeotrope forms, then the curves for Ki and K2 cross their intersection occurs at Kj = 2 = 1, which identifies the azeotrope. This possibility is shown on the left in Figure 12.2. Finally, if one component is supercritical, then the mixture may have a critical point at the fixed T if this occurs, then Ki(P) and K2(P) are two branches of the same curve. Those branches coincide at Kj = K2 = 1, which identifies the mixture critical point, as on the right in Figure 12.2. 

Because the values for all of the species are unity at an azeotrope point, a simple distillation approaches this point, at which no further separation can occur. For this reason, a azeotrope is often called a stationary or fixed or pinch point. 

Fixed-bed adsoiption experiments of laboratory-scale were earned out to remove organic solvent vsptsis by several types of adsorbents. Binaiy adsoiption equilibria of azeotropic mixture-HSZ systems showed two azeotropic points. Those experiment data were compared with molecular simulation by the Grand Canonical Mtmte Cairo (GCMC) method. Experimental results for single and binary component system, including azeotropic mixture systems, could not be in agreement with the similation result correlated satisfoctorily. But it turns out that the tendency of the simulation result is the same as the experiment result... 

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