Azeotrope pinch point

Fig. 4.8(b)). At Damkohler numbers Dac> 0.085 and Dar> 0.166, pure isobutene and pure MeOH are feasible top and bottom products, respectively. At Dar< 0.166, both pure MeOH and a kinetic azeotrope (i.e., the mixture on the branch from MTBE to the pinch point) are possible bottom products, while another kinetic azeotrope (i.e., the mixture on the branch between isobutene and the nonreactive azeotrope isobutene-MeOH) is the possible top product. 

Figure 4.33 illustrates the PSPS and bifurcation behavior of a simple batch reactive distillation process. Qualitatively, the surface of potential singular points is shaped in the form of a hyperbola due to the boiling sequence of the involved components. Along the left-hand part of the PSPS, the stable node branch and the saddle point branch 1 coming from the water vertex, meet each other at the kinetic tangent pinch point x = (0.0246, 0.7462) at the critical Damkohler number Da = 0.414. The right-hand part of the PSPS is the saddle point branch 2, which runs from pure THF to the binary azeotrope between THF and water. 

The S shape of the residue curves gives us another pinch point curve for A. It starts at the azeotrope and ends at the node for A. Above this curve, movement is again in the wrong direction. Thus, we have only the region between these two curves in which we can correctly operate the section of the column between the two feeds. 

DODS-ProPlot. This is a comprehensive profile plotting package which allows the user to plot single profiles, entire CPMs and ROMs, and their associated pinch points. There are 13 systems to choose from, each of which may be modeled either with a modified Raoult s law and the NRTL activity coefficient model, or with the ideal Raoult s law (does not model azeotropes), or with a constant relative volatility approximation where the software automatically determines the relative volatilities between components (this model also cannot account for azeotropic behavior). One also has the option to insert one s own constant volatilities. It is possible to plot the full DPE, the shortened DPE at infinite reflux (shown in Chapter 7) or the classic residue curve equation. Depending on the equation chosen, the user is free to specify any relevant parameters such as an R value, difference points, system... 

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. 

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. 

The vapor-liquid equilibrium (VLB) and liquid-liquid extraction (LLE) correlations in Aspen Plus are not always as accurate as possible. This can cause significant errors, particularly near pinch points in distillation columns. If data is available, Aspen Plus will find values of the parameters for any of the VLB or LLE correlations by doing a regression against the data you input. This is illustrated to obtain an improved fit for the non-random two-liquid (NRTL) VLB correlation for the binary system water and isopropanol (IPA). VLB data for water and isopropanol is listed in Table B-1. This system has a minimum boiling azeotrope at 80.46°C. The Aspen Plus fit to the data with NRTL is not terrible, but can be improved. 

It is interesting to note that the pinch point in Figure 6.15d is exactly the reactive azeotrope that was discovered by Doherty and Malone and discussed in their book. A mathematical approach is taken and the framework for the analysis is based on the transformed composition that is invariant from the standpoint of reaction. Let us... 

The required information about the distillation boundary is obtained from the pinch distillation boundary (PDB) feasibility test [8]. The information is stored in the reachability matrix, as introduced by Rooks et al. [9], which represents the topology of the residue curve map of the mixture. A feasible set of linear independent products has to be selected, where products can be pure components, azeotropes or a chosen product composition. This set is feasible if all products are part of the same distillation region. The singular points of a distillation region usually provide a good set of possible product compositions. The azeotropes are treated as pseudo-components. 

To study the behavior of the singular points in the vicinity of the MTBE vertex, Thiel et al. [8] used a continuation method with the Damkbhler number as continuation parameter. The results computed at p = 0.8 MPa are shown in Fig. 5.17. It can be observed that a stable node branch beginning from pure MTBE in the absence of chemical reaction moves away from MTBE vertex with rising Da. As the Damkohler number Da = 1.49 X 10 is reached, the stable node branch turns into a saddle branch. This point is called the kinetic tangent pinch [9]. The saddle branch arrives at Da = 0.0 in the binary azeotropic point between MeOH and MTBE. 

At an operating pressure p below 1.0 MPa, the curves have a qualitative shape similar to those computed for the MTBE example a stable node branch moves from the TAME vertex to the kinetic tangent pinch and reaches its maximum Damkhhler number Da here. Then, the stable node branch turns into a saddle branch and runs into the binary azeotropic point between MeOH and TAME. A second saddle branch develops if an operating pressure p = 0.8 MPa at the starting point of pure lA is chosen. This saddle branch moves away from the lA vertex and reaches the line of chemical equilibrium at Da —y oo. This point is marked with a diamond in Fig. 5.20. If p is set to 1.0 MPa the stable node branch does not turn into a saddle branch that ends in the binary azeotropic point between MeOH and TAME, but which runs into pure lA. Consequently, at p = 1.0 MPa a second saddle branch, which starts at the binary azeotropic point between MeOH and TAME and arrives in the line of chemical equilibrium at Da —y oo, can be computed. In addition, in Fig. 5.20 the branch of kinetic tangent pinches is also plotted. 

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