Azeotrope Search

After clicking the Report button on the left-hand side of this dialog box, the infonnalion of all the azeotropes is generated (see Fig. 2.14). The system contains four azeotropes 

Phy tlc 1 Pro party Modal RTl VilW Photo VAP. liQ VQ Mixtan for Aiootropos At A Prossan Of 1 ATM 

These prediction results should be compared with the experimental data for validation of the reliability of the thermodynamic model. 

For the cases when Aspen Plus built-in parameters do not exist, or when the prediction of thermodynamic behavior from built-in parameters is not satisfactory, regression to obtain suitable model parameters from experimental data needs to be done. In the following, we wiU use an example to show the step-by-step procedure to do the data 

The particular example is a design study to separate a mixture containing dimethyl carbonate (DMC) and methanol where the built-in parameters cannot be found in Aspen Plus. However, from the paper by Rodriguez et al., ° experimental data of Txy at 

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Let us now study a system in which there is more dissimilarity of the molecules by looking at the -butanol/water system. The normal boiling point of -butanol is 398 K, and that of water is 373 K, so water is the low boiler in this system. The azeotrope search results are shown in Figiure 1.16, and the Txy diagram is shown in Figure 1.17. Notice that Vap-Liq-Liq is selected in the Phases under the Property Model. ... 

Selecting the analysis type as Txy, specifying the system pressure, specifying Valid Phases as Vapor-Liquid and clicking the Go button at the bottom of the dialog box produce the Txy plot shown in Figure 2.2. Tabulated binary analysis results like activity coefficients and K values are also shown. From the example Txy plot of isopropanol-water system, we can see that this system contains an azeotrope at isopropanol mole fraction between 0.6 and 0.8 and azeotropic temperature of about 80°C. To hnd the exact prediction of the azeotropic composition and temperature, the azeotrope search tool can be used. The use of this tool will be explained later in Section 2.5.2. 

Design Azeotrope Search in the Aspen Plus menu bar. In this dialog box, all components are selected for the search, the operating pressure is selected as 1 atm, and the property method of NRTL is also selected. Because liquid-liquid splitting is observed in this system, valid phases of VAP-LIQ-LIQ should be selected. The selection is shown in Figure 2.13. 

Figure 2.14 Results of the azeotrope search for the isopropanol-water-cyclohexane system.

The composition of the azeotrope is found by going to Tools, selecting Conceptual Design and clicking Azeotropic Search. The window shown in Figure 7.4a opens on... 

Solution To determine the location of the azeotrope for a specified pressure, the liquid composition has to be varied and a bubble-point calculation performed at each liquid composition until a composition is identified, whereby X = y,-. Alternatively, the vapor composition could be varied and a dew-point calculation performed at each vapor composition. Either way, this requires iteration. Figure 4.5 shows the x—y diagram for the 2-propanol-water system. This was obtained by carrying out a bubble-point calculation at different values of the liquid composition. The point where the x—y plot crosses the diagonal line gives the azeotropic composition. A more direct search for the azeotropic composition can be carried out for such a binary system in a spreadsheet by varying T and x simultaneously and by solving the objective function (see Section 3.9) ... 

Because of the inherent difficulties of eliminating water azeotropically during nitration reactions, due to the nitric acid - nitric oxides equilibria, we searched for more convenient insitu techniques. 

Process synthesis and design of these non-conventional distillation processes proceed in two steps. The first step—process synthesis—is the selection of one or more candidate entrainers along with the computation of thermodynamic properties like residue curve maps that help assess many column features such as the adequate column configuration and the corresponding product cuts sequence. The second step—process design—involves the search for optimal values of batch distillation parameters such as the entrainer amount, reflux ratio, boiler duty and number of stages. The complexity of the second step depends on the solutions obtained at the previous level, because efficiency in azeotropic and extractive distillation is largely determined by the mixture thermodynamic properties that are closely linked to the nature of the entrainer. Hence, we have established a complete set of rules for the selection of feasible entrainers for the separation of non ideal mixtures... 

The conclusion is that the separation by simple distillation of the ternary mixture acrylo nitrile/ace to nitrile/water is not possible because of azeotropes. Changing the VLE behavior by a mass-separation agent is desirable. The literature search... 

As long ago as 1960, Tarasov et al. [121] presented some examples of the concrete three-component systems for which the existence of the azeotropic composition had already been predicted theoretically. The list of such systems was widened substantially after publication of the important paper [125], where a set of the known tabulated values of 653 pairs of reactivity ratios for a computer search of the possible multicomponent azeotropes was employed. For this aim one should, at first, reveal all the completely characterized multicomponent systems for which the values of reactivity ratios of all monomer pairs are tabulated. This problem can be formalized by reducing it to the search on the graph with 653 lines of a... 

Given a ternary azeotrope and a fourth species in the mixture, compute its infinite-dilution /(-value at this ternary azeotrope. Increase the amount of the fourth species, keeping the /(-values for the other three equal to each other. Stop where the fourth /(-value passes near 1 or shows a minimum or maximum along the trajectory being traced out. Solve directly for the four-species azeotrope. Stop searching this trajectory when it hits a side of the composition space. 

As for the synthesis of azeotropic columns, most of the developments have been based on geometric representations with residue curves (Doherty and Car-darola, 1985 Knight and Doherty, 1989 Van Dongen and Doherty, 1985). Guidelines have also been developed by Stichlmair et al. (1989), some of which Laroche et al. (1992) have questioned as general synthesis rules. Finally, Wahn-schafft et al. (1992) have developed the program SPLIT, which is based on a ruled-based system and evolutionary search method. 

Searches made by the two libraries have a direct bearing on the effective production of the patent department. Such a simple bit of information as azeotropic composition may be needed to convince the patent examiner that a separation process will function as claimed in the patent application. Most of the applications rejected are anticipated by or do not distinguish in a patentable sense over prior art. In addition to preventing the filing of applications which would be rejected on patent or literature references, good patentabihty searches are needed by the patent attorney to aid him in judging the proper scope of his claims, so that the result will be a strong patent which cannot later be proved invahd. 

A different approach to process synthesis is offered by means-ends method. It is based on the observation that the purpose of material processing is to apply various operations in such a sequence that the differences in properties between the raw materials and the products are systematically eliminated. As a result, the raw materials are transformed into the desired products. The means-ends method starts with an initial state and successively applies transformation operators to produce intermediate states with fewer differences until the goal state is reached. The hierarchy for the reduction of property differences is as follows identity, amount, concentration, temperature, pressure, and finally form. This property changing method has its limitations, as it ignores the influences and the impacts on other properties. Moreover, the search method takes an opportrmistic approach, which cannot guarantee the generation of a feasible flowsheet. The means-ends analysis approach has been used as a systematic process synthesis method for overall process flowsheet synthesis, as well as for the more detailed case of a separation system to resolve the concentration differences in nonideal systems that include azeotropes. 

When the vapor pressures are very close, searches for effective solvents involve simple measurement of the activity coefficient ratio. The searching process is easier than that for azeotropic distillation. The usual approach is to select a solvent that is similar in structure to the heavy key component. Since the light key component is of a different stracture (as it must be, or simple distillation could be employed), the solvent serves to increase the volatility of the light key relative to the heavy key. 

A listing of azeotropic data has been published by Horsley.12 General source data are included in die voluma by Reid el al.2 More inhumation on sources of equilibrium data will be found in Chapter 1. In general, a thorough search of die literature should be made before investing in laboratory measurements, but the rearoher should beware of faulty experimental data that are not tbertaodynamicatly consistent. 

Our second break came on June 30, 1999 when we were filling in a Table on the scope of the dimerization reaction for our initial communication on the subject. It was a rookie mistake - Jason Lagona set up the dimerization reaction of 17 below reflux and without the azeotropic removal of H2O. Luckily, Jason handled the outcome like a seasoned veteran and isolated 17, Civ-WC, and a new compound. This new compound turned out to be the S-shaped compound (C2/1-IIS) that we had been searching for The stereochemical assignment was... 

A collection of approximately 47,400 zeotropic and azeotropic data sets, compiled from 6600 references, are stored in a comprehensive computerized data bank (Reference 10). The references from the above-mentioned compilations and from the vapor-liquid equilibrium part of the Dortmund Data Bank (Reference 11) were supplemented by references found from CAS online searches, private communications, data from industry, etc.. Over 24,000 zeotropic data and over 20,000 azeotropic data are available for binary systems. Nearly 90% of the binary azeotropic data show a pressure maximum. In most cases (ca. 90%) these are homogeneous azeotropes, and in approximately 7-8% of the cases heterogeneous azeotropes are reported. Less than 10% of the data stored show a pressure minimum. Approximately 21,000 of the data sets stored were published after 1970. 

Indifferent situations can create problems in the trial-and-error procedures routinely used in calculations for phase and reaction equilibrium. In such calculations, we may start with an T" or T" spedfication that properly doses the problem, but during the course of the trial-and-error search, the algorithm may enter an indifferent situation that couples properties that are otherwise independent. This may occur not only when azeotropes and critical points are encountered, but also when algorithms enter metastable and unstable regions of phase diagrams [20]. 

Laboratory experiments may also be necessary to aid in the selection and preliminary design of separation operations. The separation of gas mixtures requires consideration of absorption, adsorption, and gas permeation, all of which may require the search for an adequate absorbent, adsorbent, and membrane material, respectively. When nonideal liquid mixtures are to be separated, laboratory distillation experiments should be conducted early because the possibility of azeotrope formation can greatly complicate the selection of adequate separation equipment, which may involve the testing of one or more solvents or entrainers. When solids are involved, early laboratory tests of such operations as crystallization, filtration, and drying are essential. 

The effects of these and other guidelines must be considered as each separator is designed and as the separation sequence evolves. More recently, Peterson and Partin (1997) showed that temperature sequences involving the boiling points of the pure species and the azeotrope temperatures can be used to effectively categorize many kinds of residue curve maps. This classification simplifies the search for an entrainer that has a desirable residue curve map, for example, that does not involve a distillation boundary. 

P11.4 In the free DDBSP Explorer Version, search for all mixture data for the system benzene-water. Due to very limited miscibility in the liquid phase, the system shows heteroazeotropic behavior. Estimate the azeotropic composition using the vapor pressure correlations given in Appendix A and compare the results to the experimental data. 

Haynes, W. M and D. R. Lide, eds. 2010. CRC Handbook of Chemistry and Physics, 91st ed. Boca Raton, FL CRC Press. The CRC Handbook is an excellent starting point for physical property data searches. Data tables listing both specific chemical compound data (e.g., bp, mp, mw) and physical properties (e.g., azeotropic data for binary mixtures, critical solution temperatures of polymer solutions) are provided along with literature references for additional data. 

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