High Pressure VLE

Thermodynamic constraints to the SAS process can be summarized in the required miscibility between the liquid solvent and the supercritical antisolvent and the insolubility of the solute in the antisolvent and in the solvent-antisolvent mixture. Data are available for various binary mixtures liquid-supercritical fluid and can be described as type I using the classification of van-Konynenburg and Scott. If jet break-up is obtained and mass transfer is very fast, high-pressure VLEs of the ternary system liquid-I-solute-I-supercritical fluid can control the precipitation process. 

Relationship between Particles Morphology and High-Pressure VLEs... 

Polymer-solvent VLE Vapor-hquid data, both at intermediate concentrations and at the infinite dilution of the solvent, are available in two extensive databases DECHEMA and DIPPR Polymer Project. - These databases are also available in electronic form. The data are restricted to single solvent systems and often cover various temperatures. A more recent compilation of VLE has been published by Wohlfarth. Basically low-pressure VLE data are available. Very few high-pressure VLE data exist for polymer-solvent systems (with nongasesous solvents), e.g., the work by Surana et al. ... 

P5.9 Using the free Explorer Version of DDB/DDBSP, search for mixture data for the systems COi-n-hexane and C02-hexadecane. Plot the experimental high pressure VLE data (HPV) together with the predictions using PSRK. Compare the results to those of VTPR (Figure 5.99d) and examine the results for SLE in the binary mixture C02-n-hexane. 

Specialists in the field tend to concentrate on one of two broad areas high-pressure VLE, or low-pressure VLE. My major interests are in the latter area, and hence the thrust of my talk will be in this direction the measurement and reduction of low-... 

Low-pressure VLE experimentation differs from high-pressure experimentation on two major counts. First, the problems of equipment design and operation are less formidable than for high-pressure work. Secondly, more effective use can be made of the thermodynamic equilibrium equations, both in the data reduction process and in the design of the experiments themselves. It is this second feature— the strong interplay of theory with experiment—which to me most distinguishes low-pressure from high-pressure VLE work. 

Equation (11) in fact imposes a thermodynamic constraint upon the liquid-phase activity coefficients, a constraint not necessarily satisfied by values of yi computed via Eq. (10) from real (and therefore possibly Imperfect) data. However, values of y generated from a smoothing equation for g via Eq. (6) do satisfy Eq. (11). Comparison of generated with experimental values of Yi constitutes an example of a popular exercise known as a thermodynamic consistency test. Many consistency tests have been proposed, both for low- and high-pressure VLE data. Van Ness et al (I ) and Christiansen and Fredenslund (.2) present readable discussions of such tests. 

This chapter discusses low-pressure VLE. At high pressures, VLE is different, mostiy because we approach the critical pressures of the vapor-liquid mixtures. Chapter 10 starts from what we see in this chapter and shows how the experimental behavior and our mathematical approaches to it change at high pressures. 

The description in terms of the (, /<, ) notation shown here is rarely used for high-pressure VLE, because the values of i calculated as shown above for /j-heptane are unreasonable. 

The behavior of pure species near their critical states is different from what we expect well away from those states. The same is true for mixtures, whose high-pressure VLE behavior is quite different from that at... 

Current process design computer programs mostly calculate high-pressure VLE using cubic EOSs, of which the SRK is one of the most popular. The procedure is as illustrated in Example F.5/10.3. 

Figure 10.8 shows that the BWR EOS, which contains the specific volume to the sixth power, calculates an isotherm on P-v coordinates that has one maximum and one minimum, and that can be used to calculate the PvTbehavior of both gas and liquid (but not directly the behavior in the two-phase region). The same result can be obtained with any EOS that has the volume to the third or higher power. Complex EOSs like the BWR can represent experimental PvT data more accurately than cubic equations, which have the volume only to the third power, but the latter are much easier to use in high-pressure VLE calculations and are more widely used. The cubic EOSs of interest are as follows ... 

With suitable mixing rules, these pressure-explicit EOSs can make good-to-excellent estimates of high-pressure VLE. They are very widely used for that purpose. 

We will concentrate on Vapor-Liquid Equilibrium (VLE) because of its wide applicability. In addition, the methodology involved and discussed here is typical to all types of phase equilibria. Finally, for convenience reasons that will become apparent in Section 13.4, we consider in this Chapter Low Pressure VLE and High Pressure VLE, in the next one. 

The presence, thus, of nonideality in both phases characterizes high pressure vapor-liquid equilibrium as compared to that at low pressure, where the main source of nonideality is in the liquid phase. And as a result, high pressure VLE calculations can be more complex than those at low pressures. 

The quantitative description of high pressure VLE is usually expressed through the equilibrium ratio K. ... 

Cubic EoS are the most commonly used equations of state for high pressure VLE calculations. They are relatively simple to use and provide reasonable accuracy. We confine, therefore, our discussion to such EoS and proceed to ... 

High pressure VLE calculations are carried out using K values obtained with the aforementioned methods, in a fashion similar to that used for low pressure ones. 

Cubic equations of state have become the main tool for high pressure VLE calculations. They combine simplicity with accuracy comparable to -or better than - that of other methods, including non-cubic EoS. For a comparison of the EoS approach with the Chao-Seader method, see Maddox and Erbar (1981). 

What are the main approaches used in high pressure VLE calculations ... 

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