PVT EXPERIMENTS FOR PRECISE VLE DATA FOR MIXTURES OF SIMILAR VOLATILITY
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The separation of components of similar volatility is a major industrial problem requiring precise VLE data. Classical VLE experiments provide liquid and vapor compositions, x1 and y1 to 0.2%, at best, when the important difference z y1 - x1 may be only 0.008 leaving the designer with 0.008 0.004. That is, the uncertainties can fail to exclude an azeotrope for systems where none exist. The Burnett-Isochoric (B-I) method allows precise measurement of densities along isochores leading to the isopleth for mixtures of fixed overall composition. Determination of each isopleth by the change of isochoric slope method provides complete VLE information when a series of isopleths of different compositions are overlayed on the P/T diagram. Because both densities and dew-bubble loci are measured, these data can test the accuracy of an Equation of State - Mixing Combining Rules (EOS/MCR), both as to its ability to predict densities (vapor and liquid) as well as VLE. Further, such dual data allow experimental consistency checks through the Gibbs-Duhem equation. Starting with equations similar to those of Manley and Swift (1971) we here derive further equations valid only for narrow dew-bubble gaps in pressure for isopleths. One particularly simple and useful result is that (z)2 BDP. The fractional error in z is thus one-half of that of the measured BDP; the absolute error e(z) for our apparatus is about 4 10-5 when BDP = 0.55 psi (3.8 kPa). This manuscript presents these derivations as well as an error analysis based upon simulated results. 1989.
