Isovolatility curve

TABLE 10.1 NRTL Parameters for IPA -Water-DMSO or EG System.  

i Comp, j IPA Water IPA DMSO Water DMSO IPA EG Water EG 

The main purpose of adding the entrainer into the system is to alter the relative volatility between IPA and water. Thus we would like to check this capability for using DMSO or 

Aspen Plus extended Antoine vapor pressure model is  

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Figure 16. Isoviscosity and isovolatility curves for a vinyl resin (Union Carbide Vinylite VYHH) at 20% solids.

Beens et al. [10], as part of a study on selecting column sets for a given GC xGC application, proposed a solution to this problem through the use of "isovolatility" curves. If the injection system is not sufficiently heated, compounds evaporate slowly, producing an intense tailing effect in peaks. When this tail elutes at the end of the column, the modulator will continuously inject the compound into the column. The isovolatility line includes the points in two-dimensional plots that represent retention CIr, a -axis) against retention i tR, y-axis) at the elution temperature. 

Figure 1 Simulated 2D retention behaviour for Qo to C,7 n-alkanes, showing isovolatility curves resulting from a slow injection. Based on reference [10].

Figure 2 Simulated 2D retention behaviour of a Qo to Qy n-alkane mixture, injected at 10-min intervals. Isovolatility curves (gray) are obtained by interpolation. Rl values for a compound x are obtained from its retention and from the estimated (by interpolation) retention times of the bracketing n-alkanes ( t ie and tRiy) [15].

In both cases, the mapping of the 2D space using the n-alkanes allows an easy calculation of RI values for a compound. Figure 2 shows the position in the 2D space of a compound x (black circle), having and tRx as retention times, which elutes between the Cig and C17 curves. If isovolatility curves have been obtained by a mathematical fitting process, the equations involved can be used to estimate and at RI calculation from Equation (3) only requires, besides these values, the holdup time (see Section 3.3) in order to obtain the adjusted retention times. 

The range of the isovolatility curves can be extended by extrapolation through mathematical procedures, by injections at different temperature programming rates [12] or by use of the more polar homologous series (2-methylketones and fatty acids methyl esters [11], alcohols [12]) instead of the n-alkanes. 

Beens et al. [10] describe three possible experimental approaches (1) using a plot of retention factors k of n-alkanes against temperature and extrapolating to k — 0 (2) using extrapolation of the "isovolatility" curves (see Section 3.1.2) and (3) using baseline alterations caused by the modulator operation when carrier gas is continuously doped with methane. The three methods produce similar results. 

Figure 2.10 Isovolatility curve of the acetone-methanol-DMSO system.

For comparison, Figure 10.3 displays the isovolatility curve at 1 atm using EG as entrainer. The location has been moved to xg = 0.20 (20 mol% EG) showing that EG is a less effective entrainer than DMSO. For completeness of the information for generating the above plots, the NRTL parameters and the coefficients of the Antoine equation for calculating the vapor pressure are listed in Tables 10.1 and 10.2, respectively. 

One final comment about the iso- and equivolatility curves is that we should not solely rely on the location where the isovolatility curve intercepts the IPA-entrainer edge of the triangle to determine which one is the more effective entrainer. We will show a... 

In the first part of this chapter, the important factors of an effective entrainer for separation of an azeotrope using extractive distillation have been demonstrated using IPA dehydration as an example. The isovolatility curve can be used to determine the feasible distillate produet in the extractive distillation column. The isovolatility and equivolatility curves can be used to determine which candidate entrainer is more effeetive in enhancing the relative volatility of the azeotropic mixture. The VLB information of the binary pairs between the candidate entrainer and either of the two original eomponents should also be plotted to make sure there is no problem for the separation in the leetilying section of the extractive distillation column and in the entrainer reeovery eolumn. 

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