Miscible substances

Miscible A term used to specify the degree that two substances will mix with one another. Completely miscible substances such as water and ethanol will mix uniformly no matter the proportions. 

For flashing liquids composed of many miscible substances, the flash calculation is complicated considerably, because the more volatile components flash preferentially. Procedures are available to solve this problem.16... 

Azeotropic and Partially Miscible Systems. Azeotropic mixtures are those whose vapor and liquid equilibrium compositions are identical. Their x-y lines cross or touch the diagonal. Partially miscible substances form a vapor phase of constant composition over the entire range of two-phase liquid compositions usually the horizontal portion of the x-y plot intersects the diagonal, but those of a few mixtures do not, notably those of mixtures of methylethylketone and phenol with water. Separation of azeotropic mixtures sometimes can be effected in several towers at different pressures, as illustrated by Example 13.6 for ethanol-water mixtures. Partially miscible constant boiling mixtures usually can be separated with two towers and a condensate phase separator, as done in Example 13.7 for n-butanol and water. 

On a ternary equilibrium diagram like that of Figure 14.1, the limits of mutual solubilities are marked by the binodal curve and the compositions of phases in equilibrium by tielines. The region within the dome is two-phase and that outside is one-phase. The most common systems are those with one pair (Type I, Fig. 14.1) and two pairs (Type II. Fig. 14.4) of partially miscible substances. For instance, of the approximately 1000 sets of data collected and analyzed by Sorensen and Arlt (1979), 75% are Type I and 20% are Type II. The remaining small percentage of systems exhibit a considerable variety of behaviors, a few of which appear in Figure 14.4. As some of these examples show, the effect of temperature on phase behavior of liquids often is very pronounced. 

In practice it is not possible to start marking the instant molecules start entering the reactor. Instead, a quantity of inert miscible substance is added to the feed stream in the apparatus shown in Fig. 6-3u. Provided that the tracer can be added quickly, that the tracer molecules move through the reactor in the same way as the feed stream, and that the detection of tracer in the effluent is rapid, the experimental results are satisfactory. In such an experiment the concentration of tracer in the feed takes the place of Cq... 

Gases are miscible. Miscible substances mix with one another in any proportion to form a solution. Dry air, for example, is a solution of about 18 gases. Two liquids, however, may or may not be miscible water and ethanol are, but water and gasoline are not. Two solids generally do not form a solution unless they are mixed as molten liquids and then allowed to solidify. 

Ethylene glycol has to be completely replaced by some toluene miscible substance, otherwise the gel slices start to shrink. The closely related Cellosolve, did not alone give gel slices as transparent as those obtained after additional treatment with alcohol (99.5%). This might be due to slower diffusion rates with Cellosolve. 

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