Azeotrope-like compositions

Shankland, I.R. Lund, E.A.E. (1990). Azeotrope-like compositions ofpentafluoroethane and difluoromethane. Washington, DC U.S. Patent 4,978,467. 

R.R. Singh, I.R. Shankland, R.P. Robinson, H.T. Pham, R.H.P. Thomas, and P.B. Logsdon, Azeotrope-like compositions of trifluoromethane, carbon dioxide, ethane and hexafluoroethane, US Patent 5728315, assigned to AlliedSignal Inc. (Morristown, NJ), March 17,1998. 

Example 1.9 shows that, for certain systems, deviations from Raoult s law can cause a maximum or a minimum in the vapor pressure to exist at a certain temperature and composition. At constant pressure, the boiling point or bubble point temperature curve could have a maximum or a minimum. Liquid mixtures whose vapor pressure curve or surface exhibits a maximum or a minimum are said to form azeotropes. The composition at which the azeotrope occurs is the azeotropic composition. Binaries are likely to form azeotropes if they deviate from Raoult s law and if their boiling points are not too far apart (within about 8°C). Azeotropes caused by positive deviations from Raoult s law are minimum boiling, that is, the azeotrope boils at a... 

Azeotropes act like pure substances. Ethanol and water form an azeotrope containing about 89 mole percent ethanol. Any mixture of ethanol and water containing more than 89 percent ethanol may be fractionated into ethanol and the azeotrope a mixture containing less than 89 percent ethanol can be fractionated into water and the azeotrope. The composition of an azeotrope, and its boiling point at a given pressure, are characteristics peculiar to the system. 

The more stmcturaHy, chemically similar components are, the less likely that the separation will be improved by azeotropic distillation (if an MSA-key component azeotrope is being used to alter the RCM) any azeotropes formed between one component and another similar component tend to have similar boiling points, compositions. 

The answer is like fighting fire with fire—another azeotrope is formed. When benzene is added to ethyl alcohol and water a ternary azeotrope, a mixture of three compounds that boil at a single temperature, is formed. The ternary azeotrope has the composition of 68% benzene, 24% ethyl alcohol, and 6% water, and it boils at a temperature lower than the binary ethyl alcohol/water azeotrope. So, when a little benzene is added to the ethyl alcohol/water mixture and then put through a distillation column, the ternary azeotrope, in a 68-24-6 composition will come off the top, talcing with it all the benzene, all the water, but just some of the ethyl alcohol. Out the bottom comes whats left, the rest of the ethyl alcohol in nearly pure form. Slick. None of this, by the way, is shown in Figure 13—2. 

Azeotrope. A liquid mixture of two or more substances that behaves like a single substance, but most notably, the boiling point of the mixture is higher or lower than either of the substances. The vapor from boiling has, the same composition as the azeotrope mixture. 

Except in very special cases (azeotropic copolymerizations), copolymerization via radical mechanism shows a drift in the composition of the copolymers produced through the polymerization process. Emulsion copolymerization obeys this rule too, although the special features of its mechanism can change the drift process. The most common way to obviate that composition drift is to use the semi-continuous process where, after polymerization has been initiated with a small percent of the total charge (say 10 to 20 %) like in the batch process, most of the charge is added continuously at a much smaller rate (Ra) than the rate (Rp) at the end of the batch period, so that the added charge is polymerized quite instantaneously (J, 2). Then,the composition drift is limited to the initial period and most of the product does possess actually a constant composition. 

If the components exhibit strong physical or chemical interaction, the phase diagrams may be different from those shown in Figs, 1,1 and 1,5, and more like those shown in Fig, 1.8. In such systems there is a critical composition (the point of intersection of the equilibrium curve with the 45 diagonal) for which the vapor and liquid compositions are identical, Once this vapor and liquid composition is reached, the components cannot be separated at the given pressure, Such mixtures are called azeotropes. 

So that an azeotrope with acetone does not form, the alcohol used must have a high enough boiling point. This requirement is reliably established only if vapor-liquid equilibrium data for at least two, preferably three, of the members of the series with acetone are known. The Pierotti-Deal-Derr method (4) (discussed later) or the Tassios-Van Winkle method (5) can be used in this case. In the latter method a log-log plot of y°i vs. P°i should yield a straight line. Figure 1 presents results for n-alco-hols and benzene from the isobaric (760 mm Hg) data of Wehe and Coates (6). Reliable infinite dilution activity coefficients are established for the other n-alcohols from data for at least two, and preferably three, of them. These y° values are used with equations like those of Van Laar or Wilson (7) to generate activity coefficients at intermediate compositions and to check for an existing azeotrope or a difficult separation (x-y curve close to the 45° line). 

An azeotrope is a solution that, like a pure liquid, distills at a constant temperature without a change in composition. 

Air unusual type of low-pressure VLE belravior is tlrat of double azeotropy, in wlriclr the dew and bubble curves are S-shaped, tlrus yielding at different compositions both a miniimim-pressure and a inaximum-pressure azeotrope. Assuming that Eq. (12.11) applies, detennine under wliat circumstances double azeotropy is likely to occur. 

We can predict azeotropic behavior as follows from infinite-dilution /T-values. Using a flowsheeting system, we perform a bubble-point calculation for each species in the mixture. Assuming a mixture contains the species A, B, C, and D, we wish to compute the infinite-dilution L-values for three of the species in the remaining one. For example, we perform a flash calculation where A is dominant and B, C, and D are in trace amounts, using something like a feed composition of 0.99999, 0.000003333, 0.000003333, 0.000003334. It does not... 

Suppose we have a mixture of water, pyridine, and toluene. We set the pressure to 1 atm and attempt to solve the above equations. Lacking any further insight, we set all the vapor and liquid compositions equal to 0.33333 and then attempt a solution using a Newton-based method. The problem with finding azeotropes becomes immediately evident. There are six solutions to these equations the three binary azeotropes and the three pure species. There is no ternary azeotrope. To find all azeotropes for a mixture, we must find all solutions to the above equations. Finding multiple solutions to a set of highly nonlinear equations like these is usually a very difficult task. 

The condensate that collects on the cold surface is usually a completely homogeneous, or miscible, mixture of components. In general, the relative composition of the liquid components in the condensate is different from the composition in the vapor phase (except for an azeotropic mixture, where the condensate has the same exact molar concentration ratio as the vapor phase) [194]. The film that forms is not necessarily smooth but may show the appearance of streamers (or rivulets), waves, or droplets, depending on the particular mixture and its surface tension (which depends on the local wall temperature) [25,195,196]. If the condensate mixture is heterogeneous, or immiscible (as can occur when one component, for example, is aqueous and the other is organic), the pattern can be quite complex, looking somewhat like dropwise condensation [25,193,197]. These different condensate patterns affect the resulting fluid flow and heat transfer. 

It should be noted again that in the procedure attributed to Wilson [123], as in many other suspension polymerization procedures mentioned above and in many procedures for emulsion polymerizations to be described later, reaction temperatures are given which are above the boiling point of the monomer (72.7°C at 760 mm Hg), not to mention, above the boiling point of the vinyl acetate-water azeotrope (66°C) (composition, 92.7% vinyl acetate, 7.3% water, cf. Table I). For reactions carried out in sealed ampoules or closed bottles, this reaction temperature is readily explained. How such reaction temperatures are reached in a reflux apparatus open to the atmosphere is in question. It is hardly likely that the rate of polymerization is so rapid that no free monomer exists when it is added with conventional initiators to hot water. We presume that most of the polymerizations reported to proceed at about 66 C in an aqueous medium are simply run at reflux. At such a temperature, initiation by dibenzoyl peroxide is rather slow. If the suspension polymerization is to be forced at higher temperatures, provisions will have to be made to force the monomer into the... 

A liquid mixture of two or more substances that boils at a constant minimum or maximum boiling point lower or higher than that of its constituents is called azeotropic. The liquid and the vapour produced on boiling have the same composition and, in this, the mixture acts like a single substance. Chlorotrifluoromethane and trifluoromethane forms such a mixture. 

Azeotropic Degreasing Solvent—A specific composition of two or more solvents, which does not change in the liquid and vapor phases therefore, it behaves like a single solvent in a vapor degreaser and has a constant boiling point. 

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