Vapor-liquid equilibrium ethanol-water

Pervaporation. Pervaporation differs from the other membrane processes described so far in that the phase-state on one side of the membrane is different from that on the other side. The term pervaporation is a combination of the words permselective and evaporation. The feed to the membrane module is a mixture (e.g. ethanol-water mixture) at a pressure high enough to maintain it in the liquid phase. The liquid mixture is contacted with a dense membrane. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, thus maintaining it in the vapor phase. The permeate side is often held under vacuum conditions. Pervaporation is potentially useful when separating mixtures that form azeotropes (e.g. ethanol-water mixture). One of the ways to change the vapor-liquid equilibrium to overcome azeotropic behavior is to place a membrane between the vapor and liquid phases. Temperatures are restricted to below 100°C, and as with other liquid membrane processes, feed pretreatment and membrane cleaning are necessary. 

Vapor-Liquid Equilibrium in the Ethanol-Water System Saturated with Chloride Salts... 

Many papers concerning salt effect on vapor-liquid equilibrium have been published. The systems formed by alcohol-water mixtures saturated with various salts have been the most widely studied, with those based on the ethyl alcohol-water binary being of special interest (1-6,8,10,11). However, other alcohol mixtures have also been studied methanol (10,16,17,20,21,22), 1-propanol (10,12,23,24), 2-propanol (12,23,25,26), butanol (27), phenol (28), and ethylene glycol (29,30). Other binary solvents studied have included acetic acid-water (22), propionic acid-water (31), nitric acid-water (32), acetone-methanol (33), ethanol-benzene (27), pyridine-water (25), and dioxane-water (26). 

The present work studies the vapor-liquid equilibrium of the ethanol-water system saturated with copper(II) chloride, strontium chloride, and nickel(II) chloride. 

The salt effects of potassium bromide and a series office symmetrical tetraalkylammonium bromides on vapor-liquid equilibrium at constant pressure in various ethanol-water mixtures were determined. For these systems, the composition of the binary solvent was held constant while the dependence of the equilibrium vapor composition on salt concentration was investigated these studies were done at various fixed compositions of the mixed solvent. Good agreement with the equation of Furter and Johnson was observed for the salts exhibiting either mainly electrostrictive or mainly hydrophobic behavior however, the correlation was unsatisfactory in the case of the one salt (tetraethylammonium bromide) where these two types of solute-solvent interactions were in close competition. The transition from salting out of the ethanol to salting in, observed as the tetraalkylammonium salt series is ascended, was interpreted in terms of the solute-solvent interactions as related to physical properties of the system components, particularly solubilities and surface tensions. 

The data in Tables I-XVI (see Appendix for all tables) show the isobaric vapor-liquid equilibrium results at the boiling point for potassium, ammonium, tetramethylammonium, tetraethylammonium, tetra-n-propylammonium, and tetra-n-butylammonium bromides in various ethanol-water mixtures at fixed liquid composition ratios. The temperature, t, is the boiling temperature for all solutions in these tables. In all cases, the ethanol-water composition was held constant between 0.20 and 0.35 mole fraction ethanol since it is in this range that the most dramatic salt effects on vapor-liquid equilibrium in this particular system should be observed. That is, previous data (12-15,38) have demonstrated that a maximum displacement of the vapor-liquid equilibrium curve by salts frequently occurs in this region. In the results presented here, it should be noted that Equation 1 has been modified to... 

Table I. Isobaric Vapor—Liquid Equilibrium Data for the Potassium Bromide—Ethanol—Water System at x = 0.206 0.001 (760 5 Torr)...

Several hundred plants have been installed for the dehydration of ethanol by pervaporation. This is a particularly favorable application for pervaporation because ethanol forms an azeotrope with water at 95 % and a 99.5 % pure product is needed. Because the azeotrope forms at 95 % ethanol, simple distillation does not work. A comparison of the separation of ethanol and water obtained by various pervaporation membranes and the vapor-liquid equilibrium line that controls separation obtained by distillation is shown in Figure 9.9 [40], The membranes... 

If a fluid composed of more than one component (e.g., a solution of ethanol and water, or a crude oil) partially or totally changes phase, the required heat is a combination of sensible and latent heat and must be calculated using more complex thermodynamic relationships, including vapor-liquid equilibrium calculations that reflect the changing compositions as well as mass fractions of the two phases. 

Vapor-liquid equilibrium data for the ethanol/water system (subscripts 1 and 2, respectively) at 70°C (158°F, 343 K) are given in the three left columns of Table 3.1. Check to see if the data are thermodynamically consistent. 

As an example, consider the distillation of an ethanol-water mixture at 101.3 kPa (1 atm). The initial charge is 100 mol of liqnid containing 18 mol % ethanol, and the mixture must be reduced to a maximum ethanol concentration in the still of 6 mol %. By using equilibrium data interpolated from Gmehling and Onken [Vapor-Liquid Equilibrium Data Collection, DECHEMA Chemistry Data Sen, vol. 1, Part 1, Frankfnrt (1977)], we get the following ... 

Figure 2. Vapor-liquid equilibrium data at atmospheric pressure for the boiling ethanol-water system containing potassium acetate at saturation and at various constant concentrations...

To illustrate how large the effect of a dissolved salt can be, Figure 2, calculated from the data of Meranda and Furter (I), is included to demonstrate by how much potassium acetate alters the vapor-liquid equilibrium relationship of the system, boiling ethanol-water at atmospheric pressure. The dotted curve represents the ethanol-water system alone, where the azeotrope occurs at about 87 mole % ethanol. The other curves are for various concentrations of potassium acetate, and all are... 

The Non-Random, Two Liquid Equation was used in an attempt to develop a method for predicting isobaric vapor-liquid equilibrium data for multicomponent systems of water and simple alcohols—i.e., ethanol, 1-propanol, 2-methyl-l-propanol (2-butanol), and 3-methyl-l-butanol (isoamyl alcohol). Methods were developed to obtain binary equilibrium data indirectly from boiling point measurements. The binary data were used in the Non-Random, Two Liquid Equation to predict vapor-liquid equilibrium data for the ternary mixtures, water-ethanol-l-propanol, water-ethanol-2-methyl-1-propanol, and water-ethanol-3-methyl-l-butanol. Equilibrium data for these systems are reported. 

Table V. Vapor-Liquid Equilibrium Data at 760 mm Hg Water (1)—Ethanol (2)-1-Propanol (3)...

Ternary System. The values of all binary parameters used in predicting the ternary data are shown in Table IV. The predicted values of the vapor-liquid equilibrium data—i.e.9 the boiling point, and the composition of the vapor phase, y, for given values of the liquid composition, x, are presented in Tables V, VI, and VII. Also shown are the measured boiling points for the given values of the liquid composition. The RMSD value between the predicted and measured boiling points for the systems water-ethanol-l-propanol, water-ethanol-2-methyl-l-propanol, and water-ethanol-2-methyl-l-butanol are 0.23°C, 0.69°C, and 2.14°C. It seems therefore that since the NRTL equation successfully predicts temperature, the predicted values of y can be accepted confidently. 

Isobaric vapor-liquid equilibrium data at atmospheric pressure are reported for the four systems of the present investigation in Tables I-VI. Salt concentrations are reported as mole fraction salt in the solution, while mixed-solvent compositions are given on a salt-free basis. A single fixed-liquid composition was used for potassium iodide and sodium acetate potassium acetate used three—all chosen from the region of ethanol-water composition where relative volatility is highest. In the... 

Table I. Isobaric Vapor-Liquid Equilibrium Data for the Potassium Iodide—Ethanol—Water System at x — 0.309 (758 d= 3 torr)...

Because of the wide range of fixed x values for which data had been taken, it was possible to use interpolated data from Table II to construct a family of vapor-liquid equilibrium curves for the ammonium bromide-ethanol-water system at various constant salt concentration values—the condition most closely representing that existing from tray to tray in a... 

Bedrossian and Cheh (II) studied vapor-liquid equilibrium in the sodium acetate-ethanol-water system, using constant lower values of salt concentration rather than the saturation values used by Meranda and Furter (9). Analysis of their data using Equation 1 indicates a larger variation of k with x than that observed at saturation by Meranda and Furter. Bedrossian and Cheh concluded that hydration as well as hydrotropism of ions plays a major role in this particular system. 

Axial flow pumps, 134, 136, 140 applicafion range, 150 Azeotrope separation, 387,388,420-426 Azeotropic distillation, 420-426 acetonitrile/water separation, 422 commercial examples, 421-424 design method, 424 ethanol/water/benzene process, 424 n-heptane/toluene/MEK process, 424 vapor-liquid equilibrium data, 421, 423, 425,426... 

Hall, D. J. Mash, C. 1. Pemberton, R. C. Vapor liquid equilibriums for the systems water—methanol, water—ethanol, and water—methanol-ethanol at 298.15 K determined by a rapid transpiration method. NPL Report Chem. 1979, 95, 36. 

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