Mixtures liquid + liquid equilibria with

LPG (liquefied petroleum gas) Petroleum gas stored or proeessed as a liquid in equilibrium with vapour by refrigeration or pressurization. The two LPGs in general use are eommereial propane and eommereial butane supplied to produet speeifieations, e.g. BS 4250. (These, or mixtures thereof, eomprise LPG for the purpose of the Highly Flammable Liquids and Liquefied Petroleum Gas Regulations 1972.)... 

Thus, if a gas mixture exerts 100 psia total pressure and is composed of 20% by volume (mol%) propane and 80% by volume butane, the partial pressures are 20 and 80 psia for propane and butane, respectively. The liquid in equilibrium with this mixture of vapors would have a lower percentage of propane and a higher percentage of butane. If this mixture is used as a refrigerant, the low-boiling component (propane) reaches equilibrium with a higher concentration in the condenser (as liquid) and increases the total pressure in the condenser. This requires more head and more horsepower at the compressor. 

Most measurements of densities of liquids below their normal boiling points are made in the presence of air. Densities reported here refer to liquids in equilibrium with a gas phase consisting of a mixture or air and vapor at a total pressure of one atmosphere below the normal boiling point and of vapor at the equilibrium vapor pressure above the boiling point. Thus air is not regarded as an impurity. 

Although the third component of these systems is usually a single inorganic salt, mixtures of two or more salts have been studied, and some research has been done with third components of low vapor pressure (18,19). Some qualitative studies have been done on salt effect in vapor-liquid equilibrium with salts which are either soluble in only one or both components, hygroscopic or non-hygro-scopic, etc. 

Phase equilibrium resulting in a UCST is the most common type of binary (liquid + liquid) equilibrium, but other types are also observed. For example, Figure 14.5 shows the (liquid + liquid) phase diagram for (xiH20 + jc2(C3H7)2NH. 7 A lower critical solution temperature (LCST) occurs in this system/ That is, at temperatures below the LCST, the liquids are totally miscible, but with heating, the mixture separates into two phases. 

If we calculate a liquid-liquid equilibrium for a cubic lattice mixture of molecules we find that phase-separation occurs, starting at 12/kT = 0.42 and we find a liquid-liquid phase boundary at x = 0.02 with 12/kT = 0.7. This explains why we... 

Temperature-composition phase diagrams for mixtures of solids and the liquids in equilibrium with them are very important in metallurgy and electronics. Figure 4.8 shows a simplified phase diagram for two phases that have a limited solubility for each other. 

In general, gas-air or vapour-air mixtures need a certain concentration (or partial pressure) of the burnable substance to form an explosive mixture. This concentration range is limited by LEL and UEL. If there is a liquid in equilibrium with air, the same is valid for its vapour. 

The reason why mixes with AR > 1.7 do not yield any CjjA, on independent crystallization is that the solid phases are not pure CjA, QAF and CjS. For AR = 2.71, the quaternary liquid in equilibrium with C,S, C S and CjA at 1400X contains 55.7% CaO, 27.1% AljOj, 10.0% FcjOj and 7.2% SiOj (S8). This composition can be closely matched by a mixture of aluminate (63%), ferrite (30%) and belite (7%) with the normal compositions given in Table 1.2, the bulk composition of this mixture being 54.4% CaO, 26.4% AI2O3, 9.7% Fe Oj, 5.6% SiO and 1.8% MgO, with <1% each of TiOj, Mn20j, NajO and KjO. Independent crystallization can thus yield a mixture of the three phases. The liquid composition cannot be matched by a mixture of pure CjA, C AF and CjS, which is relatively too high in CaO, so that if no ionic substitutions occurred, some C,2A7 would also be formed. A strict comparison would be with the actual composition of the clinker liquid, which is modified by minor components, but lack of adequate data precludes this. 

Activity and Activity Coefficient. —When a pure liquid or a mixture is in equilibrium with its vapor, the chemical potential of any constituent in the liquid must be equal to that in the vapor this is a consequence of the thermodynamic requirement that for a system at equilibrium a small change at constant temperature and pressure shall not be accompanied by any change of free energy, i.e., (d( )r. p is zero. It follows, therefore, that if the vapor can be regarded as behaving ideally, the chemical potential of the constituent i of a solution can be written in the same form as equation (7), where p,- is now the partial pressure of the component in the vapor in equilibrium with the solution. If the vapor is not ideal, the partial pressure should be replaced by an ideal pressure, or fugacity, but this correction need not be considered further. According to Raoult s... 

Calculate the temperature and composition of a liquid in equilibrium with a gas mixture containing lO.O mole% benzene, 10.0 mole% toluene, and the balance nitrogen (which may be considered noncondensable) at 1 atm. Is the calculated temperature a bubble-point or dewpoint temperature ... 

The dew point of a vapor mixture of A and B at 7 can be determined from the Txy diagram if there are no species other than A and B in the gas phase. Look up the specified mole fraction of A in the vapor phase, read the dew-point temperature from the corresponding ordinate value of the vapor curve, and move horizontally to the liquid curve and down to read the composition of the liquid in equilibrium with the vapor. If a noncondensable species is present in the gas phase, however, you must use Equation 6.4-6 to find the dew point, as in the previous example. 

At 100°C cyclohexane has a partial pressure of433 mm and toluene a partial pressure of 327 mm the sum of the partial pressures is 760 mm and so the liquid boils. If some of the liquid in equilibrium with this boiling mixture were condensed and analyzed, it would be found to be 433/760 or 57 mole percent cyclohexane (pointB, Fig. 2). This is the best separation that can be achieved on simple distillation of this mixture. As the simple distillation proceeds, the boiling point of the mixture moves toward 110°C along the line from A, and the vapor composition becomes richer in toluene as it moves from B to 110°C. In order to obtain pure cyclohexane, it would be necessary to condense the liquid at B and redistill it. When this is done it is found that the liquid boils at 90°C (point C) and the vapor equilibrium with this liquid is about 85 mole percent cyclohexane (point D). So to separate a mixture of cyclohexane and toluene, a series of fractions would be collected and each of these partially redistilled. If this fractional distillation were done enough times the two components could be separated. 

The above conclusions have resulted from an analysis of computer simulation data carried out on pure liquids and supercritical fluids, and on liquids in equilibrium with their vapor. One immediate question one should ask concerns thus a more general validity of the reached conclusions. Particularly important problem is to what extent they may remain valid for mixtures. Due to polarizability and other possible effects brought about by electrostatic interactions between unlike species, the pair interaction, and hence the local and, particularly, orientational arrangement may be changed considerably. With respect to a wide variety of mixtures this problem will require rather an extensive investigation. The most difficult mixtures will evidently be solutions of charged objects as e.g. electrolytes. 

The condition for equilibrium between vapor and liquid phases, expressed by Equation 1.19 as the equality of each component fugacity in the two phases, applies to equilibrium between two liquid phases or any number of phases such as two liquids and a vapor. When the deviations from ideality are large enough, mixtures can form two immiscible liquids at equilibrium with each other. It is easy to see that an ideal solution cannot form two liquid phases at equilibrium. In order for this to occur, the condition = (p,%)P, where a and P designate each liquid phase, must be satis-... 

It is required to recover 93% of component E from a binary mixture of components E and R by treating with a solvent in a single-stage extractor. What is the required solvent rate per 100 kmol of feed, and what are the product rates and compositions Assume the liquid-liquid equilibrium... 

A similar procedure is used to determine the dew point of a vapor mixture and the composition of the liquid in equilibrium with this mixture. 

Liquid-liquid equilibrium occurs when the species in a mixture are dissimilar. The most common situation is the one in which the species are of a different chemical nature. Such mixtures are best described by activity coefficient models, and that is the case considered in Illustration 11.2-3. However, liquid-liquid equilibrium may-also occur when the two species are of similar chemical nature but differ greatly in size, as in the methane-/t-heptane system, or when the species differ in both size and chemical nature, as in the carbon dioxide-n-octane system shown in Fig. 11.2-36. Since both carbon dioxide and n-octane can be described by simple equations of state, liquid-liquid equilibrium in this system can be predicted or correlated using equations of state, though with some difficulty. 

Note that in this way the complete liquid-liquid equilibrium curve and the tie lines have been obtained with very few chemical analyses. In particular, the binodal curve was obtained gravimetrically, which is generally more accurate than chemical analysis, and the lie lines were obtained for only a few mixtures, and then only by analyzing for the solute (here acetone), and not for all three components in the mixture. 

Example 15.5. The separation of benzene B from n-heptane H by ordinary distillation is difficult. At atmospheric pressure, the boiling points differ by 18.3°C. However, because of liquid-phase nonideality, the relative volatility decreases to a value less than 1.15 at high benzene concentrations. An alternative method of separation is liquid-liquid extraction with a mixture of dimethylformamide (DMF) and water. The solvent is much more selective for benzene than for n-heptane at 20°C. For two different solvent compositions, calculate interstage flow rates and compositions by the rigorous ISR method for the countercurrent liquid-liquid extraction cascade, which contains five equilibrium stages and is shown schematically in Fig. 15.22. 

The column is best pressured up with one (or more) of the condensable components of the feed mixture (Fig. 12.2a), or a gas of similar volatility, rather than with an inert gas (i.e., one which is much more volatile than the feed mixtures). Pressuring up with an inert gas will cause colder temperatures and may provide insufficient protection against metal overchilling (Fig. 12.2c). Liquid continues to flash (and chill) until boiling liquid reaches equilibrium with condensing vapor. When the column contains an inert gas, equilibrium will only be reached when the partial pressure of the evaporated liquid builds up sufficiently in the vapor space. Until then, flashing temperatures will be much lower than the feed boiling point at the column pressure (compare Fig. 12.2a and c). 

Since the sodium chloride solubility at an antisolvent fraction of 0.9 is practically zero, the binary DiPA-H20 liquid-liquid equilibrium data (see Figure 5) can be used to determine the aqueous and the organic phase compositions for the recovery of DiPA from a mixture with Xchpa = 0.9. 

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