Simple distillation liquid-vapor equilibrium

Simple distillation Partial (vapor product) condenser Partial (liquid product) reboiler 14 equilibrium stages Feed to stage 6 Properties ... 

Whereas liquid separation method selection is clearly biased toward simple distillation, no such dominant method exists for gas separation. Several methods can often compete favorably. Moreover, the appropriateness of a given method depends to a large extent on specific process requirements, such as the quantity and extent of the desired separation. The situation contrasts markedly with liquid mixtures in which the applicability of the predominant distillation-based separation methods is relatively insensitive to scale or purity requirements. The lack of convenient problem representation techniques is another complication. Many of the gas—vapor separation methods are kinetically controlled and do not lend themselves to graphical-phase equilibrium representations. In addition, many of these methods require the use of some type of mass separation agent and performance varies widely depending on the particular MSA chosen. 

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... 

The vapor-liquid equilibrium for the nitric acid / water system at atmospheric pressure is shown in Figure 9.4. This figure shows that a concentration of 68.4 weight % nitric acid is the maximum (i.e., the azeotropic point) that can be obtained by simple distillation of the weak acid220. 

In distillation operations, separation results from differences in vapor-and liquid-phase compositions arising from the partial vaporization of a liquid mixture or the partial condensation of a vapor mixture. The vapor phase becomes enriched in the more volatile components while the liquid phase is depleted of those same components. In many situations, however, the change in composition between the vapor and liquid phases in equilibrium becomes small (so-called pinched condition), and a large number of successive partial vaporizations and partial condensations are required to achieve the desired separation. Alternatively, the vapor and liquid phases may have identical compositions, because of the formation of an azeotrope, and no separation by simple distillation is possible. 

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. 

It was realized that in the water/acetic acid system, in the range of low acid concentrations, the vapor/liquid equilibrium line very nearly coincides with the diagonal, so that obtaining pure water by simple distillation would require a monstrous number of stages (in the order of 150) as well as a huge reflux ratio (in the order of 9). In view of this situation, various considerations have led the BASF to choose azeotropic distillation using butyl acetate as entrainer. 

Figure 14-12 A boiling point diagram for a solution of two volatile liquids, A and B. The lower curve represents the boihng point of a liquid mixture with the indicated composition. The upper curve represents the composition of the vapor in equilibrium with the boiling liquid mixmre at the indicated temperature. Pure liquid A boUs at a lower temperamre than pure hquid B hence, A is the more volatile liquid in this illustration. Suppose we begin with an ideal equimolar mixmre = Xg = 0.5) of liquids A and B. The point P represents the temperature at which this solution boils, Tj. The vapor that is present at this equilibrium is indicated by point Q (X = 0.8). Condensation of that vapor at temperature Ti gives a liquid of the same composition (point E). At this point we have described one step of simple distillation. The boiling liquid at point if is in equilibrium with the vapor of composition indicated by point S (X > 0.95), and so on.

An azeotrope is a mixture of two or more components that, when brought to boiling, issues a vapor with the same composition as the liquid. Hence, separation by simple distillation is not possible. Binary systems containing azeotropes have y-x equilibrium curves as shown in Figure 12.6. On either side of the azeotropic composition, separation by simple distillation is possible. 

In the simple differential distillation process, the vapor product is in equilibrium with the liquid in the reboiler at any given time but changes continuously in composition. The mathematical approach must therefore be differential. Assume that at any time during the course of the distillation there are L moles of liquid in the still of composition x mole fraction of A and that an amount dD mole of distillate is vaporized, of mole fraction y in equilibrium with the liquid. Then we have the following differential material balances ... 

A mixture of 40 mole% isopropanol in water is to be distilled at 1 atm by a simple batch distillation until 70% of the charge (on a molal basis) has been vaporized (equilibrium data are given in Problem 8.36 What will be the compositions of the liquid residue remaining in the still pot and of the collected distillate ... 

The appearance of azeotropic points has important consequences for the distillation of the mixtures concerned. First let us consider a system with a boiling point maximum (Fig. 14.23). A liquid mixture having the composition x boils at temperature Ti and its corresponding vapor is enriched by the more volatile component B (xf). If the vapor is removed continuously from equilibrium by simple distillation, meaning by condensation in a receiver, the composition of the... 

A very full bag of distillation dynamic simulation techniques has been developed and demonstrated in this chapter. The example considered is a simple binary ideal vapor-liquid equilibrium (VLB) column. As the remaining chapters in this book demonstrate, these techniques can be readily extended to much more complex flowsheets and phase equilibrium. 

Distillation is a method of separation based on the difference in composition between a liquid mixture and the vapor formed from it. The composition difference is due to differing effective vapor pressures, or volatilities, of the components of the liquid. When such a difference does not exist, as at an azeotropic point, separation by distillation is not possible. The most elementary form of the method is simple distillation in which the liquid mixture is brought to boiling and the vapor formed is separated and condensed to form a product if the process is continuous, it is called fladt distillation or an equilibrium flash, and if the feed mixture is available as an isolated batch of material, the process is a form of batch distillation and the compositions of the collected vapor and residual liquid are thus time dependent. 

Fractional distillation is used when a more efficient separation process than simple distillation is required. This type of distillation is an equilibrium process, in which the composition of the distillate is constantly changing as the distillation proceeds and is changing along the distillation column toward the outlet. The main element of the apparatus is the distillation column consisting of a series of plates located one over the other in a suitable tube that is placed under the receiver. Liquid evaporating from one (lower) plate condenses on the other (higher) plate, where the evaporation process is repeated. In each plate equilibrium between the liquid and the vapor is established. 

More than one steady state for the same set of specified variables (output multiplicity) is one of the interesting features of azeotropic distillation. Simple distillation columns with ideal vapor-liquid equilibrium, however, may also show MSS (Jacobsen and Skogestad, 1991). The existence of output multiplicities in distillation were first reported on the ternary ethanol-water-benzene (EWB) system. Earlier simulation-based studies had reported two distinct steady states depending on the starting guesses (Bekiaris et al.. 

Many mixtures of chemical components form azeotropes. An azeotropic mixture has vapor and liquid phases that have identical compositions. The occurrence of this phenomenon means that simple distillation cannot be used to achieve complete separation because distillation relies on differences in compositions between liquid and vapor phases. Azeotropes occur because of nonideal phase equilibrium resulting from the molecular interaction (either repulsion or attraction) of dissimilar chemical components. 

In this section, we will consider only solutions in which the liquid component has the majority mole fraction (the solvent) and the solid component has the minority mole fraction (the solute). We will also assume that the solid solute is non-ionic, because the presence of oppositely charged ions in solution affects the properties of the solution (which will be considered in the next chapter). There is also a consideration that is implicit in specifying a solid component It contributes nothing to the vapor phase that is in equilibrium with the solution. One way of speaking of this is to state that the solid is a nonvolatile component. Solutions of this sort are therefore easy to separate by simple distillation of the only volatile component, the solvent, rather than the more complicated fractional distillation. Figure 7.20 shows two experimental setups for simple distillation. Compare these to Figure 7.9. 

Simple Distillation. Distillation without rectification can be carried out by several methods. The two most generally considered cases are (1) continuous simple distillation and (2) differential distiUor tion. In continuous distillation, a portion of the liquid is vaporized under conditions such that all the vapor produced is in equilibrium with the unvaporized liquid. In differential vaporization, the liquid is vaporized progressively, and each increment of vapor is removed from contact with the liquid as it is formed and, although each increment of vapor can be in equilibrium with the liquid as it is formed, the average composition of all of the vapor produced will not be in equilibrium with the remaining liquid. 

Continuous Simple Distillation. Distillations that approximate this type are usually carried out on a continuous basis such that the liquid feed is added continuously to a well-mixed still in which a definite fraction is vaporized and removed and the excess unvaporized liquid is withdrawn from the still. An alternate arrangement is to preheat the feed and add it to a flash or disengaging section where vapor and liquid are separated and removed without additional heat requirements. In either case, assuming that the vapor and liquid leaving are in equilibrium with each other, the two fractions are related to each other by equilibrium constants and material balances. Thus for each component in a mixture the following material balance can be written ... 

In this simple distillation process, it is assumed that the vapor formed within a short period is in thermodynamic equilibrium with the liquid. Hence, the vapor composition xp is related to the hquid composition xb by an equiUbrium relation of the form xp = fixs)- The exact relationship for a particular mixture may be obtained from a thermodynamic analysis depending on temperature and pressure. For a system following the ideal behavior given by Raoult s law, the equilibrium relationship between the vapor composition y (or xp) and liquid composition x (or xb) of the more volatile component in a binary mixture can be approximated using the concept of constant relative volatility a), and is given by ... 

As stated earlier, distillation is a widely used separation technique for liquid mixtures or solutions. The formation of these mixtures is straightforward, and is usually spontaneous, but the separation of a mixture into its separate constituents requires energy. One of the simplest distillation operations is flash distillation. In this process, part of the feed stream vaporizes in a flash chamber, and the vapor-liquid mixture, which is at equilibrium, is separated. The vapor is rich in the more volatile component, but complete separation is usually not achieved. A simple schematic showing the necessary equipment for flash distillation is given in Figure 10.3. We will illustrate the concepts by using a simple case of the flash distillation of a binary mixture. 

A system with liquid recycle would naturally occur when the vapor-liquid equilibrium is such that a simple flash drum cannot be used and a distillation column (or... 

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