Azeotropic distillation pressure change

If the light and heavy key components form an azeotrope, then something more sophisticated than simple distillation is required. The first option to consider when separating an azeotrope is exploiting change in azeotropic composition with pressure. If the composition of the azeotrope is sensitive to pressure and it is possible to operate the distillation over a range of pressures without any material decomposition occurring, then this property can be used to... 

If an azeotropic mixture is to be separated by distillation, then use of pressure change to alter the azeotropic composition should be considered before use of an extraneous mass-separating agent. Avoiding the use of extraneous materials often can prevent environmental problems later in the design. 

If the azeotrope is not sensitive to changes in pressure, then an entrainer can be added to the distillation to alter in a favorable way the relative volatility of the key components. Before the separation of an azeotropic mixture using an entrainer is considered, the representation of azeotropic distillation in ternary diagrams needs to be introduced. 

An equimolar mixture of ethyl acetate and methanol is to be separated by distillation into relatively pure products. Data from Table 12.1 indicate that the mixture forms a minimum-boiling azeotrope. The data in Table 12.1 also show a significant decrease in mole fraction of ethyl acetate for the azeotrope as pressure increases. Sketch a flow scheme for the separation that exploits pressure change. 

If the system forms azeotropes, then the azeotropic mixtures can be separated by exploiting the change in azeotropic composition with pressure, or the introduction of an entrainer or membrane to change the relative volatility in a favorable way. If an entrainer is used, then efficient recycle of the entrainer material is necessary for an acceptable design. In some cases, the formation of two liquid phases can be exploited in heterogeneous azeotropic distillation. 

After recovering the acetonitrile the problem is breaking its azeotrope with water. VLE investigation shows that this is sensitive to the pressure change. For example, at 0.4 bar the azeotropic point is x(ACN) = 0.80 and T = 326 K, while at 6 bar this shifts to x(ACN) = 0.57 and T = 412 K. Consequently, pressure-swing distillation may be applied as indicated in a recent patent [15]. 

The composition of many azeotropes varies with the system pressure (Horsley, Azeotropic Data-Ill, American Chemical Society, Washington, 1983 Gmehling et ah. Azeotropic Data, VCH Publishers, Deerfield Beach, Fla., 1994). This effect can be exploited to separate azeotropic mixtures by so-called pressure-swing distillation if at some pressure the azeotrope simply disappears, such as does the ethanol-water azeotrope at pressures below 11.5 kPa. However, pressure sensitivity can still be exploited if the azeotropic composition and related distillation boundary change sufficiently over a moderate... 

If the vapor pressures of the two pure constituents are close together, then any appreciable positive deviation from Raoult s law will lead to a maximum in the total vapor pressure curve similarly, a negative deviation will, in the same circumstances, be associated with a minimum in the curve. In any event, even if the vapor pressures of the pure constituents are appreciably different, marked positive or negative deviations can lead to a maximum or a minimum, respectively, in the total vapor pressure curve. Such maxima and minima are the cause of the formation of the familiar constant boiling mixtures or azeotropic mixtures. A liquid mixture having the composition represented by a maximum or a minimum will distil without change of composition, for the proportions of the two constituents are then the same in the liquid and vapor phases. That this must be the case will be shown in the next section. 

Since the position of an azeotropic point is not stable, besides the azeotropic and extractive distillations a change of the external thermal conditions (temperature or pressure) in the form of a vacuum or pressure distillation may be effected to make the special point disappear. Schuberth [44] has reported relations by means of which the... 

In some situations, azeotropic points are sensitive to moderate changes in pressure. When this is the case, pressure-swing distillation can be used in place of azeotropic distillation to permit the recovery of two nearly pure species that are separated by a distillation boundary. This section introduces pressure-swing distillation. 

Extractive distillation and azeotropic distillation share as a common characteristic, in that an auxiliary solvent is added to the crude aromatics fraction to achieve better separation by distillation. Extractive distillation takes place in the presence of an extractive material with high solvent power for aromatics, which has relatively low volatility compared with the compounds which are to be separated, and is constantly added at the top of the fractionation column. The purpose of the auxiliary solvent is to change the vapor pressures of the hydrocarbon components in such a way that they can be more easily separated by distillation e.g. the vapor pressure of benzene is lowered to the point when the accompanying non-aromatics can be distilled off as an overhead fraction. 

At z in the curve, however (the minimum of vapour pressure), the solution and vapour are in equilibrium and the liquid at this point will distil without any change in composition. The mixture at z is said to be azeotropic or a constant boiling mixture. The composition of the azeotropic mixture does vary with pressure. 

The phenomenon is illustrated for HF and HCl in Fig. 17.5. Conversely, when more concentrated aqueous solutions are boiled, the concentration of HX in the vapour is greater than that in the liquid phase which thereby becomes progressively diluted by distillation until the azeotropic mixture is again reached, whereupon distillation continues without change of composition and at constant temperature. The bps and azeotropic compositions at atmospheric pressure are listed below, together with the densities of the azeotropic acids at 25°C ... 

Multicomponent distillations are more complicated than binary systems due primarily to the actual or potential involvement or interaction of one or more components of the multicomponent system on other components of the mixture. These interactions may be in the form of vapor-liquid equilibriums such as azeotrope formation, or chemical reaction, etc., any of which may affect the activity relations, and hence deviations from ideal relationships. For example, some systems are known to have two azeotrope combinations in the distillation column. Sometimes these, one or all, can be broken or changed in the vapor pressure relationships by addition of a third chemical or hydrocarbon. 

Azeotropes occur when x =yu as indicated in Figs, lie and d. Distillation of a mixture having the composition of an azeotrope is not possible since there is no difference in composition between vapor and liquid. Figure lie shows how the azeotrope composition is affected as the pressure is changed. 

When the composition of an azeotrope is sensitive to moderate changes in pressure, then pressure-swing distillation may be used to separate almost pure components. The first column separates pure A in top or in bottoms, if this is a minimum or maximum boiler, respectively. Then the second column makes possible the separation of pure B while recycling the A/B azeotrope. 

Px relation of Raoult s law, and the system therefore exhibits negative deviations. When the deviations become sufficiently large relative to the difference between the two pure-species vapor pressures, the Px curve exhibits a minimum, as illustrated in Fig. 12.96 for the chloroform/tetrahydrofuran system at 30°C. This figure shows that the Py curve also has a minimum at the same point. Thus at this point where x - y the dew-point and bubble-point curves are tangent to the same horizontal line. A boiling liquid of this composition produces a vapor of exactly the same composition, and the liquid therefore does not change in composition as it evaporates. No separation of such a constant-boiling solution is possible by distillation. The term azeotrope is used to describe this state. 

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