Partially miscible liquid distillation

Partially miscible liquid distillation, 388 Particle size... 

Figure 13.11 Temperature-composition phase diagrams of binary systems with partially-miscible liquids exhibiting (a) the ability to be separated into pure components by fractional distillation, (b) a minimum-boiling azeotrope, and (c) boiling at a lower temperature than the boiling point of either pure component. Only the one-phase areas are labeled two-phase areas are hatched in the direction of the tie lines.

Immiscible Liquids. Immiscible liquids are not an important case encountered in fractional distillation. It is much simpler to separate two liquids which are insoluble in each other by simple decantation than it is by fractionation. However, the physical-chemical laws that apply to such cases are helpful in explaining certain of the phenomena involved in the intermediate case of partially miscible liquids. 

Partially Miscible Liquids. There are a large number of systems in which the components are miscible only over limited ranges of concentration. These mixtures form a very important group for fractional distillation. The fact that the liquids are partially miscible greatly alters the normal type of vapor-liquid relationships, and the fact that two liquid phases are present requires that one less degree of freedom be available than is normal for a system of a given number of components. In general, the vapor-liquid equilibria of these systems... 

The composition of the vapour in equilibrium with a miscible liquid mixture at any temperature, e.g. on heating during distillation, will be enriched by the more volatile components. The composition of the liquid phase produced on partial condensation will be enriched by the less volatile components. Such fractionation can have implications for safety in tliat tlie flammability and relative toxicity of the mixtures can change significantly. 

As an alternative to distillation, extraetion with a eo-solvent that is poorly mis-eible with the ionie liquid has often been used. There are many solvents that can be used to extract product from the ionic liquid phase, whether from a monophase reaction or from a partially miscible system. Typical solvents are alkanes and ethers (15). Supercritical CO2 (SCCO2) was recently shown to be a potential alternative solvent for extraction of organics from ionic liquids (22). CO2 has a remarkably high solubility in ionic liquids. The SCCO2 dissolves quite well in ionic liquids to facilitate extraction, but there is no appreciable ionic liquid solubilization in the CO2 phase in the supercritical state. As a result, pure products can be recovered. For example, about 0.5 mol fraction of CO2 was dissolved at 40°C and 50 bar pressure in [BMIMJPFe, but the total volume was only swelled by 10%. Therefore, supercritical CO2 may be applied to extract a wide variety of solutes from ionic liquids, without product contamination by the ionic liquid (29). 

Apart from fractional distillation, the comparison of the three types may be looked at in another way, if the partially miscible and immiscible cases be considered too. All five cases form a series. In the case of immisc ble liquids the pressure diagram ma r be drawn a priori if at a (Fig. %o) we draw ah for the pressure of one component, and at c draw cd for the pressure of the other component all the mixtures will then have the pressure ae = a + cd. If partial mixture occurs the line heed is altered the verticals he and ed change to the gradual increase shown by he-j and and these are joined by horizontal referring to the two layers of liquid. Since the pressure of each component in the partial mixture must be less than that of the same substance by itself, the line e- e must lie below ee. 

For liquid-liquid extraction, a selective solvent is required, which shows only a partial miscibility with the liquid stream to be separated. In Figure 2, a typical multistage counter-current liquid-liquid extraction process is shown for the separation of aliphatics from aromatics. In this process, with the help of the selective solvent (extractant) the desired components (aromatics) are extracted from the feed stream. Distillation is typically used for the recovery of the selective solvent from the extract and raffinate stream leaving the extraction column. 

But since perpetual motion (distillation of a via the vapour, from one layer to the other) is impossible, it is necessary (by the Second Law of Thermodynamics) that pai = pan and hence xai — xaji That is, the molar concentration of a in both layeis is identical But this is applicable to all the molecular species present in each layer, and hence both layers are identical, and cannot therefore form two phases We must therefore conclude that the above simple vapour pressure law only holds for mixtures composed of perfectly miscible liquids and cannot apply to the cases in which partial miscibility exists The problem of liquid mixtures is in a rather rudimentary stage at present, and further discussion of it in a book of this kind must be omitted1... 

A convenient notation for classifying mixtures employed in liquid-liquid extraction is C/, where C is the number of components and the number of partially miscible pairs. Mixtures 3/1, 3/2, and 3/3 are called Type I, Type II, and Type III by some authors. A typical 3/1 three-component mixture with only one partially miscible pair is furfural-ethylene glycol-water, as shown in Fig. 3.10, where the partially miscible pair is furfural-water. In practice, furfural is used as a solvent to remove the solute, ethylene g yco, from water the furfural-rich phase is called the extract, and the water-rich phase the raffinate. Nomenclature for extraction, leaching, absorption, and adsorption always poses a problem because, unlike distillation, concentrations are expressed in many different ways mole, volume, or mass fractions mass or mole ratios and special solvent-free designations. In this chapter, we will use V to represent the extract phase and L the raffinate phase, and y and x to represent solute concentration in these phases, respectively. The use of V and L does not imply that the extract phase in extraction is conceptually analogous to the vapor phase in distillation indeed the reverse is more correct for many purposes. 

For our second nonideal system, we look at a process that has extremely nonideal VLB behavior and has a complex flowsheet. The components involved are ethanol, water, and benzene. Ethanol and water at atmospheric pressure form a minimum-boiling homogeneous azeotrope at 351K of composition 90mol% ethanol. Much more complexity is introduced by the benzene/water system, which forms two liquid phases with partial miscibility. The flowsheet contains two distillation columns and a decanter. There are two recycle streams, which create very difficult convergence problems as we will see. So this complex example is a challenging simulation case. 

If the top temperature is too cold and the bottom tenperature is too hot to allow sandwich conponents to exit at the rate they enter the column, they become trapped in the center of the column and accumulate there fKister. 20041. This accumulation can be quite large for trace conponents in the feed and can cause column flooding and development of a second liquid phase. The problem can be identified from the simulation if the engineer knows all the trace conponents that occur in the feed, accurate vapor-liquid equilibrium (VLE) correlations are available, and the simulator allows two liquid phases and one vapor phase. Unfortunately, the VLE may be very nonideal and trace conponents may not accumulate where we think they will. For example, when ethanol and water are distilled, there often are traces of heavier alcohols present. Alcohols with four or more carbons (butanol and heavier) are only partially miscible in water. They are easily stripped from a water phase (relative volatility 1), but when there is litde water present they are less volatile than ethanol. Thus, they collect somewhere in the middle of the column where they may form a second liquid phase in which the heavy alcohols have low volatility. The usual solution to this problem is to install a side withdrawal line, separate the intermediate component from the other components, and return the other components to the column. These heterogeneous systems are discussed in more detail in Chapter 8. 

Non-miscible Liquids.—Lastly, as will be shown later on, the fact that when two non-miscible liquids are distilled together, the boiling point is lower than of either component when distilled alone, may similarly be explained by the law of partial pressures, the vapour of each liquid acting like an indifferent gas towards the other. 

Evaporation into Vacuous Space.—When two volatile liquids—miscible, partially miscible or non-miscible—are placed together in a vacuous space, such as that over the mercury in a barometer tube, evaporation takes place, and, as a rule, the composition of the residual liquid differs from that of the vapour. It is only when the liquids form a mixture of maximum or minimum vapour pressure—and therefore of constant boiling point—and when it is this particular mixture that is introduced into the vacuous space, that the composition of the vapour is the same as that of the liquid. In all other cases the vapour is richer in the more volatile of the two components into which the mixture tends to separate when distilled, these components being either the original substances from which the mixture was formed, or one of these substances and a mixture of the two which has a higher or lower boiling point than that of either of the original constituents. 

Mass transport in distillation and fractionation towers can sometimes be adversely affected by the generation of unwelcome, but transient, foam, which is a product of the intrinsic properties of the relevant liquids rather than any inadvertent contaminant. Ross and coworkers have drawn attention to the role played by partial miscibility of those liquids in determining that foam behavior (see, e.g., references [134-137]). Their studies concerned both binary and ternary mixtures of low molecular weight molecules, most of which were non-aqueous. Unlike the aqueous eth-oxylated and propoxylated non-ionic surfactant and polymer systems considered in Section 4.6.3.2, these binary systems often exhibit higher critical temperatures so that miscibility occurs with increasing temperature. 

In Section 20.4.2 it was established that the mixtures of ethyl lactate with lipid type substances usually present partial hquid-liquid miscibility with upper critical solution temperature (UCST) critical point. This property could be exploited to study and develop new separation processes for the edible oil industry. In this section two particular apphcations will be reviewed the recovery of squalene from olive oil deodorizer distillates and the extraction of tocopherol from olive oil. Both case studies are based in the liquid-hquid equilibria (LLE) data presented in Section 20.4.2. 

Steam Distillation. Distillation of a Pair of Immiscible Liquids. Steam distillation is a method for the isolation and purification of substances. It is applicable to liquids which are usually regarded as completely immiscible or to liquids which are miscible to only a very limited extent. In the following discussion it will be assumed that the liquids are completely immiscible. The saturated vapours of such completely immiscible liquids follow Dalton s law of partial pressures (1801), which may be stated when two or more gases or vapoms which do not react chemically with one another are mixed at constant temperature each gas exerts the same pressure as if it alone were present and that... 

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