Close boiling mixtures, separation

Peivaporation is a relatively new process that has elements in common with reverse osmosis and gas separation. In peivaporation, a liquid mixture contacts one side of a membrane, and the driving force for the process is low vapour pressure on the permeate side of the membrane generated by cooling and condensing the permeate vapour. The attraction of peivaporation is that the separation obtained is proportional to the rate of permeation of the components of the liquid mixture through the selective membrane. Therefore, peivaporation offers the possibility of separating closely boiling mixtures or azeotropes that are difficult to separate by distillation... 

The first application of pervaporation was the removal of water from an azeotropic mixture of water and ethanol. By definition, the evaporative separation term /3evap for an azeotropic mixture is 1 because, at the azeotropic concentration, the vapor and the liquid phases have the same composition. Thus, the 200- to 500-fold separation achieved by pervaporation membranes in ethanol dehydration is due entirely to the selectivity of the membrane, which is much more permeable to water than to ethanol. This ability to achieve a large separation where distillation fails is why pervaporation is also being considered for the separation of aromatic/aliphatic mixtures in oil refinery applications. The evaporation separation term in these closely boiling mixtures is again close to 1, but a substantial separation is achieved due to the greater permeability of the membrane to the aromatic components. 

Figure 10.2 shows typical distillate composition profiles for close boiling mixtures using a solvent in a CBD column. The CBD process becomes a conventional BED process with the addition of the solvent. Due to the addition of solvent, the components can be separated at high purity using a small column with low reflux ratio. See Safrit and Westerberg (1997) and Low and Sorensen (2002) for unconventional BED processes. 

In liquid-liquid extraction (Fig. 3.6) two miscible solutes are separated by a solvent, which preferentially dissolves one of them. Close-boiling mixtures that cannot withstand the temperature of vaporization, even under vacuum, may often be separated by this technique. Like other contact processes the solvent and the mixture of solutes must be brought into good contact to permit transfer of material and then separated. The extraction method utilizes differences in the solubility of the components in the solvent. 

The extrapolation is to what is called pervaporation, where the feed mixture is a liquid, but the permeate vaporizes during permeation, induced by the relatively low pressure maintained on the permeate side of the membrane. Accordingly, the reject or retentate remains a liquid, but the permeate is a vapor. Thus, there are features of gas permeation as well as hquid permeation. The process is eminently apphcable to the separation of organics and to the separation of organics and water (e.g., ethanol and water). In the latter case, either water vapor may be the permeate, as in dehydration, or the organic vapor may be the permeate. The obvious, potential application is to the separation of azeotropic mixtures and close-boiling mixtures—as an alternative or adjunct to distillation or liquid-liquid extraction methods. 

T xtractive and azeotropic distillation in different types of chemical industry has become more important as more separations of close-boiling mixtures and azeotropic ones are encountered. Extractive distillation is used more because it is generally less expensive, simpler, and can use more solvents than azeotropic distillation. Solvent selection for azeotropic distillation has recently been discussed by Berg (I). Therefore, solvent screening for extractive distillation is discussed here. 

Azeotropes, discussed in Chapter 1, form constant boiling mixtures and are, therefore, impossible to separate unless the azeotrope is broken by the addition of an external agent, using either extractive or azeotropic distillation as in the case of close boiling mixtures. 

Membranes can be used to separate molecules that differ in size, polarity, ionic character, hydrophilicity, and hy-drophobicity.100 Their use is less energy-intensive than distillation. They can often separate azeotropes and close-boiling mixtures. They can sometimes replace traditional methods, such as solvent extraction, precipitation, and chromatography, that can be inefficient, expensive, or may result in the loss of substantial amounts of product. Thermally and chemically sensitive molecules can be handled. Membranes can be porous or nonporous, solid or liquid, organic or inorganic. 

This term usually is applied to cases where an extraneous material, called an entrainer, is added to a mixture to make a distillation separation feasible. In this way, a problem involving a close-boiling mixture is made tractable by the addition of an enltainer that will azeotrope with one of the components to give, in effect, a respectable relative volatility between the nonazeotropiag component and the azeotrope (treated as a pseudocomponent). Typically, the azeotrope has the higher volatility and becomes the distillate product. 

Applications of azeotropiaand extractive distillation have continued to expand because many very close Boiling mixtures may be separated economically by use of these techniques. The slparation of such mixtures by conventional distillation methods is usually uneconomical because of the large number of stages which would be required to effect such separations. 

As advantages of this integration, chemical equilibrium limitations can be overcome, higher selectivities can be achieved, the heat of reaction can be used in situ for distillation, auxUiary solvents can be avoided, and azeotropic or closely boiling mixtures can be more easily separated than in non-RD. Increased process efficiency and reduction of investment and operational costs are the direct results of this approach. Some of these advantages are realized by using reaction to improve separation others are realized by using separation to improve reaction. 

Distillation combined with reaction has been successfully used for separating close boiling mixtures. When used in this separation mode, the technique is frequently referred to as dissociation-extractive distillation. It can also be used in the reaction mode by continuous separation of the reaction products from the reactants. The equipment used in the latter case is often referred to as a distillation column reactor (DCR). The chief advantage of this method is that the reactants can be used in stoichiometric quantities, with attendant elimination of recycling costs. 

Pervaporation is a contraction of the terms permeation and evaporation because the feed is a liquid, and vapor exits the membrane on the permeate side. Pervaporation is a membrane process for liquid separation, and today, it is considered as a basic unit operation for the separation of organic-organic liquid mixtures because of its efficiency in separating azeotropic and close-boiling mixtures, isomers, and heat-sensitive compounds. It allows separations of some mixtures that are difficult to separate by distillation, extraction, and sorption. Pervaporation is one such type of membrane separation process with a wide range of uses such as solvent dehydration and separation of organic mixtures. When a membrane is in contact with a liquid mixture, one of the components can be preferentially removed from the mixture due to its higher affinity and quicker diffusivity in the membrane. 

Estimate the exergy efficiency of the distillation column for propylene—propane mixture, which is a close-boiling mixture. The table below shows the enthalpy and entropies of the saturated feed and saturated products from the simulation results with the Redlich—Soave equation of state for the separation of propylene and propane (Seider et al. (2004). 

Terrill, D. L., Sylvestre, L. F., Doherty, M. R (1985). Separation of closely boiling mixtures by reactive distillation. 1. Theory. Industrial Engineering Chemistry Process Design and Development, 24, 1062—1071. 

Figure VI - 28. Schematic drawing of a hybrid distillaiion/pervaporation process for the - separation of close boiling mixtures.

Another application of an entrainer addition is for the separation of a close-boiling mixture using heterogeneous azeotropic distillation. One common industrial example is the separation of acetic acid and water with an ester as the entrainer. The reason for adding the... 

Many experiments have demonstrated the ability of Ag+-exchanged Nafion membranes to separate unsaturated hydrocarbons (alkenes and arenes) from saturated organics. This ability is potentially useful in the context of hybrid distillation-membrane processes and can be further demonstrated by performing separations of bi-component mixtures containing similar boiling points, i.e with relative volatilities close to 1. Table I is a compilation of several representative close boiling mixtures which would be difficult and expensive to separate by distillation alone. Each mixture consists of one alkane and one alkene. Therefore, the separation of the mixtures should be achievable based on their difference in reactivity with Ag+. 

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