Extractive distillation example

Ionic liquids are a class of novel solvents with a melting point below 100°C and a negligible vapor pressure, which are interesting entrainers for extractive distillation. Examples of ionic liquids that have been investigated with respect to their potential as entrainers are l-R-3-methyl-imidazoHum-bis(trifluoromethyl-sulfonyl)-imides ([RMIM]+[CE3S02]2N-). 

Compounds related to benzene were obtained from similar plant extracts Eor example a pleasant smelling resin known as tolu balsam was obtained from the South American tolu tree In the 1840s it was discovered that distillation of tolu balsam gave a methyl derivative of benzene which not surprisingly came to be named... 

TABLE 13-19 Examples of Extractive Distillation/ Salt Extractive Distillation... 

In extractive distillation, the compounds which it is desired to separate are distilled in the presence of a solvent. This solvent, by reason of its greater affinity for one of the compounds, causes the other material present to exhibit an abnormally high vapor pressure which permits its removal from the system. For example, adding phenol to a mixture of toluene and close-boiling paraffins and naphthenes causes the toluene to behave as if it boiled as much as 40 °F above its normal boiling point. 

If the additional substance stays in the residue, the process is called extractive distillation of aqueous solutions. The separations of various alcohols, ethylene glycol, acetic acid, acetone, and nitric acid from water is an example in which extractive distillation has been used or proposed. References (108) and (126) describe processes. 

When butadiene is produced in olefins plants or in refinery crackers, they come mixed with relatively large volumes of the other C4 family. Sometimes the other C4S need not be separated from each other, for example if they are going to be used for allcylation plant feed. In that case, the butadiene can be separated from the other C4S by extractive distillation. This process uses a solvent that will preferentially dissolve butadiene, ignoring the other components in the stream. 

The usual procedures of fractional, azeotropic, or extractive distillation under inert gases, crystallization, sublimation, and column chromatography, must be carried out very carefully. For liquid, water-insoluble monomers (e.g., styrene, Example 3-1), it is recommended that phenols or amines which may be present as stabilizers, should first be removed by shaking with dilute alkali or acid, respectively the relatively high volatility of many of these kinds of stabilizers often makes it difficult to achieve their complete removal by distillation. Gaseous monomers (e.g., lower olefins, butadiene, ethylene oxide) can be purified and stored over molecular sieves in order to remove, for example, water or CO2. 

K and then recondensed at lower temperatures [173]. It was also demonstrated that there is a possibility of separating two ILs by distillation without their thermal decomposition. On the other hand, for example, BASF used ILs in the extractive distillation process at temperatures of 448 K and pressure of 5 kPa for more than three months without loss of the IL. The assumed nonvolatility of ILs had been a basis of their common reputation as "green" solvents. 

For example, the search for potential solvents for liquid extraction and extractive distillation is carried out through a group contribution molecular design of solvents (MOLDES) approach (Pretel et ak, 1994). 

Vapor Phase Absorption. Absorption is closely related to extractive distillation, in that a solvent is used for the separation of one or more constituents from a gaseous mixture. In absorption, however, the mixture to be treated is comprised of compounds having relatively large differences in volatility and condensation cannot be conveniently used. The various absorption processes differ primarily in the means used to separate product and absorber oil. A typical example of the application of vapor phase absorption in the petroleum industry is the recovery of gasoline from natural gas. 

In addition to the applications in extractive distillation referred to above, there are other industrial examples where electrolytes in mixed solvents occur. In many industrial situations nonvolatile electrolytes are either added to effect the separation of multicomponent process streams (e.g., the complexing agents added to enhance distribution coefficients in solvent extraction) or are present as a result of the process itself. Ex-... 

The use of a dissolved salt in place of a liquid component as the separating agent in extractive distillation has strong advantages in certain systems with respect to both increased separation efficiency and reduced energy requirements. A principal reason why such a technique has not undergone more intensive development or seen more than specialized industrial use is that the solution thermodynamics of salt effect in vapor-liquid equilibrium are complex, and are still not well understood. However, even small amounts of certain salts present in the liquid phase of certain systems can exert profound effects on equilibrium vapor composition, hence on relative volatility, and on azeotropic behavior. Also extractive and azeotropic distillation is not the only important application for the effects of salts on vapor-liquid equilibrium while used as examples, other potential applications of equal importance exist as well. 

Measurements of binary vapor-liquid equilibria can be expressed in terms of activity coefficients, and then correlated by the Wilson or other suitable equation. Data on all possible pairs of components can be combined to represent the vapor-liquid behavior of the complete mixture. For exploratory purposes, several rapid experimental techniques are applicable. For example, differential ebulliometry can obtain data for several systems in one laboratory day, from which infinite dilution activity coefficients can be calculated and then used to evaluate the parameters of correlating equations. Chromatography also is a well-developed rapid technique for vapor-liquid equilibrium measurement of extractive distillation systems. The low-boiling solvent is deposited on an inert carrier to serve as the adsorbent. The mathematics is known from which the relative volatility of a pair of substances can be calculated from the effluent trace of the elutriated stream. Some of the literature of these two techniques is cited by Walas (1985, pp. 216-217). 

Extractive Distillation Recovery of Isoprene. A typical flowsketch and material balance of distillation and solvent recovery towers for extracting isoprene from a mixture of cracked products with aqueous acetonitrile appears in Figure 13.26. A description of the flowsheet of a complete plant is given in Example 2.10. In spite of the fact that several trays for washing by reflux are provided, some volatilization of solvent still occurs so that the complete plant... 

TABLE 13.7. Examples of Extractive Distillation Processes for the Separation of Ideal, Nonideal, end Azeotropic Systems... 

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