Separation extractive distillation applicability

Extractive distillation is based on the ability of an entrainer to increase selectively the relative volatility of components. The choice of entrainer is based on its chemical affinity with one of the components to be separated. Extractive distillation can be used to separate both zeotropic and azeotropic mixtures. Table 7.31 presents a list of industrial applications. 

In normal applications of extractive distillation (i.e., pinched, closeboiling, or azeotropic systems), the relative volatilities between the light and heavy key components will be unity or close to unity. Assuming an ideal vapor phase and subcritical components, the relative volatility between the light and heavy keys of the desired separation can be written as the produc t of the ratios of the pure-component vapor pressures and activity-coefficient ratios whether the solvent is present or not ... 

Extraction (discussed in Chapter 5) uses the selective adsorption of a component in a liquid to separate specific molecules from a stream. In application extraction may be coupled with its cousins, extractive distillation and azeotropic distillation, to improve extraction efficiency. Typical refinery extraction applications involve aromatics recovery (UDEX) and lubricants processing (furfural, NMP). Extractive distillation and azeotropic distillation are rarely employed in a refinery. The only... 

Activity coefficients at infinite dilution, of organic solutes in ILs have been reported in the literature during the last years very often [1,2,12,45,64, 65,106,123,144,174-189]. In most cases, a special technique based on the gas chromatographic determination of the solute retention time in a packed column filled with the IL as a stationary phase has been used [45,123,174-176,179,181-187]. An alternative method is the "dilutor technique" [64,65,106, 178,180]. A lot of y 3 (where 1 refers to the solute, i.e., the organic solvent, and 3 to the solvent, i.e., the IL) provide a useful tool for solvent selection in extractive distillation or solvent extraction processes. It is sufficient to know the separation factor of the components to be separated at infinite dilution to determine the applicability of a compound (a new IL) as a selective solvent. 

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. 

Some Available Data. A brief list of extractive distillation processes of actual or potential commercial value is in Table 13.7 the column of remarks explains why this mode of separation is adopted. The leading applications are to the separation of close-boiling aromatic, naphthenic, and aliphatic hydrocarbons and of olefins from diolefins such as butadiene and isoprene. Miscellaneous separations include propane from propylene with acrylonitrile as solvent (DuPont, U.S. Pat. 2,980,727) and ethanol from propanol with water as solvent [Fig. 13.24(b)],... 

As usual in Sandmeyer reactions, the product, if volatile, is separated by distillation in steam if non-volatile, extraction or filtration is used. The manner in which the cuprous salt reacts is not exactly known it certainly unites at first with the diazonium compound to form a double salt (c/. Reaction CLXVI.). The method is widely applicable, and as the yields are usually good, it is a standard method for the preparation of aromatic nitriles. 

Originally, extractive distillation was limited to two-component problems. However, recent developments in solvent technology enabled applications of this hybrid separation in multicomponent systems as well. An example of such application is the BTX process of the GTC Technology Corp., shown in Figure 6, in which extractive distillation replaced the conventional liquid-liquid extraction to separate aromatics from catalytic reformate or pyrolysis gasoline. This led to a ca. 25% lower capital cost and a ca. 15% decrease in energy consumption (170). Some other examples of existing and potential applications of the extractive distillations are listed in Table 6. 

A usual solution in this case is a sequence of a reactor and several separation units (Figure 5). Another way—an integrated RD process such as shown in Figure 6— allows for simultaneous formation of methyl acetate in the reaction zone, extractive distillation and product enrichment in the upper part of the column, and methanol separation in the stripping zone. The production of esters such as methyl acetate, ethyl acetate, and butyl acetate has for years been an interesting RD application. 

Application Separation of pure C4 olefins from olefinic/paraffinic C4 mixtures via extractive distillation using a selective solvent. BUTENEX is the Uhde technology to separate light olefins from various C4 feedstocks, which include ethylene cracker and FCC sources. 

A zeotropic and extractive distillations have been used through the years in the chemical industry to separate mixtures where the relative volatility of the key components is very close, or equal, to unity. Applications from the classical dehydration of alcohol with benzene (1) to more recent ones such as the propylene-propane separation (2) and aromatics recovery from hydrocarbon mixtures with N-methylpyrrolidone (3), indicate a continuous interest through the years in this area. 

Extractive distillation has been extensively used for nearly three decades in laboratory, pilot plant, and commercial plant operations. Calculation or prediction of phase equilibria for such separations has often been discussed (I, 2, 3). Some have discussed the selection of solvents for extractive distillation (4, 5). Others have discussed its recent application to particular separations (6, 7, 8). A comparison of extractive distillation, as a separation method, with azeotropic distillation and with liquid-liquid extraction has recently been discussed briefly by Gerster (9). 

The use of digital computers to carry out complete calculations in the design of separation processes has been the goal of many. To do this effectively, suitable methods for phase equilibria and tray-to-tray distillation calculations are required. Results calculated by the application of such methods to dehydrate aqueous ethanol mixtures using ethylene glycol as the extractive distillation solvent is discussed below. A brief review of the methods used for phase equilibria and enthalpies is followed by a discussion of the results from distillation calculations. These are compared for extractive distillation with corresponding results obtained by azeotropic distillation with n-pentane. 

The results obtained in the solution of a sample problem are summarized here to illustrate the application of the method. An extractive distillation problem from Oliver (6) was used in which methylcyclo-hexane is separated from toluene by adding phenol. The column contains 11 stages (including the reboiler and condenser) and has a feed of 0.4 moles/unit time of methylcyclohexane and 0.6 moles/unit time of toluene to the fourth stage from the reboiler and 4.848 moles/unit time of phenol to the fourth stage from the condenser. We used the same physical property correlations as Oliver. The activity coefficients were obtained from a multicomponent form of the Van Laar Equation (7). 

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