Extractive distillation solvent selection

Extraction and Extractive Distillation. The choice of an extraction or extractive distillation solvent depends upon its boiling point, polarity, thermal stabiUty, selectivity, aromatics capacity, and upon the feed aromatic content (see Extraction). Capacity, defined as the quantity of material that is extracted from the feed by a given quantity of solvent, must be balanced against selectivity, defined as the degree to which the solvent extracts the aromatics in the feed in preference to paraffins and other materials. Most high capacity solvents have low selectivity. The ultimate choice of solvent is deterrnined by economics. The most important extraction processes use either sulfolane or glycols as the polar extraction solvent. 

GTC Aromatics C6-C8 hydrocarbon mixture Extractive distillation with selective solvents separates BTX from nonaromatic hydrocarbons 1 1995... 

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. 

The choice of a selective solvent is easier the more the components to be separated differ in their chemical structure. It would be difficult or impossible, for instance, to hnd a selective solvent for the separation of stereoisomers. Nevertheless, the restrictions on extractive distillation solvents are less severe than those on azeotropic distillation entrainers, because the solvent recovery problem is virtually nonexistent due to the wide gap between the boiling points of the solvent and the components to be separated. 

Approaches to the selection of an extractive distillation solvent are discussed by Beig," Ewell et al.,z and Tassios.20 In general, selection criteria follow those for azeotropic enirainers ... 

Highly Nonideal Systems. Highly nonideal systems are of principal concern the nonideality may be caused by the components present and/or by the operating conditions. This category includes subjects like critical phenomena, extractive/azeotropic distillation, solvent selection guidelines, salt effects, high molecular... 

The principal direct appHcation of furfural is as a selective solvent. It is used for separating saturated from unsaturated compounds in petroleum refining, for the extractive distillation of butadiene and other hydrocarbons in the manufacture of synthetic mbber and for the production of... 

The choice of separation method to be appHed to a particular system depends largely on the phase relations that can be developed by using various separative agents. Adsorption is usually considered to be a more complex operation than is the use of selective solvents in Hquid—Hquid extraction (see Extraction, liquid-liquid), extractive distillation, or azeotropic distillation (see Distillation, azeotropic and extractive). Consequentiy, adsorption is employed when it achieves higher selectivities than those obtained with solvents. 

A selection of industrial appHcations of extractive distillation includes (/) the separation of the / -butane—butadiene azeotrope in mixed C -hydrocarbon streams using furfural [98-01-17, as the solvent (36) (2) the dehydration of ethanol using ethylene glycol [107-21-1] (37—39) (J)... 

Extractive distillation works by the exploitation of the selective solvent-induced enhancements or moderations of the liquid-phase nonidealities of the components to be separated. The solvent selectively alters the activity coefficients of the components being separated. To do this, a high concentration of solvent is necessaiy. Several features are essential ... 

Deviations from Raonlt s law in solution behavior have been attributed to many charac teristics such as molecular size and shape, but the strongest deviations appear to be due to hydrogen bonding and electron donor-acceptor interac tions. Robbins [Chem. Eng. Prog., 76(10), 58 (1980)] presented a table of these interactions. Table 15-4, that provides a qualitative guide to solvent selection for hqnid-hqnid extraction, extractive distillation, azeotropic distillation, or even solvent crystallization. The ac tivity coefficient in the liquid phase is common to all these separation processes. 

A trade-off between selectivity and solvability should be considered when selecting a solvent. Adding co-solvents can make tough separations technically and economically feasible. Lee and Gentry (1997) have tabulated dielectric constants of selected solvents. A new class of solvents is based on the use of an aqueous. solution of hydrotropes. Gaikar and Sharma (1989) have reported the separation of close-boiling p-cresol and 2,6-xylenol with aqueous solutions of hydrotropes, such as the sodium salt of p-toluene sulphonic acid, as a novel solvent in extractive distillation. 

Affinity for solute the selectivity, which is a measure of the distribution of the solute between the two solvents (concentration of solute in feed-solvent divided by the concentration in extraction-solvent). Selectivity is analogous to relative volatility in distillation. The greater the difference in solubility of the solute between the two solvents, the easier it will be to extract. 

We would like to find a solvent that breaks the azeotrope between acetone-chloroform (or moves the azeotrope point sufficiently to one side to allow separation by distillation) so that high purity acetone and chloroform can be recovered by extractive distillation. The solvent should be more selective to chloroform than acetone. The solvent, acetone and chloroform must form a totally miscible liquid. The solvent must not form azeotrope with either acetone or chloroform. The solvent should be easy to recover and recycle. The solvent should have favorable EH S properties. 

The styrene concentrate is fed to a solvent recovery process or an extractive distillation process. The solvent selectively pulls the styrene out of the hydrocarbon mixture. The styrene raffinate, sans styrene, is sent back to be mixed with the pygas (although it can also be fractionated to pull out a high quality mixed xylene.)... 

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. 

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