Entrainers in Extractive Distillation

The VLB was also measured for binary and ternary systems of [ethanol + [C2Cilm][C2S04] and [ethanol + ethyl ferf-butyl ether + [C2Cilm][C2S04] at 101.3 kPa [151]. This ternary system does not exhibit a ternary azeotrope. The possibility of [C2Cilm][C2S04] use as a solvenf in liquid-liquid extraction or as an entrainer in extractive distillation for fhe separation of the mixture ethanol/ethyl fcrf-butyl ether was discussed [151]. 

A major highlight is the solubility of cellulose in ionic liquids, which allows functionalisation to give materials suitable for a diverse range of applications, including novel applications such as materials for sensors and specialised composites. Their use as entrainers in extractive distillation is also very promising. 

THE DETERMINATION OF ACTIVITY COEFFICIENTS AT INFINITE DILUTION FOR THE SELECTION OF ENTRAINERS IN EXTRACTIVE DISTILLATION... 

Extractive Distillation. In extractive distillation a fraction comprising compounds of similar volatility is vaporized and passed countercurrent to a liquid solvent stream in a packed or bubble cap tower. The operating conditions of temperature and pressure are regulated so that one or more of the components of the mixture are dissolved in the entrainer and removed in a liquid phase extract, while the remaining vapor is taken overhead and condensed or discharged as gaseous effluent. 

Future advances in extractive distillation may well follow the same trend as those in azeotropic distillation—that is. where the products can justify the higher processing cost, efficient entrainers will be developed and utilized in commercial operations. 

Figure 3.13 presents suitable RCMs [5]. A and B but not both must be saddles, except in extractive distillation. Two columns are sufficient, either as a direct or as indirect sequence. Entrainer and mixture can be merged in the feed, except the extractive distillation, where the entrainer goes on the top. As an example we cite the separation of acetone from its azeotrope with heptane by using benzene. Contrary to expectations, the indirect sequence has better indices of investment and energy consumption. 

Extractive distillation is a suitable distillation process for the separation of azeotropic systems or systems with separation factors tti2 close to unity. A typical extractive distillation process for the separation of aliphatics firom aromatics is shown in Figure 1. In extractive distillation processes, the high boiling selective solvent (entrainer), introduced not far from the top of the extractive distillation column, has to alter the volatilities in such a way that the separation factor attains a value very different from unity. Typical entrainers for the separation of aliphatics from aromatics are Ai-Methyl-pyrrolidone (NMP) or //-Formylmorpholine (NFM). In the presence of NMP or NFM,... 

In extractive distillation a solvent is utilized that preferentially alters the relative volatilities of the components to be separated. In contrast to the entrainer in azeotropic distillation, the solvent in extractive distillation has a considerably higher boiling point than either component in the feed and is therefore always withdrawn in the column bottoms. 

A third component, called an entrainer in azeotropic distillation and a solvent in extractive distillation, is added to increase the difference in volatility between the key components. 

Sweet Chili Miscella - Concentrates were prepared at reduced pressure only. Initial attempts to use the miscella as obtained from the supplier were unsuccessful. When the miscella was combined with distilled water and the mixture was heated with stirring, an intractable gel formed. Subsequently, a hexane fraction was first distilled from the starting material at reduced pressure. Pepper volatiles entrained in this distillate were later reclaimed by careful fractional re-distlllatlon of the hexane fraction. The residue from this fractional distillation was combined with the heptane-extracted material obtained with the steam distillation-extraction head. 

In extractive distillation a solvent is added to the mixture to be separated, its boiling point is higher than that of the components of the mixture. In the case of a binary mixture, the added solvent must interact more strongly with one of the components to lower its volatility. The other more volatile component can thus be distilled off, leaving the added solvent and the higher-boiling component at the bottom of the column. The added solvent (entrainer) must be miscible with the mixture at all temperatures, concentrations, and pressures. 

Answer the same question as in item 3 for a four-component mixture, in the feed point of which K3 > K2> K4 > Ki (in extractive distillation column, the top product is component 3 and the entrainer is mixture 1,4). 

The entrainer flow rate influences expenditures for separation not only in extractive distillation column itself, but also in the column of the entrainer recovery. In the case of separation of a multicomponent azeotropic mixture in an autoextractive distillation column (see Chapter 8), the intermediate columns can be located between this column and the column of autoentrainer recovery. In this case, the flow rate of the entrainer also influences expenditures for separation in the intermediate columns. In connection with the aforesaid, the necessity arises to carry... 

The separation of close boiling components by ordinary fractionation may require too many stages or could be practically impossible because of the proximity of the distribution coefficients (7T-values). The compositions of equilibrium vapor and liquid phases in such systems are almost identical. In general, close boiling components are likely to have different chemical structures, and would therefore interact differently with a third component. In extractive distillation, a solvent that is less volatile than the feed components is added to the mixture for the purpose of preferentially depressing the volatility of one of the feed components. In azeotropic distillation, the added component, or entrainer, forms an azeotrope with one of the components to be separated. The azeotrope may have either a higher or lower boiling temperature than the other component and may leave the separation device either in the bottoms or overhead product. 

Extractive distillation usually is preferred over azeotropic distillation, if both methods can be used to separate the feed components. The bulk of the solvent in extractive distillation is not vaporized in each cycle, as compared to azeotropic distillation where the entrainer is recovered from the overhead vapor stream. The energy input necessary to effect separation usually is lower for extractive distillation than for azeotropic distillation. Also, an extractive distillation column can operate over a wider range of pressures than an azeotropic distillation column, because the azeotropic composition is a function of pressure. Finally, there usually is a wider choice of solvents than entrainers, thus enabling the designer to minimize added component cost. 

Lladosa, E. Monton, J. B. Burguet, M. C. Munoz, R. Phase equilibria involved in extractive distillation of dipropyl ether + 1-propyl alcohol using 2-ethoxyethanol as entrainer. Fluid Phase Equilib. 2007, 255, 62-69. 

Arce, A. Martinez-Ageitos, J. Rodil, E. Soto, A. Phase equihbria involved in extractive distillation of 2-methoxy-2-methylpropane + methanol using 1-butanol as entrainer. Fluid Phase EquiUb. 2000, 171, 207-218. 

Acetonitrile serves to greatly enlarge the spread of relative volatilities so that reasonably sized distillation equipment can be used to separate butadiene from the other components in the C4 fraction. The polar ACN acts as a very heavy component and is separated from the product without much difficulty.The feed stream is carefully hydrogenated to reduce the acetylene level rerun, and then fed to the single stage extractive distillation unit. Feed enters near the middle of the extractive distillation tower, while (lean) aqueous ACN is added near but not at the top. Butenes and butanes go overhead as distillate, with some being refluxed to the tower and the rest water washed for removal of entrained ACN. 

One final point regarding extractive distillation is illustrated in Figure 12.28. The order in which the separation occurs depends on the change in relative volatility between the two components to be separated. Figure 12.28 shows both the residue curves and the equi-volatility curve for the system A-B-entrainer. This equi-volatility curve shows where the relative volatility between Components A and B is unity. On either side of the equi-volatility curve, the order of volatility of A and B changes. In Figure 12.28a, if the equi-volatility curve intersects the A-entrainer axis, then Component A should be recovered first. However, if the equi-volatility curve intersects the B-entrainer axis,... 

E Activation energy of reaction (kJ kmol 3), or entrainer flowrate in azeotropic and extractive distillation (kg-s, kmol s-1), or extract flowrate in liquid-liquid extraction (kg s-1, kmol-s-1), or stage efficiency in separation (-)... 

Two important extractive distillation processes were placed in commercial operation during World War II the recovery of butadiene from a C4 fraction using furfural as the entrainer (7, 22) and the segregation of toluene from petroleum fractions by means of phenol (14-16). 

Data of Azeotropes. The choice of azeotropic entrainer for a desired separation is much more restricted than that of solvents for extractive distillation, although many azeotropic data are known. The most extensive compilation is that of Ogorodnikov, Lesteva, and Kogan (Handbook of Azeotropic Mixtures (in Russian), 1971). It contains data of 21,069 systems, of which 1274 are ternary, 60 multicomponent, and the rest binary. Another compilation Handbook of Chemistry and Physics, 60th ed., CRC Press, Boca Raton, FL, 1979) has data of 685 binary and 119 ternary azeotropes. Shorter lists with grouping according to the major substances also are available in Lange s Handbook of Chemistry... 

Process synthesis and design of these non-conventional distillation processes proceed in two steps. The first step—process synthesis—is the selection of one or more candidate entrainers along with the computation of thermodynamic properties like residue curve maps that help assess many column features such as the adequate column configuration and the corresponding product cuts sequence. The second step—process design—involves the search for optimal values of batch distillation parameters such as the entrainer amount, reflux ratio, boiler duty and number of stages. The complexity of the second step depends on the solutions obtained at the previous level, because efficiency in azeotropic and extractive distillation is largely determined by the mixture thermodynamic properties that are closely linked to the nature of the entrainer. Hence, we have established a complete set of rules for the selection of feasible entrainers for the separation of non ideal mixtures... 

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