Entrainers, extractive distillation

There are many important industrial applications of azeotropic separations, which employ a variety of methods. In this book we discuss several of these chemical systems and demonstrate the application of alternative methods of separation. The methods presented include pressure-swing distillation, azeotropic distillation with a light entrainer, extractive distillation with a heavy entrainer (solvent), and pervaporation. The chemical systems used in the numerical case studies included ethanol-water tetrahydrofuran (THF)-water, isopropanol-water, acetone-methanol, isopentane-methanol, n-butanol-water, acetone-chloroform, and acetic acid-water. Economic and dynamic comparisons between alternative methods are presented for some of the chemical systems, for example azeotropic distillation versus extractive distillation for the isopropanol-water system. 

SEPARATIONS USING HEAVY ENTRAINER (EXTRACTIVE DISTILLATION)... 

Anhydrous Acetic Acid. In the manufacture of acetic acid by direct oxidation of a petroleum-based feedstock, solvent extraction has been used to separate acetic acid [64-19-7] from the aqueous reaction Hquor containing significant quantities of formic and propionic acids. Isoamyl acetate [123-92-2] is used as solvent to extract nearly all the acetic acid, and some water, from the aqueous feed (236). The extract is then dehydrated by azeotropic distillation using isoamyl acetate as water entrainer (see DISTILLATION, AZEOTROPIC AND EXTRACTIVE). It is claimed that the extraction step in this process affords substantial savings in plant capital investment and operating cost (see Acetic acid and derivatives). A detailed description of various extraction processes is available (237). 

In the example, the minimum reflux ratio and minimum number of theoretical plates decreased 14- to 33-fold, respectively, when the relative volatiHty increased from 1.1 to 4. Other distillation systems would have different specific reflux ratios and numbers of theoretical plates, but the trend would be the same. As the relative volatiHty approaches unity, distillation separations rapidly become more cosdy in terms of both capital and operating costs. The relative volatiHty can sometimes be improved through the use of an extraneous solvent that modifies the VLE. Binary azeotropic systems are impossible to separate into pure components in a single column, but the azeotrope can often be broken by an extraneous entrainer (see Distillation, A7EOTROPTC AND EXTRACTIVE). 

J. P. Knapp and M. F. Doherty, "Minimum Entrainer Flows for Extractive Distillation. A Bifurcation Theoretic Approach," submitted to AIChE J. (1993). 

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. 

All streams leaving the extractive distillation sections are water washed to remove entrained ACN, and the ACN is recovered by distillation. Spent Cj s from the first stage distillation tower overhead may be recycled to a steam cracking unit. This material gives excellent butadiene yields. 

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 (-)... 

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]. 

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. 

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). 

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. 

Some solvent mixtures can be very difficult and energy intensive to separate because of the closeness of boiling points and the formation of azeotropic mixtures [45]. Azeotropic or extractive distiUation can be used for azeotropic solvent mixtures and solvents which have very low relative volatihties ]43, 45]. Azeotropic and extractive distillation involves the addition of another solvent, known as an entrainer, which will form its own azeotrope with one of the components to be separated ]45]. However, the additional solvent required for azeotropic and extractive distillation can also generate more wastes depending on how easily the entrainer itself can be recycled and reused. 

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... 

The extractive distillation profits from the capacity of an entrainer (solvent) to modify selectively the relative volatility of species. Normally, the entrainer is the highest boiler, while the component to be separated becomes heavier, being carried out in bottoms. For this reason, this operation may be regarded as an extractive absorption. Extractive distillation can be used for separating both zeo-tropic and azeotropic mixtures. The entrainer is fed near the top for a zeotropic mixture or a minimum-boiling azeotrope, or mixed with the feed for a maximumboiling azeotrope. The separation sequence normally has two columns, for extraction and solvent recovery [5]. 

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. 

Description Hydrocarbon feed is preheated with hot circulating solvent and fed at a midpoint into the extractive distillation column (EDC). Lean solvent is fed at an upper point to selectively extract the aromatics into the column bottoms in a vapor/liquid distillation operation. Nonaromatic hydrocarbons exit the column top and pass through a condenser. A portion of the overhead stream is returned to the column top as reflux to wash out any entrained solvent. The balance of the overhead stream is the raffinate product, requiring no further treatment. 

Although ethanol is obtained as a top product from an extractive distillation with ethylene glycol, it is obtained as a bottom product from an azeotropic distillation column using an entrainer such as n-pentane. Based on an ethanol rate of 242.02 moles per hour, a rough comparison will be made of the two separation methods. 

If n-pentane is selected as the entrainer for an azeotropic distillation scheme, an ethanol product containing less water than that obtained in the extractive distillation method is easily obtained with entrainer-etha-nol ratios of 2.S-3.5, mole basis (10). For a ratio of 3.214, the water content of the ethanol is less than 3 ppm. Only 18 equilibrium trays are required in a column of less than 5 feet diameter. The heat loads in millions Btu/hour are about 10.7 for the reboiler and 11.3 for the condenser. A stripper is used to recover n-pentane and ethanol from the aqueous phase. The recovered n-pentane and ethanol can be recycled either to the feed or to the reflux stream of the azeotropic distillation column. 

Azeotropic distillation has often been discussed in recent literature (I, 2, 3). Methods for providing phase equilibria for azeotropic and extractive distillation have been studied extensively (I, 4, 5, 6, 7, 8, 9). Some have discussed the design (10) or calculation of azeotropic distillations (2, 3) others only discussed choosing the entrainer for azeotropic distillation processes (11). 

Diethoxymethanol-water-ethanol Minimum-boiling azeotropes Self-entraining Alternative to extractive distillation... 

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