Distillation extractive

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. 

A mixture of cyclohexane bp 80.8 C) and benzene hp 80.1 C) can, for example, be separated by distillation after adding aniline, since the interaction between benzene and aniline is greater than that between cyclohexane and aniline. Azeotropic mixtures can be separated similarly by extractive distillation (e.g.. water-ethanol by adding glycerol). Hydrocarbons with similar boiling points can be separated by extractive distillation in the presence of polar liquids (nitrobenzene, phenol, furfurol). 

Liquid-liquid extraction is also possible in which distribution takes place between a liquid eluent and a liquid (generally water) that is adsorbed on the surface of the carrier material (distribution chromatography). 

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]. 

Improvements in separation by distillation have reached the stage of being restricted to improvements in liquid/vapour contacting equipment. The application of azeotropic distillation is restricted by the comparatively small choice of effective entrain-ers and the equipment required is very similar to that needed for fractional distillation so there are few possible applications that have not been fully explored. 

however, can use an almost infinite number of entrainers, both pure materials and mixtures, but the equipment is specialized. Since it uses the same principle as gas-liquid chromatography in which a stationary phase alters the relative volatility of the compounds to be separated, the screening of entrainers that are indicated by theory as being suitable does not require a lot of laboratory work. 

ED alters the relative volatility of a binary system when the components of the system have different polarities (Fig. 12.1). It is normal to operate at 0.8 to 0.9 mole fraction of ED entrainer in the B and C sections of the extraction column (Fig. 12.2), which can raise the relative volatility to a value at which the separation is easy. Such a high proportion of the entrainer gives activity coefficient values of components 1 and 2 which approach the activity coefficients at infinite dilution. 

If the ED entrainer is highly polar the activity coefficient of any solvent which is also polar in it will be close to unity. A solvent which is not polar on the other hand will have a high activity coefficient. 

For the same mixture a non-polar entrainer will increase the activity coefficient of the polar solvent. 

An example of extractive distillation is the separation a binary mixture of acetone and methanol. These two components form a binary homogeneous minimum-boiling azeotrope. The normal boding points of acetone and methanol are 329 and 338 K, respectively, so acetone is the light-key component. The boding point of the azeotrope (328 K) is lower than the boding point of the pure light component. The composition of the acetone/methanol 

Distillation Design and Control Using Aspen Simulation, Second Edition. William L. Luyben. 2013 John Wiley Sons, Inc. Published 2013 by John Wiley Sons, Inc. 

In the numerical example studied in this section, the solvent is dimethyl sulfoxide (DMSO) whose boiling point (465 K) is much higher than either of the key components. It preferentially attracts methanol, so the bottoms from the extractive column is essentially a binary mixture of methanol and DMSO with a very small amount of impurity acetone. 

0 Input Q Rciuld 123 EOVh(i bks 123 CuftiMi StociiTk Rie Ji SOLVEtir 

U- RalK lHacfr Di ililhlior it 0 figr Spe tii Vaiy Hc ter Cookd Pump mui d  

It was found that when the given mixture (the raw solvent ) is submitted to a simple distillation, neither the head fraction nor the sump fraction shows any significant increase in diketone concentration. This is synonymous with saying that the diketone concentration in the vapor is roughly equal to the diketone concentration in the liquid, very much as in the case of distilling an azeotropic mixture. This is not surprising as the starting mixture, rep- 

In view of this situation, it was obvious that increasing the diketone concentration by distillation required the addition of a volatility modifier. It was soon discovered that adding water to the original mixture did increase the relative volatility of the diketones as the latter have polarities vastly different from that of water, so that at high water concentrations the diketone molecules are pushed out of the mixture . 

These molecules are stabilized (energetically favored) by intramolecular hydrogen bonds shown as dashed lines. Simple ketones such as acetone cannot form such intramolecular hydrogen bonds as they have only a single carbonyl group, and for this reason simple ketones do not form hydrates. 

By NMR spectroscopy [67] it was found that in aqueous solution some 68 percent of all diacetyl molecules are in the high-boiling hydrate form, and a 

The enol forms are more volatile than the keto forms as their intramolecular hydrogen bonds greatly reduce the attractive forces of neighboring molecules. In distillation, the equilibrium is upset in that the more volatile enol molecules will go more readily to the top than the less volatile keto molecules. As reestablishment of the keto/enol equilibrium is also very slow, requiring many hours [69], the time in the distillation column is not sufficient to permit regen- 

A process with comparable industrial importance is extractive distillation. Here, two distillation columns are combined with an absorption column. This process needs an entrainer (absorbent) that selectively absorbs one of the two feed components. In aqueous systems, for instance, the entrainer should be a hygroscopic hq-uid, e.g., ethylene glycol (Fig. 11.4-2). 

In a first distillation colunrn C-1 the high boiler (i.e., water) is recovered as bottoms B. The azeotropic overhead fraction t is fed (in the vapor state) into the 

To remove traces of the absorbent always present in the overhead product of the absorber, some of the ethanol is recycled as reflux at the top of the absorber A-1. Analogously, a small part of the loaded absorbent is boiled up at the bottom of A-1 to remove most of the coabsorbed ethanol. However, reboil and reflux rates are rather small. Thus, the countercurrent flow within the column A-1 is dominated by the vapor feed (low boiling fraction) at the bottom and the liquid feed (high boiling fraction) at the top of the column what is characteristic of absorption columns. Thus, the process should better be called absorptive distillation instead of extractive distillation. 

Butane/butadiene Furfural Butene/isoprene Dimethyl formamide 

Benzene/cyclohexane Aniline Ethanol/water Ethylene glycol 

In all cases, the selective solvents (entrainers) have the task of altering the partition coefficients in a way that high separation factors and selectivities for the different phase equilibria (extractive distillation vapor-liquid equilibrium (VLE), extraction liquid-liquid equilibrium (LLE), absorption gas-liquid equilibrium (GLE)) are achieved, resulting in a separation of compounds. The required partition coefficients, separation factors and selectivities can be calculated with the help of thermodynamic models (g -models, equations of state). 

As the total cost of these separation processes is strongly influenced by the selectivity of the solvent, the search for new and better solvents is important and has been a priority for many decades. 

2 Thermal Separation Processes Using a Selective Solvent as Separating Agent 

In such a process an additive or solvent of low volatility is introduced in the separation of mixtures of low relative volatilities or for concentrating a mixture beyond the azeotropic point. From an extractive distillation tower, the overhead is a finished product and the bottoms is an extract which is separated down the line into a product and the additive for recycle. The key property of the additive is that it enhance the relative volatilities of the substances to be separated. From a practical point of view, the additive should be stable, of low cost, require moderate reboiler temperatures particularly for mixtures subject to polymerization or thermal degradation, effective in low to moderate concentrations, and easily recoverable from the extract. Some common additives have boiling points 50-100°C higher than those of the products. 

Representation of a Petroleum Fraction by an Equivalent Number of Discrete Components 

TABLE 13.6. Some Rules for Design and Operation of Petroleum Fractionators 

Numbers on the streams are °F differences between the 50% points of the streams. Dashed lines are with stripping steam, full ones without [Packie, Trans. AlChE 37, 51 (1941)]. 

Operation Pressure (psia or mm) A On.) Superficial Tower Velocity (ft/sec) 

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, dilferential 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 Waias (1985, pp. 216-217). 

Represeutatiou of a Petrolenm Fraction by aa Eqaivaleut Nnmber of Discrete Components 

The control system shown holds the temperature profile in each column by manipulating heat inputs. Enough reflux is used on both columns to keep the product purities above specification. The solvent flowrate is ratioed to the fresh feed flowrate. 

Note that the level in the base of the solvent recovery column is not 

It should also be noted that many extractive distillation systems exhibit a maximum reflux ratio as well as the conventional minimum reflux ratio. For a given solvent-to-feed ratio, if too much reflux is returned to the column., the solvent is diluted and the separation becomes poorer since not enough solvent is available to soak up component B. 

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The second class of distillation operation using an extraneous mass-separating agent is extractive distillation. Here, the extraneous mass-separating agent is relatively involatile and is known as a solvent. This operation is quite different from azeotropic distillation in that the solvent is withdrawn from the column bottoms and does not form an azeotrope with any of the components. A typical extractive distillation process is shown in Fig. 3.11. ... 

As with azeotropic distillation, the separation is possible in extractive distillation because the extraneous mass-separating agent interacts more strongly with one of the components than the other. This in turn alters in a favorable way the relative volatility between the key components. 

In principle, extractive distillation is more useful than azeotropic distillation because the process does not depend on the accident of azeotrope formation, and thus a greater choice of mass-separating agent is, in principle, possible. In general, the solvent should have a chemical structure similar to that of the less volatile of the two components. It will then tend to form a near-ideal mixture with the less volatile component and a nonideal mixture with the more volatile component. This has the effect of increasing the volatility of the more volatile component. 

Sucksmith, I., Extractive Distillation Saves Energy, Chem. Engg., 88 185, June 28, 91, 1982. 

Extraction of metal ions Extraction processes Extraction resistance Extractive distillation... 

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 sulfuric acid hydrolysis may be performed as a batch or continuous operation. Acrylonitrile is converted to acrylamide sulfate by treatment with a small excess of 85% sulfuric acid at 80—100°C. A hold-time of about 1 h provides complete conversion of the acrylonitrile. The reaction mixture may be hydrolyzed and the aqueous acryhc acid recovered by extraction and purified as described under the propylene oxidation process prior to esterification. Alternatively, after reaction with excess alcohol, a mixture of acryhc ester and alcohol is distilled and excess alcohol is recovered by aqueous extractive distillation. The ester in both cases is purified by distillation. 

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. 

Butadiene Separation. Solvent extraction is used in the separation of butadiene (qv) [106-99-0] from other C-4 hydrocarbons in the manufacture of synthetic mbber. The butadiene is produced by catalytic dehydrogenation of butylene and the Hquid product is then extracted using an aqueous cuprammonium acetate solution with which the butadiene reacts to form a complex. Butadiene is then recovered by stripping from the extract. Distillation is a competing process. 

Benzene, toluene, and a mixed xylene stream are subsequently recovered by extractive distillation using a solvent. Recovery ofA-xylene from a mixed xylene stream requires a further process step of either crystallization and filtration or adsorption on molecular sieves. o-Xylene can be recovered from the raffinate by fractionation. In A" xylene production it is common to isomerize the / -xylene in order to maximize the production of A xylene and o-xylene. Additional benzene is commonly produced by the hydrodealkylation of toluene to benzene to balance supply and demand. Less common is the hydrodealkylation of xylenes to produce benzene and the disproportionation of toluene to produce xylenes and benzene. 

In France, Compagnie Europnene du Zirconium (CEZUS) now owned jointly by Pechiney, Eramatome, and Cogema, uses a separation (14) based on the extractive distillation of zirconium—hafnium tetrachlorides in a molten potassium chloride—aluminum trichloride solvent at atmospheric pressure at 350°C. Eor feed, the impure zirconium—hafnium tetrachlorides from the zircon chlorination are first purified by sublimation. The purified tetrachlorides are again sublimed to vapor feed the distillation column containing the solvent salt. Hafnium tetrachloride is recovered in an enriched overhead fraction which is accumulated and reprocessed to pure hafnium tetrachloride. 

Hydrolysis of Peroxycarboxylic Systems. Peroxyacetic acid [79-21-0] is produced commercially by the controlled autoxidation of acetaldehyde (qv). Under hydrolytic conditions, it forms an equiHbrium mixture with acetic acid and hydrogen peroxide. The hydrogen peroxide can be recovered from the mixture by extractive distillation (89) or by precipitating as the calcium salt followed by carbonating with carbon dioxide. These methods are not practiced on a commercial scale. Alternatively, the peroxycarboxyHc acid and alcohols can be treated with an estetifying catalyst to form H2O2 and the corresponding ester (90,91) (see Peroxides and peroxy compounds). 

Table 3 provides typical specifications for isoprene that are suitable for Al—Ti polymerization (89). Traditional purification techniques including superfractionation and extractive distillation are used to provide an isoprene that is practically free of catalyst poisons. Acetylenes and 1,3-cyclopentadiene are the most difficult to remove, and distillation can be supplemented with chemical removal or partial hydrogenation. Generally speaking distillation is the preferred approach. Purity is not the main consideration because high quaUty polymer can be produced from monomer with relatively high levels of olefins and / -pentane. On the other hand, there must be less than 1 ppm of 1,3-cyclopentadiene. 

The principal route for production of isoprene monomer outside of the CIS is recovery from ethylene by-product C streams. This route is most viable where ethylene is produced from naphtha or gas oil and where several ethylene plants are located in relatively close proximity to the isoprene plant. Although the yield of isoprene per mass of ethylene is quite low, there is enough ethylene produced to provide a large portion of demand. Because of the presence of / -pentane in these streams which a2eotropes with isoprene, extractive distillation must be used to recover pure isoprene. Acetonitrile is the most common solvent, but dimethylformamide is also used commercially. 

Dehydrogenation of Tertiary Amylenes, The staiting material here is a fiaction which is cut from catal57tic clacking of petroleum. Two of the tertiary amylene isomers, 2-methyl-l-butene and 2-methyl-2-butene, are recovered in high purity by formation of methyl tertiary butyl ether and cracking of this to produce primarily 2-methyl-2-butene. The amylenes are mixed with steam and dehydrogenated over a catalyst. The cmde isoprene can be purified by conventional or extractive distillation. 

The cmde wax is refined by extracting at 90—100°C with an azeotropic mixture of benzene and a mixture of alcohols, typically 85% benzene and 15% methanol (see Distillation, azeotropic and extractive). Distilling the solvent leaves a wax too daddy colored to be used without added refining. 

SASOL. SASOL, South Africa, has constmcted a plant to recover 50,000 tons each of 1-pentene and 1-hexene by extractive distillation from Fischer-Tropsch hydrocarbons produced from coal-based synthesis gas. The company is marketing both products primarily as comonomers for LLDPE and HDPE (see Olefin polymers). Although there is still no developed market for 1-pentene in the mid-1990s, the 1-hexene market is well estabhshed. The Fischer-Tropsch technology produces a geometric carbon-number distribution of various odd and even, linear, branched, and alpha and internal olefins however, with additional investment, other odd and even carbon numbers can also be recovered. The Fischer-Tropsch plants were originally constmcted to produce gasoline and other hydrocarbon fuels to fill the lack of petroleum resources in South Africa. 

Azeotropic and Extractive Distillations. Effective as they are for producing various Hquid fractions, distillation units generally do not produce specific fractions. In order to accommodate the demand for such products, refineries have incorporated azeotropic distillation and extractive distillation in their operations (see Distillation, azeotropic and extractive). 

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