Extractive distillation cyclohexane from benzene

In Section 11.1 the importance of reliable -model parameters for the synthesis and design of extractive distillation processes is demonstrated for the separation of cyclohexane from benzene using NMP as entrainer. Furthermore for the system acetone-water it is shown how default values can lead to poor separation factors or even not existing azeotropic points at the top of the column (x acetone 1) ... 

Ethanol isolation proceeds via distUlative processes. Owing to the formation of an ethanol/water azeotrope (95.5 mass% ethanol at 1 bar), the production of water-free ethanol requires the application of extractive distillation (typical entrainer benzene or cyclohexane, see Section 3.3.2.3). The isolation of water-free ethanol from fermentation is a relatively energy intense step. Even with a clever combination of several distillation columns working at different pressures, the energy input to produce 1 kg of bioethanol by fermentation is about 10 MJ (Baerns et al, 2006). In a few countries, where the production of bioethanol is particularly cheap (e.g., Brazil), there have also been attempts to convert bioethanol into chemicals in commercial scenarios. The production of ethylene from bioethanol is a potential option in this context. 

In addition. Cooper et al. [91] discussed the use of a Raman analyzer to provide feedback and feed-forward data on a number of chemical manufacturing processes originating from crude oil, where several of the production steps involved a distillation separation again, in these examples, the Raman analyzer was positioned at the outlet to the distillation tower. They claim from their work that a Raman analyzer would be useful for monitoring and controlling aromatic extraction, liquid paraffin aromatization, and the production of cumene, cyclohexane from benzene, ethylbenzene, xylene isomers, dimethyl terphthalate, and styrene. It should also be noted that in all these processes, at least one and in several cases multiple distillation columns are involved. 

Medium Boiling Esters. Esterificatioa of ethyl and propyl alcohols, ethylene glycol, and glycerol with various acids, eg, chloro- or bromoacetic, or pymvic, by the use of a third component such as bensene, toluene, hexane, cyclohexane, or carbon tetrachloride to remove the water produced is quite common. Bensene has been used as a co-solvent ia the preparatioa of methyl pymvate from pymvic acid (101). The preparatioa of ethyl lactate is described as an example of the general procedure (102). A mixture of 1 mol 80% lactic acid and 2.3 mol 95% ethyl alcohol is added to a volume of benzene equal to half that of the alcohol (ca 43 mL), and the resulting mixture is refluxed for several hours. When distilled, the overhead condensate separates iato layers. The lower layer is extracted to recover the benzene and alcohol, and the water is discarded. The upper layer is returned to the column for reflux. After all the water is removed from the reaction mixture, the excess of alcohol and benzene is removed by distillation, and the ester is fractionated to isolate the pure ester. 

The cooled contents of the 2S0-ml. flask containing ferrous chloride (Note 6) are added to the cold sodium cyclopentadienide solution while passing a stream of nitrogen through both flasks. The combined mixture is stirred for 1.25 hours at a temperature just below reflux. Solvent is removed by distillation, and the ferrocene is extracted from the residue with several portions of refluxing petroleum ether (b.p. 40-60°). The product is obtained by evaporation of the petroleum ether solution. Ferrocene may be purified by recrystallization from pentane or cyclohexane (hexane, benzene, and methanol have also been used) or by sublimation. The 3ueld is 31-34 g. (67-73%) (Note 7), m.p. 173-174°. 

B. 2,2-(Trimethylenedithio)cyclohexanone. A solution of 3.02 g. (0.02 mole) of freshly distilled 1-pyrrolidinocyclohexene, 8.32 g. (0.02 mole) of trimethylene dithiotosylate4 (Note 2), and 5 ml. of triethylamine (Note 3) in 40 ml. of anhydrous acetonitrile (Note 4), is refluxed for 12 hours in a 100-ml., round-bottom flask under a nitrogen atmosphere. The solvent is removed under reduced pressure on a rotary evaporator, and the residue is treated with 100 ml. of aqueous 0.1 N hydrochloric acid for 30 minutes at 50° (Note 5). The mixture is cooled to ambient temperature and extracted with three 50-ml. portions of ether. The combined ether extracts are washed with aqueous 10% potassium bicarbonate solution (Note 6) until the aqueous layer remains basic to litmus, and then with saturated sodium chloride solution. The ethereal solution is dried over anhydrous sodium sulfate, filtered, and concentrated on a rotary evaporator. The resulting oily residue is diluted with 1 ml. of benzene and then with 3 ml. of cyclohexane. The solution is poured into a chromatographic column (13 x 2.5 cm.), prepared with 50 g. of alumina (Note 7) and a 3 1 mixture of cyclohexane and benzene. With this solvent system, the desired product moves with the solvent front, and the first 250 ml. of eluent contains 95% of the total product. Elution with a further 175 ml. of solvent removes the remainder. The combined fractions are evaporated, and the pale yellow, oily residue crystallizes readily on standing. Recrystallization of this material from pentane gives 1.82 g. of white crystalline 2,2-(trimethylenedithio)cyclo-hexanone, m.p. 52-55° (45% yield) (Note 8). 

Cyclohexane made from benzene costs about 1.20/gal. It boils at 80.8 °C. Benzene boils at 80.1 C, and the two form an azeotrope at 53% benzene-47% cyclohexane by weight. An extractive distillation is useful in removing the benzene from the cyclohexane by the process described in Chapter 4, p. 47. 

Extractive distillation is not limited to the separation of binary mixtures, but is also capable of removing particular classes of substances from multicomponent inixtiire.s, as for instance benzene from mineral oil fractions. Mixtures of saturated and imsaturated hydrocarbons having closely similar boiling points can be separated by extractive distillation with ketoesters [73]. Recently, the sei)aration of lower hydrocarbons CyCa has been gaining ground [74]. Garner et al. [75] studied the efficiency of packed columns in the extractive distillation of the system iiictliyl cyclohexane-toluene with derived equations for this process. 

Extractive distillation uses a selective solvent (entrainer). Here the entrainer influences the ratio of the activity coefficients of the components in order to alter the separation factor far from unity. Often about 70% of the liquid phase inside the column consist of the entrainer. A typical extractive distillation process for separating aromatics (benzene) from aliphatics (cyclohexane) is show in Fig. 3.2-4. 

Fig. 3.2-4 Extractive distillation process for the separation of benzene from cyclohexane using aniline as an entrainer.

In most cases not binary but multicomponent systems have to be separated. Sometimes an additional component is needed as an entrainer e.g. for the separation by extractive distillation. As an example selected separation factors of 12 for the system benzene (l)-cyclohexane (2)-NMP (3) calculated using modified UNIFAC and default UNIQUAC parameters from a simulator are shown in Figures 11.8 and 11.9, respectively. As benzene and cyclohexane form an azeotrope, the main task of the entrainer NMP is to shift the separation factor between benzene and cyclohexane as far from unity as possible this means au 1 or au 1- In practice, typical entrainer concentrations of 50-80 mol% are employed to achieve satisfying separation factors. A higher entrainer concentration usually improves the separation factor. 

Various solvents can be used as extraction agents, for example hexane, ethanol (96%) or hexane/ethanol mixtures. Benzene, cyclohexane, acetone, chloroform, and ether are also used however. The solvent destroys the cell wall of the alga and extracts the oil from the aqueous phase (medium) owing to the higher lipid solubility in organic solvents as compared to water. The solvent recovery process takes place in a subsequent distillation stage. 

Figure 3.3.21 Extractive distillation of the azeotropic system cyclohexane and benzene at Benzene 1 bar (a) y-x diagram without (dotted line) and with (short-dashed line) addition of50mol.% dimethylformamide (DMF) note that the content of DMF is not counted, that is,xc6H6= 1 —xccHizi (b) typical process configuration. Adapted from Cmehling and Brehm (1996).

Liquid-liquid extraction can be used to recover one of the components to be separated, as well as the entrainer to be recycled. A typical process is the separation of cyclohexane/benzene, with nbp s at 80.8 °C and 80.1 °C and minimum-boiling azeotrope at 77.6 °C (Seader and Henley, 1998). If acetone is used as entrainer (nbp 56.2 °C), then an azeotrope appears between acetone and cyclohexane (nbp 53.1 °C). RCM shows a distillation boundary between the components to be separated (Fig. 9.32). To simplify the process, consider an initial azeotropic mixture. After mixing with entrainer the feed f, is separated in two products, benzene in bottoms and acetone-cyclohexane azeotrope in top. From the last mixture cyclohexane can be extracted with water. Finally acetone and water are separated by simple distillation. 

Azeotropic distillation. A further development involves the addition of an entrainer, either another solvent or water, to the mixture of liquids to be separated. The purpose of this material is to form a selected azeotrope with one of the components. This results in a difference in relative volatility between the azeotrope and the non-azeotropic component allowing separation to be achieved. Typically the azeotrope will be of higher volatility and becomes the distillate, although the azeotrope can be such that it is removed as bottoms. An effective entrainer therefore must be selective for the solvent to be recovered, stable under the conditions of use, chemically compatible with all components, relatively inexpensive, readily available and must be easily separable from the desired product. Water is an ideal entrainer when used to form azeotropes with solvents which separate on condensation. Guidelines for entrainer selection have been provided by Berg and Gerster [28,29]. Many examples of azeotropic distillation can be cited [23]. Examples include the separation of benzene from cyclohexane by the azeotrope of the latter with acetone followed by liquid-liquid extraction with water to yield the cyclic hydrocarbon. Similarly the use of methylene chloride as an entrainer for separation of an azeotropic mixture of methanol and acetone is achieved by addition of methylene chloride followed by the distillation of the selective azeotrope between the alcohol and chlorinated hydrocarbon. 

Bis(3-ethynylphenoxy)benzophenone (4) A solution containing 3-hydroxyphenylacetylene (5.3g, 0.04 mol), 4,4 -difluorobenzophenone (3.2g, 0.15 mol), anhydrous potassium carbonate (12.5g) in dimethyl-sulfoxide (250mL) was stirred under a nitrogen atmosphere at 80 C for 24 h. The reaction mixture was cooled to room temperature and poured into distilled water (250mL) and extracted with benzene. The benzene extract was filtered through a bed of silica gel and the benzene was removed under reduced pressure to give 6g (99%) of a white crystalline solid. The product on crystallization from cyclohexane exhibited a mp of 83-84 C. 

This process can be used, for instance, to separate benzene and cyclohexane, which are very close boilers. Acetone, which forms an azeotrope with cyclohexane, is added as an entrainer. The cyclohexane is separated from the acetone by extraction with water, which dissolves the acetone. The cyclohexane is practically immiscible in water. The water-acetone solution is separated by simple distillation. 

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