Benzene/cyclohexane separation

Benzene-cyclohexane separation 230 Combination of extractive distillation and one-stage pervaporation high-purity (99.2-99.5%) products estimated cost savings of 20%... 

Let us look at the benzene-cyclohexane separation more closely as we summarize how GC works. The boiling points of benzene and cyclohexane are nearly the same, 80.1 and 81.4°C respectively. Any GC separation will have to depend on differences in the intermolecular interactions between the stationary phase and these two analytes, both of which are nonpolar hydrocarbons. What differences could be exploited with GC Benzene has a -n-electron cloud, which should make it more susceptible to induction effects and perhaps dispersion attractions (Chapter 3). Therefore we should choose a stationary liquid phase that would accentuate this difference—a polar one also, using the like-dissolves-like rule we might choose an aromatic compound that would interact more with benzene than with cyclohexane. One possible liquid phase that meets these criteria is dinonylphthalate, and it has been used to separate benzene and cyclohexane. The relative retention has been found to be 1.6, which represents a very good separation.1... 

A significant number of works are concerned with the development of new membranes for the separation of mixtures of aromatic/alicyclic hydrocarbons [10,11,77-109]. For example, the following works can be mentioned. A mixture of cellulose ester and polyphosphonate ester (50 wt%) was used for benzene/cyclohexane separation [113]. High values of the separation factor and flux were achieved (up to 2 kg/m h). In order to achieve better fluxes and separation factors the attention was shifted to the modification of polymers by grafting technique. Grafted membranes were made of polyvinylidene fluoride with 4-vinyl pyridine or acrylic acid by irradiation [83]. 2-Hydroxy-3-(diethyl-amino) propyl methacrylate-styrene copolymer membranes with cyanuric chloride were prepared, which exhibited a superior separation factor /3p= 190 for a feed aromatic component concentration of 20 wt%. Graft copolymer membranes based on 2-hydroxyethyl methylacrylate-methylacrylate with thickness 10 pm were prepared [85]. The membranes yielded a flux of 0.7 kg/m h (for feed with 50 wt% of benzene) and excellent selectivity. Benzene concentration in permeate was about 100 wt%. A membrane based on polyvinyl alcohol and polyallyl amine was prepared [87]. For a feed containing 10 wt% of benzene the blend membrane yielded a flux of 1-3 kg/m h and a separation factor of 62. 

Tsubouchi and Yoshikawa [79] are concerned with pervaporation separation of benzene/cyclohexane mixtures using membranes based on polyamide/polyether block copolymers. It has been established that the separation factor increases with the increase in the polyamide component containing polar amide groups capable of forming hydrogen bonds e.g., the 1 1 block copolymer of polyamide 12 and polyoxyethylene has the benzene/cyclohexane separation factor /3p = 2.8 and the flux 2 = 300 g/m h a more rigid 3 1 polyamide 12/polyoxyethylene block copolymer has a much higher separation factor )8p = 5.0 and 2 = 80 g/m h. 

Figure 9.19 Fraction of benzene in permeate as a function of feed mixture composition for pervaporation at the reflux temperature of a binary benzene/cyclohexane mixture. A 20-qm-thick crosslinked blend membrane of cellulose acetate and polystyrene phosphate) was used [54]. Reprinted with permission from I. Cabasso, Organic Liquid Mixtures Separation by Selective Polymer Membranes, Ind. Eng. Chem. Prod. Res. Dev. 22, 313. Copyright 1983 American Chemical Society...

Water is then added, and the mixture is extracted with henzene. The organic phase is separated, dried with anhydrous sodium sulfate, filtered, and the filtrate is evaporated under vacuum. The residue is dissolved in the minimum amount of benzene and this solution is chromatographed on a column of silica gel with benzene as the mobile phase. The colored band is collected, the solvent is evaporated, and the residue is recrystallized from benzene/cyclohexane yield 1.5 g (10%) m.p. 167°. 

Before illustrating the concept for a mass spectrometric distinction of stereoisomers, let us refer to some of the problems associated with the stereoselective hydrogenation of benzene to cyclohexane this may serve as a simple and illustrative example. Metal-catalyzed hydrogenation is known to proceed with a large syn-selectivity, and the reaction involves at least three separate steps in the case of benzene cyclohexane (Scheme 3). 

Some examples of separation of organic compounds from mixtures (p-xylenes/m-xylenes, benzene/cyclohexane) were discussed, including separation of organic gases, wastewater, and organic solutions Review (Chinese)... 

Ray SK, Sawant SB, Joshi JB, and Pangarkar VG. Development of new synthetic membranes for separation of benzene-cyclohexane mixtures by pervaporation A solubility parameter approach. Ind. Eng. Chem. Res. 1997 36(12) 5265-5276. 

Inui K, Okumura H, Miyata T, and Puragami T. Permeation and separation of benzene/cyclohexane mixtures through cross-linked poly(aIkyl methacrylate) membranes. J. Membr. Sci. 1997 132(2) 193-202. 

Separation of benzene/cyclohexane mixture is investigated most extensively. This is not surprising because separation of this mixture is very important in practical terms. Benzene is used to produce a broad range of valuable chemical products styrene (polystyrene plastics and synthetic rubber), phenol (phenolic resins), cyclohexane (nylon), aniline, maleic anhydride (polyester resins), alkylbenzenes and chlorobenzenes, drugs, dyes, plastics, and as a solvent. Cyclohexane is used as a solvent in the plastics industry and in the conversion of the intermediate cyclohexanone, a feedstock for nylon precursors such as adipic acid. E-caprolactam, and hexamethylenediamine. Cyclohexane is produced mainly by catalytic hydrogenation of benzene. The unreacted benzene is present in the reactor s effluent stream and must be removed for pure cyclohexane recovery. 

Ref. [77] deals with the pervaporation separation of benzene/cyclohexane, toluene/wo-octane mixtures using DSDA-TrMPD/2,2 -diethynylbenzidine (DEB) copolyimide. The dominant role of the diffusion component /3d in the separation process has been established. It can be seen from the data in Table 9.11 that the increase in the difference between ratios of minimum cross-sections of penetrant molecules results in an increase of the separation factor /3p which is determined mainly by the diffusion component /8d-... 

Source From analysis of data presented in Fang, J., Tanaka, K., Kita, H., and Okamoto, K., Polymer, 40, 3051, 1999. Note Composition of the mixture to be separated is benzene/cyclohexane with 50-60 wt% of benzene, T = 343 K. 

FIGURE 9.27 Dependence of separation factor f)p (benzene/cyclohexane), as weU as its diffusion component /3d based on styrene/ butadienestyrene/acryUc acid copolymers on permeation flux (benzene/cyclohexane 50/50 wt% mixture, T = 293 K). (From analysis of data presented in Semenova, S.I., J. Membr. Sci., 231, 189, 2004. With permission.)... 

To increase the sorption component of the separation factor, homogeneously distributed tetracyanoethylene, a strong electron acceptor having high affinity for electron donors, was added to the polyimide matrix [77]. It can be seen from data presented in Table 9.12 that this is accompanied by an increase in the sorption component /3s (benzene/cyclohexane) by a factor of 1.5 probably as a result of selective sorption of aromatic compounds by tetracyanoethylene with a simultaneous increase in the diffusion component /3d. The prepared membranes showed good pervaporation properties with respect to benzene/cyclohexane, toluene/isooctane mixtures. For example, for a two-component 50/50 wt% benzene/cyclohexane mixture at 343 K, the flux was 2 = 0.44 kg p,m/m h, and /3p (benzene/cyclohexane) = 48 and for a two-component toluene/isooctane mixture, 45/55 wt%, at 343 K the flux was 2 = 1-1 kg p-m/m h, and /3p (toluene/wo-octane) = 330. 

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