Isomer separation permeator

Separation of the stereoisomers cis- and iraws-stilbene, or p-nitroaniline (p-NA) and o-NA was studied using 0.02—0.7 M 3-CD aqueous solution as a LM. Celgard X-10 microporous hollow fibers were used for permeators design. The characteristics of the HFCLMP module see in Ref. [93]. For nitroandine isomers separation, an equimolar 0.005 M solution of o-NA and p-NA in 80% 1-octanol and 20% heptane was used as a feed. For stilbene isomers separation, an equimolar 0.01 M solution of cis- and trans-stilbene in pure heptane was used as a feed. 

Mandal DK, Guha AK, Sirkar KK. Isomer separation by a hollow fiber contained liquid membrane permeator. J Membr Sci 1998 144(1-2) 13-24. 

Fig. 4. ZSM-5 membrane performance in xylene isomer separation, p-xylene, o-xylene permeance and mixture separation factor (SF) are plotted versus temperature of permeation for typical c-oriented (A) and b-oriented (B) films [110].

HFCLM-based Isomer Separation Processes. Armstrong and Jin (33) studied liquid membrane permeation through an aqueous SLM in a cellulose filter placed in a batch cell. The feed consisted of a 50-50 mixture of organic isomeric solute systems (structural, stereoisomers, etc.) in an organic solvent with the same solvent present in the permeate side. They incorporated p-cyclodextrin (they also tested a-and y-) in the aqueous solution to develop selectivity for one isomer over the other. Considerable selectivity was achieved initially. With time the selectivity was lost due to the inherent nature of the batch system. 

Molecular sieving effect of the membrane has been evidenced using a mixture of two isomers (i.e. no Knudsen separation can be anticipated), n-hexane and 2-2 dimethylbutane (respective kinetic diameters 0.43 and 0.62 nm). Figure 10 shows the permeate contains almost only the linear species, due to the sieving effect of the zeolite membrane (pore size ca 0.55 nm). This last result also underlines that the present zeolite membrane is almost defect-fi ee. 

It was observed that the n-PrOH concentration in the permeate through the CD/PVA membrane by pervaporation was approximately same as that in the feed solution, namely, the PrOH isomers could hardly be separated through these membranes by pervaporation. The n-PrOH concentration in the permeate obtained by evapomeation was higher than that in the... 

Liu et al. [42] reported permeation of mixture of hexanes and octanes through silicalite membranes. It was found that the permeances of the mixture could not be predicted by the single-component data. In the separation of alkane isomers, the permeance of 2,2-DMB is significantly reduced in the presence of n-hexane resulting in a permselectivity much higher than the ideal separation factor [7]. 

Krishna and Paschek [91] employed the Maxwell-Stefan description for mass transport of alkanes through silicalite membranes, but did not consider more complex (e.g., unsaturated or branched) hydrocarbons. Kapteijn et al. [92] and Bakker et al. [93] applied the Maxwell-Stefan model for hydrocarbon permeation through silicalite membranes. Flanders et al. [94] studied separation of C6 isomers by pervaporation through ZSM-5 membranes and found that separation was due to shape selectivity. 

Triglycerides. Structural analysis of the natural fat triglycerides is particularly difiicult because of the many possible molecular species which have very similar chemical and physical properties. Major advances in separation and analytical techniques since 1955 have revolutionized this field. The chemically different triglycerides are now separable by gas-liquid chromatography, by thin-layer chromatography, and/or by permeation chromatography. By use of selective enzymatic deacylation and phosphorylation techniques, the positional isomers can be separated and characterized (121). 

Miyata T, Tooru I, and Uragami T. Characteristics of permeation and separation of propanol isomers through polyvinyl alcohol membranes containing cyclodextrin. J Appl Polym Sci 1994 51 2007-2014. 

This cost differential can be tolerated only in applications in which polymeric membranes completely fail in the separation [78]. Demanding separation applications, where zeolite membranes could be justified, due to the high temperatures involved or the added value of the components, and have been tested at laboratory scale, are the following separation of isomers (i.e., butane isomers, xylene isomers), organic vapor separations, carbon dioxide from methane, LNG (liquefied natural gas) removal, olefines/paraffins and H2 from mixtures. In most cases, the separation is based on selective diffusion, selective adsorption, pore-blocking effects, molecular sieving, or combinations thereof. The performance or efficiency of a membrane in a mixture is determined by two parameters the separation selectivity and the permeation flux through the membrane. 

The separation of xylene isomers on MFl zeolite membranes can be considered one example of intracrystaUine size exclusion and competitive adsorption (strongly dependent on coverage). The difference in their kinetic diameters ( 0.58 nm for p-xylene and 0.68 nm for o- and m-xylene) indicates the possibility of an effective separation using MFl membranes (see Table 10.1). The kinetic diameter of p-xylene is close to one of the MFl channels ( 0.55 nm) whereas o- and m-xylene might be excluded. Therefore, MFl zeolite channels and crystal grain boundaries determinate the permeation characteristics [27]. 

The separation of -hexane/2,2-dimethylbutane, (DMB), another separation where size exclusion takes place, has been accomplished by Handers et al. [156], using pervaporation and vapor permeation. Both, the n-Cg and DMB fluxes are higher in the former case due to the higher driving force in pervaporation, leading to a lower selectivity compared to vapor permeation. The separation of xylene isomers on MH membranes has been described in Section 10.4.2.1 as a separation where size exclusion takes place. The results of the separation of these isomers using pervaporation with FER membranes [20] and MFl [21] were not successful, and very low fluxes of 10 and 10 mol/m s, accompanied by separation factors not greater than 16, respectively, were obtained. The best results for the pervaporation of xylenes were obtained by Yuan et al. [129] who prepared a template free sihcahte-1 membrane, the separation factor for a 50/50 wt% mixmre of p-xylene/o-xylene at 50°C was 60, and the flux of p-xylene was 13.7 x 10 kg/m h. 

Flanders CL, Tuan VA, Noble RD, and Falconer JL. Separation of C-6 isomers by vapor permeation and pervaporation through ZSM-5 membranes. J Membr Sci 2000 176 43-53. 

Table 6.11 Separation of isomers by hollow-fiber contained liquid membrane permeator (HFCLMP) using fS-cyclodextrin (P-CD) as a carrier in aqueous liquid membrane...

Product analysis of irradiated samples attains the ppb level and separation of geometrical isomers is routinely performed. Using gel permeation chromatography the product distribution of y-irradiated n-alkanes has been recently mapped (50). As can be seen in Figure 2, unprecedented resolution of the various isomers can now be achieved with modern chromatographic techniques. Capillary electrophoresis is presently used to determine the product distribution of various cresols and yields as low as 0.1 molecules/100 eV are routinely measured (51). 

Pervaporation is a contraction of the terms permeation and evaporation because the feed is a liquid, and vapor exits the membrane on the permeate side. Pervaporation is a membrane process for liquid separation, and today, it is considered as a basic unit operation for the separation of organic-organic liquid mixtures because of its efficiency in separating azeotropic and close-boiling mixtures, isomers, and heat-sensitive compounds. It allows separations of some mixtures that are difficult to separate by distillation, extraction, and sorption. Pervaporation is one such type of membrane separation process with a wide range of uses such as solvent dehydration and separation of organic mixtures. When a membrane is in contact with a liquid mixture, one of the components can be preferentially removed from the mixture due to its higher affinity and quicker diffusivity in the membrane. 

Hexane isomers and benzene-p-xylene mixtures have been separated by PV through an MFl-type zeolite membrane by Matsufuji et al. (2000). The PV tests for n-hexane, 2-methylpentane (2-MP), and 2,3-dimethylbutane (2,3-DMB) were performed using an MFl-type zeolite membrane at 303 K. -Hexane preferentially permeated through the MFI membrane in the PV tests for binary mixtures of n-hexane-2-MP and n-hexane-2,3-DMB. The separation factors (a( -hexane-2-MP) and a(n-hexane-2,3-DMB)) were always greater than their ideal selectivi-ties. The ideal selectivities of n-hexane-2-MP and -hexane-2,3-DMB were 37 and 50, respectively. It was observed that separation factor a( -hexane/2,3-DMB) was as high as 270 when the feed concentration of n-hexane was 10 mol%. Matsufuji et al. (2000) claimed that the MFI membranes have promising potential to separate n-hexane and branched hexane-isomer mixtures. 

In pervaporation, as shown in Figure 34.5, when feed mixtures are added on one side of the membrane and the other side is evacuated, a certain component in the feed mixture can be preferentially permeated through the membranes (Binning et al. 1961 Choo 1962). In the pervaporation, the difference in the solubility of permeants into the membrane, the diffusivity of permeants in the membrane, and the relative volatility of permeants from the membrane can influence the characteristics of permeation and separation. This pervaporation technique is advantageous for the separations of azeotropic mixtures, close-boiling point mixtures, and structural isomers. 

Mandal et al. (in preparation) incorporated p-cyclodextrin in an aqueous solution (0.7 M P-cyclodextrin, 7.5% NaOH, 37.5% urea, pH = 12) and employed it as a CLM in a permeator containing 300 symmetric hydrophobic feed fibers and 300 similar strip fibers. The feed was an equimolar (0.005 M) mixture of structural isomers o-nitroaniline and /7-nitroaniline in 80% octanol-20% heptane with the same solvent mixture as the strip liquid (Figure 3a). They obtained a selectivity of almost 5 for /7-nitroaniline over the ortho isomer. The operation was stable. The longest run continued for a period of almost two days with the feed solution and the strip solvent in coxmtercurrent flow. This successful separation indicated that employing a reflux arrangement at the end of the extraction cascade, it would be possible to continuously separate structural isomers into high purity (99%). ... 

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