polymer membranes for hydrocarbon separation and removal

Journal of Membrane Science 231 (2004) 189–207

Polymer membranes for hydrocarbon separation and removal

S.I. Semenova∗

Vladipor Research JSC, B. Nizjegorodskia 77, Vladimir 600016, Russia

Received 26 July 2003; accepted 20 November 2003

Abstract

The present review deals with the problem of hydrocarbon separation and removal using polymer membranes. It is demonstrated that theuse of membranes based on stiff chain glassy polymers for separation of olefins and paraffins as well as aromatic, alicyclic, and aliphatichydrocarbons is quite effective. Selective permeability in all applications based on glassy polymers is dominated by the diffusion component.Separation efficiency can be increased by modifying the structure of the polymer and introducing active groups or additives into the polymerthat change sorption and diffusion properties of the system. Application of membranes based on rubbery polymers for selective removal ofhydrocarbon vapors from their mixtures with air as well as for selective removal of hydrocarbons from their aqueous solutions, is considered.The effect of the chemical composition of rubbery polymers on their permeability to hydrocarbons is analyzed.© 2004 Elsevier B.V. All rights reserved.

Keywords:Polymer membranes; Permeability; Diffusion; Sorption; Hydrocarbons

1. Introduction and background

Separation of hydrocarbons and their removal from var-ious gas and liquid mixtures are important objectives ofchemical and petrochemical industries. These objectives areusually achieved by adsorption, rectification, and use ofcryogenics. During the past 20 years these conventionalmethods were supplemented by membrane technology. Alarge corpus of patent and literary data has been accumulatedso far that requires generalization (about 2000 documentsover the recent 20 years). The flow of patent and periodicinformation on the problem of membrane separation of hy-drocarbons is steadily growing. The flow is dominated byarticles in journals whereas the share of patents is only aboutone-third. This indicates that for the time being researchersare mainly taking a scientific, rather than commercial, in-terest in the problem. The materials of the selective layerof the membranes dominating in the data are polymers. Thepolymers used are both glassy and rubbery. It is thereforeworthwhile to consider physico-chemical regularities of hy-drocarbon mass transfer across polymer membranes.

The polymer permeation of low MW penetrants, includinghydrocarbons, is determined by both thermodynamic (sorp-

∗ Fax: +7-922-2156-74.E-mail address:[email protected] (S.I. Semenova).

tive) and kinetic (diffusive) factors. In the absence of specificpenetrant/polymer interactions, solubility of the penetrant isdetermined mainly by its chemical nature and depends oncondensability, which is represented by boiling temperature(Tb), critical temperature (Tcr) or Lennard–Jones constant(ε/k) [1,2]. It is known that in the hydrocarbon series anincrease in condensability is accompanied by a parallel in-crease in the size of molecules (Table 1, [3–11]). It is there-fore not surprising that in both glassy and rubbery polymercorrelations of hydrocarbon solubility in the polymers withcondensability and the sizes of hydrocarbon molecules areobserved.

To analyze sorption of penetrants, including hydrocar-bons, in glassy polymers, the dual-mode sorption model ismost frequently used. For a number of glassy polymers,correlations between dual-mode sorption model constantsand condensability of hydrocarbons have been established[12–14]. Temperature dependence of model constants is de-scribed by Vant-Hoff equation, where the exponent con-tains heat of penetrant sorption�Hs. This quantity is essen-tially dependent on heat of penetrant condensation�Hcond:�Hs = �Hcond + �H1, where�H1 is partial molar en-thalpy of penetrant dissolution in the polymer. The sorptionof easily condensable hydrocarbons features negative valuesof sorption heat as a result of high negative values of theircondensation heat.

0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.memsci.2003.11.022

190 S.I. Semenova / Journal of Membrane Science 231 (2004) 189–207

Table 1Physical properties of some gases and vaporsa [3–10]

Penetrant Condensability Size of molecule

Tb (K) ε/k (K) σLJ (nm) σkt (nm) aD (nm2)

N2 77.4 91.5 0.368 0.364O2 90.2 113 0.343 0.346CO 81.7 110 0.359 0.376CH4 111.7 137 0.382 0.380C2H4 169.5 205 0.423 0.390C2H6 184.5 230 0.442C3H6 225.5 303 0.468 0.450C3H8 231.1 254 0.506 0.430C4H6 281.2 0.440C4H8 266.9 330 0.528 0.560n-C4H10 272.7 410 0.534 0.430iso-C4H10 261.3 313 0.534n-C5H12 309.2 345 0.578C6H6 353.3 412 0.527 0.660 0.21C6H5CH3 383.8 0.593 0.23n-C6H14 341.9 413 0.591 0.18Cyclo-C6H12 353.9 324 0.609 0.670 0.33n-C7H14 371.6n-C8H18 398.8 320 0.745 0.18iso-C8H18 390.8 0.762 0.36

a σLJ is the molecular collision diameter calculated from the Lennard–Jones potential;σkt is the molecular kinetic diameter determined usingzeolites; aD is the minimum molecular cross-section determined fromStuart’s molecular model[4,5].

A linear correlation between solubility of various gasesin glassy polymers and distance between chains of macro-molecules has been established using X-ray structural anal-ysis. The solubility of gases, including lower hydrocarbons,increases with this distance[2,15,16]. Similarly, it has beenfound that the solubility of gases in glassy polymers in-creases with the molar fraction of free volume of the poly-mer[2,17,18]. It has been reported that the Langmuir modesaturation constant is dependent on accessible free volumeof the polymer, and the accessible free volume of the samepolymer decreases with increase in the size of penetrantmolecule[12]. The solubility of hydrocarbons in rubberypolymers can be described in more detail by several the-ories of solution using various criteria of thermodynamicaffinity [1,19–22], of which the Flory–Huggins theory is themost popular one. It takes into account the volume contentof the penetrant dissolved in the polymer and the changein the length of the polymer’s thermodynamic segment asa result of dissolution[1]. However, it should be pointedout that to describe dissolution, a refined dual-mode sorp-tion model can be used, e.g., the model by Pace and Datyner[1,23,24].

Diffusion selectivity is based on the ability of the polymermatrix to transmit molecules of a certain shape and size. Thisability is determined by the structure of the polymer and therigidity of the macromolecular ensemble as well as by theproperties of the penetrant, size and shape of its molecules.

It has been shown in[6] that, in accordance with theequation for the activation energy of diffusion proposed by

P. Meares[25], the cohesion energy density of the polymerhas a significant effect on diffusion coefficients of lower hy-drocarbons. This is especially typical of rubbery polymers:an increase in the cohesion energy density of the polymerresults in reduction of diffusion coefficients. Similar depen-dencies also exist for glassy polymers[3,26]. The effect ofmolar fraction of polymer free volume on diffusion of hydro-carbons in the polymer is especially significant in the caseof glassy polymer[2]. It has been shown in several studiesthat the diffusion coefficients of penetrants, including hy-drocarbons, decrease with the decrease in molar fraction offree volume of the polymer[3,26].

The effect of size of hydrocarbon molecules on theirdiffusion coefficients in glassy and rubbery polymers hasbeen discussed in the literature[3–6,11,26,27]. To esti-mate the molecular size of low MW penetrants permeatingthrough the polymer membrane, the following parame-ters are used: Van der Waals volume of the molecule,V; collision diameter calculated from the Lennard–Jonespotential,σLJ; kinetic diameter of the molecule,σkt, de-termined using molecular sieves; minimum cross-sectionof the molecule,aD, determined from Stuart’s molecularmodel [3–10]. Values of these parameters can differ sig-nificantly especially for molecules of oblong shapes andthe difference increases with increase in the number ofcarbon atoms in the hydrocarbon molecule (seeTable 1).It has been shown that effective cross-sections of diffusingmolecules of hydrocarbons determined from diffusion ofargon, whose molecular cross-section was derived fromStuart’s model, have different values in glassy and rubberypolymers[6,26,27]. Effective cross-sections of molecules inpolymer glasses are greater than those in rubbery polymers.Linear dependencies of effective cross-section of hydrocar-bon molecules on the number of carbon atoms have beenestablished, but these dependencies are different for alkanes,alkenes, alkynes, and dienes. The highest values of effec-tive cross-section are found in alkanes, the lowest in dienehydrocarbons.

The permeation hydrocarbons in polymer membranes istherefore governed by the basic regularities typical of per-meation of low MW penetrants, modified however by certainpeculiarities related to the structure and shape of hydrocar-bon molecules. It makes sense to consider physico-chemicalregularities of hydrocarbon separation and removal usingpolymer membranes and try to reveal the relationship be-tween the chemical structure of polymers and their separa-tion properties with respect to mixtures containing hydro-carbons. It follows from literary data that glassy polymersare used in practice for separation of olefins and paraffins aswell as for separation of aromatic, alicyclic, and aliphatic hy-drocarbons. Rubbery polymers are mainly used in gas/vaporseparation processes for selective separation of hydrocarbonvapors from their mixtures with air as well as in pervapora-tion processes for the extraction of hydrocarbons from theiraqueous solutions. These issues constitute the subject of thepresent review.

S.I. Semenova / Journal of Membrane Science 231 (2004) 189–207 191

2. Separation and removal of hydrocarbons usingmembranes based on glassy polymers

2.1. Separation of olefins and paraffins

Ethylene is found in coke-oven gas, gases resulting fromoil processing, e.g., cracking. It is produced by pyrolysisof liquid distillates of oil or lower paraffin hydrocarbons(ethane, propane, butane), or by dehydration of ethanol.Ethylene is used to produce polyethylene and its copoly-mers, oxides of ethylene, ethanol, ethyl benzene, acetalde-hyde, vinyl chloride, and vinyl acetate. Propylene is usedas feedstock in the production of polypropylene, acryloni-trile, propylene oxide, allyl chloride, acrolein, acrylic acid,and its ethers/esters, cumene, and butanols. Propylene is oneof the products of catalytic cracking of liquid hydrocarbonwaste products. It is also produced by dehydrogenation ofpropane. 1,3-Butadiene is produced by catalytic dehydrationof butane andn-butylenes present in natural gas and gas re-sulting from oil processing, as a side product of ethyleneproduction by pyrolysis of oil stock, or by catalytic decom-position of ethanol. Propylene is used in the production ofrubbers and plastics.

It is very difficult to separate the mixtures of olefins andparaffins produced in the above processes, because these va-pors have similar physical properties. The membrane methodis quite promising. When using this method the correctchoice of the membrane material is very important.

2.1.1. The effect of unsaturated bonds on the size of olefinmolecules and on the capability of olefins to enter intospecific interactions with the membrane matrix

In membrane separation of the olefin/paraffin mixture,the predominant selective separation of the olefin is evident.Firstly, the olefin molecule is smaller in size as comparedto the respective paraffin. It is known that C–C distance inparaffins is 0.1534 nm, whereas the C=C distance in olefinsis 0.1337 nm. Atoms of carbon in paraffins are characterizedby sp3 hybridization and free rotation around C–C bonds.Atoms of olefins are characterized by sp2 hybridization.The rigid C=C bond impedes internal rotation in the olefinmolecule and makes it flat. It is, therefore, clear why olefinmolecules are smaller in size compared to paraffin and whythe diffusion coefficients of olefins in polymers would behigher than those of paraffins. Secondly, the presence of un-saturated bonds in olefin molecules makes them capable ofspecific interactions with the membrane matrix. The attemptof implementing these capabilities resulted in developmentof an important field of research: facilitated transport.

Several studies are concerned with the problem of sep-arating olefins and paraffins using membrane technology,including facilitated transport using liquid membranes orion-exchange charged membranes containing ions of transi-tion metals as complexing agents[28–37]. It is known thatolefins are capable of reversible complexing with transitionmetals, including d-elements. The nature of this interaction

was first explained by Dewar in 1951 in terms of molecu-lar orbital theory. Dewar postulated that the interaction withthe atomic orbitals of the olefin determines the stability ofthe complex formed. In the course of complexing, metal andolefin act as electron donor and electron acceptor, respec-tively. � is the bond component in the complex that is formedas a result of overlapping between external s-orbitals of themetal and the bonding�-molecular orbital of the olefin (inthe case of positively charged metal ions the absent valents-electrons can be regarded as a vacant s-orbital).� is thebond component in the complex that is formed as a resultof transfer of electrons from the completely filled d-atomicmetal orbital to the vacant antibonding�∗-molecular orbitalof the olefin. In assessing the possible use of complexingagents for selective separation of olefins from gaseous mix-tures, the price of the complexing agent and the strengthof the complex should be considered formed. Such tran-sition metals as Pt(II) and Pd(II) are expensive and formtoo strong complexes with olefins, whereas Cu(I) and Ag(I)are relatively cheap and form reversible complexes. Suchmembranes have very high olefin/paraffin selectivity (up to20–60) and relatively high permeability to olefin. However,processes based on facilitated transport have several limita-tions, e.g., they should be carried out in saturated water va-por. In addition, there exists the serious problem of carrieraging. Application of membranes operating on the basis offacilitated transport for separation of olefins and paraffinsis beyond the scope of the present review and has been al-ready considered in the literature[28–37]in sufficient detail.However, the example of facilitated transfer shows that it ispossible to achieve high values of permselectivity by imple-menting specific interactions of olefins with the membranematrix.

2.1.2. Polymer membranes based on various glassypolymers

Separation of olefins and paraffins using polymer mem-branes without carrier is dealt with in several reports. Henleyand Santos[38] investigated the permeability of polyethy-lene films and found that olefin flux and olefin/paraffinselectivity are very low. Ito and Hwang[39] as well as Srid-har and Khan[40] investigated permeability of derivativesof cellulose and established that ethyl cellulose is a goodmaterial for selective extraction of olefins from its mixtureswith saturated hydrocarbons. Application of polysulfone asa membrane material for separation of olefins and paraffinsis also reported[39]. Table 2presents transport propertiesof some glassy polymers with respect to propylene/propanemixtures, C3H6/C3H8 [40]. It can be seen that among poly-mers listed inTable 2 polyphenylene oxide is the mostselective whereas ethyl cellulose and polysulfone yield thehighest flux. However, analysis of literature data shows thatfor the above purposes polyimides offer the highest fluxesand selectivities, closely followed by polyphenylene oxides.The properties of these polymers are discussed below. Idealolefin/paraffin selectivity is three times higher in polyimides


Table 2Permeability coefficients of propylene, C3H6 and propane, C3H8, as wellas selectivity C3H6/C3H8 for some glassy polymers

Polymer P (Barrer)

C3H6 C3H8 αP (C3H6/C3H8)

PPO 9 2.1 4.3EC 52 16 3.3CA 15.2 5.8 2.6PSF 25 17.8 1.4

PPO, polyphenylene oxide; EC, ethyl cellulose; CA, cellulose ac-etate; PSF, polysulfone (for mixtured vapors of C3H6/C3H8 composition55 mol%/45 mol%;p = 3 × 105–4× 105 Pa;T = 303 K) [40].

than in cellulose derivatives and polysulfone[41]. Perme-ability of various polymers to olefins rises with increasingfree volume of the polymer; the process is accompanied byparallel reduction of ideal olefin/paraffin selectivity (see,e.g., the data inFig. 1). Let us consider these effects in moredetail for the best known and promising polymer materials:polyimides and polyphenylene oxides.

2.1.2.1. Polyimides as membrane materials for separationof olefins and paraffins. Reference[2] contains a detailedanalysis of the relation between chemical composition ofpolyimides and their separation properties. It is shown thatthe molar fraction of free volume of the polymer increasesafter introduction of bulky substituents into the elementaryunit. It is therefore not surprising that the attention of re-searchers is focused on polyimides with bulky substituents,e.g., polyimides produced on the basis of such dianhydrideas 6FDA-dianhydride 4,4′-hexafluoroisopropylidene ofdiphthalic acid (containing two bulky CF3-groups in the acidfragment) and such diamines as TrMPD—2,4,6-trimethyl-1,3-phenylenediamine (containing three CH3-groups inthe amine fragment); TeMPD—2,3,5,6-tetramethyl-1,4-phenyldiamine (containing four CH3-groups in the aminefragment); 4APF—4,4′-hexafluoroisopropylydene dianiline(containing two bulky CF3-groups in the amine fragment);DDBT—dimethyl-3,7-diaminodiphenyl-thiophene-5,5′-

Table 3Permeability, diffusion, and sorption coefficients, as well as their ideal ratios, for vapors of C2H4, C2H6 (T = 323 K; pressure, 2× 105 Pa)a [26]

Polymer PC2H4 DC2H4 SC2H4 αidP (C2H4/C2H6) αid

D (C2H4/C2H6) αidS (C2H4/C2H6)

6FDA–TrMPD 58 390 1.5 2.9 2.9 1.06FDA–TeMPD 5.8 4.3

a P, Barrer;D, 10−10 cm2 s−1; S, 10−1 cm3 cm−3 cmHg−1.

Table 4Permeability coefficients of ethylene and propylene, as well as values of olefin/paraffin ideal selectivity for several polyimides (T = 308 K; pressure,3.8 × 105 Pa) [41]

Polyimide PC2H4 (Barrer) αidP (C2H4/C2H6) PC3H6 (Barrer) αid

P (C3H6/C3H8)

6FDA–m-PDA 0.3 3.3 0.13 106FDA–IPDA 1.4 3.8 0.58 156FDA–4APF 2.1 4.4 0.89 16

Fig. 1. Dependence of permeability coefficient of propylene C3H6

(�) and ideal selectivity propylene/propaneαidP (C3H6/C3H8) (�)

in various polymers on reciprocal the inverse molar fraction offree volume of the polymer: polyimides—(1) 6FDA–TeMPD; (2)6FDA–TrMPD; (3) 6FDA–DDBT; (4) 6FDA–ODA; (5) BPDA–TeMPD;(6) BPDA–TeMPD; (7) BPDA–ODA; (8) PPO, polyphenylene oxide; (9)P4MP, poly-4-methylpentene-1; (10) 1,2-PB, polybutadiene; (11) PDMS,polydimethylsiloxane. Experimental temperature, 323 K; pressure, 2 atm.From analysis of results presented in[3,26].

dioxide (industrial product of Ube: a mixture of isomershaving two methyl groups located differently in the phenylnucleus, namely, 63% in the 2,8-position, 33% in the 2,6-position, and 4% in the 4,6-position).

Permeability to vapors of ethylene and ethane: Table 3shows values of mass transfer parameters of ethyleneand ethane for 6FDA–TrMPD and 6FDA–TeMPD poly-imides, andTable 4, those for 6FDA–m-PDA, 6FDA–IPDA, and 6FDA–4APF polyimides (m-PDA—meta-phenylenediamine; IPDA—2,2′-bis(4-aminophenyl) iso-propane). It can be seen that the permeability of ethylene ishigher compared with that of ethane. The ideal permselec-tivity in these polymers isαid

P (C2H4/C2H6) = 2.9–4.4. The


Table 5Permeability, diffusion, and sorption coefficients as well as their ideal ratios for vapors of C3H6 and C3H8 (T = 323 K; pressure 2× 105 Pa)a [3,26]

Polymer Vf PC3H6 DC3H6 SC3H6 αidP (C3H6/C3H8) αid


6FDA–TeMPD 0.182 37 190 2.0 8.6 7.0 1.36FDA–TrMPD 0.182 30 130 2.3 11 8.8 1.26FDA–DDBT 0.169 0.76 4.2 1.8 27 27 1.06FDA–ODAb 0.165 0.48 8.7 0.54 11 8.9 1.3

BPDA–TeMPD 0.136 3.2 21 1.5 13 11 1.2BPDA–DDBTb 0.125 0.12BPDA–ODAb 0.121 <0.05

PPO 0.206 2.3 18 1.3 9.1 8.2 1.1P4MP 0.209 54 2100 0.26 2.0 2.3 0.91,2-PB 0.200 260 4600 0.57 1.7 1.7 1.0PDMS 0.362 6600 90000 0.73 1.1 1.3 0.9

a P, Barrer;D, 10−10 cm2 s−1; S, 10−1 cm3 cm−3cmHg−1.b At T = 373 K.

main contribution to the permeation selectivity is made bythe diffusion selectivity. Thus, for 6FDA–TrMPD polyimideαid

D (C2H4/C2H6) = 2.9, whereas the sorption selectivity isonly αid

S (C2H4/C2H6) = 1.0.Permeability to vapors of propylene and propane: It

can be seen from data shown inFig. 1 and Tables 4and 5that sufficiently high permeability to propylene andthe highest values of ideal selectivityαid

P (C3H6/C3H8) arefound in polyimides of the composition 6FDA–TeMPD,6FDA–TrMPD, 6FDA–DDBT, 6FDA–ODA, and BPDA–TeMPD (ODA, oxide aniline; BPDA, dianhydride of3,3′,4,4′-diphenyltetracarboxylic acid). Among polyimides,the highest flux to propylene is found in 6FDA–TeMPDpolyimide (PC3H6 = 37 Barrer, at 323 K and propylenepressurepC3H6 = 2 × 105 Pa), and the highest selectivityto the propylene/propane system is found in 6FDA–DDBTpolyimide (αid

P (C3H6/C3H8) = 30 at 323 K and pressure ofindividual components of 2× 105 Pa). Diffusion selectivityof polymides isαid

D (C3H6/C3H8) = 7–27, whereas sorptionselectivity is much lower, onlyαid

S (C3H6/C3H8) = 1.0–1.3.Permeability to vapors of 1,3-butadiene,C4H6, n-butane,

C4H10, and 1-butene, C4H8: The authors of[26] investi-gated the permeability of individual vapors of 1,3-butadieneand n-butane in some glassy polymers: in 6FDA–TrMPD,6FDA–DDBT polyimides, and in polyphenylene oxide(see data presented inTable 6). The investigated poly-imides had high permeability to 1,3-butadienePC3H6 =65−−110 Barrer, as well as high values of ideal permselec-tivity αid

P (C4H6/C4H10) = 67–190 (at pressure 1× 105 Paand 323 K). Diffusion selectivity wasαid

D (C4H6/C4H10) =

Table 6Permeability, diffusion, and sorption coefficients, their ideal ratios for vapors of C4H6 and C4H10 (T = 323 K; pressure, 1× 105 Pa)a [26]

Polymer PC4H6 DC4H6 SC4H6 αidP (C4H6/C4H10) αid


6FDA–TrMPD 111 225 4.1 67 45 1.56FDA–DDBT 6.5 14 4.4 190 110 1.7PPO 4.2 15 2.7 33 22 1.5

a P, Barrer;D, 10−10 cm2 s−1; S, 10−1 cm3 cm−3 cmHg−1.

Fig. 2. Dependence of 1,3-butadiene/n-butane ideal selectivity,αidP

(C4H6/C4H10) (�) and 1,3-butadiene/1-butene ideal selectivity,αidP

(C4H6/C4H8) (�) on permeability of 1,3-butadiene,PC4H6, for variouspolymers. Polyimides: 6FDA–p-ODA and 6FDA–4APF; PSF, polysul-fone; FEP, Teflon; PTFE, polytetrafluoroethylene; PE, polyethylene; BTC,4,4′-ODA-polycarbonate[42].

45–110, whereas sorption selectivity is much lower andcorresponds to onlyαid

S (C4H6/C4H10) = 1.5–1.7.The authors of[42] investigated the permeability of

1,3-butadiene,n-butane, 1-butene in various glassy poly-mers, including polyimides based on 6FDA anhydride,notably 6FDA–p-ODA and 6FDA–4APF polyimides.Fig. 2shows the dependence of ideal selectivityαid

P (C4H6/C4H10)and ideal selectivityαid

P (C4H6/C4H8) on the permeabil-ity of 1,3-butadiene. It can be seen that polymers havingthe highest permeability (PC4H6 = 102−−103 Barrer) andthe highest selectivity (αid

P (C4H6/C4H10) = 104–105 andαid

P (C4H6/C4H8) = 103–104) are polyimides based on6FDA-dianhydride.


Table 7Comparison of permeability and separation properties of the C3H6/C3H8

system for individual penetrants, as well as for their equimolar mixture,in various polyimides and in polyphenylene oxide PPO (partial pressureof penetrants, 2× 105 Pa;T = 323 K) [3]

Polymer Penetrant PC3H6

(Barrer)PC3H8

(Barrer)αid

P or αP

6FDA–TrMPD Individual 27 2.7 10.0Mixture 20 3.3 6.0

BPDA–TeMPD Individual 4.0 0.3 13.0Mixture 3.4 0.42 8.0

PPO Individual 2.9 0.32 9.0Mixture 2.7 0.50 5.4

6FDA–DDBT (373K) Individual 2.6 0.17 15.0Mixture 2.0 0.20 10.0

2.1.2.2. Polyphenylene oxides as membrane materials forseparation of olefins and paraffins.Polyphenylene oxidesare quite promising materials for separation and removal ofolefins and paraffins. By their properties these polymers areonly inferior to polyimides (seeTables 5–7andFigs. 1, 3, 4).

Separation properties of poly-2,6-dimethyl-1,4-phenyleneoxide (p-DMePO) and poly-2,6-diphenyl-1,4-phenylene ox-ide (p-DPhPO), as well as chemically modifiedp-DMePOhave been studied by a number of researchers for the purposeof using these polymers in gas separation and pervapora-tion processes[12,40,43]. p-DMePO/p-DPhPO copolymersoffer a number of advantages compared to homopolymers:

Fig. 3. Permeability and selectivity of polyimides ((�), (�)), as well asof some other polymers ((�), (�)) to individual vapors of C4H6, C4H10,and their mixtures [properties to individual penetrants were determinedat 1× 105 Pa, 323 K and are designated with open symbols ((�), (�));properties to equimolar mixtures of penetrants were determined at totalpressure of 1.5 × 105 Pa, 323 K and are designated with closed sym-bols ((�), (�))]. Polyimides: (1) 6FDA–TrMPD; (2) 6FDA–DDBT; (3)6FDA–4APF; (4) BPDA–TrMPD; (5) BTDA–TrMPD; (6) DSDA–DDBT;other polymers: (7) PSF, polysulfone; (8) PPO, polyphenylene oxide; (9)P4MP, polymethylpentene[26].

Fig. 4. Dependence of ideal selectivityαidP (C3H6/C3H8) on permeability

coefficient of propylene in polyimides at various temperatures: (1) 308 K;(2) 323 K; (3) 353 K; (4) 373 K; (5) 393 K; (6) 398 K; (7) 423 K (pressure2 × 105 Pa) [3].

better mechanical properties, higher resistance to oxidantsand radiation, which is why these copolymers have also beenstudied rather closely[12,43]. Investigation ofp-DMePOand p-DPhPO homopolymers has shown that side groupshave a significant effect on free volume of the polymer andits transport properties. It has been established that the mo-lar fraction of free volume ofp-DPhPO having phenyl sidegroups is smaller than the molar fraction of free volumeof p-DMePO having methyl side groups. Therefore, per-meability and diffusion coefficients of hydrocarbons in thep-DPhPO polymer are smaller compared to thep-DMePOpolymer (seeFig. 5). It is also not surprising that permeabil-ity and diffusion coefficients of hydrocarbons decrease withincrease in the content of the diphenyl-substituted compo-nent in the copolymer.

Thus, it follows from the above data that glassy polymersare promising materials for selective separation of olefinsfrom their mixtures with paraffins. The main contributionto the high values of permselectivity is made by the diffu-sion component,αD. However, unsaturated bonds in olefinmolecules make them capable of specific interactions with�-conjugated polar groups. Introduction of such groups intothe polymer membrane would make it possible to signifi-cantly raise sorption selectivity,αS. Unfortunately, researchactivity in this field is not sufficiently high.

2.1.3. Dependence of permeability and selectivitycoefficients on penetrant pressure: effect of plasticizationof the polymer by the penetrant

Permeability coefficients of olefins and paraffins andolefin/paraffin selectivity coefficients in polymers are es-sentially dependent on partial pressure of the penetrants.By way of example,Fig. 6 shows the dependence of thepermeability of 6FDA–TrMPD polyimide to such individ-ual components as propane and propylene on their partial


Fig. 5. Dependence of permeability coefficients of polyphenylene ox-ides to individual penetrants CH4 and C2H4 on pressure of CH4 andC2H4, respectively (T, 298 K). Dependence of permeability coefficient ofpolyphenylene oxides to C2H6 on partial pressure C2H6: (�) C2H6/He,20 mol%/80 mol% mixture; (�) C2H6/He, 35 mol%/65 mol% mixture; ()pure C2H6 (p-DMePO, polydimethylphenylene oxide;p-DPhPO, poly-diphenylphenylene oxide; CP1,p-DMePO/p-DPhPO, 97.5 mol%/2.5 mol%copolymer; CP2,p-DMePO/p-DPhPO, 75 mol%/25 mol% copolymer)[12].

Fig. 6. Dependence on pressure of permeability coefficients of propyleneand propane for 6FDA–TrMPD polyimide (T, 323 K) [3].

Fig. 7. Dependence of permeability coefficients of propylenePC3H6,propanePC3H8 and selectivity on partial pressure for the equimolar gasmixture in 6FDA–TrMPD polyimide (total pressure 3.1× 105 Pa; 323 K).For comparison, the corresponding data for individual components des-ignated with open symbols[3].

pressure. It can be seen from the figure that the permeabilitydecreases until pressure reaches 5×105 Pa, and then beginsto rise. The reduction of permeability is explained by thedual-mode sorption model and is related to sorption sites ofthe polymer being filled by the penetrant. The increase inpermeability at pressures above 5× 105 Pa is accounted forby plasticization of the polymer by the penetrant[3,26].

Permeability and diffusion coefficients of hydrocarbonsin polyphenylene oxides are also essentially dependent onpressure (seeFig. 5). It can be seen that in the case ofethylene, with increase in pressure, the permeability co-efficients first decrease, and then begin to rise. Reference[12] quotes constants of the dual-mode sorption model for anumber of hydrocarbons permeation through polyphenyleneoxide.

Table 7 compares the separation properties of variouspolyimide to individual penetrants, propane and propylene,as well as their mixtures.Fig. 7 shows dependencies of thepermeability coefficientsPC3H6 and PC3H8 and of the se-lectivity coefficientαP (C3H6/C3H8) for gaseous mixtures,as well as for individual penetrants, in 6FDA–TrMPD poly-imide on partial pressure. Comparison of permeabilities ofpenetrants in the case of individual transport to those in thecase of transport of an equimolar mixture (at equal partialpressure) shows that the permeability coefficient of propy-lenePC3H6 for mixtures is lower by 20–30%, and the per-meability coefficient of propanePC3H8 is higher by 10–40%than those for individual penetrants. As a result, the selec-tivity in the mixture is much lower than the ideal selectivity[3]. Similarly, in the case of permeation of the C4H6/C4H10mixture through polyimides, the permeation selectivity is


much lower than the ideal value which is also accounted forby the plasticization effect.

2.1.4. Dependence of permeability and selectivitycoefficients on temperature

It has been shown in[1,44–47]that for gases and vaporsstrongly sorbed by polymers, non-monotonic temperaturedependencies of mass transfer coefficients are observed.This results from negative heat of penetrant sorption andplasticization of the polymer by the penetrant in the courseof sorption. Negative heat of penetrant sorption can becaused by its specific interaction with the polymer as well ashigh values of condensation heat, which is typical of easilycondensable hydrocarbons. The possibility of existence ofnon-monotonic temperature dependencies of mass transfercoefficients is demonstrated and corroborating experimentaldata are presented, for penetrants interacting with polymermembranes in a specific way. It has been demonstrated,among other things, that in the case of variation of tem-perature, at constant concentration (pressure) of stronglysorbable penetrant in the polymer, the condition for exis-tence of a temperature corresponding to the minimum valueof diffusion coefficientTmin(D) is a negative value of sorp-tion heat�HS, and for emergence of a temperature corre-sponding to the minimum value of permeabilityTmin(P), anadditional condition should be met: |ED|>|�HS|, whereED isdiffusion activation energy. It has been established that sinceat �HS < 0, ED > ED + �HS (because the diffusion acti-vation energy is an essentially positive quantity),Tmin(D) <

Tmin(P) is observed, i.e. the minimum point in the perme-ability curveP(1/T) is shifted with respect to the minimumpoint in the diffusion curveD(1/T) towards higher temper-atures. In the curves showing the dependence of selectivityof temperature at constant penetrant pressureαP(T)p as wellas in curves showing the dependence of selectivity on pene-trant pressure at constant temperature�P(p)T , emergence ofmaxima is possible. These conclusions are quite importantfor selection of an optimum mode of using the membranes.Theoretical findings of[1,47] experimentally corroboratedfor interacting polymer-penetrant systems can certainly beapplied to systems consisting of a polymer and an easilycondensable hydrocarbon. Unfortunately, sufficient experi-mental data corroborating this conclusion are not yet avail-able in the literature, although some regularities of variationof mass exchange properties as a function of temperature areconsidered in several reports[3,26]. Fig. 4shows dependen-cies of ideal selectivityαid

P (C3H6/C3H8) on permeabilitycoefficientPC3H8 for polyimides and polyphenylene oxideat various temperatures, andTable 8lists values of perme-ability and diffusion activation energies, as well as heat ofhydrocarbon sorption. It can be seen that the increase intemperature in the range 308–423 K is accompanied by in-crease in the permeability and decrease in the selectivity co-efficient of the polymers. However, the existence of turningpoints is possible in a different range of temperatures andconcentrations.

Table 8Activation energies of permeability,�EP; diffusion, �ED; as well as heatof penetrant sorption,�HS; for C3H6 and C3H8 in polyimides (pressure,2 × 105 Pa)a [3]

Polymer C3H6 C3H8

�EP �ED �HS �EP �ED �HS

6FDA–TeMPD 2 16 −14 4 17 −136FDA–TrMPD 4 15 −11 9 20 −116FDA–DDBT 18 30 −13 29 44 −156FDA–ODA 24 43 −19 34 56 −22

BPDA–TeMPD 18 28BPDA–DDBT 23 52

PPO 11 19

a �EP, �ED, and�HS (kJ mol−1).

2.2. Separation of aromatic, alicyclic, and aliphatichydrocarbons

The separation of mixtures of aromatic, alicyclic, andaliphatic hydrocarbons, as well as their isomers, e.g., ben-zene and cyclohexane, toluene, andiso-octane, isomers ofxylol has been actively developing in recent years. At presentsuch separations are performed by fractional distillation,which is very energy expensive. In addition, this methodcannot be used for separation of organic mixtures havingsimilar boiling temperatures. Pervaporation is a good alter-native in this case. Research is this area is mainly focused ondevelopment of membranes offering high separation proper-ties and good stability to the feed mixture. The polymer ma-terials mainly used for the membranes are glassy polymers,first and foremost polyimides. The use of glassy polymershaving a rigid ensemble of macromolecules results in highseparation effectiveness. Separation effectiveness in perva-poration processes is characterized by separation factor,βP,which is determined by a diffusion component,βD, and asorption component,βS [1,48].

2.2.1. Diffusion component of separation factorβD

This parameter is determined by the ratio of molecularsizes of the penetrants being separated and the rigidity den-sity of packing the macromolecules. The greater differencebetween penetrant molecular sizes and the more dense andrigid the polymer matrix, the larger the diffusion componentof the separation factor[1,48].

2.2.1.1. Dependence ofβD on sizes of the penetrantmolecules. Section 1deals with the issue of values ofeffective diameters of hydrocarbons penetrating throughrubbery and glassy polymers (seeTable 1). As shown in[4,5], when comparing sizes of differently shaped moleculesit would be more correct to use as criterion the minimumcross-section of the diffusing moleculeaD determined fromStuart’s molecular model. This is especially important foroblong molecules, e.g.,n-hexane andn-octane. It can beseen from the data presented inTable 1that molecules of


hydrocarbons can be ordered by the size of minimum cross-section in the following series:n−hexane≤ n−octane<

benzene< toluene < cyclohexane< iso−octane. It isclear that the diffusion coefficients of these penetrantsshould vary in reverse order. This conclusion has beenconfirmed in[4] where the possibility of pervaporation sep-aration of hydrocarbon mixtures using membranes based onDSDA–DDBT polyimide (DSDA—dianhydride of 3,3′,4,4′-diphenylsulfonetetracarboxylic acid) has been examined.The diffusion coefficients of the investigated hydrocarbonsare indeed ordered in the series that is reverse to the se-ries of minimum cross-sections of penetrant molecules.The membranes had preferential permeability for aromaticover aliphatic compounds. The strongest dependence of theseparation factor and the permeability on composition ofthe mixture being separated was found in mixtures whosecomponents had the greatest difference in molecular size.

Reference[49] deals with the pervaporation separationof benzene/cyclohexane, toluene/iso-octane mixtures us-ing DSDA–TrMPD/DEB (DEB—2,2′-diethynylbenzidine)copolyimide. The dominant role of the diffusion componentin the separation process has been established. It can beseen from the data inTable 9that increase in the differencebetween ratios of minimum cross-sections of penetrantmolecules results in increase of the separation factor whichis determined mainly by the diffusion component.

2.2.1.2. Factors determining rigidity of the ensemble ofmacromolecules. An important factor that determines dif-fusion selectivity of the membrane is the rigidity of thepolymer structure, its ordered arrangement as well as sta-bility to the mixture being separated. The rigidity of thepolymer structure is determined by the make-up of indi-vidual macromolecules (primarily by potential barriers torotation around intramolecular bonds) as well as the rigid-ity of the ensemble of macromolecules (primarily, strengthof intra- and intermolecular interactions in the polymer).Intra- and intermolecular interactions in the polymer andbetween polymers produce a network of bonds, crosslinks.Intermolecular interactions can be of various types, includ-ing dipole–dipole, dispersion interactions, hydrogen bonds,ion–ion interactions, as well as chemical and radiationcrosslinking. In all cases crosslink density and strength ofbonds in the crosslinks, and, consequently, of the diameters

Table 9Pervaporation properties of the membrane based on DSDA–TrMPD/DEB polyimide containing 10 wt.% tetracyanoethylene, TCNE (content of aromaticcomponent in mixtures being separated 40–50 wt.%; 343 K; membrane thickness, 21�m) [4,5,49]

Composition of the mixture (aD)arom/(aD)ala (nm2/nm2) Qw (kg�m m−2 h−1) βP (arom/al)b

Benzene/n-hexane 0.21/0.18 2.8 9.1Toluene/n-octane 0.23/0.18 2.1 13Benzene/cyclohexane 0.21/0.33 0.44 48Toluene/iso-octane 0.23/0.36 1.1 330

a (aD)arom/(aD)al, minimum cross-section of diffusing molecule determined from Stuart’s molecular model, for aromatic and alicyclic (or aliphatic)component of the mixture being separated.

b βP(arom/al), separation factor of aromatic component with respect to alicyclic (or aliphatic) component.

of the diffusion channels in the polymer, are ultimatelyregulated by the chemical structure of the polymer[1,48].

2.2.1.3. Effect of the increase in rigidity of the polymersystem on the diffusion component of separation factor.Thermal crosslinking of unsaturated bonds of macro-molecules: Reference[49] is concerned with the perva-poration properties, with respect to benzene/cyclohexanemixture being separated, of several ethynyl-containingcopolyimides (the original composition for preparation ofpolyimides contained 2,2′-diethynylbenzidine) that werethermally crosslinked through unsaturated bonds. It can beseen from the data presented inTable 10that increasingcontent of diethynylbenzidine DEB (i.e. increasing contentof unsaturated bonds used for crosslinking) enhances theseparation factorβP (benzene/hexane).Table 11shows per-vaporation properties of crosslinked and non-crosslinkedDSDA–TrMPD/DEB polyimides. It can be seen that ther-mal crosslinking resulted in a nearly two-fold increase ofthe diffusion component of the separation factor (βD) withinsignificant change of the sorption component (βS).

Increasing cohesion energy of the polymer by introductionof active additives: In order to increase the sorption com-ponent of the separation factor for the benzene/cyclohexanemixture, the matrix of DSDA–TrMPD/DEB polyimide wasfilled with a homogeneously distributed tetracyanoethy-lene, an electron acceptor having high affinity for ethynyl-containing fragments (DEB component in polyimide) andaromatic substances[49]. As a result, there was an increasenot only of the sorption component (that will be discussedbelow inSection 2.2.2) but also of the diffusion componentof the separation factorβD (seeTable 12). This effect canbe explained by increase in rigidity of the polymer systemas a result of the rise in cohesion energy after introductionof the additive, whose active groups have�-electron accep-tor properties and can form charge transfer complexes withunsaturated fragments of macromolecules.

Increasing cohesion energy of the polymer by introduc-tion of polar groups: Introduction into a polymer of the polargroups with�-electron acceptor properties (such as ethynyl,phosphorylate, sulfone, acrylate, and phenyl groups) in-creases the cohesion energy of the polymer, which enhancesthe rigidity of the polymer ensemble, inhibits swelling, andthus increases the diffusion component of the separation


Table 10Effect of crosslinking density of polyimides containing 2,2′-diethynylbenzidine on their pervaporation properties to the benzene/cyclohexane mixture(composition of the mixture, 50 wt.%/50 wt.%; 343 K; thickness of membranes, 20–40�m; crosslinking of membranes was carried out by thermal treatmentat 350◦C for 0.5 h) [49]

Polyimide Qw (kg�m m−2 h−1) βP (benzene/cyclohexane)

DSDA–TrMPD 10.6 7.3DSDA–TrMPD/DEB (9/1) 4.1 14.0DSDA–TrMPD/ODA/DEB (6/1/1) 3.4 13.3DSDA–TrMPD/ODA/DEB (2/1/1) 1.5 21.0

Table 11Sorption and pervaporation properties of membranes based on crosslinked and non-crosslinked DSDA–TrMPD/DEB (9/1) polyimides to ben-zene/cyclohexane mixture at 343 K

Polymer XBz (wt.%) SBz (wt.%) YBz (wt.%) βS βD

DSDA–TrMPD/DEB (9/1) non-crosslinked 60 25 78.4 2.4 2.9DSDA–TrMPD/DEB (9/1) crosslinked 60 22 80.2 2.7 5.2

XBz, content of benzene in the feed mixture;YBz, content of benzene in the permeate;SBz, solubility of benzene in the polymer, g/100 g of the polymeror wt.% [49].

Table 12Sorption and pervaporation properties of membranes based on DSDA–TrMPD/DEB (9/1) polyimide after addition of tetracyanoethylene, TCNE, tomixture benzene/cyclohexane at 343 K

Polymer XBz (wt.%) SBz (wt.%) SCx (wt.%) YBz (wt.%) βS DBz × 10−8 (cm2 s−1) DCx × 10−8 (cm2 s−1) βD

DSDA–TrMPD/DEB (9/1) without TCNE additive 50 15.6 5.4 74.3 3.0 5.3 1.6 3.7DSDA–TrMPD/DEB (9/1) with TCNE additive 50 9.2 2.2 80.9 4.4 2.4 0.38 6.9

XBz, content of benzene in the feed mixture;YBz, content of benzene in the permeate;SBz, SCx, solubility of benzene and cyclohexane, respectively, inthe polymer;DBz, DCx diffusion coefficients of benzene and cyclohexane, respectively, in the polymer[49].

factor [4,51,57,59]. In addition, these groups enhance theaffinity of the membrane for the aromatic component, aswill be discussed below inSection 2.2.2.

Increase of the content of the rigid component ofcopolymers: Reference[50] deals with copolymers ofstyrene/butadiene and styrene/acrylic acid. The rigidityof the copolymer was regulated by changing the ratio ofcomponents. The rigidity of the copolymer is enhanced bythe increase in cohesion energy, which is achieved by in-creasing the content of polar groups. This is accompaniedby reduced permeability and enhanced separation factor,due to enhancement of the diffusion component of separa-tion factor up toβD ≈ 10 (seeFig. 8) while the sorptioncomponent of separation factor changes insignificantly:βS ≈ 1–1.3.

Reference[51] is concerned with pervaporation sep-aration of benzene/cyclohexane mixtures using mem-branes based on polyamide/polyether block copolymers.It has been established that the separation factor increaseswith increasing polyamide component containing polaramide groups capable of forming hydrogen bonds. Forexample, the 1:1 block copolymer of polyamide 12 andpolyoxyethylene has the benzene/cyclohexane separationfactor βP = 2.8 and flux F = 300 g m−2 h−1; a morerigid 3:1 polyamide 12/polyoxyethylene block copoly-mer has a much higher separation factor:βP = 5.0 andF = 80 g m−2 h−1.

2.2.2. Sorption component of separation factorβS

In the absence of specific penetrant/polymer interactions,the sorption component (βS) is mainly determined by theratio of boiling temperatures of the components being sepa-rated[1]. However, aromatic penetrants, as�-electron sys-tems, can have specific interactions with active groups orfragments of the polymer. If such interactions are realized,the sorption component of the separation factor is increased.

Fig. 8. Dependence of separation factorβP (benzene/cyclohexane), as wellas its diffusion componentβD based on styrene/butadienestyrene/acrylicacid copolymers on permeation flux (benzene/cyclohexane,50 wt.%/50 wt.% mixture, andT = 293 K). From analysis of datapresented in[50].


For selective removal of aromatic compounds from theirmixtures with aliphatic and alicyclic compounds, the param-eterβS can be increased by introduction into the polymerof active groups or fragments having�-electron acceptorproperties. This groups or fragments have�-electron affin-ity to aromatic compounds (a charge transfer complex canbe formed). These effects have been much investigated forpolyimides.

2.2.2.1. Introduction of a homogeneously distributed elec-tron acceptor into the polyimide matrix.To increasethe sorption component of the separation factor, homoge-neously distributed tetracyanoethylene, a strong electronacceptor having high affinity for electron donors, was addedto the polyimide matrix[49]. It can be seen from datapresented inTable 12that this is accompanied by increaseof the sorption componentβS (benzene/cyclohexane) bya factor of 1.5 probably as a result of selective sorptionof aromatic compounds by tetracyanoethylene with si-multaneous increase of the diffusion componentβD. Theprepared membranes offered good pervaporation proper-ties to benzene/cyclohexane, toluene/iso-octane mixtures.For example, for a two-component benzene/cyclohexanemixture, 50 wt.%/50 wt.%, at 343 K the productivity wasQw = 0.44 kg�m m−2 h−1, andβP (benzene/cyclohexane)= 48; and for a two-component toluene/iso-octane mix-ture, 45 wt.%/55 wt.%, at 343 K productivity wasQw =1.1 kg�m m−2 h−1, andβP (toluene/iso-octane)= 330.

2.2.2.2. Introduction ofπ-electron acceptor groups intothe polymer. Ethynyl groups: According to [52–54],acetylene fragments introduced into polyimides, e.g., 2,2′-diethynylbenzidine, DEB, have�-electron affinity for aro-matic compounds. It can be seen from the data presentedin Tables 10 and 11that increase in the content of DEBcomponent results in increase in the separation factor of thebenzene/cyclohexane mixture. This is probably caused notonly by the growth of the diffusion componentβD (resultingfrom thermal crosslinking through the unsaturated bonds)but also by enhancement of the sorption componentβS.

Phosphorylate groups: Phosphorylate (phosphonate es-ter) –P(OC2H5)2==O groups have high affinity to aromaticcompounds. It is known, e.g., that polymers with suchgroups are soluble in aromatic, but insoluble in aliphatic,hydrocarbons[5,55,56]. The authors of[55] demonstratedgood separation properties of polystyrene diethylphosphatefor mixtures of aromatic, alicyclic, and aliphatic hydrocar-bons. The separation factor of such membranes wasβP(benzene/cyclohexane)= 12–40 (50 wt.%/50 wt.% mixture,351 K), and dependence ofβP on degree of phosphoryla-tion was observed. The authors of[5] reported on mem-branes based on such glassy polymers as BPDA–TrMPDpolyimide, polystyrene, polyphenylene oxide, into whosemolecules pendant phosphorylate groups were introducedby chemical modification (treatment with methylbromidefollowed by phosphorylation resulting from reaction with

Fig. 9. Dependence of productivity for the mixture (�) and separa-tion factor of benzene/cyclohexaneβP (×) on degree of phosphorylationfor phosphorylated and thermally crosslinked BPDA–TrMPD polyimides:benzene/cyclohexane, 50 wt.%/50 wt.% mixture,T = 343 K, and mem-brane thickness 30–40�m. From analysis of data presented in[5].

triethylphosphite). The degree of phosphorylation was upto 200%, i.e. up to two phosphorylate groups per recurrentunit of the polymer. Phosphorylated polyimide membraneswere crosslinked thermally and then chemically with di-amine. The sorption properties of the prepared polymers,as well as their pervaporation properties for separation ofbenzene/cyclohexane, benzene/hexane mixtures, were in-vestigated. It has been established that the pervaporationproperties of modified polyimides depend on the degree ofphosphorylation as well as on their degree of crosslinking.Increase in degree of phosphorylation is accompanied byincrease in separation factor (seeFig. 9). Compared to theoriginal polyimide, the modified polyimide offers muchbetter separation properties and higher stability in the feedmixture. The high pervaporation selectivity of the modifiedpolyimide is due to the presence of phosphorylate groupshaving high affinity for benzene. For the modified poly-imide the sorption component of the separation factor forthe benzene/cyclohexane mixture (at 343 K) wasβS = 6–9,whereas the diffusion component was onlyβD = 2–3, andfor the benzene/hexane mixture,βD ≈ 1. The pervaporationproperties of membranes based on crosslinked phosphory-lated polyimide are similar to those of membranes based onphosphorylated polystyrene and cellulose acetate.

Sulfone groups: It is known that sulpholanes are usedfor extraction of aromatic components in petrochemicalproduction processes. This prompted the authors of[4] toinvestigate sulphonyl-containing polyimides for pervapo-ration separation of aromatic and aliphatic hydrocarbons:DSDA–DDBT polyimide whose original acid componentand original amine component both contained sulfonegroups. DSDA–DDBT polyimide displays preferential sorp-tion for aromatic, rather than aliphatic, penetrants. Thus, theamounts of sorbed benzene and toluene in this polyimidewere 11 and 15 g, respectively, per 100 g of dry poly-mer, whereas the amount of sorbed aliphatic compoundswas much lower. The investigated hydrocarbons can be


ordered by increasing boiling temperature (Tb) in the se-ries: n−octane> iso−octane> toluene> cyclohexane>benzene> n−hexane (seeTable 1). In the absence of spe-cific interactions, the sorption of these hydrocarbons shouldhave been ordered similarly. However, the experimentalsorption data follow a different series, beginning witharomatic penetrants: toluene> benzene> n−hexane>

n−octane> iso−octane> cyclohexane. This effect canbe attributed to selective sorption of aromatic penetrantsby sulfone groups. At 351 K and content of benzene ortoluene in the feed flow of the benzene toluene/cyclohexanemixture of 60 wt.%/40 wt.%, membrane productivity wasQw = 0.93 kg�m m−2 h−1, and separation factor wasβP(benzene/cyclohexane)= 32; for toluene/iso-octane mix-tures Qw = 2.8 kg�m m−2 h−1, βP (toluene/iso-octane)= 113.

Acrylate groups: The authors of[57] prepared new mem-branes for separation of mixtures of aromatic and aliphatichydrocarbons by pervaporation. A porous film based on highdensity polyethylene was used as support, onto which gly-cidinemethacrylate, having high affinity for aromatic pene-trants, was grafted using various techniques of plasma treat-ment. The prepared membranes had good separation prop-erties: permeation flux was 0.30–0.37 kg m−2 h−1 and sep-aration factorβP (benzene/cyclohexane) was 19–22, for a70 wt.%/30 wt.% composition of the mixture, at 344 K.

Phenyl groups: Styrene has high affinity for aromatic com-pounds. However, membranes based on polystyrene cannotbe used for separation of aromatic and nonaromatic hydro-carbons in practice; because good sorption of aromatic pen-etrants (much better than of aliphatic ones) accompanied bysevere swelling with consequent decrease in diffusion selec-tivity. It was proposed in[50] to use membranes prepared byemulsion polymerization of styrene and acrylic acid for sep-aration of the benzene/cyclohexane mixture. The polymerin the disperse phase of the emulsion (polystyrene) swellswell in the component being removed (benzene) whereas thepolymer in the dispersion medium (acrylic acid which doesnot swell in either component) restricts membrane swellingand ensures a high separation factor. The flux through thesemembranes wasF = 450–800 g m−2 h−1, with separationfactor, βP = 1.7–9.6, and its sorption componentβS =1.10–1.35 (50 wt.%/50 wt.% benzene/cyclohexane mixture,293 K). Reduction of the content of benzene in the feed mix-ture leads to decrease of the flux through the membrane andsignificant increase ofβS, Thus, for 10 wt.%/90 wt.% com-position of the benzene/cyclohexane mixtureβS = 1.5–2.5.With increase in temperature the permeation flux increasesand the separation factor decreases.

The dependence of the aromatics/paraffin (or alicyclichydrocarbon) separation factor on the activity of the hy-drocarbon component being separated is not consideredin detail in the present review. This dependence can benon-monotonic as a result of membrane swelling. Thisissue deserves separate consideration. It should only bepointed out that such regularities are similar to those ob-

served in hydrophilic membranes in selective separation ofwater from organics/water mixtures. The separation factorin hydrophilic membranes is strongly dependent on wateractivity in the feed mixture, and this dependence can benon-monotonic as a result of membrane swelling[48].

3. Separation and removal of hydrocarbons usingmembranes based on rubbery polymers

At present rubbery polymers are practically used ingas/vapor separation processes for selective separation ofhydrocarbon vapors from their mixtures with air as well asin pervaporation processes for the extraction of hydrocar-bons from their aqueous solutions.

3.1. Selective separation of hydrocarbon vapors from theirmixtures with air

The polymer materials mainly used for the selective sep-aration of hydrocarbon vapors from their mixtures with airare organosilicon polymers.

3.1.1. Effect of chemical composition of organosiliconpolymers on their gas separation properties with respect tohydrocarbons

This issue has been dealt with by several researchers,most comprehensively by Stern and coworkers[14,58–61]. Permeability of several gases and vapors, includ-ing hydrocarbons, in various organosilicon polymers hasbeen investigated after modification of chemical compo-sition of the polymers by introducing substituent groupsin both side and main chains. The following polymershave been investigated: [–(CH3)(R1)Si–O–]n, where R1:–CH3, –C2H5, –C3H7, –C8H17, –CH2CH2CF3, –C6H5;and [–(CH3)2Si–R2–Si(CH3)2–O–]n, where R2: –(CH2)2–,–(CH2)6–, –(CH2)8–, –m-C6H4–, –p-C6H4–. Results of theresearch are presented inTables 13 and 14andFig. 10.

It follows from analysis of these results that increase inthe volume of the substituent group results in higher rigid-ity and density of the polymer. This is manifested in higherglass transition temperatures and densities of the polymers,these parameters being influenced much more by introduc-tion of substituent groups into the side chain than into themain chain. Variation in flexibility of the polymer chain alsohas a significant effect on permeability. In this connectionit is of interest to compare polymers [–(CH3)2Si–CH2–]nand [–(CH3)2Si–(CH2)8–Si(CH3)2–O–]n. The relatively lowpermeability of the first polymer is due to the lack of flexiblesiloxane –Si–O– bonds in the main chain. The presence ofonly one siloxane bond per eight recurrent –CH2– groups inthe second polymer significantly increases the flexibility ofthe chains and results in a more than two-fold increase in per-meability as compared with the first polymer (seeTable 13).

The permeability of polymers decreases with the increasein their glass transition temperature. This dependence be-


Table 13Glass transition temperatures and permeability coefficients of organosilicon polymers with various substituents in the side and main chain with respectto various individual hydrocarbons (T = 308 K; for CH4 upstream pressurep = 6.8 × 105 Pa; for C2H6 and C3H8 the data are given at different valuesof their relative pressures)

Polymer Tg (◦C) Permeability coefficient,P × 10−2 (Barrer)

CH4 C2H6 C3H8

p/p0 p/p0

0.1 0.2 0.1 0.3 0.5

−(CH3)(R1)Si−O−R1

–CH3 −123 13.5 45 50 100 130 170–C2H5 −135 4.7 16 18 38 52 70–C3H7 −120 5.7 17 20 42 56 76–C8H17 −92 3.1 11 13 23 34 48–CH2CH2CF3 −70 2.0 3.1 3.6 4.5 6.5 8.5–C6H5 −28 0.36 1.1 1.3 1.7 2.7 4.0

–(CH3)2Si–CH2– −92 1.3 4.3 5.0 10 17 26

−(CH3)2Si−R2−Si(CH3)2−O−R2

–(CH2)2– −88 6.0 20 22 42 62 83–(CH2)6– −90 4.0 13 16 34 50 70–(CH2)8– −88 3.6 12 14 28 38 54–m-C6H4 −48 1.1 3.4 4.2 8 13 22–p-C6H4 −18 0.12 0.22 0.33 0.4 0.7 1.3

From analysis of results presented in[58].

comes more pronounced with increasing kinetic diameter ofpenetrant molecules (Fig. 10).

Variation in the structure of the organosilicon polymerhas a much stronger effect on the diffusion component ofpermeability than on the sorption component (seeTable 14).For example, the increase in glass transition temperature(Tg) from −123◦C in the case of polydimethylsiloxane, to−28◦C, in the case of polymethylphenylsiloxane, results inreduction of the diffusion coefficient (D) of propane from10.1×10−6 to 0.29×10−6 cm2 s−1, i.e. by a factor of morethan 30, whereas the sorption coefficient of propane changesfrom S = 8.49× 10−2 to 4.87× 10−2 cm3 cm−3 cmHg−1,i.e. by a factor of less than two.

The permselectivity of hydrocarbon vapors,αP, is dom-inated by the sorption component, and sorption of hy-drocarbon vapors by rubbery polymers is determined bythe condensability of their vapors. It can be seen fromdata in Table 14, that in organosilicon polymers the

Table 14Parameters of mass transfer of various organosilicon polymers to individual gases CH4 and C3H8 (T = 308 K; upstream pressure,p → 0 Pa)

Polymer Tg (◦C) D × 106 (cm2 s) S × 10−2 (cm3 cm−3 cmHg−1) P × 10−2 (Barrer) αidP αid

D aidS

CH4 C3H8 CH4 C3H8 CH4 C3H8 C3H8/CH4

(CH3)2Si–O– −123 24.5 10.1 0.59 8.49 14.5 85.8 5.9 0.41 14.4(CH3)(C3H7)Si–O– −120 7.59 2.72 0.70 9.10 5.34 29.6 5.6 0.36 13.0(CH3)(C8H17)Si–O– −92 6.54 2.60 0.48 7.81 3.14 20.3 6.4 0.40 16.3(CF3CH2CH2)(CH3)Si–O– −70 5.58 1.55 0.36 3.78 2.01 5.84 2.9 0.28 10.5(C6H5)(CH3)Si–O– −28 1.22 0.29 0.30 4.87 0.36 1.39 3.9 0.24 16.2(CH3)2Si–p-C6H4–Si(CH3)2– −18 0.44 0.07 0.23 3.69 0.10 0.27 2.6 0.16 15.8

From analysis of results presented in[58].

propane/methane sorption selectivity (αS) is 10.5–16.2,whereas diffusion selectivity (αD) is only 0.16–0.41. Refer-ences[62–66] contain values of permselectivity of hydro-carbon mixtures with nitrogen for organosilicon membranesproduced by GKSS (seeFig. 11). It can be seen that separa-tion selectivity increases with rising boiling temperature ofthe hydrocarbon, which points to domination of the sorptioncomponent of selectivity.

3.1.2. Dependence of permeability of individualhydrocarbons in rubbery polymers on pressure

With increase in relative pressure of hydrocarbon vapor,i.e. with increase in its activity, the plasticizing effect on thepolymer becomes stronger, resulting in increase in perme-ability (seeFigs. 12 and 13andTable 13).

Investigation of a wide range of block copolymers, con-sisting of various flexible (polydimethylsiloxane, polybuta-diene) and rigid (polycarbonate, polysulphone, polyarylate)


Fig. 10. Dependence of permeability coefficients of individual hydrocarbons CH4, C2H6, and C3H8 in various organosilicon polymers on theglass transition temperature of the polymer: (A) (CH3)2Si–O–; (B) (CH3)(C2H5)Si–O–; (C) (CH3)(C3H7)Si–O–; (D) (CH3)(C8H17)Si–O–; (E)(CH3)(CF3CH2CH2)Si–O–; (F) (CH3)(C6H5)Si–O–; (G) (CH3)2Si–CH2–; (H) (CH3)2Si–(CH2)2–(CH3)2Si–O–; (I) (CH3)2Si–(CH2)6–(CH3)2Si–O–; (J)(CH3)2Si–(CH2)8–(CH3)2Si–O–; (K) (CH3)2Si–m-C6H4–(CH3)2Si–O–; (L) (CH3)2Si-p-C6H4–(CH3)2Si–O–. From analysis of results presented in[58].

phases in[67], has shown that, irrespective of chemicalnature of the copolymer, content and molecular weight ofblocks, as well as pressure and composition of the hydro-carbon mixture, sorption and mass transfer of alkanes takeplace primarily in the rubbery phase. The phenomenon ofplaticization of polymers by hydrocarbons has been demon-strated by baromechanics[1,68–71]. It has been established

Fig. 11. Dependence of ideal hydrocarbon/nitrogen permselectivity on the boiling temperature of hydrocarbons for an polyorganosilicon membrane[62–66].

that the relaxation modulus of elasticity of block copoly-mers, in an environment ofn-alkane vapors, decreases inthe region of small concentrations in direct proportion tothe volume fraction of the sorbate, and the reduction of theelastic modulus (at constant relative pressure) increases withincrease in the thermodynamic affinity of the system. It hasbeen established that the deformation of block copolymers


Fig. 12. Dependence of permeability coefficients of individual hydrocar-bons in polydimethylsylmethylene on relative vapor pressure. From anal-ysis of results presented in[58].

is reversible under conditions of sorption equilibrium withvapors of paraffin hydrocarbons. Values of ideal selectivityαid

P = PCi /PC1 were calculated from data of transfer of in-dividual alkanes. With increasing molecular weight of thehydrocarbon, the concentration dependence ofαid

P becomesmore pronounced. High values of selectivity ofn-alkanes,that noticeably rise in the sequence C2–C4 and achieve, inthe case ofn-butane the order of magnitude of 102, point

Fig. 13. Dependence of permeability coefficients of individual hydrocarbons in polybutadiene on relative vapor pressure[27].

Fig. 14. Dependence of the diffusion activation energy of CO2, CH4, and C3H8 in organosilicon polymers on the kinetic diameter of the penetrantmolecules (designations of polymers are the same as inFig. 11). From analysis of results presented in[58].

to the possibility of using siloxane-containing block copoly-mers to separate vapor–gas mixtures of saturated hydrocar-bons.

3.1.3. Dependence of permeability coefficients ofhydrocarbons on temperature

As reported in studies by Stern and coworkers[14,58–61],the diffusion coefficients of hydrocarbons in organosiliconpolymers increase with rising temperature.Fig. 14 showsthe dependence of the values of diffusion activation energiesfor several polymers on the kinetic diameter of the penetrantmolecule. It can be seen that diffusion activation energyincreases with the kinetic diameter of the penetrant moleculeand the more rigid is the polymer (i.e. the higher is its glasstransition temperature) the stronger is this dependence.

3.2. Application of rubbery polymers for pervaporationremoval of hydrocarbons from their aqueous solutions

Extraction of hydrocarbons from their aqueous solutionsand emulsions resulting from a large number of processes, isnecessary for reasons of both economics and environmentalprotection. One possible way of solving the problem is per-vaporation separation of water–organic mixtures. The ma-terials used to make membranes for these applications are


Fig. 15. Dependence of productivity and separation factorβP C6H5CH3/H2O of membranes based on various rubbery polymers on the glass transitiontemperature of the polymer (pervaporation separation of saturated toluene/water mixture,T = 308 K): (1) polydimethyl siloxane; (2) polybutadiene; (3)polyoctylmethyl siloxane; (4) nitrile butadiene rubber with 18 mol% of nitrile groups; (5) the same, 28 mol% of nitrile groups; (6) the same, 38 mol%of nitrile groups; (7) ethylene/propylene copolymer; (8) polyepichlorohydrin; (9) polychloroprene; (10) polyurethane; (11) polyacrylate rubber; (12)fluorocarbon elastomer[72].

rubbery polymers. The authors of[72] investigated pervapo-ration properties of several rubbery polymers with respect tothe saturated toluene/water mixture. It has been establishedthat with increase in the glass transition temperature of thepolymer, i.e. with increase in its rigidity, a decrease in per-meability is observed. The separation factor also decreases,although the latter dependence is not very strict, probablydue to the swelling of the membrane (seeFig. 15). With in-crease in the content of nitrile groups in nitrile butadienerubber, a decrease in permeability is also observed, becauserigidity of the polymer increases with increase in its contentof polar groups (seeFig. 16). Effectiveness of pervapora-tion removal of organic substances, including hydrocarbons,from water–organic mixtures is determined by both sorptionand diffusion components of the separation factor. With in-crease in the sorption component, affinity of the polymer tothe organic component increases, the swelling of the poly-

Fig. 16. Dependence of productivity of membranes based on nitrile bu-tadiene rubber on content of nitrile groups (pervaporation separation ofsaturated toluene/water mixture,T = 308 K) [72].

mer is enhanced, which can result in reduction of the diffu-sion component of the separation factor. This issue is quitecomplicated and deserves separate consideration[48].

4. Conclusion

Selective separation of olefins and aromatic compoundsfrom their mixtures with paraffins can be quite effectivelyimplemented using membranes based on rigid-chain glassypolymers, whose permselectivity is dominated by the dif-fusion component. Especially promising materials arepolyimides with bulk substituents in both acid and aminefragments. Permselectivity of membranes with respect toolefins and aromatic compounds, whose molecules contain�-electronic bonds (unsaturated bonds in olefins and delo-calized�-bonds in aromatics) can be significantly improvedby introduction into the polymer matrix of active groupswith �-electron affinity having specific interactions withthese bonds.

In selective separation of hydrocarbons from their mix-tures with air or from their aqueous solutions, it is expedi-ent to use membranes based on rubbery polymers, whosepermeability increases with the decrease in glass transitionpoint. Permselectivity of rubbery polymers is dominated bythe sorption component, which increases with condensabil-ity of the hydrocarbon penetrant.

Higher activity of the component being separated inthe feed mixture results in plasticization of the membraneand can make it swell. This can produce a non-monotonicdependence of selective properties of the membrane onactivity of the component being separated. As a rule, perms-


electivity for mixtures of penetrants is significantly lowerthan their ideal values. Negative values of sorption heat ofeasily condensable hydrocarbons can result in existence ofnon-monotonic temperature dependencies of mass transfercoefficients.

Nomenclature

SymbolsaD the minimum molecular cross-section

determined from Stuart’s molecularmodel (nm2)

def effective diameter of penetrantmolecule (nm)

D diffusion coefficient (cm2 s−1)ED activation energy of diffusion

(J mol−1 or kcal mol−1)EP activation energy of permeation

(J mol−1 or kcal mol−1)F permeation flux (kg m−2 h−1)�H1 partial molar enthalpy of penetrant

dissolution in the polymer(J mol−1 or kcal mol−1)

�Hcond heat of penetrant condensation(J mol−1 or kcal mol−1)

�HS heat of penetrant sorption(J mol−1 or kcal mol−1)

p upstream penetrant pressure (Pa)p0 saturated pressure of the penetrant

vapor (Pa)P permeability coefficient (Barrer)Q membrane productivity (m3 m m−2 s−1,

kg�m m−2 h−1 or mol m m−2 s−1)S sorption coefficient (cm3 cm−3 cmHg−1)T temperature of experiment (◦C or K)Tb penetrant boiling temperature (◦C or K)Tg polymer glass transition temperature

(◦C or K)V molar volume of liquid penetrant

(cm3 mol−1)Vf molar fraction of free volume of

the polymerX penetrant content in the feed

mixture (wt.%)Y penetrant content in the

permeate (wt.%)

Greek lettersαD diffusion selectivity coefficientαP permeation selectivity (permselectivity)

coefficient of penetrant A overpenetrant B (αP = PA/PB)

αS sorption selectivity coefficientβD the diffusion component of the

separation factor

βP separation factor of penetrant A overpenetrant B in pervaporation process:β = (cA/cB)/(c0

A/c0B), wherecA, cB are the

component concentrations in the permeateflux andc0

A, c0B—component

concentrations in the feed fluxβS the sorption component of the

separation factorε/k Lennard–Jones force constant (K)σkt the molecular kinetic diameter determined

using zeolites (nm)σLJ the molecular collision diameter calculated

from the Lennard–Jones potential (nm)Subscriptsv volumew weightm molar;

Superscriptid ideal

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