Gas Separations with Polymer Membranes - index …index-of.co.uk/Tutorials-2/Gas Separations with Polymer...Sorption in glassy polymers can be described by eqn [7]: S"k D P# C H bP

Figure 1 Schematic representation of membrane permeation.

Gas Separations with Polymer Membranes

D. V. Laciak and M. Langsam, CorporateScience and Technology Center, Air Products andChemicals Inc., Allentown, PA, USA

Copyright^ 2000 Academic Press

Introduction

In 1996 the worldwide industrial gas market was inexcess of $29 billion (US). It continues to grow at anaverage rate of 4}5% per annum. Industrial gasesaccount for some of the largest production volumechemicals (1998 US): nitrogen (843 bcf (billion cubicfeet)), oxygen (698 bcf) and ammonia (19 700 mil-lion tons). Oxygen and nitrogen are separated frompuriRed air. Ammonia is produced by the reaction ofnitrogen and hydrogen. Certainly, the vast majorityof industrial gases are puriRed using cryogenic distil-lation or adsorption technology. However, in the last20 years there has been a growing interest in and anintense effort on the part of major gas producers toevaluate and develop membrane technology to pro-duce or purify gases. By 1999 sales of gas separationmembrane technology exceeded $100 million peryear. This article will describe basic concepts alongwith various practical aspects of polymeric gas separ-ation membranes including permeability measure-ment, membrane formation, module fabrication andapplications.

A polymeric membrane is deRned as a thin,semipermeable barrier between two gaseous phases.Gases will permeate the membrane if a difference intheir chemical potential exists between the two gas-eous phases. The chemical potential difference ismost often a result of pressure differences across themembrane. Thus, gases will solubilize into the mem-brane at the high pressure interface, diffuse across themembrane in a concentration gradient to the lowpressure interface and evolve into the low pressuregas phase (Figure 1). If a mixture of gases comprisedof components i and j is brought into contact with themembrane, the permeate stream will be enriched inthe more permeable gas i, leaving the retentate en-riched in gas j.

The realization that gases permeate through poly-mers is not new. Every child knows that a balloonRlled with air or helium deSates over time. Indeed,this phenomenon was observed by Mitchell in 1831.Balloons made of natural rubber Rlled at differentrates depending on the gaseous atmosphere they wereplaced into. Carbon dioxide Rlled the balloon fastest,air slowest. Thirty-Rve years later Graham expanded

on Mitchell’s experiments and quantitatively mea-sured the permeation rates of gases through naturalrubber. He found that the permeation rate was notrelated to the known gaseous diffusion coefRcientsand so concluded that permeation does not proceedthrough microscopic pores in the rubber but mustoccur within the rubber itself. He also demonstratedthat natural rubber could be used to produce from aira permeate which was enriched in oxygen to 46%.

A mathematical description of the permeation pro-cess was proposed by Fick. The relationship betweenpermeation rate J, gas pressure P, membrane areaA and membrane thickness l, known as Fick’s Rrstlaw, is governed by eqn [1], where �P is the pressuredifference across the membrane:

J"Po ) A ) �P/l [1]

The proportionality constant, Po, is termed the per-meability:

Po"J ) l

A ) �P"volume gas ) thickness

area ) time ) �P[2]

The customary unit of permeability is the barrerwhere:

1 barrer"10�10 cm3 ) cmcm2 ) s ) cmHg

[3]

Permeability can also be written as the product ofthe gas solubility times its diffusivity, the so-calledsolution}diffusion mechanism (eqn [4]). The per-mselectivity (�) for two gases i and j is deRned as theratio of the permeabilities:

Po"D ) S [4]

�ij"Poi/Poj [5]

II / MEMBRANE SEPARATIONS / Gas Separations with Polymer Membranes 1725

Figure 4 Coupling of Langmuir and Henry’s mode diffusioncoefficients.

Figure 3 Dual-mode sorption isotherm.

Figure 2 Schematic of free volume.

Permeation is an activated process. The effect of tem-perature on permeation is given by eqn [6], whereEp is the activation energy of permeation, R is the gasconstant and T is temperature:

Po"P�o ) exp(!Ep/RT) [6]

In typically encountered cases Ep is postive and thepermeability increases exponentially with temper-ature. Additionally, Ep is related to penetrant size andtherefore selectivity usually decreases with increasingtemperature. This treatment is not true when dealingwith gases below their critical temperature. Thereader is referred to the Further Reading section forthese special cases.

The above equations give a mathematical, phe-nomenological description of gas permeation throughpolymers but imply nothing of the molecular-levelprocesses giving rise to permeation. While we speakof gas-separating polymers as being dense Rlms, ona molecular level one must consider that the mem-brane is not ‘solid’; that is, there are molecular-sizegaps between the polymer chains. These gaps arisefrom packing defects in the solid state and also arisefrom the thermal motions of the polymer chainsthemselves. It is through pemanent and transient gapsthat gas transport is believed to occur. Solution-diffu-sion behaviour has proven adequate to describe per-meation through rubbery polymers } those whoseglass transition temperature, Tg, is below the experi-mental temperature. As a family, rubbery materialsare highly permeable but unselective for the samemolecular-level rationalization. In the rubbery statepolymer chains are highly mobile, generating a highfrequency of transient gaps which the penetrant gasescan easily diffuse through. However, these gaps arenot very selective. From a practical perspective, thepurity of the product is related to the membranepermselectivity. With some exceptions, such as the

production of oxygen-enriched air for medical ap-plications or the recovery of C4 hydrocarbons, low-selectivity membranes and hence rubbery polymershave found limited commercial utility in the puriRca-tion of industrial gases.

Dual-Mode Permeation in GlassyPolymers

Polymers in the glassy state possess ‘free volume’ asshown in Figure 2 } that is, a polymer quenchedbelow its Tg to a nonequilibrium state in which itsmolar volume is higher than the equilibrium value.This free volume can be visualized as long-lived mo-lecular-level gaps between the polymer chains. Oneaspect of free volume is that glassy polymers exhibitexcess sorption capacity. Sorption in glassy polymerscan be described by eqn [7]:

S"kDP# C �HbP1#bP

[7]

1726 II / MEMBRANE SEPARATIONS / Gas Separations with Polymer Membranes

Figure 5 Permeability test cells. A, supporting legs; B, lower plate; C, upper plate; D, adapter; E, vacuum valve.

where kD is the Henry’s law solubility constant, b isthe Langmuir equilibrium constant and C �H is theLangmuir capacity and can be related to the freevolume. Such sorption is often termed ‘dual-mode’

behavior. A typical dual-mode sorption isotherm isshown in Figure 3. At low pressures, sorption is dom-inated by the Langmuir element, while at high pres-sure sorption is described by Henry’s law.


Figure 6 Time lag diffusion experiment.

Table 1 Air separation characteristics of some commonpolymers

Polymer Po O2 (barrer) � O2/N2

Polyacrylonitrile 0.0002 '10Polyvinylidene chloride 0.0053 5.6Polyethylene terephthalate 0.059 4.5Cellulose acetate 0.78 2.8Polystyrene 2.63 3.3Poly(4-methyl-1-pentene) 32.3 4.1Silicone rubber 610 2.0Poly(trimethylsilylpropylene) 8000 1.4

Figure 7 Upper bound representation of oxygen/nitrogen permselectivity } 1990.

Dual-mode permeation can be viewed as the simul-taneous diffusion of gas molecules within and betweenthe dense polymer phase and the Langmuir gaps or‘holes’. Dual-mode permeation then is described bythe sum of permeation in these phases (eqn [8]). Thetwo diffusion coefRcients DD and DH appear to bestrongly correlated to each other (Figure 4):

Po"Po (dense)#Po (hole)"SDDD#SHDH

Po"kDDD#C �HbDH

1#bP[8]

Some consequences of dual-mode transport are:

1. The permeability of a dual-mode system decreasesas the pressure is increased.

2. Condensible vapours such as water and higherhydrocarbons are strongly adsorbed in the highenthalpy free volume sites and deleteriously affectpermeation rates and permselectivity.

3. Strongly absorbing gases such as CO2 can swellthe membrane at high pressure.

4. The free volume, a manifestation of the non-equilibrium state of the polymer, can be decreasedby annealing or upon aging.

Methods of Measuring Permeability

Conceptually, measuring the gas permeability ofpolymeric membranes is simple although fraughtwith experimental pitfalls related to establishingsteady state Sow. The experimental parameters aregiven in eqn [2]. Several methods and apparatus havebeen developed to conduct permeability measure-ments. For obtaining the permeation pure gases one


Figure 9 Upper bound representation of oxygen/nitrogen permselectivity } 1993.

Figure 8 Relationship between the upper bound slope (n) andkinetic diameter difference of gas pairs ij.

Table 2 Upper bound parameters

Pi"k�nij

Gas pair (ij) k (barrer) n

O2/N2 389 224 !5.800H2/CH4 18 500 !1.2112CO2/CH4 1 073 700 !2.6264He/N2 12 500 !1.0242

measurers either the rate of permeability pressure rise(usually from a vacuum) in a constant volume/tem-perature receiver or the volume of permeate gas ata Rxed pressure (Figure 5). Commercially availabletest cells are known as the volumetric or ‘Linde cell’and the manometric or ‘Dow’ cell. The AmericanSociety of Testing and Materials has publisheda method for measuring gas permeability (ASTMMethod D1434-82). It is possible to obtain the

permeability coefRcient Po and the diffusion coefRc-ient D through a time lag measurement as shown inFigure 6; subsequent calculation of the solubilityterm though an independent measurement of S isadvised.

Measuring the permeability of gas mixtures is morecomplex. Usually a Sow of a pre-blended source ispassed over the feed side of the membrane. Thesteady state permeate Sux can be measured by em-ploying an inert gas such as helium to sweep thepermeating components into a gas chromatograph,for example, for compositional analysis. Preferablythe experiment is conducted such that back-diffusionof the helium sweep is insigniRcant and where thefeed gas composition is not altered by permeation.Additionally, nuances in the Sow patterns within testcells not speciRcally designed for mixed gas experi-ments can lead to erroneous results.

Optimization of Polymer PermeationProperties

Table 1 illustrates the range of oxygen permeabilityand oxygen/nitrogen permselectivity for some


Table 3 Structure}property relationships in polymer membranes

Polymer Structure Density(g cm�3)

d-spacing(A�)

P (O2)(barrer)

O2/N2

PMDA-ODA 1.40 4.6 0.61 6.1

PMDA-mDA 1.35 4.9 0.98 4.9

PMDA-IPA 1.28 5.5 7.1 4.7

6FDA-ODA 1.43 5.6 3.9 5.34

6FDA-mDA 1.40 5.6 4.6 5.70

6FDA-IPA 1.35 5.7 7.5 5.60

PSF 1.24 5.0 1.4 5.60

DMPSF 1.21 5.0 0.64 7.00

TMPSF 1.15 5.5 5.6 5.28


Figure 10 Structure}property relationship: (A) FFV model(Paul). (B) Robeson model.

common polymers. Permeability spans a wide range:seven orders of magnitude. Further, as the permeabil-ity of polymers increases, their ability to differentiatebetween gases, the permselectivity, decreases. Thiscorrelation is valid for nearly all polymeric mem-branes and has been the subject of research. Whilelong recognized, this observation was Rrst formalizedby Robeson in 1991. Working with a database ofover 200 polymers, the selectivity of several gas pairs,plotted on a log}log scale against the permeability ofthe faster gas, exhibits a characteristic upper bounddeRning the combinations of permeability and perm-selectivity simultaneously achievable with polymericmaterials (Figure 7). Upper bound performance canbe described by eqn [9]:

Pi"k�nij or �ij"k�1/nP1/n

i [9]

where the values of k and n, tabulated in Table 2,are calculated from the upper bound relationship forspeciRc gas pairs. The parameter n is related to thedifference in kinetic diameters of the penetrant gaspair �dij as shown in Figure 8. This empirical treat-ment implies that the upper bound is a natural resultof the sieving characteristic of stiff chain glassy poly-mers. A fundamental theory was later developed byFreeman in which the constants could be related togas size and gas condensability and invoked justa single adjustable parameter f:

ln �ij"!�ij ln Di#�ln (Si/Sj)!�ij(b!f(1!a)/RT)�[10]

where �ij"[(di/dj)2!1] and di is the kineticdiameter; Si/Sj"N(�i/k!�j/k), where k is theBoltzmann constant and � is the potential energy welldepth in the Lennard}Jones potential energy func-tion. The constants a and b are derived from linearfree energy relationships and are independent of gastype. Moreover a is independent of polymer andhas a universal value of 0.64; b has a value of 11.5 forglassy polymers and 9.2 for rubbery polymers.The solubility selectivity among polymers is largelyconstant; consequently diffusivity considerationsdominate upper bound permselectivity.

The optimization of polymer structure to obtainupper bound properties comprises the lion’s share ofpolymer membrane research in the last 20 years andthe reader is referred to the Further Reading section.Researchers have heuristically developed the under-standing gained via the upper bound analysis. Thepermselectivity for gases i and j is given by eqns [4]and [5] as:

�ij"Poi/Poj"(Si/Sj) ) (Di/Dj) [11]

The solubility selectivity (Si/Sj) is nearly constantacross a wide variety of polymers and for O2/N2 isabout 2. Selectivity in glassy polymers is thereforedominated by the diffusive selectivity which in turnresults from the sieving properties of the imperfectlypacked polymer chains. The best trade-off in per-meability properties within a polymer family is ob-tained when both main chain mobility is limitedand intersegmental packing of polymer chains is


Figure 11 Structure units with imparting superior permselectivity.

Figure 12 Cross-section of an asymmetric membrane.

inhibited. This behaviour is illustrated (Table 3)for a family of pyromellitic dianhydride (PMDA) andhexaSuoro-isopropylidene dianhydride (6FDA)-based polyimides. Changes in the functionality leadto different packing arrangements as measured bydensity and X-ray d-spacings (average distance be-tween polymer chains). Very small changes in chainpacking result in signiRcant changes in both perme-ability and selectivity. Further, groups such as 6FDAare particularly desired because they can increasepermeability without a large loss of selectivity. A fur-ther example is that of ortho, di- versus tetra-substitu-tion patterns on aromatic polymers. It is widely recog-nized that incorporation of bulky substituents leads toan increase in permeability, usually at a loss of selectiv-ity. However, it is also noted that ortho di-substitutionpatterns result in lower permeability and higher selec-tivity than the symmetrically tetra-substituted ana-logue. Using this intuitive approach a great many newpolymers were synthesized and characterized between1990 and 1993 and as a result the empirically deter-mined upper bound has been shifted upperwards andits slope has changed (Figure 9), with many polymerslying at or near the upper bound.

Predicting Polymer Permeability

The beneRt of being able to predict a priori the rela-tionship between polymer structure or physical prop-erties and permeability is obvious. One method is tocorrelate the permeability with the reciprocal of thefractional free volume (FFV), deRned as:

FFV"V!V�

V[12]


Figure 13 Phase diagram for a ternary dope system. PP, polymer-poor; PR, polymer-rich; CP, critical point.

Figure 14 Series resistance model for defect repair.

where V is the speciRc volume of the polymer and Vo

is the volume occupied by the polymer chains. ThespeciRc volume is obtained from experimental densitymeasurement. Direct measurement of Vo is not poss-ible and therefore various computational approacheshave been developed. The most widely used is thatdeveloped by Bondi relating the zero point volume(the volume occupied at 0 K) to the van der Waalsvolume of the molecule and later modiRed by Park toaccount for the fact that different gases have access todifferent FFV depending on the speciRc gas}polymerinteraction. This group contribution approach hasyielded good correlations (Figure 10A) but it is notintuitively obvious how to relate polymer structuredirectly to free volume.

The group contribution approach by Robeson as-serts that the overall polymer permeability can berepresented by the sum of structural subunits thatcomprise the polymer in proportion to their volumefraction:

ln Po"��i ) Pi [13]

where �i is the volume fraction of subunit i and Pi itspermeability contribution. Volume fractions are cal-culated using molecular mechanics computer model-ling and the Po’s are experimentally determined.The Pi’s are found by regressing the set of simulta-neous linear equations from eqn [13]. In addition toadequately representing the experimental data(Figure 10B), this method also identiRes thosesubunits which exhibit upper bound properties

(Figure 11). Desirable moieties include the 6FDA,di-ortho substituted phenyl ethers and the spirobi-indane fragment. A superior polymer would be tetra-bromo-poly(phenylene oxide); however, no one hasas yet succeeded in its synthesis.

Membrane Formation

Most research on polymeric membrane materials isconducted on thin, solvent-cast, Rlms. The practicallimit to such membranes is about 25 �m for free-standing Rlms and perhaps 1 �m if the polymer solu-tion is cast on a microporous substrate. However, atthese thicknesses the Sux through the membrane istoo low to be of practical value. That is to say, theeconomics of a membrane process utilizing thesethick Rlms would not effectively compete with estab-lished technologies such as cryogenic distillation. Theenabling development for fabricating ultrathin, highSux membranes was the integrally skinned asymmet-ric membrane of Loeb and Sourirajan. This type ofmembrane is produced by inducing phase separation


Figure 15 Flat-sheet membrane fabrication.

Figure 16 Spiral-wound membrane element.

in a thermodynamically stable solution, usually bychanging its composition through the introduction ofa nonsolvent. These membranes are prepared by cast-ing a concentrated polymer solution (dope) incorpor-

ating a water-miscible solvent into a water coagula-tion bath. Such membranes possess an ultrathin denseskin that gradually opens into a microporous sub-structure (Figure 12). The skin layer provides the


Figure 17 Hollow-fibre membrane spinning apparatus and cross-section of spinnerette.

separation while the thicker integral substructureprovides robustness but very little resistance to gasSux.

Membrane formation processes can be groupedinto three methods:

1. Dry phase inversion processes involve thermallyquenching a dope or solvent evaporation in multi-component solvent system dopes.

2. Wet phase inversion processes involve quenchinga cast dope in a coagulation bath.

3. Dry-wet phase inversion processes combine sol-vent evaporation and a quench medium.

Most commercial gas separation membranes are pro-duced by the dry}wet process although temperature-induced phase inversion (TIPS) is also employed.

Phase diagrams illustrate the phase inversion pro-cess (Figure 13). The binodal deRnes the boundary ofthe two-phase region and is divided into two parts atthe critical point (CP). A second envelope, thespinodal line, also emerges at the critical point. Thephase inversion process involves bringing a dopesolution into the two-phase envelope. The initiallystable solution (at a composition designated by * inFigure 13) decomposes into a polymer-rich phase anda polymer-lean phase, the compositions of which are

deRned by tie lines. If the decomposition takes placein the region between the binodal and spinodal a nu-cleation and growth mechanism of the polymer-richand polymer-lean phases dominates, leading to struc-tures undesirable for gas separation. Decompositionbelow the critical point leads to a dispersion of poly-mer nodules within the polymer-lean phase; de-composition above the critical point leads to a closedcellular structure of an encapsulated polymer-leanphase. The preferred inversion path is to quicklybring the dope within spinodal envelope, thusgenerating an interpenetrating network of polymer-lean and polymer-rich phases which vitrify into a Rne-ly porous substructure.

The thermodynamic framework for a ternary dopesystem lies in Flory}Huggins theory and the Gibbsfree energy of mixing for a ternary system is givenby:

�Gmix"n1�1#n2�2#n3�3#�12n1�2

#�13n1�3#�23n2�3 [14]

where the subscripts refer to the nonsolvent (1), thesolvent (2) and the polymer (3). The notations ni and�i are the number of moles and the volume of


Figure 18 Hollow-fibre membrane element.

component i and �ij is the interaction parameter ac-counting for the nonideality of the mixture. At equi-librium the chemical potentials (u) of the componentsin all the phases are equal and are given by eqn [15],which deRnes the binodal envelope. The compositionfor the spinodal line is given by the solution toeqn [16], where vi is the pure component molar vol-ume of species i:

�ui

RT"

ni

��Gmix�RT

[15]

(1/�1#1/2�2!2�12)(1/�1#1/3�3!2�13)

�(1/�1#�231/2!�12!�13)"0 [16]

Composite Membranes

The above discussion also applies to the formation ofcomposite or multilayer membranes. Compositemembranes can be categorized as either a dense, iso-tropic or asymmetric coating of a high performanceseparating layer on a microporous substrate. Com-posite membranes are fabricated in two operations,substrate formation followed by dip coating, allow-ing for the independent optimization of coating andsubstrate properties. Gas transport through com-posite membranes is described by the series resistancemodel by analogy to an electrical circuit. For a two-layer composite consisting of a thin layer of polymerA on a substrate of polymer B, the Sux of gas i thoughthe membrane is given by eqn [17], where l is thethickness of the respective layers:

Ji"�P(lB/PoB#lA/PoA) [17]

Regardless of their method of formation a criticalelement of gas separation membranes is that the skinlayer must be as thin as possible in order to producea high Sux membrane. The practical limit to the skinlayer thickness is thought to be in the range of500}1000 A� . Concurrently, the skin layer must befree of manufacturing defects or pinholes. A defectivesurface area fraction as low as 10�5% can lowerselectivity to an extent that the membrane is notsuitable for gas separation. Conventional manufac-turing processes are not capable of achieving this levelof reliability. The second enabling development incommercializing gas separation membrane tech-nology was the demonstration of a poly(dimethyl-siloxane) defect repair coating to effectively ‘plug’manufacturing pinholes and eliminate bulk Sowthrough the defects (Figure 14). Gas permeationthrough this multicomponent system is described bythe series resistance model analogous to an electrical

circuit. The total resistance is given by the sum resist-ances in the coating layer (Rc); the parallel resistancein the defective skin layer; (Rsk,d); and the resistance ofthe substrate Rsub:

Rtot"Rc#Rsk,d#Rsub

"Rc#Rsk�Rd

Rsk#Rd#Rsub [18]

After repair, the resistance of the defect, R�d is greaterthan Rd and also, Rsk'R�d , so that the effectivepermeability of the composite approaches the intrin-sic permeability of a defect-free membrane.

Membrane Devices

Gas separation modules can be prepared from Satsheets as plate and frame assemblies and spiral-wound elements. Hollow Rbres can be Rne Rbres((1000 �m OD) or tubular. In general onlyRne Rbres and spiral-wound elements combine


Figure 19 Membrane technology: breakdown by application.

Table 4 Selected membrane applications

Category Range of operation Application Gases separated

Hydrogen recovery 95% H2 Ammonia synthesis purge gas H2/N2, Ar(5 MMSCFD Syngas ratio adjustment H2/CO

Hydrotreater off gas

Nitrogen production 95}97% N2 Inerting: fuel tanks; food transportation N2/O2, CO2

0.1}2.0 T/D Gas and oil drilling N2/O2, CO2, H2O

Oxygen 40% O2 Oxygen-enriched air O2/N2

Carbon dioxide removal 95#% CH4 Natural gas sweetening CO2/CH4

(40 MMSCFD Enhanced oil recoveryLandfill gas

Dehydration Dewpoint to !403C Instrument air H2O/air or N2

Natural gas dehydration H2O/CH4

Misc. Semiconductor process gas Perfluorocarbon from N2

performance and cost parameters providing eco-nomic viability. The beneRts of the spiral-woundconRguration include ease of fabrication and lowpressure drop but manufacturing costs are high andthe membrane surface area to module volume ratio islow (+700 m2 m�3), leading to larger, heavier sys-tems. Hollow Rbre modules can achieve very highsurface area/volume ratio ('5000 m2 m�3) and pro-vide a higher degree of countercurrent Sow but aremore difRcult to fabricate and can have high pressuredrop.

Flat Sheets and Spiral-Wound Elements

A schematic representation of a Sat-sheet asymmetriccasting device is shown in Figure 15. Flat-sheetmembranes are typically produced on a nonwovencloth which provides support for the nascent mem-brane. These nonwoven cloths are commerciallyavailable on large rolls of 24}48 inches. A thinliquid Rlm of dope solution is metered by a doctorblade onto the nonwoven cloth while the fabric ro-

tates around a stainless steel roller. The nascentmembrane is quenched in the gel tank, washed andcollected on a take-up roller. A generic spiral-wound element is shown in Figure 16. The simplestconRguration consists of a central collection pipearound which is wound and glued an envelope ofSat-sheet membrane. The envelope contains themembrane and feed and permeate spacer channels.The spacer material is an extremely porous, inertmaterial. Feed gas Sows parallel to the permeate pipe;permeating gas Sows into the permeate spacer and iscollected in the pipe. Higher membrane areas can beachieved using the multileaved method in which twoto four sheets of membrane are wrapped simulta-neously.

Hollow Fibres

A second asymmetric membrane geometry is that ofa hollow Rbre. The ultrathin skin is present on theexternal surface of the Rbre. Like the Sat sheet, thewall of the Rbre is microporous and offers low resist-ance to gas Sow. The permeating gases collect in thebore or lumen of the Rbre. A generic hollow-Rbrespinning device is shown in Figure 17. The apparatuscontains a dope reservoir, a bore Suid reservoir,pumps, coagulation baths, wash baths and a take-upwinder. The dope solution and bore Suid are co-extruded through a die, resulting in a sheath of poly-mer around the bore Suid (typically water). Thisnascent Rbre is then coagulated, washed and dried.Hollow-Rbre devices contain thousands, even hun-dreds of thousands, of individual Rbres within a singlepressure housing (Figure 18). The feed gas is usuallyfed to the outside of the Rbre (shell side) althoughsome processes route the feed gas through the Rbre


Figure 20 Triple-orifice die for co-extrusion. 1, bore fluid; 2,substrate polymer dope; 3, coating polymer dope.

bore, especially when well-deRned Sow character-istics are required.

Applications

The Rrst successful commercial activity in gas separ-ation by membranes began in 1977 with the introduc-tion of the Prism] membrane system by Monsanto(now Air Products and Chemicals, Inc.), to recoverH2 from ammonia synthesis plant purge gas. Installedmembrane capacity in 1977 was only 5 MMSCFD(million standard cubic feet per day) and grew to over3500 MMSCFD by 1996. Approximately two-thirdsof current installed membrane capacity is used forH2 separation, which includes ammonia purge gas,reRnery and petrochemical applications (Figure 19).SigniRcant growth is expected for N2 and CO2 separ-ations. While the speciRcs will vary according toapplication, a generic membrane system will includea compressor if the source gas is not available atpressure and pretreatment to remove condensible,corrosive or reactive components. The partial listingof applications (Table 4) represents established usesfor gas separations. Expanding the slate continues tobe a focus of membrane manufacturers and researchinstitutes.

Future Trends

The concurrent need for high performance polymersand low cost membranes has resulted in research intohybrid polymer systems and improved manufacturingprocesses. Four areas currently being explored are:

1. Polymer blends: A hybrid polymer system inwhich expensive, high performance polymersform a continuous phase in a multipolymer blend.

Examples include Matrimid/Ultem� 1000 andaromatic polyimides/polysulfone.

2. Coextruded composite hollow Tbres: An im-proved hollow-Rbre spinning process in whicha forming asymmetric substrate is simultaneouslycoated with a thin Rlm of separating polymer byemploying a die similar to that shown in Fig-ure 20. This is a paradigm shift away from separ-ate substrate fabrication and subsequent coatingpractices. The co-extrusion processes shouldlower manufacturing costs and relax price con-straints on new high performance polymers.

3. Organic/inorganic mixed matrix membranes:A nanocomposite composite is a hybrid in whichvery small particles of a material with high dif-fusive selectivity such as a microporous carbon isdispersed with an organic polymer matrix. Suchmaterials combine the high selectivity of the inor-ganic with the processability of the organic poly-mer matrix.

4. Computational methods: Molecular modelling ofgas transport through rubbery polymers has al-ready proven successful. Its application to glassypolymers is signiRcantly more difRcult, primarilybecause of the poorly deRned nature of the glassystate, but signiRcant progress has been made inthe last several years. As our understandingof gas}polymer and polymer}polymer inter-actions improves and merges with advancingcomputer technology, widespread use of molecu-lar dynamics should provide signiRcant insightinto gas separation with polymer membranes.

Further Reading

Ho WS and Sircar KK (1992) Membrane Handbook. NewYork: Chapman & Hall.

Hwang ST and Kammermeyer K (1975) Membranes inSeparations. New York: Wiley.

Kesting RE (1985) Synthetic Polymeric Membranes:A Structural Perspective, 2nd edn. New York: Wiley.

Koros WJ and Fleming GK (1993) Membrane-based gasseparations. Journal of Membrane Science 83: 1}80.

Mulder M (1991) Basic Principles of Membrane Tech-nology. Dordrecht: Kluwer.

Paul DR and Yampolski YP (eds) (1994) Polymeric GasSeparation Membranes. Boca Raton, FL: CRC Press.

Robeson LM (1991) Correlation of separation factor vs.permeability for polymeric membranes. Journal ofMembrane Science 62: 165.

Stern SA (1994) Polymers for gas separations: the nextdecade. J ournal of Membrane Science 94: 1}65.


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Gas Separations with Polymer Membranes - index …index-of.co.uk/Tutorials-2/Gas Separations with Polymer...Sorption in glassy polymers can be described by eqn [7]: S"k D P# C H bP