Membrane on Industrial Applications com

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Pure & A p p / . Chem.,Vol. 67, No. 6, pp. 993-1002, 1995.Printed in Great Britain.Q 1995 IUPAC

Industrial application of membrane separationprocessesHar tmut B ruschk eDe u t sc he C arb one AG, Membran Trennver fah ren GF TFriedrichsthaler Str . 19, D-66540 Neunk irchenAbstract - During the past two decades membrane separation processes have been developed andoptimized for even large scale industrial applications. The most important of theae proceasesinclude: (i) microfiltration and ultrafiltration for purification of aqueous streams, concentration andrecovery of valuable products; (ii) reverse osmosis for the production of demineralized or potablewater; (iii) electrodialysis for the concentrationor removal of dissolved ions; (iv) gas separation forsplitting gas s treams , removal or recovery of specific gases; (v) pervaporation for separation andconcentration of liquid mixtures, especially of aqueous-organic azeotropes.Whereas the first three of these processes are well established and have reached a high degree ofmaturity, the last two ones are still in developing stage, although development is fairly fast. Bothprocam, gas separation and pervaporationare very closely related with respect to their physico-chemical fundamentals. In thispaper principlesof the separation processesare outlined. Examplesof membrane performance and applications, with the emphasis on Pervaporation processas arepresented.

In general all processes used for the separation of fluid m ixtures can be splitinto two categories:separation by equilibrium distributionseparation by differences in transport rates

The most common separation processes used on large scales in the industryare based on equilibrium distribution. Evaporation, distillation, extraction,adsorption, absorption are all going back to the same principle sho wn in fig. 1.A first phase comprising a mixture to be separa ted is brought into contact witha second phase. After a certain time thermodynam ic equilibrium is establishedbetween the two phases. That means both phases show the sametemperature and all components have the same chemical potential in bo thphases. The analytical concentrations of a compo nent in the two pha ses,however, may differ e.g. a component can be highly enriched in one phaseand be depleted in the other one. If now the two phases are separated byappropriate means, the enriched component can be recovered usually byestablishing a new eq uilibrium at a d ifferent temperature or pressure.Repetition of this procedure at the end will lead to a phase in which one of thecomponents is present at the wanted purity.If separation by differences in transport rates is to be achieved, an additionalmeans is required. In fig. 2 such a means is a m embrane, separating tophases from each other. A driving force, a gradient in pressure, conce ntration,temperature, electrical field is applied and has to be maintained over themem brane. Under the influence of this driving force components from themixture to be separated, held at a higher chem ical potential, migrate throughthe membrane to the side of the lower chemical potential. Separation betweendifferent components is effected by the difference in transport rates. Thegradient in the chem ical potential has to be m aintained by continuous removal

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99 4

~

H. BRUSCHKE

C j I I

Phase I P h a s e I 1 P h a s e I 1

I 1I 1

I 1I 1

Membrane

Fig. 2. Membrane Separation

of the m igrating components from the side of the lower chem ical potential. Ifthis is not done, equilibrium would b e reached and no sepa ration wou ld occur,in most cases one of the phases would vanish.Some mechanism must exist in the membrane which is improving the transportof one component and impeding the transport of the other. Membranes ca n ina first approximation be classified according to the means of such impedimen t.

Porous membranes discriminate according to size of particles ormolecules.Non-porous mem branes discriminate according to chemical affinitiesbetween components and membrane materials.

In porous membranes diameter of pores vary in a large range, with severalmicrons in m icrofiltration and seve ral nanom eters in ultrafiltration. A gradientin hydraulic pressure acts as the d riving force. Small molecules of the solventof a solution, usually water, can pass through the pores, whereas particles orlarge molecules are retained. Whatever the size of a particle, the mechanismon which separation is bas ed is sieving or filtration.There exist a long and tedious d iscussion about the distinction betweenporous and non-porous membranes, which even tually bo ils down to thedefinition of a pore. The best answ er to this ques tion can still be found in theclassical book of Hwang and Kammermeyer (1) "There exists an unresolvedquestion as to whether any mem brane, so-called po rous or non-porous, willact to some extend as a microporous m edium. Even the membranes that wecall "non-porous" may have a number of minute pore whose diameter will be inthe 5 to 10A range. However, rightly or wrongly, such structures areconside red non-porous, and their behaviour would seem to justify this viewpoint".

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Industrial application of membrane separation processes

C( M em b r a n e )

Pi (Feed)

C F e e d I995

J . = D . .kI d x

P i ( P e r m e a t e )

C p e r m e a t e

F e e d S i d e P e r m e a t e S i d eM e m b r a n e

A b s o r p t i o n Transpor t D e s o r p t i o nFig. 3. Solution-Diffusion Mechanism

As porou s membranes are best characterized by their separating mechanism,so are non-porous m embranes. Tod ay it is generally agree d that ma sstransport thro ugh non-porous membranes can best b e described b y the so-called "Solution-Diffusion-Model". Follow ing this model (fig. 3) the overallmass transport ca n be separated into three consecutive steps:Sorption of a component out of the feed mixture and solution in th emembrane material.Transport through the m embrane alon g a poten tial gradient.Deso rption on the second side of the membrane.

Preferential solubility of a component of the feed mixture in the membranematerial may be due to a number of different interactions: ionic-ionicinteraction, ion ic-dipol interaction, dipol-dipol interaction.A membrane comprising fixed io ns in its polymer matrix (ion-exchangemembrane) will preferentially a dsorb and "dissolve" ions of opposite chargebut repe l those of the same charge. Membranes with fixed ions or dipoles willpreferentially adsorb and dissolve molecules with dipoles, e.g. water, but willrepel non-polar molecules. Therefore, a potential classification of non-porousmembranes co uld be found in the nature of the functional groups comprised inthe membrane.

Polar or hydrop hilic membranes.Non-polar or hydrophob ic membranes.

Another classification cou ld follow the driving force respo nsib le for the transport.Gradient in electrical potentialGradient in hydrau lic pressureGradient in vapor pressureGradient in concentration

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99 6 H. BRUSCHKE

As one and the same m embrane may be us ed for different processes i t hasbee n common practice to classify non-porous m embranes rather according toprocesses.The most important ones are:

Reverse OsmosisElectrolysis and ElectrodialysisGaspermeationPervaporation

Reverse Osmosis is widely used in the desalination and dem ineralization ofsaline water. The fee d may be sea water and b rackish water an d the productdrinking water or tap water may be us ed and pretreated in the production ofdeionized water. Hydraulic press ure on the feed side m aintains the neces sarydriving force, hy droph ilic membranes with h igh affinities towards water, b uthigh rejections towards ions, are us ed.Th e major application for io n exchange membranes is still fouqd in the chlor-alkali electrolysis. Cation exchange membranes, comprising fixed anions,separate anode and the cathode compartment in an electrolysis cell. Sodiu mions can pass through the membrane an d form sodium hydroxyde, whereaschloride io ns are repe lled and cannot pollute the cathode compartment.Nearly as important is the application of ion exchange membranes inelectrodialysis. Cations and anions can pass out of salt solutions through therespective oppositely charged membrane. Thus a saline s olution can b e splitinto two streams, one with a lower, the other with a higher salt content thanthe original solution. Both effects a re utilized, the product of an e lelctrodialysisprocess may be the desalinated stream, or the concentrated stream. The latteris for example important in the produ ction of salt from sea water in Japan, as apreconcentration step.Sepa ration of gaseous mixtures b y permeation thro ugh non-porousmembranes is fairly common in specific industries. Ne arly all ammoniaprodu cing plants are today equippe d with a membrane system for the recove ryand recirculation of hydrogen from purge gas. P repurif ication of natural gas bypermeation of acid components like C 0 2 or H2S through membranes isanother application. Air is separated by membranes into an oxygen and anitrogen enriched fraction. Especially the production of enriched nitrogen, withpurit ies between 95 and 99 % at small capacities has be come a seriouscompetitor for adsorption or cryog enic processes.The most recent separation process using non-porous membranes isPervaporation. This process is very closely relate d to ga s separation andvapor permeation, as in all three processes the differen ce in partial vaporpressu re is acting as the driving force for the transmembrane transport. InPervaporation the feed side partial vapor pressure is equal to the saturationpressure, in vapor permeation, it is equal or slightly below saturation and ingas permeation it is significantly below saturation conditions.Pervaporation c an be use d for a number of different applications. Hydro philicmembranes preferentially permeate water an d retain less or nor polarorganics. O rgano philic membranes, in contrast, p referentia lly permeate n on-polar organics, bu t retain water. Bo th types of membranes can a ddit ionally beuse d for the separation of m ore polar organics from less polar ones,especially w hen water is absent.

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Industrial application of mem brane separation processes

S epara t ing Layer

P o r o u s S u p p o r l

N o n - w o r e n -a b r i c997

Fig. 4. Cross Section of Composite Membrane

Pervaporation membranes are usually of the composite type (fig. 4). In acomposite membrane the different fractions, like separation an d mech anicalstability, are attributed to different layers. Thus composite mem branes cancombine very thin and highly selective separation layers with rigid,mechanically and thermally stable backing layers.From fig. 5 the basic principles of a Pervaporation process a re shown. Th emembrane separates the feed compartment from the permeate compartment.The liquid feed at a given temperature and composition is passin g over themembrane. At the permeate side partial vapor pre ssures are mainta ined wellbelow the values at the feed side. At least one of the components then istransported preferentially throu gh the membrane, the permeate sid e of which i tleaves as a vapor. Usually the perme,ating vapor is co nden sed at redu cedpressure, the presence of non-condensable gases would h inder the transportof the permeating vapor form the m embrane to the condenser. The necessaryheat for the evaporation of the permeate is taken from the sensible heat of thefee d mixture which is coo led down accordingly. In contrast to all otherseparation processes employing non-porous membranes, a phase change forthe permeating substance occurs in the transgression from fee d to permeate.Insofar Pervaporation is a unique process as not only matter, but also h eat istransported thro ugh the membrane. The retentate leaving the membrane no tonly differs in concentration but also in temperature from the o riginal feed.

Feed Compar tmen tMembrane

/Permea te Compar tmen t1

c

P e r m e a t en I LIWP

Fig. 5. Pervaporation : Basic Principles0 1995 IUPAC, Pure and Applied Chemistry, 67,993-1007


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998 H. BRUSCHKEThis temperature reduction in the Pervaporation effect has two differentdisadvantages:

The partial vapor p ressure of the better permeable component is not onlyreduced by its reduced concentration in the retentate, but also b y thedecrease i n temperature. F or the retentate the driving force is lowe r thanfor the feed.The transmembrane flux decreases with decreasing temperature.Inorder to avoid too low temperatures it has becom e common pr actic e tosplit the total membrane area into a number of stages, as shownschematically in fig. 6. The preheated feed stream enters the first stage,where heat is lost for the evapo ration of the permeate. At a lowe rtemperature the retentate is pass ed through an intermediate hea texchanger where it is reheated to the original feed temperature, and fed toa second membrane stage. This procedure is repeated as many times asare necessary to reac h the specified composition of the final retentate.Size and number of stages as well as a llowed temperature drop over astage are a matter of optimization in the design of any Pervaporation plan t.

V a c u u mP u m p

1 V a c u u n iv e s e lH ea te rP r o d u c t

Fig. 6 . PervaporationThe P ervaporation membranes used today in industrial applications a re ofthe hydrophi!ic type. They preferentially permeate water but re tain nearlyall organic molecules. The main application of these m embranes istherefore the removal of water from its mixtures with organic liquids.In fig. 7 the vapor liquid equilibrium curve for the system 2-propanol (IPA)-water is plotted. The uppe r line in the diagram indicates the composition ofthe vapor in equilibrium with the liquid, the bottom curve gives thecomposition of the permeate of a Pervaporation process. It has to be keptin mind that this composition is not in equilibrium with the liquid mixture butis obtained at a specific difference or ratio of the vapor pressures of waterat the feed and pe rmeate side of the membrane. Although water is the lessvolatile component, it is enriched in the permeate over the wh oleconcentration range. Even the azeotrope, clearly visible in the vapo r-liquidequilibrium is not reflected in the permea te composition. In contrast itseems that the Pervaporation process reaches a maximum separation w iththe feed at the azeotropic composition. From this diagram the preferredapplication of a Pe rvaporation membrane is clearly understandable. Athigh water concentration a Pervaporation process could be used, butwould not be very efficient. A large portion of water ha d to pass throughthe membrane, enrichment of IPA in the vapor is even higher thanenrichment of water in the permeate, and in a distillation process at leastthe heat of evaporation could b e partially recovered.

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Industrial application of mem brane separation processes 999

l sopropano l - WaterVapor - Liq uid Eq ui i b rium

IP A i n Liquid, Z b.w.. - * - IP A i n P e r m e a t e

Fig. 7

At low water concentration, however, distillation becomes less an dPervaporation more and more effective. For the purification of isopropanolfrom its mixture with about 50 % of water, as ob tained in normal IPAproduction, the best m eans is then a combination of distillation andPervaporation. In a d isti llat ion step the mixture is p reconcentrated close tothe azeotropic composition, then it is further dehydrated by Pe rvapora tionto a final water content of 0,5 % (fig. 8). Any lower water content could bereached, simply by adding membrane a rea to the Pe rvapo ration step. Infig. 9, costs for such a hyb rid system are presented, clearly sho wing theadvantage of the hybrid system.Similar considerations as for the IPA-water system are valid for a largenumber of other aqueous organ ic mixtures. Ethanol, for example, isdehydrated us ing the hybrid scheme today in large plants.

A z eotro D e

10% IPA

I P A

Fig. 8. Hybrid Process0 1995 IUPAC, Pure andApplied Chemistry, 87,993-1002


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1000 H. BRUSCHKEDM/lOOkg Product -~5

IH 00 kg/h 0 2 000 kglh20

15

10

5

n" Pervaporation Distillation HybridFig. 9. Operation Cost : 50%- 9,5% IPA

M a s s Frac t ionW a t e r i n Pe rmea te [ V a p o u r 1

0,2 0 ,L 46 O FM a s s F r a c t i o n W a t e r in L i q u i dFig. 10. Pyridine-Water : I , Pervaporation; 2, Liquid-Water Equilibrium

I ENRICHED I N WA TE R IP Y R l D INE WATER SEPARATIONCOMBINATION AN D DISTIL LATIO N , A Z E O T R O P E - S P L L r T l N G E Y P E R V A P O R A T I O N

Fig. 110 1995 IUPAC, Pure and Applied Chem istry, 67,993-1002


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Industrial application of memb rane separation processes 1001

In some cases where the azeotrope point is not close to one side of theVLE-diagram, final dehydration o f the organic liqu id might not be theoptimal way. An example is given in fig. 10 for the pyridine-water system.The azeotropic mixture comprises fairly equal concentrations of pyridineand water. Here, a scheme as shown in fig. 11 might be more feasible. Th eazeotrope from the top of both columns is split by Pe rvapora tion into awater rich a nd pyridine rich fractions, which are the n p assed to therespective columns. Again, optimized design of the columns and thePervaporation system are necessary.Another ap plication of hydrop hil ic Pervaporation membranes isschematically show n in fig. 12. In a chemical reaction, e.g. a nesterification, water is a by-product. Continuous re moval of this water willshift the equilibrium of the reaction to the side of the produ ct andeventually total conversion of one of the educts can b e reached. Improve dproductivity and reduc ed effort for the purification of the pro duc t are themain advantages of this scheme. A m edium size plant, opera ted in batchmode, has be en started about 16 months ago an d is pro ducing differentesters by this scheme. A muc h larger plant, produ cing an ester in astraight-forward process, using a series of reactors and Pervaporationstages, is und er construction.

Products

Educt rvaporation/ I I

Keactor 'dater

Fig. 12

Pervaporation plants employing organophilic membranes, are much lessnumerous than those with hy drophil ic membranes. The larg er number ofplants for organ ic removal are, in fact, not really P ervapo ration plants butusing air, laden with organic vapors, as feed streams. For this process theterm "vapor permeation" is used. The differences to a Pervaporationprocess are foun d in the fact that cooling of the reten tate is avoided, as thepermeating component is already supp lied as a vapor. Furthermore, thefeed side partial vapor pressure of organics may reach in a few exemptionthe saturation level, but will u sually be below this value. Vap or p ermea tionthus can be regarded as a process in between Pe rvaporation and gasseparation, par tially overlapping w ith both.0 1995 IUPAC, Pure andApplied Chemistry, 67,993-1002


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1002 H. BRUSCHKEOrganic-organic separation by non-porous membranes has bee n in thefiftieth the subject of intense investigations, especially in the US. Today,first membranes are available at least for the removal of more polarorganics, like methanol and ethanol, from their mixtures with less polarorganics. A first pilot plant for one of these applications is supposed tostart its opera tion at the end of this year.Separation of fluid mixtures by means of membrane process today alreadyplays an important role in various industries, especially in biotechnology,gas treatmen t, water treatment and chemical industry.Regard ing the fact that the first membranes app licable for industrialprocesses have been developed about 25 years ago, and new membranesand applications are produced and found, it can be expected that the roleof mem branes in separation processes will continue to improve. Membraneprocesses, however, will probab ly not displace more classical separationprocesses, but in hybrid systems will supplement them and lead totechnically, econom ically and environm entally more efficient separation.

1) S . T. Hwang, K. KammermeyerMembranes in Separation, Wiley and SonsNew York, 1975, p. 67

0 1995 IUPAC, Pure an d Applied Chemistry, 67,993-1002

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