Nuevo Membrana

8/10/2019 Nuevo Membrana

1/4

Feature32Filtration+SeparationSeptember/October 2009

Membrane filtration:

Whats newin membranefiltration?

I n his series of articles covering progress in a number of

broad classes of filtration and other separation equipment

types, Ken Sutherland looks at developments in membrane

media and membrane filtration systems.

The wide, and steadily less and less expensive,availability of the membrane, in all of its forms,

as a filter medium has made a tremendousdifference to the ability of the filtration industryto achieve very fine levels of separation.Improvements in membrane systems of allkinds have been made possible by the continualproduction of new materials from whichmembranes can be made. There is now a verywide range of materials with specific propertiesthat can be utilised to make membranes withimproved system performances.

In its early days as a separating medium, themembrane was a thin flexible sheet or a thin-walled flexible tube, rendered semi-permeable

by its production process. This characteristicof flexibility has remained associated withthe membrane, but the widespread existenceof the rigid ceramic membrane makes it verydifficult to provide a simple definition fora membrane. In fact it is probably better todefine a membrane in terms of what it does,rather than what it is: so that a separationmembrane is a semi-permeable material ableto retain suspended or dissolved substancesat levels in the region of a few micrometres,down to a few tenths of a nanometre.

The earlier applications for membranes

covered reverse osmosis, primarily for thedesalination of water (in the size range upto around 400 Dalton or about 1.5 nm), andultrafiltration, widely used for the removal ofdissolved organics and larger inorganic ions(in the size range 0.3 to 500 kD or about 1 to

Reverse osmosis membranes have been widely deployed in desalination plants.


2/4

Feature 33Filtration+SeparationSeptember/October 2009

200 nm). The two major liquid processing changes that have occurred havebeen the appearance of nanofiltration overlapping the top end of reverseosmosis and the lower end of ultrafiltration (covering around 50 D to 5nm), and the extension of membrane separations into the microfiltrationrange (from around 50 nm up to 2 m or more). This latter move, intomicrofiltration, has greatly increased the applicability of membrane mediain separation processes, and microfiltration uses are approaching the largestcomponent of the total membrane market place.

In addition to the better known liquid separation uses of membranes,they are also widely used for gas and vapour separations, which arerapidly growing as applications for membranes. The largest is in theseparation of the components of air, such that small, on-site oxygen ornitrogen plants are now commonplace, enabling the local production

of these gases. The growing emphasis in global warming abatementtechnology on carbon dioxide capture and storage is likely to providea huge application for gas permeation, as will any major move intohydrogen systems for better energy economy.

The membrane business

The manufacture of membrane modules and systems for the variousseparation processes is a sizeable business. It represents (counted at themembrane module level only) 35-40% of the total filter media market,approaching $8 billion in 2009, and is the fastest growing of the mediamarket segments. Not surprisingly, in a business of this size, there aresome large players, such as Degrmont, Dow Chemical, DuPont, GE(including Ionics, Osmonics and Zenon), Koch, Pall and Toray. Thereare many other companies involved, often a lot smaller than these, but

noteworthy for some specific feature of their product range, such as NewLogic with its vibrating membrane array, and the steadily increasingnumber of makers of ceramic membranes and membrane bioreactors.

In common with every other segment of the filtration business, corporateacquisitions among membrane suppliers have been a feature of businesslife. Of the 30 companies featured in the last of Elseviers Profiles ofthe membrane business, published in 2004, less than half have survivedunaffected by take-over activity. The two largest centrifuge companieshave each bought into the membrane business (Alfa Laval with DSSNakskov, and GEA with Membraflow), but a larger observable moveis into the fresh and wastewater applications, for which membraneseparation makes a very good entre well demonstrated by the recentacquisitions of GE and Siemens. Other business expansion moves can

be seen in Palls purchase of several of the USFilter range of companies,including Memcor; Polypores acquisition of Membrana; and the sale ofdomnick hunter to Parker Hannifin, and of Cuno to 3M. The privateequity houses have been busy in the membrane business, as elsewhere,as witness the purchase of Norit, and of the expanded Polypore, byinvestment funds, also the assisted buy-out of Novasep/Orelis.

A membrane filtration system. Image courtesy of Polymem


3/4

Feature34Filtration+SeparationSeptember/October 2009

Membrane operating problems

Most membrane processes are characterisedby two key process parameters: flux and

selectivity. The selectivity is governed by the

intrinsic nature of the membrane material,built into it by its method of manufacture, and

measured by its permeability to the species

in question. The flux is determined by the

specific resistance of the membrane materialunder a given differential pressure across the

membrane, so that the flux increases with theoperating area of the membrane and with the

applied pressure.

On the face of it, the membrane is an ideal

filter medium, able to meet the increasingly

severe demands from its marketplace forever finer degrees of separation. In practice,

there are a number of operating problems

affecting all types of membrane, and

much of the work entailed in the recentdevelopments in membrane media and

systems has been designed to reduce the

impact of these problems.

Fouling

The main operating problem of membrane

separation processes remains the ease with

which the membrane plugs, causing theresistance to flow to increase. This behaviour,

fouling, is caused by the deposit of slimy

solids, present in the feed, on the upstreammembrane surface, which eventually blocks

it. The plugging process is accentuated by the

concentration polarisation that occurs in therelatively quiescent fluid zone close to the

membrane surface, as the species separated

from previously processed fluid build up in thiszone and interfere with fresh material trying to

get to the surface.

The problems of fouling and concentration

polarisation have found some resolutionin the process arrangement that causes the

feed liquid to flow parallel to the membrane

surface, rather than perpendicular to it, soscouring the surface as the flow moves across

it, thinning the surface layer and removing

deposited material. The consequent cross-flowfiltration method has been one of the most

important equipment developments in the

filtration industry, especially when coupledwith rotating or vibrating filter systems.

Throughput

Because the resistance to flow through most

membranes is high, the consequent lowthroughput has needed large membrane areas,

and high pressures, to achieve worthwhile

fluid flow rates. The history of membranematerial and process development is very

largely one of reduction in flow resistance,

first in the diffusion membranes from reverseosmosis to ultrafiltration, and then to the

microporous materials of microfiltration. A

similar relaxation occurred in the move fromreverse osmosis to nanofiltration.

Mechanical strength

The first membranes were relatively weakmaterials, hence their use as hollow fibres,

which are intrinsically strong. As new

materials were developed, strength was oftennot the first priority, and modern membranes

can be very thin, with low tensile strengths.

The saving move from the point of view of

strength has been the development of thecomposite membrane, in which the active

separating layer is supported on one or more

substrate layers, which give the necessarystrength to the finished membrane.

Cost

The earliest membranes were expensive tomake and lasted only a short time. Coupledwith these operational costs was the cost ofhigh pressure operation. Developments bothin membrane materials and system designhave resulted in a marked reduction inmembrane cost, measured in filtration area

or process throughput, which has reducedexponentially with passing time. The overallspeed of reduction has, of course, been affectedby the development of new and initiallymore expensive materials, both organic andinorganic, but these have in turn followed asimilar cost reduction path.

Corrosion resistance

The majority of membrane applicationsfeature relatively bland conditions,but increasingly, as membranes havefound application in the chemicals andpharmaceutical sectors for example, corrosive

environments have become involved inprocessing. This has provided an opportunityfor ceramic and other inorganic membranesto take the place of less resistant polymericmembrane media, although newer membranesmade of fluorinated polymers, such as PTFEand PVDF, show marked resistance tochemical corrosion.

Temperature resistance

Just as polymeric membranes are not well suitedto highly acidic or alkaline environments, so tooare they not well suited to higher temperature

applications. Even with the use of fluorinatedpolymers, temperatures much above 150C arenot easily handled, and here again the ceramicmembrane has found ready application, althoughthe module must then take a form suited toceramic materials, such as the monolith or

Membranes have many applications in the treatment of wastewater.


4/4

Feature 35Filtration+SeparationSeptember/October 2009

coarse tube. More recently, ceramic materialsmade from fibres have enabled some flexibilityto be built in, and hollow-fibre ceramics are nowbeing made available.

Feed quality

The hollow fibre format is a very successfulone, because the fine tube structure resistshigh trans-membrane pressures very well. Adisadvantage of this format is the need for thefluid flow to occur in very fine passageways,and thus be prone to blockages caused by thepresence of large particles. As a consequence,the finer the membrane separation process,the more carefully must the feed solution befiltered to ensure the absence of blockages.Good pre-filtration is also a help incontrolling the effect of fouling, as the excessof solids that causes concentration polarisationcan be removed in the pre-filtration stages.It is not uncommon now for a membraneprocess to consist of a train of units, in orderof diminishing cut-off points.

Solids handling

Membrane systems, especially in hollow fibreand spiral wound formats, are not well suitedto the handling of feed suspensions with highsuspended solids loadings hence the needfor efficient pre-filtration. However, otherformats can accept more concentrated solids,and membrane systems are being used asthickeners as well as clarifiers.

Permeate treatmentOne of the major effects of the wider use ofmembrane systems is that impurities thatwere quite dilute in the feed stream becomeconcentrated in the retentate. This can createa toxic waste stream, which may require that aspecial retentate treatment process be built onto a membrane installation.

Energy conservation

Some membrane processes, reverse osmosis inparticular, operate with high trans-membranepressures. The need for high operating pressures

means that high energy consumptions areinvolved and the pressure in todays world isfor energy consumptions to be reduced.

In the case of filtration, this drive to moreefficient use of energy directly opposes themajor driving force represented by the need

to provide ever-finer degrees of separation.This dichotomy is not easily resolved, sincefilter media that separate more efficientlyusually need higher pressure drops to doso. Much development activity is beingput into the production of media withlower pressure drops, such as the membrane

materials formed from a surface layer of finefibres, like Donaldsons Ultra-Web and theNanoweb media supplied by Hollingsworth& Vose.

Recent developments

The membrane operational problemsalready discussed have been well knownfor some time, even in the relatively shortlifetime of membrane separations, and themethods adopted for their control are wellunderstood: composite materials, fluorinatedand ceramic membranes, agitated cross-flow,

efficient pre-filtration systems. Some morerecent developments, all aimed at bettermembrane systems, are highlighted in theremainder of this article.

Recent developments in membranematerials and formats

Polymeric media are by far the most widelyused materials for membranes, but othersemi-permeable materials are also receivingconsiderable attention, and the ceramicmembrane is growing rapidly in rangeof application. Particularly noticeable isthe availability of ceramic membranes inhollow-fibre and flexible sheet formats.They are also being made by the sinteringof very fine spherical particles, enablingthe creation of pores as low as 0.5 nm indiameter, and from very finely spun fibres.

An important development in membranematerials is the ability to make smart orfunctional membranes, such as those withappropriate chemicals grafted onto theirsurface, which can then be very selectivefor certain chemicals (such as enzymes), orwhich enable them to resist fouling moreeasily.

Although still at the small scale, thedevelopment that offers great promise is therotating or vibrating membrane unit, withmechanical movement employed to reducethe thickness of the boundary layer at themembrane surface.

Recent developments in membraneapplications

The standard membrane processes for liquidsprocessing (RO, NF, UF and MF) are nowreasonably commonplace in bulk industries, aswell as their original niche applications. The

demands for clean water, free of pathogenssuch as bacteria and viruses, have propelledmembrane processes to the first level ofconsideration in fresh and recycled waterprocessing. Sterilisation of a liquid flow is nowpossible by passage through a UF membrane.

Ultrafiltration is, in fact, becoming thepre-filter of choice ahead of reverse osmosisdesalination plants, and membranes arebeing employed with great success in themicrofiltration range, where until relativelyrecently, they could not have been considered.A prime example of the latter situation isthe rapid spread of the microfiltration-basedMBR (membrane bioreactor), combining abiological process to treat a waste liquid witha membrane separation process to clean thetreated liquid. The membranes now availablefor use in MBRs operate satisfactorily withvery low transmembrane pressures, and thishas become an important component of themembrane market.

The expansion of membrane separations intothe microfiltration range is being driven bythe markets wish for finer degrees of filtrationof its products (or waste streams), to match itscustomers demands for clearer final products

or the demands of environmental regulatorsfor less contaminating effluents. In whicheverpart of industry or commerce there exists aneed for fine filtration, in the 1 m region orbelow, then the chances are that it will nowbe met by a membrane separation process.

Contact:

Ken Sutherland

Tel: +44 (0)1737 218868

E-mail: [email protected]

Ken Sutherland has managed Northdoe

Limited, his process engineering and marketing

consultancy, for over 30 years. Northdoe is

largely concerned with the marketing and

technology of filtration and related separationprocesses. He has written numerous articles

for this journal and for its sister publication

Filtration Industry Analyst, as well as four books

on separation processes, most recently an A to

Z of Filtration and the fifth edition of the Filters &

Filtration Handbook, both for Elsevier.
mailto:[email protected]:[email protected]:[email protected]

Documents

Nuevo Membrana