Performance of new membrane material

To match the series of articles appearing through 2009 in Filtration + Separation covering separation equipment, Ken Sutherland looks at recent developments in the key component of filtration equipment, namely the various types of filter media. His examination includes ceramics, nonwovens and polymeric membranes and their place in future industry.

Johns Manville’s air filtration media uses “Assurance” self-supporting pleat technology.
Johns Manville’s air filtration media uses “Assurance” self-supporting pleat technology.
Johns Manville's self-supporting pleat technology
Johns Manville's self-supporting pleat technology

What’s new in filter media? The key component of any filtration process is the filter medium, which largely dictates the performance of the filter. In fact, any filter, from the simplest strainer to the most complex tower press, is no more than a device capable of supporting a piece of semipermeable material across a fluid stream (gas or liquid), so as to separate suspended material from the fluid. The essence of a filter medium, therefore, is its semi-permeability,which can be achieved in an ever-increasing number of ways.  

Filter media can be made from almost any material that is porous or otherwise semi-permeable or can be rendered so. The characteristics of the filter medium are a combination of those of the material from which it is made and its structure. The range of media formats include: • Loose granular solids (“deep bed” media), whose shape is dictated by their containing vessel; • Coarse porous sheets, tubes and foam, often made by compressing granules or fibres into the required shape; • Woven staple yarns, monofilament and multifilaments, made as a fabric from natural fibres or extruded synthetics; • Papers, made from cellulose or glass fibres • Felts and needlefelts, again made from natural or synthetic fibres • Spunmelt synthetic nonwovens (spun bond, melt blown, flash spun, electrospun) • Woven wire and other metallic media, fabricated from sheet or rod • Constructed filter cartridges, made from components that are not filter media in themselves, but become so when made up (such as stacks of rings or a filament wound on a core) – these components can be made from any workable material such as metal, plastic and paper • Polymeric membranes, both cellulosic and synthetic • Ceramic materials of all kinds, including membranes  

In a market once composed almost entirely of the first five of these materials, plus woven wire and constructed cartridges, the use of the other three classes (spunmelt nonwovens, membranes and ceramics) has grown rapidly, and continues growing, to the extent that these three classes now make up over half of the annual sales of all filter media (in original equipment and as spares). Polymeric membranes alone constitute more than a third of the filter media market, while the spunmelt nonwovens have approaching 15% of that market, and ceramics about 7%.  

This change in market share pattern results from constant marketplace pressure for ever finer degrees of filtration in almost all separation processes, a demand that is most easily met by the membrane (although not necessarily the cheapest solution). The other main technical market driver is the need to withstand higher operating temperatures, which is best met by ceramic media.  

Environmental regulatory pressures, regarded by many as a restricting nuisance, have played an important part in both of these technical market drivers, fresh and waste water purity and hot exhaust filtration being major examples of processes needing better filtration performance for health and environmental reasons.  

There are, of course, other market drivers than these three, but they are not specific to the filtration business, such as population growth, the current economic recession, global warming restriction, and the huge rich/poor dichotomy with its consequent pressure for growth in standards of living. There is one other, general, driver worth specific mention and that is the burgeoning energy crisis, whose effect will be to press filter manufacturers and with them the makers of filter media, to provide systems that are as energy efficient as possible (a topic addressed in Filtration& Separation, 2009, Jan/Feb p.16).  

The filter media industry has met, and continues to meet, these driving forces by means of a series of key product developments: • The adoption of a steadily widening range of synthetic polymers for special uses as filter media, especially those with higher operating temperature capabilities. • The expansion of the applications of the polymeric membrane, in particular by expanding upwards into the microfiltration range of applications (and with this growth, the parallel growth of fixed and moving cross-flow filtration systems). • The development of inorganic membrane materials, especially those made from ceramics, but also from metals, glass and carbon. • The gradual change of the membrane medium into a multi-layer composite structure, employing a very large range of substrate materials supporting a thin surface layer of membrane material. • The development of an increasing range of spunmelt fibrous materials (spun bonded, melt blown, electrospun, etc), with steadily decreasing fibre diameters. • The production of ceramic filter media capable of resisting quite high fluid temperatures, and fabricated from fibres rather than monoliths.  

It is probably the appearance of the nanofibre that has created most excitement in the filter media business over the past few years. Although the term is regularly misused to mean any fibre whose diameter is less than 1 micrometre (i.e. 1000 nanometres), there is enough experience now of fibres in the 10-100 nm range to allow employment of the word widely in terms of the basic fibre from which media are made. The importance of this size discussion is because of the relation between the fineness of the fibre making up the filter medium and that of the particle being separated by that medium, i.e. the finer the separating fibre, the finer the particle that it will capture.  

The importance of these size factors can be seen in the application for finer filtration to improve the already good quality of drinking water in most of the developed world, by the more thorough removal of bacteria and viruses. Most viruses have sizes around 100 nm, so the ability to filter at this level provides the ability to disinfect drinking water by fine filtration alone. The ability of fine filter media to clean natural water sources and even to treat brackish or salt water economically to produce drinking water is a major benefit to the filtration business. It is possible that recycled waste water will soon become another source of fresh water.  

Gas filtration is also benefiting from the availability of media with finer structures: building ventilation, like water treatment, has problems with pathogens, while the pressure is on to remove finer particulates deriving from the burning of diesel fuels. Cabin air filtration is becoming a major category for the media business.  

A development of increasing importance is the combination medium. It has been common practice to install two processing units in series (say, in drinking water production), one to remove fine particles, and one to remove taste or colour or odour, but now these two functions are combined in one material, by the embedding of carbon particles in the fibres of the filter medium, or by converting a fabric into activated carbon cloth by pyrolysis. Other materials besides activated carbon can be embedded to achieve different chemical changes, such as the inclusion of bactericides for use in pathogen reduction.  

As well as being an almost essential feature of membrane media, the composite medium is also of increasing importance in other materials, where two or more layers are joined together, at least one to provide the required degree of filtration, and the rest to provide the necessary mechanical strength. A good example of this format is the SMS sandwich medium, in which two layers of spunbonded material encase and support an internal layer of finer meltblown material, which is the active filtration layer. A similar effect is seen in Kimberley-Clark’s new “Intrepid” air filtration medium, with its graded density structure through the thickness of the material.  

Polymeric membrane media  

The general availability of the membrane, in all of its forms, as a filter medium has made a tremendous difference to the ability of filters to achieve very fine levels of separation. Improvements in membrane systems of all kinds have been made possible by the continual production of new materials from which membranes can be made, an example being the polyethersulphone membranes, which became commercially available in the late 1980s, and are now a major type of membrane in the water treatment sector. There is now a very wide range of materials with specific properties that can be utilised to make membranes with improved system performances, not just in degree of separation, but also, for example, by improving the mechanical strength of the membrane. Indeed, as a new polymer becomes available it is usually very quickly adopted for use in some particular application.  

In high technology industries such as electronics, biotechnology, and pharmaceuticals, the reliability and integrity of membranes are paramount. This presents quite a challenge to the membrane manufacturers, in which they are being aided by the development of membrane characterisation techniques that enable rapid determination of membrane properties.  

Another important development in membrane materials is the ability to make smart (or functional) membranes, such as those with appropriate chemicals grafted onto their surface, which can then be very selective for certain chemicals (such as enzymes), or which enable them to resist fouling more easily. This may mean converting the upstream membrane surface from being hydrophobic to being hydrophilic.  

In terms of membrane separation processes, it is important to note the appearance of the nanofiltration membrane as a distinct part of the filtration spectrum, used, for example, in the removal of colour from surface waters, coupled with the general acceptance of ultrafiltration as a routine polishing stage in fresh water treatment. The extension of membrane separations into the true filtration region of microfiltration has hugely widened the range of application for these media, especially in the use of membrane bioreactors for the oxidation of suspended solids and their simultaneous separation from suspension. There is also an increasing acceptance of membrane separation processes for gases, such that air separation (into nitrogen and oxygen) is increasingly undertaken on a local scale by membrane units.  

In an intriguing reversal of the trend to more complex polymeric membranes is the renewal of interest in cellulosic materials, because they are biodegradable and are thus more easily disposable. Nonwoven media  

Apart from the progress in membranes, the greatest interest in filter media development lies in the ever increasing range of nonwoven materials made from extruded synthetic fibres, known as spun bonds, meltblowns and flash spun material. The techniques of extrusion, appropriate blowing of the extruded filaments and their laying down on a moving belt or a cartridge core, are enabling the production of very fine media, capable of filtration well into the sub-micrometre range. The newer technique of electrospinning is producing even finer fibres, while these techniques are being extended to carbon and metal fibres. Du Pont’s new HMT (Hybrid Membrane Technology) polymeric materials employ a novel spinning technology in their production.  

One important aspect of finer fibre production is the blurring of the boundary between membranes and nonwoven media by the creation of a thin layer of nanofibres as a web on a thicker substrate, which then behaves for all intents and purposes like a membrane. Examples of this kind of material are Donaldson’s “Ultra-Web” and Hollingsworth and Vose’s “Nanoweb” media. This is going to create an important range of microfiltration media. Also worthy of note are: • The development of very light weight spunbonds • The appearance of polymeric media able to withstand temperatures above 125 ºC and hence able to be sterilized by steam • An increase in the availability of electrically charged media (“electrets”) Ceramics  

Polymeric materials are by far the most widely used materials for filter media and new polymers, of ever increasing complexity, are being developed to meet specific needs of modern industry. However, other semipermeable materials are also receiving considerable attention and the ceramic “membrane” is growing rapidly in range of application. Particularly noticeable is the availability of ceramic membranes in hollow-fibre and flexible sheet formats. The use of ceramic nanofibres is exemplified by the “NanoKey” media produced by Keystone, using a glass fibre substrate. Ceramic (and so-called “metallic”) membranes are now being made by the sintering of very fine spherical particles, enabling the creation of pores as low as 0.5 nm in diameter. Carbon and ceramic membranes are also being made from very finely spun fibres. Other media  

The highlighting of these three classes of filter media, admittedly the largest three, is not to be taken to imply lack of development among other classes. In fact the reverse is true, much is happening throughout the rest of the filter media industry. Thus, deep bed media are now being used for hot exhaust gas filtration, and are operating as moving beds of granules with automatic cleaning as part of the recycle circuit. Woven fabrics still have a place in filtration, especially where mechanical strength is a feature, a place that is being maintained by the use of appropriate surface coatings to improve filtration performance. Meshes woven from plastic filaments now offer the same controlled apertures as wire mesh. Paper remains a medium of choice in the laboratory, although now more likely to be made from microglass fibre than cellulose.  

Felts have been given a new lease of life by the technique of hydroentanglement (spun lacing), with Norafin claiming “membrane-like” behaviour for its new spun-laced media. Very precisely perforated metallic media are offered now, for example by Stork Veco, using the machining techniques developed for the production of silicon discs in the semi-conductor business. Even the arrangements for media support are changing: Donaldson’s “Tetratex Extreme” ePTFE media is available with a very wide range of substrates; Colbond is supplying “Colback” support material using bi-component yarns; while Johns Manville’s air filtration media now comes without support, using “Assurance” self-supporting pleat technology.