As with any successful manufacturing business, the filtration industry has achieved its success through a combination of an excellent and developing equipment portfolio and the skilled entrepreneurs able to develop and market such equipment. Of course, many of the key ideas or key items of equipment in active use today have been around for well over the lifetime of Filtration+Separation. This, however, does not imply that the industry is an old-fashioned one: progress can be seen in many parts of it, and this article aims to highlight the more important items in the past 50 years’ progress. Presence of an item on the list may be based on historic importance or estimated future value (or both).
1. Multi-media packed beds
The ‘sand bed’ is probably the oldest type of filter still in wide and everyday use, with its origins long lost in history. For most of that time, the sand bed has taken the form of a packed bed of easily cleaned inert solids, with the feed suspension flowing downwards through it. After sufficient time has elapsed the liquid flow is stopped and replaced by a backwash flow moving upwards, causing the bed of solids to expand and release the accumulated dirt into the discharge zone.
It is difficult to set the bed up in this way, to ensure that when resettled the particles are in the same relative position as before they were backwashed. A number of arrangements have been tried, and been moderately successful, but the most successful by far uses several different layers of granules, each composed of a solid with a significantly different density and milled to significantly different particle sizes, such that the whole bed of solids separated itself out into the original layers, on resettling, after backwashing. Typical solids for this purpose would be garnet, flint sand and anthracite.
This multi-media idea has extended the practical life of the sand bed by a considerable amount, which is good because it is a very simple filter.
2. Moving bed filters
The second advance in filtration equipment to be mentioned here also involves a packed bed of inert granular solids, but under very different design and operating circumstances. Two of the major driving forces acting on the filtration market are the need for finer particle sized products, and the need to be able to cope with hotter systems, especially in exhaust gas filtration.
The problem with hot gas filtration is the inability of the standard filter media: natural fibres or synthetic polymers – to stand up to the required operating temperatures. Over limited temperature ranges, some media will withstand the conditions, using, say, fluoropolymers for moderate temperatures, or sintered stainless steel meshes for more arduous conditions. Higher temperature exhaust gases pose a severe problem, which is being solved by a development of the moving packed bed of inert pebbles.
The moving bed filter involves a comparatively wide diameter filter vessel with pebbles flowing downwards counter-current to the hot dirty gas flow upwards through the filter vessel. At the base of the filter the now dirt laden pebbles are swept away by a smaller gas or water flow, which cleans the pebbles for recycling to the filter vessel. The cleaned process gas leaves from the top of the filter for re-use or for safe discharge.
Along with the multi-media systems, these two have brought the packed bed reactors into prominence again.
3. Cross-flow filtration
From the historical beginnings of filtration, the flow regimes in the filter have remained the same: a slurry of particles or liquid droplets, suspended in a gas or a liquid, has been introduced into the feed zone of the filter vessel, and from that zone is forced through the filter medium and out of the filter, leaving some or all of the suspended material trapped on or in the medium. This process has the slurry flowing at right-angles to the medium, and is known as through-flow (or dead-end flow), and the filter is chosen so as to separate a required proportion of the feed slurry, usually as free from the suspending fluid as possible, which remains in the filter until removed from it either manually or automatically.
An alternative scheme has come into prominence together with the use of membranes as filter media. It has become important because of the need to filter at high values of pressure drop across the filter, and thus to operate the membrane media with as thin a layer of filtered solid as possible. One way of achieving this has been to operate the filter with the flow of feed slurry across the filter, parallel to the surface of the filter medium, in what is known as cross-flow (or tangential flow), with most of the fed solids leaving from the high pressure side of the filter, as a concentrated slurry.
The cross-flow filters have mainly been used so far in membrane processing, but applications for them as concentrators are increasingly being developed.
4. Polyelectrolyte flocculants
The two major divisions of the mechanical separation topic are filtration and sedimentation, of which the latter, sedimentation, is much the smaller in market size, although some of its practical implementations (as water treatment plant, for example) can be quite large. The large plant size is a feature of the space required to achieve the necessary separation. For most of the hundred years or more that water treatment has been by gravity separation, the process has been aided by the addition to the feed water of a coagulant chemical, which converts the small particles of the natural suspension into the much larger flocs of a flocculated one.
For much of that time also, the coagulant would have been one based on an inorganic chemical, most likely alum or a similar polyvalent salt. This would usually have been added as a solid powder, well mixed into the impure water in the feed zone of the clarifier – not an easy process to achieve.
This process was revolutionised by the development of the organic polyelectrolyte, polyacrylamides in particular, which are themselves liquids, capable of relatively easy dispersion in water, and with the ability to produce precise dosing of the flocculating agent into the clarifier. Not only did the use of the polyelectrolytes allow major improvements in the clarification processes, but it also created process opportunities for the several types of separation equipment, none more so than with the decanter centrifuge, which has now taken a leading role in sludge dewatering.
5. Spunmelt nonwovens
It has already been postulated that the filtration business is being driven by a need to handle hot fluids, and to deal with finer particles as feed and final products. The industry has taken rapidly to the arrival of thermopolymers, and the ability to extrude finer and finer fibres from them, as one solution to the finer particle aspect, given that the finer the fibre in the filter medium, the finer will be the degree of separation in the filter. For the greater part of the history of filtration, the separation has been achieved by the use of natural fibres and filaments (wool, cotton and silk) in woven or nonwoven (felt) form, creating only the relatively coarse media corresponding to these natural materials.
With the arrival in the mid-1900s of extruded thermopolymers, and the ability to manipulate them, came a whole new range of materials, called here the spunmelts and capable of production in a wide range of sizes. The first were the spunbondeds (extruded and then dry-laid as filaments), then came the melt blowns (extruded, then broken into shorter fibres by flow downwards through an air stream, and then dry-laid as fibres), then flash spun materials (co-extruded with solvent and then flash evaporated to give fibrillated fibres that self-bond when dry laid), and then electrospun (extruded into an electrostatic gradient and pulled to form extra fine filaments).
These materials can be co-extruded to give composite fibres, and treated in other ways, to give a tremendous variety of finished media, and correspondingly a useful range of filtration applications.
6. Membranes as media
Anyone with a reasonable knowledge of filtration, if asked to nominate the one most important development in the last 50 years, would probably suggest the use of membranes as filter media. This would be difficult to refute, especially in the context of seeking finer degrees of separation.
It must be remembered that the filtration-related use of membranes started about 50 years ago with the removal of almost absolutely pure water from a range of salt waters, and it did so by means of diffusion – of the water molecules through the molecule-sized spaces in the material of the membrane – and not by filtration (hence the high trans-membrane pressure). This early usage of membranes, known as reverse osmosis, has expanded to a range of membrane processes, with progressively ‘looser’ membranes, able to separate at higher molecular and then actual particle sizes, from nanofiltration through ultrafiltration and then microfiltration. Ultrafiltration is the process of choice in the purification of very pure water, while microfiltration using membranes is fast becoming the first thought for separations in the food and beverage or the chemicals sector.
7. Ceramic media
Mention has already been made of a moving bed of ceramic granules as a successful means of providing a high temperature filter, but there are other forms of such a filter and other purposes for ceramic media in a filter. A major advantage of the ceramic filter element is that it can resist corrosive fluids, while its main problem is its brittleness, especially in unstable environments – a problem nowadays successfully solved either by the use of a coarsely porous block of ceramic material, with a fine layer of filter medium sintered onto one face, or by making the substrate out of a thick layer of ceramic fibres, which has enough flexibility to absorb any vibrations in the element.
The hexagonal ceramic brick is one form of the ceramic membrane, which can be arranged inside a cylindrical housing quite economically, and still leave space for separated solids if necessary. The brick has a number of cylindrical holes along its length, each hole being covered by a thin layer of ceramic, which is then sintered in place to provide a ceramic ‘membrane’. Fragility of the ceramic element is then not an issue.
The filtration of air, especially for building air conditioning or for combustion air intakes, requires a large volumetric intake, not easily provided in a factory or large office situation. An acceptable air filter, for this purpose, must allow for the use of the most up-to-date filter media material, which in turn must be packed as economically as possible into the available filter housing volume. For a long time this kind of air filter has been based on the replaceable square pad or panel, held in a rigid frame, which makes a tight push fit into the wall of the air filter housing. The square frame has, variously, held a simple pad of nonwoven material, or a set of bags or pockets, or an array of pleated flat material. The actual medium has been chosen to give the most efficient filtration at the time.
This style of filter has been able to match the classifications developed for air and other gas filtration by international agreement (HEPA/ULPA, ASHRAE, etc), and so make the pad filters interchangeable in their housings.
The air filter with pleated media can utilise almost any of the types of flat material currently available for filtration purposes, but the most efficient at the present time is the V-block system, where the pleated medium is arranged in a small number of flat packs held in a rigid frame at an acute angle to its neighbours. This allows good gas flow dynamics within the frame and a good collection space for trapped dust. The V-block filter would appear to be the future of air filtration.
9. The consultant
The filtration business is a complex one in many of its applications, requiring advanced technical and process knowledge from its suppliers. For equipment manufacturing companies that cannot afford to maintain a process engineering support division, or for most equipment using companies, the occasional need for sound consultancy advice, on equipment design or selection, is provided by the expert consultant, advising on the finer points of equipment choice, say, or on the particular characteristics of the separation process.
For much of his professional life, until his untimely death, Derek Purchas embodied the specialist consultant with expertise in filtration and sedimentation, starting from a process engineering background (with British Ceca), and soon able to prepare some sizeable books descriptive of all aspects of filtration technology, and especially filter media selection. He was justifiably one of the early Chairmen of the Filtration Society: the fifth in succession.
His name is included here not only because he was a good consultant in the filtration business, but also because he fought long and hard for the establishment of the role of the individual consultant within the Institution of Chemical Engineers, for which the present-day chemical engineer has a lot to thank him for.
10. Society foundation
Just making its way into the 50-year time scale (because that is partly why the period has been chosen) is the formation of the Filtration Society in the UK. Bill Norris had successfully started the forerunner to Filtration+Separation, and he saw the opportunity for a professional society related to filtration (and perhaps a little more trade-friendly than other bodies of the day). He recruited a number of like-minded individuals and the Society was formed, which has continued, with only an occasional hiccough, to this day. (Much of the formation story is included in Mike Taylor’s reminiscences in the May/June 2013 issue of Filtration+Separation.)
Whilst the formation and continued existence of the Filtration Society might have been enough to put it in this list, the full value from the Society’s creation has come from what it triggered. Within a matter of months, Wells Shoemaker and Frank Tiller had started what became the American Filtration Society. With Bill Norris’s support, Mike Taylor got the FILTECH exhibition off and running almost at the same time in the UK, and the World Filtration Congress series took off in Paris, ten years later. FILTECH and the World Congresses continue to be very successful, and do well to hold their own in the face of competition from ACHEMA.
There is a list of a half century’s developments in the filtration industry, which, of course, risks your reaction along the lines of why has a particular piece of equipment or a particular person been omitted. Where, for example, is the fully automated filter press? Or the 5-layer sintered wire mesh as filter medium? Or the needle felt materials (especially in their spun laced format)? What about developments in America? Where is Ken Ives, or the Loughborough school of people and ideas? •
Tel: +44 (0)1737 218868
Ken Sutherland has managed Northdoe, his process engineering and marketing consultancy, for over 35 years. During most of that time he has been concerned with the marketing and technology of filtration and related processes. He has written numerous articles for Filtration + Separation, and for its sister publication Filtration Industry Analyst. He has also produced four books on filtration processes for publisher Elsevier, including an A to Z of Filtration.
Do you agree with Ken’s selections?
As Ken pointed out at the end of his article a great many innovations, materials, developments, people and events could have made his ‘Top Ten’ list of notable developments in the world of filtration and separation from the past 50 years. Do you have alternative items that you feel deserves to be mentioned? If so, we would be delighted to hear your suggestions. Send your selection together with a short explanation (around 200 words) to Mark Holmes at email@example.com and we will publish your suggestions in a forthcoming issue and on F+S’s website.