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Classification of Filtration Processes

2015-12-01 By zhaozhao

The two major branches of physical separations technology, filtration and sedimentation,work by quite different mechanisms. Filtration operates entirely on particle or droplet size (and, to some extent, shape), such that particles below a certain size will pass through the barrier, while larger particles are retained on or in the barrier for later removal. The separating size is a characteristic of the barrier, the filter medium. The wide range of filter designs is very largely a consequence of the need to handle the accumulated solids that collect in the filter, although the need to pack as much filter medium area into a given equipment floor space (or volume) can be another design decider. The operation of a filter usually needs a pressure differential across the filter medium, and this can be effected by means of fl uid pressure
upstream of the medium (pressure filters) or suction downstream (vacuum filters).

Sedimentation, on the other hand, operates on the density of the particle or droplet, or, more correctly, on the density difference between the suspended particle and the suspending fluid. It is the force of gravity working on this density difference (or the much higher centrifugal force operating in a centrifuge) that causes separation by sedimentation – either of a solid from its suspension, or of a lighter solid from a heavier one. Particle size also has a part to play in sedimentation – a larger particle will settle faster than a smaller one, of the same density. Settlement area is the prime consideration in sedimentation, with throughput being directly proportional to available area, which is why the extra cost of a sedimenting centrifuge will often pay for itself because of the much smaller space that it occupies.

Solid separating technologies have two prime purposes: the removal of unwanted solids from suspension in a fluid (which may itself be a wanted product or a waste that needs cleaning prior to discharge), and the recovery of a wanted solid product from its suspension (often following a prior crystallization or precipitation step). Either kind of equipment, filter or sedimenter, may be used for either of these purposes,although it is true that most solid recovery is achieved in fi lters or sedimenting centrifuges.

The particle sizes covered by filtration range from the large pebbles of the mineral sector ’ s screens to the ultrafi ne particles and large molecules of the membrane ultrafiltration systems. Most systems involving contaminant removal are concerned with fi ne particles – fi ne enough, for example, to have stayed suspended in atmospheric
air for long periods of time.

The mean particle size, and the particle size distribution, will both have a major infl uence on the type of filter chosen to treat a suspension, a choice that would be made in order to produce the most filtration-effi cient, energy-effi cient and costeffective solution. The apparent fi ltering range of a particular type of filter can be misleading in terms of both effi ciency and cost-effectiveness. The fi ner the fi lter, the more readily it will become clogged by coarser particles, so that, where very fine filtration is required, it becomes both more effi cient and more cost-effective to filter in stages, using two or more fi lters in series with progressively decreasing cut-points. Thus, a full system might include an initial strainer, followed by a thick medium fi lter, and then an ultrafilter for the fi nal stage (or even this whole prefiltration system ahead of desalination by reverse osmosis).

The types of contaminant present, and their concentrations, will obviously depend on the environment in the case of atmospheric contamination (or on the prior history of the liquid for a suspension). Air pollution can be treated by means
of a large baghouse fi lter, capable, with appropriate bags fitted, of fi ltering down to below 0.1  m, which means just about all likely contaminants, but its use would only be justifi ed in special circumstances (such as an industrial air fl ow), and simple panel fi lters would probably be suitable for most other applications.

The size of the separated particle is used to delineate the terms used for the various filtration processes. Thus ‘ filtration ’ (or more specifi cally, nowadays) ‘ macrofiltration ’ is used for separating particles in the approximate range of 1 mm down to 5  m (with ‘ screening ’ used for particles above 1 mm, without an upper limit).  From 5  m down to about 0.1  m the process is termed microfiltration, while below that the term ultrafiltration applies. Ultrafiltration covers the fi nest distinct particles  (such as colloids), but its lower limit is usually set in molecular weight terms,measured in Daltons.

Below ultrafiltration in size terms come nanofiltration and reverse osmosis, which look just like filtration processes and are often counted in with them – they have a liquid fl ow, and a semi-permeable membrane is placed as a barrier across
this fl ow. However, NF and RO differ from UF in operating principle: the liquid treated is now a solution with no (or extremely little, if properly prefi ltered) suspended matter. The membranes have no physical holes through them, but are capable of dissolving one or more small molecular species (such as water in the case of RO desalination) into the membrane material itself. These species diffuse through the membrane, under the high trans-membrane pressure, and emerge in their pure state on the other side.

Whilst many fi lters or sedimenters have a range of applications for which they are well suited, the application ranges are by no means unique, and the engineer seeking a ‘ correct ’ separation solution may still be faced with a choice among several types from which to make the best (the most effi cient, the most cost-effective) selection.
The dewatering of sludges, from production processes or from waste treatment,for example, was, for many years, the province of the filter press. The belt press was developed very much to capture some of this market. Then good and reasonably economic coagulants were developed, which made the solid/liquid task easier, and vacuum belt fi lters and decanter centrifuges became effective tools for this task.

A final point on technology, although by no means an unimportant one, is the need for the fi lter, and especially the filter medium, to be materially compatible with the suspension and its components. This requires little attention in, for example, the filtration of ambient air, but is of vital concern where the gas is hot (as from a furnace),or the liquid is corrosive (as in mineral acid filtration), or the solids are abrasive. Filtration in the nuclear industry especially imposes problems not only of resistance to radiation, but of diffi cult or impossible access once the filter has been used.

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