The significant developments in filter media during the last recent years have mainly been achieved as a result of the demands of the end-user industry rather than the needs of the filter manufacturers. According to Ken Sutherland, the most important driver has been the demand for finer filtration degrees, occasioned by the requirements of the filter user for greater fluid clarity, heavily boosted by the impacts of environmental legislation seeking reductions in pollutant levels. A second driver has been the wish to extend upwards the operating temperature range of filter media resulting in an increased demand for chemical and abrasion resistance. This paradox of requests is quite a challenge for filter manufacturers because a finer filtration degree is mostly achieved with finer fibres or finer granulates which are less resistant to aggressive environments than coarser fibres or coarser granulates. In addition to that, the filter manufacturer is always looking for ever more reliable and reproducible media, for media giving maximum filter area per unit filter volume, and for the lowest possible media cost consistent with all of these features.
Enhanced surface filtration medium
Metal fibres used in filter media can range in diameter from 1 µm to 80 µm and are available in different alloys such as AISI 316L, Hastalloy, Inconel, Fecralloy, Ni, Titanium and so on to meet the specific application requirements.
Traditional manufacturing of sintered metal fibre media
Long fibres are randomly spread into a loose web of several layers, each layer characterised by a specific fibre diameter. The loose web is sintered at high temperature under either controlled atmosphere or high vacuum to form a rigid structure. The final filter rating is determined by the weight per layer, the characteristics of each fibre and the combination of several layers. A significant characteristic is their very high porosity, resulting in a high permeability and a low pressure drop. In most of the cases, the media is built up in such a way that particles are retained inside the filter media enhanced by its graded structure: coarser fibres at the flow inside and more and more fine fibres towards the flow outside. This provides a greater capacity to hold dirt (longer on-stream life time), combined with small pressure drops (because of high porosity) and small wall thickness for the ease of off-line cleaning. Applications can be found mostly in the polymer filtration industry to remove contaminants and to shear gels.
New development of enhanced surface filtration medium
Short fibres with an L/D ratio of +/ – 100 and with a specific c-shape are, together with a binder and additives, randomly spread and mixed into a smooth slurry. This slurry is then further processed in such a way that a homogenous layer is achieved. Several of these layers can be combined to the desired weight and thickness. After debinding and sintering under a reducing atmosphere, a final product mechanically supported by a several meshes can be obtained. This media contributes to an increased quality level through a very fine filter rating using a surface filtration layer of very fine fibres but still having a high permeability to minimise the energy requirements. The filtration layer prevents the particles from penetrating the medium with subsequent clogging and corrosion (e.g. pitting damage, stress corrosion cracking) as a result. In addition, a sufficient backwash or backpulse cleaning is achieved. In order to reduce the high surface to volume ratio which would normally lead to a decreased life time in corrosive environments, the new enhanced surface filtration medium is made of thicker fibres but still achieving this fine filter rating.
The main applications could be found in liquid filtration such as in food and beverage production, and in the chemical and pharmaceutical industries. The need in these industries – besides the very high quality and purity of the end-product – is the ability to regenerate and clean the media in place (CIP cleaning) without interrupting the filtration process. Having a medium based on sintered metal fibres broadens the types of cleaning methods used, being chemically (using NaOH, citric of nitric acids) or mechanically (using water jet or ultrasonic cleaning) without the potential danger of influencing the filtration fluid or corroding and degrading the filter medium itself. In addition, thanks to the high permeability, the flow throughput is 5 times higher than compared to traditional polymer filters (hollow fibres, plate and frame, etc.) in these applications.
Applications
Wine filtration
In current wine filtration processes, diatomaceous earth is added to the wine to adsorb the contaminant and to form a filter cake. This is done as a first step to remove contaminating colloids as well as in a second step for the retainment of tartrates (wine crystals). Adding diatomaceous earth to the wine and removing it from the filter system is a labour intensive operation and generates a considerable amount of waste. Using a sintered metal fibre medium can give several advantages to the wine manufacturers such as:
(1) an increased performance (no preparation of the pre-coat, limited interventions during the filter operations, reduced cleaning time of filter at a consistent high flow rate);
(2) more awareness for the environment and product quality (recuperation and valorisation of tartrates, no waste of used diatomaceous earth, reduced waste water treatment after filter cleaning, no impact on organoleptical performance and anticipation on future stricter disposal regulations); and
(3) a higher operational efficiency (automatic cleaning using wine or water, less transport, handling and inventory and reduced losses of wine).
In addition, this medium (with a typical filter rating of 1,3 µm) can also be further processed and manufactured in several shapes and forms such as flat sheets or tubes to be used in a cross-flow mode system. Advantages are the high flow through put (200 l/m²h compared to traditional values of 50 – 100 l/m²h) while attaining the desired purity or quality level (< 0,5 NTU and IF < 10) and no influence on the organoleptical performance of the wine (which was confirmed in a blind tasting test at a well-know wine cellar).
Beer filtration
The main filtration requirements of a beer filter are the removal of particles between 0,5 and 1 µm and the removal of floating particles like yeast, bacteria and other haze material, from the fermentation step onwards. Some 90% of breweries nowadays are still using diatomaceous earth, but due to future stricter environmental legislation and the disadvantages regarding quality, waste and cost, there is a same trend as in wine filtration towards filtration systems without filter aids, such as cross-flow systems using membranes. First of all there is no problem of waste disposal because the filter medium can be used over long periods of time (approximately 1 year). Furthermore, membrane filtration is continuous and modular in design and therefore highly flexible in operation. This makes the filtration process far less critical in the processing of beer. Thirdly, it is maintenance friendly because it can be cleaned off – and on-line. It can be reused and regenerated through Clean-In-Place (CIP) processes, such as backwash and backpulse. A fourth advantage of using membrane filtration is that capacity can be increased easily by adding extra modules.
The major problem, however, is fouling of the membrane, which can occur in three ways: by layer formation on the membrane surface caused by the accumulation of yeast cells and flocculated proteins that are too large to pass through the membrane; by blockage of the membrane pores; and by adsorption of solutes, e.g. carbohydrates, to the inner surface of the pores, thereby reducing the pore size and increasing the hydraulic resistance of the membrane. In literature, a value of the optimal pore size of the membranes of about 0.5 – 1 µm can be found back. Smaller pores result in retention of beer components like high molecular weight protein, polysaccharides, and colour. Membranes with a larger pore size result in a higher degree of fouling caused by pore blockages.
First trials using the surface filtration medium of Bekaert with pore size 1.3 µm however, gave satisfying results in the retention of yeast. Further tests are still ongoing.
Chemical process
Crossflow filtration is an ideal filtration system to be used in chemical processes for the removal or the recuperation of catalysts. A special crossflow design has been developed by BAF (Bekaert Advanced Filtration) and Inabata consisting of two concentric tubes facing each other with their respectively surface filtration layer. The distance between the two concentric tubes determines the tangential velocity and can be designed in such a way to enhance the cross-flow effect (equilibrium between retentate and filtrate) up to a factor 2. In addition, using the concentric tubes, the filtration surface area is increased with 75% within the same filter volume. Moreover, the integrated cleaning system can be operated separately for both tubes in order to optimise the cleaning performance using two separate filtrate compartments.
Labscale tests were conducted in-house using graphite powder particles with an equivalent D50 of 5 µm and a D90 of 8 µm. These particles have a special sheet shape form with a length that goes up to 10 µm and a diameter < 5 µm with a density of 2,26 g/cm³. Using Bekaert’s surface filtration medium with a pore size of 8 µm in a concentric double tube form, continuous filtration could be guaranteed at a flow rate of 165 l/m²h and a max pressure drop of 2-3 bar with a backflow every 30 min. Very rapidly (within several minutes) a stable cake of 2 mm thickness was built on the surface of the medium resulting in an excellent filtration quality.
Conclusions
Bekaert has developed an enhanced surface filtration medium based on sintered metal fibres which can be made of different alloys depending on the application. It contributes to an increased quality level through a very fine filter rating using a filtration layer of fine fibres still having a high permeability to minimise the energy requirements. The filtration layer prevents the particles from penetrating the medium with subsequent clogging and corrosion as a result. In addition, a sufficient pulse jet cleaning is achieved. Another advantage is the use of thicker fibres (8-12 µm instead of 2 µm) in order to reduce the high surface to volume ratio which would normally lead to a decreased life time while still achieving this fine filter rating and sufficient low emission values.
