Filtration Dynamic filters

Mechanical Cake Removal. This method is used in the American version of the dynamic filter described under cross-flow filtration with rotating elements, where turbine-type rotors are used to limit the cake thickness at low speeds. The Exxflow filter, introduced in the United Kingdom, is described in more detail under cross-flow filtration in porous pipes. It uses, among other means, a roUer cleaning system which periodically roUs over a curtain of flexible pipes and dislodges any cake on the inside of the pipes. The cake is then flushed out of the curtain by the internal flow. 

In some cases of pilot scale filtration, entire units have been enclosed as a secondary containment precaution (see Chapter 8). A recent commercial development is the MBR-Sultzer dynamic filter which is available in three sizes. Dynamic filtration is the same as cross flow filtration with little or no recirculation. The cross flow effect is derived from the spinning of the inner surface filter. This type of filter is more efficient, has a lower pump rate and a much higher linear velocity across the filter surface, than conventional cross flow filtration units. There is also little or no damaging effect on sensitive cells. The medium size has the same capacity as the Westfalia SA-7 separator. Van Hemert and Tiesjema concluded that the dynamic filter is suitable for work requiring strict aseptic and primary containment conditions. The use of a double mechanical seal on the rotating shaft could offer a higher degree of containment if required. 

This is a method by which the cake is prevented from forming by the mechanical action of brushes, scrapers or liquid jets. Tiller showed theoretically that the capacity of a drum or belt filter can be increased by using brushes to keep the cake off a portion of the available filtration surface, with the remaining portion used for normal filtration of the thickened slurry. The reduction in filter area available for cake formation need not lead to reduction in solids handling capacity because the thicker slurry would give faster cake build-up. This method has not caught on in practice, with the exception of the scraping action of the turbine rotors used in the American version of the dynamic filter (see section 11.6). 

The so-called axial filter , developed in the Oak Ridge National Laboratory, is remarkably similar to Morton s and Kaspar s dynamic filter in that the filter leaf is in the tubular form and the outer shell is also cylindrical. An ultra-filtration module based on this principle has also been described more recently. Unlike the European dynamic filters referred to in the previous paragraph, however, this filter is not suitable for scale-up because it poorly utilizes the available space. The Escher-Wyss pressure filter described (identical copy of this paper also appeared subsequently in Filtration and Separation ) takes the idea of axial filtration a step further in... 

Murkes has reported tests with the same American dynamic filter, except that he eliminated cake formation altogether by spinning the discs faster. No visible abrasion of the medium was observed while the filtration velocities could be maintained high even at relatively low pressures, below 5 bar. Murkes showed clearly the effect of speed and number of rotor vanes on filtration velocities and specific energy input. His results show, for any number of rotor vanes, an increase in specific energy input (kWh per m of filtrate) with rotor speed this increase can be minimized by optimization of... 

Vacuum filters are usually simulated with a Buchner funnel test or filter leaf test (54). The measured parameters are cake weight, cake moisture, and filtration rate. Retention aids are usually evaluated using the Britt jar test, also called the Dynamic Drainage Jar, which simulates the shear conditions found on the paper machine and predicts performance (55). 

The dynamics of variable-rate and -pressure filtrations can be illustrated by pressure profiles that exist across the filter medium. Figure 7 shows the graphical representation of those profiles. According to this plot, the compressed force in the cake section is ... 

The invasion of particles can be eliminated either by using solids-free systems or by formation of a competent filter cake on the rock surface. If the components forming the filter cake are correctly chosen and blended, they will form a very effective downhole filter element. This ensures that colloidal sized clays or polymeric materials are retained within the filter cake and do not enter the formation. Further protection is provided by ensuring that a thin filter cake is formed due to low dynamic and static filtrate losses. Thus, the cake may be easily removed when the well is brought into production. Additionally, the filter cake can be soluble in acid or oil. 

To date, in-bed filtration was more or less a black box as any measurement within the bed was virtually impossible. The design of such filters was based on models that could be validated only by integral measurements. However, with the MRI method even the (slow) dynamics of the filtration process can be determined. The binary gated data obtained by standard MRI methods are sufficient for the quantitative description of the system. With spatially resolved measurements the applicability of basic mass balances based on improved models can be shown in detail. 

The design of a cross-flow filter system employs an inertial filter principle that allows the permeate or filtrate to flow radially through the porous media at a relatively low face velocity compared to that of the mainstream slurry flow in the axial direction, as shown schematically in Figure 15.1.9 Particles entrained in the high-velocity axial flow field are prevented from entering the porous media by the ballistic effect of particle inertia. It has been suggested that submicron particles penetrate the filter medium and form a dynamic membrane or submicron layer, as shown in... 

Therefore, when operating in the filter cake mode, the axial velocity should be maintained at a level such that an adequate shear force exists along the filter media to prevent excessive caking of the catalyst that could cause a blockage in the down-comer circuit. For the separation of ultrafine catalyst particles from FT catalyst/wax slurry, the filter medium can easily become plugged using the dynamic membrane mode filtration. Also, small iron carbide particles (less than 3 nm) near the filter wall are easily taken into the pores of the medium due to their low mass and high surface area. Therefore, pure inertial filtration near the filter media surface is practically ineffective. 

A plethora of methods has been developed to evaluate renal function by dynamic renography and remote analysis of the excretion of renal function markers. The underlying principle is that the kidneys excrete a majority of small hydrophilic molecules and their clearance, secretion, or fixation in the kidney is quantifiable. When a renal marker in plasma is filtered through the glomeruli, the accumulation of the filtrate in the Bowman s capsule. One or more of the following events may occur in the renal tubule once a marker is filtered or is in plasma [171] ... 

Equation (6.1.4) asserts that the volumetric flow rate is a superposition of two components. They are the electro-osmotic component proportional to the electric field intensity (voltage) with the proportionality factor u> and the filtrational Darcy s component proportional to —P with the hydraulic permeability factor i>. Teorell assumed both w and t> constant. Finally another equation, crucial for Teorell s model, was postulated for the dynamics of instantaneous electric resistance of the filter R(t). Teorell assumed a relaxation law of the type... 

Dynamic filtration modules are basically of two types rotating disc filter (RDF) and vortex flow filter (VFF). In the latter, the filtration module has a cylindrical shape and has a rotating concentric cylindrical mesh in its interior. The rotational movement of the internal cylinder generates a Taylor-Couette flow in the annular gap (Roth et al., 1997), creating Taylor vortices that minimize concentration polarization and mesh fouling. Continuous perfusion processes based on this type of filter and operating continuously for up to 100 days have been reported (Mercille et al., 1994). 

It is now well known that trace-element concentrations in continental waters depend on the size of the pore filters used to separate the particulate from the dissolved fraction. This is apparent in Table 1, where results from the Amazon and Orinoco are reported using two filtration sizes the conventional 0.2 p,m filtration and filtration with membranes of smaller cutoff size (ultrafiltration). These results suggest the presence in solution of very small (submicro-metric) particles that pass through filters during filtration. The view that trace elements can be separated into particulate and dissolved fractions can thus no longer be held this has led authors to operationally define a colloidal fraction (0.20 p.m or 0.45 p.m to 1 nm) and a truly dissolved fraction (<1 nm) (e.g., Buffle and Van Leeuwen, 1992 Stumm, 1993). The existence of a colloidal phase has a major influence on the speciation calculation schemes presented above (based only on aqueous complexation), as the apparent solubihty of trace elements will be enhanced by the presence of colloids. The dynamics of colloids also completely change... 

Kroner KH and Nissinen V, Dynamic filtration of microbial suspensions using an axially rotating filter, J. Membr. Sci. 1988 36 85-100. 

Finally, the capacity of activated carbons to adsorb surfactants in dynamic conditions has been evaluated. Dynamic adsorption has been carried out in filter columns (diameter 29 nun) in conditions proposed in France [39] bed depth - 6 cm, graining of carbon 0,6—1,2 mm, filtration rate — 6 m/h. Colunuis have been filled with 3 fractions of carbon grain in equal volumes (the total volume of carbon - about 39.6 cm ). Carbons have been flooded with water and then de—aerated in vacuo during Ih. Adsorption has been carried out by passing solution of sodium laurylosulphate SLS (1 mg/dm ) through the bed. 

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