Filter, axial

The axial filter (Oak Ridge National Laboratory) (30) is remarkably similar to the dynamic filter in that both the rotating filter element and the outer shell are also cylindrical. An ultrafiltration module based on the same principle has also been described (31). Unlike the disk-type European dynamic filters described above, the cylindrical element models are not so suitable for scale-up because they utilize the space inside the pressure vessel poorly. 

Axial filtration. In most of our bench-scale filtrations cross flow was effected by use of axial filters (10). In this configuration (Figure 4), a membrane is wrapped around a rotor, which is spun in a chamber, into which feed is introduced under pressure. The rotor is perforated, and passages are provided for filtrate (e.g., by an intervening screen) from the membrane to these holes. Filtrate exits through the axis. Rotation speeds providing velocities of up to about 15 ft/sec at the membrane-feed interface can be attained in available equipment. 

Pleated ultrafiltration module. The axial filter is convenient for experiments, in that volumes small relative to ordinary ultrafiltration systems can be studied and in that pumping of viscous solutions is limited to that necessary to replace filtrate or concentrate bled from the chamber, rather than that necessary to maintain desired cross flow velocities. There is no obvious reason it could not be scaled up to moderate sizes for practical separations, but so far as we know, no large-volume axial filters are available. For the operations of interest, any of the commercial ultrafiltration systems would be candidates. We have tested one module, recently developed by Gelman, which incorporates a pleated membrane (Figure 5), with somewhat more open feed passages than those of spiral-wound membranes, and which allows backwashing. Other applications of the module were discussed at this symposium by A. Korin in a paper coauthored by G. B. Tanny, and a written account is presumably in these proceedings. 

Part was subjected to screening with a metal screen of about 125 ym apertures, mounted on an axial filter. Most of tha biomass was removed in this step, with no significant loss of viscosity. Fluxes in the coarse screening were several hundred gallons per square foot per day. 

The screened broth was then compared with unscreened broth in polishing by the axial filter wrapped with 5 ym Nuclepore membranes. Figure 8 compares the fluxes. Neither are as high as one might hope, though the screened feed values appear... 

Figure 9. Unscreened feed for axial filtration plugging of 1.2-iim Nuclepore filters by filtrate from 5-jim Nuclepore filter mounted on axial filter (plugging test pressure...

Figure 17. Plugging test results a, pilot microscreen effluents b, filtrates from polishing by crossflow filtration through Ll-pm Acropor filter in axial filter (1.5 psi, 1000 rpm) and in pleated cartridge (loop).

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... 

Blowing hot air A typical arrangement is shown in Figure 31.9. The bus duct enclosure is provided with inlet and outlet valves at suitable locations. Hot air is supplied through the inlet valve until the interior of the enclosure is completely dry. The blowing equipment comprises an axial flow fan. a heater unit, an inlet air filter unit, pressure... 

The prime design objeetive of the filter system is to proteet the gas turbine. The performanee of the gas turbine inlet-air filter system has important and far-reaehing influenees on overall maintenanee eosts, reliability, and availability of gas turbines. There are three major results of improper air filtration (1) erosion, (2) fouling of the axial-flow eompressor, and (3) eorrosion of the gas turbine hot-gas path inlets. The importanee of the inlet-air filter, as it relates to eaeh of these three phenomena, ean be appreeiated if... 

A mixture consisting of 2 grams of 2-hydroxy-3-(N,N-diethylcarboxamido)-9,10-dimethoxy-1,2,3,4,6,7-hexahydro-1 Ib-H-benzopyridocoline (OH-axial) hydrochloride (prepared by treating the base with hydrogen chloride gas in absolute ether) dissolved in 7 ml of acetic anhydride containing 3 ml of pyridine was heated at 100°C for 2 hours under a nitrogen atmosphere. At the end of this period, a crystalline precipitate had formed and the resultant mixture was subsequently diluted with an equal volume of diethyl ether and filtered. 

The crystalline hydrochloride salt so obtained, i.e., the solid material collected on the filter funnel, was then converted to the corresponding free base by distribution in 10 ml of a benzene-aqueous 5% sodium carbonate system. The product recovered from the benzene extracts was then recrystallized from diisopropyl ether to afford 1.46 grams of 2-acetoxy-3-(N, N-diethy Icarboxam ido)-9,10-dim ethoxy-1,2,3,4,6,7-hexahydro-1 Ib-H-benzopyridocoline (CHjCOO-axial), MP 130°-131.5°C. 

To a solution of 4-t-butylcyclohexanone (lmmol), tris(triphenylphos-phine)ruthenium(n) chloride (0.05 mmol) and silver trifluoroacetate (0.05 mmol) in toluene (5 ml) was added triethylsilane (1.5 mmol). The mixture was heated under reflux for 20 h, and concentrated under reduced pressure. The residue was diluted with hexane (3 ml), filtered and distilled to give a mixture of triethylsilyl ethers (0.96mmol, 96%), b.p. 70°CI 0.1 mmHg. G.l.c. analysis shows an axial (cis) equatorial (trans) ratio of 5 95—a result comparable to the best LAH results. 

The washing of filter cake is carried out to remove liquid impurities from valuable solid product or to increase recovery of valuable filtrates from the cake. Wakeman (1990) has shown that the axial dispersion flow model, as developed in Sec. 4.3.6, provides a fundamental description of cake washing. It takes into account such situations as non-uniformities in the liquid flow pattern, non-uniform porosity distributions, the initial spread of washing liquid onto the topmost surface of the filter cake and the desorption of solute from the solid surfaces. 

Program FILTWASH models the dimensionless filtration wash curves for the above case of a filter cake with constant porosity, axial dispersion in the liquid flow and desorption of solute from the solid particles of the filter bed (Boyd, 1993). 

Figure 6. Schematic outline of the first commercially available multiple collector ICPMS, the Plasma 54, after Halhday et al. (1995). This instrument uses Nier-Johnson double-focusing and is equipped with eight independently adjustable Faraday collectors. The axial collector can be wound down to provide access to a Daly detector equipped with ion counting capabilities and a second-stage energy filter for high abundance sensitivity measurements. The sample may be introduced to the plasma source by either solution aspiration or laser ablation.

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... 

Figure 15.2(a). The membrane impedes further penetration of even smaller particles through the porous filter media. In many filtration applications, this filtration mechanism is valid for an axial velocity greater than about 4 to 6 m/s. 

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. 

It is anticipated that the equilibrium filter cake mass would depend strongly on the axial velocity through the cross-flow filter assembly. The shear rate at the filter surface will increase the entrainment of the catalyst solids for a given permeate flow rate. Therefore, for each differential pressure condition, the axial velocity will be varied in order to quantify the effect of the wall shear on the filter cake resistance term. 

Ideally, the axial velocity through the cross-flow unit should be greater than about 4-6 m/s to minimize the boundary layer of particles near the membrane surface. The wax permeate flow from the filter is limited by a control valve actuated by a reactor-level controller. Hence, a constant inventory of slurry is maintained within the SBCR system as long as the superficial gas velocity remains constant. Changes in the gas holdup due to a variable gas velocity are calculated... 

Cross-flow filtration systems utilize high liquid axial velocities to generate shear at the liquid-membrane interface. Shear is necessary to maintain acceptable permeate fluxes, especially with concentrated catalyst slurries. The degree of catalyst deposition on the filter membrane or membrane fouling is a function of the shear stress at the surface and particle convection with the permeate flow.16 Membrane surface fouling also depends on many application-specific variables, such as particle size in the retentate, viscosity of the permeate, axial velocity, and the transmembrane pressure. All of these variables can influence the degree of deposition of particles within the filter membrane, and thus decrease the effective pore size of the membrane. 

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