Thick media pressure filter

The second stage of separation offers several alternatives thick media pressure filter, pleated paper roll filter or cartridge, strainer, or hydrocyclone. Only the last, the hydrocyclone, does not reqnire preventative maintenance or utilize consumables, and so seems the best choice, especially from the point of view of flexibility. The hydrocyclone is also extremely durable, due to the abrasive resistant nature of the normal material of construction, namely polyurethane, together with its high impact resistance, frequently necessary in a workshop environment. 

An emulsion of 1 (1 g, 2.77 mmol) in distilled water (150 mL) was irradiated using a 450-W medium-pressure Hanovia lamp and a Pyrex filter for 36 h with stirring. The photolyzed mixture was extracted with dichloromethane. Removal of tlie solvent in vacuo gave a thick residue. It was then stirred with methanol for about 6 h. Solvent was removed in vacuo to give a residue that was chromatographed over a silica gel column. Elution with a mixture of 15% ethyl acetate in PE gave 3 as a white solid, mp 41 °C (230 mg, 21%), which was crystallized with difficulty from a mixture of n-pentane and dichloromethane (4 1). 

Solutions of the 6-alkyl-4,4-diphenylcyclohex-2-enone (3, 0.5 mmol) in benzene (200 mL) were irradiated under Nj with a 450-W medium-pressure Hg lamp through a sodium metavanadate filter solution for 5-20 h. Photomixtures were then separated on thick layer preparative silica gel plates (MN-Kieselgel G/UV-254) yield 83-99%. 

Method C Alternatively the reaction mixture employed in method B can be irradiated (16-18 h) in a pyrex vessel, which surrounds a centrally arranged water-cooled medium-pressure mercury lamp (250 W Hanovia). A jacket containing a filter solution (thickness 1 cm 750 g NaBr and 8 g Pb(N03)2 in 1 L of water transmission > 340 nm) is placed between the lamp and the solution to be irradiated. 

At the start of filtration in a cake filter some solid particles enter the pores of the medium and are immobilized, but soon others begin to collect on the septum surface. After this brief initial period the cake of solids does the filtration, not the septum a visible cake of appreciable thickness builds up on the surface and must be periodically removed. Except as noted under bag filters for gas cleaning, cake filters are used almost entirely for liquid-solid separations. As with other filters they may operate with above-atmospheric pressure upstream from the filter medium or with vacuum applied downstream. Either type can be continuous or discontinuous, but because of the difficulty of discharging the solids against a positive pressure, most pressure filters are discontinuous. 

Preliminary filtration tests may be made with a Buchner funnel or a small filter leaf, covered with canvas or other appropriate medium and connected to a vacuum system. Usually the suspension is poured carefully into the vacuum-connected funnel, whereas the leaf is immersed in a sample of the slurry and vacuum is applied to pull filtrate into a collecting flask. The time required to form each of several cakes in the range of 3 to 25 mm (V to 1 in) thick under a given vacuum is noted, as is the volume of the collected filtrate. Properly conducted tests with a Buchner or a vacuum leaf closely simulate the action of rotary vacuum filters of the top- and bottom-feed variety, respectively, and may give the experienced observer enough information for complete specification of a plant-size filter. Alternatively, they may point to pressure-filter tests or, indeed, to a search for an alternative to filtration. Centrifugal... 

Tlie discussions of the basic features of filtration given thus far illustrate that the unit operation involves some rather complicated hydrodynamics that depend strongly on the physical properties of both fluid and particles, as well as interaction with a complex porous medium. The process is essentially influenced by two different groups of factors, which can be broadly lumped into macro- and micro-properties. Macrofactors are related to variables such as the area of a filter medium, pressure differences, cake thickness and the viscosity of the liquid phase. Such parameters are readily measured. Micro-factors include the influences of the size and configuration of pores in the cake and filter medium, the thickness of the electrical double layer on the surface of solid particles, and other properties. 

This is a method limiting cake thickness by intermittent back-flushing of the filter medium, as reported by Muira and Brociner , for example. Conventional vacuum or pressure filters can be used with some modifications for the effects of the forces of the back-flush. Thus filtering through thin cakes in short cycles can be used. 

When the space above the suspension is subjected to compressed gas or the space under the filter plate is under a vacuum, filtration proceeds under a constant pressure differential (the pressure in the receivers is constant). The rate of filtration decreases due to an increase in the cake thickness and, consequently, flow resistance. A similar filtration process results from a pressure difference due to the hydrostatic pressure of a suspension layer of constant thickness located over the filter medium. 

A slurry containing 100 kg of whiting/m3 of water, is filtered in a plate and frame press, which takes 900 s to dismantle, clean and re-assemble. If the filter cake is incompressible and has a voidage of 0.4, what is the optimum thickness of cake for a filtration pressure of 1000 kN/m2 The density of the whiting is 3000 kg/m3. If the cake is washed at 500 kN/m2 and the total volume of wash water employed is 25 per cent of that of the filtrate, how is the optimum thickness of cake affected The resistance of the filter medium may be neglected and the viscosity of water is 1 mN s/m2. In an experiment, a pressure of 165 kN/m2 produced a flow of water of 0.02 cm3/s though a centimetre cube of filter cake. 

The rotary filter breaks down and the operation has to be carried out temporarily in a plate and frame press with frames 0.3 m square. The press takes 120 s to dismantle and 120 s to reassemble and, in addition, 120 s is required to remove the cake from each frame. If filtration is to be carried out at the same overall rate as before, with an operating pressure difference of 175 kN/m2, what is the minimum number of frames that needs to be used and what is the thickness of each It may be assumed that the cakes are incompressible and that the resistance of the filter medium may be neglected. 

In addition to the pore size-particle size retention relationship problems mentioned above, other factors can influence a filter medium s retention characteristics. Absorptive retention can be influenced by the organism size, organism population, pore size of the medium, pH of the filtrate, ionic strength, surface tension, and organic content. Operational parameters can also influence retention, such as flow rate, salt concentration, viscosity, temperature, filtration duration, filtration pressure, membrane thickness, organism type, and filter medium area [52,53]. 

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