Sizing a Disc Filter

Example 5 Sizing a Disc Filter Equipment physical factors, selected from Table 18-9 Maximum effective submergence = 28% maximum portion of filter cycle available for dewatering = 45%. (High submergence versions require trunnion seals, and their use is limited to specific applications.)... 

Stock usually the best dewatering stock component used at the paper machine is selected in order to Hmit the size of the disc filter. In addition to cloudy and clear filtrate, disc filter save-aU applications also produce a superclear filtrate with very low solids content This superclear filtrate is used as a fresh water substitute for spray apphcations in the paper machine. The clear filtrate is stored in a buffer tank. The cloudy filtrate is usually fed directly back to the inlet of the disc filter, the used sweetener, including the recovered stock, is fed back to the thick stock in the approach flow system. DAF (dissolved air flotation) type save-alls are used in older machines and nowadays when a certain degree of ash and fines removal from the process is demanded. In this case, the sludge of the DAF is rejected. The advantages of a disc filter save-all compared to a DAF save-all are higher filtrate quahty, no chemical consumption and less space requirements (see Sections 4.2.62 and 4.2.9). 

Disc filters are similar in principle to rotary filters, but consist of several thin discs mounted on a shaft, in place of the drum. This gives a larger effective filtering area on a given floor area, and vacuum disc filters are used in preference to drum filters where space is restricted. At sizes above approximately 25 m2 filtration area, disc filters are cheaper but their applications are more restricted, as they are not as suitable for the application of wash water, or precoating. 

The bubble point test is a popular single-point physical integrity test for disc filter membranes based on Eq. (21). A fdter medium is wetted with a liquid, and test gas pressure is slowly raised until a steady stream of bubbles appears from a tube or hose attached to the downstream side of the filter and immersed in water (Fig. 9). The pressure at which the bubbles first appear is recorded as the bubble point and is related to the largest pores in the fdter medium. A pore size can be calculated from Eq. (21) however, it must be realized that the bubble point test does not measure the actual pore size, but only allows correla-... 

The examples which follow show how data from the correlations just presented and a knowledge of the physical characteristics of a par-ticufar filter are used to determine a filtration cycle and, subsequently, the size of the filter itself. The three examples which follow involve a disc, a drum belt, and a horizontal belt filter. 

Of all continuous filters, the vacuum disc is the lowest in cost per unit area of filter when mild steel, cast iron, or similar materials of construction may be used. It provides a large filtering area with minimum floor space, and it is used mostly in high-tonnage dewatering applications in sizes up to about 300 m (3300 ft ) of filter area. 

Determine the mass of saturated clod by subtracting the mass of watch glass and a wetted filter paper disc of the size placed below the clod. 

Adeeyinwo and Tyson [10] used a stainless-steel disc filter with 2 m pore size, 6 mm diameter, and 2 mm thickness to separate calcium from an interfering aluminium matrix by oxalate precipitation. The results were inferior to those obtained using membrane filters, giving poor reproducibility. These results are consistent with the experiences of Valcarcel et al.[7] using disc type stainless-steel filters mentioned above. ... 

The capillary disc filter looks like any other standard rotary vacuum disc filter, as can be seen in Figure 3.25, but the filter medium is a finely porous ceramic disc, which draws filtrate through the disc material by capillary action, under the applied vacuum. The filter discs are made of sintered alumina with uniform micropores less than 1 pm in size, which allows only liquid to flow through it. Despite an almost absolute vacuum, no air penetrates the filter material. The disc material is inert,... 

The filter elements should remove particles of five microns, must be water-resistant, have a high flow rate capability with low pressure drop, possess high dirt-retention capacity, and be rupture-resistant. The clean pressure drop should not exceed five psig at 100 °F (38 °C). The elements must have a minimum collapse differential pressure of 50 psig. Pleated-paper elements are preferred—provided they meet these requirements. Usually, the pleated-paper element will yield the five psig clean drop when used in a filter that was sized to use depth-type elements. This result is due to the greater surface area of the pleated element, more than twice the area of a conventional stacked disc-type or other depth-type elements. 

Ch - chopper disc, Fhl - filter holder, D - iris diaphragm and interchangeable pinholes of various sizes, Sch - sample chamber, S - sample holder, ID - iris diaphragm, L2 - receiver lens, Ph2 - detector photomultiplier, A - measuring arm, Fh 2 - filter holder, Phi - detector photomultiplier... 

SPE disc technology through rigid disc and membrane disc formats provides a way around the above-listed limitations of packed-bed SPE columns. SPE discs closely resemble membrane filters they are flat, usually 1 mm or less in thickness, with diameters varying in size. Some disks are sold loose, and users must install them in a filter holder. Others are sold preloaded in disposable holders with Luer fittings (82). 

In water studies it is standard practice to filter the sample soon after collection, usually through a 0.45p,m membrane disc (made of cellulose acetate, cellulose nitrate or polycarbonate). This process arbitrarily divides the sample components into soluble and insoluble fractions, but as shown in Table 2.3, the average size of different chemical species varies widely, and some differentiation between species can be obtained through using filter media of different pore sizes. For example, fully dissolved compounds can be separated from finer colloidal forms by using gel filtration and dialysis, and sub-division of the total content into fractions based on particle or molecular size (see Section 2.3) has been used for speciation of elements in waters. 

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