Pressure line filtration

Depth filters are usually preferred for the most common type of microfiltration system, illustrated schematically in Figure 28. In this process design, called "dead-end" or "in-line" filtration, the entire fluid flow is forced through the membrane under pressure. As particulates accumulate on the membrane surface or in its interior, the pressure required to maintain the required flow increases until, at some point, the membrane must be replaced. The useful life of the membrane is proportional to the particulate loading of the feed solution. In-line microfiltration of solutions as a final polishing step prior to use is a typical apphcation (66,67). 

Other features An increase in venous pressure owing to venous congestion will increase venous hydrostatic pressure. If the pressure on the arterial side of the capillaries is unchanged, this moves the venous end of the hydrostatic pressure line upwards and the gradient of the line decreases. This increases area A and decreases area B, again leading to net filtration. 

A. Vacuum Line Filtration. For their work on the diammoniate of dibo-rane. Parry, Schultz, and Girardot24 devised a versatile vacuum line filtration apparatus which is useful when small quantities of solid are handled and when the solvent is sufficiently volatile to be distilled on the vacuum line. The filter is attached to the vacuum system through a standard taper joint which allows it to be rocked or inverted (Fig. 9.21). Prior to filtration, any volatile contents are frozen down and the apparatus is thoroughly evacuated (Fig. 9.21a). By inversion of the apparatus, the solution is then poured onto the frit, and the solvent vapor pressure is employed to effect a suction filtration by closing the stopcock in the equalizing arm and cooling the lower tube (Fig. 9.21b). The precipitate is washed by distillation of the solvent from the lower receiver into the upper portion of the apparatus (with the stopcock in the sidearm open) and repetition... 

A simplified process used in smaller systems is in-line flocculation followed by pressure filtration. The simplified process produces water of lower quality than the lime clarification process but the equipment is smaller and simpler to operate (71). Experience with in-line filtration showed that optimal dosage of alum was rarely achieved due to fluctuating influent turbidity (72,73). 

For surface sampling with pumping systems at fixed positions, a buoy placed at some distance from the main ship may be used. From the buoy, water is drawn via polytetrafluoro-ethylene (FIFE) tubes to a pump on the vessel. Sampling is often combined with in-line filtration. A successfully tested system which delivers about 2 L/min of sample to a working height of 2 m above the sea surface with an air supply pressure of 4 bar has been described by Harper (1987) see also Tokar et al. (1981). 

For waters with much less than 0.5 mg/L SPM, the amount of material obtainable from the typical 0.5-2 L sample volume is insufficient for subsequent precise and accurate TE analyses. Off-line processing of the much larger volumes of water then necessary becomes very laborious and should be replaced by on-line filtration from appropriate samplers (Fig. 12-30). To this end, the filter holder is attached to the drain spigot, and the necessary additional pressure is applied via the sampler s air-vent. The suspended particulate matter is collected by filtering all the water held by the sampler. To remove the seasalt from the loaded filters, the on-line filter holder is disconnected from the sampler and attached to a plastic syringe. Manually, two 20 mL portions of high-purity distilled water are drained through the filters. The further treatment of the loaded, salt-free filters is identical to the procedure applied to filters obtained by off-line filtration. 

Low pressure hydraulic filters are typically designed for operating pressures up to 28 bar, with flows up to 1200 1/min. Generally they have a wide application range, particularly for industrial and mobile applications, including high flow tank top and tank top return line filters. Many versions are of the spin-on type and can be provided as dual flow assembly off-line filtration. Core-less low pressure filler elements contain no metal and are designed to minimize waste disposal. 

If the volume of filtrate is measured as a function of time, under constant pressure, then a plot of t/V against V should give a straight line, the slope of which can be used to calculate the specific resistance. 

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