Filtration, continuous factors

The depth of cut involved in precoat filtration is a veiy important economic factor. There is some disagreement as to the method required to accurately predic t the minimum permissible depth of cut. Some investigators maintain that the depth of cut can be evaluated only in a quah-tative manner during bench-scale tests by judging whether the process solids remain on the surface of the precoat beck This being so, they indicate that it is necessaiy to run a continuous pilot-plant test to determine the minimum permissible depth of cut. The use of a continuous pilot-plant filter is a veiy desirable approach and will provide accurate information under a variety of operating conditions. 

Scale-Up on Rate Filtration rates calculated from bench-scale data shouldbe multiplied by a factor of 0.8 for all types of commercial units which do not employ continuous washing of the filter medium and on which there is a possibility of filter-medium bhnding. For those units which employ continuous filter-medium washing, belt-type drum and horizontal units, the scale-up fac tor maybe increased to 0.9. The use of this scale-up fac tor assumes the following ... 

The problem of selecting the most appropriate operation will be further complicated by such factors as the concentration of liquid solution at which crystals start to form. Thus, in the separation of a mixture of ortho-, meta-, and para-mononitrotoluenes, the decision must be made as to whether it is better to carry out the separation by distillation followed by crystallisation, or in the reverse order. The same kind of consideration will arise when concentrating a solution of a solid then it must be decided whether to stop the evaporation process when a certain concentration of solid has been reached and then to proceed with filtration followed by drying, or whether to continue to concentration by evaporation to such an extent that the filtration stage can be omitted before moving on to drying. 

Continuous air monitoring for trace contaminants in ambient air has developed extensively since the mid-1960s as a result of stimulation from new air pollution measurement requirements. Workers expect that similar needs will develop as certain chemical constituents of particulate material are identified as factors in human health effects. Techniques for the continuous chemical characterization of particulate matter are slow in coming because the amounts of material sampled are small, often below the detection limits of instrumentation. In all cases so far, either a precollection method like filtration or a special detector of high sensitivity has been required. 

Sometimes, a continuous filtration system is used to clean the fryer oil. In this process, it is recommended to take out approximately 5% of the fryer oil, filtered through an in-line filter, and returned to the fryer. This system can be good, but in most cases, the flow of oil through the fryer is not uniformly distributed because of various factors. This allows fines to accumulate on the fryer pan at various spots and not be removed by the filter. In other words, a continuous filter of this type is as good as the system s capability to push all the fines through the filter, with minimum accumulation in the fryer. 

Continuous filters are most attractive when the process application is a steady-state continuous one, but the rate at which cake forms and the magnitude of production rate are sometimes overriding factors. A rotary vacuum filter, for example, is a dubious choice if a 3-mm (0.12-in) cake will not form under normal vacuum in less than 5 min and if less than 1.4 m /h (50 ft /h) of wet cake is produced. Upper production-rate limits to the practicality of batch units are harder to establish, but any operation above 5.7 m /h (200 ft /h) of wet cake should be considered for continuous filtration if it is at all feasible. Again, however, other factors such as the desire for flexibility or the need for high pressure may dictate batch equipment. 

For example, the determining factors for the filtration are the initial rate of liquid movement (6 to 8 cm s l) and the rate of the liquid rises to several centimeters during the first seconds of the ultrasonic action. On the other hand, due to adhesion of dispersed particles of solid nonmetallic inclusions onto the walls of a capillary, the effective cross-section of the latter continuously changes in the process of filtration. Finally, with actual filtration we deal not with the regularities of the flow through a capillary channel but with the statistics of melt movement through a porous medium with a host of channels. 

Concentration factor The ratio of initial feed volume or weight to the volume or weight remaining at the end of filtration. The calculations differ for batch versus other modes such as batch feed-and-bleed or continuous cascade configuration. 

Plasmapheresis. The separation of plasma from whole blood by continuous membrane filtration represents an improvement over conventional centrifugation techniques in terms of efficiency, safety and cost. In the past, plasmapheresis was carried out with blood donors by collecting their whole blood in plastic bags which were then centrifuged to separate the red cells from the plasma. The supernatant plasma was then decanted and the red cells returned to the donorenabling plasma to be drawn from the same person as frequently as three times per week. Most of this plasma is then processed to yield purified components such as albumin or anti-hemophilic factor (Factor VIII). 

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