Filter Sizing

Filter sizing based on flat disc tests usually consists of recording the filtration time, volume of suspension filtered and pressure drop across the filter. An estimate of the filter performance can be achieved by calculating the volmue of filtrate produced per unit time, for constant-pressure filtration, or the volume of filtrate per unit pressure, for constant-rate filtration. Inspection of these rates will show if the membrane is acceptable, and the investigator can decide on the volume filtered before the filtration becomes either too slow or excessive in pressure drop. This provides a volume filtrate per unit area value which can be used to proportion the areas to arrive at the required filter area. 

Finally, the process filtration will probably be performed over a difiierait time fi om that of the laboratory test, so it is usu to calculate the flux obtained in the laboratory on the basis of volume per unit time and filter area and to check that this is considerably in excess of that required by the process after having applied safety ctors to the process filtration area calculation (typically 1.5 or more). 

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Objective Determine the filter size and vacuum system capacity required to dewater 15 mtph (metric tons per hour) of dry sohds and produce a cake containing an average moisture content of 25 wt %. 

Rotation speeds to 40 r/min are possible with cakes typically 3 to 6 mm (0.12 to 0.24 in) thick. Filter sizes range from 930 cm to 19 m (1 to 207 ft") with 93 percent of the area active. The slurry is fed into a conical feed tank designed to prevent solids from settling without the use of mechanical agitators. The proper hquid level is maintained by overflow, and submergence ranges from 5 to 70 percent of the drum circumference. 

Normally the filter strueture consists of a stack of plates attached to a hollow shaft which are mounted inside a pressure vessel with eaeh plate eovered with a suitable filter medium. The slurry is fed under pressure into the vessel and the eake, which is retained by the filter medium, forms on the top of eaeh plate whilst the filtrate passes through the hollow shaft further to the proeess. Filter sizes may vary but generally the maximum is 60 m area and designed for a 6 bar operating pressure. Each circular plate in the staek is eonstructed with radial ribs that are welded to the bottom and support a horizontal eoarse mesh screen whieh is eovered with a fmer woven metal screen or filter cloth to retain the cake. The bottom of the plate slopes towards the hollow eentral shaft whieh lets the filtrate flow freely through circumferential holes and further down the shaft to the filtrate outlet. The elearanee between the plates is maintained by speeial spacers... 

Greensand filter sizing and operation are based on the following data ... 

Some designers push the RW flow rate through MM filters to as high as 15 gal/sq fit/min. This generally is too high and may lead to poor filtration. Also, the resultant cost savings due to a reduced filter size is seldom warranted because the filter is a relatively minor cost as a proportion of the entire RO scheme. A better maximum is perhaps 9 to 10 gal/sq fit/min, with 5 to 6 gal/sq ft/min being preferred. 

The trends presented in Figs. 31 and 32 qualitatively similar to those presented earlier by Agrawal et al. (2001) and Andrews et al. (2005) who, for the sake of simplicity, did simulations on much smaller domains and let the filter size be the same as the domain size. This shows clearly that the effects leading to the type of results presented in Figs. 31 and 32 are robust. 

Andrews and Sundaresan (2005) have also extracted the filtered particle-phase viscosity from these simulations and found that at low particle volume fractions (0.0-0.25), the filtered viscosity varies nearly linearly with particle volume, and that it increases monotonically (and nearly linearly) with filter size. 

Fig. 11. Evaluation of particle post-CMP performance for commercial oxide slurries with fumed and colloidal silica with and without point-of-use filtration. The filter size is 0.3 /im. The bottom denotes that the slurry used is from the bottom of the drum.

Filter Size Organisms tu2 (h) Depth (m) Rate Constant (h t-i/2 (h)... 

Although it is beyond the scope of this book to provide equipment design information, the example below is given for a first look filter-sizing exercise. 

Contact time between the hydrocarbon and the feedstock also plays an important role in asphaltene separation (Figure 3-12). Yields of the asphaltenes reach a maximum after approximately 8 hr, which may be ascribed to the time required for the asphaltene particles to agglomerate into particles of a filterable size as well as the diffusion-controlled nature of the process. Heavier feedstocks also need time for the hydrocarbon to penetrate their mass. 

Filter Preparation. The following protocol for immobilizing single-stranded DNA on nitrocellulose membrane filters is essentially that of Gillespie and Spiegelman.2 Different filter sizes may be used, but it is more efficient to immobilize DNA on large filters which can be subdivided rather than to use many smaller filters. Generally, the amount of DNA immobilized is 25 /ig/cm2. 

Dilute unsheared DNA stock in 0.1X SSC to 50 /ig/ml, starting with an amount sufficient to give 25 /ig/cm2 for the filter size to be used. 

Cut individual filters from the master membrane filter using a paper punch or other device. The filter sizes commonly used carry from 5 to 25 fig of DNA. 

Several odier MA methods have been proposed in die literature, two of die best known being die Hanning window (named after Julius Von Hann) (which for 3 points has weights 0.25, 0.5 and 0.25), and the Hamming window (named after R. W. Hamming) (which for 5 points has weights 0.0357, 0.2411, 0.4464, 0.2411, 0.0357) - not to be confused in name but very similar in effects. These windows can be calculated for any size, but we recommend diese two filter sizes. 

An example of a solid-liquid phase separation - often referred to as a mechanical separation - is filtration. Filters are also used in gas-sohd separation. Filtration may be used to recover liquid or sohd or both. Also, it can be used in waste-treatment processes. Walas [6] describes many solid-hquid separators, but we will only consider the rotary-drum filter. Reliable sizing of rotary-drum filters requires bench and pilot-scale testing with the slurry. Nevertheless, a model of the filtering process will show some of the physical factors that influence filtration and will give a preliminary estimate of the filter size in those cases where data are available. 

Filter Size Diameter ft Nominal Length ft Drum Area fl Drum Drive" hp Agitator Drive" hp... 

From Table 6.20, a standard filter has 250 fP (23.2 m ) of surface area. This choice will result in a safety factor of 29.5%. The final decision on the filter size, will require laboratory or pilot plant tests. In most cases, the filter manufacturer will provide this service. 

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