Prefilters

The use of positively-charged open-pore membranes as prefilters for a final absolute membrane has already been mentioned. In general, a prefilter serves to take the load off the final filter. The best prefilters will have a high internal surface area (high dirt-loading capacity). 

It is well known that fibrous depth filters provide enormous dirt loading capacity compared with membrane filters. Using this kind of filter media as a prefilter for the final membrane filter often provides an ideal combination (see Fig- 

The smaller the fiber diameter used in the prefilter, the greater the surface area for adsorption of particles and the better the retention of small particles. In the sixties, asbestos fibers were recognized as the best prefilter media. The individual fibrils were smaller than 0.01 ju and they had a positive zeta potential. However, when it was suspected that asbestos fibers presented a health hazard, fine diameter glass and synthetic polymer fibers were substituted. Unfortunately, neither media equals the performance of asbestos. Glass fibers are available in the finest diameters, but some users are fearful they may represent a similar health hazard. The trend has been to use polypropylene or polyester fiber prefilters. Melt blown or spun-bonded fibers are available in diameters near 1 ju. Multilayers of these media with appropriate calendering have resulted in surprisingly efficient prefilters. 

The selection of the optimum prefilter/membrane combination is best done experimentally on the user s process stream. The manufacturers give some guidance as to which prefilters should be used for a given membrane pore-size, but unfortunately, there is no standard rating system for prefilters. 

The experimental procedure used by the author places various prefilters on top of the selected membrane in 47 mm holders which are run in parallel on the process stream to be filtered. Usually, the test is carried out at constant flow rate and the rise in pressure drop is recorded versus the volume processed until the membrane plugs. The combinations providing the highest throughput are then evaluated with respect to cost of the media. 

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Inertial collectors. In inertial collectors, an object is placed in the path of the gas. An example is shown in Fig. 11.1. While the gas passes around the shutters, particles with sufficiently high inertia impinge on them and are removed from the stream. Only particles in excess of 50/um can reasonably be removed. Like gravity settlers, inertial collectors are widely used as prefilters. 

Cyclones. Cyclones are also primarily used as prefilters. These also were discussed in Chap. 3 and illustrated in Fig. 3.4. The particle-laden gas enters tangentially and spins downward and inward, ultimately leaving the top of the unit. Particles are thrown radially outward to the wall by the centrifugal force and leave at the bottom. 

Figure Bl.23.5. Schematic illustration of tlie TOE-SARS spectrometer system. A = ion gun, B = Wien filter, C = Einzel lens, D = pulsing plates, E = pulsing aperture, E = deflector plates, G = sample, PI = electron multiplier detector with energy prefilter grid and I = electrostatic deflector.

Other filters used for prefiltering account for lead- [22, 23] or drug-likeness [24-26], an appropriate ADMET profile [27-30], or favorable properties concerning receptor binding [31, 32]. 

The lesult of a gioup frequency identification can be used as a prefilter for library searches (57—59). 

In the most common production method, the semibatch process, about 10% of the preemulsified monomer is added to the deionised water in the reactor. A shot of initiator is added to the reactor to create the seed. Some manufacturers use master batches of seed to avoid variation in this step. Having set the number of particles in the pot, the remaining monomer and, in some cases, additional initiator are added over time. Typical feed times ate 1—4 h. Lengthening the feeds tempers heat generation and provides for uniform comonomer sequence distributions (67). Sometimes skewed monomer feeds are used to offset differences in monomer reactivity ratios. In some cases a second monomer charge is made to produce core—shell latices. At the end of the process pH adjustments are often made. The product is then pumped to a prefilter tank, filtered, and pumped to a post-filter tank where additional processing can occur. When the feed rate of monomer during semibatch production is very low, the reactor is said to be monomer starved. Under these... 

Analytical Methods. The official NIOSH recommended method for determining sulfur dioxide in air consists of drawing a known prefiltered volume of air through a bubbler containing hydrogen peroxide, thus oxidising the sulfur dioxide to sulfuric acid. Isopropyl alcohol is then added to the contents in the bubbler and the pH of the sample is adjusted with dilute perchloric acid. The resultant solution is then titrated for sulfate with 0.005 M. barium perchlorate, and Thorin is used as the indicator. 

The advantages of the tubular filter are that it uses an easily replaced filter medium, its filtration cycle can be interrupted and the shell can be emptied of prefilt at any time without loss of the cake, the cake is readily recoverable in dry form, and the inside of the filter is conveniently accessible. There is also no unfiltered heel. Disadvantages are the necessity and attendant labor requirements of emptying by hand and replacing the filter media and the tendency for neavy solids to settle out in the header chamber. Applications are as a scavenger filter to remove fines not removed in a prior-filtration stage with a different land of equipment, to handle the runoff from other filters, and in semiworks and small-plant operations in which the filter s size, versatility, and cleanliness recommend it. 

External-Cake Tubular Filters Several filter designs are available with vertical tubes supported by a filtrate-chamber tube sheet in a vertical cylindrical vessel (Fig. 18-115). The tubes may be made of wire cloth porous ceramic, carbon, plastic, or metal or closely wound wire. The tubes may have a filter cloth on the outside. Frequently a filter-aid precoat will be applied to the tubes. The prefilt slurry is fed near the bottom of the vertical vessel. The filtrate passes from the outside to the inside of the tubes and into a filtrate chamber at the top or the bottom of the vessel. The sohds form a cake on the outside ofthe tubes with the filter area actually increasing as the cake builds up, partially compensating for the increased flow resistance of the thicker cake. The filtration cycle continues until the differential pressure reaches a specified level, or until about 25 mm (1 in) of cake thickness is obtainea... 

Prefiltering of gas stream possibly required to remove any particulate capable of plugging the adsorbent bed... 

Pro A prefilter is placed before the main filter to extend the life of the main filter. 

Bag An extended surface filter in the form of a pocket or bag. A typical example of a bag filter is a mineral fiber bag made up of three layers, in which the first layer acts as a prefilter, the second is for fine filtration, and the third prevents fiber migration from the material used. 

Prefilter A rough filter positioned before a fine filter to reduce clogging of the fine filter. 

A slow deterioration that is not remedied by cleaning the ion source and not related to the multiplier gain is an indication that the analyzer needs to be cleaned. For quadrupole instruments, cleaning the prefilter is often sufficient to restore performance. (A prefilter is not available on all quadrupole instruments.)... 

Carbon filters find particular application as prefilters for RO and ion-exchange processes in the production of high purity FW. They are also used in clean-steam boilers and other types of steam generators where the steam is ultimately destined for application in food or beverage production, pharmaceuticals, electronics, surgical instrument sterilization, and similar processes. 

A further insight is that the best workflow depends on a combination of factors that can in many cases be expressed in closed mathematical form, allowing very rapid graphical feedback to users of what then becomes a visualization rather than a stochastic simulation tool. This particular approach is effective for simple binary comparisons of methods (e.g., use of in vitro alone vs. in silico as prefilter to in vitro). It can also be extended to evaluation of conditional sequencing for groups of compounds, using an extension of the sentinel approach [24]. 

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