Surface prefilters

Surface prefilters retain the particles on then-surface. They are made from cellulose esters or layers of polypropylene. They also have good specific retention. Prefilters are not always defined by their retention value. The value given, e.g. 3 tim, frequently corresponds to a nominal retention value. In this case, a variable proportion of particles larger than 3 p,m in diameter may pass through the 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. 

The air jet spray bar consisted of 6.4-mm copper tubing that contained 17 openings of 1.6-mm diameter spaced at 9.5 mm along its axis. The spray bar was positioned 2.2 cm above the lint batt and was moved back and forth by a small roller chain powered by an electric reversing drive. This arrangement produced a spray bar movement across the entire batt surface once every 8 s. Air from an 827-kPa conventional compressed-air source was preconditioned before delivery to the spray bar by a combination prefilter/water trap and a secondary air filter rated at 0.01 micrometer. An adjustable air regulator in the compressed-air line maintained a constant pressure on the spray bar. 

Mod i f 1 ed Mjymbrajne Viscometer Foi- the pulsed system a coil of tubing (the injection loop) was placed after the prefilter and liefore the membrane holder as shown in Figure P. Directional valves at each end of the loop controlled the flow path. Solvent or solution could be pumped directly to the UV to establish baseline absorbance or for calibration. To make P measurements the flow was directed through the membrane and then into the differential UV spectrophotometer. The flow could also be brought to the upstream portion of tlie membrane holder and then to the UV detector in an effort to measure the concentration at the membrane surface. 

In some designs, a glass fiber filter is supported above the extraction disc to prefilter samples containing particulates without clogging the extraction disc. Because of the rigid disc design, frits to support the extraction disc are not necessary. The polyethylene frits used in conventional SPEi columns may be a source of contamination (90) and have a large surface area and void volume. 

Loss of HN03 to Filter Holder Surfaces. The walls of filter holders as well as the prefilter support screens are likely sites for HN03 retention. Polycarbonate filter holders (Nuclepore Corporation) are frequently used... 

Formulation and manufacturing, at this stage of development, need to focus on the physical and chemical problems associated with biopharmaceuticals. Physical adsorption of the product on surfaces such as glass, metal, plastic, and any prefilters could mean significant loss of the product, inconsistent concentrations per unit container, poor yield, and, ultimately, rejection of a lot. 

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). 

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. 

Often preventive measures may be taken to avoid fouling the membrane. Prefilters or screens can be used to remove large particles which block thin channels or accumulate in stagnant areas of the module. High cross-flow velocities tend to sweep deposits away. Low pressures avoid compaction of gels on the membrane. Some polymers have a higher susceptibility to fouling and chemical modification of the membrane surface can have a profound effect on the propensity to foul. 

Methanol is usually utilized to prewet the Cjg Bond-Elut columns and opens the hydrophobic chains to increase the effective surface area. Water samples are also fortified with at least 1% methanol to continuously wet the stationary phase. This can improve recovery rates for a large number of herbicides, including triazines. By contrast, degradation products, which are often more polar than parent compounds, may not be retained as effectively in the presence of a modifier. Ground and surface water must always be filtered prior to the extraction of pesticides with the SPE technique. Prefiltering will not affect the determination of herbicides and their degradation products, since these compounds exhibit a log ATqc near 2 and consequently they are largely (99.5%) distributed in water in the dissolved phase. ... 

The majority of the volume of beer in the world is processed by filter aid filtration. This technology relies on adding powders to the beer that form a very porous bed when they impinge on the filter surface. This is achieved by slurrying the filter aid in water, deaeration and pumping the slurry into the beer. Thus the prefilter beer on reaching the filter surface encounters a fresh layer of filter aid as the bed develops. This prolongs the filtration mn. 

---