Polishing filters

Where iron transport takes place, accurate monitoring over a period and under different load conditions is required before filters, polishers, and other components can be properly sized. 

Like primary filters, polishing filters must be taken off line for cleaning when flow rate deteriorates, differential pressure increases, or the body of the filter fills with solids. The emphasis in fundamental analysis of the filtration of brine is on the production of clear brine at a defined rate. The efficient removal of cake from the filter, however, is of equal importance. The removal of occluded brine from the cake for recovery or for waste minimization may also be necessary. Before cleaning, the brine inside the filter body is usually removed. This involves the use of compressed air to blow the brine back to its... 

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). 

Condensate Polishing. Ion exchange can be used to purify or poHsh returned condensate, removing corrosion products that could cause harmful deposits in boilers. Typically, the contaminants in the condensate system are particulate iron and copper. Low levels of other contaminants may enter the system through condenser and pump seal leaks or carryover of boiler water into the steam. Condensate poHshers filter out the particulates and remove soluble contaminants by ion exchange. 

Cake Filters Filters that accumulate appreciable visible quantities of solids on the surface of a filter medium are called cake filters. The slurry feed may have a solids concentration from about 1 percent to greater than 40 percent. The filter medium on which the cake forms is relatively open to minimize flow resistance, since once the cake forms, it becomes the effective filter medium. The initial filtrate therefore may contain unacceptable sohds concentration until the cake is formed. This situation may be made tolerable by recycling the filtrate until acceptable clarity is obtained or by using a downstream polishing filter (clarifying type). 

Disk-and-plate assembhes somewhat resemble horizontal-plate pressure filters, which, in fact, may be used for polishing. In one design (Sparkler VR filter) both sides of each plate are used as filtering surfaces, having paper or other media clamped against them. 

Ptdp) filtei. s. These filters employ one or more packs of filtermasse (cellulose fibers compressed to a compact cylinder) stacked into a pressure case. The packs are sometimes supported in individual trays which provide drainage channels and sometimes rest on one another with a loose spacer plate between each two packs and with a drainage screen buried in the center of each pack. The liquid being clarified flows under a pressure of 345 kPa (50 psig) or less through the pulp packs and into a drainage manifold. Flow rates are somewhat less than for disk filters, on the order of 20 L/(min-m ) [0.5 gal/ (min-ft")]. Pulp filters are used chiefly to polish beverages. The filtermasse may be washed in special washers and re-formed into new cakes. 

Miscellaneous Clarifiers Various types of filters such as cartridge, magnetic, and bag filters are widely used in polishing operations, generally to remove trace amounts of suspended sohds... 

Vitreous, colourless form of free silica. Formed when quartz is heated to 870°C (1598°F). Aporous siliceous rock resulting from the decomposition of chertorsiliceous limestone. Used as a base in soap and scouring powders, in metal polishing, as a filtering agent, and in wood and paint fillers. A cryptocrystalline form of free silica. 

Polishing of solutions (as performed on a precoat filter operation). 

Vertical Pressure Leaf Filters are essentially the same as Horizontal Plate Filters except for the orientation of the filter elements which are vertical rather than horizontal. They are applied for the polishing slurries with very lov solids content of 1-5% or for cake filtration with a solids concentration of 20-25%. As with the horizontal plate filter the vertical leaf filters are also well suited for handling flammable, toxic and corrosive materials since they are autoclaved and designed for hazardous environments when high pressure and safe operation are required. Likewise, they may be readily jacketed for applications whenever hot or cold temperatures are to be preserved.The largest leaf filters in horizontal vessels have a filtration area of 300 m and vertical vessels 100 m both designed for an operating pressure of 6 bar. 

One of the differences between polishing and cake filtration is the space between the plates. For polishing applications the clearances are about 20 mm as opposed to cake filtration applications where, depending on the percentage of solids and cake build-up properties, clearances may reach 100 mm. Hence, polishing filters accommodate more plates than cake filters so for the same vessel size more effective area is available with polishers. 

Figure 9. Precoat drum filter flow scheme for polishing operations.

Another application shows the preparative purification and polishing of a therapeutic fusion protein with a humanized recombinant IgG protein. The fusion protein was expressed by the fermentation of baby hamster kidney cells. The filtered culture supernatant (155 liters) contained 2.2 g of IgG and 75.5 g of total protein. After the immunoglobulins were isolated by expanded bed adsorption and rebuffering, the IgG fraction was bound to Fractogel EMD SOj (M). This column achieved baseline separation of complete antibodies (fusion protein) from small amounts of antibodies lacking the fusion part. The resulting highly purified IgG fraction (110 ml) was diluted to 150 ml and... 

M Preparation of isopropyiidene peniciiiamine hydrochioride To the filtrate obtained In step (b) is added at 20°C to 25°C a total of 85 g of hydrogen sulfide. The precipitated HgS is filtered off and the filtrate is concentrated under reduced pressure to a volume of 200 to 500 ml. Following e polish filtration, the product-rich concentrate is mixed with 1.5 liters of isobutyl acetate. The mixture is refluxed at about 40 C under reduced pressure in equipment fitted with a water separation device. When no further water separates, the batch is cooled to 30t and filtered. The reactor is washed with 1 liter of acetone, which Is used also to wash the cake. The cake is further washed with 200 ml of acetone. The acetone washes are added to the isobutyl acetate filtrate and the mixture is refluxed for 20 to 30 minutes. After a holding period of one hour at 5°C, the crystals of isopropyiidene penicillamine hydrochloride are filtered and washed with 200 m of acetone. On drying for twelve hours at 25°C this product, containing 1 mol of water, weighs about 178 g (73%). 

The micron ratings of a cartridge are intended to indicate the smallest particle that will be retained by the pores of the filter element. Often a rough-cut pre-filter is installed ahead of a final or polishing filter in order to increase the life of the final unit. Unfortunately, the method for determining the micron rating is not a universal standard between manufacturers. Thus, one manufacturer s 50 micron filter may not perform the same as another manufacturer s with the same rating number. The only reliable approach is to send the manufacturer an actual sample of the fluid and let him test it to select the filter to do your job, or actually test the unit in your plant s field application [37]. 

---