Precoating rate

The precoating rate will depend mainly on the viscosity of the liquid used. The rate should be sufficient to keep all the filter aid in suspension but should not be fast enough to cause erosion of precoat in the filter. For water, a typical rate is from 1 to 2 gpm/ft of filter area, or 0.04 to 0.08 m /min/m of filter area. For viscous liquids, the rate may be as low as 5 gal per ft per hour (gph/ft ), or 0.02 mWm. A general rule for precoating is to precoat at that rate that gives a differential pressure of approx 2 psi (13.8 kilopascals). For water, an upward velocity of at least 4.5 ft/min (1.4 m/min) is required for proper filter aid suspension. The suspension of filter aid can be improved in the tank, or pressure leaf filter, by recirculating part of the inlet flow from the top of the filter back to the precoat tank. 

Results of test work with this filter, producing cakes of 1 mm thickness using a 3 mm clearance, have been pubUshed (33,34). The cake formed on the medium was generally stable, giving high filtration rates over long periods of time, and the precoat type cake did not blind with time. There was no evidence of any size selectivity of the process the only exception was conventional filter aids which were preferentially picked up by the rotating fluid. This... 

Where the resistance of the precoat bed is significant in comparison to the resistance of the deposited solids, the thickness of the precoat bed effec tively controls the filtration rate. In some instances, the resistance of the deposited solids is veiy large with respec t to even a thick precoat bed. In this case, variations in thickness through the life of the precoat bed have relatively little effec t on filtration rate. This type of information readily becomes apparent when the filtration rate data are correlated. 

The ability of an admix to be retained on the filter medium depends on both the suspension s concentration and the filtration rate during this initial precoat stage. The same relationships for porosity and the specific resistance of the cake as functions of suspension concentration and filtration rate apply equally to filter aid applications. 

Paper pulp and fiber cloths are excellent materials for precoats and filter aids. Paper pulp gives a high rate of flow, is easily discharged and shows little tendency to clog. 

Sometimes filtration can be improved by using filteraids. These filteraids, which are based on diatomaceous earth, improve the porosity of a resulting filter cake leading to a faster flow rate. Before filtration a thin layer is used as a precoat of tire filter (normally standard filters). After that a mix is made with the harvest broth and filtration is started. 

The most commonly used filter medium is woven cloth, but a great variety of other media is also used. The main types are listed in Table 10.2. A comprehensive discussion of the factors to be considered when selecting filter media is given by Purchas (1971) and Mais (1971) see also Purchas and Sutherland (2001). Filter aids are often used to increase the rate of filtration of difficult slurries. They are either applied as a precoat... 

Another method, which is even more successful in preventing binding of the septum, is the use of a precoat.53 Before filtration is begun a coating of 2-6 in (5-15 cm) of diatomaceous earth or perlite filter aid is deposited on the surface of the septum. During filtration operations the scraper is set so that it slowly removes the precoat and, of course, with it the materials that would have plugged the filter. Since the precoat causes a considerable pressure drop, the rate of filtration is slowed down. Flow rates may vary from 2 to 50 gal/hr/ft2 (0.025-0.60 m/hr). The precoat material costs around 3 or 40/lb and is used at the rate of 10- 15 lb/1,000 gal of feed (1,200 to 1,800 kg/m3). 

After the precoat is established the solids to be removed from the filter feed are trapped on the surface of the precoated bed. This thin layer of slime is removed by a knife which is caused to advance slowly towards the drum. The knife also removes a thin layer of the precoated bed so that a new surface of the filter medium is exposed. This procedure allows steady filtration rates to be achieved. 

The binder was applied by a distributor truck at a rate of 0 4 gal / yd (1 81 1/nrv Three hundred feet (91 4 meters) of 5/8 inch (15.87 mm) precoated rock asphalt were dropped on the binder followed by 2700 feet (823 meters) of 3/8 inch (9 5 mm) sandstone ... 

Ryu et al. (37) and Xanthos et al. (38) prepared thin films of sintered PP, 200-300 micron diameter, precoated at room temperature with POX E . The films were allowed to react in a constant-temperature oven and samples were withdrawn and analyzed to determine Mw and MWD. It was found that the reduction rates of the M v and MWD became essentially zero after six to seven half-lives of POX E as measured in dodecane. The conclusion is, since there is no mixing during reaction, the diffusion rate of the POX coating onto the PP particulates is not rate controlling, that is, CR-PP for those coated 200-300-pm PP powder particulates is not diffusion controlled. In reactive processing one should strive for process conditions and reaction kinetics where the reactive polymer processing environment is uniform, resulting in uniform product. We discuss this in Sections 11.2 and 11.3. 

FIGURE 5-45. Example of using TLC as a guide for HPLC separation of Azo dyes. Analytes shown in the figure, (a) TLC plate E Merck precoated TLC sheet, silica gel F-254. Solvent 10% methylene chloride in hexane. Development time 50 min. (b) Column Micropak Si-10 (silica, 10 /am), 2.4 mm ID x 15 cm. Mobile phase 10% methylene chloride in hexane. Flow rate 2.2 mL/min. Detector UV at 254 nm, 0.16 AUFS. (Reproduced from reference 21 with permission.)... 

When a polymer is treated with enzymes for surface modification, some of the undesired protein tends to adsorb on the polymer surface, which subsequently creates problems in the surface analysis and causes a slow down in the rate of catalysis. Adsorbed proteins can be removed from the surfaces by washing with large volumes of 1.5% Na2C03 and water (Eischer-Colbrie et al., 2006) as part of a preparation for surface analysis. Protein-resistant molecules such as polyethylene glycol can be used to prevent the nonspecific protein adsorption. Surfaces can be precoated with an inert protein such as bovine serum albumin (Salisbury et al., 2002) for increasing the rate of catalysis. 

Conventional filter systems are bound by one immutable constraint, namely, that clay particles must be separated from the clay/oil slurry by passing through a precoat of clay that is tight enough to retain all the suspended solids from the liquid to be filtered, but still permeable enough to produce economical rates of oil production. If particles become too small, they cause the filter rate to decrease so much... 

Any substances that can enhance the filtration efficiency are termed a filter aid. Diatomaceous earth is the most common filter aid for the precoat filtration system. An efficient, economical filter aid must (a) have rigid, intricately shaped, porous, individual particles (b) form a highly permeable, stable, incompressible filter cake (c) remove even the finest solids at high rates of flow and (d) he as chemically inert and essentially insoluble in the Uquid being filtered. Commercial diatomaceous earth, such as Celite diatomite, meets these requirements due to the wide variety of intricately shaped particles and inert composition that makes it practically insoluble in all hut a few liquids. 

For a given liquid, clarity of filtrate is governed principally by (a) grade and amount of filter aid for body feed (b) grade and amount of filter aid for precoat (c) length of cycle and (d) filtration rate. 

The amount of precoat should be from 10 to 15 lb of filter aid per 100 ft of filter area (4.5-6.8 kg per 9.29 m ), the higher amount being used when distribution of flow in the filter is poor, or in starting up new filters. If it is perfectly distributed, 10 lb (4.5 kg) of filter aid per 100 ft (9.29 m ) of filter area will give a precoat of approx 1.46 in. (1.6 mm) in thickness. The use of baffles or precoating at a different rate may be necessary for an even precoat at lower precoat amounts. 

In some cases, alum precoating of the diatomaceous earth can improve performance. Typical alum doses range from 1% to 2% hy weight (1-2 kg/100 kg of diatomaceous earth applied). Typical precoats of diatomaceous earth of 0.49-0.98 kg/m (0.1 to 0.2 Ih/ft ) are applied to prepare the filter. These filters also require a continuous supplemental body feed of diatomite because the filter is subject to cracking. If the filter has no body feed, there will be rapid increases in headloss due to buildup on the surface of the filter cake. Body feed rates must be adjusted for effective turbidity removal. Diatomaceous earth filters do not need a filter-to-waste cycle, because of the precoating process. 

Precoating techniques recently developed by Manville can substantially increase filtration efficiency in terms of reduced precoat usage and increased flow rate. 

---