Filter-cake Removal

At the end of the filtration cycle, the filter cake can be removed (Fig. 8) by one of the following methods  

Whatever method of cake removal is used, it is extremely important that it be complete-otherwise septa will blind and the result will be inadequate filter performance. Practical 

A wide variety of industrial filters are available for use with filter aids, each with its advantages and disadvantages. These all fall, however, into one of two basic classifications (a) those which operate under pressure and (b) those which operate under vacuum. 

Preparation of filter body feed (diatomaceous earth) and precoat Verification of proper dosages Periodic backwashing Disposal of spent filter cake 

Periodic inspection of filter septum for cleanliness and damage Verification of the effluent quality 

The drill-in fluids are typically composed of either starch or cellulose polymers, xanthan polymer, and sized calcium carbonate or salt particulates. Insufficient degradation of the filter-cakes resulting from even these clean drill-in fluids can significantly impede the flow capacity at the wellbore wall. Partially dehydrated, gelled drilling fluid and filter-cake must be displaced from the wellbore annulus to achieve a successful primary cement job. 

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J. W. Dobson, Jr. and P. D. Kayga. Magnesium peroxide breaker system improves filter cake removal. Petrol Eng Int, 68(10) 49-50, October 1995. 

J. Weaver, K. M. Ravi, L. S. Eoff, R. Gdanski, and J. M. Wilson. Drilling fluid and filter cake removal methods and compositions. Patent US 5501276, 1996. 

Fig. 6.1. Twin-belt filter press (Bellmer) 1. Prior extraction of juice, 2. Wedge zone, 3. Low-pressure pressing zone, 4. High-pressure pressing zone, 5. Juice runoff, 6. Filter cleaning, 7. Filter cake removal.

Fig. 11.8. Diagram of the circuits in a diatomaceous earth filter with continuous accretion 1, inlet of wine to be clarified 2, main feed pump 3, inspection glass for wine to be clarified 4, filtration vat with horizontal filter units 5, filter cake removal 6, external residnal filtration nnit 7, inspection glass for clarified wine 8, clarified wine outlet 9, tank containing the filtration adjnvant in snspension 10, filtration adjuvant metering pnmp...

In situations where a low concentration of suspended solids needs to be separated from a liquid, then cross-flow filtration can be used. The most common design uses a porous tube. The suspension is passed through the tube at high velocity and is concentrated as the liquid flows through the porous medium. The turbulent flow prevents the formation of a filter cake, and the solids are removed as a more concentrated slurry. 

The soiution is aliowed to cool and the crystals of the P2P-bisulfite addition compound are then separated by vacuum filtration, washed with a little clean dH20 then washed with a couple hundred mLs of ether, DCM or benzene. The filter cake of MD-P2P-bisulfate is processed by scraping the crystals into a flask and then 300mL of either 20% sodium carbonate solution or 10% HCi soiution are added (HCI works best). The soiution is stirred for another 30 minutes during which time the MD-P2P-bisulfite complex will be busted up and the P2P will return to its happy oil form. The P2P is then taken up with ether, dried and removed of the solvent to give pure MD-P2P. Whaddya think of that ... 

After 3 hours the stirring is stopped and the solution allowed to settle. By this time just about all the foil will have turned to dust, which is going to make the next step of vacuum filtration very difficult because it will plug up the filter paper in a second. So the chemist lets it settle, then pours off the liquid through the vacuum filtration setup (see methodology section). The flask is rinsed with lOOmL methanol, the methanol poured through the grey filter cake and the filter cake discarded. All of the filtrate is placed in a flask and vacuum distilled to remove all the methanol, isopropyl alcohol and water which will leave the chemist with oil and junk in the bottom of the flask. 

A filter cake from the wringer is washed to remove absorbed acid, transferred to a slurry tank of water, and quickly submerged, after which the nitrocellulose is pumped to the stabilization operation as a diluted water slurry. Exhaust systems are installed to protect personnel and equipment from acid fumes, and water sprays and cyclone separators are used for acid fume recovery before venting to the air. 

The polymer can easily be recovered by simple vacuum filtration or centrifugation of the polymer slurry. This can be followed by direct conversion of the filter cake to dope by slurrying the filter cake in chilled solvent and then passing the slurry through a heat exchanger to form the spinning solution and a thin-film evaporator to remove residual monomer. 

The scraper or knife discharge consists of a blade that removes the cake from the dmm by direct contact with the filter cake. It is normally used for granular materials with cake thickness greater than about 6 mm. In order not to damage the filter cloth, a safety distance of 1 to 3 mm between the blade and the cloth must be observed. If the residual layer is made not of filter aid but of the product, there is danger of its blocking by fine particles and by successive consoHdation by the scraper blade. 

Because gravity is too weak to be used for removal of cakes in a gravity side filter (2), continuously operated gravity side filters are not practicable but an intermittent flow system is feasible in this arrangement the cake is first formed in a conventional way and the feed is then stopped to allow gravity removal of the cake. A system of pressure filtration of particles from 2.5 to 5 p.m in size, in neutralized acid mine drainage water, has been described (21). The filtration was in vertical permeable hoses, and a pressure shock associated with relaxing the hose pressure was used to aid the cake removal. 

Mechanical Cake Removal. This method is used in the American version of the dynamic filter described under cross-flow filtration with rotating elements, where turbine-type rotors are used to limit the cake thickness at low speeds. The Exxflow filter, introduced in the United Kingdom, is described in more detail under cross-flow filtration in porous pipes. It uses, among other means, a roUer cleaning system which periodically roUs over a curtain of flexible pipes and dislodges any cake on the inside of the pipes. The cake is then flushed out of the curtain by the internal flow. 

As observed from Figure 27, the cake removal by fluid shear is also aided by centrifugal force. Other arrangements include stationary filtration media and rotating disks to create the shear effects, and rotating cylindrical elements it has also been shown how such filters can be used for cake washing. 

Reaction times can be as short as 10 minutes in a continuous flow reactor (1). In a typical batch cycle, the slurry is heated to the reaction temperature and held for up to 24 hours, although hold times can be less than an hour for many processes. After reaction is complete, the material is cooled, either by batch cooling or by pumping the product slurry through a double-pipe heat exchanger. Once the temperature is reduced below approximately 100°C, the slurry can be released through a pressure letdown system to ambient pressure. The product is then recovered by filtration (qv). A series of wash steps may be required to remove any salts that are formed as by-products. The clean filter cake is then dried in a tray or tunnel dryer or reslurried with water and spray dried. 

The softened seawater is fed with dry or slaked lime (dolime) to a reactor. After precipitation in the reactor, a flocculating agent is added and the slurry is pumped to a thickener where the precipitate settles. The spent seawater overflows the thickener and is returned to the sea. A portion of the thickener underflow is recirculated to the reactor to seed crystal growth and improve settling and filtering characteristics of the precipitate. The remainder of the thickener underflow is pumped to a countercurrent washing system. In this system the slurry is washed with freshwater to remove the soluble salts. The washed slurry is vacuum-filtered to produce a filter cake that contains about 50% Mg(OH)2. Typical dimensions for equipment used in the seawater process may be found in the Hterature (75). 

Calcite and siderite (27) are used occasionally because of their solubiUty in hydrochloric acid which offers a method of removing mud filter cake deposited on productive formations. Calcite and siderite are used most frequently in workover or completion fluids when a nondamaging fluid is required, ie, one that can be removed by acidising at a later time. 

Elemental phosphoms from the electrothermal process is a distilled product of high purity and yields phosphoric acid pure enough for most industrial uses without any further treatment. The main impurity is ca 20—100 ppm arsenic present in the phosphoms as the element and in the phosphoric acid as arsenious acid. To remove the arsenic, the phosphoric acid destined for food, pharmaceutical, and some industrial-grade appHcations is treated with excess hydrogen sulfide, filtered, and blown with air to strip out excess H2S. This treatment generally reduces the arsenic content of the phosphoric acid to less than 0.5 ppm. The small amount of filter cake is disposed of in approved chemical landfills. 

Chemical precipitation and solvent extraction are the main methods of purifying wet-process acid, although other techniques such as crystallisa tion (8) and ion exchange (qv) have also been used. In the production of sodium phosphates, almost all wet-process acid impurities can be induced to precipitate as the acid is neutralized with sodium carbonate or sodium hydroxide. The main exception, sulfate, can be precipitated as calcium or barium sulfate. Most fluorine and siUca can be removed with the sulfate filter cake as sodium fluorosiUcate, Na2SiFg, by the addition of sodium ion and control of the Si/F ratio in the process. 

In the double-neutralization process, Na2SiFg is precipitated and removed by filtration at a pH of 3—4 (9). Upon raising the pH to 7—9, insoluble phosphates of Fe, Al, Ca, and Mg form and separate. Iron can be precipitated as hydrous ferric oxide, reducing the phosphate loss at the second filter cake. Both the fluorosihcate and metal phosphate filter residues tend to be voluminous cakes that shrink when dewatered recovery of soluble phosphates trapped within the cakes is difficult. 

There are two commercial solvent crystaUi2ation processes. The Emersol Process, patented in 1942 by Emery Industries, uses methanol as solvent and the Armour-Texaco Process, patented in 1948, uses acetone as solvent. The fatty acids to be separated are dissolved in the solvent and cooled, usually in a double-pipe chiller. Internal scrapers rotating at low rpm remove the crystals from the chilled surface. The slurry is then separated by means of a rotary vacuum filter. The filter cake is sprayed with cold solvent to remove free Hquid acids, and the solvents are removed by flash evaporation and steam stripping and recovered for reuse (10). 

In another process, hypochlorite filtrate is treated with lime slurry to precipitate dibasic crystals that are filtered. The filtrate is mixed with strong caustic, chlorinated, and filtered to remove NaCl crystals. The filtrate containing Na and Ca hypochlorite is mixed with dibasic crystals and chlorinated producing a slurry of Ca(OCl)2 is filtered the cake goes to a dryer and the filtrate to the dibasic crystallizer (195). 

Several modifications of the preparation of neutral Ca(OCl)2 2H20 do not involve intermediates. In a continuous process, lime slurry containing caustic and Ca(OCl)2 mother Hquor is chlorinated under reduced pressure to remove the heat of reaction, and the resulting slurry is separated in a classifier into Ca(OCl)2— and NaCl-rich regions from which slurry is withdrawn to obtain Ca(OCl)2 filter cake and soHd salt (204). 

Purified CA (>99% assay) is produced by digesting the cmde product in 15—20% sulfuric acid. The digestion can be carried out at atmospheric pressure or above (103—105). The digested CA slurry is filtered and washed to remove residual sulfuric acid. A small portion of the filter cake (—10%) is dried conventionally at temperatures up to 200°C, granulated, and packaged for merchant sales. The remainder of the CA filter cake, which contains a small... 

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