Addition of filter aid

Filter aids can improve the permeability and sometimes porosity of a filter cake, improve filtrate clarity and help to prevent filter medium blinding. They comprise relatively porous particles such as diatomite, perlite and activated carbon and are either filtered as a precoat onto the medium or mixed as body feed with the suspension during a pretreatment stage the latter beneficially improves the porosity of a subsequently formed filter cake. Both the cost of filter aid and the need to remove filter aid from the processed solids can present problems however, the use of filter aids on rotary drum filters and in the filtration of dilute feeds (such as those found in the brewing industries) can bring undoubted benefits. More detail on filter aids is provided in Chapter 2. 

The discussion of chemical and physical pretreatments given in this chapter highlights the potential benefits (and pitfalls) of artificially changing the particle size present in a suspension. Chemical pretreatments in particular can enhance performance in all phases of a filter cycle and provide for improved filtration rates and reduced cake moistures. Some separation devices such as the deep cone thickener and belt filter press rely on the satisfactory performance of flocculants to function correctly and their use is a necessary prerequisite. The use of coagulants and flocculants with other separation devices must be assessed on an individual basis. Their introduction generally increases the operating cost of a separation which must be balanced by the potential improvements in separation characteristics. 

In this chapter practical methods of obtaining information for filtration, consolidation, cake washing, gas defiquoring and sedimentation are described. The aim is to illustrate how important data for equipment selection (see Chapter 5), scale-up and process simulafion (see Ch ters 6 and 7) can be measured in the laboratory using either weU established, manually operated apparatus or state-of-the-art, automated apparatus. In some cases details of step-by-step experimental procedures are presented to illustrate best practice. Methods of data analysis are also presented and their use is illustrated through worked examples. 

The fundamental concepts that underpin this chapter are not described in detail, as emphasis is placed on more practical issues. The interested reader is referred to Wakeman and Tarleton (2005 a,b). 

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Addition of Inert Filter Aids. FUtet aids ate rigid, porous, and highly permeable powders added to feed suspensions to extend the appheabUity of surface filtration. Very dilute or very fine and slimy suspensions ate too difficult to filter by cake filtration due to fast pressure build-up and medium blinding addition of filter aids can alleviate such problems. Filter aids can be used in either or both of two modes of operation, ie, to form a precoat which then acts as a filter medium on a coarse support material called a septum, or to be mixed with the feed suspension as body feed to increase the permeabihty of the resulting cake. 

A more flexible option from an operational viewpoint is the implementation of process-oriented enhancements that intensify particle separation. This can be achieved by two different methods. In the first method, the suspension to be separated is pretreated to obtain a cake with minimal resistance. This involves the addition of filter aids, flocculants or electrolytes to the suspension. In the second method, the period during which suspensions are formed provides the opportunity to alter suspension properties or conditions that are more favorable to... 

Filter aids may be applied in one of two ways. The first method involves the use of a precoat filter aid, which can be applied as a thin layer over the filter before the suspension is pumped to the apparatus. A precoat prevents fine suspension particles from becoming so entangled in the filter medium that its resistance becomes exces-sive. In addition it facilitates the removal of filter cake at the end of the filtration cycle. The second application method involves incorporation of a certain amount of the material with the suspension before introducing it to the filter. The addition of filter aids increases the porosity of the sludge, decreases its compressibility, and reduces the resistance of the cake. In some cases the filter aid displays an adsorption action, which results in particle separation of sizes down to 0.1 /i. The adsorption ability of certain filter aids, such as bleached earth and activated charcoals, is manifest by a decoloring of the suspension s liquid phase. This practice is widely used for treating fats and oils. The properties of these additives are determined by the characteristics... 

The addition of filter aid enables the product to be easily separated by filtration. Its omission causes the separation of product as an emulsion. 

Addition of filter aids (diatomaceous earths or perlites) to increase the porosity and to reduce the compressibility of cakes... 

Cold filtration may be performed before or after deodorization. Oil from the bleacher or the deodorizer is cooled to 12-15°C, and after settling for 12 hours, it is cold-filtrated with the addition of filter aid (perlite or diatoma) to prevent clogging caused by the wax. 

Continuous addition of filter aid (body feeding) is accomplished either by feeding filter aid as a slurry or by dry feeding. Slurry feeding is usually done with plunger or diaphragm pumps. If filtration is a batch process, the filter aid can be added directly to the batch. 

Carefully transfer the sample into the holder of the disk and apply vacuum to aspirate the sample through the disk. If the disk should become dry before addition of the sample, recondition the disk and apply the sample. A vacuum of 12 in. of mercury provides an appropriate flow rate for sample addition. If the sample contains particulates and a large volume is to be filtered, that is, > 100 rnL for a 47-mm disk, the addition of Filter Aid 400 beads (3M) is recommended. Filter Aid 400 is nonporous, inert, and resistant to leaching. It is placed on top of the disk to a depth of about 1 cm. It acts as a depth filter to prevent the suspended solids from plugging the disk. The Filter Aid 400 are silica beads of 40-pm diameter. Again do not allow the disk to go dry after addition of the sample. Because of the high efficiency of the small particle size of the C-18 (10 pm), the sample recovery is unaffected by flow rate and the sample should be past as fast as the extraction disk will allow. 

Addition of filter aid to the slurry, in the range of 1-2% of the overall slurry weight, can improve the filtration rates. Another rule of thumb is to add filter aid equal to twice the volume of solids in the slurry. By matching the particle size distribution of the filter aid to the solids to be filtered, optimum flow rates are achieved. One should also use 3% of the particles, above 150 mesh in size, to aid in filtration. 

Precoating the filter medium prevents blinding of the medium with the product and will increase clarity. Filter aid must be an inert material, however, there are only a few cases where it cannot be used. For example, waste cells removed with filter aid cannot be reused as animal feed. Filter aid can be a significant cost, and therefore, optimization of the filtration process is necessary to minimize the addition of filter aid or precoat. Another possible detriment is that filter aid may also specifically absorb enzymes. 

There are two objectives related to the addition of filter aids. One is to form a layer of second medium that protects the basic medium (filter cloth) of the system. This is commonly referred to... 

During filtration, body feed, or the continuous addition of filter aid, can help prevenr blinding of the suspended solids on the precoat. The body feed surrounds the suspended solids to provide flow around them. The body feed slurry of filter aid and oil is injected into the system prior to the filter. The suspended solids are ridged or deformable and can elongate under pressure to extrude through the filter cake and slow or block the product flow. Body feed coats the deformed solids, allowing them to be retained on the filter cake. 

The second application method involves incorporation of a certain amount of the material with the suspension before introducing it to the filter. The addition of filter aids increases the porosity of the sludge, decreases its conpressibility, and reduces the resistance of the cake. 

Rotary drum vacuum filters are a widely used piece of equipment for this filtration step, due to its simple operation and low investment costs. The drum is typically precoated (e.g., with diato-maceous earth, sUica, perlite) to improve the process performance [7]. Additional filter aid is often added to improve the filtrate qnality hence, the amount of slndge generated from this unit operation is relatively large. Several other filtration devices such as filter automates and cross-flow filtration devices are also nsed for the filtration. One of the benefits of the latter is that they can be operated withont addition of filter aids and thus, the amount of generated sludge can be reduced. 

When the filter cake itself is the product to be recovered or in case of sticky impurities (even addition of filter aid may not be of much use) filter press is to be employed. Information to be given by the purchaser regarding the liquid and solid particles is almost same as in the case of pressure leaf filter. The following shall specifically be informed slurry temperature and viscosity, nature of the cake solid and any preference for the filter medium. 

These are used for quick solid-liquid separation, generally for easily separable crystals/suspended solids. However, difficulties have been experienced in use of centrifuge in case of sticky pastes for which press leaffilter may be used with appropriate additions of filter aid powders. 

Precoat filtration can be incorporated within a wide range of pressure filters including leaf, multi-element and plate and frame types (see Section 1.4.2). Up to 700 g m of precoat is typically filtered onto the filter medium prior to introduction of the feed suspension. The feed, which may also contain a significant addition of filter aid to improve cake permeability, is filtered until the filtrate flow rate is sufficiently low to warrant cake discharge in the normal way. It is not economical to recover the feed solids from the precoat, and it follows that washing of the solids is not practised. Moreover, the filter aid tends to abrade the pumps used to promote the filtration. Precoat pressure filtration is most often used for the removal of finer particles from dilute suspension where other potential processes would require too high an investment. 

Value The product value, combined with the scale of operation, has a bearing on equipment selection. If the solid is the desired product, addition of filter aids may not be allowable. 

Cake filtration is based on passing a suspension through a permeable, relatively thin medium (chapter 8). The sohds are deposited in the form of a cake on the upstream side of the medium. As soon as the first layer of cake is formed, the subsequent filtration takes place on top of this cake and the medium provides only a supporting function. These so-called surface filters are best used for filtration of suspensions of solids concentrations in excess of 1% by volume in order to minimize medium blinding which occurs in the filtration of dilute suspensions. If cake filtration is to be applied to clarification of liquids, which imphes low feed concentrations of sohds, addition of filter aids is usually necessary—see section 2.2. 

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