Filter media properties

Pihlajamaki A. (1998), Electrochemical characterisation of filter media properties and their exploitation in enhanced filtration, PhD Thesis, Lappeenranta University of Technology, Finland. 

Some mathematical models have been developed to predict the behavior of the pressure drop over the diesel particulate trap with time during the loading/regenera-tion cycles [62, 67-69], to calculate the effect of filter-medium properties on filter performance of fibrous filters [70] or to describe the flow and filtration process [71] and the regeneration process [72-75]. An illustrative example for the performance of such a pressure drop model is provided in Fig. 15.7. 

Grace [1958] described the filtrate flow-time relationsJiip in standard law conditions in terms of the filter medium properties for constant pessure and constant filtrate rate ... 

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... 

Due to the wide variety of filter media, filter designs, suspension properties, conditions for separation and cost, selection of the optimum filter medium is complex. Filter media selection should be guided by the following rule a filter medium must incorporate a maximum size of pores while at the same time providing a sufficiently pure filtrate. Fulfilment of this rule invokes difficulties because the increase or decrease in pore size acts in opposite ways on the filtration rate and solids retention capacity. 

Filtration time. The filtration rate depends on the pressure difference, the solids content in the slurry, the particle shape and size distribution, the resistance of the filter medium to flow, and properties of the liquor. Observations show that the volume of the permeated liquor increases parabolically with time (see Fig. 5.3-26). 

In conclusion, the following experiments on filtration-washing-deliquoring should be performed to produce data (viscosity of liquids, effective solid concentration, specific cake resistance, cake compressibility, etc.) that are necessary to evaluate times of individual steps of filtration at an industrial scale, i.e. to obtain the proper basis for scale-up of filtration processes measure the filtrate volume versus time make marks on your vacuum flask and take down the time when the filtrate level reaches the mark => no more experiments are needed for preliminary evaluations of filtration properties of slurries initially fines pass the filter medium => recirculate them to the slurry,... 

Since C and C2 are unique properties of a specific slurry-cake system, it is usually more appropriate to determine their values from laboratory tests using samples of the specific slurry and filter medium that are to be evaluated in the plant. For this purpose, it is more convenient to rearrange Eq. (13-37) in the form... 

A plate-and-frame filter press contains 16 frames and operates at a constant flow rate of 30 gpm. Each frame has an active filtering area of 4 ft2, and it takes 15 min to disassemble, clean, and reassemble the press. The press must be shut down for disassembly when the pressure difference builds up to 10 psi. What is the total net filtration rate in gpm for a slurry having properties determined by the following lab test. A sample of the slurry is pumped at a constant pressure differential of 5 psi through 0.25 ft2 of the filter medium. After 3 min, 1 gal of filtrate has been collected. The resistance of the filter medium may be neglected. 

You want to filter an aqueous slurry using a rotary drum filter, at a total rate (of filtrate) of 10,000 gal/day. The drum rotates at a rate of 0.2 rpm, with 25% of the drum surface submerged in the slurry, at a vacuum of 10 psi. The properties of the slurry are determined from a lab test using a Buchner funnel under a vacuum of 500 mmHg, using a 100 cm2 sample of the filter medium and the slurry, which resulted in the lab data given below. Determine the total filter area of the rotary drum required for this job. 

Results from constant differential pressure filtration tests have been analyzed according to traditional filtration science techniques with some modifications to account for the cross-flow filter arrangement.11 Resistivity of the filter medium may vary over time due to the infiltration of the ultrafine catalyst particles within the media matrix. Flow resistance through the filter cake can be measured and correlated to changes in the activation procedure and to the chemical and physical properties of the catalyst particles. The clean medium permeability must be determined before the slurries are filtered. The general filtration equation or the Darcy equation for the clean medium is defined as... 

An alternative method of reducing the resistance to filtration is to recirculate the slurry and thereby maintain a high velocity of flow parallel to the surface of the filter medium. Typical recirculation rates may be 10-20 times the filtration rate. By this means the cake is prevented from forming during the early stages of filtration. This can be particularly beneficial when the slurry is flocculated and exhibits shear-thinning non-Newtonian properties. This method of operation is discussed by Mackley and Sherman(21) and by Holdich, Cumming and Ismail(22). 

Figure 11 shows the wet-flow properties of three hypothetical membrane filter media. Each filter medium is made of the same material and has the same thickness and total void fraction. Media A and B have the same oversized pore size, but A has a broader pore size distribution. Medium C has a pore size smaller than A and B with a narrow pore size distribution. 

Permeability The property of the filter medium that permits a fluid to pass through under the influence of a pressure differential. 

The unique part of the Universal Interface is the membrane separator or gas diffusion cell which allows the solvent vapor to be efficiently removed with essentially no loss of sample contained in the aerosol particles. In this device the aerosol is transported through a central channel bounded on the sides by a gas diffusion membrane or filter medium which is in contact with a countercurrent flow of a sweep gas. For El mass spectrometry helium appears to be most useful for both the carrier and sweep gas. The properties of the... 

Permeability describes the property of a filter medium to let liquid through at higher or lower speeds. It is expressed in Darcy units. One Darcy corresponds to the permeability of a filter material 1 cm thick with a surface area of 1 cm that lets through 1 ml/s of a liquid with a viscosity of 1 centipoise under a differential pressure of 1 bar. Filter materials have varying permeability and the following values are given as an indication ... 

The fibers are naturally drawn during the production process. The LS and HS fibers serve as filter mediums, catalyst carriers, and reinforcing fibers in composites. They have excellent ablation properties. Their relatively low electrical resistance has led to their use in flexible heating elements and wherever a bendable, nonme-tallic conductor is required. 

Conversely, by constructing the filter medium with a blend of fibres of widely contrasting triboelectric properties, it is claimed by a fibre manufacturer that superior collection efficiency can be obtained. It is further claimed that, by virtue of this enhanced efficiency, a more open structure can be used with consequent advantages in respect of the reduced power consumption required to pull the dust-laden air through the collector. However, although this effect has been used to some advantage in clean air room filtration applications, considerably more research is necessary if the triboelectric effects in industrial dust collection are to be fully understood and exploited. 

Definition Hard jet-black coal with high carbon content and calorific value Properties Bulk dens. 0.80-0.93 kg/l Uses Filler for NR, SR, adhesives domestic fuel filter medium in water treatment Trade Name Synonyms Carb-O-Fil [Shamokin Filler]... 

The fiber cloth is the deciding ctor in the success or failure of all press operations. In view of the wide range of process variables involved in the filtration process, it is virtually in ossible to select a filter medium that will satisfy all process requirements and the usual limited time scale available for cloth selection is used to find an acceptable medium, i.e. one that will satisfy most, if not all of the requirements. In this reject, one particular requirement (e.g. filtrate clarity) may have to be relaxed, if other specifications (e.g. filtrate rate, absence of blinding) are to be maximised. Thus the more open weave Mcs will be superior in nonblinding characteristics, but may have poor particle retention. The latter will in rove in the order monofilament < muldfilament < staple fibre. Tabulated information is presented in Tables 4.2,4.3 and 4.4 below on the effect of yam properties, weave patterns, etc. on the processes of cake release, productivity, resistance to blinding, etc.. 

Addition of another powdered solid, termed a filter aid, to the system can produce a significant improvemaat to a filtration operation. The filter aid may be used in two ways, either separately or in conjunction. The first of these methods is to precoat the filter medium with a layer of fiher ail cake. Precoat aids must filter quickfy without bleeding or penetrating throu the cloth and must give a uniform thickness with a reproducible filtering surfiice. The susp ntion is then fihered onto the precoat by surface and depth filtration mechanisms. Thus surface properties are inq>ortant in the choice of aid. 

---