Medium-filtering, slurry properties

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 packed bed that consists of the same medium as that in Problem 3 is to be used to filter solids from an aqueous slurry. To determine the filter properties, you test a small section of the bed, which is 6 in. in diameter and 6 in. deep, in the lab. When the slurry is pumped through this test model at a constant flow rate of 30 gpm, the pressure drop across the bed rises to 2 psia in 10 min. How long will it take to filter 100,000 gal of water from the slurry in a full-sized bed... 

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). 

Cake discharge characteristics The ease of removal of a cake from media used on either batch or continuous filters is crucial the cake should not adhere to the medium (adhesion is a result of mechanical and electrical properties of the medium and the slurry). 

First, if we ignore the filter medium and consider only the cake itself, the pressure drop versus liquid flow relationship is described by the Ergun equation [Equation (6.15)]. The particle size and range of liquid flow and properties commonly used in industry give rise to laminar flow and so the second (turbulent) term vanishes. For a given slurry (particle properties fixed) the resulting cake resistance is defined as ... 

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