Filtration of slurry

The substitution of a more logical approach to filtration of slurries yielding compressible cakes and redefinition of the specific resistance (Chapter 7). 

In the recycle process, the increase in the reaction yield is a direct result of increasing the Iso/7-chloro ratio from 1.5 1 to 3 1, which effectively increases the ratio of Iso/Mono and crystallization of Mono during the reaction. It should be pointed out that due to the difficulty of sampling and filtration of slurry at the reaction temperature, the solid phase cannot be easily assayed to quantify the percentage of Mono present. 

Fane A.G., Fell C.J.D., Hoogland M.R., McDonogh R. (1988), Crossflow membrane filtration of slurries,... 

Kaspar, J., Soudek, J. and Gutwirth, K., Improvements in or relating to a method and apparatus for dynamic filtration of slurries , British Patent No. 10577 015... 

The Precoat Discharge This is a special type of discharge of the filter cake used on the so-called precoat filters. Precoat filters are drum filters in which the filter medium is composed of a layer of diatomite, perlite, wood-flour, synthetic granulate, and so on. This layer is applied in thicknesses of approximately 100 mm before the actual filtration of slurries mostly having a very low solids content and with very fine particulate composition (Figure 18.11) commences. 

Pressure filters can treat feeds with concentrations up to and in excess of 10% sohds by weight and having large proportions of difficult-to-handle fine particles. Typically, slurries in which the sohd particles contain 10% greater than 10 ]lni may require pressure filtration, but increasing the proportion greater than 10 ]lni may make vacuum filtration possible. The range of typical filtration velocities in pressure filters is from 0.025 to 5 m/h and dry sohds rates from 25 to 250 kg nY/h. The use of pressure filters may also in some cases, such as in filtration of coal flotation concentrates, eliminate the need for flocculation. 

The ECLP tube press was originally developed for the filtration of china clay but has been used with many other slurries such as those in mining, Ti02, cement, sewage sludge, etc. The typical cycle time is about four minutes or more. 

Originally designed for the continuous filtration of conditioned sewage sludges, as were most of the filter belt presses available, the Manor Tower press is increa singly used for the treatment of paper mill sludge, coal, or flocculated clay slurries. 

Fumarates. lron(Il) fumarate [141 -01 -5], Fe(C4H20, is prepared by mixing hot aqueous solutions of sodium fumarate and iron(Il) sulfate followed by filtration of the resulting slurry. It has limited solubiUty in water but is more soluble in acid solution. The compound is red-orange to red-brown and finds uses as a hematinic. A non stoichiometric compound [7705-12-6] and iron(Ill) fumarate [52118-11-3], Fe2(C4H20 3, are also available. 

SO that the batch may be dumped into a tank for processing by successive steps, which normally include centrifugation, filtration, and/or drying. In some cases, a small heal of slurry is left in the batch ciys-tallizer to act as seed for the next batch. 

Constant-Rate Filtration For substantially incompressible cakes, Eq. (18-51) may be integrated for a constant rate of slurry feed to the filter to give the following equations, in which filter-medium resistance is treated as the equivalent constant-pressure component to be deducted from the rising total pressure drop to... 

Pressure leaf filters are used to separate much the same lands of slurries as are filter presses and are used much more extensively than filter presses for filter-aid filtrations. They should be seriously considered whenever uniformity of production permits long-time operation under essentially constant filtration conditions, when thorough washing with a minimum of hquor is desired, or when vapors or fumes make closed construction desirable. Under such conditions, if the filter medium does not require frequent changing, they may show a considerable advantage in cycle and labor economy over a filter press, which has a lower initial cost, and advantages of economy and flexibility over continuous vacuum filters, which have a higher first cost. 

This type of filter allows pressurized filtration of a slurry mixture to remove solids. A set of filter plates is sandwiched together in series with a configuration similar to the plates on a plate-and-frame heat exchanger. After the plates are compressed... 

Filters generally achieve a lower final moisture content than obtained by gravity sedimentation and are often fed from thickeners, as indicated in the schematic particulate process shown in Figure 9.2. In this chapter the principles of slurry filtration will be described and certain simplified filter design equations derived. For more complex derivations the reader is referred to specialist texts e.g. Coulson and Richardson (1991), Wakeman (1990a) and Purchas (1981). 

The operation of the eontinuous rotary vaeuum filter is illustrated in Figure 4.17. The filter drum is immersed in a bath of slurry from where the liquor is drawn through the filter medium by the pressure drop eaused by the appliea-tion of a vaeuum within the drum. During filtration eake builds up on the outside surfaee of the drum between points A and B as if by eonstant pressure bateh operation. It then travels to point C where it is removed and the eyele repeated. 

Mathews and Rawlings (1998) successfully applied model-based control using solids hold-up and liquid density measurements to control the filtrability of a photochemical product. Togkalidou etal. (2001) report results of a factorial design approach to investigate relative effects of operating conditions on the filtration resistance of slurry produced in a semi-continuous batch crystallizer using various empirical chemometric methods. This method is proposed as an alternative approach to the development of first principle mathematical models of crystallization for application to non-ideal crystals shapes such as needles found in many pharmaceutical crystals. 

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,... 

Safety. The MR is much safer than the MASR. (1) The reaction zone contains a much smaller amount of the reaction mixture (hazardous material), which always enhances process safety. (2) In case of pump failure, the reaction automatically stops since the liquid falls down from the reaction zone. (3) There is no need to filter the monolithic catalyst after the reaction has been completed. Filtration of the fine catalysts particles used in slurry reactors is a troublesome and time-consuming operation. Moreover, metallic catalysts used in fine chemicals manufacture are pyrophoric, which makes this operation risky. In a slurry reactor there is a risk of thermal runaways. (4) If the cooling capacity is insufficient (e.g. by a mechanical failure) a temperature increase can lead to an increase in reaction, and thus heat generation rate. 

J. Kieffer, M. Michaux, and P. Rae. Additive for controlling the filtrate of well cementing slurries and corresponding cementing process (ad-ditif pour le controle du filtrat des laitiers de cimentation de puits, et procede de cimentation correspondant). Patent FR 2592056,1987. 

The need to separate solid and liquid phases is probably the most common phase separation requirement in the process industries, and a variety of techniques is used (Figure 10.9). Separation is effected by either the difference in density between the liquid and solids, using either gravity or centrifugal force, or, for filtration, depends on the particle size and shape. The most suitable technique to use will depend on the solids concentration and feed rate, as well as the size and nature of the solid particles. The range of application of various techniques and equipment, as a function of slurry concentration and particle size, is shown in Figure 10.10. 

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... 

THF Conversion. Tetrahydrofuran (THF) conversion was calculated from the difference between the initial and the final solubilities of the total coal-solvent slurry in THF. It was assumed that all of the solvent and none of the starting coal was soluble in THF. THF conversions were calculated on an MAF coal basis and adjusted for the coal not recovered from the autoclaves. The filter cake resulting from filtration of the product at 250 C was continuously extracted with THF for up to 3 days. 

A slurry of CaC03 in water at 25°C containing 20% solids by weight is to be filtered in a plate-and-frame filter. The slurry and filter medium are tested in a constant pressure lab filter that has an area of 0.0439 m2, at a pressure drop of 338 kPa. It is found that 10 3m3 of filtrate is collected after 9.5 s, and 5 x 10 3 m3 is collected after 107.3 s. The plate and frame filter has 20 frames, with 0.873 m2 of filter medium per frame, and operates at a constant flow rate of 0.00462 m3 of slurry per second. The filter is operated until the pressure drop... 

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. 

The cross-flow filtration method is applied mainly to hyper- and ultrafiltration as well as to some microfiltration.8 In cross-flow filtration the slurry solution or suspension fed to the filter flows parallel to the filter medium or membrane. The filtration product (permeate or filtrate) leaves the filtration module at right angles to the filter medium (the membrane). The traditional perpendicular flow filtration (where the flow of the suspension is directed at right angles to the filter medium and the permeate leaves the filter medium in the same direction) entails filter cake buildup, whereas cross-flow filtration is intended to prevent such filter... 

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