Solid concentration, cake filtration

To keep the frequency of backwash and the washwater demand down, and to prevent undesirable cake formation on the filter surface, deep bed filtration is appHed to very dilute suspensions of solids concentrations less than 0.1% by volume. 

Separation Efficiency. Similarly to other unit operations in chemical engineering, filtration is never complete. Some soflds may leave in the hquid stream, and some Hquid will be entrained with the separated soHds. As emphasis on the separation efficiency of soHds or Hquid varies with application, the two are usually measured separately. Separation of solids is measured by total or fractional recovery, ie, how much of the incoming solids is coUected by the filter. Separation of Hquid usually is measured in how much of it has been left in the filtration cake for a surface filter, ie, moisture content, or in the concentrated slurry for a filter-thickener, ie, solids concentration. 

Pretreatment Chemicals Even though the suspended solids concentration of the slurry to be tested may be correct, it is frequently necessary to modify the sluriy in order to provide an acceptable filtration rate, washing rate, or final cake moisture content. The most common treatment, and one which may provide improvement in all three of these categories, is the addition of flocculating agents, either inorganic chemicals or natural or synthetic polymers. The main task at this point is to determine which is the most effective chemical and the quantity of chemical which should be used. 

Cake Filters Filters that accumulate appreciable visible quantities of solids on the surface of a filter medium are called cake filters. The slurry feed may have a solids concentration from about 1 percent to greater than 40 percent. The filter medium on which the cake forms is relatively open to minimize flow resistance, since once the cake forms, it becomes the effective filter medium. The initial filtrate therefore may contain unacceptable sohds concentration until the cake is formed. This situation may be made tolerable by recycling the filtrate until acceptable clarity is obtained or by using a downstream polishing filter (clarifying type). 

Filtration may also serve as the preparatory step for the operation following it. The latter stages may be drying or incineration of solids, concentration or direct use of the filtrate. Filtration equipment must be selected on the basis of their ability to deliver the best feed material to the next step. Dry, thin, porous, flaky cakes are best suited for drying where grinding operations are not employed. In such cases, the cake will not ball up, and quick drying can be achieved. A clear, concentrated filtrate often aids downstream treatment, whereby the filter can be operated to increase the efficiency of the downstream equipment without affecting its own efficiency. 

Once the precoating stage is completed the process slurry is pumped into the filter, the forming cake is retained on the plates and the filtrate flows to further processing. When the solids are fine and slow to filter a body-aid is added to the feed slurry in order to enhance cake permeability. However, it should be kept in mind that the addition of body-aid increases the solids concentration in the feed so it occupies additional volume between the plates and increases the amount of cake for disposal. Likewise, for all those applications when the cake is the product, precoat and filter-aid may not be used since they mix and discharge together with the cake. 

Vertical Pressure Leaf Filters are essentially the same as Horizontal Plate Filters except for the orientation of the filter elements which are vertical rather than horizontal. They are applied for the polishing slurries with very lov solids content of 1-5% or for cake filtration with a solids concentration of 20-25%. As with the horizontal plate filter the vertical leaf filters are also well suited for handling flammable, toxic and corrosive materials since they are autoclaved and designed for hazardous environments when high pressure and safe operation are required. Likewise, they may be readily jacketed for applications whenever hot or cold temperatures are to be preserved.The largest leaf filters in horizontal vessels have a filtration area of 300 m and vertical vessels 100 m both designed for an operating pressure of 6 bar. 

A suspension containing 50 % solids is to be filtered in a filter press, achieving a concentrated cake of 40 % water (on a weight basis). The filter capacity for the cake is G, = 700 kg/hr. Determine the amount of suspension, the filtrate and the ratio x . The density of the solid phase is 1,600 kg/m and that of the liquid phase is 900 kg/m ... 

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,... 

M — mass flow of filtrate, lbm/min (75) c = solids concentration in feed to filter, lbm/lbm filtrate (0.01) xc = mass fraction solids in dry cake... 

Benzhy dr azide.1—24 g. (0-15 mole) of ethyl benzoate (p. 141) are heated for six hours under a small reflux condenser on the water bath with 9 g. of hydrazine hydrate.2 The solid crystalline cake which is formed on cooling is, after some time, filtered as dry as possible at the pump and washed with a little ice-cold methyl alcohol. If the yield is too small the filtrate is concentrated and heated again. 

In practice, it is sometimes possible to incorporate moving blades in the filter equipment so that the thickness of the cake is limited to the clearance between the filter medium and the blades. Filtrate then flows through the cake at an approximately constant rate and the solids are retained in suspension. Thus the solids concentration in the feed vessel increases until the particles are in permanent physical contact with one another. At this stage the boundary between the slurry and the cake becomes ill-defined, and a significant resistance to the flow of liquid develops within the slurry itself with a consequent reduction in the flowrate of filtrate. 

Benefits of Prelltickeniug. The feed solids concentration has a profound effect on the performance of any cake filtration equipment. It affects the capacity and the cake resistance, as well as Uic penetration of the solids into [lie cloth which influences filtrate clarity and medium resistance. Thicker feeds lead to improved performance of most filters through higher capacity and lower cake resistance,... 

Fig. 1. Glucose concentration over time for semibatch saccharification experiments at 15% (w/w) initial insoluble solids concentration and 20 FPU/g of cellulose enzyme loading. Filtration was applied at two different times to remove glucose (vertical dashed lines). VF Only vacuum filtration was applied alone VF+UF ultrafiltration was also applied following vacuum filtration VF, Washed Vacuum filtration was applied and the filter cake was washed with water. Error bars represent averages 1 SD for two repeated experiments.

The recovery of whole cells is best explained by the manufacturing procedure for baker s yeast. This process is almost identical to the early stage of protein recovery, except that the final product is the cell instead of the filtrate. After fermentation, the cells are spun out with a centrifuge, washed with water, and recentrifuged to yield a yeast cream with a solids concentration of approximately 18 percent. Cream yeast can be loaded directly into tanker trucks and delivered to customers equipped with an appropriate cream yeast handling system. Alternatively, the yeast cream can be pumped to a plate and frame filter press or an RDVF and dewatered to a cakelike consistency with 30-32 percent yeast solids content. The press cake yeast is crumbled into pieces and packed or spray-dried for dry products. After packaging, the yeast is ready for shipping to retail. 

Yeast separation and concentration. Extraction of ethanol from biomass requires several separation steps, traditionally by centrifuging, sedimentation and cake filtration. First, after the fermentation, the yeast is removed from the fermentation broth and may be recycled. Additionally, after ethanol is stripped from the fermentation broth by steam followed by the removal of solid fractions, the remaining material called the thin stillage can be clarified for reuse upstream as the process water. 

Examples of depth filtration are sand and cartridge filtration. Solids are trapped in the interstices of the medium. As solids accumulate, flow approaches zero and the pressure drop across the bed increases. The bed must then be regenerated or the cartridge changed. For this reason, this method is not viable for high solids concentration streams as it becomes cost prohibitive. Cartridge filtration is often used as a secondary filtration in conjunction with a primary, such as the more widely used cake filtration. 

The wash can be quantified by mass or volumetric flow to ensure product quality. The intermediate and final spins are usually timed if the basket is automated, however, if out of balance conditions exist during the feed cycle, an operator is often required throughout the operation. Operator judgement is often required to determine when the liquid level on the cake disappears. This may be required, for example, to remove all mother liquor, introduce the wash, particularly for difficult filtrations, or to be sure the liquid level remains on the cake to prevent cracking and preferential channeling of the wash liquid. Variable solids concentrations or particle... 

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