Deliquoring

The filtration and washing processes leave the filter cake substantially saturated with liquid. The presence of hquid is usually a disadvantage in subsequent apphcations of the solids so there is some processing benefit in removing as much of the hquid as possible whilst the cake is on the filter. This objective may be achieved either by blowing with air 

The calculation method due to Brownell et al [1947 1949] is based on an analogy between p e flow and flow in porous media. For laminar flow a Reynolds number and a friction fltctor are defined which include the heiidty and diameter of flie particles and the poro of the filter cake. These are defined as  

The concept was extended to the two-phase flow situation by applying the relationshp to each of the phases. 

Whilst wetted porosity is a fimction of saturation, these authors fiiiled to establish a relation between wetted sphericity and saturation. For the nonwetting (gas) phase the experiments are used to det mine the wetted porosity and the wetted sphericity. Fuller details of the design method and copies of the charts can be found in the original papers and in Brown [1950]. 

The use of this approach in the calculation of gas flow in the deliquoiing of filter cakes on rotary drum vacuum filters is descried by Brownell and Gudz[1949]. However when applied to air blowing in a pressure filter the calculated gas flow under estimates 

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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,... 

Delayed coking, 28 649-650 Delayed coking technology, 22 721-722 Delignification, 22 21 chemistry of, 22 23-26 in the kraft process, 22 22 in organosolv pulping, 22 30 Deliquoring, in solid-liquid separation, 22 344... 

Deliquoring" or "dewatering" are used to designate the removal of a liquid from a product without ohanging its phase. More specifically, deliquoring refers, as a kind of filtration prooess, to the reduotion of moisture content of filter cakes by mechanical means. 

The main effeots involved in the use of high-intensity US for deliquoring are (a) alternating aooustio stresses, (b) radiation pressure, (c) acoustic streaming, (d) interface instabilities and (e) cavitation. 

In some special cases, steam filtration can be used to gain the advantages of temperature without having to heat the feed slurry. Where applicable, dry steam is passed through the deliquored cake to raise the temperature of the residual moisture, reduce its viscosity, and lower its content. The final dmng or cooling period which follows steam filtration uses the residuaf heat left in the cake to evaporate some additional moisture. 

Expression Mechanical ejqiression applies pressure directly on filter cakes rather than relying on flow frictions generated by hydraulic pressure drop to deliquor the cake. The effects of stress distribution in a compactible filter cake by these two different mechanisms are shown in Fig. 18-185. The stress distribution of an expression is more uniform than that of a pressure filtration, leading to a more uniform filter cake. Expression is therefore a better choice for deliquoring of compactible filter cakes. 

Average cake porosity (or solidosity) Deliquoring time... 

Suction filters that are similar to Buchner funnels are often used in pilot plants. These filters may be used in many operations on a small scale. With relatively thick filter cakes, extended filtration times occur. To speed the removal of liquids (deliquoring) from the filter cakes, operations may use other types of filters, such as Nutsche filters, filter presses, and centrifuges [18]. Centrifuges are often used in manufacturing operations for the efficient isolation of solids. Laboratory centrifuges are available for studies before committing a process to scale-up. 

Expression operations include mechanic squeezing of a wet filter cake with stationary diaphragms, moving belts, rollers, or screws. For further deliquoring, blowing with gas is frequently used after expression. 

For the cake, if the mother liquor in the slurry contains soluble substances, it may be necessary to wash the cake. Deliquoring is a process after cake washing. Decreasing the liquid content of cakes has become a major objective of many SLS processes. 

Membrane Filter Press. Membrane filter presses use impermeable, flexible membranes, or diaphragms to squeeze the cake for further cake deliquoring, as shown in Fig. 15. This type of filter provides less dead time in a filtration cycle, better washing, and drier cake compared to traditional plate-and-frame and recessed plate filter presses. A comparison of a recessed press operated at 100 psi and a membrane filter operated at 25psi for sludge dewatering is shown in Table 4. ... 

Fig. 28 shows the mechanically deliquoring of flocculated, fragile, and high porosity cakes by belt press filter. In the process, large floes settle out rapidly on the porous belt, and the liquid flows through quickly. Sedimentation and filtration are accomplished quickly, but at a cost of producing a very soupy cake. 

Fig. 29 shows a Klampress belt press manufactured by Ashbrook. The Klampress utilizes a combination of gravity and mechanical pressure to process porous compactible cakes. It is the type of equipment used for deliquoring of AS at many wastewater treatment plants. 

Tiller, F.M. Horng, L.L. Hydraulic deliquoring of compressible filter cakes. Part I. Reverse flow in Alter presses. AIChE J. 1983,29, 297-305. 

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