Continuous pressure filtration

When the volume of mother liquor is large and the amount of crystals small, the apparatus of Fig. II, 32, 1 may be used. The large pear-shaped receiver is supported on a metal ring attached to a stand. When the receiver is about two-thirds fuU, atmospheric pressure is restored by suitably rotating the three-way stopcock the filtrate may then be removed by opening the tap at the lower end. The apparatus is again exhausted and the filtration continued. 

There are many technical problems to be considered when developing a new commercial and viable filter. However, the filtration hardware in itself is not enough, as the control of a continuous pressure filter is much more difficult than that of its equivalents in vacuum filtration the necessary development may also include an automatic, computerized control system. This moves pressure filtration from low to medium or even high technology. Disk Filters. 

Because gravity is too weak to be used for removal of cakes in a gravity side filter (2), continuously operated gravity side filters are not practicable but an intermittent flow system is feasible in this arrangement the cake is first formed in a conventional way and the feed is then stopped to allow gravity removal of the cake. A system of pressure filtration of particles from 2.5 to 5 p.m in size, in neutralized acid mine drainage water, has been described (21). The filtration was in vertical permeable hoses, and a pressure shock associated with relaxing the hose pressure was used to aid the cake removal. 

R. Bott, H. Anlauf, and W. Stahl, Continuous Pressure Filtration ofWeyFine Coal Concentrates, Aufbereitungstechnik, Nr.5/Mai 1984, pp. 245—258. 

In cake or surface filtration, there are two primary areas of consideration continuous filtration, in which the resistance of the filter cake (deposited process solids) is veiy large with respec t to that of the filter media and filtrate drainage, and batch pressure filtration, in which the resistance of the filter cake is not veiy Targe with respect to that of the filter media and filtrate drainage. Batch pressure filters are generally fitted with heavy, tight filter cloths plus a layer of precoat and these represent a significant resistance that must be taken into account. Continuous filters, except for precoats, use relatively open cloths that offer little resistance compared to that of the filter cake. 

Vacuum or Pressure The vast majority of all continuous filters use vacuum to provide the driving force for filtration. However, if the feed slurry contains a highly volatile hquid phase, or if it is hot, saturated, and/or near the atmospheric pressure boiling point, the use of pressure for the driving force may be required. Pressure filtration might also be used where the required cake moisture content is lower than that obtainable with vacuum. 

Vacuum filtration and screen-bowl centrifuges are the principal unit operations used for mechanical dewatering of fine coal. Pressure filtration (plate and frame filter presses and continuous belt filter presses) is used to a much lesser extent to dewater fine... 

In this situation, if the pressure filtration stays unchanged, the filtrate rate will decrease with time. When unacceptable values of the filtrate rate are reached, the process must be stopped and the membrane cleaned or replaced. This mode of operation is uneconomical. One solution to this problem is to increase the transmembrane pressure in order to maintain the flow rate but, in this case, the pumping flow rate has to be reduced because pumps generally present a pre-established and characteristic flow rate-pressure relation which is, a priori, unchangeable. Consequently, when the pressure is continuously increased, the clogging rate will increase faster than when a high tangential velocity is used in the unit. 

Figure 3.10 shows the details of the numerical-solving algorithm for the monodimensional. This numerical transposition has the capacity of being related with any available software. In Fig. 3.10, we can note that only the case of constant filtrate rate has been presented. Otherwise, when we operate at constant pressure, the filtrate rate decreases with the time due to the continuous clogging phenomenon. To simulate a constant pressure filtration, some changes in the computing program of Fig. 3.10 are necessary these modifications are shown in Fig. 3.11. It is easily observable that here the stop criterion has been completed with the decreasing of the solid concentration in the recycled suspension.

In liquid-solid systems, the impaction or interception mechanism is referred to as filtration. Filtration can be classified in several ways vacuum, pressure, belch, continuous, cake, or depth. Filtration is generally considered to be a separation involving passage of liquid through a porous medium that retains most of the panicalates contained in the liquid. The porous medium mey be a wire screen, cloth, paper, or a bed oF... 

Good filtration practice requires careful regulation of the pressure at which the carbon-liquid mixture is delivered to the filter. At the start, the pressure should be low enough to permit a proper flow of clear liquid from the filter. Then as the filtration continues and the filter cake becomes thicker, the pressure from the pump should be increased to maintain the rate of flow but such an increase in pressure should be very gradual. 

CONTINUOUS FILTRATION. In a continuous filter, say, of the rotary-drum type, the feed, filtrate, and cake move at steady constant rates. For any particular element of the filter surface, however, conditions are not steady but transient. Follow, for example, an element of the filter cloth from the moment it enters the pond of slurry until it is scraped clean once more. It is evident that the process consists of several steps in series—cake formation, washing, drying, and discharging—and that each step involves progressive and continual change in conditions. The pressure drop across the filter during cake formation is, however, held constant. Thus the foregoing equations for discontinuous constant-pressure filtration may, with some modification, be applied to continuous filters. 

Up to this point we have considered distributed dilute dispersions of colloidal size particles and macromolecules in continuous liquid media. Where the particles are uncharged and of finite size, they are always separated by a fluid layer irrespective of the nature of the hydrodynamic interactions that take place. In the absence of external body forces such as gravity or a centrifugal field or some type of pressure filtration process, the uncharged particles therefore remain essentially uniformly distributed throughout the solution sample. We have also considered the repulsive electrostatic forces that act between the dispersed particles in those instances where the particles are charged. These repulsive forces will tend to maintain the particles in a uniform distribution. The extent to which a dispersion remains uniformly distributed in the absence of applied external forces, such as those noted above, is described in colloid science by the term stability, whereas colloidal systems in which the dispersed material is virtually insoluble in the solvent are termed lyophobic colloids. 

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