Reverse filtration flow

The cake may creep down along the partition due to gravity this is almost inevitable in the absence of a pressure gradient across the filtering partition. The vertical filtering partition makes these filters especially useful as thickeners, since it is convenient to remove cake by reverse filtrate flow. 

In practice, cake filtration is used more often than filter-medium filtration. Upon achieving a certain thickness, the cake must be removed from the medium. This can be accomplished by the use of various mechanical devices or by reversing the flow of filtrate back through the medium (hence, the name baclflushing). 

Tangential crossflow filtration Process where the feed stream sweeps the membrane surface and the particulate debris is expelled, thus extending filter life. The filtrate flows through the membrane. Most commonly used in the separation of high-and-low-molecular weight matter such as in ultrapure reverse osmosis, ultrafiltration, and submicron microfiltration processes. 

The oscillated backflushing mechanism accounts for the effect of the pressure waveform on TMP. When the minimum pressure of the pulsatile pressure waveform results in a negative TMP, a reverse permeate flow may occur that acts as a backwash flow that drives particles deposited on or near the membrane surface back to the bulk flow. In this way, concentration polarization and cake formation caused by the filtration operation could be limited, depending on the properties of the particles and the magnitude of the reverse pressure. 

Direct flow filtration has certain Umitations. The flux (filtration flow rate per unit membrane area) decreases over time as the process continues because the filtering media is loaded with more contaminant particles, as illustrated in Figure 14.1. Moreover, when the concentration of the contaminant in the feed stream is high, the filtering media must be replaced very frequently, which can be economically impractical. Also when the contaminant matter to be separated is small in size, requiring ultrafiltration or reverse osmosis membranes with much smaller pores, then direct filtration is less feasible as the flux declines very rapidly over time, again requiring frequent filter replacement. 

The most common process used for this is depth filtration through a bed of sand or similar material charged in a vertical vessel. The incoming water flows from top to bottom. To improve the efficiency of such filters, two or more layers of media with various particle sizes are used. Coarse and less dense material such as anthracite is located at the top of the bed, whereas finer and denser particles of sand are placed at the bottom. Such multimedia filters can remove most particles larger than 10-20 pm. Periodically the filter bed is back-washed by reversing the flow direction (from bottom to top) and by increasing the flow rate. During backwash, the captured particles are removed and sent to drain, whereas heavier particles of the filter bed remain in the vessel and settle back at the end of the cycle. 

In Figure 22.39, idealized deposition of cakes in a press is shown. Slurry enters the frame and deposits cakes on plates on either side as illustrated. The cakes, A and B, build out from the plates and meet at the center when the press is full. The cake surfaces meet at the dotted centerline, and a porosity vs. distance curve is developed as shown. Porosity varies from a maximum Sq at the unconsolidated cake surface to its minimum value Sj at the medium. In the washing mode for presses, liquid flows in one plate, passes through both cakes, and exits from the opposing plate. It traverses cake A in reverse flow with respect to the filtrate. The reversal in flow direction brings about a marked change in the frictional stresses and causes the cake to compact. 

The cross-flow filtration is often used in combination with the procedure known as backflushing, whereby the filtration flow is reversed for a short period of time so that particles or molecules very attached are removed from the membrane surface. The speed of cross-flow filtration, transmanbrane pressure, and the frequency of backflushing are important process parameters, which are optimized to obtain low fouling, high permeate flux, and low energy costs. In many such... 

Control of the back-pressure level required for efllbctive back-flushing has bear claimed with the use of flat perforated plate elements in the Cricket filter [Filtration and Separation, Sep/Oct., 1991]. Automatic, semicontinuous cleaning of tubular candle filters can also be realised by momentary reversal of the filtrate flow a resulting bump is defivered to the cake, which is dModged and Ds to the cone-shaped bottom of the unit, Bgure 11.53. The effectiveness of these filters is inproved by the use of filter media whkh can handle veiy dilute suspensions of fine (< 0.5 pm) particles, e.g. PIPE membranes). 

A number of these leaves are hung in parallel in a closed tank. The slurry enters the tank and is forced under pressure through the filter cloth, where the cake deposits on the outside of the leaf. The filtrate flows inside the hollow framework and out a header. The wash liquid follows the same path as the slurry. Hence, the washing is more efficient than the through washing in plate-and-frame filter presses. To remove the cake, the shell is opened. Sometimes air is blown in the reverse direction into the leaves to help in dislodging the cake. If the solids are not wanted, water jets can be used to simply wash away the cakes without opening the filter. 

Bickert, G., Stahl, W. and Bartsch, R., Wet classifying in the superfine range by reverse current flow centrifugal elutriation , Filtech Conference, Karlsruhe, 19-21 October, 1993, paper published in the Proceedings, Vol. 2, by the Filtration Society, Horsham, 53-61 (1993)... 

Sanjeev, G. R., and Davis, R. H. (2004). Cross-flow microfiltration with high flequency reverse filtration. AIChEJ. 41(3), 501-508. 

The most common form of cleaning is to reverse the flow of fluid through the filter mediiun, and so to blow the surface clear of coUecled contaminants. As this requires filtration to stop, a satisfactory system employs at least two filters, side by side in a duplex arrangement, with the piping and valves around the filters such that the main flow can be switched from one filter to the other when full of dirt, after which the dirty filter is backflushed to wash all of the collected contaminants off the filter element. Figure 3.59 shows such an arrangement, but with three elements. 

The inhaent flux decUne in many membrane filtration processes requires procedures to reverse the decline (for the reasons discussed above). These membrane cleaning opaa-tions can involve back-fiushing (reversal of flow direction) at... 

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