Pore blocked filtration

The transition from pore-blocked filtration to more favorable cake filtration can therefore be achieved with a suspension of low settling particles by initially feeding it to the filter medium at a low rate for a time period sufficient to allow surface accumulation. This is essentially the practice that is performed with filter aids. 

Finally, for the case of intermediate filtration, the intensity of increase in total resistance with increasing filtrate volume is less than that occurring in the case of gradual pore blocking, but greater than that occurring with cake filtration. It may be assumed that the intensity of increase in total resistance is directly proportional to this resistance ... 

A major factor that remains to be the biggest challenge in optimizing the performance of membrane processes in the dairy industry is biofouling, which is caused by specific interactions between the membrane and various proteins, as well as protein-protein interactions that can lead to pore-narrowing or pore-blocking, during the filtration process [40,153,155]. 

Summarizing, the experimental results are consistent with cyclohexenyl hydroperoxide decomposition over CrAPO-5 occuring mainly via an intramolecular pathway. High conversions and selectivities were obtained in model decomposition experiments. Deactivation of the catalyst was observed and was probably due to the formation of pore-blocking oligomeric or polymeric compounds. However, the CrAPO-5 crystals could be recovered by filtration and regenerated at 500 °C, giving the same activity and selectivity for the decomposition of cyclohexenyl hydroperoxide over three cycles. 

The Complete Blocking Model (pore blocking) is valid for particles which have a very similar size to the pores. The particles seal the pores and do not accumulate on each other. The constant pressure filtration law can be written as... 

Pore blocking requires a initial low retention of the solute. While retention may increase due to cake formation, this increase in retention especially at early stages of filtration can indicate pore fouling. 

Filtration of colloids at pH extremes served as a baseline of colloids, which are neither aggregated nor stabilised by an adsorbed organic layer. As expected from the literature review, the particle size closest to the membrane pore size (250 nm) caused largest flux decline. Rejection occurred in this case due to a combination of hydrophobic, specific, and van der Waals forces, and could not be explained by charge interaction only. Blocking law analysis showed evidence of pore blocking, cake formation, adsorption and pore closure at some stage of the filtration process. 

As the membrane pores become smaller, the standard blocking law is no longer valid (Figure 6.55B). The cake filtration model remains valid, which indicates prevention of the ferric hydroxide precipitates from entering pores and subsequent internal deposition. Membrane pore radii were tabulated in Table 6.1 as 2,6, 4.8, and 9.1 nm for the 10, 30 and 100 kDa membranes, respectively. Lo and Waite (1998) reported iron hydroxide precipitates to be as small as 10 nm. Nevertheless, complete blocking (pore blocking by the particulates) is not a valid mechanism. Possibly cake formation prevents pore blockage. 

The membrane, even in the absence of any suspended material, will have a natural flow resistance that may be determined during a clean liquid flow test. During filtration suspended matter might become attached to the pore channel of the membrane thereby reducing the flow channel dimension, or pores may become blocked off altogedier. These last two effects lead to re ance terms that are due to adsorption and pore blocking. It is not usual to be able to quantify these two resistances when filtering suspensions... 

Two operating periods lead to a reduction in permeability. During the first period, filtration primarily proceeds with pore blocking on the outer surface of the filter aid. During the second period, the filter aid pores undergo blocking within the filter aid cake. 

Filtration with gradual pore blocking is most frequently encountered in industrial practice. This process is typically studied under the operating mode of constant rate. 

An increase in the suspension s particle concentration also enhances accumulation in "dead zones with subsequent bridging. Hence, both high particle settling velocity increases and higher solids concentrations create favorable conditions for cake filtration. In contrast, low settling velocity and concentration results in favorable conditions for gradual pore blocking. 

In terms of practical applications, fouling, which occurs via formation of a cake/gel layer on the membrane surface and pore blocking by colloidal particulates and adsorption of dissolved solutes, reduces permeate flux during filtration, and is an important problem to be solved. Modification of membrane surface with a helix geometry configuration, which promoted turbulence, was found to be effective in recovering permeate flux by a factor of 6, compared with non-modified membranes [48]. 

Deep-bed filtration (or depth filtration) is known by various terms like blocking filtration, surface filtration, and clarification. Deep-bed filtration (Rajagopalan and Tien, 1979 Stamatakis and Tien, 1993 Tien and Payatakes, 1979) is normally preferred in treating large quantities of liquids containing low solid concentration (less than 500mg/liter) with particles size less than 30 pm. In this operation, the particles to be removed are often substantially smaller than the pores of the filter medium and will penetrate a considerable depth... 

The MFI test uses the same apparatus as the SDI test however, the total volume collected is recorded every 30 seconds and the data is plotted as t/V vs V (where t = time in seconds, V = volume in liters). The test covers the initial pore blocking fouling mechanism, the cake filtration mechanism, and the longer term cake blocking/cake compression mechanism (See Figure 3.9). The minimum slope (or linear part) of the curve in Figure 3.9, which corresponds to the cake filtration portion of the curve, is the MFI expressed in seconds per square liter (s/U). 

In this chapter, all mechanisms that remove polymer from the transported aqueous phase are referred to collectively as retention . A distinction will be made between individual mechanisms—including polymer adsorption, mechanical entrapment and hydrodynamic retention—in the course of this chapter. It is noted that mechanical entrapment is a filtration-like mechanism in which the larger polymer species are thought to be strained out in the smaller pores. Thus, because of the nature of filtration and the resulting pore blocking that must occur, this is not a mechanism that can persist throughout a reservoir formation. In a polymer solution, free from debris (from the... 

It has been stated that a filter medium is a porous (or at the very least semi-permeable) barrier placed across the flow of a suspension to hold back some or all of the suspended material. If this barrier were to be very thin compared with the diameter of the smallest particle to be filtered (and perforated with even sized holes), then all the filtration would take place on the upstream surface of the medium. Any particle smaller than the pore diameter would be swept through the pores, and any particle larger than that (assuming the particles to be rigid) would remain on the upstream surface. Some of the larger particles, however, would be of a size to settle into the individual pores and block them. The medium surface would gradually fill with pores blocked in this way, until the fluid flow reduced to below an acceptable level. At this point filtration would be stopped and the medium surface would be brushed or scraped clean (although many automatic filters have their surface continuously brushed or scraped). 

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