Filtrate resistance

Mathews and Rawlings (1998) successfully applied model-based control using solids hold-up and liquid density measurements to control the filtrability of a photochemical product. Togkalidou etal. (2001) report results of a factorial design approach to investigate relative effects of operating conditions on the filtration resistance of slurry produced in a semi-continuous batch crystallizer using various empirical chemometric methods. This method is proposed as an alternative approach to the development of first principle mathematical models of crystallization for application to non-ideal crystals shapes such as needles found in many pharmaceutical crystals. 

The method described here provides a convenient means of determining the specific filtration resistance of fairly dilute suspensions. Results for clay suspensions flocculated by cationic polymers show that the specific resistance gives a sensitive indication of flocculation and is a useful guide in the selection of optimum flocculant concentrations. In a series of trials not reported here, it has been shown that the specific resistance results are very well matched by re-filtration rate data, as expected. The results also agree well with other, unrelated techniques. For more concentrated suspensions, some discrepancies have been found between permeability methods and other measures of flocculation (4). 

Kim HA, Choi JH, Takizawa S (2007) Comparison of initial filtration resistance by pretreatment processes in the nanofiltration for drinking water treatment. Separ Purif Technol 56 354-362... 

The rates of filtration of microbes (particles) at a constant pressure difference decrease with time due to an accumulation of filtered microbes on the surface and inside the pores of the membrane. Hence, in order to maintain a constant filtration rate, the pressure difference across the membrane should be increased with time due to the increasing filtration resistance. Such data as are required for practical operation can be obtained with fluids containing microbes with the use of real filter units. 

Thus the filtration resistivity of the medium includes its thickness. Typical measured values of Rf are of the order of 10lom-1 for comparison, the fine filter sheet of Table 1.6, assuming it to be 1 mm thick, has LIKP =0.001/0.15(1(T12) = 0.7(1010) m1. 

As a filtration process proceeds, generally under an applied driving force of pressure, solids are removed by and begin to accumulate on the filter medium. The liquid portion continues to move through the filter medium and out of the filter system. The separated liquid is referred to as the filtrate. The amount of pressure applied to accomplish the filtration depends on the filtration resistance. Filtration resistance is a result of the frictional drag on the filtrate as it passes through the filter medium and the accumulated solids. In equation form,... 

Permeability is often referred to as a measure of liquid flow through a filter system and is the reciprocal of the filtration resistance. 

During filtration, as the particulate buildup continues on the filtration medium, the filtration resistance increases, or in other words, the filtration permeability decreases. The capacity of a system, expressed in time, volume of liquid fed, or amount of solids fed, depends on the ability of the system to maintain acceptable permeability. 

Plot p/ifJ-q) and p/(pqav) versus wc. These plots are shown in Fig. 14.3. They enable the calculation of the medium resistance Rm and the specific filtration resistance am because Eq. (14.4) can be rearranged into the form p/(pq) = amwc + Rm, and there also can be written a similar expression (valid only when aav and Rm are constant) p/(/xqm) = (am/2)wc + Rm. Both equations have the same y intercept, namely, Rm. The slope of the first is twice that of the second. It is advisable to plot both lines in order to reach a compromise on the slopes and intercept. 

It is important to take enough v and t data to generate the initial curved portions of the plots. The specific filtration resistance is smaller at the start of filtration, and the slopes of both plots have their minimum value when t and wc equal zero. If the first four points taken during the first 50 s were... 

Related Calculations. In constant-rate (as opposed to constant-pressure) filtration, v = qt and wc = cqt. Then, from Eq. (14.3), the average specific filtration resistance aav = (p — p )/(ncq2t), where Pi is the pressure at the interface of the filter medium and the cake. Constant-rate filtrations are usually operated at above 10 lbf/in2 (69 kPa), and this equation is accurate enough for most purposes. At... 

The pump has a throttling valve that relates pressure drop to flow rate Q as follows Ap (throttling) = 15(g/500)2. The temperature varies from 20 to 27°C (68 to 80.6°F). A series of constant-pressure tests yielded the data on aav and (1 — eav) shown in Fig. 14.4, aav being the average specific filtration resistance and eav the average porosity of the cake. Find cake thickness as a function of time. 

Kimura, K., Y. Watanabe, and N. Ohkuma (1998). Filtration resistance induced by ammonia oxidizers accumulating on the rotating membrane disk. Water Science Techno. Wastewater Industrial WastewaterTreatment., Proc. 199819th Biennial Conf. Int Assoc, on Water Quality. Part 4, June 21-26, Vancouver, Canada, 38,4—5,443 52. Elsevier Science Ltd., Exeter, England. 

The waveform-type 3, which had a longer negative pressure period than waveform-type 1, resulted in more than 90% reduction in filtration resistance with a minimum TMP of about 40 kPa, implying the effect of duration of the backflushing. Waveform-types 3 and 4 were long and short sinusoidal forms, providing a continuous variation of pressure. There seemed to be less effective reduction in cake resistance with the shorter negative pressure period (waveform-type 4). 

The solids distribution profile may tend to be parabolic with thicker cakes near the bottom of the basket, tapering down toward the top, since the G-field is perpendicular to the force of gravity. This is especially true with fast-sedimenting solids that will settle toward the basket bottom before the slurry is fully accelerated by the basket. The coarser solids can settle toward the basket bottom, while the finer solids deposit preferentially toward the top. This can result in uneven filtration resistance in the cake, affecting the wash pattern and efficiency of the wash. In cases where this is a concern, a rotating feed cone may be better for even distribution, or a horizontal peeler centrifuge may be better suited to the application. 

Needle-like and plate-like crystals create additional process complications. For example, these crystals generally have higher filtration resistance and poorer solid flow characteristics for formulation than cube-1 ike crystals. Therefore, it is highly desirable to grow thicker crystals. To grow thick crystals, experimentally, we should try to find the best solvent which favors the formation of such crystals. Meanwhile, solvates and hydrates may form in different solvent environments. Chemical forms, such as salt, free base, and free acid, can also be evaluated. Also, control of release of supersaturation and selection of crystallization conditions to enhance crystal growth over nucleation, which are addressed in the later chapters, would be very helpful. 

Results from the factorial study indicate that the primary input variables affecting filtration resistance were seed type and mixing intensity. When slurry seed was used, insufficient slurry seed led to... 

For the output variables, as shown in Figs. 9-15 and 9-16, the filtration time data correlated well with mean particle size (chord length) and the level of fines by Lasentec FBRM measurement and optical micrographs. This correlation enabled direct feedback of process performance (cake filtration resistance) based upon FBRM measurement. 

The poor crystalline structure of this compound prevents good growth on seed even at low supersaturation. Mixing intensity is found to be a major variable in determining filterability. The slower the mixing, the faster the filtration time and the lower the cake filtration resistance. 

FIGURE 22.59-A Wei-Ming Lu, Frank M. Tiller, et al, A New Method to Determine Local Porosity and Filtration Resistance of Filter Cakes, Journal of Chinese Institute of Chemical Engineers, vol. 1, 1970, Journal of Chinese Institute of Chemical Engineers, p. 50, Figure 8, vol. 1, 1970. 

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