Constant-pressure filtration results

It is advisable to start a constant-pressure filtration test, like a comparable plant operation, at a low pressure, and smoothly increase the pressure to the desired operating level. In such cases, time and filtrate-quantity data shoulci not be taken until the constant operating pressure is reahzed. The value of r calculated from the extrapolated intercept then reflec ts the resistance of both the filter medium and that part of the cake deposited during the pressure-buildup period. When only the total mass of diy cake is measured for the tot cycle time, as is usually true in vacuum leaf tests, at least three runs of different lengths should be made to permit a rehable plot of 0/V against W. If rectification of the resulting three points is dubious, additional runs should be made. 

An example involving the addition of a perlite filter aid to a clay suspension with a volumetric concentration of q)j = 0.058 wUl be discussed. Constant rate filtration (Camacho 1975) with different fractions of perlite was used to obtain cake resistance and cake solidosity as functions of Ap. The specific problem to be analyzed is a constant pressure filtration at 200 kPa (29 psi) of this clay suspension. For constant pressure and constant rate filtration with minimal medium resistance, maximum cycle flux is reached when the filtration time equals the time required to dump the cake, clean the filter, and start the next cycle (Rushton et al. 1996). hi this example, it is assumed that the dead time between cycles is 15 minutes. With t (filtration) = 15 minutes, a cake thickness of L = 0.014 m results when the cycle rate is maximized for this highly resistant, moderately compactible clay. 

The results of a constant-pressure filtration test are shown in a table of volume of filtrate per unit area of filter versus time. The data are presumed to agree with Eq. 12.33. Show how to plot these data so that they Will form a straight-line plot and how to find the values of k and a from this line. 

Graph of results from a constant pressure filtration test. 

When the space above the suspension is subjected to compressed gas or the space under the filter plate is under a vacuum, filtration proceeds under a constant pressure differential (the pressure in the receivers is constant). The rate of filtration decreases due to an increase in the cake thickness and, consequently, flow resistance. A similar filtration process results from a pressure difference due to the hydrostatic pressure of a suspension layer of constant thickness located over the filter medium. 

Constant-pressure drop filtration can result in saturation or blockage of the filter medium. The network of pores within the filter medium can become blocked because of one or a combination of the following situations ... 

Results from constant differential pressure filtration tests have been analyzed according to traditional filtration science techniques with some modifications to account for the cross-flow filter arrangement.11 Resistivity of the filter medium may vary over time due to the infiltration of the ultrafine catalyst particles within the media matrix. Flow resistance through the filter cake can be measured and correlated to changes in the activation procedure and to the chemical and physical properties of the catalyst particles. The clean medium permeability must be determined before the slurries are filtered. The general filtration equation or the Darcy equation for the clean medium is defined as... 

Filtration is carried out in a plate and frame filter press, with 20 frames 0.3 m square and 50 mm thick, and the rate of filtration is maintained constant for the first 300 s. During this period, the pressure is raised to 350 kN/m2, and one-quarter of the total filtrate per cycle is obtained. At the end of the constant rate period, filtration is continued at a constant pressure of 350 kN/m2 for a further 1800 s, after which the frames are full. The total volume of filtrate per cycle is 0.7 m3 and dismantling and refitting of the press takes 500 s. It is decided to use a rotary drum filter, 1.5 m long and 2.2 m in diameter, in place of the filter press. Assuming that the resistance of the cloth is the same in the two plants and that the filter cake is incompressible, calculate the speed of rotation of the drum which will result in the same overall rate of filtration as was obtained with the filter press. The filtration in the rotary filter is carried out at a constant pressure difference of 70 kN/m2, and the filter operates with 25 per cent of the drum submerged in the slurry at any instant. 

It is decided to use a rotary drum filter, 1.5 m long and 2.2 m in diameter, in place of the filter press. Assuming that the resistance of the cloth is the same in the two plants and that the filter cake is incompressible, calculate the speed of rotation of the drum which will result in the same overall rate of filtration as was obtained with the filter press. The filtration in the rotary filter is carried out at a constant pressure difference of 70 kN/m2, and the filter operates with 25 per cent of the drum submerged in the slurry at any instant. 

From Fig. 3.15 we can note that, by analogy to the 2 bar constant pressure case, example FIS shows a new special case where we have positive and small negative values in the concentrated suspension flow rate at the plant exit. This result can be explained by the background noise in the measurement of the flow of suspension. Nevertheless, the mean value of the flow rate is small but positive. If the efficiency of the filtration at constant pressure is given by the solid concentration ratio between the exit and fresh suspensions, then, as shown in Fig. 3.13, the ratio is always lower than 2 for operation case F2/1.6. For cases FI and FIS, this ratio increases permanently, non-uniformly and attains values over 12 g/1 in the proximity of the complete clogging state. 

When DE is used as an admix to produce a less resistant cake, selection of the type and amounts of filter aids depends on laboratory tests with a constant pressure filter cell. A graph illustrating the effect of filter aid addition on the average flow rate is shown in Figure 22.15 (Tiller 1978). Overdose of Alter aid resulted in a decrease of filtrate rate. 

Example 30.2. Laboratory filtrations conducted at constant pressure drop on a slurry of CaCOj in H2O gave the data shown in Table 30.2. The filter area was 440 cm, the mass of solid per unit volume of filtrate was 23.5 g/L, and the temperature was 25°C. Evaluate the quantities a and as a function of pressure drop, and fit an empirical equation to the results for a. 

The resistance of the cake of accumulated particulates, Rc, is more complicated it is a variable which increases as filtration proceeds resulting in a progressively lower filtration rate at constant pressure. This is due to the continually increasing thickness of the cake and its compaction under the pressurized conditions of filtration. If Rm is defined as above, Rc must also include the effect of pore plugging within the membrane. 

Another point of discussion in the literature is how the filtration is carried out by application of constant pressure or by using a constant rate. For silica-based particles, good results have been reported with both techniques. Thus the filtration technique can be selected freely depending on the available equiimienL It should be pointed out, thou that the filtration rate should not exce the strength of the partides as discussed in Chapter 4. 

Membrane fouling may result in a significant increase in filtration resistance, leading to unstable filtration behavior. The pressure-driven membrane processes can be operated either with constant feed pressure or in constant flux mode. For constant pressure operation where the transmembrane pressure (TMP) is maintained at a constant value during the filtration, the flux will decline with time due to the... 

We used two modes of filtration in separate experiments constant pressure and constant displacement. The results of these are shown in figure 1. The data is plotted as ln(P/J) vs. ln(X) to permit comparison of the two data sets. The two curves for the 0.125M sample are very similar to each other indicating that the pressure is not important for these samples. The various salt concentrations showed that pressing rate Increased with increasing salt content. 

For an initial determination of filtration performance the procedures described in Steps 2-9 are adequate. If data are required for filter sizing and simulation, then Steps 2-9 need to be repeated at a range of different constant pressures/vacua to establish any variation of cake resistance and solids concentration and thus cake compressibility (see also Section 4.7). It is likely that more sophisticated equipment, such as that described in Section 4.6, will give more reliable results. 

A sequence of constant pressure piston press experiments have been performed over the pressure range Ap = 0.33 -> 20.56 MPa using samples of a mineral suspension. Each experiment comprised a filtration phase followed by a consolidation phase and the results of individual analyses are shown in Table 4.6. Determine the scale-up coefficients that characterise the suspension. 

The particle size of powders can be measured with laser diffraction. In Table 3 the particle size distribution is given for 2 steam activated carbons and a chemical activated carbon. The steam activated carbons differ in particle size distribution. The SX IG contains larger particles and a smaller particle size distribution than the SX 1 caused by the absence of very small and very large particles. The DARCO KBB contains many small particles. The particle size distribution determines the filterability. Filterability can be measured in tests where in the filtration time at constant pressure is measured in a slurry of activated carbon and water. The results are also given in Table 3. The results in Table 3 show the much better filterability... 

If the suspension were a clean liquid, all the parameters in equations 9.1 and 9.2 would be constant, resulting in a constant flow rate for a constant pressure drop and the cumulative filtrate volume would increase linearly with time, as shown in Figure 9.4. 

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