Cross-flow filters

The design of a cross-flow filter system employs an inertial filter principle that allows the permeate or filtrate to flow radially through the porous media at a relatively low face velocity compared to that of the mainstream slurry flow in the axial direction, as shown schematically in Figure 15.1.9 Particles entrained in the high-velocity axial flow field are prevented from entering the porous media by the ballistic effect of particle inertia. It has been suggested that submicron particles penetrate the filter medium and form a dynamic membrane or submicron layer, as shown in... 

FIGURE 15.1 Cross-sectional view of a cross-flow filter element. 

To continuously separate FT wax products from ultrafine iron catalyst particles in an SBCR employed for FTS, a modified cross-flow filtration technique can be developed using the cross-flow filter element placed in a down-comer slurry recirculation line of the SBCR. Counter to the traditional cross-flow filtration technique described earlier, this system would use a bulk slurry flow rate below the critical velocity, thereby forcing a filter cake of solids to form between the filter media and the bulk slurry flow, as depicted in Figure 15.2b. In this mode, multiple layers of catalyst particles that deposit upon the filter medium would act as a prefilter layer.10 Both the inertial and filter cake mechanisms can be effective however, the latter can be unstable if the filter cake depth is allowed to grow indefinitely. In the context of the SBCR operation, the filter cake could potentially occlude the slurry recirculation flow path if allowed to grow uncontrollably. 

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... 

It is anticipated that the equilibrium filter cake mass would depend strongly on the axial velocity through the cross-flow filter assembly. The shear rate at the filter surface will increase the entrainment of the catalyst solids for a given permeate flow rate. Therefore, for each differential pressure condition, the axial velocity will be varied in order to quantify the effect of the wall shear on the filter cake resistance term. 

The cross-flow filtration test unit is depicted schematically in Figure 15.4 it allows several types of cross-flow filter media to be studied under simulated FTS conditions. The filtration piping and instrumentation are heated via several circuits... 

As presented in Figure 15.10, after 100 h TOS the permeate valve was closed, and thus the transmembrane pressure fell to zero. The pilot plant remained in a standby mode and unmanned for approximately 75 h. During this period, the catalyst slurry was circulated through the cross-flow filter, without permeate flow radially through the filter membrane (i.e., test conditions were constant with the... 

Application of cross-flow filtration for the removal of FT wax products can be a useful technique to maintain a constant catalyst loading in an FTS reactor in continuous operation. Addition of 1-dodecanol (at a concentration of 6 wt%) was found to decrease the permeation rate of the cross-flow filter used for the separation of simulated FT wax and activated iron catalyst slurry. However, additional... 

Tars have a tendency to cling to the filter surface and can undergo subsequent carbonization reactions that lead to fouling and plugging. Even in the absence of further decomposition, tars are difficult to remove from these materials. Examples of barrier filters suitable for biomass systems include rigid, porous-candle, or cross-flow filters constructed of metal or ceramic bag filters constructed of woven material, and packed-bed filters. 

Cross-flow filters behave in a way similar to that normally observed in crossflow filtration under ambient conditions increased shear-rates and reduced fluid-viscosity result in an increased filtrate number. Cross-microfiltration has been applied to the separation of precipitated salts as solids, giving particle-separation efficiencies typically exceeding 99.9%. Goemans et al. [30] studied sodium nitrate separation from supercritical water. Under the conditions of the study, sodium nitrate was present as the molten salt and was capable of crossing the filter. Separation efficiencies were obtained that varied with temperature, since the solubility decreases as the temperature increases, ranging between 40% and 85%, for 400 °C and 470°C, respectively. These workers explained the separation mechanism as a consequence of a distinct permeability of the filtering medium towards the supercritical solution, as opposed to the molten salt, based on their clearly distinct viscosities. 

The cellular productivity in a CSTF increases with an increase in the dilution rate and reaches a maximum value. If the dilution rate is increased beyond the maximum point, the productivity will be decreased abruptly and the cells will start to be washed out because the rate of cell generation is less than that of cell loss from the outlet stream. Therefore, the productivity of the fermenter is limited due to the loss of cells with the outlet stream. One way to improve the reactor productivity is to recycle the cell by separating the cells from the product stream using a cross-flow filter unit (Figure 6.19). 

Cross-flow filter performance is often characterized by a flux rate, which equates to the permeate flow rate per unit area of membrane surface. The flux rate in most biological separations is reduced by a fouling phenomenon called gel polarization, which tends to concentrate material at the surface of membrane to impose an additional resistance to transmembrane flow. The deterioration in flux rate must be well characterized for a commercial bioseparation process to ensure the correct size for the cross-flow filtration unit and avoid hold-ups at this processing stage. 

Cross-flow filters can be operated in three different modes according to the nature of the product to be recovered and the stage in the processing. 

Ralf Kuriyel (Millipore Corporation) addressed some of the issues related to the use of Dean vortices, formed during the flow of fluids in curved conduits, to enhance the performance of cross-flow filters by increasing the back transport of solutes. Results were presented on coiled hollow fibers with a varying radius of curvature, fiber diameter, and solution viscosity, to characterize the relationship between the back transport of solutes and hydrodynamic parameters. A performance parameter relating back transport to the Dean number and shear rate was derived, and a simple scaling methodology was developed in terms of the performance parameter. The use of Dean vortices may result in membrane systems with less fouling and improved performance. 

Ceramic cross-flow filters Ceramic cordierite monoliths... 

In filtration unit operation, especially in microfiltration, one usually differentiates between dead-end filtration (with cake formation) and cross-flow filtration [25] (Fig. 5). The cross-flow filter can have different geometries (Fig. 6) phase membranes, tubular membranes, or pleated membranes, of which the tubular and pleated ones are already accepted as cross-flow geometries in reactor technology, as mentioned above. In filtration engineering the cross-flow term means that the filtrate flows perpendicularly to the suspension stream. Cross-flow may not be considered a sufficiently illustrative term here [25]. A better term would be parallel filtration, but the term cross-flow filtration has been accepted generally and may be difficult to change at present. 

Figure 6 Different geometries of cross-flow filter membranes. (From Ref. 25.)...

MF (cross-flow filters, 0.2 pm) and RO (high salt rejection spirally wound elements)... 

In the second stage, the suspension is concentrated in a thickener. Although gravity thickeners dominate the field, cross-flow filter thickeners, hydrocyclones, and electrophoretic devices can be also used. A large fraction of the liquid can be removed economically in thickeners, thereby leading to smaller units in the third stage. 

In any separation process, three streams are involved as shown in Fig. 7 for a filter with cake formation, a cross flow filter without cake formation, and a thickener or clarifier based upon gravitational sedimentation. The filter is assumed to have a fixed volume into which slurry is fed and cake is formed as filtrate flows through the filter medium. The fraction of filtrate or overflow, compared to the liquid in the slurry, is called the fractional recovery of liquid, which can be obtained based upon material balance. The fractional recovery for cake filter, cross-flow filter, and thicker or clarifier are as follows ... 

There are basically two types of cross-flow filter. One is without rotating element in which slurry is usually pumped into the filter in the direction parallel to the filter media to produce a cross-flow. Another type of cross-flow filter is equipped with rotating elements, such as rotary filter press with agitators or turbines attached to a rotary shaft as shown in Fig. 22. ... 

Problem 3-14. Design of a Cross-Flow Filter. In cross-flow filtration, a pressure drop G forces fluid containing neutrally buoyant particles to flow between two porous plates. There is also a transverse flow that forces the particles to collect on one of the plates. A key design question is how one determines the length of the filter. 

The performance of a cross-flow filter is primarily defined by its efficiency in permeating or retaining desired species and the rate of transport of desired species across the membrane barrier. Microscopic features of the membranes greatly influence the filtration and separation performance.f ... 

From an operational standpoint, the mechanical, thermal and chemical stability ofthe membrane structure is important to ensure long service life and reliability. Table 4 summarizes the influence and significance of these features on the overall performance of a cross-flow filter. 

Polymeric membranes are prepared from a variety of materials using several different production techniques. Table 5 summarizes a partial list of the various polymer materials used in the manufacture of cross-flow filters for both MF and UF applications. For microfiltration applications, typically symmetric membranes are used. Examples include polyethylene, polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) membrane. These can be produced by stretching, molding and sintering finegrained and partially crystalline polymers. Polyester and polycarbonate membranes are made using irradiation and etching processes and polymers such as polypropylene, polyamide, cellulose acetate and polysulfone membranes are produced by the phase inversion process.f Jf f ... 

Table 6. Polymeric Cross-flow Filters Module Geometries... 

---