Cross filtrate flux

A continuous cross-flow filtration process has been utilized to investigate the effectiveness in the separation of nano sized (3-5 nm) iron-based catalyst particles from simulated Fischer-Tropsch (FT) catalyst/wax slurry in a pilot-scale slurry bubble column reactor (SBCR). A prototype stainless steel cross-flow filtration module (nominal pore opening of 0.1 pm) was used. A series of cross-flow filtration experiments were initiated to study the effect of mono-olefins and aliphatic alcohol on the filtration flux and membrane performance. 1-hexadecene and 1-dodecanol were doped into activated iron catalyst slurry (with Polywax 500 and 655 as simulated FT wax) to evaluate the effect of their presence on filtration performance. The 1-hexadecene concentrations were varied from 5 to 25 wt% and 1-dodecanol concentrations were varied from 6 to 17 wt% to simulate a range of FT reactor slurries reported in literature. The addition of 1-dodecanol was found to decrease the permeation rate, while the addition of 1-hexadecene was found to have an insignificant or no effect on the permeation rate. 

The objective of the present study is to develop a cross-flow filtration module operated under low transmembrane pressure drop that can result in high permeate flux, and also to demonstrate the efficient use of such a module to continuously separate wax from ultrafine iron catalyst particles from simulated FTS catalyst/ wax slurry products from an SBCR pilot plant unit. An important goal of this research was to monitor and record cross-flow flux measurements over a longterm time-on-stream (TOS) period (500+ h). Two types (active and passive) of permeate flux maintenance procedures were developed and tested during this study. Depending on the efficiency of different flux maintenance or filter media cleaning procedures employed over the long-term test to stabilize the flux over time, the most efficient procedure can be selected for further development and cost optimization. The effect of mono-olefins and aliphatic alcohols on permeate flux and on the efficiency of the filter membrane for catalyst/wax separation was also studied. 

Albumin solutions (1, 2, and 5wt%) are continuously ultrafiltered through a flat plate filter with a channel height of 2 mm. Under cross-flow filtration with a transmembrane pressure of 0.5 MPa, steady-state filtrate fluxes (cm min ) are obtained as given in Table P9.2. 

As stated in Chapter 9, cross-flow filtration (CFF) provides a higher efficiency than dead-end filtration, as some of particles retained on the membrane surface are swept off by the liquid flowing parallel to the surface. As shown by a solid line in Figure 14.6 [3], filtrate flux decreases with time from the start of filtration due to an accumulation of filtered particles on the membrane surface, as in the case of dead-end filtration. The flux then reaches an almost constant value, where... 

Figure 14.6 Cross-flow filtration flux of baker s yeast suspension. Cell concentration 7%. Transmembrane pressure 0.49 bar. Flow rate 0.5 ms". ...

Two process modes, namely, dead-end and cross-flow modes, are widely used for microfiltration (14). For the dead-end mode, the entire solution is forced through the membrane. The substances to be separated are deposited on the membrane, which increases the hydraulic resistance of the deposit. The membrane needs to be renewed as soon as the filtrate flux no longer reaches the required minimum values at the maximum operation pressure. This mode is mostly used for slightly contaminated solutions, e.g., production of ultra-pure water. For the cross-flow mode, the solution flows across the membrane surface at a rate between 0.5 and 5.0 m/s, which prevents the formation of a cover layer on the membrane surface. A circulation pump produces the cross-flow velocity or the shear force needed to control the thickness of the cover layer. The system is most widely used for periodic back flushing, where part of the filtrate is forced in the opposite direction at certain intervals, and breaks up the cover layer. The normal operating pressure for this mode is 1-2 bars. 

Depending on the size of cells and debris, and the desired clarity of the filtrate, microfiltration membranes with pore sizes ranging from 0.01 to 10 pm can be used. In cross-flow filtration (CFF see Figure 9.2b), the liquid flows parallel to the membrane surface, and so provides a higher filtration flux than does dead-end filtration (Figure 9.2a), where the liquid path is solely through the membrane. In CFF, a lesser amount of the retained species will accumulate on the membrane surface, as some of retained species is swept from the membrane surface by the... 

Figure 2.33 Batch cross-flow membrane filtration flux vs. concentration curve for a protein feed solution for various UF and MF membranes. Amicon polysulphone PM-10 and PM-30 membranes with MWCO of 10,000 and 30,000 Da. The "gel layer" concentration is between 30 and 40%.

Figure 2.34 Cross-flow membrane filtration flux vs. time curve for the system in Figure 2.33.

We observed earlier in dead-end cake filtration (equation (6.3.135k)) that Rcs varies inversely with the square of the particle radius therefore, in effect, the filtration flux varies with the square of the particle radius for cake dominated filtration. The larger the particle radius, the higher the filtration flux. As shown in equation (7.2.145), in cross-flow microfiltration also the averaged filtration flux increases with particle radius, here as Romero and... 

A. Zydney and C. Colton. A concentration polarization model for filtrate flux in cross-flow microfiltration of particulate suspension. Chem. Eng. Commun., 47 1, 1986. 

Effects of Membrane Filtration Method on Membrane Filtration Flux Figure 5.12 shows the changes in ozone dosage, dissolved ozone concentration of MF permeate, and membrane filtration fluxes when ozone was injected with an ejector and water was filtrated with cross-flow (Run 11) and dead-end (Run 12) methods. Membrane filtration flux for the dead-end method was higher than that of the cross-flow method, and high membrane filtration flux operation (about 6 m /m /day) was achieved with 3 mg/L ozone dosage. 

Theory Cross-flow-elecfrofiltration can theoretically be treated as if it were cross-flow filtration with superimposed electrical effects. These electrical effects include elecfroosmosis in the filter medium and cake and elecfrophoresis of the particles in the shiny. The addition of the applied electric field can, nowever, result in some quahta-tive differences in permeate-flux-parameter dependences. 

The resistances, when incorporated into equations descriptive of cross-flow filtration, yield the general expression for the permeate flux for particulate suspensions in cross-flow-electrofiltration systems. 

Cross-flow filtration systems utilize high liquid axial velocities to generate shear at the liquid-membrane interface. Shear is necessary to maintain acceptable permeate fluxes, especially with concentrated catalyst slurries. The degree of catalyst deposition on the filter membrane or membrane fouling is a function of the shear stress at the surface and particle convection with the permeate flow.16 Membrane surface fouling also depends on many application-specific variables, such as particle size in the retentate, viscosity of the permeate, axial velocity, and the transmembrane pressure. All of these variables can influence the degree of deposition of particles within the filter membrane, and thus decrease the effective pore size of the membrane. 

In order to develop a continuous flux maintenance procedure, the present study examined the transmembrane flux values from the cross-flow filtration module with a filtration media area of 0.0198 m2 (0.213 ft2), a slurry density of approximately 0.69 g/cm3 at 200°C, 17 kg of simulated FT wax with a catalyst loading of 0.26 wt%, and a TMP between 0.68 and 1.72 bar (10-25 psig). The filtration process was run in a recycle mode, whereas clean permeate was added back to the slurry mixture, thus allowing the catalyst concentration to remain approximately constant over the course of the run (given minor adjustments for about 5 ml permeate and slurry samples collected throughout the test). 

Ideally, cross-flow microfiltration would be the pressure-driven removal of the process liquid through a porous medium without the deposition of particulate material. The flux decrease occurring during cross-flow microfiltration shows that this is not the case. If the decrease is due to particle deposition resulting from incomplete removal by the cross-flow liquid, then a description analogous to that of generalised cake filtration theory, discussed in Chapter 7, should apply. Equation 8.2 may then be written as ... 

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