Scaling cross-flow filtration

Membrane modules can be configured in various ways to produce a plant of the required separation capability. A simple batch recirculation system has already been described in cross-flow filtration. Such an arrangement is most suitable for small-scale batch operation, but larger scale plants will operate as feed and bleed or continuous single pass operation (Figure 16.20). 

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 pilot-scale SBCR unit with cross-flow filtration module is schematically represented in Figure 15.5. The SBCR has a 5.08 cm diameter and 2 m height with an effective reactor volume of 3.7 L. The synthesis gas passes continuously through the reactor and is distributed by a sparger near the bottom of the reactor vessel. The product gas and slurry exit at the top of the reactor and pass through an overhead gas/liquid separator, where the slurry is disengaged from the gas phase. Vapor products and unreacted syngas exit the gas/liquid separator and enter a warm trap (373 K) followed by a cold trap (273 K). A dry flow meter downstream of the cold trap measures the exit gas flow rate. 

FIGURE 15.5 Schematic of the pilot-scale integrated SBCR unit with cross-flow filtration module. 

Laboratory Pilot-Scale Tests in SBCR with Cross-Flow Filtration Module Using FT Catalyst/Wax Slurry... 

Cross-flow helps to minimize fouling or scaling of the RO membrane. In an effort to keep the membrane surface free of solids that may accumulate and foul or scale the membrane, tangential flow across the membrane surface aids in keeping the surface clean by scouring the surface minimum flow rates across the membrane surface are required to effectively scour the surface. See Chapter 9.5 for more details about cross-flow filtration and RO system flow rates. 

The difference between conventional dead-end filtration and cross-flow filtration is the configuration of the system. For large-scale operations, only cross-flow filtration will be used. The membranes for miocrofiltration as well as ultrafiltration are commonly utilized in a variety of filtration devices. There are three basic types of tangential flow filtration devices plate and frame, hollow fiber, and spiral wound membranes. 

Novalic S, Heisler G, and Lahnsteiner J. Cross-flow filtration of latex emulsion on a pilot scale using organic and inorganic membranes with different cut-off values. J. Membr. Sci. 1997 130 1-5. 

Even if the problems of poor crystal intergrowth due to local exhaustion of reactants in the autoclave and synthesis of zeolite material in the bulk of the solution were solved, an important problem remains, related to the fact that several batch synthesis cycles (with their associated heating and cooling processes) are often required to achieve a zeolite membrane of good quality. Thus, a synthesis procedure in which reactants are continuously supplied to the synthesis vessel while this is maintained at a constant temperature would clearly be desirable not only for performance but also for the feasibility of the scale-up. This type of approaches has already been tested for inner MFI and NaA zeolite membranes [33-35], and the results obtained indicate that the formation of concomitant phases and the amount of crystals forming in the liquid phase are greatly reduced. Similarly, the continuous seeding of tubular supports by cross-flow filtration of aqueous suspensions [36-37] has been carried out for zeolite NaA membrane preparation. 

Many of the applications for MF derive from the excellent retention these membranes have for microorganisms. Indeed, the retention for bacteria or other organisms is often superior to what may be obtained from tighter UF and RO membranes. We may divide large-scale MF applications according to whether they utilize through-flow filtration (TFF) or cross-flow filtration (CFF). The former are more common. 

Continuous flow centrifuges are now available with disposable rotating bowls. However, these devices are expensive and quite complex, requiring the attendance of skilled personnel during operation. Cross-flow filtration (CFF) is potentially a less expensive and simpler separation tool for continuous plasmapheresis. Large scale plasmapheresis of animal blood is also facilitated by CFF. 

In some cases of pilot scale filtration, entire units have been enclosed as a secondary containment precaution (see Chapter 8). A recent commercial development is the MBR-Sultzer dynamic filter which is available in three sizes. Dynamic filtration is the same as cross flow filtration with little or no recirculation. The cross flow effect is derived from the spinning of the inner surface filter. This type of filter is more efficient, has a lower pump rate and a much higher linear velocity across the filter surface, than conventional cross flow filtration units. There is also little or no damaging effect on sensitive cells. The medium size has the same capacity as the Westfalia SA-7 separator. Van Hemert and Tiesjema concluded that the dynamic filter is suitable for work requiring strict aseptic and primary containment conditions. The use of a double mechanical seal on the rotating shaft could offer a higher degree of containment if required. 

Liquid Separation. Ceramic ultrafilters and nanofilters for liquid mass separation are now produced at an industrial scale and most often are synthesized by the sol-gel process. These membranes operate according to a cross-flow filtration process in which the feeding fluid flows over the surface of the membrane and the permeate flow is recovered across the membrane in a perpendicular direction to the feed flow. The driving force for liquid flow... 

Commercialized inorganic membranes exist in three configurations disks or sheets, tubes and multichannels/honeycombs. Usually, flat disks or sheets are limited to small scale industrial, medical and laboratory applications. They are used almost exclusively in flow-through filtration in contrast to cross-flow filtration in tubes and multichannel monoliths. Meanwhile, tubes and monoliths are used for various industrial applications [2, 4]. 

Cross-flow is the usual case where cake compressibility is a problem. Cross-flow microfiltration is much the same as cross-flow ultrafiltration in principle. In practice, the devices are often different. As with UF, spiral-wound membranes provide the most economical configuration for many large-scale installations. However, capillary devices and cassettes are widely employed, especially at smaller scale. A detailed description of cross-flow microfiltration had been given by Murkes and Carlsson [Crossflow Filtration, Wiley, New York (1988)]. 

The structural elements of commercial inorganic membranes exist in three major geometries disk, tube or tube bundle, and multichannel or honeycomb monolith. The disks are primarily used in laboratories where small-scale separation or purification needs arise and the membrane filtration is often performed in the flow-through mode. The majority of industrial applications require large filtration areas (20 to over 200m ) and, therefore, the tube/tube bundle and the multichannel monolithic forms, particularly the latter, predominate. They are almost exclusively operated in the cross-flow mode. 

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