Through-flow filtration

The distinction between cross-flow and dead end (also known as through-flow) filtration can be better understood if we first analyze the mechanism of retention. The efficiency of cross-flow filtration is largely dependent on the ability of the membrane to perform an effective surface filtration, especially where suspended or colloidal particles are involved. Table 2 shows the advantages and versatility of cross-flow filtration in meeting a broad range of filtration objectives, Figure 2 illustrates the differences in separation mechanisms of CFF versus dead end filtration. 

Ultrafiltration and reverse osmosis have always used a fluid management technique known as "cross-flow filtration" to sweep away deposited particles from the membrane surface. "Cross-flow filtration" (CFF) is compared with "through-flow filtration" (TFF) (sometimes called "dead-ended filtration") in Figure 2.38. 

Currently, most MF systems operate with the more conventional "through-flow filtration"-one stream in and one stream out. The particles accumulate on the membrane and are disposed of with the membrane. 

Figure 2.38 Through-flow filtration (TFF) and cross-flow filtration (CFF)...

Pleated cartridges are considerably more expensive than the equivalent membrane area in 293 mm discs (2 to 4 times as much). Some users insist that the lower replacement labor does not off-set the additional expense. However, the trend is definitely towards pleated cartridges for through-flow filtration. The movement to pleated cartridges has been accelerated with the discovery that some exhausted cartridges can be reused after backwashing. 

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. 

Prefiltration for UF. In most cases, MF is too expensive for use as a prefilter. However, there are some feed streams which severely foul UF membranes and where prefiltration with cross-flow MF is cost effective. For example, in the UF of milk or cheese whey fat, casein fines, coagulated protein, and microorganisms all cause severe membrane fouling. In the concentration of milk, the use of tubular MF as a prefilter increased the UF flux by 100% on the average. CFF is required because TFF (through-flow filtration) would plug the MF membrane immediately. 

In situations where a low concentration of suspended solids needs to be separated from a liquid, then cross-flow filtration can be used. The most common design uses a porous tube. The suspension is passed through the tube at high velocity and is concentrated as the liquid flows through the porous medium. The turbulent flow prevents the formation of a filter cake, and the solids are removed as a more concentrated slurry. 

The advantage of single-pass over cross-flow filtration is that it is an easier system to operate and can be cost effective, particularly if the product to be filtered is expensive, because very tittle of the initial fluid is lost during filtration. However, because the flow pattern of the fluid is directly through the filter, filter life maybe too short for the fluid being filtered. The minimum flow rate needed downstream of the filter must also be considered, especially when there are time constraints to the process. In some situations it may be more advantageous to use a cross-flow system where higher flow rates may be easier to obtain. 

Cross-Flow Filtration in Porous Pipes. Another way of limiting cake growth is to pump the slurry through porous pipes at high velocities of the order of thousands of times the filtration velocity through the walls of the pipes. This is ia direct analogy with the now weU-estabHshed process of ultrafiltration which itself borders on reverse osmosis at the molecular level. The three processes are closely related yet different ia many respects. 

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. 

Porous media, flow through, 11 330-332, 766-767 25 290-291 Porous pipes, cross-flow filtration in,... 

There are two principal modes under which deep bed filtration may be carried out. In the first, dead-end filtration which is illustrated in Figure 7.1, the slurry is filtered in such a way that it is fed perpendicularly to the filter medium and there is little flow parallel to the surface of the medium. In the second, termed cross-flow filtration which is discussed in Section 7.3.5. and which is used particularly for very dilute suspensions, the slurry is continuously recirculated so that it flows essentially across the surface of the filter medium at a rate considerably in excess of the flowrate through the filter cake. 

Filtration separates components according to their size. Efficiency depends on the shape and compressibility of the particles, the viscosity of the liquid phase and the driving force, which is the pressure created by overpressure or by vacuum. Filtration can be performed either as dead-end filtration, where the feed stream flows perpendicular to the filter surface (Lee, 1989) or as tangential flow filtration, where the feed stream flows parallel to the filter and the filtrate diffuses across it. Examples of the former are the continuous rotaiy vacuum dram filter, where a rotaiy vacuum filter has a filter medium covering the surface of a rotating drum and the filtrate is drawn through the dram by an... 

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. 

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