Fouling schematics

FIGt 22-67 Fouling schematics. Case A—Particles plug narrow pores and narrow larger ones. Case B—Particles plug narrow pores. Case C—Particles form a layer on the membrane. Case D—Particles or debris plug the largest pores. [Couttesy Elsevier (modified).]... 

FIGURE 8.2 Fouling schematics for different mechanisms, (a) Pore construction, (h) pore blocking, and (c) cake formation. 

However, if the particulates or solutes accumulated on the surface can be dispersed back into the bulk fluid, these membranes can be used to great advantage since there is relatively little if any "internal-fouling" of the membrane structure. There is a high probability that a molecule or particle which penetrates the skin will not be trapped within the filter structure but will pass through into the filtrate. Schematically, the pores may be represented by ever-widening cones with no internal constrictions to restrain molecules or particles. 

Hollow fine fiber modules made from cellulose triacetate or aromatic polyamides were produced in the past for seawater desalination. These modules incorporated the membrane around a central tube, and feed solution flowed rapidly outward to the shell. Because the fibers were extremely tightly packed inside the pressure vessel, flow of the feed solution was quite slow. As much as 40-50 % of the feed could be removed as permeate in a single pass through the module. However, the low flow and many constrictions meant that extremely good pretreatment of the feed solution was required to prevent membrane fouling from scale or particulates. A schematic illustration of such a hollow fiber module is shown in Figure 3.47. 

Anaiysis (a) The schematic of the heat exchanger is given in Fig, 11-11. The thermal resistance, for an unfinned shell-and-tube heat exchanger with fouling on both heat Iransfer surfaces is given by Eq. 11-8 as... 

Figure 22 Schematic representation of the fouling of a catalyst slab in a PPR by dust. A clean B partially fouled C completely fouled.

Since membrane fouling could quicldy render the system inefficient, very careful and thorough feedwater pretreatment similar to that described in the section on RO, is required. Some pretreatment needs, and operational problems of scaling are diminished in the electro dialysis reversal (EDR) process, in which the electric current flow direction is periodically (eg, 3—4 times /h) reversed, with simultaneous switching of the water-flow connections. This also reverses the salt concentration buildup at the membrane and electrode surfaces, and prevents concentrations that cause the precipitation of salts and scale deposition. A schematic and photograph of a typical ED plant are shown in Figure 16. 

Figure 9.8 Schematic representation of concentration polarization and fouling at the membrane surface. From Ref. [122] with permission.

Backflushing can remove the fouling layer both at the membrane surface and within the membrane. The forward filtration time and the duration of the backpulse need to be optimized since permeate is lost to the feed side during the backpulse. A schematic is shown in Figure 9.19. See Ref. [17] for additional details. 

In addition to high filtration rates, asymmetric membranes are most fouling resistant. Conventional symmetric structures act as depth filters and retain particles within their internal structure. These trapped particles plug the membrane and the flux declines during use. Asymmetric membranes are surface filters and retain all rejected materials at the surface where they can be removed by shear forces applied by the feed solution moving parallel to the membrane surface. The difference in the filtration behavior between a symmetric and an asymmetric membrane is shown schematically in Figure 1.10. Two techniques are used to prepare asymmetric membranes one utilizes the phase inversion process and the other leads to a composite structure by depositing an extremely thin polymer film on a microporous substructure. 

Changes in friction factor (see Chapter 5) may also be used as an indication of fouling of a flow channel. The method was used by Bott and Bemrose [1983] for estimating the extent of particulate fouling on finned tubes. Fig. 17.14 is a schematic diagram of the apparatus used in this research. 

Fig. 3.6-4 Schematic overview of the different processes leading to membrane fouling.

In practice, a variety of systems contains both amphiphilic molecules and polymers. These are, for instance, found where detergent is used to remove polymeric (proteinaceous) deposits from a fouled surface, for example, in food-processing equipment, teeth, contact lenses, and other biomedical appliances. By aggregation of the amphiphiles at the polymers, the latter ones are solubilized and subsequently released from the surface. This process is schematically depicted in Figure 12.15. 

In order to achieve a particular separation via a membrane process, the first step is to develop a suitable membrane. However, during an actual separation, e.g. a pressure driven process, the membrane performance (or better the system performance) can change ver> much with time, and often a typical flux-time behaviour may be observed the flux through the membrane decreases over time. This behaviour is shown schematically in figure VII - 1 and is mainly due to concentration polarisation and fouling. 

For industrial applications, a cross-flow operation is preferred because of the lower fouling tendency relative to the dead-end mode (figure VIII - 14b). In the cross-flow operation, the feed flows parallel to the membrane surface with the inlet feed stream entering the membrane module at a certain composition. The feed composition inside the module changes as a function of distance in the module, while the feed stream is separated into two a permeate stream and a retentate stream. The consequences of fouling in dead-end systems are shown schematically in figure VUI - 15. In dead-end filtration, the cake grows with time and consequently the flux decreases with time.Hux decline is relatively smaller with cross-flow and can be controlled and adjusted by proper module choice and cross-flow velocities. 

---