Fouling phenomena

When we talk about this subject, the term foulant or foulant layer comes to the forefront. Foulant, or fouling layer, are general terms for deposits on or in the membrane that adversely affect filtration. The term "fouling" is often used indiscriminately in reference to any phenomenon that results in reduced product rates. "Fouling" in this casual sense can involve several distinct phenomena. These phenomena can be desirable or undesirable, reversible or irreversible. Different technical terms apply to each of these possibilities. 

Cavitations generate several effects. On one hand, both stable and transient cavitations generate turbulence and liquid circulation - acoustic streaming - in the proximity of the microbubble. This phenomenon enhances mass and heat transfer and improves (micro)mixing as well. In membrane systems, increase of fiux through the membrane and reduction of fouling has been observed [56]. 

Diffusion Poisons. This phenomenon is closely akin to catalyst fouling. Blockage of pore mouths prevents full use of the interior surface area of the pellets Entrained dust par ticles or materials that can react on the catalyst to yield a solid residue give rise to this type of poisoning. 

Catalyst deactivation refers to the loss of catalytic activity and/or product selectivity over time and is a result of a number of unwanted chemical and physical changes to the catalyst leading to a decrease in number of active sites on the catalyst surface. It is usually an inevitable and slow phenomenon, and occurs in almost all the heterogeneous catalytic systems.111 Three major categories of deactivation mechanisms are known and they are catalyst sintering, poisoning, and coke formation or catalyst fouling. They can occur either individually or in combination, but the net effect is always the removal of active sites from the catalyst surface. 

As described above, the initial cause of membrane fouling is concentration polarization, which results in deposition of a layer of material on the membrane surface. The phenomenon of concentration polarization is described in detail in Chapter 4. In ultrafiltration, solvent and macromolecular or colloidal solutes are carried towards the membrane surface by the solution permeating the membrane. Solvent molecules permeate the membrane, but the larger solutes accumulate at the membrane surface. Because of their size, the rate at which the rejected solute molecules can diffuse from the membrane surface back to the bulk solution is relatively low. Thus their concentration at the membrane surface is typically 20-50 times higher than the feed solution concentration. These solutes become so concentrated at the membrane surface that a gel layer is formed and becomes a secondary barrier to flow through the membrane. The formation of this gel layer on the membrane surface is illustrated in Figure 6.6. The gel layer model was developed at the Amicon Corporation in the 1960s [8],... 

The density of liquid water decreases with decreasing temperature down to 4°C. In the fall season in the vicinity of a large population of lakes and ponds, there is often a period of one to several days in which whole geographic areas are afflicted with alarming foul odors. Based on the information contained in Figures 2.5 and 2.6, give a plausible explanation for this phenomenon. 

To achieve a high membrane rejection towards the substrate it is important that the pore size of the membrane is smaller than the size of the molecules to be retained. Nevertheless, other factors influence the separation properties of a membrane, such as the shape and flexibility of the substrate and its acid-base properties, as well as the concentration-polarization phenomenon and the membrane fouling. 

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. 

Membrane fouling is a very complex phenomenon owing to the wide variety of foulants that can be encountered in practice. Even for a specific application, fouling depends on... 

Ash fouling, the accumulation of deposits on boiler tube surfaces in utility boilers, is a severe operating problem in many power plants fired with low-rank coals. Ash fouling is a complex phenomenon the extent of which is related to the boiler design, the method of operating the boiler, and the coal properties. In extreme cases it is necessary to schedule frequent shutdowns for removing the deposits or to derate the boiler. A recent survey of six power plants estimated that total costs of curtailments due to ash-related problems were 20.6 million over a six-month period (20). 

Figures 34.16 and 34.17 depict the opposite phenomenon of the membrane fouling, i.e., salt rejection and water flux increases with the operation time during the first 3 months, in which period most reverse osmosis membranes show the beginning of the membrane fouling. The reason for this phenomenon has not been elucidated ...

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