Deposition on Fibers

Fiber-Bed Scrubbers Fibrous-bed structures are sometimes used as gas-liquid contactors, with cocnrrent flow of the gas and hqnid streams. In such contactors, both scrubbing (particle deposition on droplets) and filtration (particle deposition on fibers) may take place. If only mists are to be collected, small fibers may be used, but if solid particles are present, the use of fiber beds is limited by the tendency of the beds to phig. For dnst-cohectiou service, the fiber bed must be composed of coarse fibers and have a high void fraction, so as to minimize the tendency to plug. The fiber bed may be made from metal or... 

Fig.1 Schematic cross-section of a porous fiber preform. Reactants diffuse from the preform surfaces toward the center, and are depleted by deposition on fiber surfaces.

The theory of deposition on fibers, or cylinders, lying transverse to the direction of airflow has been studied thoroughly [9,28-32]. Fiber filters have been placed in specialized pharmaceutical aerosol sampling tubes that can... 

Deposition on Fibers/Fabrics. Following pioneering studies by Kuhn and coworkers,89 a wide variety of textile substrates have been successfully coated with... 

The dip-coating of thermally pretreated fibers in a slurry composed of catalyst-precursor seems to be a very good and easy method, for the deposition of an adequate amount of catalytic material. All catalytic fibers which were pretreated by O2/H2 mixture showed lower Pd crystallite size and higher [(Pd/Al)exp/(Pd/Al)flJ ratios than the corresponding directly calcined ones. Catalysts deposited on fibers and pretreated by O2/H2, always showed higher methane conversion than the analogous directly calcined ones. However, a low crystallite size does not seem to be the only explanation of their higher catalytic performances there could be an improved capacity of Pd to dissociate O2 molecules in ions, in those fibers pretreated under O2/H2 flows. 

Aerosol Filtration, Ph.D. dissertation, California Institute of Technology, Pasadena, 1966), forming chainlike agglomerates termed dendrites. The growth of dendritic deposits on fibers has been studied experimentally [Billings, op. cit. Bhutra and Payatakes,/. Aerosol Sci., 10, 445 (1979)], and Payatakes and coworkers [Payatakes and Tien, J. Aerosol Sci., 7, 85 (1976) Payatakes, Am. Inst. Chem. Eng. J., 23,192 (1977) and Payatakes and Gradon, Chem. Eng. Sci., 35,1083 (1980)] have attempted to model the growth of dendrites and its influence on filter efficiency and pressure drop. 

Diamond deposition on fibers Composite fibers, diamond tubes 2.2 [28]... 

Decrease in water absorption behavior is reported to occur with silanizing of banana stem and bunch fibers [26]. Ganan et al. [26] reported an increase in contact angle and decrease in surface free energy and polar component. They foxmd silane deposition on fibers surface according to Fourier transform infrared spectrometry (FTIR) analysis. 

Applications. The major use of quartemaries is related to their ability to adsorb on natural or synthetic substrates and fibers. Less-soluble long hydrophobic chain containing compounds (e.g., dialkyldimethyl ammonium chlorides) deposit on fibers and... 

Deposition on fibers or fabrics Vapor phase deposition Deposition in nanoscale matrices Photochemically initiated polymerization Enzyme-catalyzed polymerization Polymerization using electron acceptors Miscellaneous polymerization methods Routes to more processible polyanilines Emulsion polymerization Colloidal polyaniline dispersions Substituted polyanilines... 

Chem. Descrip. Mixt. of polyfunctional org. and inorg. substances Uses Peroxide bleach bath stabilizer, dispersant, chelating agent for bleaching cel-lulosic fibers and their blends in pad-steam, long liquor, or cold-pad batch applies. dispersant promoting suspension and flushing of silicates for reduction/prevention of silicate deposits on fiber and equip. 

As soon as polymer has adsorbed on the fibers, creating patches of polymer on the fibers, the colloidal particles can deposit on these polymer patches. The kinetics of colloid deposition on fibers are very similar to those of polymer adsorption on fibers, discussed above. It can be described by [13] ... 

Notice that for the smallest latex = 1.3, whereas all particles deposit. This implies that some of the particles entered the macropores of the fiber wall (estimated to have pores of about 80 nm), which have an accessible surface area about 15 times that of the external surface [29]. Interestingly, particle deposition on fibers provides a method to fractionate particles of different sizes. Adding a mixture of small and large particles to fibers and subjecting the system to shear for a short time (a few minutes) will result in the deposition of most or all large particles, with little deposition of small particles. The fibers with the large particles can be readily separated from the mixture, leaving the small particles suspended in the liquid. 

Figure 1.7 Deposition of clay on fibers, induced by PEO. In the absence of PEO or for clay pretreated with PEO for 30 min or longer, no deposition takes place. Because PEO does not adsorb on fibers, this is an example of asymmetric polymer bridging, in which PEO first from Ref [31]). adsorbs on clay, prior to clay deposition on fibers.

From the examples presented in this chapter papermaking suspensions are dearly fasdnating complex systems that show a richness of interesting phenomena. Both colloidal and hydrodynamic phenomena play a crudal role. The colloidal interactions can be modified, and thus optimized and controlled, by polymers and poly electrolytes. The time scales of polymer adsorption, partide deposition on fibers, particle detachment polymer transfer, flocculation and break-up of colloidal aggregates determine how a papermaking suspension behaves on a paper machine. These time scales can be controlled by dosage and addition points. Some of the relevant time scales can be predicted by theory, as some of the examples given here show, whereas others require experimental determination, such as polymer transfer rates, particle detachment and floe break-up rates, which are difficult to predict from first principles. Therefore, expensive pilot and mill trials are usually required to optimize and fine-tune the use of additives on a paper machine. Nevertheless, laboratory experiments can provide useful trends and help to eluddate the mechanisms by which additives function. 

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