Fouling deposit composition

TABLE 7.3. Fouling Deposit Compositions from Lab Test... 

Ash fusion characteristics are important in ash deposition in boilers. Ash deposition occurring on the furnace walls is termed slagging, whereas accumulation on the superheater and other tubes is termed fouling. A variety of empirical indexes have been developed (60,61) to relate fouling and slagging to the ash chemical composition through parameters such as acidic and basic oxides content, sodium, calcium and magnesium, and sulfur. 

These deposits responsible for fouling can block out the reactants and prevent them from reaching the active sites, or even block the internal pores of the catalyst. Hydrocarbons and aromatics are usually the cause of coking. The chemical nature of the carbonaceous deposits relies on many parameters temperature, pressure, feed composition, nature of products, and catalyst age share the responsibility of the residue formation on catalysts. 

Corrosion, fouling, and deposition processes of some degree most probably will have been steadily taking place for some time prior to the evaluation being made. If an index or model is to be used, the makeup water source should always be the starting point and cycled-up cooling water composition calculated from this point (despite the problems of accurately assessing the cycled-up water pH). 

There are no experimental data regarding the resistance of zeolitic membranes to fouling. Regeneration of zeolites after, for example, coke deposition is known to be feasible from industrial applications of zeolites. However, the influence of regeneration conditions on zeolitic, composite membranes is not known. 

Carbon deposition is one of the luost serious problems of the steam reforming catalyst process (ref 1). The deposition of carbon on naphtha steam reforming catalysts depends ori the chemical composition of the hydrocarbon oil, the steam/carbon ratio in the feedstock, as well as the pi ocesa temperature and pressure, it is also affected by tlie presence of sulfur poisons Our past research of SNG catalysts ejiamined the nature of the carbon deposits as a function of the sulfur level on the catalyst (refs, 2 4). A small amount of sulfur was found to promote the formation of carbon that is non-reactive with steam and hydrogen under steam reforming reaction conditions. The continuous accumulation of this less reactive carbon [continuous carbon deposition (CCD)l on the catalyst surface leads to coke fouling Studies of the occurrence of CCD in our laboratory tests allow ua to predict, that coke fouling is likely to occur on the same catalyst used in real Indusl.rlal applications. 

There are two different levels where fouling phenomena and related effects may interfere with performance of composite inorganic or hybrid membranes. The first and the more classically reported in literature is the one of the separation process itself, which through various interactions between solution and material (adsorption, surface deposits, pore plugging) generally leads to reduced fluxes and increased retentions. The second, much more less described by authors but of the same nature and with analogous effects, concerns membrane preparation, and the possible interactions between deposited layers. Theses two aspects are linked up with the so-called formed-in-place membranes, obtained by deposition of species onto a ceramic support through cross-flow filtration. In what follows, they will be described in a unified approach. 

The slagging and fouling tendencies of coals largely differ according to the deposit but also differ within the deposit because of widely varying coal composition. Ash properties are so complex that evaluation of coal performance is extremely difficult. The performance of coals in furnaces are further complicated by the processes which control deposition on heat transfer surfaces in boilers. Based on these complications, we must neither deceive ourselves as to the possibility of reliable estimates nor deny the possibility of making a useful estimate of the performance of brown coal in boilers. 

Feed composition, operating conditions, equipment geometry, materials and their treatments are the main variables influencing the fouling rate. Firstly, qualitative information on deposit formation is obtained by a direct analysis of the morphology of the material. Coke samples from both industrial and... 

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. 

Bott [1990] reported some qualitative data on the composition of deposits taken from the "tar side" of a tar oil heater. The appearance of the deposit was that of a very thick dark coloured paste that included fibres, iron (as rust), carbon, sand and glass particles. The deformable nature of the deposit was likely to be due to the flexible bonding of the particles by the process oil. The presence of components in the deposit other than organic related material, confirms the observation that parallel to chemical reaction fouling on process plant, there are other mechanisms involved. 

The calcareous deposit formed on cathodically protected metal surfaces will also be affected by the presence of micro-organisms and macro-fouling. Physical disruption and alteration of structure, composition and crystal form will influence the response to cathodic protection. The consequent economic implications of changes in current density requirement for protection, and the engineering design implications and associated economics may be significant [Maines 1993]. Work is therefore required to improve the understanding of the relationship between... 

Lalande and Rene [1988] have given some data on the composition of whole milk and associated deposits. These data collected from fouling resulting from milk pasteurisation in a plate heat exchanger, are reproduced in Table 16.18 and are based on Lyster [1979] and Lalande et al [1984, 1985]. 

Deposition of organic matter was determined by mass balance in feed and concentrate samples. Electron microscopy and X-ray photoelectron pectroscopy (XPSJ were used to study the morphology and composition of the fouling layer. 

The XPS analysis of unwashed membranes showed that ESCA could identify the organic fouling layers of the membranes as well as the surface structure. Peak fitting regimens determined whether the materials discovered were foulants deposited (as indicated by the N (Is) data) on the surface or materials that had become chemically bound to the surface. To be chemically bound to the surface, the foulant must show an oxidation state indicative of a bond with membrane substituents. If the foulant showed a chemical composition not associated with the membrane, it was determined to be lying on the surface (physisorbed) and not chemically attached. This was the case for calcium on LFCl (Ca (2pl, 2p3)BE 350.3-346.9 eV). However, this finding does not preclude the possibility of calcium complexation with functional groups of the RO membranes used... 

Montmorillonite may or may not break down (dissociate) into its constituent parts when coal is burned if it does dissociate, then, upon cooling, it may recombine with other elements or minerals to form mineral deposits on the inside surfaces of furnaces and boilers. This process (slagging or fouling) produces barriers to heat exchange in the affected equipment, which can substantially reduce its efficiency and require costly repairs. Illite, however, with its simpler composition, does not canse snch problems under normal furnace operating conditions. Where these two clay minerals and others occur, their relative abundances, relationships to other minerals, and exact compositions are snbjects for continued research in coal quality. 

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