Fouling mechanisms

The rate of fouling decreases with time but does not appear to approach an asymptotic maximum during the time of operation. This is the most common type of folding in the process industry and is usually the result of a combination of different fouling mechanisms occurring together. 

Flux Decline Plugging, Fouling, Polarization Membranes operated in NFF mode tend to show a steady flux decline while those operated in TFF mode tend to show a more stable flux after a short initial decline. Irreversible flux decline can occur by membrane compression or retentate channel spacers blinding off the membrane. Flux decline by fouling mechanisms (molecular adsorption, precipitation on the membrane surface, entrapment within the membrane structure) are amenable to chemical cleaning between batches. Flux decline amenable to mechanical disturbance (such as TFF operation) includes the formation of a secondary structure on the membrane surface such as a static cake or a fluid region of high component concentration called a polarization layer. 

An additional mechanism affects the deposits formation from the H-Oil reactor, rejection of vanadium and nickel sulfides from the catalyst. In the vacuum tower, asphaltene precipitation was found to be the prevalent fouling mechanism. In asphaltene... 

Bannayan, M. A., Fouling Mechanisms and Effect of Process Conditions on Deposit Formation in H-Oil Equipment, in Catalysts in Petroleum Refining and Petrochemical Industries, 1996. Kuwait, Studies in Surface Science and Catalysis, Elsevier. 100 pp. 273-281. 

Fouling effects can be hastened by reducing the flow rate or increasing the heat-transfer rate however, this may induce different fouling mechanisms than would be experienced under actual process operating conditions. 

It may be possible to do a membrane autopsy to identify the foulant(s) and fouling mechanism. For microporous membranes the blocking law analysis [1], which uses permeate volume (V) vs. time (t) data, can supplement the observations. The generalized relationship at constant pressure and in dead-end filtration mode gives,... 

This balance of forces on the approach of particles to membrane pores also has substantial consequences for rates of filtration. Of special interest is the case where the particle to be removed is comparable to or only slightly greater in dimensions than the pore diameter. In the absence of colloidal interactions, this can lead in a catastrophic loss in filtration flux as such particles can plug pores highly effectively. Such a fouling mechan-... 

Qualitative simulation is a specific KBS model of physical processes that are not understood well enough to develop a physics-based numeric model. Corrosion, fouling, mechanical wear, equipment failure, and fatigue are not easily modeled, but decisions about them can be based on qualitative reasoning. See Refs. 178 and 292. 

Zhu, X. and M. Elimelech (1997). Colloidal fouling of reverse osmosis membranes Measurements and fouling mechanisms. Environ. Science Technol. 31, 12, 3654-3662. 

Dal-Cin M.M., MeLellan F., Striez C.N., Tam C.M., Tweddle T.A., and Kumar A., Membrane performance with a pulp mill effluent Relative contrihutions of fouling mechanisms. Journal of Membrane Science 120 1996 273-285. 

Despite the numerous efforts in understanding the fouling mechanisms and the influence of operating parameters involved that lead to flux decline in filtration systems, there remain significant gaps in the present knowledge on this phenomenon. To date, none of the theoretical models, and their numerous modified forms, and empirical models sufficiently explain the influence of membrane-solute interactions and solute-solute interactions on real dairy systems. 

Another fouling mechanism that can occur is corrosion of boiler tubing and erosion of refractories due to formation of acids and their buildup in the combustion units from conversion of sulfur and chlorine present in the fuel. Fortunately, the amounts of these elements in most biomass are nil to small. The addition of small amounts of limestone to the media in fluidized-bed units or the blending of limestone with the fuel in the case of moving-bed systems are effective methods of eliminating this problem. Other sorbents such as dolomite, kaolin, and custom blends of aluminum and magnesium compounds are also effective (Coe, 1993). 

Figure 1. Six-site fouling mechanism as a parallel pathway to the main reaction mechanism ( represents a metal site) Ref [4]. Reproduced by permission of Acadonic Press, hic.

FOULING MECHANISMS AND EFFECT OF PROCESS CONDITIONS ON DEPOSIT FORMATION IN H-OIL EQUIPMENT... 

Analyses of deposit and stream samples from a commercial H-Oil unit indicate that several mechanisms influence fouling. The rejection of "vanadium- and nickel sulfides" from the catalyst and the precipitation of polycyclic aromatics appear to contribute to fouling in the reactor, reactor outlet line, and high pressure separator. "Asphaltene precipitation" is the prevalent fouling mechanism in the H-Oil vacuum tower. 

The dehydrogenation of 1-butene over a chromia-alumina catalyst is selected as a model reaction system to study the fouling mechanisms and their respective fouling precursors. The reaction and deactivation schemes can be taken as ... 

In a previous work [8], neither parallel or series coking mechanism was found to be satisfactory because the predominant fouling mechanism changes with temperature. Since the dehydrogenation reaction and coking formation takes place on the same type of active sites, a combined parallel and series mechanism is assumed in which ())( can be expressed in terms of process temperature, process time and the concentration of the respective foulents, as -c(t>T)= k(-B YcbCT) PB(t) + kcD Ycd(T) Pd(0 (2)... 

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