Models of fouling

The importance of biopolymer deposition on the membrane has been highlighted by Chu and Li [34] who noted that such material may accelerate bacterial attachment to the membrane in addition to its contribution to fouling resistance. The formation of a slime layer on the membrane, comprising an EPS matrix with embedded bacteria, may be analogous to a biofilm, the significance of which is the difficulty of its removal by nonchemical methods. The question of whether biofilm formation plays a significant role in MBRs is yet to be addressed. Indeed, a comprehensive model of fouling in MBRs is not yet available. 

A prediction model of fouling rate of the furnace was developed to decide optimal decoking cycles of the furnace. It is a very effective tool, of course, for the design of the new plant with three hours cycle of a reactor swing. 

Glen, N.F., S. Flynn, Grillot, J.M. and Mercier, P., 1992, Measurement and modelling of fouling on finned tubes, in Bohnet, M., Bott, T.R., Karabelas, A.J., Pilavachi, P.A., Semeria, R. and Vidil, R. eds.. Fouling mechanisms theoretical and practical aspects. Editions Europeennes Thermique et Industrie. 

That expectation was not borne out within genetics, it fell foul of what has aptly been called the unexpected complexity of eukaryotic genetics (Watson etal., 1992). Starting in the 1970s, five sets of developments destroyed the simple informational model of the genome in molecular biology 18... 

The basis for the experiments as carried out was that the flux decline with time, which is an easily measured quantity, could be correlated with the foulant film growth by the model of Eq. (2), or some other appropriate semi-empirical model. In this way the fouling film thickness could be deduced indirectly. To show this we write from Eqs. (5)-(7)... 

Flux. The film model (Equation 6.6) illustrates that increasing flux has an exponential effect on CP. If we accept that fouling is a consequence of CP the impact of excessive flux is obvious. As a result high flux membranes tend to be short lived and foul unless improved fluid management is able to enhance k. Selection of the appropriate flux and crossflow velocity is a trade-offbetween capital and operating costs (see cost of fouling below). 

This is a Mathcad model of the running of an oil tanker. It contains technical and cost sub-models and helps to find requirements for an anti-fouling system. The parameters are... 

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. 

In addition to the Navier-Stokes equations, the convective diffusion or mass balance equations need to be considered. Filtration is included in the simulation by preventing convection or diffusion of the retained species. The porosity of the membrane is assumed to decrease exponentially with time as a result of fouling. Wai and Fumeaux [1990] modeled the filtration of a 0.2 pm membrane with a central transverse filtrate outlet across the membrane support. They performed transient calculations to predict the flux reduction as a function of time due to fouling. Different membrane or membrane reactor designs can be evaluated by CFD with an ever decreasing amount of computational time. 

Continuons emulsion polymerization is one of the few chemical processes in which major design considerations require the use of dynamic or unsteady-state models of the process. This need arises because of important problems associated with sustained oscillations or limit cycles in conversion, particle number and size, and molecular weight. These oscillations can occur in almost all commercial continuous emulsion polymerization processes such as styrene (Brooks et cl., 1978), styrene-butadiene and vinyl acetate (Greene et cl., 1976 Kiparissides et cl., 1980a), methyl methacrylate, and chloropene. In addition to the undesirable variations in the polymer and particle properties that will occur, these oscillations can lead to emulsifier concentrations too low to cover adequately the polymer particles, with the result that excessive agglomeration and fouling can occur. Furthermore, excursions to high conversions in polymer like vinyl acetate... 

Passage of bacteria through membrane Analysis of deposits ATR, FTIR, measuring fouling in real time Measuring concentration polarization Mathematical modeling of flux decline... 

Bacchin P., Aimar P., and Sanches V., Model of colloidal fouling of membranes. AIChE Journal 41(2) 1995 368-376. 

The design tools developed for EUREKA process are very sophisticated, a) Practical Model of Thermal Cracking of Residual Oil and b) Tubular Fouling Model for Residue Cracking Furnaces made our designs very sure and reliable. 

Various treatments of Eqs. (12.5) to (12.8) are proposed in the literature for those cases where charge effects cue negligible (microfiltration, ultrafiltration or nanofiltration of neutral solutes), and low concentrated solutions are considered, i.e. with negligible non-idealities and activities assimilated to concentrations. Most of the time, membrane morphology is just considered through simple parameters such as effective pore size, tortuosity accounting for the effect of fouling on measurable transport properties. A well-known model thus obtained is due to Kedem and Katchalsky [5] ... 

Crittenden, B.C. Basic science and models of reaction fouling. In Fouling Science and Technology, Melo, L.F., Bott, T.R., Bernardo, C.A., Eds. Kluwer Academic Publishers Dordrecht, Holland, 1988. 

---