Fouling Design considerations

To counteract the typical misoneism of the world food industry, it is necessary to resort to appropriate scaling-up exercises in pilot- or industrial-plant scale to assess precisely the membrane process performance and reliability, as well as its economical feasibility. There are, however, a number of problems that have undoubtedly limited growth in ED membrane sales, like membrane-fouling problems, design considerations, cleanability, investment and membrane replacement costs, and competing technologies, such as NF and IER. 

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... 

High Water Velocities. The abiUty of high water velocities to minimize fouling depends on the nature of the foulant. Clay and silt deposits are more effectively removed by high water velocities than aluminum and iron deposits, which are more tacky and form interlocking networks with other precipitates. Operation at high water velocities is not always a viable solution to clay and silt deposition because of design limitations, economic considerations, and the potential for erosion corrosion. 

Membrane systems consist of membrane elements or modules. For potable water treatment, NF and RO membrane modules are commonly fabricated in a spiral configuration. An important consideration of spiral elements is the design of the feed spacer, which promotes turbulence to reduce fouling. MF and UF membranes often use a hollow fiber geometry. This geometry does not require extensive pretreatment because the fibers can be periodically backwashed. Flow in these hollow fiber systems can be either from the inner lumen of the membrane fiber to the outside (inside-out flow) or from the outside to the inside of the fibers (outside-in flow). Tubular NF membranes are now just entering the marketplace. 

Most of the published evidence suggests that marine fouling cover— particularly where it is continuous and well established — reduces corrosion rates of steels . Indeed, 35%o seawater is by no means the most corrosive of saline environments towards steel. Brackish water, as found in estuarine or certain other coastal areas, is considerably more aggressive towards steel, and careful design measures should be taken to ensure that effective corrosion control is achieved in such circumstances. 

This part of a plant survey is, unfortunately, seldom carried out, as it may be considered that there is little, in practice, that can be done to change the plant design or operating conditions. Nevertheless, it can often be a useful exercise to undertake, especially if it is suspected that the tower is considerably undersized or oversized, or if there is evidence of cooling tower fouling. 

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