Heat exchangers chemical reaction fouling

As with many model developments for fouling mechanisms the mathematical analysis is valuable since it draws attention to the salient effects of the variables. The present state-of-the-art models put forward, only go so far towards a complete mathematical solution that may be incorporated in the design of heat exchangers operating under chemical reaction fouling conditions. They are useful however, in suggesting a basis for an empirical formula to correlate experimental data that might so be used. 

Crittenden, B.D., 1984, Chemical reaction fouling in fouling and heat exchanger efficiency. Inst. Chem. Engrs. Course, University of Leeds, 78. 

Cooling System Corrosion Corrosion can be defined as the destmction of a metal by chemical or electrochemical reaction with its environment. In cooling systems, corrosion causes two basic problems. The first and most obvious is the failure of equipment with the resultant cost of replacement and plant downtime. The second is decreased plant efficiency to loss of heat transfer, the result of heat exchanger fouling caused by the accumulation of corrosion products. 

Slack management of operations and poor maintenance may give rise to accelerated and unexpected fouling. For instance, leaks across a heat exchanger, from one stream to the other, may introduce an unexpected and severe fouling problem, because of corrosion or chemical reactions. Again this can result in poor performance and may be expensive to rectify. 

As described in the discussion of heat exchanger fouling elsewhere in this encyclopedia, anodic and cathodic reactions occur. Chemicals may be added to prevent these reactions they are termed anodic and cathodic inhibitors. Cathodic inhibitors form a barrier at the cathode reducing or eliminating H" " or O2 transport to the cathode. They include nitrites, silicates, tannins, and orthophosphates. Anodic inhibitors prevent or restrict electron transfer and include polyphosphates, polyphosphonates, and molybdates. Some of these chemicals represent nutrients for aquatic life and may encourage the growth of microorganisms. 

The chemical reaction either forms the deposit directly on the heat transfer surface or is involved in forming deposit precursors, which subsequently result in the formation of deposits. Precursors may be formed in the bulk liquid, in the boundary layers near the heat exchanger surfaces or directly on the surface. The precursor may be soluble in the bulk fluid and only give rise to deposition when it is carried by difihision or eddy transport to the wall region. It is entirely possible that if the precursor precipitates or reacts to cause a solid to be formed remote from the wall, then the deposition process will be particulate as described in Chapter 7. If there is a reaction with the wall itself then the mechanism could be regarded as corrosion fouling. 

If fouling due to chemical reaction is anticipated on the organic liquid side of the tube, operating at the higher velocity would reduce the problem since chemical reactions are temperature sensitive. Furthermore the shear force will be increased by a factor of around 4 which is also likely to reduce the extent of the fouling. In addition, because of the increased overall heat transfer coefficient at the higher liquid velocity, a heat exchanger based on these data would require a smaller heat... 

Lalande, M., Gallot-Lavallee, T. and Corrieu, G., 1984, Chemical reactions and mass transfer associated with cleaning of heat exchange surfaces fouled by milk deposits, in McKenna. Engineering and Food, Vol. 1. Elsevier Applied Science, London, 59 - 68. 

Many exhaust streams are hot, so materials must be able to woik at elevated temperatures (see Table 6.7), and the gases are often chemically aggressive with sulphur dioxide and nitric acid, fluorine, chlorine and other components making the process difficult, altering the dew point conditions, and reducing the life of the filter medium. Reactions can occur between the carrier gas, the dust and the filter medium, and these are usually intensified by temperature effects. The temperature can rarely be reduced by heat exchange, since the heat exchangers rapidly foul up with dust deposits. 

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