Falling rate fouling

Epstein [1988] presents a mathematical analysis of falling rate fouling as exemplified by Curve B on Fig. 4.1. He assumes that 

For constant surface coefficient of heat transfer a, the heat flux is given by 

Assuming that the overall temperature difference remains constant with time a combination of Equation 4.24 and 4.25 yields 

The values of K and n that will be necessary to allow an assessment of fouling to be made, will depend on the mechanism responsible for the fouling, e.g. whether or not the fouling is caused by chemical reaction or mass transfer of particles. Such data are not in general, readily available. 

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Thermal fouling could not generally be detected for ferric oxide concentrations up to 750 mg/l even after 7 days operation. Above this concentration falling rate deposition curves were obtained with time, but these were not reproducible till a ferric oxide suspension of 1750 mg/l was exceeded, when higher concentrations produced consistently higher fouling thermal resistances. 

The boundary layer of heating surfaces represent the place of surface adsorption as well as of heat-induced reactions. Its temperature, concentration, and velocity gradients introduce anisotropic conditions for the two complex processes taking place within it, and provide an explanation for the observed fouling dynamics (induction period, constant rate period, and falling-rate period). 

Miller has shown that TBTO will prevent fouling attachment at leaching rates as low as 1.25 yg/cm2/day (18). It is thus reasonable to assume that fouling commences when the rate of release falls below 0.5 yg Sn/cm2/day. Based on this, the effective dif-fusivities are calculated, using Crank s rate equation. The calculated effective diffusivities are then substituted in the integral form of Crank s equation to estimate the amount of Sn lost. 

Crittenden observes that there are generally a minimum temperature below which fouling will not be observed and a critical upper temperature above which the deposition rate falls away because of the changes in the underlying reaction. Two aspects of fluid velocity have to be appreciated in connection with the effects of velocity on chemical reaction fouling heat and mass transfer, both of which are dependent... 

Heat transfer rates in falling film evaporators are relatively high even at low temperature differences across the liquid film thus, these evaporators are widely used for heat sensitive products because of uniform temperatures cuid short residence times. Generally, moderately viscous fluids and materials with mildly fouling characteristics can easily be handled in falling film evaporators in series for heavy evaporation loads, and part of the liquid can be pumped and recycled to the top of the unit. 

Data reported by Braun [1977] reproduced on Fig. 11.4 show that above a critical temperature fouling falls with increasing temperature. The figure also shows how the concentration of oxygen affects the rate of deposition. 

Table III details the process flowrates and concentrations during the five-hour processing period. Seventeen liters of a contaminated feed solution were processed and approximately 7 ml of toluene were extracted during the e erlment. The permeate flux decrease from 60 ml/mln to 55 ml/mln was attributed to membrane fouling. The toluene concentration In the feed decreased from 420 ppm to 380 ppm. This fall In concentration was attributed to evaporation. This 40 ppm loss may be compared to the 70 ppm loss In the first continuous run. The difference may be attributed to the 3 C air temperature change between the two days, which led to a variation in the rate of evaporation of toluene. The toluene concentration in the permeate Increased from 45 ppm to 52 ppm. In the first run there was a 30 ppm Increase In toluene concentration In the permeate. The increase in heptane volume from the run 1 to run 2 Is responsible for this difference. In the first continuous run,...

Falling film units must be plumb. Tilted units will achieve lower heat transfer rates and offer higher fouling potential. Units with long tubes must be rigidly supported to counter the effects of wind and transmitted vibrations. 

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