Fouling rating assessment

The rating assessment was conducted for heat exchangers in the preheat train and the assessment results indicated that fouling occurred, to a varying degree, to most of the... 

In order to reduce capital cost many plants are installed with the minimum of instrumentation. Under these circumstances it may be difficult or even impossible, to make a proper assessment of the fouling encountered in heat exchangers under operating conffitions. Temperatures and flow rates may have to be inferred with the problems of accuracy that this raises. 

An improvement on the assessment of individual heat exchanger performance is to pass a sidestream through some kind of monitor. In general monitors resemble the test equipment used in the laboratory. The use of monitors allows a more careful control of the fouling conditions to be exercised particularly in terms of temperature, temperature changes and flow rates, using actual plant streams. The data obtained are more reliable than those obtained directly from the operation of full scale plant. 

A miniplant produces a high-boiling residue at a rate of Ig h This residue is removedfrom the bottom of a distillation column with a distillation boiler capacity of200 ml, so that the mean residence time is 200 h. The time required to reach the steady state is consequently about 25 d (about 3 times the mean residence time). Only then will it be possible to assess this bottom residue and draw conclusions about its fouling behavior, composition, properties, and so on. 

Fouling propensity is commonly assessed by monitoring permeate flux and transmembrane pressure (TMP). Since membrane processes are generally operated either under constant TMP or constant permeate flux, a decrease in permeate flow rate or an increase of TMP is observed, respectively, once the fouling forms on the membrane. From these two parameters, calculation of hydraulic resistance (m ), membrane permeability (Lm h bar ) or specific cake resistance (mkg ) is also possible and allows further assessment of fouling conditions. The hydraulic resistance of the filtration system can be quantified by correlating TMP and permeate flux during a clean water test. This correlation is described as the Darcy s law ... 

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