Development of a Model for the Prediction of Fouling in Heat Exchangers Processing Crude Oil
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A number of models have been proposed for the prediction of fouling rates in heat exchangers forming the pre-heat trains used in crude oil distillation units. Most of these models assume that the fouling is the result of a chemical reaction and that would appear to be the case for the exchangers at the hottest part of the pre-heat train. The work of Wiehe suggests that asphaltene cores are generated as a result of the thermal decomposition of maltenes. Once these cores exceed a solubility limit they precipitate to form a deposit on the exchanger tubes.
Examination of fouling data indicates that fouling rates increase with increasing wall temperature but decrease with increasing tube velocity. This has lead workers (such as Ebert & Panchal) to suggest that there are competing mechanisms: a reaction mechanism which is dependant on temperature and a supression mechanism that is a function of wall shear. Their model then yields a fouling rate given by generation rate minus suppression rate. The suppression rate is assumed to be a linear function of wall shear. No physical explanation for this suppression term has been proposed.
In this paper we recognise that wall shear affects the time at which particles suspended in a fluid stay close to the surface of the deposit. It is suggested that this, in turn, affects the probability of the particle attaching to the surface of the deposit.
A new model consisting of a generation rate multiplied by a sticking probability is then tested against the experimental data reported by Knudsen and co-workers. The new model provides much better predictions than the model proposed by Ebert & Panchal.
The new model is then compared with data obtained from the monitoring of industrial heat exchangers.
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