Heat exchanger fouling in phosphoric acid evaporators

Multistage shell and tube evaporators are frequently used in phosphoric acid plants to increase the concentration of dilute phosphoric acid to 52-55 wt% P2O5. The concentrated phosphoric acid solution is supersaturated with respect to calcium sulfate. As a result, part of the calcium sulfate in the liquor deposits on the heat exchanger tube walls. Because the thermal conductivity of these scales is very low, thin deposits can create a significant resistance to heat transfer. Therefore, regular cleaning of heat exchangers is required, frequently at shorter than biweekly intervals. As the major costs in modern phosphoric acid plants are the cost of energy, a thorough understanding of the fouling kinetics and of the effects of various operational parameters on the behavior of calcium sulfate is required to improve operation and design of the shell and tube heat exchangers, which are extensively used. In this investigation, a large set of heat exchanger data was collected from shell and tube heat exchangers of the phosphoric acid plant of the Razi Petrochemical Complex in Iran, and the fouling deposits were analyzed with respect to appearance and composition. The overall heat transfer coefficients and fouling resistances were evaluated at different times, and a kinetic model for the crystallization fouling was developed. It is shown that the crystallization rate constant obeys an Arrhenius relationship with an activation energy of 57 kJ/mol. The average absolute error of 12.4% shows that the predictions of the suggested model are in good agreement with the original plant data.