Conference Dates

May 18-22, 2003

First Page



Multistage 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. Since 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 less 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 number of heat exchanger data were collected from shell and tube heat exchangers of the phosphoric acid plant of the Razi Petrochemical Complex (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 activation energy of 57 kJ/mol. The predictions of the suggested model are in good agreement with the plant data.