Heat exchanger fouling and cleaning in the dihydrate process for the production of phosphoric acid

Crystallization fouling and cleaning mechanisms of calcium sulphate from phosphoric acid solution have been studied experimentally and theoretically during forced convective heat transfer. The effects of flow velocity, foulant concentration, surface and bulk temperatures on the formation of scale and its removal are investigated. Close examination of the heat transfer surface at the end of a cleaning period with a cleaning agent containing 5 wt% sulphuric acid reveals that significant deposits still remained on the surface. The remaining deposits were rippled, hard and difficult to remove chemically. After clarification of the effects of operating parameters on the deposition and cleaning processes, the results of the experiments are used to develop mechanistic models for the prediction of fouling resistances of calcium sulphate during forced convective heat transfer, and for removal of the deposits by dilute sulphuric acid solutions. The predictions of the suggested models for fouling and cleaning are compared with measured data. Deviations ranging from 7% to 12% are observed which confirm the suitability of the models.