Continuous flow precipitation

Abstract

The effect of flow conditions on the nature of the precipitation curve is analyzed by applying population model concepts, under the assumption that agglomeration of precipitated particles is absent. Generalized non-dimensional precipitation curves reveal that plug flow conditions provide higher conversions of solute to precipitate, as compared to mixed flow conditions, except for the region of relatively low residence times. The enhancement of precipitation under mixed flow conditions at relatively low residence times is due to the shorter induction period occurring under mixed flow conditions. When the induction period is not taken into account, plug flow conditions are shown to provide more rapid precipitation at practically all conversion levels. A result of considerable practical significance is the prediction that the product of induction time and initial precipitation rate is a constant. Its magnitude depends solely on the measurement method for detecting the induction period and is independent of initial supersaturated solution conditions. Literature data on CaCO 3 precipitation at different temperatures are shown to support this theoretical prediction. In water treatment and desalination practice, inhibitors are added in order to extend the induction period and reduce the precipitation and scale formation rates. One of the difficulties in interpreting results of laboratory tests comparing the relative effectiveness of different inhibitors is the absence of fundamentally derived parameters to quantify the significant inhibition properties. Since the theoretically predicted expression, relating induction period to initial precipitation rate, can provide such fundamentally based characterizing parameters, further experiments were carried out to test the validity of the predicted relationship for the case of CaCO 3 precipitation in the presence of additives. Results of these tests, extending over a rather wide range of conditions, confirm the validity of the theoretical prediction and indicate the possibility of developing a fundamentally based standardized test method for characterizing important scale inhibition properties of additives.