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The mean particle size and suspension density of industrial ammonium sulfate and urea crystals were measured by a single ultrasound sensor in a saturated solution at only one frequency. The information on ultrasound velocity and attenuation were combined and related to characteristic properties of the solid phase of suspensions. By model identification, a correlation between the measured signals and mean particle size as well as suspension density were evaluated and verified by additional experiments. The measurements with different particle size fractions and different suspension densities up to 40 wt-% were carried out isothermally. The approach of coupled ultrasound velocity and attenuation provides a simple technique for inline process control on the liquid and solid state of a suspension process like solution crystallization only by a single measurement device.

The influence of Fe 3+ as an example of an impurity on the crystallization of ammonium sulfate as an example of an inorganic compound is discussed based on theory and experiments. The metastable zone width was increased up to 4.5 times by 100 ppm of impurity but the effect seems to level out at higher Fe 3+ concentrations. Fe 3+ also proves to be a very effective growth and dissolution rate suppressor. The achieved data were analyzed and described by the use of the model of Kubota-Mullin. A growth stoppage could be observed at 86.5 ppm. The effect on the dissolution rate is less pronounced. This growth suppressive behavior was not consistent for all of the individual faces of ammonium sulfate. As a consequence a change in the crystal morphology could be observed. With increasing Fe 3+ concentrations the crystal habit changes from cubic to a needle like shape.

Many processes to produce fine chemicals and precursors of pharmaceuticals are still operated in batchwise mode. However, recently, more producers have taken a change to continuous operation mode into consideration, performing studies and trials on such a change, while some have even already exchanged their production mode from batchwise to continuous operation. In this paper, the stepwise development from an initial idea to industrial implementation via laboratory testing and confirmation is revealed through the example of an organic fine chemical from the perspective of a crystallization plant manufacturer. We begin with the definition of the objectives of the project and a brief explanation of the advantages of continuous operation and the associated product properties. The results of the laboratory tests, confirming the assumptions made upfront, are reported and discussed. Finally, the implementation of an industrial plant using a draft tube baffled (DTB) crystallizer and the final product properties are shown. Product properties such as crystal size distribution, crystal shape, related storage stability and flowability have successfully been improved.

The metastable zone width (MZW) is a vital parameter for the design of industrial crystallization processes of inorganic salts and could be affected by the addition of impurities. Here, the effect of the trivalent ions Fe 3+ , Cr 3+ , and Al 3+ on the MZW of ammonium sulfate was observed. All the experiments were performed with the help of ultrasonic speed measurements. Impurity concentrations up to 100 ppm at a solution pH of 4 increase the MZW depending on the metal ion. The pH value also showed a major influence on the effect of trivalent metal impurities. The results were compared to the distribution of metal-aquo-complexes existing in aqueous solutions contaminated by trivalent metal ions. The distribution was determined by using the computer-based simulation software Hyss. The first hydrolysis product was identified to be active in widening the MZW.

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