Hans Offermann

On the solubility of sodium chloride in water

In the present experiments the crystal growth rate of sodium chloride from aqueous solution has been studied. In particular, the solubility curve of NaCl/H2O has been measured. Crystal growth and dissolution experiments were used to determine the saturation temperatures of the given solutions; the corresponding salt concentrations were measured by chemical analysis.

Experimental investigation of precipitation reactions under homogeneous mixing conditions

Abstract The theoretical description of the kinetics of precipitation reactions implicitely uses the assumption of a spatially homogeneous supersaturation in the reactor. In preliminary experiments it is shown that, since supersaturation is imposed by mixing two reactant solutions, this stands in clear contrast to reality, where a change in hydrodynamic conditions results in different spatial supersaturation patterns, which finally leads to different particle size distributions. Consequently, kinetic expressions derived herefrom will also differ when the same supersaturation, but a different mixing pattern, applies. With a newly developed experimental setup as described here it can be proven that kinetic results become independent from the mixing conditions, i.e. thermodynamically correct, when the reactants are pre-mixed at extremely high Reynolds-Numbers.

Kinetic modelling of batch precipitation reactions

Abstract In order to exploit the information contained in the particle size distributions obtained from batch precipitation experiments and derive reliable kinetic parameters estimates, a method based on reaction modelling and parameter optimization has been developed. It has been shown that the experimental reactor set-up should be modelled as a plug flow reactor followed by a fed-batch cell. The population balance equation describing this system is discretized by using a method of classes to generate a system of ordinary differential equations and the latter is solved by an adaptive step Runge-Kutta method. On top of this, a direction set optimization method, coupled to the intelligent use of some physical insight into the system, is used to iterate on the kinetic parameter values until the computed particle size distribution best fits the experimental one.

The role of hydrodynamics in precipitation

In precipitation processes the supersaturation, which is the driving force for both nucleation and crystal growth, is generated by mixing of two solutions so that the hydrodynamic conditions of the mixing process strongly influence the result of the reaction, i.e. the characteristic features of the precipitate. Due to the rapidity of the reaction, the state of mixing is quite often imperfect when nucleation occurs, so that the particles are formed under different conditions of local supersaturation. This stands in contrast to the assumption of perfect mixing in the MSMPR concept as well as in nucleation theory, which are generally used to describe and analyze the process. In this paper, the effect of different mixing conditions on the precipitate is examined in terms of the particle size distribution. It can be shown that under the same mean supersaturation, calculated from the composition of the reactant solutions, the characteristics of the size distribution will change significantly if the mixing mode is varied. Since expressions for phase transition kinetics are deduced from the particle size distribution, this will result in different expressions for nucleation and growth rates. Especially in the case of nucleation kinetics, this contradiction must not occur if the theoretical assumptions are to match experimental reality. As a consequence, an alternative experimental setup was developed which enables the investigation of the influence of the mixing process on the precipitate by a controlled manipulation of the hydrodynamic conditions and eventually the approximation of the desired spatial homogeneity.

On the difference in the growth rates of fragmented (hurt) and non-fragmented (unhurt) crystals during industrial crystallization

Abstract This paper deals with the mass crystallization of two extreme cases: the unhurt, non-fragmented “perfect” crystal and fragments. The following results were obtained for crystal sizes of 500 to 1200 μm: KA 1( SO 4 ) 2 · 12 H 2 O fragments grow 1.5 to 3 times faster than unhurt ones and K 2 SO 4 fragments grow 2.5 to 5 times faster than the unhurt ones.

On the growth of microscopic hurt and unhurt crystals

Abstract The effect of secondary nucleation depends strongly on the growth behaviour of the nuclei. Therefore the growth rate of small ( 2 SO 4 crystals grow slower and potash-alum crystals grow faster than the corresponding unhurt crystals. As more detailed experiments are still to be done, this paper makes no attempt to explain the above mentioned phenomena.