Secondary nucleation scale

Garside, J. and Jancic, S.J., 1979. Measurement and scale-up of secondary nucleation kinetics for the potash alum-water system. American Institute of Chemical Engineers Journal, 25, 948. 

As mentioned previously, scale-up of crystallization processes from the laboratory is far from straightforward. Various parameters need to be maintained to be as close to those used in the laboratory as possible in order to reproduce the results from the laboratory. For scale-up, supersolubility, agitation (and its effect on secondary nucleation throughout the vessel), fraction of solids in the slurry, seed number and sizes, contact time between growing crystals and liquid all need to be maintained. 

Note. A small response could indicate that other system properties were controlling (i.e., inherent crystal growth rate or nucleation rate). A large response would indicate sensitivity to secondary nucleation and/or crystal cleavage and require additional experimentation and evaluation of scale-up requirements. The laboratory results should be evaluated relative to each other since scale-up can be expected to make additional changes in PSD, especially when a large response is experienced in these simple experiments. 

Nienow, A.W. (1976). The effect of agitation and scale-up on crystal growth rates and on secondary nucleation. Trans. Inst. Chem. Eng. 54, 205. 

An obvious example is the surface-to-volume ratio, the value of which decreases with scale-up. Subsequently, high coohng rates can only be maintained in large-scale industrial practice if the jacket temperature is decreased substantially. This decrease may critically affect the nucleation rate however, as solution temperatures at and close to the crystalhzer wall are also lowered, potentially enhancing primary or secondary nucleation in this region, the decrease may then cause crusting on the vessel walls. 

Nienow, A.W. (1996). The Effect of Agitation and Scale-Up on Crystal Growth Rates and on Secondary Nucleation, TransIChemE. 54, pp. 205-207. 

On an industrial scale, the maximum cooling rate is often limited by the heat-transfer capacity of the equipment, which means that the theoretical cooling program cannot be followed during the final stage of the batch. A high suspension density may cause high secondary nucleation and, thus, decrease the crystal size of the product. 

Most models for the scale-up of contact secondary nucleation rates are based on the Botsaris (1976) relationship... 

Direct scale-up of kinetic information from bench-scale tests usually is unreliable since the influence of mixing intensity on secondary nucleation is difficult to assess, as is the simulation of acmal plant operating conditions. However, the exponents of the nucleation rate model shown in Eq. (11.3-12) can be evaluated in small-scale equipment in addition to the sensitivity of the nucleation rate coefficient to changes in temperature and/or level of agitation. 

Since secondary nucleation is not significantly affected by temperature, it could be concluded that the primary nucleation dominated the scaling mechanism. This is consistent with results from other researchers, who reported that in the precipitation of sparingly soluble substances, secondary nucleation either did not occui [64] or did occur only to a small extent [65, 66]. The number of crystals formed by secondary nucleation during precipitation was substantially lower than those resulting from primary nucleation [67]. 

As a result, the tip speed and, hence, the secondary nucleation rate can be altered at a constant rate of dissipated energy e. Industrial-scale experience indicates that... 

One of the primary objectives of bench scale experiments is to find out which phenomena are important in a particular process. First we have to decide for a given process whether primary or secondary nucleation prevails. When it follows from lab-experiments that meso-mixing has a strong effect on particle size, primary nucleation is probably essential (see section 63.2,1), When the particle size can be related to the slurry concentration, secondary nucleation must be taking place (see section 63.2.4). 

However, Aere is Ae risk Aat secondary nucleation would occur more on Ae larger scales, l ause Ae tip speed of Ae impeller increases wiA scale (for constant specific power input). Also, agglomeration may take place on Ae larger scale, even if it does not on Ae bench-scale, since Ae longer circulation times may give agglomeration a better chance. 

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