Ageing process Agglomerates

Each stage of particle formation is controlled variously by the type of reactor, i.e. gas-liquid contacting apparatus. Gas-liquid mass transfer phenomena determine the level of solute supersaturation and its spatial distribution in the liquid phase the counterpart role in liquid-liquid reaction systems may be played by micromixing phenomena. The agglomeration and subsequent ageing processes are likely to be affected by the flow dynamics such as motion of the suspension of solids and the fluid shear stress distribution. Thus, the choice of reactor is of substantial importance for the tailoring of product quality as well as for production efficiency. 

Since the original discovery by Lefort, selectivity in the direct oxidation of ethylene has improved from 50 to 90% for modern silver catalyst systems. However, these highly selective modern catalysts are prone to aging processes. The main reason for aging is the agglomeration of silver surface particles (Figure 6.12.1). 

Precipitation is carried out by a controlled mixing of the reactants in order to obtain a supersaturated solution from which nucleation takes place. Amorphous primary particles are formed that later crystallise into desired phases and in parallel agglomerate to larger secondary particles. Precipitation processes need in line pH meters and possibilities for automatic particle size distribution analysis coupled to the ageing vessel. 

Most processes of powder wetting operate under dynamic conditions, and improvements in their efficiency require the use of surfactants that lower the liquid surface tension Yiy under these circumstances. The interfaces involved (particles separated from aggregates or agglomerates) are freshly formed and have only a small effective age of some seconds, or even less than a millisecond. 

A clear distinction between crystallisation and precipitation is not always possible from a practical point of view [57] hence, it is more convenient to consider precipitation as a very fast crystallisation process. Crystallisation is a result of the combined effects of nucleation, nucleus growth and secondary processes inside the suspension such as agglomeration, ageing and recrystallisation. Depending on the reaction conditions, the above processes can occur together or sequentially during the crystallisation period. 

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