Category: Technical Documents
Date: 2013-02-26
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pilot size vessel is scaled up with the intentions of maintaining similar characteristics. However this isn’t easy and in order to achieve a successful scale up, it is important to consider the key points related to reaction and mixing. Therefore it is important to analyze information obtained from pilot size reactions so they can be used in the implementation of commercial size reactors.
Some factors affecting scale up include: the range of shear rate and energy dissipation in the vessel widen as volume of vessel is increased. Heat transfer surface area per unit volume will decrease and this could lead to hot spots which are bad because it could lead to unwanted chemical reactions. The manner in which chemicals (feed) are added is critical because the conditions will change between pilot size and scale up and can have an effect by factor > 10. At some critical concentration of chemicals, the chemical reaction becomes faster then mixing and this affects conversion and yield [15, 16]. Therefore chemicals need to be added in such a way little or no reaction takes place before mixing is done.
To address these concerns several methods need to be practiced such as using the same concentration of chemicals in both pilot and scale up. Also the time for addition of chemicals i increases on scale-up because of heat transfer limitations and to maintain expected molar ratio at the feed point. If the feed time is small, then the yield may be small. Even though mixing varies depending on size of vessel, the Damkoehler numbers stays the same for different vessel size, impeller rotation rate, feed location and concentration. Therefore it is a useful parameter when considering scale up because it tells us about the mixing effects that need to be considered. It is given by equation 1.
DaM DaM > 1000 Reaction rate is much faster then mixing rate
0.001
Feed pipe backmixing lowers yield by causing a slower overall mixing rate of the reactants. To avoid this, one must have sufficient feed pipe exit velocity/impeller tip speed (vf/vt) [17].
Several points need to be discussed for two-phase reaction scale up:
Liquid-Liquid Reaction. There are many uncertainties with this system when scaling up. Testing over a wide range of operating conditions in the pilot phase is required to determine interfacial differences which will help predict sensitivity of each system to changes in dispersion characteristics [18, 19, 20]. The range of operating conditions should include mixing configurations, impeller speeds and system compositions. To predict scale-up characteristics, use constant power per unit volume but look out for differences in densities for the reactants. Mass transfer limitation may be present.
Liquid-Solid Reactions. The properties of these reactions are also system dependent and at the pilot scale, these reactions need to be operated under extreme conditions to determine if there is a sensitivity to mixing. Solid suspension is required so sufficient power and speed should be applied but there is the possibility of overmixing. It’s important to maintain a good mass transfer rate to get good mixing between solids and liquid in order to achieve the expected reaction rate. The mass transfer rate can be affected when the solid is covered by another solid or liquid. This unwanted film slows mass transfer rate and needs to be removed by either chemical means (addition of surfactant) or physical means (varying mixing intensity).
