Mixing Problems

Mixing Problems

What are some pitfalls/problems you see end-users commonly encounter in fluid mixing applications? How can end-users best avoid and/or respond to such application pitfalls?

The awareness of the end-users about the different types of agitator designs available, and their applications need to be increased. Moreover, a better understanding is required on the concepts of flow and shear, produced by the impellers. More often than not, end users prefer to duplicate their operational systems, even for new requirements which may be different from the existing. The effects of improper selection are realized much later, during actual operations.

I would like to bring to you attention a very recent example, where in the works manager from a chemical company called me and asked me to quote for a “power saver” agitator for a 12,000 liters vessel. When, I inquired about his process, I was told not to worry about the technical aspects, since they felt that the “power saver” impeller was their best option. They termed the “hydrofoil impeller” as the “power saver” impeller, and chose it because someone told them that they would reduce on power consumption during agitation. Very few users know that hydrofoil impellers are available in different designs, solidity ratios, each of which is best suited for a particular kind of application. Users therefore need to consider selection of the agitator types, keeping in mind their process requirements, properties of process materials and other operating conditions. It is recommended that the selection of agitators be carried out in consultation of the agitator specialist.

Besides selection of impellers, scale-up of agitation equipment offers several challenges in fluid mixing. While scaling up, design factors such as the physical configuration of mixing vessel (off-bottom distance, spacing between impellers and the ratio of impeller diameter to tank diameter), impeller diameter, speed, power consumption, and so on need to be considered.

Application pitfalls can be avoided by ensuring that the equipment manufacturer knows everything about the process, including what could go wrong in case there is a deviation in the process conditions. In case of new processes, applications, it is recommended that laboratory, pilot scale trials be carried out to establish process objectives and equipment design and performance parameters.

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