Heat Transfer in Agitated Vessels

All over process industry agitated vessels are used for carrying out operations involving heat transfer. Both, mixing and heat transfer, are major prerequisites for safe and reliable operation in chemical reaction, fermentation, crystallization, polymerization and many other processes.

Regarding a lot of applications all over process industry, there is a lot of potential for optimization, still. Main targets for the producers can be energy savings, reduction of heating or cooling times, or more specific - to provide a narrow temperature window in a biochemical process, or to control a chemical reaction, which is a very basic requirement for safe plant operation. ?

Heat transfer in agitated vessels can be carried out either through an?external jacket?on the vessel or by?internal coils (Figure 1). Where a jacket or coils cannot provide the surface area required, a recirculation loop with an external heat exchanger may be used.

The choice between a jacket and coils is based on several considerations. Especially for large reactors with exothermic reactions, a jacketed vessel has the disadvantage that the area/volume ratio decreases with increasing scale.

For highly corrosive or highly reactive materials, a jacket has the advantage that there are no extra material of construction and no extra metal surface in contact with the process other than the normal vessel wall. There is also less risk of cooling fluid coming into contact with the reaction mass. For the manufacture of pharmaceuticals, fine chemicals and performance products, a jacket minimizes contamination as there are no extra surfaces to clean. For materials with difficult rheology the full range of agitator designs can be used with a jacket without difficulty, compared to a design with internal coils.

However, a jacket has a lower heat transfer performance than a coil as there will be a lower process side coefficient, usually a greater wall thickness, and a smaller surface area. A jacket may also require a higher service side flow. The use of an extended height/diameter ratio at larger scale can help to reduce this problem, but in many cases only to a limited extent.

For a given reactor geometry, the process side heat transfer coefficient (alpha i) will be determined by the agitator type and speed.?If this value is not sufficient for the process requirements, it can often be increased by selection or a more efficient, well-suited mixing setup.

It is important to clarify the flow conditions – turbulent, laminar, or something inbetween? – which is determined by the product rheology. A reliable measurement of the product viscosity is quite helpful, especially if it comes to higher viscosities or complex rheology, e.g. gel-like behavior or a flow limit. The impact of viscosity on the inside heat transfer coefficient is very often underestimated. Here selection of the right combination of impeller system and cooling strategy is the key to a safe and robust process.

To sum this up:

• Agitated vessels are vital for heat transfer and mixing in various processes.
• Optimization targets include energy savings and precise temperature control.
• Choosing between a jacketed vessel and internal coils depends on scale and material properties.
• Jackets offer advantages for corrosive materials and ease of cleaning, while coils provide better heat transfer efficiency.
• Proper agitator selection and understanding product rheology are crucial for a safe and robust process.

The EKATO webinar coming up August 17 will give insight into process solutions involving heat transfer in agitated vessels. It will answer the question “How to select the right mixing system to optimum serve the required heating or cooling duty for the corresponding process.”

Register now: https://www.ekato.com/ekato-group/workshops-and-seminars/mixing-basics-heat-transfer/

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