Handling Metabolic Heat Output in Pilot and Production Scale Fermentors
High density microbial cultivations often have metabolic heat outputs as large as small power plants, so appropriate cooling is required to maintain the desired cultivation temperature. In comparison with smaller fermentors, which have a higher surface-to-volume ratio, the cooling capacity of larger fermentors is comparatively poorer owing to its reduced heat exchange surface.
Of course, the heat exchange surface is only one parameter that influences adequate cooling. Other ways to increase the cooling efficiency include regulating the temperature of the cooling liquid, managing liquid flows, turbulence within the fermentor, turbulence within the jacket and so on. The basic rule for engineering such a system is by enlarging the heat exchange surface and supporting the use of colder (including supercooled) cooling liquids. In this post, Bioengineering focuses on three ways to address this challenge.
The first method is the addition of four to six hollow baffles which circulate cooling liquid. Bioengineering’s baffles fulfill all hygienic standards and do not have any limitations in terms of cleanability. The baffles can be manually flushed with cooling liquid or integrated into a fully automated system. Given that most fermentors require baffles to prevent a vortex formation, cooled baffles represent a solution that kills two birds with one stone. Cooled baffles are, however, limited in their capacity to enlarge the heat exchange area.
Cooling coils can supplement cooled baffles in circumstances that call for a larger heat exchange surface area. Cleanability may pose an issue, which Bioengineering has circumvented by combining the coils with local CIP recirculation. Cooling coils can also be custom-designed to address all kinds of concerns: for instance, they can be placed exclusively on the lower vessel, if the metabolic heat problem concerns a production phase with the lower level, or along the height of the vessel if the problem occurs at full working volume.
Direct Cooling with Cooling Liquid
As the temperature of cooling liquids influences the cooling capacity, in some cases direct supply to the vessel jacket or cooled elements is required. Supercooled liquids (below 0 °C / 32 °F) require to be handled with extra care as freezing of watery solutions might occur. Bioengineering therefore uses appropriate components and an adapted temperature control mode for its SCADA system.