Continuous Sterilization

Batch sterilization wastes energy and can overcook the medium

Batch sterilization uses steam or direct firing to elevate the temperature, and then cooling water stops the process and brings the material back toward room temperature. Both the heat and the cooling water are spent with no opportunity for energy recovery. Large volumes should be passed continuously through heat exchangers for energy economy with the hot, treated fluid heating the cold, incoming feed.

One method of continuous sterilization injects steam into the medium (no heat exchanger). The medium stays in a loop for a predetermined holding time until the entire medium is sterile.

Better heat economy comes from substituting heat exchangers for direct steam injection. Instead of having a cold water stream to cool the sterile media, the lower temperature unsterile media stream absorbs heat from the warm stream, cooling the sterile media.

system for continuous sterilization has a holding coil for detention long enough to kill all of the microorganisms. The medium from a make up vessel flows through the exchanger, is held in the coil, and passes back through the heat exchanger, heating more unsterile medium while becoming cool itself, as it is collected in a sterile fermenter.

This design would work only with an exchanger with infinite heat transfer area because there is no driving force for heat transfer as the temperatures for the two streams approach closely. A real design would have another small exchanger to raise the temperature to the setpoint after the main exchanger has done all it can do. There is no need for a cooler before entering the fermenter because it has a jacket or coils for temperature control that can easily handle this load.

Heat economy is not important for a small pilot plant unit for continuous sterilization, so direct steam injection is simpler. A heat exchanger is then needed with cooling water to bring the medium back down quickly to a temperature at which it is not over cooked. 

Advantages:

  • Uniform steam requirements throughout the duration of the sterilization
  • Simplified process control
  • Shorter sterilization time means less thermal degradation of medium

    Disadvantages:

  • High demand for steam in a shorter period of time than batch
  • Concentration of media becomes dilute due to steam condensation
  • Since steam is actually dispersed in media, steam must be clean to avoid contamination

    High temperatures for short times are used in preparing nutrient media for industrial fermentations and in pasteurizing milk, because this causes less damage to biochemicals than more prolonged times at lower temperatures. This exploits the temperature effects on activation energies because bacterial killing is affected by a temperature change more than is heat destruction of biochemicals.

    Shell and Tube Exchangers

    When the flow in a heat exchanger is countercurrent, the outlet temperature of the stream being heated can approach the temperature of the hot stream to be cooled. There is an attempt to show this in the sketch. There are gradients on the shell side as well.

    The Shell and tube exchanger is not as well-suited to continuous sterilization as the plate-and-frame type of exchanger.

    Sandwich of Plates. This sketch did not turn out well.  Much better sketch by Lawrence Bernstone, 1995 The idea is that the countercurrent flow in alternate sections gives a gradient from coolest to hottest in each plate of the sandwich.


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