生物反应工程教学课件-Bioprocess Engineering Lesson-10.pptVIP

Chapter 11 Non-ideal Flow and Behavior of Real Bioreactors So far we have restricted our discussion to two ideal flow patterns, well-mixed and plug flow. Although real bioreactors never fully follow these flow patterns, a large number of designs and operations approximate these ideals with negligible error. However, some cases involving in biological reaction systems provide the flow patterns which are obviously different from these two ideal conditions. For example, fermenters in cascade which are widely used for ethanol fermentation process; when bioreactors are scaled up, some stagnant zones may be developed, especially for high-viscous non- Newtonian biofluids. Furthermore, bypassing or short-circuiting flows also inevitably exist inside continuously operated bioreactors. The theory of nonideal flow in bioreacter engineering mainly focuses on describing all of these phenomena mathematically. 11.1 Residence Time Distribution Function In an ideal plug-flow or batch reactor, all the elements of material leaving the reactors have been inside them for exactly the same amount of time. The time reaction elements have spent in a reactor is called residence time. The ideal plug flow and batch reactors are the only two classes of reactors in which all the reaction elements present the same residence time. In all of the other reactors the elements in the feed spend different times inside the reactors before they finally leave; that is, there is a distribution of residence times for the reaction elements within the reactors. For example, consider the CSTR; the feed introduced into it at any given time becomes completely mixed with the material already inside the reactor. In other words, some of the elements entering the CSTR leave it immediately, because product stream is being continuously withdrawn from the reactor; at the same time, others maybe remain in the reactor forever because all the material is never removed from the reactor at one time.