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Unformatted text preview: 10.37 Chemical and Biological Reaction Engineering, Spring 2007 Prof. K. Dane Wittrup Lecture 10: Nonideal Reactor Mixing Patterns This lecture covers residence time distribution (RTD), the tanks in series model, and combinations of ideal reactors. NonIdeal Mixing PFR CSTR Figure 1. Ideal PFR with pulse input. A pulse input will yield an output profile that is a pulse input. Figure 2. Ideal CSTR with pulse input. A pulse input will yield an output profile that is a sharp peak with a tail. Real mixed tank stagnant bypassing mixing recirculation eddies volumes Figure 3. A real mixed tank. In a real mixed tank there are portions that are not well mixed due to stagnant volumes, recirculation eddies, and mixing bypasses. In a real PFR there is backmixing and axial dispersion. In a packed bed reactor (PBR) channeling can occur. This is where the fluid channels through the solid medium. Residence Time Distribution A useful diagnostic tool is the residence time distribution (RTD). The residence time is how long a particle stays in the reactor once entering. E ( t ) dt Probability that a fluid element entering the vessel at t=0 exits between time t and t+dt. Probability density function for exit time, t, as a random variable. Cite as: K. Dane Wittrup, course materials for 10.37 Chemical and Biological Reaction Engineering, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. t E t dt Probability that fluid element exits before time t. ( ) E t dt Probability of exiting at time later than t. ( ) t mean t = tE ( ) t dt = = E ( ) dt = 1 normalized t variance = 2 = t 2 E t dt (measures the broadness of the distribution) ( ) ( ) E after t...
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 Fall '04
 ClarkColton
 Reaction

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