lec09_03072007_w - 10.37 Chemical and Biological Reaction...

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10.37 Chemical and Biological Reaction Engineering, Spring 2007 Prof. K. Dane Wittrup Lecture 9: Reactor Size Comparisons for PFR and CSTR This lecture covers reactors in series and in parallel, and how the choice of reactor affects selectivity versus conversion. PFR vs. CSTR: Size and Selectivity Material balance: CSTR PFR F V = Ao X r A A X V A F = Ao dX A 0 r A “Levenspiel Plot” A X Ao A F r 1 st or 2 nd order reaction CSTR Volume • as X increases, decreases A C A r decreases, for 1 st and 2 nd order, A so F Ao increases r A PFR Volume Figure 1. General Levenspiel Plot. Ao A F A r o A F r V CSTR A V PFR X X A So PFR is always a smaller reactor for a given conversion when kinetics are positive order. Figure 2. Levenspiel plots for a CSTR and a PFR for positive order reactions. 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].
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Non-monotonically positive order kinetics arise: Autocatalytic reactions (e.g. cell growth) Adiabatic or non-isothermal exothermic reactions Product inhibited reactions (some enzymes) Series of Reactors Example: 2 CSTRs F Ao x 2 v 1 v 2 1 A F x 1 Figure 3. Schematic of two CSTRs in series.
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lec09_03072007_w - 10.37 Chemical and Biological Reaction...

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