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Lecture 13

Lecture 13. Chemical Reaction Engineering (CRE) is the field that studies the rates and mechanisms of chemical reactions and the design of the reactors in which they take place. Lecture 13 – Tuesday 2/22/2011. Complex Reactions A +2B  C A + 3C  D Liquid Phase PFR Liquid Phase CSTR

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Lecture 13

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  1. Lecture 13 Chemical Reaction Engineering (CRE) is the field that studies the rates and mechanisms of chemical reactions and the design of the reactors in which they take place.

  2. Lecture 13 – Tuesday 2/22/2011 ComplexReactions A +2B  C A + 3C  D • Liquid Phase PFR • Liquid PhaseCSTR • Gas Phase PFR • Gas Phase Membrane Reactor Sweep Gas Concentration Essentially Zero Sweep Gas Concentration Increases with Distance • Semi Batch Reactor

  3. Gas Phase Multiple Reactions

  4. Thenew things for multiple reactions are: • 1. Number Every Reaction • 2. Mole Balance on every species • 3. Rates: • (a) Net Rates of Reaction for every species • (b) Rate Laws for every reaction • (c) Relative Rates of Reaction for every reaction • For a given reaction i: (i) aiA+biB ciC+diD:

  5. ReactorMole Balance Summary Batch Semibatch

  6. Reactor Mole Balance Summary CSTR PFR PBR Note: The reaction rates in the abovemolebalances are net rates.

  7. Batch Flow

  8. Stoichiometry ConcentrationGas: Note: We could use the gas phase mole balances for liquids and then just express the concentration as: Flow: Batch:

  9. Example A: LiquidPhase PFR The complexliquidphasereactionsfollowelementary rate laws NOTE: The specificreaction rate k1A is defined with respect to species A. NOTE: The specificreaction rate k2C is defined with respect to species C.

  10. Example A: LiquidPhase PFR ComplexReactions Mole Balance on Each and Every Species

  11. Example A: Liquid Phase PFR Rates: Net Rates Laws Relative Rates Reaction 1

  12. Example A: Liquid Phase PFR Reaction 2

  13. Example A: Liquid Phase PFR Stoichiometry Liquid

  14. Example A: Liquid Phase PFR Others Parameters

  15. ComplexReactions Example B: LiquidPhaseCSTR Same reactions, rate laws, and rate constants as example A NOTE: The specificreaction rate k1A is defined with respect to species A. NOTE: The specificreaction rate k2C is defined with respect to species C.

  16. Example B: Liquid Phase CSTR The complexliquidphasereactionstakeplace in a 2,500 dm3 CSTR. The feed is equal molar in A and B with FA0=200 mol/min, the volumetric flow rate is 100 dm3/min and the reationvolume is 50 dm3. Find the concentrations of A, B, C and D existing in the reactoralong with the existingselectivity. Plot FA, FB, FC, FD and SC/D as a function of V

  17. Example B: Liquid Phase CSTR CSTR (1) A + 2B →C (2) 2A + 3C → D 1) Mole balance: 17

  18. Example B: Liquid Phase CSTR 2) Rates: (5)-(14) Same as PFR 3) Stoichiometry:same as Liquid PhasePFR: (15)-(18) Same as PFR 4) Parameters: 18

  19. Example B: Liquid Phase CSTRIn terms of Molar Flow Rates CSTR (1) A + 2B →C (2) 2A + 3C → D (=0) (=0) (=0) (=0) 19 19

  20. Example B: Liquid Phase CSTR In terms of Concentration CSTR (1) A + 2B →C (2) 2A + 3C → D (=0) (=0) (=0) (=0) 20

  21. Complex Reactions Example C: Gas Phase PFR, No ΔP Same reactions, rate laws, and rate constants as example A NOTE: The specificreaction rate k1A is defined with respect to species A. NOTE: The specificreaction rate k2C is defined with respect to species C.

  22. Example C: Gas Phase PFR 1) Mole balance: 2) Rates: Same as CSTR (5)-(14)

  23. Example C: Gas Phase PFR Stoichiometry Gas : Isothermal T = T0 Packed Bed with Pressure Drop

  24. Example C: Gas Phase PFR, No ΔP, (y=1) 3) Stoichmetry: 4) Selectivity:

  25. ComplexReactions ExampleD: MembraneReactorwith ΔP Same reactions, rate laws, and rate constants as example A NOTE: The specificreaction rate k1A is defined with respect to species A. NOTE: The specificreaction rate k2C is defined with respect to species C.

  26. ExampleD: Membrane Reactor with ΔP Because the smallest molecule, and the one with the lowest molecular weight, is the one diffusing out, we will neglect the changes in the mass flow rate down the reactor and will take as first approximation: • 1) Mole balance: We also need to account for the molar rate of desired product C leaving in the sweep gas FCsg

  27. ExampleD: Membrane Reactor with ΔP We need to reconsider our pressure drop equation. When mass diffuses out of a membrane reactor there will be a decrease in the superficial mass flow rate, G. To account for this decrease when calculating our pressure drop parameter, we will take the ratio of the superficial mass velocity at any point in the reactor to the superficial mass velocity at the entrance to the reactor.

  28. ExampleD: Membrane Reactor with ΔP The superficial mass flow rates can be obtained by multiplying the species molar flow rates, Fi, by their respective molecular weights, Mwi, and then summing over all species:

  29. ExampleD: Membrane Reactor with ΔP 2) Rates: Same (5)-(14) 3) Stochiometry:Same (15)-(20) 4) Sweep Gas Balance:

  30. ExampleD: Membrane Reactor with ΔP Case 1 Large sweep gas velocity Case 2 Moderate to small sweep gas velocity Varyυsg to seechanges in profiles

  31. ComplexReactions ExampleE: LiquidSemibatch Same reactions, rate laws, and rate constants as example A NOTE: The specificreaction rate k1A is defined with respect to species A. NOTE: The specificreaction rate k2C is defined with respect to species C.

  32. ExampleE: Liquid Semibatch The complexliquidphasereactionstakeplace in a semibatchreactorwhere A is fed to B with FA0=3 mol/min. The volumetric flow rate is 10 dm3/min and the initial reactor volume is 1,000 dm3. The maximum volume is 2,000 dm3 and CA0=0.3 mol/dm3 and CB0=0.2 mol/dm3. Plot CA, CB, CC, CD and SS/D as a function of time.

  33. ExampleE: Liquid Semibatch FA0 B (1) A + 2B →C (2) 2A + 3C → D • 1) Mole balance:

  34. ExampleE: Liquid Semibatch 2) Rates: Same (5)-(14) Net Rates, Rate Laws and relative rates – are the same as Liquid and Gas Phase PFR and LiquidPhase CSTR 3) Selectivity: 4) Parameters:

  35. End of Lecture 13

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