Chemical Reaction Engineering

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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 Today s lecture Complex Reactions A +2B C A + 3C D 2 Liquid Phase PFR Liquid Phase CSTR Gas Phase PFR Gas Phase Membrane Reactor Sweep Gas Concentration Essentially Zero Sweep Gas Concentration Increases with Distance Semi Batch Reactor

3 Reactor Mole Balance Summary Reactor Type Gas Phase Liquid Phase Batch Semibatch 3

4 Reactor Mole Balance Summary Reactor Type Gas Phase Liquid Phase CSTR PFR PBR 4 Note: The reaction rates in the above mole balances are net rates.

5 Reactor Mole Balance Summary The new things for multiple reactions are: Rates: 1. Rate Law for every reaction 2. Relative Rates for every reaction 3. Net Rates of Reaction 5

6 Gas Phase Multiple Reactions 6

7 Note: We could use the gas phase mole balance for liquids and then just express the concentration as: Flow: C A =F A /v 0 Batch: C A =N A /V 0 Note: The reaction rates in the above mole balances are net rates. The new things for multiple reactions are: 1. Rate Law for every reaction 2. Relative Rates for every reaction 3. Net Rates of Reaction 7

8 Net Rate of Reaction for species A For N reactions, the net rate of formation of species A is: For a given reaction i: (i) a i A+b i B c i C+d i D: 8

9 Batch Flow 9

10 Last Lecture Example A: Liquid Phase PFR The complex liquid phase reactions follow elementary rate laws NOTE: The specific reaction rate k 1A is defined with respect to species A. NOTE: The specific reaction rate k 2C is defined with respect to species C. 10

11 Complex Reactions Example B: Liquid Phase CSTR Same reactions, rate laws, and rate constants as example A NOTE: The specific reaction rate k 1A is defined with respect to species A. NOTE: The specific reaction rate k 2C is defined with respect to species C. 11

12 Example B: Liquid Phase CSTR The complex liquid phase reactions take place in a 2,500 dm 3 CSTR. The feed is equal molar in A and B with F A0 =200 mol/ min, the volumetric flow rate is 100 dm 3 /min and the reation volume is 50 dm 3. Find the concentrations of A, B, C and D existing in the reactor along with the existing selectivity. Plot F A, F B, F C, F D and S C/D as a function of V 12

13 Example B: Liquid Phase CSTR CSTR (1) A + 2B C (2) 2A + 3C D 1) Mole balance: 13

14 2) Rates: 3) Parameters: 14

15 Complex Reactions Example C: Gas Phase PFR, No ΔP Same reactions, rate laws, and rate constants as example A NOTE: The specific reaction rate k 1A is defined with respect to species A. 15 NOTE: The specific reaction rate k 2C is defined with respect to species C.

16 Example C: Gas Phase PFR, No ΔP 1) Mole balance: 2) Rates: Same as CSTR (5)-(14) 16

17 Example C: Gas Phase PFR, No ΔP 3) Stoich: 4) Selectivity: 17

18 Complex Reactions Example D: Membrane Reactor with ΔP Same reactions, rate laws, and rate constants as example A NOTE: The specific reaction rate k 1A is defined with respect to species A. 18 NOTE: The specific reaction rate k 2C is defined with respect to species C.

19 Example D: 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 in 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. The superficial mass flow rates can be obtained by multiplying the species molar flow rates, F i, by their respective molecular weights, Mw i, and then summing over all species: 19

20 Example D: Membrane Reactor with ΔP Because the smallest molecule is the one diffusing out and has the lowest molecular weight, 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 F Csg 20

21 Example D: Membrane Reactor with ΔP 2) Rates: Same (5)-(14) 3) Stoich: Same (15)-(20) 4) Sweep Gas Balance: 21

22 Example D: Membrane Reactor with ΔP Case 1 Large sweep gas velocity Case 2 Moderate to small sweep gas velocity 22 Vary sg to see changes in profiles

23 Complex Reactions Example E: Liquid Semibatch Same reactions, rate laws, and rate constants as example A NOTE: The specific reaction rate k 1A is defined with respect to species A. NOTE: The specific reaction rate k 2C is defined with respect to species C. 23

24 Example E: Liquid Semibatch The complex liquid phase reactions take place in a semibatch reactor where A is fed to B with F A0 =3 mol/min. The volumetric flow rate is 10 dm 3 /min and the initial reactor volume is 1,000 dm 3. The maximum volume is 2,000 dm 3 and C A0 =0.3 mol/dm 3 and C B0 =0.2 mol/dm 3. Plot C A, C B, C C, C D and S S/D as a function of time. 24

25 Example E: Liquid Semibatch (1) A + 2B C (2) 2A + 3C D 1) Mole balance: F A0 B 25

26 Example E: Liquid Semibatch 2) Rates: Same (5)-(14) Net Rates, Rate Laws and relative rates are the same as Liquid and Gas Phase PFR and Liquid Phase CSTR 3) Selectivity: 4) Parameters: 26

27 27 End of Lecture 13

6. Multiple Reactions

6. Multiple Reactions o Selectivity and Yield o Reactions in Series - To give maximum selectivity o Algorithm for Multiple Reactions o Applications of Algorithm o Multiple Reactions-Gas Phase 0. Types