Lecture 13 Chemical Reaction Engineering (CRE) is the field that - PowerPoint PPT Presentation

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.

Le Lect ctur ure e 13 3 – Tue uesd sday y 2/ 2/26 26/2 /2013  Complex Reactions: A +2B  C A + 3C  D  Example A: Liquid Phase PFR  Example B: Liquid Phase CSTR  Example C: Gas Phase PFR  Example D: Gas Phase Membrane Reactors Sweep Gas Concentration Essentially Zero Sweep Gas Concentration Increases with Distance  Example E: Semibatch Reactor 2

Gas Phase Multiple Reactions 3

Ne New w th things ngs for or mu multip tiple le reactions eactions are: e: 1. Number Every Reaction 2. Mole Balance on every species 3. Rate Laws (a) Net Rates of Reaction for every species N   r r A iA  i 1 (b) Rate Laws for every reaction   2 r k C C 1 A 1 A A B   2 3 r k C C 2 C 2 C A C (c) Relative Rates of Reaction for every reaction For a given reaction i: (i) a i A+ b i B  c i C +d i D: r r r r    iA iB iC iD   a b c d 4 i i i i

Reactor Mole Balance Summary Liquid Phase Reactor Type Gas Phase dN A  dC Batch r V  A r A A dt dt  dN dC C A  Semibatch   A r 0 A r V A A dt V dt   dN   dC C C   B r V F   B r 0 B 0 B B B 0 B dt dt V 5

Reactor Mole Balance Summary Liquid Phase Reactor Type Gas Phase     F F C C CSTR    A 0 A A 0 A V V 0   r r A A dF  dC PFR   A r A r A 0 A dV dV dF dC      A r A r PBR A 0 A dW dW Note: The reaction rates in the above mole balances are net rates. 6

Batch Flow N F B  C  B B C B  V N P T F P T     T 0 0 T 0 0 V V 0 0 N P T F P T T 0 T 0 N N P T F F P T   C B T 0 0 C B T 0 0 B B  N V P T F P T T 0 0 T 0 0 N P T F P T   C C B 0 C C B 0 B T 0 B T 0 N P T F P T 7 T 0 T 0

Stoichiometry Concentration of Gas:     F T          C C A y 0 F F F F F   A T 0 T A B C D F  T    T Note: We could use the gas phase mole balances for liquids and then just express the concentration as: F C  A Flow: A  0 N A  Batch: C A V 0 8

Example A: Liquid Phase PFR The complex liquid phase reactions follow elementary rate laws:     2 r k C C ( 1 ) A 2 B C 1 A 1 A A B NOTE: The specific reaction rate k 1A is defined with respect to species A.   ( 2 ) 3 C 2 A D   3 2 r k C C 2 C 2 C C A NOTE: The specific reaction rate k 2C is defined with respect to species C. 9

Example A: Liquid Phase PFR Complex Reactions A  2B  C (1) (2) A  3C  D 1) Mole Balance on each and every species ฀ dF dF   A B ( 1 ) r ( 2 ) r A B dV dV dF dF   C D ( 3 ) r ( 4 ) r C D dV dV 10 10

Example A: Liquid Phase PFR 2) Rate Laws:     ( 5 ) r r r ( 7 ) r r r Net Rates A 1 A 2 A B 1 B 2 B     ( 6 ) r r r ( 8 ) r 0 r C 1 C 2 C D 2 D   2 Rate Laws ( 9 ) r k C C 1 A 1 A A B   2 3 ( 10 ) r k C C 2 C 2 C A C r r r Relative Rates   1 1 A 1 B 1 C   Reaction 1 2 1  (11) r 2 r 1 B 1 A   (12) r r 1 C 1 A 11 11

Example A: Liquid Phase PFR r r r Relative Rates   2 A 2 C 2 D   2 3 1 Reaction 2 2  ( 13 ) r r 2 A 2 C 3 r   2 C ( 14 ) r 2 D 3 2    2 2 3 r k C C k C C A 1 A A B 2 C A C 3   2 r 2 k C C B 1 A A B   2 3 r k C C k C C C 1 A A B 2 C A C k  2 3 2 C r C C 12 12 D A C 3

Example A: Liquid Phase PFR 3) Stoichiometry Liquid   ( 15 ) C F A A 0   ( 16 ) C F B B 0   ( 17 ) C F C C 0   ( 18 ) C F D D 0   ~ F       ( 19 ) S if V 0 . 00001 then C else 0   C D F   D 13 13

Example A: Liquid Phase PFR  F Liquid – Not Needed Others T   ( 19 ) Liquid – Not Needed 0  ( 20 ) C Liquid – Not Needed T 4) Parameters  ( 21 ) k 10 1 A  ( 22 ) k 20 2 C   ( 23 ) Liquid  ( 24 ) C Liquid T 0  ( 25 ) V 2500 f  ( 26 ) F 200 A 0  ( 28 ) F 200 B 0 14 14   ( 26 ) 100 0

Example B: Liquid Phase CSTR Same reactions, rate laws, and rate constants as Example A     2 r k C C ( 1 ) A 2 B C 1 A 1 A A B NOTE: The specific reaction rate k 1A is defined with respect to species A.   ( 2 ) 3 C 2 A D   3 2 r k C C 2 C 2 C C A NOTE: The specific reaction rate k 2C is defined with respect to species C. 15 15

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 reaction 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 16 16

Example B: Liquid Phase CSTR (1) A + 2B →C (2) 2A + 3C → D   2 r k C C 1 A 1 A A B   2 3 r k C C 2 C 2 C A C 1) Mole Balance      ( 1 ) A C C r V 0 0 A 0 0 A A      ( 2 ) B C C r V 0 0 B 0 0 B B     ( 3 ) C 0 C r V 0 0 C C     ( 4 ) D 0 C r V 0 0 D D 17 17

Example B: Liquid Phase CSTR 2) Rate Laws: (5)-(14) same as PFR 3) Stoichiometry: (15)-(18) same as Liquid Phase PFR  F C   ( 19 ) S C 0 C C / D    F 0 . 0001 C 0 . 0001 D 0 D 4) Parameters:  k , k , C , C , V , 1 A 2 C A 0 B 0 0 18 18

Example B: Liquid Phase CSTR In terms of molar flow rates (1) A + 2B →C (2) 2A + 3C → D   2 r k C C 1 A 1 A A B   2 3 r k C C 2 C 2 C A C 1) Mole Balance (1 – 4) 2) Rates (5 – 14) 3) Stoichiometry: (15 – 19)      Same as   ( 1 ) f F F F r V (=0) ( 15 ) C F A A 0 A A A A 0 Example A   ( 16 ) C F      B B 0 ( 2 ) f F F F r V (=0) B B 0 B B   ( 17 ) C F C C 0      0   ( 18 ) C F ( 3 ) f F F r V (=0) D D 0 C C C F  ( 19 ) S C   C D   0   F 0 . 00001 ( 4 ) f F F r V (=0) D D D D 19 19

Example B: Liquid Phase CSTR In terms of concentration (1) A + 2B →C (2) 2A + 3C → D   2 r k C C 1 A 1 A A B   2 3 r k C C 2 C 2 C A C 1) Mole Balance (1 – 4) 2) Rates (5 – 14) 3) Stoichiometry: (15 – 19)        (=0) ( 1 ) f C C C r V Same as F A 0 A 0 0 A A  ( 15 ) S C Example A C D  F 0 . 00001 D        (=0) ( 2 ) f C C C r V B 0 B 0 0 B B       (=0) ( 3 ) f C 0 C r V C 0 C C       ( 4 ) f C 0 C r V (=0) D 0 D D 20 20

Example C: Gas Phase PFR, No ΔP Same reactions, rate laws , and rate constants as Example A:   ( 1 ) A 2 B C   2 r k C C 1 A 1 A A B NOTE: The specific reaction rate k 1A is defined with respect to species A .   ( 2 ) 3 C 2 A D   3 2 r k C C 2 C 2 C C A NOTE: The specific reaction rate k 2C is defined with respect to species C. 21 21

Example C: Gas Phase PFR, No ΔP 1) Mole Balance dF dF   A C ( 1 ) r ( 3 ) r A C dV dV dF dF   B D (2) r ( 4 ) r B D dV dV 2) Rate Laws: (5)-(14) same as CSTR 22 22

Example C: Gas Phase PFR, No ΔP 3) Stoichiometry: Gas: Isothermal T = T 0 F F   A B ( 15 ) C C y ( 16 ) C C y A T 0 B T 0 F F T T F F   ( 17 ) C C C y ( 18 ) C C D y C T 0 D T 0 F F T T     ( 19 ) F F F F F T A B C D Packed Bed with Pressure Drop       dy F T F         T T     dW 2 y F T 2 y F     T 0 0 T 0 23 23

Example C: Gas Phase PFR, No ΔP 4) Selectivity   F F            S C if V 0 . 00001 then C else 0 20   F F   D D    y 1 21 24 24