Chemical Reaction Engineering. Dr. Yahia Alhamed

Transcription

1 Chemical Reaction Engineering Dr. Yahia Alhamed 1

2 Kinetics and Reaction Rate What is reaction rate? It is the rate at which a species looses its chemical identity per unit volume. The rate of a reaction can be expressed as:- - The rate of disappearance of a reactant or - The rate of appearance of a product. 2

3 YA1 Consider species A: Reaction Rate -r A = the rate of formation of species A per unit volume r B = the rate of formation of species B per unit volume EXAMPLE: If B is being formed at 0.2 moles per decimeter cubed per second, ie, r B = 0.2 mole/dm 3 /s Then A is disappearing at the same rate: -r A = 0.2 mole/dm 3 /s The rate of formation (generation of A) is ra= -0.2 mole/dm3/s 3

4 Slide 3 YA1 Y A, 4/5/2008

5 Reaction Rate Consider species j: r j is the rate of formation of species j per unit volume [e.g. mol/dm 3 *s] r j is a function of concentration, temperature, pressure, and the type of catalyst (if any) r j is independent of the type of reaction system (batch, plug flow, etc.) r j is an algebraic equation, not a differential equation 4

6 Rate Law Basics A rate law describes the behavior of a reaction. The rate of a reaction is a function of temperature (through the rate constant) and concentration. 5

7 Reaction Rate for solid catalytic reactions For a catalytic reaction, we refer to -r A ', which is the rate of disappearance of species A on a per mass of catalyst basis. -r ' A = r A /bulk density of the catalyst (ρb) 6

8 Rate Law Basics A rate law describes the behavior of a reaction. The rate of a reaction is a function of temperature (through the rate constant) and concentration. Power Law Model k is the specific reaction rate (constant) k is given by the Arrhenius Equation: Where:E = activation energy (cal/mol) R = gas constant (cal/mol*k) T = temperature (K) A = frequency factor (units of A, and k, depend on overall reaction order) 7

9 General Mole Balance 8

10 Batch Reactor Mole Balance 9

11 Constantly Stirred Tank Reactor Mole Balance CSTR or MFR 10

12 Plug Flow Reactor (PFR) Mole Balance F df The integral form V = A is: A F A0 r A This is the volume necessary to reduce the entering molar flow rate (mol/s) from F A0 to the exit molar flow rate of F A. 11

13 Packed Bed Reactor Mole Balance PBR F A0 F A + r A dw = dn A dt The integral form to find the catalyst weight is: W = F A F A0 df A r A 12

14 F A0 = C Ao v o Space time and space velocity θ = is called space time (s) = V/v o Space velocity = 1/θ, where; F A0 = Molar feed rate of key reactant A (mol/s) C Ao = Concentration of key reactant A in the feed (mol/m 3 ) v o =Volumetric flow rate of feed to the reactor (m 3 /s) V = volume of the reactor For constant volume systems v = v o where v is volumetric flow rate leaving the reactor 13

15 Reactor Mole Balance Summary 14

16 Reactor Mole Balance Summary 15

17 Reactor Mole Balance Summary 16

18 Reactor Mole Balance Summary 17

19 Reactor Mole Balance Summary 18

20 Conversion Consider the general reaction: aa + bb - cc + dd We will choose A as bases of calculation (i.e. Key reactant) The limiting reactant is usually taken as the key reactant Then: A + (b/a)b (c/a)c + (d/a)d X A = moles reacted/moles fed 19

21 Batch Reactor Conversion dn A dt = r A V 20

22 CSTR Conversion Algebraic Form: There is no differential or integral form for a CSTR. 21

23 PFR Conversion PFR df A dv = r A ( ) F A = F A0 1 X Differential Form: Integral Form: 22

24 Design Equations V 23

25 Reactor Sizing (CSTR) Given -r A as a function of conversion, -r A =f(x), one can size any type of reactor. We do this by constructing a Levenspiel plot. Here we plot either F A0 1 as a function of X. volume of a CSTR is: r or A r A V = F A0 X 0 ( ) r A EXIT 24

26 Reactor Sizing (PFR) For PFR th evolume of the reactor needed is given by the area under the curve V PFR = 0 X F A 0 r A dx =area 25

27 Summary 26

28 Rate Law Basics A rate law describes the behavior of a reaction. The rate of a reaction is a function of temperature (through the rate constant) and concentration. Power Law Model k is the specific reaction rate (constant) 27

29 Examples of Rate Laws First Order Reactions (1) Homogeneous irreversible elementary gas phase reaction C 2 r H A 6 C = kc 2 H C 2 H H 2 with k = 0.072s 82kcal 1 mol e T 28

30 Examples of Rate Laws First Order Reactions (1) Homogeneous irreversible elementary gas phase reaction C 2 r H A 6 C = kc 2 H C 2 H H 2 with k = 0.072s 82kcal 1 mol e T (2) Homogeneous reversible elementary reaction n C 4H10 i C4H10 [ C C K ] r = n k nc4 ic4 with k = 31.1exp 7906 and C T T K C T 333 = 3.03exp T 29

31 Examples of Rate Laws First Order Reactions (1) Homogeneous irreversible elementary gas phase reaction C 2 r H A 6 C = kc 2 H C 2 H H 2 with k = 0.072s 82kcal 1 mol e T (2) Homogeneous reversible elementary reaction n C 4H10 i C4H10 [ C C K ] r = n k nc4 ic4 with k = 31.1exp 7906 and C T T K C T 333 = 3.03exp T Second Order Reactions (1) Homogeneous irreversible non-elementary reaction r = kc A ONCB C NH 3 with 3 m k = kmol.min and cal E = mol At 188 C This is first order in ONCB, first order in ammonia and overall second order. 30

32 Examples of Rate Laws Second Order Reactions (2) Homogeneous irreversible elementary reaction CNBr + CH + r = kc A 3NH2 CH3Br NCNH2 CNBr C CH 3 NH 2 with 2.2dm k = s.mol 3 31

33 Examples of Rate Laws Second Order Reactions (2) Homogeneous irreversible elementary reaction CNBr + CH + r = kc A 3NH2 CH3Br NCNH2 CNBr C CH 3 NH 2 with 2.2dm k = s.mol This reaction is first order in CNBr, first order in CH 3 NH 2 and overall second order. 3 (3) Heterogeneous catalytic reaction: The following reaction takes place over a solid catalyst: 32