1. Multiple Reactions
Chapter 6

2. Multiple Reactions
Seldom is the reaction of interest the only one occurs in the chemical reactor. Typically multiple reaction will occur, some desired and some undesired.
The key factor in economic success of chemical plant is minimizing the undesired side reactions.

3. Multiple Reactions The objectives of this chapter are
To discuss the general mole balance and reactor selection for multiple reaction.
Define different types of multiple reactions such as series, parallel, independent and complex reactions.
Define the selectivity parameters in terms of minimizing the undesired products.
Develop the algorithm to solve process with multiple reactions.

8. Desired & Undesired Reactions (Selectivity and Yield)

9. SD/U1/U2 CA

10. Example: Maximizing the Selectivity - Parallel Reactions
Determine the instantaneous selectivity, SD/U, for the liquid phase reactions:
                           
Sketch the selectivity as a function of CA.
Is there an optimum and if so what is it?

11. Solution Finding the optimum concentration:
Use CSTR with exit concentration C*A

12. Solution
In general if the order of desired product higher, then CA should be maintained higher (batch & PFR) are recommended
If the order of undesired reaction is higher then CA should be maintained at minimum (CSTR is recommended)

13. Example: Finding the Selectivity
For the elementary reactions:                     
with k1=0.1 s-1 and k2=0.2 s-1 with CAO= 2 mol/dm3.
Plot the concentration of B and selectivity of B to C as a function of space time in a CSTR.

14. Solution
CSTR
A. Balance on Species A

15. B. Balance on Species B              

16. C. Balance on Species C

17. The space time τ. is the minimum value of V (i. e. τ
The space time τ* is the minimum value of V (i.e. τ*, ) at which there is an acceptable molar flow rate of the desired product B from the CSTR.

18. What Type of Reaction is Taking Place?
Three species were found in a CSTR. The following concentration data were obtained as a function of temperature. The initial concentration of the single reactant, A, was the same at all temperatures. Both B and C are products. CA0 = 2moles/dm3
Run T (oC) CA (mole/dm3) CB (mole/dm3) CC (mole/dm3)
Explain the above data, what type of reaction is taking place, independent, complex, series or parallel ?

19. Solution Observations At low temperatures 1) Little conversion of A
                        2)   Little B formed
                        3)   Mostly C formed (but not too much because of the low conversion - 15 to 30% - of A)
            At high temperatures
                        1)   Virtually complete conversion of A
                        2)   Mostly B formed

20. Data suggest 2 reactions
                                                                                   
                            
Reaction (1) is dominant at high temperatures
                                                
                                                
Data suggest 2 reactions
Reaction (1) is dominant at high temperatures
with
Reaction (2) is dominant at low temperatures
                            
                            
Reaction (2) is dominant at low temperatures
Reaction (2) is dominant at low temperatures
                          
                          

22. Reactor selection and operation
Types of reactors
Batch reactor
PFR (with and without recycle)
CSTR (with and without recycle)
Semibatch reactors
Membrane reactor

23. Choice of reactor Example 6-3

28. 3. Algorithm for Multiple Reactions

29. 3. Algorithm for Multiple Reactions
Number of reactions
A+2B  C
3A+C2D

31. (1) A+2B  C
(2) 3A+C2D

32. (1) A+2B  C
(2) 3A+C2D

33. (1) A+2B  C
(2) 3A+C2D

34. (1) A+2B  C
(2) 3A+C2D

38. Algorithm for Multiple Reactions
1. Mole Balance

39. 2. Net Rate of Reaction for Species A

40. Elementary Multiple Reactions

41. Elementary Multiple Reactions
Species A

42. Species B

43. Species C

44. Species D

45. Case 1:Liquid Phase Reaction in a CSTR

47. Polymath Solution

48. Example: Liquid Phase Reaction

49. Example: Liquid Phase Reaction

50. Example: Liquid Phase Reaction

51. Case 3: Semibatch Liquid Phase

52. Multiple Reactions — Gas Phase
Use Molar Flow Rates!!!

53. Multiple Reactions — Gas Phase
Reaction Rate

54. Multiple Reactions — Gas Phase

55. Multiple Reactions — Gas Phase

56. Multiple Reactions — Gas Phase
Stoichiometry

57. Multiple Reactions — Gas Phase
Combine

58. Multiple Reactions — Gas Phase