1. Chapter 6
Multiple Reactions

2. Overview Multiple reactions types
Define “Selectivity”. How it can be used in the design?
Solve engineering problems with multiple reactions

3. 6.1 Definitions 6.1.1 Types of reactions
There are four types of multiple reactions
Parallel reactions (competing reactions) where the reactant is consumed by two different reaction pathways to form different products
e.g oxidation of ethylene to ethylene oxide
Series reactions (consecutive reactions) where the reactant forms an intermediate product, which reacts further to form another product
e.g EO with NH3 to form mono, di and triethanolamine

4. Complex reactions are reactions that involve a combination of both series and parallel reactions
e.g formation of butadiene from ethanol
Independent reactions are reactions that occur at the same time independently
e.g cracking of crude oil to form gasoline

5. We want to minimize the formation of undesired products and maximize the formation of desired products to reduce the cost of separating undesired from desired products

6. Selectivity
Selectivity quantifies the formation of desired wrt undesired products
Instantaneous selectivity
Overall selectivity
For a batch reactor
For CSTR

7. Yield
Yield is defined as the ratio of the reaction rate of a given product to that of the key reactant
Instantaneous yield
Overall yield
For a batch reactor
For CSTR

8. 6.2Parallel Reactions
We examine ways to maximize SD/U for parallel reactions

9. Keep the conc of reactant A as high as possible
Case 1: α1>α2
Keep the conc of reactant A as high as possible
If gas phase no inerts and high pressure are used
If liquid phase use of diluents is minimized
Batch or PFR is used because conc drops progressively while in CSTR the conc is always at the lowest exit value
Case 2: α1<α2
Keep the conc of reactant A as low as possible
Diluting the feed with inerts
Recyle reactor product
CSTR is used

10. The sensitivity of the selectivity to the temperature can be determined from the reaction rates ratio
Case 3: ED>EU
kD and rD↑ with temperature than Ku
Keep temperature as high as possible
Case 4: EU>ED
Operate at low possible temperature

11. 6.3 Reactions in series
For the sequence where B is the desired product
If the first reaction is slow and the second reaction is fast it will difficult to produce B
If the first reaction is fast and the second reaction is slow large yield of B can be achieved
For series reactions space time (in flow reactor) and real time in batch reactor is the most important variable

12. The reactions can be rewritten as
Applying on A and B mole balance, rate law, stoichiometry, combine and evaluate algorithm

13. Optimum yield of B (maximum CB) At this maximum value of CB τ, W and X can be solved for as follows

14. 6.4 Complex Reactions The algorithm for complex reactions is
Number each reaction
Write mole balance on each and every species
Write the net reaction rate for each species
Write rate law for one species in every reaction
Relate the reaction rates for each species
Combine the rates in terms of conc
Write stoichiometry (conc in terms of flow rates)
Write pressure drop in terms flow rates
Combine and solve ODE

15. The net reaction rate for species j is the sum of all reaction rates in which j appears
Where q is the number of reactions
Rate law is required for one species in each reaction
For the reaction
The reaction rate of each species can be related to each other as

16. For liquid phase
For gas phase
Then
Where represents the net rate of formation

17. 6.5 Multiple Reactions in PFR/PBR
Combining mole balance, rate laws, and stoichiometry for species 1 to j in the gas phase
Coupled j ODEs must be solved simultaneously using numerical package (Polymath)

18. 6.6 Multiple Reactions in CSTR
Recall the design CSTR equation
For q gas phase reactions with N species, the following set of algebraic equations
These equations must be solved simultaneously using numerical package (Polymath)