Chapter 5 design for multiple reactions

VIETNAM NATIONAL UNIVERSITY – HO CHI MINH CITY
UNIVERSITY OF TECHNOLOGY

FACULTY OF CHEMICAL ENGINEERING

Chemical Reaction Engineering

(Homogeneous Reactions in Ideal Reactors)
Mai Thanh Phong, Ph.D.

FCE – HCMC University of Technology

Chemical Reaction Engineering


Chapter 5. Design for Multiple Reactions
1.
•

Multiple Reactions
For multiple reactors, both the size requirement and the distribution of
reaction products are affected by the flow within the reactor.
•
The distinction between a single reaction and multiple reactions is that the
single reaction requires only one rate expression to describe its kinetic behavior
whereas multiple reactions require more than one rate expression.
•
Many multiple reactions can be considered to be combinations of two
primary types: parallel reactions and series reactions.
•
In this chapter, expansion effects are ignored, thus ε = 0

1.1. Qualitative Discussion About Product Distribution
Consider the decomposition of A by either one of two paths:
(5.1)

Mai Thanh Phong - HCMUT

Chemical Reaction Engineering

Apr 9, 2023

2


Chapter 5. Design for Multiple Reactions
with corresponding rate equations
(5.2a)
(5.2b)
Dividing Eq. 5.2a by Eq. 5.2b gives a measure of the relative rates of formation of
R and S. Thus
(5.3)
This ratio is expected to be as large as possible.
k1, k2, a1, and a2 are all constant for a specific system at a given temperature. Thus,
CA is the only factor in this equation which we can adjust and control.
Mai Thanh Phong - HCMUT

Chemical Reaction Engineering

Apr 9, 2023

3

Chapter 5. Design for Multiple Reactions
• If a1 > a2: the desired reaction is of higher order than the unwanted reaction.
Eq. 5.3 shows that a high reactant concentration is desirable since it increases the
R/S ratio. As a result, a batch or plug flow reactor would favor formation of
product R and would require a minimum reactor size.
• If a1 < a2: the desired reaction is of lower order than the unwanted reaction.
A low reactant concentration is needed to favor formation of R. But this would
also require large mixed flow reactor.
• If a1 = a2: the two reactions are of the same order, Eq. 5.3 becomes
(5.4)
Thus, product distribution is fixed by k1/k2 alone and is unaffected by type of
reactor used.
Product distribution can be controlled by varying k1/k2 in two ways:
Mai Thanh Phong - HCMUT

Chemical Reaction Engineering

Apr 9, 2023

4


Chapter 5. Design for Multiple Reactions
1. Changing the temperature. If the activation energies of the two reactions are
different, k1/k2 can be varied by changing temperature.
2. Using a catalyst. One of the most important features of a catalyst is its selectivity
in depressing or accelerating specific reactions. This may be a much more effective
way of controlling product distribution than any of the methods discussed so far.

When you have two or more reactants, combinations of high and low reactant
concentrations can be obtained by controlling the concentration of feed materials.
Figures 5.1 and 5.2 illustrate methods of contacting two reacting fluids in
continuous and noncontinuous operations that keep the concentrations of these
components both high, both low, or one high and the other low.

Mai Thanh Phong - HCMUT

Chemical Reaction Engineering

Apr 9, 2023

5


Chapter 5. Design for Multiple Reactions

Figure 5.1 Contacting patterns for various combinations of high and low
concentration of reactants in noncontinuous operations.
Mai Thanh Phong - HCMUT

Chemical Reaction Engineering

Apr 9, 2023

6


Chapter 5. Design for Multiple Reactions

Figure 5.2 Contacting patterns for various combinations of high and low
concentration of reactants in continuous flow operations.
Mai Thanh Phong - HCMUT

Chemical Reaction Engineering

Apr 9, 2023

7


Chapter 5. Design for Multiple Reactions
1.2. Quantitative Treatment of Product Distribution and of Reactor Size
If rate equations are known for the individual reactions, we can quantitatively
determine product distribution and reactor-size requirements. For convenience
in evaluating product distribution we introduce two terms, φ and Φ.
Consider the composition of A:

The instantaneous fractional yield of R (φ) is defined as:
(5.5)

Mai Thanh Phong - HCMUT

Chemical Reaction Engineering

Apr 9, 2023

8