Chapter 5 design for multiple reactions

PowerPoint Präsentation Mai Thanh Phong HCMUT FCE – HCMC University of Technology Chemical Reaction Engineering (Homogeneous Reactions in Ideal Reactors) Mai Thanh Phong, Ph D VIETNAM NATIONAL UNIVERS[.]

Chemical Reaction Engineering

(Homogeneous Reactions in Ideal Reactors)

Mai Thanh Phong, Ph.D.

VIETNAM NATIONAL UNIVERSITY – HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

FACULTY OF CHEMICAL 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)

Chapter 5 Design for Multiple Reactions

with corresponding rate equations

(5.2b) (5.2a)

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

k 1 , k 2 , a 1 , and a 2 are all constant for a specific system at a given temperature Thus,

C A is the only factor in this equation which we can adjust and control

Chapter 5 Design for Multiple Reactions

• If a 1 > a 2 : 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 a 1 < a 2: 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 a 1 = a 2 : the two reactions are of the same order, Eq 5.3 becomes

(5.4)

Thus, product distribution is fixed by k 1 /k 2 alone and is unaffected by type of

reactor used

Product distribution can be controlled by varying k /k in two ways:

Chapter 5 Design for Multiple Reactions

1 Changing the temperature If the activation energies of the two reactions are

different, k 1 /k 2 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

Chapter 5 Design for Multiple Reactions

Figure 5.1 Contacting patterns for various combinations of high and low

concentration of reactants in noncontinuous operations

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

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)