Chemical Reaction Engineering (Homogeneous Reactions in Ideal Reactors) - Chapter 5. Design for Multiple Reactions - Mai Thanh Phong, Ph.D

Nội dung text: Chemical Reaction Engineering (Homogeneous Reactions in Ideal Reactors) - Chapter 5. Design for Multiple Reactions - Mai Thanh Phong, Ph.D

1. 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 30-Dec-22 2
2. 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 30-Dec-22 4
3. 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 30-Dec-22 6
4. 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 30-Dec-22 8