Chemical Reaction Engineering (Homogeneous Reactions in Ideal Reactors) - Mai Thanh Phong, Ph.D - FCE – HCMC University of Technology

Nội dung text: Chemical Reaction Engineering (Homogeneous Reactions in Ideal Reactors) - Mai Thanh Phong, Ph.D - FCE – HCMC University of Technology

1. References 1. Octave Levenspiel, “Chemical Reaction Engineering”, John Wiley&Sons, 2002. 2. H. Scot Foggler, “Elements of Chemical Reaction Engineering”,International students edition, 1989. 3. E.B.Nauman, “Chemical Reactor Design”, John Wiley & sons, 1987. 4. Stanley M. Walas, “Reaction Kinetics for Chemical Engineers”,Int. Student Edition, 1990. 5. Coulson & Richardsons, “Chemical Engineering – Vol 6”,Elsevier, 1979. 6. Richard M. Felder, “Elementary Principles of Chemical Processes”, John Wiley & sons, 2000. Mai Thanh Phong - HCMUT Chemical Reaction Engineering 30-Dec-22 2
2. Chapter 1. Introduction I. Basic Parameter Description of the amount of a substance i: mi Number of moles: ni = Mi = molecular weight M i ni Molar concentration: c = V = volume i V ni Mole fraction: xi = n j j Mai Thanh Phong - HCMUT Chemical Reaction Engineering 30-Dec-22 4
3. Chapter 1. Introduction II. Stoichiometry of chemical reactions: Stoichiometry is based on mass conservation and thus quantifies general laws that must be fulfilled during each chemical reaction. Starting point of a quantitative analysis is the following formulation of a chemical reaction: N  i Ai = 0 i=1 This equation describes the change of the number of moles of N components A1, A2, AN. The νi are the stoichiometric coefficients of component i. They have to be chosen in such a way that the moles of all elements involved in the chemical reaction remain constant. A convention is that reactants have negative stoichiometric coefficients and products have positive stoichiometric coefficients. Mai Thanh Phong - HCMUT Chemical Reaction Engineering 30-Dec-22 6
4. Chapter 1. Introduction III. Chemical thermodynamics: Chemical thermodynamics deal with equilibrium states of reaction system. This Section will concentrate on the following two essential areas: a) The calculation of enthalpy changes connected with chemical reactions, and b) The calculation of equilibrium compositions of reacting systems. 3.1 Enthalpy of reaction The change of enthalpy caused by a reaction is called reaction enthalpy ∆HR. This quantity can be calculated according to the following equation: N H R =  i H Fi i=1 ∆HFi is the enthalpy of formation of component i ∆HR 0, the reaction is endothermic Mai Thanh Phong - HCMUT Chemical Reaction Engineering 30-Dec-22 8
5. Chapter 1. Introduction The correlation of reaction enthalpy and temperature is related to the isobaric heat capacities of all species involved in the considered reaction, cPi. N T H T = H 0 +  c T dT R ( ) R  i Pi ( ) i=1 T =298K Assuming that the reactants and the products have different but temperature independent heat capacities, the temperarue dependence of the reaction enthalpy can be estimated as follows: 0 HR (T)= HR +(T −T0 )(cP,products −cP,reactants ) Mai Thanh Phong - HCMUT Chemical Reaction Engineering 30-Dec-22 10
6. Chapter 1. Introduction In Figure 1-1 is shown the course of free Gibbs enthalpy of reaction as a function of the extent of reaction. The equilibrium is reached when the free G G Gibbs enthalpy of reaction is minimum. R 0 R 0   T ,P T ,P Thus, for the chemical equilibrium: dG R = 0 Free Gibbs enthalpy d T ,P GR Or dG =0 (or in an integrated form: ∆G = 0) = 0 R R  T ,P Thus, the equilibrium is characterized by:  N Fig. 1-1: Changing of free Gibbs enthalpy  i i = 0 for a chemical reaction i=1 Mai Thanh Phong - HCMUT Chemical Reaction Engineering 30-Dec-22 12
7. Chapter 1. Introduction 3.3.2 Equilibrium constant and temperature dependence Relationship between the free Gibss enthalpy and the equilibrium constant: 0 GR (T ) = −RT ln K 0 GR K = exp − RT Van‘t Hoff equation describing the temperature dependence of the equilibrium constant: d(ln K ) H 0 = − R dT RT 2 For a small temperature range, ∆HR is constant, thus: H 0 1 1 R ln K(T2 ) = ln K(T1 )− − R T2 T1 Mai Thanh Phong - HCMUT Chemical Reaction Engineering 30-Dec-22 14
8. Chapter 1. Introduction 5. Standard Reactors To carry out chemical reactions discontinuously operated reactors or continuously operated reactors can be used. • Discontinuously: more frequently applied to produce fine chemicals • Continuously: more advantageous for the production of larger amounts of bulk chemicals. To study the different behavior of these types of reactors another important criterion serves to distinguish two limiting cases: mixed flow and plug flow behavior For theoretical studies and to compare the different reactors, four different ideal reactors can be defined using the above classification: a) Batch Reactor (BR, perfectly mixed, discontinuous operation): Features: • All components are in the reactor before the reaction starts • Composition changes with time • Composition throughout the reactor is uniform Mai Thanh Phong - HCMUT Chemical Reaction Engineering 30-Dec-22 16
9. Chapter 1. Introduction b) Semi-batch Reactor (SBR): perfectly mixed, semi continuous operation Features: • One reactant is introduced first and then the second is dosed in a controlled manner. • Composition changes with time • Composition throughout the reactor is uniform Adv.: • Controlled reaction rate and heat generation • Disadv.: • Same as BR • More complicated than BR • Mai Thanh Phong - HCMUT Chemical Reaction Engineering 30-Dec-22 18
10. Chapter 1. Introduction d) Plug Flow Tubular Reactor (PFTR): no mixing, continuous operation A, B tubular reactor A, B, products Features: • Composition varies from point to point along a flow path Adv.: • High conversion • Easy to automate • No dead times • Better to cool (compare to stirred tanks) • Disadv.: • Complicated • Danger of “hot spot” • Mai Thanh Phong - HCMUT Chemical Reaction Engineering 30-Dec-22 20