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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[.]
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 References Octave Levenspiel, “Chemical Reaction Engineering”, John Wiley&Sons, 2002 H Scot Foggler, “Elements of Chemical Reaction Engineering”,International students edition, 1989 E.B.Nauman, “Chemical Reactor Design”, John Wiley & sons, 1987 Stanley M Walas, “Reaction Kinetics for Chemical Engineers”,Int Student Edition, 1990 Coulson & Richardsons, “Chemical Engineering – Vol 6”,Elsevier, 1979 Richard M Felder, “Elementary Principles of Chemical Processes”, John Wiley & sons, 2000 Mai Thanh Phong - HCMUT Chemical Reaction Engineering Apr 28, 2023 Chapter Introduction • Topic of the lecture „Chemical Reaction Engineering“ is the quantitative assessment of chemical reactions The selection of suitable reactor types and their design will be discussed • Reactor design uses information, knowledge, and experience from a variety of areas: thermodynamics, chemical kinetics, fluid mechanics, heat transfer, mass transfer, and economics Chemical reaction engineering is the synthesis of all these factors with the aim of properly designing a chemical reactor • Thermodynamics tell us in which direction a reaction system will develop and how far it is from its equilibrium state • Analyses of kinetics provide information about the rate with which the system will approach equilibrium Mai Thanh Phong - HCMUT Chemical Reaction Engineering Apr 28, 2023 Chapter Introduction I Basic Parameter Description of the amount of a substance i: Number of moles: mi ni Mi Mi = molecular weight Molar concentration: ni ci V V = volume Mole fraction: ni xi nj j Mai Thanh Phong - HCMUT Chemical Reaction Engineering Apr 28, 2023 Chapter Introduction Progress of chemical reactions: Conversion: ni ni Xi ni If V = const: ci ci Xi ci Extent of reaction: ni ni i Performance criteria: Productivity: Mai Thanh Phong - HCMUT produced amount of product P n P operating time Chemical Reaction Engineering Apr 28, 2023 Chapter 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 A 0 i 1 i i 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 Apr 28, 2023 Chapter Introduction As an example the stoichiometric equation for the oxidation of carbon monoxide is given by: 2CO + O2 → 2CO2 with νCO = -2, vO2 = -1, vCO2 = To calculate changes in the mole number of a component i due to reaction, the following balance has to be respected: ni ni i From this equation results the important stoichiometric balance: ni ni ni nk nk nk i i k k Using the conversion X of a component k, the above equation becomes: i ni ni nk X k k Mai Thanh Phong - HCMUT Chemical Reaction Engineering Apr 28, 2023 Chapter 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 exothermic ∆HR > 0, the reaction is endothermic Mai Thanh Phong - HCMUT Chemical Reaction Engineering Apr 28, 2023 Chapter Introduction It is simple to calculate the reaction enthalpy at a certain standard state ∆HR0 from the corresponding standard enthalpies of formation ∆HFi0 The standard enthalpies of formation are available from databases for P = P0 = bar and T = T0 = 298 K For pure elements like C, H2, O2, : ∆HFi0 = The reaction enthalpy is a state variable Thus, a change depends only on the Initial and the end state of the reaction and does not dependent on the reaction parthway 3.2 Temperature and pressure dependence of reaction enthalpy H R H R d H R dP dT P T T P The pressure dependence is usually very small For ideal gas behaviour, the reaction enthalpy does not depend on pressure Mai Thanh Phong - HCMUT Chemical Reaction Engineering Apr 28, 2023 Chapter 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 i 1 Pi T 298 K H R T H R0 i c T dT 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: H R T H R0 T T0 cP ,products cP ,reactants Mai Thanh Phong - HCMUT Chemical Reaction Engineering Apr 28, 2023 10 Chapter Introduction 3.3 Chemical equilibrium • Chemical reactions approach to an equilibrium, when the product and reactant concentrations not change anymore • A reacting system is in chemical equilibrium if the reaction rates of the forward and backward reactions are equal • The basic quantity required to indentify the equilibrium state is the Gibbs free enthalpy of reaction GR • The change of this quantity becomes zero when the equilibrium is reached (i.e dGR = 0) For constant pressure and temperature, the change of free Gibbs enthalpy of reaction can be described as follows: N dGR i i d i 1 Mai Thanh Phong - HCMUT or N dGR i i d T , P i 1 Chemical Reaction Engineering Apr 28, 2023 11 Chapter Introduction The equilibrium is reached when the free Gibbs enthalpy of reaction is minimum Thus, for the chemical equilibrium: dGR 0 d T , P Free Gibbs enthalpy In Figure 1-1 is shown the course of free Gibbs enthalpy of reaction as a function of the extent of reaction GR 0 T ,P Or dGR=0 (or in an integrated form: ∆GR = 0) Thus, the equilibrium is characterized by: N i 1 i i Mai Thanh Phong - HCMUT 0 GR T ,P GR T , P Fig 1-1: Changing of free Gibbs enthalpy for a chemical reaction Chemical Reaction Engineering Apr 28, 2023 12 Chapter Introduction 3.3.1 Reation free Gibbs enthalpy, ∆GR N G i GFi0 R GFi0 i 1 free Gibbs energy of formation Relation between ∆GR and ∆HR d GR0 T H R0 dT T2 Mai Thanh Phong - HCMUT Chemical Reaction Engineering Apr 28, 2023 13 Chapter Introduction 3.3.2 Equilibrium constant and temperature dependence Relationship between the free Gibss enthalpy and the equilibrium constant: GR0 T RT ln K GR0 K exp RT Van‘t Hoff equation describing the temperature dependence of the equilibrium constant: d ln K H R0 dT RT For a small temperature range, ∆HR is constant, thus: H R0 ln K T2 ln K T1 R Mai Thanh Phong - HCMUT 1 T2 T1 Chemical Reaction Engineering Apr 28, 2023 14 Chapter Introduction Reaction rate Based on unit volume of reacting fluid: 1 dni r i VR dt • VR is volume of the reaction mixture • ni is mole number of component i • t is reaction time If VR is constant: dci r i dt • ci is molar concentration of component i Based on unit mass of solid in solid-liquid systems: 1 dni r i W dt • W is mass of solid Based on unit solid surface of solid-liquid or solid-gas systems: 1 dni r i S dt Mai Thanh Phong - HCMUT • S is solid surface area Chemical Reaction Engineering Apr 28, 2023 15 Chapter Introduction 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 Apr 28, 2023 16 Chapter Introduction Adv.: • Simple, flexible, high conversion… Disadv.: • Dead times for charging, discharging, cleaning,… • Difficult to control and automate •… BR are applied in particular for: • Relatively slow reactions • Slightly exothermic reactions Areas of application for BR are: • Reactions in pharmaceutical industry • Polymerisation reactions • Dye production • Speciality chemicals Mai Thanh Phong - HCMUT Chemical Reaction Engineering Apr 28, 2023 17 Chapter 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 Apr 28, 2023 18 Chapter Introduction c) Continuously Stirred Tank Reactor (CSTR): perfectly mixed, continuous operation A,B A,B,products Features: • Reactants are continuously introduced, products (+ unconverted reactants) are continuously withdrawn • Composition does not change with time • Composition throughout the reactor is uniform Adv.: • Controlled heat generation • Easy to control and automate • No dead times • Constant product quality, Disadv.: • Complicated • Can become unstable • Large investmnent cost, Mai Thanh Phong - HCMUT Chemical Reaction Engineering Apr 28, 2023 19 Chapter 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 Apr 28, 2023 20