KYÕ THUAÄT PHAÛN ÖÙNG Chemical Reaction Engineering 8/22/2013 1 Chapter 1 Overview of Chemical Reaction Engineering 8/22/2013 2 8/22/2013 3 Introduction Reactor design uses information, knowledge, and[.]
Chemical Reaction Engineering 04/09/2023 Chapter Overview of Chemical Reaction Engineering 04/09/2023 Introduction Reactor design uses information, knowledge, and experience: Thermodynamics: Chemical kinetics Fluid mechanics Heat transfer Mass transfer Economics Chemical reaction engineering is the synthesis of all these factors with the aim of properly designing 04/09/2023 a chemical reactor Performance equation To find what a reactor is able to we need to know the kinetics, the contacting attern and the performance equation Performance equation relates input to output =f [input, kinetics, contacting] Contacting pattern or how materials flow through and contact each other in the reactor, how early or late they mix, their clumpiness or state of aggregation By their very nature some materials are very clumpy-for instance, solids and noncoalescing liquid droplets Kinetics or how fast things happen If very fast, then equilibrium tells what will leave the reactor If not so fast, then the rate of chemical reaction, and maybe heat and mass transfer too, will determine what will happen 04/09/2023 Classification of Reactions Homogeneous: takes place in one phase Heterogeneous: it requires the presence of at least two phases to proceed at the rate that it does More complicated: + enzyme-substrate reactions + very rapid chemical reactions 04/09/2023 Classification of Reactions Noncatalytic Homoge Most gas-phase reactions neou s Fast reactions such as burning of a flame Heterog eneo us Burning of coal Roasting of ores Attack of solids by acids Gas-liquid absorption with reaction Reduction of iron ore to iron and steel Catalytic Most liquid-phase reactions Reactions in colloidal systems Enzyme and microbial reactions Ammonia synthesis Oxidation of ammonia to produce nitric acid Cracking of crude oil Oxidation of SO2 to SO3 Variables Affecting the Rate of Reaction In homogeneous systems: temperature pressure composition In heterogeneous systems mass transfer heat transfer 04/09/2023 Definition of Reaction Rate The rate of change in number of moles of this component due to reaction is dNi /dt Based on unit volume of reacting fluid: Based on unit mass of solid in fluid-solid systems Based on unit interfacial surface in two-fluid systems or based on unit surface of solid in gas-solid systems 04/09/2023 Definition of Reaction Rate Based on unit volume of solid in gas-solid systems Based on unit volume of reactor, if different from the rate based on unit volume of fluid Speed of Chemical Reactions 04/09/2023 Reactor Classifying 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 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 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 •… 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, 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” •… To be reviewed by students 04/09/2023 15 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 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 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 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 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 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 or N dGR i i d T , P i 1 Thus, for the chemical equilibrium: dGR 0 d T , P Free Gibbs enthalpy The equilibrium is reached when the free Gibbs enthalpy of reaction is minimum Or dGR=0 (or in an integrated form: ∆GR = 0) Thus, the equilibrium is characterized by: N i 1 i i 0 GR T , P GR T ,P GR 0 T , P Changing of free Gibbs enthalpy for a chemical reaction