化学反应工程 教案.pdf

Chemical Reaction Engineering Chemical Reaction Engineering CRE History Reaction engineering first arose as a discipline to meet the needs of the rapidly growing petroleum petrochemical and chemical industry in the 1940s and 1950s What is chemical Reaction Engineering Chemical engineering chemistry engineering Reaction engineering reaction engineering CRE deals with chemically reactive systems of engineering significance Chemical reaction engineering is the discipline that quantifies the interactions of transport phenomenaand reaction kineticsin relating reactor performance to operating conditions and feed variables Why study CRE CRE is needed in the process development for both new and existing products CRE is necessary in speeding up commercialization of new specialty and commodity chemicals materials e g plastics and pharmaceuticals Development of fuel cells for automobiles Development of biochemical biotechnology processes Novel processes for synthesis gas production Novel reactors exploiting the use of metallocene catalyst Exploitation of combinatorial catalysis for economical development of new processes CRE is critical in improving environmental impact of the chemical development of new processes CRE is the key course of chemical engineering major FINAL REASON Ned to study CRE if you want chemical engineering degree So what do we expect to learn in CRE course In CRE we will develop a general methodology useful in approaching a variety of reaction systems Chemical Kinetics q q0 and V V0 Chemical Reaction Engineering For 1st order reaction rA kCA where CA CAi at t 0 as new steady state Equation 1 can only be solved analytically for 1st and zero order reactions Unsteady State PFR Mole balance over volume V dividing by dV gives the PDE Series of unsteady state CSTRs is a useful model Semi Batch Reactor One reactant A fed to reactor containing reactant B Consider liquid phase reaction Semi Batch Reactor Mole balance for species A input output consumption accumulation 1 note that V is function of time Mole balance for species B Chemical Reaction Engineering 2 Semi Batch Reactor Mass balance to determine dependence of V on time if constant density liquid 0 Now 1 2 and 3 can be solved numerically Semi Batch Reactor Alternatively rewrite mole balances in terms of conversion X NB in terms of conversion of B X 4 NA in terms of conversion of B X 5 Express rate equation in terms of conversion 6 7 Semi Batch Reactor Either sub 3 4 6 and 7 in 1 or sub 3 5 6 and 7 in 2 to obtain Chemical Reaction Engineering Transient Tank Heat Balance Applicable to both transient CSTR and semi batch reactor 3 Multiple Reactions and Selectivity Types of multiple reactions 1 parallel competing reactions e g partial oxidation reactions 2 series consecutive reactions Chemical Reaction Engineering e g halogenation reactions 3 independent reactions e g cracking of crude oil to gasoline Maximizing Selectivity in Parallel Reactions Consider again reaction with one reactant Rate laws must maximize Maximizing Selectivity in Parallel Reactions maximize if 1 2 Chemical Reaction Engineering then to keep SDU high should keep CA high How gas phase reaction do not use inerts use high pressure liquid phase reaction do not use diluents use batch or plug flow reactor Maximizing Selectivity in Parallel Reactions maximize if 2 1 then to keep SDU high should keep CA low How gas phase reaction use inerts and or low pressure liquid phase reaction usediluents use CSTR or PFR with recycle Maximizing Selectivity in Parallel Reactions maximize If ED EU Since kD increases more rapidly with temp than kU operate reactor at highest possible temp to maximize SDU If EU ED kU increases more rapidly with temp than kD low temp favours high SDU Maximizing Selectivity in Series Reactions consider An important variable to optimize is time space time for flow reactor real time for batch reactor To maximize selectivity to intermediate do not mix fluids with different conc of active species To obtain large yield of D necessary that k1 k2 if E1 E2 high temp favours high yield of D Chemical Reaction Engineering if E2 E1 low temp favours high yield of D Modelling Reactors with Multiple Reactions Often with multiple reactions a single conversion X cannot express the conc of all components Some examples Reactions Rate expressions Chemical Reaction Engineering Rule 1 Series Reactions A B C D to maximize any intermediate do not mix fluids that have different concentrations of the active ingredient reactant or intermediates optimize space time or real time Rule 2 Parallel Reactions to get the best product distribution low C favours the reaction of lowest order high C favours the reaction of highest order if desired reaction is of intermediate order then some intermediate C will give best selectivity if reactions are all of same order then selectivity is unaffected by C high temperature favours the reaction with larger activation energy low temperature favours the reaction with smaller activation energy break reaction down into series and parallel paths the dominating path will guide decisions about operation parameters eg C T mixing if no reaction path clearly dominates must rely on simulation results to choose operation parameters 4 Reactor Residence Time Distributions Reactor Non ideal behaviour Chemical Reaction Engineering Chemical Reaction Engineering The ratio C t C0 is the fraction of the exit stream that has spent less than time t in the reactor Chemical Reaction Engineering The ratio C t C0 is the fraction of the exit stream that has spent greater than time t in the reactor Chemical Reaction Engineering Mean Residence Time The mean residence time is the first moment of the RTD function Chemical Reaction Engineering Mean Residence Time where s is a dummy variable v is constant E s is continuous Integrating by parts Mean Residence Time substituting Normalized RTD function E Dimensionless time t Chemical Reaction Engineering To relate E t and E Geometrical considerations require d E dtE t using equation 1 E E t With E RTD for reactors of different size can be compared directly Chemical Reaction Engineering RTD for Non Ideal Reactor Models Non Ideal Reactor Models can be based on RTD data But example with ideal tank tube in series indicates that RTD has limitations 5 Non Ideal Reactors Building Non Ideal reactor models based on RTD data and mixing model Reactor Modeling with the RTD A reactor model to predict conversion using RTD RTD how long fluid elements spend in reactor nothing about exchange of fluid between elements mixing behaviour Good News first order reactions linear process RTD can be used directly to predict conversion Bad News higher order reactions need both RTD and knowledge of degree of mixing to predict conversion Two extremes for degree of mixing Reactor Modeling with the RTD Example to show why degree of mixing is sometimes needed Consider a reactor consisting of two equally large volumes V where reactant conc in each is C1 and C2 Reaction rate for complete segregation case Chemical Reaction Engineering Reaction rate for complete micromixing case Conclusion A first order reaction is independent of the degree of mixing Reactor Modeling with the RTD Another Example to show why degree of mixing is sometimes needed Where p s What is difference between RTD for each case Which configuration would give higher conversion Segregation Model Flow consists of tiny batch reactors each with different residence time Chemical Reaction Engineering X t conversion after time t in batch reactor For 1st order reaction Then Tanks in Series Model Consider performing a pulse experiment with 3 tanks in series RTD function is calculated Tanks in Series Model Material balance for tracer over 1st tank Integrating Material balance for tracer over 2nd tank Sub in 3 Tanks in Series Model Solving with initial condition C2 0 at t 0 Chemical Reaction Enginee