19THERMODYNAMICSINUNITPROCESSESBut what about other systems?Do the methods equally apply? Theanswer isyes,"provided that certain factors are considered.Vapor-phase Catalytic Reactions.Since a c&talyst does not change theequilibrium of areaction,the calculated composition should be the samewith & catalyst present as without.This is generally the case, but thereare exceptions to bearin mind.One is the case of apossible competingreactionwhichistoo slowtohaveany effectin thehomogeneoussystembut which is speeded up by the catalyst to a greater extent than the mainreaction.Consider,forexample,thereactionsA=R1withthepotential sidereactionA-8Suppose that the equilibrium conversion of the first reaction is 98 per centand thatthe second isnonreversiblebut infinitely slowinthevaporphase.The yield will be 98 per cent.If the catalyst speeds up both reactions sothat the second reaction is about the same speed as the first reaction, theyield will drop.The chemical-equilibrium calculation would not havepredicted this since the second reaction was not considered.Liquid-phase Reactions. There are times when the calculated equilib-rium composition ofa liquid-phasereaction doesnotcompareatallwiththe actual measured value.This does not mean that the theory is wrongjust that there is not enough of it.Here is the reason.The rigorousequilibriumequationforthereactionA+B=C+Din the liquid phase isK,acdDGAGRwhere the a's represent the activities of the components.But, since wedo notknowtheactual activities, wegenerallyuse theexpressionKTCDZAZ8where the z's are the mole fractions.This means that we are assuming themixture to be an ideal solution.If the solution really were ideal, the cal-culated composition would match the measured one.If the solutionshould deviate much from ideality,the calculated composition will not becorrect.Theoretically this could be taken care of by using the activitycoefficient of each component.Unfortunately,our knowledge of physicalequilibrium has not.progressed far enough to allow us to calculate activitycoefficients of complex liquid mixtures (more than two or three com-ponente)
20UNITPROCESSESINORGANICSYNTHESISThermodynamics Is Not Infallible. Chemical-equilibrium calculationsare useful, but they do notalwaysgive the wholeanswer.If a chemicalreaction is fast,thermodynamics calculations may tell all we need to knowabout the reaction.In these cases the rate of reaction is not important,just product distribution and temperature control. If the reaction is notpossible at all or not economically feasible because of too low a productconcentration in the reactor effluent, thermodynamics can tell us so.If, on the other hand, the reaction is not extremely fast,if the time spentin the reactor is important, or if rapid competing reactions are involved,the chemical engineer can turn to another tool-kinetics.SOURCESOFTHERMODYNAMICDATAThermodynamic data must be sought from many sources.The AmericanPetroleum Institute and Manufacturing Chemists Association have largeprojects to determine and collect thermodynamic data. Several govern-mental agencies, such as theNational Bureau of Standards and theBureauof Mines, determine, collect, and publish thermodynamic data. Some ofthe best and most readily available sources of thermodynamic data arelisted below.Bichowsky, F. R., and F. D. Roseini: "Thermochemistry of Chemical Substances,"Reinhold Publishing Corporation, New York, 1936.Din, F. (ed.): "Thermodynamic Functions of Gases," vol. 1, Ammonia, Carbon Dioxide,Carbon Monoxide; vol.2, Acetylene, Ethylene, Propane, Argon, Butterworth'sScientificPublications, London,1956.General Electric Company:"Propertiee of Combustion Gases,"vol. I,ThermodynamicProperties; vol. 2, Chemical Compoeition of Equilibrium Mixtures; McGraw-HillBookCompany,Inc.,NewYork,1956.Hilsenrath, J., and others: "Tables of Thermal Properties of Gases,"Natl. Bur.Stand.ardsCirc.564,1955.(Additional single sheets issued as NBS-NACA sponsored tables,also as NACATechnical Notes.)Hottel, H. C., G. C.Williams, and C.N. Satterfield:"Thermodynamic Charts for Com-buetion Procesees,I.Text, II.Charts, John Wiley & Sons, Inc., NewYork, 1949.Keenan, J.H., and F. G.Keyes:"Thermodynamic Properties of Steam,John Wiley&Sons, Inc.,New York,1936.Keenan, J. H., and J. Kaye: "Gas Tables," John wiley & Sons, Inc., New York, 1948.Kelley, K.K., and others: Contribution to the Data on Theoretical Metallurgy, U.S.Bur.Mines Bull.350,371,383,384,394,406,407,434,476,477,542,1932-1954Kobe, K. A., and others: "Thermochemistry for the Petrochemical Industry, GulfPublishing Co.,Houston, Tex., 1952(A collection of a series of 19 articles that appeared in the Petroleum Refiner fromJanuary,1949, toDecember, 1951.Additional papers have appeared at intervalssince 1951.Tables of heat capacity, mean heat capacity, and relative enthalpyare given in C, K, "F, and "R. Other thermodynamic data are given.)Rossini, F.D., and others:-Selected Values of Physical and Thermodynamic Prop-
21THERMODYNAMICSINUNITPROCESSESerties of Hydrocarbons and Related Compounds," Carnegie Press, Pittsburgh, Pa.,1953.(Lithographed sheets are being isued at intervals by API Project 44 to completeand extend this material.)Rossini, F. K., and others: "Tables of Selected Values of Chemical ThermodynamicProperties,"Natl.Bur.Standards Circ.500,1952.(Lithographed sheetes are being issued periodically.)Sage, B.H., and W.N. Lacey: "Some Properties of the Lighter Hydrocarbons, HydrogenSulfide, and Carbon Dioxide,"American Petroleum Institute,New York,1955.Sage, B.H., and W.N. Lacey:"Thermodynamic Properties of the Lighter ParafinHydrocarbons and Nitrogen,American Petroleum Institute,New York,1950.Stull,D.R., and G.C.Sinke:"Thermodynamic Properties of the Elements,"AmericanChemical Society,Aduances in Chemistry,1957.Zeise, H.: "Thermodynamik,Band III/1 Tabellen, S. Hirzel Verlag, Leipzig, 1954.(Tables of heat capacities, enthalpies, entropies, free energies, and equilibriumconstants.)The following special references are used throughout this volurme.The Office of Technical Services (O.T.S.), U.S. Department of Com-merce,Washington 25, D.C., has a complete record of all declassified re-ports of the British IntelligenceObjectives Sub-committee(B.I.O.S.),theCombined Intelligence Objectives Sub-committee,SHAEF (C.I.O.S.),and the Field Information Agency Technical (F.I.A.T.). The O.T.S. canarrangefordeliveryof copies,photostats,ormicrofilms of thesereportsforafee.TheOfficeof thePublicationBoard (O.P.B.)reports are also&vail-ablefromtheO.T.S
CHAPTER 2CHEMICAL KINETICSBYT.E.CORRIGANANDJOHNJ.McKETTAWhatIsChemical Kinetics?Chemical kinetics is a study of the ratesof chemical reaction and theeffectthatprocess conditions have on theserates.These process conditions are temperature, pressure, and reactantconcentration.The application of kinetics in the selection and design ofcommercial reactorsisof specificinterestinthestudyofunitprocesses.Assume in the following consecutive reaction that the desired productis B and that C is valueless:A-B→CAthermodynamic calculation may showthatthereaction isnotpracticablebecauseatequilibrium thereisan excessof C.Yetakineticstudymayshow that the rates are such that a high yield of B may be obtained byquenching the reaction at the proper time.This is shown clearly inFig.2-1.Review of Basic Principles. Themass-action law,in equation form,states that for the reactionA+B-→Rthe rate of reaction may be expressed in the following manner:rkCaCaThis law is an empirical one based on observation and, unlike the thermo-dynamic expression for the equilibrium constant,igpnota derived equationfounded on theory.OrderofReactionsOnemethodofdescribinghomogeneousreactionsis by the order of reaction, which is the sum of exponents of the concen-tration terms.In the equationr-kcach1CoRRIGAN,Chem.Eng.,61,July,1954.CoRRIGAN,Chem.Eng.,61,August,1954; LAIDLEn,"ChemicalKinetics,"MeGraw.Hill Book Company,Inc.,New York,1950;DanrELs,"Chemical Kinetics,"CornellUniversityPress,Ithaca,N.Y.,1938;FxosT andPeAnsoN,"Kineticsand Mechanism,"P.153,JohnWiley&Sons,Ine.,NewYork,1953. Nomenclature for Chaps.2 and 3 will be found at the end of Chap.2.22
23CHEMICAL KINETICSthe order of reaction is a + b. The molecularity of a reaction is the num-ber of reactant molecules in the stoichiometric equation.If the reaction isA+B-Rand the rate isT-ACICathe molecularity is 2 and the order of reaction is 1/2.The order of a re-action is not determined by the molecularity.Chemical reactions in whichthe order of reaction is the sameas the molecularity are called sim-ple-order reactions.Mechanism.A reaction usuallydoes not occur in the single stepwhich may be represented by theover-all stoichiometric equation butrather in a series of steps whichadd up to the over-all equation.These steps are the "mechanism"of the reaction.For example, thereaction2A+B=R+Smight take place as follows:A+B=AB(Step 1)A+AB=AB(Step 2)AB-AB+R(Step 3)AB=S(Step 4)2A+B=R+S01TimeTheslowest step controlsthe rateFig, 2-1. Kinetic data may show that byof the reaction and may also de-quenching a reaction economic recoveriestermine the form of the over-allare feasible.rate equation.The rateequation,rather than the mechanism, is important to engineers.First-orderReaction.If thefollowing isa first-orderreactionA-→Baplot of Ca/Atagainst Cawill givea straight linesincetherate equationfor a first-order reaction isdCA=kCA(1)dtEquation (1)integrates to the form(2)InCa--kt+I