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1、本课程的架构,第一部分：理想反应器第二部分：非理想流动 第三部分：非均相催化和 工业反应器,1.Mole balance2.Conversion and reactor sizing3.Rate laws and stoichiometry4.Isothermal reactor design5.Collection and analysis of rate data6.Multiple reaction7.Reaction mechanisms.8.Steady-state non-isothermal reactor design9.Residence time distribution(
2、RTD)10.Models for nonideal reactors,11.Catalysis and Catalytic Reactors12.External Diffusion Effects13.Internal Diffusion Effects14.Fixed-Bed Reactor15.Fluidized-Bed Reactor,CH.1 Mole balances,CH.2 Conversion&reactor sizing,CH.3 Rate laws&stoichiometry,CH.4 Isothermal reactor design,CH.5 Collection&
3、analysis of data,CH.6 Multiple reactions,CH.7Mechanisms,bioreactions.,CH.8Steady state heat effects,CH.9Unsteadystate heat effects,CH.10 Catalysis&catalytic reactors,CH.13 Residence time distribution,CH.14 Nonideal reactors,CH.11 External diffusion,CH.12 Diffusion in porous catalysts,Multiple reacti
4、ons with heat effects,Framework of Foglers bookElements of Chemical Reaction Engineering,主要参考书,H.S.Fogler,Elements of Chemical Reaction Engineering(Chapter 10,11,12,CD ROM Shelf,R12.3 Fluidized-Bed Reactor)O.Levenspiel,Chemical Reaction Engineering(Fixed-Bed Reactor)陈甘棠，化学反应工程（第三版)（固定床、流化床反应器）,3,4,C
5、hapter 11Catalysis and Catalytic Reactors,Department of Chemical EngineeringTiefeng Wang,5,Objectives,Define a catalyst,a catalytic mechanism and a rate-limiting step.Describe the steps in a catalytic mechanism and how to derive a rate law and a mechanism and rate-limiting step.Use regression to dis
6、criminate between reaction rate laws and mechanisms.Discuss the different types of catalyst deactivation and the reactor types,and describe schemes that can help offset the deactivation.Analyze catalyst decay and conversion with temperature-time trajectories.,6,10.1 Catalysis,10.1.1 Definition of ca
7、talystA catalyst is a substance that affects the rate of a reaction but emerges from the process unchanged.,Taking part in the reaction Altering the rates of reactions by promoting a different mechanism for the reactionReturning to its original form(In practice a catalyst deactivates gradually durin
8、g use)The use of catalyst DOES NOT vary DG&Keq values of the reaction concerned,it merely change the PACE of the process.,The use of catalyst DOES NOT vary DG When Pt/ZrO2 or Ni/Al2O3 is present in the reactor at the same temperature,equilibrium conversion can be achieved.,7,8,Every catalytic reacti
9、on is a sequence of elementary steps,in which reactant molecules bind to the catalyst,where they react,after which the product detaches from the catalyst,liberating the latter for the next cycle.,What is catalysis,9,Potential energy diagram of a heterogeneous catalytic reaction,with gaseous reactant
10、s and products and a solid catalyst.Note that the uncatalyzed reaction has to overcome a substantial energy barrier,whereas the barriers in the catalytic route are much lower.,10,Ammonia Synthesis,The activation of the direct reaction is 334.6 kJ/mol.In the catalytic reaction,dissociative adsorption
11、 is the rate-limiting step,and its activation energy is 70 kJ/mol.At 500oC and ambient pressure,the rate of the catalytic reaction is increased 13 orders of magnitudes.,A catalyst increases the rate by introducing new pathways with lower activation energies;the reaction profile contains no high peak
12、s and no deep troughs,Classification:Homogeneous vs.Heterogeneous Catalysis,11,Homogeneous catalysisSingle phase(Typically liquid)Low temperatureSeparations are tricky,Heterogeneous catalysisMultiphase(Mostly solid-liquid and solid-gas)High temperatureDesign and optimization tricky,Zeolite catalyst,
13、Catalyst powders,TOF(s1)Hetero.cats.101Enzymes 106,12,Important Heterogeneous Catalytic Processes,Haber-Bosch processN2+3 H2 2 NH3Fe/Ru catalysts,high pressure and temperatureCritical for fertilizer and nitric acid productionFischer-Tropsch chemistryn CO+2n H2(CH2)n+n H2O,syngas to liquid fuelsFe/Co
14、 catalystsSource of fuel for Axis in WW IIFluidized catalytic crackingHigh MW petroleum low MW fuels,like gasolineZeolite catalysts,high temperature combustorIn your fuel tank!Automotive three-way catalysisNOx/CO/HC H2O/CO2/H2OPt/Rh/Pd supported on ceria/aluminaMakes exhaust 99%cleaner,13,Active pha
15、seWhere the reaction occursSurface irregularities,dislocations,edges of crystals,cracks along grain boundariesPromoter An additive which has no catalytic properties of its own but enhances the activity of a catalystStabilization against crystal growth and sinteringSupport/carrierIncrease mechanical
16、strength and stabilityIncrease surface areaImproves the heat transfer characteristicsMay or may not be catalytically active,Catalyst composition,14,10.1.2 Catalyst properties,Large specific area for a significant reaction rate(A typical silica-alumina cracking catalyst has a pore volume of 0.6 cm3/g
17、 and an average pore radius of 4 nm.The corresponding surface area is 300 m2/g)Pore structurePorous catalyst,e.g.Raney nickel,Pt/Al2O3 Monolithic catalystMolecular sieveMost catalysts are subject to deactivation:aging,poisoning,and fouling/coking,15,Catalyst morphology vs surface area,BET surface Ar
18、ea:244.1 m2/gCrystallite size:3.7 nmPore volume:0.66 cm3/gAverage pore diameter:7.89 nm,BET surface Area:12.3 m2/gCrystallite size:44.5 nmPore volume:0.03 cm3/gAverage pore diameter:9.01 nm,Komhom et al.,Catalysis Communications,10(2008)8691,-Al2O3,-Al2O3,16,Activity-being able to promote the rate o
19、f desired reactionsSelective-being to promote only the rate of desired reaction and also retard the undesired reactions Stability-a good catalyst should resist to deactivationRegeneration-being able to be regenerated after deactivation,Performance of the Catalyst,17,Chemical adsorption,Fig.10-3 Ethy
20、lidyne as chemisorbed on platinum,Physical Adsorption Vs.Chemisorption,18,19,Rates of Catalytic Reactions,Pseudo-homogeneous reaction rater=moles/volume timeMass-based rater=moles/masscat timer=r/catReactions happen at surfaces:Area-based rater=moles/areacat timer=r/SA,SA=area/massReactions happen a
21、t active sites:Active site-based rateTurn-over frequency TOF=moles/site timeTOF=r/site,20,Turnover frequency and dispersion,Turnover frequency(TOF):the number of molecules reacting per active site per second at the conditions of the experiments.Dispersion(D):the fraction of the metal atoms deposited
22、 that are on the surface.This can be determined from chemisorption.Example 10-1Conditions:0.5 wt%Ru on-Al2O3,catalyst dispersion 49%,TOF of CH4 is 0.044 1/s.Determine the rate of formation of methane.,21,Range of TOF for different reactions,22,Effect of particle size on catalyst dispersion,The relat
23、ionship between crystallite size and dispersion of Pt supported catalyst,10.2 Steps in a catalytic reactions,Mass transport/diffusion,Chemical adsorption and reaction,Length Scales in Heterogeneous Catalysis,24,25,Steps in a catalytic reactions(contd),Mass transfer(external diffusion)of reactants fr
24、om the bulk fluid to the external surface of the catalyst pelletDiffusion of the reactant from the pore mouth through the catalyst pores to the immediate vicinity of the internal catalytic surfaceAdsorption of reactant A onto the catalyst surfaceReaction on the surface of the catalystDesorption of t
25、he products from the surfaceDiffusion of the products from the interior of the pellet to the pore mouth at the external surfaceMass transfer of the products from the external pellet surface to the bulk fluid,26,The overall rate,The overall rate of reaction is equal to the rate of the slowest step in
26、 the mechanism.(The exact meaning of“slowest”here)When the diffusion steps are very fast,they do not affect the overall rate of the reaction.When the reaction steps are very fast compared with diffusion steps,diffusion does affect the reaction rate.In this situation,changing the flow conditions past
27、 the catalysts should change the overall reaction rate.In porous catalysts,diffusion within the catalyst pores may limit the rate of reaction.,27,Step 1:Diffusion from bulk to external transport,28,Step 2:Internal diffusion,CAs is the concentration at the external surface.kr is an overall rate const
28、ant,which increases as the pellet diameter decreases.,29,Step 3:Surface adsorption,Adsorption of A on a site S,For multi-species system,the site balance is CiS:surface concentration of sites occupied by species i,mol/g cat Cv:surface concentration of vacancy sites,mol/g cat,Adsorption isotherms.Isot
29、herms portray the amount of a gas adsorbed on a solid at different pressure but at one temperature.,30,Rate of adsorption,Nondissociated adsorption and dissociated adsorption,depending on the surface,31,31,Nondissociated adsorption,The net rate of adsorption,For molecular adsorption,kA is virtually
30、independent of T,while k-A increases exponentially with increasing T.,the unit:mol/gcat s,32,Equation for the adsorption isotherm,At equilibrium,Site balance,Equation for the adsorption isotherm,This type of isotherms is called a Langmuir isothermWhat are the assumptions?,Dissociated adsorption,The
31、desorption rate is proportional to the product of the occupied-site concentration,CCSCOS,33,The adsorption rate is proportional to the pressure of CO,and square of the vacant-site concentrations.For a molecule to dissociate as it adsorbed,two adjacent vacant active sites are required.The probability
32、 of two vacant sites occurring adjacent to one another is proportional to the square of the concentration of vacant sites.,34,Equation for the adsorption isotherm,The net rate of adsorptionSite balanceEquation for the adsorption isotherm,Dissociated adsorption,For dissociative adsorption,both kA and
33、 k-A increase exponentially with increasing temperature,while KA decreases with increasing temperature.,Checking the model,One method of checking whether a predicts the behavior of the experimental data is to linearize the models equation and then pot the indicated variation against one another.,35,
34、Multi-component adsorption isotherm,36,10.2.4 Surface reaction,Langmuir-Hinshelwood MechanismSingle site:only the site on which the reactant is adsorbed is involved in the reaction.Dual site:the adsorbed reactant interacts with another site(either unoccupied or occupied)to form the product.Eley-Ride
35、al MechanismThe reaction between an adsorbed molecule and a molecule in the gas phase(Eley&Rideal,1938),37,38,Single site mechanism,Isomerization or decomposition,where Ks is the surface reaction equilibrium constant Ks=kslk-s,39,Dual site mechanism,The reaction between adsorbed A and an adjacent va
36、cant,The reaction of two adsorbed species adsorbed on the same type of site S,The reaction of two species adsorbedon different types of sites S and S.,40,Eley-Rideal mechanism,In this mechanism,proposed in 1938 by D.D.Eley and E.K.Rideal,only one of the molecules adsorbs and the other one reacts wit
37、h it directly from the gas phase,without adsorbing.,Eley-Rideal vs LH Mechanism,41,(a)An atom adsorbs onto the surface;(b)Another atom passes by which interacts with the one on the surface;(c)A molecule is formed which desorbs.,(a)Two atoms adsorb onto the surface;(b)They diffuse across the surface
38、and interact when they are close;(c)A molecule is formed which desorbs.,42,10.2.5 Desorption,The products of the surface reaction adsorbed on the surface are subsequently desorbed into the gas phase:,The rate of desorption of C is:,where KDC is the desorption equilibrium constant.,Relationship betwe
39、en rDC and rADC,between KDC and KC:,43,10.2.6 Rate-limiting step,For steady sate,the rates of each of the three reaction steps in series(adsorption,surface reaction,and desorption)are equal to one another:,The exact meaning of rate-limiting,One particular step in the series is usually found to be ra
40、te-limiting or rate-controlling.If we could make this particular step go faster,the entire reaction would proceed at an accelerated rate.,Electrical analog to heterogeneous reactions.,Langmuir-Hinshelwood approach,The approach in determining catalytic and heterogeneous mechanisms is usually termed t
41、he Longrnuir-Hinskelwood approach.It consists of the following steps:Assuming a sequence of steps in the reaction.Choose among such mechanisms as molecular or atomic adsorption,and single-or dual-site reaction.Rate Iaws are written for the individual,assuming that all steps are reversible.A rate-lim
42、iting step is postulated,and steps that are not rate-limiting are used to eliminate all coverage-dependent terms.,44,Langmuir-Hinshelwood Kinetics,45,46,Rapid,Rapid,Rate-limiting,Surface-limited reaction,Surface-limited reaction,47,Rapid,Rapid,Rate-limiting,48,10.3 Synthesizing a rate law,mechanism,
43、and rate-limiting step,C=cumene,B=benzene,P=propylene,49,50,10.3.1 Is adsorption rate-limiting?,The rate of adsorption:,For adsorption-limited reactions:,Surface reaction rate,Desorption rate,51,Site balance:,52,Initially,no products are present;consequently,PP=PB=0.The initial rate is given by,53,1
44、0.3.2 Is the surface reaction rate-limiting?,The rate of surface reaction:,Utilize the adsorption and desorption steps to eliminate CCS and CBS:,Site balance:,CCS and CBS,54,The initial rare is,At low partial pressure of C:,At high partial pressure of C:,55,10.3.3 Is benzene desorption rate-limiting
45、,The desorption rate of benzene:,Utilize the adsorption and surface reaction steps to eliminate CCS and CBS:,56,Site balance:,57,10.3.4 Summary of decomposition of cumene,Actual initial rate as a function of partial pressure of cumene,10.3.5 Reforming Catalysts,58,Platinum on alumina is a bifunction
46、al catalyst that can be prepared by exposing alumina pellets to a chloroplatinic acid solution,drying,and then heating in air 775 to 875 K for several hours.Next,the material is exposed to hydrogen at temperatures around 725 to 375 K to produce very small clusters of Pt on alumina.These clusters hav
47、e sizes on the order of 10 A,while the alumina pore sizes on which the Pt is deposited are on the order of 100 to 10,000 A(i.e.,10 to 1000 nm).,See Table 10-4,n-pentene i-penteneSurface reaction is rate limiting step.,59,60,10.3.6 Rate laws derived from the PSSH,Pseudo-Steady-State Hypothesis(PSSH):
48、Each species adsorbed on the surface is a reactive intermediate,and the net rate of formation of species i adsorbed on the surface is zero.The PSSH should be used when more than one step is limiting.,61,There is a rate-limiting step,e.g.surface reaction,There is no rate-limiting step,62,Isomerizatio
49、n of n-pentene(N)to i-pentene(I)over alumina,Solving for CNS and CIS:,63,For surface reaction-limited mechanism:,This rate law is identical to the one derived assuming that rAD/kAD0 and rD/kAD0,Site balance:,64,10.3.7 Temperature dependence of the rate law,Consider a surface-reaction-limited irrever
50、sible istlmerization,The adsorption is exothermic.Consequently,the higher the temperature,the smaller the adsorption equilibrium constant.At high temperatures,65,10.4 Heterogeneous data analysis for reactor design,Developing a algebraic rate law consistent with experimental observationsAnalyzing the
