《反应工程基础（程易）chpt11-catalysiscatalyticreactors.ppt》由会员分享，可在线阅读，更多相关《反应工程基础（程易）chpt11-catalysiscatalyticreactors.ppt（90页珍藏版）》请在三一文库上搜索。
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 balance 2. Conversion and reactor sizing 3. Rate laws and stoichiometry 4. Isothermal reactor design 5. Collection and analysis of rate data 6. Multiple reaction 7. Reaction mechanisms . 8. Steady-state non-isothermal reactor design 9. Residence
2、 time distribution (RTD) 10. Models for nonideal reactors,11. Catalysis and Catalytic Reactors 12. External Diffusion Effects 13. Internal Diffusion Effects 14. Fixed-Bed Reactor 15. Fluidized-Bed Reactor,CH.1 Mole balances,CH.2 Conversion & reactor sizing,CH.3 Rate laws & stoichiometry,CH.4 Isother
3、mal reactor design,CH.5 Collection & analysis of data,CH.6 Multiple reactions,CH.7 Mechanisms, bioreactions .,CH.8 Steady state heat effects,CH.9 Unsteadystate heat effects,CH.10 Catalysis & catalytic reactors,CH.13 Residence time distribution,CH.14 Nonideal reactors,CH.11 External diffusion,CH.12 D
4、iffusion in porous catalysts,Multiple reactions with heat effects,Framework of Foglers book Elements 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 Enginee
5、ring (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,Chapter 11 Catalysis and Catalytic Reactors,Department of Chemical Engineering Tiefeng 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 l
6、aw and a mechanism and rate-limiting step. Use regression to discriminate 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 temp
7、erature-time trajectories.,6,10.1 Catalysis,10.1.1 Definition of catalyst A 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 reaction Returning to
8、 its original form (In practice a catalyst deactivates gradually during 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 temp
9、erature, equilibrium conversion can be achieved.,7,8,Every catalytic reaction 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,Poten
10、tial energy diagram of a heterogeneous catalytic reaction, with gaseous reactants 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
11、 direct reaction is 334.6 kJ/mol. In the catalytic reaction, dissociative adsorption 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
12、 new pathways with lower activation energies; the reaction profile contains no high peaks and no deep troughs,Classification: Homogeneous vs. Heterogeneous Catalysis,11,Homogeneous catalysis Single phase (Typically liquid) Low temperature Separations are tricky,Heterogeneous catalysis Multiphase (Mo
13、stly solid-liquid and solid-gas) High temperature Design and optimization tricky,Zeolite catalyst,Catalyst powders,TOF (s1) Hetero. cats. 101 Enzymes 106,12,Important Heterogeneous Catalytic Processes,Haber-Bosch process N2 + 3 H2 2 NH3 Fe/Ru catalysts, high pressure and temperature Critical for fer
14、tilizer and nitric acid production Fischer-Tropsch chemistry n CO + 2n H2 (CH2)n + n H2O , syngas to liquid fuels Fe/Co catalysts Source of fuel for Axis in WW II Fluidized catalytic cracking High MW petroleum low MW fuels, like gasoline Zeolite catalysts, high temperature combustor In your fuel tan
15、k! Automotive three-way catalysis NOx/CO/HC H2O/CO2/H2O Pt/Rh/Pd supported on ceria/alumina Makes exhaust 99% cleaner,13,Active phase Where the reaction occurs Surface irregularities, dislocations, edges of crystals, cracks along grain boundaries Promoter An additive which has no catalytic propertie
16、s of its own but enhances the activity of a catalyst Stabilization against crystal growth and sintering Support /carrier Increase mechanical strength and stability Increase surface area Improves the heat transfer characteristics May or may not be catalytically active,Catalyst composition,14,10.1.2 C
17、atalyst properties,Large specific area for a significant reaction rate (A typical silica-alumina cracking catalyst has a pore volume of 0.6 cm3/g and an average pore radius of 4 nm. The corresponding surface area is 300 m2/g) Pore structure Porous catalyst, e.g. Raney nickel, Pt/Al2O3 Monolithic cat
18、alyst Molecular sieve Most catalysts are subject to deactivation: aging, poisoning, and fouling/coking,15,Catalyst morphology vs surface area,BET surface Area: 244.1 m2/g Crystallite size: 3.7 nm Pore volume: 0.66 cm3/g Average pore diameter: 7.89 nm,BET surface Area: 12.3 m2/g Crystallite size: 44.
19、5 nm Pore volume: 0.03 cm3/g Average pore diameter: 9.01 nm,Komhom et al., Catalysis Communications, 10 (2008) 8691,-Al2O3,-Al2O3,16,Activity - being able to promote the rate of desired reactions Selective - being to promote only the rate of desired reaction and also retard the undesired reactions S
20、tability - a good catalyst should resist to deactivation Regeneration - being able to be regenerated after deactivation,Performance of the Catalyst,17,Chemical adsorption,Fig. 10-3 Ethylidyne as chemisorbed on platinum,Physical Adsorption Vs. Chemisorption,18,19,Rates of Catalytic Reactions,Pseudo-h
21、omogeneous reaction rate r = moles / volume time Mass-based rate r = moles / masscat time r = r / cat Reactions happen at surfaces: Area-based rate r “ = moles / areacat time r “ = r / SA, SA = area / mass Reactions happen at active sites: Active site-based rate Turn-over frequency TOF = moles / sit
22、e time TOF = 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 that are on the surface. This can be determined from chemi
23、sorption. Example 10-1 Conditions: 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 relationship between crystallite size and dispersion of
24、 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 from the bulk fluid to the external surface of th
25、e catalyst pellet Diffusion of the reactant from the pore mouth through the catalyst pores to the immediate vicinity of the internal catalytic surface Adsorption of reactant A onto the catalyst surface Reaction on the surface of the catalyst Desorption of the products from the surface Diffusion of t
26、he products from the interior of the pellet to the pore mouth at the external surface Mass 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 the mechanism. (The exact meaning of “sl
27、owest” 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 the catalysts should change the
28、 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 constant, which increases as the
29、 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. Isotherms portray the a
30、mount 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 independent of T,
31、 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 isotherm What are the assumptions?,Dissociated adsorption,The desorption rat
32、e 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 of two v
33、acant 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 adsorption Site balance Equation for the adsorption isotherm,Dissociated adsorption,For dissociative adsorption, both kA and k-A i
34、ncrease 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,Multi
35、-component adsorption isotherm,36,10.2.4 Surface reaction,Langmuir-Hinshelwood Mechanism Single 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-Ri
36、deal Mechanism The 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 ad
37、jacent vacant,The reaction of two adsorbed species adsorbed on the same type of site S,The reaction of two species adsorbed on 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 othe
38、r one reacts with 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 di
39、ffuse across the surface 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 c
40、onstant.,Relationship between 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
41、 series is usually found to be rate-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 heterogen
42、eous mechanisms is usually termed the 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, assumin
43、g that all steps are reversible. A rate-limiting 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-limit
44、ing,48,10.3 Synthesizing a rate law, mechanism, 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 prese
45、nt; consequently, PP=PB=0. The initial rate is given by,53,10.3.2 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
46、pressure of C:,55,10.3.3 Is benzene desorption rate-limiting,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.
47、5 Reforming Catalysts,58,Platinum on alumina is a bifunctional 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
48、 produce very small clusters of Pt on alumina. These clusters have 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-pentene Surface reaction is rate limiting step.,59,60,10.3.6
49、Rate laws derived from the PSSH,Pseudo-Steady-State Hypothesis (PSSH): 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,Isomerization of n-pentene (N) to i-pentene (I) over alumina,Solving for CNS and CIS:,63,For s