反应工程基础(程易)chpt11-catalysiscatalyticreactors.ppt
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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 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
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