APPLICATION OF CHEMICAL KINETICS IN THE HETEROGENEOUS CATALYSIS STUDIES

Transcription

1 ALICATION OF CHEMICAL KINETICS IN THE HETEROGENEOUS CATALYSIS STUDIES L. A. ETROV SABIC Chair in Heterogeneous Catalysis Chemical and Materials Engineering Department College of Engineering, King Abdulaziz University, Jeddah

2 Kinetics of heterogeneous catalytic reactions is indispensable part of the complex study of the properties and behavior of any catalytic system i.e. catalyst plus reaction media

3 Main tass of inetics of heterogeneous catalytic reactions. Kinetics and mechanism of important in theoretical aspect catalytic processes.. Development of the theory of inetics. 3. Development of inetic models of non-stationary catalytic processes. 4. Creation of inetic models of industrially important reactions. 3

4 5. The inetics of the processes, occurring during the preparation, deactivation and regeneration of the industrial catalysts. 6. Theory and automation of inetic experiments. 7. The elaboration of laboratory methods for testing and controlling the catalytic activity and selectivity of the industrial catalysts. 8. Development of large scale tests of the catalytic properties of the obtained catalysts. 4

5 Building of inetic models Formulation of the possible reaction mechanisms. Data from literature. Chemical and physical adsorption measurements 3. Kinetic experimental methods a) Steady state experiments b) Non-steady state experiments 4. hysical methods for catalyst characterization. 5. Chemical methods for catalyst characterization. 5

6 Deriving corresponding to the proposed reaction mechanisms inetic steady state models. Method of Hougen-Watson. Method of Temin for stationary heterogeneous complex catalytic reactions 3. Method of graph theory 4. Method of group theory 5. Any other method 6

7 Mathematical treatment of inetic experimental data. Design of inetic experiments. Calculating rates on the independent reaction routes 3. Estimation of the number of independent parameters for a given inetic model 4. Kinetic parameters estimation 5. Statistical assessment of the best reaction models 7

8 The effective industrial catalysts are products, which possess very special complex of different properties. High catalytic activity;. High selectivity; 3. roper pore structure; 4. High resistance to deactivation; 5. High resistance to catalytic poisons; 6. Easy regeneration; 7. Long life time; 8

9 8. Low operational and light-off temperature; 9. High thermal stability; 0. High thermal conductivity;. High mechanical strength;. High resistance to attrition; 3. Low price. 9

10 Industrial catalysts are performance chemicals which should be offered on the maret together with information about:. Reaction inetics and inetic model;. Catalytic activity and selectivity; 3. Catalyst pre-treatment regimes; 4. Catalyst deactivation inetics with respect to different catalytic poisons; 5. Catalyst regeneration regimes; 0

11 6. Catalyst lifetime: stability, duration of operation, thermal stability; 7. hysical and mechanical properties: strength, abrasion ability, hardness, surface area; 8. Hydrodynamic characteristics of the catalyst grain and of the catalyst bed in the reactor 9. Safety transition regimes in cases of industrial accident; 0. Economy of the process.

12 The steady state regimes The steady state proceeding of heterogeneous catalytic process is realized only in open systems. The steady state regime means that all reaction parameters (concentrations of reagents and ISC, temperatures, partial and total pressure, regents flow rate) should have constant values which do not change with time. This however does not mean that the parameters should have the same value at different points of the reaction space.

13 For every ISC we can write dx ( ) i F c, X j K dt F(c j, x ) is the function which allow us to express the concentrations of ISC via the concentrations of the reagents, j - number of reagents in the system, i - number of ISC formed on the catalyst surface. At steady state regime dx i 0 F( c, ) 0 j X K dt 3

14 Conditions of Fran-Kamenetzi ). The rate of formation of the i th ISC from all possible reactions should be smaller than the rate of its consumption along all possible reactions. X i dx dt i X i r i ' i ). The life-time of the i th ISC should nearly equal to the time necessary to reach steady state τ o and smaller than the time of reaction τ r 4

15 5, r r r σ + σ + r r r + p s S S S R r r r ) ( ) ( σ 3). The reaction rates of all consecutive elementary steps should be approximately equal to each other and equal to the slowest one called limiting reaction step. For the each step we can write s n s t s n r r t t X X,, σ σ

16 Kinetic models of non-stationary catalytic processes should account for the following factors: - The rates of the elementary chemical reactions, - The rates of changing of reactant composition, - The rates of changing of the activity of the catalyst - Diffusion of the reacting species in the catalyst pores, and etc. 6

17 Catalytic processes under non-steady state regime r r r A f ( c, x, α, T ) c dx dt da dt A x r r f ( c, x, α, T ) r r f ( c, x, α, T ) A c - is stoichiometric matrix for reagents; A x is stoichiometric matrix for intermediate surface compounds (ISC) 7

18 The reasons for the deviation from steady state regimes are numerous: presence of catalyst redox cycle, presence in the reaction mechanism a nonlinear elementary steps, changes of the reaction mechanism due to changes of the degree of conversion, catalyst re-crystallisation, poisoning, deactivation,etc. Most of these factors acts spontaneously and are part of the properties of the system catalyst-reaction media and as a result different phenomena are observed lie multiple steady states, oscillating reactions, chaotic behaviour, heat explosion, etc 8

19 Use of Kinetics Models (in %) Type of company Number of companies rocess development Troubleshouting rocess optimisation Catalyst development Mechanism studies Chemical Oil Catalyst producer Engineering

20 Design of catalysts pellets 0

21 Influence of diffusion on the catalysts performance. + External diffusion regime c r C C. ed o o eff c + c C o

22 Internal diffusion regime M T S D D p g m m K eff K ρ τ θ τ θ , p g g g e S S V r ρ θ. eff eff K eff D D D,, + D,eff D.θ/τ ) 3 3 ( φ φ φ η tgh eff n s D C R. φ C s D eff r H. γ. n C S r. η.

23 Effective diffusion coefficients D eff and tortuosity factor τ obtained from the dynamic method of Wice-Kallenbach for oxide and reduced forms of the M-9 and WGSR catalysts. Catalyst T, K Gas pair D eff.x0 cm /s Tortuosity factor, τ M-9 oxid 93 O -Ar O -Ar O -Ar M-9 red 93 H -Ar H -Ar H -Ar M-9 red 93 Ar - H Ar - H Ar - H BNTK oxid 93 O -Ar O -Ar O -Ar BNTK red 93 H -Ar H -Ar H -Ar BNTK red 93 Ar - H Ar - H Ar - H

24 Influence of the reduction on the values of D eff (in cm /s) obtained from chromatographic and Wice-Kallenbach methods for M-9 and BNTK catalysts Catalyst Temperature K D oxy eff CMD red D eff Change % oxy D eff MWK red D eff Change % M BNTK

25 The tortuosity factor τ and equivalent pore radius r e obtained from the chromatographic method for oxide and reduced forms of the M-9 and WGSR catalysts. arameters Catalysts M 9 BNTK Temperature, K Tortuosity factor, τ Equivalent pore radius r e.0-7 cm

26 Catalyst deactivation 6

27 Diffusion effects in processes accompanied by catalyst deactivation.changes in the reaction rate caused by diffusion restrictions;.diffusion modified deactivation caused by diffusion restrictions; 3.Diffusion modified deactivation influence on the main reaction 7

28 Types of Deactivation. Sintering. oisoning 3. Changes of catalytic activity due to interactions with reagents: a. Strong and irreversible adsorption of some reagents; b. Interaction of reactants with catalytic centers; c. Induced diffusion of lattice components towards catalyst surface; d. Formation of coe precursors blocing the catalyst surface. 8

29 Classification of mechanisms of deactivation. arallel deactivation slow step A + Z [AZ] [BZ] B + Z [Z] coe Consecutive deactivation slow step A + Z [AZ] [BZ] B + Z [Z] coe 9

30 Kinetic description of deactivation Inseparable deactivation Coe formation is inseparable part of the reaction mechanism. This process is described by special term in the inetic equation for reaction of paraffins dehydrogenation. σ r * + + ( c c ) paraffins; reaction product; σ reaction reversibility; c coe concentration, c o threshold coe concentration; rate constant; olefins adsorption constant, * - rate constant of coe 30 formation. 0

31 Separable deactivation Coe formation is described by rate model which is uncoupled from rate equation describing main reaction. r r,, T. φ,, T in i d d i ( )( ( )) τ r in (, i, T) inetic model of main reaction at constant catalyst activity; φ, T ( ) d d i, inetic model of the reaction of catalyst deactivation. 3

32 3 ) ( ) ( ) ( ) ( 0 0 t a t a r r t r t r K d d d if t 0 then a d and a 0 ) ( ) ( t d d a a t a T a Diffusion modified deactivation caused by diffusion restrictions ( ) ( ) t r t r d

33 Dehydrogenation of i-pentenes r 0 ( x) + Q + α x 0 + 3x 4 + 5x Φ 0 +. Qexp( µ Q) + Q + α0 x + Q + α0 x Deactivation function basic reaction rate under inetic control Q C /3 C coe deposited per gram-catalyst. 33

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35 Decomposition of 4,4-dimetildioxan-,3 35

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39 Reaction mechanism ) C 6 H 5 NO + [K] [C 6 H 5 NO K] ) [C 6 H 5 NO K] + H [C 6 H 5 NOK] + H O 3) [C 6 H 5 NOK] + H [C 6 H 5 NHOHK] 4) [C 6 H 5 NHOHK] + H [C 6 H 5 NH K] + H O 5) [C 6 H 5 NH K] C 6 H 5 NH + [K] 6) [C 6 H 5 NO K] + [C 6 H 5 NHOHK] [RK] + nh 7) n[rk] coe 39

40 Kinetic model of the reaction in the absence of diffusion limitations and deactivation r in Nb + H Nb,4.0 3, 6,8.0-4, E J/mol E J/mol. The mean deviation between experimental calculated rates was 0,3%. 40

41 4 Kinetic model of nitrobenzene hydrogenation to aniline considering the catalyst deactivation in the absence of diffusion limitations + 0 * ) ( ) ( Nb H Nb H Nb H Nb in deac t r τ * 0,34 Mean deviation between experimental and calculated rates is 3,7%.

42 Kinetic model of nitrobenzene hydrogenation to aniline in presence of diffusion limitations r dif. D ( ) eff H ( t) ln( ) + Nb Nb R eff. 0.5 Mean deviation between experimental and calculated rates is,8%. 4

43 Kinetic model of the reaction of nitrobenzene hydrogenation to aniline with diffusion limitations and catalyst deactivation r dif. deac ( t) R eff.( D eff ) τ. H 0.5 [ ln( + )]. Z ( Nb ) τ ( Nb ) τ 0.5 Z * ( Nb ) K ( Nb ) 0 τ 43

44 rognosis of catalyst life time 44

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46 .rocess of gas phase hydrogenation of nitrobenzene to aniline on copper catalysts M-9 produced by Neftochim AD. Reactors used:.. Laboratory glass reactor containing g catalyst with grain size 0. mm,.. Hungarian made pilot system OL-05/0 containing 5 g catalyst with grain size mm,.3. ilot plant reactor containing g industrial pellets with size 6x6,.4. ilot plant reactor containing 0 g industrial pellets with size 6x6,.5. Industrial unit for aniline production with four paced bed reactors arranged in series. Each reactor 46 contains 4000 g catalyst with pellet size 6x6.

47 . rocess of gas phase crotonealdehyde hydrogenation to butanol on ieselguhr-supported copper catalyst produced by Neftochim AD. Reactors used:.. Laboratory glass reactor containing g catalyst with grain size 0. mm,.. Hungarian made pilot system OL-05/0 containing 5g catalyst with grain size mm,.3. Industrial unit for butanol production with four tubular reactors arranged in parallel. Each reactor contains 4000 g catalyst with pellet size 6x6. 47

48 3. Liquid phase hydrogenation of crotonealdehyde to butiraldehyde on alumina supported nicel catalyst produced by Neftochim AD. Reactors used: (i) Laboratory liquid phase glass reactor containing g catalyst with grain size 0.0 mm, (ii) Industrial unit for butyraldehyde production with four liquid phase reactors arranged in parallel. Each reactor contains 50 g powder catalyst with particle size 0.0mm. 48

49 4. Liquid phase gasoline sweetening process. Used catalyst sulfophtalocyanine produced in Neftochim AD. Reactors used: 4. Laboratory tricle-bed liquid phase metal reactor containing g catalyst with grain size 0.6- mm, 4.. ilot plant tricle-bed reactor containing 5 g catalyst with particle size 0.5- mm and 8 atm. woring pressure 49

50 Reaction in presence of selectivity promotor Ethylene oxidation ) C H 4 + O C H 4 O ) C H 4 + 3O CO + H O 50

51 R() + O O + Ethylene 3 Ethylene R() + 4 O O + Ethylene 3 Ethylene 5

52 Selectivity on ethylene oxide production S R() m R() + R() + 4 S 6. CEO 5. C +. C EO O.00 S ( F ). F Ethylene O 5

53 Ethylene inhibited oxidation S a. C + DHE b at C DCE 0 S b a sensitivity of selectivity to DCE concentration; b selectivity without presence of DCE 53

54 Sensitivity of selectivity of ethylene oxidation on DCE concentration Feed ratio Catalyst bed temperature, o C O : C H : : :

55 55 Ethylene O DCE Ethylene O Ethylene O R () + + Ethylene O DCE Ethylene O Ethylene O R () + + ) ( ) ( DCE DCE DCE m S + )) ( 6 )( (5 ) 6( DCE O Ethylene DCE O Ethylene DCE m F F F F S + +

56 Catalyst deactivation A R() R() promoted unpromoted A R() R() promoted unpromoted A 0.9 DCE A DCE 56

57 DCE critical concentrations and partial pressures Temperature C DCE, atm C DCE, ppm DCE, atm C DCE, ppm DCE, atm C DCE, ppm Selective oxidation of ethylene Complete oxidation of ethylene Complete oxidation of ethylene oxide

58 Ethylene oxidation. Influence of water vapor A concentration of 0.-6 % of water in the feed inhibits the epoxidation of ethylene to ethylene oxide and promotes the process of complete oxidations 58

59 Ethylene oxidation. Influence of carbon dioxide R() O + 5 Ethylene O + 6 O Ethylene Ethylene + 7 CO CO R() 3 O + 5 Ethylene O O Ethylene Ethylene + 7 CO CO A concentration of -33 % of carbon dioxide in the feed inhibits both the epoxidation of еthylene to ethylene oxide and complete oxidation, but the second effect is much stronger, which means that 59 it promotes the selectivity to ethylene oxide.

60 60 Ethylene oxide oxidation ( ) ) ( O EO EO O O EO O R ( ) ) ( O EO EO O O DCE EO O EO O R

61 CONCLUSIONS. Detailed study of the inetics is expensive and time consuming process and should be done on the catalysts that have already passed other chemical, physical, mechanical and physicochemical tests.. Reliable and precise data can be obtained only if the laboratory catalytic reactors are properly selected and analytical methods used for are fast, precise, and reliable. 6

62 3. Interpretation of the results from inetic studies should be done always taing into account the results obtained from the application of other characterization techniques - chemical, physical, morphological and mechanical. Very important in this sense is the opportunity to combine catalytic activity measurements with application in situ of some physical methods by which the additional information about the catalyst changes during the catalytic runs can be obtained. 6

63 4. Heat and mass transfer processes always accompany heterogeneous catalytic reactions. It is very important to properly evaluate the heat and mass transfer effect on a catalytic reaction. 63

64 It is essential that the analytical methods and equipment used to analyse reaction mixture compositions at the reactor inlet and outlet are precise, fast, and reliable. 64

65 The progress of chemical industry depends on its attitude toward chemical science and interest in research wor, while the good shape of science is determined by the fact how far it turns its face to the demand and prospects for developments in industry. 65

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