Lecture 11 Kjemisk reaksjonsteknikk Chemical Reaction Engineering

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1 Lecture Kjemisk reaksjonsteknikk Chemical Reaction Engineering Review of previous lectures Kinetic data analysis of heterogeneous reactions. Characterization of t catalysts. Kinetic study, find a kinetic expression and kinetic parameter 3. Catalytic reactor design - 7/9/

2 7-Step rocedure for CRE Data analyses (). ostulate a rate law. ower law models fro for homogeneous reactions r kc C B B. Langmuir-Hinshelwood models for heterogeneous reactions r k B ( K K BB. Select reactor type and corresponding mole balance.if batch reactor, use mole balance on Reactant r dc dt B.If differential BR, use mole balance on product ( ) F C r W W ) - 7/9/

3 7-Step rocedure for CRE Data analyses () 3. rocess your data in terms of the measured variables (e.g. N, C, or ). If necessary, rewrite your mole balance in terms of your measured variables 4. Look for simplification For example, if one of the reactants is in excess, assume its concentration is constant,. If the gas phase mole fraction of reactant is small, set ε= 5. For a batch reactor, calculate r as a function of concentration C to determine the reaction order:.differential analysis B.Integral method C.Nonlinear regression 3-7/9/

4 7-Step rocedure for CRE Data analyses (3) 6. For differential BR, calculate r as a function of C or C F r. Calculate W W as a function of reactant concentration C or partial pressure B. Choose a model, e.g., r k K C. Use nonlinear regression to find the best model and model parameters 7. nalyze your rate law model for goodness of fit. Calculate a correlation coefficient. 4-7/9/

5 Example: Scaling up of toluene hydrogen process Hydrogen and toluene are reacted over a t catalysts supported on crystalline silica-alumina to form methane and benzene. C 3 H 5 CH 3 +H C 6 H 6 + CH 4 We wish to design a packed bed reactor to process a feed consisting of % toluene and 8 % hydrogen. oluene is fed at a rate of 5 mol/min at a temperature of 64 oc and a pressure of 4 atm. Step, Characterization of t catalysts Step, Kinetic study, find a kinetic expression and kinetic parameter Step 3, Catalytic reactor design 5-7/9/

6 Determination of active site numbers he concentration of active sites on the catalyst surface is typically determined by H or CO chemisorption..5 H ads (CM 3 S) (mmhg) he chemisorption of H at 98 K on t catalysts ) determine Langmuir-isotherm. ) determine the t surface area or dispersion 6-7/9/

7 ssociative adsorption * ka H H * k a H k * D H C C t V K H * 5 Linearization of Langmuir-isotherm K H KH /Cv (g/mmmol) / (mmh - ) 7-7/9/

8 Dissociative adsorption ka H * H * H k a k H * D H ( K ( K * 5 Linearization of Langmuir-isotherm H ) H / C =.37 mmol/g cat ) / /Cv (g/mmmol) 5 5 H C C t V ( K H slop=/(c K.5 ) / ) /.5 (mmh - ) /C 8-7/9/

9 Kinetic Modeling and nalysis ) Select a proper reactor for kinetic study ) erform kinetic study and design kinetic experiments 3) Developing an algebraic rate expression consistent with experimental observations ) nalyzing the rate expression in a such manner that the rate expression parameters can readily be determined from experimental data 3) Find a mechanism and rate determining step consistent with the experimental data (for non-elementary reactions) 9-7/9/

10 We select differential fixed bed reactor for the kinetic study - 7/9/

11 Data from a differential reactor -r artial pressure (atm) mmol Run toluene/g cat.hr oluene Hydrogen Methane Benzene set Set B Set C Set D /9/

12 urn over frequency (OF) OF r C mmol g hr cat mmol g cat OF r 7.35*36 C OF.46 s - 7/9/

13 Dependence of methane and benzene r (mmol/g cat.hr) r (mmol/g cat.hr) r K M (atm) B (atm)... M M r... K K M M << K B B <<... B B /9/

14 Dependence of toluene (atm) r K /9/ -r (mmol/g cat.hr)

15 Dependence of hydrogen H (atm) 5-7/9/ -r (mmol/g cat.hr) r... K H H << H K H H

16 roposed reaction mechanism dsorption (g) + * * Surface reaction H (g) + * B* +M(g) Desorption B* B(g) +* r k H K r r D D k B k ( * S ( H K ) B K S * * 6-7/9/ M )

17 Determination of kinetic parameters /-r (gcat.hr/mmol) Slope= k H K k H.7.7 K r kh kh / (atm - ) k=4.9 g mmol atm hr cat k= atm - 7-7/9/

18 BR reactor ) Mole balance F dx r ) Rate law r kh K 3) Stoichiometry: F F X y M B C R C 4.9 H C H C X X H H, R ( x) ( x ) 8-7/9/

19 4) Combine: BR reactor F dx k H X K ( x) o K k o H ( x) dx F X 5. Evaluation W 5 W 4.9 g k H F k H X K k H dx ( K x ln( x)) o mol / min6min/ hr mmol hratm cat 3 atm8 atm ( 8 atm F atm.8 ln(.8)) 656.8g 9-7/9/

20 BR reactor with pressure drop - 7/9/

21 ressure Drop in acked Bed Reactors Ergun Equation: d dz G g D c p 3 5 D p LMINR.75G URBULEN pressure, ϕ porosity (volume of void/total bed volume) ka - ϕ (volume of solid/total bed volume) g c conversion factor.. for metric system D p diameter of particle in bed m μ viscosity of gas passing through the bed kg/m.s Z length down the packed bed m u, superficial velocity m/s ρ gas density kg/m 3 G= ρu =superficial mass velocity kg/m,s - 7/9/

22 ressure Drop in acked Bed Reactors Ergun Equation: d dz G g D c p 3 5 D p LMINR.75G URBULEN Constant mass flow: m m F F ( X) - 7/9/

23 3-7/9/ p 3 p c F F.75G D 5 D g G dz d F F Variable Density G.75 D 5 D g G p 3 p c Let ressure Drop in acked Bed Reactors 3

24 ressure Drop in acked Bed Reactors Catalyst Weight W z c b z c c Where b bulk density c solid catalyst density porosity (a.k.a., void fraction ) c, cross section area, z length of the reactor 4 Let d c c F c c F 4-7/9/

25 ressure Drop in acked Bed Reactors dy y F F y 5 We will use this form for single reactions: d dy X dy y y X X Isothermal case 5-7/9/

26 ressure Drop in acked Bed Reactors F dx k H X y K ( x) y o 6 dx f X, d dy and f X, or f y, X he two expressions are coupled ordinary differential equations. We can only solve them simultaneously using an ODE solver such as olymath. For the special case of isothermal operation and epsilon ε=, we can obtain an analytical solution. olymath will combine the mole balance, rate law and stoichiometry. 6-7/9/

27 For dy y BR dy y X When W y Initial condition 7 dy y y ( W ) ( W ) / c c 7-7/9/

Chemical Reaction Engineering

Lecture 8 Chemical Reaction Engineering (CRE) is the field that studies the rates and mechanisms of chemical reactions and the design of the reactors in which they take place. oday s lecture Block 1: Mole