Examination paper for TKP 4155 / KP 8903 REACTION KINETICS AND CATALYSIS

Transcription

1 Department of Chemical Engineering Examination paper for TKP 4155 / KP 8903 REACTION KINETICS AND CATALYSIS Academic contact during examination: Professor Magnus Rønning Phone: Examination date: Examination time (from-to): 09:00-13:00 Permitted examination support material: D No written or typed documentation is allowed. Simple, defined calculator is allowed. Other information: Language: English Number of pages: 5 Number of pages enclosed: Checked by: Date Signature

2 Problem 1. (5%) Consider the following reaction over a heterogeneous catalyst: A B (1) The catalyst is a Ni/-Al O 3 (5 wt% Ni) with a dispersion of 10 %. It is assumed that reaction (1) follows a Langmuir-Hinshelwood mechanism. a) Derive an expression for the relation between true and apparent (observed) activation energy in the case where the reaction is strongly inhibited by the product. Reaction (1) is studied in a laboratory reactor at 400C, 1 atm and with 10 mole % A in the feed. 7 % conversion of A was obtained with space velocity, GHSV = 5000 [Ncm 3 reaction gas] [g cat, h] -1. The following data are given for the catalyst: Particle diameter: d p =.4 mm Particle density: p = 1. gcm -3 Effective diffusivity: D eff = cm s -1 b) Calculate the turnover frequency (TOF) for reaction (1) at the given conditions. The molecular weight of Ni is 58,7. c) Determine if diffusion limitations are present in the catalyst at these conditions. The activity of the Ni catalyst may be increased by adding a promotor (X). The promoted catalyst Ni-X//-Al O 3 (5 wt% Ni) gives a conversion of 15 % at the above conditions in the reactor. However, it is desirable to keep the conversion level below 10%. d) Why is it an issue to keep the conversion of A below 10%? Discuss how the reaction conditions may be adjusted in order to keep the conversion at 7% also for the promoted catalyst.

3 3 Problem. (5%) Disproportion of propene is studied over a catalyst containing 9 wt% tungsten on silica at 673 K and bars: C 3 H 6 C H 4 + C 4 H 8 (1) The following mechanism is proposed: C 3 H 6 + * C 3 H 6 * () C 3 H 6 * + C 3 H 6 * C H 4 * + C 4 H 8 * rds (3) C 4 H 8 * C 4 H 8 + * (4) C H 4 * C H 4 + * (5) a) Find an expression for the reaction rate for the disproportion of propene assuming that the reaction mechanism follows a Langmuir-Hinshelwood model. b) Suggest an analogue mechanism and rate expression where the reaction follows Eley- Rideal kinetics. c) How will the Eley-Rideal model rate expression become if also the backward reaction is included? C 3 H 6 C H 4 + C 4 H 8 (6) d) What assumptions are made when deriving the Langmuir isotherm?

4 4 Problem 3. (5%) An important reaction taking place in the automotive three-way catalyst is the reaction between CO and NO to form CO and N. The reaction can be described by the following reaction mechanism where step is assumed to be the rate determining step (rds) and all other steps are in quasi-equilibrium: NO + * NO* (1) NO* + * O* + N* rds () CO + * CO* (3) CO* + O* CO * + * (4) CO * CO + * (5) N* + N* N * + * (6) N * N + * (7) a) Write down the overall net reaction and express the equilibrium constant K G in terms of the partial pressures of the involved gases. b) Give an example of at least one other elementary step that could be of relevance for this process. c) Find a rate expression based on the mechanism and assumptions given above. d) Find an expression for the surface coverage of free sites (θ * ) in terms of equilibrium constants and partial pressures alone. Problem 4. (5%) a) Make a sketch of how the heat of adsorption depends on the surface coverage for metal catalysts. Give a possible explanation for the dependency.

5 A D B C 5 b) Below is a sketch of the homogeneous Monsanto process which is a method for manufac- turing of acetic acid by catalytic carbonylation of methanol. Determine the formal oxidation state of Rh in thee four complexes A-D and determine if the complexes are coordinatively saturated. The OCCH 3 ligand has formal charge -1 and coordination number 1. c) Identify the reactions A B, B C, C D and D A. d) Show with an example how dihydrogen (H ) can be activated by the use of a transition metal complex. What is the name of such a reaction?

6 6 Appendix 1: Equation and formula chart, TKP 4155 dc Ficks law: N D [mol/s cm ] p dx Bulk diffusion: 3 1 0, T [( M1 M) / M1M] D1 cm / s Ps 1WD Effective bulk diffusivity: Knudsen diffusion: D1 q Deff cm / s t T D K, re cm / s M Effective Knudsen diffusivity: 1 q T Keff, / tksr p M D cm s Damköhler equation: Prater number: Effectiveness factor: ( H) Deff T T TS ( Cs C) k eff T ( H) Deff C S b T k T S eff s h fs tanhfs fs Mass transfer rate: r kga( Cb Cs) moles cm 1 3 Constants: Molar gas volume: 414 cm 3 /mol (STP, at 5ºC and 1 bar) The gas constant, R: 8.05 cm 3 atm/mol K (8.314 J/mol K)

7 7 Transition metals and their number of d electrons in various oxidation states: Ligands with their typical charges and coordination numbers: