Lecture 24 Chemical Reaction Engineering (CRE) is the field that studies the rates and mechanisms of chemical reactions and the design of the reactors.

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Presentation transcript:

Lecture 24 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.

Today’s lecture Normal Pentene  Iso-Pentane Langmuir Adsorption Isotherm Catalytic Converters for Automobiles

Last Lecture Catalytic Reforming Reaction (Gasoline) Normal Pentane Octane Number=62 Iso-Pentane Octane Number=90 Normal Pentane  Iso-Pentane

Steps in a Catalytic Reaction

Ethylidyne as chemisorbed on platinum (Adapted from G. A Ethylidyne as chemisorbed on platinum (Adapted from G.A.Somorjai, Introduction to Surface Chemistry and Catalysis, Wiley, New York, 1994)

Steps in a Catalytic Reaction Adsorption on Surface Surface Reaction Single Site Dual Site Desorption from Surface

Platinum on alumina (Figure from R. I Platinum on alumina (Figure from R.I. Masel, Chemical Kinetics and Catalysis, Wiley , New York, 2001, p 700)

n-pentane i-pentane 0.75 wt% Pt Al2O3 n-pentane n-pentene -H2 Pt Al2O3 i-pentene +H2 i-pentane n-pentene i-pentene Al2O3 N I

Isomerization of n-pentene (N) to i-pentene (I) over alumina N I Al2O3 1. Select a mechanism (Mechanism Dual Site) Adsorption on Surface: Surface Reaction: Desorption: Treat each reaction step as an elementary reaction when writing rate laws.

2. Assume a rate-limiting step 2. Assume a rate-limiting step. Choose the surface reaction first, since more than 75% of all heterogenous reactions that are not diffusion-limited are surface-reaction-limited. The rate law for the surface reaction step is

3. Find the expression for concentration of the adsorbed species CI. S 3. Find the expression for concentration of the adsorbed species CI.S. Use the other steps that are not limiting to solve for CI.S (e.g., CN.S and CI.S). For this reaction, From From

4. Write a Site Balance. 5. Derive the rate law. Combine steps 2, 3 and 4 to arrive at the rate law:

Automotive Catalytic Converters Placed inline with exhaust system (~$500) NOx plays big role in smog formation

Automotive Catalytic Converters 1) Propose a mechanism

Automotive Catalytic Converters (from graph data, we see CO2 is not on the surface and that the surface reaction is irreversible)

Automotive Catalytic Converters

Automotive Catalytic Converters Is this consistent with the given data? -No CO2 in rate law -rate law decreases as PN2 increases The contribution of PN2 is very small and can be neglected

Automotive Catalytic Converters optimum

End of Lecture 24