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. Lecture 21

Web Lecture 21 Class Lecture 17 – Tuesday 3/19/2013 Gas Phase Reactions Trends and Optimums 2

3 User Friendly Equations relate T, X, or F i Review Last Lecture 1. Adiabatic CSTR, PFR, Batch, PBR achieve this:

4 User Friendly Equations relate T, X, or F i 2. CSTR with heat exchanger, UA(T a -T) and a large coolant flow rate: T TaTa

5 User Friendly Equations relate T, X, or F i 3. PFR/PBR with heat exchange: F A0 T 0 Coolant TaTa 3A. In terms of conversion, X

6 User Friendly Equations relate T, X, or F i 3B. In terms of molar flow rates, F i 4. For multiple reactions 5. Coolant Balance

7 Reversible Reactions endothermic reaction exothermic reaction KPKP T endothermic reaction exothermic reaction XeXe T

Heat Exchange 8 Example: Elementary liquid phase reaction carried out in a PFR F A0 F I TaTa Heat Exchange Fluid The feed consists of both inerts I and Species A with the ratio of inerts to the species A being 2 to 1.

Heat Exchange 9 a)Adiabatic. Plot X, X e, T and the rate of disappearance as a function of V up to V = 40 dm 3. b)Constant T a. Plot X, X e, T, T a and rate of disappearance of A when there is a heat loss to the coolant and the coolant temperature is constant at 300 K for V = 40 dm 3. How do these curves differ from the adiabatic case.

Heat Exchange 10 c)Variable T a Co-Current. Plot X, X e, T, T a and rate of disappearance of A when there is a heat loss to the coolant and the coolant temperature varies along the length of the reactor for V = 40 dm 3. The coolant enters at 300 K. How do these curves differ from those in the adiabatic case and part (a) and (b)? d)Variable T a Countercurrent. Plot X, X e, T, T a and rate of disappearance of A when there is a heat loss to the coolant and the coolant temperature varies along the length of the reactor for V = 20 dm 3. The coolant enters at 300 K. How do these curves differ from those in the adiabatic case and part (a) and (b)?

Heat Exchange 11 Example: PBR A ↔ B 5) Parameters For adiabatic: Constant T a : Co-current: Equations as is Counter-current:

Reversible Reactions 12 1) Mole Balances

Reversible Reactions 13 2) Rate Laws

Reversible Reactions 14 3) Stoichiometry

Reversible Reactions 15 Parameters

3) Stoichiometry: Gas Phase 16 Example: PBR A ↔ B Reversible Reactions Gas Phase Heat Effects

17 Reversible Reactions Gas Phase Heat Effects Example: PBR A ↔ B

18 Exothermic Case: XeXe T KCKC T KCKC TT XeXe ~1 Endothermic Case: Example: PBR A ↔ B Reversible Reactions Gas Phase Heat Effects

19 Case 1: Adiabatic and ΔC P =0 Additional Parameters (17A) & (17B) Reversible Reactions Gas Phase Heat Effects

Heat effects: 20 Case 2: Heat Exchange – Constant T a Reversible Reactions Gas Phase Heat Effects

Case 3. Variable T a Co-Current Case 4. Variable T a Countercurrent Guess T a at V = 0 to match T a0 = T a0 at exit, i.e., V = V f 21 Reversible Reactions Gas Phase Heat Effects

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Conversion on temperature Exothermic ΔH is negative Adiabatic Equilibrium temperature (T adia ) and conversion (Xe adia ) X X e adia T adia T 28 Adiabatic Equilibrium

X2X2 F A0 F A1 F A2 F A3 T0T0 X1X1 X3X3 T0T0 T0T0 Q1Q1 Q2Q2 29

X T X3X3 X2X2 X1X1 T0T0 XeXe 30

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T X Adiabatic T and X e T0T0 exothermic T X T0T0 endothermic Trends: Adiabatic Gas Flow Heat Effects 32

Effects of Inerts in the Feed 33

Endothermic 34 As inert flow increases the conversion will increase. However as inerts increase, reactant concentration decreases, slowing down the reaction. Therefore there is an optimal inert flow rate to maximize X. First Order Irreversible

Adiabatic: 35 As T 0 decreases the conversion X will increase, however the reaction will progress slower to equilibrium conversion and may not make it in the volume of reactor that you have. Therefore, for exothermic reactions there is an optimum inlet temperature, where X reaches X eq right at the end of V. However, for endothermic reactions there is no temperature maximum and the X will continue to increase as T increases. X T XeXe T0T0 X T X T Gas Phase Heat Effects

Adiabatic: 36 Effect of adding inerts X T V1V1 V2V2 X T T0T0 XeXe X Gas Phase Heat Effects

Exothermic Adiabatic 37 As θ I increase, T decrease and k θIθI

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Endothermic Exothermic 40 Adiabatic

Heat Exchange Endothermic Exothermic 41

End of Web Lecture 21 End of Class Lecture 17 42