 # Lecture 19 Tuesday 3/18/08 Gas Phase Reactions Trends and Optimuns.

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Lecture 19 Tuesday 3/18/08 Gas Phase Reactions Trends and Optimuns

User Friendly Equations Relate T and X or F i

Heat Exchange Elementary liquid phase reaction carried out in a PFR 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 Elementary gas phase reaction carried out in a PBR The feed consists of both inerts I and Species A with the ratio of inerts to the species A being 2 to 1. (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? (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 Counter 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 = 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)?

Mole Balance(1) Rate Law(2) (3) (4)

Stoichiometry(5) (6) Parameters(7) – (15)

Energy Balance Adiabatic and  C P = 0 (16A) Additional Parameters (17A) & (17B) Heat Exchange (16B)

A. Constant T a (17B) T a = 300 Additional Parameters (18B – (20B): T a,, Ua, B. Variable T a Co-Current (17C) C. Variable T a Counter Current (18C) Guess T a at V = 0 to match T a = T ao at exit, i.e., V = V f

Heat Exchange Exothermic Endothermic

Gas phase heat effects Example: PBR A ↔ B 1)Mole balance: 2)Rates: 3)Stoich (gas):

4) Heat effects 5) Parameters…. - for adiabatic: U a = 0 - constant T a : - co-current: equations as is - counter-current: Co-current, (T a -T) for counter-current Exothermic Case: Endothermic Case: kCkC T XeXe T kCkC TT XeXe ~1

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. 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. Adiabatic: T X Adiabatic T and X e T0T0 exothermic T X T0T0 endothermic X T X T V1V1 V2V2 X T T0T0 optimum

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