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 13

Today’s lecture Complex Reactions

A +2B C A + 3C D

  Liquid Phase PFR   Liquid Phase CSTR   Gas Phase PFR   Gas Phase Membrane Reactor Sweep Gas Concentration Essentially Zero Sweep Gas Concentration Increases with Distance   Semi Batch Reactor

2

Reactor Mole Balance Summary Reactor Type Gas Phase Liquid Phase

Batch

Semibatch

3

Reactor Mole Balance Summary Reactor Type Gas Phase Liquid Phase

CSTR

PFR

PBR

4 Note: The reaction rates in the above mole balances are net rates.

Reactor Mole Balance Summary

5

The new things for multiple reactions are:

Rates: 1.  Rate Law for every reaction 2.  Relative Rates for every reaction 3.  Net Rates of Reaction

Gas Phase Multiple Reactions

6

Note: We could use the gas phase mole balance for liquids and then just express the concentration as:

Flow: CA=FA/v0

Batch: CA=NA/V0

Note: The reaction rates in the above mole balances are net rates. The new things for multiple reactions are:

1.  Rate Law for every reaction 2.  Relative Rates for every reaction 3.  Net Rates of Reaction

7

Net Rate of Reaction for species A

For N reactions, the net rate of formation of species A is:

For a given reaction i:

(i) aiA+biB ciC+diD:

8

Batch Flow

9

Example A: Liquid Phase PFR

NOTE: The specific reaction rate k2C is defined with respect to species C.

NOTE: The specific reaction rate k1A is defined with respect to species A.

The complex liquid phase reactions follow elementary rate laws

10

Last Lecture

Example B: Liquid Phase CSTR Same reactions, rate laws, and rate constants as example A

NOTE: The specific reaction rate k1A is defined with respect to species A.

NOTE: The specific reaction rate k2C is defined with respect to species C.

11

Complex Reactions

Example B: Liquid Phase CSTR The complex liquid phase reactions take place in a 2,500 dm3 CSTR. The feed is equal molar in A and B with FA0=200 mol/min, the volumetric flow rate is 100 dm3/min and the reation volume is 50 dm3.

Find the concentrations of A, B, C and D existing in the reactor along with the existing selectivity.

Plot FA, FB, FC, FD and SC/D as a function of V

12

CSTR (1) A + 2B →C (2) 2A + 3C → D

1) Mole balance:

13

Example B: Liquid Phase CSTR

2) Rates:

3) Parameters: 14

Same reactions, rate laws, and rate constants as example A

NOTE: The specific reaction rate k1A is defined with respect to species A.

NOTE: The specific reaction rate k2C is defined with respect to species C. 15

Complex Reactions Example C: Gas Phase PFR, No ΔP

1) Mole balance:

2) Rates: Same as CSTR (5)-(14)

16

Example C: Gas Phase PFR, No ΔP

3) Stoich:

4) Selectivity:

17

Example C: Gas Phase PFR, No ΔP

Same reactions, rate laws, and rate constants as example A

NOTE: The specific reaction rate k1A is defined with respect to species A.

NOTE: The specific reaction rate k2C is defined with respect to species C. 18

Complex Reactions Example D: Membrane Reactor with ΔP

We need to reconsider our pressure drop equation. When mass diffuses out of a membrane reactor there will be a decrease in the superficial mass flow rate, G. To account for this decrease in calculating our pressure drop parameter, we will take the ratio of the superficial mass velocity at any point in the reactor to the superficial mass velocity at the entrance to the reactor. The superficial mass flow rates can be obtained by multiplying the species molar flow rates, Fi, by their respective molecular weights, Mwi, and then summing over all species:

19

Example D: Membrane Reactor with ΔP

Because the smallest molecule is the one diffusing out and has the lowest molecular weight, we will neglect the changes in the mass flow rate down the reactor and will take as first approximation.

1) Mole Balance:

We also need to account for the molar rate of desired product C leaving in the sweep gas FCsg

20

Example D: Membrane Reactor with ΔP

2) Rates: Same (5)-(14)

3) Stoich: Same (15)-(20)

4) Sweep Gas Balance:

21

Example D: Membrane Reactor with ΔP

Case 1 Large sweep gas velocity

Case 2 Moderate to small sweep gas velocity

Vary υsg to see changes in profiles 22

Example D: Membrane Reactor with ΔP

Example E: Liquid Semibatch Same reactions, rate laws, and rate constants as example A

NOTE: The specific reaction rate k1A is defined with respect to species A.

NOTE: The specific reaction rate k2C is defined with respect to species C.

23

Complex Reactions

Example E: Liquid Semibatch The complex liquid phase reactions take place in a semibatch reactor where A is fed to B with FA0=3 mol/min. The volumetric flow rate is 10 dm3/min and the initial reactor volume is 1,000 dm3.

The maximum volume is 2,000 dm3 and CA0=0.3 mol/dm3 and CB0=0.2 mol/dm3. Plot CA, CB, CC, CD and SS/D as a function of time.

24

1) Mole balance:

(1) A + 2B →C (2) 2A + 3C → D

B

FA0

25

Example E: Liquid Semibatch

2) Rates: Same (5)-(14) Net Rates, Rate Laws and relative rates – are the same as Liquid and Gas Phase PFR and Liquid Phase CSTR

3) Selectivity:

4) Parameters:

26

Example E: Liquid Semibatch

End of Lecture 13

27