1. Chemical Reaction Engineering Asynchronous Video Series
Chapter 4, Part 5:
Selectivity
Semibatch Reactors
H. Scott Fogler, Ph.D.

2. Selectivity and Yield
Instantaneous Overall
Selectivity:

3. Selectivity and Yield
Instantaneous Overall
Selectivity:
Yield:

4. Selectivity and Yield Instantaneous Overall Selectivity: Yield:
Example: desired product ,
undesired product ,

5. Selectivity and Yield Instantaneous Overall Selectivity: Yield:
Example: desired product ,
undesired product ,
To keep the selectivity of the desired products high with respect to the undesired products carry out the reaction at high concentrations of A and low concentrations of B. If the reactor is liquid phase, a high selectivity can easily be achieved using a semibatch reactor in which B is few slowly to A.

6. Semibatch Reactors
Semibatch reactors can be very effective in maximizing selectivity in liquid phase reactions.
The reactant that starts in the reactor is always the limiting reactant.

7. Semibatch Reactors
Semibatch reactors can be very effective in maximizing selectivity in liquid phase reactions.
The reactant that starts in the reactor is always the limiting reactant.
Three Forms of the Mole Balance Applied to Semibatch Reactors:
Molar Basis:

8. Semibatch Reactors
Semibatch reactors can be very effective in maximizing selectivity in liquid phase reactions.
The reactant that starts in the reactor is always the limiting reactant.
Three Forms of the Mole Balance Applied to Semibatch Reactors:
Molar Basis:
Concentration Basis:

9. Semibatch Reactors
Semibatch reactors can be very effective in maximizing selectivity in liquid phase reactions.
The reactant that starts in the reactor is always the limiting reactant.
Three Forms of the Mole Balance Applied to Semibatch Reactors:
Molar Basis:
Concentration Basis:
Conversion:

10. Semibatch Reactors
Semibatch reactors can be very effective in maximizing selectivity in liquid phase reactions.
The reactant that starts in the reactor is always the limiting reactant.
Three Forms of the Mole Balance Applied to Semibatch Reactors:
Molar Basis:
Concentration Basis:
Conversion:
For constant molar feed:
For constant density:

11. Semibatch Reactors
The combined mole balance, rate law, and stoichiometry may be written in terms of number of moles, conversion, and/or concentration:

12. Semibatch Reactors
The combined mole balance, rate law, and stoichiometry may be written in terms of number of moles, conversion, and/or concentration:
Conversion Concentration Number of Moles

13. Semibatch Reactors Polymath Equations: Conversion Concentration Moles
d(X)/d(t) = -ra*V/Nao d(Ca)/d(t) = ra - (Ca*vo)/V d(Na)/d(t) = ra*V
ra = -k*Ca*Cb d(Cb)/d(t) = rb + ((Cbo-Cb)*vo)/V d(Nb)/d(t) = rb*V + Fbo
Ca = Nao*(1 - X)/V ra = -k*Ca*Cb ra = -k*Ca*Cb
Cb = (Nbi + Fbo*t - Nao*X)/V rb = ra rb = ra
V = Vo + vo*t V = Vo + vo*t V = Vo + vo*t
Vo = Vo = Vo = 100
vo = vo = vo = 2
Nao = Fbo = Fbo = 5
Fbo = Nao = Ca = Na/V
Nbi = Cbo = Fbo/vo Cb = Nb/V
k = k = k = 0.01
Na = Ca*V 
X = (Nao-Na)/Nao

14. Semibatch Reactors

15. Semibatch Reactors
At equilibrium, -rA=0, then

16. Semibatch Reactors
At equilibrium, -rA=0, then Xe X X t