Chemical Reaction Engineering Asynchronous Video Series

1
Chemical Reaction Engineering Asynchronous
Video Series

• Chapter 4, Part 5
• Selectivity
• Semibatch Reactors
• H. Scott Fogler, Ph.D.

2
Selectivity and Yield

• Instantaneous Over
  all
• Selectivity

3
Selectivity and Yield

• Instantaneous Over
  all
• Selectivity
• Yield

4
Selectivity and Yield

• Instantaneous Over
  all
• Selectivity
• Yield
• Example desired product ,
• undesired product ,

5
Selectivity and Yield

• Instantaneous Over
  all
• 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) -raV/Nao
  d(Ca)/d(t) ra - (Cavo)/V
  d(Na)/d(t) raV
• ra -kCaCb
  d(Cb)/d(t) rb ((Cbo-Cb)vo)/V
  d(Nb)/d(t) rbV Fbo
• Ca Nao(1 - X)/V
  ra -kCaCb
  ra -kCaCb
• Cb (Nbi Fbot - NaoX)/V rb
  ra
  rb ra
• V Vo vot
  V Vo vot
  V Vo vot
• Vo 100
  Vo 100 Vo 100
• vo 2 vo 2 vo
  2
• Nao 100 Fbo 5
  Fbo 5
• Fbo 5 Nao 100
  Ca Na/V
• Nbi 0 Cbo Fbo/vo
  Cb Nb/V
• k 0.1 k 0.01 k 0.01
• Na CaV 
• X (Nao-Na)/Nao

14
Semibatch Reactors
15
Semibatch Reactors
At equilibrium, -rA0, then
16
Semibatch Reactors
At equilibrium, -rA0, then
Xe
X
X
t