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

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chemical reaction engineering asynchronous video series

Chemical Reaction EngineeringAsynchronous Video Series

Chapter 4, Part 5:

Selectivity

Semibatch Reactors

H. Scott Fogler, Ph.D.

selectivity and yield
Selectivity and Yield

InstantaneousOverall

Selectivity:

selectivity and yield3
Selectivity and Yield

InstantaneousOverall

Selectivity:

Yield:

selectivity and yield4
Selectivity and Yield

InstantaneousOverall

Selectivity:

Yield:

Example: desired product ,

undesired product ,

selectivity and yield5
Selectivity and Yield

InstantaneousOverall

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.

semibatch reactors
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.

semibatch reactors7
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:
semibatch reactors8
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:
semibatch reactors9
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:
semibatch reactors10
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:

semibatch reactors11
Semibatch Reactors

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

semibatch reactors12
Semibatch Reactors

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

ConversionConcentrationNumber of Moles

semibatch reactors13
Semibatch Reactors

Polymath Equations:

Conversion ConcentrationMoles

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 = 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 = Ca*V 

X = (Nao-Na)/Nao

semibatch reactors15
Semibatch Reactors

At equilibrium, -rA=0, then

semibatch reactors16
Semibatch Reactors

At equilibrium, -rA=0, then

Xe

X

X

t