Chemical Reaction Engineering
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Transcript of Chemical Reaction Engineering
Chemical Reaction Engineering
Chapter 4, Part 5:
Selectivity
Semibatch Reactors
Selectivity and Yield
Instantaneous Overall
Selectivity:
Selectivity and Yield
Instantaneous Overall
Selectivity:
Yield:
Selectivity and Yield
Instantaneous Overall
Selectivity:
Yield:
Example: desired product ,
undesired product ,
rD k1CA2 CB
rU k 2CACB2
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.
rD k1CA2 CB
rU k 2CACB2
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 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:
1. Molar Basis:
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:
1. Molar Basis:
2. Concentration Basis: dN A
dt
d CA V dt
VdCA
dtCA
dVdt
VdCA
dt0CA rA V
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:
1. Molar Basis:
2. Concentration Basis:
3. Conversion:
dN A
dt
d CA V dt
VdCA
dtCA
dVdt
VdCA
dt0CA rA V
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:
1. Molar Basis:
2. Concentration Basis:
3. Conversion:
dN A
dt
d CA V dt
VdCA
dtCA
dVdt
VdCA
dt0CA rA V
For constant molar feed:
For constant density:
Semibatch Reactors
The combined mole balance, rate law, and stoichiometry may be written in terms of number of moles, conversion, and/or concentration:
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
dXdV
rA
NA 0
V
k
N A
V
N B
VV
N A 0
kN A 0 1 X N B
VN A 0
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 = 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 Reactors
C
DCBAA K
CCCCkr
Semibatch Reactors
C
DCBAA K
CCCCkr
At equilibrium, -rA=0, then
Semibatch Reactors
C
DCBAA K
CCCCkr
At equilibrium, -rA=0, then
X
t
Xe
X