Biomedical Engineering Reference
In-Depth Information
where
F
Viþ
1
is the total molar flow rate of the vapor stream into stage
i
,
F
Vi
is the total molar
flow rate of the vapor stream out of stage
i
,
y
A
i
is the mole fraction of A in the vapor stream at
stage
i
. The mass transfer flux between the two phases can be expressed as
J
A
i
¼ K
LA
i
C
A
i
C
A
i
(5.83)
where
K
LA
i
is the overall mass transfer coefficient of A at stage
i
, and
C
A
i
is the equilibrium
concentration of A in the liquid phase (in equilibrium with the gas phase
y
A
i
).
For a continuous reactive distillation column (as opposed to the stage-wise reactive distil-
lation column), the three corresponding equations become
d
ðQ
L
C
A
Þþr
A
d
V aJ
A
d
V ¼ 0
(5.84)
d
F
V
y
A
þ aJ
A
d
V ¼ 0
(5.85)
J
A
¼ K
LA
C
A
C
A
(5.86)
Therefore, the reactive distillation can be solved similar to solving a distillation column.
Reactive distillation is particularly applicable to reaction systems where 1) equilibrium
limiting the conversion of reactants to products; 2) side reactions occur to further render
one or more products to undesired products; and 3) catalyst limitation on the final product
concentration. The key for applying the reactive distillation is that there is a preferential to
evaporate one of more products into the vapor or gas stream. There are numerous examples
where reactive distillation concept can be employed. Examples of such systems include:
acetate production (equilibrium limiting); ethylene glycol production (product can further
react with reactant to form undesired product); butanol fermentation (catalyst inhibition).
In acetate production, acetates are more volatile than the acetic acid and the alcohol used.
In ethylene glycol production, water and ethylene oxide are less volatile than ethylene
glycol. In butanol fermentation, sugars are not volatile, and an inert gas can be used to strip
butanol out.
5.7.3. Membrane Reactor
Membrane reactors can be used to increase the yield of reactions that are highly reversible
(or unfavorable reactions). The membrane will preferentially allow some of the products to
permeate through, thus shifting the chemical equilibrium such that more products can be
formed.
Dehydrogenation reactions are best examples where membrane reactors can have a signif-
icant benefit. Examples of dehydrogenation reactions of commercial significance include,
C
6
H
12
%
C
6
H
6
þ 3
H
2
(5.87)
Benzene production;
C
6
H
5
CH
2
e
CH
3
%
C
6
H
5
CH
]
CH
2
þ
H
2
(5.88)
Styrene production (monomer for polymer production);
C
4
H
10
%
C
4
H
8
þ
H
2
(5.89)
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