Environmental Engineering Reference
In-Depth Information
SYMBOLS
[J.kg
-1
.K
-1
]
c
p
specific heat capacity
[J.mol
-1
]
G
Gibbs free energy
[J.kg
-1
]
H
enthalpy on mass basis
[J.mol
-1
or J.kg
-1
]
h
fg
enthalpy of evaporation
[J.mol
-1
]
H
enthalpy on mole basis
[kg.mol
-1
]
MW
i
molecular weight of species i
[kg.kg
-1
]
Y
mass yield
τ
residence time
[s]
Subscripts
c
combustion
f
formation
glc
glucose
prod
product(s)
r
reaction
Superscript
max maximum
13.1
INTRODUCTION
13.1.1 Fermentation: Biological Background
Living organisms are complex structures of mostly organic compounds. Building up
such structures from smaller molecules (
) requires energy. Photosynthetic
organisms obtain this energy from sunlight. Other organisms convert part of the
chemical compounds available to them into compounds with a lower Gibbs energy
(
“
anabolism
”
) and use this difference in Gibbs energy to drive the anabolic reactions.
The biochemistry of the cell is strictly organized for this; specific enzymes catalyze
only the necessary reactions. Generally, the catabolic reactions are such that the avail-
able Gibbs energy is not liberated as heat but used to convert adenosine diphosphate
(ADP) with inorganic phosphate (P
i
) into adenosine triphosphate (ATP). The anabolic
reactions are usually driven forward by coupling them to the reverse conversion (ATP
to ADP and phosphate). Thus, ATP is the carrier of Gibbs energy equivalents at the
biochemical level.
Carbohydrates or other compounds available to living organisms can be com-
pletely oxidized to CO
2
and H
2
O when sufficient O
2
is present. This combustion-like
catabolism generates much ATP and allows fast growth. However, in the absence of
O
2
or any other electron donor such as sulfate or nitrate, some ATP can still be
“
catabolism
”
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