66
Chapter 3
prices of oil during the oil crisis of the 1970’s. With the shortage of, and high price of fishmeal in to-
days marketplace there is no chance that the revived SCP production from methane will fail for
economic reasons. The market is likely to expand to substitute a substantial fraction of the demand for
high-grade animal protein, which in Europe alone is more than 10
6
tons per year.
Consider standard biomass (K, = 4.20) produced with a yield of 0.8 g (g CH
4
)'1. From the degree of
reduction balance and a carbon balance the stoichiometry is calculated:
-C H A
-1 .4 5 4 0 2 +O.52OA>O.48OC02 = 0
(
1
)
If the same yield of biomass could be obtained from methanol then
-
CH}OH
- O.9540J +
0.520X +
0.480
C 02 =
0
(2)
As expected half a mole of 0
2
is used to oxidize methane to methanol and consequently
Yx
in equation
(2)
is equal to
Y„
from equation (1) - 0.5._____________________________________________________
The redox balance used in the examples is often referred to as the
generalized degree o f
reduction balance,
and it was first introduced by Roels (1983) as a generalization of the earlier
work by Erickson
et a l
(1978). The degree of reduction balance can be specified as a linear
combination o f the elemental balances, where the multiplication factors in the linear combination
are the redox levels (or the degree of reduction) of the elements. It is therefore also clear that the
reduction balance does not introduce an additional balance to the system. When all the elemental
balances are satisfied then the degree of reduction balance automatically closes. As discussed
earlier the use of all the elemental balances is in practice hindered by lack of information on the
yield coefficient for water, which cannot be experimentally determined, and it is therefore
interesting to use the degree of reduction balance together with the carbon and nitrogen balances.
A major reason for applying these balances to a set of experimental rate data used to construct a
stoichiometric equation (3.15) is to check the consistency of the experimental data. Eventually
the stoichiometric equation is going to be used as a first basis for design of an industrial process
and it may have catastrophic consequences if the steady state yield coefficients are wrong from
the start.
Experiments can go wrong for many reasons of which some are listed below.
Some of the products may go undetected because it was not suspected that they would be
formed in the bioreaction.
Products that were thought to remain in the liquid phase are in fact partly stripped to the
gas phase.
The instruments used to measure the rates are wrongly calibrated or they malfunction
after some time.