Page 196 - Materials Chemistry, Second Edition

184 B. Ruggeri et al.

The insulating material is responsible for the main influence in the energy
balance of AD for several reasons. It plays a particular role in limiting the heat loss
and, at the same time, it contributes as GER to the total indirect energy consumed
for construction materials. Insulating materials are solid and usually nonhomo-
geneous materials, characterized by a very low thermal conductivity value
k, mainly due to the air enclosed in the pores of the material itself. The value of the
coefficient of conductivity k [W/(m*K)] indicates the degree of ease with which a
material allows the transport of heat, through collisions at the molecular level
(Cocchi 1993). Thermophysical properties of some insulating materials are
reported in Table 7.

3.2.4 Electrical Energy

Apart from the minor energy necessary to control the whole system, the larger
quantity of electrical energy to run a bioreactor is consumed for mixing E m and
pumping E p . A small energy input is necessary for E p for pH control. Working in
batch mode, the electrical energy is spent for filling and empting the bioreactor by
a pump. In this case, it is possible to consider, as a first approximation, E p * 0,
compared with the electrical energy consumed for the agitation, considering the
duration of the process of the order of weeks or months. However, E p depends on
the electric power of the pump:
E p ¼ P wpump Dt ð8Þ

The evaluation of the energy necessary to mix the fermenting broth could be
computed by applying a turbulence scale-up criteria, taking into account a constant
Reynolds number versus diameter. If one considers the constant ratio between the
diameter of the bioreactor and that of the impeller as a geometrical scale-up, the
following relations can be used to estimate the electrical power necessary to mix
the broth (Nagata 1975):

2
Re N 1 D ¼ N D D 2 ð9Þ
1 D
6
3
P W ¼ðP n qÞ = ð8 g pÞ N D D 4 ð10Þ
1
D
1
where
1 and D are the bench scale and actual bioreactor, respectively
is the power number and it can be evaluated by applying the procedure
P n
reported in Bailey and Ollis (1986) considering the Re of the bench
scale bioreactor
P w is the power required to have a defined Re in order to evaluate the
energy consumed for mixing
It is necessary to take into account the running time, which depends on T w :
ð
E m ¼ P W DtT w Þ =g el ð11Þ