Modeling of Biogas Reactors

188 6 Modeling of Biogas Reactors
L is the length of the baffle and g is the gravitational acceleration. The gas holdup
can be correlated from the superficial gas velocity u riser according to Weiland (1978).
Due to experiments of Reinhold et al. (1996), it can be calculated according to Eqs.
23 and 24).
Äå = 0.27 u 0.8
riser (laboratory scale) (23)
Äå = 0.76 u 0.8
riser (pilot scale) (24)
The pressure loss coefficients were determined to be î = 6.9 on the laboratory
scale and to be î = 4.8 on the pilot scale. As expected, the total friction coefficient î
slightly decreases during scale-up.
Figure 6.26 shows the exchange flow rate V · exchange as a function of the superficial
gas velocity u exchange, which can be calculated according to Eq. 25.
u exchange =
V · gas
A exchange
It is quite remarkable that the findings of Figure 6.26 are invariant with the scale
of the reactor. Because V · exchange dominates the mixing behavior of the system (indeed,
it is the only experimental parameter necessary for model B) this result is very
important for the scale-up of biogas tower reactors.
6.4.2
Distribution of Biomass within the Reactor
The wastewater is passing the BTR from the bottom to the top of the reactor. With
this feeding procedure the upflow velocity due to the feed increases – at a constant
mean residence time of wastewater in the reactor – linearly with the height of the reactor.
Therefore, in high reactors it is essential to have reliable mechanisms to retain
the active biomass. These mechanisms and the resulting distribution of biomass
within the reactor were studied by Reinhold and Märkl (1997).
6.4.2.1 Experiments
Experiments were performed with a two-modular laboratory tower reactor (height
6.5 m) and in a pilot scale tower reactor (height 20 m) as shown in Figure 6.2 and described
in Table 6.1. The laboratory tower reactor was filled with tap water and anaerobic
pelletized sludge. The sludge was biologically, inactive, as no substrate was
present in the tap water, i.e., it did not produce any fermentation gas. Air was injected
at the bottom of the column instead of real biogas. As the reactor was built out of
transparent material, visual observations were possible in addition to measurements
from samples. The following observations were made during these experiments:
• The suspended solids concentration in the lower module is always higher than in
the upper module.
(25)