Modeling of Biogas Reactors

196 6 Modeling of Biogas Reactors
V gas is the volume of the head space. Assuming steady state, the partial pressure
of H 2S in the gas bubbles rising in the biogas suspension p H2S,bubble and the partial
pressure of the produced biogas p H2S are equal.
The k La value in Eqs. 32 and 33 is not known a priori. It can be estimated by a regression
analysis, fitting the model calculation to the experimental data. The resulting
k La values are denoted in the Figures 6.32 and 6.33. It can be seen that recirculation
of biogas in the experimental reactor of Figure 6.31 increases the k La value by a
factor of 3–4 compared to the situation without gas recirculation.
Applying Eqs. 32–34 to each module of the biogas tower reactor and paying additional
regard to the liquid mixing, which is described in Section 6.4.1, the H 2S content
in the liquid phase and the gas phase in the different sections of the reactor can
be simulated as shown in Figure 6.34. It can be seen that the highest concentration
of H 2S is found in the liquid phase at the bottom of the reactor (module I). The cal-
Fig. 6.34 Effect of recycling biogas
on the H 2S concentration
in the gas phase and the liquid
phase in the pilot scale biogas
tower reactor. Wastewater was
simultaneously supplied to
modules I, II, and III. Gas was
recycled to module I: 2.3, 13, 5,
13, 1.8, 1.8 m 3 h –1 and to module
II: 0, 0, 2, 2, 3, 0 m 3 h –1 in
the sequence of the experiment
according to the denoted time
intervals; sulfate content of
wastewater: 3.4–4.5 g L –1 ;
mean residence time of wastewater
in the reactor: 2.1 d), r.,
recirc. recirculation, ext. external,
mod. module.