Modeling of Biogas Reactors

192 6 Modeling of Biogas Reactors
Fig. 6.30 Concentration of suspended solids as a function of reactor height in the 15 m 3 pilot
scale BTR. V · feed = 315 L h –1 ; total gas production of the reactor 2.3 m 3 h –1 ; gas recirculated 1.8 m 3
h –1 ; ÄTOC = 7.3 kg m –3 ; gas loading V gas, i of the connecting areas between the modules as indicated.
6.5
Mass Transport from the Liquid Phase to the Gas Phase
As demonstrated in Section 6.3, it is necessary to have quantitative data for the concentrations
of the product gases, like CO 2, NH 3, and H 2S in the fermentation broth,
to calculate the pH value within the fermentation broth and get information on the
kinetics of methane production. To calculate these concentrations it is necessary to
have additional information about the transport of these gases from the liquid phase
– where they are produced by the microorganisms – to the gas phase. Basic ideas of
this calculation will be discussed with the example of H 2S because of its strong influence
on biogas production. Polomski (1998) performed experiments on the desorption
of H 2S from the fermentation liquid broth in laboratory and pilot scale reactors.
The experimental setup of the laboratory experiment is shown in Figure 6.31. The
UASB reactor is supplied with wastewater at the bottom. A part of the generated biogas
is withdrawn at the top of the reactor and recycled to the bottom where it is injected
by means of a perforated tube. The recycled biogas is either recycled directly
or over a H 2S adsorber. The H 2S concentration in the fermentation liquid and the
partial presure of H 2S in the gas were measured by means of a silicon membrane
probe and the mass spectrometer (see also Section 6.2.1).