Modeling of Biogas Reactors

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164 6 Modeling of Biogas Reactors Fig. 6.1 Elements of a mathematical model for a biogas tower reactor (BTR). inhold (1994). The typical features of the BTR are its tower shape, the modular structure, and the internal installations. Gas collecting devices are used to withdraw the fermentation gas from different levels of the reactor. These gas collectors separate the reactor into modules along the height. By means of these devices, which are equipped with valves, the gas loading and the mixing intensity can be controlled separately in each module. To avoid flotation of active biomass due to excessive gas load, an effective biomass accumulation is generated within the reactor. The biomass is represented in this reactor by free suspended microorganisms which are associated in the form of sedimentable more-or-less loose pellets (flocs). One very important point, when modeling such a reactor, is to describe the local distribution of active biomass within the reactor (Section 6.4.2). In a mathematical model the mixing behavior of the liquid phase is of similar importance (Section 6.4.1). The mixing of the BTR is caused by internal airlift loop units. Since the reactor is designed in the shape of a tower and is only fed at the bot-
tom (Fig. 6.1), the question of mixing and supplying the microorganisms in each part of the reactor with nutrients is very important during the scale-up of such a system. Besides, hydraulic mixing is important to prevent toxic concentrations of substrate near the inlet of the reactor. As pointed out by Graef and Andrews (1973), the mass transport of product gas from the liquid phase to the gas phase is an essential element of the mathematical model. Local transport data as a function of the hydrodynamics for the example of the biogas tower reactor is given for CO 2 and H 2S in Section 6.5 of this chapter. The last element of a mathematical model which is of high importance, especially when discussing tall reactors, is represented by the influence of the local hydrostatic pressure on the kinetics of biogas generation (Section 6.6). 6.1.2 Scale-Up Strategy 6.1 Introduction Parameters of the mathematical model elements must be identified by experiments, and the models have to be evaluated with respect to their suitability for the design of technical scale biogas reactors. The scale-up strategy is demonstrated in Figure 6.2 with explanations in Table 6.1. Basic experiments are done in the laboratory. The hydrodynamic behavior and the sedimentation of non-active biomass were studied first in a small reactor unit with a height of 6.5 m. This laboratory reactor has a diameter of 0.4 m and consists of only two modules. To understand the influence of Fig. 6.2 Strategy for scale-up of the biogas tower reactor, specification of reactors according to Table 6.1. 165
Page 1: 6 Modeling of Biogas Reactors Herbe
Page 5 and 6: Table 6.2 Characteristic data of th
Page 7 and 8: desalinated water and wastewater in
Page 9 and 10: 6.3 Kinetics Fig. 6.7 Combined meas
Page 11 and 12: dx dt dc Ac dt = µ (c Ac, c i) x -
Page 13 and 14: For example, the acetic acid HAc is
Page 15 and 16: On the abscissa the concentration o
Page 17 and 18: c HAc µ = µ max · · (18) cHAc+K
Page 19 and 20: 6.4 Hydrodynamic and Liquid Mixing
Page 23 and 24: 6.4.1.1 Model A Model A, which is a
Page 29 and 30: = 0.014 m s -1 V + 3.1· · exchang
Page 31 and 32: The effect of gas recirculation can
Page 33 and 34: high (5.13 g L -1 ), the concentrat
Page 35 and 36: culated k La values in the pilot sc
Page 37 and 38: 6.7 Outlook Altogether at a higher
Page 39 and 40: References tance in the production

Modeling of Biogas Reactors

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