Modeling of Biogas Reactors

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182 6 Modeling of Biogas Reactors called a “riser” and the downflowing part is called a “downcomer”. The circulation velocity is strongly linked to the amount of gas passing through the open cross-sectional area (3). Hence, by controlling the gas outlets (2) and (6) it is possible to control the liquid circulation velocity and the mixing time in a module. If no gas is withdrawn, both the circulation velocity and the mixing intensity are high. In contrast, if all collected gas is withdrawn, the circulation slows and good settling conditions for biomass set in. In the lower zones of the reactor it might be advantageous to support the mixing characteristics, in the higher zones good settling conditions should be established. As in mixing within one module, the mixing between two neighboring modules depends, as experiments show, on the gas flow rate through the open cross-sectional area ( 3) between the two modules. If gas rises through this area, turbulence is generated, causing a convective transport between the two modules. If the gas flow rate through the connecting area (3) is low, the mass transport between the modules decreases. 6.4.1 Mixing of the Liquid Phase Reinhold et al. (1996) provided a theoretical and analytical analysis of the mixing behavior of a BTR. Experiments were performed on a laboratory (Fig. 6.19) and a pilot Fig. 6.19 Laboratory reactor representing a model of two modules of the BTR. It was filled with 0.1% ethanol in tap water, and air was injected through a sparger at the bottom to simulate biogas production (height: 6.5 m; diameter 0.4 m; volume 0.7 m 3 ; height of a module 3.5 m; length of the baffle 2.89 m).
6.4 Hydrodynamic and Liquid Mixing Behavior of the Biogas Tower Reactor scale (Fig. 6.20). To study mixing, a saline solution was injected in the form of a Dirac impulse as a tracer at positions indicated in Figures 6.19 and 6.20. The time course of the salt concentration was continuously monitored by conductivity probes at different positions. For the experiments the reactors were filled with tap water. To reduce the coalescence of bubbles and to obtain small bubbles as observed in biogas reactors, 0.1% ethanol was added. Instead of biological gas production, air was injected at the bottom of the reactors. It was assumed that the dominating effect on the circulating flow was caused by gas coming from the lower module. The results of the experiments can be summarized as follows. The mixing times for a 95% homogeneity after a tracer impulse are • mixing within one module: (1–2)·10 2 s • mixing from one module to a neighboring module: (0.5–2)·10 3 s Because the heights of a module in the laboratory and in the pilot scale reactor do not differ much, the situation on the pilot scale is more or less the same as in the laboratory scale reactor. Mixing within one module due to the axial dispersion in the circulating flow is shown in Figure 6.21, presenting laboratory scale experiments. The concentration profiles over time for the four modules of the pilot scale reactor after applying a Dirac impulse in the third module are recorded in Figure 6.22. The experimental behavior is simulated very well by a mathematical model. Fig. 6.20 Pilot scale BTR (height 20 m; diameter 1 m; volume 15 m 3 ; height of a module 4 m; length of the baffle 3 m). 183
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Page 3 and 4: tom (Fig. 6.1), the question of mix
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
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Page 23 and 24: 6.4.1.1 Model A Model A, which is a
Page 25 and 26: 6.4 Hydrodynamic and Liquid Mixing
Page 27 and 28: 6.4 Hydrodynamic and Liquid Mixing
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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