Environmental Biotechnology - Jordening and Winter

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-

6.1 Introduction 165

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 CO2 and H2S 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

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.

6.2 Measuring Techniques 167

Table 6.2 Characteristic data of the wastewater used for the experiments.

the height of a reactor, a pilot scale biogas reactor was built at the site of a company producing bakers’ yeast (Fig. 6.3). The reactor was designed on the basis of the laboratory scale experiments and investigations on the kinetics of wastewater digestion from bakers’ yeast production. The characteristic data of the wastewater are given in Table 6.2. The active volume of the reactor is 15 m3. It is divided into four modules and at the top of the reactor a settling zone is integrated to maintain a high sludge concentration. The pilot reactor is well equipped with instruments for measuring concentrations and velocities along the height of the reactor. Because the pilot scale reactor reaches almost the height of a technical reactor (in Fig. 6.2 two reactor units with a height of 25 m and a diameter of 3.5 m are shown as examples) it is assumed that the mathematical model, which is evaluated on the pilot scale with respect to hydraulics and takes into account the influence of hydrostatic pressure, will permit prediction of the behavior of a technical scale reactor.

6.2

Measuring Techniques

To understand the microbial degradation process and the system behavior of a biogas reactor and to establish reliable mathematical models, measuring techniques must be available – if possible – to analyze online special substances in the fermentation broth. Therefore, some new measuring techniques have been developed during the last years.

6.2.1

Online Measurement Using a Mass Spectrometer

The anaerobic treatment of wastewater loaded with high concentrations of sulfate is often accompanied by problems due to inhibition by hydrogen sulfide (H2S). It is well established that the inhibition is caused by undissociated hydrogen sulfide present in the liquid phase and not by the total sulfide content. This concentration of undissociated hydrogen sulfide depends on sulfate concentration in the wastewa-

168 6 Modeling of Biogas Reactors

ter, pH, and ionic strength. The concentration of undissociated hydrogen sulfide in the liquid phase undergoes great changes in the pH range used for anaerobic digestion, which is between 6.6 and 7.4. Since the actual concentration of dissolved H2S cannot be calculated on the basis of the H2S partial pressure in the biogas, because gas phase and liquid phase are not in equilibrium as is demonstrated in Section 6.5, this concentration has to be measured.

Measurement on the basis of ion-sensitive electrodes proved to be very problematic, especially with respect to their stability for a long period of time. Meyer-Jens et al. (1995) developed a new type of probe connected to a mass spectrometer. The probe consists of a small tube (outer diameter 0.9 mm, inner diameter 0.7 mm) of polydimethylsilicon. The tube can be inserted directly into the fermentation liquid of the biogas reactor. The H2S dissolved in the liquid phase penetrates the membrane and is transported by a carrier gas (technical nitrogen) to the mass spectrometer.

A very simple silicon membrane probe is shown in Figure 6.4. The silicon membrane tube is arranged in a short bypass loop outside of the reactor. The fermentation liquid flows outside of the tube while the carrier gas passes inside. In Figure 6.5 calibration of the measurement system is demonstrated. The calibration curve is perfectly linear within the relevant H2S concentrations. The different gradient for

Fig. 6.4 Configuration of a silicon membrane probe operated in a short bypass at the reactor (after Polomski, 1998).

6.2 Measuring Techniques 169

Fig. 6.5 Calibration of the silicon membrane probe coupled with a mass spectrometer measuring the concentration of dissolved H2S in desalinated water (DS water) and in the wastewater of bakers’ yeast production. The ordinate gives the value of the signal with reference to the N2 signal of the carrier gas. The abscissa is the partial pressure of H2S, with which the liquid is in equilibrium (after Polomski 1998).

desalinated water and wastewater indicates the difference in the solubility (Henry coefficient) of H2S in desalinated water (78.2 mmol L–1 bar–1) and wastewater (68.5 mmol L–1 bar–1) at 37 °C (data given by Polomski, 1998).

The measurement system can also serve well for the detection of dissolved CO2 in the biogas reactor. Figure 6.6 shows the calibration curve for CO2 in a sample of desalinated water. Also with CO2 the solubility (Henry coefficient) for wastewater (2.21 · 10–4 mmol L–1 Pa–1) is 12% smaller than the literature data given for desalinated water (2.52 · 10–4 mmol L–1 Pa–1). Matz and Lennemann (1996) used a similar silicon membrane probe for online monitoring of biotechnological processes by gas chromatographic mass spectrometric analysis. The sensitivity of the probe system could be improved considerably by an instantaneous heating of the membrane to a temperature of 200 °C. This quasi online measuring procedure had a sampling period of 15 min between two successive measuring cycles. Substances like dimethylsulfide, butanethiol, cresol, phenol, ethylphenol, and indole at concentrations between 10 and 200 ppm were detected.

6.2.2

Online Monitoring of Organic Substances

with High-Pressure Liquid Chromatography (HPLC)

Zumbusch et al. (1994) reported the simultaneous online determination of betaine (trimethylglycine), N,N-dimethylglycine (N,N-DMG), acetic and propionic acid during the anaerobic fermentation of wastewater from bakers’ yeast production. Ammonia and chloride could also be measured. The determinations were carried out by

170 6 Modeling of Biogas Reactors

Fig. 6.6 Calibration of the silicon membrane probe coupled to mass spectrometer measuring the dissolved CO2 in desalinated water (after Polomski, 1998).

means of isocratic cation exchange chromatography. For the detection of betaine and N,N-DMG an ultraviolet detector was used. All other substances were detected by conductivity.

Besides other organic components, such as acetic and propionic acid, betaine is the dominant organic substance in the wastewater. Up to 33% of the TOC is represented by betaine. The use of HPLC analysis combined with an automated ultrafiltration setup for online process monitoring gives interesting information about the dynamic behavior of the anaerobic process, as demonstrated in Figure 6.7. The graphs show the effect of hydrogen sulfide elimination from the fermentation broth. The concentration of undissociated H2S was decreased by a factor of 2 (from 300 to 150 mg L–1). During the same time biogas production increased. The concentration of acetic acid decreased while the other acids measured remained more or less stable. Betaine was completely degraded only at lower H2S concentrations. Interestingly, the concentration of N,N-DMG, which is a metabolic product of the anaerobic degradation of betaine, is also decreased.

6.3 Kinetics

The anaerobic degradation of organic substances is performed in a sequence of biological reactions in a synthrophic cooperation of different microorganisms