Environmental Biotechnology - Jordening and Winter

6.4 Hydrodynamic and Liquid Mixing Behavior of the Biogas Tower Reactor 183

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) · 102 s

•mixing from one module to a neighboring module: (0.5–2) · 103 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 m3; height of a module 4 m; length of the baffle 3 m).

184 6 Modeling of Biogas Reactors

Fig. 6.21 Mixing of a Dirac impulse in the upper module of the laboratory scale reactor (Fig. 6.19).

Fig. 6.22 Mixing of a Dirac impulse (3rd module) in the pilot scale BTR (Fig. 6.20).

The structure of the mathematical model developed is shown in Figure 6.23. The structures of the two models A and B follow the modular structure of the reactor

concept. Both models couple two neighboring modules by the flow of the feed

·

(V feed), which is constant within the tower reactor and is directed from the bottom to the top of the reactor. The central idea of both models is to predict the coupling

between two modules by the so-called “exchange flow rate”. This virtual flow rate

·

(Vexchange) accounts for the situation of a turbulent exchange of volume elements in

the area between two modules (A exchange). This turbulent flow is driven by the bub-

·

bles passing this area. V feed is kept at zero during the experiment. Also, in reality the

·

contribution of V feed is usually small compared to the turbulent exchange flow

·

Vexchange.

6.4 Hydrodynamic and Liquid Mixing Behavior of the Biogas Tower Reactor 185

Fig. 6.23 Structure of models A and B as compared to the BTR for describing the mixing behavior of the liquid phase.

6.4.1.1Model A

Model A, which is a more detailed one, is able to describe mixing within a module as well as intermixing between modules. Each module is composed of four compartments. Riser and downcomer are mathematically replaced by tube reactors with axial dispersion. At the bottom and the top of each module, riser and downcomer join

in mixing zones which are regarded as stirred vessels. The four zones are linked by

·

the circulation flow rate Vcirc. Since the cross-sectional area of the riser A riser and the

·

downcomer A downc. as well as the flow rate through the riser Vriser and the down-

·

comer Vdownc. are equal, the mean circulation velocity wm is given by Eq. 19.

Each real module of the reactor is thus replaced by a “ring structure” of four reactors. The transport and mixing mechanisms in this ring structure are convection and axial dispersion. The mathematical equations of model A can be derived from the mass balances of each of the four compartments. Model A needs three parame-

ters for calculating the mixing behavior: the liquid circulation velocity wm, the axial

·

dispersion coefficient Dax, and the exchange rate Vexchange. All these parameters depend on the gas flow rate entering the module from the lower module. For more details of the model, the mathematical structure, and the procedure of solving the partial differential equations, the publication of Reinhold et al. (1996) should be consulted.

186 6 Modeling of Biogas Reactors

6.4.1.2 Model B

Model B, which is simpler, consists of only one stirred vessel per module (Fig. 6.23). This model is able to describe the mixing behavior of the whole reactor if the mixing intensity within one module is high compared to the tracer transport from one module to another. This assumption holds true in the real situation, as shown earlier. Model B links the stirred vessels in the same way as model A. The intermixing

·

between two neighboring modules is again modeled by an exchange flow rate V ex- change going up and down. Eq. 20 shows the material balance for one module i.

For describing the mixing of one compound in a BTR with model B, the number of ordinary differential equations is equal to the number of modules. It is an initial-

value problem which can be solved by the Runge–Kutta method. The only necessary

·

parameter for calculating the mixing behavior is the exchange flow rate Vexchange.

Experiments in both laboratory and pilot scale reactors are carried out at different

·

gas flow rates Vgas to investigate the dependence of the hydrodynamic parameters on the gas loading. Although there are many investigations on airlift loop reactors and bubble columns in general, the results cannot be applied to this type of reactor because the gas and liquid loading of the BTR are far below those of the airlift loop and bubble column reactors found in the literature. Since the present BTR has

unique characteristics, it was necessary to determine the exchange flow rates

·

Vexchange. Consequently, studies on the circulation velocity wm and the axial disper-

sion Dax were carried out.

·

The characteristic parameters Dax, wm, Vexchange, are a function of the gas loading of the system. The parameters can be obtained by using the least-squares method to fit the simulation to the experimental data. The results are shown in Figures 6.24–6.26.

In Figure 6.24 the axial dispersion coefficient is plotted against the superficial gas velocity of the riser uriser which is given by Eq. 21.

·

uriser = Vgas (21) A riser

This coefficient was only determined on the laboratory scale. Figure 6.25 shows the mean circulation velocity wm of the laboratory scale and the pilot scale reactors as a function of the superficial gas velocity of the riser uriser, From a momentum balance the mean circulation velocity wm can be calculated on the basis of the difference in gas holdup between riser and downcomer Äå and the knowledge of the total pressure loss (friction) coefficient î according to Eq. 22.

6.4 Hydrodynamic and Liquid Mixing Behavior of the Biogas Tower Reactor 187

Fig. 6.24 Axial dispersion coefficient as a function of the superficial gas velocity in the riser.

Fig. 6.25 Mean circulation velocity of the liquid as a function of the superficial gas velocity of the riser.

Fig. 6.26 Effect of the superficial gas velocity on the exchange flow rate between two modules.

188 6 Modeling of Biogas Reactors

L is the length of the baffle and g is the gravitational acceleration. The gas holdup can be correlated from the superficial gas velocity uriser according to Weiland (1978). Due to experiments of Reinhold et al. (1996), it can be calculated according to Eqs. 23 and 24).

The pressure loss coefficients were determined to be î = 6.9 on the laboratory scale and to be î = 4.8 on the pilot scale. As expected, the total friction coefficient î

slightly decreases during scale-up.

·

Figure 6.26 shows the exchange flow rate Vexchange as a function of the superficial gas velocity uexchange, which can be calculated according to Eq. 25.

·

uexchange = Vgas (25) A exchange

It is quite remarkable that the findings of Figure 6.26 are invariant with the scale

·

of the reactor. Because Vexchange dominates the mixing behavior of the system (indeed, it is the only experimental parameter necessary for model B) this result is very important for the scale-up of biogas tower reactors.

6.4.2

Distribution of Biomass within the Reactor

The wastewater is passing the BTR from the bottom to the top of the reactor. With this feeding procedure the upflow velocity due to the feed increases – at a constant mean residence time of wastewater in the reactor – linearly with the height of the reactor. Therefore, in high reactors it is essential to have reliable mechanisms to retain the active biomass. These mechanisms and the resulting distribution of biomass within the reactor were studied by Reinhold and Märkl (1997).

6.4.2.1 Experiments

Experiments were performed with a two-modular laboratory tower reactor (height 6.5 m) and in a pilot scale tower reactor (height 20 m) as shown in Figure 6.2 and described in Table 6.1. The laboratory tower reactor was filled with tap water and anaerobic pelletized sludge. The sludge was biologically, inactive, as no substrate was present in the tap water, i.e., it did not produce any fermentation gas. Air was injected at the bottom of the column instead of real biogas. As the reactor was built out of transparent material, visual observations were possible in addition to measurements from samples. The following observations were made during these experiments:

•The suspended solids concentration in the lower module is always higher than in the upper module.