Combined Hydraulic Systems Minimize Mixing Costs in Biogas Plants

In biogas plants, mixers induce velocity or velocity changes. This prevents floating sludge from forming. The turbulence always involves major local consumption of energy, so the less turbulence, the less energy consumed. Complex processes generate high turbulence to saturate structural matter with liquid. However, the turbulence should be localized to minimize mixing expenditure.

Unfortunately, directional velocity is not a good substitute for real turbulence. During the startup phase, for example, floating sludge is difficult to contend with—despite the presence of adequate mean velocities throughout the reactor—because the fluid still lacks viscosity and displays a strong tendency to float. An optimal mixing process should be accommodated to the flow behavior and provide large volume flows with a global bulk flow and sufficient turbulence.

System Measurement & Positioning

Biogas plants with high volume loads exhibit a pseudoplastic, or shear-thinning, flow. The fluid begins to flow by a distinct limit—exposure to high shear rates that, after a certain minimum amount of energy is introduced, overcome its resistance to flow. This shear-thinning flow also limits successful mixing to a local scale, and submersible mixers in biogas plants recirculate the thinned material right back to their own propellers. No single hydraulic system can fulfill all the required mixing. However, all mixing functions require a certain local velocity or velocity change. Axial thrust, hydraulic system choice and operating conditions all determine how well a mixing task is fulfilled. Thrust is indispensable for good mixing and must propagate throughout all parts of the tank where there is flow. Mixer operators often have difficulty assessing mean bulk-flow velocity as a function of flow behavior. Large amounts of structural material in the substrate can quickly block sensors with stringy material, making measurement virtually impossible. Magnetic-inductive measuring systems have often yielded useful results. Directional base flow measurements in an axial direction have little practical value with respect to all mixing functions. However, ensuring locally distributed velocities or velocity changes is very important to effective mixer operation. Digesters and post-digesters with a mean bulk-flow velocity of 8 to 15 centimeters per second (cm/s), though, usually function quite well. Correspondence—when several mixers work together to make use of the thinned material from their neighbors—avoids this problem. Properly positioned and correctly spaced, the mixer configuration prevents backflow and creates favorable conditions for an optimal process. Flow accelerators with a large volume can play a supporting role in this process.

Hydraulic Systems

Depending on the task, two different mixing processes take place in biogas plants—turbulence generation and flow acceleration. Turbulence generators are mixers with small propellers running at a relatively high speed (300 rpm and above) and a low ratio of thrust and power input. In biogas plants, recirculation behavior is common, which prevents them from imposing the locally generated gain in velocity throughout the tank. Turbulence generators use energy much less efficiently than either flow accelerators or hybrid mixers. Flow accelerators use large, low-speed propellers to produce high-volume flow and very high thrust with low power consumption. This makes them energy-efficient with great correspondence and turbulence. Hybrid mixers combine the advantages of a flow accelerator with the turbulence-inducing properties of turbulence generators and can cope with critical flow behavior. In addition to good energy efficiency, they also have impressive depth of penetration and a low tendency to recirculate.

Efficient Turbulence

A flow accelerator combined with a hybrid mixer is an optimal configuration for typical biogas applications. Flow accelerators provide great thrust-to-power ratios with little energy expenditure. Shear-rate propagation throughout the tank achieves a globally successful mixing effect. In addition, the comminution of structural matter and appropriately tailored loading have positive effects when combining the two technologies. The generation of directional bulk-flow velocity alone does not suffice to simultaneously achieve all mixing functions. In many cases, the combined effect of appropriate mixer positioning, the combined deployment of different hydraulic systems and an appropriate level of turbulence will help to minimize mixing expenditure.