Simulation of mixed flow characteristics of distiller′s grains-wastewater for anaerobic fermentation

Anaerobic fermentation of distiller's grains (DG) is an effective method to treat DG. The DG separation technology, based on a hydraulic pulper, simplifies equipment and reduces energy consumption while ensuring the separation of rice husks and fermented organic matter. ANSYS Fluent, a widely used computational fluid dynamics (CFD) software, was used to simulate the two-phase flow of DG wastewater in the hydraulic pulper. The study focused on the mixed flow characteristics of rice husk, organic matter, and wastewater, investigating the velocity field, pressure field, composition field, flow lines, as well as the steady-state and transient flow fields. The simulation results showed that the slurry inside the curved blade at the top of the rotor reached a maximum velocity of 48.06 m/s and gradually decreased to zero along the radial direction. Due to the rotor's high-speed rotation, a high-pressure zone and a low-pressure zone were formed on each side of the rotor, with a maximum working pressure of 0.306 MPa. The slurry exhibited centrifugal movement, with the pressure increasing radially from the center towards the wall. The flow around the center formed a significant negative pressure, driving the central slurry downward. The highest pressure was at the wall, reaching 0.125 MPa. A region of low mixing efficiency was observed below the rotor, with the maximum solid concentration reaching 5.81%. Therefore, adjusting the rotor's height in the hydraulic pulper could be beneficial. During transient calculations, the desired mixing effect was achieved within an 8-minute crushing process. The mixed flow process of DG and wastewater can be summarized as follows: DG from the top center fell under the action of gravity and was then rotated by the rotor near the water pulper into the left or right vortex area, resulting in effective mixing with the slurry. DG from the top side quickly joined the swirling motion on both sides as they descended due to gravity. The results are important for optimizing the operating parameter.