CFD Simulation Study Of Stirred Anaerobic Digester

Anaerobic digestion is one of the core technologies to utilize waste resource and has broad prospects in application. This research systematically investigated the hydrodynamic characteristics of the mixing process of the anaerobic materials with the help of computational fluid dynamics (CFD). The results provided a basis for the selection of reasonable process parameters and industrial application. In this paper, along with the anaerobic digestion system of corn stover (CS) and cattle manure (CM) as research objectives, the numerical model of stirred anaerobic digester was established using CFD method. The characteristics of the flow field, solid distribution and energy input and output of anaerobic system were investigated in terms of material properties, impeller type and impeller design factors. The innovative work and achieved results are as follows:Firstly, a solid (F)-solid (S)-liquid three-phase CFD model in stirred anaerobic digestor was established, and the reliability of the CFD model was verified by comparing the results of velocity magnitude, vector distribution and local particle concentration. The effect of particle density, particle diameter and solid content on the solid distribution was studied. Results showed that solid with particle density greater than the liquid, was found to deposit at the bottom center of the reactor; on the other hand, the rest solid was found to float up around the agitator shaft near the liquid surface. Particle floatation and sedimentation were intensified with the increasing of particle diameter. With increasing solid content of sinking particle S from 2.5% to 10%, the standard deviation of particle concentration was lowered and overall better suspension quality was acheived.Secondly, the influence of impeller type and its design parameters(impeller diameter, off-bottom clearance, blade width) on the flow field was investigated by carring out numerical simulation of RB, HE, PBU and PBD impeller in the three-phase system. The distribution rules of flow field in different mixing systems were achieved. The results showed that different flow fields and flow patterns were formed by different impellers: RB impeller presented characteristic of typical radial impeller; HE, PBU and PBD impeller produced primary circulation and secondary circulation flow under specific geometric configuration; The velocity and flow pattern of the impeller were directly altered with the change of impeller diameter; The change of off-bottom clearance was followed by the discharging position of fluid: the secondary recirculation flow of HE and PBD impellers was inhibited, and the main recirculation flow of PBU impeller was affected. The velocity distribution was affected by changing width of impeller, but the flow pattern was not affected. Within turbulent flow, the power number Np of the RB impeller was the maximum, and which was 1.6 times that of PBU and PBD impellers and 3.2 times that of the HE impeller.Thirdly, the influence of the different impellers and their design parameters on the solid distribution of solid-solid-liquid flow field was systemically studied. The discipline of solid distribution in system of floating particle coexisting with sinking particle was as follows: the sedimentation of the particle S was occurred in the central area of the lower part of the reactor,particle F accumulated around the impeller shaft below the center of the liquid surface, and the mixing uniformity of particle S was superior than that of particle F; Axial flow was benefitial to solid suspension; With increasing rotation speed, the uniformity of suspension of particle F and S was improved.The impeller type in terms of mixing quality of solid F could be orderd as;PBU > PBD > HE > RB, while for solid S it was as; PBD ? PBU > RB > HE at the rotation speed of 210 rpm. According to the CFD results of tangent intersection the just suspension speeds of floating particle F and sinking particle S were 119 rpm and 117 rpm, respectively in the PBU mixing system.Moreover, the associated equations of just off-bottom suspension speed Njs of sinking particle and just drawdown suspension speed Njd of floating particle were deduced in the three-phase system.Fourthly, combining the CFD simulation with factorial analysis, a method was established, in which the impeller diameter, off-bottom clearance and blade width were considerd as design factors, and the suspension quality of floating particle ?F, the suspension quality of sinking particle ?s, and the power input MEL were considerd as responseds. As a result of simulation, the significant factors of the suspension quality in the four mixing systems were determined by the combination method and the correlated equations between the design factors and responses of the four impellers were developed. The further results showed that significant factor for the floating particle suspension was different from that for sinking particle. While significant factors of power input for four impellers were the blade diameter, the blade width and their interaction. Comparison among CFD results and predicted results of PBD and RB impeller gave the error ranging between 1.6% ?8.6%,which further justified the credibility of correlated equation. Based on above analysis, a dimensionless equation was established to combine three responses in solid-solid-liquid system. The optimum design of this system obtained,includes PBU impeller with impeller diameter, the off-bottom clearance and the blade width of 0.100 m, 0.078 m and 0.040 m, respectively.Finally, a numerical model suitable for anaerobic digestion with non-Newtonian anaerobic slurry was established. The flow field and power consumption of the anaerobic mono- and co-digestion of CS and CM were studied. An index "net energy output" was presented and the its corresponding correlation were provided. The uniformity of the flow field in mono- and co-digestion of CS and CM reactor was: CM> CS + CM> CS. The index "net energy output" could be used to optimize the feedstock ratio of co-digestion,and the optimum feedstock ratio for continuous mixing, intermittent mixing INTER ?, and intermittent mixing INTER ? were 1:1, 1:1 and 1:3, respectively.The energy output of anaerobic co-digestion can be improved by optimization of mixing mode and feedstock ratio.