An efficient approach to temporarily separate foulants using hydrocyclone with reflux function for thermal energy recovery from sewage

Abstract For municipal sewage, recoverable potential of thermal energy is at least 6-8 times higher than that of chemical energy, which is usually treated as the only recoverable energy in sewage. Accordingly, the sewage source heat pump with de-foulant hydrocyclone with reflux function was proposed and studied experimentally and numerically. To date, however, effect of the Pressure Difference between Overflow Outlet and Reflux Inlet (PDOORI) on its performance is not studied. Considering the significant influence of back pressure on performance of hydrocyclone, in this study, streamwise-periodic flow boundary, Reynolds Stress Model with low-Re stress-omega model, and Discrete Phase Model were utilized to numerically verify its feasibility under different PDOORI, which is caused by depressurization effect of the heat exchanger and pipeline and pressurization effect of the back water pump. Results indicate that, compared with designs of variable underflow diameter, optimizing the PDOORI can markedly enhance the de-foulant performance without increasing blockage risk of underflow, thereby improving the Coefficient Of Performance of sewage source heat pump. As a result, the recoverable potential of thermal energy in sewage will increase, and hence the amount of carbon emissions will decrease. Unlike the conventional hydrocyclone without reflux device, which is similar to a Y-shape branched pipe network, the de-foulant hydrocyclone with reflux function is similar to a loop pipe network. Therefore, effects of the pressure drop ratio on their pressure distribution, velocity distribution, and separation performance are different, even opposite. The optimum pressure drop ratio of the de-foulant hydrocyclone with reflux function is 82.14%-88.50%, which is markedly lower than that of the conventional hydrocyclone without reflux device (119.3%-135.3%). Because only at this moment, the small split ratio (6.49%-18.05%), high separation efficiency (83.00%-86.18%), and low total static pressure losses (hydrocyclone without reflux device: 1.39-1.55 kPa; hydrocyclone with reflux device: 1.99-2.25 kPa) can be simultaneously achieved.