Microplastics have been widely detected in natural and engineered water systems and removing microplastics from various water matrices has become a major challenge. Mini-hydrocyclones (MHCs) have been previously applied to separate mediums of different phases. Given MHCs' capability of separating fine particles from liquid phase, three MHCs were designed and fabricated in stainless steel with 3D printing. Microplastics of densities that were both lower (1 g·cm^-3) than water's density were used to test the separation efficiency in ultra-purified water. The separation test was performed on single-stage MHC as well as MHCs in series in a closed hydraulic circuit. A range of important operational parameters, including split ratio, feed pressure, feed flow rate, and solid concentration, were evaluated to optimize the separation efficiency. The single-stage MHC experiment revealed that >80 % microplastics >20 μm can be effectively removed at the concentration tested, and the separation efficiency peaked at the split ratio of 35 %. MHCs in series demonstrated their ability to further enhance the separation efficiency of the ones with the same density, as well as separate microplastics of different densities. Mini-hydrocyclones' were also used to separate microplastics in synthetic stormwater, and separation efficiency reached 84 % and 98.1 % for low-density polyethylene (LDPE) and polyamide (PA). The results indicated the MHCs' potential for large-scale application in microplastic separation for industrial and municipal wastewater.

The economic and efficient treatment of mixed media in offshore produced fluids is of great significance to oilfield production. Due to the small space and limited load-bearing capacity of offshore platforms, some mature multiphase media separation processes in onshore oilfields are difficult to apply. Therefore, high-efficiency processing methods with small-occupied space are required. Mini hydrocyclones (MHCs) are a potential separation method due to their simple structure, small footprint, and high separation efficiency (especially for fine particles or droplets). However, for discrete phases with different densities and sizes, the enhancement rule of the separation efficiency of MHCs is not yet clear. In this paper, numerical simulation methods were used to study the separation performance of hydrocyclones with different main diameters (including conventional hydrocyclones (CHCs) and MHCs) for discrete phases with different densities and particle sizes. Results show that MHC has the optimal enhancement range for oil–water separation when oil-droplet sizes are 60–300 μm, while the optimal enhancement range for silica particle and water separation is 10–40 μm. For other droplet/particle size ranges, the efficiency enhancement effect of MHC is not obvious compared to conventional hydrocyclones. By calculating the radial force of particles in MHC and CHC, the reasons for the enhanced efficiency of MHC are theoretically analyzed. The pressure drop of MHC is higher than CHC under the same feed velocity, which can be improved by connecting CHC with MHC. Additionally, the fluid velocity test experiments based on particle image velocimetry (PIV) were carried out to verify the accuracy of the numerical simulations. This study clarified the scope of application of MHCs to different discrete phase types, in order to provide a basis for the precise application of MHCs.

De-oiling hydrocyclones are extensively applied in oilfield wastewater purification for fluid separation, because of their efficient and rapid processing performance. However, it is difficult for a hydrocyclone to separate tiny oil droplets owing to the inadequate centrifugal force. A type of coalescence hydrocyclone (CHC) is designed based on the principle of centrifugal separation and hydraulic droplet collision coalescence. The structure, coalescence theory, and separation principle of the CHC are introduced. Based on the Euler–Euler method, the population balance model (PBM) is applied to a continuous equation to simulate the distributions of the velocities, oil volume fraction, and oil droplet size in the CHC. The simulation method is verified by conducting particle image velocimetry tests in a polymethyl methacrylate hydrocyclone. The results indicate that the CHC can improve the separation performance by enlarging the oil droplet size by collision coalescence before entering the hydrocyclone. The results of separation efficiency experiment indicate that the CHC can not only improve the oil–water separation efficiency but also clearly enhance the processing capability. The optimal inlet flow rate is 5.5 m³·h^-1 and the split ratio is 25%. The feasibility of the structure design and the reliability of the separation performance are validated by experiments.

Oil is one of the world's most crucial energy sources. In recent years, the separation of hydrocyclones coupled with multiphase or multifield has emerged as a leading trend in oilfield-produced fluid pre-separation technology. The study of complex dynamics among particles is crucial in multiphase-coupled separation systems. In this paper, we explore a novel separation approach: hydrocyclone separation coupled with particles to enhance oil–water separation, based on the composite force field. The computational fluid dynamics-discrete element method is utilized to analyze the dynamic behavior of particles and oil droplets within the coupling field, as well as the interactions among particles, oil droplets, and the flow field. Furthermore, the effects of operating parameters on the hydrocyclone coupled with particles (HCCP) and the conventional hydrocyclone (CHC) are compared through separation performance experiments. Results show that within a swirling flow field, the introduction of particles significantly exerts a pronounced influence on both the flow characteristics of the continuous-phase and the motion behavior of oil droplets. The coupling effect between particle movement and hydrocyclone separation is most pronounced when the density ratio of particles to oil ranges from 0.94 to 1. The separation performance experiments show that compared to CHC, HCCP can improve by 2.12–8.22 percentage points, and HCCP not only enhances separation efficiency but also exhibits wider applicability than CHC at lower inlet flow rates and split ratios. The numerical simulation results closely matched the experimental findings. This study may provide a reference for developing and applying hydrocyclones coupled with multiphase.