Abstract Summary (Max 250 words)
NETmix is a network mixer composed of interconnected chambers and channels that promote high heat and mass transfer rates, making it suitable for a wide range of applications, including precipitation processes. Recently, NETmix has emerged as a promising technology to produce magnesium ammonium phosphate (struvite) crystals, a sustainable alternative to conventional synthetic fertilisers. This process requires mixing three liquid streams: one containing phosphorus and nitrogen, typically originating from wastewater treatment plants; one consisting of a concentrated magnesium (Mg) solution; and one consisting of a pH-controlling solution. Two concentrated streams (Mg and pH-controlling solutions) are separately injected at low flowrate ratios relative to the main feed flowrate directly into selected chambers along the NETmix reactor. This work assesses the effect of injection chamber position on the mixing performance of the reactor. A NETmix geometry with 17 rows was used and passive tracer mixing experiments were simulated with water and two tracer species as working fluids. The topology of injection was found to have great impact on mixing. Micromixing performance was quantified using the intensity of segregation. For all configurations studied, the intensity of segregation decreased along the NETmix network, indicating progressive homogenisation of the streams. The optimised injection scheme reduced the intensity of segregation by 99.9 % and achieved comparable mixing degrees for both tracers at the reactor outlet. These results are highly relevant for crystalliser design, since mixing and in particular micromixing directly influence supersaturation, chemical reaction, and other molecular-scale processes that occur during precipitation processes.
