2 research outputs found

    Investigating the Effect of Surface Properties on Ice Scaling in Eutectic Freeze Crystallization

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    Eutectic Freeze Crystallization (EFC) is an innovative technology that can be applied to treat reverse osmosis (RO) waste streams (brines), to produce pure salt and water. Scaling of the heat exchanger (HX) surface by both ice and salt is currently one of the major drawbacks in the industrial implementation of EFC. At present scaling is controlled by the use of mechanical scraping, which is susceptible to mechanical breakdown, thus reducing the overall process efficiency. Previous studies have shown that lower surface energy materials delay the onset of freezing, and that smooth surfaces reduce nucleation and adhesion sites, thereby reducing the probability of scale formation. Therefore, this study aimed to investigate how the HX surface properties affect ice scaling in EFC, without the influence of mechanical scraping. Copper, Aluminium, Stainless Steel 316 and Brass were the selected HX materials. Ice scaling on the HX materials was investigated using a near eutectic 4 wt.% Na2SO4 aqueous solution, in a crystallization test cell uniquely designed to mimic the region near the HX wall of a crystallizer. The Differential Interference Contrast (DIC) technique was used to study the formation of the initial ice scale layer on the HX material used in the test cell. This method of observation was effective, asfor the first time in a continuous system, the crystallization of the initial ice scale layer was observable in-situ and in real-time. Therefore, with this method, it was possible to investigate the evolution of the predominantscaling modes(nucleation and growth), which differed for the different HX surfaces. The difference was proposed to be due to their distinct surface free energies and surface topographies. The effect of surface free energy and topography on the scaling induction time was investigated while operating at similar heat fluxes (similar cooling rates) for all the metals. The scaling induction time decreased with an increase in the surface free energy, with the Aluminium as an outlier. The recorded scaling induction times for Brass, primary-SS316 and Copper were 92.54, 70.95 and 54.06 min, respectively. Aluminium recorded the longestscaling induction time of 134.74 min. Both the polytetrafluoroethylene (PTFE) coated-SS316 and the primary-SS316 HX surface were used to investigate further the effect of surface free energy on the scaling induction time. The PTFE-coated-SS316 was found to increase the scaling induction times 2.79-fold at a coolant temperature of -15°C, compared to that of the primary-SS316. However, at -20°C and -25°C, the scaling induction times on both surfaces were comparable, which indicated that the benefit of using a low surface free energy material was limited by the cooling rate of the system. It was also found that the scaling induction times were shorter when using a rough-SS316 HX plate, compared to the primary-SS316, because of the larger surface area available for heat transfer. The end of the scaling induction time was characterised by the heterogeneous nucleation and subsequent growth of the ice on the HX surfaces. There was no direct correlation between the HX surface free energy and the nucleation and growth rates. This was because the Brass, Aluminium, SS316 and Copper plates all consist of different surface topographies which also influenced the nucleation and growth rates. However, the nucleation rates consistently increased when the scaling induction times were longer, regardless of the HX material used. The presence of deep sharp crevices on the primary-SS316 also enhanced nucleation rates. These deep sharp crevices created regions of high local supersaturation, where heterogenous nucleation predominated. It was, therefore, reasonable to conclude that the ice scaling induction time was increased by using smooth materials and those of lower surface free energy. The scaling mode was dependent on the surface topography and length of the ice scaling induction time, as longer ice scaling induction times resulted in heterogenous nucleation dominated scaling mode and vice versa. Materials that had a low surface free energy and were smooth minimised the nucleation rate, resulting in a reduced overall scaling rate

    Novel materials and crystallizer design for freeze concentration

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    When wastewater streams or seawater are desalinated, the waste product is a hypersaline brine stream which is usually disposed of or stored in evaporation ponds. Eutectic Freeze Crystallization (EFC) is a novel treatment technology for these hypersaline brines that can recover both water and dissolved salts. It works by cooling the stream to the Eutectic temperature, at which point both ice and salt will crystallize. The ice, being less dense than the solution, floats, and the salt, being denser, will sink – thus effecting a gravity separation.Theoretically, EFC can result in zero liquid discharge (ZLD) and has several advantages over conventional separation techniques, including low energy consumption; high quality products and it requires no additional chemicals. It can also be combined with other separation technologies to optimise the separation process.Although it sounds simple, an EFC process can be difficult to design and operate effectively, since each of the elements has its own complexity. Specifically, the issue of ice scaling is one of the major challenges in implementing EFC.Therefore, in this study, we focussed on ice scaling in a freeze concentration system i.e ice crystallization only; without the salt crystallization component that would define the process as an EFC process.We investigated two approaches to scaling reduction:The first was modification of the heat exchanger surface, where we investigated the effect of three different heat exchanger surfaces on ice scaling: a novel polypropylene graphite (PP GR) material, Stainless Steel 316 (SS316) and Aluminium. These three materials were tested in a small-scale Column Crystallizer (CC).The second method was by designing a Continuously Stirred Column Crystallizer (CSCC), which aimed to improve the hydrodynamics independent of pumping, to improve on the heat and mass transfer distribution across the crystallizer and to improve product purity by providing a larger product separation zone. The focus was on producing an efficient crystallizer design, bearing current industrial implementation hurdles in mind.This study confirmed that using materials of low surface energy has the potential to increase the scaling induction times and reduce the severity of ice scaling, hence improving process efficiency. The outcome of the initial, small-scale study provided the required evidence to support scaling up the column crystallizer using PP GR as an HX material of choice.The study also showed that the unique multicompartment mixing system of the novel CSCC led to high supercoolings and long induction times being attained. The column design provides a larger disengagement zone than a conventional column, which has the potential to improve the separation efficiency, and hence the product purity.In summary, with the correct choice of heat exchanger material and appropriate crystallizer design, we have shown that one of the major challenges i.e. ice scaling, can be overcome. This paves the way for EFC being able to be used for the treatment of hypersaline brines, not only at laboratory scale, but also at larger, commercial scales
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