Scale Minimisation in a Multi-phase Agitated Reactor Using Fluid Mechanics Approach
Creators: name: Mohd Khayry, Saddam
Publisher: name: RMIT University
Year: 2025
Subjects: classificationCode: 400411
subject: Water treatment processes
classificationCode: 400403
subject: Chemical engineering design

description: Scaling is an unwanted deposit formed on the inner walls of chemical reactors. Larger scale growth can lead to serious problems like capacity reduction, wasteful energy consumption, higher maintenance costs and production losses. Commonly used scale-mitigation techniques are injection of antiscalant chemicals, usage of the removable protective layer, and fluid mechanics approach. Antiscalants are specific to the chemical reaction involved and therefore, empirical studies are required for every chemical reaction. The extra protective layer may have limited durability which may lead to contamination of reactor contents. The scale mitigation technique that has the potential to be used in all agitated reactors is based on the fluid flow. The manipulation of fluid hydrodynamics in a reactor changes the shear force exerted on the reactor wall. The fluid shear force acting on the wall is directly influenced by the fluid velocity along the wall, which can be changed by varying the impeller rotational speed. There are other factors that can affect wall shear such as baffle arrangement, impeller type, presence of solid, solids concentration and solid suspension. The role of fluid shear on scale reduction in agitated reactors has been studied by very few researchers so far. A dismantle-able stainless-steel cylindrical reactor made up of nine parts (four wall-segments, four baffles, and a circular base) was used in experiments to study the scale formation. The scale was grown using a reaction involving calcium hydroxide solution, sodium carbonate solution and magnesium chloride powder. The experiments were run for 45 mins at 80°C to produce the optimum amount of scale. Coordinate Measuring Machine (CMM) was used to scan one of the wall segments of the reactor to determine the scale thickness, pattern, structure, and topography. Scale porosity and strength were determined from CMM data. Meanwhile the scale weight was obtained by measuring the net weight of the wall segment before and after scale growth. There are 5 different parameters experimented in relation to the scale growth which are impeller speed, baffle arrangement, impeller type, presence of solid, solids concentration and solid suspension. Experiments were conducted using 3 different impeller types (A310, Rushton R100 and Maxblend) in three different baffle arrangements namely, full baffles with no wall clearance, full baffles with 6-mm wall clearance, and partial baffles with 6-mm wall clearance. The experiments were then repeated in the presence of inert solids (glass beads) at different minimum speed to achieve just suspension (complete off bottom suspension). The concentrations of solid used were at 1, 2, 3, and 4 vol%. Experimental results showed that scale occurs mainly near the liquid surface. No significant scale was found in the lower half of the vessel. This scale pattern distribution is found in all experimental conditions and parameters. With increasing impeller speed, the scale thickness, scale coverage area, scale volume and scale weight decrease. However, the scale distribution pattern and geometrical structure remain unchanged. At the same time, the porosity of the scale also decreases with increasing impeller speed, thereby strengthening the connection between the scale particles. The partial baffles with 6-mm wall clearance led to the lowest amount of scale compared to the other two baffle arrangements. Among the impellers used, A310 with the axial flow produced the lowest scale growth compared to R100 and Maxblend impellers with the radial flow. This is only applicable when all 3 impellers are consuming at the same specific impeller power. The results indicate that fractional baffle arrangement and A310 impeller yield the weakest scale in term of scale porosity compared to other baffle arrangements and impeller types. These results lead to a notion that geometrical design of agitated tank can effectively be used to manipulate wall shear to mitigate the scale growth.In two phase system, where inert solid is present, the scale is much less compared to when no solid is present. Plainly, the liquid-solid experiments must run at minimum speed for the solid to achieve just suspension, Njs where all the solid are not stationary at the bottom of the tank (off-bottom). Depending on the mixing speed and solid suspension, increasing solid concentration can affect the total scale productions. If the mixing speeds are lower than Njs, where a fraction of the solids still settled at the bottom of the tank, increasing in solid concentration will lead to higher total scale productions in term of thickness and weight. On the contrary, when the mixing speeds are higher than Njs, where all the solid are completely suspended, an increase in solids concentrations can effectively reduce the amount of total scale produced in term of thickness and scale weight. From the findings, increase in solids concentrations can help with the scale mitigations as long as the solids are suspended at least off the tank bottom.