Heterogeneous crystallisation of calcium sulphate in the presence of silica particles

• Main Author: Lapidot, Tomer
• Other Authors: Heng, Jerry Y. Y.
• Published: Imperial College London 2015
• Subjects: 665.7
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.684320

Fouling of calcium sulphate is commonly observed in industrial heat exchangers and desalination membranes. The build-up of a fouling layer can reduce plant efficiency by a considerable extent resulting in higher energy demand, carbon footprint, and cost of operation. Traditional fouling mitigation methods are effective, but require maintenance downtime and use of hazardous chemicals. This project is aimed at exploring a novel approach in mitigating fouling through the addition of porous silica particles, which can help control the crystallisation process of calcium sulphate. Silica particles are a practicable alternative to traditional methods due to their low cost of production, tractability, and low environmental impact. The crystallisation of calcium sulphate is investigated using two primary experiments. The first experiment investigates the crystallisation of calcium sulphate inside a batch crystalliser. The progress of crystallisation is tracked online by measuring the variation of the solution's electrical conductivity in time, while the final crystal size distribution is measured externally using laser diffraction. The crystallisation process is modelled using the population balance equation, which considers the evolution of the crystal size distribution in time in the presence of nucleation, growth, agglomeration, and breakage. The model is solved numerically using the method of classes. Experiments show that porous particles can reduce the crystallisation induction time by enhancing the rate of homogeneous nucleation. Comparison between porous particles of various pore diameters and silica nanoparticles reveal that the pore volume is the significant parameter in nucleation enhancement (more so than pore diameter or surface area). Furthermore, gas sorption measurements showed a decrease in available pore volume, indicating that pores are blocked during crystallisation. Population balance modelling suggests that the confined space of the pore increases nucleation by increasing the frequency of collision between free ions. Pore deactivation is modelled through heterogeneous nucleation. The surface chemistry of the particles was functionalised to further enhance the pores' effect on nucleation. Amine functionalisation was observed to decrease the nucleation induction time by 26% at a loading of , whereas TAAcOH functionalisation was observed to increase nucleation induction time by a six-fold at a loading of . The second experiment investigates the surface crystallisation of calcium sulphate in the presence of continuous flow. The deposition of calcium sulphate crystals is measured using a microscope camera and a quartz crystal microbalance (QCM), which is capable of measuring the deposited mass in real-time by using a piezoelectric sensor. Measurements of surface crystallisation take place in a customised in-house designed flow cell that contains the QCM sensor. Raw QCM data (depicting complex frequency shifts) were translated into mass using an equivalent circuit model. A novel equivalent circuit equation was developed for the unique QCM data observed during the flow experiment. Together with imaging data, the model was used to calculate nucleation, growth, and mechanical properties of the crystals. The stiffness of the crystal was found to decrease with decreasing supersaturation.