Periodic Reporting for period 4 - SCARCE (Sustainable Chemical Alternatives for Re-use in the Circular Economy) Reporting period: 2021-10-01 to 2022-09-30 Summary of the context and overall objectives of the project In the water sector, there is a strong focus on revaluing wastewater as a resource sink, containing many chemicals of considerable value to the European economy. Crystallisation can be used by the water sector to advance recovery of these environmentally sustainable chemicals that are vital to most value chains, helping to transition Europe toward a circular economy. Classical crystallisers are characterised by poor mixing making supersaturation rate difficult to control, which is critical for determining crystal quality and downstream separation. Supersaturation rate is also limited by surface area resulting in large and costly processes. Environmental crystallisation is a high-volume activity. Existing process scale and difficulties with product control has therefore limited wide-scale use in the water industry. Membrane crystallisers (MCr) are a disruptive technology, combining process intensification (high specific surface area) and strict control over supersaturation rate, enabling good crystal product quality at a fraction of the scale. In MCr, the membrane is thought to lower activation energy, to enable crystal growth in distinct regions of the metastable zone. However, there is poor understanding of what role interfacial chemistry, hydrodynamics and geometric design can have in controlling crystal growth in MCr. Inconsistency in MCr literature is attributed to the limited description of nucleation kinetics due to a lack of suitable diagnostic tools. This research aims to enrich understanding of sustainable chemical transformations in MCr to provide definitive, scalable design of disruptive modular crystallisation technology as an enabler for the recovery of valuable resources from wastewater. The overall objectives were to:[1] Develop non-invasive diagnostic tools induction time detection (membrane and bulk), to complete kinetic description of nucleation in MCr[2] Characterise hydrodynamics to inform on the kinetics of nucleation and crystal growth, scale formation, and the mechanisms of crystal backtransport[3] Resolve the role of the solute and membrane interfacial energy in reducing the activation energy for nucleation [4] Establish how system geometry and control strategies can set nucleation kinetics as a deterministic approach to crystallisation Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far New knowledge on membrane science, scaling and crystallisation has been developed. A central tenet was to develop non-invasive techniques for detecting induction time. Induction time has rarely been measured in membrane systems. This work proves previous detection techniques to be illegitimate, and have characterised two discrete supersaturated regions, through which a mechanistic framework was established for how to suppress scaling and independently control nucleation and crystal growth. This experimentally validated model demonstrates membrane technology as a powerful platform technology for controlled crystallisation. Research results are split into three themes: 1. Interfacial chemistry. Key conclusions include: (i) This demonstrated how it is crystal interfacial energy that fixes nucleation and crystal growth, debunking critical previous literature assumptions; (iii) Evidence that it is thermal conduction and porosity which determine nucleation kinetics and not membrane surface free energy as previously proposed; (iv) Illustration of how solubility-temperature dependency determines scale formation and nucleation rate, which can be used to inform scaling mitigation strategies 2. Hydrodynamics. Key conclusions include: (i) Two discrete supersaturated regions exist which independently control the processes of nucleation and growth; (ii) Detailed boundary layer modeling shows how the boundary layer set conditions for nucleation, providing a framework to scale-up technology; (iii) Mixing phenomena have been disaggregated to show how nucleation and growth can be independently controlled. 3. Scale-up. Key conclusions include: (i) Control of supersaturation rate without adaptation of the boundary layer conditions is a viable alternative strategy for nucleation control; (ii) Control strategies can be identified that lead to deterministic nucleation events; (iii) Direct nucleation control can establish kinetic trajectories across the metastable zone to inform nucleation and growth.The exploitation of this research aligns into four areas that will benefit society: (i) Food security: e.g. resource recovery of nutrients (e.g. phosphorous) from wastewater.(ii) Environment: minimise the cost and environmental risk of brines disposal from desalination and water reuse schemes;(iii) Manufacturing: e.g. reduce supply chain risk to electronic goods manufacturing through highly selective low-cost separation of precious metals (e.g. lithium);(iv) Net zero energy production: e.g. low energy regeneration for CCS; separation and purification of ammonia as a carbon free energy source or vector for hydrogen. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) Hydrodynamics: Various theories have been previously proposed as to how surface scaling is caused. Through modelling and experimental verification, we have moved beyond the state of the art to propose a unified theory that incorporates shear and rheological conditions to elegantly describe the probability for surface crystallisation within the interfacial region of the boundary layer. Boundary layer characteristics are also related to crystal size, size distribution and habit which is contradictory to wider thinking, that suggest it is the bulk fluid which determines conditions for growth. Critically, this informs scaling mitigation strategies, process design and scale-up approaches. Solution and interfacial energy: Only limited research had been conducted on how solution chemistry informs nucleation, where many authors assume that for low molecular weight inorganic solutes, induction time is less relevant. We demonstrate the contrary but also that induction period can be predicted based upon the theoretical interfacial energy of the crystal. However, whereas for conventional crystallisation, an inverse relationship between induction time and nucleation rate is described, the opposite is shown in MCr. A validated analytical framework is developed to rationalise these phenomena, which are seemingly unique because of the system boundary layer. We have also demonstrated how interfacial energy of the membrane is insignificant in the governance of nucleation and crystal growth (including scaling), which is contrary to membrane industrial scaling literature. A mechanistic explanation is introduced and validated across two solutions of different solute characteristics. Scale-up: High packing densities have been assumed to be problematic in MCr as surface crystallisation can block interstitial volume. This can reduce the process intensification factor. Systematic analysis has found the opposite is true. Low surface area to volume induces significant scaling, whereas high surface area to volume leads to controlled downstream crystallisation, and better product quality. The research shows how nucleation can occur at the periphery of the metastable zone, while mitigating the effects of scaling - an effect which has never been seen before. Related documents Silver Sulphate formed in supersaturated interfacial condition.jpg
