Nano-Tio₂ precipitation in SDRs :experimental and modelling studies

Abstract

Precipitation is responsible for more than 70% of all solids materials produced in chemical industries. The continuous development of the chemical process industry has been accompanied by increasing demands for enhanced product quality such as crystals with a controlled size, shape, purity, and polymorphic form. The aim of the present research is to assess the TiO2 precipitation process in Spinning Disc Reactor (SDRs), where rapid mass and heat transfer rates and mixing intensity in the thin film of liquid produced as a result of centrifugal acceleration facilitate improved methods of rapid precipitation. Macro and micromixing significantly influence reaction kinetics and thus the particle formation as well as the resulting product properties. Hence the objectives of the current research are divided into three main categories. Firstly a fundamental study into the macromixing efficiency of a SDR was undertaken by characterisation of residence time distribution (RTD) of fluid flow in a 30 cm SDR. The main focus of this segment was the study of influence of the hydrodynamic conditions of the thin film flow and disc configurations on the RTD in order to determine the optimal experimental parameters for which near plug flow behaviour prevailed on the spinning disc. RTD parameters such as normalised variance, dispersion number and number of tanks in series were studied under various parameters such as flowrate, rotational speed, fluid viscosity and disc texture (smooth, grooved). The findings showed that the highest macromixing conditions are achieved at higher rotational speeds and higher flowrates with a low viscosity fluid flowing on a grooved disc. The second part of the research investigated a reactive precipitation of TiO2 from acidified water and titanium tetra isopropoxide (TTIP) precursor in 10cm and 30 cm diameter SDRs. The findings demonstrated that smaller particles of less than 1 nm mean diameter with narrower PSDs were generally formed at higher yields at higher disc speeds, higher flowrates and higher flow ratio of water to TTIP precursor on a grooved disc surface, all of which provide the best hydrodynamic conditions for intense micromixing and macromixing in the fluid film travelling across the disc surface. The results also showed that 30 cm SDR was more efficient than 10 cm SDR at producing smaller particles with narrower PSD and higher yield. Finally a population balance model was proposed to evaluate and predict the size distribution of nanoparticles in the SDR. The model accounts for nucleation and growth of the TiO2 nanoparticles, which in turn was validated against the experimental results. Such a model can be employed to optimise operating conditions based on desired product particle size distribution. The present study reveals challenges and opportunities for TiO2 precipitation in SDRs. Currently the precipitation performance is inhibited by the disc material which leads to the precipitate sticking on the disc and also inefficient collector which can result in agglomeration of nanomaterials.