Process intensification : a study of micromixing and residence time distribution characteristics in the spinning disc reactor

Abstract

Micromixing phenomena (i.e. mixing at molecular level) play a very important role in the chemical industry when the time scale of the chemical reaction involved have the same magnitude or smaller than the time scale of mixing process. The study of micromixing is very critical to the understanding of important processes such as polymerization, precipitation, crystallization and competing fast chemical reactions. It has long been recognised that the intense mixing characteristics of thin films in the spinning disc reactor (SDR), play an important role in improving the selectivity, yield, and quality of final products of a chemical reaction. However, to date, there has been no systematic study of micro and macro mixing in SDR thin films. The first part of this study reports on the fundamental study undertaken to characterise micromixing in the thin films formed in 10 cm and 30 cm SDRs operating under a wide range of operating conditions. A well-established parallel-competitive reaction test scheme was adopted to quantify micromixing in terms of the segregation index (Xs) or micromixedness ratio (α), the power dissipation (ε) and micromixing time(tm). The micromixing data obtained from 10cm and 30cm SDRs were benchmarked against both a 1.37 l conventional semi-batch reactor (SBR) and continuous tubular flow reactors in the form of narrow channels (NCRs) of 1.0 mm diameter and three different lengths namely 5 cm, 10 cm and 15 cm (Y and T shape junctions). The effects of various operating parameters such as disc rotation rates, disc size, disc surface configurations, feed flowrates, feed distribution systems, liquid feed concentrations and viscosities were investigated. It was observed that, at an acid concentration of 1 M, the lowest segregation index of 0.05 was achieved for a feed of 0.001Ns/m2 viscosity at the highest flowrate of 5ml/s (corresponding to Refilm=72) and highest rotational speed of 2400 rpm in the 10cm diameter disc. Greatly improved micromixing was obtained on the larger disc of 30 cm diameter, especially at the lower Refilm of 15 and 42, in comparison to the smaller disc. Under optimised conditions, the micromixing time(tm) in the water-like film on the 30cm diameter disc was estimated to be as low as 0.3ms with corresponding power dissipation (ε) of 1025 W/kg. In contrast, the SBR could only achieve, under optimised conditions, segregation indices of no lower than 0.13 corresponding to a micromixing time of above 1ms with power dissipation of no more than about 21 W/kg. On the other hand, the NCRs could only achieve, under optimised conditions, a micromixing time of about 19 ms corresponding to power dissipation (ε) of about 208 W/kg. Therefore, when compared with other mixing devices such as conventional SBRs or NCRs, the SDR is shown to give significantly better micromixing performance which highlights its potential as an alternative device for processes where a high degree of mixing is critically important. In the second part of this study, the residence time distribution (RTD) of the liquid flow in the 30 cm SDR was characterised for a range of operating conditions including disc rotational speeds, disc configuration (smooth vs. grooved), total flow rate of liquid and viscosity in order to determine the conditions for which plug flow profile became more prevalent in the SDR films. The dispersion number from the RTD results and Peclet number were also estimated for the purpose of further characterising the extent of axial dispersion in the thin film flow on the rotating disc. All the mentioned operating conditions were found to have a profound influence on the overall Mean Residence Time, ( ), variance, , dispersion number and Peclet number, (Pe). More specifically, the lowest value for the of 10.1 s was achieved for a feed of 0.001 Ns/m2 viscosity at the highest flowrate of 15ml/s and highest rotational speed of 1200 rpm on the smooth disc with corresponding of 2.16. The dispersion number and Pe were 0.010 and 100 respectively, showing that the degree of axial dispersion was very small. A considerable reduction in the dispersion number and Pe was observed when the smooth disc was replaced by grooved disc. Thus, under the above mentioned hydrodynamic conditions, whilst the was almost unchanged at 10.10 s on grooved disc, the corresponding variance of 1.03 was significantly lower, indicating even more reduced axial dispersion in the film on the grooved disc. This is further substantiated by dispersion number and Pe of 0.005 and 200 respectively. In general the RTD curves become narrower and the values of and decreased as the disc rotational speed and flowrates increased and as the feed viscosity decreased. For the given operating conditions used in this research, it was confirmed that the 30 cm SDR approaches plug-flow regime which had a positive influence on the micromixing intensity on the SDR.