Design of micromixers using CFD modelling

The effect of various geometrical parameters of a grooved staggered herringbone micromixer on the mixing performance has been investigated using Computational Fluid Dynamics. Mixing quality has been quantified with spatial data statistics, maximum striation thickness and residence time analyses. The results show that the number of grooves per mixing cycle does not affect the mixing quality in an important way. On the other hand, a larger groove depth and width allow the maximum striation thickness to be rapidly reduced, without increasing the pressure drop across the mixer. Wide grooves, however, create significant dead zones in the microchannel, whereas deep grooves improve the spatial mixing quality

Blending of non-Newtonian fluids in static mixers: assessment via optical methods

The performance of KM static mixers has been assessed for the blending of Newtonian and time-independent non-Newtonian fluids using planar laser induced fluorescence (PLIF). A stream of dye is injected at the mixer inlet and the distribution of dye at the mixer outlet is analyzed from images obtained across the pipe cross section. The effect of superficial velocity, scale of static mixer, flow ratio between a primary and a secondary injected flow and finally the injection position, are investigated to determine the consequences on mixing performance. Different methods are discussed to characterize mixing performance, comparing CoV and maximum striation thickness. Conflicting trends are revealed in some experiments results, leading to the development of an areal based distribution of mixing intensity and a distribution of striation with high mixing intensity. For two-fluids blending, the addition of a high viscosity stream into the lower viscosity main flow causes very poor mixing performance, with unmixed spots of more viscous component observable in the PLIF image. The final part of the work is focused on a preliminary understanding of advective mechanisms such as shearing of non-Newtonian fluid drops and stretching of a non- Newtonian fluid filaments

Use of Positron Emission Particle Tracking (PEPT) for studying laminar mixing in static mixers

Over the last few decades, static mixers have been used commonly in different industries mainly for mixing of high viscosity fluids. The performance of this group of mixers has been investigated by many groups via numerically based studies. Experimental studies, however, are limited, particularly for measurement of the flow fields with fluids of complex rheology; there is also a lack of fundamental understanding of laminar flow and mixing performance for such duties. This is essential in order to gain improved design procedures for the selection and operation of the mixers in blending applications

Modelling and experiments of microchannels incorporating microengineered structures

Microreaction technology was conceived, thanks to the advances on microfabrication by the semiconductor industry. The �first applications of microchannels used for performing reactions date back to the early nineties. Since then, many conferences dedicated to this topic are held worldwide such as the International Microreaction Technology Conference (IMRET) or the International Conference on Microchannels and Minichannels. The small dimensions of the microchannels lead to very high heat and mass transfer rates, reactions are therefore performed very efficiently on these devices. However, the small dimensions of the channels lead to high pressure drops. In addition, microchannels are very susceptible to clogging. This thesis studies the e�ffect of di�fferent microchannel configurations in terms of mixing, mass transfer, residence time distribution and reaction. The objective is to design microreactors which incorporate di�fferent structures which make them efficient in terms of heat/mass transfer, but do not have the issue of high pressure drop and channel blockage

Characterisation of flow, mixing and changeover in SMX static mixers

This thesis pursues an enhanced understanding of flow dynamics and mixing within Sulzer SMX mixers. A number of techniques were used, with the main focus on Positron Emission Particle Tracking (PEPT), as well as Particle Image Velocimetry (PIV) and high speed image capture. PEPT tracer location data was processed to derive properties such as local velocity fields, mixing efficiencies, occupancies and changeover efficiencies, for a number of model Newtonian and non-Newtonian fluids, under industrially relevant flow conditions. It was demonstrated that velocity fields within SMX mixers are not adversely affected by the fluid rheology. Comparable velocity maps were also obtained using PIV, where transparent 3D printed mixer elements were successfully used with the technique for the first time. The assessment of the mixing patterns illustrated that the concentric feed orientation offers the fastest reduction in variance across the mixer cross-section, when compared to side-by-side feed patterns. Analysis of occupancies demonstrated a sharp breakthrough front, reminiscent of plug flow, while the assessment of the changeover patterns further emphasised the resemblance to plug flow within the mixer. A model was derived predicting the time required to achieve a desired level of changeover within a system with known rheological properties and geometry

Characterisation of flow, mixing and changeover in SMX static mixers

This thesis pursues an enhanced understanding of flow dynamics and mixing within Sulzer SMX mixers. A number of techniques were used, with the main focus on Positron Emission Particle Tracking (PEPT), as well as Particle Image Velocimetry (PIV) and high speed image capture. PEPT tracer location data was processed to derive properties such as local velocity fields, mixing efficiencies, occupancies and changeover efficiencies, for a number of model Newtonian and non-Newtonian fluids, under industrially relevant flow conditions. It was demonstrated that velocity fields within SMX mixers are not adversely affected by the fluid rheology. Comparable velocity maps were also obtained using PIV, where transparent 3D printed mixer elements were successfully used with the technique for the first time. The assessment of the mixing patterns illustrated that the concentric feed orientation offers the fastest reduction in variance across the mixer cross-section, when compared to side-by-side feed patterns. Analysis of occupancies demonstrated a sharp breakthrough front, reminiscent of plug flow, while the assessment of the changeover patterns further emphasised the resemblance to plug flow within the mixer. A model was derived predicting the time required to achieve a desired level of changeover within a system with known rheological properties and geometry

Characterisation of turbulent mixing and its influence on antisolvent crystallisation

Mixing is a fundamental part of many processes in chemical engineering. In order for molecular processes to proceed there is an implicit requirement for molecular scale mixing. Many processes are so slow that they are effectively independent of mixing as mixing is fast relative to the process. However, for fast processes mixing can be the limiting step and for processes with competitive elements it can control product quality and distribution. Antisolvent crystallisation is one such process which is strongly influenced by mixing. The initial mixing controls the distribution of supersaturation which in turn controls the nucleation rate and hence many key parameters such a particle size distribution. In order to understand antisolvent crystallisation and how the initial mixing influences nucleation it is important to first understand the mixing process itself. In this thesis mixing was measured and quantified by utilising a mixing sensitive competitive reaction scheme with well understood and well defined kinetics. The reaction scheme that was chosen was the Bourne IV reaction scheme which has received considerable interest in the scientific literature as a means to quantify and characterise the mixing performance of rapid continuous mixers. The original scheme has some inherent limitations in terms of ranking mixers operating under the conditions commonly encountered in industrial applications; namely the 1:1 flow ratio and the lack of a difference in the physical properties of the streams. This original scheme has been extended in a systematic way to incorporate differences in the flow ratio and physical properties. The results have been analysed in conjunction with a model capable of allowing fair comparison between the flow ratios. Several continuous mixers of various sizes including an impinging jet mixer and a vortex mixer have been characterised over a variety of mixing conditions. The antisolvent precipitation of valine in a confined impinging jet mixer was explored and analysed in conjunction with the mixing characterisation data allowing depth to be added to the analysis of standard crystallisation experiments. It is demonstrated that the initial mixing (over the first second) controls many of the key parameters in antisolvent crystallisation which underlines the importance of designing and scaling the initial mixing process correctly. It is also demonstrated that this is true even when samples are subjected to additional shear over long timescales. The vortex mixer characterised here was utilised in an industrial scale pilot trial and the results contrasted with those obtained using an "off the shelf" cross mixer. This work underlines that controlling the initial mixing step has strong industrial relevance and is one of the single most important parameters in the process design of antisolvent crystallisation processes.Mixing is a fundamental part of many processes in chemical engineering. In order for molecular processes to proceed there is an implicit requirement for molecular scale mixing. Many processes are so slow that they are effectively independent of mixing as mixing is fast relative to the process. However, for fast processes mixing can be the limiting step and for processes with competitive elements it can control product quality and distribution. Antisolvent crystallisation is one such process which is strongly influenced by mixing. The initial mixing controls the distribution of supersaturation which in turn controls the nucleation rate and hence many key parameters such a particle size distribution. In order to understand antisolvent crystallisation and how the initial mixing influences nucleation it is important to first understand the mixing process itself. In this thesis mixing was measured and quantified by utilising a mixing sensitive competitive reaction scheme with well understood and well defined kinetics. The reaction scheme that was chosen was the Bourne IV reaction scheme which has received considerable interest in the scientific literature as a means to quantify and characterise the mixing performance of rapid continuous mixers. The original scheme has some inherent limitations in terms of ranking mixers operating under the conditions commonly encountered in industrial applications; namely the 1:1 flow ratio and the lack of a difference in the physical properties of the streams. This original scheme has been extended in a systematic way to incorporate differences in the flow ratio and physical properties. The results have been analysed in conjunction with a model capable of allowing fair comparison between the flow ratios. Several continuous mixers of various sizes including an impinging jet mixer and a vortex mixer have been characterised over a variety of mixing conditions. The antisolvent precipitation of valine in a confined impinging jet mixer was explored and analysed in conjunction with the mixing characterisation data allowing depth to be added to the analysis of standard crystallisation experiments. It is demonstrated that the initial mixing (over the first second) controls many of the key parameters in antisolvent crystallisation which underlines the importance of designing and scaling the initial mixing process correctly. It is also demonstrated that this is true even when samples are subjected to additional shear over long timescales. The vortex mixer characterised here was utilised in an industrial scale pilot trial and the results contrasted with those obtained using an "off the shelf" cross mixer. This work underlines that controlling the initial mixing step has strong industrial relevance and is one of the single most important parameters in the process design of antisolvent crystallisation processes

Microreactor engineering studies for asymmetric chalcone epoxidation.

Advances in the field of microreaction technology offer the opportunity to combine the benefits of continuous processing with the flexibility and versatility desired in the pharmaceutical and fine chemicals industry. Microreactor devices, also offer their own unique advantages over traditional continuous processing, such as improved heat and mass transfer, safer handling of exothermic reactions and easy containment of explosive and toxic materials. A reaction system can be quickly scaled-up to production levels by increasing the number of units operating in parallel, allowing significant savings in time and R&D costs. Most studies of microreactor systems to date focus on the development and performance of individual microdevices. However, a top down approach is preferred, with the focus on the requirements of the process and a suitable device design derived to meet those requirements. This work aims to demonstrate the suitability of the poly-L-leucine catalysed asymmetric epoxidation of chalcone reaction for continuous processing as well as the process and choices of designing and scaling a microchemical system. A suitable continuous reaction protocol was established for this reaction system, achieving a conversion of 88.4 % and enantioselectivity of 88.8 %. Mixing was found to be critical due to the low diffusivity ( 10"u) of the polymeric catalyst. Design criteria were established and a microstructured reactor with a footprint of 110 mm x 85 mm and production rate of - 0.5 g/day was designed for the system. An external scale-out structure was selected. The staggered herringbone mixer was selected for enhancing the mixing in the microstructured reactor. A method for characterizing the mixing performance in the staggered herringbone mixer based on stretching computations using particle tracking methods was developed, which allowed the required mixer length to be derived directly. Mixer lengths of 40 mm were provided for both deprotonation and epoxidation mixers. The effects of varying operating temperature, residence time and reactant concentrations on reaction performance in the fabricated microstructured reactor were investigated. The base case condition (13.47 g/1 PLL, 0.132 mol/1 H202, 0.0802 mol/1 chalcone, 0.22 mol/1 DBU) was found to be optimal, achieving a conversion of 86.7 % and enantioselectivity of 87.6 %. Several unexpected phenomena such as bubble clogging and increased viscosity due to the polymeric catalyst were encountered. A scaled-out system was designed and experiments carried out. Flow maldistribution, attributed to fabrication errors and bubble clogging, resulted in poor reaction performance (conversion -31.4 % and enantioselectivity 82.7 %) due to unequal residence times and imperfect mixing ratios of reactants. The commercial potential of the research was evaluated. Micro and macro level analysis of the market and industry were favourable and a suitable commercialisation route was suggested