Current methods for characterising mixing and flow in microchannels

This article reviews existing methods for the characterisation of mixing and flow in microchannels, micromixers and microreactors. In particular, it analyses the current experimental techniques and methods available for characterising mixing and the associated phenomena in single and multiphase flow. The review shows that the majority of the experimental techniques used for characterising mixing and two-phase flow in microchannels employ optical methods, which require optical access to the flow, or off-line measurements. Indeed visual measurements are very important for the fundamental understanding of the physics of these flows and the rapid advances in optical measurement techniques, like confocal scanning laser microscopy and high resolution stereo micro particle image velocimetry, are now making full field data retrieval possible. However, integration of microchannel devices in industrial processes will require on-line measurements for process control that do not necessarily rely on optical techniques. Developments are being made in the areas of non-intrusive sensors, magnetic resonance techniques, ultrasonic spectroscopy and on-line flow through measurement cells. The advances made in these areas will certainly be of increasing interest in the future as microchannels are more frequently employed in continuous flow equipment for industrial applications

Optical methods to investigate the enhancement factor of an oxygen-sensitive colorimetric reaction using microreactors

Visualization of mass transfer is a powerful tool to improve understanding of local phenomenon. The use of an oxygen-sensitive dye (colorimetric technique1) has showed its relevancy for locally visualizing and characterizing gas-liquid mass transfer at different scales2,3. At present, the occurrence of a possible enhancement of the gas-liquid mass transfer by this reaction has not been yet demonstrated. This paper aims at filling this gap by evaluating the Hatta number Ha and the enhancement factor E associated with the oxygen colorimetric reaction when implementing in milli/micro channels. For that, as data on the kinetic of the colorimetric reaction are seldom in the literature, the reaction characteristic time was firstly estimated by carrying out experiments in a microchannel equipped with a micromixer. The diffusion coefficients of dihydroresorufin and O2 were then determined by implementing two original optical methods in a specific coflow microchannel device, coupled with theoretical modelling. The knowledge of these parameters enabled at last to demonstrate that no enhancement of the gas-liquid mass transfer by this colorimetric reaction existed. Complementary information about the reliability of the colorimetric technique to characterize the gas-liquid mass transfer in milli/micro systems was also give

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

PhD ThesisMicromixing 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.Libyan Petroleum Institute (LPI)

Energy dissipation and mixing characterization in continuous oscillatory baffled reactor

The focus of this thesis is to study the macro and micromixing performance of a secondary component in the bulk flow and how it should be introduced into a COBR. The effect of the position of secondary feeds, the influence of the oscillatory conditions and power dissipation on the macro and micromixing performance is studied, using numerical simulations and experiments carried out in a commercial Nitech® OBR with smooth constrictions. Energy dissipation is calculated through CFD simulations using two different ways – via viscous energy dissipation and the mechanical energy balance, the latter being preferred due to its lower demand for a refined computational mesh. A dimensionless power density number is obtained and proposed as a useful tool in the prediction of power density in COBRs. The impact of the position where a secondary feed enters the COBR on the spatial mixing quality is studied, and shows that when the source position is chosen correctly, an increase in the velocity ratio enhances mixing performance from 2% to 87% of the perfectly mixed state. The influence of the oscillatory conditions and flow rate of a secondary feed on the micromixing quality is analysed. Micromixing performance does not appear to correlate directly with power density. However, higher amplitudes and lower frequencies are preferred over lower amplitudes and higher frequencies to have a better micromixing performance. An attempt at characterising macromixing in the COBR experimentally using a coloured tracer was made however unexpected mixing performance was observed. Some preliminary experiments therefore focused on the behaviour of the tracer upstream of the COBR as a function of the oscillatory conditions

Design and engineering of microreactor and smart-scaled flow processes

This book is a reprint of the special issue that appeared in the online open access journal Processes (ISSN 2227-9717) in 2013 (available at: http://www.mdpi.com/journal/processes/special_issues/smart-scaled_flow_processes)

Green Process Engineering as the Key to Future Processes

Microreactors are small devices with sub-millimeter internals which have superb mass and heat transfer. Initially, they were used for reactions with very high demands on the latter, e.g. very exothermic reactions, gas-liquid reactions with interfacial transport issues, reactions with very fast kinetics which demands even faster mixing, and more. In this way, the processing window was opened widely and, also due to the minute volumes only present in the reaction zone, safe processing under otherwise hazardous conditions was enabled. This includes processing of reactions which are prone to thermal runaway and in the explosive regime. Scale-up of promising reactions and products which was hindered with conventional technology is now possible using the new equipment. This has widened the process development possibilities in chemical industry. In the last years, micro process technology was not only used for the very problematic synthetic issues which formerly had a dead-end position in industry’s process development. Rather, the scope of chemical reactions to be processed in microreactors was considerably widened by exploring new process conditions with regard to temperature, pressure, concentration, solvents, and more. This is commonly referred to as flow chemistry. This allowed to reduce the processing time-scale for many reactions to the minute range or even below which fits well to the residence times of microreactors. In addition, the process integration of several reactions in one flow to a multi-step synthesis has opened a new door in molecular diversity as well as system and process complexity. The same holds for the combination of reactions and separations in micro-flow. To achieve throughputs relevant for industrial production, smart scale-out to milli-flow units has established and supplemented the num­bering-up concept (parallelization of microchannels/-reactors operated under equal conditions). New innovations and enabling technologies need anyhow evaluation and benchmarking with conventional technology on the full-system level. Yet, microreactor technology has in the last years deepened so much into process intensification on a holistic scale that the focus increasingly is given towards the process dimension—to process design and automation, real-case applications, cost analysis, life-cycle assessment, and more. The impact on cost competitiveness and sustainability becomes well assessed. Facing this very recent scientific achievement, the special issue “Design and Engineering of Microreactor and Smart-Scaled Flow Processes” of the journal Processes aims to cover recent advances in the development of microreactor and smart-scaled flow processes towards the process level — in the sense as given above

Applications of CFD Simulations on Studying the Multiphase Flow in Microfluidic Devices

Microfluidics has been extensively investigated as a unique platform to synthesize nanoparticles with desired properties, e.g., size and morphology. Compared to the conventional batch reactors, wet-chemical synthesis using continuous flow microfluidics provides better control over addition of reagents, heat and mass transfer, and reproducibility. Recently, millifluidics has emerged as an alternative since it offers similar control as microfluidics. With its dimensions scaled up to millimeter size, millifluidics saves fabrication efforts and potentially paves the way for industrial applications. Good designs and manipulations of microfluidic and millifluidic devices rely on solid understanding of fluid dynamics. Fluid flow plays an important role in heat and mass transfer; thereby, it determines the quality of the synthesized nanoparticles. Computational fluid dynamics (CFD) simulations provide an effective approach to understand various effects on fluid flows without carrying out complicated experiments. The goal of this dissertation is to utilize CFD simulations to study flow behaviors inside microfluidic and millifluidic systems. Residence time distribution (RTD) analysis coupled with TEM characterization was applied to understand the effect of reagent flow rates on particle sizes distribution. Droplet-based microfluidics, as a solution to the intrinsic drawbacks associated with single-phase microfluidics, depends on proper manipulation of the flow to generate steady droplet flow with desired droplet / slug sizes. The droplet/slug formation process inside a millifluidic reactor was investigated by both experiments and numerical simulations to understand the hydrodynamics of slug breaking. Geometric optimization was carried out to analyze the dependency of slug sizes on geometric dimensions. Numerical simulations were also performed to quantify the mixing efficiency inside slugs with the aid of mixing efficiency index. In some circumstances, the droplet sizes are difficult to control via manipulating the flow rates. By applying external electric field to the conventional droplet-based microfluidic systems, the electric force induced on the fluid interface can reduce the droplet sizes effectively. This work provides insight to understand fluid flow inside microfluidic and millifluidic systems. It may benefit the design and operations of novel microfluidic and millifluidic systems

Development of Microwave/Droplet-Microfluidics Integrated Heating and Sensing Platforms for Biomedical and Pharmaceutical Lab-on-a-Chip Applications

Interest in Lab-on-a-chip and droplet-based microfluidics has grown recently because of their promise to facilitate a broad range of scientific research and biological/chemical processes such as cell analysis, DNA hybridization, drug screening and diagnostics. Major advantages of droplet-based microfluidics versus traditional bioassays include its capability to provide highly monodispersed, well-isolated environment for reactions with magnitude higher throughput (i.e. kHz) than traditional high throughput systems, as well as its low reagent consumption and elimination of cross contamination. Major functions required for deploying droplet microfluidics include droplet generation, merging, sorting, splitting, trapping, sensing, heating and storing, among which sensing and heating of individual droplets remain great challenges and demand for new technology. This thesis focuses on developing novel microwave technology that can be integrated with droplet-based microfluidic platforms to address these challenges. This thesis is structured to consider both fundamentals and applications of microwave sensing and heating of individual droplets very broadly. It starts with developing a label-free, sensitive, inexpensive and portable microwave system that can be integrated with microfluidic platforms for detection and content sensing of individual droplets for high-throughput applications. This is, indeed, important since most droplet-based microfluidic studies rely on optical imaging, which usually requires expensive and bulky systems, the use of fluorescent dyes and exhaustive post-imaging analysis. Although electrical detection systems can be made inexpensive, label-free and portable, most of them usually work at low frequencies, which limits their applications to fast moving droplets. The developed microwave circuitry is inexpensive due to the use of off-the-shelf components, and is compact and capable of detecting droplet presence at kHz rates and droplet content sensing of biological materials such as penicillin antibiotic, fetal bovine serum solutions and variations in a drug compound concentration (e.g., for Alzheimer’s Disease). Subsequently, a numerical model is developed based on which parametrical analysis is performed in order to understand better the sensing and heating performance of the integrated platform. Specifically, the microwave resonator structure, which operates at GHz frequency affecting sensing performance significantly, and the dielectric properties of the microfluidic chip components that highly influence the internal electromagnetic field and energy dissipation, are studied systematically for their effects on sensing and heating efficiency. The results provide important findings and understanding on the integrated device operation and optimization strategies. Next, driven by the need for on-demand, rapid mixing inside droplets in many applications such as biochemical assays and material synthesis, a microwave-based microfluidic mixer is developed. Rapid mixing in droplets can be achieved within each half of the droplet, but not the entire droplet. Cross-center mixing is still dominated by diffusion. In this project, the microwave mixer, which works essentially as a resonator, accumulates an intensive, nonuniform electromagnetic field into a spiral capacitive gap (around 200 μm) over which a microchannel is aligned. As droplets pass by the gap region, they receive spatially non-uniform energy and thus have non-uniform temperature distribution, which induces non-uniform Marangoni stresses on the interface and thus three-dimensional (3D) chaotic motion inside the droplet. The 3D chaotic motion inside the droplet enables fast mixing within the entire droplet. The mixing efficiency is evaluated by varying the applied power, droplet length and fluid viscosity. In spite of various existing thermometry methods for microfluidic applications, it remains challenging to measure the temperature of individual fast moving droplets because they do not allow sufficient exposure time demanded by both fluorescence based techniques and resistance temperature detectors. A microwave thermometry method is thus developed here, which relies on correlating fluid temperature with the resonance frequency and the reflection coefficient of the microwave sensor, based on the fact that liquid permittivity is a function of temperature. It is demonstrated that the sensor can detect the temperature of individual droplets with ±1.2 °C accuracy. At the final part of the thesis, I extend my platform technology further to applications such as disease diagnosis and drug delivery. First, I develop a microfluidic chip for controlled synthesis of poly (acrylamide-co-sodium acrylate) copolymer hydrogel microparticles whose structure varies with temperature, chemical composition and pH values. This project investigates the effects of monomer compositions and cross-linker concentrations on the swelling ratio. The results are validated through the Fourier transform infrared spectra (FTIR), SEM and swelling test. Second, a preliminary study on DNA hybridization detection through microwave sensors for disease diagnosis is conducted. Gold sensors and biological protocols of DNA hybridization event are explored. The event of DNA hybridization with the immobilized thiol-modified ss-DNA oligos and complimentary DNA (c-DNA) are monitored. The results are promising, and suggests that microwave integrated Lab-on-a-chip platforms can perform disease diagnosis studies