Continuous Reactive Precipitation in a Coiled Flow Inverter: Inert Particle Tracking, Modular Design, and Production of Uniform CaCO3 Particles

Engineering advancements in microfluidic systems for enhanced mixing at low Reynolds numbers

Mixing within micro- and millichannels is a pivotal element across various applications, ranging from chemical synthesis to biomedical diagnostics and environmental monitoring. The inherent low Reynolds number flow in these channels often results in a parabolic velocity profile, leading to a broad residence time distribution. Achieving efficient mixing at such small scales presents unique challenges and opportunities. This review encompasses various techniques and strategies to evaluate and enhance mixing efficiency in these confined environments. It explores the significance of mixing in micro- and millichannels, highlighting its relevance for enhanced reaction kinetics, homogeneity in mixed fluids, and analytical accuracy. We discuss various mixing methodologies that have been employed to get a narrower residence time distribution. The role of channel geometry, flow conditions, and mixing mechanisms in influencing the mixing performance are also discussed. Various emerging technologies and advancements in microfluidic devices and tools specifically designed to enhance mixing efficiency are highlighted. We emphasize the potential applications of micro- and millichannels in fields of nanoparticle synthesis, which can be utilized for biological applications. Additionally, the prospects of machine learning and artificial intelligence are offered toward incorporating better mixing to achieve precise control over nanoparticle synthesis, ultimately enhancing the potential for applications in these miniature fluidic systems.

Coiled flow inverter mediated synthesis of activated carbon fiber-supported Ni nanoparticles

A single-stage continuous flow process for the synthesis of Ni nanoparticle-dispersed activated carbon fibers is developed using CFI technology.

Catalytic reduction of in-flow aqueous Cr(vi) using a slurry of activated carbon fiber-supported Ni nanoparticles in a coiled flow inverter

CFI technology is utilized for the in-flow reduction of Cr(vi) using formic acid as the reducing agent and an aqueous dispersion of Ni/ACF, as an alternative to the conventional packed bed reactors.

Open-source multi-purpose sensor for measurements in continuous capillary flow

Abstract

Limited applicability and scarce availability of analytical equipment for micro- and millifluidic applications, which are of high interest in research and development, complicate process development, control, and monitoring. The low-cost sensor presented in this work is a modular, fast, non-invasive, multi-purpose, and easy to apply solution for detecting phase changes and concentrations of optically absorbing substances in single and multi-phase capillary flow. It aims at generating deeper insight into existing processes in fields of (bio-)chemical and reaction engineering. The scope of this work includes the application of the sensor to residence time measurements in a heat exchanger, a tubular reactor for concentration measurements, a tubular crystallizer for suspension detection, and a pipetting robot for flow automation purposes. In all presented applications either the level of automation has been increased or more information on the investigated system has been gained. Further applications are explained to be realized in the near future.

Article highlights

• An affordable multipurpose sensor for phase differentiation, concentration measurements, and process automation has been developed and characterized

• The sensor is easily modified and can be applied to various tubular reaction/process units for analytical and automation purposes

• Simple integration into existing process control systems is possible

Graphical abstract

Continuous Cooling Crystallization in a Coiled Flow Inverter Crystallizer Technology—Design, Characterization, and Hurdles

Continuous small-scale production is currently of utmost interest for fine chemicals and pharmaceuticals. For this purpose, equipment and process concepts in consideration of the hurdles for solids handling are required to transfer conventional batch processing to continuous operation. Based on empirical equations, pressure loss constraints, and an expandable modular system, a coiled flow inverter (CFI) crystallizer with an inner diameter of 1.6 mm was designed. It was characterized concerning its residence time behavior, tested for operation with seed crystals or an ultrasonic seed crystal unit, and evaluated for different purging mechanisms for stable operation. The residence time behavior in the CFI corresponds to ideal plug flow behavior. Crystal growth using seed crystals was demonstrated in the CFI for two amino acids. For fewer seed crystals, higher crystal growth rates were determined, while at the same time, secondary nucleation was observed. Feasibility for the interconnection of a sonicated seeding crystal unit could be shown. However, the hurdles are also identified and discussed. Prophylactic flushing combined with a photosensor for distinguishing between solvent and suspension phase can lead to stable and resource-efficient operation. The small-scale CFI technology was investigated in detail, and the limits and opportunities of the technology are presented here.

Nucleation in continuous flow cooling sonocrystallization for coiled capillary crystallizers

Nucleation in continuously operated capillary coiled cooling crystallizers is experimentally investigated under the influence of ultrasound. It was found that there is no sharp boundary but rather a transition zone for nucleation under sonication. For this purpose, a tube with an inner diameter of 1.6 mm and a length of 6 m was winded in a coiled flow inverter (CFI) design and immersed into a cooled ultrasonic bath (37 kHz). The CFI design was chosen for improved radial mixing and narrow residence time distribution, which is also investigated. Amino acid l-alanine dissolved in deionized water is employed in a supersaturation range of 1.10 to 1.46 under quiet and sonicated conditions. Nucleation is non-invasive detected using a flow cell equipped with a microscope and camera.

Graphical abstract

Since the interest and demand for small-scale, continuous crystallization increases, seed crystals were generated in a coiled tube via sonication and optically investigated and characterized. No distinct threshold for nucleation could be determined in a wide range of supersaturations of l-alanine in water

Reactive crystallization: a review

Reactive crystallization is not new, but there has been recent growth in its use as a means of improving performance and sustainability of industrial processes.

Acoustophoretic focusing effects on particle synthesis and clogging in microreactors

The handling of solids in microreactors represents a challenging task. In this paper, we present an acoustophoretic microreactor developed to manage particles in flow and to control the material synthesis process. The reactor was designed as a layered resonator with an actuation frequency of 1.21 MHz, in which a standing acoustic wave is generated in both the depth and width direction of the microchannel. The acoustophoretic force exerted by the standing wave on the particles focuses them to the channel center. A parametric study of the effect of flow rate, particle size and ultrasound conditions on the focusing efficiency was performed. Furthermore, the reactive precipitation of calcium carbonate and barium sulfate was chosen as a model system for material synthesis. The acoustophoretic focusing effect avoids solid deposition on the channel walls and thereby minimizes reactor fouling and thus prevents clogging. Both the average particle size and the span of the particle size distribution of the synthesized particles are reduced by applying high-frequency ultrasound. The developed reactor has the potential to control a wide range of material synthesis processes.