Gas–liquid mass transfer in a falling film microreactor: Effect of reactor orientation on liquid-side mass transfer coefficient

Multiphasic Continuous-Flow Reactors for Handling Gaseous Reagents in Organic Synthesis: Enhancing Efficiency and Safety in Chemical Processes

The use of reactive gaseous reagents for the production of active pharmaceutical ingredients (APIs) remains a scientific challenge due to safety and efficiency limitations. The implementation of continuous-flow reactors has resulted in rapid development of gas-handling technology because of several advantages such as increased interfacial area, improved mass- and heat transfer, and seamless scale-up. This technology enables shorter and more atom-economic synthesis routes for the production of pharmaceutical compounds. Herein, we provide an overview of literature from 2016 onwards in the development of gas-handling continuous-flow technology as well as the use of gases in functionalization of APIs.

Upscaling of dispersion in gas-liquid absorption on an inclined surface

We extend the Taylor-Aris dispersion theory to upscale the gas absorption into a viscous incompressible liquid flowing along an inclined surface. A reduced-order model of advection-dispersion-reaction is developed with the aid of Reynolds decomposition and cross-sectional averaging techniques. The upscaled model allowed evaluation of the dispersion, advection, and absorption kinetics as a function of the Peclet number (Pe) and the Damköhler number (Da). The transport and kinetics parameters for the limiting cases of nonabsorption and absorption dominant are also evaluated. The upscaled model is solved analytically, and the obtained solution is used to evaluate the upscaled mass transfer between the gas and liquid. The results for the overall Sherwood number identify three regions: (i) advection dominant, (ii) transition where both advection and absorption play a role, and (iii) absorption dominant. The scaling relation between the Sherwood number (Sh) and the Da for the last region was determined to follow Sh∼Da^{1/2}. It is also revealed that in the first two regions, the Sherwood number versus the Peclet number exhibits a bell-shaped (or Gaussian) behavior, suggesting an optimal Pe that maximizes mass transfer between gas and liquid in these regions. The model and insights presented have the potential to be applied in a wide range of industrial separation processes involving the interaction of a gas exposed to a liquid flowing downward on an inclined surface under gravity.

Synthesis of cyclic carbonates from CO2 cycloaddition to bio-based epoxides and glycerol: an overview of recent development

Anthropogenic carbon dioxide (CO₂) emissions contribute significantly to global warming and deplete fossil carbon resources, prompting a shift to bio-based raw materials. The two main technologies for reducing CO₂ emissions are capturing and either storing or utilizing it. However, while capture and storage have high reduction potential, they lack economic feasibility. Conversely, by utilizing the CO₂ captured from streams and air to produce valuable products, it can become an asset and curb greenhouse gas effects. CO₂ is a challenging C1-building block due to its high kinetic inertness and thermodynamic stability, requiring high temperature and pressure conditions and a reactive catalytic system. Nonetheless, cyclic carbonate production by reacting epoxides and CO₂ is a promising green and sustainable chemistry reaction, with enormous potential applications as an electrolyte in lithium-ion batteries, a green solvent, and a monomer in polycarbonate production. This review focuses on the most recent developments in the synthesis of cyclic carbonates from glycerol and bio-based epoxides, as well as efficient methods for chemically transforming CO₂ using flow chemistry and novel reactor designs.

Degradation of dibutyl phthalate by ozonation in the ultrasonic cavitation-rotational flow interaction coupled-field: performance and mechanism

Dibutyl phthalate (DBP) is present in hydraulic fracturing flowback and produced water. Degradation of DBP in aqueous by means of ozonation in ultrasonic cavitation-rotational flow interaction coupled-field (UC-RF coupled-field) was studied. The effect of ozone dosage, ozone intake flow, operating temperature, initial pH, DBP initial concentration, liquid flow rate, and ultrasonic power on the DBP removal was investigated. Results indicated that the DBP degradation rate was strongly influenced by the liquid flow rate and the ultrasonic power over the range investigated. HCO₃^- and Cl^- revealed an inhibitory effect on the DBP removal. SO₄^2- seemed to have no effect on DBP removal. The ozone utilization efficiencies in the UC-RF coupled-field were 2.77 and 1.13 times higher than those in the conventional microporous aeration (CMA) and rotating-flow microbubble aeration (RFMA), respectively. The DBP degradation rate was diminished in the presence of tert-butyl alcohol. Cavitation bubbles are considered as innumerable microreactors. Degradation of DBP by direct ozonation, hydroxyl radical (·OH) oxidation, high pressure, and high-temperature pyrolysis was demonstrated. Finally, a possible degradation pathway of DBP is obtained on the basis of the main reaction intermediates.

Process intensification of the ozone-liquid mass transfer in ultrasonic cavitation-rotational flow interaction coupled-field: Optimization and application

A study on the intensification of ozone mass transfer in rotational flow field and UC-RF coupled-field was conducted. Two important operational parameters namely liquid flow rate and ultrasonic power, were optimized with regard to the ozone mass transfer efficiency. Results showed that the mass transfer coefficient (K_La) increased with liquid flow rate (up to 14 L min^-1) and ultrasonic power (up to 1000 W). The maximum K_La value (1.0258 min^-1) was obtained with the UC-RF coupled-field. Moreover, the reinforcement of mass transfer efficiency was promoted by the rotational flow field and UC-RF coupled-field due to the increase in the ozone-liquid contact area, intensification of turbulence, acceleration of interface renewal, and extension of residence time. Ozone microbubbles rose in the reactor in a spiral manner. In addition, the microbubbles produced in the rotational flow field served as cavitation nucleus that helped to generate the cavitation effect. The effective degradation of di-butyl phthalate (DBP) confirmed that its removal was improved by the ozone-liquid mass transfer and the promotion of hydroxyl radicals (·OH) production. The synergistic effect of DBP degradation via ultrasound-enhanced ozonation was significant.

Gas-liquid mass transfer intensification for bubble generation and breakup in micronozzles

The local gas-liquid mass transfer was characterized during bubble generation in T-contactors and in an adjacent micronozzle. A colorimetric technique with the oxygen sensitive dye resazurin was investigated to visualize gas-liquid mass transfer during slug flow, bubble deformation, as well as laminar and turbulent bubble breakup in the wake of a micronozzle. Two optimized nozzle geometries from previous studies were evaluated concerning volumetric mass transfer coefficients for low pressure loss, narrow residence time distribution, or high dispersion rates. Highest values in kla up to 60 s−1 were found for turbulent bubble breakup and an optimized micronozzle design in respect to pressure drop and dispersion rate. The achieved mass transfer coefficients were correlated with the energy dissipation rate within the micronozzles and with the inverse Kolmogorov time scale in vortex dissipation in good agreement for laminar and turbulent breakup regimes.

Graphical abstract

Application of reactor engineering concepts in continuous flow chemistry: a review

The adoption of flow technology for the manufacture of chemical entities, and in particular pharmaceuticals, has seen rapid growth over the past two decades with the technology now blurring the lines between chemistry and chemical engineering.

Mass transfer in falling film microreactors: measurement techniques and effect of operational parameters

Falling film microreactors have contributed to the pursuit of process intensification strategies and have, over the years, been recognized for their potential in performing demanding reactions. In the last few decades, modifications in the measurement techniques and operational parameters of these microstructured devices have been the focus of many research studies with a common target on process improvement. In this work, we present a review dedicated to falling film microreactors, focusing on the recent advances in their design and operation, with particular emphasis on mass transfer enhancement. Analysis of the recent techniques for the measurement of mass transfer as well as the operational parameters used and their effect on the target objective, particularly in the liquid phase (being the limiting phase reactant), are included in the review. The relationship between the hydrodynamics of falling thin liquid films and the microreactor design, the discrepancies between measured and model results, the major challenges, and the future outlook for these promising microreactors are also presented.