Microchemical Systems Research Articles

Hydrogen production in protruded millisecond microchannel reactors by catalytically reforming methanol

Flow of a fluid past a body is a very complicated phenomenon. Computational fluid dynamics is used for studying the characteristics of flow past an array of hemispherical protrusions that is disposed on the wall surfaces of a millisecond microchannel reactor. Protrusions can be used to improve the transport processes involved, but the causes of the phenomena are still incompletely understood. Parametric analyses are performed under different sets of circumstances to delineate the role of geometric features and operation conditions in reactor performance. Dimensionless quantities are used to simplify the characterization of the reactor system with multiple interacting transport phenomena. The mechanisms involved in the intensified processes are analysed, and performance improvement recommendations are presented. The results indicate that the protruded reactor behaves effectively and good yields can be obtained with only milliseconds residence of the mixtures within the channels. The reactor offers the unique advantage for hydrogen production from methanol in that process intensification is realized while preserving the energy balance between the exothermic and endothermic processes. However, the flow rates, feed compositions, and channel dimensions must be adjusted as needed to maximize production of hydrogen and minimize pressure drops. While the thermal diffusivity dominates the overall reactor behaviour, expectable compromises have to be made between hydrogen productivity and pressure drop. The results have implications for hydrogen production and beyond for the study of transport phenomena in microchemical systems.

Controllable Synthesis of Silver Nanoparticles Using a Double-Layer Y-Shaped SAR Micromixer

Based on the advantages of microfluidics in the field of nanoparticle synthesis, a controllable synthesis method for silver nanoparticles using a double-layer Y-shaped splitting and recombination (SAR) micromixer is proposed. First, the liquid phase synthesis mechanism of silver nanoparticles, the working principle of the double-layer Y-shaped SAR micromixer, and the mixing performance of micromixer at different Reynolds number (Re) are analyzed. Then, the micromixer is used to synthesize silver nanoparticles, and the effects of reductant concentration, polyvinylpyrrolidone (PVP) and inlet flow rate on the size, distribution and morphology of the synthesized silver nanoparticles are investigated comprehensively. The synthesized silver nanoparticles are characterized by UV-spectrometer and transmission electron microscopy (TEM). The experimental results show that the reductant concentration, PVP, and inlet flow rate have a direct impact on the size, distribution, monodispersity and morphology of the synthesized nanoparticles. The moderate reductant concentration makes the size of silver nanoparticles larger and the size uniformity is better. Adding PVP to the experimental reagent can prevent the aggregation of silver nanoparticles, consequently, the synthesized particles have a uniform distribution and a better morphology. The changes in inlet flow rate and Re directly affect the mixing efficiency, which in turn affect the formation of silver atoms and silver nanocrystal nuclei and have a greater impact on particle concentration. The proposed micromixer has excellent mixing performance and can be used in other fields such as controllable synthesis, biomedicine and microchemical systems.

Manipulation and Control of Structure and Size of Inorganic Nanomaterials in Microchemical Systems

AbstractMicrochemical systems exhibit a unique capacity of preparing and manipulating various nanomaterials. In this review, the state‐of‐the‐art microchemical systems and the on‐demand development for the preparation of novel inorganic nanomaterials with certain dimensions or complex structures are summarized and discussed. Multistep synthesis and manipulation in microchemical systems are presented and compared, as well as new strategies such as in‐line monitoring, feedback control, and self‐optimization. The design and strategy for tuning the nucleation, growth, functionalization, and assembly of nanomaterials are considered. The challenges are put forward and an outlook for the future directions of nanomaterial preparation and production in microchemical systems is specially provided.

Microchannels Fabrication in Alumina Ceramic Using Direct Nd:YAG Laser Writing.

Ceramic microchannels have important applications in different microscale systems like microreactors, microfluidic devices and microchemical systems. However, ceramics are considered difficult to manufacture owing to their wear and heat resistance capabilities. In this study, microchannels are developed in alumina ceramic using direct Nd:YAG laser writing. The laser beam with a characteristic pulse width of 10 µs and a beam spot diameter of 30 µm is used to make 200 µm width microchannels with different depths. The effects of laser beam intensity and pulse overlaps on dimensional accuracy and material removal rate have been investigated using different scanning patterns. It is found that beam intensity has a major influence on dimensional accuracy and material removal rate. Optimum parameter settings are found using grey relational grade analysis. It is concluded that low intensity and low to medium pulse overlap should be used for better dimensional accuracy. This study facilitates further understanding of laser material interaction for different process parameters and presents optimum laser process parameters for the fabrication of microchannel in alumina ceramic.

Millisecond methane steam reforming for hydrogen production: A computational fluid dynamics study

The potential of methane steam reforming to produce hydrogen in thermally integrated micro-chemical systems at short contact times was theoretically explored. Methane steam reforming coupled with methane catalytic combustion in microchannel reactors for hydrogen production was studied numerically. A two-dimensional computational fluid dynamics model with detailed chemistry and transport was developed. To provide guidelines for optimal design, reactor behavior was studied, and the effect of design parameters such as catalyst loading, channel height, and flow arrangement was evaluated. To understand how steam reforming can happen at millisecond contact times, the relevant process time scales were analyzed, and a heat and mass transfer analysis was performed. The importance of energy management was also discussed in order to obtain a better understanding of the mechanism responsible for efficient heat exchange between highly exothermic and endothermic reactions. The results demonstrated the feasibility of the design of millisecond reforming systems, but only under certain conditions. To achieve this goal, process intensification through miniaturization and the improvement in catalyst performance is very important, but not sufficient; very careful design and implementation of the system is also necessary to enable high thermal integration. The channel height plays an important role in determining the efficiency of heat exchange. A proper balance of the flow rates of the combustible and reforming streams is an important design criterion. Reactor performance is significantly affected by flow arrangement, and co-current operation is recommended to achieve a good energy balance within the system. The catalyst loading must be carefully designed to avoid insufficient reactant conversion or hot spots. Finally, operating windows were identified, and engineering maps for designing devices with desired power were constructed.

Design and Scaling Up of Microchemical Systems: A Review

The past two decades have witnessed a rapid development of microreactors. A substantial number of reactions have been tested in microchemical systems, revealing the advantages of controlled residence time, enhanced transport efficiency, high product yield, and inherent safety. This review defines the microchemical system and describes its components and applications as well as the basic structures of micromixers. We focus on mixing, flow dynamics, and mass and heat transfer in microreactors along with three strategies for scaling up microreactors: parallel numbering-up, consecutive numbering-up, and scale-out. We also propose a possible methodology to design microchemical systems. Finally, we provide a summary and future prospects.

Fully-Enclosed Ceramic Micro-burners Using Fugitive Phase and Powder-based Processing.

Ceramic-based microchemical systems (μCSs) are more suitable for operation under harsh environments such as high temperature and corrosive reactants compared to the more conventional μCS materials such as silicon and polymers. With the recent renewed interests in chemical manufacturing and process intensification, simple, inexpensive, and reliable ceramic manufacturing technologies are needed. The main objective of this paper is to introduce a new powder-based fabrication framework, which is a one-pot, cost-effective, and versatile process for ceramic μCS components. The proposed approach employs the compaction of metal-oxide sub-micron powders with a graphite fugitive phase that is burned out to create internal cavities and microchannels before full sintering. Pure alumina powder has been used without any binder phase, enabling more precise dimensional control and less structure shrinkage upon sintering. The key process steps such as powder compaction, graphite burnout during partial sintering, machining in a conventional machine tool, and final densification have been studied to characterize the process. This near-full density ceramic structure with the combustion chamber and various internal channels was fabricated to be used as a micro-burner for gas sensing applications.

Recent advances for serial processes of hazardous chemicals in fully integrated microfluidic systems

The development and enlargement of toxic and hazardous chemicals are severely limited by health and safety concerns. We summarize studies on fully integrated micro-chemical systems and total processes to reduce accidental exposure to various regents that are toxic, explosive, or carcinogenic, which significantly improved the safety of work involving risky compounds. This review covers the leak-free continuous-flow processes of hazardous chemicals in fully integrated microfluidic systems, specially denoted as micro-total envelope systems (μ-TESs), that are conducting a serial process of the generation of hazardous reagents, in-situ purification and separation, subsequent reaction, and product isolation with improved efficiencies. These attempts suggest safe and efficient tools and processes of useful but hazardous chemicals for researchers and manufacturing workers in the field of pharmaceutic discovery, natural products, biology as well as materials synthesis.

Fabrication of Powder-based Ceramic Micro-burners

Miniaturization of chemical system has attracted tremendous interests in chemistry and biology due to many advantages such as enhancement in heat/mass transfer rates, reduction in expensive reagents and hazardous wastes, and facilitation of massive parallelization in reaction/catalyst screening and optimization Fabrication of microchemical systems (μCSs) has matured around materials such as silicon, glass and polymers, which are not particularly suitable for harsh environment such as high pressure, high temperature, and corrosive reactants. The main objective of this paper is to introduce a new ceramic-based fabrication framework for such harsh environment. This paper demonstrates the feasibility of the new technique by constructing a micro-burner, which can be used as on-chip heater or sensor. The proposed technique overcomes one major roadblock in fabrication of ceramic-based μCSs. The unreliable joining and assembling of simple ceramic components are eliminated by utilizing a fugitive phase machined into an intricate shape of an internal combustion chamber and channels. The main fabrication strategy is to use the advantage of powder, offering the fluidity to flow into a complex cavity or around the solid phase as well as the rigidity to remove the undesirable sections. The paper demonstrates the unique method of fabricating a micro-burner that can sustain an oxy-hydrogen flame for an extensive period and under cyclic operations.

Role of Water on the Precipitation and Deposition of Asphaltenes in Packed-Bed Microreactors

Water naturally comprises some fraction of virtually all petroleum reservoirs worldwide, yet its existence introduces complexity to asphaltene precipitation and deposition mechanisms. This global problem not only impacts upstream conventional and unconventional energy production but also influences downstream chemical refining, the construction industry, and the transportation sector. A transparent packed-bed microreactor (μPBR) with inline analytics was designed to merge two fields of science, oilfield chemistry and microchemical systems, to investigate the role of water on the molecular-to-the-microscale deposition of asphaltenes in quartz porous media. Porosity loss and permeability impairment of the porous media for water mass fractions of <0.001 to 34.5 wt % were investigated. Interestingly, a switch in the mechanism of water (from 0.030 to 3.18 wt %) on the accumulation was discovered. Analyses of porosity–permeability relationships revealed competition between adsorption and desorption followed by ...

High‐throughput packed‐bed microreactors with in‐line analytics for the discovery of asphaltene deposition mechanisms

Understanding asphaltene nanoaggregation kinetics is a key to predicting the deposition in pure quartz‐grain porous media. High‐throughput quartz packed‐bed microreactors (μPBRs) were, therefore, designed to provide mechanistic insights by merging oilfield chemistry and microchemical systems. In‐line UV‐Vis spectroscopy and pressure transducer were used to characterize the stable packing of quartz particles with porosity of ∼40% and permeability of ∼5.5 × 10−13 m2. Temperature (25.0–90.0°C), n‐heptane composition (50.0–80.0 vol %), and n‐alkane (n‐C5 to n‐C9) were all observed to influence asphaltenes deposition in the porous media, and reduced dispersion was obtained in the damaged packed‐bed by estimating dispersion coefficients and the Bodenstein number. Deposition by mechanical entrapment dominated the mechanism in all scenarios, as discovered by the simplified Kozeny–Carman and Civan's permeability‐porosity relationships. The results could aid in the design of remediations that minimize production losses of considerable economic magnitude. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3534–3546, 2014

CMOS sensor-based miniaturised in-line dual-functional optical analyser for high-speed, in situ chirality monitoring

A miniaturised in-line dual-functional optical analyser equipped with a polarisation-analysing CMOS image sensor was proposed and demonstrated. A single-wavelength optical absorption measurement function was also implemented in this device. The feasibility of the proposed device was demonstrated by evaluating two functions. Quasi-simultaneous optical rotation and absorption measurements were also performed. The results suggest that the present device is feasible as an in-line, in situ monitoring device for microchemical systems.

Control of nano and microchemical systems

Many advances in the development of nano and microchemical systems have occurred in the last decade. These systems have significant associated identification and control challenges, including high state dimensionality, limitations in real-time measurements and manipulated variables, and significant uncertainties described by non-Gaussian distributions. Some strategies for addressing these challenges are summarized, which include exploiting structure within the stochastic Master equations that describe molecular interactions, manipulating molecular bonds at system boundaries, and manipulating molecules and nanoscale objects through magnetic and electric fields. The strategies are illustrated in a variety of applications that include the estimation of nucleation kinetics of protein and pharmaceutical crystals within fluidic devices, the estimation of concentration fields using DNA-wrapped single-walled carbon nanotube-based sensor arrays, the simultaneous control of nanoscale geometry and electrical activation during thermal annealing in a semiconductor material, and the control of nanostructure formation on surfaces. Promising directions for research and technology development are identified for the next decade.

Dynamics and Control of Integrated Microchemical Systems

Microchemical systems are a new generation of miniature chemical systems that carry out chemical reactions and separations in precisely fabricated three dimensional microreactor configurations in the size range of a few microns to a few hundred microns. Typical microchemical systems combine fluid handling and reaction capabilities with electronic sensing and actuation, are fabricated using integrated circuit (IC) manufacturing techniques and use silicon and related IC industry materials, polymers, ceramics, glass or quartz as their material of construction. The use of such systems for in-situ and on-demand chemical production is gaining increasing importance as the field of microreaction engineering transitions from a theoretical concept to a technology with significant industrial applications. In this paper, we provide an overview of the issues involved in modeling, design and control of microchemical systems and as examples, demonstrate some of these concepts with our work on modeling and control of microreformers for hydrogen delivery systems in micro-fuel cells. The paper concludes by suggesting possible areas of future research.

Oxygenation by a superhydrophobic slip G/L contactor

The compelling need for an efficient supply of gases into liquids or degassing of fluids within confined microchannels triggered our study on membrane assisted microchemical systems. Porous hydrophobic flat/micro-structured polyvinylidene fluoride (PVDF) membranes were fabricated and integrated in a glass G/L contacting microfluidic device with the aid of optical adhesives. The oxygen transport in microchannels, driven by convection and diffusion, was investigated both experimentally and numerically. The effects of intrinsic membrane morphology on the G/L contacting performance of the resultant membranes were studied. The experimental performance of the flat membranes are shown to obey the simulation results with the assumptions of negligible gas phase and membrane mass transfer limitations. Micro-structured membranes revealed apparent slippage and enhanced mass transport rates, and exceeded the experimental performance of the flat membranes.

Lab-on-a-Chip Systems for Microchemical Applications

Lab-on-a-chip microchemical systems were fabricated from PDMS, polyimide (PI) film and functional polymers by different lithographic techniques to look for microreactor applications in the areas of organic syntheses. In detail, efficient gas-liquid reactions in dual-channel microreactor, continuous recovery and recirculation of catalystimmobilized magnetic particles in microfluidic system, and a flexible film microreactor will be discussed.

Enhancement of water-gas-shift conversion via externally imposed temperature profiles: Comparison of linear, convex and Gaussian profiles

Microchemical systems enable the creation of reproducible and tunable temperature profiles, owing to their inherently low thermal inertia. An externally imposed decreasing temperature profile may be employed to enhance exothermic, equilibrium-limited chemical reactions by continuously balancing equilibrium limitations against kinetics along the reactor length, in comparison to isothermal operation. In this manuscript, the authors provide a side-by-side theoretical comparison of three archetypical temperature profiles (linear, convex and Gaussian) and their influence upon the water-gas-shift reaction over an appropriate range of microreactor parameters. Comparison of optimal designs identified over a broad range of temperature profile magnitude and shape with the reference case of an isothermal reactor operating at the maximum gradient temperature indicates that imposition of a temperature profile is a useful means for enhancing the performance of a pre-existing reactor design by enabling (i) an increase in CO conversion without change in capacity, or (ii) an increase in reactor capacity with no reduction in CO conversion. Comparison of optimal designs against a reference case of an isothermal reactor operating at the minimum gradient demonstrates the volume-efficiency of the optimized reactor design; lastly, analysis indicates that for each gradient shape studied (linear, convex, Gaussian) there exists a range of viable operating conditions capable of achieving CO conversions in excess of the equilibrium conversion corresponding to the average temperature of the temperature profile.

Microflow Systems for Chemical Synthesis and Analysis: Approaches to Full Integration of Chemical Process

Abstract Integrated microchemical systems on microchips, which are based on continuous microflows, are expected to become important tools for analysis and chemical synthesis applications for biological sciences and technologies. For these purposes, general integration concepts have been developed, including microunit operations (MUOs) and continuous-flow chemical processing (CFCP) to create fully functional systems for various chemical processing applications. The general methodology has enabled analysis, synthesis, and fabrication of chemical systems on microchips, and these microsystems have demonstrated superior performance (e.g., rapid, simple, easy operation, and highly efficient processing) compared to conventional methodologies. Microchemical technology has now entered the phase of practical application. In this review, we discuss the methods for integration of continuous flow-based chemical process on microchips, relevant technologies, and applications.

Scale-out of Microreactor Stacks for Portable and Distributed Processing: Coupling of Exothermic and Endothermic Processes for Syngas Production

Computational fluid dynamics (CFD) simulations are used to simulate stacks of different sizes, to understand nonlinear effects that arise in scaleout of microchemical systems. As an example process, syngas production from methane is studied using a multifunctional, parallel-plate reactor with alternating combustion and steam reforming channels. A scale-out strategy is proposed that creates larger stacks from a base unit. Stacks are evaluated in terms of efficiency, maximum wall temperature, and stability under external heat loss for both high and moderate wall thermal conductivities. We find that smaller stacks are unstable under laboratory heat-loss conditions. Stacks with high wall thermal conductivities are more stable than those with moderate wall conductivities under our conditions. At high heat-loss coefficients, significant transverse thermal gradients exist between interior and edge channels of the stacks that result in a significant loss of efficiency. A transition from all ignited to some ignited and extinguished and finally to all extinguished channels is discovered as criticality is approached in moderate size stacks. Microsystems provide 1-3 orders of magnitude larger volumetric and gravimetric throughputs than conventional technology, irrespective of model uncertainty, and such intensification is central to portable and distributed processing. They exhibit energy efficiency that is a strong function of size and heat loss but can outperform conventional processing under many conditions. However, they result in higher cost per unit syngas volume unless system optimization is performed.

Numerical and experimental mixing studies in a MEMS-based multilaminated/elongational flow micromixer

Improvement of mixing quality in microchannel mixers or reactors has been recognized as a relevant technical issue critical to the development and application of integrated microchemical processing systems. Silicon micro- electro mechanical systems (MEMS) technology was successfully used to fabricate a novel multichannel micromixer. This improved micromixer design basically used the mechanisms of fluid multilamination, elongational flow, and geometric focusing for mixing enhancement. The fabricated triple-stack (Pyrex™/silicon/Pyrex™) multilaminated/elongational flow micromixer (herein referred to as MEFM-4) was evaluated for its mixing performance using residence time distribution (RTD) measure in conjunction with UV–vis absorption spectroscopy detection technique. Using a semi-empirical model and the so-called convolution–deconvolution theorem, a model description of the experimental RTD data was obtained for the flow/mixing unit. This result was compared with numerical RTD predictions based on computational fluid dynamics (CFD) simulations. The simulation results are in good agreement with the experimental data, especially in the low flow-rate range (Reynolds number <13 in this study). However, as expected, the accuracy of the CFD simulations is generally limited at higher flow rates (high Peclet number) because of unavoidable numerical diffusion. This paper describes the efficient design, fabrication and characterization of an effective microchannel mixer for microchemical systems’ applications.