High-temperature Microreactor Research Articles

Creep Fatigue Analysis of the 316SS Monolith in the MegaPower Heat Pipe Reactor

The heat pipe reactor represents a promising high-temperature microreactor design comprising heat pipes, fuel rods, and monoliths. Prolonged operation at elevated temperatures leads to an obvious thermal creep and thermal stress within the monolith. The monolith may have structural failure due to creep damage and fatigue damage caused by temperature fatigue load. This paper presents an analysis of the creep fatigue damage in the monolith of the MegaPower heat pipe reactor using the American Society of Mechanical Engineers (ASME), Boiler and Pressure Vessel Code Section III, Division 5 (BPVC Sec. III, Div. 5) inelastic design-by-analysis rules. The research findings demonstrate pronounced stress relaxation in the monolith caused by thermal creep, resulting in a redistribution of thermal stress. The region experiencing peak thermal stress within the monolith transitions from the thinnest web between the fuel rods to the edge of the monolith after 50 000 h of operation at full power. Thermal creep results in a 40.5% decrease in peak thermal stress and a 0.023% increase in the displacement amplitude of the monolith. The creep fatigue damage in the monolith at full power for 50 cycles, each lasting 1000 h, adheres to the design rule limitation of the ASME BPVC. The damage is primarily concentrated in the thinnest web region at the edge of the monolith, predominantly attributed to creep damage. The creep fatigue damage check in the monolith should carefully consider the effect of stress relaxation.

A Structural Design Approach Tailored for the Rapid Preliminary Design of Microreactor Components

High-temperature microreactors can play a role in developing reliable, portable energy sources for off-grid remote locations, microgrid concepts, and industrial process heat. Portability and passive safety criteria tend to skew microreactor structural component designs toward complex geometries, high thermal stresses, and design bases with large numbers of startup/shutdown cycles. Current design rules, as typified by Section III of the American Society of Mechanical Engineers (ASME) Boiler & Pressure Vessel Code, are less than optimal for these conditions, particularly for preliminary component designs where developers need to rapidly consider a large number of potential component configurations. This paper presents a design method targeted toward rapid, efficient evaluation of preliminary component designs using modern finite element analysis. The new method retains key connections with the ASME Code rules and design data while streamlining the design approach. This paper presents the design method, several verification examples illustrating the similarities and differences between the new method and the current ASME rules, and the application of the new approach to the evaluation of a test article mimicking key features of a heat pipe–cooled microreactor.

Ceramic composite moderators as replacements for graphite in high temperature microreactors

Engineered composites composed of a radiation stable continuous matrix containing a highly moderating entrained phase are attractive candidates for the realization of bulk materials that are structurally and neutronically superior to nuclear graphite. Here, we explore neutronics driven selection of entrained moderating phases in MgO-based ceramic composites with a focus on the MgO-BeO system given its exceptional moderating power and high temperature stability. Using lithium-bearing salts as sintering aids, fully dense MgO-BeO composites with BeO loading up to 40 vol.% are produced through direct current sintering at markedly reduced temperatures relative to phase-pure MgO. Thermophysical properties mapped as a function of the BeO concentration are shown to align with various composite models, thus revealing the influence of underlying defects on the thermophysical property trends. From microreactor neutronics and thermal hydraulic calculations, the MgO-40BeO moderator is shown to increase both cycle length and fuel utilization relative to graphite and with steady-state temperature distributions remaining within specification. The ceramic composite moderators outperform graphite for all metrics considered with significant potential demonstrated for reducing energy costs while enabling novel microreactor designs through the replacement of graphite.

Kinetics on 1,3-butadiene dimerization by the cycloaddition reactions in high pressure and high temperature micro-reactor

The kinetics of 1,3-butadiene dimerization based on the cycloaddition reactions including Diels-Alder reaction under high temperature and high pressure were investigated in a microchannel reactor. Our findings suggest that the butadiene dimerization produced 4-vinyl-cyclohexene (VCH) and 1,5-cyclooctadiene (COD). The results indicate that COD can only be formed under high temperature conditions. The apparent kinetic parameters of butadiene dimerization under high pressure were obtained by fitting experimental data. As the reaction pressure increased, the reaction rate in the solution was found to increase slightly. Compared with the gas phase dimerization of butadiene in the literature, it can be concluded that the reaction rate constants in the liquid phase are greater than that in the gas phase. To our best knowledge, this is the first work reporting the kinetic parameters of butadiene dimerization with the radical polymerization excluded under high pressure liquid phase condition.

Monolithically Integrated Diffused Silicon Two-Zone Heaters for Silicon-Pyrex Glass Microreactors for Production of Nanoparticles: Heat Exchange Aspects.

We present the design, simulation, fabrication and characterization of monolithically integrated high resistivity p-type boron-diffused silicon two-zone heaters in a model high temperature microreactor intended for nanoparticle fabrication. We used a finite element method for simulations of the heaters’ operation and performance. Our experimental model reactor structure consisted of a silicon wafer anodically bonded to a Pyrex glass wafer with an isotropically etched serpentine microchannels network. We fabricated two separate spiral heaters with different temperatures, mutually thermally isolated by barrier apertures etched throughout the silicon wafer. The heaters were characterized by electric measurements and by infrared thermal vision. The obtained results show that our proposed procedure for the heater fabrication is robust, stable and controllable, with a decreased sensitivity to random variations of fabrication process parameters. Compared to metallic or polysilicon heaters typically integrated into microreactors, our approach offers improved control over heater characteristics through adjustment of the Boron doping level and profile. Our microreactor is intended to produce titanium dioxide nanoparticles, but it could be also used to fabricate nanoparticles in different materials as well, with various parameters and geometries. Our method can be generally applied to other high-temperature microsystems.

Energy and Resource Efficient Production of Fluoroalkenes in High Temperature Microreactors

Tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) are the most common monomers for the synthesis of fluoropolymers at industrial scale. Currently, TFE is produced via multistep pyrolysis of chlorodifluoromethane (R22), resulting in a high energy demand and high amounts of waste acids, mainly HCl and HF. In this study, a new chlorine-free process for producing TFE and HFP in a microreactor is presented, starting from partially fluorinated alkanes obtained from electrochemical fluorination (ECF). In the microreactor, high conversion rates of CHF3, which is used as a surrogate of partly fluorinated ECF streams, and high yields of fluoromonomers could be achieved. The energy saving and the environmental impact are shown by a life cycle assessment (LCA). The LCA confirms that the developed process has economical as well as ecological benefits, and is thus an interesting option for future industrial production of fluoroalkenes.

Functional Relationships between Kinetic, Flow, and Geometrical Parameters in a High-Temperature Chemical Microreactor.

Computational fluid dynamics (CFD) simulations and isothermal approximation were applied for the interpretation of experimental measurements of the C10H7Br pyrolysis efficiency in the high-temperature microreactor and of the pressure drop in the flow tube of the reactor. Applying isothermal approximation allows the derivation of analytical relationships between the kinetic, gas flow, and geometrical parameters of the microreactor, which, along with CFD simulations, accurately predict the experimental observations. On the basis of the obtained analytical relationships, a clear strategy for measuring rate coefficients of (pseudo) first-order bimolecular and unimolecular reactions using the microreactor was proposed. The pressure- and temperature-dependent rate coefficients for the C10H7Br pyrolysis calculated using variable reaction coordinate transition state theory were invoked to interpret the experimental data on the pyrolysis efficiency.

Meso-scale ceramic hotplates – A playground for high temperature microsystems

This paper presents a meso-scale low-temperature co-fired ceramic (LTCC) hotplate system. Using embedded thick-film platinum heaters, the hotplate has been able to operate at about 600°C in a reliable manner, while its operating temperature can be regulated using an automation setup. The capability of structuring in LTCC materials made it possible to create an efficient thermal decoupling to have two temperature zones in the device, an active heating area and a cold-zone (<100°C), which becomes greatly beneficial to the integration of standard fluidic and electrical interconnections. A heat dissipation model has been developed for understanding thermal characteristics of the LTCC hotplate system, while the reliability tests have shown robust mechanical structure of the device as well as durable performance of the thick-film heaters after 70 thermal cycles up to 600°C. Moreover, potential applications in the use of the LTCC hotplate have been demonstrated, including a high-temperature microreactor for a highly efficient hydrogen production and a miniaturized thermal transpiration gas pump.

Fine structuration of low-temperature co-fired ceramic (LTCC) microreactors.

The development of microreactors that operate under harsh conditions is always of great interest for many applications. Here we present a microfabrication process based on low-temperature co-fired ceramic (LTCC) technology for producing microreactors which are able to perform chemical processes at elevated temperature (>400 °C) and against concentrated harsh chemicals such as sodium hydroxide, sulfuric acid and hydrochloric acid. Various micro-scale cavities and/or fluidic channels were successfully fabricated in these microreactors using a set of combined and optimized LTCC manufacturing processes. Among them, it has been found that laser micromachining and multi-step low-pressure lamination are particularly critical to the fabrication and quality of these microreactors. Demonstration of LTCC microreactors with various embedded fluidic structures is illustrated with a number of examples, including micro-mixers for studies of exothermic reactions, multiple-injection microreactors for ionone production, and high-temperature microreactors for portable hydrogen generation.

Continuous Flow Synthesis of n-Alkyl Chlorides in a High-Temperature Microreactor Environment

Applying continuous flow processing in a high-temperature/high-pressure regime, n-alkyl chlorides can be prepared in high yields and selectivity by direct uncatalyzed chlorodehydroxylation of the corresponding n-alcohols with 30% aqueous hydrochloric acid. Optimum conditions for the preparation of n-butyl and n-hexyl chloride involve the use of a glass microreactor chip, a reaction temperature of 160–180 °C (20 bar backpressure) and a residence time of 15 min.

Microfabrication of high temperature micro-reactors for continuous flow isotope ratio mass spectrometry

Robust, high temperature micro-reactors for on- line conversion of organic compounds were microfabricated in high purity fused silica to enable stable isotopic compo- sitional analysis of individual compounds in mixtures using advanced gas chromatography (GC) separation techniques, such as fast GC and comprehensive 2D GC, coupled to isotope ratio mass spectrometry (IRMS). These micro- reactors could also be manufactured at larger channel dimensions to enable robust operations for normal GCC- IRMS applications. Photolithography was used to define the reactor channel pattern on high purity fused silica, with a protective layer of amorphous silicon. A two-step isotropic wet etching process, using 49% HF, selectively created semi-circular cross-section micro-channels of arbitrary diameters (56-209 lm), with tapered sections leading to input/output ports ((400 lm) that accept fused silica cap- illary tubing used in GC and MS peripherals. Pairs of sym- metric mirror image substrates were aligned and bonded to form enclosed circular channels. The resulting micro-reac- tors are more robust than the standard designs made of fragile-fused silica capillary or alumina tubes of relatively large bore, and are gas tight at temperatures up to 1,000C. Fast GC plugs of CH4 with and without the reactor revealed that peak shapes are minimally affected by the micro-reactor when carrier flow rate and channel diameter are optimized. Peak shapes with full widths at half maximum of 250 ms are shown for plugs of CH4 through a fast-GC-combustion- IRMS system interfaced with a micro-reactor containing a CuO/NiO combustion source, enabling carbon iso- tope ratio measurements of CH4 with a precision of SD(d 13 C) =± 0.28%. The devices enable arbitrarily nar- row bores and long path channels that can be operated as open tubes or loaded with a reactant, in a small, robust package.

テンプレート消失法を介してのセラミック前駆体ポリマーからの SiC 基セラミック微細構造体の製作 (総説)

Processes for fabricating ceramic microstructures are in demand for use in areas such as microreactors and MEMS that can be used in harsh environments requiring a high temperatures tolerance, corrosion resistance, as well as tribological properties. This review describes the manufacture of tailored porous SiC ceramic microstructures using sacrificial templates, the feasible applications of microporous structures and microchannels as a microreactor, and microfabrication of preceramic polymers using soft lithography and sterolithography. Macroporous and mesoporous SiC ceramic structures have been fabricated using various templates including packed silica sphere assemblies, porous carbon templates, and nanoporous silica structures. In addition, SiC nanotubes have also been obtained from an alumina membrane used as a template. These porous structures were generally obtained using a series of infiltration, curing and pyrolysis, and chemical or oxidative etching steps. Complicated SiC ceramic microfeatures were fabricated using near-net shape lithographic techniques to preceramic polymers. Line patterns and porous channels were produced by soft lithography, whereas the 3D woodpile structures were fabricated using stereolithography. These novel structures show great promises for use in high temperature micro-reactors for the on-demand reforming of higher hydrocarbons into hydrogen in portable power sources. It is expected that the integration of preceramic polymers into new economic manufacturing strategies such as precursor casting in templates and lithography utilization will be a break-through technology for the creation of complex ceramic nanostructures.

Fabrication and characterization of high-temperature microreactors with thin film heater and sensor patterns in silicon nitride tubes

In this paper the fabrication and electrical characterization of a silicon microreactor for high-temperature catalytic gas phase reactions, like Rh-catalyzed catalytic partial oxidation of methane into synthesis gas, is presented. The microreactor, realized with micromachining technologies, contains silicon nitride tubes that are suspended in a flow channel. These tubes contain metal thin films that heat the gas mixture in the channel and sense its temperature. The metal patterns are defined by using the channel geometry as a shadow mask. Furthermore, a new method to obtain Pt thin films with good adhesive properties, also at elevated temperatures, without adhesion metal is implemented in the fabrication process. Based on different experiments, it is concluded that the electrical behaviour at high temperatures of Pt thin films without adhesion layer is better than that of Pt/Ta films. Furthermore, it is found that the temperature coefficient of resistance (TCR) and the resistivity of the thin films are stable for up to tens of hours when the temperature-range during operation of the microreactor is below the so-called "burn-in" temperature. Experiments showed that the presented suspended-tube microreactors with heaters and temperature sensors of Pt thin films can be operated safely and in a stable way at temperatures up to 700 degrees C for over 20 h. This type of microreactor solves the electrical breakdown problem that was previously reported by us in flat-membrane microreactors that were operated at temperatures above 600 degrees C.

Fabrication of a high-temperature microreactor with integrated heater and sensor patterns on an ultrathin silicon membrane

In this paper critical steps in the fabrication process of a microreactor for high-temperature catalytic partial oxidation gas phase reactions are evaluated. The microreactor contains a flow channel etched in silicon, capped with an ultrathin composite membrane consisting of silicon and silicon nitride layers, on which on the topside thin-film heaters and sensors, and on the other side a thin-film catalyst patch are placed. The membrane is designed to have specific heat conductivity and mechanical properties. The paper focuses on three fabrication issues: definition and etching of sub-micron uniform single-crystalline silicon membranes, deposition of well-defined heater structures and temperature sensors on a thin composite membrane, and deposition of well-defined catalytic patches on the same membrane. For the latter two processes novel micromachined shadow masks were developed. Preliminary experiments on the controlled oxidation of hydrogen gas in the explosive regime are discussed, which experiments confirm that heat management in the microreactor is excellent.

Micromachined reactors for catalytic partial oxidation reactions

AbstractSilicon‐based microfabrication of a novel chemical reactor (microreactor) having submillimeter flow channels with integrated heaters, and flow and temperature sensors is described. The potential application of this reactor to partial‐oxidation reactions is explored by using Pt‐catalyzed NH3 oxidation as a model reaction. Investigation of reactor behavior as a function of operating conditions shows that conversion and selectivity behavior of conventional laboratory reactors can be reproduced and demonstrates the feasibility of conducting chemical reactions in microfabricated systems. Ignition‐extinction behavior is explored, along with high‐temperature microreactor materials degradation. Potential applications and scale‐up of microreactors are also discussed.