Waveguide Reactor Research Articles

Miniature glass-metal coaxial waveguide reactors for microwave-assisted liquid heating

<abstract> <p>Microwave (MW) irradiation is recognized as an effective tool in industries related to pharmaceuticals, chemistry, nanoparticle synthesis, food, etc. In the hardware field, some research efforts are concentrated on creating miniature reactors using low-cost technologies aimed at on-demand chemistry or parallel synthesis of many drugs.</p> <p>This paper reports on the development and characterization of novel miniature chemical-resistant glass-metal coaxial reactors based on a modified Liebig condenser. It is composed of two concentric glass tubes, one for the central conductor carrying MW current, and the other for the copper-foiled cylinder surrounding the first pipe. The gap between them is filled with a liquid that is pumped and evacuated by using shielded thin inlet/outlet glass tubes, which are melted and opened into this cylindrical cavity. The reactor's geometry allows for the direct soldering of miniature MW SMA coaxial connectors of 50-Ω impedance.</p> <p>The developed components are studied analytically, numerically and experimentally. The frequency properties of reactors are measured with a network analyzer. The temperature trends are explored by using a variable high-power MW generator, power meters and temperature sensors.</p> <p>These reactors demonstrate their relative insensitivity toward variations in the permittivity of filling liquids in the range of $3.75 < \varepsilon < 30$, as shown in simulations and measurements. They demonstrate the increase by two orders in the longitudinal modal penetration depth and a more homogeneous heating along reactors as compared to their hollow coaxial prototypes.</p> <p>These glass-metal miniature reactors can be used in on-demand continuous-flow accelerated liquid heating, chemistry and pharmacy.</p> </abstract>

Study of microwave heating of reference liquids in a coaxial waveguide reactor using the experimental, semi-analytical and numerical means

In this work, a new application of coaxial waveguides for microwave heating of individual liquid substances and reaction mixtures is considered. The studied reactor is a small hollow coaxial waveguide in which liquids (methanol or ethanol) are exposed to microwaves generated by a miniature semiconductor source of 2.45-GHz frequency. The temperature of a liquid in the coaxial chamber is measured using an IR temperature sensor, thermocouple and optic fiber probes through non-radiating (in microwave range) openings in the thick copper reactor's wall, with the satisfactory accord between their readings at the medium-level power (10 W). The dynamics of microwave heating is studied using the semi-analytical and numerical simulations, which prove to be in good agreement with the experimental data. Two semi-analytical models of microwave heating in the coaxial waveguide as particular cases of the general electromagnetic-thermal problem are developed. One of them is the lumped system model in which uniformity of the temperature field is assumed. It gives good agreement with the measurements and is physically justified at late heating times. Another one is a simplified non-uniform heating model which is based on consideration of a convective turbulent boundary layer in a short horizontal coaxial waveguide. This model furnishes a quantitative explanation of the measured temperature difference at opposite ends of this waveguide over the entire heating period. The results of this study are interesting for development of miniature reactors for on-demand chemistry and pharmacy.

Stacked waveguide reactors with gradient embedded scatterers for high-capacity water cleaning.

We present a compact water-cleaning reactor with stacked layers of waveguides containing gradient patterns of optical scatterers that enable uniform light distribution and augmented water-cleaning rates. Previous photocatalytic reactors using immersion, external, or distributive lamps suffer from poor light distribution that impedes scalability. Here, we use an external UV-source to direct photons into stacked waveguide reactors where we scatter the photons uniformly over the length of the waveguide to thin films of TiO2-catalysts. We also show 4.5 times improvement in activity over uniform scatterer designs, demonstrate a degradation of 67% of the organic dye, and characterize the degradation rate constant.

Novel waveguide reactor design for enhancing algal biofilm growth

Algal film photobioreactors have the potential to reduce dewatering costs and have a comparable productivity to raceway ponds. Current designs have focused on maximizing productivity and nutrient removal through material selection and design. However, fewer studies have focused on analyzing the effects of illumination and carbon dioxide (CO2) concentration on algal film growth. The transport of light in suspended photobioreactors is considered a serious problem due to cell shading, light scattering and non-uniform optical absorption. To enhance light delivery to algal biofilms, five different waveguide designs were developed Light emitting waveguides are capable of distributing a bright light source over a larger surface area. The light emission from these waveguide designs was characterized and the algal biofilm growth on waveguides was characterized in a parallel plate airlift reactor (17L). The light intensity and CO2 concentration were varied in a 32 factorial design experiment to determine if there were any interaction effects. Light emission from the waveguides could be altered by changing the tapper angle or by notching the surfaces. Algal film growth kinetics on the waveguides were overall non-linear, but had linear regions of growth. The waveguides achieved an algal biofilm surface area productivity of 2.8g/m2day and an aerial productivity of 33.6g/m2day. Algal film productivity displayed saturation kinetics with respect to light intensity and CO2 concentration, but the interaction effect of light and CO2 on productivity is likely non-linear. Algal film biomass per photon consumed decreased with increasing light intensity when the reactor was not CO2 limited. The results indicate the potential for algal biofilms to be grown on light emitting waveguides which opens up the opportunity to explore new algal film photobioreactor configurations.

Microwave-induced temperature fields in cylindrical samples of graphite powder – Experimental and modeling studies

Microwave heating of graphite powder is the key process of a number of promising applications and emerging technologies ranging from kiln crucibles to synthesis of graphene. However, this process is typically studied in terms of structural parameters of the resulting products, whereas the prime electromagnetic and associated temperature characteristics remain insufficiently studied. In this paper, we investigate microwave heating of graphite powder in a batch (non-resonant) and waveguide (resonant) reactors using multiphysics (electromagnetic–thermal) modeling and experimentation. Temperature-dependent material parameters of graphite power are determined from measurements and appropriate physical models. Non-uniform temperature fields and their trend for relatively quick homogenization are shown to be conditioned by high values of the loss factor and thermal conductivity. These parameters are also found to be responsible for the effect of post-microwave heating. It is demonstrated that processing of graphite power in a resonant reactor may be convenient for controlling the heating rate and the level of temperature uniformity.

Microwave-assisted continuous flow synthesis on industrial scale

A continuous flow microwave system based on a transmission line short-circuited waveguide reactor concept is described. The continuous flow reactor is capable of operating in a genuine high-temperature/high-pressure process window (310°C/60 bar), applying a microwave transparent and chemical resistant cylindrical γ-Al 2 O 3 tube as reaction zone. The system can be operated in an extremely energy efficient manner, utilizing 0.6–6 kW microwave power (2.45 GHz). The application of the reactor for processing four chemical transformations in a high-temperature/high-pressure regime with a throughout of 3.5–6.0 l/h is demonstrated.

In situ SAXS and WAXS of zeolite microwave synthesis

In situ small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) studies were carried out during microwave and conventional synthesis of zeolites using a custom waveguide reactor. In situ SAXS studies showed that the crystallization of NaY zeolite was preceded by a reorganization of nano-sized particles in their precursor solutions or gels. In situ WAXS was used to observe the rate of formation of crystalline zeolite products.

In situ SAXS/WAXS of Zeolite Microwave Hydrothermal Synthesis

ABSTRACTThe microwave hydrothermal synthesis of silicalite zeolite was monitored in situ using small and wide angle X-ray scattering (SAXS and WAXS). A custom-built waveguide reactor provided the microwave heating at 2.45 GHz frequency. Microwave power up to 300 W was supplied by a SAIREM generator via a coax connection. A sample port in the waveguide section allowed a thin-walled glass sample vessel to be held in the microwave field. The sample vial was sealed with a fiber optic temperature probe used for monitoring the temperature. Silicalite zeolite precursor solutions were heated temperatures between 100 and 130°C for up to 120 minutes. The SAXS or WAXS patterns were simultaneously scanned during the reaction under microwaves via X-ray beam through slots in the side of the waveguide. The progress of silicalite zeolite formation was followed and the analysis of SAXS patterns showed the primary particles of 2 nm shifted to aggregates of 5.8 nm in diameter, in 120 minutes at 130°C.

Langmuir probe diagnostic studies of pulsed hydrogen plasmas in planar microwave reactors

Langmuir probe techniques have been used to study time and spatially resolved electron densities and electron temperatures in pulse-modulated hydrogen discharges in two different planar microwave reactors (fmicrowave=2.45 GHz, tpulse= 1 ms). The reactors are (i) a standing-wave radiative slotted waveguide reactor and (ii) a modified traveling-wave radiative slotted waveguide reactor, which generate relatively large plasmas over areas from about 350 to 500 cm2. The plasma properties of these reactor types are of particular interest as they have been used for basic research and for plasma processing; for example, for surface treatment and layer deposition. In the present study the pressures and microwave powers in the reactors were varied between 33 and 55 Pa and 600 and 3600 W, respectively. In regions with high electromagnetic fields, shielded Langmuir probes were used to avoid disturbances of the probe characteristic. Close to the microwave windows of the reactors both the electron density and the electron temperature showed strong inhomogeneities. In the standing-wave reactor the inhomogeneity was found to be spatially modulated by the position of the slots. The maximum value of the electron temperature was about 10 eV, and the electron density varied between 0.2 and 14×1011 cm−3. The steady-state electron temperature in a discharge pulse was reached within a few tens of microseconds, whereas the electron density needed some hundreds of microseconds to reach a steady state. Depending on the reactor the electron density reached a maximum between 80 and 200 μs after the beginning of the pulse.

Efficient multiphoton dissociation of CF2HCl in waveguide reactors with high selectivity

The feasibility of carbon-13 enrichment with high enrichment factors (> 102) in waveguide reactors (WGRs) is investigated using IR laser chemistry of neat CF2HCl. The performance of WGRs made of pyrex glass and gold-coated copper is evaluated under various experimental conditions and compared with that of a conventional cylindrical pyrex reactor under similar conditions. The decomposition efficiency is found to be several times better in the WGRs compared with that in the normal cell, while both the types of WGR preserve the dissociation selectivity well. The pyrex WGR seems to have an edge over the metallic version by virtue of its lower transmission losses.

Isotope selective IR multiphoton dissociation in an efficient laser waveguide reactor

Isotope selective IR multiphoton dissociation (MPD) was investigated in a simple Pyrex glass waveguide (WG) reactor. Important theoretical and experimental studies were performed concerning the excitation-dependent transmission properties of the WG with a transverse electric atmosphere (TEA) CO 2 laser. From these studies, the WG parameters can be selected for efficient MPD. Various important aspects such as laser beam quality excitation of various WG modes, beam coupling in the WG and wall-induced breakdown or reaction, etc. are discussed. The WG reactor was initially operated with well-known systems such as SF 6 and CF 3Br. The dissociation yield was several times larger than that obtained with a conventional cylindrical reactor, whereas the separation factor remained unchanged. Therefore the WG reactor can be used for isotope selective dissociation and the process parameters can be determined using fewer laser pulses even in a poor absorptive medium. Using this reactor, we report, for the first time T—D separation in CTF 3—CDF 3 mixtures by exciting at the wings of CTF 3 absorption. Optimization of these reactors will eventually lead to a better utilization of laser photons and the reactor may then be integrated into full-scale separation plants.

A waveguide reactor for IR multiphoton dissociation and its application to13C laser isotope separation

A waveguide reactor for infrared multiphoton dissociation reactions has been presented and applied to laser isotope separation of13C. The reactor is a sealed vessel containing a hollow waveguide for a CO2 laser, and the laser beam is condensed in the waveguide. The waveguide is a pipe of total-reflection glass or metal-coated glass with a funnelled inlet; it is 50 cm long with a 3 mm inner diameter. The isotope separation of13C has been made by multiphoton dissociation of CHF2Cl using a TEA CO2 laser. The result has shown that the waveguide reactor increases the dissociation yield by four to ten times as much as that without the waveguide at the pulse rate of up to 150 Hz, while the separation factor remains on the same level. It is also found that an optical breakdown which is induced at the wall surface plays a significant role to reduce the isotope selectivity.

Identification of dioxirane (H 2[formula omitted]) in ozone-olefin reactions via microwave spectroscopy

Dioxirane (H 2 COO ) has been identified as a reaction product in the low temperature reaction of ozone with ethylene. The identification comes as the result of a microwave experiment in which the oxonolysis reaction is carried out at −196°C in a waveguide reactor.

Acoustic Measurements in the EBR-II Reactor Primary Sodium Tank

Two sodium-immersed detectors, a microphone and an acoustic waveguide, have been developed, installed, and used for monitoring acoustic background of Experimental Breeder Reactor No. II (EBR-II). The microphone signal is free of structural resonances which appear in the signals from accelerometers mounted on the waveguide and various reactor components. Sound pressure level decreased by an order of magnitude when the primary sodium coolant flow was reduced from 100% to 50%. At full flow and independent of reactor power, the 1-millibar r.m.s. sound pressure as sensed by the sodium-immersed microphone had the characteristics of wideband noise over the frequency range 0.6 to 40 kHz.