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• 1. [Article] Microchannel steam-methane reforming under constant and variable surface temperature distributions

  Steam-methane reforming is a well understood industrial process used for generating hydrogen and synthesis gas. The reaction is generally carried out with residence times on the order of one second. ...

  Citation × Citation Citation Abstract Copy Title: Microchannel steam-methane reforming under constant and variable surface temperature distributions Author: Eilers, Benn J. Steam-methane reforming is a well understood industrial process used for generating hydrogen and synthesis gas. The reaction is generally carried out with residence times on the order of one second. By performing this reaction at microscales it is possible to take advantage of increased heat transfer rates and low diffusion times allowing equipment size and residence times to be decreased by an order of magnitude. The energy required for the steam-methane reforming reaction could be supplied by solar energy through the use of a solar collector/receiver, thus providing a "cleaner" pathway for hydrogen generation. In such reactors, it is expected that the heat flux and temperature distribution is non-uniform. This study presents a first step in characterizing reforming performance under such variable temperature conditions. The design of the channel was such that the non-uniform heat flux profile expected inside of a solar receiver could be simulated by a controllable temperature profile along the reactor surface. Experiments were conducted over four catalyst configurations and the effects of average reactor temperature, temperature distribution, residence time, steam-methane ratio, and long duration testing were all evaluated. Results demonstrated over 60% conversion of methane at 900°C and a residence time of 26 milliseconds. Methane conversion was found to be strongly dependent on reactor temperature. Ramping temperature distributions demonstrated a 46% greater hydrogen output than isothermal reactions performed at the same average temperature. This work was performed in conjunction with development of a CFD model of reacting flow through a microchannel [1]. The experiments were used to determine the pre-exponential constants of the global reaction rates. Title: Microchannel steam-methane reforming under constant and variable surface temperature distributions Author: Eilers, Benn J. Steam-methane reforming is a well understood industrial process used for generating hydrogen and synthesis gas. The reaction is generally carried out with residence times on the order of one second. By performing this reaction at microscales it is possible to take advantage of increased heat transfer rates and low diffusion times allowing equipment size and residence times to be decreased by an order of magnitude. The energy required for the steam-methane reforming reaction could be supplied by solar energy through the use of a solar collector/receiver, thus providing a "cleaner" pathway for hydrogen generation. In such reactors, it is expected that the heat flux and temperature distribution is non-uniform. This study presents a first step in characterizing reforming performance under such variable temperature conditions. The design of the channel was such that the non-uniform heat flux profile expected inside of a solar receiver could be simulated by a controllable temperature profile along the reactor surface. Experiments were conducted over four catalyst configurations and the effects of average reactor temperature, temperature distribution, residence time, steam-methane ratio, and long duration testing were all evaluated. Results demonstrated over 60% conversion of methane at 900°C and a residence time of 26 milliseconds. Methane conversion was found to be strongly dependent on reactor temperature. Ramping temperature distributions demonstrated a 46% greater hydrogen output than isothermal reactions performed at the same average temperature. This work was performed in conjunction with development of a CFD model of reacting flow through a microchannel [1]. The experiments were used to determine the pre-exponential constants of the global reaction rates. Close

• 2. [Article] A technique for spatially resolved wall temperature measurements in microchannel heat sinks using infrared thermography

  A non-intrusive measurement technique for direct quantitative thermal visualization of channel wall temperatures in two phase microchannel flows using infrared thermography (IRT) is presented. Specifically, ...

  Citation × Citation Citation Abstract Copy Title: A technique for spatially resolved wall temperature measurements in microchannel heat sinks using infrared thermography Author: Krebs, Daniel P. A non-intrusive measurement technique for direct quantitative thermal visualization of channel wall temperatures in two phase microchannel flows using infrared thermography (IRT) is presented. Specifically, the measurement of top channel wall temperatures in a fractal-like branching microchannel silicon heat sink during flow boiling is demonstrated and thoroughly documented. Obtaining quantitative local temperature measurements involving IRT poses significant challenges and requires careful consideration with regard to heat sink design, fabrication and calibration as well as data acquisition, reduction and analysis procedures -- all of which are addressed and discussed in detail. Also discussed and rigorously quantified are the potentially significant measurement uncertainties associated with obtaining non-intrusive, local data of this nature. Results indicate that temperature maps of the microchannel top wall during flow boiling can be obtained with a spatial resolution of 10 μm and an uncertainty varying from 0.82°C-1.67°C at 25 °C and 0.96°C-2.80°C. Title: A technique for spatially resolved wall temperature measurements in microchannel heat sinks using infrared thermography Author: Krebs, Daniel P. A non-intrusive measurement technique for direct quantitative thermal visualization of channel wall temperatures in two phase microchannel flows using infrared thermography (IRT) is presented. Specifically, the measurement of top channel wall temperatures in a fractal-like branching microchannel silicon heat sink during flow boiling is demonstrated and thoroughly documented. Obtaining quantitative local temperature measurements involving IRT poses significant challenges and requires careful consideration with regard to heat sink design, fabrication and calibration as well as data acquisition, reduction and analysis procedures -- all of which are addressed and discussed in detail. Also discussed and rigorously quantified are the potentially significant measurement uncertainties associated with obtaining non-intrusive, local data of this nature. Results indicate that temperature maps of the microchannel top wall during flow boiling can be obtained with a spatial resolution of 10 μm and an uncertainty varying from 0.82°C-1.67°C at 25 °C and 0.96°C-2.80°C. Close

• 3. [Article] Intermetallic microlamination for high-temperature microreactors

  Chemical microreactors offer opportunities for portable power generation, on-site waste remediation and point-of-use chemical synthesis. Much of the existing development of microreactor devices involves ...

  Citation × Citation Citation Abstract Copy Title: Intermetallic microlamination for high-temperature microreactors Author: Dewey, Tyler Chemical microreactors offer opportunities for portable power generation, on-site waste remediation and point-of-use chemical synthesis. Much of the existing development of microreactor devices involves silicon-based microfabrication techniques. It is recognized that new refractory materials are important to realizing high-temperature microreactors. Requirements of these materials include high-temperature resistance, chemical inertness and low-cost microfabrication. Advances in multilayer ceramics hold promise for the fabrication of microreactor structures from ceramic tape. Problems include creep, moderate levels of densification, and volumetric shrinkage, all of which can lead to dimensional instability. Intermetallics are another class of refractory materials which may hold some promise for high-temperature microreactor development. In this paper, a new method of forming microchannel arrays from thin layers of intermetallics is demonstrated. This method has the advantage of eliminating volumetric shrinkage due to binder removal. Various iterations of NiAl intermetallic conversion and bonding are presented. Results show that the NiAl system may be suitable as a substrate for microchannel reactor designs. Title: Intermetallic microlamination for high-temperature microreactors Author: Dewey, Tyler Chemical microreactors offer opportunities for portable power generation, on-site waste remediation and point-of-use chemical synthesis. Much of the existing development of microreactor devices involves silicon-based microfabrication techniques. It is recognized that new refractory materials are important to realizing high-temperature microreactors. Requirements of these materials include high-temperature resistance, chemical inertness and low-cost microfabrication. Advances in multilayer ceramics hold promise for the fabrication of microreactor structures from ceramic tape. Problems include creep, moderate levels of densification, and volumetric shrinkage, all of which can lead to dimensional instability. Intermetallics are another class of refractory materials which may hold some promise for high-temperature microreactor development. In this paper, a new method of forming microchannel arrays from thin layers of intermetallics is demonstrated. This method has the advantage of eliminating volumetric shrinkage due to binder removal. Various iterations of NiAl intermetallic conversion and bonding are presented. Results show that the NiAl system may be suitable as a substrate for microchannel reactor designs. Close

• 4. [Article] Experimental Characterization of a Supercritical Carbon Dioxide Microchannel Solar Thermal Receiver

  Characterization of a microchannel solar thermal receiver for supercritical carbon dioxide (sCO₂) is presented. The receiver design is based on conjugate computational fluid dynamics and heat transfer ...

  Citation × Citation Citation Abstract Copy Title: Experimental Characterization of a Supercritical Carbon Dioxide Microchannel Solar Thermal Receiver Author: L'Estrange, Thomas Characterization of a microchannel solar thermal receiver for supercritical carbon dioxide (sCO₂) is presented. The receiver design is based on conjugate computational fluid dynamics and heat transfer simulations as well as thermo-mechanical stress analysis. Two lab scale receivers with a 2x2 cm² absorber area are fabricated and experimentally characterized - a parallel microchannel design and a microscale pin fin array design. Static pressure experiments are used to demonstrate receiver integrity at the design pressure of 125 bar at 750°C surface temperature. A concentrated solar simulator was designed and assembled to characterize the thermal performance of the lab scale receiver test articles. Results indicate that, for a fixed exit fluid temperature of 650°C, increase in incident heat flux results in an increase in receiver and thermal efficiency. At a fixed heat flux, efficiency decreases with an increase in receiver surface temperature. The ability to absorb flux of up to 100 W/cm² at thermal efficiency in excess of 90 percent and exit fluid temperature of 650°C using the microchannel receiver is demonstrated. Pressure drop for the pin array at the maximum flow rate for heat transfer experiments is less than 0.64 percent of typical line pressure of 200 bar. Following the successful test of the 2 x 2 cm lab-scale receiver, a larger 8x8 cm² integrated receiver was designed and fabricated. The integrated receiver is shown to withstand 200 bar of static pressure at 600°C. Furthermore, infrared imaging is used to glean the flow distribution from the headers into the microscale region of the receiver. While the integrated receiver could not be tested in the solar simulator due to limitations of the facility, receiver scale-up from 2 x 2 cm to 8 x 8 cm with more effective material usage is demonstrated. Title: Experimental Characterization of a Supercritical Carbon Dioxide Microchannel Solar Thermal Receiver Author: L'Estrange, Thomas Characterization of a microchannel solar thermal receiver for supercritical carbon dioxide (sCO₂) is presented. The receiver design is based on conjugate computational fluid dynamics and heat transfer simulations as well as thermo-mechanical stress analysis. Two lab scale receivers with a 2x2 cm² absorber area are fabricated and experimentally characterized - a parallel microchannel design and a microscale pin fin array design. Static pressure experiments are used to demonstrate receiver integrity at the design pressure of 125 bar at 750°C surface temperature. A concentrated solar simulator was designed and assembled to characterize the thermal performance of the lab scale receiver test articles. Results indicate that, for a fixed exit fluid temperature of 650°C, increase in incident heat flux results in an increase in receiver and thermal efficiency. At a fixed heat flux, efficiency decreases with an increase in receiver surface temperature. The ability to absorb flux of up to 100 W/cm² at thermal efficiency in excess of 90 percent and exit fluid temperature of 650°C using the microchannel receiver is demonstrated. Pressure drop for the pin array at the maximum flow rate for heat transfer experiments is less than 0.64 percent of typical line pressure of 200 bar. Following the successful test of the 2 x 2 cm lab-scale receiver, a larger 8x8 cm² integrated receiver was designed and fabricated. The integrated receiver is shown to withstand 200 bar of static pressure at 600°C. Furthermore, infrared imaging is used to glean the flow distribution from the headers into the microscale region of the receiver. While the integrated receiver could not be tested in the solar simulator due to limitations of the facility, receiver scale-up from 2 x 2 cm to 8 x 8 cm with more effective material usage is demonstrated. Close

• 5. [Article] Performances and kinetic studies of hydrotreating of bio-oils in microreactor

  Hydrotreating reaction of vegetable oil is an alternative method for the production of renewable biodiesel fuel. This reaction involves conversion of triglycerides into normal alkanes, leads to a deoxygenated ...

  Citation × Citation Citation Abstract Copy Title: Performances and kinetic studies of hydrotreating of bio-oils in microreactor Author: Attanatho, Lalita Hydrotreating reaction of vegetable oil is an alternative method for the production of renewable biodiesel fuel. This reaction involves conversion of triglycerides into normal alkanes, leads to a deoxygenated and stable product, which is fully compatible with petroleum derived diesel fuel. The hydrotreating process uses hydrogen to remove oxygen from triglyceride molecules at elevated temperature in the presence of a solid catalyst. This work focused on the development of microtechnology-based chemical reaction process for liquid biofuel production from oil-based biofuel feedstock. A hydrotreating reaction of oleic acid and triolein as model compounds and jatropha oil as real feedstock was studied in a continuous flow microchannel reactor of inner diameter 500 μm and of varied length; 1.5 - 5 m. The microchannel reactor was fabricated from SS-316. The walls of the microreactor were coated with a thin Al₂O₃ film, which was then impregnated with Ni-Mo catalyst containing phosphorus as promoter. The reactions were carried out in the temperature range of 275-325 °C, residence time in the range of 11-40 s and at constant system pressure of 500 psig. The results showed that the microchannel reactor was suitable for the hydrotreating process. Complete conversion of the fatty acid hydrotreating reaction was achieved at a reaction temperature of 325 °C. Hydrotreating of fatty acids occurred primarily via hydrodeoxygenation and the main liquid products were octadecane and heptadecane. Fatty alcohol, fatty acid and long chain esters were formed as reaction intermediates. Hydrotreating of triglycerides proceeded via the hydrocracking of triglycerides into diglycerides, monoglycerides and fatty acids. Then fatty acids were subsequently deoxygenated to hydrocarbons. The conversion of fatty acids and triglycerides increased with increasing temperatures. A detailed mathematical model was developed to represent this two-phase chemical reaction process. The mathematical model was entirely based on first principles, i.e. no adjustable or correlation parameters were used. Kinetic parameter estimation was performed and the predicted results were in good agreement with experimental results. Title: Performances and kinetic studies of hydrotreating of bio-oils in microreactor Author: Attanatho, Lalita Hydrotreating reaction of vegetable oil is an alternative method for the production of renewable biodiesel fuel. This reaction involves conversion of triglycerides into normal alkanes, leads to a deoxygenated and stable product, which is fully compatible with petroleum derived diesel fuel. The hydrotreating process uses hydrogen to remove oxygen from triglyceride molecules at elevated temperature in the presence of a solid catalyst. This work focused on the development of microtechnology-based chemical reaction process for liquid biofuel production from oil-based biofuel feedstock. A hydrotreating reaction of oleic acid and triolein as model compounds and jatropha oil as real feedstock was studied in a continuous flow microchannel reactor of inner diameter 500 μm and of varied length; 1.5 - 5 m. The microchannel reactor was fabricated from SS-316. The walls of the microreactor were coated with a thin Al₂O₃ film, which was then impregnated with Ni-Mo catalyst containing phosphorus as promoter. The reactions were carried out in the temperature range of 275-325 °C, residence time in the range of 11-40 s and at constant system pressure of 500 psig. The results showed that the microchannel reactor was suitable for the hydrotreating process. Complete conversion of the fatty acid hydrotreating reaction was achieved at a reaction temperature of 325 °C. Hydrotreating of fatty acids occurred primarily via hydrodeoxygenation and the main liquid products were octadecane and heptadecane. Fatty alcohol, fatty acid and long chain esters were formed as reaction intermediates. Hydrotreating of triglycerides proceeded via the hydrocracking of triglycerides into diglycerides, monoglycerides and fatty acids. Then fatty acids were subsequently deoxygenated to hydrocarbons. The conversion of fatty acids and triglycerides increased with increasing temperatures. A detailed mathematical model was developed to represent this two-phase chemical reaction process. The mathematical model was entirely based on first principles, i.e. no adjustable or correlation parameters were used. Kinetic parameter estimation was performed and the predicted results were in good agreement with experimental results. Close

• 6. [Article] Deep desulfurization of diesel fuel using a single phase photochemical microreactor

  There is an urgent need to lower the concentration of sulfur in diesel fuels. Growing concern over environmental effects caused by burning sulfur-containing diesel has led the world to higher standards ...

  Citation × Citation Citation Abstract Copy Title: Deep desulfurization of diesel fuel using a single phase photochemical microreactor Author: Jones, Jacob Aaron There is an urgent need to lower the concentration of sulfur in diesel fuels. Growing concern over environmental effects caused by burning sulfur-containing diesel has led the world to higher standards in fuel refinement. Because of these lower standards, many techniques have been researched to remove sulfur containing compounds or otherwise reduce the sulfur content. Hydrodesulfurization remains the primary method to reduce most light sulfur-containing compounds. This process removes sulfides, sulfates, and thiols using high temperature and pressure reactions but is unable to remove aromatics and long chains containing sulfur molecules. Biodesulfurization has also been considered, utilizing microorganisms to target specific sulfur-containing compounds to remove the sulfur while leaving the high energy fuel intact. Oxidative reactions have also been considered including photocatalytic oxidation in an attempt to oxidize these aromatic compounds so that they may be removed using a polar solvent. An ultraviolet light assisted oxidative reaction occurring in a microreactor was studied. Dibenzothiophene was used as a model refractory organic compound mixed in Decane as a solvent. Dibenzothiophene undergoes a series of oxidative reactions which produce an intermediate, Dibenzothiophene Sulfoxide, and a final product, Dibenzothiophene Sulfone. This reaction only proceeds in the presence of Tert-Butyl Hydroperoxide and ultraviolet light. These reactions are reversible and an equilibrium is established between Dibenzothiophene, Dibenzothiophene Sulfoxide, and Dibenzothiophene Sulfone. The equilibrium is strongly affected by the molar ratios of the reactants. Increasing the molar ratio of Tert-Butyl Hydroperoxide to Dibenzothiophene reduces the concentration of Dibenzothiophene in the product stream or causes the reaction to shift its equilibrium towards the products. Equilibrium may not be affected by Temperature. Previous studies have shown that equilibrium concentrations have a temperature dependence. However, under the conditions of this research there was no indication of a temperature dependence. It is possible that because of the molar ratios used the reaction did not proceed appreciably regardless of temperature so the temperature dependence was not apparent. A Taylor dispersion apparatus was assembled to measure infinite dilution diffusion coefficients for Dibenzothiophene, Dibenzothiophene Sulfoxide, Dibenzothiophene Sulfone, and Tert-Butyl Hydroperoxide were measured. Values were close to those calculated using the Wilke-Chang equation. These coefficients along with measured absorption coefficients were important in developing an accurate mathematical model. A mathematical model was developed to include convective and diffusive flux, fluid transport, and reaction kinetics. COMSOL™ Multiphysics was used to numerically solve the mathematical model. Experimental data is fitted to the model to determine the reaction rate constants for each of the reversible reactions. The model was compared to previously reported data taken at 22°C and 40°C using a reactor thickness of 50 um and 100um. The model fit the data very well and can be implemented in predicting concentrations within the reactor. Title: Deep desulfurization of diesel fuel using a single phase photochemical microreactor Author: Jones, Jacob Aaron There is an urgent need to lower the concentration of sulfur in diesel fuels. Growing concern over environmental effects caused by burning sulfur-containing diesel has led the world to higher standards in fuel refinement. Because of these lower standards, many techniques have been researched to remove sulfur containing compounds or otherwise reduce the sulfur content. Hydrodesulfurization remains the primary method to reduce most light sulfur-containing compounds. This process removes sulfides, sulfates, and thiols using high temperature and pressure reactions but is unable to remove aromatics and long chains containing sulfur molecules. Biodesulfurization has also been considered, utilizing microorganisms to target specific sulfur-containing compounds to remove the sulfur while leaving the high energy fuel intact. Oxidative reactions have also been considered including photocatalytic oxidation in an attempt to oxidize these aromatic compounds so that they may be removed using a polar solvent. An ultraviolet light assisted oxidative reaction occurring in a microreactor was studied. Dibenzothiophene was used as a model refractory organic compound mixed in Decane as a solvent. Dibenzothiophene undergoes a series of oxidative reactions which produce an intermediate, Dibenzothiophene Sulfoxide, and a final product, Dibenzothiophene Sulfone. This reaction only proceeds in the presence of Tert-Butyl Hydroperoxide and ultraviolet light. These reactions are reversible and an equilibrium is established between Dibenzothiophene, Dibenzothiophene Sulfoxide, and Dibenzothiophene Sulfone. The equilibrium is strongly affected by the molar ratios of the reactants. Increasing the molar ratio of Tert-Butyl Hydroperoxide to Dibenzothiophene reduces the concentration of Dibenzothiophene in the product stream or causes the reaction to shift its equilibrium towards the products. Equilibrium may not be affected by Temperature. Previous studies have shown that equilibrium concentrations have a temperature dependence. However, under the conditions of this research there was no indication of a temperature dependence. It is possible that because of the molar ratios used the reaction did not proceed appreciably regardless of temperature so the temperature dependence was not apparent. A Taylor dispersion apparatus was assembled to measure infinite dilution diffusion coefficients for Dibenzothiophene, Dibenzothiophene Sulfoxide, Dibenzothiophene Sulfone, and Tert-Butyl Hydroperoxide were measured. Values were close to those calculated using the Wilke-Chang equation. These coefficients along with measured absorption coefficients were important in developing an accurate mathematical model. A mathematical model was developed to include convective and diffusive flux, fluid transport, and reaction kinetics. COMSOL™ Multiphysics was used to numerically solve the mathematical model. Experimental data is fitted to the model to determine the reaction rate constants for each of the reversible reactions. The model was compared to previously reported data taken at 22°C and 40°C using a reactor thickness of 50 um and 100um. The model fit the data very well and can be implemented in predicting concentrations within the reactor. Close

• 7. [Article] Oxidative desulfurization of dibenzothiophene with tert-butyl hydro peroxide in a photochemical micro-reactor

  Sulfur content in fuels is an increasingly critical environmental issue. Hydrodesulfurization removes sulfur from hydrocarbons; however, further desulfurization is necessary in fuels. New methods are required. ...

  Citation × Citation Citation Abstract Copy Title: Oxidative desulfurization of dibenzothiophene with tert-butyl hydro peroxide in a photochemical micro-reactor Author: Hebert, Eilleen M. Sulfur content in fuels is an increasingly critical environmental issue. Hydrodesulfurization removes sulfur from hydrocarbons; however, further desulfurization is necessary in fuels. New methods are required. In this study, micro-scaled single phase oxidative desulfurization is explored. An ultraviolet light-assisted micro-reactor is introduced for desulfurization of dibenzothiophene. Dibenzothiophene is mixed with decane and oxidant, tert-butyl hydro peroxide. The mixture forms a thin layer inside the micro-reactor which has a rectangular window to facilitate ultraviolet light irradiation. Thin spacers, 100 μm and 50 μm, establish fluid thickness within the micro-channel. Ultraviolet light reacts with oxidant to create hydroxyl radicals and convert dibenzothiophene to sulfoxides and sulfones. These products are easily extracted. The overall desulfurization process is a pseudo first-order reaction. Experiments using the 100 micron spacer are conducted at steady state conditions and three temperatures; 22 oC, 40 oC, and 60 oC. For the 50 micron spacer, temperature conditions were 22 oC and 40 oC. Micro-reactor residence times range from 2 to 30 seconds. The desulfurization study includes a comparison between theoretical model and experimental results, significance of spacer thickness, significance of temperature, and comparison between outcomes of this study to those obtained from other researchers. A mathematical model of the micro-reaction system is developed to predict dibenzothiophene conversion. The model includes convection, diffusion, and pseudo first order reaction kinetics in the presence of ultraviolet light. The model is solved numerically using COMSOL software. Experimental data is fitted to the model equations to determine the pseudo first order reaction rate constant. The dibenzothiophene reaction rate constants are 7.76*10-4 s-1 and 1.47*10-3 s-1 at temperatures of 22 oC and 40 oC, respectively. The mathematical model fit the experimental data well and therefore, may be used to predict concentrations within the micro-reactor for various operating conditions. Overall, desulfurization of dibenzothiophene using tert-butyl hydro peroxide and ultraviolet light-assisted micro-reactor was more efficient compared to other processes. The micro-reactor was capable of achieving 79 % conversion in about 30 seconds, thus reducing the sulfur content from 125 ppm to about 30 ppm. Title: Oxidative desulfurization of dibenzothiophene with tert-butyl hydro peroxide in a photochemical micro-reactor Author: Hebert, Eilleen M. Sulfur content in fuels is an increasingly critical environmental issue. Hydrodesulfurization removes sulfur from hydrocarbons; however, further desulfurization is necessary in fuels. New methods are required. In this study, micro-scaled single phase oxidative desulfurization is explored. An ultraviolet light-assisted micro-reactor is introduced for desulfurization of dibenzothiophene. Dibenzothiophene is mixed with decane and oxidant, tert-butyl hydro peroxide. The mixture forms a thin layer inside the micro-reactor which has a rectangular window to facilitate ultraviolet light irradiation. Thin spacers, 100 μm and 50 μm, establish fluid thickness within the micro-channel. Ultraviolet light reacts with oxidant to create hydroxyl radicals and convert dibenzothiophene to sulfoxides and sulfones. These products are easily extracted. The overall desulfurization process is a pseudo first-order reaction. Experiments using the 100 micron spacer are conducted at steady state conditions and three temperatures; 22 oC, 40 oC, and 60 oC. For the 50 micron spacer, temperature conditions were 22 oC and 40 oC. Micro-reactor residence times range from 2 to 30 seconds. The desulfurization study includes a comparison between theoretical model and experimental results, significance of spacer thickness, significance of temperature, and comparison between outcomes of this study to those obtained from other researchers. A mathematical model of the micro-reaction system is developed to predict dibenzothiophene conversion. The model includes convection, diffusion, and pseudo first order reaction kinetics in the presence of ultraviolet light. The model is solved numerically using COMSOL software. Experimental data is fitted to the model equations to determine the pseudo first order reaction rate constant. The dibenzothiophene reaction rate constants are 7.76*10-4 s-1 and 1.47*10-3 s-1 at temperatures of 22 oC and 40 oC, respectively. The mathematical model fit the experimental data well and therefore, may be used to predict concentrations within the micro-reactor for various operating conditions. Overall, desulfurization of dibenzothiophene using tert-butyl hydro peroxide and ultraviolet light-assisted micro-reactor was more efficient compared to other processes. The micro-reactor was capable of achieving 79 % conversion in about 30 seconds, thus reducing the sulfur content from 125 ppm to about 30 ppm. Close

• 8. [Article] Flow characterization of multiple-tube reactors for synthesis of nano-sized silicon nitride powder via silicon monoxide ammonolysis

  Concentrically arranged multiple-tube reactors with different dimensions were built for synthesizing nano-sized silicon nitride powder via the ammonolysis of SiO vapor. The reaction was operated at ...

  Citation × Citation Citation Abstract Copy Title: Flow characterization of multiple-tube reactors for synthesis of nano-sized silicon nitride powder via silicon monoxide ammonolysis Author: Vas-Umnuay, Paravee Concentrically arranged multiple-tube reactors with different dimensions were built for synthesizing nano-sized silicon nitride powder via the ammonolysis of SiO vapor. The reaction was operated at temperatures ranging from 1350 to 1400°C and pressure slightly above atmospheric pressure. Silicon monoxide (SiO) vapor was generated by charging argon gas through a bed of SiO particles packed in the innermost feeding tube and reacted with ammonia gas introduced through a separate feeding tube into the concentrical arrangement. Additional argon was fed into the annular space between SiO feeder and ammonia feeder to prevent the two reactant gases from reacting instantly at the feeder outlets and forming whiskers. None of the reactors with micro-scale dimensions (less than 1 mm) could be operated stably due to whisker formation at the SiO feeders. On the other hand, a meso-scale reactor with a 3.25 mm ID innermost tube for feeding SiO vapor was successfully operated for synthesizing nano-sized powder at measurable quantities. The velocity feed ratio of argon through a bed of SiO particles/NH₃ was maintained at 8.5 for effective production, maximizing nano-sized powder production and minimizing whisker formation. The arrangement of the feeding tubes of reactor was one of the most important parameters that have a direct influence on nano-sized powder formation. When a SiO generating tube was extended out of the annular Ar feeder, the highest efficiency of nano-sized powder production was obtained. The mean residence time of the reactants in the reacting zone was adjusted by the length of product collection tube placed in the uniform temperature zone and set to be about 0.04 seconds. High reaction temperature promoted the SiO utilization, leading to more nano- sized Si₃N₄ powder as well as Si₃N₄ whiskers at the outlet of the SiO generator. The apparent activation energy for the formation of nano-sized powder was 211 kJ/mol, based on an assumption of first-order with respect to the SiO concentration. The average particle size of nano-sized powder decreased with an increase in the reaction temperature, lying in the same range as obtained in the preceding work. The flow of a gaseous mixture in the reactor was simulated with a math software as an effective aid for identifying a reactor structure and operating variables suitable for synthesizing nano-sized silicon nitride powders. The simulated results suggested the optimal configuration of reactor and operating conditions to be employed in the experimental runs. It also revealed that locations where whiskers form corresponded to those appeared in the experimental runs. Title: Flow characterization of multiple-tube reactors for synthesis of nano-sized silicon nitride powder via silicon monoxide ammonolysis Author: Vas-Umnuay, Paravee Concentrically arranged multiple-tube reactors with different dimensions were built for synthesizing nano-sized silicon nitride powder via the ammonolysis of SiO vapor. The reaction was operated at temperatures ranging from 1350 to 1400°C and pressure slightly above atmospheric pressure. Silicon monoxide (SiO) vapor was generated by charging argon gas through a bed of SiO particles packed in the innermost feeding tube and reacted with ammonia gas introduced through a separate feeding tube into the concentrical arrangement. Additional argon was fed into the annular space between SiO feeder and ammonia feeder to prevent the two reactant gases from reacting instantly at the feeder outlets and forming whiskers. None of the reactors with micro-scale dimensions (less than 1 mm) could be operated stably due to whisker formation at the SiO feeders. On the other hand, a meso-scale reactor with a 3.25 mm ID innermost tube for feeding SiO vapor was successfully operated for synthesizing nano-sized powder at measurable quantities. The velocity feed ratio of argon through a bed of SiO particles/NH₃ was maintained at 8.5 for effective production, maximizing nano-sized powder production and minimizing whisker formation. The arrangement of the feeding tubes of reactor was one of the most important parameters that have a direct influence on nano-sized powder formation. When a SiO generating tube was extended out of the annular Ar feeder, the highest efficiency of nano-sized powder production was obtained. The mean residence time of the reactants in the reacting zone was adjusted by the length of product collection tube placed in the uniform temperature zone and set to be about 0.04 seconds. High reaction temperature promoted the SiO utilization, leading to more nano- sized Si₃N₄ powder as well as Si₃N₄ whiskers at the outlet of the SiO generator. The apparent activation energy for the formation of nano-sized powder was 211 kJ/mol, based on an assumption of first-order with respect to the SiO concentration. The average particle size of nano-sized powder decreased with an increase in the reaction temperature, lying in the same range as obtained in the preceding work. The flow of a gaseous mixture in the reactor was simulated with a math software as an effective aid for identifying a reactor structure and operating variables suitable for synthesizing nano-sized silicon nitride powders. The simulated results suggested the optimal configuration of reactor and operating conditions to be employed in the experimental runs. It also revealed that locations where whiskers form corresponded to those appeared in the experimental runs. Close

• 9. [Article] Experimental Investigation of Supercritical Heat Transfer of Carbon Dioxide in Parallel Square Microchannels with a Single-Wall Constant Heat Flux Boundary Condition

  In the vicinity of the pseudocritical point, supercritical carbon dioxide (sCO₂) undergoes a steep change in properties from “liquid-like” to “gas-like” as it is heated at a constant pressure. At the same ...

  Citation × Citation Citation Abstract Copy Title: Experimental Investigation of Supercritical Heat Transfer of Carbon Dioxide in Parallel Square Microchannels with a Single-Wall Constant Heat Flux Boundary Condition Author: Zada, Kyle R. In the vicinity of the pseudocritical point, supercritical carbon dioxide (sCO₂) undergoes a steep change in properties from “liquid-like” to “gas-like” as it is heated at a constant pressure. At the same time, there is a large spike in specific heat which can yield high heat transfer coefficients and heat capacity rates. These unique properties have made sCO₂ an attractive working fluid in next generation power and HVAC&R technologies. Microchannel heat exchangers are being used to safely and efficiently utilize the high pressure fluid in these applications. However, prior investigation of heating of supercritical CO₂ has primarily focused on circular, uniformly heated channels at relatively low heat flux for nuclear power applications. Thus, it is unclear if models and correlations developed from large circular tube data can be scaled down to the smaller, non-circular channels, with non-uniform heating. In the present work, a methodology is developed to experimentally characterize heat transfer for multiple parallel microchannels with a hydraulic diameter of 0.75 mm and aspect ratio of 1. Experiments are conducted over a range of heat fluxes (20 ≤ q” ≤ 40 W cm⁻²), mass fluxes (500 ≤ G ≤ 1000 kg m⁻² s⁻¹), reduced pressure (1.03 ≤ P[subscript R] ≤ 1.1), and inlet temperatures (20 ≤ T[subscript in] ≤ 100°C) in a parallel square microchannel test article with a single-wall constant heat flux boundary condition. Local and average heat transfer coefficients are experimentally measured and the results are compared to previously developed correlations. The predictive capabilities for the supercritical models were poor, with the lowest mean absolute percent error (MAPE) of 55.3% for the range of bulk fluid temperatures, heat fluxes, and mass fluxes. Interestingly, subcritical correlations were also investigated and yielded much lower MAPE than 80% of the supercritical correlations even though the effects of variable fluid properties were not taken into account. The subcritical correlations did not incorporate property ratios to account for the variability in fluid properties; in some supercritical correlations it was found to add additional uncertainty for the case of the present study. The effects of buoyancy and flow acceleration were also evaluated. Based on dimensionless criteria, buoyancy was expected to play a role in heat transfer, especially when the bulk fluid temperature is below the pseudocritical temperature. However, the relative importance of flow acceleration was inconclusive. Despite the apparent importance of buoyancy effects, heat transfer was not degraded, as would be expected in larger, circular, uniformly heated tubes. The mixed convection could be inducing a density driven swirling with the stratification of low-density fluid near the top (unheated). This would ultimately improve the heat transfer at the bottom portion of the test section channels. Therefore, the flow geometry and the non-conventional heated boundary could be improving the heat transfer even with buoyancy driven effects under supercritical conditions. Title: Experimental Investigation of Supercritical Heat Transfer of Carbon Dioxide in Parallel Square Microchannels with a Single-Wall Constant Heat Flux Boundary Condition Author: Zada, Kyle R. In the vicinity of the pseudocritical point, supercritical carbon dioxide (sCO₂) undergoes a steep change in properties from “liquid-like” to “gas-like” as it is heated at a constant pressure. At the same time, there is a large spike in specific heat which can yield high heat transfer coefficients and heat capacity rates. These unique properties have made sCO₂ an attractive working fluid in next generation power and HVAC&R technologies. Microchannel heat exchangers are being used to safely and efficiently utilize the high pressure fluid in these applications. However, prior investigation of heating of supercritical CO₂ has primarily focused on circular, uniformly heated channels at relatively low heat flux for nuclear power applications. Thus, it is unclear if models and correlations developed from large circular tube data can be scaled down to the smaller, non-circular channels, with non-uniform heating. In the present work, a methodology is developed to experimentally characterize heat transfer for multiple parallel microchannels with a hydraulic diameter of 0.75 mm and aspect ratio of 1. Experiments are conducted over a range of heat fluxes (20 ≤ q” ≤ 40 W cm⁻²), mass fluxes (500 ≤ G ≤ 1000 kg m⁻² s⁻¹), reduced pressure (1.03 ≤ P[subscript R] ≤ 1.1), and inlet temperatures (20 ≤ T[subscript in] ≤ 100°C) in a parallel square microchannel test article with a single-wall constant heat flux boundary condition. Local and average heat transfer coefficients are experimentally measured and the results are compared to previously developed correlations. The predictive capabilities for the supercritical models were poor, with the lowest mean absolute percent error (MAPE) of 55.3% for the range of bulk fluid temperatures, heat fluxes, and mass fluxes. Interestingly, subcritical correlations were also investigated and yielded much lower MAPE than 80% of the supercritical correlations even though the effects of variable fluid properties were not taken into account. The subcritical correlations did not incorporate property ratios to account for the variability in fluid properties; in some supercritical correlations it was found to add additional uncertainty for the case of the present study. The effects of buoyancy and flow acceleration were also evaluated. Based on dimensionless criteria, buoyancy was expected to play a role in heat transfer, especially when the bulk fluid temperature is below the pseudocritical temperature. However, the relative importance of flow acceleration was inconclusive. Despite the apparent importance of buoyancy effects, heat transfer was not degraded, as would be expected in larger, circular, uniformly heated tubes. The mixed convection could be inducing a density driven swirling with the stratification of low-density fluid near the top (unheated). This would ultimately improve the heat transfer at the bottom portion of the test section channels. Therefore, the flow geometry and the non-conventional heated boundary could be improving the heat transfer even with buoyancy driven effects under supercritical conditions. Close

• 10. [Article] Half Array Architecture for Advancing Microchannel Heat Recuperation to Market

  High temperature microchannel heat recuperators (HTμRs) offer an efficient means of improving reactor process thermal efficiency by offering smaller form factors due to increased heat transfer surface ...

  Citation × Citation Citation Abstract Copy Title: Half Array Architecture for Advancing Microchannel Heat Recuperation to Market Author: Brannon, Samuel T. High temperature microchannel heat recuperators (HTμRs) offer an efficient means of improving reactor process thermal efficiency by offering smaller form factors due to increased heat transfer surface area density and volumetric heat transfer rate over traditional heat exchanger designs. To date, HTμRs face cost and reliability challenges. In this work, a HTμR design was developed capable of utilizing high volume manufacturing methods to improve raw material utilization and cost. The design utilizes a microchannel flow insert (MFI) to resist differential pressures within low pressure channels eliminating the need for photochemical machining. The MFIs are placed within a laser-welded microchannel half array which is inserted within a metal housing to establish the counter flow path. Analytical and computational models were developed to guide the structural, thermal and fluidic design of the sub-millimeter MFI for waste heat applications. A forming process was developed for producing the MFIs and experimental investigations were used to validate structural, thermal and fluidic performance. A robust microforming fabrication process was developed capable of providing average MFI heights of 529 μm with 0.5% standard deviation. MFIs were analyzed for out-of-plane compressive stiffness and compressive strength over multiple loading cycles to validate both analytical and finite element models. A finite element model provided a model error of 6.8% for compressive stiffness. The unbonded μPTN design presented yields an 88% decrease in structural panel size compared to macro-pyramidal truss networks for a given loading condition while maintaining similar panel area. Single layer fluid flow experiments were conducted to determine the average Nusselt number and pressure drop in microchannels both with and without an MFI installed. The MFI increased the average Nusselt number by 2.9x due to increased fluid mixing. The pressure drop through the MFI was found to be on average five times higher than the empty channel due to reduced flow cross-section area. The network pressure drop model yielded a percent error at low Reynolds numbers of 5.2%. Subsequently, a 3.5 kW HTμR made of Inconel 625 was designed for waste heat recovery with an inlet temperature of 850°C. A subscale HTμR was fabricated and tested to validate the 3.5 kW design. The sub-scale five layer HTμR was tested with a hot side inlet temperature of 300°C. The sub-scale device performed as expected in terms of effectiveness (4.3% error), duty (4.3% error), and MFI channel pressure drop (-2.6% error). These results were used to scale a 3.5 kW Inconel 625 HTμR with a heat transfer surface area density of 1,633 m²/m³, a volumetric heat transfer coefficient of 2.3 MW/(m³K), and a volumetric heat transfer rate of 32 MW/m³ providing improved results versus high temperature recuperator designs in literature. Title: Half Array Architecture for Advancing Microchannel Heat Recuperation to Market Author: Brannon, Samuel T. High temperature microchannel heat recuperators (HTμRs) offer an efficient means of improving reactor process thermal efficiency by offering smaller form factors due to increased heat transfer surface area density and volumetric heat transfer rate over traditional heat exchanger designs. To date, HTμRs face cost and reliability challenges. In this work, a HTμR design was developed capable of utilizing high volume manufacturing methods to improve raw material utilization and cost. The design utilizes a microchannel flow insert (MFI) to resist differential pressures within low pressure channels eliminating the need for photochemical machining. The MFIs are placed within a laser-welded microchannel half array which is inserted within a metal housing to establish the counter flow path. Analytical and computational models were developed to guide the structural, thermal and fluidic design of the sub-millimeter MFI for waste heat applications. A forming process was developed for producing the MFIs and experimental investigations were used to validate structural, thermal and fluidic performance. A robust microforming fabrication process was developed capable of providing average MFI heights of 529 μm with 0.5% standard deviation. MFIs were analyzed for out-of-plane compressive stiffness and compressive strength over multiple loading cycles to validate both analytical and finite element models. A finite element model provided a model error of 6.8% for compressive stiffness. The unbonded μPTN design presented yields an 88% decrease in structural panel size compared to macro-pyramidal truss networks for a given loading condition while maintaining similar panel area. Single layer fluid flow experiments were conducted to determine the average Nusselt number and pressure drop in microchannels both with and without an MFI installed. The MFI increased the average Nusselt number by 2.9x due to increased fluid mixing. The pressure drop through the MFI was found to be on average five times higher than the empty channel due to reduced flow cross-section area. The network pressure drop model yielded a percent error at low Reynolds numbers of 5.2%. Subsequently, a 3.5 kW HTμR made of Inconel 625 was designed for waste heat recovery with an inlet temperature of 850°C. A subscale HTμR was fabricated and tested to validate the 3.5 kW design. The sub-scale five layer HTμR was tested with a hot side inlet temperature of 300°C. The sub-scale device performed as expected in terms of effectiveness (4.3% error), duty (4.3% error), and MFI channel pressure drop (-2.6% error). These results were used to scale a 3.5 kW Inconel 625 HTμR with a heat transfer surface area density of 1,633 m²/m³, a volumetric heat transfer coefficient of 2.3 MW/(m³K), and a volumetric heat transfer rate of 32 MW/m³ providing improved results versus high temperature recuperator designs in literature. Close