Anna Lee Tonkovich

Experimental investigations of inorganic membrane reactors: A distributed feed approach for partial oxidation reactions

An inorganic membrane reactor (MBR) is investigated for the oxidative dehydrogenation of ethane to ethylene. A tube and shell configuration is used where ethane is fed at the top of an annular packed catalyst bed and air permeates radially outward into the reaction zone along the length of the permeable reactor wall (porous α-alumina membrane). Using a magnesium oxide catalyst doped with lithium and samarium oxide, the effects of the overall ethane to oxygen feed ratio, temperature, and residence time are evaluated. The results are compared to a tubular reactor (PFR) operated with the same catalyst and conditions. The MBR outperforms the PFR in both ethylene yield and selectivity at low feed ratios. At 873 K, an ethane to oxygen feed ratio of 0.5, and a residence time of 4 s, per pass ethylene yields of 50.5 and 8.1% are produced in the MBR and PFR, respectively. The corresponding ethylene selectivities are 53 and 8.4%. As the feed ratio increases, the performance of the MBR and PFR merges. In addition, periodic counter-propagating blue flames are observed in the tubular reactor for some experiments with feed ratios between 0.25 and 1.0. No flames are observed in the MBR.

Inorganic membrane reactors for the oxidative coupling of methane

Inorganic membrane reactors have been investigated for the oxidative coupling of methane to improve yields over those produced in a fixed-bed reactor. The membrane reactor produces small yield improvements using a samarium oxide doped magnesium oxide catalyst. The magnitude of the yield increase is less than reported values for other partial oxidation reactions. These differences are shown to be a function of the intrinsic reaction kinetics with the aid of an isothermal membrane reactor model.

Enabling cellulosic diesel with microchannel technology.

The advantages of producing transportation fuels using microchannel technology for Fischer–Tropsch (FT)-based biomass to liquids are demonstrated using the Oxford Catalyst Group’s catalyst and reactor technology. Tests performed with high-activity catalysts in microchannel FT reactors across multiple scales with C5+ productivities, ranging from approximately 0.004 gallons per day to approximately 1.5 gallons per day, demonstrated equivalent process performance, as determined by the metrics of CO conversion, selectivity to byproducts and the chain growth probability (α). The catalyst and microchannel reactors showed excellent performance under conditions likely to be observed in a biomass-to-liquids facility (e.g., low H2:CO ratio and high dilution) and also demonstrated very good stability and excellent robustness to process upsets. With these significant advantages, the microchannel FT technology is poised at the cusp of commercialization for enabling biomass-derived FT fuels.

Passive Heat Transfer Enhancement in Microchannels Using Wall Features

The two important considerations in the design of a heat exchanger are — the total heat transfer rate and the allowable pressure drop. The allowable pressure drop defines the maximum flow rate through a single microchannel and economics drives the design towards this flow rate. Typically the flow rate in the microchannel is in laminar flow regime (Re < 2000) due to smaller hydraulic diameter. The laminar flow heat transfer in a smooth microchannel is limited by the boundary layer thickness. Commonly the heat transfer rate is enhanced by passively disrupting the laminar boundary layer using protrusions or depressions in the channel walls. More often these methods are best applicable at small range of Reynolds number where the heat transfer rate enhancement is more than the pressure drop increase and break down as the flow rate is changed outside the range. The benefit of a flow disruption method can be reaped only if it provides higher heat transfer enhancement than the increase in the pressure drop at the working flow rates in the microchannel. A heat transfer efficient microchannel design has been developed using wall features that create stable disrupted flow and break the laminar boundary layer in a microchannel over a wide range of flow rates. The paper experimentally investigates the developed design for the heat transfer enhancement and pressure drop increase compared to a smooth wall microchannel. A simple microchannel device was designed and fabricated with and without wall features. The experiments with single gas phase fluid showed promising results with the developed wall feature design as the heat transfer rate increase was 20% to 80% more than the pressure drop increase in the laminar regime. The wall feature design was an important variable to affect the magnitude of performance enhancement in different flow regime. A general criterion was developed to judge the efficacy of wall feature design that can be used during a microchannel heat exchanger design.© 2007 ASME

Enhancing Two-Phase Flow Mixing and Mass Transfer in Microchannel With Surface Features

Slug or plug flow is generally considered as major flow pattern in microchannels in gas-liquid two-phase flow. A new microchannel design has enabled experimental interfacial surface area density exceeding 10,000 m2 /m3 based on the two-phase volume in bubbly flow, and mass transfer coefficients exceeding 10sec−1 . Numerical simulations as well as experiments are presented with the new microchannel design. The velocity components of secondary flow induced by specially designed angled microgrooves break the gas phase into small bubbles, where otherwise much larger gas pockets/slugs would dominate in flat or smooth wall microchannels. As such, mixing of the two phases and mass transfer are greatly enhanced as a results of increased interfacial surface area density and reduced average mass transfer distance. The Volume-Of-Fluid (VOF) method is used in the numerical computations for different surface feature patterns, gas and liquid flow rates, liquid viscosity and surface tension. In the experiments, nitrogen, carbon dioxide and water are used as the two phase media. The two-phase superficial velocity in the channel is in the range 0.45–2.75 m/s. The results show that the two-phase flow in the microchannel with the angled microgrooves leads to enhanced mass transfer relative to the flat microchannel. Higher flow rates and higher liquid viscosity lead to smaller gas bubbles and in turn enhanced mixing. Opportunities for additional improvement exist with increasing flow rates and optimized processing conditions.Copyright

Non-Newtonian Flow Behavior in Microchannels for Emulsion Formation

Emulsion formation within microchannels enables smaller mean droplet sizes for new commercial applications such as personal care, medical, and food products among others. When operated at a high flow rate per channel, the resulting emulsion mixture creates a high wall shear stress along the walls of the narrow microchannel. This high fluid-wall shear stress of continuous phase material past a dispersed phase, introduced through a permeable wall, enables the formation of small emulsion droplets — one drop at a time. A challenge to the scale-up of this technology has been to understand the behavior of non-Newtonian fluids under high wall shear stress. A further complication has been the change in fluid properties with composition along the length of the microchannel as the emulsion is formed. Many of the predictive models for non-Newtonian emulsion fluids were derived at low shear rates and have shown excellent agreement between predictions and experiments. The power law relationship for non-Newtonian emulsions obtained at low shear rates breaks down under the high shear environment created by high throughputs in small microchannels. The small dimensions create higher velocity gradients at the wall, resulting in larger apparent viscosity. Extrapolation of the power law obtained in low shear environment may lead to under-predictions of pressure drop in microchannels. This work describes the results of a shear-thinning fluid that generates larger pressure drop in a high-wall shear stress microchannel environment than predicted from traditional correlations.Copyright