Contents. 11. Microreactors. Jens Ducrée and Roland Zengerle. Praxis beispie l:ausa rbeitu ngspha A usarbe itungd erstan dard-z - PDF

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Contents 1. Introduction 2. Fluids 3. Physics of Microfluidic Systems 4. Microfabrication Technologies 5. Flow Control 6. Micropumps 7. Sensors 8. Ink-Jet Technology 9. Liquid Handling 10.Microarrays 11.Microreactors

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Contents 1. Introduction 2. Fluids 3. Physics of Microfluidic Systems 4. Microfabrication Technologies 5. Flow Control 6. Micropumps 7. Sensors 8. Ink-Jet Technology 9. Liquid Handling 10.Microarrays 11.Microreactors 12.Analytical Chips 13.Particle-Laden Fluids a. Measurement Techniques b. Fundamentals of Biotechnology c. High-Throughput Screening 11. Benefits of Miniaturization h Transport phenomena Well-predictable / reproducible in microstructures Intrinsic driving force - No actuation required - Processes to large extent controlled by passive structures h Fast diffusion Fick s laws 11. Benefits of Miniaturization Characteristic diffusion time - D approx m 2 / s for solvated molecules Macro: l approx. 1 cm = 10-2 m - t approx s Micro: l approx. 100 µm = 10-4 m - t approx. 10 s h Brownian motion 11. Diffusion Coefficient Fourier mass number Particle diffusion / concentration 11. Benefits of Miniaturization 11. Benefits of Miniaturization h Fast heat exchange Fourier s law 11. Benefits of Miniaturization Power associated with heat transport Characteristic time - Same scale ~ l 2 as diffusion Fourier number - Heat diffusion / stored heat 11. Thermal Conductivity 1. Micromixers 2. Heat Exchangers 3. Chemical Microreactors 4. Splitting of Flow 5. Fuel-Based Power Supplies 11.1. Micromixers h Mixing of two phases Liquid liquid Liquid gas Gas gas Particles with fluids (suspensions) h High accuracy E.g. Bubble (gas, liquid) sizes h Mixing of reactants Key step in chemical process engineering Fast response times Low dead volumes 11.1. Mixing of Reactants h Speed depends on surface-to-volume ratio of interfaces h Two consecutive processes h Generation of heterogeneous mixture Time critical step - Macroscopic time scales Finely dispersed structure Large surface-to-volume ratio to Enhanced diffusion h Mixing on molecular level Reaction kinetics Fast step, molecular time scales 11.1. Mixing in Macro- and Microdevices h Macrodevices Fast mixing by turbulences - Shaking - Stirring h Microdevices Laminar conditions Turbulences hard to induce Static mixing by diffusion enhanced by passive structures 11.1. Micromixers: Unique Features h Ultrafast mixing as well as defined slow mixing by control over mixing time h Geometrically defined mixing processes by control over fluid layer thickness h Integrability of mixing units as parts of complex assembled systems h Generation of dispersions with small particle sizes and narrow size distributions h Synthesis of uniform-sized micro- to-nanoscale powder particles 11.1. Mixing Principles for Miscible and Immiscible Fluids h Miscible fluids after contact Miscible fluids mixed fast by means of diffusion due to small thickness of generated lamellae h Immiscible fluids after contact Due to parabolic flow profile strong periodical velocity gradients generated at exit of each channel Velocity gradients lead to unstable flow configuration Thin fluid lamellae finally decompose into microdroplets or microbubbles surrounded by continuous phase IMM Mainz 11.1. Multilamination of Layers IMM Mainz h Fluids to be mixed introduced into mixing element as two counter-flows h Fluids stream into interdigital channel configuration h Periodical flow configuration consisting of lamellae of two fluids generated by means of slit-shaped interdigital channels h Stratified flow leaves device perpendicular to direction of feed flows 11.1. Multi-Lamination of Sheaths IMM Mainz h Fluids to be mixed introduced into boreholes of mixer h Fluids entering single platelets divided into streams h From, e.g., three openings streams are guided through channel system to central position h These streams merge to fluid sheaths of small thickness h Alternating assembly of platelets results in alternating stack of sheaths of two different fluids h Mixing performed by interdiffusion within this multi-laminated sheath structure h Caterpillar Mixer Split and Recombine Fluids to be mixed introduced into the device as two parallel streams By special surface shaping of channel walls, referred to as caterpillar structure, each fluid stream split into two substreams Four such substreams recombined to a multi-laminated fluid system Process repeated several times IMM Mainz 11.1. Split and Reunify h Two vertically stacked flows Moebius Mixer h Flow split in horizontal plane h Each subflow rotated by 90 h Rejoining of flows Four lamellae 11.1. Mixing by Lamination of Flow 11.1. Mixing by Lamination of Flow 11.1. Mixing by Lamination of Flow 11.1. Mixing by Micro-Plumes 11.1. Mixing by Micro-Plumes 11.1. Low-Aspect Ratio Mixers Throughput: function of aspect ratio 11.1. Example: Drum Mixer on Bio-Disk 11.1. Mixing by Hydrophobic Barriers 11.1. Mixing by Hydrophobic Barriers 11.1. Coanda-Effect Mixer 11.1. Dispersions of Immiscible Fluids IMM Mainz h Liquid / liquid dispersions including emulsions h Gas / liquid dispersions including foams h Suspensions of nano-scale particles 11.1. Mixers for Liquid-Liquid Dispersion h Liquid-liquid dispersion including emulsions h Production of small, regularly sized droplets h Droplets of narrow size distribution IMM Mainz h Droplets of narrow size distribution 4 to 50 µm Dependent on operating conditions 11.1. Mixers for Liquid-Liquid Dispersion h Critical parameters Smaller droplets generated at higher volume flows Smaller droplets generated at higher ratios of flow rates Smaller droplets generated using mixing elements at smaller channel width IMM Mainz 11.1. Mixers for Liquid-Liquid Dispersion h Critical parameters Smaller droplets generated at higher volume flows Smaller droplets generated at higher ratios of flow rates Smaller droplets generated using mixing elements at smaller channel width IMM Mainz 11.1. Mixers for Liquid-Liquid Dispersion h Critical parameters Smaller droplets are generated at higher volume flows Smaller droplets are generated at higher ratios of flow rates Smaller droplets generated using mixing elements at smaller channel width IMM Mainz 11.1. Mixers for Gas-Liquid Dispersions h Production of small, regularly sized bubbles IMM Mainz h Under certain operating conditions, production of highly regular bubble trains h Formation of segmented (slug) and annular flow patterns, when combined with miniaturized residence time channels 11.1. Mixers for Gas-Liquid Dispersions h Bubble sizes in model and real systems: Diameter - 50 to 500 µm Ratio of bubble length to diameter - 1 to 4 Average bubble size deviation - Typically 50 µm IMM Mainz 11.1. Mixers for Suspension IMM Mainz h Production of small micro-scale particles of uniform size h Model system: precipitation of copper oxalate Particle size: 5 µm 11.1. Active Mixers: Ultrasonic 11.1. Active Mixers: Chaotic Advection 1. Micromixers 2. Heat Exchangers 3. Chemical Microreactors 4. Splitting of Flow 5. Fuel-Based Power Supplies 11.2. Physics of Heat Exchange h Heat transmission 11.2. Physics of Heat Exchange h Heat transition 11.2. Convection and Heat Diffusion h Convection Collective transport of macroscopic volumes - Buoyancy - Pumping h Peclet number Ratio convective / diffusive particle transport 11.2. Convection and Heat Diffusion h Nußelt number Ratio convective / diffusive heat transport Boundary region: wall / fluid Turbulences enhance heat transport h Note Deviations from macro-theory Turbulence formation in wall layers 11.2. Physics of Heat Exchange h Newton s law of cooling Temperature decay Characteristic time 11.2. Heat Exchangers h Mixing and reaction devices often connected to heat transfer systems h Essential for effective heat management of reaction processes IMM Mainz 11.2. Flow Schemes 11.2. Co-Current Heat Exchanger 11.2. Counter-Current Heat Exchanger h Power of heat transfer With h Effective surface (cross section) Required to establish given power P Measure for efficiency 11.2. Crossed-Flow Heat Exchanger 1. Micromixers 2. Heat Exchangers 3. Chemical Microreactors 4. Splitting of Flow 5. Fuel-Based Power Supplies 11.3. Introduction h Production of Chemicals Contacting of phases - Liquid liquid - Gas - gas - Gas liquid - Solid gas - Solid liquid Contact on molecular level to enable reactions Controlled mixing of phases - E.g. by microfabricated nozzles h Important parameters - Fast mixing - Uniform heat distribution IMM Mainz 11.3. Chemical Microreactors h Objectives of miniaturization Process intensification - Running processes at more aggressive conditions, e.g. higher temperatures Automated gathering of information - Generation of comprehensive libraries High-quality chemicals - Control of heat management - Uniform heat distributions 11.3. Gas-Liquid Reactions 11.3. Gas-Liquid Reactions 11.3. Catalytic Gas Reactor 1. Micromixers 2. Heat Exchangers 3. Chemical Microreactors 4. Splitting of Flow 5. Fuel-Based Power Supplies 11.4. Throughput h Usually pressure-driven flow (PDF) EO-pumping strongly depends on - Chemical to be transported - Interaction between surface and liquid - No universal transport mechanisms Law of Hagen-Poisseuille - I V ~ A 2 h Numbering up Throughput I V,0 given N channels Channel radius r I V,0 ~ I V (r) N Throughput IMM Mainz h Flow resistance Throughput: Flow Resistance Single channel N channels in parallel h Ratio of flow resistances h Throughput h Velocity profile Velocity and Residence Time h Ratio of velocities h Minimum residence time Center of channel 11.4. Numbering-Up Concepts h Numbering-up aims at an increase of throughput by parallel operation of a multitude of mixing units. h Feeding of these mixing units may be achieved by using a hierarchic distribution route, e.g. based on system / platelet / element. h When numbering up, the mixing conditions remain virtually the same, because mixing elements are simply added. IMM 11.4. Numbering-Up Concepts h Numbering up, hence, is advantageous, because the step from lab-scale processing to industrial processing is faster and more reliable. h As demonstrated in industrial case studies, simple combination of several micromixers paves the way to numbering-up. IMM h Numbering up Numbering-Up: Problem Source often single stream h Splitting up Distribution of one source flow to multiple small streams h Uniformity problem Like electrical resistors in parallel Highest flow rate in channel with smallest flow resistance h Non-uniformity of resistances Fabrication tolerances Flow patterns in flow splitter Backpressures caused by air pockets... 1. Micromixers 2. Heat Exchangers 3. Chemical Microreactors 4. Splitting of Flow 5. Fuel-Based Power Supplies 11.5. Micro Fuel Cells h A fuel cell operates like a battery in principle, producing electricity by electrochemically combining hydrogen and oxygen without combustion. h Unlike a battery, a fuel cell does not run down as long as fuel is supplied. The Micro Fuel Cell from Energy Related Devices is fueled by methanol or ethanol and is constructed with multiple layers of thin films. h The micro-fuel cell is like the cell of any living thing: Input fuel is a hydrocarbon that is broken down catalytically, producing energy and harmless waste products of heat, carbon dioxide, and water vapor. The collection and energy distribution scheme of this microfuel cell system can be likened to the capillaries and veins in an animal. 11.5. Micro Fuel Cells h The alcohol side of the film contains a catalyst that breaks the alcohol down into hydrogen ions and carbon dioxide. h This generates a negative electrical potential on the alcohol electrode. The hydrogen ions move through the film to the oxygen electrode on the air side of the cell. h The hydrogen ions combine with oxygen from the air to produce water vapor. h This produces a positive charge on the oxygen electrode. The oxygen and alcohol electrodes form the positive and negative terminals of the fuel cell; this is equivalent to the positive and negative terminals on a conventional battery. 11.5. Advantages of Fuel Cells h Inexpensively manufacturable Printing process similar to manufacture of computer chips h Environmentally responsible energy source Non-polluting and does not involve use of toxic chemicals h Instant refuel with methanol or ethanol Safe, low-flammability, inexpensive, renewable, plant-based energy source h Always On power source h Lightweight and compact size h 20 year estimated shelf-life PNNL 11.5. Microchannel Combustor / Evaporator h Lightweight and compact sources of electric power and microclimate control Carry-on power source for soldiers Thermal energy could drive electrical generation and provide the heating or cooling Combustion of hydrocarbon fuels produces highdensity thermal energy - Energy density (W/kg or W/m 3 ) of fuels factor of 100 greater than most advanced batteries Current heat generation - Power transfer of 25 W / cm 2 Low air-toxic emissions during operation - Emissions of both NO x and CO reduced to below regulatory limits Developing an approach to mass production - Diffusion bonding of laminates for low cost fabrication PNNL 11.5. Microchannel Reactors for Automotive Fuel Processors Praxis beispie l:ausa rbeitu ngspha h Microchannel reactors reduce size of conventional reactors Without lowering throughput h Heat and mass transport limitations slow observed reaction rates in conventional reactors Minimized in microchannel reactors h Distance between heat generation and removal reduced Tens of centimeters in conventional reactors Tens of microns in microchannel reactors h As distance shrinks, corresponding contribution of slow conduction and diffusion to heat exchange or catalyst surface is reduced. h Fast heat and mass transfer increases process efficiency, enabling process miniaturization without sacrificing productivity. 11.5. Microchannel Reactors for Automotive Fuel Processors Praxis beispie l:ausa rbeitu ngspha h Fuel processor critical reactor technology for deployment of PEMbased fuel cells for automotive applications h Fuel processor produces hydrogen rich streams from gasoline or methanol in multi-step process Fuel vaporizer Primary conversion reactor to produce synthesis gas Water gas shift reactor CO clean-up reactor h Conventional fuel processing technology based on fixed-bed reactors Not scaling well with small modular nature of fuel cells h Microchannel reactor-based fuel processors Small, efficient, modular, lightweight and potentially inexpensive h Based upon our results with other component investigations, we project a complete system volume of less than 9 liters to produce hydrogen at a sufficient rate and quality to produce 50-kW from a PEM fuel cell. 11.5. Microchannel Reactors for Automotive Fuel Processors Praxis beispie l:ausa rbeitu ngspha
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