Micro Reactor

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A report on Microreactor as seminar course at IIT-Hyderabad... Author: P V CHANDRA SEKHAR
  vcspalla@gmail.com MICROREACTOR DEPARTMENT OF CHEMICAL ENGINEERING | IIT HYDERABAD Page 1 MICROREACTOR INTRODUCTION   In accordance with the term ³microsystem´, microreactors usually are defined as miniaturizedreaction systems fabricated by using, at least partially, methods of microtechnology andprecision engineering. The characteristic dimensions of the internal structures of microreactorslike fluid channels typically range from the submicrometer to the sub-millimeter range.The material used for the manufacturing of the microstructured reactors is heavily dependent onthe desired application. Factors such as temperature and pressure range of the application, thecorrosivity of the fluids used, the need to have catalyst integration or to avoid catalytic bindactivities, thermal conductivity and temperature distribution, specific heat capacity, electricalproperties as well as some other parameters have a large influence on the choice of material.Finally, the design of the microstructures itself is an important consideration [1].There are two manufacturing processes available for the development of microstructuresdepends on the material used: 1. Metal microstructures and 2. Ceramic microstructures [2].1. Manufacturing Techniques for Metals:Metals and metal alloys are the most often used materials for conventional devices inprocess engineering, and thus applied in microprocess or technology as well. The range of materials spread from noble metals such as silver, rhodium, platinum or palladium via stainlesssteel to metals such as copper, titanium, aluminum or nickel based alloys. Most manufacturingtechnologies for metallic microstructures have their roots either in semiconductor deviceproduction or in conventional precision machining. The techniques may adapt are:a. Etchingb. Machiningc. Generative method: Selective Laser Machining (SLM) a. EtchingFor many metals, etching is a relatively cheap and well-established technique toobtain freeform structures with dimensions in the sub-millimeter range. Two techniques arethere namely dry and wet etching. In this technique a photosensitive polymer mask material isapplied on the metal to be etched. The mask is exposed to light via a primary mask withstructural layers. The polymer is then developed. This means that the non-exposed parts arepolymerized in such a way that they cannot be diluted by a solvent that is used to remove therest of the polymer covering the parts to be etched. Thus, a mask is formed, and the metal isetched through the openings of this mask.When etching techniques are used, two main considerations should be followed.First, the aspect ratio (the ratio between the width and depth of a structure), for wet chemicaletching, can only be < 0.5 at the optimum. As a result of the isotropic etching of the wet solvents,the minimum width of a structure is two times the depth plus the width of the mask openings.Dry etching (e.g. laser) is not limited to this aspect ratio, but it shows other limitations and israther expensive. Second, wet chemical etching always results in semi-elliptic or semicircular   vcspalla@gmail.com MICROREACTOR DEPARTMENT OF CHEMICAL ENGINEERING | IIT HYDERABAD Page 2 structures, which is again due to the isotropic etching. Dry etching often leads to other channelgeometries. Here, rectangular channels are also possible. In Figure 1.1, a stainless steelmicrochannel structure manufactured by wet chemical etching is shown. The microchannels areused to build a chemical reactor for heterogeneously catalyzed gas-phase reactions. They areabout 360 mm wide and 130 mm deep. Figure 1.2 shows the entrance area of such amicrochannel. The semicircular structure is clearly seen. b. Machining Not all materials can be etched in an easy and cheap way. In those cases precisionmachining will be used to generate microstructures from standard metal alloys such as stainlesssteel or hastelloy. Depending on the material, precision machining can be performed by sparkerosion (wire spark erosion and countersunk spark erosion), laser machining or mechanicalprecision machining. In this case, mechanical precision machining means milling, drilling,slotting and planning. Although the machining technology used is comparable to the techniqueswell known from conventional dimensions in the millimeter range or above, the tools used aremuch smaller. Spark erosion and laser machining are suitable for any metal. The use of mechanical precision machining and the tools suitable for this type depend on the stability of thealloy. For brass and copper, natural diamond microtools are suitable and widely used, while for stainless steel and nickelbased alloys, hard metal tools are needed.The range of surface quality reached with the different techniques is widespreaddepending on the material as well as on the machining parameters. Spark erosion techniqueslead to a considerably rough surface. The surface quality obtained with laser ablation heavilydepends on the material to be structured and on the correct parameter settings. Values betweensome 10 mm and about 1 mm are common. By using brass or copper structural material, thebest surface quality is achievable with mechanical precision machining. However, anelectropolishing step must follow the micromechanical machining. A surface roughness rangingdown to 30nm can be reached.  vcspalla@gmail.com MICROREACTOR DEPARTMENT OF CHEMICAL ENGINEERING | IIT HYDERABAD Page 3 c. Generative Method: Selective Laser Melting (SLM) A special method to manufacture metallic manufacture metallic microstructures isSLM. It is one of the rare generative methods for metals and is normally taken into the list of rapid prototyping technologies. On a base platform made of the desired metal material, a thinlayer of a metal powder is distributed. A focused laser beam is ducted along the structure linesgiven by a 3D CAD model, which is controlled by a computer. With the laser exposure, themetal powder is melted, forming a welding bead. The first layer of welding beads forming a copyof the 3D CAD structure is generated. After this, the platform is lowered by a certain value, newpowder is distributed and the process is repeated. Thus, microstructures are generated layer bylayer. In principle, any metal powder can be used for SLM as long as the melting temperaturecan be reached with the help of the laser. For metal alloys, some problems might occur withdealloying by melting.  vcspalla@gmail.com MICROREACTOR DEPARTMENT OF CHEMICAL ENGINEERING | IIT HYDERABAD Page 4 2. Ceramic devicesMicrostructure devices made from ceramic and glass can be used for processes withreaction parameters that are reachable neither with metals nor with polymers. Hightemperatures measuring above 1000 0 C, absence of catalytic blind activity and some easy waysto integrate catalytic active materials make ceramics a very interesting material. Glass ischemically resistant against almost all chemicals and also provides good resistivity at elevatedtemperatures. In addition, optical transparency of glass leads to some very interestingpossibilities such as photochemistry or a closer look into several fluid dynamics and processparameters with online analytical methods using optical fibers.The conventional way to obtain ceramic microstructures is to prepare a feedstock or slurry, fluid or plastic molding, injection molding or casting (CIM, HPIM and tape casting),demolding, debinding and sintering. Most ceramic materials will   shrink during the sinteringprocess, thus a certain tolerance to the dimensions have to be added. Solid free-formtechniques such as printing, fused deposition or stereo lithography are also possible withceramic slurry.   The most crucial point is the correct microstructure design. Owing to the specificproperties of ceramics, it is not suitable simply to transfer the design of metallic or polymer devices to ceramic devices. Special needs for sealing, assembling and joining as well asinterconnections to metal devices have to be considered.a. Joining and Sealing Joining of ceramic materials should only involve materials with similar properties.Especially, the thermal expansion coefficient is a crucial point while either joining ceramicmaterials to each other or, even worse, joining ceramics to metals. The ideal joining of ceramicsto each other is done in the green state before the firing process. When the firing process takesplace, the ceramics are bound together tightly to form a single ceramic body from all parts. Another possibility is the soldering with, for example, glass±ceramic sealants. Here, the workingtemperature of the device is limited by the melting temperature of the sealant. Reversibleassembling and sealing with clamping technologies or gluing are also possible. Conventionalseals such as polymer o-rings or metal gaskets may be used in metal technology as well. Theadaptation of ceramic microstructure devices to metallic process equipment should be done asfar away from high temperatures as possible. Owing to the very different thermal expansioncoefficients of both material classes, problems will most likely occur here. Then the sealing usedshould be designed to minimize tensile stresses as far as possible.
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