micro reactor

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[email protected] MICROREACTOR

MICROREACTOR

INTRODUCTIONIn accordance with the term “microsystem”, microreactors usually are defined as miniaturized reaction systems fabricated by using, at least partially, methods of microtechnology and precision engineering. The characteristic dimensions of the internal structures of microreactors like 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 on the desired application. Factors such as temperature and pressure range of the application, the corrosivity of the fluids used, the need to have catalyst integration or to avoid catalytic bind activities, thermal conductivity and temperature distribution, specific heat capacity, electrical properties 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 microstructures depends 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 in process 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 stainless steel to metals such as copper, titanium, aluminum or nickel based alloys. Most manufacturing technologies for metallic microstructures have their roots either in semiconductor device production 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 to obtain freeform structures with dimensions in the sub-millimeter range. Two techniques are there namely dry and wet etching. In this technique a photosensitive polymer mask material is applied on the metal to be etched. The mask is exposed to light via a primary mask with structural layers. The polymer is then developed. This means that the non-exposed parts are polymerized in such a way that they cannot be diluted by a solvent that is used to remove the rest of the polymer covering the parts to be etched. Thus, a mask is formed, and the metal is etched through the openings of this mask.

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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 chemical etching, 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 is rather expensive. Second, wet chemical etching always results in semi-elliptic or semicircular structures, which is again due to the isotropic etching. Dry etching often leads to other channel geometries. Here, rectangular channels are also possible. In Figure 1.1, a stainless steel microchannel structure manufactured by wet chemical etching is shown. The microchannels are used to build a chemical reactor for heterogeneously catalyzed gas-phase reactions. They are about 360 mm wide and 130 mm deep. Figure 1.2 shows the entrance area of such a microchannel. The semicircular structure is clearly seen.

b. Machining Not all materials can be etched in an easy and cheap way. In those cases

precision machining will be used to generate microstructures from standard metal alloys such as stainless steel or hastelloy. Depending on the material, precision machining can be performed by spark erosion (wire spark erosion and countersunk spark erosion), laser machining or mechanical precision machining. In this case, mechanical precision machining means milling, drilling, slotting and planning. Although the machining technology used is comparable to the techniques well known from conventional dimensions in the millimeter range or above, the tools used are much 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 the alloy. For brass and copper, natural diamond

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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 widespread depending on the material as well as on the machining parameters. Spark erosion techniques lead to a considerably rough surface. The surface quality obtained with laser ablation heavily depends on the material to be structured and on the correct parameter settings. Values between some 10 mm and about 1 mm are common. By using brass or copper structural material, the best surface quality is achievable with mechanical precision machining. However, an electropolishing step must follow the micromechanical machining. A surface roughness ranging down to 30nm can be reached.

c. Generative Method: Selective Laser Melting (SLM)A special method to manufacture metallic manufacture metallic

microstructures is SLM. 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 thin layer of a metal powder is distributed. A focused laser beam is ducted along the structure lines given by a 3D CAD model, which is controlled by a computer. With the laser exposure, the metal powder is melted, forming a welding bead. The first layer of welding beads forming a copy of the 3D CAD structure is generated. After this, the platform is lowered by a certain value, new powder is distributed and the process is repeated. Thus, microstructures are generated layer by layer. In principle, any metal powder can be used for SLM as long as the melting temperature can be reached with the help of the laser. For metal alloys, some problems might occur with dealloying by melting.

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2. Ceramic devicesMicrostructure devices made from ceramic and glass can be used for

processes with reaction parameters that are reachable neither with metals nor with polymers. High temperatures measuring above 10000C, absence of catalytic blind activity and some easy ways to integrate catalytic active materials make ceramics a very interesting material. Glass is chemically resistant against almost all chemicals and also provides good resistivity at elevated temperatures. In addition, optical transparency of glass leads to some very interesting possibilities such as photochemistry or a closer look into several fluid dynamics and process parameters 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 sintering process, thus a certain tolerance to the dimensions have to be added. Solid free-form techniques such as printing, fused deposition or stereo lithography are also possible with ceramic slurry. The most crucial point is the correct microstructure design. Owing to the specific properties 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 as interconnections to metal devices have to be considered.

a. Joining and SealingJoining of ceramic materials should only involve materials with similar

properties. Especially, the thermal expansion coefficient is a crucial point while either joining ceramic materials to each other or, even worse, joining ceramics to metals. The ideal joining of ceramics to each other is done in the green state before the firing process. When the firing process takes place, 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 working temperature of the device is limited by the melting temperature of the sealant. Reversible assembling and sealing with clamping technologies or gluing are also possible. Conventional seals such as polymer o-rings or metal gaskets may be used in metal technology as well. The adaptation of ceramic microstructure devices to

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metallic process equipment should be done as far away from high temperatures as possible. Owing to the very different thermal expansion coefficients of both material classes, problems will most likely occur here. Then the sealing used should be designed to minimize tensile stresses as far as possible.

Structural hierarchy of MicroreactorsThe smallest units of a miniaturized continuous flow system are microstructures. A typical single or multiple flow channel configuration of distinct geometric nature is named element. A typical example for a mixing element is an interdigital channel configuration. In some cases, elements can consist of chambers too, e.g. carrying additional microstructures such as pores. A combination of an element, connecting fluid lines and supporting base material, is termed unit. For instance, a mixing platelet with an interdigital structure and feed lines is a micromixing unit. In order to increase throughput, units may form a stack, e.g. a stack of catalytic platelets in a chamber of a gas phase microreactor. Alternatively, identical devices can be arranged in parallel in a plane, e.g. a micromixer array consisting of thousands of unit cells. Neither units nor stacks can be operated alone. Hence, they are not real microreactors, since they need housings or, at least, top and bottom plates for fluid connection to external periphery. A device refers to a unit embedded either in housing or between two end caps. The build-up of complex systems can be performed by integration of several units withn one common housing. A system can also be based on a connection of devices, in this case referred to as components. Any parallel or serial interconnection of components, systems or mixed combinations may be termed set-up or plant, dependent on the type of application, being lab- or industrial scale oriented, respectively. These set-ups or plants consist of either only microdevices or -systems, or, more likely, may contain microreactors next to conventional larger equipment [1].

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MicroreactorsMikroglas is an International company working on developing the microreactor technology. This company developed many products related to microreactor. Some of the products are [3]:

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Design of Mikroglas T-mixerMikroglas chemtech GmbH developed and manufacturing the micro fluidic components made of glass, such as static mixers, heat exchangers, or a combination of both. The material glass makes the modules resistant against corrosive chemicals. Due to its optical transparency it is possible to investigate the flow regime under the microscope. The T-mixers shown here represent the simplest mixing principle. Different channel geometries are available which can be used for a wide variety of experiments.

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Applications of Microreactor• Micromixers• Micro heat exchangers• Microseparators• Gas phase reactors• Liquid phase reactors

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• Gas/liquid reactors

Advantages of Microreactor Optimal mixing is possible in this reactor because of Laminar Flow, Fast

diffusion and mixing within seconds. High surface-to-volume ratio. Effective heat exchange in the reactor resulting in a good heat transfer. Reaction volume in the range of milliliters. Small reaction volumes allows handling of dangerous reactions, e.g. with

explosives or toxic components. Exact control of reactions and therefore suppression of unwanted side

products. By numbering up instead of scale up, they can be applied to synthesize kg or

even ton- amount, particularly in parallelized arrays.

Direct nitration of aromates (e.g. naphthalene) with dinitrogen pentoxideThe reaction observed is in liquid phase. The reaction is highly exothermic (∆HO = -500 kJ/mol), and needs a temperature of -500C for the reactants to react. The reaction time is very small less than 10sec. The reaction system is complex because of side reactions [4].

The results studied in the case of conventional and microreactors are:

Conventional technology Microreaction technology

Reaction tends to explosive Reaction is controlable

Temperature : -50°C Temperature: +30°C

Mixture of mono-, di-, tri-nitro-product

Only mono-nitro-product

The results show that microreactor technology has the better feasibility while compared with the conventional technology.

Challenges

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Microreactors generally do not tolerate particulates well, often clogging. Clogging has been identified by a number of researchers as the biggest hurdle for microreactors. Only the so called microjetreactor is free of clogging by precipitating products. Gas evolved may also shorten the residence time of reagents by pushing out material much faster than anticipated.

Mechanical pumping may generate a pulsating flow which can be disadvantageous. Much work has been devoted to development of low pulse or pulse less pumps. A continuous flow solution is electro osmotic flow (EOF).

Reactions performing very well in a microreactor encounter many problems in vessels, especially when scaling up. Microreactors are well suitable for fast and exothermic reactions but cause trouble as soon as particles like unsoluble/ unexpected side-products are involved.

To made the microreactors for manufacturing in large scale.

Fabrication of microreactor is not so easy, lack of straightforward approaches for creating integrated, flexible systems, still exist but notable advances are being made.

In the case of multiphase reactions, such as those involving gas–liquid, gas–liquid–solid, and gas–liquid–liquid systems, MRT is in a much earlier stage of development [5].

References[1]. Microreactors - New Technology for Modern Chemistry: W. Ehrfeld, V.

Hessel & H. Lowe[2]. Brandner, J.J. Fabrication of Microreactors Made from Metals and

Ceramics. [3]. Microreaction Technology Chemical Process Technology of Tomorrow

(www.mikroglas.com).[4]. Microreaction Technology (www.dusemund.com).[5]. Mills, P.L.; Quiram, D.J.; Ryley, J.F. Microreactor technology and process

miniaturization for catalytic reactions – a perspective on recent developments and emerging technologies. Chemical engineering science 62(2007) 6992-7010.

http://www.dusemund.com/

http://www.mikroglas.com/

micro reactor

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