PROFILE

Micro chemical processing at IMM—from pioneering work to customer-specific services

Micro chemical processing at IMM—from pioneering work to customer-specific services

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A brief history

Some ten years ago the government of the German federal state of Rhineland-Palatinate decided to invest a considerable amount of money into the newly emerging field of microtechnology. Until then this south western part of Germany had been known primarily for its renowned wine. With this in mind, the Institut für Mikrotechnik Mainz GmbH (IMM), founded in the late 1990s as a private non-profit making company, has become internationally recognised as one of the most fertile regions for developments in microtechnical devices. Funded in part by the local Ministry of Economics, the IMM has more than a dozen spin-offs and, all combined, employs some 300 staff members.

If one endeavours to describe the distinguishing factors of microtechnology à la IMM one needs to point out the broad technological applications available at its premises in the state capital of Mainz. This basis, which ranges from microprecision engineering techniques via the different variants of LIGA (the German acronym for a combination of Lithography, Electroforming and Moulding) to typical thin film processes, not only allows one to make noteworthy contributions to varied fields such as bio- and medical technology, chemistry, optical datacom or metrology, it also facilitates the realisation of microstructures in materials ranging from stainless steel and metal alloys to silicon, ceramics or polymers. In addition, the IMM strives to develop the necessary production techniques, which are at the heart of its prototypes. Examples of this latter activity are the realisation of an X-ray scanner, a specialised electroplating unit, tools to automate microassembly steps or, more generally speaking, advances in the processes required to treat wafers.

Paramount as this approach may seem, the IMM focuses on mainly three application areas, namely optical datacom, microfluidics and microreaction technology. The distinction between the latter two may be regarded as somewhat artificial for they have a good deal in common. On the other hand, it is not mere arbitrariness to think of the two domains as the two sides of the same coin. Their difference basically lies in their potential applications. When work on microfluidic devices began not long after the IMM had become operational, the development of valves and of a self-priming micropump, first attempted in silicon, but eventually realised in plastic, were among the earliest projects in this domain which at the time started to evolve quickly. With the advent of the concept of Lab on a Chip and the advances in microinjection moulding and hot embossing, the focus eventually shifted to microstructured polymer chips in the second half of the 1990s. However, microfluidics, as the term has come to be used, generally relates to the handling of small fluid volumes—an asset or even a prerequisite in the domains of analytics. The IMM, however, is also renowned for its developments in the field of microreaction technology, which we shall relate in more detail hereafter.

The term ‘microreaction technology’ was forged a number of years ago to describe the application of microfabrication methods to chemical process engineering, and the need for miniaturisation in an area in which synthesis and thus high volume production are concerned is much less evident than in the targeted fields of Lab on a Chip developments.

Still, the field is evolving rapidly and industrial interest is considerable. The reason for this is distinctly different from the one which drives the evolution of miniaturisation in analytics: it is the possibility to design reactions by controlling the crucial parameters.

‘Chemical microfluidics’—microreaction technology at the IMM

IMM′s Microreaction Technology Department has two main focuses of research and one which only emerged very recently. The Reaction Engineering Group is mainly concerned with the development of micromixing and special gas/liquid-contacting devices, their combination to systems and set-ups, and the testing of all this equipment for selected applications. Since a number of microdevices already are available and mixing processes are involved in practically every chemical process, the corresponding R&D at IMM is necessarily technology driven, implying microfabrication and chemical processing knowledge.

In turn, the Chemical Engineering Group presently is more market driven, targeting specific microreformer and fuel cell applications. These are currently highly requested by industrial customers, particularly in the fields of automobiles and heat managing as well as energy generating systems. From the reaction and process engineering side, this requires, for instance, the development and operation of gas phase reactors, heat exchangers and evaporators or stacked membrane modules. Accordingly, the group has developed specific know-how concerning catalyst coating of microchannels and testing in home-made catalyst screening devices and set-ups. This technological basis opens up other market segments which also shows a demand for these types of process modules.

A third and smaller group named Electrochemical Microreactors explores the market potential of microdevices of the same name. In addition, research on sensors and analytical devices is conducted, in particular, with respect to the needs of the chemical microdevices mentioned above.

Services of the microreaction technology department

Generally, R&D of the Microreaction Technology Department aims at identifying innovative microfluidic developments in line with market requirements, realising technical breakthroughs therein and arousing the appropriate industrial interest.

Accordingly, the activities fall into three categories. In government funded projects basic or applied research is carried out. For instance, new principles of microfluidic flow are evaluated and design studies result in novel device prototypes. The ground for commercial exploitation of these results is paved in contract research projects with industry. Fortunately, the fabrication of some selected microdevices and their use in processing meanwhile has become a matter of routine. ‘Routine services’, such as the supply of off-the-shelf devices and the conducting of certain experimentation services, help new customers to get into the field, broaden the scope of applications targeted and, more particularly, serve to demonstrate that micro chemical processing has established itself.

With regard to this working model, the services of the Microreaction Technology Department of IMM cover: (i) chemical process engineering using plants equipped with microstructured components, (ii) process engineering for automotive and energy applications, (iii) other types of process engineering, e.g. related to cosmetics, food, and health care, (iv) special proprietary development of microstructured chemical components, systems and plants, (v) consultation and training services, and (vi) purchase of standard microstructured chemical components.

Microfabrication for microreactors

IMM particularly focuses on the manufacture of steel and metal microreactors (see Fig. 1).¹ For this purpose, wet chemical etching technologies are often employed. At present, the majority of current microchannel platelets with parallel, straight channels are made by this technique. Other microstructured parts such as nozzles, feeding slits or complex microchannel arrangements are realised preferably by means of μ-EDM. In particular, die sinking and thin-wire erosion techniques as well as EDM-drilling techniques are applied. Modern precision engineering methods such as milling, turning and drilling complete the list of metal micromachining methods applied.

Fig. 1 Gas phase microreactor with two steel end caps enclosing a stack of microstructured steel platelets manufactured by wet chemical etching. For better insulation of this stack, two ceramic blocks (material: Macor) are placed between the end caps and the stack. The microdevice was specifically developed for carrying out reactions with periodic changes of reactant concentration. However, it may also be employed as a heat exchange module.

IMM usually prefers a reversible assembly for steel microreactors if they serve as application-unique special tools. In this case, the level of experience about the micro chemical processing is generally low and the cost of the microdevice is rather high. Hence disassembly, for various reasons such as removal of fouling layers or inspection of corrosion, is crucial and desired by most customers. Flat-sealing, for instance by means of O-rings or home-made graphite foils, is one preferred way to reversibly tighten the device. For standard (off-the-shelf) devices, irreversible sealing, e.g. by welding or diffusion bonding, is an alternative.

The second largest class of IMM microreactors are glass devices (see Fig. 2).² They are made by employing a photoetching technique, which makes use of a special glass material, termed Foturan, and consist of several bonded microstructured layers. The most obvious advantage of these devices is that they are transparent and allow a perfect monitoring of fluid flows. Hence IMM regards these devices as ideal diagnostic and process development tools, chiefly to be used at the start of experimental investigations. In turn, it is advised to rely on steel devices for process optimisation studies and chemical production. As a matter of fact, steel is the preferred material for chemical processing apparatus on the macro-scale.

Fig. 2 Transparent cyclone micromixer realised by photoetching of a special glass, termed Foturan. This gas/liquid contacting device is built from 13 microstructured layers, containing 30 μm wide gas nozzles and 30 μm wide liquid nozzles.

Silicon etching, either by wet or dry methods, have been applied in selected cases for the fabrication of microfluidic devices at IMM. By means of the Advanced Silicon Etching (ASE) process, silicon micromixer inlays were derived.³ Moreover, silicon is undoubtedly the ideal material for fabrication of sensors and small process control units.

Referring to inexpensive mass production, injection moulding of polymeric microfluidic devices is recommended, as far as there are no other objections to this choice of material.⁴ In this context, the first interdigital micromixer inlays were made by injection moulding.

Micromixers

Facing mixing tasks as diverse as, e.g., emulsification, foaming and intermixing of reactants, has caused a large number of micromixers to be developed. The class of so-called interdigital devices performs multilamination of fluid layers, generally characterised by uniform concentration profiles and predictable mixing physics. Specific interdigital designs refer to performing multilamination exclusively, to additionally rely on geometric focusing or to include secondary mixing effects such as jet mixing.

A new focusing mixer, termed SuperFocus, was optimised with respect to mixing speed based on predictions of theoretical calculations.⁵ The total width of the 124 lamellae, fed by inlets which are arranged on a circle of an arc, is reduced from 20 mm initially to 0.5 mm. This corresponds to a decrease of individual lamellae width from 160 to 4 μm. As a consequence of this large focusing ratio of about 40, liquid mixing is completed in a few milliseconds, and hence even at high fluid velocities in the m s⁻¹ range a mixing channel length of a few mm is sufficient. As a further benefit of the large number of fluid inlets, flow rates of about 8 l h⁻¹ for liquids of the viscosity of water at a pressure loss of 1.5 bar are accessible.

Even higher flow rates of about 100 l h⁻¹ for similar conditions are achieved by a special slit-recombine mixer, termed caterpillar mixer due to its internal shape created by step-like, and rather large, microstructures.⁶

Cyclone mixers are a further category of multilamination mixers which generate a vortex-flow pattern and thereby focus the fluids. These devices were used for gas/liquid contacting besides liquid mixing.²

Since fouling is one of the technical hurdles of utmost importance to be cleared for chemical processing in microchannels, special contactors were developed which either retard mixing or perform mixing in a wall-free fluidic environment.⁷ Separation layer mixers rely on an ‘inert’ solvent layer inserted between two lamellae consisting of reactant solutions. By setting the widths of all layers, the onset and completion of mixing can be ‘delayed’ until a fluidic unit is reached which is less prone to fouling. Impinging jet mixers lead to a collision of two streams in a large chamber filled with air, an inert gas, or an immiscible liquid (see Fig. 3). Either way, wall contact is completely avoided. The mixed stream can then be introduced in any process equipment capable to transport particle-containing solutions.

Fig. 3 Impinging jet micromixer for performing liquid reactions with strong fouling. The colliding jets have no wall contact and thus the mixed solution can enter a further processing apparatus attached to the mixer which is not so sensitive to fouling. Most likely this is a tubular reactor/heat exchanger having a diameter in the mm-range.

Process engineering based on micromixers

With respect to the mixing of reactive miscible liquids, the use of IMM interdigital mixers for carrying out a metallo-organic reaction (Merck, Darmstadt, 1998),⁸ and radical polymerisation of acrylates (Aventis, Frankfurt am Main, 1998),⁹ has been documented in the past. In the first case, the yield was improved by about 20% while lowering the energy expenditure. In the second case, a plugging-free operation using a tubular reactor was enabled by proper pre-mixing in the microdevice whereas the former process (without it) suffered from intense fouling. Both processes were envisaged for the production-scale; the Merck plant has been running since August 1998.

Although the opposite occurred for the Aventis process, one has to be aware that fouling in micromixers, as well as in other types of microdevices, may pose a significant hurdle for a wide acceptance of micro chemical processing in total. Every organic chemist knows that a large proportion of organic reactions are accompanied by either the introduction of solid reactants or the precipitation of intermediates or products, including auxiliary reagents such as acids and bases. The choice is to process only the ‘nice’ candidates, to rewrite all synthesis books with new experimental protocols, or to look for special microdevices which also allow one to process solid-containing fluid flows.

As a result, IMM immediately decided to follow the latter strategy. With respect to this topic, Clariant, Frankfurt am Main, searched for a continuous flow alternative route for a particle generating process, presently carried out batch wise at a production scale.¹⁰ With interdigital micromixers, the gathering of information was possible; however, after a few minutes of operation they were blocked up. This came as no surprise since it was known beforehand that the intermediate to the product precipitates quantitatively. Using a microdevice of larger internal dimensions, the caterpillar minimixer, processing for acceptably long operation times was allowed. Thereby, a scale-up set-up was developed which showed an increase by nearly 25% in yield as compared to the former industrial batch process.

While for this example a simple and moderate increase in characteristic dimensions—without loss of the microfluidic advantages—was sufficient, it is foreseeable that special devices are needed to process reactions with strong fouling, retaining a large freedom in choosing process parameters. Such a reaction is the synthesis of acetyl n-butyl-amide via the acid chloride route usually involving auxiliary bases.⁷ This leads to instantaneous, heavy flocculation of ammonium precipitates. Using separation layer and impinging jet mixers, even such particle-rich solutions, respectively at molar concentrations of the reactants, can be injected into mini-tubular reactors for completion of reaction and heat exchange (see Fig. 3). The amide formation is only a model reaction, but this opens up the door for other sensible well-known reactions such as the Wittig reaction, esterifications, metallo-organic reactions etc.

Recent work of IMM has also focused on the theoretical understanding of the droplet formation when immiscible liquids are contacted in interdigital micromixers. Previous investigations already showed the dependence of droplet size on flow rate, analysed the corresponding frequency distributions, and were concerned about determining the decrease of energy input needed.¹¹ In the past two years, high-quality flow imaging, in particular by using transparent glass micromixers, was established.¹² Therefore, and by using modern simulation techniques, it could be confirmed that, at least for certain mixer designs and process parameters, the Rayleigh-Plateau instability, driven by statistical fluctuations of the liquid/liquid interface, induces a decay to droplets (see Fig. 4).¹² The formation of droplets of uniform size was thus predicted and experimentally confirmed. Knowing the physical background of the droplet generation, this now opens the door to more widespread predictions and to identify favourable operation parameters, e.g. to achieve a uniform droplet decay.

Fig. 4 Top: 3D simulation of the decay of a cylindrical stream of silicon oil in water. Due to the Rayleigh-Plateau instability fluctuations occur and droplets are generated. Bottom: corresponding experimental result. Uniform droplets are observed for certain experimental parameters.

BASF has used the emulsification of immiscible liquids for extraction processes in the framework of the miniplant technique.¹³ For three recommended test processes, among them the transfer of succinic acid in the system water/n-butanol, it was shown that by only one fluid passage, equivalent to a residence time of only a few milliseconds, the thermodynamic equilibrium could be reached. It was found further that the quality of dispersion in the interdigital mixers is actually a delicate interplay between generation of specific interface and residence time which leads to maximised mass transfer both at low and high flow rates. The theoretical description of the corresponding function is in excellent accordance with experiment.

Newer investigations use the fast generation of large specific interfaces for the extraction of radioactive isotopes in the framework of the MicroSISAK process (Short-Lived Isotopes Studied by the AKUVE technique), carried out by the Institut für Kernchemie, Mainz (see Fig. 5).

Fig. 5 Extraction process of a short-living isotope by means of the SISAK process (Short-Lived Isotopes Studied by the AKUVE technique). By using a steel interdigital micromixer, a variant of this process termed MicroSISAK was realised. The process investigated was the extraction of ⁹⁹Tc (Eγ = 141 keV ) from 1% NaCl solution containing 2 × 10⁻⁴ TPAC into chloroform.

Interdigital micromixers were used also for gas/liquid contacting. Early investigations have shown that foams with extremely regular and small bubble sizes can be generated, if surface tension and density are adjusted. In the framework of a current EU project (partners: BP, Rhodia, Rogalandsforskning, CNRS Lyon, University College London, University of Strathclyde, University of Dublin), these foaming properties as well as the emulsification are applied for fast serial screening of homogeneous catalysts in multiphase systems.¹⁴ Most often transition metal–ligand complexes are utilised as catalysts which are available only in small quantities and hence demand screening processes with low sample volume. The latter is realised by a pulse injection of a catalyst probe into a flowing multiphase system. High-quality emulsions and foam make the intrinsic kinetics of this probe accessible by setting mass transfer to a high level and may also affect the signal dispersion of the catalyst pulse which in turn has an impact on the signal-to-noise ratio, the reproducibility and the test throughput frequency.

Gas/liquid contactors

Although simple micromixers can be used for some tasks concerning gas/liquid contacting, as discussed above, special devices for that purpose are undoubtedly needed. In this context, IMM developed two devices which rely on types of gas/liquid contacting also known on the macro-scale—a falling film microreactor (see Fig. 6) and a micro bubble column.¹⁵

Fig. 6 Falling film microreactor made of stainless steel. The microstructured reaction plate is realised by wet chemical etching of steel. The housing is fabricated by μ-EDM and precision engineering.

Both devices were applied to the fluorination of toluene using elemental fluorine. This reaction is strongly accompanied by explosions and uncontrolled polymerisations when carried out with conventional equipment. Operation in both types of special microreactors mentioned above, however, was safe and resulted in a yield of about 25% of the mono-substituted o- and p-products.¹⁶

Recent work of IMM focused on the determination of the hydrodynamics in the micro bubble column.¹⁷ For instance, frequency distributions of the bubbles and flow pattern maps were derived (see Fig. 7). Using this information, a reaction model was developed for the example of the oxidation of butyl alcohol to yield butyl aldehyde.

Fig. 7 Flow pattern map of gas/liquid flows in a micro bubble column with a single microstructured channel. For both water and isopropanol, flow patterns are obtained similar to that known from large and mesoscopic channels.

Concerning the falling film microreactor, present work concerns the prolongation of residence time by realising more compact designs with longer fluidic paths.⁴ Moreover, by CFD simulation novel designs presently are evaluated in order to reduce the signal dispersion of the falling film flow.⁴

Tools for reforming

The Microreaction Technology Department also is involved in European and national projects, which aim at the development of gas phase microreactors/heat exchangers for methanol steam reforming and partial oxidation of methane. One project is focusing on the development of systems for generating energy in the Watt-range (see Fig. 8), i.e. devices alternative to batteries (partners: Shell, ECN, Gastec, University of Eindhoven, MESA, ENSIC Nancy). In another project, starting only recently, automotive applications are envisaged (partners: Peugeot, ACA, CIRIMAT, IRC, ICT Prague, POLITO, INFRAGAS). In industrial projects, supplementary reformer devices and novel fuel cell devices are fabricated.

Fig. 8 Hydrogen generation by means of methanol steam reforming in a catalyst test reactor, being operated at short residence times. A Cu-ZnO (γ-Al₂O₃) catalyst layer coated on stainless steel platelets was employed.

All these reformer applications rely on three knowledge bases, previously developed at IMM. Firstly, various gas phase reactors were designed and manufactured and tested at external sites^18,19 (see Fig. 1). Secondly, together with partners (INM, University of Kaiserslautern, BASF, Axiva) catalyst coating of microchannels has been investigated at IMM for about two years. Thirdly, micro heat exchangers have been previously built and tested²⁰ (see Fig. 1).

First time experience with gas phase reactions was acquired when building microreactors for HCN¹⁸ and ethylene oxide¹⁹ syntheses. Moreover, special microreactors were manufactured for other industrial gas phase processes, for instance, concerning partial oxidations.⁴ The first catalysts employed were mostly pure metals. In recent efforts, know-how was achieved which now allows one to coat carrier-supported catalysts of virtually any composition, morphology and thickness (up to practical limits of about 100 μm) within microchannels. In most cases, the layers derived were uniform in terms of layer thickness; at least the exact profiles now are precisely known.

Despite the manufacture of special gas phase microreactors designated for one chemical process, reforming developments, in particular, demanded for the design of more widely applicable catalyst testing and screening microreactors. Several prototypes for different tasks meanwhile were realised, including serial and parallel operation of catalyst coated platelets (see Fig. 9⁴ and 10, HCN¹⁸). A construction kit for an 48-parallel catalyst screening reactor was manufactured and currently is being tested²¹ (see Fig. 10).

Fig. 9 Catalyst test reactor, allowing optionally serial or parallel processing. Thereby, screening tasks, numbering-up investigations and increase in fluidic passage length are accessible.

Fig. 10 One type of assembly of an 48-parallel catalyst screening microreactor which is built of a construction kit of modules. By various assembly of these modules, different kinds of operations are possible, including primary screening, kinetic investigations and scale-up studies.

Recently, work on evaporation systems has been initiated to close one of the last prerequisites needed for a complete reformer design. The development of these complex systems is to be undertaken over the next few years.

Future research topics

The development of screening systems will remain a dominant topic of IMM’s microreaction technology research. In particular, the investigation of catalysts for multiphase systems will be a matter of future investigation. Specially adapted gas–liquid contactors, preferably of falling film design, are currently being developed and will be evaluated in the up-coming months. The minimisation of signal dispersion will be one of the most important quality parameters thereof.

For all applications, complete system development will be the main focus. The construction of turn-key chemical plants has already been started in the past two years, but will steadily gain in importance. The integration of process control devices therein is one issue to be addressed. Generally, it is not planned to rely only on standardised systems built up of a kit of only a few modules. It is IMM’s philosophy that the complex nature of chemical processes will always demand the use of advanced unique solutions which require considerable technical skill in order to be realised. Nevertheless, systems with standardised architecture will have their place and will attract customers.

Particularly the complete system approach will be transferred to reforming applications. Additionally, the respective tool box has to be further completed, demanding new types of microdevices. The mass fabrication aspect is of utmost importance for reformer applications as they target the production of large numbers in the automotive and energy sector. Here, IMM will forge alliances with partners capable of handling this request (see below).

Finally, a development process still considered to belong to R&D must always prove that it is beyond teething troubles. Establishing reliability, practicability and quality will remain a continuous IMM research topic. Initiatives to minimise fouling are only the start, precise process control will be the future’s focus. The motto is: ‘not as small as possible, but as small as needed in the true sense of chemical process engineering’.

Alliances

Needless to say, work in an innovative, interdisciplinary field such as micro chemical processing cannot be undertaken by one party alone. IMM therefore has forged alliances to broaden the technological base for future microfluidic developments.

Heatric (Poole, UK) and IMM aim at using wet chemical etching processes for the professional production of steel microfluidic devices. Heatric, a division of Meggitt (UK) Ltd., has supplied chemically etched and diffusion bonded compact PCHEs (Printed Circuit Heat Exchangers) to the process industries for over 15 years. PCHEs are widely used in offshore gas processing applications as well as in the chemical industry. Recently Heatric introduced the PCR (Printed Circuit Reactor), a range of reactor designs integrating chemical reaction, fluid mixing and heat exchange.

Together with Symyx (Santa Clara, USA) IMM develops special catalyst screening reactors.

Naturally, close links have been established with companies which originated from IMM and which are commercially active in the field of microfluidics. For instance, novel glass microreactors are jointly developed with Mgt Mikroglas Technik AG (Mainz, Germany). These devices can be either integrated in the mikroglas mikroSyn system or used in freely configured IMM plants. Similar efforts are undertaken with CPC GmbH (Frankfurt, Germany) with regard to the realisation of special steel microreactors. These devices manufactured by IMM will be employed by CPC for contract research with regard to chemical applications.

Assessing the market needs

Besides its bilateral projects with and for industrial customers, the IMM also strives to gain a more complete picture of what the market needs are at present and what they are likely to be in the future. To this end, a market study called PAMIR (the acronym stands for ‘Potential and Applications of MIcroReaction technology’) was recently conducted together with the French consulting company YOLE Développement. Funded to a major part by the European Commission under the GROWTH programme, some 90 companies and institutions were interviewed over the past few months.

The results of the PAMIR study will soon become available (for more details see www.imm-mainz.de/pamir). In this context it may suffice to point out that the companies show an almost equal interest in unit operations and reactors. Looking more closely at the former, more than half of the present or potential users request mixers and heat exchangers. A more qualitative result may be established by plotting the time that certain industrial sectors have been aware of microreaction technology versus the expectation when it will have a first major impact within each of these sectors (see Fig. 11). As can be seen, a cluster of four sectors is likely to make significant use of microreaction technology within the next few years.

Fig. 11 A result from the PAMIR study: while microreaction technology may be regarded as established on the laboratory level, a first larger impact on production is to be expected in the domains of the ovoid. (The diagram is based on 48 interviews with companies from the industries mentioned in the figure.)

References

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V. Hessel , H. Löwe and T. Stange
Institut für Mikrotechnik Mainz GmbH, Carl-Zeiss-Str. 18–20, 55129, Mainz, Germany

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