On the Applications of Alternative Energy Forms and Transfer

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Ind. Eng. Chem. Res. 2007, 46, 4232-4235

On the Applications of Alternative Energy Forms and Transfer Mechanisms in Microprocessing Systems Andrzej Stankiewicz* Delft UniVersity of Technology, Process & Energy Department, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands

Alternative energy forms and transfer mechanisms have been shown to have dramatic intensification effects on processes carried out in conventional, “macroscale” equipment. The present paper focuses on possible applications of those alternative energy forms in microprocessing systems and brings a review of the relevant publications in the field. Comparison of the investigated applications of alternative energy forms in conventional equipment and in microprocessing systems shows clear differences. At the same time, microtechnology is expected to become the primary tool for exploring the alternative energy forms in chemical processing to gain more understanding of the underlying mechanisms and related phenomena. Introduction Microprocessing systems are often seen as the most extreme form of process intensification. The application of microtechnology not only minimizes the inventory of the potentially dangerous chemicals in the equipment but also allows for excellent thermal control of the processes. On the other hand, alternative sources and forms of energy are broadly investigated in the “conventional-scale” processing and some of them have already been applied commercially. To the microprocessing world, the alternative forms and transfer mechanisms of energy are quite new and have been investigated to a limited extent so far. The present paper brings a review of those investigations, trying to lay down the link between the mechanisms, phenomena, and effects observed on the conventional scale and those expected in the microequipment. In particular, four forms of energy are discussed: high-gravity fields, electric fields, electromagnetic fields (microwave and light), and acoustic fields. High-Gravity Fields. The use of high-gravity fields (HiGee) generated by the centrifugal operation dates actually to the beginning of the industry with such physical transport operations as pumping, compression, and solid-liquid separations. The first use of high-gravity fields to intensify mass-transfer operations took place in 1945 when a centrifugal liquid extractor based on earlier patents by Podbielniak was commercially applied to recovery of penicillin. In recent years, two HiGee technologies attracted particular attention: the rotating packed beds (RPB) and the spinning disk reactors (SDR). In rotating packed beds, Ramshaw and Mallison1 reported a 27-44-fold increase of the liquid-side mass-transfer coefficient and a 4-9-fold increase of the gas-side mass-transfer coefficient, with respect to the stationary bed of 1/2-in. Intalox saddles. Lin et al.2 reported the HETP (height equivalent of theoretical plate) values of 3-9 cm, compared to 30-40 cm for the conventional structured packings. On the other hand, the mass- and heat-transfer performance of the spinning disk reactors is also impressive, with local heattransfer coefficients ranging from ca. 10 000 to ca. 30 000 W/m2K and local mass-transfer coefficients between ca. 4E-04 and 1E-03 m/s, much higher than predicted by the Higbie model * To whom correspondence should be addressed. Tel.: +31 15 27 82147. Fax: +31 15 27 86975. E-mail: [email protected].

(Aoune and Ramshaw3). Such excellent transfer rates allow significant shortening of the processing times and equipment miniaturization, as it was shown by Oxley et al.4 on a phasetransfer-catalyzed Darzen’s reaction. The processing time was shortened by a factor of 1000 compared to the conventional batch processing, while the inventory was reduced by a factor of 100. In the microprocessing systems, additional increase of the transfer rates by using high-gravity fields appears to be of less importance. Here, high-gravity fields are primarily used to achieve an intensified mixing at high throughputs as well as to realize specific contacting patterns. In the comix Coriolis microreactor developed at the University of Freiburg (Haeberle et al.5), liquid is pumped through radial microchannels of a microreactor rotating with frequencies between 20 and 120 Hz and reaching high-gravity conditions of up to 10 000g. This allows fast mixing at high throughputs up to 1 mL/s/channel. Ducre´e et al.6 report shortening of the mixing times by up to 2 orders of magnitude in cocurrent centrifugal flow through straight microchannels. Centrifugal force can also be applied to increase the separation efficiency in the microchromatographs (Penrose et al.7). Finally, high-gravity fields can also be used to realize countercurrent phase contacting in microstructures. An example here is the spinning microchannel multiphase contactor developed and patented by the University of Sheffield (Allen et al.8), which is claimed to enable efficient countercurrent contacting between gas and liquid phases in such processes as distillation. Electric Fields. The ability of superimposed electric fields to improve transport phenomena in chemical processes has been known for many years. Mass-transfer enhancement by the electric field in the liquid-liquid extraction has been investigated by numerous authors. Usually, enhancement factors between 2 and 10 are reported (e.g., Weatherley9). Scott10 describes the surface area formation via the electric-field-induced emulsification. The method may lead to a 200-500 times increase in the surface area per unit volume, compared to the millimeter-sized droplets in the conventional extraction processes. Similarly, droplet size manipulation by the use of electric field has been investigated in microprocessing systems. Chabert et al.11 developed an electric field-based system for the coalescence of microfluidic droplets. Interesting observation was that coalescence was not accompanied by the mixing of the

10.1021/ie0612764 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/10/2007

Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 4233 Table 1. Effect of Microwave Heating on Product Yield in Capillaries and Microreactors reaction type

reaction time

product yield no MW irradiation

product yield with MW irradiation

ref

Suzuki coupling of 4-bromobenzonitrile and phenyloboronic acid Suzuki-Miyaura coupling of 4-iodooct-4-ene and 4-methoxyboronic acid benzyl alcohol oxidation to benzaldehyde

15 s 168 s 17 s

1% 0% 8.4%

37% 59% with Au coating 65% 75.4%

23 24 25

Table 2. Effect of Microreaction Environment on the Rate of Photochemical Reactions

reaction type

product yield conventional

product yield microreactor

reaction time conventional

reaction time microreactor

intensification factor

ref

photocycloaddition photocycloaddition redox-combined synthesis

56% 8% 22%

55% 88% 22%

180 min 120 min 60 min

3.4 min 120 min 0.86 min

53 11 70

35 36 37

droplet contents. Also, Link et al.12 used electric fields to create, recombine, split, and sort droplets in a microfluidic device, while Li et al.13 produced a trimodal aspirin droplet size distribution with the most significant size of ca. 6 µm by applying an electric field to a needle. Electric fields were shown to intensify mixing processes in microfluidic devices. Tsouris et al.14 reported shortening of the mixing length by ca. 35 times, when applying electric field to alcohol mixing in microchannels. El Moctar et al.15 investigated the influence of both DC and AC electric fields on the mixing in microreactors and reported substantial increase of the mixing index (up to ca. 70 times) with increasing field intensity. Electric fields are also reported in microprocessing systems as a method to induce electrostatic repulsions between the macromolecules (e.g., proteins) and the inner surface of the microchannel (Blanchard and Lee16). Such reduced adsorption of macromolecules enhances separation resolution and efficiency (e.g., in electrophoresis) and can also help prevent fouling in the microsystems. Obviously, microsystems can also be used for carrying out electrochemical reactions, although most of the processes investigated so far concern the area of combinatorial (bio)organic synthesis and chemical analysis (Suga et al.,17 Mengeaud et al.,18 Yoon et al.,19 and Schultze and Tsakova20). Electromagnetic Fields, Microwaves. The literature concerning the enhancement effects of the microwaves on chemical reactions, homogeneous liquid-phase organic reactions in particular, is exceptionally rich. Authors generally agree about the ability of microwave heating to accelerate organic reactions, and acceleration factors from several to more than a thousand are reported (Gedye et al.,21 Larhed et al.22). In many cases, increase of the product yield has also been observed. Only a few papers on the use of microwaves in continuousflow capillary reactors or microreactors have been published so far (see Table 1). Two of them report clear benefits associated with rate enhancements, higher yields, and greater product selectivity for Suzuki cross-coupling reactions (He et al.,23 Comer and Organ24). In another paper, an isothermal operation for oxidizing benzyl alcohol to benzaldehyde was performed in a continuous microchannel reactor placed in a microwave oven (Jachuck et al.25). In the latter study, the influence of microwave intensity and residence time was studied and the authors found microwave to accelerate the reaction rate (up to ca. 10 times) and claimed to have observed an unexplained nonthermal microwave effect. Dramatic effects of microwave irradiation have also been reported in heterogeneous catalysis. For example, Zhang et al.26 investigated microwave-assisted oxidative coupling of methane on alumina-supported La2O3/CeO2 catalyst and observed that in the absence of oxygen conversion into the C2 products occurred at temperatures about 200 °C lower than during the

conventional heating. Zhang et al.26 ascribed this dramatic effect to the CH4 plasma formation and arcing. In other heterogeneous processes investigated, the phenomenon of selective microwave heating of catalytically active nanoparticles well above the support temperature was reported (Thomas,27 Zhang et al.28). Depending on particle size and microwave frequency used, temperature hot spots exceeding 200 °C could be reached. Regardless of the nature of the observed effects, use of microwaves in heterogeneous catalysis may open an extremely interesting new research area in microreaction engineering. Coupling of very fast (pulsed) microwave heating with very fast product quenching in the microchannels could possibly bring significant improvement in the selectivity of some heterogeneously catalyzed processes. First studies in this area are on the way, for example, at the Technical University of Clausthal, where new microreactors for microwave-assisted heterogeneous processes have been developed (Cecilia et al.29). The reactors include a microwave-transparent polymeric support containing catalytically active noble metal (Pd) nanoparticles. Electromagnetic Fields, Light. The use of light, either artificial or solar, can drastically improve the selectivity of many industrially relevant chemical reactions. Examples here include complete (100%) selectivity of toluene oxidation to benzaldehyde (Sun et al.30) or cyclohexane oxidation to cyclohexanone (Sun et al.31). The most important hurdle in photocatalytic reactors is their low energetic efficiency because of light absorption and dissipation between the source and the catalytic site. Second, the illuminated specific surface of the catalyst in such reactors remains relatively low. Recently, attempts were undertaken to eliminate those drawbacks. A monolithic photoreactor with catalytically coated optical fibers in the monolith channels was shown to deliver apparent quantum yield 2 orders of magnitude higher than the conventional units (Lin and Valsaray32). However, an exponential decay of light intensity along the channels was still observed. On the other hand, the illuminated specific surface of a microstructured reactor usually clearly surpasses that of the conventional-scale photoreactors, and factors of up to 400 are reported (Gorges et al.33). Also, the amount of TiO2 catalyst per unit reactor volume is more than an order of magnitude higher in microreactors than in conventional photoreactors (Barthe et al.34). Using microreactors can lead to a much shorter reaction time and more intensive processing, as shown in Table 2 for several recent Japanese studies. Practically all photocatalytic microreactors use external lightemitting elements (LED, tungsten lamps, etc.), which brings up again the light decay problem, especially when multilayer structures have to be illuminated in a uniform way. The challenge here would be to either design LED-based lightemitting layers or to imbed the light-emitting elements directly in the structure of the catalyst layer. First studies in this direction,

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Table 3. Main Applications of Alternative Forms of Energy in Conventional Equipment and in Microprocessing Systems Investigated So Far energy source/form high-gravity field electric field electromagnetic field, microwave electromagnetic field, light acoustic field

applications in conventional equipment

applications in microprocessing systems

improved heat transfer improved gas-liquid and liquid-liquid mass transfer droplet size manipulation for improved liquid-liquid mass transfer

improved processing capacity improved mixing droplet manipulation for improved product quality

increased reaction rates and (sometimes) yields increased yields increased reaction rates and (sometimes) yields improved gas-liquid and liquid-solid mass transfer

for example, using luminescence in porous silicon structures (Gole et al.38), are already underway. Acoustic Fields, Ultrasound. Similarly to microwaves, the use of ultrasound can speed-up chemical reactions by factors of >100 and can also dramatically increase the product yield (Thompson and Doraiswamy39). Acoustic irradiation appears to be able not only to boost chemical reactions via microcavity implosions but also to intensify gas-liquid (up to 5 times, Kumar et al.40) and liquid-solid (up to 20 times, Neis41) mass transfer in multiphase systems. A reduction of the boundary layer thickness because of the microscale turbulence and reduction of the viscosity in the boundary layer was postulated as the mechanism behind the observed phenomena. In microsystems, the ultrasonic fields are mainly used for flow measurements (Khuri-Yakub and Degrtekin42) or for agitation by inducing rotating vortex flows in microchannels (Bengtsson and Laurel43). Carrying out sonochemical reactions in microsystems may open up new possibilities here and may help to overcome some problems existing in the conventionalscale sonochemical reactors, such as low-energy efficiency because of small irradiation depths, poorly described energy fields in the reacting medium, and so forth. On the other hand, creating microcavities in the microchannels could intensify mass-transfer phenomena there. In some cases, it might also help to avoid the nightmare of any microsystem user, that is, fouling. Conclusions As can be seen from the summary in Table 3, the research on the use of alternative energy forms and transfer mechanisms in microprocessing has often focused on different functions and applications than in the conventional macroscale processing. There is little doubt that alternative sources and forms of energy can intensify a process carried out in microprocessing systems. However, contrary to the conventional-scale equipment, the process intensification incentive here is not to miniaturize the devices further. Using alternative forms and transfer mechanisms of energy may significantly enlarge the applicability range of microtechnology by accelerating chemical processes to “fit” in microsystems, by reaching higher product yields by combining alternative energy-transfer mechanisms with microprocessing features (e.g., fast heating up of the reactants and a fast quenching of the products), by manufacturing products difficult to produce using conventional methods, and by reducing or preventing some basic problems in the microprocessing system operation, such as fouling. On the other hand, alternative energy forms and transfer mechanisms still present a field full of scientific unknowns and tremendous research challenges. Microtechnology, with its highly structured, well-defined geometries, can become the

fouling prevention increased reaction rates increased yields or reaction rates improved mixing measuring liquid characteristics (pressure, density, viscosity, flow rate) pumping liquids

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ReceiVed for reView October 3, 2006 ReVised manuscript receiVed December 3, 2006 Accepted December 5, 2006 IE0612764