The Use of Microchannels for Separation and Preconcentration: Comparison Between Three-Dimensional and Planar Configurations (original) (raw)
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Review on gas–liquid separations in microchannel devices
Chemical Engineering Research and Design, 2013
Separations are indispensable to most chemical processes, regardless of operational scale. In the field of chemical microprocess engineering, separations have so far attracted rather limited attention compared to reactive processes, although increasingly more research is directed towards this area. Microstructured devices offer the opportunity of intensifying transport processes associated with separation operations by providing high ratios of contact areas to volume, short transport distances and high driving force gradients. These attributes have so far been exploited for various microseparations. In this review, we focus on separations where gas and liquid phases are present, and in particular on absorption, stripping and distillation. Two main approaches for contacting the two phases have been employed: continuous and dispersed phase contactors. Removal of components from gas mixtures by absorption and stripping of volatiles from liquid mixtures have been achieved in falling film devices, thin porous plate microcontactors, as well as in dispersed flow systems. Distillation has been performed in microchannel devices, utilizing capillary, centrifugal and gravity forces, vacuum or carrier gas.
Analyst, 2004
This paper describes the micro-fabrication and preliminary assessment of a miniature polydimethylsiloxane (PDMS) device for performing rapid, parallel liquid phase chromatographic separations driven by centrifugal force in microchannels. Device components include a main separating channel, into which a high performance liquid chromatography (HPLC) particulate stationary phase was packed under pressure by application of centrifugal force, in addition to solvent and sample reservoirs. Also described are methods for sealing such devices based upon partial polymerisation of PDMS. The mobile phase flow rate through a typical device was measured and several important chromatographic parameters determined from a test separation. An expression describing mobile phase flow through packed channels was also developed, based upon work on liquid flow in open micro-channels. Good agreement between predicted and measured flow rates were observed. Some predictions for potential uses of such devices and possibilities for further miniaturisation are discussed.
Micro-separation of fluid systems: A state-of-the-art review
Separation and Purification Technology, 2013
Micro-separation technology is based on the application of micro-structured equipment units. Such units contain structured elements with micro-scale dimensions. They are necessary for the implementation of classical downstream unit operations within the overall micro-scale production process. However, the development of micro-separation units for fluid mixtures faces substantial difficulties. This article gives an overview of current research activities in the field of micro-separation technology focusing on the unit operations extraction, absorption/desorption, distillation, as well as particle or droplets sorting. In addition, some examples are given for a combined application of different micro-separation operations. Both advantages and limitations of micro-separation operations are highlighted.
Journal of Separation Science, 2007
The chromatographic characteristics were determined for a set of microfabricated separation channels structured with cylindrical and diamond-shaped pillars with a characteristic size of 5 lm. Channels with different structures and porosities were etched in a silicon wafer using lithographic techniques. The permeability for flow of the channels was shown to increase strongly with the overall porosity. Diamondshaped pillars appeared to yield a slightly higher permeability than cylindrical pillars at the same channel porosity. Compared to packed columns, permeabilities were higher by a factor of up to 5. Band dispersion in the channels was measured with an unretained fluorescent probe compound using a fluorescence microscope. A relatively large variation in the observed plate heights between channels was found, which was mainly attributed to the inaccurate geometry of the structure close to the side walls. Reduced plate heights between 0.2 and 1.0 were obtained. The lowest plate heights were found for channels with low porosity. The chromatographic impedances were calculated and compared to the values for the traditional chromatographic systems. For one of the structured microchannels the impedance was found to be more than ten times lower than for a column packed with nonporous spherical particles. With the data collected, predictions are given on the possibilities in terms of efficiency and speed offered by structured microchannels for pressure-driven separations, taking practical constraints into account.
Three-dimensional microchannels as a simple microreactor
Sensors and Actuators B: Chemical, 2009
The aim of this work is the production of a simple and compactable device able to remove hydrocarbons from a gaseous sample. The device is composed of a microstructure, inlet/outlet, heating and detection systems. The microstructure (microreactor) corresponds to a manifold formed by an array of 192 threedimensional microchannels, 40 m wide and 8 mm long each. Microchannels surface was modified by electroless plating copper deposition in order to promote catalysis. The structure can be heated up to 300 • C in a few seconds. The flow mechanisms in the structure and the heating properties were simulated using FEMLAB 3.2b and COSMOS ® 5.0 packages, respectively. The microchannels were exposed to volatile organic compounds during catalysis. An inexpensive tin oxide sensor and correspondent electronic coupled to computer storage data provided detection. Catalysis occurred and could remove at least 10 g of n-hexane in a single batch, but not 2-propanol. This simple miniaturized device is compact, low-cost and can be used not only for sample pretreatment in microanalysis but also in synthesis of new chemical compounds.
Analytical Chemistry, 2011
We report on the results of an experimental study established to optimize the design of microfabricated flow distributors for use in pressure-driven separations and reactions in flat-rectangular channels. For this purpose, the performance of a wide variety of possible flow distributor designs etched in glass/silicon wafers was compared, using CCD camera detection to study the shape and variance of the bands eluting from them. The best performance was obtained with radially interconnected distributors with a diverging inlet section and filled with diamond-shaped pillars, oriented perpendicular to the main flow direction and with a high transversal over axial aspect ratio. It was found that the best distributor designs start with a diverging section containing some 10-12 subsequent rows of high aspect ratio pillars (with a transversal width making up 10-15% of the final channel width) and with a divergence angle selected such that the sloped side-walls run parallel with the sides of the diamond-shaped pillars. After this zone, one or more regions with pillars with a smaller aspect ratio should be provided to increase the number of exit points. To prevent the formation of dead zones in these subsequent zones, so-called distributor wedges can be used to prevent the formation of any dead zones in the wake of the large aspect ratio pillars of the preceding section.
Journal of Chromatography A, 2003
We present a new stationary phase coating method, yielding a monolayer of densely arrayed porous HPLC beads (d 54 mm) for use in a disposable shear-driven flow LC system. The system is inherently suited for whole-column detection p through the small voids between the individual particles of the layer. The chromatographic performance of the system has been characterized by performing a series of coumarin dye separation experiments (reversed-phase mode) and by measuring the theoretical plate height as a function of the mobile phase velocity. The resulting Van Deemter curve, yielding a value of about 90 000 plates / m near the u5u velocity, shows good agreement with the theoretical expectations, and hence opt constitutes the first full validation of the theory of shear-driven chromatography.
Sensors and Actuators B Chemical, 2013
Adsorbent-packed preconcentrators are essential elements of most microanalytical systems designed for monitoring airborne volatile organic compounds (VOC) at low concentrations. These devices also serve as thermally desorbed injectors that transfer focused bands of captured VOCs to downstream separation and/or detection modules. Despite their importance, the factors affecting the capture efficiency of such devices have not been extensively or systematically studied. In this study, the dynamic retention capacities of four deep-reactive-ion-etched Si micropreconcentrator-focusers (PCF) packed with a commercial graphitized carbon, Carbopack X (C-X), were characterized for several VOCs and compared to those of a reference capillary preconcentrator-focuser (cPCF). Devices were challenged with ∼100 parts-per-billion of benzene, 2-butanone, toluene, or n-heptane in dry N 2 over a range of volumetric flow rates, Q. The relationships between the bed residence time, , and the 10% breakthrough volume and breakthrough time (V b−10 and t b−10 , respectively) were evaluated in the context of the modified Wheeler Model. Both V b−10 and t b−10 decrease monotonically with decreasing , in accordance with the model. The performance of the largest PCF, packed with 2.3 mg of C-X, was comparable to that of the reference cPCF packed with a similar quantity of C-X. The critical flow rates, Q c−10 , corresponding to immediate breakthrough, ranged from 70 to 290 mL/min and varied directly with the affinity of the vapor for the adsorbent. As a practical operating guideline, it is recommended that flow rates be limited to <0.4Q c for reliable performance of any PCF used for quantitative analysis. Estimated preconcentration factors range from 730 to 39,000. Challenges to predicting device performance via the modified Wheeler Model are illustrated.
Microchannel Devices for Efficient Contacting of Liquids in Solvent Extraction
Separation Science and Technology, 1999
Microchannel devices were designed and tested for efficient contacting of two liquids in solvent extraction and the results are presented. This study is part of an overall effort to produce and demonstrate efficient compact devices for chemical separations. Engineering these devices at the microscale offers many technical advantages. Achieving high contact area per unit system volume, thin film contacting, and establishing uniform flow distribution results in substantially higher throughput per total system volume over conventional technologies. This extended summary provides a general overview of background, theory, experimental procedures, results, and potential applications of microchannel devices for solvent extraction. BACKGROUND Developments in microfabrication techniques, based on techniques founded in the electronics industry, are facilitating development of novel hardware for performing traditional chemical processing at a miniaturized scale. Features can now be designed with resolutions approaching 1-10 µm or less in microchemical components. At this scale, the distances for heat and mass transfer can be uniformly minimized, giving extremely rapid transport rates that result in very short residence times and high throughput per unit hardware volume. In addition, process conditions can be more tightly controlled. Eventually, it may be possible to miniaturize entire chemical processes including most of the elements of traditional chemical processes, (e.g., heat exchangers, reactors, separators, and actuators).
Utilization of Micromixers for Extraction Processes
Chemical Engineering & Technology, 2001
The aim of the investigation was to evaluate the extraction performance of a mixer settler set-up for miniplant technology using static micromixers as an alternative to conventional stirring apparatuses. A comprehensive experimental study was conducted at BASF AG to broaden the technology base for the ªextractionº unit operation which is well established for miniplants in order to be utilized for microplant systems. The work proved that micromixers, or micromixer arrays, are highly efficient apparatuses for extraction purposes. The extraction efficiency was found to be a function of volume flow, which could be explained in light of the volume flow dependence of the mean droplet size and, hence, the specific surface area of the dispersions intermediately formed. At optimal flow conditions, one practical mixer settler stage was found to yield one theoretical plate for most systems investigated. . Phase partition as a function of volume flow using a 300 channel micromixer array: A) system 3, water (succinic acid)/ n-butanol, and B) water (dl-pantolactone)/methyl-i-butylketone. The solid horizontal lines indicate thermodynamic equilibrium.