Flow switching

FIGURE 3.24 Schematic diagram of a microfluidic device that creates a concentration gradient and a temperature gradient. The inset shows the channel layout design [447]. Reprinted with permission from Reprinted with permission from the American Chemical 

It was found that Sylgard 184 provided a higher tensile strength and elongation than RTV 615, and so the former was selected for fabrication of the microvalve control layer [449]. Moreover, the control channel must be filled with water before use, in order to prevent bubble formation in the fluidic channel due to the gas permeability of PDMS [449]. 

FIGURE 3.27 An elastomeric peristaltic pump using three valve control channels. The fluidic channel is 100 pm wide and 10 pm high. Dimension of microvalve control channels = 100 pm by 100 pm by 10 pm applied air pressure = 50 kPa. Peristalsis was typically actuated by the pattern 101, 100, 110, 010, Oil, 001, where 0 and 1 indicate valve open and valve closed, respectively [167]. Reprinted with permission from the American Association for the Advancement of Science. 

There are other reports on the use of PDMS valves [124,357,368,450]. A thin PDMS valve membrane (3 pm) allowed for a high valve density (300 valves/cm2). Actuation of the valves was caused by expansion of the air in the air-filled cavities below the valves. PDMS prepolymer diluted with toluene reduced the solution viscosity, allowing for spin-coating of very thin layers [124]. 

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Flow switch Siemens Electric al Engineering Handbook, Siemens AG. Munich,... 

PW circulation pumps B/A mechanical failure PW circulation pumps B/A stopped by operator PW collecting tanks pmnps B/A failure PW flow switch failure PW supply pumps B/A controller 2 failure PW supply pumps B/A mechanical failure PW supply pumps B/A stopped by operator PW surge tank instrument failure -15 C glycol storage tank leak PW level valve fails... 

Fire detection For efficient operation, control of the ventilation must be linked to a fire-detection system or sprinkler flow switch to ensure that the ventilators are operated as early as possible. Natural ventilators must... 

Water flow switch, to stop the machine if flow stops in the chilled water circuit... 

Flow switch open, in load or condenser water... 

As C-C bond formation is an important step in organic synthesis, particularly for pharmaceutical applications, it is useful to look for operation modes of chemical micro processing that allow one to carry out combinatorial chemistry investigations. As such, the serial introduction of multiple reactant streams by flow switching was identified [66,67]. The wide availability of precursors for acyiiminium cations has led to the expression cation pool [66, 67]. 

OS 30] [R 30] [P 22] By simple flow switching, serial combinatorial synthesis for creating a cation pool from diverse carbamates and silyl enol ethers was accomplished (Figure 4.46) [66, 67]. The conversions and selectivities were comparable to continuous processing using three feed streams only (see Conversion/yield/selec-tivity, above). 

A six-port valve was used in both manual and semi-automated SPME interfaces and PEEK tubing used to connect the HPLC system to the SPME probe. A Cohesive HTLC 2300 with dual pumps along with a Sciex API 3000 mass spectrometer was used for LC/MS/MS and a Symmetry Shield RP-18 (5 ji, 50 x 2.1 mm) for HPLC. A quaternary pump with flow switching was used for desorption chamber flushing along with MS make-up flow and a binary pump for LC/MS/MS. Acetoni-trile/0.1% acetic acid in water (90 10, solvent B) and 10 90 acetonitrile/0.1% aqueous acetic acid (solvent A) were used, with 10% B for 0.5 min ramped to 90% B in 2 min and held at this concentration for 1.5 min before returning to 10% B for 1 min at a flow rate of 0.5 mL/min. 

An outstanding feature of the cation flow method is that the method enables continuous sequential combinatorial synthesis by simple flow switching as shown in Figure 6. In the first step, the cation flow generated from precursor A1 is allowed to react with nucleophile Bl. Then, the cation flow is allowed to react with the nucleophile B2. In the third step, the cation flow is allowed to react with nucleophile B3. Then, the precursor of the cation is switched to A2, and the cation flow generated from A2 is allowed to react with nucleophiles Bl, B2, and B3 sequentially. Then, the precursor of the cation is switched to A3, and the... 

Valve-based modulators rely on flow switching mechanisms, and traditionally discrete (sub)sampling of effluent has been performed for subsequent separation on... 

Flow switches, pressure relief valves, and high pressure switches, all tend to be less than compeltely reliable, and while we have in no way checked all available models and manufacturers, our experience Indicates need for additional attention. Components related to switches, and pressure relief valves, could be of particular concern in semi-attended and fully automated installations. 

T H ) Temperature switch ( P L ) Pressure switch ( PSH ) Pressure switch (jsT) Flow switch ( MP Motor protection switch (PT100 Temperature sensor For DRYVAC with LIMS ( C Current sensor ( EPS ) Exhaust pressure sensor... 

Behavior of the fluids in the microfabricated channels are different from those in the millimeter scale channels. Miniaturization of micro flow devices opens a new research field, microfluidics which represents the behavior of the fluid in the micro channel [8]. Since the Reynolds number in the micro channel is usually below 200, the flow is laminar and special design concepts are necessary for the fluidic elements of mixers, reaction coils etc. in the pTAS. Some components of flow switches and fluid filters were developed using laminar flow behavior. 

Flow switches using fluidics in small Reynolds number are fabricated. The principle of the flow switch is shown in Fig. 4 [21]. Mixing of the sample stream and carrier liquid is negligible when the contact area is small and the contact time within subsecond range. The width of the sample stream is controlled by two carrier flows. This structure can be applied for a valveless sample injection in FIA and for sorting of particles and living cells in flow cytometry. A flow switch having 5 outlets has also been obtained by this method. 

Flow switching in a PDMS chip can be achieved by deflection from two side channels (see Figure 3.34) [359]. When there is zero hydrostatic pressure, the two streams of fluorescent beads flow equally to two channels. When the hydrostatic pressure from the bottom channel is greater (i.e., 2 mm liquid level differ-... 

FIGURE 3.36 Schematic representation of a micromachined pre-focused 2x6 flow switch with six inlet ports and six outlet ports. The flow switch integrates two important microfluidic phenomena, including hydrodynamic focusing and valveless flow switching. The pre-focused samples can be injected into desired outlet ports precisely [263]. Reprinted with permission from the Institute of Physics Publishing. 

Lee, G.B., Hwei, B.H., Huang, G.R., Micromachined pre-focused MxN flow switches for continuous multi-sample injection. J. Micromech. Microeng. 2001, 11, 654-661. 

Blankenstein, G., Scampavia, L., Branebjerg, J., Larsen, U.D., Ruzicka, J., Flow switch for analyte injection and cell/particle sorting. Micro Total Analysis Systems 96, Proceedings of 2nd International Symposium on pTAS, Basel, 19-22 Nov. 1996, 82-84. 

Large-scale Sorbex processes have been developed for a variety of different bulk separations a brief summary is given in Table IV. In recent years, the same principle has been applied also to a wide range of chiral separations and other difficult separations that are important in the pharmaceutical industry. Several novel system configurations have been developed. In one system, a carousel of 12 small columns rotates between two stationary circular headers, which act as the switch valve, thus effectively incorporating the adsorption and the flow switching functions within a single unit. 

For the preparation of samples of used catalyst, the conventional flow apparatus previously described was used. In order to collect the catalytic bed, the o-xylene/air mixture was interrupted and a nitrogen flow switched on. This was maintained until the reactor reached the room temperature. 

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