Flow device, continuous

Continuous flow devices have undergone careful development, and mixing chambers are very efficient. Mixing is essentially complete in about 1 ms, and half-lives as short as 1 ms may be measured. An interesting advantage of the continuous flow method, less important now than earlier, is that the analytical method need not have a fast response, since the concentrations are at steady state. Of course, the slower the detection method, the greater the volumes of reactant solutions that will be consumed. In 1923 several liters of solution were required, but now reactions can be studied with 10-100 mL. 

In much the same manner as pumps, compressors are classified as one of two general classes positive displacement or dynamic (see Figure 3-68) [23]. These two general classes of compressors are the same as that for pumps. The positive displacement class of compressors is an intermittent flow device, which is usually a reciprocating piston compressor or a rotary compressor (e.g., sliding vane, screws, etc.). The dynamic class of compressor is a continuous flow device, which is usually an axial-flow or centrifugal compressor (or mix of the two). 

Micro reactors are continuous-flow devices consuming small reaction volumes and allowing defined setting of reaction parameters and fast changes. Hence they are ideal tools for process screening and optimization studies to develop solution-based chemistries. 

Figure 2 Drawings of commonly used mixers in continuous-flow and stopped-flow devices. Mixing in the four-jet tangential mixers occurs in the orifice of the Pt inlay (MHQ) or in a Teflon gasket (RFQ). Ri and R2 denote reactant 1 and reactant 2, respectively...

Stopped flow and continuous flow methods [11] have been used to follow proton transfer reactions with half-lives in the millisecond range. The stopped flow method which is more popular is essentially a device for mixing the reactants rapidly (typically in one millisecond) together with some means of observing the fast reaction which follows. Proton transfer from p-nitrobenzyl cyanide to ethoxide ion in ethanol/ether mixtures at —77 °C was studied in this way [12]. The reaction was followed spectrophotometrically. The most rapid reaction occurred with ti/2 ca. 2 x 10 2 sec although the equipment was suitable for following reactions with f1/2 ca. 2 x 10 3 sec. A similar method has been used to measure rates of proton transfer between weak carbon acids (for example, triphenylmethane) and bases (for example, alkoxide ions) in dimethyl sulphoxide [13], A continuous flow apparatus with spectrophotometric detection was used [14] to measure rates of ionization for substituted azulenes in aqueous solution (4), reactions for which half-lives between 2 and 70 msec were observed. 

In addition, stagnation point (it is a point at which the net magnitude of velocity is zero, such as at the midpoint of a cylinder or a sphere in cross flow) devices, such as the dough roll mill, cross slot flow, and opposing jet devices can be used in which an area of intense extensional flow is created without the need to sustain a continuous filament. The latter technique is illustrated in Figure 3-27. Clark (1997) obtained extensional viscosity data on syrups and food gums dispersions using creation of... 

Recent inventions in micro and nano-scale systems and the development of micro and nano-scale devices continues to pose new challenges, and the understanding of the fluid flow and heat transfer at such scales is becoming more and more important. In Chapter 6, microscale heat transfer is presented as a Topic of Special Interest. 

Pudjiono, P.I. Tavare, N.S. Garside, J. Nigam, K.D.P. Residence time distribution from a continuous-couette flow device. Chem. Eng. J. 1992, 48, 101. 

The ex pans ion-type CPC is cyclic in operation, and this may pose a problem in measuring concentrations from steady flow devices such as the electrical mobility analyzer and diffusion battery discussed later. Continuous-(low CPCs have been developed in... 

The heat produced during the growth of microorganisms can be also be used for biomass concentration estimation. Different calorimetric devices (external-flow, twin-type, and heat-flux calorimeters) and different calorimetric techniques (dynamic and continuous calorimetry) have been used for on-line biomass estimation [8j. In most cases, the experimental setup is complicated and measurements are restricted to relatively small volumes (less than 1 L). Larger devices (continuous calorimeters for volumes up to 14 L) were studied by Luong and Volesky [123-125]. One of the best devices seems to be the heat-flux calorimeter developed by Marison and von Stockar. Several applications to bioprocess monitoring are given by the authors [126-129]. 

Figure 8.41 Rapid Mixing Continuous Flow Device. Biocatalyst E and substrate S are combined in a mixing zone (hatched area) and the mixture ejected along a common outlet tube. Distance from the mixing zone determines time t from mixing. Spectroscopic monitoring of mixture as a function of distance generates first order relaxation curves for analysis.

In the case of the stopped flow device (Figure 8.42), two compressed syringes are set up to express small volumes (50-200 pi) at any one time. These syringes are individually loaded with samples as above and small volumes of these samples are then fired together into a mixing chamber and expelled down a flow-tube at constant flow rate (approximately 10 m s ) prior to a mechanical stop. A fixed detector at a distance of 1 cm from the mixing chamber is then positioned to observe solution that is initially 1 ms old at the end of continuous flow. Compression converts continuous into static flow, so spectroscopic and/or physical changes... 

Fig. 5. Setup for the i Flow device. The chemoeffector to be tested is introduced at two concentrations (high and low) into the gradient-generation (upstream) portion of the device. Bacteria are introduced into the device via the central inlet. The entire device is mounted on a resonant-scanning confocai microscope for continuous imaging.

EN 14594 2005 Respiratory protective devices - Continuous flow compressed air line breathing apparatus - Requirements, testing, marking. 

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