Flow time-pulsing mixing

Another paper describes the stretching and folding of material lines yielded by simulation and experimental imaging, induced by time-pulsing mixing via unsteady cross-flow injection in a steady-flow main channel [48],... 

In a typical pulse experiment, a pulse of known size, shape and composition is introduced to a reactor, preferably one with a simple flow pattern, either plug flow or well mixed. The response to the perturbation is then measured behind the reactor. A thermal conductivity detector can be used to compare the shape of the peaks before and after the reactor. This is usually done in the case of non-reacting systems, and moment analysis of the response curve can give information on diffusivities, mass transfer coefficients and adsorption constants. The typical pulse experiment in a reacting system traditionally uses GC analysis by leading the effluent from the reactor directly into a gas chromatographic column. This method yields conversions and selectivities for the total pulse, the time coordinate is lost. 

M 83] [P 72] If the amplitude for time pulsing is doubled, the degree of mixing remains unchanged, being 22% in both cases [26], The increase in amplitude amounts to a maximum flow rate of 17.0 mm s 1 instead of 8.5 mm s 1 used formerly. 

Figure 4.19 (A) Time courses of the bioluminescence at three measurement points (2, 10, 46 mm) downstream of the Y-shaped junction. The measurement was started immediately after flow stopping. (B) Typical pattern of the driving signal for alternate pulse flow. (C) Photograph of the alternate pulse flow generated by switching at 3 Hz. This frequency is selected for the visualization of thin skins, which enhances the diffusion-based mixing along the flow axis. (D) Mixing performance of the alternate pulsed flow at a frequency ranging from 1 Hz to 1 kHz. The intensity of bioluminescence is measured at points A ( ) and B ( ) [72] (by courtesy of RSC).

The limit for the measured rate constants is determined by the mixing rate and the instrument s dead time, defined as the time required for the solution to travel from the mixing chamber to the observation point. Nowadays, half-times in the millisecond range can be measured routinely. An extension of accessible rates up to 2000 s through algebraic corrections for mixing effects was discussed [11]. Under the assumption that the behavior of the solution at short times after mixing in the stopped-flow is described by the same equations that were found applicable for pulsed-accelerated flow, the precise rate constant can be obtained from a set of experiments carried out under pseudo-first-order conditions by use of Eq. 10. 

The time resolution of these methods is detennined by the time it takes to mitiate the reaction, for example the mixing time in flow tubes or the laser pulse width in flash photolysis, and by the time resolution of the detection. Relatively... 

Siemes and Weiss (SI4) investigated axial mixing of the liquid phase in a two-phase bubble-column with no net liquid flow. Column diameter was 42 mm and the height of the liquid layer 1400 mm at zero gas flow. Water and air were the fluid media. The experiments were carried out by the injection of a pulse of electrolyte solution at one position in the bed and measurement of the concentration as a function of time at another position. The mixing phenomenon was treated mathematically as a diffusion process. Diffusion coefficients increased markedly with increasing gas velocity, from about 2 cm2/sec at a superficial gas velocity of 1 cm/sec to from 30 to 70 cm2/sec at a velocity of 7 cm/sec. The diffusion coefficient also varied with bubble size, and thus, because of coalescence, with distance from the gas distributor. 

Figure. 1. Schematic of essential components of the Exxon group cluster laser vaporization source and fast flow tube chemical reactor. On the far left is a 1 mm diameter pulsed nozzle that emits an -200 ysec long pulse of helium which achieves an average pressure of approximately one atmosphere above the sample rod. Immediately before the sample rod position the tube is expanded to 2 mm diameter. The length of this extender section can be varied form 6 mm to 50 mm depending upon the desired integration time for cluster growth. The reactor flow tube is 10 mm in diameter and typically 50 mm long. The reactants diluted in helium are added and mixed with the flow stream via the second pulsed valve.

In studying residence time distribution in a tank-flow electrolyzer, a tracer injected into it is recovered in a mixing tank placed downstream. If both tanks are perfectly stirred, then with a tracer pulse, the mole balance for the tracer can be written as... 

Fig. 3.2 The operation of flow methods. The distance x and the combined flow rate govern the time that elapses between mixing and when the combined solutions reach the observation, or quenching, point. In the stopped flow method, observation is made as near to the mixer as is feasible, and monitoring occurs after the solutions are stopped. In the pulsed accelerated flow method, observation is within the mixer.

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