Pulsed continuous flow

Pulsed continuous flow is a method in which continuous flow is established for a short time. This method can reduce reagent consumption to 5 ml, and fast jet mixers have lowered the accessible reaction half-time to the 10 ps range. Pulsed accelerated flow may be viewed as an adaptation of pulsed continuous flow in which the flow rate through the mixer and observation chamber is varied during the course of one kinetic run. This method can be used for reactions with half-times down to 10 ps. This method is limited to first-order reaction conditions. 

In bofh CW and pulsed lasers fhe dye solution musf be kepf moving to prevenf overheating and decomposition. In a pulsed laser fhe dye is continuously flowed fhrough fhe confaining cell. Alternatively, magnetic stirring may be adequate for low repetition rates and relatively low power. In a CW laser fhe dye solution is usually in fhe form of a jef flowing rapidly across fhe laser cavify. 

In the continuous hydrovinylation experiments, the ionic catalyst solution was placed in the reactor R, where it was in intimate contact with the continuous reaction phase entering from the bottom (no stirring was used in these experiments). The reaction phase was made up in the mixer from a pulsed flow of ethylene and a continuous flow of styrene and compressed CO2. 

The use of IR pulse technique was reported for the first time around the year 2000 in order to study a catalytic reaction by transient mode [126-131], A little amount of reactant can be quickly added on the continuous flow using an injection loop and then introduce a transient perturbation to the system. Figure 4.10 illustrates the experimental system used for transient pulse reaction. It generally consists in (1) the gas flow system with mass flow controllers, (2) the six-ports valve with the injection loop, (3) the in situ IR reactor cell with self-supporting catalyst wafer, (4) the analysis section with a FTIR spectrometer for recording spectra of adsorbed species and (5) a quadruple MS for the gas analysis of reactants and products. 

Figure 4.11. Pulse of 20 jxl of CO in a 1000-ppm NO in He continuous flow on a Pt/Si02 catalyst T = 498 K. (a) IR spectra of the gas phase (one spectrum per 2 s). (b) IR spectra of the surface species (one spectrum per 2 s). (c) Correlation between the surface species and the gas phase IR analysis [126],...

For gas phase heterogeneous catalytic reactions, the continuous-flow integral catalytic reactors with packed catalyst bed have been exclusively used [61-91]. Continuous or short pulsed-radiation (milliseconds) was applied in catalytic studies (see Sect. 10.3.2). To avoid the creation of temperature gradients in the catalyst bed, a single-mode radiation system can be recommended. A typical example of the most advanced laboratory-scale microwave, continuous single-mode catalytic reactor has been described by Roussy et al. [79] and is shown in Figs. 10.4 and... 

Figures 7-9 show the fractional conversion of methanol in the pulse as a function of temperature for the three catalysts and the three methanol feeds. Evidently the kinetic isotope effect is present on all three catalysts and over the complete temperature range, indicating that the rate limiting step is the breaking of a carbon-hydrogen bond under all conditions. From these experiments, the effect cannot be determined quantitatively as in the case of the continuous flow experiments, but to obtain the same conversion of CD,0D, the temperature needs to be 50-60° higher. This corresponds to a factor of about three in reaction rate. The difference in activity between PfoCL and Fe.(MoO.), is larger in the pulse experiments compared to tHe steady stateJ results.

The Incentive to modify our existing continuous-flow microunit to incorporate the square pulse capability was provided by our work on perovskite-type oxides as oxidation-reduction catalysts. In earlier work, it had been inferred that oxygen vacancies in the perovskite structure played an important role in catalytic activity (3). Pursuing this idea with perovskites of the type Lai-xSrxFeg 51 10 503, our experiments were hampered by hysteresis effects which we assumed to be due to the response of the catalyst s oxygen stoichiometry to the reaction conditions. 

Figure 3. The square pulse method combined with a conventional continuous flow...

Figure 7. Pulse flow and continuous flow measurements of the conversion of CO over prereduced (see Fig. 6 for conditions) Sr,tFeMnOs. Flow of 100 mL/60 s of 5% CO and 5% 02 in He over 177 mg of catalyst. Key , points taken during stepwise decrease of temperature and O, A, during stepwise increase.

Epr is most effective for detecting free radicals that may occur as intermediates in oxidation and reduction reactions involving transition metal ions. Since these transients are invariably quite labile, epr is combined with continuous flow, (more conveniently) stopped-flow, flash photolysis, and pulse radiolysis. 

The need to use multiple extraction to achieve efficient extraction required the development of new types of continuously working extractors, especially mixer-settlers and pulsed columns, which were suitable for remotely controlled operations. These new extractors could be built for continuous flow and in multiple stages, allowing very efficient isolation of substances in high yield. A good example is the production of rare earth elements in >99.999% purity in ton amounts by mixer-settler batteries containing hundreds of stages. These topics will be further developed in Chapters 6 and 7. 

Figure 6. TAP mess intensity versus time curve showing the time sequence of HCN formation when continually flowing oxygen and pulsing a methanol/ammonia blend.

V205/Si02 catalysts at 550-650°C in batch, pulse and continuous flow reactors at 1.7 10 kPa. The HCHO productivity (Shcho Scat ) results in the order... 

Comparison between the periodic-pulse and continuous-flow reactions for propene oxidation... 

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