Stop-flow reactors

More popular for reactions in the millisecond range is the "stopped-flow" technique [4], which consumes less fluid and for which commercial equipment is available (e.g., see Figure 3.7). Over an extremely short time span, liquids are injected into and mixed in a small reaction chamber, and the composition of the mixture is then monitored continuously or analyzed after short, preset reaction times. In contrast to the Hartridge-Roughton reactor, a stopped-flow reactor functions essentially as a micro-batch reactor. 

Goldfinger et have studied the reaction between CI2 and HBr to form HCl and BrCl in the gas phase. Using a stopped-flow reactor and absorption spectroscopy they conclude, but not convincingly, that the mechanism is bimo-lecular, four-centered and molecular. 

These systems are designed for the endpoint determination of substrate concentrations, and do not provide a straightforward means for making kinetic measurements. Stopped-flow enzyme reactor systems have been designed for automated kinetic assays. A diagram of a stopped-flow reactor that uses a postcolumn chemical indicator reaction is shown in Figure 4.12.31 In this system, the flow rate of the... 

In some specific cases, analytical solutions for the population balances can also be derived. For instance, Soares and Hamielec [73, 74] obtained analytical dynamic solutions to describe how the CLD of polyolefins varied as a function of time in stopped-flow reactors commonly used for mechanistic studies on olefin polymerization kinetics and mechanism [75-78]. These analytical solutions combine the power of full population balance numerical solutions with the ease and convenience of using closed form equations they are, unfortunately, difficult to attain for more complex cases. 

Nanomaterial growth can be controlled by selective microwave heating as exemplified by the synthesis of CdSe and CdTe in microwave transparent alkane solvents. The high microwave absorptivity of the chalcogenide precursors allowed instantaneous activation and subsequent nucleation and is a clear example of the specific microwave effect. Regardless of the desired size, narrow dispersity nanocrystals could be isolated in less than 3 min. The reaction did not require a high temperature injection step, a problem encountered with conventional approaches. In addition, the use of a stop-flow reactor allowed for automation of the process for scale-np. 

Chemical kinetics the absolute values of kRi and kR2. The magnitude of the rate constant, kRi, will determine how much A can be converted during the time required to achieve molecular mixing. The extent of the conversion will determine the amount of R that is subject to excess B concentration and hence overreaction to S as determined by kR2. In some cases the kinetics can be determined by use of a stopped-flow reactor or similar device. For... 

A system has been constructed which allows combined studies of reaction kinetics and catalyst surface properties. Key elements of the system are a computer-controlled pilot plant with a plug flow reactor coupled In series to a minireactor which Is connected, via a high vacuum sample transfer system, to a surface analysis Instrument equipped with XFS, AES, SAM, and SIMS. When Interesting kinetic data are observed, the reaction Is stopped and the test sample Is transferred from the mlnlreactor to the surface analysis chamber. Unique features and problem areas of this new approach will be discussed. The power of the system will be Illustrated with a study of surface chemical changes of a Cu0/Zn0/Al203 catalyst during activation and methanol synthesis. Metallic Cu was Identified by XFS as the only Cu surface site during methanol synthesis. 

For the Michael addition of 2,4-pentanedione enolate to ethyl propiolate, improvements in conversion were determined. This example serves also to demonstrate that proper process conditions are mandatory to have success with micro-reactor processing. A conversion of only 56% was achieved when using electroosmotically driven flow with a two-fold injection, the first for forming the enolate and the second for its addition to the triple bond (batch synthesis 89%) [151]. Using instead a stopped-flow technique to enhance mixing, a conversion of 95% was determined. 

The conversions observed followed the sequence of reactivity known from batch experiments carried out in advance. For example, only 15% conversion was found for the less reactive reagent benzoylacetone in the micro reactor experiment, while 56% was determined when using the more reactive 2,4-pentanedione (batch syntheses 78% and 89%, respectively) [8]. Using the stopped-flow technique (2.5 s with field applied 5.0 s with field turned off) to enhance mixing, the conversions for both syntheses were increased to 34 and 95%, respectively. Using a further improved stopped-flow technique (5.0 s with field applied 10.0 s with field turned off), the conversion could be further enhanced to 100% for the benzoylacetone case. For the other two substrates, diethyl malonate and methyl vinyl ketone, similar trends were observed. 

The titration of an acid with a base, or vice versa, and the precipitation of an ion in an insoluble compound are examples of chemical methods of analysis used to determine the concentration of a species in a liquid sample removed from a reactor. Such methods are often suitable for relatively slow reactions. This is because of the length of time that may be required for the analysis the mere collection of a sample does not stop further reaction from taking place, and a method of quenching the reaction may be required. For a BR, there is the associated difficulty of establishing the time t at which the concentration is actually measured. This is not a problem for steady-state operation of a flow reactor (CSTR or PFR). 

A stream of fully suspended fine solids (v = 1 mVmin) passes through two mixed flow reactors in series, each containing 1 m of slurry. As soon as a particle enters the reactors, conversion to product begins and is complete after two minutes in the reactors. When a particle leaves the reactors, reaction stops. What fraction of particles is completely converted to product in this system ... 

Figure 3.7 — (A) Cross-sectional view of the McPherson stopped-flow mixer unit. The outer aluminum housing (a) and quartz windows b) are press-fitted with three bolts. Mixing occurs at e, where the streams meet at 90° to each other one stream is in the figure plane and the other normal to it. The immobilized enzyme reactor is placed inside d. With the reactor in place, the observation cell is 1.75 cm in length. The dashed arrow represents the lightpath inside the cell. (B) Flow-cell used to accommodate enzymes on CPG. (Reproduced from [48] and [49] with permission of Elsevier Science Publishers).

The complex interplay of basic and acidic sites in the deamination and disproportionationn of amines is the probable cause of the stop-effect which has been observed in the reaction of triethylamine on alumina [155] and, more recently, of other amines [149], When the steady state on the catalyst surface in a flow reactor is rapidly changed by substituting the amine feed for a nitrogen stream, a rapid temporary increase in olefin production is observed. This phenomenon has been explained as the result of the increased availability of basic centres which were previously blocked by adsorbed molecules [149,155]. 

The apparatus used are mostly stirred-tank-, tubular-, and differential recycle reactors. Also, optical cells are used for spectroscopic measurements, and differential thermal-analysis apparatus and stopped flow devices are applied at high pressures. 

The stopped-flow method uses syringe-type pumps, (a), to feed the components, A and B, through a mixing cell, (c), into the reaction cell, (d), which can be an optical cell (Fig. 3.3-5). The pumps, mixing cell, and reactor are well thermostatted. The flow is stopped when the syringe, (e), is loaded and operates a switch, (f), to start the monitoring device. The change in concentration is detected either by spectroscopy or conductivity measurement. 

Products from the reactor passed through a back-pressure regulator into a separator maintained at 75°F and 200 psig. Tail gas from the separator passed through a second back-pressure regulator and was metered and sampled. Liquid products were drained from the separator after each 24-hr period of operation and washed with water to remove ammonia and hydrogen sulfide before a sample was taken for analysis. At the conclusion of each experiment the oil and hydrogen flows were stopped, the reactor was depressurized, and steam was introduced at the rate of 0.41 lb/lb/hr while the reactor was cooled to 700°F. Air was then introduced at the rate of 1.1 scf/lb/hr, and these conditions were maintained until the coke bumoff was completed. 

A chip-based integrated precolumn microreactor with 1 nl reaction volume has been explored by Jacobson et al. [67]. The reactor is operated in a continuous manner by electrokinetically mixing of sample (amino acids) and reagent (o-phthaldialdehyde) streams. The reaction time is adjusted via the respective flow velocities. By switching of potentials, small plugs of the reaction product were injected into a 15.4 mm separation channel in a gated injection scheme (< 1.8% RSD in peak area). The separation efficiency achieved was relatively poor, however, electrokinetic control of reaction time (and yield) permitted to monitor the kinetics of the derivatization under pseudo first-order conditions. A similar integrated precolumn reactor operated in a stopped flow mode has been described by Harrison et al. [68]. 

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