Laboratory continuous-flow reactor

Mixing of product and feed (backmixing) in laboratory continuous flow reactors can only be avoided at very high length-to-diameter (aspect) ratios. This was observed by Bodenstein and Wohlgast (1908). Besides noticing this, the authors also derived the mathematical expression for reaction rate for the case of complete mixing. 

Batch reactors often are used to develop continuous processes because of their suitabiUty and convenient use in laboratory experimentation. Industrial practice generally favors processing continuously rather than in single batches, because overall investment and operating costs usually are less. Data obtained in batch reactors, except for very rapid reactions, can be well defined and used to predict performance of larger scale, continuous-flow reactors. Almost all batch reactors are well stirred thus, ideally, compositions are uniform throughout and residence times of all contained reactants are constant. 

The use of glutaric dialdehyde as a coupling agent bound the enzymes trypsin or glucose-6-phosphate dehydrogenase to the surface. A large part of the enzymic activity was retained (Fig. 4), and the activity was such that the particle-enzyme conjugate could be used in laboratory scale continuous-flow reactors. 

Various laboratory reactors have been described in the literature [3, 11-13]. The most simple one is the packed bed tubular reactor where an amount of catalyst is held between plugs of quartz wool or wire mesh screens which the reactants pass through, preferably in plug flow . For low conversions this reactor is operated in the differential mode, for high conversions over the catalyst bed in the integral mode. By recirculation of the reactor exit flow one can approach a well mixed reactor system, the continuous flow stirred tank reactor (CSTR). This can be done either externally or internally [11, 12]. Without inlet and outlet feed, this reactor becomes a batch reactor, where the composition changes as a function of time (transient operation), in contrast with the steady state operation of the continuous flow reactors. 

One disadvantage of this, and indeed all continuous flow reactors, is that if the catalyst is very sensitive to poisons there is a risk that a small amount of poison in the reactant would accumulate on the catalyst during the experiment and cause major changes in activity and/or selectivity. In the laboratory this problem can usually be avoided by using very pure reactants. 

Continuous flow reactors can also be found also at a laboratory scale, where the feed is constant and the output stream is constantly monitored by means such as GC-MS (Alberici and Jardim, 1997 Sun et al., 2007), GC-FID (Doucet et al., 2006), GC-TCD (Yamazaki-Nishida et al., 1996), or even FT-IR (Nimlos et al., 1993). In certain cases (for example Doucet et al., 2006), continuous one-pass flow reactors are used for performing kinetic measurements later to be used for scaling up. 

Microwave irradiation generates high temperatures and corresponding high pressures, and reactors should be equipped with pressure relief valves. This concern has limited the use and scale-up of microwave-enhanced reactions. A continuous microwave reactor has been described [32], and continuous flow reactors are available commercially for laboratory use [33]. 

Owing to the complex and often dedicated equipment required to perform gas-liquid phase reactions within research laboratories, this area of synthetic chemistry is somewhat underutilized. Over the past decade, however, numerous research groups have developed an array of continuous flow reactors capable of conducting such reactions in a safe and efficient manner, including microchannel contactors, falling film micro reactors, and packed-bed reactors [68, 69]. 

With numerous researchers investigating the advantages associated with the thermal or biocatalytic control of asymmetric reactions, Ichimura and co-workers [89] considered the potential of photochemical asymmetric syntheses performed in continuous flow reactors. To investigate the hypothesis, the authors employed the asymmetric photochemical addition of MeOH to (R)-( + )-(Z)-limonene (159) as a model reaction, comparing three quartz micro reactors, with a standard laboratory cell as a means of highlighting the synthetic potential of this approach. 

Although photochemical transformations provide the synthetic chemist with an attractive, atom-efficient approach to the synthesis of complex molecules, the inability to increase the scale of reactions beyond the bench-scale has hampered the adoption of this technique. As can be seen from the examples described below, the use of continuous flow reactors affords a facile means of increasing the throughput of photochemical reactions while employing laboratory-scale light sources such as low-energy LEDs. 

Chloroform in drinking water may be aerobically biodegraded to carbon dioxide (Speitel et al. 1989). Bacterial cultures from contaminated sites produced efficient degradation of chlorinated hydrocarbons in laboratory-scale continuous-flow reactors (Kaestner 1989). Woods and coworkers... 

In addition to the continuous-flow reactors, we developed a stopped-flow apparatus to study slow reactions of the NO3 radical. In this apparatus, a series of solenoid valves is used to divert and isolate a flow of gas that contains the reaction mixture. These valves were designed and fabricated in this laboratory by the PI, and ensure that only glass is in contact with the flow. Concentrations of NO3 are then followed as a function of time after the flow is cut off, the data being captured by computer. Figure 1 shows the apparatus in schematic form, while Fig. 2 illustrates decay curves for [NO3] in the absence and presence of C2H4. 

Delueze and coworkers reported a polymer-supported titanium alkoxide catalyst for the transesteriflcation. They evaluated the efficiency and stability of the catalyst and tested it in a laboratory-scale continuous-flow reactor under equilibrium conditions [129]. The average metal leaching was estimated to less than 1% of the total amount of titanium engaged. 

The Suzuki-Miyaura coupling reaction is one of the most versatile, chemically robust pathways to form carbon-carbon bonds. It is usually associated with the use of low-hazard reactants and is thus being widely applied in academic and industrial laboratories [46]. Several groups have described results of Suzuki reactions in continuous-flow reactors [40,47-49]. As an example, a simple... 

Specific reactor characteristics depend on the particular use of the reactor as a laboratory, pilot plant, or industrial unit. AH reactors have in common selected characteristics of four basic reactor types the weH-stirred batch reactor, the semibatch reactor, the continuous-flow stirred-tank reactor, and the tubular reactor (Fig. 1). A reactor may be represented by or modeled after one or a combination of these. SuitabHity of a model depends on the extent to which the impacts of the reactions, and thermal and transport processes, are predicted for conditions outside of the database used in developing the model (1-4). 

This chapter treats the effects of temperature on the three types of ideal reactors batch, piston flow, and continuous-flow stirred tank. Three major questions in reactor design are addressed. What is the optimal temperature for a reaction How can this temperature be achieved or at least approximated in practice How can results from the laboratory or pilot plant be scaled up ... 

A process for the depolymerisation of Nylon 6 carpet fibre in the presenee of steam under medium pressure (800 to 1500 KpA, 100 to 200 psig) is described. The feasibility of the seheme was demonstrated using a small laboratory apparatus and the best run produced a 95% yield of crude eaprolaetam. The data obtained were used to construct a computer model of the process for both batch and continuous flow stirred reactors. 6 refs. 

Choose the right type of reactor for testing There are quite a number of different reactors. The above-mentioned plug flow reactor and the continuously stirred tank reactor are usually preferred for research laboratory use, but other set-ups may also be of interest for simulating real industrial conditions. 

There is an additional point to be made about this type of processing. Many gas-phase processes are carried out in a continuous-flow manner on the macro scale, as industrial or laboratory-scale processes. Hence already the conventional processes resemble the flow sheets of micro-reactor processing, i.e. there is similarity between macro and micro processing. This is a fimdamental difference from most liquid-phase reactions that are performed typically batch-wise, e.g. using stirred glass vessels in the laboratory or stirred steel tanks in industrial pilot or production plants. 

Chakrabarti, T. and Subrahmanyam, P.V.R., Biological hydrolysis of urea in a continuous flow stirred tank reactor under laboratory conditions—a bench scale study, Proc. 36th Industrial Waste Conference, Purdue University, pp. 477, 1981. 

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