Continuous, one-pass flow reactors

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. 

One of the major advantages of flow reaetors is the short residence time of reaetions in the reaetor. This allows seleetive reaetions to pass through the system and out again before any side reaetion ean take plaee. This is very well illustrated in the synthesis of dithioketal and -aeetals, where the seleetive reaetion resulted in superior conversion using eontinuous flow when eompared to bateh synthesis. A dramatic increase of yield was noted in the hydrogenation reaetion performed by Kobayashi et al. when a residence time of less than 1 min was used. The yield inereased from 1% to 97% using the continuous-flow reactor. 

Several continuous stirred tank reactors are often operated in series or cascade as shown in Fig. 13. In this way, the disadvantages of the relatively low reactant concentration on the one hand, and by-passing on the other, may be partially off-set. As the number of tanks in series increases, the performance of the complete system approaches that of a plug-flow reactor and, in the limit of an infinite number of tanks, becomes equal to it. 

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. 

The most common type of tubular flow reactor is the single-pass cylindrical tube. Another type is one that consists of a number of tubes in parallel. The reactor(s) may be vertical or horizontal. The feed is charged continuously at the inlet of the tube, and the products are continuously removed at the outlet. If heat exchange with surroundings is required, the reactor setup includes a jacketed tube. If the reactor is empty, a homogeneous reaction—one phase present—usually occurs. If the reactor contains catalyst particles, the reaction is said to be heterogeneous. 

The Direct Fluorlnatlon of Polyethylene. About 1 gram of finely powdered linear polyethylene (140 mesh) was placed in a nickel boat in the one inch nickel tube flow reactor system. The reaction chamber was flushed with helium at 20 cc/min for 3 minutes. The helium flow was then terminated and a 3 1/2 - 4 cc/min fluorine flow was passed over the sample at room temperature. The reaction was continued for 24 or 48 hours if a high purity sample was desired. A white perf1uorocarbon polymer was produced which resembles its hydrocarbon precursor in appearance. 

There is one report of triphase liquid/liquid/solid catalysis in a continuous flow reactor. Ragaini and Saed l passed a mixture of 1-bromooctane in o-dichlorobenzene and aqueous potassium iodide upward through a bed of polystyrene-bound phosphonium ion catalyst. 

One interesting membrane reactor uses a homogeneous catalyst that cannot pass through an ultrafiltration membrane. Reagents flow continuously toward the membrane, but the catalyst is injected only at the start of the experiment. It forms the... 

In continuous processes, the reactants are added and products are removed at a constant rate from the reactor, so that the volume of reacting material in the reactor (reaction vessel) remains constant. Two types of reactors, either (1) a continuous stirred tank or (2) a pipe reactor, are generally used. A continuous stirred tank reactor is similar to the batch reactor described above. A pipe reactor typically is a piece of tubing arranged in a coil or helix shape that is jacketed in a heat-transfer fluid. Reactants enter one end of the pipe, and the materials are mixed under the turbulent flow and react as they pass through the system. Pipe reactors are well-suited for reactants that do not mix well, because the tiu--bulence in the pipes causes all materials to mix thoroughly. 

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