Equipment continuous microwave reactor

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]. 

No rate enhancement was observed when the reaction was performed under microwave irradiation at the same temperature as in conventional heating [47]. Similar reaction kinetics were found in both experiments, presumably because mass and heat effects were eliminated by intense stirring [47]. The model developed enabled accurate description of microwave heating in the continuous-flow reactor equipped with specific regulation of microwave power [47, 48]. Calculated conversions and yields of sucrose based on predicted temperature profiles agreed with experimental data. 

Should the economics of the above-described new process favor a switch from the present AIC process (see Case Study 2) to one based on dihydrohypoxanthine new investment might be justified in new production equipment.81 A switch to the microwave option versus the conventional heating option would depend on the outcome of process optimization work on both. If a rapid clean reaction in near-quantitative yield were realized by optimizing the microwave conditions, then it would prove worthwhile to engineer a simple continuous flow reactor for further development work. The case in favor of adopting the microwave-assisted process would be further enhanced if the solution from the microwave reactor could be used directly in the next process step (thus eliminating the need for isolation equipment). 

Specialized microwave reactors for chemical synthesis are commercially available from such companies as CEM [20], Lambda Technologies [21], Microwave Materials Technologies (MMT), Milestone [22], PersonalChemistry [23], and Plazmatronika [24] which are mostly adjusted from microwave systems for digestion and ashing of analytical samples [25]. They are equipped with built-in magnetic stirrers and direct temperature control by means of an IR pyrometer, shielded thermocouple or fiber-optical temperature sensor, and continuous power feedback control, which enable one to heat reaction mixture to a desired temperature without thermal runaways. In some cases, it is possible to work under reduced pressure or in pressurized conditions within cavity or reaction vessels. 

Every efficient microwave reactor to perform chemical syntheses requires reliable temperature measurement as well as continuous power feedback control. Most of the reactors are equipped with temperature monitoring systems, which enable heating of reaction mixtures to a desired temperature without thermal runaways. Moreover, power feedback control systems that are operated in most of the microwave reactors enable a synthesis to be carried out without knowing the dielectric properties and/or conductive properties of all the components of the reaction mixture in detail. An overview of microwave equipment manufacturers and detailed descriptions of microwave reactors can be found in recent review papers and chapters. 

Despite more than 20 years of study, the application of microwave irradiation to chemical process development is still in relative infancy. Microwave equipment companies continue to address the requirements for large-scale continuous flow and other reactors.80 The availability of versatile equipment, and preferably a champion in a chemical process development department, would encourage evaluation of the technology to identify those reactions where the main advantage, enormous reduction in reaction times (often with cleaner reactions and yield increases beyond those achievable using conventional conditions), can be harnessed in practical terms. 

Recently, a wide range of organic reactions have been promoted by microwave irradiation," but in the field of Heck chemistry only a limited number of papers have appeared. " " "" Two types of microwave heating equipment have been used, a multimode reactor or a monomode reactor.The latter is more expensive but allows the placement of the reaction mixture at a fixed position of much higher continuous electric field strength than can be obtained in a multimode reactor." This is particularly important with Pd-catalyzed reactions since the reaction mixture must be heated to a high temperature in a reproducible and homogeneous fashion. 

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