IR pyrometer

These results provide clear evidence for the existence of selective heating effects in MAOS involving heterogeneous mixtures. It should be stressed that the standard methods for determining the temperature in microwave-heated reactions, namely with an IR pyrometer from the outside of the reaction vessel, or with a fiber-optic probe on the inside, would only allow measurement of the average bulk temperature of the solvent, not the true reaction temperature on the surface of the solid reagent. 

Fig. 3.1 Modified domestic household microwave oven. Inlets for temperature measurement by IR pyrometer (left side) and for attaching reflux condensers (top) are visible. A magnetic stirrer is situated below the instrument.

Most forms of carbon interact strongly with microwaves. When irradiated at 2.45 GHz, amorphous carbon and graphite in powdered form rapidly reach ca. 1000 °C within 1 min of irradiation. An example of a solvent-free Diels-Alder reaction performed on a graphite support is shown in Scheme 4.5. Here, diethyl fuma-rate and anthracene adsorbed on graphite reacted within 1 min of microwave irradiation under open-vessel conditions to provide the corresponding cycloadduct in 92% yield [14]. The maximum temperature recorded by an IR-pyrometer was 370 °C. In other cases, it was necessary to reduce the microwave power and therefore the reaction temperature in order to avoid retro-Diels-Alder reactions [13]. 

Hot spots as large scale nonisothermalities, which can be detected and measured by optical pyrometers, fiber optic or IR pyrometers, i.e. these are macro scale hot zones. 

Fig. 10.1 Microwave batch reactor 1. micro-wave cavity, 2. magnetron, 3. stirring bar, 4. alir minum plate, 5. magnetic stirrer, 6. IR pyrometer, 7. switch on/off, 8. watercooler.

Chemat and his coworkers [92] have proposed an innovative MW-UV combined reactor (Fig. 14.7) based on the construction of a commercially available MW reactor, the Synthewave 402 (Prolabo) [9[. It is a monomode microwave oven cavity operating at 2.45 GHz designed for both solvent and dry media reactions. A sample in the quartz reaction vessel could be magnetically stirred and its temperature was monitored by means of an IR pyrometer. The reaction systems were irradiated from an external source of UV radiation (a 240-W medium-pressure mercury lamp). Similar photochemical applications in a Synthewave reactor using either an external or internal UV source have been reported by Louerat and Loupy [93],... 

The sample of desorbed tritide is placed inside a quartz tube that is connected to a gas-handling manifold by a TorrSeal . A quartz sleeve with Silicon Carbide (SiC) in the annular space is placed around the end of the quartz tube, surrounding the sample with microwave susceptor. The quartz tube and susceptor sleeve are thermally insulated from the rest of the microwave cavity. An internal thermocouple measures the temperature of the sample and provides the temperature signal for process control of the desired temperature. A shine block (alumina foam), attached to the thermocouple, blocks radiant heating of the TorrSeal and the upper area of the quartz tube and manifold. An IR pyrometer is used as a secondary measure of the temperature of the susceptor, and therefore of the sample. A stainless steel shield reflects microwaves from the quartz tube not in the susceptor sleeve, eliminating the production of a plasma at low pressure in the quartz tube. 

The 1000-watt conventional microwave oven is capable of generating over 1000°C inside the sample quartz tube in about 12 minutes with the hybrid configuration. A plot of the thermocouple and the IR pyrometer measurement as a function of time is in Figure 3. The pyrometer cuts off at about 600°C the two measurements are just about coincident throughout the operation. Notice at about 900°C the SiC susceptor begins to loose susceptibility and the temperature rise begins to slow down. 

The metals were either packed in high-purity graphite and heated in an Ar atmosphere or heated in a pure-nitrogen atmosphere up to 2500 K in a cold-wall autoclave. The temperature was recorded by thermocouples or two-color IR pyrometers. Gas pressures (mbar range up to 40 bar) were measured with piezoelectric gauges. The conditions were recorded on a PC equipped with an A/D converter. Extended heating periods could be maintained without any change in experimental conditions. 

IR pyrometers offer a very viable noncontact method to measure temperatures all the way up to 3,600°C (6,500°F) and can be the best choice for most applications. When high temperature is to be detected in hard-to-reach locations, IR radiation pyrometers are combined with the use of optical fibers. These applications will also be discussed later. 

Fig. 5.22. Kinetic plots of the reactions shown in Scheme 5.30 (A) under the action of microwave irradiation at 180 °C (IR pyrometer) (B) conventional thermal heating at 200 °C (DSC).

Yield of isolated product relative to the minor reagent Continuous MW irradiation (CMWI) applied incident pow er irradiation time maximum temperature indicated by IR pyrometer Sequential MW irradiation (SMWI) applied incident pow er time and number of irradiations interval between two irradiations 2 min (entries 2-5), 1 min (entries 6-9)... 

Dubac and coworkers have observed, by using a thermocouple at the end of a reaction, that the temperature inside the graphite powder was twenty to fifty degrees higher than that indicated by an IR pyrometer. During MW irradiation of G-supported catalysts in the absence of nitrogen carrier. Bond and coworkers [16] observed very small bright spots within the catalyst bed. They proposed that these... 

Fig. 16 Experimental setup for inductively triggering SME in an alternating magnetic field consisting of a high-frequency generator, a water cooled coil with six loops having a diameter of 4 cm and an IR pyrometer...

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. 

Optical (IR) pyrometers allow the determination of the temperature if the surface emissivity and absorption in the optics is constant and known. When they are not known, the IR pyrometer can be used to establish a reproducible temperature even if the value is not known accurately. 

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