**1. Introduction**

### **1.1. Microwave heating on organic reactions**

As many noticeable studies are introduced in this book, microwave heating technique has gained high expectation for utilizing various chemical processes including material synthesis, organic synthesis and conversion of energy resource with high reaction and energy efficiency. For organic synthesis, Gedye et al.[1] and Giguere et al.[2] prove effectiveness of microwave heating to accelerate organic reactions. Comparison of the energy efficiency between a conventional oil bath synthesis and a microwave-assisted synthesis has indicated that a significant energy savings of up to 85-fold can be expected using microwaves as an energy source on a laboratory scale[3]. It was also shown that microwave heating on organic chemical reaction had much higher yields within short reaction times for some products[4]. These high efficiency would be kept for a pilot scale plant and the plant must be greener chemical process because of pre-workup reduction and associated energy savings.

#### **1.2. Importance of kinetic data and** *in-situ* **observation**

To develop an industrial process for a new chemical reaction, one has to grasp a correct kinetics of the reaction as a fundamental data to design a reactor and set an operating condition. Kinetic data can be measured using either batch reactor, semi batch reactor and flow reactor. However, the important points to evaluate kinetics are to keep operating condition constant and to know reaction time correctly. Particularly, it is quite difficult for a batch and a semi-batch reactor to be achieved to a desired temperature rapidly and/or cooled down to enough low temperature spontaneously.

© 2012 Watanabe et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Watanabe et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Furthermore, chemical reaction is affected with mass transfer occurring at phase boundary. Ionic liquid, which is widely investigated as reaction media for biomass conversion, is high viscous liquid depending on a solute concentration, amount of additive and temperature and a reaction in it is sometimes controlled by flow dynamics in a reactor. To know the effect of mass transfer on reaction kinetics, *in-situ* observation often provides a meaningful hint.

#### **1.3. Concept for development of a new microwave heating apparatus**

Advantage of microwave irradiation is capable of rapid heating *via* absorption of a heated target, mainly dielectric substance like water, carbon, some kinds of metal oxides, and so on. Now we consider microwave irradiation for high pressure vessel containing water. That is, to keep liquid phase of water over 100 C, a pressure vessel has to be used to resist higher pressure than atmospheric one. In the high pressure vessel for microwave irradiation, rapid cool down is typically impossible due to lower heat transfer of a material (ceramics and plastics are used in a commercial set up) for the high pressure vessel. To cool down rapidly using air blow, which is a simple way for cooling, a metal material is favorable owing to its high heat conductivity (heat conductivity of stainless steel is around 20). But a metal material reflects microwave and it is not heated up by microwave irradiation. On the other hand, a microwave-transparent material has low heat conductivity (heat conductivity of ceramics is single digit and that of plastics is one digit smaller than ceramics) and rapid cooling by air blow can not be expected. In addition, high pressure vessel for microwave irradiation is composed of visible light-proof materials (which does not allow visible light pass through), for example, alumina, Teflon, and so on, and *in-situ* observation of reaction behavior in the vessel is basically impossible. To overcome these disadvantages, we developed a novel high-pressure reactor for microwave irradiation, which is capable of rapid heating, rapid cooling and *in-situ* observation. The key points of the reaction vessel are three as follows: (1) a commercial Pyrex glass cup and polycarbonate (PC) tube, of which transparency for visible light are quite high, are employed to compose the vessel as inner and outer tube, respectively, (2) heat insulating space between the inner glass and outer PC tube allow a reaction fluid in the glass cup to be heated up to 200 C (PC is engineering plastics, however its heat tolerance is low and PC can normally be used up to 100 C), and (3) the heat insulating space can be used for cooling unit after the reaction by introducing cooling water. The detail of the microwave setup and a typical procedure are describe below.

#### **2. Microwave apparatus**

#### **2.1. Setup of microwave apparatus**

Figure 1 shows a schematic diagram of the microwave apparatus. Figures 2 and 3 show cover shot of the setup and photograph of high pressure vessel (2 ~ 6 in Figure 1), respectively. The setup consists of a multimode microwave generator (1) -Reactor, SMW-087, 2.45 GHz, maximum power 700 W, Shikoku Keisoku, Takamtsu, Japan) with a K-type thermocouple (2), a stirring system (5 and 15), a control box (16:, a high pressure reactor (consists of 3, 4, 6 and 7), a pressure gauge (8), an inert gas (Ar or N2) cylinder (13), a cooling water tank (18) and a vacuum pump (14). The reactor was composed of an inner thick-wall Pyrex glass tube (4: HPG-10, volume 10 ml, maximum supporting pressure 10 MPa, TaiatsuTechno. Corporation, Tokyo, Japan), an outer PC tube (3) and two PEEK (Teflon or PC is OK depending on operating temperature) screw caps (6 and 7) with special seal joint (consists of stainless steel connectors, Teflon O-ring, and Viton O-ring) used to fix glass tube and PC tube. The thermocouple (2) is inserted into the glass reactor (4) through a stainless steel sleeve and fixed with the inner wall of microwave oven to avoid microwave leakage from the oven (for this reason, an aluminum plate is also placed on a hole opened at the top of the microwave oven) and sparks produced from the thermocouple. Sparks from the thermocouple have never observed during all the experiments. The leakage of microwave was monitored by a microwave survey meter (Holiday Industries Inc., Model HI-1501) for safety and found to be less than 1 mW/cm2 at distance of 5 cm far away from the microwave oven. The temperature inside the reactor is monitored and controlled by the control box (16). The temperature and power of microwave were monitored and recorded using a computer (21).

1- Microwave oven, 2- K-type Thermocouple 3- Polycarbonate outer tube, 4- Thick-walled glass reactor, 5- Stirrer bar, 6- PEEK, Tefron, or PC cap, 7- PEEK, Tefron, or PC bottom, 8- Pressure indicator, 9- Stainless steel connectors, 10- Aluminum plate, 11- PEEK line, 12- Stainless steel line, 13- inert gas (Ar or N2) cylinder, 14- Vacuum pump, 15- Stirrer controller, 16- Power controller, 17- Observation window, 18- Cooling water tank, 19- Thermocouple connecting line, 20- Controller connecting line, 21- Computer, 22- Computer connecting line, V1~V5- stop valves

**Figure 1.** Microwave heating experimental setup

164 The Development and Application of Microwave Heating

hint.

Furthermore, chemical reaction is affected with mass transfer occurring at phase boundary. Ionic liquid, which is widely investigated as reaction media for biomass conversion, is high viscous liquid depending on a solute concentration, amount of additive and temperature and a reaction in it is sometimes controlled by flow dynamics in a reactor. To know the effect of mass transfer on reaction kinetics, *in-situ* observation often provides a meaningful

Advantage of microwave irradiation is capable of rapid heating *via* absorption of a heated target, mainly dielectric substance like water, carbon, some kinds of metal oxides, and so on. Now we consider microwave irradiation for high pressure vessel containing water. That is, to keep liquid phase of water over 100 C, a pressure vessel has to be used to resist higher pressure than atmospheric one. In the high pressure vessel for microwave irradiation, rapid cool down is typically impossible due to lower heat transfer of a material (ceramics and plastics are used in a commercial set up) for the high pressure vessel. To cool down rapidly using air blow, which is a simple way for cooling, a metal material is favorable owing to its high heat conductivity (heat conductivity of stainless steel is around 20). But a metal material reflects microwave and it is not heated up by microwave irradiation. On the other hand, a microwave-transparent material has low heat conductivity (heat conductivity of ceramics is single digit and that of plastics is one digit smaller than ceramics) and rapid cooling by air blow can not be expected. In addition, high pressure vessel for microwave irradiation is composed of visible light-proof materials (which does not allow visible light pass through), for example, alumina, Teflon, and so on, and *in-situ* observation of reaction behavior in the vessel is basically impossible. To overcome these disadvantages, we developed a novel high-pressure reactor for microwave irradiation, which is capable of rapid heating, rapid cooling and *in-situ* observation. The key points of the reaction vessel are three as follows: (1) a commercial Pyrex glass cup and polycarbonate (PC) tube, of which transparency for visible light are quite high, are employed to compose the vessel as inner and outer tube, respectively, (2) heat insulating space between the inner glass and outer PC tube allow a reaction fluid in the glass cup to be heated up to 200 C (PC is engineering plastics, however its heat tolerance is low and PC can normally be used up to 100 C), and (3) the heat insulating space can be used for cooling unit after the reaction by introducing cooling

water. The detail of the microwave setup and a typical procedure are describe below.

Figure 1 shows a schematic diagram of the microwave apparatus. Figures 2 and 3 show cover shot of the setup and photograph of high pressure vessel (2 ~ 6 in Figure 1), respectively. The setup consists of a multimode microwave generator (1) -Reactor, SMW-087, 2.45 GHz, maximum power 700 W, Shikoku Keisoku, Takamtsu, Japan) with a K-type thermocouple (2), a stirring system (5 and 15), a control box (16:, a high pressure reactor

**2. Microwave apparatus** 

**2.1. Setup of microwave apparatus** 

**1.3. Concept for development of a new microwave heating apparatus** 

**Figure 2.** Cover shot of microwave heating experimental setup

**Figure 3.** Photograph of high pressure vessel

#### **2.2. Typical procedures for microwave heating experiments**

166 The Development and Application of Microwave Heating

**Figure 2.** Cover shot of microwave heating experimental setup

**Figure 3.** Photograph of high pressure vessel

A typical workup procedure for an experiment is as follows: An sample solution which must contain dielectric substance (water, polar solvents, ionic liquids, carbon, some kinds of metal oxides, etc) and sometimes a given amount of a catalyst are loaded into the glass tube (4) with a stirrer bar (5). The glass tube (4) is mounted into a PC tube (3) that is closed with PEEK screw caps (6 and 7: Teflon and PC caps are sometimes used depending on reaction temperature). This assembly is placed into the microwave oven (1) as shown in Figure 1. An inert gas (such as Ar and N2) is used for purging air inside the reactor at a pressure of about 1.2 MPa. Then a vacuum pump (14) evacuates air from the space between the inner glass tube (4) and the outer PC tube (3) to minimize conductive heat losses. Introduction of cooling water into this vacuum space provided a method for rapidly cooling of the reactor. When microwave irradiation is started, the reaction mixture could be heated up to 200 C within 60 s depending on the kind and amount of materials and substrates. Figure 4 shows a temperature and power of microwave profile at the experiments of fructose conversion in water in the presence of TiO2 at 200 C for 2 min 30 s. In this experiment, the reaction fluid was rapidly heated up to 200 C for 30 s. After a desired reaction time passed, microwave irradiation was turned off, stop valve (V1) located at the line between the PC cap (6) and the vacuum pump (14) is closed, the vacuum pump (14) is stopped. After that, stop valve (V2) connected the line between the PC cap (6) and the cooling water tank (18) is opened to admit introduction of cooling water from tank (18) into the PC tube. By the cooling process, the reaction solution could rapidly be cooled down to below 80 C within 60 s. As shown in Figure 4, at the fructose conversion experiment in water, the solution in the reaction vessel was rapidly cooled down to 80 C within 30 s. After cooling, stop valve (V4) at the opposite site of the inert gas cylinder is opened and the inert gas inside the glass tube (4) is discharged. The reactor is disassembled and the reaction solution is collected by washing

Reaction condition: 5 mL fructose aqueous solution, 0.05 g TiO2, set temperature 200 C, maximum power 700 W **Figure 4.** Temperature and power profile during a reaction

the glass tube (4) with an amount of an appropriate solvent. During a reaction, reaction behavior can be observed from a observation window (17) and we sometimes record the images of some experiments with a digital video camera. One example is shown in the section of ionic liquid system concerning cellulose hydrolysis in an ionic liquid.
