9.1.1 MicrowaveMicrowave (MW) is el

9.1.1 Microwave

Microwave (MW) is electromagnetic waves with wavelengths in the range of 0.01e1 m and a corresponding frequency ranging from 0.3 to 300 GHz. The MW region in the whole electromagnetic spectrum is between the infrared and radio frequencies. As a kind of electromagnetic wave, MW has both electric and magnetic fields. Based on the interaction be-tween MW irradiation and materials, a material can be categorized into three types [1]: (1) absorbing materials such as water and glycerol, which can absorb MW, are also called MW dielectric; (2) conductors (mainly metals), which MW cannot penetrate, and most of which is reflected; and (3) insulators or MW-transparent materials, including quartz and Teflon, which allow MW to pass through without loss. MW dielectric is commonly used as the heating medium [2].


9.1.2 MW Heating Mechanism

MW heating is also called dielectric heating, because of the dielectric employed to absorb MW irradiation. To avoid interference with telecommunications and cellular phone fre-quencies, 915 MHz (896 in the United Kingdom) and 2450 MHz are two frequencies usually used for MW heating for industrial, scientific, and medical applications [3]. A domestic MW oven normally operates at 2450 MHz. The way a material is heated by MW depends on its shape, size, and dielectric constant, and the nature of the MW equipment used. The main MW heating mechanisms consist of dipolar polarization, conduction, and interfacial polari-zation [4e6]: (1) Dipolar polarization is responsible for most MW heating in the solvent systems. Polar molecules characterized with an electrical dipole moment will align them-selves in an electromagnetic field. In an electromagnetic wave with rapid oscillation, the po-lar molecules will rotate continuously, aligning with it. This is called dipolar polarization. As


Pretreatment of Biomass 157
http://dx.doi.org/10.1016/B978-0-12-800080-9.00009-8
Copyright 2015 Elsevier B.V. All rights reserved.



158 9. MICROWAVE PRETREATMENT
TABLE 9-1 Comparison of conventional and MW heating

MW irradiation Conventional heating

Conversion of energy Transfer of energy
In corn volumetric and uniform heating Superficial Heating via Convection/conduction
Rapid and efficient Slow, inefficient
Selective Non selective
Hot spots No hot spot
Dependent of the properties of the material Less dependent
Precise and controlled heating Less controllable


the field alternates, the molecule reverses direction to align itself with it. This causes energy to be lost from the dipole by molecular friction and collisions, giving rise to dielectric heating. For this reason, MW heating is always called dielectric heating. (2) Conduction mechanism happens when the dissolved charged particles in a sample (electrons, ions, etc.) oscillate back and forth under the influence of the electric component of MW irradiation. They collide with the adjacent molecules or atoms, which cause agitation or motion, and heat is thus created. (3) Interfacial polarization is a phenomenon viewed as a combination of conduction and dipolar polarization. It is an important mechanism for systems composed of conducting and nonconducting materials.


9.1.3 MW Effects

The action of MW irradiation results from materialewave interactions leading to thermal effects, specific MW effects and nonthermal effects [7e10]. A combination of these contribu-tions is responsible for the observed effects.

Heat from conventional heating is transferred from the surface toward the center of the material by conduction, convection, and radiation. It is relatively slow and inefficient, and depends on the thermal conductivity of the material and convection currents. MW heating is characterized by converting electromagnetic energy into thermal energy, which is a kind of energy conversion rather than heating. Compared with conventional heating, the heat produced by MW irradiation is throughout the volume of the materials rather than an external source.

The differences between conventional and MW heating are presented in Table 9.1 [11].


9.2 MW APPLICATION

Since the first MW oven built in 1947 by Raytheon Corporation and an early commercial model introduced in 1954, MW heating has gained popularity in food processing because of its ability to achieve high heating rates, significant reduction in cooking time, more uniform heating, safe handling, ease of operation, and low maintenance [12,13]. Over the decades, this




A. LIGNOCELLULOSIC BIOMASS


9.2 MW APPLICATION 159

technology has spread widely to such applications as analytical chemistry, heating and vulcanization of rubber, plasma processing, chemical synthesis and processing, and waste remediation. MW irradiation has acquired a great deal of attention in domestic, industrial, and medical applications. It has been used in many applications including mineral and met-a

Kết quả (Việt) 1:

9.1.1 lò vi sóngLò vi sóng (MW) là sóng điện từ với các bước sóng trong khoảng 0.01e1 m và một tần số tương ứng từ 0.3 tới 300 GHz. MW, vùng quang phổ điện từ toàn bộ là giữa tia hồng ngoại và tần số vô tuyến. Là một loại sóng điện từ, MW đã có điện và từ trường. Dựa trên sự tương tác-tween MW chiếu xạ và vật liệu, vật liệu có thể được phân loại thành ba loại [1]: (1) hấp thụ nguyên liệu như nước và glycerol, mà có thể hấp thụ MW, còn được gọi là lưỡng điện MW; (2) dây dẫn (chủ yếu là các kim loại) mà MW không thể xâm nhập, và đặt trong đó được phản ánh; và chất cách điện (3) hoặc vật liệu MW, minh bạch, bao gồm thạch anh và Teflon, cho phép MW đi qua mà không mất. MW lưỡng điện thường được sử dụng như là hệ thống sưởi trung [2].9.1.2 MW chế hệ thống sưởiMW, Hệ thống sưởi cũng được gọi là hệ thống lưỡng điện sưởi, vì lưỡng điện sử dụng để hấp thụ bức xạ MW. Để tránh can thiệp với viễn thông và điện thoại di động fre-quencies, 915 MHz (896 ở Vương Quốc Anh) và 2450 MHz được hai tần số thường sử dụng MW, Hệ thống sưởi cho các ứng dụng công nghiệp, khoa học và y tế [3]. Một lò MW trong nước thường hoạt động ở 2450 MHz. Cách một vật liệu được đun nóng bởi MW phụ thuộc vào hình dạng, kích thước và liên tục lưỡng điện của nó, và bản chất của các thiết bị MW sử dụng. Cơ chế chính hệ thống sưởi MW bao gồm độ phân cực dipolar, dẫn truyền, và interfacial polari-zation [4e6]: (1) Dipolar độ phân cực là chịu trách nhiệm về hầu hết các hệ thống sưởi MW trong hệ dung môi. Phân cực phân tử đặc trưng với một moment lưỡng cực điện sẽ sắp xếp họ bản thân ở một trường điện từ. Trong một làn sóng điện từ với dao động nhanh chóng, các phân tử po-lar sẽ xoay liên tục, việc xếp thẳng với nó. Điều này được gọi là dipolar phân cực. NhưPretreatment sinh khối 157 http://DX.Doi.org/10.1016/B978-0-12-800080-9.00009-8 Bản quyền 2015 Elsevier B.V Tất cả các quyền. 158 9. LÒ VI SÓNG PRETREATMENT BẢNG 9-1 so sánh các thông thường và hệ thống sưởi MW Chiếu xạ MW thông thường hệ thống sưởi Chuyển đổi năng lượng truyền năng lượng Trong bắp thể tích và thống nhất, Hệ thống sưởi, Hệ thống sưởi bề ngoài thông qua đối lưu/dẫn Nhanh chóng và hiệu quả chậm, kém hiệu quả Selective Non selective Hot spots No hot spot Dependent of the properties of the material Less dependent Precise and controlled heating Less controllable the field alternates, the molecule reverses direction to align itself with it. This causes energy to be lost from the dipole by molecular friction and collisions, giving rise to dielectric heating. For this reason, MW heating is always called dielectric heating. (2) Conduction mechanism happens when the dissolved charged particles in a sample (electrons, ions, etc.) oscillate back and forth under the influence of the electric component of MW irradiation. They collide with the adjacent molecules or atoms, which cause agitation or motion, and heat is thus created. (3) Interfacial polarization is a phenomenon viewed as a combination of conduction and dipolar polarization. It is an important mechanism for systems composed of conducting and nonconducting materials.9.1.3 MW EffectsThe action of MW irradiation results from materialewave interactions leading to thermal effects, specific MW effects and nonthermal effects [7e10]. A combination of these contribu-tions is responsible for the observed effects.Heat from conventional heating is transferred from the surface toward the center of the material by conduction, convection, and radiation. It is relatively slow and inefficient, and depends on the thermal conductivity of the material and convection currents. MW heating is characterized by converting electromagnetic energy into thermal energy, which is a kind of energy conversion rather than heating. Compared with conventional heating, the heat produced by MW irradiation is throughout the volume of the materials rather than an external source.The differences between conventional and MW heating are presented in Table 9.1 [11].9.2 MW APPLICATIONSince the first MW oven built in 1947 by Raytheon Corporation and an early commercial model introduced in 1954, MW heating has gained popularity in food processing because of its ability to achieve high heating rates, significant reduction in cooking time, more uniform heating, safe handling, ease of operation, and low maintenance [12,13]. Over the decades, this A. LIGNOCELLULOSIC BIOMASS 9.2 MW APPLICATION 159technology has spread widely to such applications as analytical chemistry, heating and vulcanization of rubber, plasma processing, chemical synthesis and processing, and waste remediation. MW irradiation has acquired a great deal of attention in domestic, industrial, and medical applications. It has been used in many applications including mineral and met-a