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RF Heating of Dry Food Products

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Introduction

Radio Frequency (RF) heating (or Capacitive dielectric heating) process is considered as a promising low energy heating technology for food products thanks to its rapidity that is associated to the large penetration depth and the uniform heating behavior. It has been successfully applied for many food needs like baking, drying and thawing and even for pasteurization and sterilization.

The conventional RF heating process involves placing the food sample between two parallel electrodes connected to an Alternating Electric field excitation. The electromagnetic waves are emitted from the metallic electrodes at frequencies of 13.56MHz or 27.12 MHz or 40.68 MHz. These frequencies are well selected by the US National Telecommunications and Information Administration [1] for domestic, industrial and scientific use. Those frequencies had to be well selected since RF and MW lie in the radar range and can interfere with communication systems.

The alternating electric field between the capacitor electrodes is induced by a rapid change in polarity; the molecules are then oscillating (trying to orient themselves with Electric Field) by a flip-flop motion. Consequently, this kinematic movement and friction between adjacent molecules generate then heat withing the dielectric material. RF heating is primarily influenced by the dielectric properties of the food product which determine the conversion rate of the electromagnetic energy into heat energy.

An FEM analysis using HFWorks allows to simulate the RF heating process of a dry food sample in a 6 kW parallel plates RF system at 27.12MHz frequency. This study is attempting to quantify the effects of dielectric properties of a surrounding food container for a high-rate food heating. A schematic illustration of the studied process is shown in the next figure.

a)-Effect of electrodes polarity on the Molecules polarity [2] b)- Schematic illustration of the RF heating process [3]

Figure 1 - a)-Effect of electrodes polarity on the Molecules polarity [2] b)- Schematic illustration of the RF heating process [3]
 

a)- 3D design and b)- Cross section front view of the used RF heating process

Figure 2 - a)- 3D design and b)- Cross section front view of the used RF heating process

Problem description

The studied model is formed by an RF cavity chamber limited by two parallel Aluminum electrodes. A polystyrene sheet deposed on the bottom electrode and supporting another Polystyrene container which is containing a three Soybean flour layers to be RF heated. An S-Parameters study of HFWorks coupled to the thermal solver will be used to predict the electric field intensity and temperature distribution in the Soybean flour material. The detailed dimensions are demonstrated by Table 1.

Table 1 - Dimensions of the studied model
Part Dimension (mm)
Top Electrode Width:400 Depth:830 Height:1
Bottom Electrode Width:590 Depth:990 Height:1
Polystyrene container Width:260 Depth:340 Height:80
Polystyrene sheet Width:400 Depth:830 Height:10
Soybean flour layers Width:220 Depth:300 Height:60
Air gap 30
Table 2 - Material properties
Material Relative permittivity Dielectric loss tangent Thermal conductivity (W/m. K)
Soybean Flour 3.96 0.38 0.097
Polystyrene 2.6 0.0003 0.036
Aluminum 1 0 160

Electromagnetic boundary conditions

Wave port: The wave port boundary is applied to the both Entry/Exit WG faces and fed with a 6 kWatt microwave power at 27.12 MHz
Perfect Electric conductor: A PEC boundary condition is applied to the top and bottom face of the Waveguide box.

Applied wave port BC
Figure 3 - Applied wave port BC

Mesh

A fine mesh control is needed for the dielectric parts to be heated for more accurate results. The whole meshed model is showed by the next figure.

 The meshed model
Figure 4 - The meshed model

Results

The HFWorks simulation revealed the next results: the first figure is showing the fringe plot results of the Electric and Magnetic field distribution across the food layers for an excitation power of 6 kW. High electric field concentrations occurred at the corners and edges due to the reflection and refraction of field at these zones.

a)-Electric and b)-Magnetic field distribution for 6Kw, 27.12 MHz  
Figure 5 - a)-Electric and b)-Magnetic field distribution for 6Kw, 27.12 MHz
 

The next GIF animation demonstrates the alternating electric field behavior between the top and bottom electrode which is due to the rapid change of polarity like detailed in the introduction.

 GIF animation of the Electric field between top and bottom electrode versus phase
Figure 6 -  GIF animation of the Electric field between top and bottom electrode versus phase
 

The next table summarizes the dielectric loss quantity for each food layer for an excitation power of 6kW.

Table 3 - Dielectric loss results
Soybean Flour layer Top layer Middle layer Bottom layer
Dielectric loss (Watt) 27.79 30.73 31.13

The next figures show the Dielectric and conductor loss densities across the Flour sample and the metallic electrodes. The first figure confirms that the maximum volume loss density follows the electric field intensity; it is concentrated at the corners and the side walls in contact with the container. The conductor losses are neglected compared to the dielectric ones for the applied excitation power.

a)-Volume loss and b)-Surface loss densities
Figure 7 -  a)-Volume loss and b)-Surface loss densities
 

For the thermal analysis; a convection boundary condition is applied to the upper top surface of the sample at an ambient temperature of 20°C and a convection coefficient set to 20 W/ .C .The simulated spatial temperature profile of Soybean over the three layers is figured below: it attains a maximum steady state value of 431°C at the bottom layer which is in a direct contact with the Polystyrene container.

 a)-Fringe plot of Temperature distribution across the Flour layers b)- Sectional-plane view.
Figure 8 - a)-Fringe plot of Temperature distribution across the Flour layers b)- Sectional-plane view.
 
 Heat flux distribution across the Polystyrene container and the Soybean flour layers.
Figure 9 - Heat flux distribution across the Polystyrene container and the Soybean flour layers.

Conclusion

RF heating or Capacitive dielectric heating is an innovative technique including radiative and ohmic heating aspects. The numerical coupling between Electromagnetic and heat transfer equations allowed to confirm the performance of RF heating as a safe substitute for traditional heating of food products.

References

[1]. https://www.ntia.doc.gov/page/2011/manual-regulations-and-procedures-federal-radio-frequency-management-redbook
[2]. http://hfwoodmachinery.com/2-high-frequency-edge-gluer/187641/
[3]. Huang, Zhi, Francesco Marra, and Shaojin Wang. "A novel strategy for improving radio frequency heating uniformity of dry food products using computational modeling." Innovative Food Science & Emerging Technologies 34 (2016): 100-111.