direct ohmic heating
TRANSCRIPT
Electroheat
Rudolf Žitný, Ústav procesní a zpracovatelské techniky ČVUT FS 2010
HEAT PROCESSESHP12
Electroheat. Direct ohmic heating, radiofrequency heating and microwave heating.
Direct Ohmic HeatingHP12
Wagner L.M.
Ohmic heating is one of these new technologies, which consists of the direct passage of electric current through the product. The permanent motion of electrical charges creates heat in the product in agreement with Joule's law
)U( = Q 2Gradient of electric potential [V/m]
Specific electrical conductivity [S/m]
Volumetric heat source [W/m3]
Continuous ohmic heater APV
Direct Ohmic HeatingHP12
Obr.1 Kontinuální ohřev APVAseptic cooler
Holding tube
Cooler
Aseptic tank
Pump
Electrical insulation
Electrode
Transformer
Waste
Product
Filling line
APV Baker Ltd. power 75 kW up to 300 kW, mass flowrates 750 kg/h up to 3000 kg/h. Pasterization of acidic products from 20°C to 90°C and sterilization of less acidic products from 65°C to 140°C (the whole line operates at overpressure 4 bars so that the boiling point temperature will be increased)
Direct ohmic heating of foods. Examples
Aseptic processing line for apricots and plums
Tomatoes
Direct Ohmic Heating ExampleHP12
Continuous direct ohmic heater for liquids in our laboratories
S. Jun, S. Sastry: Reusable pouch development for long term space missions: A 3D ohmic model for verification of sterilization efficacy. Journal of Food Engineering 80 (2007) 1199–1205
Direct Ohmic Heating ExampleHP12
Simulations suggest the presence of significant hot and cold zones, suggesting the need to further optimize pouch design for more uniform heating. In particular, the zones within the V-formed by metal foil electrodes, and the edge of the pouch, where current densities are lowered, are identified as points of concern
applications: space (Mars), earth-based in-package sterilization of foods, or for reheating of military rations
Ohmic Heating-corrosionHP12
Electrode corrosion
Metal that is migrating into the medium can be oxidized and can start new secondary reactions. For example Fe2+ or Fe3+ can be responsible as catalyzers.The corrosion effects can be suppressed either by using a noble material like gold or platinum for the electrodes, or by using an increased frequency above 50 Hz.
Samaranayake, C.P., Sastry, S.K., & Zhang, Q.H. (2005). Pulsed ohmic heating A novel technique for minimization of electrochemical reactions during processing. Journal of Food Science, 70(8), 460-465.
Stainless steel electrodes
TiN coating electrodes
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100,00
0 500 1000 1500 2000 2500 3000 3500
t [s]
P[W
]
0,00
20,00
40,00
60,00
80,00
100,00
3000 3200 3400 3600 3800 4000
t [s]
P [W
]
V=45,5 ml/s
Sel=30x40 mm
H=10 mm
Istart=3 A
Tvsádky=70 °C
V=43,5 ml/s
Sel=30x40 mm
H=10 mm
Istart=3 A
Tvsádky=70 °C
Ohmic Heating-corrosionHP12
See also: M., Ayadi et al. , Innovative l'ood Science and Emerying Technologies 5 (2004) 465-473
Ohmic Heating-Milk foulingHP12
Model of thermal milk fouling in continuous ohmic heater Jong (1996), Toyoda et al. (1994)
Rate of the denaturation and agglomeration of milk proteins
Denaturation of native protein -lactoglobulin
Agglomeration of denatured -lactoglobulin
Mesh of finite
elements T
CNativní CDenat. CAgglom
Ohmic Heating-Milk foulingHP12
Model of fouling in continuous ohmic heater using Finite Element Program FEMINA
temperature distribution
0)()(
yU
yxU
x
Laplace equation for electric potential distribution
F.K equation heat transfer with ohmic heating source
22
2
2
2
2
)()()(yU
xU
yT
xT
tTcp
Ohmic Heating-Solid in LiquidHP12
density of generated heat
[W/m3]2)( UQ
What is heated faster: liquid (conductivity l) or solid (S)?
Ohmic Heating-Solid in LiquidHP12
2 1 sin 0sin
U Urr r
cos, 2
r
brarU f
ff
3
0 03 cos , 1 cos .
2 2l l s
S Ll s l s
RU E r U E rr
2 20
0 21
3 91, ,0 cos , sin ,0 .2 2
s s l l ss s s s s
l s s
U U EU E Q U U
r r
Laplace equation for voltage in spherical coordinate system
f=l,sContinuity of voltage at the
sphere surface
Remarkable result: Intensity of heat production (Q) is constant
(independent of r,)!!!
sl
sl
s
l
92 2
.41 l
s
Conclusion: spherical particle will be heated faster than liquid if
kappa L=0.04 kappa S=0.1 potato heats faster than liquid
kappa L=0.04 kappa S=0.01 potato heats slower than liquid
Model FEMINA-Solid in LiquidHP12
+100V -100V
SL
30oC 30oC
The solution is more complicated for solid particles having form of cubes or platelets.
Heat generated inside the particles is not uniform and depends upon particle orientation.
Numerical solution using FEM is shown as an example
Minced meat sample between electrodes with adjustable contact pressure
1 2
4
5
6
7
3
8
Description:1 Moving electrode2 Fixed elecrode3 Carrier4 Frame 5 Roller6 Weight 7 Silon fibre 8 Stop
Ohmic Heating-Minced meatHP12
Ohmic Heating-Minced meatHP12
Meat sample
Contact layer
Tmmm 10
nkkkk pT 210
Electrode
1, , ,( )EF k
m
HR T H p R p TT S
Contact pressure
Thickness of sample
Ohmic Heating-Minced meatHP12
Contact surface and contact pressure
B
Hz
F
]21)1(
21)1ln()1)[((8)( 222
Hz
Hz
HzEBzF
0102030405060708090
100110
0 50 100 150 200 250 300t[s]
T[°C
]
P [W]
T [°C](průměr T2-T3)
T1 T2 T3
0,00
10,00
20,00
30,00
40,00
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60,00
70,00
80,00
90,00
0 20 40 60 80 100t[s]
T[°C
]
T1expT2expT3expT1modT2modT3mod
Comparison of temperatures T1 -T2 -T3 for pressure p5=8,175kPa.
Results
Ohmic Heating-Minced meatHP12
Modular Heating systemHP12
Applicators
Boiling pot Kettle
Toaster combined heating
Control unit
Power control
Control unit
Filter Rectifier
Pulse generator
Microprocessor
Pulse control
Power measurementConnector
Direct ohmic heating
Indirect ohmic heating
Temperature/power control
Modular Heating systemHP12
Direct ohmic heating
Contact heating
power control (triacs)
rectifier
pulse contro
l
MOSFET
MicrowaveHP12
Veneziano
Microwave heating operates at higher frequencies of electromagnetic waves 900 MHz or 2500 MHz. Polar molecules are heated due to excitation of vibrational mode and viscous friction
Intensity of electric field [V/m]
Frequency f , Permeability
Volumetric heat source [W/m3]
20 "Q f E
MicrowaveHP12
xkxkAfEfQ '20
2*0 2cos"2cosh"2"
E A e e A kxikx ikx* cos . 2
2 *2 *
2 0d E k Edx
Maxwell equation for distribution of electromagnetic field results to Helmholtz equation for intensity of electric field.
Example: One dimensional Helmholt equation describing E in a plate of meat in microwave oven
Nonuniform distribution of absorbed energy inside the plate of meat (6 cm thick) at microwave oven with different frequencies