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Sakamon DevahastinDepartment of Food Engineering
King Mongkut's University of Technology Thonburi (KMUTT)Bangkok, Thailand
International Workshop on Drying of Food and Biomaterials
June 6-7, 2011
Superheated Steam Drying of Foods and Biomaterials
OUTLINE
• Introduction to Superheated Steam Drying (SSD)
• Basic principles of SSD
• SSD of foods and biomaterials
• Low-Pressure Superheated Steam Drying (LPSSD)
SUPERHEATED STEAM DRYING (SSD)
• Proposed over 100 years ago; received serious attention only during the past 20 years
• Uses steam in place of hot air or combustion/flue gases in a direct dryer
• More complex than hot-air drying system
• Lower net energy consumption (if exhausted steam can be used elsewhere in the process)
• Better product quality (in most cases)
Closed steam drying system
Recycled steam
Fan/blower
Direct use of steam
Energy recovery via heat exchanger
Removal of condensate
Heaterpurged steam
steam from boiler
Typical SSD set-up
SUPERHEATED STEAM DRYER
Saturated Steam FeedSaturated Steam FeedAssume 100°C, 1 bar; H = 2,690 kJ/kg
Steam Superheater
Superheated SteamSuperheated SteamAssume 110°C, 1 bar; H = 2,720 kJ/kg
Drying chamber
Saturated Steam ExhaustSaturated Steam ExhaustBack to 100°C, 1 bar; H = 2,690 kJ/kg
Bleeding off for other uses
SSD & ENERGY
• If exhausted steam can be used elsewhere or can be recycled, latent heat is not charged
• Net energy consumption is 1000-1500 kJ/kg water removed
• Reduced net energy consumption is a clear advantage of SSD!
Change in Energy Use*
ConventionalConventional
Ann
ual E
nerg
y In
put (
GJ)
Ann
ual E
nerg
y In
put (
GJ)
0
80,000
SSDSSD
Thermal
Electricity
*Centra Gas Manitoba, Inc.
SOME ADVANTAGES OF SSD
• Dryer exhaust is steam so it is possible to recover all latent heat supplied to SSD
• No oxidative reactions possible due to lack of O2;color and some nutrients are better preserved
• Higher drying rates possible in both CRP and FRP depending on steam temperature (above the so-called inversion temperature SSD is faster than air drying)
• Toxic or organic liquids can be recovered easily
SOME ADVANTAGES OF SSD
• Casehardened skin is unlikely to form in SSD
• SSD yields higher product porosity due to evolution of steam within the product - bulk density is thus lower while rehydration behavior is better
• Sterilization, deodorization or other heat treatments (e.g. blanching, boiling, cooking) can be performed simultaneously with drying
SOME DISADVANTAGES OF SSD
• SSD system is more complex than its hot-air counterpart
• Initial condensation is inevitable – sometimes desirable though
• Products that may melt, undergo glass transition or be damaged at saturation temperature of steam cannot be dried in SSD
• Limited knowledge and experience on SSD
BASIC PRINCIPLES OF SSD
• Drying rate in CRP depends only on heat transfer rate since there is no resistance to diffusion in its own vapor
• If sensible heat effects, heat losses and other modes of heat transfer are neglected, CRP drying rate is then:
λλ)( surfacesteam TThqN
−==
BASIC PRINCIPLES OF SSD
• In hot-air drying ΔT is higher at low drying temperatures; reverse is true at higher drying temperatures
• These counter-acting effects lead to phenomenon of inversion; beyond inversion temperature SSD is faster than hot-air drying
CRP drying rate
Temp.inversion temp.
hot-air drying
SSD
An inversion phenomenon
λλ)( surfacemedium TThqN
−==
Air drying: Tsurface = Twet-bulbSSD: Tsurface = Tsaturation
BASIC PRINCIPLES OF SSD
• FRP drying rate of SSD is sometimes higher than that of hot air - mechanisms responsible are different, however!
• FRP drying rate of SSD is sometimes higher than air drying rate since product temperature is higher. Casehardening is unlikely to form and product is likely to be more porous as well
Prachayawarakorn et al., Drying Technol., 20, 669-684 (2002)
Drying rates of shrimp dried in superheated steam and hot air
Vacuum steam dryers for wood*
Vacuum steam dryers for silk cocoons**
Fluidized bed dryersfor coal*
Impingement and/orthrough dryer for textiles, paper***
Flash dryers for peat (25 bar)****
Conveyor dryers for beet pulp (5 bar)****
Fluidized bed dryers for pulps, sludges*
* Extensive commercial applications** Laboratory scale testing*** Pilot scale testing****At least one major installation
Superheated Steam Dryers
Near AtmosphericPressure High PressureLow Pressure
Classification of superheated steam dryers basedon their operating pressure
RotarySpray
Fluid bed
Flash
Conveyor
POSSIBLE TYPES OF SSDs• Flash dryers with or without indirect
heating of walls• FBDs with or without immersed heat
exchangers• Spray dryers• Impinging jet dryers• Conveyor dryers• Rotary dryers• Impinging stream dryers
Pressurized Steam FBD (Niro A/S)
PRESSURIZED STEAM FBD
• Closed system
• Used to dry materials produced in brewery, food and sugar processing, wood-based bio-fuels
• Close to 90% energy recovery as steam at 2-4 bar
• No product oxidation
Exergy Steam Dryer (GEA Exergy AB)
Exergy Steam Dryer with Backmixing (GEA Exergy AB)
• Residence time of 5-60 sec
• Generated excess steam at 1-5 bar - can be reused either directly or after re-boiling
• If there is no external use, excess steam can be recompressed to 10-20 bar and used as heating media. Power consumption is 150-200 kWh/ton evaporated water
• 70-90% energy recovery is possible
Exergy Steam Dryer (GEA)
SSD OF FOOD PRODUCTS
• Received serious attention during the past 10 years
• Possesses several advantages that are of special interest to food processors e.g. lack of oxidative reactions, ability to maintain color, nutrients, yields product of higher porosity
• Ability to inactivate microorganisms
• Many heat treatments can be performed simultaneously with drying
HIGH-PRESSURE SSD OF FOODS
• Drying of pressed beet pulp after extraction of sugar
• Operates at pressure ~ 5 bar
• Consumes 50% less energy than conventional air dryer
• Product quality i.e. appearance, texture, digestability by cattle is better than air drying
• Pilot tests with spent grain from brewery, alfalfa, fish meal, pulp from citrus, etc.
NEAR-ATM PRESSURE SSD OF FOODS
• Most SSDs operate in this range of pressure
• Wide variety of products dried successfully e.g. potato chip, tortilla chip, shrimp, paddy, soybean, noodles
• Better product quality (in some cases) than air drying
Experimental set up of Iyota et al. (2001)
Iyota et al., Drying Technol., 19, 1411-1424 (2001)
initial condensation
Drying curves for both SSD and hot air drying of potato slices
Iyota et al., Drying Technol., 19, 1411-1424 (2001)
SEM photos of cross section near the surface of potato slices
SSD
Hot air
SEM photos of cross section near the surface of potato slices
SSD
Hot air
second-layer crust
Microbial inactivation using saturated
steamHot air drying
Raw material Product
– Temp. 120˚C– 10 min
– Temp. 80˚C– Air velocity 2 m/s
SSD & FOOD SAFETY
Decontamination of pepper seeds (Method # 1 –Conventional industrial method)
Saturated steam treatment
Raw material 1.1 ± 0.1 × 1021.3 ± 0.1 × 1040.71 ± 0.0312.03 ± 0.07
n.d.n.d.*0.85 ± 0.0217.75 ± 0.69
Yeasts and MoldsTPCaw
Moisture content(% w.b.)
Microbial survival, moisture content and aw after saturated steam treatment
* Not detectable
Higher MC & aw after treatment means higher drying load!
n.d.n.d.*0.213 ± 0.0035.01 ± 0.17
Yeasts and MoldsTPCaw
Moisture content(% w.b.)
Microbial survival, moisture content and aw after hot air drying
* Not detectable
Total drying time = 210 min and Total process time = 220 min (rather long??)
– Temp. 120, 130, 140˚C– 5, 10 and 15 min– Pressure ∼ 1 bar
Microbial inactivation using superheated
steamHot air drying
Raw material Product
– Temp. 80˚C– Air velocity 2 m/s
SSD & FOOD SAFETY
Decontamination of pepper seeds (Method # 2)
Moisture content and aw after superheated steam treatment
7.87 ± 0.13
9.09 ± 0.04
10.61 ± 0.17
8.53 ± 0.04
9.25 ± 0.39
11.68 ± 0.27
10.35 ± 0.11
11.65 ± 0.10
12.92 ± 0.09
Moisture (% w.b.) aw*Time (min)Temp.
0.31 ± 0.0115
0.38 ± 0.0210
0.48 ± 0.025140°C
0.35 ± 0.0115
0.43 ± 0.0210
0.55 ± 0.015130°C
0.47 ± 0.0115
0.52 ± 0.0210
0.63 ± 0.025120°C
* Initial aw ~ 0.71
Microbial survival after superheated steam treatment
n.d.n.d.15
n.d.n.d.10n.d.n.d.5140°C
n.d.n.d.15
n.d.n.d.10n.d.3.1 ± 0.2 × 1025130°C
n.d.n.d.15
n.d.n.d.10n.d.5.0 ± 0.4 × 1025120°C
1.1 ± 0.1 × 1021.3 ± 0.1 × 104Raw material
Yeasts and Molds (CFU/g)
Total plate count (CFU/g)Time (min)Temp.
* Not detectable
Hot air drying time
90
130
200
140
180
200
190
210
290
Drying time(min)
15
10
5
15
10
5
15
10
5
SHS treatmenttime (min)
105
140
205140 °C
155
190
205130 °C
205
220
295120 °C
Total process time (min)Temp.
SSD & FOOD SAFETY
Decontamination of pepper seeds (Method # 3)
– Temp. 120, 130, 140˚C– Pressure ∼1 bar
Microbial inactivation and drying using SSD
Raw material Product
Microbial survival, moisture content and aw after superheated steam drying
5.77 ± 0.01
5.67 ± 0.09
5.94 ± 0.08
Moisture content(% w.b.)
0.245 ± 0.04
0.242 ± 0.02
0.227 ± 0.01
aw
n.d.
n.d.
n.d.*
TPC
n.d.
n.d.
n.d.
Yeasts and Molds
140°C
130°C
120°C
Temp.
* Not detectable
Total process time: 120oC = 180 min130oC = 65 min140oC = 30 min
Much shorter than that of the previous two methods! Taste, odor and color are comparable to conventionally treated product
OTHER FOODS DRIED IN SSD
• Potato chip, tortilla chip
• Shrimp, pork, chicken, fermented fish
• Sugar beet pulp, spent grain from brewery, okara
• Paddy, soybean, sunflower seed, cacao bean
• Asian noodles
• Vegetables, fruits, herbs - problems here!
IF YOU REMEMBER...
• Products that may be damaged at saturation temperature of steam cannot be dried in SSD
Need exists for a low-pressure superheated steam drying system for
heat-sensitive products
LOW-PRESSURE SSD (LPSSD)
• Combines ability to dry product at low temperature with some advantages of SSD
• Dryer is operated at reduced pressure
• Steam becomes saturated (and superheated) at lower temperature
• Suitable for heat-sensitive products, e.g., herbs, fruits and vegetables and other biomaterials
Devahastin et al., Drying Technol., 22, 1845-1867 (2004)
Photographs of carrot cubes underwent LPSSD and vacuum drying
Devahastin et al., Drying Technol., 22, 1845-1867 (2004)
Devahastin et al., Drying Technol., 22, 1845-1867 (2004)
Relationship between β-carotene content and
MC of carrot during drying
Suvarnakuta et al., J. Food Sci., 70, S521-S526 (2005)
Methakhup et al., Lebensm.-Wiss. u.-Technol., 38, 579-587 (2005)
Methakhup et al., Lebensm.-Wiss. u.-Technol., 38, 579-587 (2005)
Nimmol et al., J. Food Eng., 81, 624-633 (2007)
Léonard et al., J. Food Eng., 85, 154-162 (2008)
(a) (b)
(c) (d)
(a) Fresh sample(b) HAD(c) VD(d) LPSSD
SEM images of Salmonella on cabbage surfaces
MATH MODELING OF LPSSD
• No resistance to mass (moisture) transfer at product surface
• Concept of mass transfer coefficient is not valid
• Earlier model: Assuming that free MC equals zero at the surface – not appropriate!
• Initial condensation not considered
MATH MODELING OF LPSSD
• Simple 3-D liquid diffusion model
• Pressure gradient is the driving force for external mass transfer
• Net rate of evaporation or condensation per unit droplet area was estimated by the modified Hertz–Knudsen Equation
Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, 2644-2650 (2009)
MATH MODELING OF LPSSD
Energy equation
When Ts < Tsat:
Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, 2644-2650 (2009)
Convective heat transfer neglected!
MATH MODELING OF LPSSD
When Ts < Tsat (cont’d):
Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, 2644-2650 (2009)
Condensing vapor is superheated steam!
MATH MODELING OF LPSSD
When Ts = Tsat:
Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, 2644-2650 (2009)
MATH MODELING OF LPSSD
When Ts > Tsat:
Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, 2644-2650 (2009)
MATH MODELING OF LPSSD
Mass transfer equation
When Ts < Tsat:
Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, 2644-2650 (2009)
MATH MODELING OF LPSSD
When Ts = Tsat:
Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, 2644-2650 (2009)
When Ts > Tsat:
1 3
2
Initial condensation (only Model 1 can capture this!)
Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, 2644-2650 (2009)
132
Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, 2644-2650 (2009)
IN SUMMARY...
There is still much room to play with...