Superheated Steam Drying of Foods and Biomaterials

Transcription

1 Superheated Steam Drying of Foods and Biomaterials Sakamon Devahastin Department 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

2 OUTLINE Introduction to Superheated Steam Drying (SSD) Basic principles of SSD SSD of foods and biomaterials Low-Pressure Superheated Steam Drying (LPSSD)

3 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)

4 steam from boiler Recycled steam Fan/blower Direct use of steam Energy recovery via heat exchanger Heater Closed steam drying system purged steam Removal of condensate Typical SSD set-up

5 SUPERHEATED STEAM DRYER Saturated Steam Exhaust Back to 100 C, 1 bar; H = 2,690 kj/kg Drying chamber Saturated Steam Feed Assume 100 C, 1 bar; H = 2,690 kj/kg Bleeding off for other uses Steam Superheater Superheated Steam Assume 110 C, 1 bar; H = 2,720 kj/kg

6 SSD & ENERGY If exhausted steam can be used elsewhere or can be recycled, latent heat is not charged Net energy consumption is kj/kg water removed Reduced net energy consumption is a clear advantage of SSD!

7 Change in Energy Use* 80,000 Annual Energy Input (GJ) Thermal Electricity 0 Conventional SSD *Centra Gas Manitoba, Inc.

8 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

9 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

10 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

11 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: N = q = λ h ( T steam Tsurface) λ

12 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

13 CRP drying rate SSD hot-air drying N = q = λ h( T ) medium Tsurface λ Air drying: Tsurface = Twet-bulb SSD: Tsurface = Tsaturation inversion temp. Temp. An inversion phenomenon

14 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

15 Drying rates of shrimp dried in superheated steam and hot air Prachayawarakorn et al., Drying Technol., 20, (2002)

16 Superheated Steam Dryers Low Pressure Vacuum steam dryers for wood* Vacuum steam dryers for silk cocoons** Near Atmospheric Pressure Fluidized bed dryers for coal* Impingement and/or through dryer for textiles, paper*** High Pressure 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 Classification of superheated steam dryers based on their operating pressure

17 POSSIBLE TYPES OF SSDs Conveyor Spray Flash Fluid bed Rotary 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

18 PRESSURIZED STEAM FBD Closed system Used to dry materials produced in brewery, food and sugar processing, wood-based biofuels Close to 90% energy recovery as steam at 2-4 bar No product oxidation Pressurized Steam FBD (Niro A/S)

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21 Exergy Steam Dryer (GEA Exergy AB)

22 Residence time of 5-60 sec Generated excess steam at 1-5 bar - can be reused either directly or after reboiling If there is no external use, excess steam can be recompressed to bar and used as heating media. Power consumption is kwh/ton evaporated water 70-90% energy recovery is possible Exergy Steam Dryer with Backmixing (GEA Exergy AB)

23 Exergy Steam Dryer (GEA)

24 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

25 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.

26 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

27 Experimental set up of Iyota et al. (2001) Iyota et al., Drying Technol., 19, (2001)

28 initial condensation Drying curves for both SSD and hot air drying of potato slices Iyota et al., Drying Technol., 19, (2001)

29 SSD Hot air SEM photos of cross section near the surface of potato slices

30 SSD second-layer crust Hot air SEM photos of cross section near the surface of potato slices

31 SSD & FOOD SAFETY Decontamination of pepper seeds (Method # 1 Conventional industrial method) Raw material Microbial inactivation using saturated steam Hot air drying Product Temp. 120 C 10 min Temp. 80 C Air velocity 2 m/s

32 Microbial survival, moisture content and a w after saturated steam treatment Moisture content (% w.b.) a w TPC Yeasts and Molds Raw material ± ± ± ± Saturated steam treatment ± ± 0.02 n.d.* n.d. * Not detectable Higher MC & a w after treatment means higher drying load!

33 Microbial survival, moisture content and a w after hot air drying Moisture content (% w.b.) a w TPC Yeasts and Molds 5.01 ± ± n.d.* n.d. * Not detectable Total drying time = 210 min and Total process time = 220 min (rather long??)

34 SSD & FOOD SAFETY Decontamination of pepper seeds (Method # 2) Raw material Microbial inactivation using superheated steam Hot air drying Product Temp. 120, 130, 140 C 5, 10 and 15 min Pressure 1 bar Temp. 80 C Air velocity 2 m/s

35 Moisture content and a w after superheated steam treatment Temp. Time (min) Moisture (% w.b.) a w * 120 C ± ± ± ± ± ± C ± ± ± ± ± ± C ± ± ± ± ± ± 0.01 * Initial a w ~ 0.71

36 Microbial survival after superheated steam treatment Temp. Time (min) Total plate count (CFU/g) Yeasts and Molds (CFU/g) Raw material 1.3 ± ± C ± n.d. 10 n.d. n.d. 15 n.d. n.d. 130 C ± n.d. 10 n.d. n.d. 15 n.d. n.d. 140 C 5 n.d. n.d. 10 n.d. n.d. 15 n.d. n.d. * Not detectable

37 Hot air drying time Temp. SHS treatment time (min) Drying time (min) Total process time (min) 120 C C C

38 SSD & FOOD SAFETY Decontamination of pepper seeds (Method # 3) Raw material Microbial inactivation and drying using SSD Product Temp. 120, 130, 140 C Pressure 1 bar

39 Microbial survival, moisture content and a w after superheated steam drying Temp. Moisture content (% w.b.) a w TPC Yeasts and Molds 120 C 5.94 ± ± 0.01 n.d.* n.d. 130 C 5.67 ± ± 0.02 n.d. n.d. 140 C 5.77 ± ± 0.04 n.d. n.d. * Not detectable Total process time: 120 o C = 180 min 130 o C = 65 min 140 o C = 30 min Much shorter than that of the previous two methods! Taste, odor and color are comparable to conventionally treated product

40 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!

41 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

42 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

43 Devahastin et al., Drying Technol., 22, (2004)

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45 Devahastin et al., Drying Technol., 22, (2004) Photographs of carrot cubes underwent LPSSD and vacuum drying

46 Devahastin et al., Drying Technol., 22, (2004)

47 Suvarnakuta et al., J. Food Sci., 70, S521-S526 (2005) Relationship between β-carotene content and MC of carrot during drying

48 Methakhup et al., Lebensm.-Wiss. u.-technol., 38, (2005)

49 Methakhup et al., Lebensm.-Wiss. u.-technol., 38, (2005)

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52 Nimmol et al., J. Food Eng., 81, (2007)

53 Léonard et al., J. Food Eng., 85, (2008)

54 (a) Fresh sample (b) HAD (c) VD (d) LPSSD (a) (b) (c) (d) SEM images of Salmonella on cabbage surfaces

55 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

56 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, (2009)

57 MATH MODELING OF LPSSD Energy equation When T s < T sat : Convective heat transfer neglected! Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, (2009)

58 MATH MODELING OF LPSSD When T s < T sat (cont d): Condensing vapor is superheated steam! Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, (2009)

59 MATH MODELING OF LPSSD When T s = T sat : Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, (2009)

60 MATH MODELING OF LPSSD When T s > T sat : Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, (2009)

61 MATH MODELING OF LPSSD Mass transfer equation When T s < T sat : Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, (2009)

62 MATH MODELING OF LPSSD When T s = T sat : When T s > T sat : Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, (2009)

63 Initial condensation (only Model 1 can capture this!) Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, (2009)

64 1 2 3 Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, (2009)

65 IN SUMMARY... There is still much room to play with...

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