Heat Pump Assisted Drying - Freeze Drying Technology

Transcription

1 Heat Pump Assisted Drying - Freeze Drying Technology Trygve M. Eikevik Professor Norwegian University of Science and Technology (NTNU) Department of Energy and Process Engineering trygve.m.eikevik@ntnu.no

2 2 Content Introduction Open system Closed loop system Vacuum freeze drying Atmospheric freeze drying Conclusions

3 Production Cost Quality vs. production costs 3 High Vacuum freeze drying Heat Pump Drying Hot Air Low Low Quality High

4 4 Comparing different drying methods -40 o C -15 o C 0 o C +60 o C Vacuum Freeze driers Heat pump driers Spray driers Tunnel driers Belt driers Frozen particles Solid particles Drum driers Etc. Immobilisated particles

5 5 Challenges in drying systems Optimal product quality (colour, flavour, texture, density and shrinkage) are influenced by the dryer type and dryer conditions, temperature and relative humidity Knowledge of product characteristics such as drying curve, water activity and properties / quality attributes To achieve high energy efficiency, high SMER-ratio Environmental aspects

6 6 Content Introduction Open system Closed loop system Vacuum freeze drying Atmospheric freeze drying Conclusions

7 Drying of cod at the clips of west Norway 7

8 Traditional tunnel dryers (Hordetørke) 8 A L Inlet fresh air Discharge air Recirculation air L o E B Product C Side view

9 9 Traditional tunnel dryers (Hordetørke) Drying air Product A B C Top view

10 Hot air dryers 10 Max. temperature B C B C E L Outdoor air A L o

11 Influence of recirculation of air 11 1, % 1,05 90 % SMER (kgw/kwh) 1,00 0,95 0,90 0,85 0,80 0,75 0,70 SMER Capacity Energy 80 % 70 % 60 % 50 % 40 % 30 % 20 % Enegy and Capacity reduction (%) 0,65 10 % 0, % Recirculation of air (%)

12 Reduction in thermal efficiency Thermal efficiancy, Dh/DY (kj/kgw) Recirculation of air (%)

13 13 Content Introduction Open system Closed loop system Vacuum freeze drying Atmospheric freeze drying Conclusions

14 14 Heat pump drying - Status HP tunnel dryers Have successfully been designed, dimensioned and installed in Norway for drying of fish products (70 industrial plants in Norway) HP fluidized bed dryers By using drying temperatures below and above the freezing point, the rehydration, colour, taste and bulk density of food products can be regulated. Medicine, pharmaceutical and biotechnological products can be dried without loss in biological activity (5 units in Norway, 1 very large HPD plant constructed in Hungary) Non-adiabatic HP fluidizes bed dryers HP condenser is put in the fluidized bed to increase the drying capacity (1 unit in Norway)

15 15 Advantages with heat pumps in combination with driers Product quality Drying temperatures from -20 o C to +100 o C secure high quality for heat sensitive products Environmental aspects Recirculation of drying gas (air or inert gas) eliminates waste gas from the dryer. Electric driven machinery have not emissions of CO 2, SO 2 or NO x Energy aspects Energy consumptions for a heat pump dryer are typical 20% to 35% compared to conventional dryers

16 Fluidized bed heat pump dryer 16

17 Tunnel dryer with heat pump 17

18 Schematic layout of a heat pump fluidised bed dryer 18 Air flow Fluidized bed chamber External condenser M Compressor Receiver A Expansion valve Condenser C Evaporator B Water

19 19 Water activity, a w (product in equilibrium with the ambient air) 100 C a w = p p w Water activity, a w B A), B) and C) shows different types of materials/products A 0 0 Moisture content, X wb % 100

20 20 Water activity vs. relative humidity Water activity a w = p p p w p p : vapor pressure close to product surface Relativ humidity φ = p a p w p a : vapor pressure in free air p w : saturated vapor pressure for water at t a w > φ Product will dry (loosing moisture/weight) a w = φ Product is in equilibrium (is not loosing or gaining weight - stable) a w < φ Product will gain water/moisture from air (weight is increasing)

21 Unbound moisture (kg H2O/kg d.m.) General drying curve with different drying phases 21 A A B A B Initial drying period B C Period with constant drying rate C D E Period with falling drying rate C D E Time, t (h)

22 Drying rate [kg H 2 O/(s m 2 )] General drying rate curves 22 Falling drying rate Constant drying rate A R C C B A D E X C Unbound moisture [kg H 2 O/kg d.m.]

23 Change in initial freezing point with reduction of water content 23 Apple s enthalpy family curves

24 Enthalpy, h (kj/kg) Enthalpy as a function of temperature and water content 24 Zone A: Atmospheric drying unfrozen Zone A Zone B. Atmospheric drying partly frozen Zone B Zone C: Atmospheric drying - frozen Zone C t if Temperature, t ( o C)

25 25 Design rules for heat pump dryers Continuous operation Countercurrent operation (air product) Inlet air temperature to the drying chamber as high as possible Avoid over dimensioning the refrigeration capacity Provide for the air flows over product without possibility to by-pass Optimal SMER

26 Design of heat pump dryers Lengthwise countercurrent drying 26 φ = crosswise φ = 100% φ = lengthwise SMER = COP dh/dx (kg H 2 O/kWh) dh dh dx dx lengtwise crosswise

27 Design of heat pump dryers As high inlet temperature as possible 27 t high φ = 100% t low SMER = COP dh/dx (kg H 2 O/kWh) dh dh dx dx high low

28 Design of heat pump dryers Heat pump capacity: Q o 28 Qo high Qo low φ = 100% SMER = COP dh/dx (kg H 2 O/kWh) dh dh dx dx Qo low Qo high

29 29 Design of heat pump dryers The choice of t o, t c φ = 100% t o,high SMER = COP dh/dx (kg H 2 O/kWh) t o,low dh dh dx dx to low to high Influence of COP = Q o / W

30 30 Mollier diagram a Temperature, t [oc] d c t s dx b dh min dh opt φ = 1 dh max Absolute humidity, x [kg/kg dry air]

31 31 Equations Specific Moisture Extraction Rate SMER = COP / (dh/dx) [kg water/kwh] Coefficient Of Performance COP = Q o / (W compr + W fan ) [-] Evaporator capacity Q o = m a * (h b - h c ) Compressor power W compr = (Q o / dh re ) * (dh is /η is ) [kw] [kw]

32 32 SMER (40/60%, R717) SMER ta = -5 oc ta = 10 oc ta = 25 oc ta = 45 oc ta = 75 oc Opt.SMER Temperature difference, tb - td [oc]

33 33 Optimal temperature difference (R717) 20 Optimal temp.diff., tb - td [oc] /50 % 40/60 % 50/70 % Air inlet temperature drying chamber, ta [oc]

34 34 Optimal SMER (R717) 7 6 Optimal SMER [kg/kwh] /50 % 40/60 % 50/70 % Air inlet temperature drying chamber, ta [oc]

35 35 Content Introduction Open system Closed loop system Vacuum freeze drying Atmospheric freeze drying Conclusions

36 Pressure, p (mmhg) 36 Phase diagram for water Solid phase Liquid phase Vapour phase Sublimation Evaporation Temperature, t ( o C)

37 Freeze drying, solid structure 37 Vacuum freeze drying are carried out by first freeze the product to achieve a solid structure and than dry at pressure lower than the triple point. The drying will occur by sublimation ice crystals evaporates directly without melting Only water vapor will be transported in the product and cause no internal transport of dissolved components like sugar, salt etc. Conventional convective drying at air temperatures above 0 o C will a larger part of the water removal be done by evaporation from the surface of the product. Liquid will be transported from internal out to the outer layers of the product. This will cause a internal transport of dissolved components.

38 Benefits with vacuum freeze drying 38 Tastes as fresh Will retain taste and nutrition's. Only water will be removed. (some light volatiles will be lost) Look fresh Will retain shape and color Lower weight than fresh product Freeze drying will normally remove up to 98% of the water. That means the weight of the product will be reduced with 90% Can be stored like fresh Can be stored at room temperatures

39 Benefits and drawbacks compared with traditionally conservation methods 39 Retention of morphologic, biochemical and immunological properties High viability / activity of biological components Lower temperatures, oxygen level and operational conditions than other comparable methods High recovery of volatile components Retention of structure, surface area High yield Long shelf life Reduced weight at storage, transport and handling Drawbacks Higher investment costs Higher energy consumption Higher maintenance costs

40 Vacuum freeze drying system based on heat radiation 40 Rack for stacking of trays Movable trapdoor Product trays Heating plates Vacuum pump Condenser during defrosting Active condenser Melt water chamber

41 41 Content Introduction Open system Closed loop system Vacuum freeze drying Atmospheric freeze drying Conclusions

42 Advantages with Atmospheric Freeze Drying 42 Constant or step vice temperature program -15 o C to +30 o C Product quality control of Shrinkage Rehydration ability Color Taste Aroma Close to vacuum freeze drying qualities Peas (Industrialized) Corn (Industrialized) Vegetables, fruits, berries, herbs (Tested) Biological active components (Tested)

43 Actual products 43 Fish/meat (snacks, fast food etc.) Rehydration, colour, taste, aroma Peas, corn (bag food, quick preparation) Rehydration, formation of cracks, colour, taste Herbs (ethereal oils, spice) Extraction level, taste, aroma, appearance, biological effect Berry, fruits (breakfast cereals ingredients, topping ) Rehydration, colour, taste, aroma Dairy (bioactive products, aroma products) Biological activity, rancidity, taste

44 Scrimps dried at low and high temperature 44

45 Pieces of Cod dried at 5 o C and +30 o C 45

46 Drying curves - temperature program Pieces of Cod (5*5*5 mm) Dried at -5 oc in 1 hour Dried at -5 oc in 3 hour Dried at -5 oc in 5 hour Dried at -5 oc in 10 hour Water content (%) Drying time (hour)

47 Bulk density of cod after drying 47 0,4 0,4 Bulk density [g/ml] 0,3 0,2 0,1 Bulk density [g/ml] 0,3 0,2 0, Temperature in drying chamber [oc] Drying time at -5 oc [hours]

48 Energy consumption in a heat pump dryer operated at a continued mode drying 48 Assumptions: LT HT Air inlet drying temperature -5 C 30 C Air inlet relative humidity 40 % 40 % Air outlet relative humidity 80 % 80 % Surface temperature of air cooler t dp 5 C t dp 5 C Heat pump evaporating temperature air cooler surface temperature 2 C Heat pump condensing temperature air inlet drying temperature +5 C air cooler surface temperature 2 C air inlet drying temperature + 5 C Heat pump refrigerant NH 3 NH 3 SMER = COP LT (dh/dx) LT ( t LT ttot ( ) * ) + COP HT ) * (dh/dx) HT ( t HT ttot ( )

49 Energy consumption in a heat pump dryer 0perated at a continued mode drying 49 Energy consumption [kwh/kg dried product] Drying time at -5 oc air inlet temperature, [hours]

50 SMER for a heat pump dryer operated at a continued mode drying SMER, [kg water/kwh] Drying time at -5 o C air inlet temperature, [hours]

51 Production line for instant products 51 CO 2-35 o C Product Pure/jam Cold expansion equipment Freezing from cold expansion Heat Pump Drying

52 Bulk density for different potatoes and extrusion with CO Bulk density [g/l] Extruded beate Beate Pimpernel Mandel

53 Standard rehydration (30 sec) for different potatoes and CO 2 extrusion REHYDRATION INDEX Extruded beate Beate Pimpernel Mandel

54 Rehydration in cold water of dried products (after 2 minutes) Ra = (M water / M dry matter ) * Dried at -5 oc only Dried at +30 oc only Rehydration [%] Cod Meat Apples Carrot

55 55 Content Introduction Open system Closed loop system Vacuum freeze drying Atmospheric freeze drying Conclusions

56 Conclusions 56 New generations of high quality low density dried products can be produced by heat pumps operated in a combined drying mode at drying temperatures below/above the freezing point. There is an optimal SMER-ratio. This gives an optimal inlet temperature difference for the air cooler SMER values for heat pumps operated in combined modes will be in the order of 1,5 to 4,5 kg H 2 O/kWh depending on temperature and time at freeze drying conditions.

57 Thank you for your attention! Спасибо за внимание!