Energy Issues and Use of Renewable Source of Energy for Drying of Foods

Transcription

1 Energy Issues and Use of Renewable Source of Energy for Drying of Foods Prof. Arun S. Mujumdar / Sachin Jangam National University of Singapore International Workshop on Drying of Food and Biomaterials Bangkok June 6 7, 2011

2 Why Energy Saving Needed? Rising energy costs may even end up with carbon tax Greenhouse gas emission related directly to energy consumed Industrial dryers operate at 20 70% efficiency lot of scope to improve operation Globalization will force competition This part includes global energy status, Energy in Drying. possible options for energy savings in general way (Not specific criteria)

3 General observations Drying a highly energy intensive operation due to 1. High vaporization latent of water 2. Affinity of solids to water 3. Difficulty in getting heat to where water is; resistances to heat/ mass transfer, etc Some obvious options to save energy 1. Avoid drying not feasible! 2. Displace water with solvent with low heat of vaporization and high vapor presence 3. Use mechanical means (filtration, ultra centrifugation, etc) 4. Use evaporation (for dilute liquids)

4 Some indices used to define dryer efficiency Energy efficiency η η = E E ev = t Energy used for moisture evaporatio Total energy given to dryer n Energy efficiency η T (range from 0 to 1) η T = T T 1 1 T T 2 0 T 1 = Inlet air temperature; T 2 = Outlet air temperature; T 0 =Ambient temperature Max η T is when T 2 =T wb T wb = wet bulb temperature at exit conditions

5 Think Drying System instead of Dryer!! Always look at full flowsheet and the whole drying system not just dryer! Pre drying ( mechanical dewatering etc) and post drying ( cooling, granulation, blending etc) should be considered in overall strategy to save energy Mechanical dewatering is 10 times cheaper than evaporation which is 10 times cheaper than drying! (Rough rules of thumb!) Not always possible to use minimum energy option as it may not be optimal for quality etc.

6 Techniques for energy savings How energy is produced for drying direct firing is most efficient; convective drying can be more rapid Electric heating technologies some new technologies have emerged to be cost competitive Heat recovery various options Control of dryer New developments e.g. superheated steam pulsed combustion, adsorption drying etc.

7 Direct firing why? CDDET recommends direct drying with natural gas to reduce chain of heat transfer equipment. Claims advantages such as: Reduced maintenance; Ease of installation Uniform heating, better control of temperature Potential for integration into existing control system. Limitations Loss of energy in exhaust air Heat recovery from low temperature exhaust is not inexpensive Product maybe contaminated by combustion gas; not allowed in some countries for foods Flammable solids/vapors require self inertizing (low oxygen atmosphere).

8 Electric Heating Higher grade energy typically an expensive source for low grade heat; use only if there is quality advantage MW drying can be more energy efficient than solar energy in energy consumed, not cost effective always look at bottom line! IR heating, MW/RF, induction heating can deliver energy direct to productno air needed hence more efficient thermally. Advantages Precise temperature control Reduced dryer size Clean, easy to maintain Better quality, no contamination Short start up, shut down times.

9 Heat recovery Recycle ratio of exhaust air, if drying rates are high due to high air velocity and short contact time in dryer e.g. Yankee dryer for tissue paper 80 90% of exhaust needs to be recycled! Consider cost associated with recycle ducting and some reduction in driving potential for drying Can cause problems of condensation etc as weather changes significantly.

10 Use of Process Control Refer to chapter on Control of Dryers in Handbook of Industrial Drying, 3rd Edition for details of control strategies Here we look only at potential for energy savings % Energy savings depends on the quality of base case used!!.

11 Use of Process Control Refer to chapter on Control of Dryers in Handbook of Industrial Drying, 3rd Edition for details of control strategies Here we look only at potential for energy savings % Energy savings depends on the quality of base case used!!.

12 Some heat recovery methods Use of heat pumps Heat Pipe heat exchangers Gas/gas plate heat exchangers; fin tube exchangers Heat Wheels can have short paybacks Control improve quality, reliability, energy consumption etc Good housekeeping; good insulation etc.

13 General guidelines for saving energy Use indirect heating( Conduction, IR) to avoid loss of energy in exhaust air Use intermittent, multi mode heating, where possible for batch drying Use less air, higher temperature, where possible Use waste heat, renewable energy( solar, wind, etc), where feasible Recover exhaust heat or recycle Optimize operating conditions; model based control Some options involve extra capital costs

14 Energy Efficiency for Different Dryers Dryer type η range% Dryer type η range% Direct continuous Indirect continuous Tower Drum flash Rotary Sheeting (stenters) Cylinder conveyor Batch Rotary (bundle) Agitated pan Spray Vacuum rotary Up to 70 Tunnel Vacuum tray Fluidized bed Infrared Batch Dielectric Tray 85

15 Methods to improve energy efficiency Methods Use of heat exchangers including heat pipes Use of heat pumps Model based control Optimized operation Superheated steam drying with utilization of excess steam Multi stage drying More efficient heat source (e.g. pulse combustion) Potential M M L M L H M L M L M L H: high M: medium L: low

16 Solar Drying

17 Background on Solar Drying Numerous food products require moisture removal for many reasons To minimize the microbial damage To increase the shelf life and make the product available throughout the year Various drying methods are described so far Slow drying / suitable temperatures are necessary to retain the nutrients and texture At the same time Minimize cost Renewable Energy Answer to tomorrow s energy needs? Solar energy potential in Developing and Under developed countries Solar Dryers (most obvious option for foods)

18 Solar Cabinet Drying Solar cabinet dryers are the choice for conventionally used opensun drying Advantages Solar drying provides desired reduction of losses together with improved quality of dried products Time of drying can be significantly reduced. Farmers have a greater income by the production of marketable crops. The additional costs involved in installing solar dryers can be returned by increased profits greater control over drying characteristics

19 Solar Dryers Classification SOLAR DRYERS Conventional dryers Solar energy drying Bulk or storage (low temperature) dryers Batch and continuous flow (high temperature) dryers Natural open to sun drying Solar energy dryers Crops dried insitu Post harvest drying Active dryers Passive dryers Distributed type dryers Mixed mode dryers Integral type dryers Drying on ground mats concrete and floor Drying on trays Distributed type dryers Mixed mode dryers Integral type dryers Horizontal trays Cabinet type dryers Green house dryers

20 Solar Dryers (Variants) General classification Solar natural dryers (using ambient energy only) Semi artificial solar dryers using a fan driven by an electric motor Solar assisted artificial dryers able to operate using conventional energy sources if needed Tent type dryer (natural convection) Natural convection cabinet dryer Terrace type (natural convection)

21 Solar Dryers (Variants) Static bed type solar dryer with chimney (Natural Convection) Shelf type dryer with separate collector (Natural Convection) Tray and chimney type solar dryer with heat storage (Natural Convection)

22 Solar Dryers (Variaants) Solar dryer with rock bed heat storage (Forced Convection) Forced convection solar dryer Solar dryer with physical heat storage (Forced Convection)

23 Key elements / Issues in Solar drying Solar collectors Efficient designs for better heat transfer to air Drying chamber For uniform product quality Control Strategy Presence of micro organisms in the inlet air Use of efficient filters to process the air before it enters the dryer Increasing the efficiency Various possibilities including partial recirculation of air

24 Important challenge in cabinet dryers Advanced computational tools such as Fluent can help understand flow of drying air in cabinets Simple ideas such as use of mesh / net in the air path can make the air distribution uniform This will result in pressure drop and ultimately the higher pumping cost; however this can be compensated by uniform product quality whichwill reflect in better returns Existing Modified

25 How to save energy in drying? (Different Options) Do regular energy audits find out where heat is being lost. Temperature and humidity measurements at inlet, outlet ; IR thermography canhelp detect leaks Typically 20 30% energy can be saved at no cost! Using advanced/modified drying systems 1. Superheated steam drying 2. Heat Pump Drying 3. Multi stage drying system 4. Low pressure spray drying 5. Pulse combustion drying

26 Heat Pump Drying

27 Schematic of a typical HPD system Heat sink External condenser Auxiliary heater Refrigerant mixing Condenser Expansion valve Compressor Dryer Evaporator

28 How heat pump + dryer works? dehumidifying atmospheric air in evaporator and reheating to a desired temperature in the condenser of heat pump can be used beneficially in drying for better drying rate and product quality Absolute humidity (kg kg 1 ) Saturation line 2 1 3' 4 5 4' 5' Dry bulb temperature ( C)

29 More about heat pump drying technique Any dryer that uses convection as the primary mode of heat input to the dryer can be fitted with a suitably designed heat pump system Batch shelf and tray dryers are more common but other types of dryers can be used in conjunction with heat pump Dryers requiring large amount of air (Flash and Spray dryers) are not suited for heat pump operation Typical cost of heat pump dryer is 1.5 to 2 times the cost of FBD but substantially lower than freeze dryer In several applications HPD can replace Freeze dryer

30 Heat pump options Heat Pump Dryer (possibilities) Mode of operation HPD Cycle Drying media Supplementary heating Heat pump dryer operation Number of heat pump stages Temperature for drying Batch (tray, shelf, fluid bed) Continuous (fluid bed, vibrated bed) Open loop Closed loop Air N 2 CO 2 other inert gases Superheat ed steam Convection Conduction Microwave, infrared, radio frequency Continuous operation Intermittent Cyclic operation Single stage Multiple stages Below freezing point Above freezing point

31 Principle advantages of HPD Highest Specific Moisture Extraction Rate Can significantly improve product quality by drying at low temperature. At low temperatures drying potential of air can be maintained by further reduction of air humidity Wide range of drying conditions typically 20 0 C to C and relative humidity from very low to 80% (with humidification systems Excellent control of environment for high value products and reduced electrical energy consumption for low value products

32 Some limitations Chlorofluorocarbons (CFC s) are used in refrigeration cycles, some of which are not environment friendly Requires regular maintenance of components and charging of refrigerant May incur higher capital costs

33 Comparing HPD with other Drying Systems Type of Heat pump Hot air dryer Vacuum Freeze dryer dryer dryer dryer Parameter SMER (kj kg 1 ) and lower Operating 10 to to very to >50 temperature ( C) high Operating humidity Varies Low Low (% RH) depending on temperature Dryer Efficiency (%)* Up to Up to 70 Very low Product quality Very good Average Good Excellent Capital cost Moderate Low High Very high Drying rate Faster Average Very slow Very slow Operating cost Low High Very High Very high Control Very good Moderate Good Good

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35 Multi stage HPD Multi stage heat pumps can be used to produce multiple air streams of different drying conditions which can enter at different sections of the dryer depending on the drying requirement Multi stage heat pumps can, moreover, reduce energy consumption of the drying process as the COP values for such systems are high

36 Cascade HPD Drying chamber Auxiliary heating Condenser Evaporator High pressure compressor Low pressure compressor Evaporator Condenser

37 Heat Pipe Finned pipe partially filled with liquid and vapor Here heat recovered from return air both provides free reheat and also reduces load on the main coil This saves energy and reduces plant capacity

38 Chemical heat Pump drying Chemical heat pumps Principle reversible chemical reaction is used to absorb and reject heat, mainly endothermic reaction absorbs heat (cooling effect) which is then released during an exothermic reaction (heating effect) Heat Storing step B (g) (AB) S + Qh = C (s) + B (g) = A (s) + B (g) (BC) S + QC B (g) heat releasing step Pros No environment impact; for high temperature applications; possibility of using it for heat storage (with PCM) etc Cons Difficulty in scale up

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40 Solar assisted heat Pump drying Use of solar heat with or without heat storage solar energy is used to heat the drying air either before or after the condenser of a conventional heat pump especially useful when high drying temperatures are required (grain drying) Advantages effective utilization of renewable energy for direct heating as well as storage with easy control strategy Limitations Higher capital cost (solar panels) and maintenance Solar radiation s Air Dryer Solar Collector Condenser Evaporator Preheater

41 Solar assisted heat Pump drying Use of phase change materials such as paraffin wax for discharging sensible energy to the drying air higher drying temperature in comparison with conventional heating system Solar assisted chemical heat pump dryer Solar assisted heat pump dryer and water heating system

42 Time varying Operation and quality aspects Auxiliary heater 3 Compressor Dryer 3 Dryer 2 Auxiliary heater 2 External condenser Condenser Evaporator Auxiliary heater 1 Dryer 1 Air blower Expansion valve Multiple drying chambers with time varying cycle in heat pump dryer

43 Time varying Operation and quality aspects Reduction in drying time Maximum retention of sugar in food products Avoiding color change due to non enzymatic browning (example: Drying of banana (Chau and co workers) The color and aroma herbs can be improved when compared with the commercial products. Example: drying of Ginger (maximum retention of Gingerol), because of the closed operation of heat pump dryer The sensory values in the literature are almost double for the heat pump dried products Use of modified atmosphere to replace air allows new dry products to be developed without oxidative reactions

44 Some more options with HPD Modified Atmosphere HPD This includes use of inert drying medium such as CO 2, N 2 in order to avoid unnecessary reactions High quality products; has been used for various food products Need to use closed loop HPD to reduce the cost associated with the use of drying medium Solar assisted chemical HPD Very challenging option which can be used to upgrade the solar energy Very efficient tool for energy storage and has lot of potential in drying Needs lot of R&D to make it commercially viable Heat pump assisted spray drying No commercial application so far Theoretically proved to give very high efficiency compared to conventional spray drying

45 HPD Applications in Foods Product dried Observations Reference Mushroom and chili Vacuum heat pump dryer; Temperature and pressure have significant effect on color. Temperature has little effect on rehydration but decreasing pressure increases rehydration capacity. Artnaseaw et al., 2010a Tom Yum herbs Thin layer vacuum heat pump drying; pressure of 0.2 bar and drying temperatures ranging from 50 Artnaseaw et al., 2010b C to 65 C. High moisture paddy Heat pump dryer can save 40% energy compared to far infrared low temperature diesel powered Jinjiang and Yaosen, 2010 hot air drying for the same capacity and final product quality. Longan Far infrared assisted heat pump drying and hot air drying were used to reduce the drying time; Porous structure of longan was obtained. Nathakaranakule et al., 2010 Maize (Grain drying) Solar assisted heat pump drying was used with reduced drying time, uniform drying of maize and Li et al., 2010 very high energy efficiency. The COP of heat pump was found to be 5.4. Ginger Tray, heat pump, two stage drying and mixed mode solar drying; Heat pump dried ginger had Phoungchandang et al., 2009; shorter drying time, better rehydration and maximum retention of the active component 6 gingerol. Phoungchandang and Saentaweesuk, 2010 Apple Heat pump dryer and solar dryer were used. It was recommended to use solar dryer in day time and HPD during night time to make the operation energy efficient. Aktas et al., 2009 Olive leaves Heat pump continuous dryer was successfully used; optimum conditions for minimum antioxidant Erbay and Icier, 2009 activity loss and maximum exergetic efficiency were obtained Green sweet paper More retention of total chlorophyll content and ascorbic acid content using heat pump dryer. Pal et al., 2008 Hazelnut PID controlled HPD was used with energy utilization as high as 65%. Ceylan and Aktas, 2008 Shrimp Peeled, headed and whole shrimps were dried using HPD; shrimps dried using HPD had better Zhang et al., 2008 rehydration characteristics and better water holding capacity. Herbs Heat pump drying was used for drying of Jew's mallow, spearmint and parsley with low specific Fatouh et al., 2006 heat consumption value. Apple, guava, potato slices Modified atmosphere heat pump drying using nitrogen and carbon dioxide at around 45 C and 10% Hawlader et al., 2006a relative humidity, resulted in better structure and faster drying. Carrot slices Heat pump in combination with fluidized bed for granular carrot, SMER was observed to be 2.35 Zhang et al., 2006 kg/kwh. Guava and papaya slices Drying using HPD, vacuum dryer and freeze dryer; Using modified atmosphere resulted in 44% Hawlader et al., 2006c increase in diffusivity of guava and 16% increase for papaya with less browning, better rehydration and better nutrient retention.

46 Introduction to pulse combustion drying Pulse combustion is intermittent ; can be subsonic or supersonic Mach Number >1.0 (supersonic) Features Steady Pulsed Combustion intensity (kw/m 3 ) Efficiency of burning (%) Temperature level ( K) CO concentration in exhaust (%) NOx concentration in exhaust (mg/m 3 ) Convective heat transfer coefficient (W/m 2 k) Time of reaction (s) Excess air ratio

47 More about pulse combustion drying High drying rates Increased turbulence and flow reversal in the drying zone promote gas/materials mixing Decreased boundary layer thickness of materials Increased heat and mass transfer rates High driving force because of high gas temperature short contact time Suitable for some heat sensitive materials High energy efficiency and economic use of fuels Environmentally friendly operation

48 Energy consumption between PC and conventional dryer Dryers Typical evaporation capacity Typical consumption (kj/kgh 2 O) PC dryers kg H 2 O/h Tunnel dryer Impingement dryer 50 kg H 2 O/hm Rotary dryer kg H 2 O/hm Fluid bed dryer Flash dryer kg H 2 O/hm Spray dryer 1 30 kg H 2 O/hm Drum dryer (pastes) 6 20 kg H 2 O/hm

49 Use of Advanced Software for Energy Optimization Simprosys Simprosys is a Windows based process simulator developed by Simprotek Corporation ( It can be used for flowsheet design and simulation of drying and evaporation systems. Simprosys 1.01 covers 19 unit operations including solid and liquid dryer. Simprosys is extremely user friendly. Mainly solves heat and mass balances However does not use drying kinetics Simprosys 2.1 can simulate 11 nonaqueous drying systems Can be used to study the effect of recycle, pre heating, indirect heating etc

50 Use of Advanced Software for Energy Optimization A typical Simprosys flowsheet

51 Concluding Remarks Energy is a important point to be considered in drying which is highly energy intensive unit operation Different routes can be used to minimize the energy losses Carbon foot prints can be minimized by making the drying system energy efficient Developing energy intensive methods with sustainability need to develop innovative drying techniques Some of the advanced/innovative ways of drying can substantially reduce the energy consumption Use of renewable sources of energy need to be employed in order to make dryers sustainable

52 Books /monographs by Prof. Arun S. Mujumdar During 2010 Visit us at:

53 Selected books and journals in drying and transport phenomena edited / authored by Prof. Arun S. Mujumdar, Visit us at:

54 Visit us at

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