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Lecture‐3.December 17‐2007
Kamaruddin AbdullahLaboratory of Solar Conversion TechnologyFaculty of EngineeringDarma Persada [email protected]
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Outlines (Lecture‐3)IntroductionBasic drying theoryThermophysical properties as basis for drying system designSolar dryingTypes of developed solar dryersTest PerformancesConclusions
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IntroductionHigh air temperature and humidityultural and in the tropics make many agricultural and marine products are susceptible to rapid decomposition and therefore, are not suitable for human consumption.Due poor post harvest handling around 12‐20% of harvest are reported loss every yearDrying can extend shelf life of products by reducing its moisture to a level save for long storage
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Drying theoryThe term drying refers generally to the moisture removal from a substance. For example a wet solid such as wood, grains such as coffee, cocoa, rough rice etc. may be dried by evaporation of the moisture either into gas stream or without the benefit of the gas to carry away vapor, but mechanical removal of such moisture by expression or centrifuging is not ordinarily considered drying.
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Terminology (Treybal,1968)Moisture content, wet basis, X, Mass of moisture within a substance devided by mass of that substance, expressed in % w.b.Moisture content, dry basis, M, is the ratio between the mass of water with the mass of dry matter in solid and is expressed in percent.(% d.b.)Equilibrium moisture, Me, expressed commonly in terms of % dry basis is the moisture content of a substance when at equilibrium with a given temperature and RH surrounding a substance.
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TerminologyBound moisture. Is the moisture contained by a substance which exerts an equilibrium vapor pressure less than that of pure liquid at the same temperature. Bound moisture is the type of moisture held by chemical solution and in capillary within the solid.
Unbound moisture. Is the moisture contained by a substance which exerts an equilibrium vapor pressure equal to that of pureliquid at the same temperature. Such condition can be in the form free water on the surface of substance.
Free moisture, is the portion of moisture not being held by chemical reaction within the substance. Only free moisture can be evaporated, and the free moisture content of a solid depends upon the vapor concentration in the gas.
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The Psychrometric chartPsychrometric chart describes the all important properties of dry and moist air used during drying. Using the psychrometric chart one can study the drying process in a simple way.
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1 2
3
Absolute Humidity Kg/kg dry air
Dry bulb temperature
Wet bulb temperature
Enthalpy kJ/kg
RH
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Drying rate(%/h)
Moisture content (% wb)
Constant rate period
Falling rate period
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Moisture content (% db)
Drying rate (%/min.)
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S
Basic equationM‐Me/Mo‐Me = A exp (‐ kt) ……..(10a)M(A, Me, K)= A (Mo‐Me) exp (‐ kt)+ Me…….(10b)
Linearization using Taylor expansion
Mi( A, k, Me) = Mi (A, k, Me) + dMi/dA (∆Ai) + dMi/dk (∆Ki) + dMi/dMe (∆Mei)..(10c)i=1,2.3..nwheredM/dA= (Mo‐Me) exp (‐kt)dM/dk= ‐A k (Mo‐Me) exp (‐kt)dM/dMe=‐A exp (‐kt) +1
Least Square MethodThe 1st iteration Let A=Ai, Me=Mei, K=KiSubstituting into eqs. (10a, 10c) will give the respective values of ∆Ai, ∆Ki, and ∆Mei. Me,new= Mei‐∆Mei, A new=Ai‐∆AiKnew=Ki‐∆Ki
∆Ai, ∆Ki, and ∆Mei≤ 0.004?
No Print Final value of A,K, and Me
End
Determination of drying parameters(Nishiyama, 1973)
yes
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Drying parameters (cont.)Equilibrium M.C.
The drying constant
&
Commodity Specific heat,Cp (kJ/kg)
Latent heat of evaporation,∆Hfg (kJ/kg)
Equilibrium m.c.Me (%,db)
Drying constant, k (1/min.)
1. Coffee berriesDyah W. (1997) Jusuf
(1990)
Cp=0.02125 M + 1.8175For 0.5<M<0.67
∆Hfg/∆Hfgw= (1+ 0.597exp(-0.19427Me))for RH>57%; Me>8%
Me = 3.7045+0.11716 ∆t+0.007679 ∆t2
K= exp( 15.432-5976.4 t )
2. Cocoa berriesNelwan, 1998.
∆Hfg/∆Hfgw= (1+ 0.7297 exp(-0.1361 t Me))at t =55 C and 7%<M<49%wb∆Hfg= 2411.7-3236.4 kJ/kg
(!-RH)=exp(-0.1936 t Me 1.1487)
K= exp( 15.432-5976.4 t)
3. Rough riceIR-36(finite cylinder model)
Thahir (1986)
∆Hfg/∆Hfgw= 1.298 atMe= 9.7%wb, and t= 30-50 oC
Me=17.89exp(-0.061∆t)∆t=tdb-twb
K=exp( 1.9283-2803.4/T)
3. Corn (Sadewavar.)
Sphere model, Thahir(1986)
∆Hfg/∆Hfgw= 1.298 atMe= 8.8%wb, and t= 30-50 oC
Me=12.46exp(-0.035∆t)∆t=tdb-twb
K=exp 1.9283-2803.4/T)
3. Black pepperSphere model(Prayudi, 1992)
∆Hfg=(2500-2.34 t)x (1+0.4132exp(-0.224 Me)
Me=16.86exp(-0.224∆t)∆t=tdb-twb
K=0.167exp(13.277-4900/T)
3. Mackerel(Fasirun, 2003)
∆∆Hfg/Hfg/∆∆HfgwHfgw==1.478 (at 55% db, 45 o C)1.478 (at 55% db, 45 o C)
Me=516.79[(-ln (1-RH)/T]0.556
K=exp(7.549-4503.8/T)313<T<323
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Solar dryingMain componentsSolar heat collector as hot air generatorDrying chamberAir moving devices to convey hot air into the drying chamber and later move the resulting moisture from the product due to drying process out of the drying chamberProducts holder, of different forms, where products to be dried are placed or distributed
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Optimization of solar drying system
Objective function
C= Σ xi ‐‐‐ min. .(1)
i=1,2,3,..n
Constraints, for i=3
Φ1(x1,x2,x3)=0 ..(2)
Φ2(x1,x2,x3)=0 .(3)
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Lagrange multiplierOptimum condition:
∇ C‐λ1 ∇ϕ1‐λ2 ∇ϕ2= 0 ……(4)
here∇C (x1,x2,x3) = Σιi (∂C/ ∂xi) (i=1,2,3) …(5)∇ϕ1 (x1,x2,x3) = Σιi (∂ ϕ1 / ∂xi) =0 ….(6)∇ϕ2 (x1,x2,x3) = Σιi (∂ ϕ2 / ∂xi) =0 …(7)
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ResultsCommodityCommodity RiceRice Ac(mAc(m22)) 60.960.9W (kg)W (kg) 20002000 Pw (Watt)Pw (Watt) 11141114ρρ(kg/m(kg/m33)) 599599 AbAb (m(m22)) 6.686.68Q(mQ(m33/s)/s) 1.21.2 θθdd(hrs(hrs)) 88Initl.Initl. costcost K (1/h)K (1/h) 0.150.15X1 (X1 (mill.Rpmill.Rp)) 6.0856.085 Td (Td (ooCC)) 46.546.5X2 (X2 (mill.Rpmill.Rp)) 0.3870.387X3 (X3 (mill.Rpmill.Rp)) 1.3391.339C (C (mill.Rpmill.Rp)) 7.8117.811!US$=Rp. 9100
GHE Solar drying system simulation
Solar heat collector is placed within the drying chamber to reduce the construction cost
Drying floor can also painted black to enhance solar heating
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Ap
Solar drying simulation
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Energy balance in heat collector to calculate the value of plate temperature Tp,
mp Cpp dTp/dt = τ α Ap(600 sin(π t/8)+300) - 1.5 h Ap (Tp - Tr) (1)
Drying chamber temperature, Tr
mr Cpa dTr/dt =mu Cp (Ta-Tr)+1.5 h Ap (Tp -Tr)+hw Aw(Tw-Tr)
-U Ad (Tr-Ta)-Wd (dM/dt) ∆Hfg+hf Af β (Tf-Tr) .. (2)
Temperature rice of the floor , Tf, which is painted black
mf Cpf dTf/dt= τ α β Ap(600 sin(π t/8)+300)-hf Af (Tf-Tr) -k Af (Tf-Tso)/xf (3)
Temperature of hot water tank
mw Cpw dTw/dt= [d(mb)/dt] η CV-Uw At(Tw-ta)-hw Aw (Tw-Tr) (4).
The ambient temperature change, Ta
Ta = 4 sin (π t/8) + 28 (5)
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Daytime drying: W=300 kg; mu=0.05 kg/s
0
20
40
60
80
100
0 1
1.5 2
2.5 3
3.5 4
4.5 5
5.5
Time (h)
Tp(C)Tr(C)Ta(C)I(t)/10Series1
GHE Solar drying system
Integrated the function of solar heat collector and drying chamber into one compartment to reduce construction costCan be design with different configurations according the geometry and product holder:Stationary:
House type with : flat bed, cabinet, drums, trolleys, etc.Ventury type or truncated pyramids or cones
Recirculation type: bunker, inclined drying chamber‐collector
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Recirculation dryerDryer with inclined collector‐drying chamber
Air in $
$$
Grain in
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Working principle: inclined collector‐drying chamber
Uses pneumatic conveyor to recirculate the grainsDrying process occurs when the grain falls into the collector‐drying chamber section of the dryerSeveral drying cycles are needed to accomplish drying processBiomass stove can be operated for day and night dryingGood for small to medium capacity drying of grains (rough rice, corn, soybeans, etc.)
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Integrated collector‐drying chamberThe Energy balance
mg Cpg dTg/dz= hr W (Tr‐Tg) ‐m ∆Hfg dM/dz ‐ α τ I C2 ... (1)
dTg/dz= z1(Tr‐Tg)+z2 dM/dz‐z3I........................................(2)
Wa Cpa dTr/dz =‐ hc W (Tc‐Tr)+(Ua+Ub)W(Tr‐Ta)+hr W (Tr‐Tg)+m ∆Hfg dM/dz ......(3)
dTr/dz= z4(Tc‐Tr)+z5(Tr‐Ta)+z6(Tr‐Tg)+ z7dM/dz)............(4)
Mass balance
mM|z‐mM|z+∆z = ‐ (ma Cp T|z ‐ma CpT|z+∆z )/∆Hfg orLim ∆z‐‐‐>0
dmM/dz = ma dTr/dz /∆Hfg ....................................................(5)
M dm/dz= ma dTr/dz /∆Hfg ‐mdM/dz ............................(6)
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∆z
z
z+∆z
Ua
Ub
I(t) mg
ma
Drying of corn in recirculation dryer‐inclined drying chamber‐collector system
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Solar recirculation dryer
GHE type with bunker for temporary instore dryer
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Air in
Working principle: Bunker type
Uses pneumatic conveyor to recirculate the grains and vortexDrying process occurs when the grain falls into a cyllinrical type heat exchanger located at the center of thedrying chamberSeveral drying cycles are needed to accomplish drying processBiomass stove can be operated during the night of bad weatherGood for medium to large capacity drying of grains (corn, rough rice, soybeans, etc.)
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Experiments: ‐Temporary instore dryer
Capacity : 266.4 kg/hDimension : 2,28 m, height 3,09 mConveyor : Centrifugal Blower
Model : CZR – 200Voltage : 220 Volt, 50 HzVolume : 450 m3/hSize : Ø 60 mmPressure : 1200 PaPowerType : Electric Motor, 0.25 kWModel : YY 632 ‐2RPM : 2840 rpmVoltage : 220 Volt, 50 Hz, 1,9 A,
Auxiliary heaterType :Biomass stoveFuel : Charcoal, saw dust
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Sample: Rough rice –IR64Average MC : 23,6 % wbDegree of cleanliness : 97,2 %Dimension (L x W x th) : 9,87 × 1,94 × 2,37 mmBulk density : 463,92 gr/cm3
Intact grains : 97,8 %Persentage of cracked grains : 2,2 %
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Test results conducted by Ministry of Agriculture: Bunker type solar dryer (Prototype)1. Intial mass of grain
2. Drying time (hr)
3. Mass of grain after 10 hr drying
4. Drying rate
5. Average temperature at top section
6. Average temperature at the middle section
7. Temperature of polycarbonate wall
8. Increase in cracked grains
9. Homogenity in m.c.
10. Fuel use
~ charcoal (18 kg)
~ Solar irradiation (1252,67 Wh)
: 155 kg
: 10
: 140 kg
: 0,74 %/hr
: 40,34 0C
: 38,12 0C
: 40,65 0C
: 1,4 %
: 0.05% – 0.51 %
: 43710,19 kJ/hr
: 99%
:1%
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Experiment 2: inclined collector‐drying chamber (Prototype)
Over all Dimension: : L:2630 mm,W: 1530 mm,H: 2520 mmHolding capasity : 93,31 kgRecirculation capacity : 42,12 kg/hPneumatic conveyor : Centrifugal Blower, 0,25 kW, 22 V,AC Type
YY 632‐2, RPM: 2840Coveyor pipe:
Diameter : 420 mmLength : 1080 mm
Drying chamber:Dimension : 2270 × 1080 mmPolicarbonate thickness: 1,2 mmBlackened metal sheet thickness : 0,5 mmInclination from horizontal : 200
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Test results‐inclined collector‐drying chamber
1. Initial mass
2. Effective drying time
3. Final mass after 7 hrs drying
4. Drying rate
5. Temperature in heating chamber
6. Temperature of drying air
7. Temperature of absorber
8. Increase of cracked grains
9. Homogenity in m.c.
10. Energy consumption
~ Charcoal (12 kg)
~ Solar radiation (161,26 Wh)
: 24 kg
: 7 hr
: 12 kg
: 1,03 %/hr
: 67,41 0C
: 47,11 0C
: 51,56 0C
: 2,4 %
: 0.04% – 0.41 %
: 34690,63 kJ/hr
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Conclusions1) Several design configurations of GHE solar dryer
have been developed in Indonesia and many have been distributed throughout the country
2) The dryers may be used as the main component of a SPU
3) Laboratory and field test results have shown that the developed GHE solar dryers can be used to dry food crops (rough rice, corn, soybeans), estate crops (coffee and cocoa beans, cloves, pepper, etc., marine products (a variety of fish, sea weeds, fish crackers)
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