MOISTURE CONTENT AND DRYING RATE CALCULATIONS
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Transcript of MOISTURE CONTENT AND DRYING RATE CALCULATIONS
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MOISTURE CONTENT MOISTURE CONTENT AND DRYING RATE AND DRYING RATE
CALCULATIONSCALCULATIONSSOLIDS SOLIDS
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MOISTURE CONTENT RELATIONSHIPS
MOISTURE/SOLID EQUILIBRIUM RELATIONSHIPS
FIGURES 9.4-1 AND 9.4-2 FOR SEVERAL TYPES OF SYSTEMS
DEFINED ON THE BASIS OF RELATIVE HUMIDITY AT A SPECIFIC TEMPERATURE
EQUILIBRIUM AMOUNT OF MOISTURE TENDS TO DECREASE WITH INCREASING TEMPERATURE
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MOISTURE CONTENT MOISTURE CONTENT VARIABLESVARIABLES
BASED ON THE MASS OF MOISTURE RELATIVE TO THE MASS BONE DRY SOLID
)(
.
%100@
)25.9(
15.9
*
*
MoistureSurface
XXBoundAboveContentMoistureX
MoistureUnbound
HwithSaturationContentMoistureX
XXContentMoistureFreeX
ContentMoisturemEquilibriuX
BDSSolidDryMass
LiquidMassX
BtU
RB
t
t
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DRYING RATE CURVES
DEPEND ON WHETHER HEAT OR MASS TRANSFER CONTROLS• FREE MOISTURE VS. TIME• DRYING RATE VS. MOISTURE CONTENT
http://www.ias.ac.in/sadhana/Pdf2005Oct/PE1280.pdf
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DRYING REGIMESDRYING REGIMES CONSTANT RATE - NO LIMIT TO MASS
TRANSFER IN SOLID PHASE • SURFACE MOISTURE• TRANSFER NEAR SURFACE
FALLING RATE –MOISTURE FLUX THROUGH THE SOLID IS HINDERED
• CRITICAL POINTS OCCUR BETWEEN CONSTANT RATE AND FALLING RATE WITH A CHANGE IN THE FALLING RATE DRYING MECHANISM
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DRYING MODELSDRYING MODELS RATES FROM EMPIRICAL DATA
CONSTANT RATE DRYING• CONTROLLED BY HEAT TRANSFER TO
VAPORIZE THE MOISTURE OR MASS TRANSFER
)35.9(
tA
XLR S
76.9)(
HHMkTTh
R WByW
WC
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HEAT TRANSFER HEAT TRANSFER CORRELATIONSCORRELATIONS
TO PREDICT CONSTANT RATE DRYINGTO PREDICT CONSTANT RATE DRYING
RADIATION CAN ALSO BE A FACTORRADIATION CAN ALSO BE A FACTOR
)106.9(37.017.1
:
)96.9(0128.00204.0
:
37.02
37.02
8.02
8.02
GRft
BTUhG
Km
Wh
SURFACETOLARPERPENDICU
GRft
BTUhG
Km
Wh
SURFACETOPARALLEL
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FACTORS THAT FACTORS THAT AFFECT h
AIR VELOCITY (G)AIR VELOCITY (G) GAS HUMIDITY (T – TGAS HUMIDITY (T – TW)W) AND (H AND (HWW-H)-H) GAS TEMPERATURE (T – TGAS TEMPERATURE (T – TW)W) AND (H AND (HWW-H)-H) SOLID THICKNESS - NO EFFECT ON RATE SOLID THICKNESS - NO EFFECT ON RATE
FOR SURFACE MOISTUREFOR SURFACE MOISTURE MATERIALS SURFACE FINISH OR ANY MATERIALS SURFACE FINISH OR ANY
CONDITION THAT STIMULATES CONDITION THAT STIMULATES TURBULENCETURBULENCE• J. E. SUGARMAN & T. J. VITALE, J. E. SUGARMAN & T. J. VITALE, OBSERVATIONS ON THE OBSERVATIONS ON THE
DRYING OF PAPER: FIVE DRYING METHODS AND THE DRYING DRYING OF PAPER: FIVE DRYING METHODS AND THE DRYING PROCESSPROCESS Journal of the American Institute for Journal of the American Institute for ConservationConservation ,, 1992, Volume 31, Number 2, Article 3 (pp. 175 1992, Volume 31, Number 2, Article 3 (pp. 175 to 197) to 197) http://www.jstor.org/stable/3179491?seq=1
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CONSTANT RATE DRYING TIMECONSTANT RATE DRYING TIME
DRYING TIME CAN BE CALCULATED BY INTEGRATING (9.5.-3)• LOWER VALUE OF X > XC (CRITICAL
POINT)
1
22
X
X CC
SR XXWHERE
R
dX
A
Lt
C
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FALLING RATE DRYING
CONTROLLED BY• GAS PHASE MASS TRANSFER FROM
SOLID• OR HEAT TRANSFER INTO THE SOLID TO
VAPORIZE THE MOISTURE.• GENERAL FORM OF THE EQUATION:
X1 < XC
16.9)(
1
2
X
X
SF XR
dX
A
Lt
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FALLING RATE DRYING NUMERICAL CALCULATION FOR NUMERICAL CALCULATION FOR
COMPLEX SYSTEMS COMPLEX SYSTEMS • SEE EXAMPLE (9.7-1) FOR NUMERICAL SEE EXAMPLE (9.7-1) FOR NUMERICAL
INTEGRATIONINTEGRATION SIMPLIFICATIONS FOR LINEAR SIMPLIFICATIONS FOR LINEAR
RELATIONSHIPS: R(X) = aX + bRELATIONSHIPS: R(X) = aX + b
FOR b = 0, LINEAR THRU ORIGINFOR b = 0, LINEAR THRU ORIGIN
)47.9(ln)(
)(
2
1
21
21
21
21
R
R
RRA
XXLtSO
XX
RRa S
F
)87.9(lnln22
X
X
AR
XL
R
R
AR
XLtSOaXR C
C
CSC
C
CSF
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FALLING RATE EXAMPLEFALLING RATE EXAMPLE
Shibata, H.; Iwao, Y., Vacuum Drying of Sintered Spheres of Glass Beads,Ind. Eng. Chem. Res.; 1999; 38(9); 3535-3542
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FALLING RATE EXAMPLEFALLING RATE EXAMPLE
Carmen Rossello, Jaime Canellas, Susana Simal, Angel Berna, Simple mathematical model to predict the drying rates of potatoes, J. Agric. Food Chem.; 1992; 40(12); 2374-2378.