07 heat&mass
TRANSCRIPT
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Nq
T
x
evaporation
q
T
x
condensation
NN
T
x
drying
q
liquid vapour
Three times Mass & Heat Transfer
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T
T
2COCO2
H2O H2+ CO
H2O + CO
H2+ CO2
q
Reactions and Heat Transfer
gasification of coal
endothermic
reaction
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1
2
3
1 mol of iat T,p
add heat
samep
H
dq
H dHenthalpy change
Enthalpy
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H
0
TTref
liquid H c T T pL ref
molar heat capacity
H= 0 at the reference temperature
H
Hrefvap
0
TTref
gas H c T T H pG ref refvap
enthalpy of vaporisation,
(depends on choice of Tref)
Effect of Temperature
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mixture H x Hi iH f T x i i ( , )species enthalpies
effect of composition often small: H Hi i
pure i
Enthalpy in Mixtures
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Enthalpy of
Reaction
12N H NH2
32 2 3
(1) (2) (3)
H
T
Tref 298c cp p1 2 29
J mol K
1 1
cp 3 41 J mol K1 1
12 1 32 2c c T T p p ref
NH3(3)
reactants
H Treaction
Hrefreaction 46 1 1. (mol )kJ NH3
H H c T T refreaction
p ref3 3
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changes are not
very important
Temperature Effects
MS-equation
)termsdiffusionthermal()(, jijjii uuxFsmall
driving force
dz
dx
x
RT
dz
dF i
iT
ii
difference form:
zx
xTRF i
i
i
at constant
temperature
average film
temperature
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E
Ni
T
driftthermal
conduction
iiHNdz
dTE
thermal conductivity species enthalpies
difference equation:
E h T N H i i
at the average Theat transfer coefficient hz
differential equation:
z
T
Energy Flux Relation
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E EE E
calculated with the
energy transport relation
calculated by
other means
the film other phase
Continuity of Energy Flux
a Bootstrap
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Condensation with Inert Gascondensate wall coolant
T
y1
vapour
inert (1) accumulates at interface
y H
R T T
vap
b2
1 1
exp
y2
interfacial equilibrium of (2)
low temperature for condensation
boiling point
E E
E N H T h T T L2 2
methanol
nitrogen
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Reaction with Convection
(1) (2) (3)
catalyst
1
2 2N
32H2
NH3
bootstraps:
N N N N 2 1 3 13 2
no net energy flux through film:
h T T N H T N H T N H T
1 1 2 2 3 3 0
1
2N H NH23
2 2 3
T T T 0 5.
conduction thermal drift
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water (1)
air (2) + ammonia (3)
gas
liquid
10 30 50T
o
C
condensation of 1
co-diffusion
absorption of 2
evaporation of 1
counter diffusion
Ammonia Absorber
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T
T
y1
y1
E
N
N H T E L1 1
water hot air (2)
water (1)
N2 0
THNTThE G11
y y N y N
k c1
2 1 1 2
1 2,
bootstraps transport relations
(much) lower than
bulk temperature
T
y1
Drop in a Spray Drier