Drying Foods. Geankoplis Singh&Heldman. Bound and unbound water in solids. If the equilibrium moisture content of a given material is continued to its intersection with the 100% humidity line, the moisture is called bound water.
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(kg /kg dry solids)
Drying rate, dW/dt (kg/h)
Fig. 1a &b Typical drying rate curve of for food solids.
AB = Settling down period where the solid surface conditions come into equilibrium with the drying air.
BC = Constant rate period which the surface of the solid remains saturated with liquid because the movement of water vapour to the surface equals the evaporation rate. Thus the drying rate depends on the rate of heat transfer to the drying surface and temperature remains constant.
C = Critical moisture content where the drying rate starts falling and surface temperature rises.
CD = Falling rate period which surface is drying out and the drying rate falls. This is influenced by the movement of moisture within the solid and take time.
X = free moisture (kg water/kg dry solid)
R = drying rate (kg water/h.m2)
Ls = kg of dry solid
A = exposed surface area for drying (m2)
= 1.80 h.
R = drying rate (kg dry solid/h.m2)
Rc = drying rate for constant drying rate period = constant
Rc = 1.51 kg/h.m2
MA NA A=m=RcA
q = mw
NA = mass flux = kg mol/s.m2, M = molecular weight, H = humidity ratio
y = mole fraction of water vapor in gas,
yw = mole fraction of water vapor in gas at surface,
A = water, B = air, ky = mass transfer coefficient, w = latent heat of vaporization
Relate drying rate with heat transfer and mass transfer to determine the coefficient
For air temperature of 45-150 periodC and mass velocity G of 3900-19500 kg/h.m2 or a velocity of 0.9-4.6 m/s and air is flowing perpendicular to drying surface.
For air temperature of 45-150C and mass velocity G of 2450-29300 kg/h.m2 or a velocity of 0.61-7.6 m/s and air is flowing parallel to drying surface.
Mass velocity (G) = v
To estimate time of drying during constant-rate period:
ma = air flow rate (kg dry air / hr)
mp = product flow rate (kg dry solids / hr)
W = absolute humidity (kg water / kg dry air)
w = product moisture content (kg water / kg dry solid)
ma W2 + mp w1 = ma W1 + mp w2
ma Ha2 + mp Hp1 = ma Ha1 + mp Hp2 + qloss
q = heat losses
Ha = heat content of air(kJ/kg dry air)
Hp = heat content of product(kJ/kg dry solids)
H occurs but is controlled by convective processes.a = CS (Ta – TO )+ WHL
CS = humid heat (kJ/kg dry air.K)
= 1.005 + 1.88 W
Ta = air temperature (C)
TO = reference temperature (0C)
HL = latent heat of vaporization of water(kJ/kg water)
HP = CPP (TP – TO )+ wCPw (TP – TO)
CPP= specific heat of dry solid (kJ/kg.K)
TP = product temperature (C)
CPW = specific heat of water