Lecture 5
Lecture 5
Lecture 5
Objectives
Introduce drying of solids
Introduce Psychometric Charts and key parameters
Explain the drying process and drying curves
Introduction
Drying is the removal of moisture (either water or other volatile
compounds) from solids, solutions, slurries, and pastes to give solid
products. In the feed to a dryer, moisture may be:
Introduction
Drying can be expensive, especially when large amounts of water (which
has a large heat of vapourisation) must be evaporated.
Water and energy conservation measures, and advances in equipment
design, have broadened the use of pre-feed dewatering operations by
mechanical means, which also diminish the length of drying cycles.
gravity expression,
pressure filtration,
settling, and
centrifugation.
Introduction
As drying involves vapourisation of moisture, heat must be transferred to
the material being dried. The common modes of heat transfer are:
Psychometric Charts
If moisture is to evaporate from a wet solid, it must be heated to a
temperature at which its vapour pressure exceeds the partial pressure of
the moisture in the gas in contact with the wet solid.
Calculations involving the properties of moisture-gas mixtures for
application to drying are most conveniently carried out with psychrometric
charts.
Psychometric Quantities
A moisture
Psychometric Quantities
A moisture
Psychometric Charts
Dryer Temperatures
The important parameter for the design is the temperature at which the
moisture evaporates.
When convection from a hot gas is employed and the moisture is on the
surface or rapidly migrates to the surface from the interior of the solid, the
rate of evaporation is independent of the properties of the solid and is
governed by the rate of convective heat transfer from the gas to the
surface. Then, the evaporating surface is at the wet-bulb temperature of
the gas if the dryer operates adiabatically.
Dryer Temperatures
If the convective heat transfer is supplemented by radiation, the
temperature of the evaporating surface is then higher than the wet-bulb
temperature.
In the absence of contact with a convective-heating gas, and when a sweep
gas is not present, such that the dryer operates non-adiabatically, the
evaporating moisture is at its boiling point temperature at the pressure in
the dryer.
If the moisture contains dissolved, non-volatile substances, the boiling-
Wet-Bulb Temperature
The temperature at which moisture evaporates in a direct heat dryer is
difficult to determine and varies from the dryer inlet to the dryer outlet.
When the dryer operates
isobarically
adiabatically
all energy for moisture evaporation supplied from the hot gas by convection
Wet-Bulb Temperature
If it is further assumed that the moisture being evaporated is free liquid
exerting its full vapour pressure at the surface of the solid, this
temperature of evaporation is called the wetbulb temperature, Tw,
Wet-Bulb Temperature
At steady state, the rate of convective heat transfer from the gas to the wet
solid must be equal to an enthalpy balance on the moisture evaporated,
The molar rate of mass transfer of evaporated moisture from the wet
surface of the solid, A, is,
Wet-Bulb Temperature
Combing these
assume that
=1
the mole fraction of moisture in the bulk gas and at the wet solid-gas interface is small
the bulk flow effect is small
Wet-Bulb Temperature
From the definition of absolute humidity, replacing pA with yAP,
Wet-Bulb Temperature
Solid Types
Granular or crystalline solids that hold moisture in open pores between
particles.
These are mainly inorganic materials,
crushed rocks,
titanium dioxide,
sand,
zinc sulphate,
catalysts, and
sodium phosphates.
Solid Types
Fibrous, amorphous, and gel-like materials that dissolve moisture or trap
moisture in fibers or very fine pores.
These are mainly organic solids,
wood,
eggs,
starch,
leather,
glues,
cotton,
soap,
cereals, and
wool.
category under agitated conditions such that all particle surfaces are in
direct contact with the gas - the