Drying Curves
Drying Curves
Drying Curves
College Of Engineering
Chemical Engineering Department
Submitted to:
Engr. Rubi Rudgi
Experiment No. 2
DRYING CURVES
ABSTRACT
The objective of this work is to analyze the possibility of applying the drying curves
generalization methodology to the conductive/convective hot plate drying of cellulose. The
experiments were carried out at different heated plate temperatures and air velocities over the
surface of the samples. This kind of approach is very interesting because it permits comparison
of the results of different experiments by reducing them to only one set, which can be divided
into two groups: the generalized drying curves and the generalized drying rate curves.
TRANSMITTAL LETTER
October 6, 2015
Engr. Albert Evangelista
Chemical Engineering Department
Adamson University
Ermita, Manila
Engr. Evangelista:
In compliance with the fulfillment of the requirements on the subject Unit Operations Lab
2, the group would like to present this experiment report entitled DRYING CURVES in
accordance with your instructions. The main purpose of this experiment report is to
determine heat flow rate through the bare and lagged pipes, to determine the thermal
conductivity of lagging material by assuming the heat input to be that heat flow rate through
lagged pipe and to determine the efficiency of insulating materials. We hope that this
experiment report will meet you approval.
Respectfully Yours,
Briones, Neil Ryan V.
Paguia, Nikka C.
Rodriguez, John Paul
Magleo, Gelo
Sangalang, Joey
Modina, Ma. Betina V.
I.
Objectives
1. To produce drying and drying rate curves for a wet solid being dried with air of
fixed temperature and humidity
2. To determine the critical and equilibrium moisture contents, bound and unbound
moisture of the wet solid being dried.
II.
Materials/Equipment Needed
1. Tray Drier
2. Sieve shaker
3. Sand sieved to approximately 500 microns
4. 2-Sling Psychrometers
5. 7-Thermometers
6. Weighing Instrument
7. Oven (if necessary)
8. Stop watch
9. Water
10. Container/pale
11. Anemometer/Wind Speed meter
12. Boiler
III.
Equipment Set up
IV.
Theory
In the process of drying heat is necessary to evaporate moisture from the grain and a
flow of air is needed to carry away the evaporated moisture. There are two basic
mechanisms involved in the drying process; the migration of moisture from the interior
of an individual grain to the surface, and the evaporation of moisture from the surface
to the surrounding air. The rate of drying is determined by the moisture content and the
temperature of the grain and the temperature, the (relative) humidity and the velocity of
the air in contact with the grain.
Drying only takes place if the wet material contains more moisture than the equilibrium
value for its environment. The earliest ideas on convective drying implied that liquid
moisture diffuses to the exposed surface of a wet body where it evaporates, the vapor
diffusing through the boundary layer into the bulk of the surrounding air. This view is
clearly unsatisfactory, except for drying of homogeneous materials in which the
moisture is effectively dissolved. Mechanisms of moisture movement are generally
more complex. Most materials are composed of sub entities, such as particles and fibers,
which may be loose or held in some kind of matrix. The number and nature of the voids
between these entities and the pores within them govern the quantity of moisture
retained and the extent of bonding to the solids. If the openings form a capillary
exponential expression can be fitted over a more limited range at higher relative
humidities. The general shape of the isotherm reflects the nature of the moist material,
as illustrated in Figure 3 An exception to this kind of behavior is that of inorganic
crystalline solids which have multiple hydrates. With these materials, relative humidity
falls in stepwise fashion with loss of moisture as each hydrate disappears.
(with C, k and X1 as the adjustable parameters) has been tested for sorption of water
vapor on 29 materials at room temperature over a wide range of relative humidity (from
0.07 to 0.97) and for some of these materials, over a narrower range of temperatures
between 45 and 75%C [Jaafar and Michalowski (1990)]. In most cases, experimental
data could be fitted to within 8% up to a relative humidity of 0.7, and in some
instances over the whole humidity range. To cope with the hygroscopic behavior at high
relative humidities with colloidal material, which swells with increasing moisture
content, Schuchmann et al. (1990) have recommended that ln (1 ) be chosen as the
dependent variable rather than y itself in the correlation. It is unwise to extrapolate
sorption correlations beyond their tested range of relative humidities, owing to changes
in hygroscopic behavior at extremes in this range compared with that at intermediate
values.
The manner in which a material dries out depends not only on its structure but also on
its physical form. The drying of small wood chips is controlled essentially by moisturevapor transport through the boundary layer; veneers and thin slats of the same wood by
the dry fraction of the exposed surface; while the drying of board timber, by the internal
moisture-transport mechanisms within the timber itself. Early experiments on drying
materials in sample trays in an air stream have noted that initially, the drying rate was
almost the same as that of a free liquid surface under the same conditions and remained
relatively constant as the material dried out [Keey (1972)]. This period of drying is
followed by one in which the drying rates fell off sharply as moisture content was
reduced to the equilibrium value even though the drying conditions remain unchanged.
This marked difference in behavior has led to the division of drying into the constantrate period and falling-rate period, respectively. The "knee" in the drying curve
between these two periods is known as the critical point. Sometimes, these periods are
referred to as unhindered dryingand hindered drying, respectively, to indicate whether
the material itself plays a controlling role in restricting moisture loss. Appearance of the
initial period can be masked by the induction effects at the start of drying as a moist
solid warms up or cools to a dynamic equilibrium temperature, which is the Wet-bulb
Temperature, if the surface is only heated convectively. This surface temperature is
maintained as long as the surface is sufficiently wet to effectively saturate it. An
example of a drying curve is shown in Figure 4.
is:
where pG is the partial pressure of the moisture vapor in the bulk of the gas and p S is the
value of the gas adjacent to the moist surface. In drying calculations, it is more useful to
use humidities (ratios of the mass of moisture vapor to that of dry gas), and the above
expression transforms to
never be observed in colloidal material drying that reaches unit relative humidity only at
very high moisture contents.
To a first approximation, the drying kinetics in the falling-rate period may be regarded
as of the first-order, and thus the drying rate is directly proportional to the difference
between the mean moisture content of the wet material (X) and its equilibrium value
(Xe):
Integration of this equation yields the time t to dry from a moisture content of X 1 to
X2:
to its maximum value (mW) in the constant-rate period, against a characteristic moisture
contentdefined as the ratio of the freely evaporable moisture content (X X e) to the
free moisture content at the critical point (Xcr Xe). These nondimensional parameters
thus become
and
Examples of the use of this expression to describe industrial drying processes are given
by Keey (1978, 1992).
The intensity of drying (I) is given by the ratio of the maximum unhindered drying rate
(mW) to the maximum moisture-transfer rate through the material assuming a diffusionlike process:
where D is a moisture diffusion coefficient, X0 is the initial moisture content and b is the
effective thickness or radius of the material. This suggests that the product mWb is a
useful property; it is called the flux parameter, F. If ln F is plotted against moisture
content X, a separate curve is found for each initial moisture content X 0 in the
penetration period as the moisture content and temperature profiles develop within the
material. In the regular regime, there is a common curve independent of initial moisture
content and drying flux.
Under certain circumstances, drying curves can appear with both negative and positive
gradients in drying rate as moisture is lost. The drying of layers of soluble dyestuffs can
show discontinuities due to the build up and cracking of surface crusts, particularly if
the crust is removed periodically. The drying of porous bodies containing a mixed
volatile solvent may also show periods of falling and rising rates due to selective
evaporation of the moisture with changes in relative volatility as the composition alters.
References to nikka:
http://www.thermopedia.com/content/711/
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S010466322003000100015