Page 399 - Biosystems Engineering

376 Cha pte r T h i r tee n

silicate. Anhydrous silica gel and activated carbon are also used as
bleaching adsorbents to a limited extent. The desired color of the oil
can be obtained by adjusting the amount and type of earth used and
the physical conditions under which the bleaching operation is carried
13
out. Synthetic adsorbents and attapulgites have also been used.
Because the necessity of using fuller’s earth involves not merely
an expense but also loss of oil, and because the costs of decolorizing
rise with the amount of earth used, the price paid for oil, such as cot-
tonseed oil in the United States, among other factors, also depends on
the ease with which the oil may be decolorized or bleached. Further-
more, if too much fuller’s earth has to be used, the oil acquires an
earthy flavor. The American trade has, therefore, established two
classes of cottonseed oil: bleachable and unbleachable. A bleachable
oil is one that may be reduced to a very definite color standard when
treated in a specifically prescribed way. 14
10
Nag et al. have studied the effect of temperature and cost
required to bleach oil. Kinetic studies on bleaching edible oil reported
using a low-cost adsorbent (e.g., charred sawdust) at different tem-
peratures. The changes in energy, enthalpy, and entropy during this
process have been calculated as follows.
Nag et al. have calculated the change in free energy (ΔG) due to
11
the adsorption of inorganic salts from the solution to the surface of
the solid [Eq. (13.1)]:
C p
G
Δ= RTM/ ∫ Γ p / C dC p + RTM/ ln C p (13.1)
p
0
The first term on the right-hand side of Eq. (13.1) is based on the
Gibbs adsorption equation and represents the change in free energy
(KJ/mole) due to the transfer of Γ (mg/m ) of the adsorbate to an
2
interface when the solute concentration is changed from zero to C .
p
The value of this term can be obtained by graphical integration using
the experimental data. The value of Γ /C has been plotted for vari-
p p
ous values of C within the concentration range from zero to a fixed
p
value of adsorbate concentration, and the graphical value of the area
has been estimated. Here, M is the molecular weight of the oils, and
the second term on the right-hand side represents the free-energy
change due to hypothetical dilution of the bulk solution from the con-
centration range C to one molal value, keeping Γ (mg/m ) hypo-
2
p p
thetically fixed during the dilution process. ΔG for different values
of C has been estimated from the experimental data. For a diluted
p
solution, the difference between molarity and molality has been
13
neglected. ΔG is the free-energy change of the crude oil per gram of
the adsorbent, that is, the saturated surface area of charred saw dust
referred as Φ, where Φ = surface area = 1 . From kinetic studies, it
is observed that adsorption values will decrease, which show that
colorants and impurities in the oils are adsorbed by charred sawdust.