Advancements in Oil and Oilseed Processing - Food and Industrial Bioproducts and Bioprocessing - page 132

4.4.3 Bleaching
Chlorophyll or pheophytin and their degradation products give oil a greenish color and
carotenoids contribute to yellow and red tints that may be undesirable. Thus, oils are usually
bleached after deacidification/refining and before deodorization. Originally bleaching was
used to remove color compounds. Today, bleaching is designed to remove undesirable oil
components, including peroxides, aldehydes, ketones, phosphatides, oxidative trace metals,
soaps and other contaminants, such as pesticides and polycyclic aromatic hydrocarbons.
Clays used for bleaching are commonly called “Bentonites”. Activated carbon, alumina,
silicic acid, aluminum- and magnesium-silicate, silica gel and synthetic silicates are also
used to adsorb impurities from refined oil (Neuman and Dunford, 2004). Although natural
clays do not remove color, they are very effective in cleaning up residual soap from alkali
refining and gums from degumming, and normally retain lower amount of oil as compared
to activated clays (Rini, 1960). Bleaching is normally carried out under vacuum (20-30 mm Hg)
to minimize oxidation reactions and control moisture levels. Preheated oil (90 °C) is pumped
into a slurry tank and adsorbent is added to the tank simultaneously. After mixing, the clay/
oil system is fed into a vacuum bleacher. Bleaching process takes 15-30 min in a temperature
range of 80-120 °C. Although high temperature increases adsorption efficiency, bleaching
at very high temperatures is not recommended because it promotes undesirable reactions.
Temperature should be high enough to maintain a low oil viscosity, which improves diffusion
and mass transfer rates.
Wet bleaching is practiced when processing oils containing PLs. Water will act as a
carrier for PLs into the bleaching clay particles. The optimal amount of water used for wet
bleaching is about 50-100% of the adsorbent used for the process. Initially, oil (about 0.5%
moisture) is treated with water and adsorbent (8-15% moisture) at 70-90°C for 20min
under atmospheric conditions. Then bleaching is carried out under vacuum for 15-30 min.
The amount of adsorbent required for bleaching depends on the types of adsorbent and the
oil and its pretreatment. The adsorbent dosage range varies significantly, usually 0.1-2.0%
(of oil processed) depending on the oil quality and type of adsorbent used. In some cases it
can be as high as 5%. Physically refined oils require a higher amount of adsorbent than
chemically refined oils. After bleaching, oil is filtered and separated from the adsorbent.
Currently bleaching is performed in slurry tanks. However, fixed-bed bleaching using
granular clays is an option. At present, there are a few applications of fixed-bed post-bleach
“polishing” before storage or shipment. Fixed-bed bleaching requires highly purified feed
streams, that is, no soaps, no phospholipids and low particulate content. Development of
regenerable bleaching clays could reduce the impact of oil refining on the environment.
Patents do exist covering bleaching clay regeneration technologies but currently these
technologies are not economically feasible.
There are several relatively new techniques being examined to replace conventional
bleaching or at least to combine bleaching with other processing steps. In one study the
bleaching step was incorporated into a SC-CO 2 extraction process by integrating an
adsorbent-containing high-pressure vessel after the oil extractors in the process cycle
(Woerlee and Pellikaan, 2003; Ambrogi et al ., 2003). Although the contact time between oil
and the adsorbent in the reactor was very short, a significant bleaching effect was achieved
using this process. Furthermore, the used adsorbent did not retain oil. During the process the
peroxide value of the oil was significantly reduced, indicating that catalytic decomposition
of hydroperoxides on bleaching earth takes place under high pressure. Ketones, aldehydes
and FFAs were not adsorbed and they were detected in the bleached oil. Oil extracted with