earth was taken when the filter was discharged into the slurry
vessel. One aliquot of this sample was immediately extracted
with hexane in a Soxtec Avanti automatic Sohxlet apparatus,
which also provides a drying cycle. The weight loss on hexane extraction was 38.7%; this illustrates that the cake had not
been blown exhaustively. The dried sample was then placed
in a muffle furnace and heated at 600°C for four hours. This
treatment caused all water present to be evaporated and all
organics that had not been extracted by hexane to be burnt
off, resulting in a weight loss of the dried sample of 18.6%;
when the measurements were repeated with another sample,
a value of 18.4% was found.
To mimic what happens in the plant, another aliquot of
the sample bleaching earth was slurried with crude soybean
oil in a ratio of 2: 3 and stored at 52°C for a period of 12 hours
before it was analyzed. It turned out to be difficult to extract
this slurry in the Soxtec Avanti apparatus, so the slurry was filtered first, and the filter cake was then extracted in the Soxhlet
apparatus. The dried sample obtained was treated in the muffle furnace and, again, its weight loss was determined. If there
had been no additional degradation catalyzed by bleaching
earth, this loss should have been 18. 4—18.6%, as it was in the
evaporated samples. Oil degradation increases the weight loss
on ashing, and if this weight loss had reached some 40% (
calculated on the hexane-extracted sample) this would have indicated that more oil had been degraded than would have been
present in an exhaustively blown filter cake. That would, of
course, have defeated the purpose of the whole exercise.
Fortunately, the weight losses for the duplicate deter-
minations were 20.2% and 21.6%, respectively, leading to
additional degradation product formation of 1.6% and 3.4%,
respectively. So, while a significant (p < 0.004) increase in the
amount of degradation products was established, it was so
small that it would hardly affect the profitability of the process
in which non-exhaustively blown filter cake is slurried in crude
oil and the slurry is fed to an extractor, where the oil in the
spent earth is recovered.
Refiners in vertically integrated oil mills who recover oil
from spent bleaching earth by slurrying the earth in oil and
pumping the slurry to the extractor can therefore heave a sigh
of relief. Refiners who dispose of their spent earth in another
way should compare this way with the above spent-clay
extraction. If they conclude that their method is more profitable, I would like to hear from them.
Albert Dijkstra started his career by working as chief chemist
in the chemical industry. In 1978 he switched to edible oils and
fat and became R&D director of the Vandemoortele Group in
Belgium. Since 1997 he enjoys working from his home in S-W
France as consultant, author, inventor, reviewer, and editor. He
has won several awards and is a Fellow of the AOCS. He can be
contacted at email@example.com.
1. Dijkstra, A. J. (2007) “Bleaching of oils and fats.” In:
Gunstone, F.D., Harwood, J.L., Dijkstra, A. J. (Eds.) The
Lipid Handbook 3rd Edition. Taylor & Francis Group LLC,
Boca Raton, FL, pp. 212–231.
2. Jalalpoor, M. (2010) “Staggered filtration system and
method for using the same for processing fluids such as
oils.” US Patent Application Publication 2010/0233335.