Environmental Engineering Reference
In-Depth Information
CHAPTER 10
Spent Catalysts from Dewaxing Operations
Apparently, n-paraffins are desirable components of diesel fuels because their cetane number
is the highest among all hydrocarbon groups. Similarly, n-paraffins possess the highest
viscosity index and, as such, they appear to be suitable components of lubricating oils.
Unfortunately, n-paraffins have to be removed from both fuels and lubricants because of their
undesirable cold flow behavior, i.e., high cloud point, pour point, and freezing point. By
converting n-paraffins to i-paraffins, cetane number of diesel fuel, and viscosity index of lube
base oil are affected, but they are still much higher than those of aromatic and naphthenic
compounds. At the same time, melting point of n-paraffins is significantly decreased, i.e., from
+35
◦
Cto
15
◦
C for C
20
n-paraffin for the C
20
iso-paraffin containing 5-methyl substituent,
respectively. To achieve desirable cold flow properties, the n-paraffins must be removed from
the corresponding petroleum fractions.
−
Conventional dewaxing involved the removal of aromatics by the extraction using solvents
such as furfural, phenols, etc. prior to the removal of n-paraffins. In a concentrated form,
aromatics were separated from the solvent by distillation. The extraction of aromatics usually
followed by the removal of long chain paraffins. Removal of the former was necessary to
improve viscosity behavior of lube base oil. The paraffins were removed by mixing the feed
with a solvent, e.g., acetone-benzene, barisol (ethylenedichloride-benzene), trichloroethylene,
etc., and cooling the mixture until most of the paraffins solidified in a crystalline form.
Subsequently, the solid paraffins were filtered off in rotary drum filters and solvent distilled off
for reuse.
During catalytic dewaxing, n-paraffins are converted to i-paraffins. The main reactions
occurring during dewaxing include hydrocracking (HCR) and hydroisomerization (HIS). The
relative contribution of these reactions to the overall dewaxing depends on the catalyst
structure, temperature, and the origin of the feed. For example, HCR is more important
reactions when production of diesel fuel and gasoline from vacuum gas oil (VGO) and
deasphalted oil (DAO) are considered. A high activity of the catalyst for HIS is desirable to
maximize the conversion of VGO and DAO to the lube base oil by converting n-paraffins to
iso-paraffins. It is advantageous to use a catalyst possessing a high HIS activity when naphtha
fraction is used for gasoline production. In fact, in some cases, the coproduction of
transportation fuels (gasoline, jet fuels, and diesel) with lubricants may be part of the refinery
strategy
[572]
. The catalyst possessing a good HCR activity besides the adequate HIS activity
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