Hydrocarbon Processing - August 2021 - 40

Process Optimization
ports are not allowing vessels to discharge
washwater near their facilities.
Refinery
challenges for
VLSFO
bunker fuel production. Most fuels
used in the market after the start of the
IMO's sulfur regulation are VLSFO (fuel
names are listed in TABLE 1). The switch
to VLSFO has affected refinery production,
since it is unlikely that HFO products
can simply be desulfurized to create
compliant fuels. Instead, VLSFO will
consist of blends of fuels produced from
different refinery streams. These blends
are expected to have higher fractions of
paraffinic products, which will affect the
properties of VLSFO. It is important to
state that VLSFO must be compliant
with ISO 8217:20171
and must fit within
the same residual marine (RM) grades
as used prior to 2020. The following are
some important parameters that present
huge challenges while adhering to the
ISO 8217:2017 specification of VLSFO.
Compatibility and/or stability. Depending
on the origin and production
process used, VLSFO blend streams can
be aromatic or paraffinic in nature. This
could lead to compatibility problems between
different fuel streams.
Blending of aromatic streams with paraffinic
streams results in sludge formation
(after prolonged tank storage, even if initially
stable), due to a change in the solubility
properties of the subsequent blend.
Viscosity. The viscosity of fuel
oils must be in accordance with ISO
8217:2017.1. VLSFO fuels are expected
to have a broad range of viscosities, even
within the same grade. Some types of
VLSFO will have low viscosity levels,
such as distillate marine (DM) fuel.
Cold flow properties. With VLSFO
having high fractions of paraffinic components,
exposure to prolonged cold conditions
may lead to wax formation, which,
in turn, could affect the cold flow properties
of the fuel.
TABLE 2. Typical properties of blending components
Description
Specific gravity
Sulfur, wt%
Viscosity, centistokes
(cSt) at 50°C
Asphaltenes, wt%
Vanadium, wt%
ARD residual
0.9302
0.46
175
1.7
7.5
TABLE 3. Blend component and aromaticity
Blend component
Vacuum tower bottoms
Gasoil
VR residual
0.989
0.7
5,000
3.1
36
Calculated carbon aromaticity index
(CCAI). The CCAI provides an indication
of the ignition properties or ignition
delay of fuels based on fuel viscosity
and density. In view of the broad range of
density and viscosity values of VLSFO,
resulting CCAI values can also vary considerably.
Catalytic
fines. The level of catalytic
fines in VLSFO are unknown and might
vary, depending on the refinery streams
from which the fuel was produced.
The blending of VLSFO. The major
challenge that refineries face is to keep the
asphaltene micelles in colloidal solutions
of fuel oil. The high aromaticity of fuel oil
helps achieve the same. In addition, if an
aromatic cutter is poured into a paraffinic
base, then the initial small volume of aromatics
in a paraffinic medium will disturb
the low aromaticity of the paraffinic base,
thereby increasing the probability of asphaltene
sludging (FIG. 2).
In the opposite situation, where parGasoil
0.845
0.0005
4
-
-
Trim
gasoil (TGO)
0.9068
0.16
96
-
-
affinic blendstock is poured into an aromatic
medium, the aromaticity at the
interface of the two liquids will be predominantly
by the aromatics and will always
keep asphaltenes in the solution.
Based on typical examinations of
VLSFO blendstock properties, they can
be classified as a high aromaticity base
(50%-60%)-i.e., vacuum bottoms and
visbreaker tar bottoms-and two types of
cutter stocks (aromatics and paraffinic).
Aromatic cutters are typically from
Aromaticity
Nearly 80%
30%-40%
TABLE 4. Any aromatic blend will have an excess of aromaticity (more than 50%)
and be stable
Blend component
Visbreaker tar bottoms
LCO and HCO
Aromaticity
47%-56%
More than 80%
TABLE 5. A hybrid blend is not guaranteed to always have an excess of aromaticity
(more than 50%) and be stable
Blend component
Atmospheric tower bottoms and vacuum tower bottoms
Gasoil
Cycle oil
40 AUGUST 2021 | HydrocarbonProcessing.com
Aromaticity
50%-60%
30%-40%
More than 80%
fluid catalytic cracking units (FCCUs)
and include light-cycle oil (LCO), heavycycle
oil (HCO), and slurries or clarified
oils (CLOs). These cutters are highly aromatic
liquids (80% or higher). Paraffinic
Blending
distillate to
VR bottoms is
acceptable
VR bottoms
Distillate
FIG. 2. Blending direction.
Reverse blend
will result in
sludging
ARD unit
ARD distillate
Bunker tanks
ARD residue
FIG. 3. KNPC's ARD unit.
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