Ara Barsamian, President/CEO, Refinery Automation Institute, LLC. Image Credit: Ara Barsamian
Bunker fuel asphaltenes are the cause of compatibility problems, defined as their precipitation in the form of sludge, clogging filters, decreasing effectiveness of purifiers, and discarding asphaltene sludge energy you paid for. Stability is basically a time-dependent compatibility issue, whereby the asphaltenes precipitate during the ship voyage.
Come 2020, you'll have a nasty surprise: asphaltene content of bunker residual fuel is not included in ISO8217-2017 specs. So what? See below…
Role of Asphaltenes in Bunker Fuel Oil
The basis of most residual fuels are atmospheric and vacuum tower bottoms, essentially residues with a "drop" of diluent such as gasoil or slurry, forming a highly unstable colloidal mixture. All crude petroleum residues contain asphaltenes, so you cannot avoid them unless you switch to marine gasoil, an uneconomic alternative.
In view of widely varying blend recipes for 2020 ULSFO, some paraffinic, some aromatic, and some of highly variable composition, there is a "high probability" of compatibility and stability issues.
Some bunker blend examples are:
Paraffinic: atmospheric or vacuum tower bottoms (VTB) "cut" with LS VGO or HAGO
Aromatic: atmospheric or vacuum tower bottoms(VTB) "cut" with slurry or LCO
Mixed paraffinic-aromatic: VTB or VBB+LS VGO+Slurry+LCO
Where VGO=vacuum gasoil; LCO=Light Cycle Oil; VBB=visbreaker bottoms
Measuring and Predicting Compatibility/Stability of Bunker Fuel Oil
Compatibility and stability issues have been thoroughly investigated over the last 30+ year, and well known methods to measure the propensity of bunker fuel to become incompatible/unstable is to measure the aromatics solvent power of asphaltenes in bunker fuel, in the form of Toluene or Xylene Equivalence (TE or XE). This refers to the percentage of aromatics, Toluene or Xylene, required to keep asphaltenes in the bunker fuel oil colloidal solution without precipitation, and most oil companies have filed patents based on these fundamental principles , , .
Following the ExxonMobil patent US9,803,152, , the Toluene Equivalence is directly proportional to asphaltene content of bunker fuel.
Compatibility/stability is defined by bunker fuel oil asphaltene "solubility reserve" K, as the ratio of BMCI (Bureau of Mines Correlation Index) divided by TE or XE
K=(BMCI/TE)=>1.15 or alternately, K=(BMCI-TE)=>10
The higher the K ratio of available aromaticity (BMCI) to required aromaticity (TE), the greater the "solubility reserve", and hence the more compatible and stable the bunker fuel.
Other Desirable Spec/Tests Additions to ISO 8217-2017
To minimize uncertainty and risk on the part of bunker buyers, ISO should consider including the following changes and additions to ISO 8217 RM grade residual fuels:
Asphaltene content; justification: compatibility is a function of Asphaltene content
Compatibility test using ASTM D4740 Spot Test: a simple and an approximate indication of compatibility and stability
Example of "Satisfactory" Lab Certificate Of Analysis (COA)
What is a satisfactory bunker fuel analysis? The example COA below illustrates the key non-traditional tests which are the closest thing to guarantee 2020 bunker fuel oil compatibility:
Compatibility/stability spot test (ASTM)
Toluene equivalent (Exxon)
Xylene equivalent (BP)
COA for RMK 700 (used to "cut" to RMG 380 with either RMG 180, MGO, etc.)
For further information or learn more about 2020 bunker blending, please contact the author at firstname.lastname@example.org or attend RAI upcoming 2020 Bunker Blending Course in Houston, March 11-12, 2019.
 US Patent 9,803,152 B2 Kar et al, assignee: ExxonMobil, 'Modifications of Fuel Oils for Compatibility", October 31, 2017
 US Patent US20040121472/ EP 1573315 A1, assignee: BP, 'Predictive Crude Oil Compatibility Model", September 14, 2005
 US Patent 8,916,041 B2 Van Den Berg et al, assignee: Shell Oil Company, 'Blending Hydrocarbon Streams to Prevent Fouling", December 23, 2014