Stability and fouling potential
WTI May 2021 #2, Azeri March 2021
ASI
SBn
In
LSCI #
LSCI concern
WTI Azeri
-1 37 24
0.437 0.476 0.465
0.441 0.348 0.376
-2.1
Severe
0.1
High
70/30 Azeri/WTI
-0.5
Severe
Table 1
Compared to medium or heavy crude oils, the heating of light crudes like WTI does result in a faster and more marked decrease in the solvency power of the crude matrix and its ability to keep resins and asphaltenes in solution. Consequently, in the WTI crude, all the asphaltenes and a portion of the resins, can precipitate, thus increasing the risk of deposition and fouling of heat transfer equipment. For these reasons, WTI is incompatible with heavier crude oils as the resulting blend has a very low asphaltene stabil- ity and, consequently, a high fouling potential. Finally, WTI crude oil can contain high concentrations of finely dispersed solids that can interact with polar resins and precipitated asphaltenes, resulting in a tight water- in-oil emulsion, which is hard to resolve at the desalter. Therefore, it is important to ensure that the WTI cargo does not have a high solids content and/or employ a suit- able mitigation strategy, such as injecting a tank pretreat- ment demulsifier, in order to facilitate good solids wetting and removal. This will minimise the risks of a desalter upset and the potential to foul the cold preheat exchangers, as well as co-deposit with asphaltenic resin and asphaltenes downstream from the desalter. By assessing the crude blend stability, Baker Hughes can advise if injecting the antifoulant upstream from the desalter is required to avoid asphaltenes precipitation and maintain its good desalter performance. Dealing with light crude oil (LTO) problems As part of a continued commitment to operational excel- lence and emissions reduction, the RAM looked to collab- orate with experts experienced in WTI processing. This is when the Lifespan program was deployed to provide a holistic approach to fouling mitigation, utilising the Crude Compatibility Model to predict and assess fouling risks in conjunction with chemical antifoulants.
The first step was characterisation of cargoes of WTI crude, as it is known to show a high variability level. Several key parameters were analysed, including crude stability, compatibility with other light crudes, fouling potential, and filterable solids content (see Table 1 ). Determining the stability and crude compatibility param- eters, provides insights into how the crudes can be blended safely without risking asphaltene and resin precipitation, which is key to minimising the risks of fouling and associ- ated economic penalties. Stability measurements To determine the stability of crude oil and blends therein, Baker Hughes developed the proprietary ASIT Asphaltene Stability Index Test. This instrument utilises near-infrared turbidimetry to detect the onset of asphaltene floccula - tion. This allows for the determination of key parameters to assess crude stability, namely the solubility blending number (SBn). Indeed, the capability of molten to keep the asphaltenes in solution, and the insolubility number (In), indicates the tendency of the asphaltenes to precipitate out of solution. The method is automated, quick, and extremely accurate (error of only ±1%). It can be used to measure the stabil - ity of heavy fuel oils as well as crude oils, including very light crude oils such as WTI, where the asphaltene content is very low. Figure 1 shows the instrument’s main result where the asphaltenes’ flocculation onset is determined by the peak in the intensity curve. The method was extended by a proprietary procedure to be able to evaluate crude stability with an extremely low content of asphaltenes, less than 0.05%, similar to WTI, which does not present a clear flocculation onset. One field ASIT unit is used at RAM to determine the stability of any crude oil and build a local database that is used to evalu- ate the compatibility among processed crudes. The same instrument is also used to monitor the stability before and after daily desalting and will eventually be used to check crude tank stabilities. Measurement of these stability indexes (SBn, In, and ASI) and methods allows for precise calculations and replacement of any other stability index known in the literature/industry (ASTM methods and p-value). Furthermore, they represent the main parameters utilised to evaluate the overall fouling potential and Lifespan Stability Concern Index (LSCI), so that the proper antifoulant dose rate can be predicted and/or adjusted to correctly mitigate fouling. Lifespan blending model Based on the field ASIT stability indexes (SBn, In, and ASI)
1.0
0.8
0.6
0.4
0.2
0.0
0
50
100
150
200
Increasing ASI = Increasing stability
Figure 1 ASIT main results
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