Apr-2025

Thermodynamic solution for evaluating crude compatibility

Case studies emphasise the power of advanced analytical tools such as predective modelling in crude compatibility assessment and blend management

Asok Tharanivasan and Michelle Wicmandy
KBC (A Yokogawa Company)

Article Summary

Global crude oil demand is projected to tip 103 million bpd in 2025, according to the International Energy Agency (IEA). As refiners aim to maximise output while keeping costs in check, many are turning to opportunity crudes from various geographical locations throughout the world. This shift presents the refining industry with challenges in optimising crude oil blends driven by the growing demand for diverse crude sources, stringent environmental regulations, and economic pressures. While opportunity crudes are economically attractive, they are unpredictable – you never know what you are going to get in terms of composition and properties.

One critical issue that refineries face when grappling with opportunity crudes is asphaltene stability. Specifically, this refers to how well the asphaltenes are dissolved in the crude oil at a given operating temperature and pressure in refinery processes. Asphaltenes, the heavy and complex molecules in crude oil, can precipitate to either remain suspended or settle, depending on the conditions. The presence of precipitated asphaltenes can alter the physical properties of the fluid stream, leading to fouling and equipment damage, which causes significant operational inefficiencies and increased maintenance and repair costs.¹

Problem
Asphaltene precipitation, the precursor of asphaltene- related problems, is a thermodynamic phenomenon that depends on the fluid composition, temperature, and pressure. Additional factors such as the residence time, equipment configuration, chemical reactions, and high process temperatures can produce deposition in streamlines, pump plugging, and equipment fouling. Hence, asphaltene precipitation is the primary trigger for these issues. 

Typically, refiners assess asphaltene precipitation in a particular crude oil using standardised titration tests. These tests determine the onset point, which represents the minimum amount of titrating solvent required to initiate asphaltene precipitation.

Usually, the onsets are reported in mL of solvent per g of oil. Alternatively, the onsets are also reported as P-Value (ASTM D71122, ASTM D82533), P-Ratio (ASTM D70604), and S-Value (ASTM D71575). Wiehe6 presents the measured onsets in terms of solubility blending number (SBN) and the insolubility number (IN).

The onset point indicates the severity of asphaltene precipitation; for instance, higher onset values indicate more stable crude oils. Crude oils containing precipitated asphaltenes are considered unstable, where the onset cannot be measured. Test conditions (pressure and temperature) and titrants, typically n-heptane and n-hexadecane, vary by procedure, with most tests conducted at ambient conditions. Notably, onset values do not correlate to bulk asphaltene content of the crude oil.

Blending crude oils from various sources is unavoidable during transportation, pre-refining, and refinery processing. Additionally, mixing processed fluid streams containing asphaltenes occurs during processing, and product streams are blended in certain scenarios during post-refining operations. In all these cases, the source oils are incompatible when blending compositionally different stable source oils that lead to asphaltene precipitation. Consequently, using the correct proportion of source oils is crucial to avoid any unstable blends while ensuring compatibility.

While onset tests assess blend compatibility, testing all possible blends is impractical and expensive. Simple averaging is unreliable, as asphaltene solubility depends on fluid composition. This underscores the need for a predictive tool to assess the compatibility of different source oils or streams. In today’s fast-paced environment, seamless integration of such a predictive tool into a process simulator is vital for efficient planning and operations.

Solution
To address the crude oil compatibility challenges, a Multiflash Crude Compatibility Tool (MFCCT) was developed and incorporated into KBC’s proprietary Petro-SIM process simulator. Adapted from KBC’s proprietary Multiflash asphaltene model, the MFCCT predicts asphaltene precipitation onsets for blends, and it has been validated using data from several refineries. Its predictive capabilities eliminate the need for extensive measurements in identifying the extent of compatibility of source oils or streams. Since the tool is based on the thermodynamic model, compatibility assessments can be extended to changing operating conditions, providing a reliable and efficient solution for blending and process optimisation. This enables refiners to make informed decisions quickly and effectively.

Asphaltene model
At the core of the MFCCT is a robust thermodynamic framework that considers vapour, liquid, and asphaltene as equilibrium phases.7 The model employs the cubic plus association equation of state (CPA-EOS) for phase equilibrium calculations. Asphaltene precipitation is modelled by considering two key mechanisms: asphaltene-asphaltene self-association and asphaltene-resin cross-association. At least one onset data is required to parameterise the model to account for the association behaviour of asphaltenes.

In terms of fluid characterisation, the refinery assay and the properties are represented as a defined number of pseudo-components, including the assignment of component properties for phase equilibrium calculations. The key steps involved in the characterisation are generating a boiling curve from the crude assay, translating the boiling curve into single carbon number (SCN) fractions, and lumping of SCN fractions into a specific number of pseudo-components using appropriate estimation methods and correlations. The inclusion of saturates, aromatics, resins, and asphaltenes (SARA) or paraffins, naphthenes, and aromatics (PNA) data will enhance the characterisation accuracy.

Note that this asphaltene phase behaviour model applies only to a single-source oil when calculating precipitation amounts upon dilution with a pure precipitating solvent (for example, n-heptane). When a source oil is blended with other source oils, the association behaviour and the solubility of asphaltenes are affected.

Adaptation of the model for oil blends
Figure 1 summarises the methodology used for phase behaviour modelling of oil blends. Fluid characterisation and parameterisation are first performed individually for each source oil. Each source oil requires a crude assay with asphaltene content and onset data. Simulated distillation data and bulk density are acceptable when full crude assay is absent.

Second, assays for the source oils are blended based on their proportions, followed by fluid characterisation. Third, model parameters for each source oil are blended using appropriate mixing rules to determine the blend’s parameters. Finally, the phase equilibrium calculations use the characterised fluid and the blend parameters. The calculations simulate the titration experiment, thereby predicting the onset for the blend. If the blend is unstable, the precipitation amounts are calculated. Note that the Petro-SIM simulator automates these calculations once the source oil streams are defined.