Oct-2024

Proactive management reduces crude preheat fouling risks

Predictive modelling allows for fouling risks to be predicted and effectively managed, improving energy efficiency while reducing emissions and operating costs.

Giuseppe Della Sala and Marco Respini
Baker Hughes

Article Summary

Crude distillation unit (CDU) preheat exchangers are critical to CDU heat recovery and energy efficiency. However, it is commonplace for the heat transfer efficiency of these exchangers to be impacted by fouling due to the deposition of asphaltenes. The consequence of this fouling is that the CDU heater must be fired harder to achieve the necessary coil outlet temperature, resulting in a significant fuel increase in oil consumption and carbon dioxide (CO₂) emissions and, consequently, a decrease in EII performance. Overall fuel consumption increases, and CO₂ emissions can typically be up to 10% of the total crude unit heater energy consumption.

Against this backdrop, successful application of Baker Hughes’ proprietary Lifespan program at the Refinery at Milazzo (RAM) T3 unit will be discussed. The unit processes high percentages of West Texas Intermediate (WTI) crude in blends with other crudes, resulting in a very high fouling tendency crude feed and high rates of crude preheat fouling.

The key component of the Lifespan program is the Lifespan blending model allowing the refiner to determine if any of the processed crude blends are incompatible with a high likelihood of asphaltene instability. This then allows prediction of the crude oil fouling potential, providing the basis for optimising the antifoulant dose rate to minimise the actual fouling of the preheat exchangers.

By utilising the program, crude preheat exchanger fouling rates were reduced by 78%, providing an overall saving of €2.7 million/yr with a net fuel gas reduction of 3.8K t/yr and a decrease of CO₂ emissions due to fouling of 11.7K t/yr.

Asphaltene fouling
Crude unit fouling, typically at preheat exchangers, can result in severe heat recovery efficiency losses. Preheat train heat transfer losses due to fouling need to be compensated with more heater duty, predicating higher fuel consumption and CO₂ emissions. The annual worldwide cost associated with CDU fouling is estimated at 4.5 billion $/yr. CDU fouling increases the refinery carbon footprint by 10% on average.

Very severe fouling cases, apart from higher fuel consumption, can result in processed throughput limitations. Mechanical cleaning removes fouling, leading to reduced throughput during the process or the need for a turnaround. In both cases, there is a loss in production and additional costs. Crude oil asphaltenes are the main source of fouling. These are high molecular weight polar molecules with a high content of condensed polynuclear aromatic (PNA).

Asphaltenes have an extremely low solubility in crude oil and a high tendency to agglomerate and phase separate, yielding fouling deposits. They are kept dispersed in a colloidal form by petroleum resins. The solvation by resins is a weak equilibrium, and several operational aspects can result in their precipitation, including:
• Blending of crude oils (compatibility)
• Temperature increase: The impact of temperature is different depending on crude properties
• Presence of solids and/or water in oil emulsions.

Fouling is also impacted by the velocity that can result in mitigation due to partial removal of deposit by shear stress. Solids not removed by the desalter can induce destabilisation of asphaltenes and tend to increase the fouling and hardness of the deposits. The inorganics typically account for 10-30% of the total deposit mass.

WTI challenges
RAM started processing WTI crude over the past two years, blending it with Azeri and other light crudes. Based on experience at other locations, it has been observed that WTI is potentially a highly fouling crude, posing an elevated level of risk of fouling to the crude preheat exchangers.

WTI, as pure crude, has an extremely low content of asphaltenes. However, it does contain a high concentration of polar resins that are close to the asphaltene solubility range. These resins can be destabilised at the oil film temperatures found in hot preheat exchangers (>200°C), as the paraffinic matrix of this crude is a poor solvent for polar and polynuclear molecules like asphaltenes and resins.

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 stability 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 suitable mitigation strategy, such as injecting a tank pretreatment 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 excellence and emissions reduction, the RAM looked to collaborate 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 parameters, 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 associated economic penalties.