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Feb-2025

Desalter optimisation strategies: Part 2

Analysis and optimisation strategies implemented for a two-stage desalter processing light to medium API crude blends at a Southeast Asian refinery.

Venkatesan Mani
Veolia Water Technologies and Solutions

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Article Summary

As discussed in Part 1, published in PTQ Q4 2024, crude oil desalting plays a pivotal role in refinery operations by removing salts and impurities that can wreak havoc on downstream equipment through corrosion, fouling, catalyst poisoning, and product quality degradation. Despite significant technological advancements in desalter design, optimising desalter performance remains an intricate challenge due to the complex interplay of multiple factors, including crude oil composition, operational conditions, equipment design, and chemical treatment. This interdependence among factors often hinders achieving overall performance goals through isolated optimisation efforts.

Optimisation of key parameters: Southeast Asian refinery case study
Desalter operation overview
From the crude tank farm, the feed crude blend (light to medium API) is pumped to the crude distillation unit (CDU). The feed undergoes preheating in a cold preheat train to a targeted desalter temperature of 138-145°C to reduce viscosity. The feed is evenly distributed to the two-stage electrostatic desalters in Trains A and B (see Figure 1).

At the crude unit battery limit, a wash water concentration of 2.5-3.0% is introduced, while 4.5-5.0% is fed before the mix valve. The wash source is a blend of raw water and stripped sour water. The crude and water are mixed in the first-stage mix valve at a pressure drop of ~1.2-1.4 bar, while the second-stage mix valve operates at a pressure drop range of ~0.8-1.0 bar. The resulting emulsion downstream of the mix valve facilitates the removal of water-soluble impurities from the crude.

The water-in-oil emulsion is distributed between the electric grids at an applied field of 150 kilovolt-amps (KVA) with an operating voltage of 400-360 volts and an amperage of 80-120 amps for most crude blends. The design residence time for the desalter is 12 minutes for crude oil and 160 minutes for water.

The dosage strategies followed split feed technologies.1 The proprietary emulsion breaker (Embreak) feeding rate will be 5-10 ppm, with the dosage split into two locations: one before the cold preheat and another at the mix valve of each stage. The solids wetting agent feed rate will be 3-5 ppm based on incoming crude solids (>60 ppm in raw crude). These chemistries help reduce interfacial film tension, promoting oil and water separation.

The desalted crude achieves the key performance indicators (KPIs). Specifically, it achieves a crude outlet salt concentration of <0.5 per thousand barrels (PTB) and 0.2 vol% basic sediment and water (BS&W) as free water, with 8% wash water and <12 ppm chloride in the wash water.

The crude oil inlet is designed for 20 PTB salt and 0.5 vol% BS&W. However, most of the crude blends processed have a salt concentration of <10 PTB. Chloride in the overhead is controlled to <30 ppm without any caustic injection into the desalted crude, ensuring sodium levels are maintained below 1 ppm in the atmospheric residue.

The following section covers the basics of key parameters and how each key parameter optimisation approach was followed, considering interdependent variables.

Crude oil characteristics and benchmarking study
Any desalter optimisation strategy begins with understanding the characteristics of the crude oil before moving on to operational parameter optimisation. A detailed crude blend quality analysis was conducted periodically based on crude blend changes and whenever KPIs were not in control for any short period. The crude characterisation testing focused on desalter impact variables, such as crude oil API/density, viscosity, salt content, water content, metal content, conductivity, compatibility, fouling potential, and filterable solids.

An emulsion potential study was also conducted using a portable electric desalter (PED) to validate process operating conditions and optimise chemical dosages for the emulsion breaker and solids wetting agents. The impacts of wash water quality, pH, and chlorides were also assessed. Additionally, desalter emulsion layer samples were analysed when there was growth in the desalter emulsion layer. The brief test results are summarised in Table 1.

Crude samples were subjected to compatibility and fouling assessments using field-proven technologies, such as the proprietary CrudePlus tools (see Figure 2).
The emulsification tendency of the crude was evaluated in the PED. The crude was confirmed to have high emulsion potential based on poor water separation with no chemical treatment. With the right emulsion breaker dosage, the emulsion broke down, and water separation was observed with treatments using solids wetting agents. These results were used for desalter optimisation, as shown in Figure 3.

Desalter temperature
Desalter temperature = f {crude oil viscosity and density, water solubility in the crude}
To achieve maximum desalting efficacy, a widely followed approach is to increase the desalter temperature, which can enhance salt removal efficiency. However, this method typically adopted and pushed the desalter’s maximum operating temperature limit (typically 155°C) or the limit based on the transformer bushing design temperature and sometimes corrosion risk limits. While higher temperatures can reduce crude oil viscosity and aid water separation, beyond certain temperature increases, water yield diminishes the returns in separation efficiency. Hence, increasing to the maximum desalter temperature is not a cure-all and comes with limitations and risks.

Temperature optimisation often starts based on the recommended operating viscosity of the desalter. Even though there are no well-defined design limits for desalter operating viscosity, the desalter is targeted to operate at a viscosity <2 cSt based on best practices guidelines for effective desalting. Hence, with the analysis of crude blend viscosity and API, the temperature required to achieve the desired operating viscosity can be estimated from the ASTM standard viscosity temperature charts for liquid petroleum products (D341 Chart VII).

It is important to remember that emulsion viscosity increases exponentially with lower temperatures, particularly in oil-water emulsions with varying water-to-oil ratios, as referenced in emulsion journals. Typically, emulsion viscosity >100 centipoise will lead to a stable emulsion layer in the desalter. Increasing the emulsion breaker dosage can help resolve short-term issues. However, with intense solids and destabilised asphaltenes stabilisation conditions, increasing temperature will be the best approach for long-term sustainable operations. Hence, without temperature optimisation based on crude viscosity, optimisation of other parameters will not help achieve consistent KPIs.


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