March 2018

Process Optimization

Flexibility in desalting operations for opportunity crudes

Typically, opportunity crudes require more rigorous desalting to yield high-quality products due to high levels of naphthenic acids, basic sediment and water (BS&W) and filterable solids, among others.

Mejias, I., Liu, Y., Boul, J., Collins, T., Forum Process Technologies

Typically, opportunity crudes require more rigorous desalting to yield high-quality products due to high levels of naphthenic acids, basic sediment and water (BS&W) and filterable solids, among others. These crudes differ greatly in quality, as does the quality of the desalted crude from such feeds. The benefits from low-priced feedstock are balanced against capital investments for desalting units and higher operating costs. The key is ensuring that the desalting process has the operational flexibility to process these types of crudes.

The impact from emulsion sensitive crudes is prevalent; upsets from the desalter elevate downstream corrosion. Cold liquid carryover to downstream units and corrosion, among many others, are examples of issues caused by water and water-soluble salts in crude oil. High oil content in the desalter effluent water can cause environmental constraints. Opportunity crudes, with emulsions that are difficult to resolve, can also consume significantly higher amounts of demulsifier and create high grid voltage fluctuations. Subsequent fluctuations in voltage and higher power consumption may lead to electromechanical failures, further reducing run time between maintenance periods. This leads to poor desalting and low-power efficiencies that inevitably result in higher operating costs. How can a refiner take advantage of opportunity crudes? An improved desalter design can mitigate these impacts.

The dynamic design of a desalter/dehydrator to process different types of crudes can significantly reduce the carryover of inorganic contaminants and lead to economic savings in refinery operations. If the desalter can process or water concentration fluctuations in the crude feedstock, desalting would reduce operating costs significantly. These fluctuations would not inhibit the electrostatic coalescence process within the treater.

Desalting opportunity crudes

To offer flexibility in electrostatic separation, refiners need a technology-based solution to address higher chemical and energy consumption to improve the quality of opportunity crudes and reduce operating costs. Refiners need a technology option to process different crude slates, a unique design that incorporates two different types of distribution systems into the same vessel to provide a dynamic design. As the properties of the crude change, the two distribution systems in one vessel offer a significant advantage to process a wide variety of crudes.

The direct-distribution desalting mode is typically used to process medium to heavy feedstock or for any emulsion-sensitive crude high in solids, such as opportunity crudes. The crude is introduced horizontally between the electrode grids (FIG. 1). Improved laminar flow provides an enhanced water droplet environment for emulsion-sensitive crudes. The dual horizontal flow distribution provides quick, complete coalescence, thereby ensuring larger and faster settling droplets. The high-level distribution between the grids allows for two- to three-times the volume of water/oil emulsion, compared to other vertical flow desalters. Furthermore, the distribution improves the control of the interface (rag) emulsion and minimizes oil carryunder for improved effluent water quality.

FIG. 1. Direct-distribution desalting mode for heavy feeds and opportunity crudes.
FIG. 1. Direct-distribution desalting mode for heavy feeds and opportunity crudes.

 

FIG. 2 shows data for six units operating in the direct-distribution desalting mode for heavy feeds and opportunity crudes. These units have been installed in the US, Canada, Australia and Egypt. The data shows desalting efficiencies obtained with different crude charge rates and API gravities. All units achieved above 95% desalting efficiency. The heaviest feed, with 23 API gravity crude, presented a 98% desalting efficiency. All of the units continue to exceed the salt specifications to date.

FIG. 2. Operating desalting units in direct-distribution mode.
FIG. 2. Operating desalting units in direct-distribution mode.

 

Desalting light- and high-water content feeds

The low-velocity desalting mode is typically used for lighter crudes. This operation is typical in applications where water content in the feed may be very high, or during tank switches. The crude is introduced below the electrodes to create the maximum oil residence time for improved dehydration (FIG. 3). The low-intensity electric field is then used to treat the bulk emulsion and interface, whereas the high-intensity electric field (between the electrodes) is used for removal of the final traces of water and smaller droplets that remain. FIG. 4 shows data for four units operating in this mode that were installed in the US, Canada and Brazil. The data shows desalting efficiencies obtained with different crude charge rates and API gravities. All units achieved above 95% desalting efficiency. Even the heaviest feed, with 20 API gravity crude, presented a 99% desalting efficiency. Higher dehydration efficiencies for heavier crudes are seen with a crude distribution between the grids (FIG. 5) for a 27 API crude. All of the units continue to exceed salt specifications to date.

FIG. 3. Low-velocity desalting mode for light crudes or feeds with high water content.
FIG. 3. Low-velocity desalting mode for light crudes or feeds with high water content.

 

FIG. 4. Operating desalting units in low-velocity mode.
FIG. 4. Operating desalting units in low-velocity mode.

 

FIG. 5. Operating data of a desalter treating a 27 API gravity crude in direct-distribution mode with controller.
FIG. 5. Operating data of a desalter treating a 27 API gravity crude in direct-distribution mode with controller.

Reaching desalter steady state

When processing opportunity crudes, which may be highly conductive and/or refinery slops and tank bottoms, the desalter transformer may reach near its overloading state. However, the output voltage can be automatically changed to minimize the current draw. A desalter/dehydrator controller, which is a bolt-on type, pre-programmed, power electronics device, provides the optimum voltage gradient/electrical field inside the desalter vessel to help resolve stable emulsions and mitigate issues caused by water excursions in the dehydrator/desalter feed. To show the improved operational performance with a desalter/dehydrator controller, the following case study is presented.

A four-train, two-stage desalting system operating in direct-distribution mode was installed in the Middle East to process a 27 API gravity crude. The crude inlet salt content was 460 parts per millions (ppm), and the BS&W was up to 2.5 vol%. After the operation of the unit was stabilized, samples of the undesalted and desalted oil were collected. The samples were then analyzed for BS&W and salt content with ASTM standard methods. The results confirmed that the direct distribution mode achieved an average of 80% dehydration efficiency and 92% desalting efficiency over a period of 3 mos, exceeding the specifications established for this unit (FIG. 5).

The desalter/dehydrator controller has the ability to alter the voltage gradient based on dynamic feedback from the treater. The desalter/dehydrator controller can be programmed with parameters on the timing and how power is delivered to each grid within the vessel based on emulsion, API gravity, flowrate, etc. This technology mates up to the primary of any existing standard transformer, with only a few minor modifications. It can be bypassed, returning the desalter to its pre-controlled state via a no load selector switch.

The four-train, two-stage desalter system operating in direct-distribution mode was analyzed before and after the installation of the desalter/dehydrator controller with the same operating parameters (FIG. 5). Before the controller installation, the overall average desalting efficiency was 90%. After the installation of the controller, the desalting efficiency increased to 94%. It is important to note that after the controller installation, the desalting efficiency was stabilized, even though the system experienced fluctuations in the incoming BS&W content. Thus, the controller optimized the operation performance, ultimately stabilized the desalting efficiency and increased the dehydration efficiency of the system.

Because the controller provides real-time monitoring and control adjustability without system shutdown, it proves to be an excellent tool for additional flexibility when operating opportunity crudes or feeds where the physicochemical properties may change. Therefore, two different crude distribution designs, selectable in operation with a voltage controller, can improve the efficiency of production and refining operations, while reducing OPEX.

Gaining flexibility with redundant electrode configuration

The electrode and transactor design enhances the desalting/dehydration process for opportunity crudes, as well. The independently energized electrode grid design provides flexibility to handle upsets that can come with heavy or emulsion-sensitive crudes and facilitates long run cycles for maintenance (FIG. 6). Independently energized grid layers provide a redundant configuration and allow for continuous desalting, even upon loss of a grid due to the loss of level control or other upsets. Transactor connection and phasing allow for little to no loss in performance due to the loss of an energized electrode. This design facilitates chemical reduction, since the lower electrode provides a low-intensity electric field for continuous resolution of rag emulsion, thereby enhancing the ease of interface control. Multiple transactor secondary voltage levels provide the flexibility to optimize performance at varying operating conditions. In high-conductivity environments, such as with very conductive crudes, lower voltages provide longer service life for electrical components.

FIG. 6. Redundant electrode configuration.
FIG. 6. Redundant electrode configuration.

 

The heart of desalting opportunity crudes: Efficient mixing

The quality of mixing imparted to the oil and water prior to reaching the desalter vessel is key to maximize the desalting and dehydration efficiencies when processing opportunity crudes. It is desirable to disperse the fresh process water in the crude as thoroughly as possible without forming an emulsion that is difficult or impossible to break due to its high stability. This may be a challenge with opportunity crudes that may already contain emulsions that are difficult to resolve. If the size distribution of the water droplets in the crude feed is not relatively constant, the water droplet coalescence is lower, thus the desalting and dehydration efficiencies suffer. Because of this, an inline modulated, multiphase mixing technology is key to improving the droplet size distribution for enhanced desalting, reducing operating costs while increasing plant operations efficiency (FIG. 7).

FIG. 7. An example of an inline mixing device and traditional droplet size distributions.
FIG. 7. An example of an inline mixing device and traditional droplet size distributions.

 

The mixing device should be designed to exert a homogeneous shear force, orientation-free, to the process flows running through. The mixing element should generate moderate, yet efficient, turbulent flow conditions to facilitate the mixing process with low pressure drop (up to 5 psi), and to accommodate water content fluctuations in opportunity crudes. Traditional inline mixing technologies, such as mixing valves, yield high pressure drop (≥ 25 psi) combined with non-homogeneous shear forces, and may lead to undesired stable emulsions and low desalting efficiency. For example, a specific inline mixing device utilizes the main process flow momentum to create turbulent eddies inside the internal mixing element to enhance the mixing process with evenly sized droplets.b

The operating data presented here demonstrates the ability of a desalter/dehydrator controller, several units operating under direct distribution and low-velocity modes, and a specific mixing device to provide flexibility for processing different crude slates, from light to heavy feeds, particularly opportunity crudes. These technologies ultimately result in operational flexibility and a dynamic design, which improves the crude-water separation process. HP

NOTES

     a Refers to Forum Process Technologies’ Edge II technology.

     b Refers to Forum Process Technologies’ ForuMIX inline mixing device.

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