Analysis of the treatment processes for salts exceeding the specified limits and the emulsion layer after electrodialysis desalination

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Recently, the electrodialysis unit has shown a persistent issue of excessive salt content in the crude oil after desalination, with a relatively thick emulsion layer at the oil-water interface within the tank. Due to limitations in the equipment design, this unit does not have a dedicated emulsion removal pipeline for the middle level. Conventional water separation methods are insufficient to eliminate the emulsion layer; by periodically lowering the level to allow the emulsion layer to be discharged, the salt content can be quickly brought back to acceptable levels. The process is analyzed as follows. I. Operating phenomena: Under normal water injection and power supply conditions, the salt content in the crude oil after dehydration remains high for an extended period. There is a thick, aged emulsion layer at the oil-water interface; this layer is enriched with gums, asphaltenes, mechanical impurities, and organic calcium and magnesium salts, and its structure is stable, making demulsification difficult. Daily bottom water removal can only discharge the clear water from the lower layer, and cannot remove the salts contained within the emulsified colloids. The long-term accumulation of the emulsion layer occupies the effective space of the electric field, resulting in a reduced separation effect of the electric field, which is the direct cause of excessive salt content. II. Causes 1. Fluctuations in the properties of crude oil, with an increase in the content of heavy components and acidic substances, lead to the formation of a dense and rigid emulsification film at the oil-water interface. As a result, tiny saltwater droplets find it difficult to coalesce and settle, thereby enhancing the stability of the emulsion. 2. The equipment design is limited; there is no intermediate milk emulsification pipeline, so it is not possible to discharge waste in thin layers on a continuous basis – the waste can only accumulate naturally. 3. Sludge at the bottom of the tank continues to accumulate; solid impurities form an emulsified framework that promotes the thickening and solidification of the emulsion layer, leading to gradual aging over time. III. On-site handling procedures: In line with the existing production conditions, the facility carries out periodic centralized emulsion removal operations, with specialized treatment being conducted once every one to two months. First, the oil-water interface is artificially lowered, pushing the suspended emulsion layer in the middle down to the bottom of the tank; discharge continues for 1 to 2 hours until oil appears in the wastewater, ensuring that the aged emulsion paste is completely removed. After the discharge is complete, keep the liquid level in the tank high, and transport the discharged emulsified waste oil to the wastewater treatment plant for recycling. This operation can quickly remove the high-salt emulsion layer, restore the electric field sedimentation conditions, and enable the desalination parameters to meet the standards promptly. IV. Optimizing prevention and control measures: To reduce the formation rate of the emulsion layer and stabilize production conditions, optimization measures have been formulated. In the event of fluctuations in crude oil quality or abnormal current levels, emulsification removal operations should be scheduled in advance, without adhering to a fixed schedule. Slight perturbations of the interface level during the production process are employed to prevent the emulsion layer from becoming compacted and hardened. Reasonably control the water injection temperature, mixing pressure difference, and amount of demulsifier added to weaken the strength of the oil-water interface film. Regularly discharge the sludge at the bottom of the tank to reduce the accumulation of solid impurities. V. Conclusion: The fundamental cause of the excessive salt content this time is the accumulation of large amounts of salt in the aged emulsification layer at the interface, which cannot be removed by conventional water separation methods. In the absence of dedicated sewage pipelines, regularly lowering the level to discharge the emulsified layer in bulk is the most direct and effective method of disposal. By optimizing operational control and flexibly adjusting the oil drainage schedule, it is possible to maintain a stable desalination efficiency, ensuring that the salt content levels after desalination remain within acceptable limits.