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What's the philosophy of desalting system in Crude Distillation Unit, with respect to High Voltage & Demulsifier?

Jan-2023

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Raju Raju Thakare, KBC ADVANCED TECHNOLOGIES PVT LTD, Raju.Thakare@kbc.global

Desalter is used for dehydration & desalting purpose in crude processing system. The objective of desalting process is to remove chloride salts and other minerals from the crude oil by water-washing. Depending on the desired salt content in the desalted crude oil, a one- or two-step process could be applied. For refining purposes, a salt concentration of < 1 PTB (pound of salt measured as NaCl per thousand barrels) is desired. By desalting, a considerable percentage of suspended solids (sand, clay, or soil particles, or even particles product from corrosion of pipelines and other upstream equipment) are removed. It is measured as BS&W (Bottom Sediments & Water) in inlet & outlet of crude sample from desalter (Typical values in crude inlet of desalter is 0.5 % Vol and outlet should be less than 0.1% Vol).

The philosophy of desalting system in Crude Distillation Unit with respect to High Voltage & Demulsifier:
Demulsifiers also called as Dehydration chemicals are chemical compounds used to destabilize, and assist in coalescence of crude-oil emulsions. The chemicals counteract the emulsifying agent, allowing the dispersed droplets of the emulsion to coalesce into larger drops and settle out of the crude.

To work efficiently, the dosing of demulsifiers must be correctly done in crude oil or/ desalter wash water based on the type of demulsifier being used (oil based or water based) and it must be intimately mixed with the emulsion. At refinery end, demulsifier is added at crude feed pump inlet (battery limit) as well as near mixing valve. As a refinery best practice, crude tank is given a settling time of min. 24 hours before it is charged to crude unit. In case, still there is water carry over along with crude to unit, desalter gets upset leading to rise in water/ crude interface level, sometimes tripping of transformers as well. In such event, demulsifier dosing rate is increased to ensure more coalescence and removal of water from desalter (It is observed by increased brine flow from desalter). It should be noted that demulsification chemical can act more efficiently in the presence of electrostatic fields.

Electrostatic grids are often used to enhance the effect of demulsification. When crude oil containing a dispersed water phase is subjected to an electrostatic field, one of three physical phenomena causes the conductive particles or droplets to combine.
A) The water droplets become polarized and align themselves with the lines of electric force. Electrical attraction between oppositely charged ends forces the water droplets together and makes them to coalesce.
B) An induced electric charge attracts the water droplets to an electrode. In a direct current (DC) field, the droplets tend to collect on the electrodes or bounce between the electrodes, interacting and coalescing, forming larger and larger droplets until eventually they settle by gravity.
C) The electric field distorts and weakens the film of emulsifiers surrounding the water droplets. Water droplets dispersed in oil that are subjected to an alternating-current (AC) field become elongated along the lines of force as voltage rises during the first half-cycle. As the droplets are relaxed during the low-voltage part of the cycle, the surface tension pulls them back toward a spherical shape. This effect repeats with each cycle, weakening the film so that it breaks more easily when droplets collide.

Depending upon the mechanism used, the electrical field causes the droplets to act rapidly, which increases the probability of collision with other droplets. Droplets coalesce when they collide at the proper velocity. The greater the voltage gradient, the greater the forces that cause coalescence; however, actual in practice, at some voltage gradient, rather than coalescing, the water droplets can be pulled apart leading to formation of tight emulsion. This is referred as critical voltage.

By using the high Voltage gradient, there is a reduction in the need for emulsion-breaking chemicals. The one limitation of electrostatic dehydration is shorting/arcing, which will negate the electrostatic field. When we process highly conductive crude, the amperage of transformers increases leading to tripping of desalter sometimes. Therefore, crude acidity to be monitored carefully and crude blend to be prepared accordingly.

Jan-2023
Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

The desalting of crude oil is one of the most important processes in a refinery to ensure the reliability and the operational availability of the refining hardware. During the crude oil extraction processes the petroleum drag sediments and water beyond inorganic salts (carbonates, calcium, chlorides, etc.) which are responsible for fouling heat exchangers leading to efficiency reduction, raise in energy consumption and reduce the operation campaign of the process units.

The presence of the dissolved salts in the crude oil is still responsible for catalysts deactivation in conversion process units (FCC and Hydrotreating), furthermore, these compounds can accumulate in the top of atmospheric crude distillation columns leading to corrosion and loss in separation efficiency.
The desalting process involves the mixture of crude oil with water aiming at the dissolution of the salts considering the higher solubility of these compounds in the aqueous phase.

The crude oil is pumped from the storage tanks through the heating battery where it is heated and mixed with dilution water, the mixture is made by a mixing valve that promotes an intense mixture through pressure drop. The major part of water is under the free form and is removed by decantation due to the difference of density between the aqueous and oil phases, however, part of the water is emulsified in the oil phase and are required actions to broke the emulsion and allow the decantation of this water and the dissolved salts.

The emulsion breaking is carried out with the application of high-intensity electric field (close to 3,0 kV/cm) that provokes the polarization of water droplets, his agglutination and consequently his decantation. Desalting heavy crude oils is a greater challenge to refiners once the lower difference of density between the aqueous and oil phases makes the separation hard, beyond the higher content of compounds which stabilize the emulsions in heavier crudes (asphaltenes), in these cases the refiners operate under higher desalting temperatures and are used demulsifiers to facilitate the emulsion breaking.

Demulsifiers are normally a combination of surfactants with hydrophilic and hydrophobic bands in the same molecule which normally have their formulation protected by patents and his dosage needs to be accompanied by a specialist (chemical vendor). Regarding the electrical field, higher electrical intensity tends to improve the desalting efficiency considering the other variables fixed once improve the mixing effect and intensity of water droplets, collision with consequent coalescence and decantation, but it's necessary to consider that there is an optimal point for achieve this effect, once mixing in excess can promote collisions but without adequate conditions of coalescence.

It's important to consider the whole desalting process and all operating variables and not only the demulsifier and electrical field. The desalting temperature is a key parameter of the process once impact the viscosity of the crude and consequently the sedimentation velocity, it's important to realize an study including all operating variables like content of dilution water, pressure drop in the mixture valve, electrical field and desalting temperature. It's important to take into account the compatibility of the crude oils processed, which can lead to asphaltenes precipitation in some cases, especially in blends of high paraffinic crudes with heavier crudes.

Jan-2023