The Hidden Costs of Saline Crude Oil: Why Desalting Pretreatment Cannot Be Ignored
All crude oils extracted from underground contain substantial amounts of water, inorganic salts (such as sodium chloride, calcium chloride and magnesium chloride), organic salts and other impurities. Most of the inorganic salts dissolve in the water, forming a saline-water system. Although crude oil undergoes preliminary desalting and dewatering at the oilfield, a certain proportion of salts and water still remains when it is delivered to the atmospheric and vacuum distillation unit. These residual salts and water not only disturb unit stability and push up energy consumption, but also significantly aggravate equipment corrosion risk, impair final product quality and adversely affect the efficiency of downstream units.
For this very reason, every major refining enterprise attaches great importance to crude desalting. Desalting is no longer merely a link in the "one removal, four injections" anti-corrosion practice of atmospheric and vacuum distillation — it has been elevated to an indispensable key step in crude oil pretreatment.
How Metal Salts Interfere with Crude Processing
Metals in crude oil exist mainly in two forms: oil-soluble metal compounds or organic salts dissolved in the oil, and water-soluble alkali-metal or alkaline-earth-metal salts — only a tiny fraction exists as suspended crystals, while the vast majority dissolves in water and disperses inside the crude as emulsions. Statistics show that about 90% of sodium salts and about 10% of calcium and magnesium salts belong to the water-soluble category.
1. Higher system energy consumption. Residual water in desalted crude evaporates during heating, absorbing large amounts of heat and raising fuel consumption. In addition, as the crude flows through heat exchangers, furnaces and other equipment, the evaporating water leaves salts deposited on tube walls, markedly reducing heat-transfer efficiency.
2. Disturbed unit operation. Water in the crude gradually vaporizes as temperature rises in the heat-exchange train, increasing pipeline pressure drop and easily causing static-seal leaks in piping and heat-exchange equipment, affecting long-term stable operation. Heavy vaporization also increases the vapor load on the columns, causing operating fluctuations in the atmospheric and vacuum unit and, in severe cases, even column flooding accidents.
3. Accelerated equipment corrosion. Inorganic salts in crude oil (sodium chloride, calcium chloride, magnesium chloride) hydrolyze at high temperature to form corrosive hydrogen chloride, which attacks the overhead systems of the atmospheric and vacuum columns. Research shows that magnesium chloride begins to hydrolyze at about 120 °C, and in the presence of naphthenic acid, sodium chloride also hydrolyzes at about 300 °C. These reactions take place both in aqueous solution and with the crystal water of the salts themselves. Dry hydrogen chloride is not corrosive, but once it meets water it forms hydrochloric acid, which strongly corrodes the overhead systems of the primary (flash) column, the atmospheric column and the vacuum column.
4. Damaged catalyst activity in downstream units. Metal impurities such as nickel, sodium and iron in crude oil become enriched in residual oil after atmospheric and vacuum distillation. As these metals enter downstream processing units with the residual oil, they seriously impair catalyst performance: sodium destroys the molecular-sieve (zeolite) structure of FCC catalysts; nickel deposited on catalyst surfaces promotes non-selective cracking reactions, increasing hydrogen and coke yields; and iron salts can raise the pressure drop across hydrotreating catalyst beds.
With process advances, explicit limits on crude salt content have now been established. For corrosion control, the salt content of desalted crude is required to be below 5 mg/L; to safeguard catalyst stability in secondary processing, deep desalting must bring it below 3 mg/L.
