Crude oil decalcification technology

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1 Introduction Besides hydrocarbons, crude oil also contains impurity elements such as oxygen, sulfur, nitrogen, and calcium, magnesium, sodium, nickel, vanadium, etc. Although their concentrations are low, they have a significant impact on the petroleum processing process and the quality of the products. Among them, metal elements such as calcium, magnesium, sodium, nickel, and vanadium exist in crude oil mainly in the form of salts. If these metal compounds in crude oil cannot be removed during the desalination process, they can cause problems in the crude oil processing stage; for example, they deposit on the walls of heat exchangers and heating furnaces, reducing heat transfer efficiency, accelerating corrosion in the condensation systems at the top of atmospheric and vacuum distillation units, and having an adverse effect on the electrodesalination process of crude oil. Furthermore, due to the high boiling points of metal compounds in crude oil, after vacuum distillation, the vast majority of these metal compounds end up in the vacuum residue, thereby increasing the concentration of these salts further. This has a severe impact on the processing processes that use vacuum residue as a raw material; it contaminates catalytic cracking and hydrocracking catalysts, leading to a reduction in their activity, a decrease in their surface area, pore volume, and specific surface area, catalyst caking, and catalyst deactivation as a result of poisoning. All of this results in lower quality petroleum coke. In the petroleum processing process, the most significant hazard posed by calcium in crude oil to refining processes is its ability to contaminate catalytic cracking and hydrocracking catalysts. Impact on hydrocracking catalysts. The metal calcium impurities present in the hydrogenation feed react with hydrogen sulfide to form sulfides that deposit on the catalyst bed. These deposits not only block the active sites of the catalyst, causing it to become inactive or to develop fouling, but they also reduce the surface area, pore volume, and specific surface area. Worse still, these deposits tend to accumulate easily between the catalyst particles, especially at the top of the fixed-bed layer, blocking the reactor bed and causing a significant increase in the reactor pressure drop. This severely hinders the normal flow of feed oil and recycled hydrogen through the catalyst bed, resulting in the inability of the plant to operate properly. Impact on catalytic cracking. Calcium causes various adverse effects on all types of cracking catalysts, such as reduced activity, decreased accessibility to active sites, diminished specific surface area, and catalyst agglomeration. Different types of cracking catalysts are affected to varying degrees; 1000 μg/g of calcium in the balance agent reduces its activity by 0.3% to 2%. The extent of this reduction depends on the regeneration temperature – the higher the regeneration temperature, the more severe the impact of calcium on the catalyst. Since the vast majority of calcium in crude oil is concentrated in atmospheric and vacuum residue, and the proportion of residue used in catalytic cracking is increasing increasingly, the harm caused by calcium to catalytic cracking catalysts is becoming more and more apparent. 2 Summary of crude oil decalcification technologies: In recent years, as the calcium content in crude oil has increased and its harmful effects have become better understood, there has been significant interest in the development of decalcification technologies. Numerous new methods have been developed, including chelation precipitation decalcification, hydrocatalytic decalcification, membrane separation decalcification, CO2-based decalcification, resin-based decalcification, biological decalcification, hydrogen peroxide-based decalcification, and filtration-based decalcification. 2.1 Chelation precipitation method for decalcification
In the chelation precipitation method for decalcification, under conditions where water is injected into crude oil, a decalcifying agent is thoroughly mixed with the crude oil. This allows the water-soluble decalcifying agent to come into full contact and react with calcium at the oil-water interface, resulting in the formation of precipitates, chelates, etc. These products either dissolve in water or disperse into the aqueous phase; subsequently, under the influence of a high-voltage electric field and demulsifiers, they are discharged along with the desalting wastewater, thereby achieving the purpose of decalcification. This technology is easy and flexible to use, and it has been widely developed and applied. Decalcification by chelation precipitation is, abroad, a series of methods developed by Chevron Research Company for removing metallic calcium from crude oil using chelating agents (or precipitants). The chelating agents they have developed fall into two main categories: inorganic acids and their salts, and organic acids and their salts. Such as carbonic acid, sulfuric acid, monocarboxylic acids, dicarboxylic acids, aminocarboxylic acids, hydroxycarboxylic acids, and their salts. In this field, research in our country mainly utilizes inorganic phosphorus-containing compounds and organic phosphonic acids and their salts as chelating agents to remove calcium from crude oil. The process involves mixing the crude oil or heavy oil from which metal removal is desired with an aqueous solution of the selected chelating agent, and adjusting the pH to a value greater than 2, preferably between 5 and 9; this causes the calcium in the oil to be bound together to form water-soluble ionic chelates. Separating the aqueous phase from the oily phase can remove calcium from crude oil. The decalcifier is the core technology of this method, and its active ingredients mainly include three types: strong acids, chelating agents, and precipitants, which will be discussed in detail in the next section. 2.2 Hydrogenation catalytic decalcification: Chevron developed a staged catalytic process in the 1980s that enables the removal of calcium and sodium from hydrocarbon feedstocks containing 1 μg/g or more of calcium and 1 μg/g or more of sodium ； A hydrogenation decalcification catalyst system includes at least two different catalyst sections, and preferably more than two; each section may have one or multiple layers of catalyst. The raw materials suitable for this method include crude oil distillate, atmospheric and vacuum residue, and synthetic crude oil. The catalyst systems they have successfully developed include the Ni/SiO2 system and the K/Al2O3 system containing 2.8 wt% potassium, among others. In 1982, Chevron Corporation developed another highly efficient hydrogenation catalyst, ICR122; there are a total of 7 types of this catalyst. In 1990, a Type V catalyst was successfully developed, with the Type II ICR122 catalyst being primarily used to remove oil-soluble elements such as calcium, iron, and nickel. The ICR122 catalyst is used in the deasphalting oil hydrogenation unit at the Chimney Point refinery in the United States; its high effectiveness in removing calcium and iron extends the operating cycle and doubles the metal capacity of the catalyst. The new catalyst recently developed by Chevron, possibly named the Type VIII catalyst, has a better decalcification effect than the ICR122 and Type VII catalysts. 2.3 Membrane separation for decalcification: The membrane separation method is a technique developed by Canadian Patents and Developments Ltd. for removing high-boiling fractions and inorganic substances (including calcium) from hydrocarbon feedstocks. This method involves separating hydrocarbon liquids with a viscosity of less than 0.6 Pa·s by forcing them to pass transversely through the high-pressure side of a microporous membrane under a pressure difference of 0.1 to 10 MPa. The pore structure of the microporous membrane allows molecules with lower molecular weights to pass through, while larger molecules and impurities remain on one side of the membrane. These residues are rich in inorganic substances such as metals like calcium, nickel, vanadium, iron, and magnesium; therefore, membrane separation can effectively remove metals. Membrane separation experiments using Venezuelan crude oil showed that the metal removal rate was generally above 30%, with some metals achieving a removal rate of around 60%. It should be noted in particular that membrane separation is very effective for calcium removal. Although membrane separation can remove various metals, the removal efficiency for calcium is low (up to 60% at most). Moreover, for crude oils with high viscosity, membrane separation requires a large driving force to maintain permeation; in addition, selecting appropriate membrane materials and preparing the membranes presents many challenges, making industrial application difficult. Currently, there are no reports of industrial applications of this method. 2.4 CO2 decalcification: The principle of CO2 decalcification is that when CO2 gas is introduced into crude oil, it reacts chemically with Ca2+ at the oil-water interface to form CaCO3 precipitates, and calcium is removed through centrifugation. 　　Some research has been conducted on carbon dioxide gas as a decalcifying agent. In the experiment, crude oil and distilled water were placed in an autoclave, with carbon dioxide introduced into the autoclave. Under the conditions of a water injection rate of 10%, a pressure of 0.4 MPa, a temperature of 80°C, and an agitator speed of 300 r/min, the decalcification rate of Caofeidian crude oil reached 60.7%, while that of Nantong crude oil and Dagang crude oil reached 55.7% and 50.9% respectively. Experiments also showed that, due to the formation of naphthenic acids as a result of the reaction between the hydrogen ions produced during carbonation with carbon dioxide and the ionized naphthenate ions, the acid value of crude oil increases to some extent, which in turn has an impact on equipment corrosion. Although CO2 decalcification can achieve a certain decalcification rate, the calcium content after decalcification remains quite high. CO2 decalcification must be carried out slowly using dry ice in an autoclave; this slows down the processing speed of crude oil and reduces the amount of crude oil that can be processed, while also increasing additional costs for equipment and maintenance. Therefore, using CO2 for decalcification is neither economical nor reduces process complexity, and it is not suitable for industrial application at present. 2.5 Resin decalcification: Resin decalcification involves bringing crude oil into contact with a resin containing carboxyl, sulfonic, or phosphoric groups in order to remove calcium from the oil. These groups have a strong affinity for calcium, allowing them to transfer calcium from crude oil into the resin. After the reaction, the crude oil and the resin can be separated, resulting in decalcified crude oil. The used resin can be regenerated by treatment with acid. More suitable resins include styrene-divinylbenzene copolymer, methacrylic acid-divinylbenzene copolymer, polyacrylic acid, etc. The temperature for the decalcification reaction should be as high as possible without causing the resin to decompose, so as to reduce the viscosity of the crude oil and facilitate handling; generally, a suitable temperature ranges from 50 to 150°C. However, in resin decalcification technology, the separation of crude oil from resin is far less convenient and economical than the separation of saltwater from crude oil. Moreover, resin separation requires additional acid treatment equipment for resin regeneration, which increases the costs of crude oil processing. Furthermore, the acid value of the crude oil increases after decalcification; therefore, the equipment needs to have excellent corrosion resistance ； The decalcification time is long, which makes it less effective for decalcifying crude oil with high calcium content; the calcium level in the crude oil remains high after decalcification. 2.6 Biological Decalcification The American company Energy Biosystems uses biological catalysts to remove metals from crude oil, such as Ni, V, Ca, and Zn, primarily for removing metals that are tightly bound to cyclic porphyrin structures. They already possess bacterial enzymes that can be used to produce such biocatalysts. Biocatalysts are mixed with crude oil under certain conditions (an emulsified aqueous solution at a temperature of 5–40°C and a pH of 5–9). The metal ions released as a result of the reaction are water-soluble, making them easy to remove from the crude oil; subsequent extraction is carried out using conventional metal recovery techniques. The operating conditions for biological demetallization are relatively mild, but the operational costs are high. 2.7 Decalcification using hydrogen peroxide: Research conducted by the Department of Chemical Engineering at Beijing Institute of Petrochemical Technology has shown that washing crude oil containing calcium with an acidic hydrogen peroxide solution can reduce its calcium content, bringing it below 10 μg/g, thereby meeting the requirements of subsequent crude oil processing steps. This is probably because the acidic hydrogen peroxide oxidation destroys the aromatic carbon groups and aromatic carbon-carbon double bonds in asphaltenes, causing the asphaltenes to lose their surface activity. As a result, the W/O microemulsion is disrupted, leading to the separation of calcium that had been solubilized in the microemulsion. However, there are no reports yet on the application of this method. 2.8 Filtration for decalcification: Professor Yen from Southern California Institute of Technology has recently invented a filter that can effectively remove impurities such as sulfur, nitrogen, and metals from crude oil. The main objective of current research is sulfur removal; in laboratory tests, this filter manages to eliminate 60% of the sulfur after just one filtration step. The technical core of this filter lies in its filtering material, which is formed by spraying two types of metals in a molten state at 538°C from a nozzle to create fine metal powder. This metal powder is then bonded to an inert matrix such as carbon fiber, and after being placed inside a glass tube, a filter with a very large surface area is obtained. The principle behind the filtration process is that the grains of this metal powder contain numerous defects; when the size of these defects is equal to that of sulfur atoms, nitrogen atoms, or metal atoms, selective adsorption of them occurs. After filtration is complete, applying a voltage to the filter element allows the impurities trapped in it to be removed; the filter element can be reused multiple times, which results in lower processing costs for this process. Currently, Professor Yen is able to control the size of these defects through chemical methods, allowing for the selective removal of impurities such as S, N, and metals from crude oil. In summary, the current methods for decalcification mainly include chelate precipitation decalcification, hydrogenation catalytic decalcification, resin-based decalcification, CO2 decalcification, membrane separation decalcification, biological decalcification, as well as hydrogen peroxide and filtration decalcification. Hydrogenation-catalyzed decalcification is an effective method for protecting highly active hydrodesulfurization and hydrodenitration catalysts. This approach is suitable for treating materials with low calcium content, as it enables the calcium level in the processed material to be reduced to very low levels; however, it is not economical for dealing with materials that have high calcium content (such as those with a calcium content of over ten several μg/g). The operation process of the resin and CO2 decalcification methods is relatively complex; it can achieve a certain degree of calcium removal from crude oils with high calcium content. No reports exist regarding the decalcification effect on crude oils with low calcium content, and the acidity of the crude oil increases after decalcification. The conditions for biological decalcification are mild, but the technology is not yet mature and the costs are high. The new filtration technology in Southern California operates under mild conditions and has low operational costs; it has the potential to become an advanced process for desulfurization, denitrification, and demetallization. Further development and scale-up are needed, as none of these processes have yet been put into industrial use. Although the chelate precipitation decalcification method has some drawbacks, it is easy and flexible to use. Given current technological conditions, it remains an effective decalcification method. The chelate precipitant added in this process is referred to as a decalcifying agent. 3 Looking ahead, the deterioration and increasing heaviness of crude oil worldwide are leading to higher calcium contents in crude oil. The rise in calcium levels in crude oil limits its further processing, which in turn drives the development of decalcification technologies. Although there are many decalcification techniques, few of them are currently industrialized; most remain at the laboratory stage. At present, since chelation decalcification, chemical precipitation decalcification, and chemical acid treatment decalcification do not require any changes to the existing desalination processes, the decalcifiers are simply added as additives together with demulsifiers into the electrodialysis units. This approach requires less investment and yields quick results, which is why it is widely used in refineries. However, these methods generally suffer from poor compatibility with crude oil, a high ratio of agents to oil, and consequently high decalcification costs. Other methods, such as biological decalcification, have a mild temperature and meet environmental requirements; however, this method still needs further improvement and is not yet ready for industrial application ； Carbon dioxide decalcification leads to an increase in the acid value of the crude oil after decalcification, thereby exacerbating equipment corrosion. Developing hydrogen decalcification technology is the most effective way to completely solve the problem of crude oil decalcification. At present, due to the high pressures involved and the stringent requirements for equipment, this technology results in high processing costs, posing difficulties for large-scale industrial application. However, it is believed that with the advancement of catalyst technology and the development of new, inexpensive, and efficient catalysts, this issue will surely be resolved.