Causes and analysis of catalyst poisoning in the refining process

Thread Content

During the petroleum refining process, trace impurities present in the reaction feedstock can significantly reduce or even eliminate the catalyst’s activity and selectivity, a phenomenon commonly known as catalyst poisoning. The essence of catalyst poisoning is a chemical reaction between trace impurities and the active centers of the catalyst, resulting in the formation of inactive species. Adsorption complexes are formed in gas-solid multiphase catalytic reactions. One type is when the toxin has a weak effect on the active component, and the activity can be restored using simple methods; this is known as reversible poisoning or temporary poisoning. The other category is irreversible poisoning, for which it is not possible to restore activity using simple methods. Crude feedstocks contain many heavy metals and other pollutants such as nickel, vanadium, iron, silicon, arsenic, phosphorus, calcium, and sodium. In the processed crude oil, these pollutants are present in concentrations of only ppm levels, or even ppb levels, yet they have a significant impact, as they deposit irreversibly on the active catalysts through various mechanisms, resulting in permanent deactivation that cannot be reversed even through regeneration. Heavy distillates containing high levels of metal contaminants are typically used as feedstocks for catalytic cracking, hydrocracking, and lubricant hydrogenation units; therefore, they represent an important issue for the long-term operation of these facilities. Vanadium and nickel are mainly contained in the asphaltenes of crude oil. Generally, these metal species are concentrated in residue, but some also exist in heavy distillates above 350°C. Feed oils containing vanadium, nickel, or iron entering the vacuum gas oil hydroprocessing unit can severely affect the operating cycle of the unit. Because the relatively high space velocity of these units (compared to residue hydroprocessing) may cause metals to enter the highly active main catalyst bed. Iron mainly deposits on the surface of the catalyst, while nickel and vanadium primarily deposit within the pore structure of the catalyst. To address this issue, it is necessary to install a highly active demetallization catalyst with strong metal capture capabilities above the main catalyst bed. Catalysts require a high surface area to volume ratio and large average pore sizes to effectively remove metals. Arsenic is a veritable catalyst poison, as it causes reactions at the catalytic active centers (such as nickel and cobalt in catalysts) to turn into NiAs or CoAS. The poisoned active centers cannot be revived upon regeneration, and even a small amount of them on the catalyst can affect its activity. Silicon is occasionally found in distillate oils, coming from defoamers used in refinery delayed coking units and chemicals used in oil transportation and tertiary oil recovery. The surface reaction between silicon and the catalyst produces silica gel, which hinders the utilization of the catalyst’s active centers and thus deactivates the catalyst. Silicon enters the pore structure of the catalyst, and the deactivation of the catalyst accelerates as its amount increases. Phosphorus is rarely found in common crude oils, but it is often present in higher amounts in certain type of crude oils, especially those that are renewable sources. Furthermore, phosphorus containing preservative additives was found in the diesel and vacuum gas oil fractions. Phosphides are decomposed during hydrogenation, and phosphates react with the alumina carrier to form highly stable aluminum phosphate. The accumulated phosphates reduce the accessibility of the hydroprocessing catalyst, thereby lowering its activity. Phosphides usually originate from the injected corrosion inhibitors, such as thiophosphates, thiophosphites, and tributyl phosphate. Controlling organic phosphides when feeding vacuum gas oil into the hydrogenation pretreatment unit for catalytic feedstock or the pretreatment reactor of a hydrocracking unit is a major challenge. Phosphorus causes conventional catalysts to become inactive rapidly, **shortening their operational life. Sodium is occasionally found in crude oils subjected to catalytic cracking and hydrocracking pretreatment, usually as a result of inadequate desalting of the crude oil. Such inorganic sodium does not easily penetrate into the pores of the catalyst; it usually deposits on the outer surface of the catalyst, forming a solid layer between the catalyst particles, which reduces the catalyst’s activity and increases the pressure drop. Inorganic iron is a common pollutant; rust results from the corrosion of equipment upstream, and it can consist of both larger flakes as well as very small rust particles from oil tanks. Large particles of rust can easily get trapped in filters, receivers, and catalytic materials with large pores. However, 5–10 micron inorganic iron particles are difficult to control, as they can pass through the feed oil filter and enter the main catalyst bed without a suitable bed grading system. Inorganic iron also preferentially deposits on the outer surface of the catalyst, unless a demetallized catalyst with large pores is used as a protective catalyst, which will ultimately lead to an increase in pressure drop. Calcium, zinc, and magnesium are all foreign metals that have also been found in some devices. These metals may originate from different additives, and are occasionally found in the porphyrin structure (asphaltenes) of crude oil. These pollutants are common in devices that process used lubricating oil into new base oils; they adhere to the surface of the catalysts, hindering the use of the pore structure, and thus are harmful to the catalysts in the main bed. By testing, analyzing, and evaluating the collected waste catalyst samples using modern instruments such as inductively coupled plasma mass spectrometry (ICD-MS), scanning electron microscopy (SEM), electron microprobe analyzer (EMPA), and energy-dispersive spectroscopy (EDS), it is possible to identify differences in contamination caused by various metals or impurities: Source of contaminant, mechanism of contamination, degree of severity. Nickel: Deposits in the form of sulfides, blocking the pores of the catalyst ☆; Vanadium: Deposits in the form of sulfides, blocking the pores of the catalyst ☆; Iron (raw material): Deposits in the form of sulfides, blocking the pores of the catalyst ☆; Arsenic: Enters the active sites to form arsenides, thereby truly poisoning the catalyst ☆☆☆; Iron (during processing): The corrosion process reduces the voids in the catalyst, increasing the pressure drop across the catalyst bed ☆; Silicon additives: Are absorbed on the surface of the catalyst and react with phosphorus ☆☆; Phosphorus additives: Coat the outer surface of the catalyst ☆☆☆; Sodium salts: Deposit on the outer surface of the catalyst, forming a solid layer between the catalyst particles, which affects the catalyst’s activity and increases the pressure drop ☆☆. In summary, the demetallization capability of hydrogenation units depends on factors such as pressure, temperature, residence time (liquid space velocity), the catalyst itself, and the quality of the feed oil. Demetallization is a catalytic reaction, and therefore depends to a large extent on the activity of the catalyst and the operating temperature of the reactor. The metal capacity of hydrogenation catalysts is primarily determined by the porosity of the catalyst. Using the appropriate hydrogenation demetallization agent ensures that the hydrogenation unit has a catalyst bed with a high metal capacity. During the operation of the device, the actual metal capture efficiency of the catalyst is determined by the sum of all the factors mentioned above.