Catalyst heavy metal pollution

Catalyst poisoning in catalytic cracking has a fatal impact on the catalytic reaction, as there is no other solution other than replacing the catalyst promptly once poisoning occurs. The most direct way to determine whether a catalyst is poisoned is by checking the heavy metal content in the regenerant; if this content exceeds the standard, it indicates that the catalyst is poisoned and prompt action must be taken. However, in some refineries such conditions are not available, so other methods must be used for judgment: 1. Judge based on the color of the regenerant; for example, if the catalyst turns yellow, it indicates iron poisoning, while if it becomes shiny, it indicates calcium poisoning ； 2. Based on the contents of hydrogen and methane in the dry gas, if these two components have similar concentrations or if the concentration of hydrogen exceeds that of methane, caution is warranted ； 3. Judgment is made based on the temperature difference between the dense and lean phases in the regenerator; if there is no temperature difference between these phases for an extended period of time or if the order of the temperatures reverses, poisoning is likely to have occurred ； 4. Judging from the yields of oil slurry and dry gas, if the yields of oil slurry and dry gas remain high while the yield of light oil is low, it is possible ； 5. Additionally, when abnormal conditions occur during the operation of the equipment and no cause can be identified, it is also worth considering whether catalyst poisoning has taken place. Of course, there are other methods as well; I hope my colleagues can introduce them for everyone’s reference.

Alkaline nitrogen poisoning: red color; V poisoning: dark color

The effect of iron poisoning on FCC catalysts is similar to that of nickel, but at lower levels of poisoning, the impact on the catalysts is mild; it is only at higher levels of poisoning (＞ 5000 ug/g) that the damage to the catalysts becomes severe. The toxic effects of iron manifest in two ways: firstly, iron poisoning also leads to a decrease in the acidity of the catalysts and a reduction in their activity； Secondly, Fe itself also has a dehydrogenation effect, which reduces the yield of light oil and increases the yields of coke and hydrogen. The design principle of iron passivators is similar to that of nickel passivators; a substance that can react with Fe is added to form large-grained solid solutions with iron, thereby reducing the activity of iron. The phenomena associated with high iron content generally include: an increase in the regenerated dilute-phase density, low slurry density, a decrease in light yield; in severe cases, the catalyst turns yellow. The most obvious sign of iron poisoning is that the catalyst turns yellow or even red. If the catalyst appears reflective and shiny, it’s a sign of calcium poisoning. A soil-yellow color indicates iron poisoning. Poisoning caused by other metals cannot be detected by color. After heavy metal poisoning, the selectivity of the catalyst decreases, resulting in increased coke and gas production, reduced liquid yield, and increased unsaturation of the products. Additionally, the yields of C3 and C4 in the gas decrease. Heavy metals in the raw materials can cause varying degrees of poisoning to the system catalysts, thereby reducing their selectivity. Generally speaking, in cases of severe calcium poisoning, the catalyst appears shiny to the naked eye; upon shutdown, glassy aggregates of the catalyst can be found at the bottom of the regenerator. In more severe instances, these aggregates can even block the sulfidation lines! If iron poisoning is severe, the catalyst takes an earthy yellow color; the effects of iron and nickel poisoning on the catalyst are essentially the same, leading to dehydration and a significant increase in the hydrogen-to-methane ratio in the dry gas ； If vanadium and sodium poisoning is severe, the resulting sodium vanadate directly destroys the catalyst framework, causing the catalyst to be damaged! Therefore, no matter which heavy metal has a high level, it should be given great attention! ! System catalyst poisoning directly affects the product distribution and yield, leading to increased yields of coke and dry gas! Therefore, to carry out catalytic cracking, it is necessary to first prepare the feedstock properly and ensure quality from the source; only then can yield be considered! ! ! ! ! ! ! Primarily affected by iron poisoning. Current measures mainly involve enhancing water separation from crude oil tanks and adding desalting agents in electrodesalination processes to keep the total iron level below 15 ppm. Even so, the catalyst is occasionally poisoned, even though the iron ion concentration is not very high. I would like to ask all the experts: what is the reason? Sulfur exists on the catalyst in two forms: reversibly adsorbed sulfur and irreversibly adsorbed sulfur. The irreversibly adsorbed sulfur is primarily adsorbed on the metal surface and on certain strong acid sites; the amount of adsorption is independent of the H2S partial pressure in the gas stream during sulfidation, and depends only on the active metal components. Reversible adsorption mainly occurs on the carrier, and the amount of adsorption is related to the number of aluminum ions (Al3+) on the surface area of the carrier. The presence of chloride ions inhibits the adsorption of this portion of sulfur. Irreversibly adsorbed sulfur can suppress the hydrogen activity of the catalyst, thereby improving its selectivity. Under certain conditions, sulfur will be the most persistent poison for modern binary (poly)metal reformation catalysts. However, under certain conditions, sulfur can also be beneficial for reforming catalysts. For example, when the reforming unit is brought online, the catalyst needs to be pre-sulfurized in order to suppress the excessive number of active centers on the catalyst at the beginning of its operation, prevent excessive carbon deposition, and shorten the operating cycle of the unit! I. Hazards caused by the process: When the reforming catalyst becomes sulfur-poisoned, its platinum activity decreases, leading to a reduction in dehydrogenation and dehydrocyclization reactions (metal-catalyzed functions), while hydrocracking reactions (acid-catalyzed functions) increase. In such cases, the following phenomena generally occur: (1) A decrease in hydrogen production ； (2) Decrease in the purity of recycle hydrogen ； (3) Increased C3 and C4 yields, enhanced cracking performance ； (4) The total temperature drop of the reaction (∑△T) decreases ； (5) Decrease in the yield of C5+ liquid products ； (6) Increasing the operation intensity does not improve catalyst activity ； (7) The catalyst coking rate increases (stability decreases). The trace elements that may be present in distillate oil hydrogenation units include heavy metals such as Fe, Ni, V, Hg, Pb, as well as soluble organometallic compounds like Ca and As. In addition, there are additives used during processing in upstream units, such as defoamers like Si, preservatives like P, desalting agents like Na, and metal contaminants from oil storage and transportation such as Fe. All of these can deposit on the surface and within the pores of the catalysts, ultimately leading to blockages of the catalyst pores and poisoning of the active centers, thereby rendering the catalysts inactive ； Additionally, chlorine contained in the feedstock oil and make-up hydrogen can also have a significant impact on the operation and safe running of the hydrogenation unit. 1. Na is primarily introduced through the addition of alkali (NaOH) in atmospheric and vacuum distillation units. Na can cause rapid deactivation of catalysts; when crude oil contains 1%–3% Na, catalyst activity decreases by 50%. Na can penetrate into the microporous structure of catalysts. In catalysts with cracking functionality, Na neutralizes acidic sites, thereby reducing their cracking ability. Additionally, it also decreases the mechanical strength of the catalysts. The most serious consequence of Na poisoning is that, during catalyst regeneration, Na can promote sintering at high temperatures and penetrate into the catalyst to cause sintering, thereby reducing the catalyst’s surface area and leading to the aggregation of active metals. Generally, if the Na content on the catalyst exceeds 0.25%, it cannot be regenerated ； Furthermore. Na contamination can also cause an increase in system pressure drop. 2. As is present in certain crude oils; for example, the Daqing crude oil in China has a high arsenic content, reaching 2800 ppb, and chemicals used in oil field drilling can also introduce it. As exists in the form of organic compounds and is primarily found in the light fractions such as steam, coal tar, and diesel from atmospheric and vacuum distillation units. Its toxic mechanism lies in the combination of As with the metals present in metal sulfides. The catalyst contains 0.1% As, resulting in a 50% loss of activity. The maximum allowable As content in the catalyst is 500–1000 μg/g. Arsenic poisoning of hydrogenation catalysts is permanent poisoning. Before replacing the catalyst contaminated with arsenic, the hydrogenation reactor, heating furnace tubes, high-pressure heat exchangers, etc., must undergo arsenic removal treatment to prevent the newly installed catalyst from becoming contaminated again. 3. Si comes from defoamers or anti-coking agents, and is commonly present in light fractions such as gasoline and diesel produced by delayed coking and visbreaking processes. Si poisoning occurs mainly because it deposits evenly within the catalyst’s pores during the hydrogenation process, blocking these pores and reducing the catalyst’s specific surface area and pore volume, thereby causing poisoning of the catalyst ； During regeneration, SiO2 combines with MoO3 to form an inactive phase. Therefore, catalyst Si poisoning is a permanent form of poisoning, and the silicon content in the hydrogenation feedstock should be reduced as much as possible. Given the destructive effect of silicon on catalysts, non-silicon antifoamers or anti-coking agents are now widely used in delayed coking units and visbreaking units, reducing the impact of silicon on catalysts to an negligible level. 4. Ni and V are present in the heavier fractions of crude oil, existing in the form of macromolecules (resins and asphalts) in fractions with temperatures above 500°C. During the processing of heavy oil, such as catalytic cracking and coking, most of the macromolecular compounds containing Ni and V turn into coke; however, a small portion of them breaks down into smaller molecules containing heavy metals, such as vanadium porphyrins and nickel porphyrins, which end up in the diesel fuel produced in secondary processing and are then fed into hydrogenation units. The mechanism of Ni and V poisoning is the blockage of the pores in the catalyst. When the catalyst contains 3–4% (Ni+V), its activity is reduced by 50%. 5. Trace elements such as P, Ca, K, and Mg may also be present in the hydroprocessing feed distillate oil. These elements are present in lower amounts in atmospheric residue and vacuum residue, and even less so in straight-run diesel and diesel produced through secondary processing. They can have a negative impact on the activity of catalysts, and they can increase the system pressure drop, thereby raising the operating costs of the facility. However, due to its extremely low content, the effect can be neglected. 6. The iron content in the crude oil is the main cause of bed blockage and an increase in pressure drop. Table 2 shows the relationship between the iron content of the raw materials and the operation cycle of the plant. As can be seen from Table 2, the iron content in the crude oil directly affects the start-up cycle. Table 2 Relationship between iron content in the feedstock and the operation cycle. Units: A, B, C. Iron content in the feedstock in μg/g; average number of operating days: 0.36 for unit A, 1595 for unit B; 2–3.5 for unit C, 258 for unit D; 3.2–9 for unit E, 102 for unit F. There are two main sources of iron in the hydrogenation feedstock: iron originally present in the crude oil, and iron generated during the processing process. Iron in crude oil or distillates may exist in the form of suspended inorganic particles, or as oil-soluble salts and complexes. Most of the existing iron can be removed by deep electrodeionization. Most of the iron in the hydrogenation feedstock is process iron. Especially when the material choice for equipment such as atmospheric and vacuum distillation units is inappropriate, the iron content can increase by several dozen times. The relevant literature provides a detailed examination of the sources of iron in hydrodesulfurization feed oils, and it is concluded that the iron content is directly related to the acid value of the crude oil, particularly to the content of naphthenic acids. As the amount of naphthenic acid increases, the iron content in the hydrogenation feed doubles. Naphthenic acids are present in most crude oils, with typical concentrations ranging from 0.02% to 2.0%. Their molecular weight is around 200–350, and they are mainly found in kerosene, diesel, and heavier fractions. In environments above 230°C, naphthenic acid exhibits strong corrosivity, with the highest level of corrosion occurring at 270–280°C; the corrosivity increases again when the temperature reaches 350°C. Naphthenic acid causes iron corrosion to produce oil-soluble iron naphthenate. Especially when highly active hydrogen sulfide, elemental sulfur, and thiols are present in the crude oil, it can accelerate the corrosion of iron by naphthenic acids. Iron naphthenate readily undergoes hydrogenolysis in the reactor and reacts with hydrogen sulfide to form iron sulfide. This iron sulfide is a non-stoichiometric, polymeric “cluster” that contains Fe–S, Fe–Fe, and S–S bonds. Typically, the number of iron atoms is less than that of sulfur atoms, denoted as Fesx. The attraction between iron sulfide clusters is strong, allowing them to easily aggregate and cover the upper layer of the catalyst. At high temperatures, this iron sulfide can promote the coking reaction of the coking precursor, which in turn accelerates bed blockage and an increase in pressure drop. Experts recommend that hydrogenation feedstocks should meet the following specifications: particles larger than 0.8 μm < 0.2 μg/g; organic and inorganic chloride content < 1.0 μg/g; iron content < 1.0 μg/g; nickel and vanadium content < 1.0 μg/g. The phenomenon in which trace impurities present in the reaction feedstock significantly reduce or even eliminate the activity and selectivity of the catalyst. The essence of poisoning is a certain 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. Another type is irreversible poisoning, for which it is impossible to restore activity using simple methods. To reduce the activity of side reactions, it is sometimes necessary to selectively poison the catalyst. 　　Catalysts can lose their activity during use due to various factors, one of the important ones being poisoning. There are several possible causes of catalyst poisoning: minor impurities present in the raw material, strong adsorption (usually chemical adsorption) on the active sites, or chemical reactions with the active sites that result in the formation of other substances – all of these can poison the active sites. Additionally, such toxic substances may also be present in the reaction products ； During the preparation of catalysts, impurities contained in the support interact with the active components and may also poison the active sites. 　　Toxins come in various forms; for a given catalyst, it is only by considering the reaction it catalyzes that it can be determined what substances are toxic. In other words, toxins affect not only the catalyst itself but also the reaction that this catalyst facilitates. During the period of stable activity, catalysts often experience a significant decline in their activity due to contact with small amounts of impurities; this phenomenon is known as catalyst poisoning. Substances that render a catalyst inactive are called catalyst poisons. If activity can be restored after the toxic factor is removed, it is called temporary poisoning; otherwise, it is called permanent poisoning. 　　Some catalysts are poisoned in certain reactions; some of these poisons are temporary, while others are permanent. For example, in the synthesis of ammonia, iron-based catalysts are used, and water and oxygen act as poisons. When such poisoning occurs, the catalyst can be reactivated by reduction or heating. This type of poisoning is temporary, or reversible poisoning ； Compounds of sulfur or phosphorus are also poisons for this catalyst and this reaction. When poisoning occurs due to them, it becomes very difficult to reactivate the catalyst; this is known as permanent poisoning, or irreversible poisoning. The latter type of poisoning can be prevented. 　　Poisoning not only affects the activity of catalysts, leading to a decrease in their efficiency, but it also impacts their selectivity. Poisoning is a practical problem that often arises when catalysts are used in chemical manufacturing, yet it is not yet fully understood. As for how to prevent poisoning and remove its effects, this must be addressed through practical experience

What level should the heavy metal content be above? It is generally considered that a nickel+vanadium content of over 10,000 ppm is not considered low

Or the total metal content must not exceed 20,000 ppm; generally, the quality of the raw materials used is poor, resulting in high reagent consumption, and a large part of this is due to heavy metals