What is a special vanadium scavenger for fluidized bed catalytic cracking catalyst?

In modern refining industry, fluidized bed catalytic cracking (FCC) unit is a crucial core unit, responsible for efficiently converting heavy oil (such as conventional residue oil and vacuum gas oil) into high-value light products such as gasoline, diesel, liquefied gas, etc. However, with the increasing heavy and inferior quality of crude oil resources, the content of heavy metal impurities, especially vanadium (V), in raw oil has significantly increased, posing a serious threat to the activity and selectivity of FCC catalysts. In this context, special vanadium scavenger for fluidized bed catalytic cracking catalysts (hereinafter referred to as "specialized special vanadium scavenger for fluidized bed catalytic cracking catalysts") have emerged as a key functional additive and play an irreplaceable role. It is not only the "guardian angel" for protecting FCC catalysts, but also the technical cornerstone for ensuring the long-term stable, efficient, and economical operation of the entire device.

1. The root cause of the problem: the toxic mechanism of vanadium on FCC catalysts

To understand the importance of specialized special vanadium scavenger for fluidized bed catalytic cracking catalysts, it is first necessary to clarify the mechanism of vanadium's damage to FCC catalysts. The core active component of FCC catalysts is molecular sieves, especially Y-type molecular sieves, which have a regular porous structure and strong acidic centers, and are the "main battlefield" for cracking reactions. The toxic effects of vanadium are multifaceted and deadly:
1. Destruction of molecular sieve crystal structure (the most lethal destruction): In the high temperature (especially above 700 ℃ in the regenerator) and water vapor environment of the FCC reaction regeneration system, vanadium compounds (mainly in the form of porphyrin vanadium) in the raw oil deposit on the catalyst in the reactor, and then are oxidized to vanadium pentoxide (V ₂ O ₅) in the regenerator. V ₂ O ₅ has high mobility at high temperatures and can diffuse into the crystal interior of molecular sieves. More seriously, V ₂ O ₅ will react with the aluminum in the molecular sieve skeleton to form aluminum vanadate (AlVO ₄). This reaction directly destroys the silicon aluminum framework structure of the molecular sieve, resulting in a decrease in its crystallinity, a sharp reduction in specific surface area and pore volume, and permanent loss of cracking activity.
2. Neutralization of acidic centers: Vanadium oxide itself has Lewis acidity, but its acidic characteristics are different from the Br ø nster acidic centers of molecular sieves, which cannot effectively catalyze cracking reactions. On the contrary, they will cover or neutralize the effective acidic centers of the molecular sieve, leading to a decrease in the selectivity of the catalyst and a decrease in the yield of expected products such as gasoline.
3. Catalytic dehydrogenation reaction, exacerbating carbon deposition: Some valence states of vanadium (such as V ⁴⁺) have dehydrogenation activity and promote non selective dehydrogenation reactions, resulting in a significant increase in hydrogen yield (H ₂ yield). This not only increases the load on the gas separation device, posing safety hazards, but also further reduces the efficiency of the catalyst due to the increase in coke production, forming a vicious cycle.

Therefore, the direct consequences of vanadium pollution are: rapid decline in overall catalyst activity, decrease in gasoline yield, increase in dry gas and coke yield, decrease in plant processing capacity, ultimately forcing refineries to frequently replace fresh catalysts and significantly increase operating costs.

2. Definition and core functions of specialized special vanadium scavenger for fluidized bed catalytic cracking catalysts

Vanadium capture agent is a solid additive designed to specifically capture and fix vanadium impurities in raw oil. It usually exists in the form of microspheres and has similar physical properties (such as density and particle size distribution) to the main FCC catalyst, allowing for uniform mixing and co circulation with the main catalyst in a fluidized bed.
Its core functions can be summarized as "interception, fixation, and passivation":
1. Interception: In the reactor, the special vanadium scavenger for fluidized bed catalytic cracking catalyst competes with the main catalyst to adsorb and capture the vanadium species released from the raw oil, which is equivalent to setting a "priority adsorption target" for vanadium, reducing the opportunity for vanadium to migrate to the main catalyst molecular sieve.
2. Fixed: Vanadium capture agents, through their special chemical composition, can undergo strong chemical reactions with captured vanadium to generate highly stable, non-volatile, and non migratory vanadate compounds under FCC regeneration conditions.

3. Passivation: By immobilizing vanadium, the special vanadium scavenger for fluidized bed catalytic cracking catalyst effectively prevents vanadium from existing and migrating in the destructive form of V ₂ O ₅, thereby passivating its ability to damage the molecular sieve structure and catalyze dehydrogenation reactions.

3. The working principle and key components of specialized special vanadium scavenger for fluidized bed catalytic cracking catalysts

The effectiveness of specialized special vanadium scavenger for fluidized bed catalytic cracking catalysts comes from their carefully designed chemical composition. Its principle of action is mainly based on the high-temperature solid-state reaction between alkaline earth metals or rare earth metals and vanadium oxides.
At present, the most mainstream and effective special vanadium scavenger for fluidized bed catalytic cracking catalysts are based on alkaline earth metal oxide systems, especially calcium based compounds such as calcium oxide (CaO) and calcium hydroxide (Ca (OH) ₂).
Calcium based vanadium capture mechanism: In the oxygen rich high-temperature environment of the regenerator, the active calcium component (such as CaO) in the special vanadium scavenger for fluidized bed catalytic cracking catalyst will undergo irreversible chemical reactions with the toxic substance V ₂ O ₅, generating stable calcium vanadate (Ca ∝ (VO ₄) ₂, also in the form of Ca (VO ∝) ₂, etc.).
3CaO+V₂O₅→Ca₃(VO₄)₂
The Gibbs free energy of this reaction is very negative, indicating a strong reaction trend. The generated calcium vanadate has a very high melting point (over 1400 ℃) and is very stable at the FCC regeneration temperature. It neither decomposes nor evaporates, thus firmly "locking" vanadium inside the particles of the special vanadium scavenger for fluidized bed catalytic cracking catalyst, preventing it from migrating to the main catalyst and causing harm.
In addition to the core vanadium capture component, a high-performance dedicated special vanadium scavenger for fluidized bed catalytic cracking catalyst typically includes the following key elements:
1. High specific surface area and suitable pore structure: In order to fully contact with vanadium species, special vanadium scavenger for fluidized bed catalytic cracking catalysts need to have a developed multi-level pore structure (macropores, mesopores, micropores) to ensure that vanadium can quickly diffuse into the interior of particles and react with active components.
2. Balance between reactivity and stability: The active component (such as CaO) needs to have sufficient reactivity, but at the same time, its precursor (such as Ca (OH) ₂) needs to maintain good structural stability during preparation and hydrothermal environment to avoid rapid sintering deactivation.

3. Good fluidization and anti-wear performance: Its physical properties must be matched with the main catalyst to ensure that excessive fine powder will not be generated due to wear and tear in severe fluidization environments, causing running damage.

4. Technical advantages and usage strategies of specialized special vanadium scavenger for fluidized bed catalytic cracking catalysts

The use of specialized special vanadium scavenger for fluidized bed catalytic cracking catalysts has significant technical and economic advantages compared to traditional "passivators" or simply increasing the amount of fresh catalyst supplementation
1. Strong targeting and high efficiency: Specifically targeting vanadium without interfering with the cracking function and product distribution of the main catalyst.
2. Protecting the main catalyst and extending its lifespan: By sacrificing oneself to protect the more valuable main catalyst, the replacement rate of the main catalyst caused by vanadium poisoning is significantly reduced, resulting in significant economic benefits.
3. Improve product distribution: By inhibiting the dehydrogenation activity of vanadium, the dry gas (especially hydrogen) yield is effectively reduced, and the liquid yield (gasoline and light cycle oil) is increased.
4. Flexible operation: Refineries can flexibly adjust the addition ratio of special vanadium scavenger for fluidized bed catalytic cracking catalysts based on fluctuations in vanadium content in the feed, achieving precise control and cost optimization.

In terms of usage strategy, special vanadium scavenger for fluidized bed catalytic cracking catalysts are usually continuously added to the catalyst storage tank of the FCC unit through an automatic feeding system, and supplemented into the system together with fresh main catalyst. The amount of vanadium added is usually calculated based on the "agent consumption ratio" (i.e. the proportion of special vanadium scavenger for fluidized bed catalytic cracking catalyst to the total system inventory) or the vanadium content in the feed, with the goal of maintaining the total vanadium content on the catalyst below a safe threshold.

In summary, special vanadium scavenger for fluidized bed catalytic cracking catalysts are a key innovation in modern refining technology to address the challenge of low-quality heavy crude oil. It is based on a profound understanding of the mechanism of vanadium toxicity, and through sophisticated chemical design, achieves efficient, selective fixation and passivation of vanadium impurities. Although it is not the "protagonist" directly involved in the cracking reaction, it is the "gold guard" that ensures the long-term stable performance of the "protagonist" - FCC main catalyst. With the further development of global crude oil resources towards heavy and high metallicity, the research and application of high-performance, low-cost, and environmentally friendly specialized special vanadium scavenger for fluidized bed catalytic cracking catalysts will continue to play an increasingly important role in improving refining efficiency, achieving long-term operation of facilities, and efficient utilization of resources.