Catalyst technologies for improved FCC yields
Boron based nickel passivation enables refiners to reduce contaminant hydrogen and coke production and improve FCC yields.
CARL KEELEY, VASILIS KOMVOKIS, ALEXIS SHACKLEFORD, MELISSA CLOUGH, SABEETH SRIKANTHARAJAH, BENJAMIN O’BERRY and BILGE YILMAZ
BASF Refining Catalysts
Viewed : 3245
To improve refinery margins, there is a trend to process heavier, more contaminated crude oil. Consequently, fluid catalytic cracking (FCC) feedstock is getting heavier (see Figure 1).1 One of these contaminants that can be detrimental is nickel (see Figure 2).2 Nickel (Ni) deposits on the FCC catalyst particle and catalyses dehydrogenation reactions, which increase contaminant hydrogen and coke production. Hydrogen and coke are low value products, resulting in reduced profitability. Thus, there is strong demand for improved nickel passivation technology. Nickel passivation is especially beneficial when the FCC is at regenerator temperature limit or air limit. By removing the limit, a substantial increase in unit profitability is immediately possible, either by increasing the FCC feed rate or the severity.3 Nickel’s detrimental effect is reduced using passivators such as antimony (Sb) or specialty alumina and, more recently, a Boron Based Technology (BBT) developed by BASF.
Prior art of nickel passivation technologies
Nickel passivation using antimony compounds was first patented by Phillips Petroleum Company,4 and has since been used for more than 40 years. Numerous studies show that antimony injection alone can reduce the detrimental dehydrogenation effect of nickel by 20-40%, depending on the specific situation. Antimony based nickel passivator chemicals are liquids that are injected into the FCC feedstock. After injection, there is typically an immediate, sharp reduction in dry gas volume production and delta coke yield. However, apart from passivating nickel, antimony will also poison carbon monoxide (CO) combustion promoters and can increase nitrogen oxide (NOx) emissions. In addition, there are environmental and safe handling concerns, and because of these concerns the use of antimony is not permitted in some parts of the world. There is also the concern of slurry circuit fouling due to excessive antimony injection. Furthermore, depending on several factors, the ‘antimony pick-up’ by the FCC catalyst can vary, which impacts the efficacy of this nickel passivation option.
Specialty aluminas are commonly used in resid-FCC catalysts and are effective at trapping nickel to reduce the contaminant hydrogen and coke. The mechanism for this trapping/passivation is debated in the literature;5,6 however, what is certain is that the efficacy of this route is limited by the immobility of both nickel and the alumina trap. It is known that nickel deposition on resid-FCC catalysts typically follows a gradient, where higher amounts of nickel accumulate on the outer surfaces of the catalyst particle. Using the Peripheral Deposition Index (PDI) to quantify this observation, it was revealed that the measured PDI values for nickel are typically high, confirming its extremely limited mobility.7,8 Since nickel is typically concentrated on the outer surfaces of a resid-FCC catalyst, it can only be trapped by alumina when it is in sufficient proximity. It can be concluded that the lack of both alumina trap and nickel mobility within the catalyst particle limits nickel passivation by this method.
Given the need for improved metal passivation technologies, BASF’s aspirational goal was to innovate the next generation of nickel passivation technology. The solution was a boron based chemistry to passivate nickel.
Boron based nickel passivation
As was already mentioned, specialty aluminas have a fixed position in the catalyst particle, thus nickel passivation is only possible when a nickel-containing compound deposits close to the passivating alumina. The advantage of boron based technology is its mobility under FCC conditions; therefore, the boron-containing compound migrates to the nickel, increasing its effectiveness in passivating nickel. It has been shown that the boron functionality is complementary to speciality alumina materials and provides an added benefit. Furthermore, multiple spectroscopic studies show that with BBT, nickel is at a less reducible state, which lowers its potential to participate in dehydrogenation reactions. In these studies, CO is used for the characterisation of nickel species. The nature of bonding between CO and nickel depends on parameters like the degree of metals dispersion, the type of interaction between the metal and the support, and the resulting oxidation state of the nickel.9 CO DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) was used to observe the effect of boron on passivating nickel. This infrared spectroscopy method identifies the nickel oxidation state:
• Bands with wave numbers between 2150 and 2100 cm-1 are associated with nickel species that are easily reduced. Thus, these species have the potential to participate in detrimental dehydrogenation reactions in the FCC riser.
• Bands with wave numbers between 2255 and 2175 cm-1 are associated with nickel species that are more difficult to reduce. Thus, these species have less potential to participate in detrimental dehydrogenation reactions in the FCC riser.
This technique was used to analyse two FCC catalysts:
• ‘Catalyst A’ is a catalyst designed for resid applications, while
• ‘Catalyst B’ is from the BBT platform.
Figure 3 shows that BBT lowers the proportion of nickel that is easier to reduce (wave numbers 2150–2100 cm-1), which are more active in detrimental dehydrogenation reactions.
To investigate boron’s nickel passivation mechanism further, additional catalyst samples were deactivated and tested. Depending on a variety of factors, refiners’ catalyst testing laboratories will typically use one of three methods to deactivate fresh FCC catalyst samples to mimic commercial equilibrium catalyst (Ecat). These methods are cyclic propylene steaming (CPS), cyclic metals deactivation unit (CMDU) or steam deactivation. CMDU is a laboratory deactivation unit used to deactivate fresh catalyst to resemble Ecat where metals are cracked onto the catalyst. This deactivation method is usually the best at mimicking Ecat with high amounts of nickel and vanadium (V),5,10,11 because it better simulates the conditions in the FCC unit that result in the laydown of contaminant metals Ni and V. Consequently, the nickel is concentrated on the outer surfaces of the deactivated catalyst.
Based on the testing, it was found that BBT reduces hydrogen by around 25% and coke by around 13% or better, depending on the deactivation method used (see Table 1). Several successful commercial trials were conducted at multiple refineries. These product commercialisation trials demonstrated the much improved nickel passivation performance using BBT.
Solutions for refiners
In 2015, BASF introduced to the market BoroCat, the first FCC catalyst based on the BBT platform. BoroCat is engineered to provide maximum metals passivation and superior product yields, leading to more effective utilisation of heavy resid feeds. The benefits of this catalyst include high conversion, high gasoline and light cycle oil (LCO) yields, and minimum bottoms yield. Since its introduction, BoroCat has been successfully introduced in refineries all over the world. Results have shown that refineries using BoroCat can process heavier, more contaminated feedstocks and improve their profitability.
Building on the successes of BoroCat, in 2017 BASF launched Borotec, the newest addition to its resid-FCC catalysts portfolio. Borotec is the latest technology using BASF’s unique BBT platform to provide mild and moderate resid feed FCC units with more flexibility in crude selection, which results in increased yields of high value products.
Add your rating:
Current Rating: 4