Mar-2013

Advances in FCC pretreatment catalysis

Performance data for a new FCC pretreat catalyst show a substantial advance in HDS and HDN activity

BILL GILLESPIE, ALEXEI GABRIELOV, THOMAS WEBER and LARRY KRAUS
Criterion Catalysts & Technologies

Article Summary

Catalysts applied in FCC feed pretreat (FCC PT) service must operate robustly with the wide range of feeds charged to these units. These feeds’ origins include crude atmospheric and vacuum towers, cokers, ROSE/deasphalters, FCC units, lube units and others. FCC PT units often process synthetic feeds. Catalysts that operate in FCC PT service must obtain optimised sulphur, nitrogen and aromatic saturation performance to drive the FCC process’s economic performance. They must also be able to operate stably in an environment where significant levels of contaminants such as nickel, vanadium, silicon, sodium, arsenic and asphaltenes are present. Often, several catalysts are utilised in a tailored package to fulfill the demands placed on the FCC PT unit. The new-generation Centera catalyst DN-3651 combines commercially demonstrated stability and the contaminant tolerance features of Ascent DN-3551, an industry-proven NiMo FCC PT catalyst,1 
with Centera’s active site 
architecture. This allows for step-out hydrodesulphurisation (HDS), hydrodenitrogenation (HDN) and aromatics saturation (ASAT) performance, plus high catalyst stability in FCC PT applications.

Catalyst development and testing
A significant aspect of 
the DN-3651 development programme is that the catalyst was developed specifically for FCC PT applications utilising high throughput experimentation. The high throughput reactor system utilises multiple tubular flow reactors with automated process control and sampling. The use of this multi-tube reactor system allows significant acceleration of catalyst development relative to conventional testing techniques. Leads generated with high throughput equipment were confirmed by conventional-scale pilot plant testing. The reactor conditions and feedstock properties used in high throughput experimentation are listed in Tables 1 and 2, respectively.

The target performance level for DN-3651 was an improvement in HDN activity of at least 10°F (5.5°C) relative to DN-3551 (~20 RVA [relative volume activity]) with equivalent or better HDS activity. Figure 1 illustrates the high throughput experimentation data for HDN activity obtained during the prototype development phase, as well as during the scale-up and commercialisation phase, while Figure 2 illustrates HDS activity. The activity as presented in these plots is the decrease in temperature required to achieve the target sulphur or nitrogen level in the product relative to that required by DN-3551. Approach 4 shown in Figures 1 and 2 represents the approach that ultimately led to the final formulation of DN-3651. The points enclosed in the highlighted area in Figures 1 and 2, labelled DN-3651, represent samples tested as part of scale-up and manufacturing optimisation. Use of high throughput experimentation allowed rapid development of a reliable manufacturing process, yielding the highest practical catalyst performance. These data, coupled with pilot plant data and discussed below, clearly demonstrate success in exceeding the 10°F (5.5°C) HDN activity improvement target, as well as providing significantly improved HDS activity.

Pilot plant testing was conducted in trickle-flow, fixed-bed reactors using 50 cc catalyst loads. The catalyst load consisted of whole pellets diluted with an equal volume of an 80-60 mesh fraction of silicon carbide. Plug flow and wetting requirements were assured for each test. A variety of commercial FCC PT feedstocks were used to demonstrate the advantage of DN-3651 in various operations.

In the first example, the commercial unit processes high-sulphur, high-nitrogen synthetic feedstocks at high pressure. The process conditions employed and the test feed properties are given in Tables 3 and 4. Data were obtained at 700, 720 and 740°F (371, 382 and 393°C), producing typical product nitrogen levels in the ranges 200-900, 50-500 and 6-350 wppm, respectively, depending on the catalyst used and the time on stream. Product sulphur levels were typically 800-2300, 200-1200 and 30-70 wppm, respectively, at the three temperatures.

Figure 3 illustrates temperature advantages of 20-25°F (11-14°C) in HDN (temperature required to achieve 260 wppm product nitrogen) and 12-20°F (7-11°C) in HDS (temperature required to achieve 1000 wppm product sulphur) for Centera DN-3651 over Ascent DN-3551 catalyst. The activity advantage of DN-3651 was maintained throughout the duration of the extended pilot plant test, which included high-temperature operation, where catalyst deactivation is typically accelerated. This indicates that DN-3651 has stability equivalent to that of DN-3551, which has consistently demonstrated high stability in commercial FCC PT service.

Another example illustrates the significant activity advantage of DN-3651 and stable operation at 380°C (716°F) in a moderate hydrogen pressure of 70 barg (1000 psig) using an Asia-Pacific VGO feed. The process conditions employed and the test feed properties are given in Tables 5 and 6.

Figure 4 demonstrates a temperature advantage of 5-10°C (9-18°F) for Centera DN-3651 over the previous-generation Ascent DN-3551 catalyst under moderate pressure conditions. These data also demonstrate that DN-3651 has the same high stability as DN-3551 at moderate pressure. This is attributable to the use of the commercially proven Ascent catalyst support technology for DN-3651.

Summary data for four more examples, labelled as Tests 1-4 and demonstrating the performance of DN-3651 processing a variety of feedstocks of different origins over a wide range of operating pressures (1250–1600 psig, 86-110 bar), are presented in Table 7 and Figure 5. The feeds processed and process conditions employed in these tests are shown in Table 7. 

In Figure 5, DN-3651 shows significant activity advantages over DN-3551. DN-3651 has HDN RVA of 120–135 and HDS RVA of 120–130 compared with DN-3551 (RVA = 100).

The advantages in performance of DN-3651 over DN-3551 are enhanced when operating at high hydrogen availability (a hydrogen-rich environment). Figure 6 illustrates the relationship between DN-3651’s HDN RVA advantages over DN-3551 and H2 availability. While DN-3651 shows advantages in performance in all process conditions, these data show that it has an even greater advantage in hydrogen-rich environments. This allows the capabilities of units in this operating environment to be more fully utilised.

Catalyst characterisation
Commercial DN-3651 catalyst was characterised using transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) and compared with DN-3551. TEM images of sulphided DN-3651 and DN-3551 are shown in 
Figure 7.