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Pretreatment improves FCC performance

Feedstock hydrotreatment catalyst provides improved HDS and HDN activity at various unit operating pressures

Per Zeuthen, Haldor Topsøe
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Article Summary
The latest diesel and gasoline specifications require the reduction of sulphur to very low concentrations. The FCC unit is a major source of gasoline in a FCC refinery. In general, more than 90 per cent of the sulphur compounds in the gasoline pool originate from FCC gasoline.
FCC feed pre-treatment can improve the performance of the FCC unit and is an excellent way of meeting the required sulphur levels in gasoline. The key is to maintain a high level of nitrogen removal while extending the degree of desulphurisation. Selection of the best catalyst for a given service will depend on the operating pressure, which sulphur compounds are present in the feed, and the nature and amount of inhibitors to the desulphurisation reaction.

Better dispersion
New catalysts prepared with significantly better dispersion of the active phases, such as the latest-generation proprietary TK-558 (Co-Mo catalyst) and TK-559 (Ni-Mo catalyst), provide higher hydrodesulphurisation (HDS) and hydrodenitrogenation (HDN) activities. Both of these catalysts are characterised by a high HDS activity, making them suitable for the increased demand for sulphur reduction in the FCC feedstock to meet the future gasoline sulphur specification of 10 ppm. Furthermore, the requirements for high HDN activities are maintained.

Figure 1 illustrates the relative volume (RVA) HDS/HDN performance activities of the new-generation catalysts compared with previous-generation catalysts in a typical FCC pre-treatment service treating a VGO/CGO blend at 80 bar pressure. Since the type of FCC feed used and other process conditions influence the activities, the relative activity of each catalyst will differ among the dozens of refineries where these catalysts are currently in use. The new catalysts will in many cases enable refiners to meet the future specifications for ULS gasoline without any post-treatment. The economic benefit is gained through better product yield structures, cleaner FCC products and longer pre-treatment cycles.

New insight
Researchers discovered the existence of the Co-Mo-S (or Ni-Mo-S) active sites in the late 1970s. Much of the subsequent scientific and industrial research has focused on understanding the catalytic function of the Co-Mo-S-type structure and on finding preparation procedures and methods for modifying the structures to give enhanced activity.

In the 1980s, Topsøe researchers found that there existed at least two types of Co-Mo-S. Type I is thought to be strongly bound to the alumina carrier, and Type II, which exhibits higher activity, is less strongly bound to the carrier. Type II can occur, for example, when the slabs are stacked, in which case the upper layers will be Type II and the bottom layer may be Type I.

Recent research has provided fresh insight into the hydrogenation function of hydrotreating catalysts. It has been shown that new types of metallic sites located on top of the molybdenum disulphide slabs close to the edges play an important role (Figure 2). Also, the relative importance of the direct and hydrogenation routes depends strongly on the composition of the feed.

A preparation technology using standard sulphiding procedures gives very high-activity catalysts for many refining services. The proprietary BRIM technology has several key features:
- The hydrogenation activity of the BRIM sites is enhanced
- The frequency of sites is increased by not stacking the molybdenum disulphide slabs. Although stacking can be an advantage when it comes to the occurrence of Type II structures, it also reduces the number of BRIM sites (because the sites are only available on the upper surface of the stack)
- The bonding of molybdenum disulphide to the carrier is optimised, giving more Type II direct desulphurisation sites.
These advantages are achieved without having to reduce the catalyst’s strength or change its porosity or bulk density.

Catalyst choice
The choice between the Ni-Mo (TK-559) and Co-Mo (TK-558) catalysts is dictated by unit pressure, temperature and desired product properties. The TK-558 will provide the best performance in low- to moderate-pressure units, and it further exhibits a HDN activity that is unique for a Co-Mo-based catalyst. This is the catalyst of choice for the refiner who wishes to produce low-sulphur FCC products without changing yields of gasoline and other high-quality products. In many low- to moderate-pressure applications, the TK-558 even exhibits identical or better HDN activity than the TK-559. Figure 3 displays performance evaluations for the new catalysts in a FCC pre-treatment service treating an American West Coast VGO/CGO blend at 50 bar pressure. In these conditions, the TK-558 performs better than the TK-559 catalyst.

In high-pressure FCC pre-treatment plants, and in cases where nitrogen is the main concern, the preferred choice is the TK-559 catalyst.

PNA saturation
Aromatic compounds are not easily cracked in a FCC unit, and the limited amount of cracking achieved produces a large amount of coke. However, by saturating the polynuclear aromatic (PNA) compounds present in typical FCC feeds and converting them into naphthenes, they are more easily cracked into valuable products. PNA saturation is negatively affected by increased FCC pre-treater severity, as a high reactor temperature does not favour PNA saturation, and the severe operating conditions will result in higher PNA content in the feed to the FCC unit.
Running existing pre-treaters at higher severity (that is, at a high temperature) to meet the stringent sulphur levels in the gasoline pool will decrease the saturation of polyaromatics in the pre-treater, because saturation is equilibrium limited. The benefits of FCC pre-treating in terms of higher conversion and increased yields of high-value products will therefore decrease. Figure 4 shows the product polyaromatics at different temperatures in FCC pre-treatment VGO services at low and high operating pressures.
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