January 2018

Catalyst Technology Update

A new generation of dewaxing catalysts

The refining industry has been considering incorporating dewaxing in ultra-low sulfur diesel (ULSD) and hydrocracking services to increase the flexibility of hydroprocessing units and to process more barrels by increasing the endpoint of the feed when diesel margins are favorable.

Rasmussen, H., Haldor Topsoe

The refining industry has been considering incorporating dewaxing in ultra-low sulfur diesel (ULSD) and hydrocracking services to increase the flexibility of hydroprocessing units and to process more barrels by increasing the endpoint of the feed when diesel margins are favorable. Processing higher endpoint feed makes it more difficult to meet product specifications for cold flow properties and the T90. Dewaxing is one solution, but because today’s dewaxing catalysts also have a significant cracking functionality, some of the extra diesel barrels will be lost to naphtha and light ends, making dewaxing less attractive in many cases.

The key is to use a dewaxing catalyst that is very selective towards isomerization and, consequently, will result in a minimal reduction of the diesel yield. Topsoe has recently launched a series of dewaxing catalysts that are true isomerization catalysts and have very little activity toward the cracking of diesel molecules. These catalysts are called TK-910 D-wax, TK-920 D-wax, TK-930 D-wax and TK-940 D-wax, and their application areas are shown in TABLE 1.

Cold flow properties

These catalysts will enable true molecule management. In other words, refiners can increase the endpoint of the diesel feed, even during the winter months, to produce extra diesel barrels and still meet cold flow property specs, easily justifying the cost of the dewaxing catalyst.

ULSD cold flow properties are defined as cold filter plugging (CFPP) and cloud point (CP), or pour point (PP). These properties are negatively affected by the presence of waxy molecules, such as normal paraffins, in the diesel feed.

Without the use of a dewaxing catalyst, only a few remedies can get the cold flow properties on spec during the winter months:

  1. Feed management: Endpoint reduction and consequently a loss of barrels
  2. Kerosine blending: Diluting the wax molecules
  3. Use of additives: An increased operating cost to the refinery.

These options are often not commercially attractive. The case study here will show that the use of the TK-930 D-wax catalyst in ULSD service provided the refiner with a substantial increase in profitability during both the winter and summer periods, easily justifying the cost of the dewaxing catalyst.

The dewaxing catalysts are based on a proprietary zeolite structure promoted with nickel and tungsten, or noble metals, to provide an unmatched selectivity towards isomerization and an exceptional yield structure of the product.

Another key feature of the TK-930 D-wax and TK-940 D-wax catalysts is that they exhibit good desulfurization (HDS) and denitrification (HDN) activity in their sulfided state, which means that they work well in a sour environment. Since the catalysts work in a sour environment, they may be installed in the same reactor as the hydrotreating catalyst.

FIG. 1 shows that the new D-wax series catalysts generate only 1/8 of the yield loss per degree of CP improvement, compared to the company’s previous generation dewaxing catalyst. Furthermore, 90% of the diesel yield loss experienced with the D-wax series is lost to naphtha—which still provides value for the refiner—and 10% is lost to light ends (C1–C4).

Fig. 1. Yield loss as a function of dewaxing.
Fig. 1. Yield loss as a function of dewaxing.

Case study

A US refinery operating a 25-Mbpd ULSD unit selected a load of NiMo catalyst TK-611 HyBRIM on top of a layer of TK-930 D-wax for use in the higher pressure ULSD unit. This was based on a detailed pilot plant study confirming the high activity of TK-611 HyBRIM and the selectively of the dewaxing catalyst. The feed properties of this unit are listed in TABLE 2.

Using the TK-930 D-wax catalyst enables the refinery to increase the feed rate during the five winter months by more than 600 bpd while still meeting cold flow property specifications. In addition, the catalyst system reduces the T90 point of the product by 12 degrees–15 degrees, depending on the level of dewaxing. Most refineries are limited on the feed endpoint because they are up against the T90 spec in the product. The reduction in the T90 enabled the refinery to process an additional 1.5 Mbpd during the summer months and still meet all ULSD specifications.

The additional profit from the enhanced winter and summer modes of operation correspond to approximately $6 MM/yr–$8 MM/yr, or approximately $30 MM over the catalyst cycle.

Renewable fuels dewaxing with TK-920 D-wax

Renewable diesel generated by the hydrotreating of triglycerides is mainly n-paraffins and will have very poor cold flow properties. Unless the renewable diesel is used in a very warm climate or as blendstock, it is necessary to dewax the product significantly. Since the renewable feedstocks are typically very sweet, the use of noble metal-based TK-920 D-wax is recommended. This catalyst is already installed in several renewable fuels reactors and is providing the required cloud point reduction with a very low diesel yield loss.

Takeaways

The new dewaxing catalysts have been proven to provide 1/8 of the yield loss experienced with a traditional dewaxing catalyst per degree of cloud point improvement. The outstanding selectivity allows refiners to maintain high feed endpoint during the winter months and, due to a reduction in the T90 point of the product, it is also possible to process more barrels in the summer months. HP

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