Maximising premium distillate by catalytic dewaxing
Advances in technology for catalytic dewaxing of distillates by isomerisation provide improved activity
Tim Hilbert, Mohan Kalyanaraman, Bill Novak, Joesph Gatt, Béatrice Gooding and Stephen McCarthy, ExxonMobil Research and Engineering Company (EMRE)
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Distillate fuel (diesel, home heating oil and kerosene) needs to be fluid at the engine or burner when temperatures are low. There are various specifications that cover fluidity at low temperatures, including cloud point, pour point, freeze point and cold filter plugging point. Various approaches can be employed to improve the cold flow properties of an off-spec fuel. The simplest can be blending a light stream, such as kerosene, into diesel. However, this requires a lighter stream that is fully compatible with all specifications (sulphur, for example), which may require further processing of the diluent stream. The diluent stream may need to be diverted from its optimal disposition, and there are often limitations as to the amount of diluent that can be blended. Catalytic dewaxing has the advantage that it actually converts the “bad” molecules in the distillate — those with poor cold flow properties (the wax) — to better molecules. This technology has been deployed since the 1970s.
As the air temperature drops, the wax naturally found in diesel begins to form crystals and solidify. These crystals can prove problematic by clogging fuel filters and preventing engine start-up.
Improvements in diesel’s cold flow are tracked by either cloud point or pour point: cloud point is the temperature at which wax crystals begin to precipitate from a petroleum sample, while pour point is the temperature at which oil ceases to flow.
Many different approaches exist for improving cold flow properties in diesel; for instance, additives, undercutting, diluting with kerosene and solvent dewaxing. All of these have significant disadvantages, including high costs and significant diesel yield loss, while catalytic dewaxing is a competitive alternative.
Mobil developed the original catalytic dewaxing process (called MDDW), based on the ZSM-5 catalyst. ZSM-5 has a pore structure that allows only straight-chain hydrocarbons (wax) to enter the cage. Once inside, the wax molecules are cracked to lighter materials. The remaining diesel has significantly improved cold flow properties. This process was widely commercialised, but, while successful, a small amount of distillate-range material was cracked to naphtha and lighter material during the process. For those who wanted maximum diesel, higher selectivity was desired.
Research work continued, with the goal of increasing the distillate yield. This resulted in the development of catalysts based on new zeolites and a process called MIDW. In MIDW, when normal (n-)paraffin enters the zeolite pore, it is isomerised rather than cracked. This results in a higher retention of distillate material compared with the original MDDW process.
Cold flow improvement chemistry
MIDW technology incorporates a metal capable of dehydrogenation/hydrogenation reactions and a zeolite for shape-selective skeletal isomerisation of n-paraffins to iso-paraffins. The result is fewer losses of feed material to form light products.
A gas chromatographic study of the product shows a high conversion of n-paraffins to iso-paraffins (see Figure 1). Iso-paraffins have significantly better cold flow properties. In fact, the technology has been able to produce “Arctic” diesel from rather poor diesel feeds.
During the process, n-paraffin is thought to be dehydrogenated on a metal site. It is then isomerised on the acidic zeolite site within a zeolite pore and hydrogenated back to the saturated iso-paraffin (see Figure 2).
MIDW technology has been employed in seven commercial installations in North America, Europe and Asia, while two more are at the design stage. MIDW can be installed as a standalone process operating on a distillate feed; however, it can also be used in combination with a hydrotreating unit. The technology enables 300 000-plus b/d of diesel already meeting the Euro-IV specification to further meet the low cloud point specification, including the stringent specifications for Arctic classes of diesel applied to Northern Europe.
Some examples of commercial data, where MIDW technology is used in combination with hydrotreating to produce premium, ultra-low sulphur diesel, follow. An additional feature of the technology is that it can lower the back-end distillation of the distillate, in effect converting some material heavier than diesel into the diesel-boiling range (see Figures 3 and 4).
A second commercial example shows the utility of the process. MIDW operates mostly in the wintertime unless the feed is inherently waxy. The data in Table 1 demonstrate typical summer and winter operations.
Thus, for a small naphtha loss, the process produces good cold flow properties and an equivalent cetane uplift, as well as a lower 90% point, which allows an additional feed of VGO. This kind of configuration is shown in the next section.
An alternative sour service configuration (see Figure 5) used for very waxy stocks processes a small amount of heavy or very waxy material and connects the effluent to an existing hydrotreater. Commercially, the process dewaxes and desulphurises just enough to meet the product total run, while minimising naphtha make. Ultra-low cloud points have been achieved in sweet configurations (see Figure 6) with good yields.
Continued Improvements in catalytic dewaxing
EMRE continues to make improvements in catalytic dewaxing. One area of improvement has been to increase distillate retention across the dewaxing unit. The distillate yield has grown with the progression from MDDW to MIDW, and improved versions of MIDW (see Figure 7). The current catalyst has only a slight loss of distillate to lighter products. This has become increasingly important with regard to the increasing demand for diesel in many parts of the world.
Another focus of research has been improving the activity of the catalyst to enable dewaxing at lower reactor temperatures. The technology can be employed within a hydrotreater to produce a premium diesel.
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