Feb-2013

Understanding cloud point and hydrotreating relationships

As the distillate markets allow for increased distribution worldwide, refiners need to be more aware of the additional product specifications that can be present at the various locations.

Brian Watkins and Meredith Lansdown
Advanced Refining Technologies

Article Summary

One product property that is difficult to modify with general hydrotreating is the ability to improve (lower) the cloud point and cold filter plugging point of the diesel. Cold flow properties are determined by the wax or crystals that are formed as the diesel is cooled. The formation of these crystals can plug filters and lead to poor engine performance.

There are various approaches to meeting cold flow targets, the simplest of which is the blending of lighter material (kerosene or jet) into the fuel. Other options include the use of additives, solvent dewaxing or adding a separate isomerisation reactor. All of these options have disadvantages, including high costs or yield losses. Diluting with blending stocks, such as kerosene, has the added complication that the blending stocks must separately meet all of the same requirements, such as sulphur, of the finished ultra-low-sulphur diesel (ULSD) and could require that the blending stock undergo additional hydrotreating. This also involves taking a higher value fuel, degrading its value by blending it into diesel, and will have volume limits in order to stay within distillation and flash point specifications for diesel fuel.

The ability to modify the cold flow properties of the diesel in the hydrotreater can have significant economic advantages that the other options do not provide. Use of a speciality catalyst is required in order to do catalytic or hydro-dewaxing (HDW) to improve the cold flow product properties within the ULSD hydrotreater complex, and to avoid making changes to naphthenes or iso-paraffins that already have acceptable cold flow properties. Understanding the cold flow requirements first is necessary to create an individually tailored process and avoid the pitfalls associated with inappropriate quantities of HDS catalyst such as yield losses and not having the flexibility to meet market demands. Figure 1 shows some simple reactions that can effectively improve the cold flow properties of the diesel product. The resultant products contain some olefinic material due to the cracking mechanism and require proper catalyst staging to achieve process goals.

The typical process of dewaxing utilises a ZSM-5-type catalyst. The structure of ZSM-5 is such that only straight-chained hydrocarbon molecules (normal paraffins or n-paraffins) fit inside the cage structure and are cracked into smaller, lighter molecules. These molecules have significantly lower cloud and pour point characteristics.

Figure 2 shows some of the various n-paraffins present in a typical diesel boiling range. The melting point is what influences the cloud point of the diesel if left unconverted and, of course, the higher the carbon content, the higher the boiling point.

Due to the nature and structure of the zeolite, the catalysis choice is important, as these structures can easily be poisoned by nitrogen and olefins present in the feed. Even in high-pressure applications, hydrotreating has only a small impact on product cloud point. Figure 3 shows the effect of hydrotreating on cloud point at 1400 psi hydrogen partial pressure and with a feed containing 50% cracked material. This figure examines the cloud point improvement across a wide range of product sulphur and operating temperatures. Even at high temperatures, well beyond that required to produce ULSD, there is little change in product cloud point.

Typically, the target market for these products requires more than several degrees decrease in cloud point below the value of the feed. This suggests the use of hydro-dewaxing in conjunction with the ULSD unit is desirable.

As mentioned previously, for HDW catalyst to perform most efficiently requires some hydrotreating first, since it is susceptible to poisoning from the organic sulphur and nitrogen present in the feed. Figure 4 shows pilot plant results from operating a system using untreated feed over dewax catalyst at 5.75 LHSV to simulate the feed rate over a dewax bed in a hydrotreater. This work was completed over the entire range of temperatures for a ULSD hydrotreater from start of run to end of run. As the figure shows, there is very little change in product values. The cloud point of the product is improved only slightly and this corresponds with a small increase in bromine number, which is expected based on the reactions listed in Figure 1. It is important to note that it also shows that there is very little sulphur and nitrogen removal, and interestingly enough there is actually a slight increase in the volume per cent for mono and poly-aromatic species in the product, as indicated by a negative per cent change.

Decreasing the LHSV over the dewax catalyst bed produces similar trends for the total and poly-aromatics conversion, as well as the HDN and HDS conversions. There are only a few degrees change in cloud point and almost no change in the bromine number from the higher LHSV. This clearly indicates that it is important to provide some level of hydrotreating in advance of the HDW catalyst in order to be able to utilise the zeolitic acid function. It is also important to note that there needs to be enough hydrotreating catalyst available in the system in order to meet the other product specifications such as sulphur and aromatics, since the HDW catalyst provides no sulphur or nitrogen removal.

One of the keys to successfully combining a dewaxing catalyst with an HDS system is an understanding of the trade-offs between dewaxing and HDS activity as the amount of dewax catalyst is changed. ART completed a number of pilot plant tests with two different amounts of dewax catalyst and ART’s NDXi, a premium nickel molybdenum catalyst for ULSD applications. The pilot plant work consisted of testing loadings of 10% and 20% dewax.

The first set of data examines the ability of the system to meet 10 wppm sulphur in the diesel at both low pressure (500 psi hydrogen partial pressure) and at a higher pressure (975 psi hydrogen pressure). Figure 6 compares the two systems, and the base case is the 10% dewax system at low pressure. This base condition is for producing 10 wppm product sulphur and is the zero point on the temperature axis. As expected, the higher pressure system outperforms the low-pressure application by almost 30°F (~17ËšC). At lower pressure, the difference between the systems shows a 10°F (~6ËšC) higher temperature required with the increased dewax catalyst volume. The additional temperature required to meet ULSD also needs to be considered, as this could mean a debit of 4-8 months in cycle life if too much HDW catalyst is loaded into the hydrotreater.

The ability to determine product cloud point and how it is expected to change over time is also important. Similar to a hydrocracking reactor, as the temperatures are increased over the bed of HDW catalyst, the ability to break the n-paraffins increases. There is a clear difference in the ability to make a cloud point change, based on not only LHSV over the dewax bed, but also the operating pressure of the unit, much like that for producing ULSD. In Figure 7, the base case is again the 10% dewax bed, and the zero point on the chart is the point at which 10 ppm sulphur is produced. Moving from left to right is changing the WABT relative to the expected SOR temperature for 10 wppm sulphur.