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Jun-2012

Maximise yields of high-quality diesel

Global growth in distillate demand has driven refiners to investigate ways to increase middle distillate yields. Figure 1 shows the diesel-gasoline spot price differential for the Gulf Coast.

Greg Rosinski, BrianWatkins and Charles Olsen
Advanced Refining Technologies
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Article Summary
As the figure demonstrates, diesel prices have remained strong relative to gasoline prices, which are incentivising refiners to look for more opportunities to increase diesel yield. Options for refiners include increasing diesel yield from FCC pretreat units by operating in a mild hydrocracking (MHC) mode or extending the endpoint of the feed to a diesel unit and converting the heavy fraction into distillate-range material.

There are many challenges associated with both approaches utilising the MHC approach, with a significant one being the need to minimise production of excess light ends and naphtha to avoid overloading the existing downstream fractionation system. These issues become even more important as end of run approaches and the reactor temperatures are higher, which increases the conversion of the MHC catalyst. Another potential obstacle is dealing with the possible negative impacts on other diesel from product properties such as diesel product colour and cold flow properties.

Many of these concerns can be alleviated through the proper design and selection of a catalyst system for the hydrotreater. Advanced Refining Technologies (ART) has conducted extensive pilot plant work in an effort to better understand these potential options for refiners wishing to increase diesel yields. This work has shown that a catalyst system design approach incorporating a MHC component is a viable option for the production of higher yields of middle distillate.

The use of an MHC catalyst component in the FCC pretreater allows for an opportunity to achieve incremental conversion above that typically experienced with a hydrotreating catalyst system. In addition, the operating mode of the FCC pretreater can be used to adjust overall yields of gasoline and middle distillate, giving the refiner more flexibility to change the product mix to meet market demands. In order to assess the effectiveness of the MHC catalyst, it is important to understand the level of boiling range shift that can be achieved simply through hydrotreating. Table 1 shows a variety of sulphur, nitrogen and aromatic compounds and the corresponding change in boiling point that occurs upon removal of sulphur and nitrogen, and through saturation of aromatic rings. The table demonstrates that a fair amount of boiling point shift occurs through hydrotreating.

Understanding the different catalyst attributes and their impact on unit performance is important to designing the best catalyst system. Most hydroprocessing catalysts have an acidic and an active metals component. For typical hydrotreating catalysts, these metals are either cobalt-molybdenum or nickel-molybdenum on an alumina or silica-alumina support. In a hydrocracking catalyst, a significant amount of the support is made with amorphous silica-alumina or zeolite. This substantially increases the acidity of the catalyst and is what gives hydrocracking catalyst cracking activity. In a typical hydrocracker, NiMo hydrotreating catalyst is used to convert organic nitrogen to ammonia, since hydrocracking catalysts are poisoned by organic nitrogen. This allows the hydrocracking catalyst to effectively perform its cracking function.

In MHC, the goal is to achieve a lower amount of cracking conversion compared to a full-conversion hydrocracker, so organic nitrogen levels can be relaxed. It does, however, require properly balancing of the amount of hydrocracking catalyst with hydrotreating catalyst in order to achieve the optimum in terms of yields and product qualities.

One recent study completed by ART investigated the trade-offs observed by adding MHC catalyst to the catalyst system in an FCC pretreater. The feedstock used in this work is shown in Table 2. The feed is full-range vacuum gas oil (VGO) with just over 1600 wppm nitrogen and 2.75 wt.% sulphur. The breakdown of the aromatic compounds present in the feed is also given. The feed is relatively aromatic and has a significant quantity of polyaromatic compounds. One of the goals of this work was to investigate the trade-off in HDS/HDN activity with cracking activity as more MHC component was incorporated in the catalyst system. The hydrotreating catalysts selected were current-generation Type II catalysts from ART’s DX catalyst series. The MHC components were selected based on cracking activity and selectivity to diesel products.

Figure 2 shows how the HDS activity of the catalyst system changes as the fraction of the MHC component increases. Not surprisingly, the base case hydrotreating catalyst system, which contains no MHC catalyst, shows the highest activity for HDS. Adding in 10% MHC catalyst results in essentially the same HDS activity as the base case system. Increasing the MHC component to 20% results in a slight decrease in HDS activity at low temperatures, but the same HDS activity at higher temperatures. Finally, at the highest fraction of MHC catalyst (35%), the HDS activity is consistently lower than the base case activity, suggesting that the system contains too much MHC catalyst relative to hydrotreating catalyst.
A comparison of the HDN activity of the different catalyst systems is shown in Figure 3. Again, the base case system shows the highest activity, but only for lower HDN conversion (higher product nitrogen). Interestingly, the data show that adding in MHC catalyst results in a system that has higher HDN activity relative to the base case for higher nitrogen conversions. This suggests that the stronger acidic function of the MHC catalyst improves nitrogen removal of the catalyst activity. Note that increasing the fraction of MHC catalyst too far does result in a decrease in HDN activity except for very high nitrogen conversions.

In terms of HDS and HDN activity, there is an interesting interaction between the hydrotreating and MHC catalyst components. HDS activity is not significantly affected by the addition of MHC catalyst until higher percentages are reached. For HDN activity, there is an optimum level of MHC catalyst where HDN activity is improved over the base case performance. Figure 4 summarises how the cracking conversion is impacted by the addition of MHC catalyst to the system. The conversion is defined as the difference between the amount of 680°F+ material in the feed and product. The figure shows that at lower temperatures all the catalyst systems behave similarly and show the same level of boiling range shift as achieved with hydrotreating alone. As the temperature is increased, there is a point at which the conversions for the MHC catalyst systems start to differentiate themselves from the base case.

The conversion observed for the base case increases only gradually as temperature is increased. Adding only 10% MHC catalyst results in a slightly faster increase in conversion with increasing temperature, and adding 20% MHC catalyst results in an even faster increase in conversion. The 20% system provides 10 numbers higher conversion relative to the base case, with only about a 10-15°F increase in temperature.
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