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Mar-2009

Increasing ULSD production with current assets

An understanding of the detailed kinetics in ultra-low sulphur diesel hydrotreaters yields an optimal catalyst system for higher throughput, increased cycle length and lower reactor temperature

Bob Leliveld, Albemarle Catalyst Company BV
Steven Mayo, Albemarle Corporation
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Article Summary
A wide variety of catalyst solutions for ultra-low sulphur diesel 
(ULSD) service are available to the refining industry. The introduction of Albemarle’s patented Stars technology more than a decade ago marked the beginning of the Type II catalysts era, which includes the application of these catalysts with super active catalytic sites in ULSD operations. Increasing the throughput in a ULSD unit or processing low-quality feedstocks without additional capital investment are both great ways of increasing margins and financial returns. Based on new detailed insight into the chemistry taking place in a ULSD unit, new catalysts and catalyst technology are being introduced that can help boost performance.
Recent developments are able to improve hydrodesulphurisation (HDS) unit performance by up to 35% over the most active catalysts in the current hydroprocessing marketplace. For example, the new Ketjenfine (KF) 770 HDS catalyst, built on Stars technology, will enable ULSD refiners to substantially improve their operating margins by extending catalyst cycle length, increasing throughput or running cheaper, lower quality feedstocks, all without additional capital expenditure.

The KF 770 HDS catalyst’s functionality focuses on reaching breakthrough performance in low- and medium-pressure range ULSD. It takes the place of KF 757 Stars in Albemarle’s ULSD catalysts (Figure 1). KF 757 is still available for refiners running less severe operations.

The catalytic removal of refractory sulphur species occurs through a complex reaction scheme comprising at least two routes: through direct route HDS or direct desulphurisation (DDS) and hydrogenation (HYD) sites.

A complicating factor at low and medium partial pressures of hydrogen is the slow rate of organic nitrogen removal. As the nitrogen species inhibit the HYD sites, the catalyst’s effectiveness is largely determined by the number of DDS sites. KF 770, developed through high throughput experimentation, has precisely those sites gearing its performance at low and medium pressures to a start-of-run (SOR) performance activity benefit of more than 20%, as shown in Figure 2. 

Figure 3 compares the activity of KF 770 vs KF 757 in a 20 bar pilot plant test. Over a period of more than 40 days, KF 770 continuously showed an advantage in HDS, achieving an extra volume activity of 20–25%.

Prolongation of the test at 45 bar (Figure 4) clearly demonstrates that the throughput in a unit with KF 770 can be increased to 20% over the same unit loaded with KF 757. These and other pilot plant results confirm that the stability of KF 770 is equal to or better than KF 757.

Throughout the develop-ment of KF 770, the formulation of the catalyst was tailored to achieve maximum stability under the conditions of low and medium partial pressure of hydrogen.

35% cycle length increase
In addition to 20% extra SOR activity, detailed forecasting for specific cases demonstrates that KF 770 can increase cycle length by up to 35% when compared to KF 757 (Figure 5).

The lower SOR temperature favours catalyst stability, as the deactivation rate is a function of reactor temperature. In addition, the high HDN activity of KF 770 provides an extra boost in HDS activity by releasing HYD sites from inhibition by nitrogen species. It further suppresses the needed reactor temperature for reaching ULSD specifications and builds to an overall 35% increase in cycle length.

Economic benefits
In addition to a longer cycle length, KF 770 can add significant value by enabling higher throughputs or intakes of sour feeds. For a 30 000 bpd distillate hydrotreater, running at a 20% higher throughput, this can add an additional margin per year of up to $22 million. This figure is based on an upgrading margin of $10/bbl.

This advantage is illustrated further in Figure 6, showing the extra margin obtained with KF 770 as function of the additional throughput. Three levels of upgrading margin have been chosen to reflect the various qualities of feed.

Catalyst systems: single or mixed loads
For most of the history of hydrotreating in refineries, a single catalyst loaded in a unit was sufficient to provide the expected level of performance. There were exceptions, of course, such as fixed-bed resid, but the majority of applications did not need a different approach. As the demands placed on hydrotreating units have increased and the requirements for hydrotreated product have become more stringent, the opportunities for a more skilful approach to catalyst system design have become apparent.

The traditional approach to understanding the chemistry of hydroprocessing is to analyse feed and product properties, measure average reactor temperature (WABT) and use basic kinetic expressions to analyse unit performance. Obviously, this approach only provides an average view of how the unit is operating. In recent years, more sophisticated sulphur- and nitrogen-specific analysis as well as inter-bed temperature measurements have given more insight into unit 
performance. However, the information is still not detailed enough to fully understand the reactions taking place inside the unit.

Fully understanding the chemistry of hydroprocessing inside the reactor is key towards designing a catalyst system for maximum performance. The problem, of course, as shown in Figure 7, is that there are many independent variables changing at every point in the reactor. The chemistry taking place at any point both affects and is affected by these variables.

No two slices of the reactor along the vertical axis have the same reaction environment, which means the best catalyst for optimising system performance could be different at each point. This has radically changed the way we think about the application and development of ULSD catalysts. While in certain conditions a single catalyst fill is still the best solution, in other cases a combination of catalysts is preferred.
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