Jul-2009

Hydrocracking catalyst and processing developments

Improvements to hydrocracking catalyst activity and selectivity at various operational and feedstocks conditions are discussed

Robert Wade, Jim Vislocky, Theo Maesen and Dan Torchia, Chevron Lummus Global

Article Summary

Refiners currently find themselves in a challenging environment as regulations continue to increase demands on refining processes, while high-quality refining feedstocks become scarcer and consequently more expensive. This combination of increasing raw material cost (usually of lesser quality), coupled with more stringent finished product quality requirements, emphasises the need to utilise the latest technology to remain competitive and maintain safe unit operation.

Additionally, recent world events have resulted in reduced capex and thereby increased focus on hydrocracking catalyst and know-how solutions. In this effort, Chevron Lummus Global (CLG) is involved in operating dozens of pilot plants and micro units. There are also annual programmes in progress for each of the following proprietary hydroprocessing technologies: Resid Hydrotreating, LC-Fining, Isotreating, Isocracking, Isodewaxing and Isofinishing. These programmes focus on catalyst improvements and process improvements, along with optimising catalyst offerings for existing customers.

Chevron invented the modern hydrocracking process in 1959. The first licensed unit started up in 1962, followed by the first commercialised Isocracking process within Chevron’s own system at the Pascagoula, Mississippi, refinery in 1963. Three years later, a two-stage Isocracking plant was commissioned at its Richmond, California, refinery to upgrade vacuum gas oil (VGO) to naphtha and jet fuel. At the same time, a single-stage once-through (SSOT) unit was also commissioned at the Richmond refinery to hydrocrack deasphalted oil (DAO). These early hydrocracking projects added ten high-pressure reactors to the Richmond refinery. Isocracking technology was further applied by Chevron with a second unit at its Pascagoula refinery in 1969, and one at its El Segundo, California, refinery in 1971.

Hydrotreating catalyst design
It is well understood that support and active metals are two key ingredients critical to optimising performance for any hydroprocessing catalyst. These key ingredients determine the density of active sites and pore size distribution. The optimum activity is achieved by maximising the density of active sites while maintaining access for the critical molecules of a particular feed.1 This optimum will be different for the larger molecules in a VGO feed than for the smaller molecules in a diesel feed. CLG has focused on improving hydrotreating catalysts tailored to a full-range VGO hydrocracking service. Figure 1 shows the relative hydrodenitrogenation (HDN) activity advantage on a full-range VGO for the latest version, ICR D179, along with its predecessors.

The activity gains shown for ICR 134 to ICR 154 and ICR 154 to ICR 178 were achieved through the optimisation of support and active site density, as previously described. Greater than 10°F gains shown for ICR 178 to ICR 179 and ICR 179 to ICR D179 were achieved through the use of a process that increases the density of the more active (Type 2) catalyst sites.1 

Hydrocracking catalyst design
The principles for optimising hydrotreating catalyst design extend to hydrocracking catalyst design. As compared to hydrotreating catalysts, hydrocracking catalysts exhibit a larger fraction of active sites that selectively reduce the average size of the feed molecules to shift the boiling range of the feed into the desired product boiling range. Balancing the density and accessibility of these so-called cracking sites with that of the hydrogenation sites is critical to manufacturing fuels with the lower levels of sulphur, nitrogen and aromatics required to meet or exceed current and future standards.

Commercialisation of multiple new generations of hydrocracking catalysts has been  achieved through optimising the catalyst formulation, optimum choice of raw materials, enhanced characterisation, more efficient testing techniques, optimised synthesis steps and improved manufacturing processes. In addition, CLG has been able to include elements of unit operability into catalyst designs, based on feedback from its operation of hydrocrackers in many different markets across the globe.

Figure 2 shows CLG’s base metal Isocracking catalysts, covering the full range of hydrocracking applications. The curve represents the trade-off between activity and selectivity, which characterises a generation of catalysts. The goal of hydrocracking catalyst development is to move to a next generation of catalysts that operate at higher selectivity and activity. Higher selectivity produces more of the desired product, while higher activity allows the refiner to extend catalyst run lengths, increase throughput or process more difficult feeds. The catalysts that are commercially available and discussed in further detail include ICR 177, ICR 180, ICR 160*, ICR 183 and ICR 240.

Figures 3 and 4 illustrate how improved catalytic performance is achieved through modification of the cracking (acid) function. These figures show the relative cracking rate constant as a function of carbon number for catalysts of varying activity.

Figure 3 shows that with an increase in activity of the cracking component of the catalyst, the cracking rate constant for molecules in the middle distillate boiling range increases considerably faster than that for molecules in the VGO boiling range. Thus, the middle distillate product molecules are preferentially adsorbed and overcracked, resulting in the selectivity decline with increasing cracking activity shown in Figure 2.

Figure 4 shows how the accessibility to the cracking function can be 
modified to reduce the amount of overcracking, which results in a 
catalyst with higher activity while maintaining mid-distillate selectivity. CLG has recently developed three new catalysts that have been modified to attenuate overcracking of AGO and increase diesel yield selectivity in this fashion. The formulation of each of these catalysts retains the best characteristics of their respective predecessor with the addition of performance enhancements that increase diesel selectivity by attenuating AGO overcracking.

Bottoms cracking and distillate production
For many years, ICR 142 has been the catalyst of choice for both maximum bottoms cracking and maximum mid-distillate production. As feeds become more difficult and process severity increases, the need for a more active catalyst to replace ICR 142 became apparent; hence, the advent of ICR 177. ICR 177 provides a significant increase in diesel yield as conversion is increased, without reducing kerosene and naphtha selectivity. ICR 177 is 10°F more active than ICR 142, with no increase in light gas make.