Optimizing hydroprocessing unit performance

Steve Mayo, Global Application Manager of Hydroprocessing Catalysts with Albemarle, explains why a skilful approach to catalyst system design is becoming ever more apparent and how new technology is improving unit performance.

Use of a single catalyst in hydrotreaters has been the norm for many years. Apart from exceptions, such as fixed-bed resid, this approach was sufficient for the majority of units to achieve the expected performance. As demands on hydrotreating units have increased and product requirements have become more stringent, the advantages of a more skilful approach to catalyst system design become apparent.

Albemarle’s STAX technology is the application of an understanding of hydroprocessing chemistry to the optimization of unit performance. Catalyst selection and catalyst system design are made according to reaction chemistry and process conditions in each part of the reactor, tuning it to performance and process objectives.

Understanding the chemistry occurring inside the reactor is key to designing a catalyst system for optimum performance. The problem is that there are many independent variables. The chemistry taking place at any point in the reactor both affects and is affected by these variables. No two slices of the reactor along its vertical axis will have the same reaction environment. The best catalyst for each slice could be different.

In ULSD a conceptual reaction zone model with three zones can be envisaged. The size and position of each zone varies as a function of feedstock properties, operating conditions and process objectives.

The three zones have different reaction conditions affecting catalyst performance. In zone 1, the primary reaction occurring is direct route HDS. The rate of desulfurization is fast and sulfur content drops rapidly. As the rate of direct route HDS slows, the feedstock moves into zone 2, removal of nitrogen becomes the key reaction. Zone 2 ends when organic nitrogen has been almost completely removed. The feedstock enters zone 3 where hydrogenation route HDS increases. The ppH2 is at its lowest in zone 3 but because the catalyst is operating in a nitrogen-free environment the rate of hydrogenation increases. Finally the feedstock exits the reactor as ULSD.

Understanding where reaction conditions change presents an opportunity to improve system performance. Rather than an average level of performance from a single catalyst across all zones, an optimized catalyst system can be designed using catalysts that perform well in a particular regime. This is the essence of the STAX approach – superior performance through skilful design of catalyst systems.

Key to the application of STAX technology is a model to predict reaction conditions at any point in the hydrotreater. Albemarle has developed techniques to effectively see into the reactor and develop models based on those observations. Changes in sulfur, nitrogen and aromatics are tracked, slice by slice. GCxGC techniques are used to segregate heteroatoms by structure.

Stacked beds have been used for decades to enhance HDS or HDN by applying CoMo and NiMo catalysts. Performance of these stacked systems is the average of the HDS and HDN activities of the catalysts used. The performance of a STAX catalyst system is better than the performance of any catalysts in the system, individually or on average.

Meeting objectives
Key performance and process objectives include:
•  Maximum HDS with constrained hydrogen consumption
•  Maximum cetane uplift up to unit hydrogen limit
•  Maximum cycle length at specific S target
•  Simultaneous sulfur and volume gain target