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Apr-2018

Hydroprocessing rate increase using shaped charges

Catalyst size and shape are critical contributors to hydroprocessing reactor performance.

ADRIENNE VAN KOOPEREN, Criterion Catalysts & Technologies/Zeolyst International
JAMES ESTEBAN, Criterion Catalysts & Technologies
BRANDON MURPHY, Marathon Petroleum Company
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Article Summary
Achieving increases in hydrocracker capacity of 20-35% without any capital investment sounds impossible; however, novel changes in catalyst design have enabled just that for two North American refiners. Hydrocracking units have rapidly become one of the highest profile units in the modern refinery with increasing pressure to maximise charge rates up to a multitude of constraints including reactor system pressure drop. This article provides two examples of hydrocracking units processing significantly higher rates as a direct result of Criterion’s new hydrocracking catalyst shape.

Criterion and its customers have seen that a fine balance of activity and pressure drop has long since created a challenge when considering the maximisation of performance for hydroprocessing and hydrocracking units. It is especially a critical balance for high profile units in hydrocracking service that receive large margins for product upgrades and also have high incentives for incremental processing capacity. Recent margins have placed a great deal of pressure on refiners to maximise hydrocracking unit throughput up to hydraulic limitations which in many cases is a limit set by reactor pressure drop. Limitations in reactor pressure drop can be mitigated by many means, but ultimately catalyst selection is the most critical factor in hydrocracker optimisation. Criterion developed the Advanced Trilobe eXtra (ATX) catalyst shape to allow hydrocracking units to reduce pressure drop and improve activity simultaneously. There are several significant advantages of the ATX shape (see Figure 1), but it is first important to reflect on how catalyst shape affects reactor performance to understand fully the benefits of this revolutionary product.

Catalyst shape and size
All hydroprocessing reactor systems operate with a few standard objectives that apply from the smallest of naphtha hydrotreating applications to the largest of hydrocracking operations. While this list may seem rudimentary, every hydroprocessing unit must provide the desired catalytic activity, protection from feed poisons and the filtration of feed contaminants (though not generally a desired function for catalytic solutions). A properly designed catalyst system should employ a wide variety of shaped and sized particles to support this set of target objectives for each specific hydroprocessing unit large or small.
 
Grading catalysts
For several decades, the industry has capitalised on the advantages offered from graded bed solutions to enable improved performance with respect to increasing system pressure drop throughout the catalyst cycle life. This has employed the use of a multitude of materials that have varying void fractions and structures with a common objective to provide the optimum available bed void space and transition layers to remove contaminants from the feed stream over an extended portion of the catalyst bed. The application of grading materials and layers is common to hydroprocessing units as pressure drop across the leading bed remains a challenge for many units in the industry. This deep bed filtration phenomenon has led to the development of several extruded shapes with varying degrees of catalytic activity including, but not limited to, hollow cylinders, macroporous lobed particles, and specialty shaped extrudates (see Figure 2). In many cases, these materials developed as top bed grading are not suitable for a large volume of the reactor due to low inherent activity. In addition, many refiners are beginning to capitalise on new technologies with regard to reactor internals to further improve the filtration of feed contaminants and extend catalyst life cycles.1

Main bed catalysts
Historically, main bed catalysts used in hydroprocessing reactors were manufactured in the form of cylindrical shapes of varying diameters, but in the early 1970s American Cyanamid Company pioneered the production of shaped catalysts with the introduction of trilobe (TL) catalysts for residual oil and gasoil hydrodesulphurisation.2 To this day, a large majority of the hydroprocessing market’s catalysts are still manufactured in this same shape, and the transition in the past to the use of shaped catalyst particles is one of extreme importance because of the impact it has had on overall reactor performance. Currently, the two most common main bed catalyst shapes offered by hydroprocessing catalyst manufacturers are TL and quadlobe (QL) extrudates of varying particle diameters (see Figure 3). The particle length of commercial catalysts offered is variable to some degree within tolerances set by each manufacturer, but ultimately is determined by properties of the substrate mixture, operating conditions, and particle diameter as the weight of the extruded mixture drives the length by breakage of the extrudate simply as a function of gravitational force. While both catalyst shapes are common in industry, each offers a distinct set of advantages and disadvantages.

Multi-lobed catalyst shapes offer significant advantages in general over historical conventional shapes and have higher particle surface area (Sp) to particle volume (Vp) ratios when compared to a standard cylinder of equal particle length (Lp) and particle diameter (Dp). This increase in surface area results in greater activity as a result of reactions that occur on the catalyst particle surface, and those that occur within the pore structure of the catalyst pellets. Since many of the reactions that occur in the hydroprocessing reactor are governed by mass transfer, the advent of shaped particles has provided increased access for reactants to contact active sites in the pore structure closer to the centre of the pellet by reducing diffusion distance limitations. Inherently, this also provides greater selectivity when considering reactions that continue to propagate, such as the formation of light ends from over-cracking. Furthermore, an increase in the number of lobes is associated with increasing the overall catalyst effectiveness factor, which implies a catalyst with more lobes is capable of providing higher activity per volume, assuming that the catalyst is manufactured in the same manner as its comparison. Figure 4 illustrates this concept as a function of catalyst pellet effective diameter (dpe) as defined in Equation 3 for a typical hydrodesulphurisation operation.3,4 The effectiveness factor (η) and Thiele modulus (Φ) for the catalyst pellets are determined from the relationships in Equations 1 and 2:5
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