Single Guard Layer of Moderate Activity, High Nickel Based Catalyst is Proven to be Effective when Processing Opportunity Crudes and Cracked Feedstocks Contaminated with Arsenic and Silicon
Charles Olsen, Advanced Refining Technologies
Detection of unexpected levels of hydroprocessing catalyst poisons such as arsenic (As) and silicon (Si) may coincide with the processing of heavier opportunity crude feeds. In recent years, Advanced Refining Technologies (ART) has seen an increase in the number of units experiencing poisoning from arsenic and silicon as more of these opportunity feeds are processed.
Arsenic is found in certain crudes including some from West Africa, Russia and Venezuela (e.g., 20-284 ppb As for certain Venezuelan crudes). This contaminant is also found in many synthetic crudes. Unlike silicon from antifoams, evidence suggests that arsenic is distributed throughout the whole boiling range.
Arsenic is a permanent catalyst poison, which means once it is on the catalyst it cannot be removed via regeneration or other means. It is a severe poison since a small amount results in significant activity loss. The arsenic is believed to bind with the metal sulfide sites, and in particular the active nickel on the catalyst, forming nickel arsenide.
To demonstrate the impact of arsenic on catalyst activity, ART obtained a series of spent catalyst samples from a refiner. The samples had different levels of arsenic because of their location in the reactor. These samples were carefully regenerated in the laboratory and then activity tested using a diesel containing 50% FCC LCO under conditions producing less than 500 ppm sulfur (< 500 ppm S).The catalyst had lost 15°F of activity after only 1000 ppm of arsenic poisoning (0.1 wt% As poisoning), and the activity loss quickly increases to about 60°F at 1.0 wt% arsenic on the catalyst.1,2
The ultimate arsenic capacity of a catalyst is strongly dependent on temperature, similar to silicon pick up. To show how arsenic pick up varies as a function of catalyst temperature for an ART NiMo catalyst, an analysis was performed on spent samples retrieved from a three-reactor unit processing 100% cracked naphtha from a synthetic crude source.
The first reactor was operated at very low temperature (~275°F) in order to saturate diolefins; the second reactor was designed to saturate mono-olefins and operated at about 430°F; the last reactor had an inlet of 570°F and an outlet temperature of approximately 650°F. The nearly linear increase in arsenic content on the catalyst correlated with reactor temperature, thus demonstrating the temperature dependence of arsenic pick up, further indicating that a high nickel catalyst can pick up very high arsenic levels if the operating temperature and feed concentration are high enough.
Combined Arsenic and Silicon Control
A number of catalyst basket tests and spent catalyst data have provided additional evidence that catalysts containing nickel are much more effective for trapping arsenic. ART researchers found that the relative arsenic pick up observed on a variety of catalysts correlates well with catalyst nickel content. These observations, combined with the increasing incidence of catalyst poisoning from arsenic, prompted ART to develop a high surface area arsenic guard catalyst utilizing the highly successful AT724G catalyst technology platform.
The goal of the development was to provide a single catalyst for applications suffering from both silicon and arsenic poisoning. In those types of units catalyst suppliers have typically recommended two guard catalysts, one each for silicon and arsenic. With the commercialization of AT734G catalyst, one guard layer is sufficient for both contaminants leaving more reactor volume available for the active catalyst. For example, AT734G picks up about four times more arsenic compared to AT724G, and even picks up more than some high nickel catalysts like NDXi and AT580.
Data from another catalyst basket that was installed in a unit processing cracked naphtha from a synthetic crude again demonstrated how the AT734G catalyst significantly outperforms AT724G and AT535 in terms of arsenic capacity. While the catalyst basket data confirms the superior arsenic pick up of AT734G, the silicon pick up of AT734G and AT724G was also compared by installing catalyst baskets in coker naphtha units as well as diesel units suffering from silicon poisoning. The collected data show that the silicon pick up of AT734G is on average the same as the silicon pick up of AT724G.
Processing Cracked Stocks
A few basket samples showed extremely high silicon pick up, in excess of 25 wt% Si. These were installed in diesel units and experienced much higher temperatures than some of the baskets showing lower silicon values. The data documented from the basket samples demonstrates that ART has successfully commercialized a combined silicon and arsenic guard catalyst, AT734G. Similar to AT724G, it has moderate activity making it suitable for activity grading in applications processing cracked stocks. Several refiners have already selected AT734G because of its high capacity for both silicon and arsenic. It has quadruple the arsenic capacity of the AT724G silicon guard, but with the same silicon capacity making it an excellent new addition to the StART™ Catalyst System.
Editor’s Note: This discussion was summarized from a more detailed report published with supporting graphics and tables in a prior issue of ART Catalagram : “AT734G: A Combined Silicon and Arsenic Guard Catalyst.”
- Olsen, Charles (“Chuck”), “AT734G: A Combined Silicon and Arsenic Guard Catalyst,” ART Catalagram, Issue No. 108, Special Edition, 2010, pp.s 9-13.
- Olsen, Chuck, “Processing Challenging Feedstocks,” Refinery Operations, Vol. II, No. 3, February 2, 2011.
Charles “Chuck” Olsen is Worldwide Technical Services Manager, Advanced Refining Technologies (ART) in Chicago, Illinois, USA (Chuck.firstname.lastname@example.org). ART is the joint venture of Chevron Products Company and W. R. Grace & Co.’s Grace Davison catalysts business unit (www.grace.com).