Oct-2024

Understanding the decomposition of TBPS for efficient catalyst sulphiding

Identifying optimal operating conditions for sulphiding without negatively impacting the performance of hydroprocessing catalysts.

Jennifer A Jackson, The Lubrizol Corporation
Tiago Vilela, Avantium R&D Solutions

Article Summary

Hydroprocessing plays a crucial role in the refining industry. To ensure the catalytic activity of hydrotreating catalysts, sulphiding is employed. Sulphiding involves the conversion of initially inactive metal oxides present in the catalysts into active metal sulphides, significantly enhancing their performance in hydroprocessing. However, achieving efficient catalyst activation through sulphiding is not without challenges.

The sulphiding process requires careful selection of sulphiding agents that can effectively convert metal oxides into sulphides under specific operating conditions. One notable sulphiding agent is tert-butyl polysulphide (TBPS), a commercially available compound provided by The Lubrizol Corporation and under the proprietary tradename SulfrZol 54.

In contrast to traditional sulphiding agents like dimethyl disulphide (DMDS), TBPS offers several advantages. It has more ideal health and safety characteristics, a higher flash point (DMDS: 16°C vs TBPS: 100°C), reduced odour (DMDS: foul, TBPS: diesel-like), and improved emissions profile. When TBPS is used as a sulphiding agent, it produces butane and hydrogen sulphide (H₂S) as by-products, while DMDS yields methane (and methylmercaptan) and H₂S. This distinction is significant as it contributes to a safer, cleaner, and more effective catalyst activation process in refineries.

Decomposition profile
The primary objective of this study is to investigate and illustrate the decomposition profile of TBPS as a sulphiding agent. To achieve this objective, an experimental programme was conducted involving the sulphiding of three commercial catalysts commonly used in diesel and naphtha hydrotreating processes: CoMo, NiMo, and NiW.

The experimental tests covered a wide range of operating conditions, including varying temperatures, pressures, space velocities, and gas-to-oil ratios. These conditions were carefully selected to simulate real-world hydrotreating processes and to evaluate the impact of different parameters on the sulphiding process. By exposing the catalysts to TBPS under selected operating conditions, we aimed to facilitate the decomposition of the sulphiding agent into H₂S and isobutane, with intermediary components consisting of butyl mercaptans and iso-butene.

Relatively low temperatures used in this study (ranging from 150°C to 240°C) were chosen to reflect typical operating conditions in sulphiding hydrotreating processes. At these temperatures, only a fraction of the surface metals on the catalysts are likely to convert into sulphides. The transformation from metal oxides to sulphides typically occurs sequentially as operating temperature increases. Therefore, understanding the TBPS decomposition profile is crucial for optimising the sulphiding process.

This investigation focused on determining the decomposition profile of TBPS under different operating conditions. The aim was to identify the temperature at which TBPS starts decomposing and the influence of catalysts on its decomposition. Additionally, we analysed the formation and consumption rates of the intermediate components (isobutene and C₄ mercaptans) and the final products (H₂S and isobutane), considering factors such as temperature, catalyst type, H₂ partial pressure, gas-to-oil ratio, and liquid hourly space velocity (LHSV).

By comprehensively studying the decomposition profile of TBPS and its effects on catalysts, the sulphiding process can be optimised to ensure efficient activation without compromising catalyst performance. Results of this study will provide valuable insights for refiners, helping advance hydroprocessing techniques and contributing to the development of cleaner and more sustainable refining practices.

Experimental
This study was conducted in a 16-parallel fixed-bed reactor system with stainless-steel reactors with a diameter of 2mm and a total length of 561mm, including an isothermal section of 300mm. The gaseous product was analysed using an online gas chromatograph (GC), while the liquid reaction product from each reactor was collected in individual sample vials for subsequent offline analyses.

Two liquid samples per 24 hours were collected, and two were selected for offline analysis. To maintain stable conditions during gas-liquid separation, the sample vials were temperature-controlled. Gas exiting the vials was analysed using the online GC, which provided measurements of hydrocarbons, H₂, He, and H₂S.

Avantium’s Flowrence technology relies on the use of single-string reactors, wherein catalyst particles are arranged in a specific order. This arrangement allows for the decoupling of axial dispersion phenomena from fluid dynamics, facilitating ideal plug flow reaction conditions near the reactor inlet.

For a more detailed understanding of the theory supporting the use of single-pellet-string reactors, refer to the publications by Moonen et al1 and Ortega et al.²

Test design
The experimental programme aimed to evaluate the decomposition profile (by-products yield vs temperature) of the sulphiding agent TBPS at typical operating conditions used for the sulphiding of hydroprocessing catalysts. By studying the presence of the different by-products formed during the catalyst sulphiding, the objective was to identify the operating conditions that favour the use of TBPS as a sulphiding agent without affecting the catalyst’s performance. The experimental programme for studying the decomposition of TBPS was conducted in three different runs performed at a testing unit with 16 reactors in parallel.

The catalyst loading was carried out using three commercial catalysts: CoMo, NiMo, and NiW 1/16in trilobe extrudates obtained from Topsoe. Before loading, no crushing, sieving, or other structural changes were performed on the catalyst particles.

In all three runs, the reactors were operated under constant conditions for a duration of 12 hours. Following four hours of operation at each condition, the analysis of gas effluent commenced, proceeding sequentially from reactor 1 to 16. This sequential analysis spanned a total duration of four hours, allowing approximately 15 minutes per reactor. During this time, a single cumulative liquid sample was collected. This sampling procedure was repeated during the final four hours of operation for each testing condition, with the second sampling phase considered more representative due to the longer stabilisation time.