Fuel oil desulphurisation is a mature and critical process within the petroleum refining industry. Long before widespread environmental regulations, desulphurisation of oil stocks was essential for a variety of operational and commercial reasons, including: preventing platinum catalyst poisoning in reforming processes; reducing sulfurous acid corrosion in heating equipment; demetalizing crude stocks; recovering pure sulfur; and controlling final product odor. The foundational technology for this is hydrodesulfurization (HDS), a process that has been refined into several variants to handle different oil fractions.

2.1 The Hydrodesulfurization (HDS) Process

Hydrodesulfurization is fundamentally defined as the catalytic reaction of sulfur compounds with hydrogen to form hydrogen sulfide (H₂S), which can then be easily separated and processed. The process is complex, as it must treat a wide range of sulfur-containing molecules—such as mercaptans, sulfides, and thiophenes—all of which react at different rates.

The core chemical reactions are exothermic, meaning they release heat. Key examples include:

• Desulfurization of ethyl mercaptan: C₂H₅SH + H₂ → C₂H₆ + H₂S
• Desulfurization of diethyl sulfide: (C₂H₅)₂S + 2H₂ → 2C₂H₆ + H₂S
• Desulfurization of thiophene: C₄H₄S + 4H₂ → C₄H₁₀ + H₂S

The workhorse catalyst for these reactions is cobalt molybdate, whose chemical composition can be considered CoO · MoO₃. This catalyst is highly valued for its high activity, excellent heat stability, and long operational life, which typically ranges from two to five years. It facilitates the cleavage of sulfur-to-carbon bonds, converting the sulfur compounds into hydrogen sulfide and saturated hydrocarbons with minimal breaking of carbon-to-carbon bonds. Catalyst regeneration is accomplished at 700 to 1200°F using air with steam or flue gases.

2.2 Technologies for Light Oil and Gas Oil Desulphurisation

For lighter oil fractions, such as gas oils, several established fixed-bed reactor technologies are commercially deployed.

The GO-Fining Process

The GO-Fining process is designed for the relatively complete desulfurization of vacuum gas oils (VGO), cycle oils, and coker gas oil. In a typical application, atmospheric residuum is fed into a vacuum fractionation unit. The resulting VGO is then desulfurized in a fixed-bed reactor system and subsequently reblended with the vacuum bottoms to produce a lower-sulfur final fuel oil. A representative performance example involves processing a 3% sulfur Middle East atmospheric residuum feed to produce a 1.72% sulfur fuel oil. According to 1989 economic data, a 50,000 barrel per stream day (BPSD) unit required a total investment of approximately $16.3 million, with operating costs averaging 60¢ per barrel of fuel oil.

The UOP Gas Oil Desulphurisation Process

The UOP process offers an alternative for light oil desulfurization. Unlike GO-Fining, which processes a fraction of the residuum, the UOP scheme is designed for the near-complete (approximately 90%) desulfurization of a blend of light and vacuum gas oils and does not involve vacuum residuum. This commercially established process shares many operational parallels with GO-Fining. Its costs are of the same order of magnitude, differing primarily in the initial capital investment, partly because the UOP facility does not require an atmospheric residuum fractionation unit.

2.3 Technologies for Heavy Residuum Desulphurisation

Processing heavy residuum presents significant challenges not encountered with lighter distillates. The high concentration of asphaltenes and organo-metallic compounds—primarily vanadium and nickel—makes these feedstocks particularly difficult to treat in conventional fixed-bed reactors. These systems are prone to operational problems such as hot spots, coking, pressure build-up, and rapid catalyst deactivation from metals deposition, often requiring shutdowns for catalyst replacement as frequently as six times per year.

The H-Oil Process

To overcome the limitations of fixed-bed systems for residuum processing, the H-Oil process utilizes an ebullated bed reactor. In this design, feed oil and hydrogen flow upward through the reactor, suspending the catalyst particles in a liquid phase. This creates a back-mixed, isothermal environment that prevents hot spots and coking. Key advantages of this system include:

• Isothermal Operation: Temperature gradients within the reactor are minimal (no greater than 5°F).
• Constant Pressure Drop: Solids in the feed pass through the reactor without causing blockages.
• Continuous Catalyst Replacement: Catalyst can be added and removed daily without requiring a system shutdown, ensuring a steady state of activity.

The effectiveness and cost of the H-Oil process are highly dependent on the feedstock. Processing a low-metals Kuwait Residuum (Case 1) allows for operating conditions that maximize desulfurization (to 0.9% sulfur) while minimizing hydrocracking and hydrogen consumption. In contrast, treating a high-metals Venezuelan feed (Case 3) requires higher catalyst addition rates to compensate for deactivation, leading to significantly increased operating costs to achieve a similar 0.9% sulfur product. This data illustrates a critical strategic trade-off in residuum processing: feedstocks with lower upfront cost, such as high-metals Venezuelan crudes, directly translate to higher lifecycle costs due to accelerated catalyst consumption and more severe operating conditions.

The Isomax Processes

A range of fixed-bed processes, collectively known as Isomax, are also widely used for desulfurizing various crude and residual stocks. The four major variants offer different capabilities and economic profiles depending on the specific feedstock and desired product, as summarized in the table below.

Having reviewed the mature technologies for liquid fuels, the analysis now turns to the distinct processes used for the desulphurisation of natural gas.