2.1 Strategic Importance and Motivations for Fuel Oil Desulfurization

Fuel oil desulfurization is a cornerstone of modern refining. In terms of environmental impact, a fuel oil with 3% sulfur produces approximately the same sulfur dioxide emissions as a coal with 2% sulfur on a comparable energy-release basis, making its treatment a high priority. Interestingly, the need for oil desulfurization was recognized long before the ecological imperative became a primary driver. Historically, oil stocks were treated for several key operational and economic reasons:

1. Catalyst Protection: To avoid the poisoning and deactivation of expensive platinum catalysts used in catalytic reforming processes.
2. Corrosion Prevention: To reduce the formation of sulfurous acid, which causes corrosion in heating equipment like home burners.
3. Demetalization: To remove trace metals such as sodium, vanadium, and nickel, as their removal often accompanies sulfur removal and improves fuel quality.
4. Sulfur Recovery: To recover pure, elemental sulfur as a valuable commercial byproduct.
5. Odor Elimination: To reduce or eliminate the unpleasant odors associated with sulfur compounds in the final product.

2.2 The Chemistry of Hydrodesulfurization (HDS)

The primary technology used to remove sulfur from fuel oil is hydrodesulfurization (HDS). By definition, HDS is the removal of sulfur through a catalytic reaction with hydrogen to form hydrogen sulfide (H₂S). It is not a single, specific chemical reaction but rather a complex treatment process applied to a wide variety of sulfur-containing organic compounds, including mercaptans, sulfides, polysulfides, and thiophenes. These different compounds react at varying rates, making the process a challenge of chemical engineering design.

The fundamental reactions are exothermic, meaning they release heat, as indicated by their negative enthalpy change (ΔH).

• General Reaction: CₙHₘSₚ + x H₂ → CₙHₘ₊₂ₓ₋₂ₚ + p H₂S
• Desulfurization of Ethyl Mercaptan: C₂H₅SH + H₂ → C₂H₆ + H₂S (ΔH = -19.56 kg cal/mole)
• Desulfurization of Diethyl Sulfide: (C₂H₅)₂S + 2H₂ → 2C₂H₆ + H₂S (ΔH = -36.54 kg cal/mole)
• Desulfurization of Thiophene: C₄H₄S + 4H₂ → C₄H₁₀ + H₂S (ΔH = -73.26 kg cal/mole)

The reaction mechanism is postulated to occur on the surface of the catalyst. Hydrogen molecules undergo activated absorption, dissociating into hydrogen atoms on the catalyst’s surface sites. The sulfur-bearing organic molecules, which are more strongly absorbed, then interact with these hydrogen atoms. This interaction cleaves the carbon-sulfur bonds, converting the sulfur into hydrogen sulfide and saturating the hydrocarbon molecule without breaking carbon-carbon bonds.

2.3 General HDS Process Flow and Challenges

For processing lighter distillates, the flow design of an HDS system is relatively simple. Preheated oil and hydrogen are pressurized and brought into contact with the catalyst in a reactor. The reactor effluent is then passed through one or more separators to remove hydrogen and light hydrocarbon gases, which are often recycled. The remaining liquids are then treated (e.g., stripped) to remove the dissolved hydrogen sulfide and yield the final, desulfurized product.

This simplicity vanishes when processing heavy residuum stocks. Residuum properties vary widely in terms of viscosity, Conradson carbon content (a measure of coking tendency), and, most critically, metal content. Vanadium and nickel are particularly problematic as they deposit on the catalyst surface, causing poisoning and deactivation. This rapid deactivation means the catalyst must be frequently regenerated or replaced, adding significant complexity and cost to the process.

2.4 Light Oil Desulfurization Technologies

Several commercial processes have been developed to desulfurize the lighter fractions of crude oil.

The GO-Fining Process

The GO-Fining process is designed for the deep desulfurization of vacuum gas oils (VGO), thermal and catalytic cycle oils, and coker gas oils. The process begins by feeding atmospheric residuum to a vacuum fractionation unit. The resulting VGO is then desulfurized in a fixed-bed reactor system and reblended with the vacuum bottoms to create a final, lower-sulfur fuel oil.

A typical 50,000 barrels per stream day (BPSD) operation using a 3% sulfur Middle East atmospheric residuum feedstock would work as follows:

• The initial 50,000 BPSD of residuum is fractionated, yielding an intermediate stream of 33,400 BPSD of VGO with 2.33% sulfur.
• This VGO is desulfurized in the GO-Fining unit.
• The treated VGO is then blended with the untreated vacuum bottoms to produce a final product of 49,700 BPSD of fuel oil with a sulfur content of 1.72%.
• Based on 1989 figures, the total investment for such a plant was approximately $16.3 million, with operating costs around 60¢ per barrel of fuel oil.

UOP’s Gas Oil Desulfurization Process

This process is a commercial alternative to GO-Fining, designed for nearly complete (~90%) desulfurization of a 630°F to 1050°F blend of gas oils with an initial sulfur content of about 1.5%. The key difference from GO-Fining is that vacuum residuum is not involved in the process, meaning a dedicated atmospheric residuum fractionation unit is not required. This distinction can significantly lower the initial capital investment compared to a GO-Fining facility.

Catalyst Technology: Cobalt Molybdate

The effectiveness of these processes hinges on the catalyst. The Union Oil Company developed a highly effective cobalt molybdate (CoO · MoO₃) catalyst capable of handling a full range of petroleum stocks. Its key attributes include:

• High Activity: Effective at removing even refractory sulfur compounds like thiophenes.
• Durability: Excellent abrasion resistance and high heat stability, retaining activity even after calcination at temperatures up to 1470°F.
• Long Life: A typical catalyst life is between two and five years.
• Regenerability: It can be regenerated by treating with air and steam or flue gases at 700-1200°F to burn off poisons like carbon and sulfur.

2.5 Alternative Strategy: Low-Sulfur Crudes and Blending

An alternative to technological desulfurization is to source and refine naturally low-sulfur crude oils. Major sources for these crudes include North African fields (primarily Libya and Nigeria) and some Far Eastern fields (Sumatra). Fuels derived from these crudes can meet very stringent regulations of 0.5% sulfur or less.

However, these crudes present a significant operational challenge. They are typically highly waxy and paraffinic, which makes handling them difficult and costly, often requiring specialized heated storage and transport infrastructure. Therefore, a more practical and “palatable” course of action for meeting moderate sulfur regulations (1-2% S) is to blend these premium low-sulfur oils with more readily available high-sulfur fuel oils.