3.1 The Unique Challenge of Residuum Processing

Desulfurizing residual oils—the heavy, viscous “bottoms” of the refining process—is far more challenging than treating lighter distillates. The primary reason for this difficulty lies in a fraction known as asphaltenes. Asphaltenes are characterized by very high molecular weights (several thousand) and are the fraction where organo-metallic compounds are concentrated. While many metals are present in trace amounts, vanadium and nickel are by far the most abundant and problematic. It is the significant variation in asphaltene and metal content between residual oils from different crudes that causes them to behave so differently during hydrodesulfurization.

3.2 The H-Oil Process: An Innovative Reactor Design

The H-Oil process, developed by Cities Service, was engineered specifically to overcome the challenges of residuum processing.

Problems with Conventional Fixed-Bed Reactors

When used for residual oil, conventional fixed-bed reactors encounter several major difficulties:

1. Hot Spots and Coking: The high heat release from exothermic reactions can lead to localized “hot spots” within the stationary catalyst bed, causing thermal cracking and the formation of coke, which deactivates the catalyst.
2. Pressure Drop Buildup: Solids present in the feed, along with coke deposits, gradually plug the catalyst bed, leading to a continual buildup of pressure drop across the reactor.
3. Frequent Shutdowns: The rapid deactivation of the catalyst by metals and coke necessitates frequent shutdowns for catalyst replacement—often as many as six times per year—which severely impacts plant availability and economics.

The Ebullated Bed Solution

The H-Oil process solves these problems by using an ebullated bed reactor. In this design, feed oil and hydrogen flow upwards through the reactor, suspending the catalyst particles in random motion within the liquid phase. This creates a back-mixed, isothermal environment where the temperature gradient between any two points is no greater than 5°F, eliminating hot spots.

This design offers two crucial advantages:

• Constant Pressure Drop: Since the catalyst is suspended and not packed, any solids in the feed pass directly through the reactor, and the pressure drop remains constant.
• Continuous Operation: Catalyst can be added and withdrawn on a daily basis while the reactor is operating, eliminating the need for periodic shutdowns and maintaining a steady state of catalyst activity.

Performance Analysis

The performance of the H-Oil process is highly dependent on the feedstock, as shown in the data in Table 2.

Table 2: H-Oil Desulfurization Performance with Atmospheric (A) and Vacuum (V) Residuals

Let’s analyze this table. A comparison of Case 1 (Kuwait) and Case 3 (Venezuela) clearly illustrates the impact of metals. Pay close attention to the metals column here, as it’s the key driver of the economic difference. The Venezuelan crude, with its very high metals content (320 PPM), deactivates the catalyst much more rapidly, requiring higher catalyst addition rates and thus increasing operating costs. Similarly, a comparison of atmospheric (Cases 1-3) versus vacuum (Cases 4-6) residua shows that desulfurization is more difficult for vacuum bottoms. The concentration of asphaltenes and metals in vacuum residua leads to lower desulfurization rates and significantly higher hydrogen consumption, as more hydrocracking occurs alongside desulfurization.

3.3 The Isomax Family of Processes: A Comparative Overview

Isomax refers to a broad spectrum of fixed-bed desulfurization and hydrocracking processes used globally. While they share the fixed-bed design, they are tailored by different licensors to handle a wide range of feedstocks, from atmospheric residuum to whole crude oil, each with different economic trade-offs.

The RCD Isomax process, licensed by UOP, is designed for atmospheric residuum. As shown in the process diagram (Figure 4), it’s a relatively straightforward system. For a typical Kuwaiti atmospheric residuum feed with 3.9% sulfur, it can produce a fuel oil product containing 1.0% sulfur. Its relative investment cost is comparatively low, making it a viable option for targeting the heaviest fraction of atmospheric distillation.

Chevron’s RDS Isomax process also targets atmospheric residuum (Figure 5), but the economics presented suggest a significantly higher investment than the RCD process for a similar application. For an Arabian Light residuum feed (3.1% sulfur), it also achieves a 1.0% sulfur product, but its higher capital cost may reflect a more robust design for handling different crudes or achieving longer catalyst cycles.

A far more ambitious and capital-intensive option is Chevron’s CDS Isomax process (Figure 6), which is designed to desulfurize whole crude oil. By treating the entire crude stream (e.g., Arabian Light at 1.7% S) before distillation, it produces an entirely desulfurized synthetic crude from which low-sulfur products, including a 1.0% S fuel oil, are fractionated. The key takeaway here is the economic scale: its relative investment cost is more than 15 times that of the RCD Isomax, reflecting the massive scale of processing the entire crude barrel at high pressure.

Finally, the HDS process from Gulf R&D (Figure 7) is a residuum desulfurization technology that, in the example provided, tackles a very high-sulfur feed (5.5%) and reduces it to 2.2%. Its investment cost is on par with the RCD Isomax, positioning it as a competitive technology for heavy oil treatment where the goal is significant, but not necessarily deep, desulfurization.

Table 3: Comparative Summary of Major Isomax Processes

3.4 Concluding Economic Perspective on HDS

Hydrodesulfurization of heavy oils is an effective but relatively expensive process. By 1989 standards, costs often exceeded 75¢ per barrel. Several factors contribute to this cost:

• Capital Investment: The need for large, robust reactors capable of operating at high pressures and high temperatures.
• Hydrogen Consumption: Hydrogen is a key reactant and represents a significant operational cost.
• Catalyst Use: Large volumes of catalyst with a relatively short life are required, especially for high-metal feedstocks.

Processing costs are highly dependent on the characteristics of the feedstock. However, when one considers the awesome annual alternative of 30 million tons of sulfur dioxide being pumped into the atmosphere, the cost of these essential technologies seems trifling indeed.