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Membrane separation for clean fuels

An illustration of how membrane separation technology can be integrated into a clean fuel strategy at low capital cost relative to hydrotreating

Xinjin Zhao and Gautham Krishnaiah, Grace Davison
Todd Cartwright, CB&I Process and Technology
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Article Summary
Refiners worldwide are rushing to develop their strategy for economically and reliably meeting clean fuel regulations. Many of the North American refiners need to meet the average 30ppm and 300ppm sulphur cap requirements on gasoline by the end of 2004. Although a large number of refiners have already chosen a primary strategy for sulphur reduction, there are still many refiners who are finding ways to postpone decisions, such as with sulphur credits, utilisation of sulphur reduction catalysts etc.

Refiners granted small refinery or hardship status have a little more time to evaluate emerging technologies and options. Even refiners who have already made their decisions are re-evaluating their strategy with the changing market or financial situation. With their clean fuel regulations lagging, most refiners outside Europe and North America have a little longer to make their eventual investment.

Whatever the process solution chosen by a refiner, all the technical requirements must be met. In addition to the technical requirements, however, business and operational concerns often dictate the decision-making process leading to the chosen process solution. Issues include low capital investment for compliance, ease of implementation and flexibility for future changes in regulations. Capital investment is very tight and refiners view the investment in sulphur reduction technology as regulatory driven and not revenue and profit generation driven. As a result, capital avoidance and capital spending delay are common.

In general, there are two classes of technology for refiners’ clean fuel solutions: stand-alone technology and complementary technology. Hydrotreating based technologies are readily accepted partly due to refiners’ familiarity with the technology. While there are several excellent hydrotreating technology options available to address the gasoline sulphur compliance technical requirements, they are not without problems, including high capital costs, gasoline quality degradation and operability challenges, especially when the gasoline sulphur requirement drops below 30ppm. For example, one of the challenges associated with hydrotreating-based technology has been the octane loss issue due to olefins saturation. The more selective hydrotreating technologies have partially alleviated the octane loss. However, this comes with additional capital and operating cost requirements.

In addition to the capital requirement for the hydrotreater itself, refiners very often need to expand or build grassroots hydrogen capacity to meet the hydrogen needs, further increasing capital requirements. The proprietary S-Brane membrane separation technology is designed to fill this gap and provide a complementary technology for achieving gasoline sulphur compliance requirements.

The core of the technology provides a low capital cost tool while also improving the refiner’s capital spending and future regulatory compliance flexibility [Krishnaiah G and Balko J, Reduce ulta-low sulfur gasoline compliance costs with Davison clean fuels technologies;  NPRA 2003 annual meeting, San Antonio, Texas, USA.

Zhao X et al, S-Brane membrane technology for gasoline sulfur reduction; 21st Annual Membrane/Separations Technology Planning Conference, Newton, Massachusetts, USA, 1-2 December 2003]. The S-Brane process removes sulphur-containing molecules (along with a fraction of the total gasoline volume) from FCC gasoline. This relatively sulphur-free stream (typically about 75%+ of the total volume) can be blended directly into the gasoline pool, while the 25% (or less) stream containing all the sulphur represents a significantly lower volume requiring hydrotreating before it is blended back into the gasoline pool. Primary advantages of the technology include:
- A substantial reduction in the amount of octane-sensitive, high olefin gasoline that requires capital-intensive hydrodesulphurisation
- Low capital cost (about 20 to 25% the cost/capacity bbl of other gasoline desulphurisation technologies)
- Simplicity in design and operation
- Modular design allowing for simple unit expansions and revamps for future regulation change
- Low operating cost (process runs at low temperatures and pressures relative to other gasoline desulphurisation processes)
- An inherently safer sulphur removal process asset, owing to the fact that the operating pressures and temperatures are much lower than traditional desulphurisation processes and no chemical reactions occurring.

Process description
S-Brane is a simple physical separation process that separates a feed gasoline stream into two product streams. A feed stream (ideally a C5-350°F FCC gasoline stream) is passed over a specially formulated polymeric membrane that is selective for sulphur-containing hydrocarbon molecules. As the gasoline passes over the membrane, sulphur-containing molecules and some other hydrocarbon molecules dissolve into the membrane and subsequently diffuse (or permeate) through the membrane.

The resulting low sulphur stream known as retentate (about 70 to 80% of the inlet gasoline volume) can be directly blended into the gasoline pool. A smaller stream (permeate) contains the majority of the sulphur and must be processed further before being blended into the gasoline pool. This separation is illustrated in Figure 1 (previous page). Note that no chemical reactions take place in this process and all molecules are essentially preserved.

S-Brane operates as a pervaporation mode membrane process. In pervaporation mode membrane processes, the driving force that moves molecules through the membrane is a vacuum on the backside (permeate side) of the membrane that causes a vapour pressure gradient for each type of molecule. The shell side of the assembly is operated under vacuum conditions (about 0.75– 1.5psia). Sulphur and hydrocarbon permeate through the membrane. Once through the membrane, these molecules are then condensed and passed along for further processing to remove the now concentrated sulphur species.

As mentioned previously, the relatively small volume of sulphur-rich gasoline that moves through the membrane is called permeate while the high volume of ultra-low sulphur gasoline that does not pass through the membrane is called retentate.

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