Apr-2004

Reducing gasoline sulphur with additives

Evolution of regulations in an era of limited capital has led to the development of catalyst additive technologies to reduce FCC gasoline sulphur

Michael K Maholland
Intercat (Johnson Matthey)

Article Summary

In many of the world’s refining markets, the implementation of ultra-low sulphur gasoline and diesel fuels is resulting in processing changes. For example, the US Environmental Protection Agency’s (EPA) Tier 2 gasoline sulphur control programme requires gasoline sulphur levels to average 30ppm, with a cap of 80ppm, by 2006. This represents a 90% reduction in gasoline sulphur from baseline levels. These low sulphur levels are the basis for many of the processing changes affecting FCC-based refiners, not only in the USA, but also in other areas throughout the world. In Europe, for example, gasoline sulphur must be reduced to 50ppm in 2005 and 10ppm by 2009.

To achieve these reductions in gasoline sulphur, the refiner has a number of options to choose from. Most options involve tradeoffs between high capital costs and gasoline yield loss. However, the use of sulphur-reducing additives, such as those of Intercat’s LGS family of additives, may offer an economically attractive alternative. These additives can provide the refiner with a capital-free option to reduce gasoline sulphur without the loss of gasoline yield.

Gasoline specifications in the USA have undergone substantial evolution over the past 30 years. Prior to the 1970s, there was essentially no environmental regulation of gasoline composition in the USA. Virtually all motor gasoline contained tetraethyl lead (TEL) or a blend of TEL and MMT (methyl-cyclopentadienyl manganese tricarbonyl) as octane-enhancing additives. Gasoline compositions between brands were fairly consistent during the various seasons, and specifications were set up to meet fungibility requirements for pipelines and inter-company exchanges.

Blending gasoline was a relatively easy task. Refiners typically added three or more grams of TEL per gallon of gasoline for octane enhancement. Seasonal adjustments for vapour pressure (depending on geographic location) varied between summer-grade gasoline vapour pressure of between 10.0 and 11.5Rvp, and winter-grade gasoline with a typical vapour pressure of 15.0Rvp. Product grades at that time were generally limited to regular and premium.

By the late 1980s, refiners were getting a taste of what was to come. Products were no longer readily fungible due to the evolution of fuel regulations and reformulations over the course of about 20 years, and specialty blends had to be made to accommodate local and state requirements. The supply and logistics system that was initially developed on a two-grade product slate was now under pressure.

In 1989 the EPA began implementing a two-stage programme to meet lower volatility standards in gasoline. This resulted in restricting the content of normal butane, with its high RVP of 52. For refiners, normal butane is an economically attractive blend stock because it is produced in relatively significant quantities in the refining process, has a high research octane number of 93.0, and has few alternative uses other than refinery fuel. The restriction on the use of normal butane as a blending additive had a negative economic impact on refiners.

Complications
The US Clean Air Act Amendments of 1990 resulted in implementation of the Reformulated Gasoline (RFG) programme. Under this programme, certain areas of the country were mandated to use a cleaner burning fuel called RFG. These regions were designated non-attainment areas, as they did not meet the ambient air quality standards established by the EPA. Attainment areas could continue to use conventional gasoline. Anti-backsliding rules were enacted to prevent the conventional gasoline from becoming “dirtier” than it was in the past. The programme had a two-step implementation schedule. Phase I was implemented in 1995 and Phase II in 2000.

Phase I RFG mandated a reduction in VOC (volatile organic compounds) by 17%, NOx (nitrogen oxides) by 2%, and toxic pollutants (air toxics) by 17%. Now, computer models were required to determine compliance with Phase I reductions. Blend-by-blend reporting was required and penalties for even minor errors were enormous. Gasoline blending became extremely complicated.

Refiners began feeling the economic pinch as gasoline components required additional treatment, extraction or removal from the gasoline pool altogether. Benzene levels in gasoline were limited. Oxygenates were mandated in gasoline but could not exceed 2.7 wt% oxygen in the blend. Vapour pressures were lowered and limitations on gasoline distillation became necessary to meet the guidelines. Not only did the produced gasoline have to meet EPA specs, it had to still meet general industry specs.

Refiners serving regions that had both RFG and conventional customers now had to blend and segregate six or more grades of gasoline. Under the rules, certain areas even had different oxygen content requirements, so it was possible that a refiner had to blend up to nine different grades at any one time.

Phase II RFG took effect on 1 January   2000. The established reduction levels were 27% for VOC, 7% for NOx and 22% for toxics. Refiners must meet pool (average) emission reductions and individual blend reductions. The record-keeping under this programme continues to be a substantial burden. The use of EPA’s emissions calculation model is mandated. It is appropriately called the Complex Model.

As previously stated, the EPA RFG programme mandated the use of oxygenates in gasolines sold in non-attainment areas. MTBE was typically the refiner’s choice of oxygenate. However, because of recent concerns in America about leaking underground gasoline storage tanks and the public health risk of MTBE-contaminated groundwater, many US states have banned the blending of MTBE into gasoline in part or entirely. As a result, refiners are phasing out MTBE in favour of ethanol. Removal of MTBE from the gasoline pool requires not only the replacement of the lost volume but also the oxygen content, octane, and emissions-reducing properties it provides to reformulated gasoline.

Aromatics and olefins are generally high-octane stocks but they are also “bad actors” in that they produce air toxics. The RFG programme required benzene reductions in all grades of gasoline. The principle source of benzene in gasoline comes from catalytic reformers.