Apr-1998

Catalytic distillation to enhance gasoline quality: Part I

Initiatives towards the improvement of gasoline motor fuel quality means refiners must modify their processing facilities to meet higher standards

Kerry L Rock, Richard M Foley, Hugh M Putman, Amarjit S Bakshi and Monoj Som, CDTech

Article Summary

Gasoline quality improvement initiatives in the USA are at the most advanced level of implementation. Lead has already been removed and reformulated gasoline (RFG) reduces emissions of ozone precursors and toxics through limits on RVP and benzene. Oxygenates are added to reduce carbon monoxide emissions.

This RFG programme was applied only to those geographic areas with the worst emissions, and currently affects about one third of US gasoline. The Complex Model relates emissions to gasoline properties and provides blending flexibility for refiners.

Further reductions in hydrocarbon and toxics emissions along with new restrictions on NOx will begin in 2000. Recently, the Environmental Protection Agency (EPA) introduced greater reductions on ozone levels and particulate matter, which will affect diesel fuel as well as gasoline in future years.

Another developing issue relates to sulphur in gasoline. The EPA continues to push auto manufacturers to increase gasoline mileage in new vehicles. The auto manufacturers claim they have done all that can be done to improve mileage without improvements in gasoline composition. Specifically, they see lower sulphur content as necessary to enable the emissions detectors to function correctly. If this issue proceeds as expected, the US will have 100 per cent RFG with less than 50ppm sulphur by 2010.

In Europe, the European Parliament has issued a directive to the Council of Ministers to legislate a legal and possibly fiscal framework for the improvement of European motor fuel quality. Their proposed specifications represent substantially greater improvements than have previously been proposed by the European Auto-Oil Programme (EPEFE) (Table 1), and discussions continue on the appropriate levels. Virtually all are agreed on the need for full lead removal, but the required levels of volatility reduction, aromatics and/or benzene reduction, and sulphur removal are still undecided. EU inclusion of eastern European nations will eventually expand these EU quality standards throughout much of the continent.

In the Asia-Pacific area, a broad diversity of gasoline specifications exists in this market, reflecting the spectrum of economic development levels represented in the region. Gasoline production is already more than 70 per cent unleaded, an indication of the high percentage of gasoline consumption represented by South Korea and Japan. Continuing diversity is expected. However, lead reduction, benzene reduction, and sulphur removal will all progress at varying speeds, depending on many regional factors.

Refinery changes required
In each of these regions, refiners will have to modify their facilities to meet the new quality specifications. Each refiner will have a somewhat different solution depending on his existing processing facilities and the market that he serves.

CDTech offers a set of CD refining technologies that may be applied to any hydroprocessing refinery to allow production of gasoline meeting the improved quality specifications:

FCC gasoline desulphurisation
FCC gasoline is the most significant contributor of sulphur to the gasoline pool in any hydroprocessing refinery. The octane contribution of this component is partially offset by the volatility of the light olefins and the olefin content relationship to emissions. This article discusses a catalytic distillation processing option for significant reduction of sulphur in FCC gasoline while retaining its high-octane value.

Reformer feed and product hydrogenation
Benzene levels in gasoline are most readily controlled by reformer feed and product fractionation and hydrotreating. Reduction of C6 benzene precursors in the gasoline reformer feed reduces benzene production to the low levels formed in the reformer by dealkylation. Saturation of the benzene in the distillate from this prefractionation column (debenzeniser) eliminates another benzene contributor to the gasoline pool.
A new application, CDHydro, may be integrated with an existing de-isohexaniser to saturate benzene in the overhead product in addition to removing benzene precursors from the reformer feed. Multiple sources of reformer feed may limit the benzene reduction of this approach.
If higher levels of benzene reduction are required, reformer product hydrogenation may be the best option. Use of the process on the reformer product provides high benzene saturation while distillation removes toluene from the reaction zone to minimise its loss. These options for benzene reduction using the CDHydro process will be discussed in the Summer issue of PTQ.

Oxygenates production improvements
Use of MTBE and TAME to replace the octane lost from gasoline by the removal of lead is a well-established option. The concurrent advantage of removing light FCC olefins from gasoline blends is also fully demonstrated. Improvements have been made in the CD processes for feed treatment and coproduction of MTBE and TAME.

Retrofits to fixed-bed processes demonstrate increased olefins removal/TAME production. Hexene ether co-production with TAME has also been demonstrated commercially. These options for enhanced oxygenates production will be included in the Summer PTQ.

FCC gasoline sulphur removal
FCC C4 streams have typically been separated and utilised for higher value (and more emissions friendly) applications than direct gasoline blending. However, CDHydro applications can be utilised to saturate diolefins in the mixed C4 product via retrofits to existing FCC C4 production and in MTBE debutanisers.

FCC C5 streams may be routed to gasoline blending or used as feed to TAME and/or alkylate production.