Jul-2008

Reducing benzene in gasoline

Cost-effective solutions for the reduction of benzene in gasoline, particularly with regard to MSAT II requirement

Maarten J Almering, Kerry L Rock and Arvids Judzis, CDTech

Article Summary

In the current global quest for the production of cleaner fuels, benzene has been identified as a gasoline component that should be reduced. As a result, the Environmental Protection Association placed a limit of maximum 1.0 vol% of benzene in reformulated gasoline (RFG) in 1995. The recently issued MSAT II regulations further reduce benzene to less than 0.62 % in all US gasoline by 2011. Europe, Canada, Australia, Japan and others have also adopted limits on benzene. Many other countries are currently in the process of reducing benzene in gasoline.

For most refiners, benzene reduction in RFG was easily accomplished using existing options. Many refiners simply adjusted the C6 content of the naphtha feed to their reformer by prefractionation and produced reformate with reduced benzene content. Refiners with integrated chemical operations were able to send their light reformate to extraction facilities and move benzene into the petrochemical market. Others were also able to take advantage of this option by exporting the light reformate fraction over the fence for outside processing. Several refineries installed facilities for the hydrogenation of benzene. Another possibility is the reaction of propylene with benzene to produce cumene. However, this approach requires a significant clean-up of the impurities in refinery propylene, considerably increasing the total capital cost.

The removal of benzene from the gasoline pool represents the loss of one of the highest octane components. The current programme of adding ethanol to the gasoline pool will eventually make most refiners long on octane. As a result, the benzene/octane issue is not a significant factor.

The benzene reduction required by MSAT II is clearly a much larger quantity to deal with, and refiners may be forced to make more substantial changes in refinery operations to achieve these levels. For many refiners, prefractionation of the reformer feed will not provide a sufficient benzene reduction. The high capital cost of benzene extraction expansions or new facilities will also not be attractive to all refiners, especially smaller refineries or those in locations remote from any petrochemical benzene users. Assuming hydrogen availability is not a problem, benzene hydrogenation may provide the answer. Even if the hydrogen supply is a problem, it may well have to be addressed by the need to remove sulphur from FCC gasoline as well as other refinery products, as the increased pressure to reduce sulphur emissions comes into play in the near future. The refiner who is looking ahead and making plans probably has a hydrogen plant increasing in priority.

Benzene sources
In a typical refinery, benzene comes from several sources, including:
- Light straight-run gasoline
- Reformate
- Hydrocrackate
- FCC gasoline
- Coker gasoline.

However, the benzene from the reformer usually represents 50-80% of the total (Figure 1). As a result, reformate is the natural place to focus benzene reduction. In most cases, the desuphurised light straight can be co-processed with reformate for benzene reduction. 

Conventional benzene saturation
The conventional approach to removing benzene from reformate is shown in Figure 2. Reformate is fractionated in a reformate splitter to take benzene and lighter components overhead. The toluene content of the light reformate fraction is limited to minimise its loss due to hydrogenation. This benzene-containing fraction is sent to a hydrogenation reactor, where benzene is converted into cyclohexane in a highly exothermic, high-pressure, fixed-bed catalytic reactor. A cooled recycle stream is normally required to modulate reactor temperature. The reactor effluent is sent to a stripper, where light ends are removed by fractionation. This approach requires many equipment items at high capital cost and is susceptible to upsets in operation.

Combining fractionation with hydrogenation
In 1994, a new process for hydrogenation was commercialised. The patented CDHydro process combines fractionation with hydrogenation. Proprietary distillation devices (CDModules) containing catalyst are installed in the top section of the fractionation column (Figure 3). Hydrogen is introduced beneath the catalyst zone. Fractionation carries light components into the catalyst zone where the reaction with hydrogen occurs. Fractionation also sends heavy materials to the bottom. In addition, clean hydrogenated reflux continuously washes the catalyst zone. These factors combine to provide a long catalyst life.

The heat of reaction evaporates liquid, and the resulting vapour is condensed in the overhead condenser to provide additional reflux. The natural temperature profile in the fractionation column results in a virtually isothermal catalyst bed rather than the temperature increase typical of conventional fixed-bed reactors.

Light ends control can be achieved by the addition of a pasteurisation section at the top of the fractionation column. The overhead product is taken as a sidedraw with reduced content of hydrogen and other light ends.

The process can operate at much lower pressure than conventional processes. Pressures for the CDHydro process are typically set by the fractionation requirements. Additionally, the elimination of a separate hydrogenation reactor and hydrogen stripper offers a significant capital cost reduction relative to conventional technologies. The resulting process simplification can also reduce manpower requirements.

Another factor that is important in the design of refining units is safety. The combination of integral reaction heat removal, low operating pressure and fewer equipment items enhances the safety of the process relative to conventional technologies.

Benzene hydrogenation
The CDHydro process offers several advantages when applied to the hydrogenation of benzene. The typical process scheme is presented in Figure 4. The requirements of MSAT II can be met in a fractionation column, which is similar to that required to produce light reformate for the conventional fixed-bed benzene hydrogenation process.