Benzene Reduction Strategies for Refineries
The US EPA’s most recent clean fuels regulations (MSAT II) will have significant impact on small refineries and/or refineries that are not located near the petrochemical beltway. The MSAT II regulations further reduce benzene to less than 0.62 vol-% (on an annual basis) in all U.S. gasoline by 2011.
For most refiners, the reduction of benzene in reformulated gasoline (RFG) (1995 regulations) 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 hydrogenation of benzene. Another possibility is reaction of propylene with benzene to produce cumene. However, this approach requires significant clean up of the impurities in refinery propylene considerably increasing the total capital cost.
Further, removal of benzene from the gasoline pool represents the loss of one of the highest octane components of gasoline. The current program of adding ethanol to the gasoline pool will eventually replace the lost octane from benzene reduction. As a result, in the long run, the benzene/octane issue will not be a significant factor.
However, the benzene reduction now required by MSAT II will likely force refineries to invest in additional technologies to achieve these levels. For many refiners, prefractionation of the reformer feed will not provide a sufficient benzene reduction to achieve the 0.62 vol-%.
The high capital cost of benzene extraction expansions or new facilities will also not provide an attractive answer for all refiners, especially smaller refineries or those in locations remote from the petrochemical benzene users. Assuming hydrogen availability is not a problem, benzene hydrogenation may provide the answer. Even if hydrogen supply is a problem, it may well have to be addressed by the need to remove sulfur from FCC gasoline as well as other refinery products, as increased pressure to reduce sulfur emissions comes into play in the near future.
If the refinery is already short on hydrogen, then a hydrogen plant will almost assuredly be required to achieve the MSAT II levels under most reduction strategies.
Typically, reformate is the largest process stream contributing benzene to the gasoline pool, making up about 75 to 80 vol-% of the benzene. The second largest contributor is fluid catalytic cracking (FCC) unit naphtha, which contributes 10 – 15 vol-% of the benzene.
Reformate is the natural place to focus benzene reduction. In most cases the desulfurized light straight run can be co-processed with the reformate for benzene reduction.
The two basic approaches to reduce benzene from the reformer and thereby reduce the benzene content of the gasoline pool are prefractionation and postfractionation.
- Prefractionation – In this approach, benzene precursors are removed from the reformer feed by using a reformate splitter to fractionate benzene, isohexane and lighter components overhead, to reduce benzene formation in the reforming unit. The toluene content of the light reformate fraction is limited to minimize its loss due to hydrogenation. This benzene containing fraction is sent to a hydrogenation reactor where benzene is converted to 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.
However, removing all of the benzene from the light straight run naphtha will not be sufficient to achieve 0.62 vol-% benzene with a fixed bed reforming unit, but may be sufficient with the lower pressure continuous catalytic reforming units.
In any event, in order to assure compliance with the 0.62 vol-% level (compliance margin), refineries should also consider adding other nonaromatic (nonreformate) octane, which will reduce the required reforming unit octane resulting in lower benzene. One way of doing this is to add non-aromatic octane by isomerizing the light naphtha, which will also saturate benzene (see Figure 2). Isomerization of light naphtha will provide compliance margin for low pressure continuous catalytic reforming units, but will likely still not remove sufficient benzene from the higher pressure reforming units.
- Postfractionation – Postfractionation offers the greatest control over gasoline benzene. In post fractionation, reformate is split into a light stream and a heavy stream in a reformate splitter. Further, achieving 0.62 vol-% benzene with the higher pressure reforming units requires Postfractionation. Postfractionation requires investment in a reformate splitter and processing to remove benzene. Options for managing the benzene in the light reformate produced in postfractionation are: saturation and isomerization (as described above), plus alkylation and extraction. The gasoline benzene levels achieved with postfractionation are relatively insensitive to the type of reforming unit or the processing technology chosen to remove benzene.
Summary
Refineries with low pressure reforming units may be able to use one or more of the above described prefractionation techniques (saturation, isomerization) to meet the new MSAT II regulations. Some additional operating room (compliance margin) can be achieved with additional non-aromatic octane that can help reduce reformer severity needed to meet overall gasoline pool octane. This additional octane can come from isomerizing the light naphtha produced in removing benzene and benzene precursors from the feed to the reforming unit and by blending more ethanol into the gasoline pool or purchasing other high octane blendstocks.
Refineries with high pressure reforming units are less likely to meet the new benzene limits with prefractionation alone and may require postfractionation techniques, or recovering benzene from the FCC naphtha unit (although costly). Solutions for benzene reduction will be refinery specific and are determined by the individual refinery configuration, type of reformer, amount of benzene contributed from other blendstocks, and the amount of ethanol blended.