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MSAT II and Benzene Management Solutions

The U.S. EPA MSAT II (Mobile Sources Air Toxics Phase 2) regulation limits the annual average U.S.

gasoline pool benzene content to 0.62 vol% for most refiners. Steps to meet this limit are refinery-specific

and determined by the current refinery configuration, the amount of benzene contributed from other

blendstocks, and the use of credits. A brief review of the options refiners have to manage their benzene pool

follows.

NAPHTHA PRE-FRACTIONATION

This case represents the typical first step a refinery takes to reduce benzene, which is pre-fractionating the

incoming naphtha into light and heavy fractions. The light naphtha leaving the overhead of this column

contains the aromatic precursors, and bypasses the reformer. The heavy naphtha goes to the Reformer,

where due to the lack of the aromatic precursors in the feed benzene production is reduced. However some

benzene is still in the final product due to dealkylation of heavy aromatics.

The bypassed light naphtha stream is blended back into the reformate to produce a mixed stream going to the

gasoline pool. There is a drop in RON as a result of the lower benzene level as well as a loss in hydrogen

make. However, the reduction in benzene for this scheme is typically insufficient for the overall gasoline pool

to meet the MSAT II limit.

NAPHTHA PRE-FRACTIONATION FOLLOWED BY ISOMERIZATION

In this case, light naphtha leaving the Naphtha Splitter overhead is sent to an Isomerization unit prior to mixing

with the reformate. The Isomerization unit eliminates any benzene present in this stream and increases the

RON. There is an overall reduction in hydrogen make due to the reduced reformer feed benzene saturation,

though the loss in RON is not as high. The reduction in benzene is typically insufficient for the overall gasoline

pool to meet the MSAT II limit.

REFORMATE SPLlTTER OPERATION – KEY TO BENZENE MANAGEMENT

With or without naphtha pre-fractionation, the bulk of the benzene in the gasoline pool comes from

reformate. As a result, any successful benzene management solution will involve some form of reformate

processing. To minimize the size of downstream benzene processing equipment, the Reformate Splitter can

be run to create a concentrated benzene stream, after which numerous process options can be considered.

REFORMATE FRACTIONATION FOLLOWED BY ALKYLATION

If the refinery has a source of light olefins such as ethylene and propylene, it can react them with the benzene-

rich stream leaving the Reformate Splitter to produce alkyl aromatics. This is a proven commercial process

(BENZOUT) that uses a platinum zeolite catalyst. No hydrogen is required. The resulting alky aromatic

blendstock increases the volume and the octane of the gasoline pool and incurs no hydrogen penalty. Very

low gasoline benzene levels can be reached with this process.

REFORMATE FRACTIONATION FOLLOWED BY EXTRACTION AND SEPARATION

A number of well-established commercial aromatic extraction processes exist that separate benzene, toluene,

and xylenes from reformate. These processes can reduce the benzene level in the gasoline pool to very low

levels, though this comes at the cost of octane and volume reduction. It is unlikely that MSAT II regulations

alone can justify the significant capital outlay for the addition of an aromatics extraction unit to a refinery, but

they have resulted in a renewed appreciation for the importance of these units. As a result, there is

considerable interest in debottlenecking and operational improvement projects for improving both the

throughput and product quality for existing extraction units.

REFORMATE FRACTIONATION FOLLOWED BY SATURATION

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Saturating benzene with hydrogen in a catalytic reactor is fast and typically close to stoichiometric, which

makes this a good, though expensive, option for benzene reduction. A number of benzene saturation

technology suppliers have been identified, including UOP (Bensat), GTC (BENzap), Axens (BENFREE), RHT-

Hydrogenation, and CDTech (CDHydro). Process conditions are moderate, and only a slight excess of

hydrogen above the stoichiometric level is required. The high heat of reaction associated with benzene

saturation is carefully managed to control the temperature rise across the reactor. These units can guarantee

very low gasoline pool benzene levels, though at the cost of octane loss and hydrogen consumption.

WHICH SOLUTION TO GO WITH?

It is difficult or impossible to meet the gasoline benzene of 0.62 vol% with pre-fractionation of the naphtha

stream alone. Gasoline benzene reduction by saturation

requires hydrogen, reduces gasoline octane, and does not have an investment payback. Gasoline benzene

alkylation does not require hydrogen, increases gasoline octane, and does have an investment payback, but

requires a source of light olefins. Post-fractionation is energy intensive, but for most refineries will be

necessary step to reduce gasoline benzene to the required level. Aromatic extraction is expensive but

produces high purity benzene and other aromatic products that generate incremental revenue to pay back the

investment. The ultimate solution chosen will depend on many factors that are specific to the individual

refinery. C&I has the experience to help with this selection process.

© 2014 C&I Engineering, Inc. All rights reserved.

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