MSAT II and Benzene Management Solutions | C&I Engineering
Page Page 1 of 2
Welcome to C&I: C&I: Engineerin g, Procur ement and Total Project Management Management
MSAT II II and Benzene Management Management Solu tio ns 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 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 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 B Y 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 REFORMATE SPLl TTER OPERATION – KEY TO B ENZENE 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 benzenerich 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 REFORMATE FRACTIONATION FOLLOWED B Y 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 REFORMATE FRACTIONATION FOLLOWED B Y SATURATION
http://www.cieng.com/a-111-380-ISBL-MSAT-II-and-Benzene-Management-Solution...
4/22/2015
MSAT II and Benzene Management Solutions | C&I Engineering
Page 2 of 2
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), RHTHydrogenation, 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.
http://www.cieng.com/a-111-380-ISBL-MSAT-II-and-Benzene-Management-Solution...
4/22/2015
