Reid Vapor Pressure Control Process (United States Patent Application 20150329443 ExxonMobil)

CATEGORY: OLEFINS 
Reid Vapor Pressure Control Process (United States Patent Application 20150329443 ExxonMobil)
United States Patent Application 20150329443
November 19, 2015
Assignee: Exxonmobil Research And Engineering Company
Abstract
A process for removing cyclopentene from the C.sub.5 fraction of a light olefin feed useful in an isoparaffin/olefin alkylation process redistributes fragments of C.sub.5 olefins formed by ring opening metathesis (ROM) in the presence of a catalyst. The higher molecular weight olefins produced in the reaction can be blended into the gasoline blend pool without imposing a significant or any vapor pressure penalty. Cyclopentene present in the C.sub.5 portion of the feed undergoes various ring opening reactions while other pentenes are converted to hydrocarbon products of lower and higher molecular weight relative to pentene. The reduction in cyclopentene results in a reduced tendency for the formation of acid soluble oil (ASO) during alkylation.
FIELD OF THE INVENTION
[0002] This invention relates to an integrated process for use in a petroleum refinery for improving utilization of FCC olefins and providing additional feedstocks which can be used in the isoparaffin-olefin alkylation process as well as additional low RVP blendstock for gasoline production.
BACKGROUND OF THE INVENTION
[0003] Vapor pressure is an important physical property of volatile liquids, particularly in the case of motor gasoline, where the vapor pressure of gasoline and gasoline-oxygenate blends is regulated by various government agencies; the specifications for volatile petroleum products generally include vapor pressure limits to ensure products of suitable volatility performance and these limits are becoming an ever more important problem for refineries with more stringent emissions regulations. Vapor pressures for motor gasolines are typically measured and expressed in terms of the Reid Vapor Pressure, ASTM D5191 (Standard Test Method for Vapor Pressure of Petroleum Products (Mini Method)). Complicating the issue is the fact that there is an increasing abundance of light virgin naphtha in the North American supply pool; C.sub.5 molecules are typically responsible for over 70% of gasoline vapor pressure, and consequently, there is great interest in removing a significant portion of C.sub.5's from the gasoline blending pool in order to meet government specifications: to make gasoline complying with the complex model refineries will require the RVP reduction that pentene alkylation can provide. The problem may be exacerbated by biofuel mandates in the United States which may require an increase in the ethanol content of gasoline: any further increase the ethanol mandate will put further pressure on removing C.sub.5's from gasoline to maintain distillation product specifications, notably the summer RVP limit.
[0004] C.sub.5's are one of the most prevalent FCC cracking products by mass and as a result, refineries produce large quantities of C.sub.5 olefins. Alkylation units are well-integrated to FCCUs and have the ability to upgrade light olefins to high-value alkylate product with its low RVP, low sulfur, and high octane value which is consequently is a valuable gasoline blending component. While for these reasons, C.sub.5 olefins are a useful feed source for alkylation units, their utilization is generally limited due to the high level of contaminants in the C.sub.5 boiling range which are detrimental to the alkylation process. Several chemical species found in the C.sub.5 feed form a polymer byproduct in the alkylation process known as acid soluble oil (ASO) which forms as an undesirable by-product in both the HF and sulfuric acid alkylation processes. ASO builds up in the acid catalyst and degrades the catalyst activity. As the acid activity is reduced by ASO, ASO is produced at even higher rates, which can lead to an "acid runaway" incident where the desired alkylation reaction completely stops and ASO is produced at an uncontrollable rate. An acid runaway is a very costly incident for a refinery which normally leads to severe rate reductions or unit shutdown. In severe acid runaway incidents the acid runaway could be carried to downstream equipment causing extensive damage. For these reasons, feeds containing high levels of ASO forming contaminants are often treated to remove the ASO precursors. Sulfur or diene contaminants can be removed by existing feed pretreatment technologies, such as Merox.TM. and selective diene hydrogenation, respectively. Unfortunately, the only method to limit the proportion of cyclopentene in C.sub.5 olefin feeds has been distillation. Cyclopentene is the highest boiling C.sub.5 olefin, so the cyclopentene concentration can be limited by distilling off only the lighter portion of the C.sub.5 stream for use in the alkylation unit. The relatively small temperature difference between the boiling points of cyclopentene and the other C.sub.5 olefins makes separation by fractional distillation approaches difficult and imposes practical limits on the volume of C.sub.5 olefins that can be alkylated while excluding cyclopentene.
[0005] Cyclopentene is thought to form ASO at nearly a weight-for-weight basis. Detailed chemical analysis of ASO has shown it to be an unsaturated polycyclic structure, consisting of 5 and 6 member rings. Cyclopentene likely preferentially forms ASO over alkylate due to its cyclic structure and the introduction of relatively small quantities of cyclopentene into the alkylation feed can markedly increase ASO production, which will have a proportional impact on acid consumption. Concerns for operational expense often limit C.sub.5 olefin content to less than 10% of the olefin feed, which often corresponds to less than 20% of the total FCC C.sub.5 olefins. A feed treatment process for selectively removing cyclopentene could significantly increase the maximum volume of C.sub.5 olefins that can be alkylated without incurring dramatic increases in acid consumption.
[0006] U.S. Pat. No. 6,566,569 (Chen) discusses the problem of reducing the pentane content of the gasoline blend pool and points to the difficulties encountered in disposing of pentane. The patent is directed to a process of producing C.sub.2-4 and C.sub.6+ paraffins from the pentane fraction by dehydrogenation to form pentenes which are then subjected to metathesis and rehydrogenation to form alkanes; all three processes are preferably carried out in the same reactor with unconverted pentanes being recycled and converted to incremental lighter and heavier alkanes.
[0007] U.S. Pat. No. 6,677,495 (Schwab) relates to a process for converting cyclopentene to oligomer mixtures by metathesis of a hydrocarbon mixture containing cyclopentene and acyclic monoolefins using a homogeneous or heterogeneous catalyst.
SUMMARY OF THE INVENTION
[0008] The present process for removing cyclopentene from the C.sub.5 fraction of catalytic cracking products comprises redistributing fragments of C.sub.5 olefins by ring opening metathesis (ROM) in the presence of a catalyst. The cyclopentene present in the C.sub.5 portion of the feed undergoes various ring opening reactions while other pentenes are converted to hydrocarbon products of lower and higher molecular weight relative to pentene. The lower molecular weight olefins may be used in the absence of cyclopentene in the alkylation unit with a reduced tendency to form ASO or alternatively, in chemicals production or directly for LPG sales; the higher molecular weight olefins produced in the reaction can be blended into the gasoline blend pool to make a positive contribution to gasoline yield without imposing a significant or any vapor pressure penalty.
[0009] In an integrated refinery FCCU-alkylation sequence, the isoparaffin-olefin alkylation process will be operated using a light C.sub.4-C.sub.6 isoparaffin reactant and a light C.sub.2-C.sub.6 olefin reactant which are reacted in the presence of an acid catalyst to form a higher molecular weight hydrocarbon product including branch chain hydrocarbons in the conventional manner. When the olefin reactant includes pentene, typically obtained from the depentanizer column of the FCCU or by increasing the temperature of the overhead cut point of the FCCU debutanizer, the potential for an undesirable degree of ASO exists but according to the present invention, a significant reduction in the proportion of cyclopentene is effected by the metathesis reaction. The improvement provided by the present invention enables C.sub.5 olefinic feeds including cyclopentene to be used as a component of the light olefin reactant with a reduced propensity for ASO formation from cyclopentene during the alkylation process. By converting the pentenes to higher gasoline blend components in this way, the RVP specification for the gasoline blend can be more readily achieved while, at the same time, making effective use of the pentenes with reduced risk of ASO formation in the alkylation unit.
Free Full Text Source:  http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=5&f=G&l=50&co1=AND&d=PG01&s1=catalyst&s2=exxonmobil.AS.&OS=catalyst+AND+AN/exxonmobil&RS=catalyst+AND+AN/exxonmobil