Desulfurization Of Naphtha Blends (ExxonMobil)

CATEGORY: DESULFURIZATION
Desulfurization Of Naphtha Blends (ExxonMobil)
United States Patent Application 20160046881
February 18, 2016
Assignee: ExxonMobil Research and Engineering Company
Abstract
A process for desulfurizing a delayed coker naphtha in a catalytic naphtha desulfurization process in which the feed comprising the delayed coker naphtha is passed over a silicon trap comprising a high surface area inert alumina of low metals content prior to being hydrodesulfurized in an olefin-retentive, catalytic naphtha hydrodesulfurization process. Unpromoted (no intentional metals content), inert alumina is preferred for the silicon trap since it will not affect the olefin-retentive qualities of the hydrodesulfurization catalyst and for maximal silicon capture, a high surface area alumina is employed.
FIELD OF THE INVENTION
[0001] This invention relates to a method of using thermally cracked naphtha feedstocks effectively in the refinery gasoline pool by catalytic desulfurization in a naphtha blend.
BACKGROUND OF THE INVENTION
[0002] A large proportion of the gasoline pool in the United States, Europe and elsewhere is filled by naphtha from fluid catalytic cracking units. While this naphtha has good octane as a result of a relatively high olefin content resulting from the catalytic cracking process, it also tends to have an unacceptably high sulfur content under the regulatory standards for motor gasoline, the United States currently sets a standard with a maximum of 30 ppm sulfur but in 2017 the Tier 3 standard of not more than 10 ppm which has been the European standard since 2009. Over the same time period, the progressive decreases in permitted sulfur content have been accompanied by a requirement to reduce mobile source emissions, especially carbon monoxide, by the addition of oxygenates, usually provided in the form of bioethanol since the use of ether type oxygenates such as MTBE became controversial in the US and legislation favoring ethanol was became effective.
[0003] Ethanol has a high octane number, commonly reported as about 108 RON (Research Octane Number) although some reports from ethanol manufacturers place it as high as 113 RON although not specifying the MON (Motor Octane Number). However, it is accepted that blending ethanol with gasoline will have a favorable effect on the RON so that there is usually no difficulty for the refiner to meet the PON (Pump Octane Number) requirements of most vehicles with ethanol blends. The octane boost provided by the use of ethanol in gasoline blends, typically the E10 blend with 10 percent ethanol has provided an opportunity to bring feedstocks previously considered of marginal acceptability into the refinery pool for processing into motor gasoline.
[0004] Coker naphtha, being derived by the thermal cracking of residual feedstocks is generally high in sulfur as well as olefins although its octane is typically low. Retention of olefins is therefore necessary if octane targets in the gasoline pool are to be met; in addition, sulfur must be significantly reduced to meet current and future sulfur limits in the gasoline pool.
[0005] A number of sulfur reduction techniques for gasoline blend components have been developed, summarized briefly in U.S. Pat. No. 7,837,861. The ExxonMobil selective naphtha naphtha hydrofining process, SCANfining.TM., developed for deep hydrodesulfurization with maximum preservation of the olefins (octane), which is commercially available under license from ExxonMobil Research and Engineering Company, is a very effective naphtha desulfurization process which incorporates aspects of the processes described in the following patents: U.S. Pat. No. 5,985,136; U.S. Pat. Nos. 6,231,753; 6,409,913; U.S. Pat. No. 6,231,754; U.S. Pat. No. 6,013,598; U.S. Pat. No. 6,387,249 and U.S. Pat. No. 6,596,157. The single stage version of the process can be used with a full range catalytic naphtha or with an intermediate catalytic naphtha (ICN), for example a nominal 65-175.degree. C. (150-350.degree. F.) or a heavy catalytic naphtha (HCN), for example, a nominal 175.degree. C.+(350.degree. F.+) naphtha, or both. The two-stage version of the process, as described in U.S. Pat. No. 6,231,753, WO 03/048273 and WO 03/099963, adds a second reactor and inter-stage removal of H.sub.2S allowing very deep HDS with very good olefin retention. The operation of this process relies on a combination of a highly selective catalyst with process conditions designed to achieve hydrodesulfurization with minimum olefin saturation.
[0006] While the octane retention afforded by the SCANfining process would be useful if the process were applied to the desulfurization of the olefinic coker naphthas, the difficulty encountered in many cases results from the fact that coker naphthas frequently contain silicon which has been found to have a deleterious effect on the SCANfining catalyst. This silicon, in various combined forms, is generated from the silicone oil anti-foaming additives typically based on polydimethylsiloxane, used in the delayed coking process to suppress foaming caused by gas formation; pilot plant testing has shown that anywhere from 25 to 75% of the silicon from the anti-foamant will be found in the naphtha draw. In fact, it has been reported that 70-80% of the silicon at the coker ends up in the coker naphtha fraction. The silicone oils decompose in the coker to form modified silica gels and silicone fragments. The fragments are typically in the naphtha boiling range though some will boil at higher temperatures. The deactivating silica gels which have been found on catalyst surfaces have been primarily SiO.sub.2, Si(OSi).sub.4, Si(OSi).sub.3OH and methylated forms such as .dbd.Si(CH.sub.3).sub.2.
[0007] In hydroprocessing units downstream of the coker fractionator, silicon-containing fragments from the antifoam agent undergo a condensation reaction with alumina surfaces of the catalyst, forming a strong chemical bond. Once the silicon is bound to the alumina surface, it cannot be removed by regeneration or other means. It is a more moderate poison compared to contaminants like sodium or arsenic, but it nonetheless results in activity loss of the order of 5-10.degree. F. (3-6.degree. C.) for each 1.0 wt % Si deposited on the catalyst.
[0008] For these reasons, coker naphthas coming from the delayed coking process have not usually been considered suitable feedstocks for the process even though their desulfurization in this way and subsequent incorporation into the gasoline pool would be desirable. While alternative desulfurization processes for coker naphthas are, of course, at hand, they are generally hydrogenative with a high hydrogen consumption resulting from olefin saturation. Thus, the refiner is faced with a true dilemma, he may use SCANfining and retain olefins but inhibit the catalyst or he may use other hydrodesulfurization methods and spend money on hydrogen. Neither option is attractive.
SUMMARY OF THE INVENTION
[0009] We have now found that it is possible to process coker naphthas as a naphtha blend component in the feedstocks for olefin retentive selective catalytic naphtha hydrodesulfurization processes if a suitably selected silicon trap is used. To minimize the octane loss which occurs in the process a high surface area inert alumina with a controlled low metal content is required. In particular, the levels of metals with hydrogenation capability such as the base metals Ni, Co and Mo should be held at low levels and preferably should be absent.
[0010] According to the present invention the delayed coker naphtha is desulfurized in a catalytic naphtha desulfurization process in which the feed comprising the delayed coker naphtha is passed over a silicon trap comprising a high surface area inert alumina of low metals content prior to being hydrodesulfurized in an olefin-retentive, catalytic naphtha hydrodesulfurization process. Unpromoted (no intentional metals content), inert alumina is preferred since it will not affect the olefin-retentive qualities of the hydrodesulfurization catalyst and for maximal silica capture, a high surface area alumina is employed.
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