Mineral oils: processes and products – Refining – Sulfur removal
Reexamination Certificate
1999-12-03
2002-06-25
Myers, Helane (Department: 1764)
Mineral oils: processes and products
Refining
Sulfur removal
Reexamination Certificate
active
06409913
ABSTRACT:
BACKGROUND OF THE DISCLOSURE
1. Field of the Invention
The invention relates to a naphtha desulfurization process with reduced formation of reversion mercaptans. More particularly, the invention relates to selectively removing sulfur compounds from an olefin-containing naphtha feed, with reduced reversion mercaptan formation and olefin saturation, by reacting the feed with a hydrogen treat gas in the presence of a hydrodesulfurization catalyst, within specific constraints of high temperature, low pressure and a high treat gas ratio.
2. Background of the Invention
Future mogas sulfur specifications are being regulated through legislation to increasingly lower levels, due to environmental considerations. Sulfur specifications on the order of less than 150 wppm of total sulfur are likely near term, with values of no greater than 30 wppm of total sulfur possible in the not too distant future. Such sulfur specifications are without precedent and will require the production of low sulfur blend stock for the mogas pool. The primary sulfur sources in the mogas pool are the blend stocks derived from FCC naphthas, whose sulfur content can fall in the range of 500-7000 wppm depending upon crude quality and FCC operation. Conventional fixed bed hydrodesulfurization can reduce the sulfur level of FCC naphthas to very low levels, but the severe conditions of temperature, pressure and hydrogen treat gas ratio results in significant octane loss, due to olefin loss from saturation. Selective hydrodesulfurization processes have been developed to reduce olefin saturation and concomitant octane loss. Such processes are disclosed, for example, in U.S. Pat. Nos. 4,149,965; 5,525,211; 5,423,975, and 5,906,730. However, in these and in other processes, in the hydrodesulfurization reactor the H
2
S formed as a consequence of the hydrodesulfurization reacts with the feed olefins, to form mercaptan sulfur compounds, which are known as reversion mercaptans. The amount of these mercaptans formed during the process typically exceeds future fuel specifications for mercaptan sulfur and, in some cases, total sulfur. Accordingly, it is desirable to have a desulfurization process, with reduced mercaptan reversion and octane loss.
SUMMARY OF THE INVENTION
The invention relates to a process for removing sulfur compounds from an olefin-containing naphtha feed, with reduced reversion mercaptan formation, by reacting the feed with a hydrogen treat gas in the presence of a hydrodesulfurization catalyst, within specific constraints of high temperature, low pressure and high treat gas ratio. Thus, the process of the invention comprises reacting a naphtha feed containing organic sulfur compounds and olefins, with a hydrogen treat gas in the presence of a hydrodesulfurization catalyst at a temperature of from 550-425° C., a pressure of from 60-150 psig., and a hydrogen treat gas ratio of from 2000-4000 scf/b, to remove most of the organic sulfur compounds, with reduced olefin saturation. Temperature and pressure ranges of from 340-425° C. and 60-125 psig. are preferred, with from 370-425° C. and 60-100 psig. particularly preferred. Similarly, a preferred treat gas ratio range is from 2500-4000 scf/b, with from 3000-4000 scf/b particularly preferred. From 90-100 wt. % sulfur removal may be achieved to form a desulfurized naphtha product having a total sulfur level of from 5-500 wppm, including from 5-200 wppm of mercaptan sulfur, with a feed olefin loss of from 5-60 wt. %. The catalyst can be any catalyst known to be useful for hydrodesulfurization. Such catalysts typically comprise at least one catalytically active metal component of a metal from Group VIII, on a suitable support. More typically the catalyst will comprise a component of at least one metal of Group VIII and at least one metal of Group VI on a suitable catalyst support, with non-noble Group VIII metals preferred. Low catalytic metal loadings are preferred. Particularly preferred is a low metal loaded catalyst comprising CoO and MoO
3
on a support having a Co/Mo atomic ratio of from 0.1 to 1.0, as is explained in detail below. The catalyst may be either fresh, partially deactivated or severely deactivated. The catalyst may be presulfided or it may be sulfided in-situ, using conventional sulfiding procedures. By hydrogen treat gas is meant a gas comprising hydrogen, which may or may not contain one or more diluent gases which don't adversely affect the desulfurization catalyst, process or product. It is preferred that the treat gas comprise from 60-100 vol. % hydrogen. By organic sulfur compound is meant any sulfur bearing organic compound.
Thus, in a broad sense the invention comprises a process for desulfurizing a sulfur and olefin-containing naphtha feed, by reacting the feed with hydrogen, in the presence of a hydrodesulfurizing catalyst, at reaction conditions including a temperature of from 290-425° C., a pressure of from 60-150 psig, and a hydrogen treat gas ratio of from 2000-4000 scf/b. In a more narrow embodiment, the invention comprises a process for desulfurizing a sulfur and olefin-containing naphtha feed with reduced mercaptan formation, by reacting the feed with hydrogen, in the presence of a hydrodesulfurizing catalyst, at reaction conditions including a temperature of from 290-425° C., a pressure of from 60-150 psig, and a hydrogen treat gas ratio of from 2000-4000 scf/b, wherein the catalyst comprises a Mo catalytic component, a Co catalytic component and a support component, with the Mo component being present in an amount of from 1 to 10 wt, % calculated as MoO
3
and the Co component being present in an amount of from 0.1 to 5 wt. % calculated as CoO, with a Co/Mo atomic ratio of 0.1 to 1, wherein the naphtha feed contains up to 0.7 wt. % total sulfur and up to 60 wt. % olefins, and wherein the hydrodesulfurization reduces said feed sulfur by at least 90%, with no more than 60% olefin saturation.
DETAILED DESCRIPTION
The selective hydrodesulfurization process of the invention removes organic sulfur compounds, including mercaptan and non-mercaptan sulfur compounds from a naphtha feed, with reduced formation of reversion mercaptans, by conducting the hydrodesulfurization reaction within the specific process constraints of high temperature, low pressure and high treat gas ratio of the invention. While the actual value of any one of these three hydrodesulfurization process constraints of temperature, pressure and hydrogen treat gas ratio used in the process of the invention, may fall within the extremely broad windows of hydrodesulfurization process conditions that one might piece together from a multiplicity of various disclosures, the combination thereof and the unexpected and unique results obtained, have not heretofore been known to be used for naphtha hydroprocessing. It has been found that these three specific operating constraints improve the hydrodesulfurization selectivity, by favoring hydrodesulfurization with less olefin saturation (octane loss). At these selective hydrodesulfurization conditions of high temperature, low pressure and high treat gas ratio, the high concentration of H
2
S formed in the hydrodesulfurization reactor, resulting from the deep desulfurization, along with high concentrations of retained feed olefins, would be expected to inherently promote mercaptan reversion. Surprisingly, the opposite has been observed. We have unexpectedly found that, at a given level of retained feed olefins and H
2
S formed by the hydrodesulfurization (as found in the effluent of the hydrodesulfurization reactor), significantly less mercaptans are formed by reversion using the process constraints of the invention, than that which occurs at conditions of temperature, pressure and treat gas ratio outside the ranges of the invention. Such conditions outside the constraints of the invention are typically one or more of, a lower temperature, higher pressure and low or lower treat gas ratio. The combination of deep desulfurization of the naphtha feed, along with the relatively high olefin retention and reduced mercaptan formation, ac
Baird Jr. William C.
Brignac Garland B.
Clark Janet R.
Cook Bruce R.
Halbert Thomas R.
ExxonMobil Research and Engineering Company
Hughes Gerard J.
Myers Helane
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