Mineral oils: processes and products – Refining – Sulfur removal
Reexamination Certificate
1999-11-24
2001-05-15
Myers, Helane (Department: 1764)
Mineral oils: processes and products
Refining
Sulfur removal
C208S220000, C208S210000, C208S217000, C208S21600R
Reexamination Certificate
active
06231753
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a two stage naphtha desulfurization process with reduced mercaptan formation. More particularly, the invention relates to a two stage, vapor phase naphtha hydrodesulfurization process with interstage H
2
S removal, wherein the second stage temperature and space velocity is greater than the first, for deep, selective sulfur removal with reduced olefin loss and mercaptan formation.
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 100-7000 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 treat gas ratio results in significant octane loss, due to extensive loss of olefins by saturation. Selective one and two-stage hydrodesulfurization processes have been developed to avoid massive olefin saturation and octane loss. Such processes are disclosed, for example, in U.S. Pat. Nos. 4,149,965; 4,243,519; 5,525,211; 5,423,975, and 5,906,730. However, some of these processes, including two-stage processes, aren't suitable for use with high (e.g., >1000 wppm) levels of feed sulfur. Further, in these and in other processes, in the hydrodesulfurization reactor the liberated H
2
S reacts with the retained olefins, to form mercaptan sulfur compounds. These mercaptans are formed as a consequence of the H
2
S formed during the hydrodesulfurization process and 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, a selective naphtha desulfurization process is needed for reducing the total sulfur level, particularly for high sulfur content naphtha feeds, with minimal mercaptan reversion, while retaining a high level of feed olefins.
SUMMARY OF THE INVENTION
The invention relates to a two-stage, vapor phase naphtha hydrodesulfurization process with interstage H
2
S removal, wherein most of the sulfur is removed in the first stage and wherein the percent desulfurization, temperature and space velocity are greater in the second stage than in the first. The process selectively removes sulfur, with reduced mercaptan formation, from a naphtha feed containing olefins and organic sulfur compounds. The process comprises (i) passing a sulfur and olefin-containing naphtha feed and hydrogen into a first vapor reaction stage in which the feed reacts with the hydrogen, in the presence of a hydrodesulfurization catalyst, to remove most of the sulfur from the feed and produce an effluent comprising a mixture of reduced sulfur naphtha and H
2
S, (ii) separating the reduced sulfur naphtha and H
2
S, (iii) passing hydrogen and the reduced sulfur naphtha into a second vapor reaction stage, in which the percent desulfurization is greater than that in the first stage and in which naphtha reacts with the hydrogen, in the presence of a hydrodesulfurization catalyst, to remove most of the remaining sulfur in the naphtha and form an effluent comprising a mixture of H
2
S and a desulfurized naphtha product and (iv) separating the naphtha product from the H
2
S. The second stage reaction temperature is preferably higher than that in the first stage. Both reaction stages are vapor reaction stages, for increased selectivity of the hydrodesulfurization reaction for sulfur removal. The process is useful for deep and selective sulfur removal, with reduced olefin loss and mercaptan formation, particularly with a high sulfur content naphtha feed. By high feed sulfur content is meant from 0.1-0.7 wt. % (1000-7000 wppm) of sulfur in the form of organic sulfur bearing compounds. At least 80, preferably at least 90 and more preferably at least 95 vol. % of the H
2
S formed in the first stage is separated from the first stage sulfur reduced naphtha, before it is passed into the second stage. The second stage effluent comprises a desulfurized product naphtha having less than 5 and preferably less than 2 wt. % of the feed sulfur, with at least a 40 vol. % feed olefin retention. The desulfurization reaction conditions are adjusted to achieve respective desulfurizations of at least 70 and 80 wt. % in the first and second stages and preferably respective desulfurizations of at least 80 and 90 wt. %. The reaction conditions include respective temperatures, pressures, treat gas ratios and space velocities broadly ranging from 450-800° F., 60-600 psig., 2000-4000 scf/b and 1-10 v/v/hr. Interstage H
2
S separation and removal may be achieved by any suitable means, such as cooling the first stage vapor effluent to condense the naphtha, separating the condensed naphtha liquid from the remaining gas, which contains most of the H
2
S, followed by stripping the separated naphtha, if necessary. Amine scrubbing may also be used to remove any remaining H
2
S in the naphtha. The hydrodesulfurization 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 and more typically at least one from both Group VIII and Group VI, preferably Group VIB, on a suitable catalyst support, with non-noble Group VIII metals preferred. Low catalytic metal loadings of less than 12 wt. %, based on the weight of the catalytic metal oxide, are preferred. Particularly preferred is a low metal loaded catalyst comprising CoO and MoO
3
on a support, in which the Co/Mo atomic ratio ranges from 0.1 to 1.0, as is explained in detail below. The hydrodesulfurization catalyst in each stage may be the same or different, and may be fresh or partially spent. The catalyst may be presulfided or it may be sulfided in-situ, using conventional sulfiding procedures.
DETAILED DESCRIPTION
It is anticipated that environmentally driven regulatory pressure on motor gasoline (mogas) sulfur levels, will result in the widespread production of 150 ppm total sulfur mogas by the year 2000 and 30 wppm or less, perhaps shortly thereafter. Further, gas oil and other feeds for a fluid cat cracker used to produce cat cracked naphthas, are increasingly using ever more amounts of poorer quality, high sulfur content components, due to the dwindling supply of higher quality crude oil sources. This results in higher sulfur contents in the cat cracked naphthas, which are the major source of naphtha stocks for mogas pools. Thus, the reduction in mercaptan reversion achieved by the two stage, selective hydrodesulfurizing process of the invention, is important with respect to the desulfurized product meeting both low total sulfur and mercaptan sulfur specifications, while preserving the olefins valuable for octane. At such deep levels of desulfurization of from 90-100 wt. % feed sulfur removal, particularly with relatively high sulfur content naphtha feeds (e.g., >1000-7000 wppm sulfur), the contribution of sulfur from reversion mercaptans to the total sulfur, can be significant. Therefore, the control of mercaptan formation is necessary to reach these very low sulfur levels of ≦150 wppm, especially ≦30 wppm.
In the process of the invention, both reaction stages are vapor reaction stages, in which the naphtha is present as vapor, to increase selectivity of the hydrodesulfurization catalyst and reaction for sulfur removal, with reduced olefin loss by saturation, to maximize feed olefin retention in the desulfurized napht
Brignac Garland B.
Cook Bruce R.
Demmin Richard A.
Greeley John P.
Halbert Thomas R.
Exxon Research and Engineering Company
Hughes Gerard J.
Myers Helane
Naylor Henry E.
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