Mineral oils: processes and products – Refining – Sulfur removal
Reexamination Certificate
2001-09-17
2004-01-13
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Refining
Sulfur removal
C208S213000, C208S20800M, C208S209000, C208S211000, C208S217000, C585S834000, C585S841000, C585S850000, C585S867000
Reexamination Certificate
active
06676830
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for the desulfurization of a light boiling range (C
5
-350° F.) naphthas, such as fluid catalytic cracked naphtha. More particularly the present invention employs a combination of steps which include catalytic distillation to reduce sulfur to very low levels, makes more efficient use of hydrogen and causes less olefin hydrogenation for a full boiling range naphtha stream.
2. Related Information
Petroleum distillate streams contain a variety of organic chemical components. Generally the streams are defined by their boiling ranges which determine the composition. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated materials (diolefins). Additionally, these components may be any of the various isomers of the compounds.
The composition of untreated naphtha as it comes from the crude still, or straight run naphtha, is primarily influenced by the crude source. Naphthas from paraffinic crude sources have more saturated straight chain or cyclic compounds. As a general rule most of the “sweet” (low sulfur) crudes and naphthas are paraffinic. The naphthenic crudes contain more unsaturates and cyclic and polycylic compounds. The higher sulfur content crudes tend to be naphthenic. Treatment of the different straight run naphthas may be slightly different depending upon their composition due to crude source.
Reformed naphtha or reformate generally requires no further treatment except perhaps distillation or solvent extraction for valuable aromatic product removal. Reformed naphthas have essentially no sulfur contaminants due to the severity of their pretreatment for the process and the process itself.
Cracked naphtha as it comes from the catalytic cracker has a relatively high octane number as a result of the olefinic and aromatic compounds contained therein. In some cases this fraction may contribute as much as half of the gasoline in the refinery pool together with a significant portion of the octane.
Catalytically cracked naphtha gasoline boiling range material currently forms a significant part (≈⅓) of the gasoline product pool in the United States and it provides the largest portion of the sulfur. The sulfur impurities may require removal, usually by hydrotreating, in order to comply with product specifications or to ensure compliance with environmental regulations. Environmental concerns suggests that the sulfur of the final product should be below 50 wppm.
The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The following equations illustrate the reactions in a typical naphtha HDS unit:
RSH+H
2
→RH+H
2
S (1)
RCI+H
2
→RH+HCI (2)
RN+2H
2
→RH+NH
3
(3)
ROOH+2H
2
→RH+2H
2
O (4)
Typical operating conditions for the naphtha HDS reactions are:
Temperature, ° F.
500-650
Pressure, psig
300-800
H
2
recycle rate, SCF/bbl
300-3000
Fresh H
2
makeup, SCF/bbl
100-400
After the hydrotreating is complete, the product may be fractionated or simply flashed to release the hydrogen sulfide and collect the now desulfurized naphtha. The loss of olefins by incidental hydrogenation is detrimental by the reduction of the octane rating of the naphtha and the reduction in the pool of olefins for other uses.
In addition to supplying high octane blending components, the cracked naphthas are often used as sources of olefins in other processes such as etherifications. The conditions of hydrotreating of the naphtha fraction to remove sulfur will also saturate some of the olefinic compounds in the fraction reducing the octane and causing a loss of source olefins.
Various proposals have been made for removing sulfur while retaining the more desirable olefins. Since the olefins in the cracked naphtha are mainly in the low boiling fraction of these naphthas and the sulfur containing impurities tend to be concentrated in the high boiling fraction, the most common solution has been prefractionation prior to hydrotreating. The prefractionation produces a light boiling range naphtha which boils in the range of C
5
to about 250° F. and a heavy boiling range naphtha which boils in the range of from about 250-475° F.
The predominant light or lower boiling sulfur compounds are mercaptans while the heavier or higher boiling compounds are thiophenes and other heterocyclic compounds. The separation by fractionation alone will not remove the mercaptans. However, in the past the mercaptans have been removed by oxidative processes involving caustic washing. A combination oxidative removal of the mercaptans followed by fractionation and hydrotreating of the heavier fraction is disclosed in U.S. Pat. No. 5,320,742. In the oxidative removal of the mercaptans, the mercaptans are converted to the corresponding disulfides.
U.S. Pat. No. 5,597,476 discloses a two step process in which naphtha is fed to a first distillation column reactor which acts as a depentanizer or dehexanizer with the lighter material containing most of the olefins and mercaptans being boiled up into a first distillation reaction zone where the mercaptans are reacted with diolefins to form sulfides which are removed in the bottoms along with any higher boiling sulfur compounds. The bottoms are subjected to hydrodesulfurization in a second distillation column reactor where the sulfur compounds are converted to H
2
S and removed.
In a catalytic distillation column reactor, the reaction temperature is limited by the boiling point of the material in the catalyst bed which is a function of the boiling range of the naphtha and the pressure in the reactor, which may require using higher pressures than desired to obtain the necessary reaction temperatures of desulfurizing light naphtha.
SUMMARY OF THE INVENTION
The present process is particularly useful for desulfurizing light naphtha. The central elements of the process comprise a catalytic distillation hydrodesulfurization step in combination with a distillation step and a straight pass hydrogenation step. In the catalytic distillation hydrodesulfurization step mercaptans in the feed are contacted with hydrogen in the presence of hydrodesulfurization catalyst preferably at pressure of >250 psig and fractionated into a first overheads and a first bottoms. In the distillation step the first bottoms are fractionated into a second overheads and a second bottoms. In the straight pass hydrogenation step the second bottoms is contacted with hydrogen in the presence of a hydrodesulfurization catalyst at pressure of >250 and temperature >400° F. to further reduce the sulfur content. The light naphtha feed is composed primarily of C
4
-C
10
hydrocarbons (typically C
5
-350° F.) which contain mercaptans and diolefins in amounts generally less than 1000 ppm before processing. The sulfur compounds are treated in the straight pass hydrogenation to extinction (<50 ppm).
Briefly the present process comprising:
a. feeding a naphtha stream containing diolefins and organic sulfur compounds comprising mercaptans to a reaction distillation zone;
b. concurrently in said reaction distillation zone:
i. contacting said naphtha feed with hydrogen in the presence of a hydrodesulfurization catalyst, preferably at a pressure of >250 psig, to produce a reaction mixture containing H
2
S and
ii. fractionating said reaction mixture into a first overheads containing H
2
S and a naphtha fraction of less than C
8
and a first bottoms of a C
6
+ naphtha fraction and organic sulfur compounds boiling in the range of the C
6
+ naphtha fraction;
c. fractionating the first bottom
Klussman Bertrand
Maraschino Mario J.
Vichailak Montri
Arnold Jr. James
Catalytic Distillation Technologies
Griffin Walter D.
Johnson Kenneth H.
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