Targeted desulfurization apparatus integrating oxidative desulfurization and hydrodesulfurization to produce diesel fuel having an ultra-low level of organosulfur compounds

Abstract

Deep desulfurization of hydrocarbon feeds containing undesired organosulfur compounds to produce a hydrocarbon product having low levels of sulfur, i.e., 15 ppmw or less of sulfur, is achieved by flashing the feed at a target cut point temperature to obtain two fractions. A first fraction contains refractory organosulfur compounds, which boil at or above the target cut point temperature. A second fraction boiling below the target cut point temperature is substantially free of refractory sulfur-containing compounds. The second fraction is contacted with a hydrodesulfurization catalyst in a hydrodesulfurization reaction zone operating under mild conditions to reduce the quantity of organosulfur compounds to an ultra-low level. The first fraction is contacted with an oxidizing agent and an active metal catalyst in an oxidation reaction zone to convert the refractory organosulfur compounds to oxidized organosulfur compounds. The oxidized organosulfur compounds are removed, producing a stream containing an ultra-low level of organosulfur compounds. The two streams can be combined to obtain a full range hydrocarbon product having an ultra-low level of organosulfur compounds.

Claims

1. An apparatus for processing a hydrocarbon feed containing undesired organosulfur compounds comprising: a flashing column operable to flash the hydrocarbon feed at a temperature cut point of about 320 C. to about 360 C., the flashing column including an inlet for receiving the hydrocarbon feed, a low boiling temperature outlet for discharging a low boiling temperature fraction containing labile organosulfur compounds, and a high boiling temperature outlet for discharging a high boiling temperature fraction containing refractory organosulfur compounds; a hydrodesulfurization zone having an inlet in fluid communication with the low boiling temperature outlet and an outlet for discharging hydrotreated effluent; and an oxidative desulfurization zone containing an oxidation catalyst and an oxidizing agent, oxidative desulfurization zone having an inlet in fluid communication with the high boiling temperature outlet and an outlet for discharging oxidized effluent; and a solvent extraction zone having a product inlet in fluid communication with the outlet for discharging oxidized effluent, a solvent inlet in fluid communication with a source of polar solvent, an extract outlet for discharging a mixture of solvent and oxidized sulfur-containing compounds, and a raffinate outlet for discharging a solvent extracted hydrocarbon product stream.

2. The apparatus as in claim 1, further comprising a distillation column having an inlet in fluid communication with the extract outlet, a byproduct outlet for discharging oxidized sulfur-containing compounds, and a solvent outlet, wherein the solvent outlet is the source of polar solvent and is in fluid communication with the solvent inlet of the solvent extraction zone.

3. The apparatus as in claim 1, further comprising an adsorption zone having an inlet in fluid communication with the raffinate outlet of the solvent extraction zone, and a product outlet for discharging an adsorbent treated hydrocarbon product stream.

4. The apparatus as in claim 1, further comprising a decanting vessel between the oxidative desulfurization zone and the solvent extraction zone having an inlet in fluid communication with the outlet for discharging oxidized effluent and an outlet in fluid communication with the product inlet of the solvent extraction zone.

5. The apparatus of claim 1, wherein the oxidizing agent is selected from the group consisting of hydrogen peroxide, organic peroxides such as ter-butyl hydroperoxide, peroxo acids, oxides of nitrogen, oxygen, and air.

6. The apparatus of claim 1, wherein the oxidizing catalyst is selected from the group consisting of homogeneous catalysts and heterogeneous catalysts.

7. The apparatus of claim 6, wherein the oxidizing catalyst includes a metal from Group IVB to Group VIIIB of the Periodic Table.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The foregoing summary as well as the following detailed description of preferred embodiments of the invention will be best understood when read in conjunction with the attached drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and apparatus shown. In the drawings the same numeral is used to refer to the same or similar elements, in which:

(2) FIG. 1 is a graph showing cumulative sulfur concentrations plotted against boiling points of three thiophenic compounds;

(3) FIG. 2 is a schematic diagram of an integrated desulfurization system and process of the present invention that includes a flashing column upstream of the hydrodesulfurization and oxidative desulfurization zones; and

(4) FIG. 3 is a schematic diagram of a separation apparatus for removing oxidized organosulfur compounds from a fraction boiling at or above the target cut point temperature according to the system and process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(5) The present invention comprehends an integrated desulfurization process to produce hydrocarbon fuels with an ultra-low level of sulfur which includes the following steps:

(6) a. Flashing the hydrocarbon feedstock at a target cut point temperature of about 300 C. to about 360 C., preferably about 340 C., to obtain two fractions. The two fractions contain different classes of organosulfur compounds having different reactivities when subjected to hydrodesulfurization and oxidative desulfurization processes.

(7) b. The organosulfur compounds in the fraction boiling below the target cut point temperature are primarily labile organosulfur compounds, including aliphatic molecules such as sulfides, disulfides, mercaptans, and certain aromatics such as thiophenes and alkyl derivatives of thiophenes. This fraction is contacted with a hydrodesulfurization catalyst in a hydrodesulfurization reaction zone under mild operating conditions to remove the organosulfur compounds.

(8) c. The organosulfur compounds in the fraction boiling at or above the target cut point temperature are primarily refractory organosulfur compounds, including aromatic molecules such as certain benzothiophenes (e.g., long chain alkylated benzothiophenes), dibenzothiophene and alkyl derivatives, e.g., 4,6-dimethyldibenzothiophene. This fraction is contacted with an oxidizing agent and an active metal catalyst in an oxidation reaction zone to convert the organosulfur compounds into oxidized sulfur-containing compounds.

(9) d. The oxidized organosulfur compounds are subsequently removed in a separation zone by oxidation product removal processes and apparatus that include extraction, distillation, adsorption, or combined processes comprising one or more of extraction, distillation and adsorption.

(10) e. The resulting stream from the hydrodesulfurization reaction zone and the low sulfur stream from the separation zone can be recombined to produce an ultra-low sulfur level hydrocarbon product, e.g., a full-range diesel fuel product.

(11) Referring to FIG. 2, an integrated desulfurization apparatus 6 according to the present invention is schematically illustrated. Apparatus 6 includes a flashing column 9, a hydrodesulfurization reaction zone 14, an oxidative desulfurization reaction zone 16 and a separation zone 18. A hydrocarbon stream 8 is introduced into the flashing column 9 to be fractionated at a target cut point temperature of about 300 C. to about 360 C., and preferably about 340 C., into two streams 11 and 12. The hydrocarbon stream 9 is preferably a straight run gas oil boiling in the range of about 260 C. to about 450 C., typically containing up to about 2 weight % sulfur, although one of ordinary skill in the art will appreciated that other hydrocarbon streams can benefit from the practice of the system and method of the present invention.

(12) Stream 11 boiling below the target cut point temperature is passed to the hydrodesulfurization reaction zone 14 and into contact with a hydrodesulfurization catalyst and a hydrogen feed stream 13. Since refractory organosulfur compounds are generally present in relatively low concentrations, if at all, in this fraction, hydrodesulfurization reaction zone 14 can operate under mild conditions. The hydrodesulfurization catalyst can be, for instance, an alumina base containing cobalt and molybdenum.

(13) As will be understood by one of ordinary skill in the art, mild operating conditions is relative and the range of operating conditions depend on the feedstock being processed. According to the present invention, these mild operating conditions as used in conjunction with hydrotreating a mid-distillate stream, i.e., boiling in the range of about 180 C. to about 370 C., include: a temperature of about 300 C. to about 400 C., preferably about 320 C. to about 380 C.; a reaction pressure of about 20 bars to about 100 bars, preferably about 30 bars to about 60 bars; a hydrogen partial pressure of below about 55 bars, preferably about 25 bars to about 40 bars; a feed rate of about 0.5 hr.sup.1 to about 10 hr.sup.1, preferably about 1.0 hr.sup.1 to about 4 hr.sup.1; and a hydrogen feed rate of about 100 liters of hydrogen per liter of oil (L/L) to about 1000 L/L, preferably about 200 L/L to about 300 L/L.

(14) The resulting hydrocarbon stream 15 contains an ultra-low level of organosulfur compounds, i.e., less than 15 ppmw, since substantially all of the aliphatic organosulfur compounds, and thiophenes, benzothiophenes and their derivatives boiling below the target cut point temperature, are removed. Stream 15 can be recovered separately or in combination with the portion boiling at or above the target cut point temperature that has been subjected to the oxidative desulfurization reaction zone 16.

(15) Stream 12 boiling at or above the target cut point temperature is introduced into the oxidative desulfurization reaction zone 16 for contact with an oxidizing agent and one or more catalytically active metals. The oxidizing agent can be an aqueous oxidant such as hydrogen peroxide, organic peroxides such as ter-butyl hydroperoxide, or peroxo acids, a gaseous oxidant such as oxides of nitrogen, oxygen, or air, or combinations comprising any of these oxidants. The oxidation catalyst can be selected from one or more homogeneous or heterogeneous catalysts having metals from Group IVB to Group VIIIB of the Periodic Table, including those selected from of Mn, Co, Fe, Cr and Mo.

(16) The higher boiling point fraction, the oxidizing agent and the oxidation catalyst are maintained in contact for a period of time that is sufficient to complete the oxidation reactions, generally about 15 to about 180 minutes, in certain embodiments about 15 to about 90 minutes and in further embodiments about 30 minutes. The reaction conditions of the oxidative desulfurization zone 16 include an operating pressure of about 1 to about 80 bars, in certain embodiments about 1 to about 30 bars, and in further embodiments at atmospheric pressure; and an operating temperature of about 30 C. to about 300 C., in certain embodiments about 30 C. to about 150 C. and in further embodiments about 80 C. The molar feed ratio of oxidizing agent to sulfur is generally about 1:1 to about 100:1, in certain embodiments about 1:1 to about 30:1, and in further embodiments about 4:1 to about 1:1. In the oxidative desulfurization zone 16, at least a substantial portion of the aromatic sulfur-containing compounds and their derivatives boiling at or above the target cut point are converted to oxidized sulfur-containing compounds, i.e. sulfones and sulfoxides and discharged as an oxidized hydrocarbon stream 17.

(17) Stream 17 from the oxidative desulfurization zone 16 is passed to the separation zone 18 to remove the oxidized sulfur-containing compounds as discharge stream 19 and obtain a hydrocarbon stream 20 that contains an ultra-low level of sulfur, i.e., less than 15 ppmw. A stream 20a can recovered, or streams 15 and 20a can be combined to provide a hydrocarbon product 21 that contains an ultra-low level of sulfur that is recovered. A stream 20b can be recycled back to the hydrotreating zone 14 if the sulfur content of the oxidative desulfurization zone products remains high and needs to be further reduced. Stream 19 from the separation zone 18 is passed to a sulfones and sulfoxides handling unit (not shown) to recover hydrocarbons free of sulfur, for example, by cracking reactions, thereby increasing the total hydrocarbon product yield. Alternatively, stream 19 can be passed to other refining processes such as coking or solvent deasphalting.

(18) Referring to FIG. 3, one embodiment of a process for removing sulfoxides and sulfones from oxidized hydrocarbon stream 17 is shown. Stream 17 containing oxidized hydrocarbons, water and catalyst is introduced into is introduced into a decanting vessel 35 to decant water and catalyst as a discharge stream 58 and separate a hydrocarbon mixture stream 25. Stream 58 which can include a mixture of water (e.g., from the aqueous oxidant), any remaining oxidant and soluble catalyst, is withdrawn from the decanting vessel 35 and recycled to the oxidative desulfurization zone 16 (not shown in FIG. 3), and the hydrocarbon stream 25 is passed generally to the separation zone 18. The hydrocarbon stream 25 is introduced into one end of a counter-current extractor 46, and a solvent stream 47 is introduced into the opposite end. Oxidized sulfur-containing compounds are extracted from the hydrocarbon stream with the solvent as solvent-rich extract stream 49.

(19) The solvent stream 47 can include a selective solvent such as methanol, acetonitrile, any polar solvent having a Hildebrandt value of at least 19, and combinations comprising at least one of the foregoing solvents. Acetonitrile and methanol are preferred solvents for the extraction due to their polarity, volatility, and low cost. The efficiency of the separation between the sulfones and/or sulfoxides can be optimized by selecting solvents having desirable properties including, but not limited to boiling point, freezing point, viscosity, and surface tension.

(20) The raffinate 48 is introduced into an adsorption column 62 where it is contacted with an adsorbent material such as an alumina adsorbent to produce the finished hydrocarbon product stream 20 that has an ultra-low level of sulfur, which is recovered. The solvent-rich extract 49 from the extractor 46 is introduced into the distillation column 55 for solvent recovery via the overhead recycle stream 56, and the oxidized sulfur-containing compounds, i.e., sulfones and/or sulfoxides are discharged as stream 19.

(21) The addition of a flash column into the apparatus and process of the invention that integrates a hydrodesulfurization zone and an oxidative desulfurization zone uses low cost units in both zones as well as more favorable conditions in the hydrodesulfurization zone, i.e., milder pressure and temperature and reduced hydrogen consumption. Only the fraction boiling at or above the target cut point temperature is oxidized to convert the refractory sulfur-containing compounds. This results in more cost-effective desulfurization of hydrocarbon fuels, particularly removal of the refractory sulfur-containing compounds, thereby efficiently and economically achieving ultra-low sulfur content fuel products.

(22) The present invention offers distinct advantages when compared to conventional processes for deep desulfurization of hydrocarbon fuel. For example, in certain conventional approaches to deep desulfurization, the entire hydrocarbon stream undergoes both hydrodesulfurization and oxidative desulfurization, requiring reactors of high capacity for both processes. Furthermore, the high operating costs and undesired side reactions that can negatively impact certain desired fuel characteristics are avoided using the process and apparatus of the present invention. In addition, operating costs associated with the removal of the oxidized sulfur-containing compounds from the entire feedstream are decreased as only a portion of the initial feed is subjected to oxidative desulfurization.

EXAMPLE

(23) A gas oil was fractionated in an atmospheric distillation column to split the gas oil into two fractions: A light gas oil fraction (LGO) that boils at 340 C. and less with 92.6 W % yield and a heavy gas oil fraction (HGO) that boils at 340 C. and higher with 7.4 W % yield were obtained. The LGO boiling 340 C. or less was desulfurized, the properties of which are given in Table 4.

(24) TABLE-US-00004 TABLE 4 SR Gas Oil 340 C. 340 C. + Property Unit Value Value Value Yield W % 100 92.6 7.4 Sulfur W % 0.72 0.625 1.9 Density g/cc 0.82 0.814 0.885 5% C. 138 150 332 10% C. 166 173 338 30% C. 218 217 347 50% C. 253 244 355 70% C. 282 272 363 90% C. 317 313 379 95% C. 360 324 389

(25) The LGO fraction was subjected to hydrodesulfurization in a hydrotreating vessel using an alumina catalyst promoted with cobalt and molybdenum metals at 30 Kg/cm.sup.2 hydrogen partial pressure at the reactor outlet, weighted average bed temperature of 335 C., liquid hourly space velocity of 1.0 h.sup.1 and a hydrogen feed rate of 300 L/L. The sulfur content of the gas oil was reduced to 10 ppmw from 6,250 ppmw.

(26) The HGO fraction contained diaromatic sulfur-containing compounds (benzothiophenes) and triaromatic sulfur-containing compounds (dibenzothiophenes) with latter one being the most abundant species (80%) according to speciation using a two dimensional gas chromatography equipped with a flame photometric detector. Further analysis by gas chromatography integrated with a mass spectroscopy showed that benzothiophene compounds are substituted with alkyl chains equivalent to four and more methyl groups.

(27) The heavy gas oil fraction, the properties of which are given in Table 4, was oxidized in a reactor at 80 C. and 1 atmosphere for 1.5 hour. 0.5 W % of Na.sub.2WO.sub.4, 2H.sub.2O and 13 W % of acetic acid are used as catalytic system. A 30% H.sub.2O.sub.2/H.sub.2O mixture is used as oxidizing agent targeting peroxide to sulfur molar ratio of 4. After the oxidation reaction, the reaction medium was cooled to room temperature and the layers were separated. The oil layer that contained the oxidized sulfur-containing compounds underwent an extraction step using methanol (1:1 V/V % ratio of oil to solvent ratio) at room temperature. Adsorption of remaining sulfur-containing compounds over -Al.sub.2O.sub.3 in an oil layer after solvent extraction was carried out at room temperature in a chromatography column, equipped with a coarse bottom frit (10:1 ratio of oil and adsorbent).

(28) The sulfur content of the oil layer after oxidation was reduced to 1.03 wt % from 1.9 wt % in the original heavy gas oil fraction. It was then further reduced to 0.31 wt % after methanol extractions and to 0.28 wt % after adsorption. The oil fraction, which is free of refractory sulfur-containing compounds but still contains labile sulfur-containing compounds, was recycled back to the hydrotreating unit for desulfurization. The process yielded a diesel product with a sulfur content of 10 ppmw.

(29) The method and system of the present invention have been described above and in the attached drawings; however, modifications will be apparent to those of ordinary skill in the art and the scope of protection for the invention is to be defined by the claims that follow.