Method for the production of very low sulfur diesel

Abstract

A process for the deep desulfurization of diesel range feedstock to produce low sulfur diesel fuels by contacting a sulfur containing diesel range feedstock with a cobalt molybdenum (CoMo) catalyst followed by a nickel containing catalyst, such as nickel molybdenum (NiMo), nickel tungsten (NiW), nickel tungsten molybdenum (NiWMo) and nickel cobalt molybdenum (NiCoMo), under a combination of elevated temperature and superatmospheric hydrogen pressure to convert the sulfur in the sulfur-containing feedstock to inorganic sulfur compounds and produce a desulfurized product having a sulfur content below SO ppm by weight. The process can include a dual catalyst system, wherein the sulfur containing diesel range feedstock is desulfurized with a cobalt molybdenum (CoMo) catalyst and then the sulfur compounds can optionally be stripped from the stream prior to contacting with the nickel containing catalyst. The preferred desulfurized product contains less than 11 wt. % polycyclicaromatics and has an increased cetane number.

Description

BACKGROUND OF INVENTION

[0002] The present invention relates to the desulfurization of diesel fuels, and, in particular, to reducing the sulfur content of diesel fuels to a very low level, while also yielding a high cetane value.

[0003] The sulfur impurities in diesel fuels require removal, usually by hydrotreating, in order to comply with product specifications and/or to ensure compliance with environmental regulations. The current U.S. specification for diesel fuels permits a maximum sulfur content of 50 ppmw. However, the EPA is expected to propose new diesel fuel specifications that will become effective in 2004. The new specification is likely to require further reduction of sulfur content in diesel fuels to below 50 ppmw. Recently, the European Union published new diesel specifications, which limit the sulfur content of diesel fuels to a maximum of 350 ppmw after the year 2000, and to 50 ppmw maximum after the year 2004. In addition, new specifications have been proposed which will increase the cetane number of diesel fuels to 58 in the year 2005, and reduce the polyaromatics content.

[0004] With the enactment of stricter diesel specifications, the processes presently being used for producing diesel fuels may not sufficiently reduce the sulfur content. This will require a modification of existing processes or the introduction of new processes. However, many of the new diesel specifications also require a higher cetane value and so any modified or new desulfurization process will also have to decrease the sulfur content while maintaining or increasing the cetane value. Diesel fuels can be hydrotreated by passing the feed over a hydrotreating catalyst at an elevated temperature and a somewhat elevated pressure in a hydrogen atmosphere. One suitable family of catalysts which has been widely used for this service is a combination of a Group VIII metal and a Group VI metal of the Periodic Table, such as cobalt and molybdenum, on a substrate such as alumina. After the hydrotreating operation is complete, the product can be fractionated, or simply flashed, to release the hydrogen sulfide, remove low flash light gases and collect the sweetened diesel fuel.

[0005] Various proposals have been made for removing sulfur while retaining the more desirable paraffinic components. For example, U.S. Pat. No. 3,546,103 teaches hydrodesulfurization with a catalyst of cobalt and molybdenum on an alumina base.

[0006] Although the art of hydroprocessing has been known for a long time and is a highly developed art, there exists today even greater need for efficient and economical means for hydrodesulfurizing diesel fuels in order to comply with more stringent diesel fuel specifications and environmental regulations. Therefore, a process for the deep desulfurization of diesel fuels that will also maintain a high cetane value, or increase the cetane value, is essential to the upgrading of such stocks. None of the prior art mentioned above nor any prior art known to applicant discloses a catalyst which is capable of efficiently hydrodesulfurizing diesel fuels while providing an improved cetane value and polyaromatics saturation.

SUMMARY OF THE INVENTION

[0007] In accordance with the present invention, a process is provided for the desulfurization of diesel boiling range feedstocks to produce low sulfur diesel fuels. The process includes contacting a sulfur containing diesel boiling range feedstock with a cobalt molybdenum (CoMo)-containing catalyst, followed by a nickel (Ni)-containing catalyst under a combination of elevated temperature and superatmospheric hydrogen pressure to convert the sulfur in the sulfur-containing feedstock to inorganic sulfur compounds and produce a desulfurized product having a sulfur content below 50 ppm by weight (ppmw). The feedstock can be desulfurized prior to contacting with the nickel containing catalyst or a desulfurized diesel fuel containing less than 0.2 wt. % can be used as the feedstock. The nickel-containing catalyst includes a desulfurization component of nickel molybdenum (NiMo), nickel tungsten (NiW), nickel tungsten molybdenum (NiWMo) or nickel cobalt molybdenum (NiCoMo); and a support of alumina, silica-alumina, titania, magnesia, zirconia, silica, zeolite, non-zeolite molecular sieve or combinations thereof.

[0008] In one embodiment, the process is carried out in at least two stages, wherein the feedstock is desulfurized in a first stage before being contacted with a nickel containing catalyst to achieve deep desulfurization in a second stage. In a preferred embodiment, the process is carried out using a dual catalyst system, wherein the feedstock is contacted with a cobalt molybdenum/alumina (CoMo/Al2O3) catalyst in the first stage to produce a desulfurized feedstock prior to contacting the feedstock with the nickel containing catalyst. After the feedstock is desulfurized in the first stage, the feedstock can be stripped of sulfur and, nitrogen compounds (primarily H2S and NH3) before it is contacted with the nickel containing catalyst in the second stage. The dual catalyst system process can be carried out in one reactor vessel or in multiple vessels. The feedstock can be desulfurized by a CoMo catalyst system in a first reactor, and then sent to a stripper, before being contacted with a nickel containing catalyst, preferably NiMo/Al2O3, in the second reactor.

[0009] In addition to desulfurizing the feedstock, the process improves the cetane number so that the cetane number of the product is at least equal to, and in preferred embodiments greater than, the cetane number of the diesel range feedstock. In preferred embodiments, the process reduces the polyaromatics in the desulfurized product to less than 11 wt. %.

[0010] The process of the present invention provides deep desulfurization of diesel range feedstocks by using nickel containing hydrotreating catalysts to remove the last refractory sulfur compounds from diesel fuels. This process significantly reduces the necessary volume of the reactor and provides substantial savings due to the high activity of the nickel containing catalysts compared to conventional CoMo catalysts. The increased activity of these catalysts requires smaller reactors and, thus, can save as much as 50-70% of the catalyst volume compared to processes that use CoMo catalysts.

BRIEF DESCRIPTION OF THE FIGURES

[0011] Other objects and many attendant features of this invention will be readily appreciated as the invention becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

[0012] FIG. 1 is a graph showing sulfur reduction using a CoMo catalyst and a NiMo catalyst.

[0013] FIG. 2 is a graph showing polyaromatics saturation using a CoMo catalyst and a NiMo catalyst.

[0014] FIG. 3 is graph showing the cetane number for desulfurized product using a CoMo catalyst and a NiMo catalyst.

[0015] FIG. 4 is a flow diagram of a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0016] Diesel fuels currently being used will require additional desulfurization in order to meet the stricter government regulations that are expected to be enacted in the next few years. The present invention provides a method for desulfurizing diesel fuel feedstocks to below 50 ppmw (parts per million by weight) sulfur to comply with these regulations by using a nickel containing catalyst to desulfurize refractory sulfur-containing components of the feedstocks. The process can also be used to provide deep distillate desulfurization of diesel fuels currently being produced that do not comply with the stricter regulations.

[0017] Most desulfurization processes for diesel fuel feedstocks use cobalt molybdenum (CoMo) based catalysts instead of nickel based catalysts because CoMo based catalysts are more active when treating feedstocks with a relatively high sulfur content. The present invention is based on the discovery that NiMo catalysts are more active than CoMo, catalysts for deep desulfurization of diesel fuels having, relatively low sulfur levels. Using the method, of the present invention, diesel fuels, containing low sulfur levels are desulfurized to produce diesel products containing <50 ppm sulfur. In addition, NiMo catalysts are more effective than CoMo catalysts in improving the cetane number and polyaromatics saturation.

[0018] In a preferred embodiment of the present invention, a NiMo/Al2O3 catalyst is used for post desulfurization of current 0.05 wt. % S diesel to less than 50 ppmw sulfur. In another embodiment, a dual catalyst system consisting of a CoMo catalyst followed by a nickel based catalyst, such as a NiMo catalyst, is used for desulfurization of raw diesel range feedstocks. The CoMo/NiMo catalyst system has been found to be more active for deep distillate desulfurization than a reverse NiMo/CoMo catalyst system, or a CoMo catalyst system alone. While CoMo catalysts are effective in reducing the sulfur level of raw diesel range feedstocks to below 0.2 wt. %, it has been found that NiMo catalysts are more active for desulfurization of residual refractory sulfur-containing compounds. CoMo catalysts are effective in desulfurizing most of the organic sulfur compounds in diesel fuel feedstocks but, by comparison, have been found to be relatively ineffective in desulfurizing refractory sulfur-containing compounds that have at least one alkyl (primary methyl group) substitute adjacent to the sulfur atom of dibenzothiophene type compounds, such as 4,-methyl or 4,6-dimethyl dibenzothiophene and their alkyl homologs. Conversely, nickel containing catalysts, which are not as active as CoMo catalysts for desulfurizing many organic sulfur-containing compounds, are particularly well suited for desulfurizing refractory sulfur-containing compounds and have a significantly higher activity when processing these compounds than CoMo catalysts.

[0019] Sulfur compounds that have at least one methyl group adjacent to the sulfur atom, such as 4-methyl dibenzothiophene, 4,6dimethyl dibenzothiophene and related compounds, are very difficult to remove in a desulfurization process. Generally, these are the primary sulfur compounds that remain in diesel fuels desulfurized to less than 0.2 wt. % sulfur using conventional CoMo catalyst based desulfurization processes. These CoMo catalyst processes are marginally effective in reducing these refractory sulfur-containing compounds and, therefore, they usually require high temperatures and/or prolonged residence times to efficiently reduce sulfur levels to below about 50 ppmw as required by the new regulations. The process of the present invention uses a nickel containing catalysts, such as a NiMo catalyst, in a second “deep desulfurization” stage to reduce sulfur levels to below 50 ppmw. The nickel containing catalysts are more active than CoMo catalysts for desulfurizing these refractory sulfur-containing compounds and can more efficiently produce diesel fuels with low sulfur levels than the diesel fuels produced by known processes.

[0020] The deep desulfurization nickel containing catalysts can also be used as combination dual catalyst system, such as a CoMo/NiMo dual catalyst system, for desulfurizing raw diesel range feedstocks. In the first stage of the process, raw diesel range feedstocks are desulfurized to below 0.2 wt. % sulfur using a CoMo catalyst and then the second stage uses a nickel containing catalyst to desulfurize the feedstocks to below 50 ppmw sulfur. The dual CoMo/nickel containing catalyst process can be carried out in separate reactors or in a single reactor. For example, in a downflow reactor, the top section of the catalyst bed can contain CoMo based catalyst and the bottom section can contain NiMo catalyst. The method of operating this type of dual bed reactor is well known to those skilled in the art. The diesel fuel product will have a boiling point range of about 350° F. to about 650° F. (about 175° C. to about 345° C.). The process of the invention can be used to upgrade a feedstock within the diesel fuel boiling point range to a higher cetane diesel fuel.

[0021] Cetane number is calculated by using either the standard ASTM engine test or NMR analysis. Although cetane number and cetane index have both been used in the past as measures of the ignition, quality of diesel fuels, they should not be used interchangeably. Cetane index can frequently overestimate the quality of diesel fuel streams containing cracked stocks. Thus, cetane number is used herein.

[0022] The diesel boiling range feedstocks product can generally be described as high boiling feeds of petroleum origin. In general, such feedstocks include gas oils distilled from various petroleum sources, boiling point from about 350° F. to about 750° F. (about 175° C. to about 400° C.), preferably about 400° F. to about 700° F. (about 205° C. to about 370° C.). Catalytic cracking cycle oils, including light cat cycle oil (LCCO) and heavy cat cycle oil (HCCO), clarified slurry oil (CSO), other catalytically cracked products, and thermally cracked products, such as coker light gas oil, are potential sources of feeds for the present process. If used, it is preferred that these cycle oils make up a minor component of the feed. Cycle oils from catalytic and thermal cracking processes typically have a boiling range of about 400° F. to 750° F. (about 205° C. to 400° C.), although light cycle oils may have a lower end point, e.g. 600° F. or 650° F. (about 315° C. or 345° C.). Because of the high content of aromatics found in such cycle oils, as well as undesirable amounts of nitrogen and sulfur, they require more severe process conditions. Lighter feeds may also be used, e.g. about 250° F. to about 400° F. (about 120° C. to about 205° C.). However, the use of lighter feeds will result in the production of higher value, lighter distillate products, such as kerosene.

[0023] The feed to the process can be rich in naphthenic species, such as found in a hydrocrackate product. The naphthenic content of the feeds used in the present process generally will be at least 5 wt. %, usually at least 20 wt. %, and in many cases at least 50 wt. %. The balance will be divided among paraffins and aromatics according to the origin of the feed and its previous processing.

[0024] The nickel containing catalyst stage of the process operates with a relatively low sulfur feed, generally less than about 1.0 wt. % sulfur by weight and preferably less than 0.2 wt. %. Hydrotreated or hydro cracked feeds are preferred, because both processes remove sulfur and nitrogen compounds from hydrocarbon streams without substantial boiling range conversion. In addition, for some feeds hydrotreating saturates aromatics to naphthenes, and hydrocracking produces distillate streams rich in naphthenic species.

[0025] In a preferred embodiment, the selected sulfur-containing, diesel boiling range feed is desulfurized in two stages. In the first stage, sulfur compounds present in the feed are converted to the inorganic form (i.e., H2S) in the presence of a CoMo catalyst. In the second stage, desulfurization is carried out in the presence of a nickel containing catalyst. In one embodiment, the inorganic compounds are separated from the diesel fuel stream between the first and second stage. However, in another embodiment the first stage effluent can be cascaded directly into the second stage reactor without the need for interstage separation.

[0026] When a two stage process is used, the particle size and the nature of the catalysts used in both stages will usually be determined by the type of process used, such as: a down-flow, liquid phase, fixed bed trickle flow process; an up-flow, fixed bed countercurrent process; an ebullating, fluidized bed process; or a transport, fluidized bed process. All of these different process schemes, which are well known, are possible although the down-flow fixed bed arrangement has the advantage of simplicity of operation.

[0027] The present invention can be operated as a single stage process, wherein a diesel fuel feedstock having a sulfur level of less than 1.0 wt. %, preferably less than 0.2 wt. %, is contacted with a nickel containing catalyst to reduce the sulfur content to below 50 ppmw. For feedstocks with a higher sulfur content, a preferred embodiment of the invention uses a two stage process, wherein the feedstock is desulfurized in two stages. The following description is for a two stage process. When a one stage process is used, the first stage of the two stage process is bypassed and the feedstock is desulfurized in accordance with the description for the second stage processing.

[0028] First Stage Processing

[0029] The first stage of the process desulfurizes the diesel fuel feedstock using any one of several well known hydrodesulfurization processes, preferably but not necessarily a process which employs a CoMo catalyst. Similar processes are disclosed in U.S. Pat. No. 4,568,448 Angevine et al., U.S. Pat. No. 5,011,593 Ware et al., U.S. Pat. No. 5,401,389 Mazzone et al. and U.S. Pat. No. 5,865,988 Collins et al., all of which are incorporated by reference herein in their entirety.

[0030] The catalyst used in the first stage hydrodesulfurization can be a conventional desulfurization catalyst made up of a Group VI and/or a Group VIII metal on a suitable substrate. The Group VI metal is usually molybdenum or tungsten and the Group VIII metal usually nickel or cobalt. Combinations such as Ni—Mo or Co—Mo are typical. Other metals which possess hydrogenation functionality are also useful in this service. The support for the catalyst is conventionally a porous solid, usually alumina, or silica-alumina but other porous solids such as magnesia, titania or silica, either alone or mixed with alumina or silica-alumina may also be used, as convenient.

[0031] The conditions used in the first stage of the process are those which result in the controlled formation of inorganic sulfur compounds, such as H2S. Typically, the temperature of the first stage reactor will be from about 300° F. to 850° F. (about 150° C. to 455° C.), preferably about 350° F. to 800° F. (about 177° C. to about 427° C.). Since the desulfurization of the diesel fuel feedstocks normally takes place readily, low to moderate pressures can be used. The pressure, therefore, will depend mostly on operating convenience. Pressure will typically be about 50 to 1500 psig (about 445 to 10445 kPa), preferably about 300 to 1000 psig (about 2170 to 7000 kPa) with space velocities typically from about 0.5 to 10 LHSV (hr−1), normally about 1 to 6 LHSV (hr−1). Hydrogen to hydrocarbon ratios typically of about 200 to 5000 SCF/Bbl (36 to 890 n.1.11.), preferably about 500 to 2500 SCF/Bbl (about 89 to 445 n.1.1−1) will be selected to minimize catalyst aging.

[0032] Second Stage Processing

[0033] The feed to the second stage can be the first stage effluent or the effluent from another desulfurized diesel boiling range stream and should contain less than 1.0 wt. % sulfur by weight, and preferably less than 0.2 wt. %. Diesel fuels conforming to the current regulatory standards have a sulfur content of less the 0.05 recent by weight and can be used as the feed to the second stage of the process. In the second stage, the desulfurized diesel boiling range stream undergoes deep desulfurization by contacting the stream with a nickel containing catalyst, such as nickel molybdenum (NiMo), nickel tungsten (NiW), nickel tungsten molybdenum (NiWMo) and nickel cobalt molybdenum (NiCoMo).

[0034] After the first stage and before being sent to the second stage of the process, the desulfurized diesel boiling range effluent stream can be stripped of H2S, NH3 and light gases, which were formed in the first desulfurization stage. Stripping the effluent of H2S and NH3 makes the second stage desulfurization more efficient.

[0035] The temperature of the second hydrodesulfurization step is from about 400° F. to 850° F. (about 220° C. to 454° C.), preferably about 500° F. to 750°F. (about 260° C. to 400° C.), with the exact selection dependent on the desulfurization required for a given feed with the chosen catalyst. A temperature rise occurs under the exothermic reaction conditions, with values of about 20° F. to 100° F. (about 11° C. to 55° C.) being typical under most conditions and with reactor inlet temperatures in the preferred 500° F. to 750° F. (260° C. to 400° C.) range.

[0036] Since the desulfurization of the diesel boiling range effluent stream normally takes place readily, low to moderate pressures maybe used, typically from about 50 to 1500 psig (about 445 to 10443 kPa), preferably about 300 to 1000 psig (about 2170 to 7,000 kPa). Pressures are total system pressure, reactor inlet. Pressure will normally be chosen to maintain the desired aging for the catalyst in use. The space velocity is typically about 0.3 to 10 LHSV (hr.−1), preferably about 1 to 6 LHSV (hr.−1). The hydrogen to hydrocarbon ratio in the feed is typically about 500 to 5000 SCF/Bbl (about 90 to 900 n.1.1.−1), usually about 1000 to 2500 SCF/B (about 180 to 445 n.1.1.−1). The extent of the desulfurization will depend on the feed sulfur content and on the product sulfur specification, with the reaction parameters selected accordingly. Normally, the process will be operated under a combination of conditions such that the second desulfurization stage yields a product having less than 50 ppm sulfur.

[0037] The catalyst used in the second hydrodesulfurization step is suitably a nickel containing desulfurization catalyst including one or more Group VI and/or a Group VIII metals on a suitable substrate. The Group VI metal is usually molybdenum or tungsten and the Group VIII metal usually nickel or cobalt. The preferred combinations are NiMo, NiW, NiWMo and NiCoMo. The support for the catalyst is conventionally a porous refractory metal oxide, such as alumina or silica-alumina but other porous solids such as magnesia, zirconia, titania or silica, either alone or mixed with alumina or silica-alumina may also be used. Under certain conditions, it may be desirable to include a zeolite or other molecular sieve in the support to provide improved dispersion of the hydrogenation component or enhanced resistance to poisons, such as H2S. Other metals which possess hydrogenation functionality are also useful in this service. In a preferred embodiment, a high hydrogenation catalyst is combined with the NiMo catalyst to provide maximum desulfurization. The high hydrogenation activity catalysts include Pt or Pt/Pd promoted alumina or zeolite containing catalysts.

[0038] The particle size and the nature of the catalyst will usually be determined by the type of conversion process which is being carried out, such as: a trickle flow fixed bed process; a counter-current (liquid phase downflow, gas phase upflow) fixed bed process, or an ebulatted, bed process, as noted above, with the down-flow fixed-bed trickle flow type of operation typically preferred.

EXAMPLE 1

[0039] Two commercial catalysts (NiMo/Al2O3 and CoMo/Al2O3) were used at the same conditions to desulfurize a partially desulfurized distillate stream with sulfur-compounds that contained at least one methyl group on the aromatic ring adjacent to the sulfur atom. The feedstock was a 0.05 wt. % sulfur diesel fuel obtained from a commercial desulfurization unit rundown (Table 1). This feed contained 429 ppmw sulfur associated with the refractory sulfur compounds. Both catalysts were tested separately in a fixed bed, down flow, trickle-bed reactor at the same conditions, i.e., 288-370° C., 2 LHSV, 42.2 kg/cm2 G total pressure and 2670 Nm3/m3 hydrogen circulation rate.

TABLE 1
DESULFURIZED DIESEL WITH 0.05 WT. % SULFUR
PARAMETER                     VALUE
Gravity, ° API                31.8
Nitrogen, ppmw                230
Cetane Index                  38.7
Sulfur, ppmw
4-methyl DBT(a)               6
4,6-dimethyl DBT              122
3+ Carbon DBT(b)              301
2,3-dimethyl DBT              23
Total sulfur                  452
Total refractory sulfur, ppmw 429
Aromatic Content, wt. %
Monoaromatics                 29.1
Polyaromatics                 13.8
Total aromatics               42.9
Distillation (D86), ° F.
IBP                           346
10%                           411
50%                           494
90%                           606
EBP                           668
(a)DBT = dibenzothiopene
(b)Having two methyl groups located in the 4, 6 positions.

[0040] The desulfurization results are shown in FIG. 1 and clearly show that the NiMo catalyst is more active for desulfurization <50 ppmw sulfur. At reaction temperatures between 325° C. and 375° C. (the range within which the sulfur content is below 50 ppmw for the NiMo catalyst), the NiMo catalyst reduces the sulfur level of the diesel fuel from 50 to 125 ppmw more than the conventional CoMo catalyst. The activity advantage of the NiMo catalyst over the CoMo catalyst can be translated to 50-70% catalyst volume saving. Because less NiMo catalyst is required to provide the same activity as the CoMo catalyst currently being used, the catalyst cost is reduced and a smaller reactor vessel can be used.

[0041] The higher activity of the NiMo catalyst not only resulted in more efficient desulfurization, but also was more active for saturation of polycyclic aromatics, which is an important catalytic function to meet the new government regulations for diesel fuels. The new European diesel fuel specifications are expected to limit the polycyclicaromatic content of diesel fuels to less than 11 wt. %. Therefore, it is desirable for a catalytic desulfurization process to also provide some polycyclicaromatic saturation. FIG. 2 shows a comparison of the polycyclicaromatics saturation for the diesel fuel products treated with the NiMo catalyst and the CoMo catalyst. Between 325° C. and 360° C. (the range within which the sulfur content is below 50 ppmw), desulfurization with the NiMo catalyst produces a product with polyaromatics content lower than the CoMo product and significantly below the expected regulatory limit of 11 wt. %. At the higher temperatures (e.g., 360° C. and above), the catalysts' performances are roughly equivalent as they are limited by thermodynamic equilibrium.

[0042] Aromatics saturation generally increases the cetane number of a diesel fuel. Consequently, the NiMo catalyst with its better polycyclicaromatics saturation produces a desulfurized diesel fuel product with a higher cetane number than the CoMo catalyst. FIG. 3 is a graph which compares the cetane number of desulfurized diesel fuel products produced using NiMo and CoMo over temperatures between 275° C. and 375° C. Between 325° C. and 360° C. (the range within which the sulfur content is below 50 ppmw), the cetane number for the NiMo catalyst products is higher than the cetane number for the CoMo catalyst products.

[0043] A simplified flow diagram (which omits ancillary equipment) for a preferred embodiment of the present invention is shown in FIG. 4. In this embodiment, the effluent from the first desulfurization stage (using a CoMo catalyst) is sent to a stripper, which removes H2S, NH3 and light hydrocarbons (e.g., methane, ethane, propane and butanes), before the effluent is fed into the second stage reactor where it is contacted with a nickel containing catalyst (NiMo) to produce a diesel fuel product containing less than 50 ppmw sulfur.

EXAMPLE 2

[0044] This example compares the desulfurization activity of NiMo and CoMo catalyst systems in two different loading configurations using a diesel fuel containing refractory sulfur compounds. The tests were conducted in a fixed-bed, down-flow trickle-bed pilot unit. For the first test, the reactor was loaded with a CoMo catalyst in the top section of the reactor and NiMo catalyst in the bottom section in a 50/50 volumetric ratio. For the second test, the catalyst loading sequence was reversed, with a CoMo catalyst in the bottom section and NiMo catalyst in the top section. Both tests were carried out under the same conditions, using the same raw distillate as the feedstock (Table 2) for each test. The results listed in Table 3 show that the A CoMo/NiMo catalyst configuration was more active than the NiMo/CoMo catalyst configuration for the desulfurization of the diesel fuel. These results indicate that the CoMo catalyst in the top section is more active for the initial desulfurization than the catalyst system using the NiMo catalyst and the NiMo catalyst system is more active for the deep desulfurization of the refractory sulfur compounds.

TABLE 2
RAW DIESEL DISTILLATE
PARAMETER           VALUE
Gravity, ° API      28.0
Nitrogen, ppmw      250
Total Sulfur, wt. % 2.0
Aromatic Content, wt. %
Monoaromatics       19.7
Polyaromatics       31.4
Total aromatics     51.1
Distillation (D86), ° F.
IBP                 316
10%                 415
50%                 506
90%                 605
EBP                 642

[0045]

TABLE 3
DUAL CATALYST CONFIGURATION PERFORMANCE
	DUAL CATALYST	CoMo/NiMo		NiMo/CoMo
	Temperature, ° C.	342	370	343	370
	Desulfurization, wt. %	86.5	95	84	93.5	I

[0046] Operating conditions: 3 LHSV, 35.2 kg/cm2 G pressure, 178 Nm3/m3 H2 circulation.

EXAMPLE 3

[0047] This experiment shows that other nickel containing catalysts (in addition to the NiMo catalyst), such as NiCoMo catalysts (and by extension, also NiW and NiWMo catalysts) can be successfully used to desulfurize refractory sulfur compounds. In this example a NiCoMo catalyst was compared with a NiMo catalyst for desulfurizing a raw diesel distillate that contains 1.38 wt. % sulfur. The test results in Table 4 show the NiCoMo catalyst is more active than the CoMo catalyst in attaining less than 50 ppmw. The NiCoMo attained 43 ppmw sulfur at 366° C., while the CoMo only attained 56 ppmw sulfur at the same temperature.

TABLE 4
NiCoMo CATALYST PERFORMANCE
	NiCoMo	NiMo Catalyst
Temperature, ° C.	Residual Sulfur, wt. %	Residual Sulfur, wt. %
299	0.48	0.48
316	0.21	0.28
366	0.043	0.056
378	0.006	0.006
Feed: 1.38 wt. % sulfur
Conditions: 1 LHSV, 28 kg/cm2 G pressure, and 178 Nm3/m3 H2 circulation

[0048] Thus, while there have been described the preferred embodiments of the present invention, those skilled in the art will realize that other embodiments can be made without departing from the spirit of the invention, and it is intended to include all such further modifications and changes as come within the true scope of the claims set forth herein.

Claims

1. A process for the desulfurization of diesel boiling range feedstocks to produce low sulfur, low polynuclear diesel fuels comprising: contacting a sulfur-containing diesel boiling range feedstock with a refractory supported cobalt molybdenum containing catalyst, then stripping said feedstock, followed by a second stage containing a refractory supported nickel cobalt molybdenum catalyst under a combination of elevated temperature and superatmospheric hydrogen pressure, to convert said sulfur in said sulfur-containing feedstock to inorganic sulfur compounds and produce a desulfurized product having a sulfur content below 50 ppm by weight, wherein (i) the first stage's temperature ranges from about 150° C. to about 455° C. and the first stage's pressure ranges from about 50 to 1500 psig, and (ii) the second stage's temperature ranges from about 220° C. to about 454° C. and the second stage's pressure ranges from about 50 psig to about 1500 psig.

2. The process of claim 1, wherein said process is carried out in one reactor vessel.

3. The process of claim 1, wherein said diesel boiling range feedstock has a first cetane number and said desulfurized product has a second cetane number, and wherein said second cetane number is greater than said first cetane number.

4. The process of claim 1, wherein said desulfurized product contains less than 11 wt. % polyaromatics.

5. The process of claim 1, wherein said diesel range feedstock contains less than 0.05 wt. % sulfur.

6. The process of claim 3, wherein said diesel range feedstock has a polycyclicaromatics content of less than 11 wt. %.

7. The process of claim 5, wherein said desulfurized product contains less than 11 wt. % polycyclicaromatics.