CA1320167C - Process for upgrading distillate feedstock material - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A process for upgrading a distillate feedstock material is provided. The process comprises hydrogenating the distillate feedstock material in the presence of a hydrogenation catalyst to produce from about 90 to 96 percent by weight of a hydrogenated product, the balance being refractory aromatic compounds. The refractory aromatic compounds are then removed by an extraction technique. The process is particularly suitable for upgrading the middle distillate of a synthetic crude oil to produce i) diesel fuel having a high cetane number and;
ii) jet fuel having suitable combustion characteristics.

Description

13~01~
The present invention is related to a process for upgrading a distillate feedstock material. More particularly, the present invention is related to a process for producing diesel and jet fuels.

A well known source of distillate feedstock, particularly synthetic crude oil, for the production of fuel products is bitumen, which is typically upgraded by coking processes. These coking processes result in the production of polycyclic aromatic ring structured materials which are then: cracked to a lighter product;
fractionated to produce coker distillates and upgraded further by severe hydrotreating which ~erves to remove sulphur and nitrogen while simultaneously saturating olefins and some aromatics. The distillate streams which are thus produced through partial upgrading are then blended to produce a synthetic crude oil.

~ Synthetic crude oil generally comprises a mlddle distillate fraction having a boiling point range of from about 140 to about 350 C. The boiling point range of this middle distillate fraction embodies that of diesel fuels, Jet fuels (also known as kerosene) and light fuel oils. Unfortunately, this synthetic crude middle distillate fraction is of low quallty and thus, iR not useful for the production of these fuels and oils becauso it possesses a high concentration of aromatic hydrocarbons (eg. as much as 45 percent by weight~
subsc~uent to bitumen upgrading. A hi~h aromatic hydrocarbon content is undesirable in both diesel and jet fuels since it has a deleterious affact on the ig~ition and combustion qualities of these fuels.
Diesel and ~et fuels heretofore produced from conventional crude oil generally contain a lower . . .

, ~ . ~

13~" ~ 67 concentration of aromatic hydrocarbons (eg. less than about 30 percent by weight).

A key property of diesel fuel which i9 usually assessed in determining whether the ~uel is suitable for use ls the cetane number, which ls a measure o~ the ignition quality of the fuel. The cetane number is determined by a comparative test using a standard test engine which measures the delay period between fuel in~ection and ignitien for a blend of reference fuel-~, and also for the fuel under test. The refarence fuels are n-cetane (excellent diesel ignition propert~es~ and heptamethylnonane (poor diesel ignition properties), which are blended to produce a fuel having the same ignition delay period as the test fuel. On the cetane numbsr scale, pure n-cetane and heptamethylnonane are arbitrarily assigned 100 and 15, raspectively. Thus, the cetane number of the blend is a function of the percentage of n-cetane which is given by:
cetane # = % n-cetane + 0.15(~ heptamethylnonane) ~igher cetane numbers are synonymous with short ignition delay periods and generally good fuel ignition properties. Low cetane numbers usually result in poor engine perPormance and problematic environmental emissions.

The most lmportant characteristics relating to the quality of Jet fuel are those in~luenclng energy content and combustion ch~racteristics. These characteristics determine the vverall performance of the Jet engins when the fuel is burned in the gas turbine.
The combustion charac~eristics of jet fuel are particularly related to its hydrogen/carbon (hereinafter .. . .
~ . .
~ ' ' ~20~7 referred to as H/C) ratio. Generally, fuels which are hydrogen deficient, such as those with a high concentration of aromatic hydrocarbons, are more difficult to burn than those which are richer in paraffins (ie. straight and branched hydrocarbon chains). Further, fuels having a low H/C ratio burn with more radiant flames which tends to raisa combustor liner tamperatures and thereby shorten engine life. The combustion performance of jet fuel is usually deined by one of the following standard tests: measurement of luminometer number (ASTM D1740); smoke point test (ASTM
D1322); and smoke point plus maximum naphthalenes content (ASTM D1840).

The percenta~e by mass of the various hydrocarbon groups contained in a typical middle distillate fraction of synthetic crude oil are provided in Table 1. For comparison, the percentage by mass of the various hydrocarbon groups contained in a typical middle distillate fraction of conventional crude oil are also provided in Table 1.

Hydrocarbon Synthetic Crude Conventional Crude Group Oil, mass ~ Oil, mass~
. ., _ . .
paraffins 17 39 naphthenes 37 34 alkylbenzenss 36 18 2 ring aromatic~ 8 8 3-ring aromatic 2 - _ _ cetane number of fuel produced 31 4 .. _ .... _ _ .
, . . .

.

1~201~

As illustrated, a typi~al middle distillate fraction of synthetic crude oil is low in paraffins and high in aromatics when compared to a typical middle distillate fractlon of convantional crude oil. Hydrocarbon groups with the most favourable cetane numbers are paraffins having long straight chains up to about C20. Branched chain paraffins yield lower cetan~ numbers which decrease with increassd branching. Napthenes (eg. cycloparaffins), which generally have lower cetane numbers than paraffins, have better lgnition properties than aromatics, which generally have the lowest cetane numbers. The deficiencies of processes known heretofore to produce diesel fuel from synthetic crude oil are e~emplified by a comparison of the cetane numbers shown in Table 1.
Thus, it would be desirable to have an efflclent process which could be used to upgrade a low grade distillate feedstock, more particularly the middle distillate fraction of synthetic crude oil, such that the upgraded product possessed the necessary properties to enable it to be used as diesel or jet fuals.

It is an obJect of the present invention to obviate or mitigate the above-mentioned disadvantages.
~ Accordingly, the present invention provides a process for upgrading a distillate feedstock material which compri~es:
i) hydrogenating the distillate ~eedstock material in the presenca of a hydroganation catalyst thereby producing from about 90 to about 95 percent by weight of a hydrogenatad product comprising paraffins and naphthenes, and from ~bout 4 to about 10 percent by weight of residual refractory aromatic compounds; and ~ 3 ~ 7 ii) removing substantially all of said residual refractory aromatic compounds by an extraction technique.

Thus, the process of the present invsntion may be used to improve the cetane number of diesel fuels by removing the refractory aromatic compounds contained therein. Moreover, the process disclosed herein may be used to further improve the cetane number of diesel fuels by removing at least a pvrtion oP the naphthenes (ie. cyclic hydrocarbons) which would otherwise be formed through hydrogenatlon of their aromatic precursors.

Although embodiments of the invention relating to upgrading the middle distillate of synthetic crude oil to proAuce diesel fuel will be disclosed hereinafter, Appllcant believes that the scope of the present invention may be applicable to the upgrading of a variety of distillate materials.

Pre~erably, the distillate feedstock material suitable for use is synthetic crude oil middle distillate derived from the conversion o a material selected from the group comprising heavy patroleum oil, coal and mixtures of coal and an oil of petroleum origin. Further, heavy petroleum oils may chosen rom the group comprising petroleum oils known as bitumens, heavy crude oils and refinery residual oils. For the purpose of the present process, the above-mentioned conversion may be performed by techniques known in the art, namely: coking, hydrocracking and ~oking/hydrocracking in the casa of heavy petroleum oil;
direct liquefication in th~ case of coal; and coprocessing in the case of mixt~res of coal and an oil of petroleum orlgin. A typical middle distillate fraction of synthetic crude oil has a boiling point in the ~ange of from about ~32~:~67 140 to about 350 C and may be derived as described hereinbefore.

The hydrogenation catalyst suitable for use in the process of the present application is preferably selected fxom i) conventional sulphides of molybdenum or tungsten comprising an inert support compound (also known as conventional hydrotreating catalysts) and ii) non-sulphided transition metals comprising an inert support compound.
Non-limiting examples of inert support compounds suitable for use with either type of hydrogenation catalyst are alumina and silica~alumina.

Non-limiting examples of conventional hydrotreating catalysts suitable for use in the procesq of the present invention may be selected from sulphides of the group comprising Co-Mo, Ni-Mo and Ni-W. Because these catalysts are relatively inactive toward saturating aromatic hydrocarbons, vigorous reaction conditions are usually employed such that hydro~enation of the aromatic hydrocarbons is thermodynamically favoured. Typical reaction conditions which are used with thase catalysts are:
reaction temperature of from about 340 to about 420 C and hydrogen pressure of at least about 10 MPa, more preferably from about 10 MPa to about 15 MPa. The use of such severe reaction conditions necessitates the use of ralatively hi~h-cost reactors. However, the use of such catalyst systems and reaction conditions enhances the removal of refractory arom tic hydrocarbons which results in an improvement in the cetane number of the dieqel fuel.

Although, conventional hydrotreating catalysts will be oparative in the process of the present invention, the most preferred catalysts are the non-sulphided transition metals. Non~limiting examples of non-sulphided .... . .

~32~167 transition metals suitable for use in the process of the present invention are nickel, palladium and platinum. The advantage of using this type of catalyst is that only relatively mild reaction condltions are required to achieve hydrogsnation o~ the distillate feedstock material. Typlcal reaction conditions which would be required using thl~ type of catalyst are reaction temperature of from about 160 to about 300 C and a hydrogen pressure of at least about 1.5 MPa, more preferably from about 2.5 to about 3.5 MPa. Thus, the use of non-qulphided transition metals as hydrogenation catalysts enables the process of the present invention to be carried out in relatively inexpensive reactors due to the mild reaction conditions which are required~

The hydrogenation o~ the distillate feedstock material results in the production of from about 90 to about 96 percent by weight of hydrogenated hydrocarbons. Thus, the re~ractory aromatic compounds are present in an amount of from about 4 to about 10 percent by weight.
The hydrogenated distillate feedstock material is then subJected to extraction which serves to remove substantially all of the remaining refractory aromatic compounds. Examples of suitable extraction techniques include solvent extraction~ sulphonation, sorption extraction, membrane extraction and extraction with salts.

The preferred extraction technique is one of solvent extraction and sulphonation.
If solvent extraction is used to remove the re~ractory aromatic hydrocarbons, the preferred solvents may be selected from the group comprising sulphur dioxide, ~ulfolane and glycols.

., .,,, ... , = , .
, 2~6~

Sulphonation involves treating the hydrogenated distillate feedstock material with oleum (concentrated H2 S04 comprising S03 in solution as an oily corrosive liquid).
The sulphonated refractory aromatic compounds so-produced may then be removed from the desired fuel product using standard separa-tory techniques.

Embodiments of the present invention will now be illustrated in the following non-limiting example with refersnce to the accompanying drawing in which:
Figure 1 is a plot of aromatic content (mass %) versus cetane number for a number of upgraded distillate materials.

EXAMPLE

A middla dlstillate fraction of synthetic crude oil was derived from Athabasca bitumen by fluid coking. The distillate undarwent primary hydrotreating prior to synthetlc crude blending and plpelining. Some of the properties o~ the middle distillate fraction are provided in Table 2.

Relative Density, 15/15C 0.862 Carbon content, ~ by weight 87.2 Hydrogen aontent, ~ by weight 11.7 Sulphur content, ppm 97 Nitrogen content, ppm 37 Ave. molecular weight 200 Aromatic carbon, ~ 16.9 Cetane number 31 .

-1~2~1 ~7 Hydrogenation of the middle distillate fr~ction was conducted using a supported elemental nickel hydrogenation catalyst and a bench-scale continuous-10w hydrotreating unit. The reactor volume was 100 cm3 and the nickel catalyst loading was 84.4 g. The reactor was operated in the up-flow mode, the liquid feed and hydrogen were mixed, passed through a preheater and then over the fixed catalyst bed. The hydro~enation wa~ carried out at a reaction temperature of 160 to 300 C, liquid space velocities of 0.75 to 2.25 h~1 and a hydrogen flow rate at standard temperature and pressure of 530 L (hydrogen) L~1 (feedstock). A11 runs were performed at a hydrogen pressure of 3.5 MPa and the hydrogen was vented without recycle.

A series of synthetic diesel fuel products were analyzed using low resolution mass spectrometry. The samples were also analyzed by a W method in thosa cases where the aromatics were at low concentrations (less than ~). Analysis for aromatic carbon content was also done by C-13 NMR.

Extraction of refractory aromatic hydrocarbons from the hydrogenated middle distillate samples was achieved using sulphonation. Quantities of 700 mL of the ~amples were treated with 30 mL portions of 15-20~ oleum at room temperature. After removal of the sulphonated aromatics u~ing a separatory funnel, the oil layer was washed with concentrated sulphuric acid and then neutralized with a mlxture of sodium hydroxide and isopropy~ alcohol. Excess sulphonates and water were ramoved by shaking the mixture in a saparatory funnel and removing the bottom layers. Tha remaining solvent wa~ str~pped in a rotary e~aporator and the emul ified naphthenic oil was treated with 15 g of activated alumina, stirred vigorously us1ng the W method.
Compositional analysis was carried out by mass spectrometry _ g _ . .,. ~, - ,- :

.
.

-~2~:~67 to confirm the selectivity of aromatics sepaxation from the hydrogenated product. Analysis of the latter provided absolute yields of paraffins and naphthenes.

Cetane numbers for two series of fuel products (hydrogenated and extracted samples) were datermined by means of an engine test using the standard Cooperative Fuels Research ( CFR ) test engine.

Figure 1 illustrates the relationship between aromatic hydrocarbon content (mass %) and cetane number for the diesel fuel samples. The solid, left-hand curvs is referred to as the Hydrogenation Line and is representative of fuel samples which were sub;ected to hydrogenation only using the narrow hydroprocessing range described above. The curvature of the Hydrogenation Line is believed to be a unique feature of the middle distillate raction of synthetic crude which was used and indicates that cetane numbsr improvement is related to the types o~ aromatics removed and their resistance to hydrogenation. A3 indlcated by the decraasing aontent of aromatics in the fuel samples with increasing cetane number, it is apparent that a significant amount of the rafractory aromatic hydrocarbons have heen converted to their naphthene analogs, which have generally higher cetane numbers than the aromatic hydrocarbons. However, in order to ensure ~ery low concentrations of aromatic hydrocarbons in the fuel, the ~everity o~ the hydrogenation process must be increased.

Accordingly, an essential feature of the process is the inclusion of an extraction stsp after hydrogenation of the di~till~te ~eedstsck material. Because extrac~on will remove the refractory aromatic hydrocarbons, it is actualIy desirable not to allow their removal by conversion to naphthenes at the hydrogenation stage of the process.

. , , ~2~ ~7 Rather, it is desirabls to have from about 4 to about 10 percent by weight of refractory aromatic hydrocarbons in the feedstock prior to the extraction step. Thus, not only are the aromatics removed but also the naphthenes which would have otherwisa been formed during hydrogenation. The Extraction Line is shown in Figure 1 as the broken, right-hand curve and is representative of a series of hydrogenated fuel samples which where subsequently subjected to an extraction step using the sulphonation step as described above.

To illustrate the co~parlson of propertie betwsen hydrogenated (only) and hydrogenated/axtractad fuel samples, both were plotted on the same axis as shown in Figure 1.
The Extraction Line was constructed by plotting the aromatic hydrocarbon contant of the hydrogenated fuel samples (prior to extraction) against their cetane numbers which were determined after extraction. Accordingly, the e~traction line is "imaginary" in terms of the aromatic hydrocarbon content since the hydrogenated/extracted samples did not contain any aromatic hydrocarbons. As an example, Figure 1 show~ that a distillate material upgraded to a cetane number of 40 has an aromatic hydrocarbon content of about 4 mass %.
The extraction of these refractory aromatic hydrocarbons re ults in a product having a cetane number of about 45, which would not have been attainable (according to Figure l) using hydrogenation alone.

Thus, the 2-step process o~ the present invention is useful in producing improved diesel (and ~et) fuels which have higher cetane numbers than those which are subjeGted only to severe hydro~enation. While not wishing to be bound by any partiaular theory, Applicant believes that the extraction step results not only in the removal of the refractory aromatic hydrocarbon component in the ~eedstock ~2~1~7 but also the corr0sponding naphthene which would otherwise remain if a hydrogenation step alone was used. Since naphthenes generally have cetane numbers which intermediate between normal hydrocarbon paraffins and aromatlcs, their removal would, conceivably, enhance further the cetane number of diesel fuel.

Claims (15)

1. A process for upgrading a feedstock comprising a middle distillate fraction of synthetic crude oil, the process comprising the steps of:
(a) hydrogenating the feedstock in the presence of a hydrogenation catalyst to hydrogenate aromatic hydrocarbons in the feedstock to provide from about 90 to about 96 weight percent of a hydrogenated product comprising paraffins and naphthenes, and from about 4 to about 10 weight percent refractory aromatic compounds; and (b) removing the refractory aromatic compounds by extraction.

2. The process defined in claim 1, wherein the middle distillate fraction of synthetic crude oil comprises from about 30 to about 45 percent by weight aromatic hydrocarbons.

3. The process defined in any one of claims 1-2, wherein the middle distillate fraction of synthetic crude oil has a boiling point of about 140° to 350°C.

4. The process defined in any one of claims 1-2, wherein the hydrogenation catalyst comprises a sulphide of molybdenum or tungsten and an inert support compound.

5. The process defined in claim 4, wherein the sulphide of molybdenum or tungsten in the catalyst is selected from the group comprising Co-Mo, Ni-Mo and Ni-W sulfides.

6. The process defined in claim 4, wherein the inert support compound is selected from the group comprising alumina and silica-alumina compounds.

7. The process defined in claim 4, wherein Step (a) is conducted at a temperature of about 340 to 420°C and at a hydrogen pressure of at least about 10 MPa.

8. The process defined in any one of claims 1-2, wherein the hydrogenation catalyst comprises a non-sulphided transition metal comprising an inert support compound.

9. The process defined in claim 8, wherein the transition metal of the catalyst is selected from the group comprising nickel, palladium and platinum metals.

10. The process defined in claim 8, wherein Step (a) is conducted at a temperature of about 160 to 300°C and at a hydrogen pressure of at least about 1.5 MPa.

11. The process defined in claim 9, wherein Step (a) is conducted at a pressure of from about 2.5 to about 3.5 MPa.

12. The process defined in claim 8, wherein the support compound of the catalyst is selected from the group comprising alumina and silica-alumina compounds.

13. The process defined in any one of claims 1-2, wherein the extraction of the refractory aromatic compounds comprises a step selected from the group comprising solvent extraction, sulphonation, sorption extraction, membrane extraction, extraction with salts and combinations thereof.

14. The process defined in any one of claims 1-2, wherein the extraction of the refractory aromatic compounds comprises solvent extraction using a solvent selected from the group comprising liquid sulfur dioxide, sulfolane and glycols.

15. The process defined in any one of claims 1-2, wherein the extraction of the refractory aromatic compounds comprises sulphonation using oleum.