Process for producing middle distillates and middle distillates produced by that process

Description of equivalent WO 0020535 A1

PROCESS FOR PRODUCING MIDDLE DISTILLATES AND MIDDLE DISTILLATES

PRODUCED BY THAT PROCESS

0003Field of the Invention

0004This invention relates to middle distillates having good cold flow properties, such as the Cold Filter Plugging Point (CFPP) measured in accordance with the IP method 309, and a high Cetane number, as well as to a process for production of such distillates. More particularly, this invention relates to middle distillates produced from a mainly paraffinic synthetic crude which is produced by the reaction of CO and H₂ , typically by the Fischer-Tropsch (FT) process.

0005Background to the invention

0006Waxy products of a FT hydrocarbon synthesis process, particularly the products of a cobalt and/or iron based catalytic process, contain a high proportion of normal paraffins. Primary FT products provide notoriously poor cold flow properties, making such products difficult to use where cold flow properties are vital, e.g. diesel fuels, lube oil bases and jet fuel. It is known in the art that cold flow properties of a middle distillate, such as jet fuel, can be improved by increasing the branching of the paraffins of distillates within the proper boiling range, as well as by hydrocracking and hydroisomerising heavier components. Hydrocracking, however, produces smaller amounts of gases and light products, which reduce the yield of valuable distillates. There remains an incentive for a process to maximize middle distillates obtained from FT waxes having good cold flow properties and a high Cetane number.

0007The middle distillate fuel described in this invention is produced from a highly paraffinic synthetic crude (syncrude) obtained from synthesis gas (syngas) through a reaction like the FT reaction. The FT primary products cover a broad range of hydrocarbons from methane to species with molecular masses above 1400; including mainly paraffinic hydrocarbons and much smaller quantities of other species such as olefins, and oxygenates.

0008The prior art teaches in US 5,378,348 that by hydrotreating and isomerizing the products from a Fisher-Tropsch reactor one can obtain a jet fuel with freezing point of -34°C or lower due to the isoparaffinic nature of this fuel. This increased product branching relative to the waxy paraffin feed corresponds with a Cetane rating (combustion) value less than that for normal (linear) paraffins, depicting that an increase in branching reduces the Cetane value of paraffinic hydrocarbon fuels. Surprisingly, it has now been found by the applicant, that a hydroprocessed middle distillate, such as diesel. may be produced having a high Cetane number as well as good cold flow properties. The middle distillates of the present invention could be used on their own or in blends to improve the quality of other diesel fuels not meeting the current and/or proposed, more stringent fuel quality specifications.

0009Summary of the invention

0010Thus, according to a first aspect of the invention, there is provided a process for the production of a middle distillate or distillate blend, such as diesel, having a high Cetane number as well as good cold flow properties.

0011The synthetic middle distillate cut may comprise more than 50% isoparaffins. wherein the isoparaffins are predominantly methyl and/or ethyl and/or propyl branched.

0012The gradient of an isoparaffins to n-paraffins mass ratio profile of the synthetic middle distillate cut may increase from about 1: 1 for C_s to 8.54: 1 for C₁5 and decrease again to about 3: 1 for C_]8.

0013Typically, a fraction of the synthetic middle distillate cut in the Cjo to Cj₈ carbon number range has a higher ratio of isoparaffins to n-paraffins than a Cs to Co fraction of the synthetic middle distillate cut.

0014The isoparaffins to n-paraffins mass ratio of the do to C₁₈ fraction may be between 1: 1 and 9: 1.

0015The isoparaffins to n-paraffins mass ratio may be 8.54:1 for a C₁5 fraction of the synthetic middle distillate cut.

0016A Cjo to C₂ fraction of the middle distillate cut may have a narrow mass ratio range of isoparaffins to n-paraffins of between 3.3: 1 and 5: 1, generally between 4: 1 and 4.9: 1.

0017The mass ratio of isoparaffins to n-paraffins may be adjusted by controlling the blend ratio of hydrocracked to straight run components of the synthetic middle distillate cut. Thus, the isoparaffins to n-paraffins mass ratio of the do to ₈ fraction having 30% straight run component may be between 1 : 1 and 2:5: 1.

0018The isoparaffins to n-paraffins mass ratio of the do to d_s fraction having 20% straight run component mav be between 1.5: 1 and 3:5: 1. The isoparaffins to n-paraffins mass ratio of the Cio to Cj_S fraction having 10% straight run component may be between 2 3 1 and 4 3 1

0019The isoparaffins to n-paraffins mass ratio of the do to Cj_g fraction having substantially only a hydrocracked component may be between 4 1 and 9 1

0020At least some of the isoparaffins may be methyl branched At least some of the isoparaffins may be dimethyl branched At least 30% (mass) of the isoparaffins are typically mono-methyl branched

0021Some of the isoparaffins may however be ethyl branched

0022Table A: Comparison of the Branching Characteristics of Blends of SR .HX and SPD Diesels

0023<img file="WO0020535A1_D0001.tif" /> In the table: SPD - Sasol Slurry Phase Distillate SR - Straight Run HX - Hydrocracked

0024Table B: Branching Characteristics of Blends of SR &amp; HX Diesels

0025<img file="WO0020535A1_D0002.tif" />

0026Branching Characteristics of FT Diesel

0027<img file="WO0020535A1_D0003.tif" />

C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24

0029.0% SR —B— 10% SR —ή— 20% SR —©— 30% SR

0030According to a further aspect of the invention, there is provided a synthetic middle distillate cut having a Cetane number above 70 and a CFPP. in accordance with IP 309. of below -20°C. said distillate having an isoparaffinic content substantially as described above

0031In one embodiment, the synthetic middle distillate cut is a FT product

0032The invention extends to a diesel fuel composition including from 10% to 100% of a middle distillate cut as descπbed above

0033Typically, the diesel fuel composition may include from 0 to 90% of one or more other diesel fuel

0034The diesel fuel composition may include at least 20% of the middle distillate cut, the composition having a Cetane number greater than 47 and a CFPP, in accordance with IP 309, below -22°C The diesel fuel composition may include at least 30% of the middle distillate cut. the composition having a Cetane number greater than 50 and a CFPP, in accordance with IP 309, below -22°C The diesel fuel composition may mlcude at least 50% of the middle distillate cut, the composition having a Cetane number greater than 52 and a CFPP, in accordance with IP 309, below -25°C

0035The diesel fuel composition may include at least 70% of the middle distillate cut, the composition having a Cetane number greater than 60 and a cold flow plug point, in accordance with IP 309, below -30°C

0036The diesel fuel composition may further include from 0 to 10% additives

0037The additives may include a lubricity improver

0038The lubricity improver may comprise from 0 to 0 5% of the composition, typically from 0 00001% to 0 05% of the composition In some embodiments, the lubricity improver compπses from 0 008% to 0 02% of the composition

0039The diesel fuel composition may include, as the other diesel, a crude oil derived diesel, such as US 2-D grade (low sulphur No 2-D grade for diesel fuel oil as specified in ASTM D 975-94) and/or CARB (California Air Resources Board 1993 specification) diesel fuel

0040According to yet another aspect of the invention, there is provided a process for producing a synthetic middle distillate having a Cetane number higher than 70, the process including (a) separating the products obtained from synthesis gas via the FT synthesis reaction into one or more heavier fraction and one or more lighter fraction, (b) catalytically processing the heavier fraction under conditions which yield mainly middle distillates,

0041(c) separatmg the middle distillate product of step (b) from a light product fraction and a heavier product fraction which are also produced in step (b), and

0042(d) blending the middle distillate fraction obtained in step (c) with at least a portion of the one or more lighter fraction of step (a), or products thereof

0043The catalytic processing of step (b) may be a hydroprocessing step, for example, hydrocracking

0044The process for producing a synthetic middle distillate may include one or more additional step of fractionating at least some of the one or more lighter fraction of step (a), or products thereof, pπor to step (d) The process for producing a synthetic middle distillate may mclude the additional step of hydrotreatmg at least some of the one or more light fraction of step (a), or products thereof, pπor to step (d)

0045The one or more heavier fraction of step (a) may have a boiling point above about 270°C. however, it may be above 300°C

0046The one or more lighter fraction may have a boiling point in the range C₅ to the boiling point of the heavier fraction, typically in the range 160°C to 270°C

0047The product of step (d) may boil in the range 100°C to 400°C The product of step (d) may boil in the range 160°C to 370°C

0048The product of step (d) may be a diesel fuel

0049The product of step (d) may have a CFPP below -20°C. typically below -30°C. and even below -

005035°C

0051The product of step (d) may be obtained by mixing the middle distillate fraction obtained in step (c) with at least a portion of the one or more lighter fraction of step (a), or products thereof, m a volume ratio of between 1 1 and 9 1, typically 2 1 and 6 1, and in one embodiment, in a volume ratio of 84 16

0052The invention extends further to a process for the production of middle distillate fuels from FT pnmary products, comprising predominantly long chain linear paraffins

0053In this process, the waxy product from the FT process is separated into at least two fractions, a heavier and at least one lighter fraction The lighter fraction may be subjected to mild catalytic hydrogenation to remove hetero-atomic compounds such as oxygen and to saturate olefins, thereby producing matenal useful as naphtha, solvents, diesel and/or blending components therefor The heavier fraction may be catalytically hydroprocessed without pπor hydrotreatmg to produce products with good cold flow characteristics This hydroprocessed heavier fraction could be blended with all or part of the hydrogenated and/or unhydrogenated light fraction to obtain, after fractionation, naphtha and a diesel fuel characterised by a high Cetane number The catalysts suitable for the hydroprocessmg steps are commercially available and can be selected towards an improved quality of the desired final product

0054Detailed Description

0055This invention descπbes the conversion of primary FT products mto naphtha and middle distillates, for example, diesel having a high Cetane number in excess of 70, while also having good cold flow properties, as described above

0056The FT process is used industπally to convert synthesis gas, derived from coal, natural gas, biomass or heavy oil streams, mto hydrocarbons ranging from methane to species with molecular masses above 1400

0057While the main products are linear paraffinic materials, other species such as branched paraffins, olefins and oxygenated components form part of the product slate The exact product slate depends on reactor configuration, operating conditions and the catalyst that is employed, as is evident from e g Catal Rev -Sci Eng . 23(1&amp;2). 265-278 (1981)

0058Preferred reactors for the production of heavier hydrocarbons are slurry bed or tubular fixed bed reactors, while operating conditions are preferably in the range of 160°C - 280°C. in some cases 210- 260°C. and 18 - 50 Bar, in some cases 20-30 bar

0059Preferred active metals in the catalyst compπse iron, ruthenium or cobalt While each catalyst will give its own unique product slate, m all cases the product slate contains some waxy, highly paraffinic mateπal which needs to be further upgraded into usable products The FT products can be converted into a range of final products, such as middle distillates, gasoline, solvents, lube oil bases, etc Such conversion, which usually consists of a range of processes such as hydrocracking, hydrotreatment and distillation, can be termed a FT work-up process

0060The FT work-up process of this invention uses a feed stream consisting of C₅ and higher hydrocarbons derived from a FT process This feed is separated mto at least two individual fractions, a heavier and at least one lighter fraction The cut point between the two fractions is preferably less than 300°C and typically around 270°C

0061The table below gives a typical composition of the two fractions with a ±10% accuracy Table 1 : Typical Fischer-Tropsch product after separation into two fractions (voI% distilled)

0062<img file="WO0020535A1_D0004.tif" />

0063The >270°C fraction, also referred to as wax, contains a considerable amount of hydrocarbon material, which boils higher than the normal diesel range. If we consider a typical diesel boiling range of 160- 370°C, it means that all material heavier than 370°C needs to be converted into lighter materials by means of a catalytic process often refeπed to as hydroprocessing, for example, hydrocracking. Catalysts for this step are of the bifunctional type; i.e. they contain sites active for cracking and for hydrogenation. Catalytic metals active for hydrogenation include group VIII noble metals, such as platinum or palladium, or a sulphided Group VIII base metals, e.g. nickel, cobalt, which may or may not include a sulphided Group NI metal, e.g. molybdenum. The support for the metals can be any refractory oxide, such as silica, alumina, titania, zirconia, vanadia and other Group III, IN, NA and VI oxides, alone or in combination with other refractory oxides. Alternatively, the support can partly or totally consist of zeolite. However, for this invention the preferred support is amorphous silica- alumina.

0064Process conditions for hydrocracking can be varied over a wide range and are usually laboriously chosen after extensive experimentation to optimize the yield of middle distillates. In this regard, it is important to note that, as in many chemical reactions, there is a trade-off between conversion and selectivity. A very high conversion will result in a high yield of gases and low yield of distillate fuels. It is therefore important to painstakingly tune the process conditions in order to limit the conversion of >370°C hydrocarbons. Table 2 gives a list of the preferred conditions. Table 2: Process conditions for hydrocracking

0065<img file="WO0020535A1_D0005.tif" />

0066Nevertheless, it is possible to convert all the >370°C material in the feedstock by recycling the part that is not converted during the hydrocracking process.

0067As is evident from table 1, most of the fraction boiling below 270°C is already in the typical boiling range for diesel. i.e. 160-370°C. This fraction may or may not be subjected to hydrotreating. By hydrotreating, hetero-atoms are removed and unsaturated compounds are hydrogenated. Hydrotreating is a well-known industrial process, catalyzed by any catalyst having a hydrogenation function, e.g. Group VIII noble metal or sulphided base metal or Group VI metals, or combinations thereof. Preferred supports are alumina and silica.

0068Table 3 gives typical operating conditions for the hydrotreating process.

0069Table 3: Operating conditions for the hydrotreating process.

0070<img file="WO0020535A1_D0006.tif" />

0071While the hydrotreated fraction may be fractionated into paraffinic materials useful as solvents, the applicant has now surprisingly found that the hydrotreated fraction may be directly blended with the products obtained from hydrocracking the wax. Although it is possible to hydroisomerise the material contained in the condensate stream, the applicant has found that this leads to a small, but significant loss of material in the diesel boiling range to lighter material. Furthermore, isomerisation leads to the formation of branched isomers, which leads to Cetane ratings less than that of the coπesponding normal paraffins.

0072The combination of highly linear paraffins derived from the <270°C fraction and mainly branched paraffins derived from the >270°C fraction results in a superb diesel.

0073Important parameters for a FT work-up process are maximization of product yield, product quality and cost. While the proposed process scheme is simple and therefore cost-effective, it produces High Performance Diesel, having a Cetane number >70, and naphtha in good yield. In fact, the process of this invention is able to produce a diesel of hitherto unmatched quality, which is characterized by a unique combination of both high Cetane number and excellent cold flow properties. This is believed to be related to a low degree of isomerisation in the 160-270°C fraction of the diesel and contrary to this, a high degree of isomerisation in the 270-370°C fraction of the diesel.

0074The total amount of isomers in the light boiling range of the diesel (160-270°C fraction) and the heavier range of the diesel (270°C-370°C) are shown in the following table 4.

0075Table 4: Isoparaffins: n- Paraffins of Middle Distillate Fractions

0076<img file="WO0020535A1_D0007.tif" />

0077The relatively high percentage of normal paraffins in the light boiling range contributes to the high Cetane number of the diesel fuel, without affecting the cold flow properties. On the other hand, in the heavier range of the diesel, branching is of utmost importance because the linear hydrocarbons in this range provide very poor cold flow properties and in some cases, may even crystallize. Therefore, the amount of iso-paraffins in this range is maximised during hydroprocessing under the process conditions described herein.

0078It is this unique composition of the synthetic fuel, which is directly caused by the way in which the FT work-up process of this invention is operated, that leads to the unique characteristics of said fuel.

0079The applicant has also found, that from the perspective of fuel quality, it is not necessary to hydrotreat the &lt;270°C fraction, adding said fraction directly to the products from hydrocracking the wax. While this results in the inclusion of oxygenates and unsaturates in the final diesel fuel specifications usually allow for this Circumventing the need for hydrotreatment of the condensate results in considerable savings of capital and operating costs

0080The described FT work-up process of Figure 1 may be combined in a number of configurations The applicant considers these an exercise m what is known in the art as Process Synthesis Optimisation

0081However, the specific process conditions for the Work-up of FT primary products , the possible process configurations of which are outlmed in Table 5, were obtained after extensive and laboπous experimentation and design

0082Table 5 - Possible Fischer-Tropsch Product Work-up Process Configurations

0083<img file="WO0020535A1_D0008.tif" />

0084Numbers reference numerals of Figure 1

0085FT Fischer-Tropsch

0086The basic process is outlined m the attached Figure 1 The synthesis gas (syngas), a mixture of Hydrogen and Carbon monoxide, enters the FT reactor 1 where the synthesis gas is converted to hydrocarbons by the FT reaction

0087A lighter FT fraction is recovered in line 7, and may or may not pass through fractionator 2 and hydrotreater 3 The product 9 from the hydrotreater may be separated in fractionator 4 or, alternatively, mixed ith hydrocracker products 16 sent to a common fractionator 6

0088A waxy FT fraction is recovered in line 13 and sent to hydrocracker 5 If fractionation 2 is considered the bottoms cut 12 are be sent to hydrocracker 5 The products 16, on their own or mixed with the lighter fraction 9a. are separated in fractionator 6 Depending on the process scheme, a light product fraction, naphtha 19, is obtained from fractionator 6 or by blending equivalent fractions 10 and 17. This is a C₅-160°C fraction useful as naphtha.

0089A somewhat heavier cut, synthetic diesel 20, is obtainable in a similar way from fractionator 6 or by blending equivalent fractions 11 and 18. This cut is recovered as a 160-370°C fraction useful as diesel.

0090The heavy unconverted material 21 from fractionator 6 is recycled to extinction to hydrocracker 5. Alternatively, the residue may be used for production of synthetic lube oil bases. A small amount of Cj-C gases are also separated in fractionator 6.

0091The following examples will serve to illustrate further this invention.

0092Examples

EXAMPLE 1

0094A commercially available hydrocracking catalyst was used for hydrocracking of a non-hydrotreated FT hydrocarbon fraction with an initial boiling point of about 280°C. The active metals on the catalyst comprised cobalt and molybdenum, while the support was amorphous silica-alumina. Operating conditions were temperatures between 375 and 385°C, pressure of 70 bar and hydrogen flow rate of 1500 m³_n/m³ feed. The experiment was carried out in a pilot plant reactor. The conversion of >370°C material to lighter material ranged between 65 and 80%. Diesel component A is obtained after fractionation of the reactor products. The properties of this diesel component are given in table 1.

EXAMPLE 2

0096A non-hydrotreated FT hydrocarbon fraction with a final boiling point of ca 285 °C and alcohol content of ca. 4.3 mass%, expressed as n-hexanol, was rigorously hydrotreated using a commercially available catalyst. The active metals on the catalyst comprised molybdenum and cobalt, while the support was alumina. The process conditions were temperatures around 250°C, pressure of 68 bar and hydrogen flow rate of 1070 mVm³ feed. The test was carried in a commercial scale fixed bed reactor. Diesel components B and C were obtained after fractionation of respectively the reactor feed and reactor product. The properties of these diesel components are given in table 6. Table 6: Diesel Blending Components

0097<img file="WO0020535A1_D0009.tif" />

EXAMPLE 3

0099The diesel fraction obtained from hydrocracking a heavy FT material (component A) was blended with a hydrogenated lighter FT material (component B) in a volume ratio of 84: 16. The properties of the final blend, called Blend I, are given in table 7.

0100Those skilled in the art will realize that Blend I may be used on its own. but also as a blending feedstock. The combination of a high Cetane numbers, above 70, and excellent cold flow properties, with CFPP substantially better than -20°C, make Blend I an ideal blending feedstock to upgrade crude oil derived diesels.

EXAMPLE 4

0102The diesel fraction obtained from hydrocracking a heavy FT material (component A) was directly blended with a lighter non-hydrogenated FT material (component C) in a volume ratio of 84: 16. The properties of the final blend, called Blend II. are given in table 7 Similar to example 3, Blend II may be used on its own, but also as a blending feedstock. In addition to a high Cetane numbers, above 70, and excellent cold flow properties, with CFPP substantially better than -20°C. Blend II contains alcohols and smaller quantities of other oxygenates, the level of which depend on the blending ratio used to prepare the blend.

0103Table 7: Diesel Blends

0104<img file="WO0020535A1_D0010.tif" />

EXAMPLE 5

0106The diesel Blend I of Example 3 was blended with US 2-D grade diesel having desired Cetane number and CFPP properties, as shown in Table 8 and charts 1 and 2 below, were obtained.

0107Table: 8 Performance properties of Sasol SPD diesel, 2D diesel and blends

0108<img file="WO0020535A1_D0011.tif" />

0109Cold flow properties of SPD diesel, 2D diesel and blends

0110-20

0111-22 -24 -26 -28

0112U g- -30 υ

0113-32 -34 -36 -38 -40 <img file="WO0020535A1_D0012.tif" />

0114SPD Diesel Volume, % Cetane number of SPD diesel, 2D diesel and blends

011580

011675

011770

011865

011960 a> c ra O

012055

012150

012245

012340 <img file="WO0020535A1_D0013.tif" />

01240% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

0125SPD Diesel Volume %

0126Chart 2: Cetane Number of Applicants Diesel and Blends Thereof.