ZA200306842B - Process for the preparation of middle distillates. - Google Patents

Description

by WO 02/070628 PCT/EP02/02336

PROCESS FOR THE PREPARATION OF MIDDLE DISTILLATES

. The present invention relates to a process for the preparation of one or more hydrocarbon fuel products . boiling in the kero/diesel range from a stream of hydrocarbons produced in a Fischer-Tropsch process and to hydrocarbons so produced.

Today the energy requirements of the transport sectors are dominated by liquid fuels derived from the fractionation and processing of crude oil. The dominance of liquid fuels is expected to continue.

Crude oil derived liquid fuels usually are not clean.

They typically contain significant amounts of sulphur, nitrogen and aromatics. Diesel fuels derived from crude oil show relatively low cetane values. Clean distillate fuels can be produced from petroleum based distillates through (severe) hydrotreatment at great expense. For diesel fuels, however, these treatments usually hardly improve the cetane number.

Another source for distillate fuels, especially middle distillates, i.e. kerosene and diesel, is the

Fischer-Tropsch process, especially using cobalt catalysts. During the last two decades this process has evolved as a key process for the conversion of natural gas into especially middle distillates of high quality.

In this process synthesis gas is converted in several steps into middle distillates. First, natural gas in converted into synthesis gas by means of a (catalytic) partial oxidation process and/or steam reforming process. . In a second step the synthesis gas is converted into long chain paraffins (the average Cg+ hydrocarbon usually ’ 30 comprising 25 to 35 carbon atoms). In a third step the long chain hydrocarbons are hydrocracked into molecules [

¥ v WO 02/070628 PCT/EP02/02336 of the desired middle distillate fuels. In this respect reference is made to EP 161 705, EP 583 836, EP 532 116,

WO 99/01218, US 4,857,559 and EP 1 004 746. Further ’ reference is made to HMH van Wechem and MMG Senden,

Conversion of Natural Gas to Transportation Fuels, ‘ Natural Gas Conversion II, HE Curry-Hyde and RF Howe (editors), Elsevier Science B.V. pages 43-71.

In general, the quality of the middle distillates prepared by the Fischer-Tropsch process is excellent. The mainly paraffinic products are free from sulphur, nitrogen and aromatic compounds. The kerosene and diesel have excellent combustion properties (smoke point and cetane number). The cold flow properties meet the relevant specifications. If necessary, additives may be used to meet the most stringent cold flow specifications.

In addition, also the usual additives may be added.

In view of the continuously increasing requirements of the middle distillate properties, there is a need to further improve the middle distillate properties, especially the cold flow properties of the middle distillates. Thus, there is a need for middle distillates with improved intrinsic cold flow properties, i.e. these properties are to be obtained without using any further treatment of the fuels (e.g. dewaxing) or without the use of any additives. In addition, for the diesel fraction it is desired that 'Y5, the temperature at which Y5 vol amount of diesel boiling, is 380 °C or less, preferably 370 °C or less, more preferably 360 °C or less, the density (15 °C) should be 840 kg/m3 or less, preferably \ 30 800 kg/m3 or less, more preferably 780 kg/m3 or less and the amount of (poly)aromatic compounds should be zero. , It has now been found that hydrocracking/hydro- isomerising a relatively heavy Fischer-Tropsch hydrocarbon product (a Cg+ product, preferably a Cqig+ product) at a relatively low conversion per pass rate,

0) WO 02/070628 PCT/EP02/02336 i.e. less than 80% conversion of a fraction boiling above a certain boiling point (e.g. 370 °C) which is fed into the reactor into a fraction boiling below that boiling ’ point, and subjecting most of the material boiling above the kero/diesel boiling range to a second, similar : hydrocracking/hydroisomerising reaction followed by a recycle of the main part of the material boiling above the kero/diesel boiling range to a hydrocracking/hydro- isomerising reaction, results in middle distillates showing cxceplionally youd cold [low properties, maklng any further treatment (to improve the cold flow properties) and/or the use of additives in principle superfluous. Compared with Fischer-Tropsch product which is less heavy (for example the amount of C3g+ is e.g. 10 %wt less) the cold flow properties (pour point, CFPP) may be 5 or even 10 °C better. In addition, T95, density and (poly)aromatic content satisfy the ranges as mentioned above. The process is preferably carried out in a continuous way.

The present invention thus relates to a process as described in claim 1.

The process of the present invention results in middle distillates having exceptionally good cold flow properties. These excellent cold flow properties could perhaps be explained by the relatively high ratio iso/normal and especially the relatively high amount of di- and/or trimethyl compounds. Nevertheless, the cetane number of the diesel fraction is more than excellent at values far exceeding 60, often values of 70 or more are obtained. In addition, the sulphur content is extremely low, always less than 50 ppmw, usually less than 5 ppmw and in most case the sulphur content is zero. Further, the density of especially the diesel fraction is less than 800 kg/m3, in most cases a density is observed between 765 and 790 kg/m3, usually around 780 kg/m3 (the u hy WO 02/070628 PCT/EP02/02336 viscosity for such a sample being about 3.0 cSt).

Aromatic compounds are virtually absent, i.e. less than 50 ppmw, resulting in very low particulate emissions. The

K polyaromatic content is even much lower than the aromatic content, usually less than 1 ppmw. T95, in combination : with the above properties, is below 380 °C, often below 350 °c.

The process as described above results in middle distillates having extremely good cold flow properties.

For instance, the cloud point of any diesel fraction is usually below -18 °C, often even lower than -24 °C. The

CFPP is usually below -20 °C, often -28 °C or lower. The pour point is usually below —-18 °C, often below -24 °C.

Due to the relatively heavy Fischer-Tropsch product which is used in the process, the overall conversion of the process is extremely high. This holds for the carbon conversion as well as for the thermal conversion. The carbon conversion for the Fischer-Tropsch process and the hydrocracking/hydro~isomerising reaction is above 80%, preferably above 85%, more preferably above 90%. The thermal conversion for the process will be above 70%, preferably is above 75%, more preferably is above 80%. It is an extremely advantageous situation that such high conversions can be coupled with the extremely good product properties. In addition, the selectivity to Coy hydrocarbon is usually above 85 wt%, preferably above 90 wt%, of all hydrocarbons made in the Fischer-Tropsch process.

The kero/diesel boiling range in general may vary slightly, depending on local conditions, availability of specific feed streams and specific practices in refineries, all well known to the man skilled in the art.

For the purposes of this specification the kero/diesel boiling range suitably has an initial boiling point between 110 and 130 °C, preferably at least 140, more

3 WO 02/070628 PCT/EP02/02336 _ preferably at least 150 °C, still more preferably at least 170 °C. The final boiling point for the purposes of this specification is suitably between 400 and 410 °C, ‘ preferably at most 390 °C, more preferably at most 5 375 °C, still more preferably at most 360 °C. The end of ' the kerosene boiling range may be up to 270 °C, usually up to 250 °C, but may also be up to 220 °C or even 200 °C. The start of the diesel boiling range may be 150 °C, dis usually 170 °C but may also be 190 °C or even above 200 °C. The 50% recovered Lemperalure of Lhe diesel fraction is preferably between 255 and 315 °C, preferably between 260 and 300 °C, more preferably around 285 °C.’

It will be appreciated that the one or more hydrocarbon fuel products of the present invention suitable is a full range boiling product in the diesel/kero range as defined above, but also very suitably may be two fractions, one boiling in the diesel range, the other boiling in the kerosene range. In addition, three or more fractions, for instance a kerosene fraction, a light diesel fraction and a heavy diesel fraction, may be considered as a commercially attractive option. In principle, the number of fractions and the boiling ranges will be determined by operational and commercial conditions.

The synthesis gas to be used for the Fischer-Tropsch reaction is made from a hydrocarbonaceous feed, especially by partial oxidation and/or steam/methane reforming. The hydrocarbonaceous feed is suitably methane, natural gas, associated gas or a mixture of C14 hydrocarbons, especially natural gas.

To adjust the H,/CO ratio in the syngas, carbon . dioxide and/or steam may be introduced into the partial oxidation process. The H»/CO ratio of the syngas is suitably between 1.3 and 2.3, preferably between 1.6 and 2.1. If desired, (small) additional amounts of hydrogen

~ WO 02/076628 PCT/EP02/02330 may be made by steam methane reforming, preferably in combination with the water gas shift reaction. The additional hydrogen may also be used in other processes, ’ e.g. hydrocracking.

In another embodiment the Hp/CO ratio of the syngas obtained in the catalytic oxidation step may be decreased by removal of hydrogen from the syngas. This can be done by conventional techniques as pressure swing adsorption or cryogenic processes. A preferred option is a separa- tion based on membrane technology. Part of the hydrogen may be used in the hydrocracking step of especially the heaviest hydrocarbon fraction of the Fischer-Tropsch reaction.

The synthesis gas obtained in the way as described above, usually having a temperature between 900 and 1400 °C, is cooled to a temperature between 100 and 500 °C, suitably between 150 and 450 °C, preferably between 300 and 400 °c, preferably under the simultaneous generation of power, e.g. in the form of steam. Further cooling to temperatures between 40 and 130 °C, preferably between 50 and 100 °C, is done in a conventional heat exchanger, especially a tubular heat exchanger. To remove any impurities from the syngas, a guard bed may be used.

Especially to remove all traces of HCN and/or NH3 specific catalysts may be used. Trace amounts of sulphur may be removed by an absorption process using iron and/or zinc oxide. : The purified gaseous mixture, comprising pre- dominantly hydrogen; carbon monoxide and optionally . 30 nitrogen, is contacted with a suitable catalyst in the catalytic conversion stage, in which the normally liquid . hydrocarbons are formed.

The catalysts used for the catalytic conversion of the mixture comprising hydrogen and carbon monoxide into hydrocarbons are known in the art and are usually referred to as Fischer-Tropsch catalysts. Catalysts for use in this process frequently comprise, as the catalytically active component, a metal from Group VIII ' of the Periodic Table of Elements. Particular catalytically active metals include ruthenium, iron, - cobalt and nickel. Cobalt is a preferred catalytically active metal in view of the heavy Fischer-Tropsch hydrocarbon which can be made. As discussed before, preferred hydrocarbonaceous feeds are natural gas or associated gas. As these feedstocks usually results in synthesis gas having Ho/CO ratio’s of about 2, cobalt is a very good Fischer-Tropsch catalyst as the user ratio for this type of catalysts is also about 2.

The catalytically active metal is preferably supported on a porous carrier. The porous carrier may be selected from any of the suitable refractory metal oxides or silicates or combinations thereof known in the art.

Particular examples of preferred porous carriers include silica, alumina, titania, zirconia, ceria, gallia and mixtures thereof, especially silica, alumina and titania.

The amount of catalytically active metal on the carrier is preferably in the range of from 3 to 300 pbw per 100 pbw of carrier material, more preferably from 10 to 80 pbw, especially from 20 to 60 pbw.

If desired, the catalyst may also comprise one or more metals or metal oxides as promoters. Suitable metal oxide promoters may be selected from Groups IIA, IIIB,

IVB, VB and VIB of the Periodic Table of Elements, or the actinides and lanthanides. In particular, oxides of magnesium, calcium, strontium, barium, scandium, yttrium, lanthanum, cerium, titanium, zirconium, hafnium, thorium, uranium, vanadium, chromium and manganese are very suitable promoters. Particularly preferred metal oxide promoters for the catalyst used to prepare the waxes for use in the present invention are manganese and zirconium a WO 02/070628 PCT/EP02/02336 oxide. Suitable metal promoters may be selected from

Groups VIIB or VIII of the Periodic Table. Rhenium and

Group VIII noble metals are particularly suitable, with ‘ platinum and palladium being especially preferred. The amount of promoter present in the catalyst is suitably in : the range of from 0.01 to 100 pbw, preferably 0.1 to 40, more preferably 1 to 20 pbw, per 100 pbw of carrier. The most preferred promoters are selected from vanadium, manganese, rhenium, zirconium and platinum.

The catalytically active melal and Lhe promoter, if present, may be deposited on the carrier material by any suitable treatment, such as impregnation, kneading and extrusion. After deposition of the metal and, if appropriate, the promoter on the carrier material, the loaded carrier is typically subjected to calcination. The effect of the calcination treatment is to remove crystal water, to decompose volatile decomposition products and to convert organic and inorganic compounds to their respective oxides. After calcination, the resulting catalyst may be activated by contacting the catalyst with hydrogen or a hydrogen-containing gas, typically at temperatures of about 200 to 350 °C. Other processes for the preparation of Fischer-Tropsch catalysts comprise kneading/mulling, often followed by extrusion, drying/calcination and activation.

The catalytic conversion process may be performed under conventional synthesis conditions known in the art.

Typically, the catalytic conversion may be effected at a temperature in the range of from 150 to 300 °C, preferably from 180 to 260 °C. Typical total pressures ' for the catalytic conversion process are in the range of from 1 to 200 bar absolute, more preferably from 10 to 70 bar absolute. In the catalytic conversion process especially more than 75 wt% of Cg+, preferably more than 85 wt% Cg+ hydrocarbons are formed. Depending on the

A) WO 02/070628 PCT/EP02/02330 catalyst and the conversion conditions, the amount of heavy wax (C20+) may be up to 60 wt%, sometimes up to 70 wt%, and sometimes even up till 85 wt%. Preferably a ) cobalt catalyst is used, a low Ho/CO ratio is used (especially 1.7, or even lower) and a low temperature is ) used (190-240 °C), optionally in combination with a high pressure. To avoid any coke formation, it is preferred to use an Hp/CO ratio of at least 0.3. It is especially preferred to carry out the Fischer-Tropsch reaction under such conditions that the ASF-alpha value (Anderson-

Schulz-Flory chain growth factor), for the obtained products having at least 20 carbon atoms, is at least 0.925, preferably at least 0.935, more preferably at least 0.945, even more preferably at least 0.955.

Preferably the Fischer-Tropsch hydrocarbons stream comprises at least 40 wt% C3g+, preferably 50 wt%, more preferably 55 wt%, and the weight ratio Cgp+/C3p+ is at least 0.35, preferably 0.45, more preferably 0.55.

Preferably, a Fischer-Tropsch catalyst is used, which yields substantial quantities of paraffins, more pre- ferably substantially unbranched paraffins. A most suitable catalyst for this purpose is a cobalt-containing

Fischer-Tropsch catalyst. Such catalysts are described in the literature, see e.g. AU 698392 and WO 99/34917.

The Fischer-Tropsch process may be a slurry FT process or a fixed bed FT process, especially a multitubular fixed bed.

The term "middle distillates", as used herein, is a reference to hydrocarbon mixtures of which the boiling ‘ 30 point range corresponds substantially to that of kerosene and diesel fractions obtained in a conventional ’ atmospheric distillation of crude mineral oil.

Any normally liquid Fischer-Tropsch hydrocarbons mentioned in the present description are in general Cs.13

Ll WO 02/070628 PCT/EP02/02336 hydrocarbons or mixtures thereof, although certain amounts of Cy- or Cjg+ hydrocarbons may be present. These hydrocarbons or mixtures thereof are liquid at ) temperatures between 5 and 30 °C (1 bar), especially at 20 °C (1 bar), and are paraffinic of nature, although . considerable amounts of olefins and/or oxygenates may be present. Suitably up to 20 wt%, preferably up to 10 wt$%, of either olefins or oxygenated compounds may be present.

Any heavy Fischer-Tropsch wax comprises all hydrocarbons or mixtures thereor which are solid at ZU °C, especially

C18-300, more especially Cig9-250. Any normally gaseous

Fischer-Tropsch hydrocarbons are Cj to C4 hydrocarbons, although small amounts of Cg+ may be present.

The Fischer-Tropsch step of the present process is followed by a step in which at least part of the heavy paraffins-containing hydrocarbon mixture produced in the first step is hydrocracked and hydroisomerized. In this step a catalyst is used which preferably contains a catalytically active metal component as well as an acidic function. The metal component can be deposited on any acid carrier having cracking and isomerisation activity, for example a halogenated (e.g. fluorided or chlorided) alumina or zeolitic carrier or an amorphous silica/alumina carrier.

The catalyst used in the hydrocracking/hydro- isomerising step of the process according to the invention may contain as catalytically active metal components one or more metals selected from Groups VIB,

VIIB and/or VIII of the Periodic System. Examples of such . 30 metals are molybdenum, tungsten, rhenium, the metals of the iron group and the metals of the platinum and . palladium groups. Catalysts with a noble metal as catalytically active metal component generally contain 0.05-5 parts by weight and preferably 0.1-2 parts by weight of metal per 100 parts by weight of carrier y WO 02/070628 PCT/EP02/02336 material. Very suitable noble metals are palladium and platinum. Catalysts with a non-noble metal or a combination of non-noble metals as catalytically active : metal component generally contain 0.1-35 parts by weight of metal or combination of metals per 100 parts by weight : of carrier material. Very suitable hydrocracking catalysts contain a combination of 0.5-20 parts by weight and in particular 1-10 parts by weight of a non-noble metal of Group VIII and 1-30 parts by weight and in particular 2 20 parts by weight of a metal of Croup VIB and/or VIIB per 100 parts by weight of carrier material.

Particularly suitable metal combinations are combinations of nickel and/or cobalt with tungsten and/or molybdenum and/or rhenium. Likewise very suitable as hydrocracking catalysts are catalysts which contain 0.1-35 parts by weight and in particular 1-15 parts by weight of nickel per 100 parts by weight of carrier material.

If the present hydrocracking catalysts contain a non-noble metal or combination of non-noble metals as catalytically active metal component, they are preferably used in their sulphidic form. The conversion of the hydrocracking catalysts to their sulphidic form can very suitably be carried out by contacting the catalysts at a temperature below 500 °C with a mixture of hydrogen and hydrogen sulphide in a volume ratio of 5:1 to 15:1. The conversion uf the cvalalysts intu Lhe sSulpliidic fourm may also be carried out by adding to the feed, under reaction conditions, sulphur compounds in a quantity of from 10 ppmw to 5% by weight and in particular in a quantity of from 100 ppmw to 2.5% by weight. ’ The isomerisation/hydrocracking step (2) or (5) of the present process may be carried out using a catalyst comprising a zeolite having a pore diameter in the range from 0.5 to 1.5 A. The silica:alumina ratio of the zeolite is preferably in the range from 5 to 200. A very

Claims (13)

1. A process for the preparation of one or more hydrocarbon fuel products boiling in the kero/diesel range from a stream of hydrocarbons produced in a Fischer-Tropsch process, in which process synthesis gas is converted into liquid hydrocarbons, at least a part of the hydrocarbons boiling above the kero/diesel range, comprising the following steps: (1) hydrocracking/hydroisomerising at least a part of the Fischer-Tropsch hydrocarbons stream at a conversion per pass of at most 80 wt% of the material boiling above 370 °C into material boiling below 370 °C, (2) separating the product stream obtained in step (1) into one or more light fractions boiling below the kero/diesel boiling range, one or more fractions boiling in the kero/diesel boiling range and a heavy fraction boiling above the kero/diesel boiling range, (3) hydrocracking/hydroisomerising the major part of the heavy fraction obtained in step (2) at a conversion per pass of at most 80 wt% of the material boiling above 370 °C into material boiling below 370 °C, (4) separating the product stream obtained in step (3) into one or more light fractions boiling below the kero/diesel boiling range, one or more fractions boiling in the kero/diesel boiling range and a heavy fraction boiling above the kero/diesel boiling range, (5) hydrocracking/hydroisomerising the major part of the heavy fraction obtained in step (4) in the . hydrocracking/hydroisomerising process described in step (1) and/or step (3), P) 30 in which process the Fischer-Tropsch hydrocarbons stream comprises at least 35 wt% C3p+ (based on total amount of hydrocarbons in the Fischer-Tropsch hydrocarbons stream) and in which stream the weight ratio Cgg+/C3p+ is at least 0.2. .

2. A process as described in claim 1, in which the Fischer-Tropsch hydrocarbons stream is obtained in a . Fischer-Tropsch reaction using an iron or cobalt catalyst, preferably a cobalt catalyst comprising a carrier and optionally one or more promoters selected from vanadium, manganese, rhenium, zirconium and platinum.

3. A process as described in any of claims 1 or 2, in which the Fischer-Tropsch hydrocarbons stream is obtained in a Fischer-Tropsch reaction which is performed under such conditions that the ASF-alpha value for the obtained products having at least 20 carbon atoms is at least

0.925, preferably at least 0.935, more preferably at least 0.945, even more preferably at least 0.955.

4. A process as described in any of the claims 1 to 3, in which at least a part of the full product of the Fischer-Tropsch reaction is separated into a light product stream, preferably comprising all components boiling below the kero/diesel boiling range, and a heavy Fischer-Tropsch hydrocarbons stream, which stream is used in step (1), preferably the light products stream comprising unreacted synthesis gas, carbon dioxide, inert gasses as uilroyen and sleam, and the C1-Cy4 hydrocarbons, preferably the Cq1-Cq1g hydrocarbons.

5. A process as described in any of claims 1 to 4, in which the Fischer-Tropsch hydrocarbons stream comprises at least 40 wt% C3p+ (based on total hydrocarbons stream), preferably 50 wt%, more preferably 55 wt$%, and ’ in which stream the weight ratio Cgg+/C3p+ is at least

0.35, preferably 0.45, more preferably 0.55.

6. A process as described in any of claims 1 to 5, in which the product boiling in the kero/diesel boiling range has a boiling range within the range of 110 °C and } 400 °C, preferably within the range of 140 °C and 375 °C, more preferably between 150 °C and 360 °C.

’ 7. A process as described in any of claims 1 to 6, in which the conversion per pass in steps (1) and/or (3) of the material boiling above 370 °C into material boiling below 370 °C is between 30 and 70 wt$, preferably between 40 and 60 wt%, more preferably about 50 wt%.

8. A process as described in any of claims 1 to 7, in which the first and the second hydrocracking/hydro- isomerisation step are carried out at a temperature between 290 and 375 °C, preferably between 310 and 350 °C, a pressure between 15 and 200 bar, preferably between 20 and 80 bar, more preferably between 30 and 50 bar and a WHSV between 0.5 and 3 kg/1l/h, preferably between 1 and 2.5 kg/l/h.

9. A process as described in claim 13, in which the first and the second hydrocracking/hydroisomerisation step are combined.

10. A process as described in any of claims 1 to 8, in which the amount of heavy fraction obtained in step 2 which is used in step (3) is at least 70 wt%, preferably at least 85 wt%, more preferably at least 95 wt% of the total heavy fraction, and in which the amount of heavy fraction obtained in step (4) which is used for step in step (1) and/or step (3), is at least 70 wt%, preferably at least 85 wt%, more preferably at least 95 wt% of the total heavy fraction. :

11. A hydrocarbon product boiling on the kero/diesel boiling range obtainable by a process as defined in any ’ of the claims 1 to 15.

12. A hydrocarbon product, which has not been subjected to a dewaxing treatment, boiling in the diesel boiling a p y WO 02/070628 PCT/EP02/02336 range having the following properties: cetane number at least 50, preferably 60, more preferably 70, iso/normal ratio between 2.5 and 10, especially between 3.5 and 6, ) more especially between 4 and 5, the amount of mono-iso . 5 compounds being at least 70 wt$% (based on total product boiling in the diesel boiling range), preferably 75 wt%, cloud point below -10 °C, preferably -20 °C, pour point below -15 °C and preferably below -22 °C.

13. A hydrocarbon product as claimed in claim 12, in which the amount of dimethyl compounds is between 23 and 28 wt$% based on total product boiling on the diesel boiling range.