Patent Application
20070227946
The invention relates to a process to prepare a gas oil, in combination with a gasoline, by catalytic cracking of a Fischer-Tropsch product.
It is known that paraffinic products boiling in the gas oil range can be prepared from a Fischer-Tropsch derived synthesis product. However, Preparing a gasoline having an acceptable octane number, and a paraffinic gas oil, from a Fischer-Tropsch product, using a single conversion process, is not straightforward. This because the Fischer-Tropsch product as such consists for a large portion of normal paraffins which have a low octane value or contribution. Various publications are known which describe catalytic cracking as a process to prepare a gasoline having an acceptable octane value from a Fischer-Tropsch product. For example U.S. Pat. No. 4,684,756 discloses a process to prepare a gasoline fraction directly by catalytic cracking of a Fischer-Tropsch wax as obtained in an iron catalysed Fischer-Tropsch process. The gasoline yield is 57.2 wt %.
A disadvantage of some of the above processes involving catalytic cracking is that the cetane number of the gas oil fraction, which is produced in combination with the gasoline, is too low, and the gas oil yield is low.
The object of the present invention is to prepare a high quality paraffinic gas oil in a catalytic cracking process of a Fischer-Tropsch product which process has as the main product a gasoline.
Process to prepare a gas oil, by
Applicants found that the first gas oil fraction, obtained in step (a), will improve the cetane number of the second gas oil obtained by catalytically cracking a Fischer-Tropsch synthesis product. In a preferred embodiment, a relatively heavy Fischer-Tropsch product is used as feed to the catalytic cracking step (b). The enrichment of the catalytically cracked gas oil fraction with paraffins, as obtained in step (a), increases the cetane number to the level that makes the gas oil suitable as a diesel fuel blend component. Another advantage is that use can be made of well-known processes known for fluid catalytic cracking (FCC), step (b).
The Fischer-Tropsch synthesis product may in principle be any reaction product as obtained when performing the well know Fischer-Tropsch synthesis reaction. Preferably use is made of a relatively heavy Fischer-Tropsch product in step (b). This heavy feed preferably has at least 30 wt %, preferably at least 50 wt %, and more preferably at least 55 wt % of compounds having at least 30 carbon atoms. Furthermore the weight ratio of compounds having at least 60 or more carbon atoms and compounds having at least 30 carbon atoms of the Fischer-Tropsch product is at least 0.2, preferably at least 0.4 and more preferably at least 0.55. Preferably the Fischer-Tropsch product comprises a C20+ fraction having an ASF-alpha value (Anderson-Schulz-Flory chain growth factor) of at least 0.925, preferably at least 0.935, more preferably at least 0.945, even more preferably at least 0.955.
The initial boiling point of the Fischer-Tropsch product used in step (b) may suitably range from below 200 up to 450° C. Preferably the initial boiling point is between 300 and 450° C. in case all compounds having a boiling point in the gas oil range are separated from the Fischer-Tropsch synthesis product before the Fischer-Tropsch synthesis product is used in step (b). Applicants found that a high yield to gas oil can be achieved starting from such a Fischer-Tropsch product, thus excluding the Fischer-Tropsch fractions boiling in the gas oil range. The relatively heavy Fischer-Tropsch synthesis product can be obtained by any process, which yields a relatively heavy Fischer-Tropsch product. Not all Fischer-Tropsch processes yield such a heavy product. Preferred processes are the cobalt catalysed Fischer-Tropsch processes. An example of a suitable Fischer-Tropsch process is described in WO-A-9934917 and in AU-A-698391. These processes may yield a Fischer-Tropsch product as described above.
A preferred catalyst to be used to obtain the relatively heavy Fischer-Tropsch product is suitably a cobalt-containing catalyst as obtainable by (aa) mixing (1) titania or a titania precursor, (2) a liquid, and (3) a cobalt compound, which is at least partially insoluble in the amount of liquid used, to form a mixture; (bb) shaping and drying of the mixture thus obtained; and (cc) calcination of the composition thus obtained.
Preferably at least 50 weight percent of the cobalt compound is insoluble in the amount of liquid used, more preferably at least 70 weight percent, and even more preferably at least 80 weight percent, and most preferably at least 90 weight percent. Preferably the cobalt compound is metallic cobalt powder, cobalt hydroxide or an cobalt oxide, more preferably Co(OH)2 or Co3O4. Preferably the cobalt compound is used in an amount of up to 60 weight percent of the amount of refractory oxide, more preferably between 10 and 40 wt percent. Preferably the catalyst comprises at least one promoter metal, preferably manganese, vanadium, rhenium, ruthenium, zirconium, titanium or chromium, most preferably manganese. The promoter metal(s) is preferably used in such an amount that the atomic ratio of cobalt and promoter metal is at least 4, more preferably at least 5. Suitably at least one promoter metal compound is present in step (aa). Suitably the cobalt compound is obtained by precipitation, optionally followed by calcination. Preferably the cobalt compound and at least one of the compounds of promoter metal are obtained by co-precipitation, more preferably by co-precipitation at constant pH. Preferably the cobalt compound is precipitated in the presence of at least a part of the titania or the titania precursor, preferably in the presence of all titania or titania precursor. Preferably the mixing in step (aa) is performed by kneading or mulling. The thus obtained mixture is subsequently shaped by pelletising, extrusion, granulating or crushing, preferably by extrusion. Preferably the mixture obtained has a solids content in the range of from 30 to 90% by weight, preferably of from 50 to 80% by weight. Preferably the mixture formed in step (aa) is a slurry and the slurry thus-obtained is shaped and dried by spray-drying. Preferably the slurry obtained has a solids content in the range of from 1 to 30% by weight, more preferably of from 5 to 20% by weight. Preferably the calcination is carried out at a temperature between 400 and 750° C., more preferably between 500 and 650° C. Further details are described in WO-A-9934917.
The Fischer-Tropsch process is typically carried out at a temperature in the range from 125 to 350° C., preferably 175 to 275° C. The pressure is typically in the range from 5 to 150 bar abs., preferably from 5 to 80 bar abs., in particular from 5 to 70 bar abs. Hydrogen (H2) and carbon monoxide (synthesis gas) is typically fed to the process at a molar ratio in the range from 0.5 to 2.5. The gas hourly space velocity (GHSV) of the synthesis gas in the process of the present invention may vary within wide ranges and is typically in the range from 400 to 10000 Nl/l/h, for example from 400 to 4000 Nl/l/h. The term GHSV is well known in the art, and relates to the volume of synthesis gas in Nl, i.e. litres at STP conditions (0° C. and 1 bar abs), which is contacted in one hour with one litre of catalyst particles, i.e. excluding interparticular void spaces. In the case of a fixed catalyst bed, the GHSV may also be expressed as per litre of catalyst bed, i.e. including interparticular void space. The Fischer-Tropsch synthesis can be performed in a slurry reactor or preferably in a fixed bed. Further details are described in WO-A-9934917.
Synthesis gas may be obtained by well known processes like partial oxidation and steam reforming and combinations of these processes starting with a (hydro) carbon feedstock. Examples of possible feedstocks are natural gas, associated gas, refinery off-gas, residual fractions of crude oil, coal, pet coke and biomass, for example wood. Partial oxidation may be catalysed or non-catalyzed. Steam reforming may be for example conventional steam reforming, autothermal (ATR) reforming and convective steam reforming. Examples of suitable partial oxidation processes are the Shell Gasification Process and the Shell Coal Gasification Process.
The Fischer-Tropsch product will contain no or very little sulphur and nitrogen containing compounds. This is typical for a product derived from a Fischer-Tropsch reaction, which uses synthesis gas containing almost no impurities. Sulphur and nitrogen levels will generally be below the detection limits, which are currently 5 ppm for sulphur and 1 ppm for nitrogen. The Fischer-Tropsch product can advantageously be directly used in step (a) without having to hydrotreat the feed to remove olefins and/or oxygenates.
The catalyst system used in step (b) will at least comprise of a catalyst comprising of a matrix and a large pore molecular sieve. Examples of suitable large pore molecular sieves are of the faujasite (FAU) type as for example Zeolite Y, Ultra Stable Zeolite Y and Zeolite X. The matrix is preferably an acidic matrix. The acidic matrix will suitably comprise amorphous alumina and preferably more than 10 wt % of the catalyst is amorphous alumina. The matrix may further comprise, for example, aluminium phosphate, clay and silica and mixtures thereof. Amorphous alumina may also be used as a binder to provide the matrix with enough binding function to properly bind the molecular sieve. Examples of suitable catalysts are commercially available catalysts used in fluid catalytic cracking processes which catalysts comprise a Zeolite Y as the molecular sieve and at least alumina in the matrix.
The temperature at which feed and catalyst contact is between 450 and 650° C. More preferably the temperature is above 475° C. and even more preferably above 500° C. Good gasoline yields are seen at temperatures above 600° C. However higher temperatures than 600° C. will give rise to thermal cracking reactions and the formation of non-desirable gaseous products like for example methane and ethane. For this reason, the temperature is more preferably below 600° C. The process may be performed in various types of reactors. Because the coke make is relatively small, as compared to an FCC process operating on a petroleum-derived feed, it is possible to conduct the process in a fixed bed reactor. In order to be able to regenerate the catalyst more simply, preference is nevertheless given to either a fluidised bed reactor or a riser reactor. If the process is performed in a riser reactor, the preferred contact time is between 1 and 10 seconds and more preferred between 2 and 7 seconds. The catalyst to oil ratio is preferably between 2 and 20 kg/kg. It has been found that good results may be obtained at low catalyst to oil ratios of below 15 and even below 10 kg/kg.
This is advantageous because this means a higher productivity per catalyst resulting in, e.g. smaller equipment, less catalyst inventory, less energy requirement and/or higher productivity.
The catalyst system may advantageously also comprise of a medium pore size molecular sieve such to also obtain a high yield of propylene and other lower olefins next to the gasoline fraction. It has also been found that the yield to gas oil increases when such medium pore molecular sieves are present. Preferred medium pore size molecular sieves are zeolite beta, Erionite, Ferrierite, ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23 or ZSM-57. The weight fraction of medium pore crystals on the total of molecular sieves present in this process is preferably between 2 and 20 wt %. The medium pore molecular sieve and the large pore molecular sieve may be combined in one catalyst particle or be present in different catalyst particles. Preferably, the large and medium pore molecular sieves are present in different catalyst particles for practical reasons. For example, the operator can thus add the two catalyst components of the catalyst system at different addition rates to the process. This could be required because of different deactivation rates of the two catalysts. A suitable matrix is alumina. The molecular sieve may be dealuminated by for example steaming or other known techniques.
It has been found that the combination of the large pore molecular sieve, more preferably of the FAU type, in combination with the medium pore size molecular sieve, results in a high selectivity to the lower olefins. Applicants have found that, by performing the process according the invention with a large pore molecular sieve, more preferably of the FAU type, in combination with the medium pore size molecular sieve, as described above, not only lower olefin yield improves, but also the yield to the iso and normal pentenes and hexenes increases. In such an embodiment these pentenes and hexenes are preferably oligomerised to compounds boiling in the gas oil range. This is preferred for at least two reasons, namely that the ultimate yield to gas oil increases and also because low octane contributing compounds are removed from the gasoline. Oligomerisation is a well known process and is for example exemplified in US-A-20020111521.
In step (c) a second gas oil fraction is isolated from the product of step (b) from the main gasoline product. Isolation of said fractions is suitably performed by means of distillation. In this invention a gasoline or gasoline fraction is a fraction boiling for more than 90 wt % between 25 and 215° C., preferably boiling for more than 95 wt % in said boiling range. A gas oil or gas oil fraction is a fraction boiling for more than 90 wt % between 200 and 370° C., preferably boiling for more than 90 wt % between 215 and 350° C.
The first and second gas oil fraction may separately or in a mixture be subjected to an additional catalytic dewaxing step in order to reduce the pour point to an acceptable level if required. Such a treatment is not only advantageous for reducing the pour point but will also decrease the content of any aromatic compounds formed in step (a). The pour point is preferably below −10° C. and even more preferably below −15° C. Catalytic gas oil dewaxing may suitably be performed using a catalyst comprising a binder, a molecular sieve and a hydrogenation metal component. The binder may be any binder, suitably alumina, silica-alumina or silica. The molecular sieve is preferably a zeolite or a silica-aluminophosphate (SAPO) material. The zeolites preferably have a pore diameter of between 0.35 and 0.8 nm. Suitable intermediate pore size zeolites are mordenite, Zeolite Beta, ZSM-5, ZSM-12, ZSM-22, ZSM-23, MCM-68, SSZ-32, ZSM-35 and ZSM-48. Preferred silica-aluminophosphate (SAPO) materials are SAPO-11. The hydrogenation component is preferably a Group VIII metal, more preferably nickel, cobalt, platinum or palladium. Most preferably the noble metal Group VIII metals are used. Catalytic dewaxing conditions are known in the art and typically involve operating temperatures in the range of from 200 to 500° C., suitably from 250 to 400° C., hydrogen partial pressures in the range of from 10 to 200 bar, preferably from 15 to 100 bar, weight hourly space velocities (WHSV) in the range of from 0.1 to 10 kg of oil per litre of catalyst per hour (kg/l/hr), suitably from 0.2 to 5 kg/l/hr, more suitably from 0.5 to 3 kg/l/hr and hydrogen to oil ratios in the range of from 100 to 2,000 litres of hydrogen per litre of oil. Examples of suitable dewaxing processes and catalysts are described in WO-A-200029511 and EP-B-832171.
A Fischer-Tropsch product having the properties as listed in Table 1 was contacted with a hot regenerated catalyst at different temperatures and contact times at a catalyst to oil ratio of 4 kg/kg. The catalyst was a commercial FCC catalyst comprising an alumina matrix and Ultra Stable Zeolite Y, which had been obtained from a commercially operating FCC unit. The Zeolite Y content was 10 wt %. The operating conditions are presented in Table 3.
A Fischer-Tropsch product having the properties as listed in Table 2 was contacted with a hot regenerated catalyst at different temperatures and contact times as in Examples A-D. The Fischer-Tropsch product was obtained according to Example VII using the catalyst of Example III of WO-A-9934917. The operating conditions are presented in Table 3.
(*) Gasoline fraction defined as the distillation cut boiling between 25 and 215° C.
(**) Middle distillate defined as the distillation cut boiling between 215 and 325° C.
From Table 4, it can be derived that the process according to the invention will provide high yields to gasoline and middle distillate, or gas oil. In Examples 1-4, gas oil yields are lower than in Examples B-D, but the gas oil content in the feed to experiments B-D is 42.2 wt % (Table 1), which is higher than the gas oil yield in any of experiments B-D. In addition, the gasoline fractions from experiments 1-4 contain considerable amounts of normal and iso-pentenes, which can be oligomerised to gas oil.
Table 4 also shows that a high gasoline yield is obtained at high contact times and relatively mild temperatures (Examples B and 2).
Examples 2-4 were repeated with the Fischer-Tropsch product having the properties as listed in Table 5 and the conditions of Table 3. The feed in Table 5 can be obtained from the feed in Table 2, by removing 22 wt % of the gas oil and lighter fraction of Table 1. The yields are presented in Table 6. The gas oil yields are higher than the yields in Examples 2-4, but considerably lower than the sum of the gas oil yields from Examples 2-4 and the 9 wt % (on total feed) gas oil that can be recovered from the fraction of Table 1, and blended with the gas oil fractions obtained in Examples 2-4, according to the invention.
(*) Gasoline fraction defined as the distillation cut boiling between 25 and 215° C.
(**) Middle distillate defined as the distillation cut boiling between 215 and 325° C.
Example 6 was repeated except that part of the catalyst was exchanged for a 25 wt % ZSM-5 containing catalyst. The content of ZSM-5 based catalyst on the whole catalyst charge was 20 wt % (as calculated on the total catalyst weight). The gasoline yield was 47.99 wt %, and the middle distillate yield 9.27 wt % on total product. The content of normal and iso-pentenes was 54.61 wt % in the gasoline fraction.
Example 2 was repeated except that part of the catalyst was exchanged for a 25 wt % ZSM-5 containing catalyst. The content of ZSM-5 based catalyst on the whole catalyst charge was 20 wt % (as calculated on the total catalyst weight). The results are presented in Table 7.
Example 3 was repeated except that part of the catalyst was exchanged for a 25 wt % ZSM-5 containing catalyst. The content of ZSM-5 based catalyst on the whole catalyst charge was 20 wt % (as calculated on the total catalyst weight). The results are presented in Table 7.
(*) Gasoline fraction defined as the distillation cut boiling between 25 and 215° C.
(**) Middle distillate defined as the distillation cut boiling between 215 and 325° C.
Example 8-10 show that the addition of ZSM-5 increases oil yields.
| Number | Date | Country | Kind |
|---|---|---|---|
| PCT EP04 050931 | May 2004 | EP | regional |
| 04106189.6 | Nov 2004 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP05/52391 | 5/25/2005 | WO | 11/22/2006 |
