FLUID CATALYTIC CRACKING PROCESS
A fluid catalytic cracking process for the preparation of cracked products by contacting in a reactor a hydrocarbon feedstock with a cracking catalyst, wherein the hydrocarbon feedstock comprises a paraffinic feedstock and triglycerides.
FIELD OF THE INVENTION
The present invention relates to a fluid catalytic cracking process.
BACKGROUND OF THE INVENTION
In fluid catalytic cracking processes a preheated hydrocarbon feedstock of a high boiling point range is brought into contact with a hot cracking catalyst in a catalytic cracking reactor, usually a riser. The feed is cracked into lower boiling products, such as dry gas, LPG, gasoline, and cycle oils. Furthermore, coke and non-volatile products deposit on the catalyst resulting in a spent catalyst. The reactor exits into a separator wherein the spent catalyst is separated from the reaction products. In the next step the spent catalyst is stripped with steam to remove the non-volatile hydrocarbon products from the catalyst. The stripped catalyst is passed to a regenerator in which coke and remaining hydrocarbon materials are combusted and wherein the catalyst is heated to a temperature required for the cracking reactions. Hereafter the hot regenerated catalyst is returned to the reactor.
As hydrocarbon feedstock a feedstock comprising a large portion of paraffins can be cracked. However, cracking such a paraffin rich hydrocarbon feedstock, such as for example a Fischer-Tropsch product, is not straightforward.
U.S. Pat. No. 4,684,756 describes a process to prepare a gasoline fraction by fluid 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 the process disclosed in U.S. Pat. No. 4,684,756 is that the yield to gasoline is relatively low.
EP-A-454256 describes a process to prepare lower olefins from a Fischer-Tropsch product by contacting this product with a ZSM-5 containing catalyst at a temperature of between 580 and 700° C. in a moving bed reactor at a catalysts to oil ratio of between 65 and 86 kg/kg.
WO-A-2004/106462 describes a process wherein a relatively heavy Fischer-Tropsch product and a catalyst system comprising a catalyst, which catalyst comprises an acidic matrix and a large pore molecular sieve, are contacted, yielding a gasoline product having a high content of iso-paraffins and olefins, compounds which greatly contribute to a high octane number.
A disadvantage of processing such a paraffinic feed in an FCC unit is that the coke make is too low. Coke on the catalyst is removed by oxidation in a so-called FCC regenerator. In such a process step the catalyst temperature increases due to exothermic reactions and reaches a temperature that makes it suitable for use in the actual catalytic cracking step. If the coke content of the catalyst is too low additional fuel is to be added to the regenerator and this situation is obviously not desired.
NL-A-8700587 describes catalytic cracking of water-free butter to hydrocarbon products, like C4 gases and lighter gases, gasoline (C5-216° C.), light cycle oils and coke, over a type RE-USY catalyst further comprising an active crystalline aluminium oxide matrix.
It is the object of the present invention to achieve a process which is better heat balanced than the prior art processes.
SUMMARY OF THE INVENTION
It has now been found that the above can be achieved by performing the fluid catalytic cracking of the paraffinic feedstock in the presence of triglycerides.
Accordingly, the invention provides a fluid catalytic cracking process for the preparation of cracked products by contacting in a reactor a hydrocarbon feedstock with a cracking catalyst, wherein the hydrocarbon feedstock comprises a paraffinic feedstock and triglycerides.
It has been found that by cracking a mixture of a paraffinic feedstock and triglycerides, more coke is formed on the cracking catalyst. An additional advantage of cracking the mixture is that a gasoline is obtained having a higher octane number. Applicant further found that by choosing the right balance between the paraffinic feedstock on the one hand and the triglycerides on the other hand, a gasoline product may be obtained having a sulphur content of less than 10 ppm, an aromatic content of lower than 35 vol %, preferably lower than 25 vol %, and an octane number of higher than 87. The triglycerides present in the hydrocarbon feedstock are cracked and the products formed result in improved RON octane numbers of the total product.
DETAILED DESCRIPTION OF THE INVENTION
Triglycerides are glycerides in which the glycerol is esterified with three fatty acids. Preferably, the triglycerides that are being used in the process according to the invention comprise fatty acids wherein the fatty acid moiety ranges from 4 to 30 carbon atoms, the fatty acids most commonly being saturated or containing 1, 2 or 3 double bonds. Triglycerides are the main constituent in vegetable oil, fish oil and animal fat.
Preferably, the hydrocarbon feedstock comprises vegetable oil, animal fat or fish oil to provide the triglycerides. The vegetable oil, animal fat or fish oil does not need to be in anhydrous or pure form or to be subjected to prior hydrogenation. The oil or fat may contain variable amounts of free fatty acids and/or esters both of which may also be converted to hydrocarbons during the process of this invention. The oil or fat may further comprise carotenoids, hydrocarbons, phosphatides, simple fatty acids and their esters, terpenes, sterols, fatty alcohols, tocopherols, polyisoprene, carbohydrates and proteins.
Suitable vegetable oils include rapeseed oil, palm oil, coconut oil, corn oil, soya oil, safflower oil, sunflower oil, linseed oil, olive oil and peanut oil. Suitable animal fats include pork lard, beef fat, mutton fat and chicken fat. Mixtures of oils or fats of different origins may be used as feed to the catalytic conversion step. Thus, mixtures of the vegetable oils, animal fats, fish oils, and mixtures which include vegetable oil, animal fat and/or fish oil may be used. Preferred oils are rapeseed oil and palm oil, in particular palm oil. It has been found that the use of palm oil results in a higher conversion to cracked products and in higher yields to gasoline.
The hydrocarbon feedstock may further comprise natural fatty acids and esters other than triglycerides, for example fatty acid methyl esters derived from transesterification of the above plant oils and animal oils.
Without wishing to be bound to any theory, we found that catalytic cracking of triglycerides seems to be a stepwise process where in the first step fatty acids molecules and the glycerol backbone are being formed. The fatty acid molecules are subsequently cracked into lighter components. We found that, in the presence of a cracking catalyst, the conversion of the triglycerides into fatty acids is almost instantaneous, while the next step, being the conversion of fatty acids, depends on factors such as catalyst to oil ratio, type of catalyst, temperature and residence time.
Typically, one expects that the oxygen present in the triglycerides is being converted to CO2 in the catalytic cracking step. We, however, found that most of the oxygen is converted to water as by-product. This water will already function as stripping gas and will be separated from the valuable products in the stripping step of the fluid catalytic cracking process.
Examples of suitable paraffinic feedstocks are a Fischer-Tropsch derived hydrocarbon stream or hydrowax.
Hydrowax is the bottoms fraction of a hydrocracker. With a hydrocracker in the context of the present invention is meant a hydrocracking process of which the main products typically are naphtha, kerosene and gas oil. The conversion, expressed in the weight percentage of the fraction in the feed to the hydrocracker boiling above 370° C. to hydrocarbons boiling below 370° C., is typically above 50 wt %. Examples of hydrocracking processes which may yield a bottoms fraction that can be used in the present process, are described in EP-A-699225, EP-A-649896, WO-A-97/18278, EP-A-705321, EP-A-994173 and U.S. Pat. No. 4,851,109.
By “Fischer-Tropsch derived hydrocarbon stream” is meant that the hydrocarbon stream is a product from a Fischer-Tropsch hydrocarbon synthesis process or derived from such product by a hydroprocessing step, i.e. hydrocracking, hydro-isomerisation and/or hydrogenation.
The Fischer-Tropsch reaction converts carbon monoxide and hydrogen into longer chain, usually paraffinic, hydrocarbons:
in the presence of an appropriate catalyst and typically at elevated temperature, for example 125 to 300° C., preferably 175 to 250° C., and pressure, for example 5 to 100 bar, preferably 12 to 80 bar. Hydrogen:carbon monoxide ratios other than 2:1 may be employed if desired.
The carbon monoxide and hydrogen is typically derived from a hydrocarbonaceous feedstock by partial oxidation. Suitable hydrocarbonaceous feedstocks include gaseous hydrocarbons such as natural gas or methane, coal, biomass, or residual fractions from crude oil distillation.
The Fischer-Tropsch derived hydrocarbon stream may suitably be a so-called syncrude as described in for example GB-A-2386607, GB-A-2371807 or EP-A-0321305. Other suitable Fischer-Tropsch hydrocarbon streams may be hydrocarbon fractions boiling in the naphtha, kerosene, gas oil, or wax range, as obtained from the Fischer-Tropsch hydrocarbon synthesis process, optionally followed by a hydroprocessing step.
Preferably, the Fischer-Tropsch hydrocarbon stream product has been obtained by hydroisomerisation of hydrocarbons directly obtained in the Fischer-Tropsch hydrocarbon synthesis reaction. The use of a hydro-isomerised hydrocarbon fraction is advantageous because it contributes to a high yield in gasoline due to the high content of iso-paraffins in said fraction. A hydro-isomerised fraction boiling in the kerosene or gas oil range may suitable be used as the Fischer-Tropsch derived hydrocarbon stream. Preferably, however, a higher boiling hydro-isomerised fraction is used as feed.
A particularly suitable hydro-isomerised hydrocarbon fraction is a fraction which has a T10 wt % boiling point of between 350 and 450° C. and a T90 wt % of between 450 and 600° C. and a wax content of between 5 and 60 wt %. Such fraction is typically referred to as waxy raffinate. Preferably, the wax content is between 5 and 30 wt %. The wax content is measured by solvent dewaxing at −27° C. in a 50/50 vol/vol mixture of methyl ethyl ketone and toluene. Examples of such a hydrocarbon streams are the commercially available Waxy Raffinate product as is marketed by Shell MDS (Malaysia) Sdn Bhd snf the waxy raffinate product as obtained by the process described in WO-A-02/070630 or in EP-B-0668342.
The paraffinic feedstock comprises preferably at least 50 wt % paraffins, more preferably at least 70 wt % paraffins. With paraffins both normal and iso-paraffins are meant. The paraffin content of the paraffinic feedstocks in the context of the present invention are measured by means of comprehensive multi-dimensional gas chromatography (GC×GC), as described in P. J. Schoenmakers, J. L. M. M. Oomen, J. Blomberg, W. Genuit, G. van Velzen, J. Chromatogr. A, 892 (2000) p. 29 and further.
The hydrocarbon feedstock according to the present invention comprises both a paraffinic feedstock and triglycerides. Preferably, the weight ratio between the amount of paraffinic feedstock and the amount of triglycerides present in the hydrocarbon feedstock is between 20:1 to 1:5, more preferably between 5:1 to 1:2.
The hydrocarbon feedstock may optionally also comprise a component not being a triglyceride or a paraffinic feedstock. Suitable components are so-called conventional FCC feedstocks, which are typically derived from crude oil refining and which are less paraffinic than the above described paraffinic feeds. The conventional FCC feedstock that can be used in the process according to the invention includes high boiling non-residual crude oil fractions, such as vacuum gas oil, straight run (atmospheric) gas oil, coker gas oils and residues from atmospheric or vacuum distillation of crude oil. These feedstocks have boiling points preferably ranging from 220° C. to 650° C., more preferably ranging from 300° C. to 600° C.
The quantity of the conventional FCC feedstock relative to the paraffinic feedstock and triglycerides may vary depending on feedstock availability and on the quality of the desired product. In the process according to the invention the hydrocarbon feedstock may comprise up to 90 wt % of the conventional FCC feedstock, preferably up to 70 wt % of the conventional FCC feedstock, more preferably up to 50 wt % of the conventional FCC feedstock, even more preferably up to 40 wt % of the conventional FCC feedstock. An advantage of processing a mixture of conventional FCC feedstock, paraffinic feedstock and triglycerides is for example that gasoline with a reduced aromatic content is produced. Another advantage is that when triglycerides and a paraffinic feedstock are added to a heavy conventional FCC feedstock, an increased yield of lower olefins is obtained. The advantages of the present invention become more pronounced at lower content of the conventional FCC feedstock in the feed.
Thus, by choosing the right balance between the paraffinic feedstock and triglycerides on the one hand and the conventional FCC feedstock on the other hand a gasoline product may be obtained having the desired properties such as an acceptable octane number, a low sulphur content and a desired aromatic content. The properties of the cracked products can be adjusted.
In the process according to the invention, the cracking catalyst comprises a large pore zeolite. With a large pore zeolite, a zeolite is meant comprising a porous, crystalline aluminosilicate structure having a porous internal cell structure on which the major axis of the pores are in the range from 0.62 to 0.8 nanometer. Axis of zeolites are depicted in the ‘Atlas of Zeolite Structure Types’, of W. M. Meier, D. H. Olson, and Ch. Baerlocher, Fourth Revised Edition 1996, Elsevier, ISBN 0-444-10015-6. Examples of such large pore zeolites are FAU or faujasite, preferably synthetic faujasite, like zeolite Y, USY, Rare Earth Y (=REY) or Rare Earth USY (REUSY). According to the present invention preferably USY is used as the large pore zeolite.
The cracking catalyst preferably further comprises a medium pore zeolite if a high yield of propylene is desired. By a medium pore zeolite that can be used in the present invention is understood a zeolite comprising a porous, crystalline aluminosilicate structure having a porous internal cell structure on which the major axis of the pores are in the range from 0.45 to 0.62 nanometer. Examples of such medium pore zeolites are of the MFI structural type such as ZSM-5, the MTW type, such as ZSM-12, the TON structural type such as theta one, and the FER structural type such is ferrierite. According to the present invention preferably ZSM-5 is used as the medium pore zeolite.
The weight ratio of large pore zeolite to medium pore size zeolite in the cracking catalyst is preferably in the range from 99:1 to 70:30, more preferably in the range from 98:2 to 85:15.
The total amount of large pore size zeolite and/or medium pore zeolite that is present in the cracking catalysts is preferably in the range from 5 to 40 wt %, more preferably in the range from 10 to 30 wt %, even more preferably in the range from 10 to 25 wt % relative to the total mass of the catalyst.
Next to the large or medium pore size zeolite, the catalysts may comprise one or more porous, inorganic refractory metal oxide binder materials or supports and/or active matrix materials. These binder materials or supports may or may not contribute to the cracking reaction. Examples of such binder materials are silica, alumina, titania, zirconia and magnesium oxide, or combinations of two or more of them. Also organic binders may be used.
The temperature at which the hydrocarbon feedstock and the cracking catalyst are contacted is preferably between 450 and 650° C. More preferably, the temperature is above 475° C., even more preferably above 500° C. Good gasoline yields are seen at temperatures above 600° C. However, temperatures above 600° C. will also give rise to thermal cracking reactions and the formation of non-desirable gaseous products like methane and ethane. For this reason the temperature is preferably below 600° C.
The process may be performed in various types of reactors. In order to simplify catalyst regeneration, preference is given to either a fast 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 (hydrocarbon feedstock) ratio is preferably between 2 and 20 kg/kg. It has been found that good results may be obtained at a catalyst to oil ratio above 6 kg/kg, since a higher catalyst to oil ratio results in a higher amount of coke on the catalyst.
The invention is further illustrated by the following Examples. The most important properties of hydrowax are shown in table 1.
Catalytic cracking experiments were carried out in a micro-riser reactor that operates in an isothermal plug-flow regime. The micro-riser reactor is a once-through bench-scale fluid catalytic cracking reactor that simulates the hydrodynamics of an industrial FCC reactor. The reactor temperature was set to 525° C. The length of the reactor was in these experiments 21.2 meters. The catalyst used was a commercial silica sol based FCC equilibrium catalyst (e-cat), containing 11 wt % USY zeolite crystals. Before each experiment, the catalyst was regenerated in a fluidised bed reactor, where coke was combusted in air at 600° C. for three hours. The catalyst was fed to the reactor by means of a catalyst feeder. Nitrogen was used to facilitate the catalyst flow. The oil feed was fed through a pulse-free syringe pump to the pre-heated oven where it was partially evaporated. In the last part before the injection point the oil was completely evaporated and adopted the reaction temperature, as well as the catalyst. The feed was injected perpendicularly into the catalyst stream. The feed consisted of pure hydrowax, or hydrowax blended with 20 wt % or 40 wt % of crude degummed rapeseed oil.
Sample collection started when the system had reached steady-state operation. Separation of the catalyst and gaseous product took place by means of a cyclone. During the steady-state operation the catalyst was stored under reaction conditions and was afterwards stripped with nitrogen. The effluent gas was led through three condensers in series operating at 25, −60, and −60° C., respectively. Any uncondensed products were captured in a gas bag. The C1-C4 hydrocarbon components in the gas bag were determined by means of gas chromatography. The entrained C5 and C6 hydrocarbons were detected as two separate lumps by this analysis method and added to the gasoline fraction. The liquid product was analysed by simulated distillation. This gave the amounts of product in terms of lumps of boiling ranges: gasoline (C5-215° C.), Light Cycle Oil (LCO, 215-325° C.), and Heavy Cycle Oil and Slurry Oil (HCO+SO, 325+° C.). The coke on the catalyst was determined with a LECO C-400 carbon analyser. The results are presented in table 2.
In comparison with 100% hydrowax, addition of rapeseed oil (RSO) results in increasing amounts of coke and LCO. Furthermore, a clear increase in the calculated RON is observed for the catalytically cracked blend of hydrowax with 40 wt % rapeseed oil as compared to 100% hydrowax.
In a small-scale fluidised bed reactor the catalytic cracking blends of hydrowax, with rapeseed oil and palm oil (at 5, 10, 25 wt %) using a equilibrium catalysts, e-cat2, was performed. The experiments were done in a reactor in which 10 grams of the commercial e-catalyst was constantly fluidised with nitrogen. Dependent on the cat/oil ratio an amount of 1.25 to 3.33 grams of oil was injected in the reactor. During stripping the liquid products were collected in glass vessels (receivers) in a bath at a temperature of −15° C. The gas produced was analysed online with a gas chromatograph. After stripping for 660 seconds, the amount of coke formed on the catalyst was determined by burning the coke from the catalyst in a regeneration step. During 40 minutes the temperature of the reactor was at 650° C. in an air environment. The coke was converted to CO2 and measured online. After regeneration the reactor was cooled to the reaction temperature and a new injection was started. The results are presented in tables 3 and 4.
1. A fluid catalytic cracking process for the preparation of cracked products by contacting in a reactor a hydrocarbon feedstock with a cracking catalyst, wherein the hydrocarbon feedstock comprises a paraffinic feedstock and triglycerides.
2. A process according to claim 1, wherein the paraffinic feedstock comprises at least 50 wt % of paraffins.
3. A process according to claim 1, wherein the weight ratio between the paraffinic feedstock and the triglycerides present in the hydrocarbon feedstock is in the range of from 20:1 to 1:5.
4. A process according to claim 1, wherein the hydrocarbon feedstock comprises up to 90 wt % of a conventional FCC feedstock.
5. A process according to claim 1, wherein the hydrocarbon feedstock comprises vegetable oil.
6. A process according to claim 1, wherein the paraffinic feedstock is a hydrowax.
7. A process according to claim 1, wherein the paraffinic feedstock is a Fischer-Tropsch derived hydrocarbon stream.
8. A process according to claim 1, wherein the cracking catalyst comprises a large pore size zeolite.
9. A process according to claim 8, wherein the large pore zeolite is USY.
10. A process according to claim 1, wherein the cracking catalyst further comprises a medium pore zeolite.
11. A process according to claim 10, wherein the medium pore size zeolite is ZSM-5.
12. A process according to claim 1, wherein the paraffinic feedstock comprises at least 70 wt % of paraffins.
13. A process according to claim 1, wherein the weight ratio between the paraffinic feedstock and the triglycerides present in the hydrocarbon feedstock is in the range of from 5:1 to 1:2.
14. A process according to claim 1, wherein the hydrocarbon feedstock comprises up to 70 wt % of a conventional FCC feedstock.
15. A process according to claim 1, wherein the hydrocarbon feedstock comprises palm oil.
16. A process according to claim 1, wherein the hydrocarbon feedstock comprises rapeseed oil.
17. A process according to claim 2, wherein the weight ratio between the paraffinic feedstock and the triglycerides present in the hydrocarbon feedstock is in the range of from 20:1 to 1:5.
18. A process according to claim 2, wherein the hydrocarbon feedstock comprises up to 90 wt % of a conventional FCC feedstock.
19. A process according to claim 3, wherein the hydrocarbon feedstock comprises up to 90 wt % of a conventional FCC feedstock.
20. A process according to claim 2, wherein the weight ratio between the paraffinic feedstock and the triglycerides present in the hydrocarbon feedstock is in the range of from 5:1 to 1:2.
International Classification: C10G 11/05 (20060101);