Patent Application
20080064770
The invention may be carried into practice in various ways and some embodiments will now be described by way of example.
Support Materials
A morphous catalyst support materials typically have surface areas between 50 and 500 m2/g, more typically between 100 and 300 m2/g. The alumina supports ALU-A to ALU-D applied in embodiments of the present invention are at least predominately, of the γ-alumina type with surface areas between 100 and 200 m2/g. These supports are prepared by spray-drying techniques using appropriate solutions in order to obtain essentially spherical particles of appropriate size, e.g. 80% in the range between 30-120 μ. After spray-drying, the material is calcined at a high temperature to give the appropriate crystal size and pore structure. These calcinations can be performed at temperatures above 800° C.
Further, it is essential that the pore volume is sufficiently high, above 0.4 cm3/g or better, above 0.6 cm3/g. This will give a light material suitable for operation in a slurry environment and ease the impregnation by minimising the number of impregnation steps required. At the same time the support, and the final catalyst, should have sufficient strength for extended operation of months or years with minimal attrition of the materials. This can be tested in a slurry environment or by the ASTM method applicable for testing FCC (fluid catalytic cracking) catalysts.
The various support materials are as follows, and further information is set out in Table 4.
ALU-A
This standard γ-alumina is available under the trade name PURALOX from Condea of Germany (recently changed to Sasol GmbH) with the code SCCa-40/195.
ALU-A*
This is the same as ALU-A, but without the additional drying and precalcination step at 500° C. usually applied before impregnation, see below. From Table 4 it is seen that the properties of ALU-A and ALU-A* are indistinguishable, as expected. This is also a verification of the reproducibility of the analytical methods used.
ALU-B
This is a developmental spray-dried and calcined alumina provided by a second supplier.
ALU-C
A specially design alumina support for the purpose of the present invention of the same PURALOX SCCa series as for ALU-A.
ALU-D
A second specially design alumina support for the purpose of the present invention of the PURALOX SCCa series of materials.
ALU-E
A third specially design PURALOX SCCa type alumina support for the purpose of the present invention.
ALU-X
A specially designed alumina support (CPR 11 type) provided by a third supplier, Akzo Nobel.
ALU-Y
A specially designed alumina support provided by a fourth supplier, Alcoa, containing a few percent lanthanum and intended as a washcoat material for exhaust catalysts.
ALU-Z
A catalyst support of the type Catapal B.
FCC
A particularly hard alumina with an attrition resistance suitable for use in an FCC (Fluid catalytic cracking) refinery process.
From Table 4 it is clear that the surface areas are in a conventional and fairly narrow range for all the supports. For example, ALU-A and ALU-C have practically the same surface area. ALU-C and ALU-D are special in that they have a particularly high pore volume. Most noticeable is the gradual increase in mean pore diameter from ALU-A to ALU-D.
Upon high temperature treatment, the γ-aluminas of the different alumina hydrates will be converted to transition phase aluminas, denoted δ, θ, η, χ or κ-aluminas, that will all finally will be converted to α-alumina, with gradual decrease in surface areas. These aluminas may also be suitable as support materials for cobalt for the Fischer-Tropsch synthesis, even for surface areas in the range 10-50 m2/g, although a surface area higher than this number is advisable in order to obtain sufficiently high cobalt metal loading and dispersion. It is also possible to increase the high temperature stability of aluminas by adding certain stabilising agents like lanthanum (lanthanum oxide). Thus, the γ-phase can be retained, even above 1000° C. Other stabilising agents have also been reported, such as magnesia or ceria. Different support materials are also frequently used as support materials for active metals or metal salts in catalytic reactions. Some materials reported for use in the Fischer-Tropsch synthesis are silica and titania (anatase or rutile). Other options include silica-aluminas, zirconia and zeolites.
Three other properties that are important for the support, and for the final catalyst, in particular when used in a slurry reactor environment like a slurry bubble column, are the particle density, the particle size, and the abrasion or attrition resistance. The density is important to be able to secure a suitable distribution (dispersion) of the catalyst particles in the reactor; a light material is particularly advantageous for avoiding settling or an excessive concentration of particles in the lower part of the reactor. The particle size is also related to settling and the catalyst concentration profile, but should not be excessively small, to facilitate separation of the liquid product from the reactor slurry and prevent particles being transported with the gas phase at the top of the reactor. Attrition should be minimised to prolong the lifetime of the catalyst and avoid contamination of the liquid hydrocarbon product. Examples of these parameters for selected γ-alumina supports are shown in Table 0. There is a tendency for a light support and catalyst to be weak material. Further, impregnation and calcinations reduce the attrition resistance somewhat. The FCC catalyst is a very attrition resistant reference catalyst intended for use in the fluidized-bed riser of a Fluid Catalytic Cracking unit at an oil refinery, and therefore should be regarded as a limit for an, in this respect, particularly hard catalyst.
Catalyst Preparation
Unless otherwise stated, the catalysts all contain a nominal amount of cobalt of 20 wt % and 1 wt % Re, as calculated assuming reduced catalysts with complete reduction of cobalt. The actual metal loading as determined by XRF or ICP may vary by up to 12%, i.e. for cobalt between 18 and 22 wt % of the total reduced catalyst weight.
Before impregnation, the catalyst support is precalcined at about 500° C. Impregnation is in a single or multiple steps from a mixed aqueous solution of appropriate metal salts, generally of cobalt nitrate and perrhenic acid. The impregnation technique is by the pore filling or “incipient wetness” method that implies that the solution is mixed with the dry support until the pores are filled. The definition of the end point of this method may vary somewhat from laboratory to laboratory, giving an impregnated catalyst that has a completely dry appearance to one which appears sticky or snow-like. In no instance is there any free flowing liquid present.
The impregnated catalyst is dried, typically at 80-120° C., to remove water from the catalyst pores, and then calcined at typically 300° C.
The above description represents a standard way of preparing the catalysts. However, there are a number of variations of these procedures that will not influence the essence of the invention. It has been found that if the catalyst support is already calcined, e.g. at a supplier's facility, to a higher temperature than 500° C., recalcinations at this temperature prior to impregnation have minimal effect (Table 4). Further, calcination after impregnation of the metal salts at different conditions are also appropriate, Table 1. It is obvious that large variations in the calcination conditions can be applied to achieve good activity and selectivity of the catalyst. However, it is known that calcinations for a prolonged time at a sufficiently high temperature will cause agglomeration of the cobalt crystallites and hence a reduced catalytic activity. Using low calcination temperatures and times will result in an incomplete decomposition of the cobalt nitrate, and a residual nitrogen content that might cause problems in the subsequent reduction step. The calcinations in the present case are performed in a stationary oven with a certain temperature ramping speed of 2° C./min. It should be understood that the ramping speed could be varied and that any standard or specially designed calcination equipment could be applied by adjusting the conditions properly. Examples of such calcination equipment are continuous or batch wise operated rotational calciners and conveyor belt type calciners. Additional data for the performance of the catalysts CAT-D3 and CAT-D4 are given in Table 4.
Another important step in the catalyst preparation is the impregnation of the metal salts. A number of different procedures have been described in the literature, including the case of alternative solvents and chemicals. The preferred procedure involves aqueous incipient wetness with solutions of cobalt nitrate (Co(NO3)2) and perrhenic acid (HReO4). Alternatives include using cobalt acetate(s), cobalt halide(s), cobalt carbonyl(s), cobalt oxalate(s), cobalt phosphate(s), organic cobalt compounds, ammonium perrhenate, rhenium halide(s), rhenium carbonyl(s), industrial metal salt solutions and organic solvents. However, the impregnation technique may encompass all available methods besides incipient wetness, such as precipitation, impregnation from slurry with surplus liquid, chemical vapour deposition etc. It is well known that the impregnation method may influence the dispersion of the active metal (cobalt) and hence the catalytic activity, but as the Fischer-Tropsch reaction is believed to be non-structure sensitive, the dispersion should not influence the selectivity. Table 2 compares catalysts prepared by the incipient wetness method, but using different amounts of water. The appearance of the catalysts prior to calcination will then vary from completely dry and free flowing to lumpy, like wet snow. Again, excellent and consistent performance in terms of activities and selectivities is achieved. The chemicals used were also varied, see Table 4, catalysts CAT-D5 and CAT-D6.
Catalyst Materials
Note that the X in CAT-Xn denotes the support material applied.
CAT-A1
Catalyst prepared in our laboratory (L1) with standard procedures as described above and one-step impregnation.
CAT-A2
Catalyst prepared in the laboratory of a catalyst supplier (L2) with standard procedures as described using two-step impregnation with calcinations in between.
CAT-A3
Catalyst prepared in the laboratory of a catalyst supplier (L2) with standard procedures as described using three-step impregnation with drying in between.
CAT-B1
Catalyst prepared in our laboratory (L1) with standard procedures as described above and one step impregnation, but using 12 wt % Co and 0.5 wt % Re.
CAT-B2
As CAT-B 1, but standard Cobalt and Rhenium loading.
CAT-B3
Catalyst prepared in the laboratory of a second catalyst supplier (L3) with standard procedures as described above using one-step impregnation.
CAT-C1
Catalyst prepared in our laboratory (L1) with standard procedures as described above and one-step impregnation.
CAT-C2
Reproduction of CAT-C1.
CAT-D1
Catalyst prepared in our laboratory (L1) with standard procedures as described above and one-step impregnation.
CAT-D2
Catalyst prepared in the laboratory of a catalyst supplier (L2) with standard procedures as described using one-step impregnation.
CAT-D3
Catalyst prepared in the laboratory of a catalyst supplier (L2) with standard procedures as described using two-step impregnation with drying in between.
CAT-D4
As CAT-D3, but the final calcinations is performed at 400° C.
CAT-D5
Catalyst prepared in the laboratory of a second catalyst supplier (L3) with standard procedures as described above using one-step impregnation.
CAT-D6
Catalyst prepared in the laboratory of a second catalyst supplier (L3) with standard procedures as described above using one-step impregnation, but from an industrial cobalt solution and ammonium perrhenate
CAT-E1
Catalyst prepared as D1, but on the E-support.
Additional materials tested include those using alternative promoters to rhenium, specifically, platinum, iridium or ruthenium, that all are beneficial. Another option to add a second promoter such as lanthanum oxide or a mixture of oxides of the lanthanides or other difficult reducible compounds, salts and oxides, as well as the alternative support materials mentioned above.
Catalyst Testing and Characterization
One critical step before testing is the activation of the catalyst, involving reduction of cobalt oxide(s) to cobalt metal. This reduction can be performed by flowing a suitable reducing gas over the catalyst particles. Particularly suitable are hydrogen or carbon monoxide or mixtures thereof. The reducing gas can be mixed with inerts such as nitrogen, noble gases or steam and suitable temperatures and pressures should be applied. If a fluidised bed reactor is used for activation, it might be convenient to use a recycle of (part of) the reductive gas and a slight atmospheric total overpressure just to secure a suitable gas flow. It is also possible to use elevated total pressures, for example up to 8 bar (8×105 Pa) or higher, or even the Fischer-Tropsch reactor pressure. Selection of the reduction temperature strongly depends on the actual catalyst formulation, and in particular on the presence and nature of promoters. For one set of catalysts, the reducibilities shown in Table 3a were found to be determined by back oxidation with pure oxygen gas. It is verified that the Re promoter is highly efficient in achieving high reducibilities at a convenient temperature. Some exploratory performance data for activated catalysts are given in Table 3b. It can be seen that acceptable gas velocities (GHSV) can be applied for a moderate period of time. There is, however, a lower limit of 1/10 of the highest GHSV is unsuccessful, unless the reduction period is sufficiently long.
A particularly important method for characterisation of the present catalysts and support materials is related to the determination of the pore characteristics; pore volume, pore size distribution, surface area and average pore size. There are a number of variations of the techniques applied, but most are variations of the so-called BET method using the adsorption or desorption isotherms of a gas that is adsorbed to the surface of the pores, typically nitrogen, but also certain noble gases or mercury can be used for specialised purposes. In the present invention, the nitrogen desorption isotherm has been used, measured by standard procedures on an ASAP 2000 instrument from Micromeretics and the standard included software for calculation of the pore characteristics.
The values reported in the tables are the BET surface area, the BJH desorption cumulative pore volume between 17 and 3000 A (17 and 3000×10−10 m) diameter and the BJH desorption average pore diameter (4V/A). Specific values will vary depending on the method, but the general findings of this invention are expected to be valid regardless of any standard method employed, and characterisation data compared therefore should be harmonized to a given method. This means, of course, that values cited depend on the method, and should be adjusted accordingly if another method and procedure is applied.
The particle size distribution was measured by laser light scattering of a water dispersion using a Malvern type instrument. It should be noted that somewhat different results will be obtained if a different dispersion medium is employed, e.g. an alcohol will give a slightly lower average particle size.
The modified ASTM type equipment for testing attrition consists of two main parts, one air feeding system and one reactor where the attrition takes place. Compressed air passes through a pressure regulator at 5 bar (5×105 Pa) to a moisture chamber where the air is moisturised to approximately 30% relative humidity. This is done to avoid static electricity in the system. The amount of air is then adjusted in a mass flow controller. The humid air is then entering the reactor (ID=1.4″, L=28″) through a sieve tray where the holes have a diameter of 0.4 mm. Because of these holes, the gas reaches sonic velocity, which causes the “wear and tear” on the particles in the reactor. The pressure is approximately 1.8 bar (1.8×105 Pa).
After passing through the reactor, the velocity is reduced in the separation chamber (ID=4.4″ (117.8 mm), L=12″ (304.8 mm). Conical connections: L=8″ (203.2 mm) between reactor and separation chamber, L=4″ (101.6 mm) between separation chamber and u-tube) above the reactor. Particles >40 μm will fall back down into the reactor, while smaller particles <40 μm (fines) will enter a Soxhlet-filter through a u-formed tubing. A vibrator is mounted on the separation chamber, to loosen any particles on the inside walls.
50 g of powder or catalyst, sieved to >40 μm before testing, is loaded to the reactor, and the reactor is connected to the separation chamber. The air is turned on, and the fines produced in the reactor and collected in the Soxhlet filter are weighed every 15 minutes during the first 2 hours, and every 30 minutes during the next 3 hours. A normal run lasts 5 hours and the amount of fines produced can be plotted against time.
The catalysts were tested in an isothermal fixed-bed microreactor. The reactor was 25 cm long and had an inner diameter of 1 cm. Each catalyst was given a pretreatment consisting of reduction by passing hydrogen over the catalyst while heating the catalyst at a rate of 1° C./min to 350° C. and maintaining this temperature for 16 h at a pressure of 1 bar. In the tests, synthesis gas consisting of 2.1:1 H2:CO (+3 vol % N2) was passed over 1-2 g of the catalyst diluted 1:5 with SiC at 20 bar at 210° C. and at the desired space velocity. The space velocity was varied to keep the CO conversion between 40 and 45% after stable operation was obtained, and the activity and selectivity was measured for the 90-100 h on stream time interval.
First, it can be seen that the surface areas, pore diameters and pore volumes are reduced upon impregnation/calcinations, but the trends of the supports are maintained concerning pore diameters and volumes. The surface areas of the catalysts are confined in a rather narrow range.
CAT-B1 naturally has a low relative activity due to the reduced cobalt content, whereas the activities of the other catalysts are 0.90±14% (excluding the catalyst with 12% cobalt and the catalyst made from a more impure cobalt solution). This range is normal in view of the different impregnation techniques used, the skills practiced at three separate laboratories, and the reproducibility of the test method.
However, what is highly surprising and significant is the increase in C5+ selectivity as one goes from the catalysts of type A, to C and flirter to the D-type catalysts. Even the lowest selectivity of the D catalysts is 3% higher than the highest selectivity of the A type. The data suggest a clear relationship between the selectivities and an increase in mean pore diameter, alternatively an increased pore volume. This is perhaps more clearly seen in
In a separate series of experiments, catalysts made on two different supports were compared with different levels of the promoter, in this case rhenium or platinum. The data are summarised in Table 5. For un-promoted or rhenium promoted catalysts, it is again surprisingly found that the selectivity is systematically high for the catalysts prepared on the high pore volume, large pore diameter alumina support, C, and also for a variety of promoter (second metal) loadings. It can also be seen that the metal promoter has a significant effect on the catalyst activity for both supports. However, the platinum promoted catalysts do not show the expected result of changing the support material, in fact, platinum lowers the selectivity for both support C and Z to an equal and low level. This observation may be due to a special effect of platinum as this metal is known for easy dissociation of hydrogen molecules, which again may spill over and hydrogenate intermediate hydrocarbon species that are formed on cobalt.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0226514.8 | Nov 2002 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB03/04873 | 11/10/2003 | WO | 00 | 3/15/2006 |
