METHOD FOR PREPARING A CATALYST SUPPORT

Abstract

A process for preparing a powder support containing alumina and silica or their derivatives for a catalyst of a Fischer-Tropsch type reaction, including stage (a) of preparing a first reactant containing an alumina compound or precursor including a reaction for peptization of an alumina compound or precursor in the presence of an acid, to form solid particles in suspension, stage (b) of preparing a second reactant based on silicic acid and/or on a compound or precursor of silicic acid, including a controlled aging treatment of the silicic acid targeted at its polymerization up to a degree of conversion of the silicic acid of at most 70%, stage (c) of mixing the two reactants in a mixer, and the pH of the first reactant is adjusted to a value not exceeding a given maximum pH threshold.

Claims

1. A process for the preparation of a support for a catalyst of a reaction of Fischer-Tropsch type in the form of a powder, said support comprising alumina and silica or their derivatives, starting from: a first reactant comprising an alumina compound or precursor, and optionally alumina, in particular an aluminum oxyhydroxide, in suspension in a solvent, and a second reactant based on silicic acid and/or on a compound or precursor of silicic acid, and optionally silica, in suspension or dissolved in a solvent, characterized in that the process comprises: a stage (a) of preparation of the first reactant in a first reactor (10), including a reaction for peptization of an alumina compound or precursor, in particular of an aluminum oxyhydroxide, in the presence of an acid, so as to form solid particles in at least partially colloidal suspension, a stage (b) of preparation of the second reactant in a second reactor (20), including a controlled aging treatment of the silicic acid targeted at its polymerization up to a degree of conversion of the silicic acid of at most 70%, said second reactant resulting from stage (b) being at a pH of at most 3.4, a stage (c) of mixing the two reactants resulting from the two reactors in a mixer or group of mixers (40, 41), in which the reaction between the two reactants takes place, and in that the pH of the first reactant is adjusted to a value not exceeding a given maximum pH threshold (pH.sub.max), said threshold having a value equal to 3.4, before its introduction into the mixer in the mixing stage (c), in order to obtain a mixture in the form of particles in suspension in a liquid phase.

2. The process as claimed in claim 1, characterized in that the controlled aging treatment is carried out at a temperature of between 5° C. and 90° C., in particular 5° C. and 20° C. or greater than 20° C. and at most 85° C., in particular between 35° C. and 75° C., preferably between 45° C. and 65° C.

3. The process as claimed in claim 1, characterized in that the controlled aging treatment is carried out up to a degree of conversion of the silicic acid of at least 10%, in particular of between 10% and 40%.

4. The process as claimed in claim 1, characterized in that the controlled aging treatment is carried out at a concentration of silicic acid in the liquid phase of the second reactant of between 30 g/l and 200 g/l, in particular between 40 g/l and 60 g/l.

5. The process as claimed in claim 1, characterized in that the controlled aging treatment is carried out batchwise, in a closed reactor, or continuously, in particular with a thermostatically controlled plug-flow reactor.

6. The process as claimed in claim 1, characterized in that the pH of the first reactant is adjusted to a value equal to +/−15%, in particular +/−10%, of the isoelectric point of silicic acid, preferably to a pH of between 2.8 and 3.2.

7. The process as claimed in claim 1, characterized in that the pH of the first reactant is adjusted during all or part of stage (a) of its preparation and/or at the end of stage (a) and/or after stage (a) and before stage (c).

8. The process as claimed in claim 1, characterized in that the peptization reaction uses boehmite, in the presence of an acid, in particular of nitric acid.

9. The process as claimed in claim 1, characterized in that the second reactant, resulting from stage (b), is at a pH of at most 3.2.

10. The process as claimed in claim 1, characterized in that the mixing stage (c) takes place continuously in the mixer or the group of mixers (40, 41) in which the reaction between the two reactants takes place.

11. The process as claimed in claim 1, characterized in that the mixing stage (c) takes place with regulation of the input flow rates of the first and second reactants at the inlet of said mixer/group of mixers, in particular carried out by subjecting one of the flow rates of reactants to the control of the other.

12. The process as claimed in claim 1, characterized in that the viscosity of the mixture produced in stage (c) is less than or equal to 300 cP, in particular less than or equal to 250 cP, preferably at most 30 or 40 cP.

13. The process as claimed in claim 1, characterized in that the viscosity of the second reactant on conclusion of stage (b) remains, before mixing of stage (c), at a viscosity of less than or equal to 250 cP, in particular of less than or equal to 100 cP.

14. The process as claimed in claim 1, characterized in that it comprises: a stage (d) which takes place continuously following the mixing stage (c), stage (d) being a stage of drying by atomization of the mixture resulting from the mixing stage (c) in order to obtain a powder.

Description

DESCRIPTION OF THE EMBODIMENTS

[0078] The invention will be described in detail below with the help of nonlimiting examples illustrated by the following figures:

[0079] List of the Figures

[0080] FIG. 1 represents a diagrammatic representation of the complete process for the synthesis of particles of catalyst support in the form of a block diagram according to the invention.

[0081] FIG. 2 represents a graph representing, for examples 1 and 2, the change in the viscosity, expressed in Pa.Math.s, as a function of time, expressed in seconds, of the mixture obtained at the end of stage (c).

[0082] FIG. 3 represents a graph representing, for examples 3 and 4, the change in the viscosity, expressed in Pa.Math.s, as a function of time, expressed in seconds, of the mixture obtained at the end of stage (c).

[0083] FIG. 1 very diagrammatically represents the different stages of a process for the synthesis according to the invention of a Fischer-Tropsch catalyst support based on alumina and silica starting from boehmite and silicic acid, by dividing it into 5 blocks numbered 1 to 5 in FIG. 1, from the most upstream stage to the most downstream stage of the process:

[0084] Block 1: stage (a), with a stirred vessel for the preparation of alumina from a first reactant, boehmite [0085] Block 2: stage (b), with a vessel for the preparation of the second reactant, silicic acid [0086] Block 3: intermediate stage of regulation of injection of boehmite and silicic acid in order to respect a target ratio [0087] Block 4: stage (c) of injection of the streams originating from blocks 1 and 2 into a group comprising a disperser mounted in series with a static mixer [0088] Block 5: stage (d) of drying by atomization the suspension resulting from block 4.

[0089] It should be noted that the term “vessel”, in this instance, denotes a reactor in general, which can be a simple batch treatment vessel or a continuous treatment reactor, in particular.

[0090] Details are now given of these blocks successively:

[0091] Block 1: stage (a) of peptization of boehmite by addition of nitric acid. The peptization is carried out in a vessel 10 provided with mechanical stirring means 11 which ensure the suspension of the boehmite in powder form in water and sufficient shearing to keep the boehmite in suspension and to peptize it.

[0092] An example of a vessel is as follows: [0093] the vessel 10 has a capacity of 30 liters, the liquid height in the vessel is equal to the diameter of the vessel [0094] the vessel 10 can, if appropriate, be thermostatically controlled. The peptization reaction can be carried out at ambient temperature and does not need to be thermostatically controlled.

[0095] However, it might be necessary to control the temperature and to thermostatically control the jacketed vessel. [0096] the stirring means 11 are stirring impellers sold by Lightnin or turbines with inclined blades: preferably, the height of the impeller with respect to the bottom of the vessel is in a 1/3 ratio. The motor driving these stirring means 11 can ensure a wide range of stirring speeds, for example between 50 and 500 rpm. [0097] baffles can be provided in the vessel, for example in the form of baffles detached from the wall and held by the lid of the vessel (not represented) [0098] a pump 12 is also provided, which is used both to feed the static mixer of block 4 (flow rate, for example, 5-20 I/h) but also to ensure optional recycling around the vessel, via the recycling line 13, which can constitute a means of regulating the exiting flow rate of the vessel (maximum flow rate 50 I/h). High-pressure pumps, of screw pump type or other, are preferable because they make it possible to control the flow rate, to overcome the pressure drop of the downstream line (flow meter, mixer and disperser, atomization device). The objective is to have a pressure of approximately 10 bar (5 to 15 bar) in front of the atomization device of block 5. [0099] the optional recycling line 13 has a return to the top of the vessel 10 dipping into the liquid (in order to avoid injecting bubbles). [0100] a valve is also provided, for example on a roundabout (at blocks 1 and 2 in the figure). This control valve makes it possible to adjust the pressure upstream of the atomization device of block 5. [0101] a discharge valve can be provided (at block 3 in the figure) in order to reduce the time required to stabilize the system.

[0102] Operating mode: The action of the nitric acid on the boehmite modifies its surface state and makes possible its at least partial peptization. In this instance, the reactant is boehmite, an alumina precursor. It is also possible to add alumina, which is thus already transformed, to the boehmite at this stage. The peptization takes place in the following way: a water+HNO.sub.3 mixture is prepared in the vessel 10, then the boehmite is gradually added and the mixture is kept stirred in the vessel for approximately 30 minutes.

[0103] Block 2: Stage (b) with Vessel (20) for Storage of the Silicic Acid [0104] the silicic acid is prepared upstream from sodium silicate, then passed through an ion exchange resin in order to remove the sodium ions.

[0105] This two-stage preparation is carried out in the following way: Sodium (meta)silicate is a strong base forming highly alkaline solutions (pH 13 in 1% solution). It is formed naturally by reaction of silica (silicon dioxide) with sodium carbonate in the molten state. Sodium silicate and carbon dioxide are obtained according to the following reaction:

Na.sub.2CO.sub.3+SiO.sub.2.fwdarw.Na.sub.2SiO.sub.3+CO.sub.2

[0106] It is found in two main forms: an anhydrous form (it then exists as a translucent to white crystalline solid of formula Na.sub.2SiO.sub.3) and a hydrated form (Na.sub.2SiO.sub.3.nH.sub.2O); it is then sometimes described as “liquid glass” or “water glass”.

[0107] The silicate is subsequently neutralized by an acid and leads to the formation of silanol groups by hydrolysis (silicic acid): the acidification is carried out by passing through an ion exchange resin which makes it possible to remove the sodium ions and replace them with H.sup.+ ions.

[0108] Consequently, the silicic acid monomers group together to form Si—O—Si bonds and constitute the silicic acid which is used in stage (b).

[0109] During stage (b) and in accordance with the invention, the controlled aging of the silicic acid obtained will be carried out as indicated above: this is a thermal aging, which can be carried out directly in the vessel 20, a closed reactor, of block 2 or which can be carried out in a different reactor which subsequently feeds the vessel 20, for example. It is also possible to envisage that the vessel 20 is a reactor operating continuously in which the aging of the silicic acid takes place, with feeding at the outlet to the means of mixing with the other reactant.

[0110] In the nonlimiting case described below, the controlled aging and the optional storage before mixing with the other reactant take place in a single closed reactor, which is the vessel 20, in block 2.

[0111] The targeted degree of conversion must be less than 70% and in particular less than or equal to 50%, in order to prevent the rise in gel, and preferably between 10% and 40%.

[0112] This conversion is monitored by the measurement of viscosity, determined as follows:

In(n.sub.r)=(2.50)/(1−K.sub.1C)  (B)

with C the gel fraction value. C can be estimated by virtue of Mooney's equation, it being assumed that the gel domains are spherical and of uniform size, C being determined by virtue of the relationship (B) above, with K.sub.1 a constant equal to 1.43 and with n.sub.r the ratio of the dynamic viscosity (μ, in cP) of the sol during gelation to the initial viscosity of the sol (μ.sub.0, in cP), as indicated in the relationship (C) below:

n.sub.r=μ/μ.sub.0  (C)

[0113] The aging times are determined in order to achieve the target conversions at a given temperature. Thus, the silicic acid can be aged for a time 4 to 6 times shorter at 60° C. than at 10° C. for one and the same degree of conversion.

[0114] This aging operation can be carried out in closed mode or in continuous mode, in a way known to a person skilled in the art.

[0115] In closed mode, this operation can, for example, be carried out directly in the reactor/vessel 20 of block 2. Block 2 is, for example, regulated at 10° C. because, at this temperature, the conversion of the silicic acid is slow. The aging is then carried out by a temperature window making it possible to bring the silicic acid to the desired temperature (greater than 10° C.) for a given time and then to bring the temperature of block 2 back to the regulation temperature of 10° C.

[0116] In continuous mode, the silicic acid can, for example, pass through a plug-flow reactor thermostatically controlled at the desired temperature with a residence time equal to the desired aging time. [0117] The concentration of the silicic acid in the vessel 20 before mixing with the other reactant is of the order of 50 g/l and the pH is set in the vicinity of 3. [0118] the vessel 20 has, for example, a capacity of 10 l, with a height of liquid equal to the diameter of the vessel. [0119] the vessel 20 is thermostatically controlled with a cold unit making it possible to maintain a temperature between 5° C. and 10° C., in particular 10° C., except during the controlled aging. This is because, the silicic acid having undergone controlled aging beforehand, it is thus useful to have a cooled vessel in order to stabilize the aging of the silicic acid solution at the desired degree of conversion. [0120] the vessel 20 is stirred, in order to have good temperature homogeneity, by being provided with stirring means, for example in the form of a marine propeller or of the type sold by Lightnin:

[0121] preferably, the height of the impeller with respect to the bottom of the vessel is in a 1/3 ratio. The motor driving the impeller is capable of ensuring a wide range of stirring speeds between 50 and 500 rpm. [0122] baffles can be provided in the vessel 20, for example baffles detached from the wall and fixed to the lid of the vessel. [0123] a pump 22 is provided: it is used both to feed the static mixer of block 4 (flow rate of between 0.5 and 5 I/h) but also to ensure optional recycling around the vessel 20 (which is a means for flow regulation of the coinjection of the two effluents from the vessels 10 and 20 of blocks 1 and 2 to the mixer of block 3, with a maximum flow rate of 50 I/h). High-pressure pumps, of screw pump type, are preferable because they make it possible to control the flow rate, to overcome the pressure drop of the downstream line (flow meter, mixer and disperser, atomization device). [0124] the objective is to have a pressure of approximately 10 bar (5 to 15 bar) at the inlet of the atomization device of block 5. [0125] a valve is provided on a roundabout. This control valve is used to adjust the pressure upstream of the atomization device of block 5. An optional discharge valve is desirable in order to reduce the time required to stabilize the system. [0126] An insulated recycling line is optionally provided (line diameter approximately 1 cm, for example) with a return to the top of the reactor 20 dipping into the liquid (in order to avoid injecting bubbles).

[0127] This stage (b) is thus a stage of aging the silicic acid, followed by an optional cold storage (maintaining the solution at approximately 10° C. in this instance) of the latter.

[0128] Block 3: Regulation of the Injection Flow Rates of Boehmite and Silicic Acid

[0129] This type of regulation is based on servo-controlled valves and requires two flow meters and a valve on the servo-controlled line (in this instance, that of the silicic acid). A manual control valve is, for example, arranged in parallel with the regulating valve so as to widen the operating valve coefficient.

[0130] The regulation of the flow rates is important because it makes it possible to maintain a ratio between the two reactants (suspension of boehmite and silicic acid), despite the possible change in the viscosity of the silicic acid.

[0131] The flow rate of boehmite is fixed and that of silicic acid is servo-controlled in order to maintain a ratio R, for example between 2.8 and 3.5, in particular in the vicinity of or equal to 3.

[0132] Block 4: Injection of the Two Flows of the Lines 30 and 31 Originating from Block 3 into a Static Mixer

[0133] At the inlet of the mixer, each flow of the lines 30 and 31 can be interrupted by quarter-turn valves (in order to avoid a return from one line to the other during the start-up and shut-down phases).

[0134] The configuration envisaged involves two consecutive static mixers 40, 41 serving respectively: [0135] as zone for dispersion of the silicic acid in the boehmite suspension for the reaction [0136] as zone for mixing the polysilicic acid/boehmite product

[0137] A pressure measurement is provided upstream and downstream of the static mixers 40, 41.

[0138] The static mixers of block 4 are positioned as close as possible to the atomization device of block 5, so as to reduce to a minimum the residence time after the mixing zone preceding the drying. If the reaction requires a longer residence time, the static mixers are placed further from the atomization device, so as to leave an additional length, for example of 1 to 3 m, of pipe before atomization.

[0139] Block 5: Spray Drying of the Suspension

[0140] Once the suspension has been prepared, it is conveyed by a common line 42 via the static mixers 40, 41 to the chamber 50 of the atomization dryer so as to be sprayed as fine droplets which, after evaporation of the solvent, form solid particles. The spraying process is targeted at obtaining liquid droplets or filaments by passing through a local deformation which leads to the atomization. The main parameters which influence the spraying are: the properties of the fluid (density, viscosity, surface tension); the properties of the nozzle (spray angle, spray shape, size of the orifice); the operating parameters (pressure and flow rate of the fluid, temperature). The drying chamber 50 exhibits all the items of equipment known to a person skilled in the art necessary for the generation of the spray fed with hot air; it is followed by a line of cyclones in order to carry out the gas/solid separation.

[0141] Two devices for spraying the suspension were used: [0142] Turbine (preferred mode). The peripheral speed at the atomization wheel conventionally varies between 10 and 200 m/s. Such spray systems make it possible to obtain a homogeneous jet. In this case, the distribution in drop sizes depends predominantly on the rotational speed of the turbine and very little on the pressure. It is thus pointless to add a pump upstream of the atomizer. With this type of spraying, which radially ejects the particles, it will be necessary to control the severity of the drying in order to ensure that the particles have formed a crust before touching the wall and thus to avoid fouling. [0143] Monofluid nozzle: the flow rate is adjusted by a screw pump in order to maintain an appropriate pressure. It is possible to adopt a cocurrent configuration or, preferentially, to pass in countercurrent, also called fountain mode, in order to increase the residence time.

[0144] For this atomization, the aim is to dry the suspension below the bubble point (to avoid internal boiling in the drop). The atomization is preferably carried out between 200 and 400° C. The residence times can vary from a few seconds to one minute (for example, conventionally 25 seconds). The atomization is carried out if the viscosity of the suspension is below 800-1000 cP at the atomization nozzle, ideally below 300 cP, which is the case of the suspension prepared according to the invention and originating from the preceding blocks.

EXAMPLES

[0145] The examples make use of the results of a rheokinetic study (shear 30 s.sup.−1) which made it possible to measure the viscosity of the reactive mixture between the peptized boehmite and the silicic acid over time. The ratio of the flow rates by volume is 3.22 (Q boehmite/Q silicic acid). The cell is a controlled strain rheometer (TA Instruments ARES) equipped with a single helical ribbon and with a 30 ml flat-bottomed vessel which is thermostatically controlled by the Peltier effect at 10° C.

[0146] Tests were carried out to define the viscosity windows where the mixture is atomizable. In the following examples, the preparation of the reactants, peptization of the boehmite, on the one hand, and preparation of the silicic acid from silicate on ion exchange resins, on the other hand, strictly follows the same protocol (see below). [0147] Peptization of the boehmite (block 1): [0148] Nature of the boehmite: boehmite sold by Axens under the trade name GA7001 (any other boehmite also being able to be suitable) [0149] Amount of boehmite: 13% by weight (the rest is made up of water). Preferably, for the boehmite GA7001, it remains less than 22%. [0150] Nitric acid: 9% equivalent weight/boehmite (other acids may also be suitable, such as weak or strong acids, inorganic or organic acids).

[0151] The mixture is stirred at 10° C. for 45 minutes. At the end of the peptization, the pH of the solution is equal to 3. By adsorption of H+ ions on the crystal faces, without any particular action, the pH tends to rise over time: after 5 hours, the pH has gone from 3 to 3.5, then it tends toward 3.8 after 10 hours.

[0152] According to the examples in accordance with the invention indicated below, at the end of the peptization, the pH of the solution is maintained in the vicinity of 3 during its preparation until the mixing with the silicic acid. Alternatively, it is brought back to the vicinity of 3 before mixing with the silicic acid if it has not been maintained at this pH during the peptization (if the pH during the peptization is markedly greater than 3 while remaining acidic): in both scenarios, it is maintained at/reduced to 2.86, a value close to the isoelectric point of silicic acid, in the following two examples. (The same results are obtained by maintaining the pH at/bringing the pH back to the value of 3, which is the isoelectric point, or values a little higher than this isoelectric point and of at most 3.1 or 3.2).

Example 1 (Comparative)

[0153] In example 1, the silicic acid is mixed with the other reactant once peptized without prior aging according to the invention. This is then referred to as “fresh” silicic acid, that is to say without intermediate storage or at most intermediate storage of a few hours at a temperature of at most 10° C. (between 0 and 10° C.). The silicic acid solution is at 60 g/l and its degree of conversion/condensation is thus equal to or in the vicinity of 0%.

Example 2 (The Invention)

[0154] In this example 2, the silicic acid is aged beforehand at 10° C. for 24 hours, with a conversion of the silicic acid of 15%.

[0155] FIG. 2 illustrates the rise in viscosity over time of the mixtures according to example 1 (curve C1) and according to example 2 (curve C2): In the case of example 1, the mixture leads to a low viscosity, below 30 cP. The viscosity does not exceed 50 cP after 3000 s. In the case of example 2, the mixture leads to a slower rise in the viscosity, which does not exceed 50 cP for the duration of the experiment. It is thus demonstrated that the same viscosity is obtained whether “fresh” silicic acid or silicic acid aged according to the invention is used, with the advantage, for the aged silicic acid, of a lower viscosity after 3000 seconds, optionally making it possible to further defer, if need be, the subsequent atomization stage. The powder grains obtained for examples 1 and 2 are regular and approach a spherical shape, their silica content is 9.75% by weight and their alumina content is 90.25% by weight. It is by virtue of the invention that it is possible to carry out a drying by atomization which makes it possible to obtain, in comparison with other drying processes, particles of more spherical and more regular shape and of more reproducible mean size.

Example 3 (According to the Invention)

[0156] This example 3 reproduces example 2, while modifying the conditions of the aging: in this instance, the silicic acid is aged beforehand at 60° C. for 4 hours, with a conversion of the silicic acid of 15%.

[0157] The results obtained are comparable to those obtained with example 2.

Example 4 (According to the Invention)

[0158] This example 4 reproduces example 3, while modifying the conditions of the aging: in this instance, the silicic acid is aged beforehand at 80° C. for 3 hours, with a conversion of the silicic acid of 15%.

[0159] The results obtained are comparable to those obtained with example 3.

[0160] In examples 5 and 6 below, unlike examples 1 to 4, at the end of the peptization, the pH of the boehmite solution is equal to 3.86 and is not maintained at 2.86. They show the impact of the aging of the silicic acid mixed with a first reactant peptized at higher pH, at a pH markedly greater than the isoelectric point (equal to 3) of silicic acid.

Example 5 (Comparative)

[0161] The conditions are the same as for example 1, with the use of a “fresh” silicic acid (0% conversion), except that the first reactant is at pH 3.86 during the mixing of the two reactants.

Example 6 (Comparative)

[0162] The conditions are the same as in the case of example 3, with silicic acid aged at 60° C. for 4 hours, to reach 15% conversion of the silicic acid, except that the first reactant is at pH 3.86 during the mixing of the two reactants.

[0163] As illustrated in FIG. 3, for these examples 5 (curve C3) and 6 (curve C4), the mixture rises rapidly in viscosity and reaches in a few seconds the range of “non-atomizability” (it is impossible to atomize the mixture at such viscosity levels) because the pH of the mixture in both cases is too high, above the isoelectric point of silicic acid. The plateau reached is, however, lower in the case of the converted silicic acid of example 4.

[0164] In conclusion, the aging (the partial conversion) of the silicic acid makes it possible both to extend the time for rise in viscosity of the reactive mixture of boehmite and silicic acid and to lower the viscosity of the final viscosity plateau of this, which is what makes it possible to be able to use the technology of atomization to make the grains from this mixture. In addition, it has been shown that, to do this, it is also important to control the pH of the mixture and that the most effective way of doing it is to control/regulate the pH of the alumina-based reactant, in particular in the vicinity of the isoelectric point of silicic acid (pH=3).

[0165] The invention thus makes it possible to control the viscosity of each of the reactants, one by controlled aging and the other by adjusting its pH, in order for the mixture to react without exceeding the maximum viscosity desired for the continuation of the process. The viscosity of the mixture is thus lowered, in comparison with a process without aging, and it also tends to “rise” later in time.