PROCESS FOR CONTINUOUSLY SYNTHESIZING ZEOLITE CRYSTALS

Abstract

The present invention relates to a process for preparing zeolite crystals continuously, comprising the continuous introduction of a composition capable of generating zeolite crystals into at least one crystallization reaction zone subjected to stirring means, giving said composition a flow characterized by a relative Reynolds number Re.sub.r of between 40 and 50 000, and the continuous recovery of the crystals formed according to a flow characterized by a net Reynolds number Re.sub.n of between 1 and 1500.

Claims

1. Process for preparing zeolite crystals continuously, comprising at least the following steps: a) continuous supply of a composition capable of generating zeolite crystals; b) continuous introduction of said composition into at least one crystallization reaction zone subjected to stirring means, giving said composition a flow characterized by a relative Reynolds number Re.sub.r of between 40 and 50 000, limits included, c) continuous recovery of the crystals formed in step b) according to a flow characterized by a net Reynolds number Re.sub.n of between 1 and 1500, limits included.

2. Process according to claim 1, wherein the difference between the relative Reynolds number Re.sub.r and the net Reynolds number Re.sub.n is greater than 50.

3. Process according to claim 1, for preparing crystals of a zeolite of MFI type, a zeolite of MOR type, of OFF type, of MAZ type, of CHA type and of HEU type, a zeolite of FAU type, a zeolite of EMT type or a zeolite of LTA type, and also the other zeotypes.

4. Process according to claim 3, for preparing zeolite crystals, where the zeolite is chosen from zeolites of MFI type, of FAU type, of LTA type, the zeolites of CHA type and the zeolites of HEU type.

5. Process according to claim 1, comprising at least the following steps: 1) continuous preparation of a composition capable of generating zeolite crystals, in order to obtain a synthesis medium, 2) addition of seed to the synthesis medium obtained in step 1), 3) continuous operation, under stirring, of the reaction for forming zeolite crystals, according to steps a), b) and c) according to claim 1, 4) continuous filtration in order to separate the crystals obtained from mother liquors, and 5) optionally recycling of the mother liquors.

6. Process according to claim 1, wherein the crystallization step is carried out at a temperature of between 60 C. and 200 C.

7. Process according to claim 1, further comprising the addition, in one or more stages, before, after or during the crystallization step, of one or more seeding agents.

8. Process according to claim 8, wherein the seeding agent is chosen from a nucleating gel, a crystal, a mineral particle and mixtures thereof.

9. Process according to claim 1, which process is carried out in a tubular reactor optionally provided with restrictions, and equipped with a stirring system, an oscillating or pulsating system, and also two or more of these techniques combined.

10. Process according to claim 1, carried out in a tubular reactor provided with internal systems of restrictions and with a pulsating device, and operated under particular conditions, namely: an oscillation amplitude of the pulsating device of between 20 mm and 400 mm, and an oscillation frequency of between 0.1 Hz and 2 Hz.

Description

CHARACTERIZATION TECHNIQUES

Qualitative and Quantitative Analysis by X-Ray Diffraction (XRD)

[0079] The purity of the zeolite crystals synthesized is evaluated by x-ray diffraction analysis, known to a person skilled in the art under the acronym XRD. This identification is carried out on a Bruker XRD apparatus.

[0080] This analysis makes it possible to identify the various zeolites present in the adsorbent material since each of the zeolites has a unique diffractogram defined by the positioning of the diffraction peaks and by their relative intensities.

[0081] The zeolite crystals are ground and then spread and levelled out on a sample holder by simple mechanical compression.

[0082] The conditions under which the diffractogram is acquired on the Bruker D5000 machine are as follows:

Cu tube used at 40 kV30 mA;
slit size (divergent, scattering and analysis slits)=0.6 mm;
filter: Ni;
sample device rotating at: 15 rpm;
measuring range: 3<2<50;
increment: 0.02;
counting time per increment: 2 seconds.

[0083] Interpretation of the diffractogram obtained is performed with the EVA software with identification of the zeolites using the ICDD PDF-2 release 2011 base.

[0084] The amount of crystals, by weight, is determined by XRD analysis; this method is also used to measure the amount of noncrystalline phases. This analysis is performed on a Bruker machine, and the amount by weight of the zeolite crystals is then evaluated using the TOPAS software from the company Bruker. The purity is expressed as a weight percentage of desired crystalline phase relative to the total weight of the sample.

EXAMPLE 1 (ACCORDING TO THE INVENTION): CONTINUOUS SYNTHESIS OF ZEOLITE A

[0085] The continuous synthesis of zeolite A consists in supplying a tubular reactor (internal diameter=1.5 cm and length 20 m) with a synthesis medium defined below added to which is 1% by weight of nucleating gel clarified below.

[0086] The nucleating gel is prepared by adding a solution of sodium silicate at 35 C. to a solution of sodium aluminate at 35 C. so as to obtain a gel with the composition: 2.66 Na.sub.2O/Al.sub.2O.sub.3/1.92 SiO.sub.2/65 H.sub.2O.

[0087] The solution of sodium aluminate is prepared by dissolving alumina in a solution of sodium hydroxide at boiling point then cooling to 35 C. This solution contains 938.7 g of alumina, 1539.0 g of an aqueous solution of sodium hydroxide at 50% by weight and 1542.6 g of water.

[0088] The solution of sodium silicate is prepared by mixing 2601 g of sodium silicate with 486 g of an aqueous solution of sodium hydroxide at 50% by weight and 2160 g of water, then heating to 35 C.

[0089] The nucleating gel is kept at 35 C. for 2 hours and then cooled to 25 C. and stored for 20 hours at 25 C. This solution may then be used as seed in the synthesis of zeolite A by continuously adding it to the synthesis medium with a content equal to 1% by weight relative to the weight of the synthesis medium.

[0090] The synthesis medium is prepared using a rotor-stator shear mixer, of which the diameter of the rotor is 38.1 mm and the distance of the gap between the rotor and the stator is 0.2 mm. A solution of sodium aluminate at 35 C. and a solution of sodium silicate at 35 C. are mixed simultaneously so as to obtain a gel with the composition: 3.5 Na.sub.2O /Al.sub.2O.sub.3/2.0 SiO.sub.2/175 H.sub.2O.

[0091] The solution of sodium aluminate is prepared by dissolving alumina in a solution of sodium hydroxide at boiling point then cooling to 35 C. This solution contains 31 824 g of alumina, 86 402 g of an aqueous solution of sodium hydroxide at 50% by weight and 273 600 g of water.

[0092] The solution of sodium silicate is prepared by mixing 91 152 g of sodium silicate with 8641 g of an aqueous solution of sodium hydroxide at 50% by weight and 254 880 g of water, then heating to 35 C.

[0093] In order to prepare the synthesis medium, the chamber of the Silverson shear in-line mixer is simultaneously supplied using two peristaltic pumps: the flow rate of the aluminate solution is equal to 220.5 g.Math.min.sup.1 and that of the silicate solution is equal to 211.5 g.Math.min.sup.1. The supply lines are first filled with water.

[0094] The mixing is carried out with a rotor speed of 550 rev.Math.min.sup.1, which corresponds to a shear rate of 54 800 s.sup.1. The synthesis medium continuously supplies the static mixer into which the nucleating gel is introduced at 25 C. with a flow rate of 4.32 g.Math.min.sup.1 (1% by weight of the synthesis medium).

[0095] The stream that leaves the static mixer consisting of the mixture of the synthesis medium and the nucleating gel has a density of 1200 kg.Math.m.sup.3 and a viscosity of 5 mPa.Math.s. A weight fraction of 1/9 of this stream directly supplies the pulsed tubular reactor.

[0096] The pulsed tubular reactor is initially heated by operating the oscillator (amplitude 50 mm and frequency 0.4 Hz). Water is firstly circulated while adjusting the temperature of the oil bath to 108 C. so as to obtain a temperature on the inside of 100 C. The pilot plant is then supplied with the mixture of the synthesis medium and the nucleating gel produced using a 17-element static mixer made of 316 stainless steel having a length of 125 mm sold by Fisher Scientific (reference 1174-4119) while maintaining an oscillation having an amplitude of 50 mm and a frequency of 0.4 Hz. With these operating conditions, the net Reynolds number (Re.sub.n) is equal to 13.7 and the relative Reynolds number (Re.sub.r) is equal to 240.

[0097] After 24 hours of continuous operation, no pressure increase is observed in the equipment, as indicated on the pressure manometer installed at the inlet of the tubular reactor. The feed flow rate of 2.4 L.Math.h.sup.1 makes it possible to obtain a residence time in the tubular reactor of 90 minutes, a sufficient residence time to obtain a crystallized zeolite LTA. The reaction medium withdrawn at the outlet of the tube is filtered and washed with water until a neutral pH is obtained, then dried at 80 C. and activated at 550 C.

[0098] In order to empty the equipment, cold water is circulated with a flow rate of 60 L.Math.h.sup.1 for 1 hour. The crystals of zeolite LTA obtained have a crystallinity of 99%.

EXAMPLE 2 (COMPARATIVE): CONTINUOUS SYNTHESIS OF ZEOLITE A

[0099] When the continuous synthesis is carried out according to the operating conditions of example 1, but by reducing the amplitude of the oscillator to 20 mm, then, as indicated on the pressure manometer installed at the inlet of the tubular reactor, pressure increases are observed after only 2 hours of operation, which result in a plugging of the equipment after 3 hours and 15 minutes of synthesis. With these operating conditions, the net Reynolds number is equal to 13.7 and the relative Reynolds number is equal to 38.

EXAMPLE 3 (ACCORDING TO THE INVENTION): CONTINUOUS SYNTHESIS OF ZEOLITE X

[0100] By reproducing the operating conditions of example 1, and by modifying the nature of the composition of the starting reaction medium, a synthesis of zeolite of faujasite X type is carried out continuously for 24 hours. The crystals of zeolite X obtained at the outlet of the tubular reactor have a crystallinity of 99%. The diffractogram of these crystals is presented in the FIGURE.