Zeolite adsorbents containing strontium

Abstract

The present invention relates to zeolite adsorbents based on agglomerated crystals of zeolite(s) comprising barium and strontium These adsorbents have applications in the separation of fractions of aromatic C8 isomers and in particular xylene.

Claims

1. An adsorbent comprising an agglomeration of zeolite crystals, the adsorbent comprising: crystals of FAU zeolite(s), with an Si/AI molar ratio of between 1.00 and 1.50, limits included; a weight content of barium ions (Ba.sup.2+), expressed by weight of barium oxide (BaO), greater than 30%, relative to the total weight of the adsorbent; and a weight content of strontium ions (Sr.sup.2+), expressed by weight of strontium oxide (SrO), greater than 0.1% and less than 3%, relative to the total weight of the adsorbent.

2. The adsorbent according to claim 1, having an Si/AI atomic ratio of between 1.00 and 2.00, limits included.

3. The adsorbent according to claim 1, wherein the total weight content of alkali or alkaline-earth metal ions other than Ba.sup.2+ and Sr.sup.2+, expressed by weight of alkali or alkaline-earth metal oxides, is less than 5%, relative to the total weight of the adsorbent.

4. The adsorbent according to claim 1, wherein the weight content of sodium ions (Na.sup.+), expressed by weight of sodium oxide (Na.sub.2O), is less than 0.3%, relative to the total weight of the adsorbent.

5. The adsorbent according to claim 1, wherein the weight content of potassium ions (K.sup.+), expressed by weight of potassium oxide (K.sub.2O), is less than 9%, relative to the total weight of the adsorbent.

6. The adsorbent according to claim 1, having a barium/strontium weight ratio of greater than 15:1.

7. The adsorbent according to claim 1, having a weight content of a non-zeolite phase (NZP) of less than 15%, relative to the total weight of the adsorbent, when the adsorbent is anhydrous.

8. A process for liquid-phase or gas-based production of purified para-xylene from a feedstock of aromatic hydrocarbons containing isomers comprising 8 carbon atoms that include para-xylene, the process comprising contacting the feedstock with the adsorbent according to claim 1.

9. A process for recovering purified para-xylene from a feedstock comprising fractions of aromatic isomers comprising 8 carbon atoms that include para-xylene, the process comprising the following successive steps: a) bringing the feedstock into contact with the adsorbent according to claim 1, b) bringing the adsorbent into contact with a desorbent in liquid phase or in gas phase.

Description

EXAMPLES

(1) The examples which follow illustrate the invention without, however, limiting it in any way whatsoever, and the scope of protection of which is specified by the appended claims.

(2) General Method for Preparing an Adsorbent According to the Invention Based on Zeolite X with an Si/Al Molar Ratio=1.25

(3) A homogeneous mixture is prepared and 800 g of zeolite NaX crystals are agglomerated according to the procedure described in patent application WO 2014/090771 (synthesis of example B) with 105 g of kaolin (expressed in calcined equivalent) and 45 g of colloidal silica sold under the trade name Klebosol®30 (containing 30% by weight of SiO.sub.2 and 0.5% of Na.sub.2O) with the amount of water which allows extrusion of the mixture.

(4) The extrudates are dried, crushed in such a way as to recover the grains of which the number-average diameter is equal to 0.5 mm, then calcined at 550° C. under a nitrogen stream for 2 hours.

(5) The agglomerate obtained (200 g) is placed in a glass reactor equipped with a jacket regulated at a temperature of 100° C.±1° C. 1.5 l of an aqueous sodium hydroxide solution having a concentration of 2.5 M are then added and the reaction medium is left to stir for a period of 4 hours.

(6) The agglomerates are then washed in water in 3 successive washing operations, followed by emptying of the reactor. The washing is known to have been efficient when the final pH of the washing waters measured is between 10.0 and 10.5.

(7) The sodium cations of the agglomerates obtained are exchanged at 95° C. with barium and strontium ions. For this, various amounts of strontium salt, of formula SrCl.sub.2.6H.sub.2O, are added to the barium salt, of formula BaCl.sub.2.2H.sub.2O, (pure, containing at most 0.2% by weight of strontium chloride), such that the mass percentage of SrCl.sub.2 salt is equal to the percentage indicated in Table 1 below.

(8) For example, an exchange solution is prepared by dissolving 150 g of BaCl.sub.2.2H.sub.2O salt with 1.4 g of SrCl.sub.2.6H.sub.2O salt in 1 l of water. The amount of strontium salt corresponds to 0.9% by weight relative to the total mass of salt. Next, 10 g of agglomerates prepared above are brought into contact with this solution in order to carry out the cation exchange.

(9) The exchange is carried out in 4 steps. At each step, the volume of solution to mass of solid ratio is 25 ml.Math.g.sup.−1 and the exchange is continued for 4 hours each time. Between each exchange, the solid is washed several times so as to remove the excess salt therefrom. The agglomerates are then dried at 80° C. for 2 hours and, finally, activated at 250° C. for 2 hours under a nitrogen stream

(10) The loss on ignition measured, as described above, is 5.5%±0.1% for each sample. The degree of barium+strontium exchange of the adsorbents is calculated from the elemental analyses by X-ray fluorescence of the barium, strontium and sodium oxides as described in the characterization techniques.

(11) In the example given above, the degree of barium exchange is 95.1%, and the degree of strontium exchange is 4.0%.

(12) The other examples are carried out starting from 150 g of barium salt, to which is added the amount of SrCl.sub.2 salt which makes it possible to obtain the weight % values that are indicated for the examples according to the invention and the comparative examples.

Reference Example A

(13) This example corresponds to Example 1 of application WO 2014/090771 and corresponds to an adsorbent exchanged with barium alone, with a very low sodium content (cf. Table 1).

(14) The degree of barium exchange calculated according to the X-ray fluorescence analysis of this agglomerate is 99.1% and the loss on ignition is 5.4%.

Reference Example B

(15) This second reference example is carried out in a manner identical to Example 1 of application WO 2014/090771, but is stopped at the second exchange. The agglomerates are involved in a reaction of cation exchange through the action of a 0.5 M aqueous barium chloride solution at 95° C. in only 2 steps. At each step, the volume of solution to mass of solid ratio is 20 ml.Math.g.sup.−1 and the exchange is continued for 4 hours each time. Between each exchange, the solid is washed several times so as to remove the excess salt therefrom. The agglomerates are then dried at 80° C. for 2 hours and, finally, activated at 250° C. for 2 hours under a nitrogen stream.

(16) The degree of barium exchange of this agglomerate is 92.6% and the loss on ignition is 5.5%.

(17) Examples 1 to 5 and the comparative example are carried out according to reference example A using solutions of barium chloride containing increasing contents of strontium chloride, in order to simulate solutions of barium chloride containing strontium as impurity.

(18) The details of each of Examples 1 to 5 and of the Comparative Example are reproduced in Table 1 below:

(19) TABLE-US-00001 TABLE 1 Example % SrCl.sub.2 % BaO % SrO % Na.sub.2O Ba/Sr Example A 0 37.6 0 <0.1 0 Example B 0 34.3 0 1.1 0 Example 1 0.4 35.8 0.5 <0.2 75.8 Example 2 0.9 34.9 1.0 <0.2 35.0 Example 3 1.7 34.8 1.7 <0.2 21.1 Example 4 2.0 34.8 2.0 <0.2 18.4 Example 5 2.5 34.2 2.4 <0.2 15.1 Comparative 4.8 33.8 3.7 <0.2 9.7 Example

(20) In Table 1 above: % SrCl.sub.2 denotes the weight percentage of strontium chloride hexahydrate in the barium chloride solution (comprising barium and impurities) used to carry out the cation exchange; % BaO denotes the weight percentage of barium oxide relative to the total weight of the anhydrous adsorbent; % SrO denotes the weight percentage of strontium oxide relative to the total weight of the anhydrous adsorbent; % Na.sub.2O denotes the weight percentage of sodium oxide relative to the total weight of the anhydrous adsorbent; Ba/Sr denotes the barium/strontium weight ratio in the anhydrous adsorbent.
Breakthrough Test

(21) A breakthrough test (frontal chromatography) is then carried out on the agglomerates obtained in Example 1 in order to evaluate their efficiency. The amount of adsorbent used for this test is approximately 30 g.

(22) The procedure for obtaining the breakthrough curves is the following: Filling the column with the sieve and placing in the test bench. Filling with a solvent (toluene) at ambient temperature. Gradual increase to the adsorption temperature under a stream of solvent (2 cm.sup.3.Math.min.sup.−1). Injection of solvent at 2 cm.sup.3.Math.min.sup.−1 when the adsorption temperature is reached. Solvent/feedstock permutation to inject the feedstock (2 cm.sup.3.Math.min.sup.−1). Injection of the feedstock is then maintained for a time sufficient to reach thermodynamic equilibrium. Collection of the breakthrough product in a single flask then analysis of the composition of the product by GC.

(23) The pressure is sufficient for the feedstock to remain in the liquid phase, i.e. 1 Ma. The adsorption temperature is 175° C. The composition of the feedstock used for the tests is the following: Para-xylene: 18% by weight Meta-xylene: 18% by weight Ortho-xylene: 18% by weight Ethylbenzene: 18% by weight Para-diethylbenzene: 18% by weight Isooctane: 10% by weight (this is used as a tracer for estimating the non-selective volumes and is not involved in the separation).

(24) The binary selectivities of the compounds in pairs, denoted binary selectivities α.sub.l/k, are calculated from the adsorbed amounts q.sub.i and q.sub.k of the compounds i and k, the latter being determined by the material balance from the analysis of the composition of the breakthrough product and of the composition of the feedstock (in which feedstock the mass fraction of the compounds i and k is y.sub.i and y.sub.k):

(25) ${\alpha }_{i/k}=\frac{q_i{y}_k}{q_k{y}_i}$

(26) The evaluation of the potential of these adsorbents during the simulated counter-current implementation is carried out on the basis of the equilibrium theory applied to multicomponent systems with constant selectivities, as described by Mazotti, Storti and Morbidelli in “Robust Design of Countercurrent Adsorption Separation Processes: 2. Multicomponent Systems”, AIChE Journal, (November 1994), Vol. 40, No. 11.

(27) In particular, reference is made in this case to equation 8, which describes the conditions to be met with regard to the reduced flow rates m.sub.j of the 4 sections (j=1 to j=4) of a counter-current separation unit as represented diagrammatically in figure 1 of the article cited, in order to obtain complete separation.

(28) $\begin{array}{cc}\mathrm{Section}1\text{:}{K}_{ss}<m_i{\delta }_1<+\infty \mathrm{Section}2\text{:}{K}_{\mathrm{wk}}<m_2{\delta }_2<K_{sk}\mathrm{Section}3\text{:}{K}_{\mathrm{wk}}<m_3{\delta }_3<K_{sk}\mathrm{Section}4\text{:}-\frac{{ϵ}_p{\delta }_4}{\sigma \left(1-{ϵ}_p\right)}<m_4{\delta }_4<K_{\mathrm{ww}}.& \left(8\right)\end{array}$

(29) This equation 8 refers to the adsorptivities K.sub.i of the various constituents, and also to the parameter δ.sub.j of each section j defined by equation 7:

(30) $\begin{array}{cc}{\delta }_j=\underset{I=1}{\overset{\mathrm{NC}}{.Math.}}{K}_{I}y\{\left(j=1,\mathrm{.Math.},4\right),& \left(7\right)\end{array}$

(31) It should be noted here that, by definition, the binary selectivity α.sub.i/k between the compounds i and k is equal to the ratio of the adsorptivities K.sub.i/K.sub.k.

(32) The reduced flow rate “m” of each section of the unit is defined as being the ratio of the flow rate of the liquid phase to the flow rate of the adsorbed phase. Equation 8 indicates which are the limiting reduced flow rates for each section. In a 4-section counter-current separation unit, the feedstock flow rate corresponds to the difference between the reduced flow rate in zone 3 and the reduced flow rate in zone 2.

(33) Consequently, when it is desired to evaluate the maximum productivity that can be achieved with a given adsorbent, it is sought to evaluate the maximum amount of feedstock that it will be possible to treat, that is to say to evaluate the difference between the maximum reduced flow rate in zone 3 and the minimum reduced flow rate in zone 2.

(34) It will be possible to compare the performances in terms of maximum productivity of two adsorbents by comparing their maximum reduced flow rate of feedstock determined from the reduced flow rates of zones 2 and 3, respectively m.sub.2 and m.sub.3, according to the relationship:
max(m.sub.Feedstock)=max(m.sub.3)−min(m.sub.2).

(35) If a constant-selectivity system is considered, the composition of the liquid phase which gives the highest stress in zone 2 and in zone 3 is the composition of the liquid phase at the point of injection of the feedstock into the unit. Indeed, starting from this point, the concentration of para-xylene, which is the compound most adsorbed, increases in the direction of circulation of the solid in zone 2 and decreases in the direction of circulation of the liquid in zone 3. The composition at this point can be approximated to the composition of the feedstock to be treated, and it is this composition that will be used to evaluate the terms δ.sub.2 and δ.sub.3 of equation 8. The terms δ.sub.2 and δ.sub.3 being defined by equation 7 mentioned above.

(36) For each adsorbent, this max reduced flow rate (m.sub.Feedstock) is calculated from the values of binary selectivities measured experimentally. Table 2 makes it possible to compare the maximum reduced flow rate of feedstock “max(m.sub.Feedstock)” for each of the adsorbents tested. The maximum reduced flow rate of feedstock “max(m.sub.Feedstock)” is representative of the productivity; the higher its value, the better the productivity.

(37) TABLE-US-00002 TABLE 2 α.sub.PX/MOX α.sub.PX/EB max(m.sub.Feedstock) Example A 3.10 2.30 1.17 Example B 2.92 2.23 1.11 Example 2 3.26 2.26 1.18 Comparative 2.85 2.19 1.08 example

(38) It is noted that the max(m.sub.Feedstock) reduced flow rate remains substantially the same whether it is an adsorbent of which the ion exchange has been carried out with a solution of barium chloride that is “pure” (i.e. free of strontium impurity) (Example A) or an adsorbent prepared from a solution of barium chloride that also contains strontium chloride (Example 2).

(39) On the other hand, when the barium chloride contains sodium at contents comparable to strontium (owing to the partial exchange with barium), it is noted that the max(m.sub.Feedstock) productivity is reduced. This observation effectively confirms, in addition to the effect on selectivity, the advantage of barium exchange for separating xylenes, which is perfectly known to those skilled in the art.

(40) Likewise, it has been observed that, when the content of strontium impurities in the barium chloride solution is too high, in particular greater than 4% (cf. Comparative example, strontium chloride content >4.8%), the max(m.sub.Feedstock) productivity drops drastically.

(41) These examples confirm in all respects the subject of the present invention and make it possible to demonstrate that it is entirely possible to envisage the presence of strontium ions in barium-exchange adsorbents that can be used for separating xylenes, without however affecting the para-xylene productivity. Moreover, an improvement in selectivity for para-xylene is obtained.