Title of Invention

AGGLOMERATED ZEOLITIC ADSORBANTS AND A PROCESS FOR THEIR PREPARATION

Abstract

Agglomerated zeolite adsorbents and a process for their preparation The present invention relates to Agglomerated zeolite adsorbents comprising at least 70% of faujasite with an Si/Al atomic ratio such that 1≤Si/Al ≤ 1.15, in which the exchangeable sites are at least 70% occupied b barium ions (the remainder generally being made up of alkali metal i y ons or alkaline-earth metal ions other than barium) and agglomerated with a binder and which adsorbents have a loss on ignition, measured at 900°C, of between 4.0 and 7.7%.

Full Text

The field of the invention is that of zeolite adsorbents for the separation of xylenes, in particular with a view to the industrial production of para-xylene. High-purity para-xylene is reclaimed industrially, inter alia, in order to be converted into terephthalic acid intended for the manufacture of PET. The prior art has recognized that adsorbencs consisting of zeolites X or Y exchanged using ions gulch as barium, potassium or strontium, alone or as a mixture, are efficient at selectively adsorbing para-xylene in a mixture containing at least one other aromatic Ce isomer. US Patents 3,558,730, 3,558,732, 3,626,020 and 3,663,638 disclose adsorbents comprising aluminosilicates exchanged with barium and potassium, which efficiently separate para-xylene from a mixture of aromatic Ce isomers. These adsorbents are used as adsorption agents in liquid-phase processes, preferably of the simulated counter-current type similar to those described in US 2,965,589, which are applied, inter alia, to the aromatic C^ fractions derived, for example, from processes for the dialkylation of benzene, in gas-phase processes. The performance of the industrial process for the separation of para-xylene depends largely on the ■ adsorbent, on its adsorbing capacity and on the selectivity which it shows for para-xylene in a medium consisting of Cg aromatics, typically para-xylene itself (PX), met^-xylene (MX), ortho-xylene (OX) and ethylbenzene (EB), as well as on the ability of desorbing, such as toluene and para-diethylbenzene, to desorb the adsorbed para-jcylene therefrom. The selectivity Sel(B/A) of the adsorbent for a compound (B) relative to a compound (A) is defined as the ratio of the concentrations of the compounds in the adsorbed phase divided by the ratio of concentrations of the compounds in the non-adsorbed phase at equilibrium. The selectivity equation is as follows: Sel{B/A) = (B)Z / (A)2 {B)s / (A)S where {B}^ and (B)s represent the concentrations of B in the zeolite and In the solution, respectively, where (A)^ and {A)s represent the concentrations of A in the zeolite and in the solution. The method for evaluating these magnitudes is outlined later. The zeolites encountered in the prior art for the separation of xylenes belong to the faujasiCe structural type, first described in US 2,882,244 and US 3,130,007, which are crystalline silicoaluniinates having cages of fully defined size connected three-dimensionally. The faujasites correspond to the general formula: (1 ± 0.1) M2,,nO ; AI2O3 ; W SiOj ; y HzO in which M represents at least one alkali metal cation or alkaline-earth metal cation of valency n, Y is less than or equal to 8 depending on the nature of M and the degree of hydration of the crystal, W is the factor which makes it possible to distinguish between silica-rich faujasites (faujasites Y) and alumina-rich faujasites (faujasites X]. Faujasites X should be placed in the range W 3, which is interpreted, rather, according to the Si/Al ratio on either side of 1.5. For the purposes of the present invention, a.n additional convenient distinction is introduced with faujasites with a low silica content (which will be referred to by the abbreviation LSX used by those skilled in the art, meaning Low Silica X), for W The prior art has recognized, as adsorbent for the separation of xylenes, only those faujasites exchanged with barium with Si/Al atomic ratios of between 1.2 and 2.6. in this matter, a person skilled in the art does not appear to have available any simple and sufficiently reliable criterion for predicting their behaviour. Thus, the search for increasingly improved adsorbents proceeds by more or less random research, such as, for example, the result disclosed and claimed by US 3,878,127 with an actual barium exchange of zeolite X pretreated with a sodium hydroxide solution. The Applicant proposes adsorbents which offer para-xylene/meta-xylene or para-xylene/ ortho-xylene selectivities of at least 2.0 and, advantageously, of least 2.5, measured according to the test described in the examples. The invention achieves this result, with a number of advantages which will become apparent in the description. The subject of the present invention is agglomerated zeolite adsorbents comprising at least 70% and preferably at least 80% of faujasite with an Si/Al atomic ratio such that 1 preferred faujasites being those in which the overall level of exchange for barium alone or for barium + potassium is greater than or equal to 90%. The agglomerated zeolite adsorbents according to the invention have a loss on ignition, measured at 900° C, between 4.0 and 7.7 and preferably between 5.2 and 7.7%. One process for the preparation of the agglomerated zeolite adsorbents according to the invention consists, firstly, in agglomerating zeolite powder with an Si/Al ratio such that 1 The term agglomeration is understood to refer to the production of solid particles from a mixture of zeolite(s) and binder{s) using any technique known to those skilled in the art, such as extrusion, granulation, compacting or spraying. The practical content of binder in the agglomerate generally does not exceed 30%, and preferably 20%, of the total mass of the adsorbent. The efficiency of these adsorbents is improved substantially by selecting a clay of the kaolin family as agglomeration binder, in practice kaolinite or halloysite, and by subjecting the granules to zeolitization. Zeolitization of the binder is performed by immersing the agglomerate in an alkaline liquor, sodium hydroxide or a mixture of sodium hydroxide and potassium hydroxide whose concentration is preferably at least 0.5 M, after the grains have been calcined, the first result of this first calcination being to, harden the grain, but also to activate the clay by converting it into meta-kaolin. The zeolitization is— preferably carried out under hot conditions since working at the higher temperature improves the kinetics of the process and reduces the inrnersion times. Zeolitizations of at least 50% of the binder, that is to say that the resulting adsorbent generally consists of at least 85% and preferably at least 90% zeolite of active faujasite type and of not more than 15%, preferably not more than 10% of material which is inactive for the adsorption, are thus readily obtained. The barium exchange is performed in an entirely conventional manner, preferably by successive exchanges so as to reach a minimum target exchange level of at least 70% and preferably at least 90%. The potassium exchange can be performed before or after the barium exchange, but it is also possible to agglomerate faujasite LSX powder already containing potassium ions. The activation is the final step in the production of the adsorbents of the invention. Its aim is to fix the water content, more simply the loss on ignition of the adsorbent, within optimal limits. The most practical way of proceeding is by thermal activation, which is preferably carried out between 180 and 2S0°C. The invention also consists of an improvement to the process for recovering para-xylene from aromatic Cs isomer fractions, which consists in using, as adsorption agent, a zeolite adsorbent based on faujasite with an Si/Al ratio such that 1 S Si/Al The desired product can thus be separated out by preparative adsorption liquid chromatography (batchwise), advantageously in a simulated fluid bed, i.e. one with a simulated counter-current or a simulated co-current, and more particularly with a simulated counter-current. The operating conditions of an industrial unit for adsorption of simulated counter-current type are generally as follows: Number of beds 6 to 3 0 Number of zones at least 4 Temperature 100 to 250^0, preferably 150 to igcc Pressure 0.2 to 3 MPa Flow rates of desorbent l to 2.5 to feedstock [for example 1.4 to 1.8 for an adsorption unit alone (stand-alone) and 1.1 to 1.4 for an adsorption unit coinbined with a crystallization unit) Level of recycling 3.5 to 12, preferably 4 to 6 Reference may be made to US patents 2,985,589, 5,284,992 and 5,629,467. The operating conditions for a simulated co-current adsorption, industrial unit are generally the same as those operating in. simulated counter-current, except for the level of recycling, which is generally between 0.8 and 7. Reference may be made to US patents 4,402,832 and 4,498,991. The desorption solvent can be a desorbent whose boiling point is less than that of the feedstock such as toluene, but also a desorbent whose boiling point is greater than that of the feedstock, such as para-diethylbenzene (PDEB). Paujasites with an Si/Al ratio more or less equal to 1, which are prepared according to the production method described in European patent EP 486,384 or US patent 5,173,462, are preferred here. The selectivity of the adsorbents according to the invention for the adsorption of the p-xylene contained in Cg aromatic fractions Is optimal when rheir loss on ignicion, measured at 900"=C, is between 4.0 and 7.7%, and preferably between 5.2 and 7.7%. Water and a small amount of carbon dioxide form part of the loss on .ignition . The non-limiting examples which follow will allow the invention to be understood more clearly. EXAMPLES These examples involve the measurement or evaluation of certain characteristic magnitudes of the adsorbents of the invention. in order to evaluate the selectivity afforded by the adsorbent in a process for the separation of para-xylene, it is subjected to a test which makes it possible to measure its separating power between para-xylene (PX) and the aromatic Cg isomers thereof (MX, OX), but also between para-xylene and ethylbenzene {EB), which is important since certain fractions can be rich in ethylbenzene and not in other Ca isomers, and also between para-xylene and the desorbent, since it is Just as important to have available a low px/desorbent selectivity, which is the condition for the desorption to be effective. The test consists in immersing an adsorbent (17 grams), thermally preactivated and cooled in the absence of air, in 80 g of a mixture of aromatics dissolved in 2,2,4-trimethylpentane. The exact composition of the mixture is as follows PX 2% MX 2% OX 2% EB 2% desorbent (toluene or 2% p-diethylbenzene) 2,2,4-trimethylpentane remainder The mixture is autoclaved at 150°C for 4 hours, which is long enough to ensure that the adsorption reaches equilibrium. Some of the liquid is then removed, condensed at -30°C and analysed by gas chromatography. It is then possible to return to the concentrations in the adsorbed phase and in the non-adsorbed phase and to express the amount of para-xylene adsorbed and the selectivities for para-xylene relative to the other aromatics and to the desorbent. The 2,2,4- trimethylpentane does not disrupt these results since it is adsorbed very little. For Examples 1 to 9, the desorbent used is toluene, and paradiethylbenzene for Example 10. Accordingly the present invention relates to anAgglomerated zeolite adsorbents comprising at least 70% of faujasite with an Si/Al atomic ratio such that I EXAMPLE 1: Preparation of a control adsorbent An industrial zeolite NaX, with an Si/Al ratio = 1.25 and an Na/Al ratio = 1, is agglomerated by intimately mixing 850 grams of zeolite x powder (expressed as calcined equivalent), 150 grams of Charentes kaolinite (expressed as calcined equivalent) and 6 grams of carboxymethyl cellulose (retention adjuvant intended to retain the water during the extrusion operation) with the appropriate amount of water for the extrusion. The extrudate is dried, crushed so as to recover grains whose equivalent diameter is equal to 0.7 mm, and then calcined at 550°C under a stream of nitrogen for 2 hours. Its toluene-adsorbing capacity, determined at 25°C and under a partial pressure of 0.5, is 20.2%; this is interpreted as a micropore volume of 20.2/0.86 = 0.23S cm^/g (in, calculating the pore volume, it is considered that the density of the liquid phase is identical to the density of the adsorbed toluene, i.e. 0.86) . This granule is exchanged using a 0.5 M/1 barium chloride solution at 95°C in 4 steps. At each step, the ratio by volume of solution to the mass of solid is 20 ml/g and the exchange is continued for 4 hours each time. Between each exchange, the solid is washed several times in order to free it of the excess salt. It is then activated at a temperature of 2S0°C for 2 hours under a stream of nitrogen. Its toluene-adsorbing capacity is 14.8%, which equates to a micropore volume of 0.17 cm"/g. The loss on ignition is also measured, which is an important magnitude since it gives an estimate of the residual water present on the adsorbent: a loss on ignition of 4.5% is found here. Application of the selectivity test leads to the following results: F Isomers Selectivity PX/OX 2.25 PX/MX 2.12 PX/EB 1.77 1.52 The amount of para-xylene adsorbed is equal to 0.054 cm^/g- The effective zeolite content in this adsorbenC is close to 85%. EXAMPLE 2: Preparation of an adsorbent according to the invention 950 grams (calcined eqiiivalent) of a zeolite X with an Si/Al ratio = 1.01, obtained according to the process described in European Patent EP 0,486,384 or US Patent 5,173,462, are agglomerated with 170 grams (calcined equivalent) of Charentes kaolinite, 6 grams of carboxymethylcellulose and the appropriate amount of water to correctly extrude the paste obtained. The extrudates are then dried, after which they are calcined at a temperature o£ 600°C for 2 hours under a stream of dry nitrogen. They are then crushed in order to bring the equivalent diameter of the particles to 0.7 mm. The crushed material thus obtained is subjected to the barium exchange treatment already desciribed in Example 1 and is heat-activated at a temperature of 22Q°C. The product thus obtained has a loss on ignition of 5* and a toluene-adsorting capacity of 13% (0.15 cm^/g micropore volume}- The adsorbent satisfies the selectivity test with the following values: Isomers ^ 1^ ^ Selectivity PX/OX 2.60 PX/MX 2.55 PX/EB 2.80 PX/Tol 2.00 The amount of para-xylene a.dsorbed during the test is 0.057 cm""/g, i.e. the same as that measured on the adsorbent of Example 1 despite the difference in micropore volume. Better selectivity towards ethylbenzene is also noted, which may be advantageous when loads rich in this isomer need to be treated. The value for the selectivity of para-xylene with respect to toluene is entirely favo-urable to desorption of the para-xylene by means of a reasonable consumption of desorbent. EXAMPLE 3: Preparation of an adsorbent according to the invention As above, 950 grams of a zeolite X with an Si/Al ratio = 1.01 are agglomerated with 170 grams of Charentes kaolinite, 6 grams of carboxymethylcellulose and the appropriate amount of water. The mixture is extruded. The extrudates are dried, calcined at a temperature of 600®C for 2 hours under a stream of dry nitrogen and are then crushed so as to bring their equivalent diameter to 0.7 mm. 10 grams of these agglomerates are then immersed in 17 ml of a 220 g/1 sodiiim hydroxide solution for 3 hours at 95°C- They are then washed four times with water. In order to estimate the effectiveness of the zeolitization, a. small portion of the product is heated to SSO"^C under a stream of dry nitrogen and a toluene-adsorbing capacity of 21,6% is determined. The overall active zeolite content is estimated at 95%, i.e. greater than its initial content in the agglomerated adsorbent. The solid is then exchanged with barium under the same conditions as those outlined in Example 1. After activation under dry nitrogen at 220°C for 2 hours, a toluene-adsorbing capacity of 15% {micropore volume: 0.175 cmVg) and a loss on ignition of 5.2% are measured. The adsorbent thus prepared is evaluated by the selectivicy tesc. The following are obtained: Isomers Selectivity | PX/OX 2.64 PX/MX 2.60 PX/EB 2.75 PX/Tol 1.94 1 The amount of para-xylene adsorbed during the test is equal to 0.066 cm/g. The seleccivities with respect to the various isomers compare favourably with that of the adsorbent of Example 2, which reflects that the active element of the two products is a zeolite hSX, The substantial gain in para-xylene absorbed is the consequence of the richness in LSX on account of the zeolitization of the binder. EXAMPLES 4_to 2= Preparation and test of adsorbents according to the invention which have undergone various final calcinations The sample preparation is repeated a.s in Example 3, the only difference being that the activation temperature was varied between 180® and 300°C: 180°C for Example 4, 200°C for Example 5, 220°C (Example 3 repeated), 250°C for Example 6 and 300°C for Example 7, The compared characteristics of these products are given in the table below. ] E^camples 4 180 5 200 3 220 _ 6 ^^^ 2S0 7 1 1 Activation temperature CC) 300 1Microporos i ty 0.132 0.17 0.175 0.17S 0.185 Para-icylene adsorbed 0-0506 0,065 0.066 0.062 0.051 PX/OX 2.91 4.21 2.64 2.17 1.45 PX/MX 3.06 3.11 2.60 2.01 1.55 PX/EB 2.00 2.37 2.75 2,76 2.34 PX/Tol 2.28 1.55 1.94 1.90 1.19 Loss on ignition (%) 7.7 6.6 5.2 4.4 2.4 1 The preferred PX/OX and PX/MX target selectivity of at least 2.5 is satisfied for these Zeolites when the loss on ignition (roughly speaking, the water content) measured after the step of activation of the exchanged product is from 5.2 to 7.7%, which is achieved in practice by heat-activation at a temperature of between 180 and 220°c. The preferred PX/EB target selectivity of at least 2.5 is satisfied for these zeolites when the loss on ignition (roughly speaking, the water content) is £rom 4.4 to 5.2%, which is achieved in practice by heat-activation at a temperature of between 220 and 250 The activation at 250°C has a mildly adverse effect on these performance levels but the product nevertheless remains advantageous for treating fractionfi t:hat are relatively rich in ethylbenzone, SXAMPt^E 8 Agglomerated samples are prepared as indicated in Example 3, which are exchanged with potassium and then according to the procedure below: A rigorous potassium exchange is performed on an agglomerate whose binder has been zeolitized, using IM K.C1 solution at 25°C in four successive steps. At each step, the volume of solutiun/mass of solid ratio is 20 ml/g and the exchange is continued for 4 hours each time. Between each exchange, the solid is washed several times so as to free it of the excess salt. The product obtained has a potassium exchange level of 97-5%. It is then subjected, to 2 barium exchange operations identical to those described in Example 1. After all of these operations, the solid is finally activated at a temperature of 200°C for 2 hours under a stream of nitrogen. It has the following characteristics: Bariiim exchange level 74.3% Potassium exchange level 24% Toluene-adsorption capacity 15% Micropore volume 0.174 cm^/g Loss on ignition at 900°C 6.4% The adsorbent thus prepared satisfies the selectivity test with tho following values; ^PX/Tol The amount of para-xylene adsorbed during the test is 0.06 cm^/g. EXAMPLE 9 950 grams (calcined equivalent) of a zeolite X with an 3i/Al ratio - 1.12, whose synthesis is inspired £rom "Investigation on the crystallization of X-type zeolites" by H. Lechert, Zeolites, 1991, vol. 11, pp. 720-728, are agglomerated with 170 grams (calcined equivalent) of kaolinite from Charentes, 6 grams of carboxymethylcellulose and a suitable amount of water to allow the paste obtained to be extruded correctly. The sxtrudates are subsequently dried and then calcined at a temperature of 600°C for 2 hours under a stream of dry nitrogen. Crushing is then carried out so as to bring the equivalent diameter of the particles to 0.7 mm. The crushed material thus obtained is subjected to the barium exchange treatment described in Example 1 and heat-activated at a temperature of 220="C for 2 hours. The product thus obtained has a loss on ignition of 5.2% and a toluene-adsorption capacity of 13.7% (0.159 cra^/g micropore volume). The adsorbent satisfies the selectivity test with the following values: Isomers Selectivity PX/OX 2.52 PX/MX 2.50 PX/EB 2.62 PX/Tol 1.78 The amount of para-xylene adsorbed during the test is 0.059 cmVg. EXAMPLE 10 The adsorbent prepared in Example 3 is subjected to an identical selectivity test using para-diethylbenzene as desorbent, and the following values are found: Isomers Selectivity PX/OX 2.65 PX/MX 2.58 PX/EB 2 .70 PX/PDEB 1.12 WE CLAIM l. Agglomerated zeolite adsorbents comprising at least 70% of faujasite with an Si/Al atomic ratio such that 1 2. The adsorbents according to Claim 1, wherein the exchangeable sites are at least 70% occupied by barium ions and upto 30 % by potassium (any remainder generally being made up of alkali metal ions or alkaline-earth metal ions other than barium or potassium). 3. The adsorbents according to Claim 1 or 2, comprising at least 80% of the said faujasite. 4. The adsorbents according to any one of the preceding claims, wherein said adsorbent is agglomerated with zeolitizable binder. 5. The adsorbents according to Claim 4, wherein the zeolitzable binder is clay of the kaolin family. 6. The adsorbents according to Claim 5, wherein the clay is gadolinite or halloysite. 7. The adsorbents according to anyone of the preceding claims, wherein the overall level of exchange for barium alone or for barium + potassium is greater than or equal to 90%. 8. The adsorbents according to anyone of the preceding claims, wherein the loss on ignition, measured at 900°C, is between 5.2 and 7.7%. 9. ^Process for obtaining adsorbents as claimed in anyone of the Claims 1 to 8, which process comprises the following steps: a) agglomeration of zeolite powder with a binder, b) calcination of the agglomerate, d) barium exchange and e) activation. 10. The process according to Claim 9, wherein the activation in step e) is a thermal activation performed at a temperature of 180 to 250°C. 11. The process according to Claim 9 or 10 which comprises the following step c) zeolitization of the binder by immersing the agglomerate in an alkaline liquor, sodium hydroxide or a mixture of sodium hydroxide and potassium hydroxide. 12. The process according to Claim 11, wherein there is used a zeolitizable binder and the alkaline solution in step c) has a concentration of at least 0.5 M. 13. The process according to any one of the claims 9 to 12, wherein in step d), a potassium exchange is also carried out. 14. A process for the recovery of para-xylene from aromatic Cg isomer fractions in the liquid phase or gaseous phase, which process comprises adsorption of the para-xylene using an adsorbent as claimed in anyone of Claims 1 to 8 in the presence of a desorbent. 15. ^Agglomerated zeolites adsorbents substantially as hereinbefore described and exemplified. 16. Process for obtaining adsorbents substantially as hereinbefore described and exemplified.

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