Molecular sieves agglomerated based on zeolite, and processes for preparing them, drying organic compounds, alcohols and especially ethanol and esters in gas or liquid phase

Abstract

"MOLECULAR SIEVES AGLOMERATED WITH A ZEOLY BASE, AND PROCESSES FOR PREPARING THE SAME, DRYING ALCOHOLIC ORGANIC COMPOUNDS AND ESPECIALLY ETHANOL AND ESTERS IN GASEOUS OR LIQUID PHASE". The present invention relates to agglomerated molecular sieves based on a molecular sieve of approximately 3 <134> pores, the process of preparing them and their use for drying organic compounds, particularly alcohols and / or esters , in gas or liquid phase by passing the compounds to be dehydrated through an adsorbent bed based on said agglomerated sieves.

Description

Agglomerated adsorbents, their preparation process and their use for drying organic compounds The present invention relates to a process for dehydrating organic compounds, particularly alcohols or esters, using agglomerated adsorbents based on a molecular sieve with a pore opening of approximately 3 Å. Many organic molecules are produced in a water-containing medium or synthesized by hydrolysis. For example, alcohols produced by the hydration of an olefin, such as ethanol from ethylene or isopropanol from propylene, contain water once the reaction is complete. Alcohols obtained by fermentation of agricultural materials such as beetroot, corn or sugar cane also have a significant water content. Esters synthesized from alcohol and carboxylic acid can also be contaminated by traces of water once their synthesis is complete. However, many uses of organic compounds require that they be free of or at worst contain only minute traces of water, such as applications in the pharmaceutical field; it is necessary to find effective methods to remove as much water as possible. There are dehydration methods that involve passing the organic compound to be dehydrated through a bed of adsorbent (desiccant). For drying organic molecules using these methods, zeolites with a pore size of approximately 3 Å are known to be used as adsorbents. Water can theoretically penetrate these zeolites, unlike larger organic molecules. Among zeolites with a pore size of approximately 3 Å, type A zeolites can be cited as an example, in which 28 to 60% (expressed in equivalents) of the exchangeable cationic sites are occupied by potassium ions, with the remaining sites being primarily occupied by Na⁺. These are also called 3A zeolites. Zeolites occur as very small crystals, typically less than 10 µm. Their use most often requires shaping them into objects, such as beads, filaments, or extrudates, with a larger particle size, typically between 0.5 mm and 5 mm. This is primarily to avoid pressure losses during the handling of sieves, particularly during the loading and unloading of adsorption columns. The shaping of these objects, referred to hereafter as agglomerates, is generally carried out using binders, particularly clays such as sepiolite, attapulgite, montmorillonite, or kaolin clays. FR 2.495.007 or GB 2.088.739 describe a process for dehydrating organic solvents by slow passage (surface velocity of the solvent in the column less than 15 cm / min) through a column containing a dehydrated molecular sieve based on zeolite 3 A where the ratio between the length of the column and the mass transfer zone is greater than or equal to 4. US 4,407,662 describes a VPSA (Vacuum Swing Adsorption) water adsorption process comprising a gas-phase adsorption step at the distillation outlet on a 3A molecular sieve column, followed by a regeneration step at a pressure lower than the adsorption pressure with purging of some of the anhydrous ethanol. The advantage of this adsorption process is that it allows for much faster cycle times since the entire process is normally isothermal, with the exception of the heats of adsorption and desorption, which generate a temperature variation that the applicant estimates to be less than 140°C. However, the adsorption capacity used is much lower than the saturation capacity. One of the drawbacks observed when using conventional commercial 3A agglomerated molecular sieves for the dehydration of 25 organic compounds, whether in the gas or liquid phase, concerns the formation of undesirable molecules catalyzed by the molecular sieve. In the case of ethanol dehydration, the formation of acetaldehyde, ethylene, diethyl ether, and even more complex molecules such as paraldehyde (formed by the cyclic condensation of acetaldehyde), acetals, and hemiacetals (formed by the reaction of ethanol with acetaldehyde) is observed. These parasitic reactions also depend on the temperature and therefore on the adsorption pressure which, in the PSA or VPSA processes, is imposed (liquid condensation must be avoided). Besides the quantity of water which must be very low, some applications of organic compounds, particularly in the pharmaceutical field, require organic compounds of very high purity where the total of undesirable organic species does not exceed 10 ppm. WO 00 / 34217 describes a process for drying organic liquids in which the liquid to be dehydrated is treated using a 3A molecular sieve that has previously undergone a treatment to reduce its concentration of acidic sites to less than 18 mmol / g, measured by TPD (Temperature Programmed Desorption) of NH3. The pretreatment consists of contacting the molecular sieve with a solution of an alkali metal salt, preferably potassium nitrate, followed by several washes. Using this pretreatment performed on two conventional, agglomerated commercial 3A sieves, WO 00 / 34217 shows that it is possible to reduce both the formation of propylene during the drying of isopropanol and the formation of diethyl ether during the drying of ethanol.Besides the fact that this process involves several solid-liquid contact steps, which complicates it and increases its cost, it is limited in its ability to significantly reduce the content of acidic sites in the molecular sieve, which are responsible for acidic reactions such as intramolecular or intermolecular dehydration, or acetal formation. In support of this observation, see in particular the examples in WO 00 / 34127, where the reduction in the content of acidic sites in two commercial zeolites is less than 50%. Description of the invention The present invention relates to new molecular sieves based on agglomerated 25 zeolite 3 A which, when used in a process for drying liquid or gaseous organic compounds by passing the compound(s) to be dehydrated over a bed based on said agglomerated sieves, have the advantage of limiting the formation of undesirable species obtained by partial conversion of the organic compound(s) to be dried. The agglomerated sieves according to the invention, with an average particle size generally between 1.6 mm and 5 mm, are characterized by: * an iron content, expressed as Fe2O3 relative to the total anhydrous mass of the agglomerate, less than or equal to 0.5%, and preferably less than or equal to 0.3%; * a titanium content, expressed as TiO2 relative to the total anhydrous mass of the agglomerate, less than or equal to 0.2%, preferably less than or equal to 0.1%. They can be prepared by agglomeration, according to known techniques, of zeolite powder 3A, obtained, for example, by hydrothermal synthesis, with an agglomeration binder selected from clays such as kaolin, silica, and / or alumina. In general, the agglomerated sieves contain less than 25% inert binder (in the sense of adsorption) and preferably up to 20% by weight. The binders suitable for the present invention will be chosen from among the usual agglomeration binders; a person skilled in the art will easily select those whose iron and titanium content will allow the agglomerated sieves to be obtained according to the invention. Agglomeration can, for example, be carried out by mixing a crystalline zeolite powder with water, the binder (usually also in powder form) and possibly additives to aid agglomeration, then extruding or pressing the mixture thus obtained into filaments, or spraying this mixture onto zeolite agglomerates acting as a seed for agglomeration. During pulverization, the zeolite agglomerates are subjected to continuous rotation. This can be achieved by arranging the agglomerates in a reactor rotating around an axis of rotation, this axis of rotation preferably being inclined with respect to the vertical direction. By this process, commonly referred to in the art as the "snowball" process, agglomerates in the form of spheres are obtained. The agglomerated sieves thus shaped are then subjected to firing at a temperature between approximately 400 and 7000C. The sieves according to the invention are preferably subjected to a treatment consisting of introducing a basic species: either by impregnating the agglomerated and baked sieves in aqueous phase with alkali metal hydroxide(s) at room temperature (15-30°C) or at a temperature between 70°C and boiling, followed by washing to remove excess ionic species and then drying, or by incorporating these alkali metal hydroxides and / or carbonates and / or salts and organic acids such as acetate, lactate, oxalate, citrate, etc., during the agglomeration and shaping stage. This second variant, particularly preferred by the applicant, does not require rewetting and washing of the sieve after shaping. The sieves according to the invention which have been subjected to this treatment preferably have an alkali metal content (preferably sodium and / or potassium), expressed in M20 (M= preferably Na and / or K) relative to the total anhydrous mass of the agglomerate, greater than or equal to 0.5% and less than or equal to 1.4%, and preferably greater than or equal to 0.7% and less than or equal to 1.1%. Examples - description of the experimental setup We test the ability to promote the formation of acetaldehyde during an ethanol drying operation on agglomerated sieves based on zeolite 3A (type A zeolite of which 46% of the CEC (cation exchange capacity) is occupied by potassium ions, the remainder being occupied by sodium ions and agglomerated with 20% by weight (relative to the total mass of the agglomerate) of different binders 20 in the form of extrudates of average particle size of about 1.6 mm using the apparatus consisting of the following elements: - a reserve of ethanol at 96% by weight (the remaining 4% being water) connected to a peristaltic pump, - a vertical cylindrical reactor (volume = 200 mi) placed in a furnace 25 fed by the pump;This reactor is filled to one-third of its height with inert glass beads with respect to the reactions considered and contains above it the molecular sieve to be tested, i.e. a quantity of approximately 70 ml, - a nitrogen inlet allowing the reactor to be purged of the air initially contained therein, - a condenser at the reactor outlet. The reactor is loaded with sieves and then swept with nitrogen for 1 hour; the furnace temperature is programmed to reach 140°C. The ethanol is then pumped in liquid form, vaporizes in the reactor, and recondenses at the outlet (temperature = 14°C). Gas chromatography (GC) analysis of the resulting fractions is then performed to determine the quantities formed. EXAMPLE 1 We test several agglomerated sieves for their ability to convert ethanol into acetaldehyde and we report in Table 1 the weight contents of minor elements of these sieves as well as their rate of conversion of ethanol into acetaldehyde. Sieve 1 contains 20 parts by weight of binder containing calcium, iron, magnesium, and titanium in the following proportions: CaO = 0.09%; Fe2O3 = 0.77%; MgO = 1.15%; TiO2 = 0.1%; sieve 2 contains 20 parts by weight of binder containing calcium, iron, magnesium, and titanium in the following proportions: CaO = 0.6%; Fe2O3 = 4.7%; MgO = 12.5%; TiO2 = 0.5%; and sieve 3 contains 15 to 20 parts by weight of binder containing calcium, iron, magnesium, and titanium in the following proportions: CaO = 0%; Fe2O3 = 1.3%; MgO = 0.2%; TiO2 = 2%. Sieve 4 is obtained from sieve 1 by impregnating 50 g of sieve 1 per ml with an aqueous solution containing 12.64 g of iron nitrate nonahydrate; this quantity corresponds to 2.5 g of Fe2O3 retained on the solid. 20 Table 1 Agglomerated sieve CaO MgO Fe2O3 TiO2 Conversion (ppm) (%) (%) (%) 1 according to the invention 0.018 0.23 0.15 0.018 0 2 comparative 0.12 2.5 0.94 0.1 50 3 comparative 0 0.04 0.26 0.4 7 4 comparative 0.017 0.21 5 0.017 50 EXAMPLE 2 Several sieves are tested for their ability to form ethylene and diethyl ether during the dehydration of ethanol and the weight contents of minor elements of these agglomerated sieves and the results of the catalytic tests are reported in Table 2. Sieve 5 contains 20 parts by weight of binder which contains calcium, iron, magnesium and titanium in the following proportions: CaO = 1%; Fe2O3 = 0.95%; MgO = 5.65%; TiO2 = 0.05%. Sieve 6 is obtained by impregnating 40 g of sieve 5 with 40 ml of a 100% aqueous potassium hydroxide solution containing 0.8 g of KOH without washing with water. The residual amount of potassium on sieve 6, expressed as the amount of K2O, is 0.8%. Sieve 7 is obtained by incorporating 0.9 g of K2CO3 per 41 g of zeolite powder and 9 g of binder during the agglomeration step. The residual quantity of potassium on sieve 7, expressed as K2O, is 1.16%. We also measure the amount of acetaldehyde formed on sieves 5 to 7; for these three samples, it is on the order of 3 ppm. Table 2 Sieve CaO MgO Fe2O3 TiO2 K2O Conversion Conversion (%) (%) (%) (%) Residual Ether (ppm) Ethylene (%) (ppm) 0.2 1.13 0.19 0.01 O 15 15 according to the invention 6 0.2 1.13 0.19 0.01 0.8 2 2 according to the invention 7 0.2 1.13 0.19 0.01 1.13 3 <1 according to the invention EXAMPLE 3 A sieve (sieve 8) is formed into 1.6 mm diameter extrudates by agglomerating 80% zeolite 3A powder with 20% of a binder containing calcium 5, iron, magnesium, and titanium in the following proportions: CaO = 0.09%; Fe2O3 = 0.77%; MgO = 1.15%; TiO2 = 0.1%. This zeolizable binder is similar to that used in Example 1 (sieve 1 according to the invention). After calcination at 550°C for 2 h, half of the sieve is immersed in a sodium hydroxide and potassium hydroxide solution at 100°C for 2 h (L / W = 4; NaOH = 80 g / L; KOH = 30 g / L), then the sieve is washed with water until pH = 11. A measurement of the H2O adsorption capacity on this sieve under partial pressure of 0.5 at a temperature of 25 C shows an improvement of approximately 13% compared to the sieve not having undergone treatment in basic medium at 100 C. The catalytic test described in the previous examples is carried out on the two 15 sieves, having or not undergone treatment in the sodium / potash solution, and the values ​​indicated in the table below are found. Table 3 Conversion (ppm) without basic treatment with basic treatment sieve 1 sieve 8 Acetaldehyde (%) 0 0 Ether (%) 7 3 Ethylene (%) 20 8

Claims

Demands 1. Agglomerated molecular sieves based on zeolite 3 A and one or more agglomeration binders, characterized in that their iron content expressed as Fe2O3 relative to the anhydrous sieve mass is less than or equal to 0.5%, and preferably less than or equal to 0.3%, by weight and their titanium content, expressed as TiO2 relative to the total anhydrous sieve mass, is less than or equal to 0.2%, preferably less than or equal to 0.1%.

2. Sieves according to claim 1, characterized in that their average particle size is between 1.6 mm and 5 mm.

3. Sieves according to claim 1 or 2 characterized in that their content of alkali metal(s), expressed in M20 (M being preferably sodium and / or potassium) in relation to the total anhydrous mass of sieve is greater than or equal to 0.5% and less than or equal to 1.4%, and preferably greater than or equal to 0.7% and less than or equal to 1.1%.

4. Method of preparing sieves as defined in any one of claims 1 to 3 by agglomerating zeolite powder 3 A with an agglomeration binder and then baking.

5. A process according to claim 4 characterized in that it comprises a treatment consisting of introducing at least one basic species: > either by impregnation in aqueous phase of the agglomerated and baked sieves, by means of alkali metal(s) hydroxide(s) at ambient temperature or at a temperature between 70°C and boiling, followed by washing and then drying, > or, preferably, by incorporation of alkali metal(s) hydroxide(s) and / or carbonate(s) and / or salt(s) and organic acid(s), during the agglomeration and shaping step.

6. A process for drying organic compounds in gas or liquid phase by passing through an adsorbent bed based on agglomerated sieves as defined in any one of claims 1 to 3.

7. A process for drying alcohols, and in particular ethanol, in gas phase or in liquid phase by passing through a bed of adsorbent based on agglomerated sieves as defined in any one of claims 1 to 3.

8. Process for drying esters in gas or liquid phase by passing them through a bed of adsorbent based on agglomerated sieves as defined in any one of claims 1 to 3.