Optimized mesoporous adsorbents to ensure in-situ drying of a separated gas mixture in a VSA unit

Abstract

Use of a mesoporous adsorbent material in a VSA process for separating a gaseous mixture, said gaseous mixture comprising, in addition to the principal constituents to be separated, water vapor to be retained on a guard bed prior to passing said mixture over the main adsorbent mass, in order to protect said main adsorbent mass from moisture, said adsorbent material being characterized by: - ​​a Henry's constant relative to water measured at 30°C of between 6000 and 80000 mmol / bar / g, preferably between 8000 and 50000 mmol / bar / g, - an isosteric heat of adsorption of water in the Henry's range of between 60 and 80 kJ / mol, - an isosteric heat of adsorption of water greater than 40 kJ / mol, preferably between 50 and 65 kJ / mol for a relative humidity greater than 30%, - an average pore diameter between between 2 and 8 nm, preferably between 2 and 5 nm. Abbreviated figure: Fig. 3

Description

Title of the invention: Optimized mesoporous adsorbents to ensure in-situ drying of a separated gas mixture in a VS A unit

[0001] The present invention relates to the drying of a gaseous mixture by a guard bed prior to its separation by the main adsorbent mass into a first fraction comprising the most adsorbable compounds and a second fraction comprising the least adsorbable compounds. It relates to VSA-type processes, that is, adsorption separation processes using a vacuum during regeneration. More specifically, it relates to the replacement of adsorbents conventionally used for this purpose for water removal, namely activated alumina, silica gel, zeolite X, etc., with new, more efficient materials. The production of oxygen from ambient air is taken as an example, but the use of these new materials as guard beds for moisture removal is more general than this single application.

[0002] In general, a process implementing the phenomenon of adsorption makes it possible to separate one or more molecules from a mixture containing them, by exploiting the difference in affinity of one or more adsorbents towards the constituent molecules of the mixture.

[0003] Adsorption processes are of several types depending on whether the adsorbent is regenerable in situ or not. Three main families of adsorption processes can then be distinguished: lost-charge processes, TSA (Temperature Swing Adsorption) processes, and finally PSA (Pressure Swing Adsorption) processes.

[0004] In lost charge processes, a new charge is put in place when the one in use is saturated by impurities or more generally when it can no longer play its protective role sufficiently.

[0005] In contrast, TSA or PSA type processes are cyclic processes that continuously link adsorption and regeneration phases. The cycle time Te is the duration after which an adsorber returns to the same state as before, for example at the beginning of an adsorption step.

[0006] In TSA-type processes, the adsorbent at the end of its use is regenerated in situ; that is, the trapped impurities are removed so that the adsorbent recovers most of its adsorption capacity and can begin a new purification cycle. The essential regeneration effect is due to a temperature increase. Regeneration often takes place under low pressure, or even possibly under The vacuum promotes the desorption of impurities. It is also common practice to push the heat front present in the adsorber at the beginning of the cooling phase to complete the desorption of impurities from the area not yet regenerated. In these latter cases, the acronyms TPSA or TEPSA are sometimes used instead of TSA.

[0007] Finally, in PSA-type processes, the adsorbent at the end of the production phase is regenerated by desorption of impurities achieved by reducing their partial pressure. This pressure reduction can be obtained by lowering the total pressure and / or by purging with a gas free of or containing few impurities. This gas may, in some cases, be external to the unit. Generally, the regeneration of a PSA is incomplete, and the adsorbent mass still contains a significant amount of adsorbed constituents at the end of this step. Attempting to eliminate all of these constituents by simply reducing the partial pressure would require the use of a high vacuum and / or very large quantities of elution gas, which are incompatible with economical industrial operation.

[0008] It should be noted that a PSA-type unit is not free from thermal effects. The adsorption and desorption of impurities create localized heating and cooling, the amplitude (fluctuation) of which can reach several tens of °C when the adsorbed quantity is significant. There may also be a temperature profile between the inlet and outlet with areas that are warmer and / or colder than average. These thermal effects generally have a negative impact on regeneration. Pressure-modulated adsorption processes are used both to remove trace impurities—for example, those with a content of less than 1% in the feed gas—and to separate mixtures containing tens of percent of different gases. In the first case, the process is generally referred to as purification (for example, gas drying), and in the second case, as fractionation (for example, the production of oxygen or nitrogen from atmospheric air).In the most complex cases, purification and fractionation can of course be combined in the same unit. It is common to give more specific names depending on the pressure levels involved or the time required for an adsorber to return to its initial point (cycle time): .

[0009] VSA processes in which adsorption takes place substantially at atmospheric pressure, preferably between 0.95 and 1.25 bar abs and the desorption pressure is lower than atmospheric pressure, typically from 50 to 400 mbar abs.

[0010] MPSA or VPSA processes in which adsorption takes place at a high pressure above atmospheric pressure, typically between 1.3 and 6 bar abs, or even 10 bar abs, and desorption at a low pressure below atmospheric pressure, generally between 200 and 750 mbar abs.

[0011] The PSA processes proper in which the high pressure is substantially greater than atmospheric pressure, typically between 3 and 50 bar abs and the low pressure substantially equal to or greater than atmospheric pressure, generally between 1 and 9 bar abs.

[0012] RPSA (Rapid PSA) processes for which the duration of the pressure cycle is typically less than one minute.

[0013] URPSA (Ultra Rapid PSA) processes for which the duration of the pressure cycle is on the order of a few seconds maximum.

[0014] It can also be noted that the gaseous fraction recovered in a PSA process can correspond to the fraction produced at high pressure but also to the fraction extracted at low pressure as long as the constituent(s) sought are the most adsorbable of the mixture.

[0015] It should be noted that these various names (TSA, TEPSA, TPS A, RTS A PSA, VS A, VPSA, MPSA, RPSA, URPSA) are not standardized and that the boundaries vary depending on the author. Other names exist in the literature highlighting one or another specific aspect of the process.

[0016] This document, according to our definitions, focuses on VSA, MPSA, and VPSA units used to separate initially humid gas mixtures. These units comprise, within a single casing hereafter referred to as an adsorbent, an adsorbent mass designed to retain water located upstream of the main adsorbent mass. This main mass separates the gas mixture into a first fraction consisting of the most adsorbable components and a second fraction consisting of the least adsorbable components. The adsorbent recommended here for drying is particularly suitable, but not limited to, cycles in which the High Pressure adsorption step and / or the regeneration step lasts one minute or less. For simplicity, all these units (VSA, MPSA, VPSA) covered by this document will henceforth be referred to as VSA.

[0017] The need to stop the water before the gaseous mixture to be separated penetrates the main adsorbent mass is a constraint well known to those skilled in the art. This is particularly relevant in cases where moisture acts as a poison to the main adsorbent due to its almost irreversible adsorption, as well as in cases where it alters the internal structure, for example by reducing the mechanical resistance of the particles, or even by decreasing the stability of the microcrystals themselves. The adsorbent mass intended here to stop the water and, more generally, the constituents considered poisonous to the main adsorbent mass, is referred to interchangeably as a guard bed or pretreatment bed.

[0018] It should be noted in general that to treat by adsorption a gaseous mixture comprising on the one hand a poison for the main adsorbent, for example of the type acidic gas, constituent capable of polymerizing, reacting by making deposits, of being irreversibly adsorbed (here the case of H2O) and on the other hand main constituents (for example and without limitation H2, CH4, CO, O2, N2) which are subject to separation into a most adsorbable fraction and a least adsorbable fraction, it is necessary to use a guard bed intended to protect the main adsorbent mass, this guard bed being located in the same adsorber as the main bed and upstream of it.

[0019] In this general context, the present document is only concerned with the stopping of water vapor in a guard bed placed upstream of the main adsorbent mass in a process of separating a gaseous mixture by VS A, VS A being understood in the sense specified above.

[0020] A classic example of this problem concerns VSA O2. It should be noted that a VSA oxygen process typically consists of two cylinders, each containing a fixed bed (axial or radial depending on the geometry chosen for the adsorber). This bed generally contains an activated alumina layer ensuring the complete removal of water and partial removal of CO2, and a LiLSX zeolite bed ensuring the N2 / O2 separation to produce a gas with an oxygen purity of approximately 93%. A VSA 2 adsorber cycle generally comprises, in addition to intermediate balancing steps, two main stages: - an air supply phase including the pressurization and adsorption phases at pressures between 1.1 and 1.5 bar abs; - a regeneration phase including the depressurization and vacuum elution phases, the minimum pressure of which can be between 0.5 and 0.35 bar abs at the end of the stage.The first adsorbent bed used for air pretreatment must prevent water contamination of the LiLSX. Extremely hydrophilic, a LiLSX zeolite bed exposed to water would see its initial fraction lose a considerable amount of nitrogen adsorption capacity. Similarly, for some sieves used for N2 / O2 separation, exposure to humid air leads to a loss of crush resistance that can be critical for the process: the adsorbent could then disintegrate rapidly over time. The pretreatment step is therefore essential for the proper functioning of oxygen production. The pretreatment layer is generally composed, as already mentioned, of activated alumina (2 mm or 3 mm diameter beads) and / or sometimes a 13X zeolite layer.

[0021] Without going into detail, it can be observed that, generally during the adsorption step, the transfer of gas molecules to the adsorption sites occurs on only a portion of the adsorbent bed. Three zones can then be distinguished: the upstream zone, called the saturated zone, where the adsorbent is completely loaded with the constituent; the downstream zone, which has not yet been in contact with the constituent in question and is called the unsaturated zone; and, between the two, the zone where the transfer of the constituent contained in the gas takes place. Towards the still partially free adsorption sites, a zone variously called the adsorption front, mass front, frontal zone, mass transfer zone, or by the acronyms MTZ (Mass Transfer Zone). This zone moves in the direction of gas flow. During its co-current movement, it generally has an inverted S shape when plotting the composition of the constituent in both the gaseous phase and the adsorbed phase (adsorbate). The term "front tip" is generally used to define the interface between the front and the free zone.During regeneration, the adsorbed constituents are pushed back against the current towards the adsorber inlet. For a given constituent, a similar end of the regeneration front is obtained at the end of the regeneration process. This end is upstream of the point where (by reference to the direction of gas flow in the adsorber during the regeneration stage) the constituent in question has been completely desorbed. Stable PSA operation corresponds to operation in which the extreme positions of the adsorption and regeneration fronts no longer change from one cycle to the next. There is then neither accumulation nor loss of molecules in the adsorbent mass over a complete cycle, and what enters with the feed gas exits in the production or residual stream of the treatment unit.

[0022] It is then understood that the stopping efficiency of a constituent of an adsorbent mass will depend both on its efficiency in stopping this constituent during the adsorption phase and on its efficiency in desorbing during the regeneration step.

[0023] The regeneration efficiency is characterized in particular by what is called the regeneration power (RP) of the PSA, which is defined as the ratio of the gas volumes flowing counter-currently through the adsorbent mass during a cycle to the gas volumes flowing co-currently through said mass, the respective volumes being measured in actual m³. Regeneration power (RP) is understood here in its most comprehensive definition, corresponding to the theoretical "Purge Factor," meaning that it is determined locally (here, for example, in the pretreatment adsorbent mass) and takes into account all gas flows circulating at that level during all stages of the cycle. For information, there is another, simpler, global definition that only considers the feed gas during the adsorption stage (for the co-current) and the elution gas during the elution stage (for the counter-current).This is not the definition used here, but depending on the cycles considered for PSAs, this simplification can indeed be a good approximation of the more rigorous formulation. The regeneration power (FP) must be greater than 1 and is often found to be between 1.1 and 1.3 for the majority of PSAs.

[0024] It should be noted that an increase in regeneration power will generally allow a reduction in the adsorbent masses required and therefore the investment, but at the expense of the unit's performance in terms of consumption. Energy efficiency and extraction efficiency are key factors. Given this, the effective regeneration power is that required for—or optimized for—the primary separation. The guard adsorbent mass, corresponding to the water arrest point in the example cited, must then be sized according to this regeneration power. A useful adsorption capacity for this adsorbent mass is sometimes defined as equal to the amount of constituent actually arrested per cycle per unit mass of adsorbent. A useful capacity ratio is also sometimes defined by comparing this effective arrest capacity to the amount of constituent that could theoretically be arrested to saturation under operating conditions. This ratio, generally low, for example on the order of 30%, takes into account, in particular, the fact that the adsorbent is not completely regenerated, as already explained, and that the mass transfer zone is, by definition, not saturated.It depends in a very complex way on numerous parameters characteristic of adsorption and desorption. Without going into detail, it is worth noting that what is favorable to adsorption is often unfavorable to desorption, i.e., to regeneration, and vice versa. For example, an isotherm with a "favorable" shape and a downward curvature tends to shorten the frontal mass during the adsorption phase but prolong regeneration; or a strong affinity between the constituent and the adsorbent facilitates adsorption but hinders desorption. Conversely, high kinetics can improve both aspects, adsorption and regeneration, and is an important parameter to control.

[0025] Experience has shown since the commissioning of the first VS A systems incorporating a moisture-retaining guard bed within the adsorbent, that the effective capacity of the adsorbent materials used for this purpose—whether activated alumina, zeolite X or A, silica gel, or multi-beds formed from these adsorbents—was particularly low and well below the usual expected values. This still results in large guard beds today. For example, the volume of activated alumina or zeolite 13X in a VS A O2 represents almost 30% of the volume of LiLSX, even though the only objective is to retain traces of water with adsorbents that are supposedly very effective for this purpose, instead of performing, as with the main adsorbent, an O2 / N2 separation, which is notoriously difficult and covers almost the entire flow rate.

[0026] This low efficiency of the guard bed results in a significant dead volume containing compressed air that is wasted during the adsorption stage and nitrogen that is not removed at the end of regeneration. This has a negative impact on the oxygen extraction yield and therefore on energy consumption. Furthermore, this adsorbent mass located at the inlet of the adsorber creates additional pressure losses that degrade the specific energy of the main O2 / N2 separation.

[0027] There is therefore a definite interest in finding alternative adsorbents to activated alumina, silica gel, zeolite X or A in order to dry more efficiently a gaseous mixture intended to be separated in a VS A type unit.

[0028] A solution according to the invention consists of using a silica-based mesoporous adsorbent material in a VS A process for separating a gaseous mixture, said gaseous mixture comprising, in addition to the main constituents to be separated, water vapor to be stopped on a guard bed prior to passing said mixture over the main adsorbent mass, in order to protect said main adsorbent mass from moisture, said adsorbent material being characterized by: - a Henry's constant for water measured at 30°C between 6000 and 80000 mmole / bar / g, preferably between 8000 and 50000 mmole / bar / g, - an isosteric heat of water adsorption in the Henry range of between 60 and 80 kJ / mol, - an isosteric heat of water adsorption greater than 40 kJ / mol, preferably between 50 and 65 kJ / mol for a relative humidity greater than 30%, - an average pore diameter between 2 and 8 nm, preferably between 2 and 5 nm (20 and 50 angstroms), these characteristics relating to the powder (microcrystals) from the synthesis of the adsorbent material, before shaping from this powder and by adding a binder of an adsorbent in an agglomerated form (agglomerated adsorbent).

[0029] According to one variant, the adsorbent, still considered to be in powder form (microcrystals), also has at least one of the following characteristics:

[0030] - a specific surface area greater than 600 m² / g, preferably including between 900 and 1200 m2 / g,

[0031] - a mesoporous volume of between 0.6 and 1.0 cm3 / g,

[0032] - microcrystals with dimensions ranging from 0.1 to 50 microns, plus generally between 1 and 20 microns.

[0033] According to one variant, the agglomerated adsorbent, formed from the powder obtained from the synthesis to which a binder and additives are added, is in the form of beads, extrudates, rods, grains or pellets, this agglomerated adsorbent then having at least one of the following characteristics:

[0034] - presence of a binder in proportions of 5 to 30% by weight of the mass of adsorbent material (powder, microcrystals), this binder being essentially inert or partially zeoliticized,

[0035] - an average diameter (balls, grains...) or a length, that is to say the most large dimensions (sticks, extruded products, etc.) ranging from 0.2 to 10mm, preferably between 0.5 and 5mm,

[0036] - a macropore porosity plus a mesopore porosity representing more than 70%, of preferentially more than 80%, even more preferentially more than 90% of the total particle porosity.

[0037] According to one variant, the adsorbent material is a silica-based mesoporous material (generally referred to in publications as "silica-based mesoporous" or "silicate mesoporous" material).

[0038] According to one variant, the adsorbent material is a mesoporous material obtained by reaction between an alkaline solution of quaternary alkyiammoniums (surfactants) and silicates and then after reaction, by removal of the surfactant.

[0039] According to one variant, the adsorbent material undergoes aluminum grafting and cationic doping in particular with sodium, magnesium, potassium, calcium, lithium, barium or strontium cations.

[0040] According to one variant, the internal surface of the pores of the adsorbent material is functionalized by means of amino, carboxylic, phosphate groups.

[0041] The inventors observed, by studying the water content profiles in the guard bed adsorbent mass installed upstream of the main LiLSX layer in a pilot unit of type MPSA 02, representative of industrial operation, that these profiles did not correspond to the previously given academic description. Specifically, the classic configuration described above, of a well-defined mass transfer zone advancing during the adsorption stage and retreating during the regeneration stage, was not observed. Instead, a moist adsorbent mass was found that changed very little during a complete cycle. The local quantity of water adsorbed and then desorbed is very small, explaining the surprisingly large volume required to trap all the water upstream of the main bed. This phenomenon is true for both a guard bed composed of activated alumina and zeolite X, even though the profiles are not identical.

[0042] It should be noted that the need to install a large protective adsorbent mass, greater than could be estimated, has been known since the very beginning of the commissioning of the VSA 02. Such oversizing was, for example, already recommended by Bayer in the 1980s to protect the CaX-type adsorbents offered for this application. These dimensions, which effectively protected the main adsorbent from any moisture ingress in practice, have not fundamentally been called into question for various reasons, some more justified than others:

[0043] - low cost of activated alumina and then of zeolite X compared to the adsorbent The main one, which has always been a zeolite, is a more complex manufacturing process involving expensive products (lithium) with environmental constraints.

[0044] - secondary energy consumption taking into account initial production relatively low oxygen levels of a few tens of tons per day maximum.

[0045] Although the impact of the guard bed on performance has increased due to significant improvements in the main adsorbent (the guard bed / main bed ratio is now considerably higher), and the use of small-diameter activated alumina particles in response to the cycle shortening which creates additional pressure drops, it appears that no specific studies have been conducted on this point, despite the implementation of large-capacity production units for which energy consumption has become a significant factor. Optimization studies have focused primarily on the performance of the main adsorbent, the cycles implemented, the machinery used in the process, and energy management during reduced operation.

[0046] In order to search for adsorbents that would be more favorable than those conventionally used for the guard volume, the inventors focused on two points.

[0047] The first is to increase the adsorption and desorption kinetics for a given particle size. Laboratory tests show that kinetics is a sensitive parameter for the shape of moisture profiles in a guard bed when rapid cycles are implemented. These results can be compared to those known in thermal engineering regarding the operation of regenerators: longitudinal displacement between the inlet and outlet of well-defined temperature fronts, or simply local thermal respiration depending on the duration of the heating / cooling stages. The first mode of operation, with the advance and retreat of the fronts, corresponds to the description given above of the movements of the frontal mass during adsorption and then regeneration. This can be understood in light of the similarities between the equations governing thermal engineering and adsorption.It is therefore logical to attribute at least part of the phenomenon observed in the VSA on-call beds to insufficient kinetics relative to the duration of the stages.

[0048] By analogy, it appears that a substantial gain on this parameter is necessary to hope for a significant effect on the shape of the adsorption and regeneration profiles and thus on the efficiency of the guard bed, a gain which cannot be obtained by a simple improvement of the macroporosity of a conventional particle given the progress already made in this area by adsorbent suppliers.

[0049] The second point of improvement identified concerns the optimal isotherm that the guard bed adsorbent should have. This is generally a theoretical problem often raised and usually resulting in hypothetical adsorbents with very high adsorption capacities, selectivities, and kinetics depending on the cycles involved.

[0050] [Fig.1] Fig.1 represents the quantity adsorbed in % weight, as a function of relative humidity;

[0051] [Fig.2] Fig.2 represents the pore or micelle diameter (in nanometers) in function of the number of carbon atoms in the alkyl chain of the surfactant; and

[0052] [Fig.3] Figure [Fig.3] represents the effect of cations on the water adsorption isotherm of a silica-based mesoporous adsorbent.

[0053] A more empirical approach consisted of making the assumption, supported both by the shape of the water profiles in the current guard adsorbent masses and by the isotherms shown in [Fig. 1], that zeolite X had too high an affinity for water (resulting in an excessively high Henry's constant and possibly isosteric heat of sorption / desorption), making desorption difficult under the operating conditions of a VS A, whereas activated alumina had, on the contrary, too low a Henry's constant and an isotherm with an unfavorable shape (upward final curvature) to be effective over short cycles on the order of a minute. The hysteresis phenomenon (greater amount of water at equilibrium in regeneration than in adsorption) is also an unfavorable effect for alumina. It is largely related to the pore size.

[0054] Figure 1 shows the typical isotherms of water adsorption on activated alumina and on zeolite X at room temperature. It should be noted that each adsorbent will have its own isotherm, with some variations around its typical representation depending on the base products, manufacturing procedures, and even the degree of activation before measurement. Nevertheless, the conclusions drawn from this comparison cannot be called into question.

[0055] The figure on the left shows the entire isotherm for a relative humidity of 0% to 100% (saturation) on the x-axis, with the y-axis representing the quantities adsorbed as a percentage by weight. The figure on the right is a close-up of the region of low relative humidity, which in practice corresponds to the point at which the last ppm of water contained in the gas mixture ceases to be adsorbed. It should be noted that Henry's constant is the slope at the origin of the isotherms and that the isosteric heat corresponds to the heat released by adsorption. This last one generally varies by a few tens of percent depending on whether we are interested in the adsorption of the very first molecules in a monolayer on the wall (we then speak of isosteric heat in the Henry zone, therefore in the case where the water vapor pressure - or Relative Humidity - tends towards 0) or in the adsorption of molecules when the walls of the pores are already totally lined by a first layer, which corresponds to the so-called multilayer adsorption.The methods for determining Henry's constant and isosteric heats are described later in the document.

[0056] This [Fig.1] highlights the large difference in behavior of activated alumina and zeolite X with respect to water adsorption.

[0057] Based on values ​​from the literature and internal databases, Henry's constants were calculated for a large number of activated aluminas, silica gels, and zeolites, particularly zeolite X, from various suppliers. As expected, the results show, as previously stated, a certain dispersion due primarily to the material itself (raw materials, manufacturing processes), its activation, and partly to the accuracy of the available data. Since the measurements were often performed on zeolite particles, a correction was applied to account for a percentage of inert binder.

[0058] However, the very different ranges of Henry's constant are clearly visible between, on the one hand, activated aluminas and silica gels and, on the other hand, X zeolites. To visualize these differences, at this stage, a Henry's constant normalized to 1 was used for the lowest values ​​of Henry's constants of standard activated aluminas and for a temperature of 30°C.

[0059] This yields normalized Henry's constant ranges of 1 to 3 for standard activated alumina and 1 to 2 for standard silica gel, but 100 to 500 times higher, and even higher, for zeolite X. These values ​​reinforce the hypothesis that currently used are either adsorbents that are inefficient at trapping traces of water, or adsorbents with a very high affinity for water and therefore difficult to regenerate in rapid VSA cycles. It is therefore highly probable that an intermediate material with a normalized Henry's constant between approximately 5 and 60 will have better efficiency with sufficient affinity for water in adsorption while also being easily regenerable.

[0060] By standard activated alumina, standard silica gel, zeolite X, we mean products marketed in large quantities for this type of drying application (VSA 02, purification at the head of a cold box, in particular cryogenic air separation) and not specific or laboratory products.

[0061] The determination of Henry's constant is known to the person skilled in the art, although there are still developments on this subject and some differences in the procedures for obtaining it.

[0062] Here, we recommend the method described in FR 3 005 873, the main points of which are recalled below.

[0063] The Henry's constant of a gas for a given adsorbent is defined, as already indicated, as the slope at the origin of the curve representing the quantity of gas adsorbed at a given temperature as a function of the adsorption pressure or, in the case of water, as a function of the relative humidity, when this pressure or relative humidity tends towards 0. As it is not possible to perform Based on measurements under the strict assumption of zero pressure, a general procedure has been established to evaluate this value from a series of measurements that approximate this assumption while remaining within the measurable range. It should be noted that the initial state of the adsorbents strongly impacts the amount of gas adsorbed as well as the Henry constant. For this reason, the measurement of adsorption isotherms and the estimation of the Henry constant of a given gas are performed after the activation or complete regeneration of the adsorbent. The regeneration conditions depend heavily on the nature of the adsorbent, and samples are generally regenerated between 200°C and 450°C, depending on the type of adsorbent, under secondary vacuum (10⁻⁵ mbar) for 8 hours.Approximately twenty regularly spaced adsorption points of the constituent under study are determined using the most suitable measuring instruments, particularly for the range of partial pressure of interest. A mathematical model is developed to relate the amount of gas adsorbed to the pressure. This model is preferably based on well-known adsorption models, and its parameters are determined using a mathematical solver by minimizing the relative error between the measured and calculated values. The slope at the origin of the resulting equation gives the value of Henry's constant. Established methods are used to validate or determine the uncertainty of the chosen value (robustness of the result when additional measurement points are added, testing with different models).

[0064] With the determination of the Henry constant by this method, a silica-based mesoporous adsorbent material is then selected, characterized by a Henry constant relative to H2O measured at 30°C between 6000 and 80000 mmole / bar / g, preferably between 8000 and 50000 mmole / bar / g, corresponding to the range allowing better initial adsorption than with activated alumina and better regeneration than with a zeolite X.

[0065] For the record, the Henry's constants of standard aluminas and silica gels are around 1500 to 5000 mmol / bar / g, while those of zeolite X are on the order of 250,000 mmol / bar / g and often well above. As a first approximation, we can say that we are selecting an adsorbent whose Henry's constant for water will be 2 to 3 times higher than that of a standard activated alumina and 5 times lower than that of a non-exchanged zeolite X (Na cation).

[0066] The variations in the initial isosteric heat of water adsorption between the different adsorbents are much less than those related to Henry's constant and vary in a range of about 50 to 90 KJ / mol, the minimum values ​​generally corresponding to the activated alumina and silica gel group and the higher values ​​to zeolite X.

[0067] Thus, with the same concern for a compromise between adsorption and regeneration, a material with an initial isosteric heat of sorption, i.e., in the Henry range, between 60 and 80 kJ / mol, will be selected. Outside the Henry range, particularly for a relative humidity RH greater than 30%, a material with an isosteric heat greater than 40 kJ / mol, preferably between 50 and 65 kJ / mol, will be chosen.

[0068] It is recalled that isosteric heat is the energy released by the action of intermolecular attractive forces between water molecules and the pore surface (or required to break these same forces during regeneration). These forces must be sufficient and not excessive both at the very low partial pressures corresponding to the cessation of the last traces of water in the gas mixture, and at partial pressures or relative humidities close to those of the inlet gas mixture.

[0069] A person skilled in the art can determine the isosteric heat of sorption at a given surface coverage of the adsorbent material. Without going into the details of the equations, it can be calculated from the experimental measurement of the adsorption isotherms by applying the Van't Hoff equation, or from the Clausius-Clapeyron equation, assuming that the gas phase behaves as an ideal phase and that the volume of the adsorbed phase is negligible compared to that of the gas phase. It is this latter method, known for correctly estimating the isosteric heat of adsorption as a function of the surface loading of the material for a pure substance, that is used here, particularly within the framework of the values ​​used for the invention. The isosteric heat in the Henry zone corresponds to the coverage by a monolayer of molecules.The initial value, that is, when the loading tends towards zero, can be estimated by a method identical to that used for Henry's constant, namely the limiting value of the function correlating the values ​​determined as a function of relative humidity. The variations in isosteric heat are relatively small, and therefore this method is particularly well suited.

[0070] As already noted, a significant increase in kinetics can only be obtained by a change in the nature of the porosity, that is to say by moving towards mesoporous materials having a majority of pores of size greater than or equal to 2 nanometers favorable to the circulation of molecules.

[0071] More precisely, it can be estimated that an optimum between kinetics and adsorption capacity concerns mesoporous materials having relatively small diameter pores within the range covered by this term (mesopores ranging from 2 to 50 nm), which have a significant impact on kinetics but sufficient internal surface area. It should be noted that, for a given pore volume, crystals with smaller diameter pores develop a greater internal surface area. The selected adsorbent will therefore have A porosity in the range of 2 to 8 nm, preferably 2 to 5 nm, i.e., 20 to 50 Angstroms, meaning at least twice the size of current adsorbents. At the low pressures used in VSAs, this particularly alters the diffusion mechanisms and their relative importance.

[0072] It is recalled that nanoporous materials consist of an amorphous or crystalline structure with void spaces, which can be essentially cylindrical or cage-like. Most nanoporous materials belong to three main categories: microporous, mesoporous, and macroporous.

[0073] Microporous materials—such as MOFs, zeolites, activated alumina, most activated carbons, silica gels, and certain amorphous glasses—exhibit narrow pore size distributions in the 0.5 to 2 nm range, offering high affinities for gas adsorption due to the confinement effect, but simultaneously limiting the diffusion of species within these pores. In contrast, macroporous materials with pore sizes between 50 and 1000 nm, such as porous polymer beads, allow easy access to internal pores at the expense of adsorption capacity and selectivity. These drawbacks have led in recent years to the development of mesoporous materials with an intermediate pore size range, between 2 and 50 nm.This class of materials is characterized by a narrow pore size distribution, relatively high specific surface areas (generally greater than 600 m² / g), and the possibility of performing structure / wall modifications with functionalization strategies to modulate the interaction energy of the pore walls with gas molecules. These strategies include the introduction of cations into the structure and / or the grafting of functional groups, both methods allowing modification of the electric fields at the wall.

[0074] Several pore topologies are possible for mesoporous materials. Cylindrical topologies, such as MCM-48, AMS-6, MCM-41, SBA-15, and NFM-1, have a uniform pore diameter and show potential applications in catalysis and adsorption. In contrast, cage-like solids, such as FDU-1, SBA-1, and AMS-8, consist of spherical or ellipsoidal cages that are connected in 3D by connecting windows or smaller cages and are used to control the mass transfer of active agents, for example, in pharmaceuticals.

[0075] The most developed synthesis of mesoporous materials is carried out by mixing alkaline solutions of quaternary alkylammonium compounds (surfactants) and silicates. The resulting nanomaterials resemble liquid crystals or assemblies of micelles. After removal of the surfactant, generally by calcination, the inorganic phase exhibits porosity with variable but well-defined pore sizes. determined, in particular between 2 and 5nm, these different porosities can be created using surfactants of variable chain length as shown in [Fig.2] where the diameter of the porosities before calcination (black dots and solid black line) and after calcination (white dots surrounded by black and dashed black line) shown on the ordinate is a linear function of the number of atoms of the surfactant.

[0076] High-alkalinity syntheses produce materials with wall thicknesses as small as 0.5 nm, corresponding to a silica bilayer. After removal of the surfactant, also called the "structuring element" or "template," the material exposes a highly reactive surface that tends to rearrange itself toward a more stable configuration through silicate migration and recondensation. This rearrangement, which leads to a decrease in specific surface area and a broadening of the pore size distribution, is enhanced by increased temperature and the presence of water vapor. A stable configuration is obtained for wall thicknesses between 1 and 2 nm.

[0077] As an example, one of the most studied mesoporous materials is MCM-41.

[0078] Its purely silicic network is electronically neutral due to the charge (+4) of the Si and four (-1) oxygen atoms. However, substituting another element, such as aluminum, affects the charge density of the structure. Consequently, purely silicic MCM-41 loses its neutrality when Si+4 cations are replaced by Al+3 cations, and the whole structure becomes negatively charged. The distribution of the silicon and aluminum tetrahedral elements in the framework is generally governed by Loewenstein's rule. Thus, according to this principle, Al-MCM-41 consists of alternating silicon and aluminum tetrahedra, which imposes an overall negative charge. The negatively charged lattice is balanced by charge-compensating cations such as Na+, Mg++, K+, Ca++, Ba++, Li+, and Sr++ in the system.

[0079] With regard to mechanical strength, the crushing mechanism of MCM-41, for example, subjected to uniaxial pressure, is in all respects similar to that of macroscopic honeycomb structures. In the case of well-structured solids with wall thicknesses greater than 1 nm, the measured yield strength—which varies with the square of the ratio between the wall thickness and the pore size—corresponds to that of the microcrystals of conventional adsorbents. By applying the shaping procedures known and used for zeolites, for example, it will be possible to obtain millimeter-sized particles perfectly usable in adsorption processes, particularly in VSAs. These agglomeration procedures, intended to obtain, for example, beads or rods, are discussed below.

[0080] More generally, there are families of mesoporous materials other than MCM-41 that also have uniform cylindrical pores or pores with a regular geometric cross-section of the hexagonal type, for example, with adjustable size, and that also allow for the post-treatment incorporation of heteroatoms such as aluminum into the silica structure. This implies, as we have seen, the addition of charge-compensating cations whose specific characteristics (charge / ionic radius) will allow for the modulation of the interaction energy of water molecules with the pore walls of the material. More precisely, cationic doping of a mesoporous material that has undergone aluminum grafting allows for the modulation of the Henry's adsorption constant of water in this material.Selective cationic doping can indeed lead in particular to an increase in water adsorption capacity at low relative humidity, i.e. to obtain a Henry's constant higher than that of activated alumina or silica gel.

[0081] Figure 3, taken from literature data, is intended solely to illustrate the effect of different cations (Li, Na, Mg) on ​​the shape of the isotherm, particularly at low partial pressure, on an MCM-type material, and does not constitute a limiting selection of materials for the application envisaged in this document. This effect of the nature of the cations on water adsorption is not limited to this particular material but is more general, with effects that vary in intensity.

[0082] Indeed, by choosing a mesoporous material as a guard bed, a large number of levers are available to obtain the adsorbent exhibiting the right adsorption / desorption compromise, based in particular on the values ​​of the Henry constant and the isosteric heats, and a significantly improved kinetics. These levers concern the initial synthesis which, after removal of the template, leads to the base material (composition, shape, volume, and pore dimensions, etc.), the surface density of the aluminum groups grafted onto the pore surface, the nature of the charge-compensating cations to modulate the interaction energy (isosteric heat in the Henry region and the Henry constant KH), and possibly the degree of functionalization of the internal pore surface with charged groups.

[0083] In order to be effective, among the large number of possible materials having the chosen characteristics (Henry constant, isosteric heats, pore diameter), the selected adsorbent material will have a high specific surface area, at least on the order of conventional adsorbents, i.e., greater than 600 m² / g and preferably between 900 and 1200 m² / g. The technique used to determine the specific surface area of ​​an adsorbent is well known to those skilled in the art (BET method). Similarly, it will have a high pore volume, greater than that of most conventional adsorbents and preferably between 0.6 and 1 cm³ / g. The mesoporous volume is determined by the Barrett-Joyner-Halenda method (also known as the BJH method).

[0084] Since the materials are produced in powder form with microcrystals ranging in size from 0.1 to 50 microns, more generally between 1 and 20 microns with variable size distributions depending on the synthesis, it is necessary to process them into millimeter-sized particles suitable for use in industrial processes. Agglomeration processes, of the type used for conventional zeolites, consist of forming a paste using a suitable binder, generally clay-based such as kaolin, halloysite, attapulgite, sepiolite, montmorillonite, bentonite, etc. One or more additives may also be added. These additives are preferably organic, for example, methylcellulose, polycarboxylic acids, surfactants, etc.These modifications are designed to facilitate the handling of the paste formed by the microcrystals and the binder by altering its rheology and stickiness, or to give the final agglomerates satisfactory properties, particularly macroporosity and crush resistance. The relative porosity of the binder must not, of course, negate the benefits provided by the mesoporous material. Newly developed high-kinetic binders containing, for example, polymers, can be used for this purpose. The mechanical crush resistance of the particles is conventionally determined using instruments developed for this purpose, such as the "Grain Crushing Strength" device marketed by Vinci Technologies, according to the relevant ASTM standards or equivalent procedures.

[0085] The agglomerated form of the material, formed from the powder obtained from the synthesis, the binder and the additives as usable in a VS A process, can be in the form of beads, extrudates, rods, grains, pellets... These particles then have the following characteristics: - a binder in proportions of 5 to 30% by weight of the mass of adsorbent material (powder, microcrystals), this binder being essentially inert or partially zeoliticized, - an average diameter (beads, grains, etc.) or a length, i.e. the largest dimension (sticks, etc.) between 0.2 and 10 mm, preferably between 0.5 and 5 mm, - a macropore porosity plus a mesopore porosity representing more than 70%, preferably more than 80%, even more preferably more than 90% of the total porosity of the particle.

[0086] It is recalled here that there are several methods for determining pore volumes and pore size, from simple procedures called intrusion (mercury, N2, Ar, He...) and their associated treatments (BET, BJH) to sophisticated methods such as Density Functional Theory (DFT). This allows, for example, the determination of the size and distribution of mesopores, and potentially, with software that allows for a detailed definition of the characteristics of the crystal structure. In addition, direct observation via electron microscopes (SEM, TEM) combined with image processing is used.

[0087] Mesopores are characterized here by their diameter. While many materials do indeed have cylindrical pores, some have different geometries, such as hexagons. Here, the pore cross-section is approximated as a circle of the same area, and the diameter is also referred to in these cases. This is a common approximation (cylindrical pore model) which, moreover, yields a result close to that of a circle whose circumference would be equal to the perimeter of the polygon.

[0088] Apart from the description of the material recommended for stopping water in a VSA, it is interesting to discuss the means now available to those skilled in the art, on the one hand, to restrict the number of candidates and on the other hand, to select the one or those corresponding to the best compromise between adsorption and regeneration, the latter selection being a priori dependent on the VSA process envisaged (high pressure, low pressure, temperature, cycle, regeneration power).

[0089] Molecular modeling / simulation, which encompasses all techniques for simulating the behavior of atoms, ions, and molecules in a given environment, is increasingly used when real-world experiments are costly, time-consuming, or dangerous. Here, it allows for the selection of hundreds of theoretical possibilities arising from potential ranges of composition, size, and post-processing, and makes it possible to obtain a theoretical isotherm for materials meeting the selected criteria. This type of approach is, for example, used for the study of MOFs, which also involve a very large number of possible arrangements.

[0090] Once isotherms have been established for materials exhibiting a Henry's constant, isosteric heats, and pore dimensions that meet the criteria defined in this document, these are tested for the intended application using adsorption process simulation software. Those skilled in the art currently use dynamic simulation software for cyclic adsorption processes. This simulation software is based on a spatial segmentation of the adsorber (into slices called meshes) and a temporal segmentation of the cycle, and performs successive sequential calculations until a stable state is found that satisfies the mass and heat balances as well as all the imposed constraints.

[0091] The combined use of molecular modeling and dynamic simulation of adsorption processes makes it possible, a priori, to determine the few best candidates to test for the intended application and to considerably limit the sample production and the number of pilot tests. The screening of potential materials will begin with the structure corresponding to the family of MCMs that have undergone aluminum grafting and cationic doping, a structure that can lead to adsorbents corresponding to the selection criteria.

[0092] For economic reasons, it will most likely be necessary to select a single material suitable for the different VSA cycles, at least for a given application (VSA 02, CO, CO2, CH4...).

Claims

Demands

1. Use of a silica-based mesoporous adsorbent material in a VS A process for separating a gaseous mixture, said gaseous mixture comprising, in addition to the principal constituents to be separated, water vapor to be retained on a guard bed prior to passing said mixture over the principal adsorbent mass, in order to protect said principal adsorbent mass from moisture, said adsorbent material being characterized by: - ​​a Henry's constant relative to water measured at 30°C of between 6000 and 80000 mmol / bar / g, preferably between 8000 and 50000 mmol / bar / g, - an isosteric heat of adsorption of water in the Henry's range of between 60 and 80 kJ / mol, - an isosteric heat of adsorption of water greater than 40 kJ / mol, preferably between 50 and 65 kJ / mol, for a relative humidity greater than at 30%, - an average pore diameter between 2 and 8 nm, preferably between 2 and 5 nm,These characteristics relate to the powder resulting from the synthesis of the adsorbent material, before shaping from this powder and by adding a binder to create an adsorbent in an agglomerated form.

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