FR2794993A1 - Adsorption process for oxygen and nitrogen production, hydrogen purification, separating hydrocarbons, removing solvents, mercury vapor, etc. uses adsorbent with some non=homogeneous particles - Google Patents

Abstract

An adsorption process for separating components of a gas mixture uses an adsorbent where at least some of the adsorbent particles is non-homogenous. Independent claims are included for the following: (a) An adsorption process for separating components of a gas mixture uses a heterogeneous adsorbent where at least some of the adsorbent particles have a core made of a first material at least partly surrounded by an adsorbent layer that has a different composition. (b) Making particles as in (a). Preferred Features: The particles are spherical. oval or ellipsoidal and are 0.5 - 5.0 mm. The first material forms 15 - 95% of the particle. There may be either a discrete or a gradual change between the two materials. The first material includes metal(s) or ceramic(s).

Description

The present invention relates to a heterogeneous adsorbent, its method of manufacture and its use in a PSA type adsorption process.

Adsorption processes are widely used for purification or separation of fluids. Examples that may be mentioned include the production of oxygen or nitrogen, the drying of atmospheric air, the purification of hydrogen, the separation of hydrocarbons, the stopping of various impurities such as solvents, mercury vapor, carbon dioxide ...

Among these methods, some are lost charge, that is to say that after saturation of the adsorbent by the impurity or impurities to be removed, it is replaced by a new charge.

In the majority of cases, however, the adsorbent is regenerable, often in situ, this regeneration being done by pressure (PSA or pressure swing adsorption), temperature (TSA or Temperature Swing Adsorption) or a combination of the two effects ( PTSA or Pressure Swing Adsorption), usually accompanied by a sweep with a fluid that promotes the transfer of heat and / or material, usually a gas.

The favorable effect of the heterogeneous adsorbent according to the invention will be illustrated on a PSA type process, but its interest and its embodiment are not limited to this case alone, as will be shown by the various other examples of application. cited later in the description.

In general, increasing the productivity of such PSA units is a key factor in reducing their cost because it has a direct effect on the volume of adsorbent to be involved.

For this purpose, a known way to proceed is to reduce the cycle time to produce more gas with a given volume of adsorbent.

To do this, it is essential to use an adsorbent having sufficiently fast adsorption kinetics.

As a first approximation, if the kinetics are improved by a factor of 2, all things being equal, the cycle of such a PSA can be accelerated by a factor of 2 and thus reduce by <B> 50% </ B> the volume of adsorbent to treat or produce the same amount of gas as before.

Since the kinetics are generally controlled by the diffusion of the constituents in the porous network of the adsorbent, a known way of accelerating mass transfer kinetics for the majority of processes of this type is to reduce the hydraulic radius of the adsorbent. for example up to bead sizes of less than 1.5 mm.

However, the use of small particles causes, consequently, a decrease in the permeability of the system.

As a result, when the adsorption and / or regeneration is carried out at low pressure, the increase in the resulting pressure drop leads to a significant increase in energy consumption or, conversely, to a decrease in energy consumption. performance that can counterbalance the investment gain.

In addition, the small diameter particles are more easily set in motion by circulating fluids and to avoid attrition phenomena, their use requires either adsorber geometries or complex and therefore expensive maintenance systems.

An example of such an approach to complex systems is given by the design of radial adsorbers for the separation of gases from air, as described in US-A-5,232,479 or EP-A-820798.

In an attempt to overcome these disadvantages, new adsorbent shaping is being developed combining small thickness of active material and sufficient free passage section for the fluids.

Examples of such novel forms are the monolithic adsorbents perforated as shown diagrammatically in FIG. 1 or, moreover, the deposits or impregnations of active materials on solid porous structures constituting semicircular, triangular, sinusoidal or analogous channels, as described in particular in US-A-5,733,451.

 However, until now, these new products are limited to a few special applications because they lead to investments greater than those corresponding to simple beds of conventional adsorbent particles.

From there, the problem is to be able to have improved high kinetics adsorbent particles, easily used in fluid separation processes, particularly gas mixtures, and produced at an acceptable cost at the industrial level.

The present invention thus relates to a process for separating a gaseous mixture containing at least one preferably adsorbable compound and at least one less preferably adsorbable compound, wherein said preferentially adsorbable compound is adsorbed on adsorbent particles, characterized in that at least a portion of the adsorbent particles have a non-homogeneous composition throughout its volume.

According to another aspect, the invention also relates to a process for separating a gaseous mixture containing at least one preferably adsorbable compound and at least one less preferably adsorbable compound, wherein said preferentially adsorbable compound is adsorbed on heterogeneous adsorbent particles. at least a part of said heterogeneous adsorbent particles consisting of a central core formed of at least a first material and at least one adsorbent peripheral layer encapsulating at least a portion of said central core, said adsorbent peripheral layer being composition different from that of said first material.

Generally speaking, in the context of the present invention, composition refers to the characteristics that define the properties of the adsorption from the point of view of equilibrium and kinetics.

The equilibrium is dependent on the chemical composition of each of the phases constituting the adsorbent, their crystalline structure and their proportion, as well as their degree of mixing. The kinetics is dependent on how the molecules will diffuse to the adsorption sites; it is controlled by the geometry of the porous networks that must be crossed from the gas phase to the adsorption sites. The geometry of a porous network is defined in the first approach by porosity and tortuosity.

Preferably, the method of the invention may comprise one or more of the following features - at least a portion of the adsorbent particles, preferably all the adsorbent particles, comprise a central or internal portion and a peripheral portion or external, the composition of said central or internal portion being different from the composition of said peripheral or outer portion; the composition variation between said inner or central portion and said external or peripheral portion is continuous; the composition variation between said inner or central part and said external or peripheral part is discontinuous; the size of the central nucleus or of the central or internal part represents from 5 to 95% of the size of the heterogeneous particle, preferably from 10 to <95% </ B>; the particles have a general spherical, ovoid or ellipsoidal shape; the central core or the central or internal part of the particles consists of a non-adsorbent inert material; the central core or the central or internal part of the particles is porous or non-porous; the central core or the central or internal part consists of one or more metals, or one or more ceramics; the central core or the central part and the peripheral layer or the external or peripheral part contain at least one binder, the proportion of said binder in the central core being different from the proportion of binder in the peripheral layer; the central core or the central part consists of an adsorbent material, preferably having adsorption kinetics greater than that of the peripheral layer or the outer or peripheral adsorbent part; the peripheral layer or the external or adsorbent peripheral part contains at least one zeolite, preferably the zeolite is exchanged with metal cations chosen from the alkaline, alkaline-earth, divalent transition and trivalent lanthanide metals; the particles have a size of between 0.5 mm and 5 mm, preferably between 0.8 and 3 mm; the gaseous mixture is air, the less preferentially adsorbable compound is oxygen and the preferentially adsorbable compound is nitrogen; the gaseous mixture is a synthesis gas, the less preferably adsorbable compound is at least hydrogen and / or carbon monoxide and the preferable adsorbable compound is at least water vapor and / or carbon dioxide; the gaseous mixture is air, the preferentially adsorbable compound is at least one compound selected from CO 2, water vapor, hydrocarbons, sulfur oxides and nitrogen oxides. the gaseous mixture is a hydrocarbon stream, in particular olefins, the preferable adsorbable compound is at least one compound chosen from CO 2, water vapor; selected from TSA, PSA, PTSA or VSA methods; - selected from non-regenerable adsorption purification processes in situ.

The invention also relates to a process for producing heterogeneous adsorbent particles, in which central cores are coated with a composition of a different nature from that of said central cores.

Preferably, the core is formed of at least one material selected from ceramics, metals, silicas, clays and aluminas and / or in that the composition forming the peripheral coating layer contains at least one zeolite.

Depending on the case, the size of the central core is at least <5% </ B> of the particle size, preferably at least 30%.

According to a first embodiment, the present invention consists in depositing an active material no longer on more or less complex organized solid structures, but in forming adsorbent particles by coating a core having characteristics different from those of the peripheral active material.

An example of such a manufacture will be given below by way of illustration but not limitation.

The majority of the adsorbents used in the PSA units are conventionally synthesized in the form of micron-sized crystal powder.

To be used industrially, a step of shaping by extrusion or bead formation is required. An example of manufacture consists in preparing an internal mixture of zeolite and a powdery binder in which water and, if necessary, pore-forming or fluidifying agents are added.

From this mixture, balls of various sizes can be obtained via turntables.

In a method according to the invention, it is proposed to carry out the progressive deposition of active material on a core of different nature.

Heterogeneous beads are thus obtained consisting of a central part of considerable size in front of the diameter of the particle, for example in volume of 5 to <B> 95%. </ B>

In a first embodiment, the central portion may be inert with respect to the adsorbable compounds of the fluid.

Then, the diameter of the inert core and the outside diameter of the particle are called di (see Figure 2.a).

This heterogeneous particle can be compared to the homogeneous bead of adsorbent of the same outside diameter (see FIG. 2.b), in particular as regards the kinetics.

It is known that the kinetics of adsorption of a gas in an adsorbent particle is often controlled by the transport of this gas in the porous networks and that this kinetics can be reasonably well represented by a model of the Linear Driving Force type, as explained in EP-A-785020.

In general, the overall kinetic coefficient hg is given for particles of various shapes by the following relation (1) hg = (z + 1). (z + 3). Of ff / Rz (1) where.

- R is the characteristic dimension of the particle, - Deff is the effective diffusion coefficient, - z = 0 for a plane, - z = 1 for a cylinder, and - z = 2 for a sphere.

One can, in particular, refer to the thesis of L.M. SUN "Contribution to the study of the kinetics of adsorption of the gases", 1988, UNIVERSITY PARIS VI, FRANCE.

In the selected case (for example) of a spherical particle we find the classical relation (2) hg = 15.Deff / RZ (2) We can refer to "Principles of adsorption and adsorption processes" by DM RUTHVEN, 1984, paragraph 8.5, to confirm the validity of such a relationship.

This means that to improve by a factor of 2 the kinetics of such a ball, it is necessary to reduce its diameter by a factor of 1.41.

At constant gas flow velocity, the linear pressure drop increases by a factor of 2 for the laminar term and by a factor of 1.41 for the turbulent term.

An integral type calculation in the case of the heterogeneous ball defined above makes it possible to determine the increase in global kinetics (Hg / hg) as a function of the ratio (a) of the diameter of the inert central portion to the external diameter of the the particle, ie a = di / de <B> (3) </ B> The results obtained are shown in the table below Table

a <SEP> 0.3 <SEP> 0.4 <SEP> 0.5 <SEP> 0.6 <SEP> 0.7 <SEP> 0.8
<tb> Hg / hg <SEP> 1.10 <SEP> 1.25 <SEP> 1.51 <SEP> 2.03 <SEP> 3.14 <SEP> 6.22 It should be noted in particular that for a ratio a = 0.6, the kinetics of adsorption is twice faster than in the case of a homogeneous adsorbent particle.

The reduction of active material is only 0.63, that is to say less than 22% relative to the homogeneous particle.

At a diameter of unchanged, this type of heterogeneous particles significantly improved kinetics thus allows an acceleration of cycles P.S.A. and a substantial reduction in adsorbent volumes.

More generally, the value of Hg / hg gives an indication of the possible acceleration of the PSA cycle and the product (1-a3). Hg / Hg makes it possible to estimate the potential volume reduction that can be expected with such a heterogeneous adsorbent.

The same type of particles can be used in a TSA process to improve its performance, in particular by replacing the smaller diameter adsorbent layer recommended in US-A-5,728,198.

The same effect as that sought in this document is obtained without reducing the diameter of the adsorbent avoiding, thus the problems of attrition and increase of pressure losses described in this document.

In another embodiment, only the proportion of the zeolite-binder mixture is changed during granulation.

The central part rich in binder and poor in active crystals is weakly adsorbent, which generally leads to higher kinetics. More generally, it is possible to obtain by modifying, continuously or discontinuously, the composition of the zeolite-binder mixture between the core and the periphery of the adsorbent materials having improved physical characteristics.

The outer layer may be, for example, of greater mechanical strength than the inner portion thus making it possible to improve locally the properties of said particle with respect to the resistance to crushing and / or attrition.

The heterogeneous adsorbents obtained according to any method of manufacture may have other advantages over conventional conventional adsorbents.

We have previously seen the interest of an inert core to improve the kinetics of an adsorbent for a PSA process.

It is known, moreover, that the rise in temperature following the adsorption of a constituent and then the cooling subsequent to its desorption adversely affect the performance of the PSA units.

However, the presence of a heart increasing the heat capacity of the particles with a given volume of active substance decreases the temperature fluctuations during the cycle and can, therefore, improve the operation of the PSA unit.

In another embodiment, the nature of the adsorbent or that of the binder may be changed during manufacture.

The thin outer layer of different nature of the core can thus stop a first constituent A, allowing the inner core thus protected to have improved performances for the adsorption of a second component B, the negative effect of A being able to be due to coadsorption, pore blocking, structural modification ... (see Figure 3a and 3b).

 Example <B> <U> of adsorbent preparation </ U> </ B> A granulator for balls is used, consisting of a plate rotating on its axis inclined with respect to the vertical.

Firstly, cooked fine clay particles, obtained by grinding and sieving larger particles, of a size of the order of, for example, 0.1 mm, are introduced.

By keeping the plate in rotation, a flow of powdered clay is brought in as well as water. The powdered clay agglomerates on the pre-existing particles and it stops adding clay when the average size of the balls reaches for example 0.5 mm. It may be possible to allow the plate to turn thus a certain time to refine the homogeneity of the material.

Then, a mixture of zeolite and clay powder is made in the proportions, for example 80/20, and the accretion of the particles present is thus continued until, for example, an average size of 1.0 mm .

Then, the plate can be kept under rotation for a time sufficient to refine the homogeneity of the material.

Finally, the plate is emptied, the filler optionally sieved, then fired at 600 C. The product obtained consists of an inert core of clay binder surrounded by a layer of zeolite bound by the binder.

The relative size of the two agglomerated materials can be adjusted to any value by varying the size of the precursor, the conditions of agglomeration and the proportion of materials deposited during the two different steps.

Also, it is not necessary to use the same binder during the two stages, the choice depending on the desired characteristics, density, mechanical strength, kinetics, porosity, etc.

The starting precursor can be an untreated binder powder, or raw agglomeration particles, or even a third material.

The starting precursor may also consist of particles that are ground or not obtained by a process other than the formation of beads, for example extrusion or the so-called "spray drying" process, or particles are dropped into a stream of water. hot air.

It is also possible to introduce an intermediate step of cooking the inert core before the agglomeration of the active phase.

The material addition sequence can also be reversed, leading to an active heart surrounded by an inert phase.

The two phases can also be active in varying degrees, or vis-à-vis different compounds.

Claims (23)

claims Process for the separation of a gaseous mixture containing at least one preferably adsorbable compound and at least one less preferably adsorbable compound, in which said preferentially adsorbable compound is adsorbed on adsorbent particles, characterized in that at least a part adsorbent particles have a non-homogeneous composition throughout its volume. 2. Method according to claim 1, characterized in that at least a portion of the adsorbent particles, preferably all the adsorbent particles, comprises a central or internal part and a peripheral or external part, the composition of said part central or internal being different from the composition of said peripheral or external portion. 3. Method according to one of claims 1 or 2, characterized in that the composition variation between said inner or central portion and said outer or peripheral portion is continuous. 4. Method according to one of claims 1 or 2, characterized in that the composition variation between said inner or central portion and said outer or peripheral portion is discontinuous. 5. A process for separating a gaseous mixture containing at least one preferably adsorbable compound and at least one less preferably adsorbable compound, wherein said preferentially adsorbable compound is adsorbed on heterogeneous adsorbent particles, at least a part of said particles of heterogeneous adsorbent consisting of a central core formed of at least a first material and at least one adsorbent peripheral layer encapsulating at least a portion of said central core, said adsorbent peripheral layer being of different composition from that of said first material. 6. Method according to one of claims 1 to 5, characterized in that the size of the central core or the central or internal portion is from 5 to 95 s of the size of the heterogeneous particle, preferably 10 to 95%. 7. Method according to one of claims 1 to 6, characterized in that the particles have a generally spherical, ovoid or ellipsoidal shape. 8. Method according to one of claims 1 to 7, characterized in that the central core or the central or internal part of the particles consists of a non-adsorbent inert material. 9. Method according to one of claims 1 to 8, characterized in that the central core or the central or internal part of the particles is porous or non-porous. 10. Method according to one of claims 1 to 9, characterized in that the central core or the central or internal part consists of one or more metals, or one or more ceramics. 11. Method according to one of claims 1 to 10, characterized in that the central core or the central portion and the peripheral layer or the outer or peripheral portion contain at least one binder, the proportion of said binder in the central core being different the proportion of binder in the peripheral layer. 12. Method according to one of claims 1 to 11, characterized in that the central core or the central portion consists of an adsorbent material, preferably having adsorption kinetics greater than that of the peripheral layer or the part external or adsorbent device. 13. Method according to one of claims 1 to 12, characterized in that the peripheral layer or the outer or adsorbent peripheral part contains at least one zeolite, preferably the zeolite is exchanged with metal cations selected from alkaline metals, alkalino -terrous, divalent transition and trivalent lanthanides. 14. Method according to one of claims 1 to 13, characterized in that the particles have a size between 0.5 mm and 5 mm, preferably between 0.8 to 3 mm. 15. Method according to one of claims 1 to 14, characterized in that the gaseous mixture is air, the less preferably adsorbable compound is oxygen and the preferentially adsorbable compound is nitrogen. 16. Method according to one of claims 1 to 14, characterized in that the gaseous mixture is a synthesis gas, the less preferably adsorbable compound is at least hydrogen and / or carbon monoxide and the preferentially adsorbable compound is at least water vapor and / or carbon dioxide. 17. Method according to one of claims 1 to 14, characterized in that the gaseous mixture is air, the preferably adsorbable compound is at least one selected from CO 2, water vapor, hydrocarbons, hydrocarbons and the like. oxides of sulfur and oxides of nitrogen. 18. Method according to one of claims 1 to 14, characterized in that the gaseous mixture is a hydrocarbon stream, especially olefins, the preferably adsorbable compound is at least one compound selected from CO2, water vapor . 19. Method according to one of claims 1 to 18, characterized in that it is selected from the methods TSA, PSA, PTSA or VSA. 20. Method according to one of claims 1 to 18, characterized in that it is selected from non-regenerable adsorption purification methods in situ. 21. A process for producing heterogeneous adsorbent particles, in which central cores are coated with a composition of a different nature from that of said central cores. 22. Manufacturing method according to claim 21, characterized in that the core is formed of at least one material selected from ceramics, metals, silicas, clays and aluminas and / or in that the composition forming the layer of peripheral coating contains at least one zeolite. 23. The manufacturing method according to one of claims 21 or 22, characterized in that the size of the central core is at least 5% of the particle size, preferably at least 30%.