FR2830462A1 - Adsorber with at least one adsorption bed containing granular adsorbant, where the grain size at the edge of the bed is smaller than the rest of the granules to avoid edge effects - Google Patents

Abstract

The adsorption bed (2, 3) has around its edge relative to the general direction of fluid circulation, granules (2B, 3B) with a smaller grain size than those (2A, 3A) in the rest of the adsorption bed Adsorption device for adsorbing at least one components of a fluid entering a box (1) of apparatus via a fluid inlet (11). This box contains at least one adsorption bed (2, 3) made of adsorbant material(s) in the form of granules placed so they can be penetrated by the fluid and so the fluid flows in the adsorption bed overall through the thickness of the bed. At least one adsorption bed contains a single material in granules of different grain size, or it can contain two materials of different grain size and nature. Alternatively, the adsorber contains at least two adsorption beds for adsorbing different components. The ratio (dg(B)/dg(A)) of the diameter of the smaller granules to the normal granules is at most equal to the ratio (Cap(B)/Cap(A))<2> of the squares of the respiration capacities of the two parts of the bed.

Description

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The invention relates to adsorption devices or adsorbers, and in particular adsorbers for cleaning the air of cryogenic air separation units.

In this application, the adsorbers follow a so-called TSA (Temperature Swing Adsorption) operating cycle, in which regeneration is obtained mainly by circulation of a hot regeneration fluid (waste nitrogen).

In industrial cryogenic air separation units, the adsorbers used as air scrubbers at the top of a cold box have significantly lower performance than what could be expected from extrapolations of results. '' laboratory tests under ideal conditions, regardless of the technique used (vertical, horizontal or annular beds, etc.).

As a result, it is necessary to significantly oversize the adsorbers, with respect to what there is good reason to assume to be sufficient from laboratory tests, and that by at least 20%.

It appears from various researches that an important cause of the limitation of the performance which is obtained from industrial adsorbers is the inhomogeneity of the operating conditions of the beds and more particularly that which exists near the walls as a result of the imperfection of flow distribution, heat losses during regeneration, etc., leading to poor regeneration of the bed near the wall of the adsorber. These unfavorable phenomena are known as edge effects.

The object of the invention is to remedy this drawback and to this end relates to an adsorption device for adsorbing at least one component of a fluid entering a housing of the device through a fluid inlet thereof, this housing containing at least one adsorption bed made of adsorbent material (s) in the form of granules arranged so as to be penetrated by the fluid and so that this fluid flows in the adsorption bed generally in the direction in which the thickness of the bed extends, characterized in that the bed comprises, in its marginal region with respect to the general direction of circulation of the fluid, granules having a particle size smaller than that of the granules of the rest of the fluid adsorption bed.

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With this arrangement, homogeneity defects near the walls are effectively compensated.

The adsorber can also have one or more of the following characteristics: it comprises at least one adsorption bed made up of a single material in granules of different particle sizes; - It comprises at least one adsorption bed consisting of at least two granular materials of different natures and particle sizes; - it has at least two adsorption beds for adsorbing different components; - The ratio of the diameter of the granules of larger particle size is at most equal to the ratio of the squares of the breathing capacities of the two parts of the bed.

Other characteristics and advantages of the invention will emerge from the following description of embodiments of the invention described by way of nonlimiting examples and illustrated by the accompanying drawings, in which: FIG. 1 is a schematic section of part of an apparatus according to the invention; - Figure 2 is a schematic sectional view of part of another embodiment of an apparatus according to the invention; and - Figure 3 is a graph illustrating the characteristics of the granules constituting at least one adsorption bed of the devices partially schematized in Figures 1 and 2.

The adsorption devices or adsorbers of Figures 1 and 2 are intended to extract from a fluid such as for example compressed air intended for an air distillation installation, the gaseous impurities formed, in the case of air, mainly by water vapor and carbon dioxide, but also by other impurities present in atmospheric air such as nitrogen oxides, ozone and hydrocarbons.

To this end, conventionally, these adsorbers comprise a housing 1 equipped with an inlet 11 for introducing the impure fluid into the device and an outlet 12 for discharging the purified fluid. The housing 1 contains one or more successive adsorption beds 2, 3 consisting of adsorbent material (s) in the form of granules.

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Typically, these adsorbers comprise a bed of alumina 2 for adsorbing water vapor and a bed of zeolite 3 for adsorbing carbon dioxide, or else a single bed suitable for adsorbing these two impurities.

When the fluid to be purified comprises several undesirable components, it is also possible to have several adsorbers in cascade, each intended to fix a different component, or one more adsorber each intended to fix several components by means of one or more beds. adsorption.

The adsorber can also contain several adsorption beds intended to fix the same component, made of the same material and having equal or different particle sizes, or of different materials.

The granular materials constituting the adsorption beds are conventionally regenerable materials, for example thermally; their regenerative thermal energy is supplied to them by a stream of hot gas (for example nitrogen) which is passed through the adsorption bed, for example in the same direction as the fluid to be purified, in the same direction or, more frequently, in the reverse direction, to desorb from the granules the component previously adsorbed.

In Fig. 1, schematically illustrating a conventional bunk bed adsorber 2, 3, each bed is constituted as a volume of granules extending in an approximately planar layer having a certain thickness; the bed is arranged so as to be penetrated by the fluid to be treated and so that this fluid flows therein generally in the direction in which the thickness of the bed extends, that is to say here vertically and from bottom up.

In FIG. 2, schematically illustrating an adsorber with annular and concentric beds 2,3, each bed is constituted in the form of a volume of granules extending in an approximately tube-shaped layer, for example cylindrical having a certain radial thickness. ; the bed is also arranged so as to be penetrated by the fluid to be treated and so that this fluid flows therein generally in the direction in which the thickness of the bed extends, that is to say here radially and centripetally.

In both cases, in the absence of special precautions, in steady state, the adsorption is weaker in the marginal region of the bed with respect to the general direction of flow of the fluid.

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More precisely, in the case of FIG. 1, the adsorption is weaker in the vicinity of the periphery of the layer of adsorbent granules, and in the case of FIG. 2, it is weaker in the vicinity of the slices of end of the granule tube. By in the absence of particular precaution is meant: when the material is conventionally identical in nature and particle size throughout the volume of the bed.

It has been assumed and verified that this lower adsorption is due to less complete regeneration in said marginal regions of the bed (s), as a result of thermal losses through the walls and / or an uneven distribution of the circulating fluids. .

Also, according to the invention, in order to increase the adsorption in these regions, it has been planned to reduce therein the flow rate per unit area (the flow) of the fluid to be purified. The pressure drop being, as a first approximation, proportional to the diameter of the particles, according to the invention, in the marginal regions of the bed 2, 3, the granules 2B, 3B are available having a particle size smaller than that of the granules 2A. , 3A from the rest of the bed, so that the flow of fluid generally in the direction of the thickness of the bed has, in these marginal regions, a lower flow rate than in the rest of the bed.

More precisely, in the case of FIG. 1, the particle size is chosen to be smaller in the vicinity of the periphery of the layer of granules, and in the case of FIG. 2, smaller in the vicinity of the ends of the tube of granules.

In practice, for certain adsorbents and certain geometries of the bed, it is possible to choose, for the same bed, two adsorbent materials of different natures and of different particle sizes, the material used in the marginal region being of better quality, within the limits, of course. , at an acceptable cost.

Thus, in the case of two concentric beds (figure 2), including an outer bed 2 of alumina to fix the water and an inner bed 3 of molecular sieve to fix the C02: - bed 2 consists entirely of granules 2A , 2B of the same quality of alumina; but - bed 3 consists, for the current granules 3A, of 13X type zeolite having a high Si / Al ratio, typically greater than 1.15 and, for the marginal granules 3B, of 13X type zeolite having an SI ratio / AI low, typically less than 1.15.

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By way of indicative, but not limiting, example, in a case such as that of FIG. 1, for a circular bed 2, 3 having a diameter of approximately 2 meters, the secondary adsorbent can extend in the form of a ring. circular over a width measured radially, from ten to twenty centimeters, this width depending in particular on the particle size and the nature of the granules, heat losses, the thickness of the bed, etc.

In any event, the two types of granules will hereinafter be designated by the two adsorbents, whether the granules are of the same material or of different materials.

In FIG. 3, a given bed is considered, for example bed 2. Ads (A) and Reg (A), respectively, denote the adsorption and regeneration capacities of the main adsorbent granules 2A constituting the most large part of the bed, and Ads (B) and Reg (B), respectively, the adsorption and regeneration capacities of the secondary adsorbent granules 2B constituting the marginal part of the bed. These values are masses of adsorbal impurities per unit volume of adsorbent, denoted M / m3, and the corresponding temperature values (in kelvins), for the TSA cycle considered, are shown on the abscissa.

In figure 3, the quantities Cap (A) and Cap (B) denote the breathing capacities of the two parts of the bed: Cap (A) = Ads (A) -Reg (A), and Cap (B) = Ads ( B) -Reg (B).

According to the invention, to minimize the edge effects, adsorbents are chosen having adsorption and regeneration characteristics such that the difference between the adsorption and regeneration capacities of the granules (2B, 3B) of smaller particle size is at least equal to the difference between the adsorption and regeneration capacities of the other granules (2A, 3A) multiplied by the ratio of the corresponding respective volume flow rates. Indeed, if this relationship is verified, the marginal region of the bed is at least as effective in adsorption as the current region.

Moreover, if we denote by Qads (A) and Qads (B), the quantities of impurities to be adsorbed per unit of volume of adsorbent during each cycle, expressed in kg / m3, respectively for the two adsorbents , and by dg (A) and dg (B), the respective diameters of the granules of the two adsorbents expressed for example in millimeters, these quantities are linked, at constant pressure, by the relation:

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Therefore, so that, between two regenerations, the adsorption per unit volume of secondary adsorbent in the vicinity in the marginal region of the bed, is at least equal to the adsorption per unit volume of primary adsorbent in the remainder of the bed , it is necessary that :

Thanks to the invention, the imperfection of the known adsorbers, usually causing them to oversize, due to the edge effects of the beds near the walls which delimit the beds is, not really eliminated, but effectively treated, which allows for substantial savings.

This invention, according to which the main adsorbent corresponds to the normal function of the bed, and the secondary adsorbent has a smaller particle size limiting the flow rate therethrough, preferably with the quantitative relationships stated above, is generally usable, in many separation or purification processes with adsorber (s) with temperature modulation (TSA) or pressure modulation (PSA), with one or more beds.

Claims (5)

1. Adsorption apparatus for adsorbing at least one component of a fluid entering a housing (1) of the apparatus through a fluid inlet (11) thereof, said housing containing at least one adsorption bed (2,3) consisting of adsorbent material (s) in the form of granules arranged so as to be penetrated by the fluid and so that this fluid flows in the adsorption bed generally in the direction in which the thickness of the bed extends, characterized in that the bed (2,3) comprises, in its marginal region with respect to the general direction of circulation of the fluid, granules (2B, 3B) having a particle size more lower than that of the granules (2A, 3A) of the rest of the adsorption bed. 2. Apparatus according to claim 1, characterized in that it comprises at least one adsorption bed (2,3) consisting of a single material in granules of different particle sizes. 3. Apparatus according to any one of claims 1 and 2, characterized in that it comprises at least one adsorption bed (2,3) consisting of at least two granular materials of different natures and particle sizes. 4. Apparatus according to any one of claims 1 to 3, characterized in that it comprises at least two adsorption beds (2,3) for adsorbing different components. 5. Apparatus according to any one of claims 1 to 4, characterized in that the ratio (dg (B) / dg (A)) of the diameter of the granules of smaller particle size to that of the granules of larger size is at most equal to the ratio [Cap (B) / Cap (A)] of the squares of the breathing capacities of the two parts of the bed.