EP1638669A1

EP1638669A1 - Method for prepurifying air in an accelerated tsa cycle

Info

Publication number: EP1638669A1
Application number: EP04767845A
Authority: EP
Inventors: Celina Hemeryck; Christian Monereau
Original assignee: Air Liquide SA; LAir Liquide SA a Directoire et Conseil de Surveillance pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 2003-06-27
Filing date: 2004-06-18
Publication date: 2006-03-29
Also published as: WO2005000447A1; FR2856607B1; US20060254420A1; FR2856607A1; JP2007516059A; CN1812827A; WO2005000447A8

Abstract

The invention relates to a method for prepurifying air by means of adsorption, using two adsorption recipients operating alternately, in parallel, and in a TSA cycle, each recipient containing an adsorbent arranged on a radial adsorption bed, and each adsorption cycle consisting of (a) an adsorption stage during which the impurities are eliminated on the adsorbent at an adsorption temperature (rads), the air crossing the adsorption bed in a centripetal manner; (b) a regeneration stage during which the adsorbent is regenerated by flushing with a regeneration gas at a regeneration temperature (Treg), such as Treg > Tads, in order to desorb the impurities; et (c) an adsorbent cooling stage during which the temperature of the regenerated adsorbent is reduced. The maximum duration (rads) of the adsorption is 120 minutes, preferably between 60 and 120 minutes. The regeneration gas is introduced in such a way that it flushes the bed containing the adsorbent in the centrifugal direction. The maximum regeneration rate is 35 % of the adsorption rate. The regeneration temperature is reached by means of a heat exchanger arrange outside the adsorber.

Description

AIR PREPURIFICATION PROCESS BY ACCELERATED TSA CYCLE

The present invention relates to a TSA process for the prepurification of air intended to be subsequently fractionated by cryogenic means. Conventionally, the air to be distilled by cryogenic means is previously dried, decarbonated and at least partially freed from the secondary atmospheric pollutants which it contains, such as hydrocarbons, nitrogen oxides or the like, by passing the air through one or more adsorbent masses arranged in one or more adsorption zones of an air prepurification unit. This process is commonly called the air prepurification process or more simply the overhead cleaning process. The main object of such a prepurificatjon is to stop and eliminate the various atmospheric impurities likely to be present in the gas flow, until obtaining contents compatible with the good functioning of the cryogenic unit supplied with this air, whether in terms of performance or equipment safety. Indeed, in the absence of such a pre-treatment, there is a condensation and / or a solidification of these impurities, during the cooling of the air to cryogenic temperature, from which it can result from problems of clogging of equipment, in particular heat exchangers, distillation columns, etc. Among the main impurities to be removed are carbon dioxide and water vapor which are always present in the air and which solidify before reach cryogenic temperatures of the order of -180 ° C to -193 ° C, since the water vapor begins to solidify in ice at about 0 ° C, and carbon dioxide crystallizes as soon as

- 56 ° C at its triple pressure and around -130 ^β C at its partial pressure in the air. Furthermore, it is also customary to eliminate, at least partially, so-called secondary impurities, such as certain hydrocarbons (C _π H _m ) saturated or not and nitrogen oxides (NxOy), inevitably present in the air in order to avoid any dangerous concentration of these products in the cold box of the cryogenic separation unit located downstream, in particular to absolutely prevent their concentration in the liquid oxygen vaporizer of this unit, up to a content such that the operational safety of the unit is no longer ensured without other precautions. To do this, a prepurification process can use a single or more adsorbent beds, located in one or more adsorbers, commonly called adsorption bottles. The adsorbents used to remove these impurities are in particular exchanged or non-exchanged zeolites, silica gels, activated and / or doped aluminas, or their combinations or mixtures. Certain adsorbents can contain, in addition to the active phase, a more or less significant quantity of binder, making it possible in particular to reinforce the mechanical resistance of the adsorbent particle, such as its resistance to attrition. Industrial adsorbents are generally used in the form of balls, more or less spherical, ovoid or ellipsoidal, or sticks, such as extrudates, or even more complex shapes. The diameter of these beads (or equivalent dimension when it is a stick) is generally between 1.5 and 4 mm and preferably between 2 and 3 mm. Certain particles are formed from a mixture of several adsorbents, for example a mixture of several compounds of the same nature, such as a mixture of a Nax zeolite with a NaLSX or CaX zeolite, a X zeolite with a A zeolite, or formed from several compounds of different natures, such as a mixture of a zeolite with an activated alumina. Likewise, an adsorption bed can be formed of a single type of adsorbent or of several distinct adsorbents either distributed in juxtaposed or superposed layers, or in intimate mixture and this, in variable proportions. In addition, the adsorbers can be, depending on the case, with a vertical or horizontal axis, or alternatively of the radial type, that is to say that the flow of gas to be purified flows therein either vertically (from bottom to top), horizontally (from right to left, or vice versa), either centripetally (radially towards the axis of the adsorber) or centrifugal (radially away from the axis). When fluids, in particular gases, circulate through masses of adsorbents whose surface is free, it is necessary to ensure that the circulation speed remains lower than the speed setting in motion the particles, to avoid or minimize any attrition or mechanical erosion of the adsorbent particles. This constraint generally fixes the passage section of the adsorber, that is to say, conventionally, its diameter. However, this constraint no longer exists in the case of radial adsorbers in particular, or any other type of adsorber in which the mass of adsorbent is maintained by specific means, such as grids. Indeed, in this case, the dimensions chosen are solely the result of an economic optimization between the cost of the equipment and the energy consumption directly linked to the pressure drops across the system and, to a lesser extent, to the dead volumes. Conventionally, the mass of adsorbent is cyclically regenerated within the adsorber itself by heating and / or gas sweeping, before being used again in the adsorption phase. Currently, two types of process are more particularly used for this purpose, namely the PSA (Pressure Swing Adsorption) processes where most of the regeneration power is provided by a variation or a pressure effect, and TSA (Temperature Swing Adsoφtion = Modulated Temperature Adsoφtion) processes for which the regeneration power is provided by a temperature effect. Less conventionally, hybrid solutions are also proposed for this purpose, in particular in document US-A-5,614,000. Usually, a TSA process cycle of air prepurification comprises the following stages: a) supply of air to an adsorber and purification of the air by adsorption of the impurities at super-atmospheric pressure and at temperature close to ambient, that is to say typically between 0 ° C. and 40 ° C. approximately, b) depressurization of the adsorber up to the pressure of the regeneration gas, generally atmospheric, c) regeneration of the adsorbent at a pressure lower than the adsorption pressure, in particular by a waste gas, typically impure nitrogen at atmospheric pressure originating from the air separation unit and heated to a higher temperature at room temperature by means of one or more heat exchangers. It may also happen that the regeneration gas is at a pressure substantially higher than atmospheric pressure due to the upstream process, for example due to pressure distillation, the regeneration gas being used downstream of the regeneration, for example recompressed. In this case, a short phase can be envisaged during which a regeneration step will be carried out by lowering the pressure of the regeneration gas to atmospheric pressure. d) cooling to ambient or sub-ambient temperature of the adsorbent, in particular by continuing to introduce said residual gas therefrom from the air separation unit, but not reheated, e) repressurization of the adsorber with purified air from, for example, another adsorber in the production phase or from storage capacity. The nitrogen in the air is adsorbed on the adsorbent (s) causing the gas contained in the adsorber to heat up during the repressurization phase. The rise in temperature will depend on the quantity of energy released by the nitrogen addition, therefore by the quantity of nitrogen adsorbed, itself dependent on the conditions of regeneration and absorption. f) inversion of the adsorbers. At this stage, the energy contained in the form of heat in the adsorber is evacuated with the air flow, causing a momentary rise in temperature at the outlet of the adsorber (temperature peak). The amplitude and duration of this temperature peak depends on the amount of energy stored during the repressurization phase as well as on the air flow rate passing through the adsorber. The prepurification unit can be preceded by a step of possible pre-cooling of the air to be purified by means, for example, of one (or more) cold water exchanger, mechanical refrigeration unit or any other similar system. A PSA process cycle for air purification comprises, for its part, substantially the same steps a), b) and e), but is distinguished from a TSA process by an absence of heating of the waste gas (es) during the regeneration step (step c), and therefore also by an absence of step d). Less conventionally, the regeneration can be carried out at a pressure substantially different from atmospheric pressure, either higher as already mentioned (generally in the case of ASD), or even lower than the latter.

(generally in the case of PSA) by using in this case suitable vacuum pumping means. Likewise, the waste gas can be a gas very enriched in oxygen, in the case where the recovered product of the air separation unit is nitrogen, since, in this case, the oxygen is a waste product Not used. Generally, the air pretreatment devices comprise two adsorbers, operating in alternating fashion, that is to say that one of the adsorbers is in the production phase, while the other is in the regeneration phase. Such TSA air purification methods are described in particular in documents US-A-3,738,084 and US-6,093,379. In a prepurification process, the cycle time is defined as the sum of the durations of the addition and regeneration stages. In general, for a cycle with 2 adsorbers and continuous production, the addition time is equal to half the cycle time. The adsorption time in a PSA cycle is of the order of 3 to 30 minutes, while in a TSA cycle, it is of the order of 2 to 8 hours. The volume of adsorbent used in a PSA cycle is generally lower than in an TSA cycle because the effect of the reduction of the cycle, that is to say the least amount of impurities to be stopped per phase, l prevails over the fact that the regeneration of the adsorbent is only partial, which is equivalent to a lower capacity for stopping impurities. Furthermore, the PSA does not require either a heater or a cooler because the regeneration is carried out by sweeping at ambient temperature. However, the drawback of the PSA process is that regeneration requires a large flow of gas. Thus, as a first approach, we can estimate that the volume of gas which must sweep the bed to completely desorb the impurities must correspond to n times the volume of gas treated in the adsorption phase. The value of n is of the order of 1.15 to 1.35 to take into account the different deviations from ideality. Thus, if a flow rate of 100 Nm ³ / h of air is purified at 5 bar absolute for 10 minutes, it will be necessary to regenerate the adsorbent in 7 minutes (and 3 minutes for depressurization and recompression), at 1.25 bar abs, a flow rate of the order of 100x1.25 / 5 x10 / 7 x1.25 = 45 Nm ³ / h, i.e. 45% of the air flow. With more favorable conditions, for example a higher adsorption pressure, this ratio can be reduced but for the most common air separation devices, we will generally be in the zone 30 to 50%. In the case of ASDs, the desorption energy is very generally supplied in the form of heat conveyed within the adsorbent by the regeneration gas heated beforehand. The quantity of gas required is then significantly lower, of the order of 5 to 25% of the gas flow rate to be treated depending on the temperature level adopted and the details of the cycle. It can be seen that the choice between PSA and TSA is generally made on the quantity of regeneration gas available. From there, the air separation units intended to produce a high percentage of valued products, i.e. oxygen, medium pressure and low pressure nitrogen ... will have little residual gas, typically low pressure impure nitrogen, and will therefore necessarily be provided with an overhead purification of the TSA type. Conversely, units intended to produce only a single product, such as oxygen or medium pressure nitrogen, will generally have sufficient waste gas to use a PSA system. In the case of ASD units, recent developments have essentially consisted in wanting to reduce either the cost of the unit (adsorber technologies, choice of adsorbent, etc.), or to minimize the regeneration rate in order to be able to produce even more pure products in the cold box of the cryogenic air separation unit (choice of adsorbent ...). It is theoretically possible to shorten the cycle time of a TSA treatment and, consequently, the volumes of adsorbents to be used, but in doing so, an increase in the rate of regeneration gas is observed. This increase in the regeneration gas flow results from the various thermal inertias of the system, independent of the cycle time and the duration of the heating and cooling stages which are not directly proportional to the cycle time. Indeed, before starting to efficiently heat a bed of adsorbent to the fixed temperature level, it is necessary to take into account the inertia as well as the thermal losses of the heater itself, the heating of the connection pipes and of the equipment that they support, namely the valves, the instrumentation, etc., the heating of the end of the adsorber and possibly of the holding or filtration systems. As these elements are dimensioned as a function of the flow rate of gas to be treated and or of regeneration gas, they are constant or increase in size, when the prepurification cycle time is reduced. Similarly, for cooling, in addition to the "adsorbent" part, it is necessary to take into account the elements to be cooled upstream and downstream. Similarly, the volumes to be depressurized and then to be re-pressurized are not proportional to the volume of adsorbent because a non-negligible, even preponderant, part consists of 'dead' volumes located at the bottom of the adsorbers and in the pipes up to '' to the isolation valves. In practice, for a total regeneration time of 3 hours, we will have 2 hours really effective for heating the adsorbent, desorbing the impurities and cooling. Wanting to go through a 2-hour adsorption (and regeneration) cycle will approximately double the gas flow required. However, in view of the explanations given above, such a speed will generally not be available because otherwise we would then have chosen, a priori, a PSA cycle and not a TSA cycle. Furthermore, the percentage of effectively useful heat decreases because the metallic masses relating to the pipes, valves, measuring and control instruments at the bottom of the adsorber remain constant, while the mass of adsorbents and the impurities to be desorbed decrease during a reduction in the cycle, which amounts to drastically increasing the energy consumption of the overall purification process. Another drawback resulting from the increase in the regeneration gas flow rate is the increase in pressure drops which, here too, results in an increase in energy consumption, unless the diameter of the adsorber is increased by causing so an increase in thermal inertia and dead volumes .... We see that so far the potential improvement in TSA process cycles goes, directly or indirectly, through a reduction in the flow rate required for regeneration. To this end, numerous methods have been proposed for more efficiently supplying heat to the adsorbent beds. Thus, document EP-A-766989 proposes heating only part of the adsorbent bed thanks to a partial PSA type operation. Furthermore, document EP-A-815920 describes the introduction of a heat pulsation in the intermediate part of the adsorber. In addition, document EP-A-884085 proposes a method which implements internal heating of the adsorbent. We can also cite the document US-A-4,312,641 which uses microwaves to provide the energy necessary for the desoφtion; document US-A-4,094,652 which recommends applying an electric current to effect electro-desoφtion of the impurities retained by the adsorbent; and document US-A-2003/00337672 which claims the installation of adsorbent in the tubes of an exchanger thus authorizing cycles of 2 hours instead of a conventional cycle of 6 hours. In this context, the problem which arises is to improve the known air prepurification processes so as to substantially reduce the prepurification cycles by TSA process, in particular the cycle time, and the volume of adsorbent ( s) to be implemented, while retaining both a simple and inexpensive heating system, that is to say by using a standard electric or steam heater and a regeneration rate compatible with the required productions of pure products , that is to say oxygen, nitrogen and / or argon. The solution of the invention is then a process of air prepurification by adsorption using two adsorption containers operating in parallel, alternately and in TSA cycle, each container containing at least one adsorbent arranged in at least one bed. adsorption, each additive cycle comprising at least: a) an additive step during which at least a portion of the impurities contained in the air is removed by adsorption on said adsorbent, at an additive temperature (Tads) , the air passing through the adsorption bed in a centripetal manner, b) a regeneration step during which the adsorbent used in step a) is regenerated by sweeping with a regeneration gas at a regeneration temperature (Treg), such as Treg> Tads, the regeneration gas passing through the adsorption bed in a centrifugal manner, so as to desorb the impurities adsorbed in step a), c) a step of cooling the adso rbant during which there is a reduction in the temperature of the adsorbent having been regenerated in step b), characterized in that: - in step a), the adsorption time (Tads) is between 60 and 120 minutes, - in steps b), and optionally in step c), the regeneration gas is introduced into one or other of the mixing containers so as to sweep the bed containing in the centrifugal direction the adsorbent used in step a), the regeneration flow rate being during these steps less than or equal to 35% of the adsorption flow rate, and - in step b), the regeneration temperature is reached by means a heat exchanger arranged outside the adsorbers. Depending on the case, the method of the invention may include one or more of the following technical characteristics: - before sending the regeneration gas to an adsorber to be regenerated during a step b), an operation is carried out. the regeneration temperature of the regeneration heater used to heat the regeneration gas and of all or part of the heating circuit located between said heater and the adsorber to be regenerated. in step b), at least one heating parameter chosen from the group formed by the heating time, the temperature level and the flow rate of the regeneration gas is controlled so that the maximum temperature at the outlet of each adsorber is lower than minus 30 ° C at the inlet temperature of the adsorber under consideration, preferably at least 60 ° C, preferably at least 90 ° C. For example, for a maximum temperature of adsorption between 50 and 70 ° C, for a maximum regeneration temperature at input of 120 to 160 ° C - in step a), it is used as adsorbent to at least one zeolite and preferably at least one alumina. - In step b), the regeneration gas is nitrogen or a gas rich in nitrogen. - It includes a step of filtration of the gas produced by means of a filtration means located downstream of the adsorbers. - In step b), at least one heat exchanger is used to heat the regeneration gas and at least one bypass circuit arranged so as to allow a bypass of the heat exchanger. - as an adsorbent, a faujasite zeolite of the LSX type without binder is used. - the regeneration flow is between 20 and 30% of the additive flow. - In step a), the adsorption time is between 90 and 120 minutes. a step of distillation or fractionation at cryogenic temperature of the purified air so as to produce nitrogen, oxygen and / or argon. In the context of the present invention, the reduction in thermal inertia is obtained by using one or more adsorbers 1 of the radial type with centrifugal circulation of the regeneration gas, that is to say from the center of the adsorber 1 towards the periphery , and with, on the other hand, centripetal circulation of the air to be purified, that is to say from the periphery towards the center of the adsorber 1, as shown diagrammatically in FIG. 1 appended, which is a view in transverse section of an adsorber 1 of axis AA, usable within the framework of the present invention. More specifically, the air to be purified under pressure is introduced, via a first orifice 10 located in the bottom 12 of the adsorber 1, on the side of the outer peripheral wall 2 of the adsorber containing the adsorbent arranged in a bed 3 of three-dimensional cylindrical shape with hollow central volume 16, that is to say that the air to be purified is sent to the level of the external lateral periphery 8 of the bed 3 of adsorbent. The adsorbent particles constituting the adsorbent bed 3 are retained by two lateral grids 4, 5 perforated with gas passage orifices, situated on either side of the bed 3 of addition so as to keep the particles of said bed 3 in their initial position during the life of the equipment. In addition, the bed 3 rests on a support structure 6 of flat, curved or other shape as appropriate. The air passes through, in a centripetal manner, successively the grid 5, the bed 3 of adsorbent and the grid 4 until it reaches the center 16 of the adsorber 1. The impurities present in the air flow at the temperature d adsorption, typically between 5 and 50 ° C. are adsorbed, during the adsorption phase, on the adsorption bed 3 which is formed of one or more adsorbents, preferably bed 3 contains a layer of alumina and a layer of zeolite, in particular of the faujasite type, in particular a zeolite X or LSX exchanged or not with metal cations. The type of zeolite to be used is chosen according to the impurities to be removed. The purified air is recovered at the center 16 of the adsorber 1 and is evacuated to a place of storage or use, via a second orifice 9 located at the level of the ceiling 11 of the adsorber 1. After a given adsorption time, the adsorber 1 is regenerated by introducing a regeneration gas at a temperature higher than the adsorption temperature, for example nitrogen at a temperature of 50 to 250 ° C, the gas regeneration being introduced into the adsorber 1 through the second orifice 9 and passes through the bed 3 in a centrifugal manner, that is to say that it is introduced into the center 16 of the adsorber and then sweeps the bed 3 of adsorption in the direction of the external wall 2, before being discharged through the first orifice 10. By crossing the bed 3, the regeneration gas becomes charged with impurities, said impurities being desorbed from the bed 3 on which they were trapped during the previous adsorption step. Thanks to this type of configuration, the hot regeneration gas is no longer in contact with the wall 2 of the external envelope of each adsorber 1, which is intended to withstand the pressure mechanically, and the internal metallic mass upstream of the or beds 3 of adsorbents can be reduced significantly. To go beyond, we reduce the size of an internal filter 7 located in the center of the adsorber

1, using for example a filter 7 of height substantially half the height of the beds 3 without conical distribution part, or even this filter 7 is moved outside of the adsorber 1, that is to say by placing it downstream of the prepurification unit so that it is not arranged, inside the adsorber, on the regeneration circuit. Alternatively, as shown in Figure 2, to reduce thermal inertia, one can install a bypass 14 around the heater 13, whether the latter is steam, electric or any other type. Thus, it becomes possible to switch very quickly from the heating phase (for regeneration) to the cooling phase (for addition) with temperature variations of the niche type. Likewise, it is possible to keep the heating circuit 15 warm, for example by leaving the steam inlet open or more generally by keeping the heating means whatever it is at an adequate level, and possibly by creating a small circulation of fluid through this heater 13 and the connection to the adsorbers 1. On the side of the outlet of the bed 3 of each adsorber 1, it is advisable to avoid heating the outer wall 2 more than necessary because this means, on the one hand, that the duration of the heating was too long and, on the other hand, that the cooling will also be longer than necessary. To do this, an energy control system should be installed in order to adjust the heating to what is strictly necessary. One can for example take into account the thermal profile at the exit of the heat front to correct the estimate made a priori according to the operating conditions, as explained in the document EP-A-1080773. Thus, the maximum temperature obtained at the outlet can be significantly lower than the regeneration temperature at the adsorber inlet. This difference depends on the adsorption and regeneration conditions (ie temperature and pressure) but it is generally at least 20 ^° to 30 ° C. Another advantage of using a bed with a radial configuration, in this case, is that the heat losses are reduced to a minimum since the heat front circulating from the inside to the outside is not in contact with the external walls. when heating the adsorbent. The cooling must also be limited compared to what is conventionally done, and all the more so since the adsorber will be provided with an internal isolation device, which will reduce the possibility of heat loss by unnecessary heating of the metal constituting the structure, in particular the walls, of the adsorber. In particular, during the addition stage, the heat of addition of the water increases the air temperature appreciably (temperature variation of 10 ° C for example), which implies that the addition of the C0 ₂ does not happen at the air inlet temperature but at a sum corresponding to this temperature increased by the effect of the heat of addition of water. As the heat front moves faster than the material fronts, it can be shown that it suffices to cool the beds to a temperature equal to or moreover even above this sum of temperatures to obtain identical performance. The final cooling is effected by means of the gas to be treated itself. The continuous improvement of the adsorbents also makes it possible to reduce the thermal capacity of the bed for a given quantity of impurities adsorbed. Therefore, preferentially, the adsorbents developed specifically for this type of purification are used. When the duration of the adsorption step is reduced, the mass transfer zone, that is to say the mass of adsorbent in which it there is no equilibrium between the concentrations of gas phases and adsorbed phases, becomes increasingly important and it is then possible to reduce the volume of this mass transfer zone by using adsorbents of smaller equivalent diameter. Indeed, the limitation of the kinetics being essentially due to the diffusion in the macropores, a reduction in size is accompanied by an increase in the kinetics, and promotes the achievement of the phase equilibria As the diameter of each particle of adsorbent intervenes squarely in determining the adsorption kinetics, even a moderate reduction in diameter leads to a significant improvement in kinetics Thus, it is generally not useful to go down to diameters less than 1 to 1.5 mm even for ASD fast cycle, as described in this document. Certain adsorbents have seen their intrinsic kinetics increase thanks to the recent developments carried out on these products, this is in particular the case of the so-called X zeolites. without binder for which a better particle size has been found to have better kinetics in the form known as without binder than in the older form with binder. In this case, no reduction in diameter is necessary to keep good performance with shortened cycles. As we said, the choice of the size of the balls, the geometry of the adsorber ... is part of the normal work of optimizing the unit and takes into account both investment and energy Reducing the flow of regeneration and therefore the reduction of the cycle time also involves optimizing the depressurization and repressurization stages. The regeneration rate may not be constant for the duration of the regeneration. For example, one can have a higher flow rate during the cooling phase than during the heating phase. This makes it possible to decrease the power of the heater at the outlet temperature of the set heater or at constant installed power to obtain a higher outlet temperature. This reduction in flow rate can also occur only at the end of heating in order to pass a higher heat peak through the adsorbent and thus obtain a better quality of the regeneration. It is noted that this temperature peak can be pushed through the adsorbent with a large flow of regeneration gas, that corresponding to the cooling phase. In order to shorten these steps as much as possible without creating attrition or erosion problems, on the one hand, and without disturbing the operation of the cold box, on the other hand, by cyclic sampling of purified air, we use valves with ramp for opening and / or closing and on the cold box side of the cryogenic distillation unit located downstream, an advanced regulation system playing on the various liquid and gaseous storages to erase or at least flatten the flow disturbances. Thus, contrary to the usual practice, the process of the present invention makes it possible to carry out TSA cycles of duration less than or equal to 240 minutes, that is to say with an adsorption phase less than or equal to 120 minutes, while not requiring only regeneration flows of less than 35%, or even 30% of the air flow, which does not allow, for the same application, to use a PSA type cycle. Comparative example A comparison was made between a conventional ASD cycle with overhead beds, with thermal inertia and current inversion time, that is to say ranging from approximately 30 minutes for the longest cycles to 15 minutes for the longest cycles. short cycles, and an accelerated cycle TSA unit according to the invention, according to the principle of FIGS. 1 and 2. In absolute value, the regeneration flows indicated as a percentage of the air flow to be purified, for the various cases envisaged, are obviously depending on the operating conditions, the internal dimensioning rules specific to each installation considered, the technologies used, the quality of the insulation, the layout, the type of heater ..., but the very mechanism of the comparison remains general and helps to understand the different effects. The TSA cycle is an air cleaning cycle for which the addition stage is carried out at 6 bar abs and at 25 ° C. Table I below gives for different durations of addition the necessary regeneration flow expressed in% of the air flow. Table I

As can be seen in Table I, the notable increase in the gas flow rate (nitrogen for example) necessary for regeneration reflects the preponderant importance of thermal inertias and dead times when the cycle time is shortened since switching from a duration of 180 min to 60 min implies approximately a doubling of the regeneration flow. However, for such high flow rates, it becomes advantageous to use a PSA type cycle rather than TSA but with the drawbacks which result therefrom, as explained above. Starting from test C of Table I, a test D was carried out using a radial type adsorber and a total bypass of the heater, as recommended in the context of the present invention. Similarly starting from this test D, a test E was carried out by introducing, in addition, a shortening of 30% of the transient stages, essentially of the depressurization stage (high flow rate made possible by the radial solution), a system of advanced regulation of the type described in document EP-A-1080773 and improved cooling, that is to say limited to temperature due to the adsorption of water. The results of tests D and E are given in Table II below; test C given for comparison. Table II

We note that by proceeding in this way, we return to a necessary regeneration rate of the type corresponding to that of cycles 2 to 3 times longer (test A and B in Table I) and this, for a duration of addition. only 60 minutes, therefore 2 to 3 times shorter than that of tests A and B. As mentioned above, these flow rates are a function of the operating conditions and would be lower in the event of a lower adsorption temperature or higher pressure, by example if the air should contain less water to desorb. Using a higher regeneration temperature would also reduce the necessary regeneration rate. The process of the invention therefore proves to be advantageous on 'short' or 'very short' cycles, that is to say those of 120 minutes to 60 minutes, respectively, but it must be emphasized that it can be also be for longer cycles, in particular up to 180 minutes or more, because it reduces the regeneration rate and produces a maximum amount of purified air. The technology of 'radial' adsorbers, that is to say with adsorbent bed arranged in the shape of a three-dimensional cylinder, allows great flexibility in dimensioning because, on the one hand, it overcomes the constraints of speed limit, which makes it possible to reduce the passage sections offered to the gases in circulation and, on the other hand, it also makes it possible to install large passage sections associated with thin beds if it is desired to favor the reduction of pressure drops and thereby energy. By way of example, two examples of TSA unit sizes of 'radial' types according to the invention capable of ensuring the purification of a high air flow, namely a flow of 860,000 Nm3 / h at a pressure of 7.5 bars abs. and at a temperature of 21 ° C, are given in Table III below. The regeneration flow in short cycle represents approximately 30% of this flow, or 1.05 bar abs at the outlet of the TSA unit for a regeneration temperature of 150 ° C. Depending on the local cost of energy, one can for example adopt one or the other of the following sizing. Table III

Note, however, that even with conventional adsorbers, with supeφosed beds, the implementation of certain aspects of the process of the invention, such as bringing the heater and regeneration circuit to the prior regeneration temperature, makes it possible to reduce the regeneration rate and thereby the given available regeneration rate cycle time. However, in this case, the adsorbent beds are necessarily thin and for large air flow rates to be purified, that is to say of the order of at least 100,000 Nm ³ / h, the adsorbers will be difficult to industrialize and will present significant dead volumes, ie that they will be “cheese box” type adsorbers. Rather, spherical or preferably cylindrical adsorbers with a horizontal axis will be used.

Claims

claims

1. Method of air prepurification by adsorption using two adsorption containers operating in parallel, alternately and in TSA cycle, each container containing at least one adsorbent arranged in at least one adsorption bed, each cycle d adsorption comprising at least: a) an adsorption step during which at least part of the impurities contained in the air is removed by adsorption on said adsorbent, at an adsorption temperature (Tads), the air passing through the bed of centripetal adsorption, b) a regeneration step during which the adsorbent used in step a) is regenerated by sweeping with a regeneration gas at a regeneration temperature (Treg), such as Treg> Tads, the regeneration gas passing through the adsorption bed centrifugally, so as to desorb the impurities adsorbed in step a), c) a step of cooling the adsorbent during which a d decrease in the temperature of the adsorbent having been regenerated in step b), characterized in that: - in step a), the duration of adsorption (Tads) is between 60 and 120 minutes, - in steps b), and optionally in step c), the regeneration gas is introduced into one or other of the adsorption containers so as to sweep in the centrifugal direction the bed containing the adsorbent used in step a), the regeneration flow rate being during these stages less than or equal to 35% of the adsorption flow rate, and - in step b), the regeneration temperature is reached by means of a heat exchanger arranged at the outside of the adsorbers.

2. Method according to claim 1, characterized in that, before the regeneration gas is sent to an adsorber to be regenerated during a step b), the regeneration heater serving for regeneration is brought to the regeneration temperature. reheat the regeneration gas and all or part of the heating circuit located between said heater and the adsorber to be regenerated.

3. Method according to one of claims 1 or 2, characterized in that in step b), at least one heating parameter chosen from the group formed by the duration of heating, the temperature level and the flow rate of the regeneration gas so that the maximum temperature at the outlet of each adsorber is at least 30 ° C. lower than the temperature at the inlet of the adsorber under consideration, preferably at least 60 ° C.

4. Method according to one of claims 1 to 3, characterized in that in step a), is implemented as an adsorbent at least one zeolite and, preferably, at least one alumina.

5. Method according to one of claims 1 to 4, characterized in that in step b), the regeneration gas is nitrogen or a gas rich in nitrogen.

6. Method according to one of claims 1 to 5, characterized in that it comprises a step of filtering the gas produced by means of a filtration means located downstream of the adsorbers.

7. Method according to one of claims 1 to 6, characterized in that in step b), at least one heat exchanger is used to heat the regeneration gas and at least one bypass circuit arranged so as to allow a heat exchanger bypass.

8. Method according to one of claims 1 to 7, characterized in that a faujasite zeolite of the LSX type without binder is used as adsorbent.

9. Method according to one of claims 1 to 8, characterized in that the regeneration flow is between 20 and 30% of the adsorption flow and / or in step a), the duration of adsorption (Tads) is between 90 and 120 minutes.

10. Method according to one of claims 1 to 9, characterized in that it further comprises a step of distillation or fractionation at cryogenic temperature of the purified air so as to produce nitrogen, oxygen and / or argon.