RU2574439C2 - Air purification - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: in method of reduction of water, CO₂ and N₂O in raw material air applied is first adsorbent, such as aluminium oxide (25-40 vol %), and second adsorbent, such as zeolite X (60-75 vol %); time of adsorbent work is determined by determination of concentration, measured by means of analyser for concentration of CO₂ in position within the range of second adsorbent length, when maximal N₂O level is obtained simultaneously on lower by stream end of second adsorbent in direction of supply, where time of work is the time from the start of raw material air passage into first and second adsorbents before measuring by analyser of certain CO₂ concentration; at least, second adsorbent is regenerated by means of heated regeneration gas at temperature from 140 to 220°C and molar ratio of regeneration gas to raw material air, supplied for one cycle iteration, constitutes 0.08-0.5.

EFFECT: increase of air purification degree.

27 cl, 11 tbl, 5 dwg

Description

BACKGROUND

[0001] The invention relates to the removal of water, carbon dioxide and nitrous oxide, and in some cases also hydrocarbons, from the air stream before cryogenic separation of air.

[0002] Cryogenic separation of air requires a preliminary purification step to remove both high boiling and hazardous materials. The main high boiling air components include water and carbon dioxide. If the removal of these impurities from the surrounding raw air is not achieved, then water and carbon dioxide can freeze in cold sections of the separation method, such as heat exchangers and liquid oxygen collectors (LOX). This can lead to pressure drops, flow changes and operational problems. Various hazardous materials must also be reduced, including acetylene and other hydrocarbons. High boiling hydrocarbons, if not removed, can be concentrated in the liquid oxygen section of the column, which leads to a potential explosion hazard.

[0003] It is known that nitrogen oxides must also be removed. The minor component of the air is N ₂ O nitrous oxide, which is present in ambient air at about 0.3 ppm. It has similar physical properties to carbon dioxide and, therefore, poses a potential operational problem due to the formation of solids in the column and heat exchangers of the cryogenic distillation unit. In addition, nitrous oxide is known to improve the combustion of organic materials and is shock-sensitive.

Therefore, nitrous oxide also poses a safety hazard. Hydrocarbons, such as ethylene, acetylene, butane, propylene and propane, are additional impurities that it is desirable to remove before cryogenic separation of air.

[0004] Preliminary air purification is usually carried out using adsorption getter methods. They can operate by means of variable temperature adsorption (TSA), as described in US Pat. Nos. 4,541,851 and 5,137,548 or U. Gemmingen ("Designs of Adsorption Dryers in Air Separation Plants" Technology Reports 54/1994, Linde), by means of Variable Pressure Adsorption (PSA), as described in US patent 4711645, 5232474 or C. In. Skarstrom (“Heatless fractionation of gases over solid adsorbents” vol. II, 95, N.V. Lee (U), CRC Press, Cleveland, Ohio 1972), or through variations of these methods, such as thermally enhanced pressure swing adsorption (TEPSA ), as described in US Pat. No. 5,614,000, or adsorption with variable temperature and pressure, as described in US Pat. No. 5,855,650.

[0005] Typically, air pre-treatment is carried out by adsorption of polluting gas components from air by adsorption on a solid adsorbent with periodic regeneration of the adsorbent. In such methods, air is supplied in contact with at least two layers of solid adsorbents in order to adsorb components in order to remove them; the concentration of these components gradually increases in the adsorbents. The concentration of each of the removed gas components in the adsorbent will not be uniform, but it will be the highest at the upstream end of the adsorbent layer and will decrease gradually along the mass transfer zone in the adsorbent. If the method is carried out for an indefinite time, the mass transfer zone will gradually move downstream in the adsorbent bed until the component to be removed leaves the downstream end of the bed. Before this, it is necessary to regenerate the adsorbent.

[0006] In variable pressure adsorption (PSA) systems, this is done by stopping the flow of gas to be treated in the adsorbent, depressurizing the adsorbent, and typically by passing a flow of regenerating gas with a low content of the component adsorbed on the bed, through the bed countercurrent to the direction of flow of the product. Since the component, which is removed, is adsorbed while the layer is in operation, the adsorption process will generate heat of adsorption, causing a heat pulse to move downstream through the adsorbent. During the regeneration process, heat must be supplied to desorb the gas component that has been adsorbed on the bed. In PSA variable pressure adsorption systems, one is directed towards the start of regeneration before the heat pulse reaches the downstream end of the layer; the direction of movement of the heat pulse is changed using the countercurrent flow of the regenerating gas, and the heat obtained from the adsorption of the gaseous component in question is used to desorb that component in the regeneration process. One thus avoids the need for heat supply during the regeneration step. However, the short cycle time (usually 10-15 minutes), used to avoid the heat pulse leaving the adsorbent bed, requires frequent depressurization of the bed during which the feed gas is released and lost (“distribution loss”). In addition, usually two layers of adsorbent are used, one of which is in operation, while the other is regenerated. Pressure relief and regeneration of one layer should take place within a short time, during which the other layer is in the process of operation, and rapid restoration of pressure can lead to changes in the transient process in the raw materials and product flows, which can adversely affect the operation of the installation.

[0007] An alternative procedure is known as variable temperature adsorption (TSA). In the TSA, the cycle time is increased (the delivery time is usually 2-12 hours) and the heat pulse mentioned above is allowed to exit the downstream end of the adsorbent layer during the supply of raw materials or during operation. To achieve regeneration, therefore, heat must be supplied to desorb the adsorbed gas component. To this end, the regeneration gas used is heated for a period to produce a heat pulse moving through the bed, countercurrent to the normal flow direction. This heated regenerating gas stream is usually accompanied by a cold regenerating gas stream that continues to move the heat pulse through the bed toward the upstream end. TSA has a longer cycle time than PSA. TSA is energy-intensive since it is necessary to supply regenerative gas heated to a high temperature, such as 150-200 ° C, to ensure the desorption of the most strongly adsorbed component from the layer. Typically, the air to be treated is also precooled in order to minimize the amount of water that must be adsorbed on the bed, which further increases production and energy costs.

[0008] In the conventional TSA air pre-treatment method, a double layer is used to remove substantially all of the water and carbon dioxide present in the feed air stream. Since water is more heavily adsorbed by the two compounds, the layers are usually used until carbon dioxide begins to break through the adsorbent layer. More CO ₂ than N ₂ O is present in the raw air stream, but since 13X has a greater ability for CO ₂ than for N ₂ O, if the layers work before the CO ₂ breakthrough, a significant amount of N ₂ O will break out of the layer, and can cause downstream problems in a cryogenic distillation unit.

[0009] US patents 4249915 and 4472178 disclose an adsorption method in which moisture and carbon dioxide are removed from atmospheric air by adsorption in separate respective layers. The moisture-containing layer is regenerated by PSA for a relatively short duty cycle, while the layer containing carbon dioxide is thermally regenerated at significantly longer time intervals. Although there are certain advantages of this device, production costs are high due to duplication of columns and the need for additional equipment to implement both systems of regeneration of the corresponding layers.

[0010] Wenning ("Nitric Oxides in Air Separation Units" W. Wenning, Transactions of MUST 96, paragraphs 32-36) describes how carbon dioxide can displace already adsorbed nitrous oxide from a zeolite adsorbent, causing a breakthrough of nitrous oxide at a higher concentration than that in the surrounding air.

[0011] US Pat. No. 5,919,286 teaches that a zeolite layer (17% by volume) at the product (downstream) end of an alumina layer can be used to remove nitrogen oxides in the PSA method.

[0012] EP 0992274 describes a method for removing carbon dioxide, water and nitrous oxide from air, preferably in the TSA method, which uses a three-layer adsorbent, with a first layer, for example, of aluminum oxide, predominantly adsorbing water, a second layer, for example, of 13X, predominantly adsorbing carbon dioxide, and a third layer, for example, of CaX, predominantly adsorbing nitrous oxide.

[0013] US Pat. No. 5,846,295 describes a TSA method for removing CO ₂ and H ₂ O, using impregnated alumina, in some cases in combination with a zeolite, such as 13X, at the product end of the layer. The method works before the breakthrough of CO ₂ from the end of the layer, and the ratio of the heating time to the time required for the desorption of CO ₂ and water adsorbed on the layer is between 54% and 38%.

[0014] US Pat. No. 5,614,000 describes a method for removing water and CO ₂ from air, in which an adsorbent layer, preferably containing only alumina, can be regenerated in part by TSA and partly by PSA, with a part of the adsorbent that adsorbs water (upstream ), being regenerated by PSA, while the residue is regenerated by TSA, using a temperature of the regenerating gas of about 70 ° C. This method is known by the acronym TEPSA. This method works before the CO ₂ breakthrough from the end of the layer, and the ratio of the heating time to the operating time required for the desorption of CO ₂ and water adsorbed on the layer is usually about 33% (Tables 2 and 3 show the heating time / operating time - 10 / 30 = 0.33).

[0015] US Pat. No. 5,855,650 describes a method for removing water and CO ₂ from air, in which an adsorbent layer comprising an alumina layer and a 13X zeolite layer, or one full alumina layer, is regenerated by TSA using a gas temperature of about 100 ° C at the bottom upstream, while the upstream portion on which water is adsorbed is partially regenerated by TSA and partially by PSA. Such a process is known by the acronym TPSA. This method works before the breakthrough of CO ₂ from the end of the layer, and the ratio of the heating time to the time required for the desorption of CO ₂ and water adsorbed on the layer is 46% and 35% in examples 2 and 3, respectively.

[0016] PCT / EP 2012/060317 describes a method for removing nitrous oxide, carbon dioxide and water from a feed air stream, in which the feed air stream is passed through a first adsorbent having Henry selectivity to CO ₂ compared to N ₂ O at least 12.5, and a second adsorbent, occupying from 25% to 40% by volume of the total volume of the first and second adsorbent, whose constant according to Henry's law for the adsorption of CO _{2 is} less than 1020 mmol / g / atm and whose selectivity Henry's law with respect to CO ₂ over N ₂ O, is not more than 5, wherein Adsorbents are regenerated by means of a first regenerating gas having a temperature between 20 ° C and 80 ° C and from 10 ° C to 60 ° C higher than the temperature of the feed gas, and then by means of a second regenerating gas having a lower temperature than the first regenerating gas .

[0017] WO 2005/000447 describes a method in which the use of an adsorbent through which radial flow paths are provided, reduces cycle time for the TSA method to remove CO ₂ and H ₂ O from the feed air stream, and also reduces heat loss, and therefore improving process efficiency. The use of a radial layer plays an important role in preventing heat loss to the outer parts of the absorbent apparatus.

[0018] CA 804391 relates to a method of drying air, and teaches that a moisture-absorbing layer can be used effectively, despite fluctuations in the moisture level in the raw air, by monitoring the position of the water adsorption front within the layer and regenerating the layer as soon as the front reaches the selected position inside layer.

[0019] It is known that in some places, the ambient level of CO ₂ present in the air has grown significantly compared to levels that prior art methods needed to be addressed. For example, when an air separation unit is located in an area where there is heavy industry, it often happens that elevated levels of CO ₂ will be observed in the air.

[0020] The selectivity represented by the adsorbent for one gas compared to the selectivity for another gas can be expressed as the ratio of constants according to Henry's law (initial angular coefficients of isotherms) for two gases at 30 ° C.

[0021] The present invention is directed to a method for removing high levels of water, carbon dioxide and nitrous oxide, and preferably also hydrocarbons, such as propane, ethylene, propylene, acetylene and / or butane, present in ambient air. In particular, it is an object of the present invention to provide higher levels of N ₂ O removal that are obtained in the methods described in US Pat. Nos. 5,846,295, 5,614,000, 5,855,650 and WO 2005/000447. Without using CaX as the final adsorbent layer, if TSA systems in the literature work as described before the CO ₂ breakthrough, then the removal of N ₂ O will be less than 30-70% depending on the operating conditions and layer schemes.

[0022] An additional object of the present invention is to provide a method in which the level of breakthrough of nitrous oxide and, if present, hydrocarbons depends on the level of carbon dioxide inside the adsorbent, so that ensuring that the level of carbon dioxide at a selected point in the adsorbent layer is below the required threshold, ensures that the level of nitrous oxide and, if present, hydrocarbons are also below the required threshold.

[0023] An additional object of the present invention is to provide a more economical method of processing large volumes of air per unit volume of a layer than is provided in PCT / EP 2012/060317.

[0024] An additional objective of some embodiments of the present invention is to reduce the ratio of heating time to operating time compared with known prior art methods using thermal regeneration of at least a portion of the adsorbent layer.

[0025] An additional object of the present invention is to avoid the use of extremely moisture-sensitive adsorbents, such as CaX. Since CaX’s ability is very dependent on water load, the use of this adsorbent requires great care when taken during loading and operation to ensure that it does not come in contact with water, especially where a steam heater is used to supply heated regenerating gas . Additionally, since a temperature above 220 ° C is used to regenerate CaX in order to remove any adsorbed water, the exclusion of this adsorbent provides for the use of an electric heater in addition to the steam heater, with proper cancellation.

[0026] An additional object of the present invention is to enable an existing installation configured for TSA using three layers to remove H ₂ O, CO ₂ and N ₂ O, to be adequately improved, to ensure N ₂ O removal without using CaX and without the need to increase the size of the layer adsorbent.

[0027] An additional objective of some embodiments of the present invention is to reduce the molar ratio of purge to air used, i.e. reduce the amount of regeneration gas needed compared to the amount of feed gas supplied during operation for the adsorbent bed, compared to those of conventional PSA or TEPSA methods.

[0028] Another additional objective of some embodiments of the present invention is to provide a method for determining the conditions under which the removal of N ₂ O, CO ₂ and water can be carried out with a given set of adsorbents to provide the desired degree of removal of CO ₂ , N ₂ O and H ₂ O.

[0029] Another additional objective of some embodiments of the present invention is to provide a range of operating conditions allowing an increase in effective working time for the adsorbent layer, thereby reducing distribution losses, and / or reducing the required regeneration flow rate.

[0030] An additional objective of some embodiments of the present invention is to provide an improvement to an existing plant so that it can provide improved nitrous oxide removal.

[0031] An additional object of the present invention is to provide an installation and conditions under which the elevated levels of CO ₂ and / or N ₂ O present in the feed air stream can be removed.

SHORT DESCRIPTION

[0032] The authors of the present invention sought to provide conditions under which the removal of CO ₂ and water from the feed air stream, plus reduction to the desired degree of N ₂ O level in this feed air stream, can be achieved without the use of highly sensitive to water and expensive adsorbents and not incurring large distribution losses or not requiring excessively high regeneration temperatures. With this task in mind, the inventors are proceeding to further define, although effective conditions could be achieved to reduce the levels of CO ₂ , N ₂ O and water on a layer having a first adsorbent, such as alumina, and a second adsorbent. such as 13X. To do so contrary to the teaching, for example, EP 0992274, which teaches that the third layer of adsorbent having a high selectivity of N ₂ O / CO _2, such as CaX, in addition to the first two adsorbents is required in order that the nitrogen oxides can be removed .

[0033] Referring to FIG. 1, the inventors have determined breakthrough curves for nitrous oxide and carbon dioxide on an adsorbent layer containing 320 mm alumina and 705 mm 13X using a feed concentration of 500 ppm CO ₂ and 320 ppm N ₂ O. Analyzers measuring the concentration CO ₂ and N ₂ O were placed at the output (i.e., downstream end) of the 13X layer. You can see that if this adsorbent remains in operation until the carbon dioxide breakthrough (1 part per million) at about 390 min, as described in the prior art, a concentration of N ₂ O exiting the layer of at least 475 ppb, which is greater breakthrough concentration than the raw material concentration, as carbon dioxide, when co-adsorption with nitrous oxide is known to displace already adsorbed N ₂ O, causing a high concentration pulse N ₂ O, to leave the layer directly lane d CO ₂ breakthrough.

[0034] The present inventors have realized that if the space is available in 13X only for the adsorption of N ₂ O, this displacement by means of CO ₂ does not occur, and therefore as CO ₂ and N ₂ O can be effectively removed at 13X, and the characteristics of mass transfer two adsorbates can be modeled as if both are adsorbed independently. When the amount of 13X that would be required to adsorb 95% of the expected N ₂ O in a conventional feed stream of air was calculated from known adsorption capacities and adsorbent parameters and compared with the amount of CaX needed for the same purpose, it was found that for this particular set process conditions, the volume of 13X provided only for the adsorption of N ₂ O should be approximately twice as large as the CaX provided at the prior art installation in accordance with EP 0992274. Therefore, it is possible to calculate the necessary portions of alumina and 13X (or alternative suitable adsorbents as defined below) to remove the required degree of water, carbon dioxide and nitrous oxide, based on the expected initial concentrations of those components and the estimated operating time of the installation, by calculating the amount of alumina plus 13X required to remove all water (only on alumina) and 99% CO ₂ (on alumina and 13X), and then add the amount of 13X calculated to remove the expected amount of N ₂ O.

[0035] The inventors of the present invention then faced the problem of determining when the adsorbent layer would need to be regenerated, that is, the case in which the expected maximum concentration of N ₂ O breaking through the layer would be exceeded. Typically, prior art methods work until the CO ₂ breaks out of the layer, which is not advisable here, as explained above. A determination of a N ₂ O breakthrough would be possible, but it would not be advisable to make a precisely predetermined significantly lower concentration of N ₂ O in the feed air stream (concentration of parts per billion, not parts per million). At the expected level of N ₂ O removal, the concentration of CO ₂ breaking through the layer would be zero or very small (see, for example, Fig. 1 for 240 min), and thus, it is not advisable to determine the operating time based on one concentration of CO ₂ at the output. Thus, the authors of the present invention decided to control the concentration of CO ₂ in the selected position inside the adsorbent layer by comparing the maximum concentration of N ₂ O at the outlet of the layer with the concentration of CO ₂ in the selected position inside that layer. The operating time can then be determined by measuring the associated concentration of CO ₂ using an analyzer inside the adsorbent layer. The ratio of the concentration of CO ₂ and the concentration of N ₂ O can be determined by placing the analyzer for CO ₂ inside the adsorbent layer, and the analyzer for N ₂ O at the outlet of the adsorbent layer, and correlating the concentration determined using each analyzer at a given operating time. On the other hand, the operating time for a given adsorbent can be selected based on the level of N ₂ O breakthrough shown in the study as shown in Fig. 1, and the CO ₂ analyzer can be placed inside the layer, and the concentration measured by that analyzer in end of time, determined. The corresponding position of the CO ₂ analyzer can be deduced from the expected relative amounts of adsorbents that can be occupied by CO ₂ and N ₂ O based on breakthrough curves for a given system, as in FIG. one.

[0036] In a first aspect, the present invention provides a method for determining conditions to reduce, before cryogenic distillation, the level of water, carbon dioxide and nitrous oxide in a feed air stream, the method comprising: (a) passing said feed air stream at a supply pressure in the direction feed through the first adsorbent, which has Henry law selectivity for carbon dioxide compared to nitrous oxide, measured at least 12.5 at 30 ° C, and sequentially through a second adsorbent, whose Henry's law constant of the by adsorbing N ₂ O, as measured at 30 ° C is less than 500 mmol / g / atm and whose Henry's law selectivity with respect to CO ₂ over N ₂ O is not more than 5;

(b) stopping the passage of said raw air stream into said first and second adsorbents after a first time period;

(c) depressurizing the gas in contact with the first and second adsorbents to a second pressure less than the supply pressure;

(d) passing the regenerating gas at a second pressure and at a first temperature in the range of 140 to 220 ° C. into at least a second adsorbent in a direction opposite to the feeding direction for a second time period and then passing the regenerating gas at a second pressure and a second temperature less than the first temperature in the second and first adsorbents in the opposite direction to the feed direction during the third time period;

(e) stopping the transmission of regenerating gas to the first and second adsorbents;

(f) restoring gas pressure in contact with the first and second adsorbents to the supply pressure;

(g) repeating steps (a) to (f);

where the first adsorbent occupies from 25% to 40% by volume of the total volume occupied by the first and second adsorbents; and where the molar ratio of regenerating gas to feed air supplied during each iteration of steps (a) to (f) is from 0.08 to 0.5; whose method contains:

(I) placing an analyzer for measuring the concentration of CO ₂ at a selected position within the length of the second adsorbent;

(Ii) determining the threshold concentration measured by the CO ₂ analyzer when the expected maximum N ₂ O level is simultaneously obtained at the downstream end of the second adsorbent in the feed direction;

(III) using the result of stage (II) to determine the maximum duration of the first time period, such that the maximum duration of the first time period is the time from the beginning of the passage of the raw air stream into the first and second adsorbents until the analyzer determines a certain threshold concentration of CO ₂ .

[0037] In a second aspect, the present invention provides a method for reducing the level of water, carbon dioxide and nitrous oxide in a feed air stream, before cryogenic distillation, comprising:

(a) passing said feed stream of air at a feed pressure in the feed direction through a first adsorbent that has Henry law selectivity to carbon dioxide compared to nitrous oxide, measured at 30 ° C. of at least 12.5, and sequentially through a second adsorbent, whose constant according to Henry's law for adsorption of N ₂ O, measured at 30 ° C, is less than 500 mmol / g / atm and whose selectivity according to Henry's law with respect to CO ₂ compared to N ₂ O is no more than 5;

(b) stopping the passage of said feed air stream into said first and second adsorbents after a first time period;

(c) depressurizing the gas in contact with the first and second adsorbents to a second pressure less than the supply pressure;

(d) passing the regenerating gas at a second pressure and at a first temperature in the range of 140 to 220 ° C. into at least a second adsorbent in a direction opposite to the feeding direction for a second time period and then passing the regenerating gas at a second pressure and a second temperature less than the first temperature in the second and first adsorbents in a direction opposite to the feed direction during the third time period;

(e) stopping the transmission of regenerating gas to the first and second adsorbents;

(f) restoring gas pressure in contact with the first and second adsorbents to the supply pressure;

(g) repeating steps (a) to (f); where the first adsorbent occupies from 25% to 40% by volume of the total volume occupied by the first and second adsorbents; where the method further comprises:

(I) placing an analyzer for measuring the concentration of CO ₂ at a selected position within the length of the second adsorbent;

(Ii) determining the threshold concentration measured by the CO ₂ analyzer when the expected maximum N ₂ O level is simultaneously obtained at the downstream end of the second adsorbent in the feed direction;

where the maximum duration of the first time period is the time from the beginning of the passage of the raw air flow into the first and second adsorbents to the measurement with the analyzer of a certain threshold concentration of CO ₂ ; and where the molar ratio of the regenerating gas to the feed air supplied during each iteration of steps (a) to (f) is from 0.08 to 0.5.

[0038] The first adsorbent and the second adsorbent may be provided in separate tanks, but preferably provided as, respectively, layers upstream and downstream (with respect to the flow direction) from the single-layer adsorbent.

[0039] Accordingly, the first time period is equal to the time from the beginning of the passage of the raw air stream into the first and second adsorbent until the analyzer determines a certain threshold concentration of CO ₂ .

[0040] The method and process comprises measuring a concentration of CO ₂ at a selected point within an adsorbent layer, the selected point of which is located upstream of the exit from the layer. This ensures that the expected degree of removal of N ₂ O and CO _{2 is} obtained by monitoring only the concentration of CO ₂ and not (significantly lower) the concentration of N ₂ O. The control point is chosen so as to make sure that, with a detected threshold of CO ₂ , there sufficient ability in the remaining zeolite, located downstream from the control point, to adsorb the amount of N ₂ O necessary to obtain the expected degree of removal of N ₂ O. This ensures effective removal of N ₂ O and CO ₂ without using an increase in the volume of adsorbent (allowing installation (without increasing the size of the layer) compared to conventional three-layer TSA systems designed to remove N ₂ O, CO ₂ and water from the feed air stream, and without the use of costly and highly moisture-sensitive adsorbents such as CaX to remove N ₂ O.

[0041] The conditions under which the method is carried out are selected so that the expected degree of removal of water, CO ₂ and N ₂ O is obtained, making the most efficient use of the adsorbent. These conditions vary depending on the initial concentration of CO ₂ and N ₂ O. Typically, the initial concentration of CO _{2 is} 100-600 ppm, since 300-500 ppm is considered typical, accompanied by an initial concentration of N ₂ O of about 0, 3 ppm. However, it has recently been noted that in some places the initial concentration of CO _{2 is} 400-1000 ppm, since 400-600 ppm is typical, accompanied by an initial concentration of N ₂ O of 300-800 ppm, for example, 300-600 parts per billion. The conditions under which the method of the present invention is carried out can be adapted to provide these different ranges of initial concentrations of CO ₂ and N ₂ O such that the expected low content of both of these components can be obtained downstream of the adsorbents.

[0042] A suitable position for the analyzer can be determined as described above with reference to breakthrough curves for CO ₂ and N ₂ O on adsorbents and under conditions of interest. For example, using FIG. 1, it can be seen that if the layer works before the CO ₂ breakthrough (approximately 390 min), the entire layer is used for adsorption of CO ₂ ; if the layer works for only 180 minutes, then about 45% of the layer is used to remove CO ₂ , and the remaining 55% is available to remove 95% of the original N ₂ O.

Thus, the analyzer can be located at or close to the expected boundary between the volume of the layer intended for adsorption of CO ₂ and that for adsorption of N ₂ O, although this will also depend to some extent on the concentration of CO ₂ , it is designed to detect with using the analyzer; a higher concentration will be detected further upstream of the boundary point. Accordingly, the selected position of the analyzer for CO ₂ within the length of the second adsorbent is at a distance equal to or less than 50%, preferably equal to or less than 45%, for example, 41% of the path along the length of the second adsorbent in the downstream direction when the layer is working.

[0043] Accordingly, as soon as the selected maximum concentration of CO _{2 is} reached, the first and second adsorbents regenerate. Accordingly, the selected maximum concentration of CO ₂ is 200 ppm, preferably 100 ppm, more preferably 20 ppm, for example 1 ppm; from a practical point of view, the concentration of CO ₂ must be at least 20 parts per billion in order to be detected. These values are current CO ₂ concentrations.

[0044] In a specific embodiment, the present invention provides a method for reducing the level of water, carbon dioxide and nitrous oxide in a feed air stream, before cryogenic distillation, comprising:

(a) passing said feed stream of air at a feed pressure in the feed direction through a first adsorbent that has Henry law selectivity to carbon dioxide compared to nitrous oxide, measured at 30 ° C. of at least 12.5, and sequentially through a second adsorbent, whose constant according to Henry's law for adsorption of N ₂ O, measured at 30 ° C, is less than 500 mmol / g / atm and whose selectivity according to Henry's law with respect to CO ₂ compared to N ₂ O is not more than 5;

(b) stopping the passage of said feed air stream into said first and second adsorbents after a first time period;

(c) depressurizing the gas in contact with the first and second adsorbents to a second pressure less than the supply pressure;

(d) passing the regenerating gas at a second pressure and at a first temperature in the range of 140 to 220 ° C. into at least a second adsorbent in a direction opposite to the feeding direction for a second time period and then passing the regenerating gas at a second pressure and a second temperature less than the first temperature in the second and first adsorbents in a direction opposite to the feed direction during the third time period;

(e) stopping the transmission of regenerating gas to the first and second adsorbents;

(f) restoring gas pressure in contact with the first and second adsorbents to the supply pressure;

(g) repeating steps (a) to (f); where the first adsorbent occupies from 25% to 40% by volume of the total volume occupied by the first and second adsorbents; where the maximum duration of the first time period is the length of time from the beginning of the passage of the raw air stream into the first and second adsorbents to the detection of 1 part per million CO ₂ at a point equal to or less than 45% of the path along the length of the second adsorbent in the supply direction; and where the molar ratio of the regenerating gas to the feed air supplied during each iteration of steps (a) to (f) is from 0.08 to 0.5.

[0045] In the specific aforementioned embodiment, using an analyzer that detects 1 ppm CO ₂ , it is possible to remove 95% N ₂ O from the feed air stream when the analyzer is present at 43% of the second adsorbent path; remove 96% when the analyzer is at 41% of the path along the second adsorbent, remove 97% when the analyzer is at 38% of the path along the second adsorbent, remove 98% when the analyzer is at 35% of the path along the adsorbent, and 99% when the analyzer - equal to or less than 27% of the path along the adsorbent.

[0046] A person skilled in the art can set the desired position of the analyzer to obtain the expected degree of nitrous oxide removal, since the analyzer detects different concentrations of carbon dioxide. Alternatively, the position of the analyzer can be maintained at a selected position within the layer, and the N ₂ O breakthrough level, controlled by changing the threshold level of CO ₂ detected by the analyzer, causes layer regeneration; for a given position of the analyzer, there will be a higher threshold concentration of CO ₂ , which must be detected by the analyzer in order to cause regeneration of the layer, there will be a higher level of N ₂ O breakthrough from the layer. This can be designed to give an acceptable N ₂ O content in the air downstream of the adsorbent for the treated air.

[0047] In addition, the measurement of the concentration of CO ₂ can only be monitored from time to time, and not during each period of time during which the raw air stream is passed into the adsorbents to set a fixed time during which the raw air stream is passed in the first and a second adsorbent prior to regeneration, whose fixed time is checked and changed, if necessary, in response to a subsequent measurement of the concentration of CO ₂ .

[0048] In order to ensure that the N ₂ O and CO ₂ levels at the outlet of the second adsorbent remain below the required levels, based on measuring the CO ₂ level at a selected point within the adsorbent layer, the properties of the first and second adsorbent with respect to the three H ₂ components O, CO ₂ and N ₂ O must be balanced to ensure that a N ₂ O breakthrough does not occur before the measured CO ₂ level exceeds a selected threshold value. On this adsorbent, the speed at which the component of the feed stream of air moves through the bed depends on its adsorption strength. From water, CO ₂ and N ₂ O, water is most strongly adsorbed on alumina or zeolites, and thus moves very slowly through the adsorbent bed. CO _{2 is} less strongly adsorbed than water, but, on adsorbents used in the present invention, it is more strongly adsorbed than N ₂ O. It is desirable that at least part of CO ₂ , as well as all H ₂ O present in the raw air stream was adsorbed on the first adsorbent, and so that the second adsorbent was used to adsorb N ₂ O and the rest of CO ₂ , as well as any hydrocarbons present in the raw air stream. Preferably, the first adsorbent is capable of adsorbing at least 99% of the water present in the raw air stream, as well as 10-60% CO ₂ , and more preferably 20-40% CO ₂ present in the raw air stream. And if so, it is preferable to use the first adsorbent having a high ability for CO ₂ and H ₂ O and selectivity according to Henry's law with respect to CO ₂ compared to N ₂ O, measured at 30 ° C, at least 12.5. This allows N ₂ O to quickly pass through the first adsorbent, since it is not strongly adsorbed on it, and is believed to prevent CO ₂ from displacing the N ₂ O pulse to break out of the first adsorbent, since insufficient N ₂ O is adsorbed on it for any pulse to cause a problematic high concentration at the outlet after the second adsorbent. Additionally, the first adsorbent must be capable of desorption of H ₂ O and CO ₂ under PSA conditions.

[0049] Preferably, said first adsorbent contains activated alumina, since this adsorbent has a high water absorption capacity, thereby preventing water from coming into contact with a generally more water-sensitive second adsorbent, and can simultaneously adsorb water and carbon dioxide with by the formation of bicarbonates on the surface of alumina. More preferably, said first adsorbent comprises activated alumina impregnated with alkali and / or alkaline earth carbonates, bicarbonates, hydroxides and / or phosphates. Preferably, the activated alumina is impregnated with potassium carbonate. Impregnated alumina may be as described in US Pat. No. 5,656,064, in which the starting alumina is treated with a stock solution having a pH of at least 9, for example, KHCO ₃ solution, and dried at a temperature that is low and sufficient (e.g. , below 200 ° C), to avoid decomposition of the impregnating compound, to a form that absorbs CO ₂ so that the compound does not regenerate under the expected conditions of regeneration. Preferably, when the pH of the impregnation solution is associated with zero charge (ZPC) of alumina in accordance with the formula: pH ≥ ZPC-1,4, or more preferably: ZPC + 2 ≥ pH ≥ ZPC-1,4. In addition, the impregnated alumina may be as described in US Pat. The impregnating agent is preferably an alkali metal or ammonium hydroxide, carbonate, bicarbonate, phosphate or an organic acid salt. More preferably, when the impregnating substance is selected from the group consisting of K ₂ CO ₃ , Na ₂ CO ₃ , KHCO ₃ , NaHCO ₃ , NaOH and KOH. It has been found that such impregnated aluminas have an increased absorption of CO ₂ , but their ability with respect to nitrous oxide is similar to that of non-impregnated aluminas.

[0050] Preferably, the first adsorbent has a BET specific surface area of 400 m ² / g or less. Preferably, the first adsorbent has a relatively low specific surface so that water and CO ₂ can be desorbed under PSA conditions.

[0051] Preferably, said first adsorbent layer is from 25% to 40% by volume, for example, from 25% to less than 40% by volume, and more preferably from 28% to 37% of the total volume of the first and second adsorbent for example, 31 or 32%.

[0052] The second adsorbent is provided in an amount sufficient to adsorb N ₂ O, and possibly also any hydrocarbon impurities present in the feed air stream, as well as a sufficient amount of carbon dioxide so that the level of carbon dioxide at a selected point in the adsorbent layer reaches the selected threshold level before the concentration of nitrous oxide and, if present, hydrocarbon impurities breaking through the layer exceeds the selected value. The second adsorbent is chosen in order to have a high ability with respect to N ₂ O, and possibly also with respect to hydrocarbon impurities, so that the layer size can be kept as small as possible, which gives an advantage for cost reasons, since the second adsorbent , as a rule, much more expensive than the first adsorbent. The second adsorbent comprises from 60% to 75% by volume of the total volume of the first and second adsorbent, preferably from 65% to 72% by volume, with lower values within each range in most cases, being preferred for cost reasons, since the level of removal N ₂ O of this range is within acceptable limits for the intended use.

[0053] The second adsorbent has a higher absorption capacity with respect to both CO ₂ and N ₂ O than the first adsorbent under supply conditions.

[0054] The second adsorbent should also be able to desorb N ₂ O, and, where applicable, also hydrocarbon impurities, effectively under the selected regeneration conditions. The authors of the present invention tried to provide a method that can operate at a moderate regeneration temperature so that the costs of providing heaters for the installation and energy consumption can be minimized. This requires the ability of the second adsorbent with respect to carbon dioxide to be balanced with respect to the expected regeneration temperature, proper use. If CO ₂ and / or N ₂ O is adsorbed too much on the second adsorbent, the second adsorbent will not regenerate sufficiently under the selected regeneration conditions. The inventors of the present invention have found that CaX trained to remove N ₂ O during TSA regeneration at high temperatures, furthermore having a high N ₂ O ability, cannot desorb N ₂ O and CO ₂ efficiently under the preferred regeneration conditions of the present invention if contaminated with water. The inventors of the present invention have found that an aggregated zeolite adsorbent based on LSX zeolite or zeolite X, one way or another containing no binders, is suitable for use as a second adsorbent in the present invention. Mentioned second adsorbent has a constant according to Henry's law for adsorption of N ₂ O, measured at 30 ° C, less than 500 mmol / g / ATM. The inventors of the present invention also found that alumina is not suitable for use as a second adsorbent, and therefore the second adsorbent has maximum Henry law selectivity for CO ₂ / N ₂ O, measured at 30 ° C, 5, and preferably a constant according to Henry's law for CO ₂ adsorption, measured at 30 ° C, is greater than 5.6 mmol / g / atm. Preferably, when said second adsorbent is selected from bound or not containing binders: NaX having a Si / Al ratio of 1.25 to 1.0, zeolite 4A, zeolite 5A, mordenite, chabazite and clinoptilolite; more preferably, when said second adsorbent is selected from bound or not containing binders: NaX having a Si / Al ratio of 1.25 to 1.0, zeolite 4A, zeolite 5A; and most preferably, said second adsorbent is selected from bound or not containing binders: NaX having a Si / Al ratio of 1.25 to 1.0, for example, zeolite 13X or NaLSX, and zeolite 5A. Most preferred adsorbents are relatively cheap and have good N ₂ O ability under the conditions used in the present invention.

[0055] Constants according to Henry's law were measured at 30 ° C for a number of adsorbents for N ₂ O and CO ₂ , selectivity according to Henry's law with respect to CO ₂ compared to N ₂ O was calculated, and the specific surface area by BET was determined as set out below in Table 1:

Table 1 Adsorbent K _H CO ₂
(mmol /
g / atm) K _H N ₂ O (mmol /
g / atm) Selectivity CO ₂ / N ₂ O Selectivity N ₂ O / CO ₂ BET specific surface (m ² / g) Aluminium oxide
Axen AA-300 5,6 0.45 12.5 0.08 320 UOP 13X 162 63 2,57 0.39 726 UOP 5A 145 54 2.69 0.37 540 Binder-free CaX 1031 1035 1.00 1.00 742 Na mordenite 366 185 1.98 0.51 421 Ca mordenitis 374 113 3.31 0.30 448 Cax 1020 503 2.03 0.49 712 K ₂ CO ₃ - impregnated alumina
(Axen AA-320) 22.8 0.42 54.3 0.018 260 NaLSX 310 102 3.04 0.33 712

[0056] You can see that both alumina in the above table satisfy the preferred selectivity and specific surface conditions for the first adsorbent, and that all adsorbents except CaX, which does not contain CaX binders and alumina, satisfy the preferred K _H CO ₂ and selectivity conditions according to Henry's law for CO ₂ / N ₂ O for the second adsorbent.

[0057] As described above, it is possible to determine an operating time suitable for obtaining the expected degree of N ₂ O removal based on the behavior of the adsorption of CO ₂ and N ₂ O under the selected conditions and the selected position of the analyzer inside the second adsorbent layer.

[0058] Accordingly, the time during which the feed air stream is passed into the first and second adsorbent before regeneration is from 100 minutes to 350 minutes, for example, from 100 minutes to 300 minutes, for example, from more than 100 to less than 250 minutes preferably from 120 minutes to 240 minutes, for example, from more than 120 minutes to less than 200 minutes, for example, 180 minutes Accordingly, the feed air stream is passed into the first and second adsorbent at a feed temperature of from 5 ° C to 50 ° C, preferably from 10 ° C to 30 ° C. Accordingly, the supply pressure is from 1 bar abs. (100 kPa) up to 30 bar abs. (2000 kPa), preferably from 4 bar abs. (400 kPa) up to 7 bar abs. (700 kPa).

[0059] Preferably, the ratio of the time during which the heated gas is passed into the adsorbent bed during the regeneration process (heating time, or hot purge time, or second time period) to the operating time (first time period) for the adsorbent layer is less than 35% When the initial concentration of CO ₂ is within the normal range, the ratio is preferably 30% or less, for example, 25%. This allows a lower ratio of heating time to operating time than is observed in prior art methods for removing normal levels of CO ₂ and water, and thus saves energy. However, for higher initial concentrations of CO ₂ , a higher ratio close to 35% is preferred.

[0060] Periodic regeneration mainly occurs while the second set of the first and second adsorbent is used to continue the cleaning process, and each set of two adsorbents is in operation during the cleaning process and is regenerated during alternation.

[0061] Preferably, the regeneration of the first and second adsorbent comprises passing the heated regenerating gas (purge gas) countercurrently through the second and first adsorbent for a period of from 20 minutes to 100 minutes, preferably from 25 minutes to 80 minutes. The temperature of the heated gas is from 140 ° C to 220 ° C, for example, from more than 140 ° C to less than 200 ° C, preferably from 140 ° C to 180 ° C, for example, from more than 140 ° C to less than 160 ° C, or from more than 150 ° C to 200 ° C. Further, the regeneration comprises passing a second regenerating gas at a temperature within the range given above for the supply temperature, but colder than the one used for the temperature of the heated gas, countercurrently through the second and first adsorbent. Preferably, the second regenerating gas has a temperature of not more than 5 ° C higher than the supply temperature. Accordingly, the second regenerating gas may be passed into the adsorbents for a period of from 50 to 220 minutes, preferably from 80 to 180 minutes. The regenerating gas should not contain water, carbon dioxide, nitrous oxide, or, since hydrocarbons should be removed from the feed air stream, hydrocarbons, and may accordingly consist of N ₂ , O ₂ , CO, Ar, He, product air depleted in water and CO ₂ and mixtures thereof. In a preferred embodiment, the regenerating gas would consist of either product N ₂ or, more preferably, waste from the N ₂ plant (60% O ₂ /40% N ₂ ). Preferably, when the selected regeneration conditions lead to the regeneration of the second adsorbent by means of a hot gas purge, i.e. in a TSA type method, and the regeneration of the first adsorbent by means of a cold gas purge, that is, in a PSA method, this provides sufficient desorption of CO ₂ and N ₂ O from the second adsorbent in order to lead to acceptable performance while minimizing the energy consumption of desorption, to what extent alumina can be regenerated efficiently under PSA conditions. However, if desired, the first adsorbent can also be fully or partially regenerated by blowing with heated gas in a TSA type process. This allows the adsorbent to be completely "cleaned" of all adsorbed components of the air, which may periodically be desirable. It can also be used as a means to restore the ability of the CO ₂ and / or N ₂ O in the case of early breakthrough of impurities from the adsorbent.

[0062] Accordingly, the molar ratio of regenerating gas to feed gas is from 0.08 to 0.5, preferably from 0.1 to 0.3, more preferably from 0.2 to 0.25. A shorter heating time is correlated using a higher molar ratio of regenerating gas to feed gas, as is known to those skilled in the art.

[0063] Accordingly, at least 75% of N ₂ 0 present in the feed air is removed by the method of the present invention, preferably at least 80% or at least 90% and more preferably 95%, even more preferably 96%, 97% or 98%, and most preferably 99%. That is, the level of N ₂ O is preferably reduced by no more than 25% from its original level in the raw air stream, preferably by no more than 20%, more preferably 10% and even more preferably not more than 5%, 4% , 3%, 2%, and most preferably 1%.

[0064] If the feed air stream further comprises at least one hydrocarbon selected from the group consisting of ethylene, acetylene, butane, propylene and propane, the level of the at least one hydrocarbon is reduced by adsorption on a second adsorbent. Preferably, at least 60% of or at least one hydrocarbon is removed by the method of the present invention, and preferably at least 75%, for example at least 85%, and more preferably 90%. The inventors of the present invention have found that hydrocarbon (s) are co-adsorbed on a second adsorbent with CO ₂ and N ₂ O, and that there is no additional need to provide a second adsorbent to accommodate the hydrocarbon (s).

[0065] Accordingly, the method further comprises the step of conducting cryogenic distillation of the purified air stream to separate the nitrogen rich stream and / or the oxygen rich stream.

[0066] In a third aspect, the present invention provides an apparatus for reducing the level of water, carbon dioxide and nitrous oxide in a feed air stream prior to cryogenic distillation, comprising:

in the adsorbent layer, a layer of the first adsorbent, which has Henry Henry selectivity with respect to carbon dioxide compared with nitrous oxide, measured at 30 ° C of at least 12.5 and a second adsorbent, whose Henry law constant for adsorption N ₂ O, measured at 30 ° C, is less than 500 mmol / g / atm and whose Henry law selectivity for CO ₂ compared to N ₂ O is not more than 5, where the first adsorbent takes from 25% to 40% by volume of the total volume occupied by the first and second adsorbent; carbon dioxide analyzer located within the length of the second adsorbent.

[0067] The apparatus used in the second aspect of the present invention may accordingly further comprise one or more of the following: entrance to the adsorbent bed for supplying a feed air stream in the feed direction in contact with adsorbents and a feed gas outlet, valves for controlling the feed stream air, valves for depressurizing gas in contact with adsorbents to a lower pressure, entering the adsorbent layer to supply regenerating gas to the adsorbents in the opposite direction to the supply direction, and outlet for regenerating gas, valves for controlling the flow of regenerating gas, a heater for raising the temperature of the regenerating gas, and control systems for controlling the valves and heater. Accordingly, the installation may be in accordance with that described in accordance with FIG. 2 and 3.

[0068] In a fourth aspect, the present invention provides the use of a device in accordance with a third aspect of the present invention to remove nitrogen oxides from a feed air stream prior to cryogenic distillation.

[0069] In a fifth aspect, the present invention provides a method for improving an apparatus for reducing levels of carbon dioxide, nitrous oxide and water in a feed air stream using a variable temperature adsorption method so that it can reduce the level of nitrous oxide in a feed stream air without using an adsorbent having Henry selectivity measured at 30 ° C with respect to nitrous oxide compared to carbon dioxide, 0.5 or more, for example, CaX or CaX, not containing a bond substances in which the adsorbents contained in the installation are removed and replaced with the same total volume of adsorbent, consisting of a layer of the first adsorbent, which has Henry law selectivity with respect to carbon dioxide compared with nitrous oxide, measured at 30 ° С, according to at least 12.5 and a second adsorbent whose constant according to Henry's law for adsorption of N ₂ O, measured at 30 ° C, is less than 500 mmol / g / atm and whose selectivity according to Henry's law with respect to CO ₂ compared with N ₂ O is not more than 5, wherein the first adsorbent animaet from 25% to 40% by volume of the total volume occupied by the first and second adsorbent, and wherein the analyzer to carbon dioxide is placed within the length of the second adsorbent.

[0070] All features described in connection with any aspect of the invention may be used with any other aspect of the invention.

LIST OF FIGURES OF DRAWINGS WITH EXPLANATION BY EACH OF THEM

[0071] The present invention will be further described with reference to preferred embodiments and examples, and with reference to the accompanying drawings, in which:

[0072] FIG. 1 shows breakthrough curves obtained on a 31% alumina / 69% 13X adsorbent layer for CO ₂ and N ₂ O over 480 minutes;

[0073] FIG. 2 shows a plant suitable for carrying out the method according to the present invention;

[0074] FIG. 3 shows in more detail the installation device of FIG. 2, surrounding the adsorption layer;

[0075] FIG. 4 shows the effect of runtime on the removal rate of N ₂ O;

[0076] FIG. 5 shows the effect of the arrangement in the adsorption layer of 1 ppm of the CO ₂ front on the N ₂ O removal level.

DETAILED DESCRIPTION

[0077] Referring to FIG. 2, this schematically depicts an apparatus suitable for use in the present invention. Proper purification air is supplied to the main system of the air compressor 10 at the inlet 12, in which it is compressed by a multi-stage compressor with cooling between and after by heat exchange with water. Cooled compressed air is supplied to the intake manifold 14, comprising an adjustable intake valve 16 and 18, to which a pair of reservoirs 20 and 22 containing an adsorbent layer are connected. The intake manifold is connected downstream of the adjustable valves 16 and 18 using a ventilation manifold 24 containing ventilation valves 26, 28, which serve to close and open the connections between the upstream end of the corresponding adsorption tanks 20 and 22 and the breathing valve 30 through a muffler 32 Each of the two layers of adsorbent 20 and 22 contains two adsorbent. The adsorbent of the feed end (i.e., the adsorbent at the upstream or inlet collector end of the layer) is denoted by 34, 34 ′ in the respective layers and the adsorbent at the end of the product (i.e., adsorbent at the downstream or outlet collector end) - reference number 36, 36 ′.

[0078] The installation has an outlet 38 connected to the downstream ends of the two adsorbent reservoirs 20, 22 by means of an exhaust manifold 40 containing adjustable exhaust valves 42, 44. The exhaust manifold 40 is connected using a regenerative gas manifold 46 containing adjustable regenerative gas valves 48 and 50. Upstream of the regeneration gas manifold 46, a line 52 containing an adjustable valve 54 also builds a bridge through the exhaust manifold 40.

[0079] The regenerative gas inlet is provided at 56 positions, which are connected through adjustable valves 58 and 60 to pass through either the heater 62 or through the bypass line 64 to the regenerative gas manifold 46.

[0080] The operation of the valves can be monitored by appropriate means programming a time frame and controlling the valves, as is known in the art, not shown.

[0081] During operation, air is compressed in the main system of the air compressor 10 and fed to the intake manifold 14, and also passes through one of two reservoirs containing the adsorbent. Starting from the position in which air passes through the open valve 16 to the adsorption tank 20, and through the open valve 42 to the outlet 38, the valve 18 in the intake manifold will immediately be closed to cut off the tank 22 from the cleaning air supply. Valve 44 will also be immediately closed. At this stage, valves 48, 50, 54, and 26 are closed. Layer 20 thus works, and layer 22 needs to be regenerated.

[0082] The air to be cleaned enters the upstream end of the layer 20, and water and carbon dioxide from the air are adsorbed on the adsorbent 34. The layer 20 is designed so that water will not be adsorbed on the downstream adsorbent 36 from the fact that this the adsorbent is a water-sensitive zeolite, and because the length of the upstream adsorbent (alumina) 34 is chosen so that the mass transfer zone for water never goes beyond the boundary between the adsorbent and 34 adsorbent 36 during normal operation. Some of the CO ₂ must also be adsorbed on the upstream adsorbent 34, and the remaining part of CO ₂ will be adsorbed together with N ₂ O and any hydrocarbons on the downstream adsorbent 36. The concentration of CO ₂ at a selected distance along the adsorbent 36 from the boundary with the adsorbent 34 is measured, either at each operating phase of the layer 20, or periodically, to ensure that the time period during which the layer remains in operation leads to an acceptable level of N ₂ O breakthrough. CO ₂ does not break through the layer. As soon as the measured concentration of CO ₂ reaches the selected threshold, or the predetermined time period has expired, the valve 16 is closed so as to stop the additional passage of raw air through the layer 20, and the regeneration of the layer can begin, as described below for layer 22.

[0083] The regeneration of layer 22 occurs while layer 20 is operating. To begin depressurizing the layer 22, the valve 28 is opened, and as soon as the pressure in the reservoir 22 has dropped to the expected level, the valve 28 is left open while the valve 50 is opened to start the movement of the regenerating gas. The regenerating gas will usually be a stream of dry nitrogen, free of CO ₂ and N ₂ O, obtained from the refrigerating chamber of an air separation device, possibly containing a small amount of argon, oxygen and other gases, into which the air purified in the present installation is passed. The valve 60 is closed and the valve 58 is opened so that the regenerating gas is heated to a temperature in the range from 140 ° C to 220 ° C before being transferred to the reservoir 22. Although the regenerating gas enters the reservoir 22 at a selected elevated temperature, very slightly cooled by transferring heat to the desorption of nitrous oxide and some of the carbon dioxide from the upper, downstream adsorbing position 36 ′ in the tank. Since the heat pulse is stored in the system, the purge exhaust gas leaves the vent outlet in a cooled state. Gradually, the heat wave travels through the upper adsorbent 36 ′, while nitrous oxide and, to some extent, carbon dioxide are purified. After the expected period, while the heat pulse is part of the path through the upper adsorbent 36 ′, the valve 58 is closed and the valve 60 is opened so that the flow of regenerating gas now becomes cold. The cooled regenerating gas moves the heat pulse further through the upper adsorbent 36 ′.

[0084] While the upper adsorbent was thus regenerated by TSA, the cold regenerating gas continued to flow through the lower adsorbent and, through its reduced pressure, stripped water and carbon dioxide from the upper adsorbent 34 ′ using PSA. At the end of the allotted period of regeneration, the valve 50 can be opened to move nitrogen from the adsorbent layer and, after closing the valve 28, to restore the pressure of the vessel 22 with purified air. After that, valve 54 may be closed and valves 18 and 44 may be opened to return reservoir 22 to operation. Residual heat left in the layer can be removed by means of purified air in the form of a temperature pulse, which can be removed in the downstream heat exchanger. The reservoir 20 can then be regenerated in a similar way and in the entire sequence, which continues with the vessels, being in operation, depressurizing, regenerating, restoring pressure and returning to work, in phased operating cycles.

[0085] FIG. 3 shows in more detail a control algorithm that provides for the removal of contaminants at the expected level. As in the description of FIG. 2, compressed air enters the vessel 22 through the inlet pipes 12 and passes through the first adsorbent layer 34 and through the second adsorbent layer 36, 36a before leaving the tank through the outlet pipe 40, which is connected to the downstream part of the process .

[0086] The second adsorbent layer responsible for the removal of CO ₂ and N ₂ O can be divided into two subsections: 36, in which CO ₂ and N ₂ O are simultaneously removed in accordance with the selectivity of the adsorbent, and 36a, in which only N is removed ₂ O, since the air flow is already free of CO ₂ . The dashed line 38 corresponds to the virtual separation of the two subsections, as well as the height of the layer at which an equal or smaller selected maximum level of only CO ₂ can be detected using the analyzer 20 at any time while the layer is in operation, providing a sufficient level of removal of N ₂ O.

Suitable process conditions are set forth in Table 2 below:

table 2 Method conditions Preferred More preferred Working hours 100 to 350 min 120 to 240 min Purge / Air Ratio 0.08 to 0.5 0.1 to 0.3 The initial concentration of CO ₂ 300 to 1000 ppm 400 to 700 ppm Hot regeneration temperature 140 to 220 ° C 140 to 180 ° C Feed temperature 5 to 50 ° C 10 to 30 ° C Feed pressure 1 to 30 bar absolute pressure 4 to 7 bar absolute pressure Hot purge duration From 20 to 100 min 25 to 80 min Raw N ₂ O (parts per billion) 200 to 600 300 to 400

[0087] In the case where the initial concentration of CO ₂ is within the normal range, the preferred process conditions are set forth in Table 3 below:

Table 3 Method conditions Preferred More preferred Working hours 100 to 250 min 120 to 200 min Purge / Air Ratio 0.1 to 0.5 0.1 to 0.3 The initial concentration of CO ₂ 300 to 600 ppm 400 to 500 ppm Hot regeneration temperature 140 to 220 ° C 140 to 160 ° C Feed temperature 5 to 50 ° C 10 to 30 ° C Feed pressure 1 to 30 bar absolute pressure 4 to 7 bar absolute pressure Hot purge duration From 20 to 70 min 25 to 50 min Raw N ₂ O
(parts per billion) 200 to 600 300 to 400

[0088] When the initial concentration of CO ₂ rises, for example, when the air separation unit is located close to plants that emit CO ₂ and / or N ₂ O, preferred process conditions are set forth in Table 4 below:

Table 4 Method conditions Preferred More preferred Working hours 100 to 300 min 140 to 240 min Purge / Air Ratio 0.08 to 0.3 0.08 to 0.12 The initial concentration of CO ₂ 300 to 1000 ppm 400 to 600 ppm Hot regeneration temperature 140 to 220 ° C 120 to 180 ° C Feed temperature 5 to 50 ° C 10 to 30 ° C Feed pressure 1 to 30 bar absolute pressure 4 to 7 bar absolute pressure Hot purge duration From 20 to 100 min 25 to 80 min Raw N ₂ O
(parts per billion) 200 to 600 300 to 400

[0089] Thus, it is possible to use a multilevel layer containing an upstream layer of alumina followed by a downstream layer of zeolite. The alumina section can be improved with this kind of alumina which can be produced by impregnating from about 5% to 10% by weight of potassium carbonate onto alumina by treating the starting alumina with a solution of potassium carbonate and drying at high temperatures to about 125 ° C. Such aluminas have a particularly high carbon dioxide ability.

[0090] Of course, it should be understood that the reservoirs 20, 20 ′ and 22, 22 ′ can each, if desired, be divided into smaller reservoirs arranged in series, and the references to the “layers” of adsorbents given above include arrangements in which individual adsorbents are placed in separate tanks arranged in series.

EXAMPLES

[0091] Typically, these examples work for 20 cycles to achieve unchanging conditions in order to obtain the following results:

Example 1: Creating Loop Conditions

[0092] This example is an initial study conducted to determine if conditions suitable for TPSA can be adapted to remove N ₂ O. The cyclic, dynamic breakthrough curves of carbon dioxide and nitrous oxide were measured on a two-level layer. The first layer on the raw side is K ₂ CO ₃ AA330 activated alumina (37% by volume), and the second layer on the product side is Heng Ye Na-free LSX exchange agent (63% by volume). Water saturated at 20 ° C (68 ° F) and 6 bar abs. (290 pounds per square inch) and with a typical initial concentration of CO ₂ 400 ppm by volume and N ₂ O 350 ppm by volume. The gas flow rate was 4.924 kg / m ² h. Data were obtained on a column with a diameter of 203 mm (8 inches) and 2740 mm (9 feet) in height. The analyzer was located at 41% of the path along the length of the LSX zeolite. Under these conditions, no carbon dioxide was detected by the analyzer during the operating time. The conditions and results used regarding the removal of CO ₂ and N ₂ O were as follows:

Table 5 Working hours 120 min Purge / Air Ratio (molar) 0.25 Hot Purge Temperature 160 ° C Hot purge time 30 minutes Cold purge time 70 min Same pressure recovery / pressure relief time 20 minutes Feed temperature 19.3 ° C Feed pressure 6.04 bar abs. Ratio of hot purge time to run time 0.25 Cycle view Hybrid PSA / TSA The number of adsorbent layers 2 Removal of CO ₂ on alumina 30-40% Removal of N ₂ O on zeolite 93-94%

[0093] Table 5 shows that high removal of N ₂ O is possible without the use of CaX material. The degree of N ₂ O removal obtained was comparable to that for the TSA method in operation for 180 minutes, as described in EP0992274.

Example 2: Changing cycle conditions

[0094] Example 1 was repeated using a cycle time of 130 minutes and a hot purge time of 25 minutes (the ratio of hot purge time to operating time 0.19), and more than 95% N ₂ O was removed.

[0095] In the present invention, the method is not executed until the CO ₂ breakthrough, and thus allows most of the CO _{2 to} be adsorbed on the first adsorbent (40% compared to 10% in the standard TSA method). This reduces the amount of CO ₂ appropriate to be removed by zeolite, and therefore improves the degree of N ₂ O removal on the zeolite.

Example 3: the action of the zeolite

[0096] The 13X zeolite material was tested under the same conditions as in Example 1, except that the feed temperature was reduced to 18.5 ° C and the adsorbent proportions were 31.7% alumina and 68.3% zeolite 13X by volume. The resulting removal rate of N ₂ O was 98%.

[0097] A small difference is noticeable between zeolites 13X and LSX with respect to the removal of N ₂ O, although US Pat. No. 7,935,177 teaches that the Na-exchange LSX gives better performance for both CO ₂ removal and N ₂ O removal, as shown in Table 6:

Table 6 Zeolite CO ₂ relative dynamic adsorption capacity N ₂ O relative constant according to Henry's law for adsorption 13X with 15% binder 1.00 1.00 NaLSX with 15% binder 2,30 1.31

[0098] It was assumed that, given the high adsorption capacity of NaLSX with respect to CO ₂ , only a small section of the NaLSX layer would be used for adsorption of CO ₂ , especially since the removal of CO ₂ with alumina as the first adsorbent is high. However, only a fraction of NaLSX is located within the equilibrium zone under such short-cycle conditions, meaning that NaLSX is not used to its full capacity. This explains why material with lower performance, such as 13X, shows similar results and, therefore, is a much more cost-effective option. In addition, NaLSX is known to be more sensitive to water and requires a higher regeneration temperature in order to restore its full capacity.

Example 4: Increased Regeneration

[0099] Another way to increase the level of removal of N ₂ O is to increase the amount of energy supplied to the layer during the regeneration process. This is done due to the higher power of the heater, but it also provides the opportunity to extend the cycle time.

[0100] Using the same equipment as described in Example 3, the results shown in Table 7 were obtained.

Table 7 unit of measurement Test 1 Test 2 Baseline CO ₂ Ppm 400 400 Working hours Min 120 140 Hot purge time Min 25 35 Feed temperature ° C 18,4 fifteen Hot Purge Temperature ° C 140 140 Blowdown to air ratio (molar) 0.23 0.17 Removal of N ₂ O % 98 99 Ratio of hot purge time to run time % twenty 25 Excess heat 0.72 0.82

[0101] This shows that the cycle time can be extended by 20 minutes, provided that additional heat is supplied to the layer. The additional heat is measured in units of excess heat, being the ratio of the heat supplied by the heater during the regeneration process to the heat of desorption of water, CO ₂ and N ₂ O.

Example 5: Optimizing Effective Work Time

[0102] Under the above conditions for Test 2 shown in Table 7, the cycle time was extended to 180 minutes and then to 200 minutes, and the N ₂ O removal level was measured at 93% and 88%, respectively. The results are shown in FIG. four.

[0103] It is advisable to extend the cycle time in order to reduce the amount of gas loss when switching layers from the current position to the inactive one and vice versa, especially in large plants in which the volume pumped after each cycle is significant. In addition, in order to avoid excessive heating of the product air stream, the heat pulse generated by adsorption must either be used or released from the bed during the regeneration process. A short cycle time requires a higher flow rate of regenerating gas to achieve this than a longer cycle time, and therefore it is preferable to extend the cycle time to reduce pressure drop losses.

Example 6: Control Algorithm

[0104] Under test conditions 2 in Table 7, the position of the CO ₂ front (typically when the analyzer would measure 1 part per million by volume of CO ₂ ) is determined based on the gas velocity in the bed. The second layer (zeolite) is composed of 13X standard material and occupies a column height of 820 mm. This position is then correlated with the amount of N ₂ O removed, as shown in FIG. 5. Thus, if the analyzer is located at the boundary between the first and second layers (zero on the horizontal axis in Fig. 5), while the analyzer detects 1 ppm CO ₂ , the removal of N ₂ O at the end of the layer will be greater. than 99%. If it is desired to obtain 99% removal of N ₂ O, this can be done by placing the analyzer at any distance between 0 and 222 mm from the boundary between the first and second layers (less than or equal to 27% of the distance along the second layer); 98% removal can be obtained up to 290 mm from the border (less than or equal to 35% of the distance along the second layer); removal of 97% to 315 mm from the border (less than or equal to 38% of the distance along the second layer); removal of 96% to 335 mm from the border (less than or equal to 41% of the distance along the second layer); and the removal of 95% to 349 mm from the border (less than or equal to 43% of the distance along the second layer). Thus, it is possible to balance the cycle time with the degree of removal of N ₂ O, as expected. After CO _{2 is} detected by the analyzer, the layer is removed from the regeneration line. For comparison, the standard TSA is designed so that CO ₂ must break somewhere through the end of the second layer at the end of the operating time, which makes the CO ₂ removal zone close to 100% of the total zeolite height, whereas, for 95%, N ₂ removal O, the CO ₂ removal zone in the present invention is about 43% of the height of the zeolite (second layer).

[0105] Examples 7-11: Removal of CO ₂ and N ₂ O from raw air streams containing a high concentration of CO ₂ .

[0106] The cyclic, dynamic breakthrough curves of carbon dioxide and nitrous oxide were measured on a two-level layer. The first layer on the raw side is 8% of the mass. K ₂ CO ₃ co-molded Heng Ye activated alumina (31% by volume) and the second layer on the food side is Heng Ye HO 13X (69% by volume). Water saturated at 7 ° C and 5.64 bar abs. and with a typical starting concentration of N ₂ O 325 ppmv. The gas flow rate was 5.224 kg / m ² h. Data were obtained on a column with a diameter of 210 mm (8 inches) and 1050 mm in height. The conditions used were as follows:

Table 8 Purge / Air Ratio (molar) 0.11 Hot Purge Temperature 150 ° C Feed temperature 7 ° C Feed pressure 5.64 bar abs. Cycle view Hybrid PSA / TSA The number of adsorbent layers 2 Analyzer Position 13X 41%

The results obtained are shown in Table 9:

Table 9 Example Working hours
(min) Cycle time (heating time / cooling time / switching time) Average inlet CO ₂ % CO ₂ removal with alumina % total removal of N ₂ O % total CO ₂ removal Hot Purge Time / Run Time 7 240 67/147/30 500 10% 75% one hundred% 0.28 8 240 77/137/30 500 13% 85% one hundred% 0.32 9 240 77/133/30 400 24% 87% one hundred% 0.32 10 210 70/110/30 500 17% 90% one hundred% 0.33 eleven 180 60/90/30 500 twenty% 96% one hundred% 0.33

[0107] For examples 7 to 10, the analyzer measured a concentration of more than 20 ppm CO ₂ at the end of the operating time, and as you can see, this corresponds to a N ₂ O removal level of 75-90%. In Example 11, the analyzer measured a concentration of 20 ppm CO ₂ at the end of the operating time, which corresponds to a 96% N ₂ O removal rate. Thus, the operating time can be extended to as much as 240 minutes, with some reduction in the level of removal of N ₂ O, which may in some cases be acceptable.

Example 12: Removal of hydrocarbon impurities

[0108] The conditions set forth in Examples 1 and 8 were compared with the conditions set forth in US Pat. No. 5,855,650 using alumina / 13X or alumina / 13X / CaX adsorbent configurations to determine their relative ability to remove a number of hydrocarbons from a feed air stream. All experiments were conducted with a water-saturated feed air stream containing 400 ppm CO ₂ and 320 ppm N ₂ O, with the exception of the experiment in Example 1, but using NaLSX, for which a concentration of 500 ppm CO was used. ₂ .

Table 10 Comparison of TPSA impurity removal cycles
(expressed as% of initial concentration) Impurity Standard TPSA (13X) US Patent 5,855,650 Short-cycle TPSA (NaLSX) Example 1 of the current invention Short cycle TPSA (NaX) Example 8 of the current invention Bilayer TPSA (CaX) US Patent 5,855,650
with 3 layers Ethane fifteen% fifteen% eleven% fifteen% Ethylene 60% 95% 97% one hundred% Acetylene one hundred% one hundred% one hundred% one hundred% Propane 35% 80% 67% 45% Propylene one hundred% one hundred% one hundred% one hundred% Butylene one hundred% one hundred% one hundred% one hundred% N ₂ O thirty% > 85% > 99%

Comparative Example 1 - Conditions Reported in US Patent 5,885,650

[0109] Two-layer cyclic experiments to remove microimpurities from the air were carried out in tanks 0.2 m in diameter. The height of the total adsorbent layer was 2 m. The raw air was saturated with water under supply conditions and contained 380 ppm CO ₂ and 0.3 ppm N ₂ O. In all cases, the raw end of the adsorbent layer contained activated alumina (Alcan AA -300), and the product end of the layer contained 13X zeolite (Zeochem Z10-02, Si / Al = 1.15). The experiments were carried out under the conditions shown in Table 11:

Table 11 Feed time 145 min Heating time 32 min at 165 ° C Cooling time 98 min at 35 ° C Switching time (pressure relief and pressure recovery) 15 minutes Feed temperature 35 ° C Feed pressure 8.9 bar abs. (890 kPa) % removal of N ₂ O at a time-average value of a CO ₂ breakthrough _of 20 parts per billion 32.6% Regeneration pressure 1.05 bar abs. (105 kPa) Purge / Air (Molar) 0.3 Feed rate 4.1 kmol / h

[0110] The layering scheme was 75% by volume of K ₂ CO ₃ impregnated activated alumina (Alcan AA-320AP) at the raw end of the layer and 25% by volume of 13X zeolite (Zeochem Z10-02) at the product end of the layer.

[0111] As you can see, the present invention provides a significantly increased degree of removal of N ₂ O compared with the conditions used in US patent 5885650.

[0112] Although the invention has been described based on a preferred embodiment, it should be understood that various modifications are possible within the scope of the present invention.

[0113] In this description, unless specifically indicated otherwise, the word "or" is used in the sense of an on-duty word that returns true when one or both of these conditions are satisfied, in contrast to an on-duty word "exclusive or" which requires that only one of the conditions was satisfied. The word “comprising” is used in the sense of “including” and not “consisting of.” All previous teachings recognized above are incorporated herein by reference. Any recognition of any previously published document in this document should not be construed as recognition or submission, because his teaching was publicly known, publicly available in Australia or elsewhere on the date of this description.

Claims (27)

1. The method of determining the conditions for reducing, before cryogenic distillation, the level of water, carbon dioxide and nitrous oxide in the feed air stream in a process including:
(a) passing said feed stream of air at a feed pressure in the feed direction through a first adsorbent that has Henry law selectivity to carbon dioxide compared to nitrous oxide, measured at 30 ° C. of at least 12.5, and sequentially through a second adsorbent, whose constant according to Henry's law for adsorption of N ₂ O, measured at 30 ° C, is less than 500 mmol / g / atm and whose selectivity according to Henry's law with respect to CO ₂ compared to N ₂ O is not more than 5;
(b) stopping the passage of said feed air stream into said first and second adsorbents after a first time period;
(c) depressurizing the gas in contact with the first and second adsorbents to a second pressure less than the supply pressure;
(d) passing the regenerating gas at a second pressure and at a first temperature in the range of 140 to 220 ° C. into at least a second adsorbent in a direction opposite to the feeding direction for a second time period and then passing the regenerating gas at a second pressure and a second temperature less than the first temperature in the second and first adsorbents in the opposite direction to the feed direction during the third time period;
(e) stopping the transmission of regenerating gas to the first and second adsorbents;
(f) restoring gas pressure in contact with the first and second adsorbents to the supply pressure;
(g) repeating steps (a) to (f);
where the first adsorbent occupies from 25% to 40% by volume of the total volume occupied by the first and second adsorbents; and where the molar ratio of the regenerating gas to the feed air supplied during each repetition of steps (a) to (f) is from 0.08 to 0.5; wherein the method includes:
(I) placing an analyzer for measuring the concentration of CO ₂ at a selected position within the length of the second adsorbent;
(Ii) determining the threshold concentration measured by the CO ₂ analyzer when the expected maximum N ₂ O level is simultaneously obtained at the downstream end of the second adsorbent in the feed direction;
(III) using the result of stage (II) to determine the maximum duration of the first time period, such that the maximum duration of the first time period is the time from the beginning of the passage of the raw air stream into the first and second adsorbents until the analyzer determines a certain threshold concentration of CO ₂ . 2. The method according to claim 1, where the analyzer for measuring the concentration of CO _{2 is} placed at a point equal to or less than 45% of the path along the length of the second adsorbent in the feed direction. 3. The method according to p. 1, where the level of nitrous oxide, downstream of the second adsorbent, is not more than 20% of its initial level in the raw air stream. 4. A method of reducing the level of water, carbon dioxide and nitrous oxide in the feed stream of air, before cryogenic distillation, including:
(a) passing said feed stream of air at a feed pressure in the feed direction through a first adsorbent that has Henry law selectivity to carbon dioxide compared to nitrous oxide, measured at 30 ° C. of at least 12.5, and sequentially through a second adsorbent, whose constant according to Henry's law for adsorption of N ₂ O, measured at 30 ° C, is less than 500 mmol / g / atm and whose selectivity according to Henry's law with respect to CO ₂ compared to N ₂ O is not more than 5;
(b) stopping the passage of said feed air stream into said first and second adsorbents after a first time period;
(c) depressurizing the gas in contact with the first and second adsorbents to a second pressure less than the supply pressure;
(d) passing the regenerating gas at a second pressure and at a first temperature in the range of 140 to 220 ° C. into at least a second adsorbent in a direction opposite to the feeding direction for a second time period and then passing the regenerating gas at a second pressure and a second temperature less than the first temperature in the second and first adsorbents in the opposite direction to the feed direction during the third time period;
(e) stopping the transmission of regenerating gas to the first and second adsorbents;
(f) restoring gas pressure in contact with the first and second adsorbents to the supply pressure;
(g) repeating steps (a) to (f);
where the first adsorbent occupies from 25% to 40% by volume of the total volume occupied by the first and second adsorbents; where the method further includes:
(I) placing an analyzer for measuring the concentration of CO ₂ at a selected position within the length of the second adsorbent;
(Ii) determining the threshold concentration measured by the CO ₂ analyzer when the expected maximum N ₂ O level is simultaneously obtained at the downstream end of the second adsorbent in the feed direction;
wherein the maximum duration of the first time period is the time from the beginning of the feed air flow passing the first and second adsorbents to measurement by analyzer certain threshold concentration of CO ₂ and wherein the molar ratio of regeneration gas to the feed air supplied during each iteration of steps (a) to (f) is from 0.08 to 0.5. 5. The method of claim 4, wherein the second adsorbent is selected from the group consisting of NaX having a Si / Al ratio in the range of 1.25 to 1.0, zeolite 4A, zeolite 5A, mordenite, chabazite and clinoptilolite. 6. The method according to claim 4, in which the analyzer for measuring the concentration of CO _{2 is} placed at a point equal to or less than 45% of the path along the length of the second adsorbent in the feed direction. 7. The method of claim 4, wherein the ratio of the second time period to the first time period is less than 35%. 8. The method according to p. 4, in which the first time period is from 100 to 300 minutes 9. The method according to p. 4, in which the second time period is from 20 to 100 minutes 10. The method according to p. 4, in which the temperature of the heated regenerating gas is from 140 to 180 ° C. 11. The method according to p. 4, in which the third time period is from 50 to 220 minutes 12. The method according to p. 4, in which the first adsorbent occupies from 28% to 37% by volume of the total volume occupied by the first and second adsorbents. 13. The method according to p. 5, in which the second adsorbent is selected from zeolite NaX having a Si / Al ratio in the range from 1.25 to 1.0, and zeolite 5A. 14. The method of claim 4, wherein the first adsorbent is selected from alumina or impregnated alumina. 15. The method according to p. 4, in which the level of nitrous oxide downstream of the second adsorbent is not more than 20% of its initial level in the raw air stream. 16. Installation to reduce the level of water, carbon dioxide and nitrous oxide from the raw air stream before cryogenic distillation, containing: in the adsorbent layer a layer of the first adsorbent, which has selectivity according to Henry's law with respect to carbon dioxide compared to nitrous oxide, measured at 30 ° C of at least 12.5 and a second adsorbent whose constant according to Henry's law for adsorption of N ₂ O, measured at 30 ° C, is less than 500 mmol / g / atm and whose selectivity according to Henry's law with respect to CO ₂ compared to N ₂ O is not more than 5, where the first adsorbent occupies from 25% to 40% by volume of the total volume occupied by the first and second adsorbents; and a carbon dioxide analyzer located within the length of the second adsorbent. 17. The apparatus of claim 16, wherein the analyzer for measuring the CO ₂ concentration is placed at a point equal to or less than 45% of the path along the length of the second adsorbent in the feed direction. 18. The apparatus of claim 16, wherein the second adsorbent is selected from the group consisting of NaX having a Si / Al ratio in the range of 1.25 to 1.0, zeolite 4A, zeolite 5A, mordenite, chabazite and clinoptilolite. 19. The apparatus of claim 18, wherein the second adsorbent is selected from a NaX zeolite having a Si / Al ratio in the range of 1.25 to 1.0, and a zeolite 5A. 20. The method of claim 16, wherein the first adsorbent is selected from alumina or impregnated alumina. 21. The method according to p. 16, in which the first adsorbent occupies from 28% to 37% by volume of the total volume occupied by the first and second adsorbents. 22. The way to improve the installation, designed to reduce the level of carbon dioxide, nitrous oxide and water in the feed air stream, using a variable temperature adsorption method so that it can reduce the level of nitrous oxide in the feed air stream without using an adsorbent having selectivity according to Henry's law, measured at 30 ° C, with respect to nitrous oxide compared to carbon dioxide of 0.5 or more, in which the adsorbents contained in the installation are removed and replaced with the same total adsorbed volume entent, consisting of a layer of the first adsorbent, which has Henry law selectivity with respect to carbon dioxide compared with nitrous oxide, measured at 30 ° C, at least 12.5 and a second adsorbent, whose constant according to Henry's law for adsorption N ₂ O, measured at 30 ° C, is less than 500 mmol / g / atm and whose selectivity according to Henry's law with respect to CO ₂ compared to N ₂ O is not more than 5, where the first adsorbent occupies from 25% to 40% by volume of the total volume occupied by the first and second adsorbents, and in which the analyzer for carbon dioxide is placed within the length of the second adsorbent. 23. The method according to p. 22, in which the analyzer for measuring the concentration of CO _{2 is} placed at a point equal to or less than 45% of the path along the length of the second adsorbent in the feed direction. 24. The method of claim 22, wherein the second adsorbent is selected from the group consisting of NaX having a Si / Al ratio in the range of 1.25 to 1.0, zeolite 4A, zeolite 5A, mordenite, chabazite and clinoptilolite. 25. The method according to p. 24, in which the second adsorbent is selected from zeolite NaX having a Si / Al ratio in the range from 1.25 to 1.0, and zeolite 5A. 26. The method of claim 22, wherein the first adsorbent is selected from alumina or impregnated alumina. 27. The method according to p. 22, in which the first adsorbent occupies from 28% to 37% by volume of the total volume occupied by the first and second adsorbents.

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