Method and apparatus for gas purification

N 2 Coadsorption Effects

Adsorbents were tested as described above to determine the effect of N 2 coadsorption upon the adsorption of N 2 O. The results of the saturation capacity of N 2 O (1.0 ppm) in N 2 and in He on various adsorbents are compared in Table 1 below. These results were determined for feed conditions of 6.0 bar, 300° K. and 0.08 kmol/m 2 s molar flux using a 22.9 cm or 5.6 cm adsorbent bed length. The SiO 2 /Al 2 O 3 ratio is specified for some adsorbents in the table, e.g. NaX (SiO 2 /Al 2 O 3 =2.3). FIG. 2 shows N 2 O breakthrough curves for adsorbent 13X (NaX 2.5) for N 2 O in N 2 and N 2 O in He.

These results clearly show the substantial effect of N 2 coadsorption, resulting in a decrease in N 2 O capacity from 36% to 96% compared to the single component saturation capacity (1.0 ppm N 2 O in He). The fourth column of Table 1 also indicates examples of preferred adsorbents of the invention (i.e. adsorbents with high N 2 O loading in the presence of N 2 ).

N 2 Isotherms

Isotherms for N 2 at 300° K were determined for various adsorbents over a range of pressures which included typical feed pressures to a prepurifier of a cryogenic air separation unit. Example isotherms are shown in FIG. 3 . The pure component N 2 loadings from these isotherms are compared in Table 2 for various adsorbents at 6.0 bar.

Breakthrough at 50 ppb N 2 O

The average N 2 O loading (IBL) of the adsorbent bed was determined at 50.0 ppb N 2 O from the same N 2 O/N 2 tests reported in Table 1 above. Typical breakthrough results are illustrated in FIG. 4 . The IBL values (reflecting not only the 50 ppb breakthrough, but also N 2 coadsorption) are compared for various adsorbents in Table 3 and shown in FIG. 5 . Separation factors were also computed using Equation 3. The N 2 loadings at 6 bar from Table 2 are used in Equation 3 to represent the working capacity of N 2 in the process.

N 2 O working capacities can be computed either from the saturation loadings of N 2 O/N 2 or from the average loadings of N 2 O at the IBL. The separation factor computed from the average loadings at the IBL is the preferred method since it reflects both the equilibrium and dynamic effects of the adsorbent. Nevertheless, the former method (which reflects equilibrium effects only) is acceptable when only isotherms and no breakthrough data are available. Both separation factors are tabulated in Table 3 for various adsorbents. Both methods identify clinoptilolite (TSM-140) as having the highest N 2 O/N 2 separation factor, as well as establishing the same group of six adsorbents with higher separation factors compared to the prior art choice of CaX. The order of effectiveness amongst the adsorbents in the preferred group is affected by which separation factor method is used.

Using the methodology of the invention, clinoptilolite TSM-140 is the most preferred adsorbent for removing N 2 O from air. This adsorbent has the highest N 2 O working capacity at IBL, the highest working N 2 O/N 2 separation factor and a moderate N 2 working capacity. It is evident from Table 3 that TSM140 has approximately 6.6 times the average N 2 O breakthrough capacity of CaX. Thus, clinoptilolite TSM140 provides a surprisingly superior solution to the problem.

N 2 O and CO 2 Coadsorption Effects

In the present invention, the adsorber is preferably configured so that the water vapor and CO 2 contaminants in the feed air are substantially removed from the gas mixture prior to the final clean up of the stream in which N 2 O is adsorbed. Sufficient removal of N 2 O must be affected to prevent breakthrough beyond 50 ppb N 2 O during the adsorption step of the prepurifier cycle. In this situation, it is estimated that low levels of CO 2 (less than 10.0 ppm, most likely less than 1.0 ppm) could be present with 100 ppb or more N 2 O with the remaining bulk gas being of air composition (N 2 /O 2 ). In order to verify the effectiveness of the adsorbent and to determine the competitive coadsorption effects of CO 2 upon N 2 O, breakthrough tests were performed with clinoptilolite TSM140 using a feed mixture with 1.0 ppm CO 2 and 1.0 ppm N 2 O in N 2 . The results are shown in Table 4 for the average IBL loading and saturated loadings of N 2 O. The loadings are comparable within the experimental error with the N 2 O loadings from Tables 1 and 3. Thus, it is evident that CO 2 and N 2 O do not compete with each other at these low concentrations, i.e. each competes individually only with N 2 . This can be explained by the fact that the number of adsorbed molecules of N 2 (Table 3) is far greater than either those adsorbed molecules of N 2 O or CO 2 . As N 2 O or CO 2 enter the adsorbent the adsorption sites are predominantly occupied by N 2 molecules.

These results suggest that other low-level contaminants present in the gas stream can also be removed simultaneously with N 2 O. In the case of clinoptilolite, adsorbates with a kinetic diameter less than about 4.5 Å and with an interaction potential energy greater than that of N 2 are likely to be completely or partially removed. Such adsorbates include but are not limited to acetylene, ethylene and propane.

Prepurifier Operation

A two-bed TSA prepurifier was designed to evaluate the adsorbent requirement for complete N 2 O removal. The inlet air flow was 569,000 NCFH at a pressure of 72 psig, and an ambient temperature of about 78 F. The ambient N 2 O level was about 400 ppb and the CO 2 level was about 400 ppm. Average temperature of air going into the prepurifier was about 44.6 F. Each bed has an internal diameter of 8.0 ft.

Initially the bed has two layers: First layer of Alumina (9.9 in) and the second layer of 13X APG (46.5 in). The breakthrough level of N 2 O was monitored for this two-layered bed system. It was found that about 80% of the entering N 2 O in each cycle is retained in the bed. About 20% of the incoming N 2 O breaks through the bed. The thickness of the clinoptilolite TSM 140 layer needed to ensure complete N 2 O removal if this layer is added downstream of 13 X layer was calculated to be about 9 in. Therefore, a very thin layer of clinoptilolite TSM140 added downstream of 13X layer substantially eliminates the problem of N 2 O leaking into the cold box. The resulting prepurifier with the three layers could substantially eliminate all of water vapor, CO 2 and N 2 O and most of the hydrocarbons entering the bed.

The Si/Al ratio and the composition (% of exchangeable cations) of major cations in clinoptilolite TSM140 and CS400 are given in Table 5. It is evident from the working capacity in Table 1 and the selectivity in Table 3 that TSM 140 has superior ability to remove N 2 O in the presence of high concentrations of N 2 compared to CS400. Although both materials are of clinoptilolite structure with nearly the same Si/Al ratio, the efficiency of adsorption of N 2 O is quite different. A primary difference in the composition of these materials is in the amount of Na cation present. We have found that a preferred clinoptilolite would have sodium in an amount between about 30 to about 80% of the exchangeable cations.

As indicated above, clinoptilolite (most preferably TSM140 and CS400), chabazite and LiX (most preferably having a greater than 86% Li exchange and a SiO 2 /Al 2 O 3 ratio of (2.3) or (2.0)) were found to be the preferred adsorbents for removing N 2 O from air prior to cryogenic air separation.

The natural zeolites clinoptilolite and chabazite, having higher Si/Al ratio (3.0 to 5.0 in this invention) than type X zeolites, are “weaker” adsorbents compared to LiX, CaX and NaX. These natural zeolites also have a smaller micropore volume than type X. These factors contribute to the lower N 2 adsorption capacity and selectivity; however, overall capacity is typically not a critical issue when removing trace quantities of contaminants. Conversely, because of the higher Si/Al ratio of clinoptilolite and chabazite, these materials have fewer cations. This generally means weaker energetics in relation to polar adsorbates. While this favors weaker attraction for N 2 it also means weaker attraction for N 2 O as well.

The adsorption characteristics of zeolites are strongly dependent upon their cation composition. Both the equilibrium and kinetic adsorption properties can be altered by ion exchange. Cation type, location and number can completely alter adsorption behavior. Acid washing of small pore natural zeolites may remove impurities that block the pores, progressively eliminate cations and finally dealuminate the structure as the strength and duration of the treatment increases. Alkali washing has been shown to modify both the pore size and pore volume of clinoptilolite. The method and extent of dehydration is important in determining the adsorption properties and structural stability of activated zeolites. Dehydration and thermal treatment can result in cation migration, thereby influencing cation location and pore openings. Any of the methods may be used to further improve the adsorption characteristics of the preferred adsorbents of this invention, i.e. equilibrium and/or kinetic adsorption properties may be effected.

As indicated previously, the invention relates to the removal of N 2 O from air in which the H 2 O and CO 2 have already been substantially removed. By “substantially removed” we mean removed to levels of less than 10 ppm, preferably less than 1.0 ppm. However, the invention may be applied in removing N 2 O in the presence of higher concentrations of CO 2 and/or H 2 O. In the most direct application of the invention, a layer of the N 2 O-selective adsorbent is located downstream (as determined by the direction of feed flow during adsorption) of those adsorbents that are used to remove H 2 O and CO 2 Typical TSA prepurifiers have either a single layer of zeolite to remove both H 2 O and CO 2 or a layer of activated alumina (for H 2 O removal) followed by a layer of zeolite for CO 2 adsorption. In general, adsorbents useful for the water and CO 2 adsorption are known to those skilled in the art and include cation containing zeolites (synthetic or natural), activated alumina, silica gel and activated carbon.

In such configurations, the N 2 O-selective layer would represent either a second or third layer of adsorbent, respectively, in the prepurifier adsorber. Other alternative configurations contemplated by this invention are described below. According to the invention N 2 O is adsorbed onto at least one adsorbent selected from natural clinoptilolite, natural chabazite and Li exchanged X zeolite. The removal of N 2 O from the gas stream is achieved by passing the gas stream through a bed of clinoptilolite, chabazite or LiX or a mixture of these in the temperature range of about −70° C. to 80° C., preferably 0° C. to 40° C. While the invention is directed at the removal of low concentrations of N 2 O from air, it may also be used to remove higher concentrations of N 2 O from air or other gas mixtures. Typical gases that can be purified by this process include air, nitrogen, oxygen, argon, methane etc.

The process of N 2 O removal is carried out preferably in a cyclic process such as pressure swing adsorption (PSA), temperature swing adsorption (TSA), vacuum swing adsorption (VSA) or a combination of these. Such processes can be used for removing ppm or ppb levels of N 2 O present in air prior to cryogenic separation. The process of the invention may be carried out in single or multiple adsorption vessels operating in a cyclic process that includes at least the steps of adsorption and regeneration. The adsorption step is carried out at pressure range of 1.0 to 25 bar and preferentially from about 3 to 15 bar. The temperature range during the adsorption step is −70° C. to 80° C. When a PSA process is used, the pressure during the regeneration step is lower than the adsorption pressure, preferably in the range of about 0.20 to 10.0 bar, and preferably 1.0 to 2.0 bar. For a TSA process, regeneration is carried out at a temperature greater than the adsorption temperature; preferably in the range of about 50° C. to 400° C., more preferably between 100° C. to 300° C. In cryogenic air separation processes, the regeneration gas is typically taken from the product or waste N 2 or O 2 streams.

In the cyclic process, the gas containing N 2 O is introduced at one end of an adsorption vessel that contains at least a layer of N 2 O-selective adsorbent. As the gas passes through the bed, N 2 O is adsorbed and an essentially N 2 O-free gas is obtained at the other end of the bed. As the adsorption step proceeds, a N 2 O front develops in the bed and moves forward through the bed during the adsorption step. When the front reaches the end of the bed, which is determined by the concentration of N 2 O acceptable in the outlet gas, the adsorption step is terminated and the vessel enters the regeneration mode. The method of regeneration depends upon the type of cyclic process. For a PSA process, generally the vessel is countercurrently depressurized. Subatmospheric pressure levels can be additionally employed during the regeneration steps using a vacuum pump. For a TSA process, regeneration of the adsorbent bed is achieved by passing heated gas countercurrently through the bed. Using the thermal pulse method, a cooling purge step follows the hot purge step. The heated regeneration gas may also be provided at a reduced pressure (relative to the feed) so that a combined TSA/PSA process is affected. For removal of N 2 O from air, the TSA/PSA method is preferred.

In some cases, passing an inert or weakly adsorbed purge gas countercurrently through the bed can further clean the adsorbent bed. In a PSA process, the purge step usually follows the countercurrent depressurization step. In a TSA process, the heated purge gas can be used for regeneration of adsorbent. In case of a single vessel system, the purge gas can be introduced from a storage vessel, while for multiple bed system, purge gas can be obtained from another adsorber that is in the adsorption phase.

The adsorption system can have more steps than the two basic fundamental steps of adsorption and desorption. For example, top to top equalization or bottom to bottom equalization can be used to conserve energy and increase recovery.

In a specific embodiment, the prepurification process operates as follows with reference to FIG. 6 . Referring to FIG. 6 , feed air fed to the system via conduit 23 is compressed in compressor 10 and cooled by chilling means 11 prior to entering one of two adsorbers ( 16 and 17 ) where at least the contaminants H 2 O, CO 2 and N 2 O are removed from the air. The purified air exits the adsorber via conduit 24 and then enters the air separation unit (ASU) (not shown) where it is then cryogenically separated into its major components N 2 and O 2 . In special designs of the ASU, Ar, Kr and Xe may also be separated and recovered from the air. While one of the beds is adsorbing the contaminants from air, the other is being regenerated using purge gas provided via conduit 25 . A dry, contaminant-free purge gas may be supplied from the product or waste stream from the ASU or from an independent source to desorb the adsorbed contaminants and thereby regenerate the adsorber and prepare it for the next adsorption step in the cycle. The purge gas may be N 2 , O 2 , a mixture of N 2 and O 2 , air or any dry inert gas. In the case of thermal swing adsorption (TSA), the purge gas is first heated in heater 22 prior to being passed through the adsorber in a direction countercurrent to that of the feed flow in the adsorption step. TSA cycles may also include a pressure swing. When only pressure swing adsorption (PSA) is utilized, there is no heater.

The operation of a typical TSA/PSA cycle is now described in reference to FIG. 6 for one adsorber 16 . One skilled in the art will appreciate that the other adsorber vessel 17 will operate with the same cycle, only out of phase with the first adsorber in such a manner that purified air is continuously available to the ASU. Feed air is introduced via conduit 23 to compressor 10 where it is pressurized. The heat of compression is removed in chilling means 11 , e.g. a mechanical chiller or a combination of direct contact after-cooler and evaporative cooler. The pressurized, cool and H 2 O-saturated feed stream then enters adsorber 16 . Valve 12 is open and valves 14 , 18 and 20 are closed as the adsorber vessel 16 is pressurized. Once the adsorption pressure is reached, valve 18 opens and purified product is directed to the ASU with conduit 24 for cryogenic air separation. When the adsorber 16 has completed the adsorption step, valves 18 and 12 are closed and valve 14 is opened to blow down the adsorber 16 to a lower pressure, typically near ambient pressure. Once depressurized, valve 20 is opened and heated purge gas is introduced into the product end of the adsorber 16 . At some time during the purge cycle, the heater is turned off so that the purge gas cools the adsorber to near the feed temperature. The other adsorber 17 will operate with the same cycle with valves 13 , 15 , 19 and 21 .

One of ordinary skill in the art will further appreciate that the above description represents only an example of a typical prepurifier cycle, and there are many variations of such a typical cycle that may be used with the present invention. For example, PSA may be used alone wherein both the heater 22 and the chilling means 11 may be removed. Pressurization may be accomplished with product gas, feed gas or a combination of the two. As indicated above, bed-to-bed equalization may also be used and a blend step may be incorporated where a freshly regenerated bed is brought on line in the adsorption step with another adsorber nearing completion of its adsorption step. Such a blend step serves to smooth out pressure disturbances due to bed switching and also to minimize any thermal disturbances caused when the regenerated bed is not completely cooled to the feed temperature. Furthermore, the invention may be practiced with a prepurifier cycle not limited to two adsorber beds.

As indicated above, the most preferred embodiment of the present invention is the removal of trace amounts of N 2 O from gaseous streams, particularly from air prior to cryogenic separation. The method of the invention is particularly applicable to the removal of low to intermediate (e.g. ppb to ppm) concentrations of N 2 O from a feed stream. For example, the methodology is particularly useful in air prepurification (prior to cryogenic distallation) where the N 2 O concentration is on the order of 350 ppb. The adsorbents are also especially effective for the removal of N 2 O from gas streams containing 100 ppm or less N 2 O. If the gas contains water vapor, this should most preferably be removed to a level of less than 100 ppb prior to passing it through the N 2 O adsorbent. If the gas contains CO 2 , CO 2 should be removed to levels less than 10 ppm, preferably less than 1 ppm, however, removal of CO 2 is not as essential as removal of water vapor. The relative thickness of the N 2 O layer depends upon the pressure, temperature, composition and flow of the feed gas and the desired purity of the purified gas, but could be determined by one of ordinary skill in the art.

As indicated above, in a preferred air prepurification embodiment of the invention, water vapor and CO 2 are substantially removed from air on at least one layer of activated alumina or zeolite, or by multiple layers of activated alumina and zeolite prior to passing the air stream through the N 2 O adsorbent layer. Optionally, the N 2 O-selective adsorbent layer may be extended and used to remove part or all of the CO 2 from air. Alternately, in an adsorption vessel, a first layer of alumina can be used to remove water vapor and a next layer comprised of a mixture of the N 2 O -selective adsorbent and 13X (or other zeolite) can be used to remove both CO 2 and N 2 O from the air. Such an adsorbent mixture may be composed of physically separate adsorbents or of different adsorbents bound together in the form of a composite. Additionally, the N 2 O selective adsorbent may be deposited in the form of fine particles on a substrate such as a monolith.

In the existing prepurifier beds with water adsorbent layer and CO 2 adsorbent layer, the method of the invention allows for replacing 10-100% of CO 2 adsorbent layer with the N 2 O adsorbent at the most downstream end.

As illustrated in FIGS. 7 a and 7 b various layered arrangements of a bed 30 are possible. FIG. 7 a shows for example a bed 30 arrangement comprising a layer of a. first adsorbent for water removal 31 ; a layer 32 of a second adsorbent for CO 2 removal; and a third layer 33 that is the N 2 O adsorbent.

In FIG. 7 b , a bed 30 arrangement having a first layer of water adsorbent 31 ′ and a second layer 34 that is a mixture or composite of a CO2 adsorbent and the N 2 O adsorbent

Some chemically modified forms of the adsorbents used in the process of this invention would also be appropriate for N 2 O removal purposes. Thus, the method of the invention can be carried out wherein said clinoptilolite and chabazite are natural or synthetic, and have exchangeable cations from ions of Group 1A, Group 2A, Group 3A, Group 3B, the lanthanide series of the Periodic Table, as well as mixtures of these. According to the invention, at least nitrous oxide gas contained in a gas stream is separated, whereby N 2 O is adsorbed on at least one adsorbent or a mixture of these selected from the following: natural clinoptilolite, natural chabazite and Li exchanged X zeolite. The mixture can be made with any ratio of these adsorbents namely 0-100% clinoptilolite, 0-100% chabazite and 0-100% LiX, wherein the total of these is 100%.

The adsorbent beds used in the method of the invention can have a variety of configurations, such as vertical beds, horizontal beds or radial beds and can be operated in a pressure swing adsorption mode, temperature swing adsorption mode, vacuum swing adsorption mode or a combination of these.

Clinoptilolite has excellent thermal stability at very high temperatures up to 700° C. Thus, it can be regenerated at very high temperatures if needed.

Since clinoptilolite and chabazite are natural zeolites mined from the earth, they should be thermally treated before being used in the method of the invention. These natural zeolites should also be ground to a suitable average grain size, for example, 4 to 50 mesh, preferably 8 to 12 mesh (US Sieve Series), although smaller or larger average sizes may be employed depending upon the requirements of the application.

In the method of the invention, the ground natural mineral with predetermined grain size distribution is thermally treated at a temperature of 250° C. to 700° C. It is important to dehydrate the zeolites to less than 1.0 % (wt) H 2 O. Those skilled in the art are familiar with such sizing and calcination procedures.

The adsorbents in this method may be shaped by a series of methods into various geometrical forms such as beads and extrudates. This might involve addition of a binder to zeolite powder in ways very well known to prior art. These binders might also be necessary for tailoring the strength of the adsorbents. Binder types and shaping procedures are well known to prior art and the current invention does not put any constraints on the type and percentage amount of binders in the adsorbents.

The N 2 O adsorbent could also potentially adsorb some hydrocarbons from air. To ensure complete removal of hydrocarbons, the N 2 O adsorbent can be physically mixed in a layer 35 with a hydrocarbon selective adsorbent such as 5A (Bed 30 in FIG. 7 c ). Alternately, an additional layer of hydrocarbon selective adsorbent can be placed upstream (layer 36 ) or downstream (not shown) of the N 2 O adsorbent layer 33 ″ (Bed 30 in FIG. 7 d ). Note that 31 ″ and 31 ′″ refer to a layer of water adsorbent and 32 ′ and 32 ″ refer to a layer of CO 2 adsorbent in FIGS. 7 c and 7 d respectively.

The term “comprising” is used herein as meaning “including but not limited to”, that is, as specifying the presence of stated features, integers, steps or components as referred to in the claims, but not precluding the presence or addition of one or more other features, integers, steps, components, or groups thereof.

Specific features of the invention are shown in one or more of the drawings for convenience only, as each feature may be combined with other features in accordance with the invention. Alternative embodiments will be recognized by those skilled in the art and are intended to be included within the scope of the claims.