WO2000040332A1 - Zeolites a base de lithium contenant de l'argent et du cuivre, et leur utilisation pour l'adsorption selective - Google Patents

Zeolites a base de lithium contenant de l'argent et du cuivre, et leur utilisation pour l'adsorption selective Download PDF

Info

Publication number
WO2000040332A1
WO2000040332A1 PCT/US1999/029666 US9929666W WO0040332A1 WO 2000040332 A1 WO2000040332 A1 WO 2000040332A1 US 9929666 W US9929666 W US 9929666W WO 0040332 A1 WO0040332 A1 WO 0040332A1
Authority
WO
WIPO (PCT)
Prior art keywords
zeolite
cation
silver
composition
copper
Prior art date
Application number
PCT/US1999/029666
Other languages
English (en)
Inventor
Ralph T. Yang
Nick D. Hutson
Original Assignee
The Regents Of The University Of Michigan
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by The Regents Of The University Of Michigan filed Critical The Regents Of The University Of Michigan
Priority to US09/869,623 priority Critical patent/US6780806B1/en
Priority to AU20532/00A priority patent/AU2053200A/en
Publication of WO2000040332A1 publication Critical patent/WO2000040332A1/fr

Links

Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/02—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by adsorption, e.g. preparative gas chromatography
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/02—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material
    • B01J20/10—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising silica or silicate
    • B01J20/16—Alumino-silicates
    • B01J20/18—Synthetic zeolitic molecular sieves
    • B01J20/186—Chemical treatments in view of modifying the properties of the sieve, e.g. increasing the stability or the activity, also decreasing the activity
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2253/00—Adsorbents used in seperation treatment of gases and vapours
    • B01D2253/10—Inorganic adsorbents
    • B01D2253/106—Silica or silicates
    • B01D2253/108—Zeolites
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2256/00—Main component in the product gas stream after treatment
    • B01D2256/12—Oxygen
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/10—Single element gases other than halogens
    • B01D2257/102—Nitrogen

Definitions

  • This invention relates to a process and adsorbents for selective adsorption of a gas component, and particularly, selective adsorption of nitrogen.
  • zeolites Since their introduction in the late 1950 's, synthetic zeolites have been used in numerous applications such as catalysis, ion exchange, drying, and separation by selective adsorption. In the separation of air, zeolites of type A and X have typically been used. (See U.S. Patent No. 5,551,257, Jain).
  • the A and X type zeolites are composed of silica and alumina tetrahedra which are joined together to form the truncated octahedral or sodalite structure. These sodalite units are connected with tertiary units to form the structured zeolite unit cell.
  • Type X zeolites contain between 77 and 96 Al per unit cell. The unit cell, including cation sites, for the X zeolite is shown in Figure 1.
  • the extra-framework cations in the zeolite are largely responsible for the nitrogen selectivity of these materials.
  • These zeolites adsorb nitrogen preferentially to oxygen (usually at a ratio of about 4:1) due primarily to the interactions between the charge compensating cations of the zeolite and the quadruple moment of the adsorbing gas (N 2 or 0 2 ) .
  • the quadruple moment of N 2 is approximately four times that of 0 2 .
  • X-zeolite which is typically available as the Na + form (known commercially as 13X) , is not aluminum saturated and contains 86 aluminum atoms per unit cell, while the low silica X zeolite contains 96 aluminum atoms per unit cell.
  • the zeolite contained lithium and the alkaline earth cations in a mixture of 10% to 70% alkaline earth and 30% to 90% lithium.
  • These mixed cation zeolites provide good adsorption capacity and good thermal stability.
  • the cost of separation still remains high. Therefore, there remains the need for improved methods and adsorbents to effectively and economically separate nitrogen from a gaseous mixture.
  • the invention provides new methods for separating nitrogen from a mixture.
  • the invention provides adsorbents specifically for accomplishing nitrogen separation.
  • the adsorbents and separation methods are particularly useful for the selective adsorption of nitrogen from air.
  • the adsorbent comprises an ion exchange zeolite X and preferably zeolite LSX (low silica zeolite X) .
  • the zeolite is most preferably a lithium-based zeolite.
  • the zeolite has exchangeable cationic sites, with silver cation or copper cation occupying at least some of the exchangeable cationic sites.
  • the presence of the silver cation or copper cation at any of the sites will provide an improvement over the non-exchanged zeolite. Therefore, the minimum amount of silver cation or copper cation is greater than zero.
  • the inclusion of silver cation and/or copper cation at the exchangeable cationic sites provides such an improvement in strength of adsorption of nitrogen, that any amount is helpful. However, consideration is given to the strength of such adsorbent capacity when optimizing the amount, in view of subsequent desorption.
  • Type 5A zeolite, and type 13X zeolite are described for nitrogen adsorption in U.S. Patent No. 5,551,257, also incorporated herein by reference in its entirety.
  • zeolite characteristics are also described in the reference book entitled “Gas Separation by Adsorption Processes” by R.T. Yang (1987 Butterworth Publishers) . To the extent that zeolite characteristics are pertinent to the present invention, they will be described further hereinbelow.
  • the important characteristic desired is imparted by the presence of silver and/or copper cation in a zeolite which has been previously exchanged to provide a lithium X zeolite or a lithium LSX zeolite.
  • the desirable X zeolite has a silicon to aluminum ratio (Si/Al) of about 1 to about 1.3.
  • the more desirable lithium LSX has the preferred silicon to aluminum ratio of 1.0. Therefore, the adsorbents of the invention are essentially silver or copper ion exchanged Li + zeolites.
  • the presence of the silver cation or the presence of the copper cation in combination with the lithium cation provides the desired characteristic for improved nitrogen adsorption.
  • the zeolite may also include minor amounts of other commonly found cations which occur in zeolite including, but not limited to besides lithium, potassium, sodium, rubidium, caesium, and mixtures thereof which are alkali metal cations; and alkaline earth metal cations beryllium, magnesium, calcium, strontium, barium, and mixtures thereof.
  • alkali metal cations include lithium, potassium, sodium, rubidium, caesium, and mixtures thereof which are alkali metal cations; and alkaline earth metal cations beryllium, magnesium, calcium, strontium, barium, and mixtures thereof.
  • the presence of a trivalent cation is also possible, however, such is not preferred in order to provide available sites for occupancy by the preferred silver, copper, and lithium.
  • the adsorbents of the invention are used in a method for separating nitrogen from a gaseous air mixture, by accomplishing adsorption at a first select pressure and temperature and then accomplishing release or desorption by changing at least one of the pressure and temperature.
  • Preferential adsorption of nitrogen is preferably achieved by pressure swing adsorption. Conveniently, this may be carried out and is preferably carried out at about ambient room temperature conditions. Therefore, special temperature treatment is not required.
  • the preferred range for adsorption is about 1 to about 10 atmospheres
  • the preferred range for desorption is about 0.2 atmospheres to about 1 atmosphere.
  • lithium zeolites are prepared by ion exchange using lithium chloride. Then these lithium- zeolites were used to prepare Li x Ag y -zeolites and Li x Cu y - zeolites. For convenience, these will be referred to as mixed cation zeolites containing lithium, and transition metal capable of a +1 valence state (targeted metal ion) .
  • the preparation of the Li x Ag y -zeolites is exemplary and is accomplished by ion exchange of a Li-zeolite, prepared as described earlier, with a solution of silver nitrate. The copper ion exchange is accomplished in a comparable manner.
  • Ion exchange of zeolite is easily accomplished by mixing the zeolite in an aqueous solution of metal salt.
  • the metal of the salt is the metal to be exchanged into the cationic site.
  • the concentration of the solution is varied according to the desired level of ion exchange.
  • the ion exchanged zeolite is then removed by filtration from the aqueous solution and washed free of the soluble salts.
  • the Cu-zeolites of the invention are prepared by ion exchanging with a copper salt solution preferably copper chloride or copper nitrate, followed by reduction of any copper +2 to copper +1.
  • the mixed cation material is dried at room temperature and atmospheric conditions. Dehydration in vacuo may follow later, and prior to use and/or analysis. Zeolites have a strong affinity for water; and some molecules are tenaciously held. The presence of water in the zeolite. affects measurement. In the process of the present invention, specific conditions for heat treatment are used beyond the treatment required for mere dehydration. In the present invention, specific heat treatment is used to optimize performance of the mixed cation zeolite of the invention. The heat treatment, after ion exchange, of the mixed cation zeolite is above a minimum temperature of approximately 400°C. A temperature of 400°C or greater is required in order to form crystal clusters of silver and/or copper.
  • the upper limit to the heat treat temperature is 700°C; and preferably is below 700°C, as this is determined to be the point at which destruction of the zeolite itself occurs.
  • the heat treatment is able to be accomplished in air, in vacuum, in inert atmosphere such as argon, nitrogen, or in reducing atmosphere. Desirably, the heat treatment is in a non-oxidizing atmosphere such as in vacuo, in inert atmosphere, or reducing atmosphere. An air atmosphere is less desirable.
  • the non-oxidizing atmosphere is selected to produce partially metallic clusters, and provide the cluster formation and character of the zeolite product described herein.
  • inert means inert with respect to the metal ions, cluster formation and character of the zeolite.
  • the atmosphere needs to be unreactive with the zeolite, and not interfere with formation of desired ion clusters.
  • Treatment temperatures on the order of 20 to 30 minutes are thought to be a minimum. There is no real maximum to the duration of heat treatment time and such time has been extended to 5 hours without any difficulty. Typical heat treatment time varies from about 1 to about 4 hours; and more typically 1 to 2 hours.
  • the preferred lithium content of the zeolite is such that, of the available cationic sites, 70% or more and preferably 80% or more of such sites contain lithium. It is preferred that the proportion of cationic sites occupied by the silver and/or copper be up to about 10%, although up to about 20% is workable as described earlier.
  • Compositions as described hereinbelow were prepared and found to be operable for a variety of ranges including 0.5 to 5% of the cationic sites occupied by silver; over 88% of cationic sites occupied by lithium; and with other alkali and alkaline earth metals constituting the balance.
  • the compositions contain the aforesaid metallic clusters where the metal (M) , copper or silver, is desirably partially metallic.
  • n-1 This is exemplified by clusters of n number of metal atoms collectively having a charge represented by n-1. This is expressed as M n (n_1) where n is 2 or more, and examples are Ag 3 2+ and Ag 6 5+ .
  • the invention provides substantial advantages over conventional methods for separating nitrogen from an air mixture due to the effective and economical processes and adsorbents provided by the invention.
  • Objects, features, and advantages of the invention include an improved method for separating nitrogen from a gaseous mixture, and particularly for separating nitrogen from air. Another object is to provide new adsorbents used in such new separation method.
  • Figure 1 shows unit cell of faujasite-type (X and Y) zeolites including cation sites.
  • FIG. 2 shows adsorption isotherms for N 2 , 0 2 and Ar measured at 25°C for Li 96 -X-1.0 Si/Al zeolite dehydrated in vacuo at 350°C.
  • This nominal Li 96 -X-1.0 is Li 94 _ 5 Na 1-5 -X-l .0.
  • Figure 3 shows N 2 adsorption isotherms, measured at 25°C for (a) nominal Li 86 X-1.25 Si/Al which is Li 77 Na 9 X-1.25 and (b) nominal Li 96 -X-1.0 Si/Al which is
  • Figure 4 shows adsorption isotherms measured at 25°C for N 2 , 0 2 and Ar on nominal Ag 96 -X-1.0 Si/Al zeolite which is Ag 95 . 7 Na 0-3 -X-l .0 dehydrated in vacuo at 450°C.
  • Figure 5 shows N 2 adsorption isotherms, measured at 25°C, for (a) nominal Ag 86 -X-1.25 Si/Al which is Ag 85 . 7 Na 0 . 3 -X-1.25 and (b) nominal Ag 96 -X-1.0 Si/Al which is Li 95 . 7 Na 0-3 -X-l.0. Both materials were dehydrated in vacuo at 450°C.
  • Figure 6 shows N 2 adsorption isotherm, measured at 25°C, for Li x Ag y -X-1.0 Si/Al (Li x Ag y -X-l .0) zeolites dehydrated in vacuo at 450°C. This shows the addition of increasing amounts of Ag results in a change in the general aspect of isotherm toward that of the fully Ag + - exchanged material .
  • Figure 7 shows N 2 adsorption isotherm, measured at 25°C, for nominal Ag 96 -X-1.0 Si/Al which is Ag 95 . 7 Na 0 . 3 -X-1.0 (left) and nominal Li 96 -X-1.0 Si/Al which is Li 94 . 5 Na 1>5 -X-l.0 (right) . The materials were dehydrated in vacuo at (a) 350°C and (b) 450°C.
  • Figure 8 shows plots of ln(P) vs 1/T at different coverages for nominal Li 96 -X-1.0 Si/Al which is Li 94 . 5 Na 1 . 5 -X-1.0 (left) and nominal Li ⁇ Ag ⁇ X-l .0 Si/Al which is Li 94 . 2 Na 0 _ 7 Ag 1-1 -X-l .0 (right).
  • Figure 9 shows isosteric heats of adsorption of
  • Figure 10 shows N 2 and 0 2 isotherms for nominal LigsAg- L -X-l .0 Si/Al which is Li 94 2 Na 0 _ 7 Ag 1-1 -X-l .0 dehydrated in vacuo at 450°C and for nominal Li 96 -X-1.0 Si/Al which is Li 94 . 5 a . 5 -X-!.0 dehydrated in vacuo at 350°C. All isotherms were measured at 25°C.
  • the invention provides type X zeolites comprising varying mixtures of Li and one or more transition metal having +1 valence state and selected from Ag (silver) , Cu (copper) , and mixtures thereof.
  • the invention is demonstrated using silver.
  • the experimental results show that even very small amounts of the +1 transition metal leads to beneficial results.
  • Small amounts of Ag on the order of less than 5 Ag per unit cell (UC) were very effective.
  • the addition of very small amounts of Ag and the proper dehydration conditions resulted in the formation of silver clusters and enhanced adsorptive characteristics and increased energetic heterogeneity as compared to those of the fully exchanged Li-zeolites.
  • Silver has been found to have very strong effects on the adsorption characteristics of zeolites.
  • This sorbent utilized the very strong adsorptive properties of the Ag + ion which provided for increased capacity over that of the Li-X while maintaining some degree of the advantageous isotherm linearity that is seen with Li-X.
  • Ab inito molecular orbital calculations showed the adsorption of nitrogen was enhanced by weak chemical interaction (through a classical ⁇ -complexation bond) with the Ag + cation on the zeolite framework.
  • Transition metal ions were reduced in zeolites for the purpose of forming highly dispersed metallic clusters for use as catalysts. This generally involved treatment at elevated temperatures and/or in reducing atmospheres (e.g., sodium vapor, hydrogen gas, carbon monoxide gas) .
  • reducing atmospheres e.g., sodium vapor, hydrogen gas, carbon monoxide gas
  • color changes upon vacuum dehydration of silver-exchanged A-type zeolites were found to be related to the formation of metallic clusters within the sodalite cage or the 6-prism of the zeolite. Using volumetric sorption techniques and temperature programmed desorption, it was possible to relate these color changes to an autoreductive process involving framework oxygens.
  • Autoreduction is the reduction of the transition metal ion and the oxidation of water or lattice oxygen; this has been observed for both Ag + and Cu 2+ ions in zeolites A, X and Y, and has been shown to take place by two mechanisms in two clearly defined temperature regions: (i) autoreduction in the presence of zeolite water (25-250°C) (See Equation 1; and (ii) autoreduction by oxygen from the zeolite lattice (127- 380°C) (See Equation 2) .
  • octahedral hexasilver metal clusters stabilized by coordination to six silver ions (Ag + ) 6 (Ag°) 6 ) is proposed from x-ray structural determinations of a dehydrated silver- exchanged zeolite A.
  • the formation of such large metal clusters is improbable since color changes are seen even at low temperatures and low silver loadings where extensive migration of neutral silver atoms and subsequent sintering into Ag 6 metal clusters is highly unlikely.
  • the present invention provides type X zeolites containing varying amounts of Li and along with the Li, one or more of Ag and Cu. These materials are heat treated in a way which promotes the formation of intracrystalline silver clusters or copper clusters .
  • the resulting adsorptive characteristics were evaluated with respect to the gases which are of primary interest in the separation of air: N 2 , 0 2 and Ar. The performance of the best of these sorbents was compared to that of the fully Li + -exchanged zeolite using a numerical simulation of a standard five-step PSA cycle, that is used in industry, and the results are given below. Experimental Section Materials
  • Two type-X zeolites differing only by the Si/Al ratio, were used in this work. These were: (1) X- type zeolite with a Si/Al of 1.0 (Praxair, #16193-42, sometimes referred to as LSX, low silica X-zeolite) , and (2) X-type zeolite with a Si/Al of 1.25 (Linde, lot 945084060002) . Both of these materials were binderless, hydrated powders.
  • Helium (99.995%, prepurified) , oxygen (99.6%, extra dry), nitrogen (99.998%, prepurified) and argon (99.998%, prepurified) were obtained from Cryogenic Gases. All water used was deionized.
  • the lithium zeolites were prepared by 5 consecutive static ion-exchanges using a 6.3-fold excess (over that necessary for full ion-exchange) of a 2.2 M solution of LiCl . This was done in a 0.01 M solution of LiOH at a pH ⁇ 9. The lithium ion-exchange solution was heated to a mild boil and then allowed to cool and settle. The solution was decanted, a fresh 6.3X LiCl solution was added, and the procedure was repeated for a total of 5 exchanges.
  • the material was vacuum filtered and washed with copious amounts of deionized water until no free ions were present in the filter water (i.e., no AgCl precipitation upon treatment with Ag + ) .
  • the resulting lithium exchanged zeolites were dried overnight at 100°C in a conventional oven before being dehydrated in vacuo prior to measurement of adsorption isotherms.
  • the silver zeolites were prepared by 2 consecutive- ion-exchanges using a 0.05 M solution of
  • Each silver solution contained a cation content which was double that required for 100% exchange.
  • the silver ion-exchange solution was heated to a mild boil and immediately allowed to cool and settle. As with the lithium ion-exchange, the solution was decanted, fresh
  • the Li x Ag y -zeolites were prepared by ion exchange of a Li-zeolite (prepared as described above) with a 0.05 M solution of AgN0 3 .
  • This silver solution contained a cation content which was equivalent to the targeted amount. This was possible with silver ion- exchange because the silver cation is quickly and easily exchanged.
  • the silver ion-exchange solution was heated to a mild boil and immediately allowed to cool and settle. The resulting material was vacuum filtered and washed with copious amounts of deionized water. Complete incorporation of the targeted silver ions was verified when no precipitation was observed in the filtered water upon treatment with Cl " .
  • Li x Ag y - zeolites are more accurately referred to as Li x Na y Ag z - zeolites since ion exchange is rarely exhaustive and there is almost always some residual Na + present in the starting Li-zeolite.
  • zeolites Prior to measurement of the adsorption isotherms or uptake rates, it is necessary to dehydrate the zeolite sample. Zeolites have a strong affinity for water; and some molecules are tenaciously held. The presence of water in the zeolite significantly affects the validity of the adsorption measurement. Prior to analysis, all samples were heated in order to remove water. Differential thermal analysis (DTA) of zeolite X demonstrated a continuous loss of water over a broad range of temperatures, starting at slightly above room temperature up to 350°C with a maximum at about 250°C. Specific dehydration conditions varied from sample to sample and are given for each sample.
  • DTA Differential thermal analysis
  • the ion-exchanged, mixed LiAgX zeolite samples were analyzed for Ag and Li contents using an inductively coupled plasma mass spectrometer (ICP-MS, Hewlett Packard HP 4500) .
  • the samples were first digested in concentrated nitric acid solution at 100°C for 20 minutes. At the end of digestion, the samples were further diluted and filtered. The filtrates were subjected to ICP-MS analyses.
  • the adsorption isotherms were measured using a static volumetric system (Micrometrics ASAP-2010) . Additions of the analysis gas were made at volumes required to achieve a targeted set of pressures. A minimum equilibrium interval of 9 seconds with a tolerance of 5% of the target pressure (or 0.007 atm, whichever was smaller) was used to determine equilibrium for each measurement point.
  • the pressure transducers in the ASAP-2010 are accurate to ⁇ 0.2% for the pressure range of 0-1 atm.
  • the sample weights were obtained using a digital laboratory balance which is accurate to ⁇ O.Olg. The isotherm measurements and the samples themselves were found to be highly reproducible .
  • the samples were compositionally characterized using neutron activation analysis (NAA) in the research nuclear reactor of the Phoenix Memorial Laboratory at the University of Michigan.
  • NAA neutron activation analysis
  • the samples were irradiated sequentially for one minute at a core- face location with an average thermal neutron flux of 2xl0 12 n/cm 2 /s.
  • Two separate gamma-ray spectra were then collected for each sample with a high resolution germanium detector: one after a 13 minute decay to determine the concentrations of Al and Ag, and a second after a 1 hour and 56 minute decay to analyze for Na and K; both were for 500 seconds real time.
  • NBS-SRM-1633a coal fly ash
  • silver foil were used as standard reference materials and check standards. Absorbent parameters and PSA simulation results are described with reference to Tables 1-3. Results of compositional characterization (NAA and ICP-MS) are given in Table 4. The unit cell compositions for those analyzed samples are given in Table 5.
  • the zeolites herein are referred to for convenience by the nominal amount of Li and Ag substituted for the Na. More precise values are given for each of the zeolites.
  • Li 96 -X-1.0 is more precisely Li 94 . 5 Na 1 . 5 -X-l .0, since replacement of the Na by Li is not fully accomplished.
  • Figure 2 shows the N 2 , 0 2 and Ar adsorption isotherms, measured at 25°C, for Li 96 -X-1.0 (Li 94 . 5 Na 1 . 5 -X-1.0) after vacuum dehydration at 350°C.
  • This zeolite was used in adsorptive air separation because of its very high N 2 capacity and very favorable N 2 :0 2 selectivity (approximately 6:1 at 1 atm) as well as its
  • N 2 isotherm linearity.
  • Figure 3 shows the enhancement in the N 2 adsorption capacity for Li 96 -X-1.0 (Li 94 . 5 Nai. 5 -X-! .0) over that of the Li 86 -X-1.25 (Li 77 Na 9 X-l .25) .
  • Figure 4 shows N 2 , 0 2 and Ar adsorption isotherms for Ag 96 -X-1.0 (Ag 957 Na 0 . 3 -X-l .0) , all measured at 25°C, after vacuum dehydration at 450°C for a minimum of 4 hours. These samples were all initially gray in color, but after vacuum dehydration turned to a deep golden yellow, indicating the formation of silver clusters.
  • Figure 5 shows the enhancement in the N 2 adsorption capacity for Ag 96 -X-1.0 (Ag 8S . 7 Na 0 . 3 -X-l .25) over that of the Ag 86 -X-1.25 (Li 85 . 7 Na 0 . 3 -X-l .0) .
  • the fully exchanged Ag-zeolites like their Li-zeolite analogs, have very high N 2 capacity and favorable N 2 :0 2 selectivity, they are not favorable for use in adsorption-based separations .
  • the working capacity i.e., the ⁇ Q, the change in the adsorptive capacity from the typically used adsorption pressure of 1.0 to a desorption pressure of 0.33 atm
  • the sorbent must be exposed to very low pressure conditions in order to increase that working capacity.
  • Some Ag-zeolites are demonstrated to have a selectivity for Ar over 0 2 .
  • the present examples and data reveal that the Ag-zeolites which had been dehydrated in vacuo at 350°C also showed a selectivity for Ar over 0 2 .
  • the Ag-zeolites which had been dehydrated in vacuo at 450°C had approximately the same adsorption capacity for Ar and 0 2 (as shown in Figure 4) . This is probably due to increased interaction between the charged Ag-clusters and the quadrupole moment of the 0 2 molecule (whereas, the Ar has no quadrupole moment) .
  • the comparative lithium exchanged sodium zeolite is highly selective for the absorption of nitrogen and also preferentially absorbs oxygen as compared to argon. This is undesirable for the purification of oxygen since the less selectively absorbed argon will remain with the oxygen product.
  • the silver exchanged sodium zeolite has high selectivity for nitrogen but is not preferentially selective for oxygen as compared to argon. This means that the purified oxygen stream will not be relatively argon rich as compared to the pre-absorbed argon content.
  • the mixed lithium silver zeolite of the present invention is adaptable for selectivity for argon as compared to oxygen.
  • the lithium silver zeolite of the invention is most favorable for oxygen production because the argon will be removed along with the nitrogen, providing an oxygen stream after absorption which has a smaller fraction of argon. Accordingly, the mixed lithium silver zeolite of the invention is more favorable for oxygen production because it provides absorption of nitrogen without the disadvantage of also being highly selective for absorption of oxygen over argon.
  • zeolites have a strong affinity for water; and some molecules are held tenaciously.
  • the zeolites must be completely dehydrated prior to measurement of the adsorption isotherms in order to guarantee the validity of the result.
  • the dehydration conditions have a very significant effect on the formation of silver clusters.
  • the atmosphere and temperature of the dehydration were found to be the most important.
  • Figure 7 shows N 2 adsorption isotherms for the fully exchanged Ag 96 -X-1.0 (left) and the fully exchanged Li 96 -X-1.0 (right), both after vacuum dehydration at 350°C and 450°C.
  • N 2 adsorption isotherms were measured for Ag 96 -X-1.0 and Li 86 -Ag 10 -X-1.0 after partial or full dehydration in vacuum at various temperatures.
  • the results for both zeolites showed a continual increase in the N 2 adsorption capacity (at 1 atm) with increasing dehydration temperature up to about 450 to about 500°C.
  • Samples which had been dehydrated in vacuum at 550°C and 600°C had N 2 capacities which were considerably lower than those dehydrated in vacuum at the 450 - 500°C range.
  • Heterogeneity in zeolites may result from a number of causes.
  • the existence of different cation sites is one of them. If the intra-crystalline cation population is mixed, sites in the vicinity of a cation will differ for each cation whether or not they occupy equivalent crystallographic positions. Further, in a mixed cation population the proportion of one cation to another may vary from one cavity to another so that the behavior of the cavities as multiple sorption sites may vary throughout the crystal .
  • the presence of energetic heterogeneity of a sorbent can be determined by plotting the isoteric heat of adsorption versus the amount adsorbed. Energetic heterogeneity of the system will result in a decrease in the isoteric heat of adsorption as the amount sorbed increases. For small uptakes, the isoteric heat may decrease rather strongly with the amount adsorbed. This would be an indication that there are some local intracrystalline positions where the guest molecules are preferentially sorbed more strongly than in the rest of the intracrystalline volume. At intermediate uptakes, the slope of this plot will usually decrease and become nearly constant.
  • the measurement of adsorption isotherms at different temperatures permits the calculation of the heat of adsorption as a function of surface coverage.
  • the differential isoteric heat of adsorption is determinable .
  • the isoteric heat of adsorption is calculated from a series of isotherms by application of the Clausius-Clapeyron equation given below as Equation (3) .
  • the isoteric heats of adsorption were determined by evaluating the slope of a plot of ln(P) versus (1/T) at several coverages.
  • the plots of ln(P N2 ) versus (1/T) at several coverages for Li 96 -X-1.0 and Li ⁇ Ag ⁇ X-l .0 (Li 94 . 2 Nag. 7 Agi.i-X-! .0) are shown in Figure 8.
  • the isoteric heats of adsorption at different coverages were calculated for each of these materials and are shown in Figure 9.
  • the cation site designations are conventionally designated as SI (the center of the hexagonal prism), SI' (opposite SI but located in the cubooctahedron), SII (single six-ring in the supercage) , SII' (opposite SII but inside the cubooctahedron) , and SIII (near the four-ring windows in the supercage) .
  • the clusters, in these mixed cation zeolites are instead formed at the N 2 and 0 2 accessible SII and/or SIII locations due to competition with the Li + cations for the preferred SI and SI' locations.
  • Ag-cluster formation at the SII sites would most enhance the adsorptive characteristics of mixed Li x Ag y -X zeolites since these sites have been shown to be non-interactive when occupied with Li + ions.
  • the location of Ag in mixed Na x Ag y -A zeolites was investigated and it was found that the Ag ions prefer six-ring sites (such as the SI, SI' and SII in the X zeolites) .
  • the total energy of physical adsorption ⁇ ⁇ is the result of the interactions between the adsorbate molecules and interactions between the adsorbate molecules and the zeolite cavity wall.
  • the ⁇ ⁇ is comprised of dispersive (D) , repulsive (R) , polarization (P) , field-dipole (FD) interactions, field-quadruple (FQ) interactions, and adsorbate-adsorbate energies and can be written as per Equation (4) .
  • the adsorbates of interest in this evaluation do not have permanent dipoles; and the coverages are low.
  • Equation 4 can be reduced to Equation (5) .
  • N 2 and 0 2 molecules are very similar in size and have comparable polarizabilities, the dispersive, repulsive and polarization energies between the adsorbate and the extra-framework cations are very similar.
  • the quadruple moment of the N 2 molecule is approximately four times that of the 0 2 molecule and is primarily responsible for the difference in the adsorptive capacity for N 2 over that of 0 2 .
  • Argon which does not have a quadruple moment, is more affected by the polarization energy; and for most zeolites, the Ar capacity is about the same as that of 0 2 .
  • step I pressurization with the feed gas, namely 22% 0 2 (mixture of 0 2 and Ar) and 78% N 2 ;
  • step II high pressure adsorption with the feed gas, or feed step;
  • step III co-current depressurization;
  • step IV countercurrent blowdown;
  • step V countercurrent low pressure purge with the product of the feed step (oxygen) .
  • the PSA bed characteristics and the operating conditions used are summarized in Table 2.
  • the pressure ratio which is the ratio of the feed pressure (P H ) to the desorption pressure (P L ) , is an important operating characteristic and it has been shown that a value of 3 suffices for an optimal PSA performance using the Li 96 -X-1.0 sorbent.
  • the same pressure ratio was employed for the comparison of the Li 96 -X-1.0 (Li 94 . 5 Nai. 5 -X-l .0) and Li ⁇ Ag ⁇ X-l .0 (Li 94 . 2 Na 07 Agi.i-X-l .0) sorbents in this work.
  • Sorbent Soi bate ki k 2 k 3 k 4 - ⁇ H
  • Feed gas composition 78% N 2 , 22% 0 2
  • LigsAgi-X-l.O 1.0 0.33 0.69 0.60 0.42 96.42 62.74 5.40xl0 "2
  • the examples above demonstrate new methods for separating nitrogen from a mixture, and more specifically for accomplishing nitrogen separation from an air mixture using new and improved adsorbents.
  • the zeolites of the invention are lithium-based zeolites with silver cation or copper cation present at at least some of the exchangeable cationic sites. The presence of the exchanged cation at any of the sites demonstrated improved performance for adsorption over the non-exchanged zeolites. Without wishing to be held to any particular theory, it is believed that the lithium- based silver and copper exchanged zeolites of the invention achieve selective adsorption of nitrogen enhanced by weak chemical interaction through a type of pi-complexation bond.
  • the effectiveness is clearly demonstrated in he above examples using the Ag + cation and the same results are obtainable with the Cu + cation.
  • the Ag + ion and Cu + ion have the same electronic structure. That is, in the outer shell orbitals, the s- orbital (5s for Ag and 4s for Cu) is empty, whereas the d-orbitals (4d for Ag and 3d for Cu) are filled (with 10 electrons) .
  • This unique electronic structure is the reason that they can form the pi-complexation bonds with molecules that contain pi-electrons, such as olefins.
  • Ag + and Cu + have the same adsorption properties for olefins.
  • Ag + in the AgX zeolite can also form pi-complexation with nitrogen (N 2 ) molecules because N 2 has a triple bond. Based on these two observations, it is evident that CuX zeolite forms pi-complexation with N 2 molecules.
  • the X-zeolite unit cell can be represented by (A10 2 ) 96 (Si0 2 ) 96 . There are 96 charges to each unit cell.
  • the X zeolites, usable for the invention, are not limited to this formula and may range from Si/Al ratio of about 1 to about 1.3. In one aspect the LSX zeolite having Si/Al ratio of about one, is preferred.
  • the unit cell contains the preferred Li and Ag + or Cu + ions and has the formula
  • the adsorbent is represented by this formula, Li x M +1 y (A10 2 ) 96 (Si0 2 ) 96 , where y is in a range of about 0.5 to about 10 and x(Li) is 96-y.
  • the ratio of Si/Al may range from about 1 to about 1.3, with good results using such Ag + and Cu + substituted lithium-based zeolites .
  • alkali metal (A + ) and alkaline metal (Z +2 ) may also be included, as represented by the formula Li x M +1 y Z +2 a A +1 b (A10 2 ) 96 (Si0 2 ) 96 .
  • the atomic portion of M is desirably less than 20, and preferably less than or equal to 10; and the portion of Li is desirably greater than 70 and preferably greater than 80.
  • the atomic portion of cationic sites occupied by other metals (Z, A) is less than that occupied by Li . In any case, the largest number of cationic sites is occupied by Li; then after Li, by Ag or Cu, and by Z and/or A, if any.

Abstract

L'invention concerne de nouveaux procédés permettant de séparer l'azote d'un mélange, ainsi que des adsorbants spécifiques de ladite séparation. Les adsorbants et les procédés de séparation de l'invention sont particulièrement utiles pour l'adsorption sélective de l'azote à partir de l'air. Dans l'un des modes de réalisation, l'adsorbant comprend un zéolite X échangeur d'ions, de préférence le zéolite LSX (zéolite X à faible teneur en silice) et idéalement un zéolite à base de lithium. Le zéolite possède des sites cationiques échangeables, dont au moins certains peuvent être occupés par le cation d'argent ou le cation de cuivre. Le zéolite échangé Ag/Cu est traité thermiquement dans les conditions spécifiques de l'invention. La présence du cation d'argent ou du cation de cuivre sur n'importe quel site constitue une amélioration par rapport au zéolite non échangé.
PCT/US1999/029666 1998-12-30 1999-12-13 Zeolites a base de lithium contenant de l'argent et du cuivre, et leur utilisation pour l'adsorption selective WO2000040332A1 (fr)

Priority Applications (2)

Application Number Priority Date Filing Date Title
US09/869,623 US6780806B1 (en) 1998-12-30 1999-12-13 Lithium-based zeolites containing silver and copper and use thereof for selective absorption
AU20532/00A AU2053200A (en) 1998-12-30 1999-12-13 Lithium-based zeolites containing silver and copper and use thereof for selective adsorption

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US11437198P 1998-12-30 1998-12-30
US60/114,371 1998-12-30

Publications (1)

Publication Number Publication Date
WO2000040332A1 true WO2000040332A1 (fr) 2000-07-13

Family

ID=22354820

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/US1999/029666 WO2000040332A1 (fr) 1998-12-30 1999-12-13 Zeolites a base de lithium contenant de l'argent et du cuivre, et leur utilisation pour l'adsorption selective

Country Status (2)

Country Link
AU (1) AU2053200A (fr)
WO (1) WO2000040332A1 (fr)

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6432170B1 (en) 2001-02-13 2002-08-13 Air Products And Chemicals, Inc. Argon/oxygen selective X-zeolite
US6544318B2 (en) 2001-02-13 2003-04-08 Air Products And Chemicals, Inc. High purity oxygen production by pressure swing adsorption
EP1316357A1 (fr) * 2001-11-19 2003-06-04 Air Products And Chemicals, Inc. Procédé de récuperation du krypton et du xénon contenus dans un courant gazeux ou liquide
WO2003080236A1 (fr) * 2002-03-25 2003-10-02 Council Of Scientific And Industrial Research Procede pour l'elaboration d'un adsorbant de tamis moleculaire pour l'adsorption selective d'azote et d'argon
US6658894B2 (en) 2001-11-19 2003-12-09 Air Products And Chemicals, Inc. Process and adsorbent for the recovery of krypton and xenon from a gas or liquid stream
FR2853257A1 (fr) * 2003-04-02 2004-10-08 Air Liquide Systeme embarque de production de flux gazeux enrichi en oxygene et procede pour alimenter les voies aeriennes d'occupants d'un aeronef
US7300905B2 (en) 2001-01-05 2007-11-27 Questair Technologies Inc. Methods for their manufacture of adsorbent

Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB937750A (en) * 1958-09-24 1963-09-25 Union Carbide Corp Metal loading of molecular sieves
US3626020A (en) * 1969-03-12 1971-12-07 Universal Oil Prod Co Separation of paraxylene from mixture of c aromatic utilizing crystalline aluminosilicate adsorbent
JPS5964510A (ja) * 1982-10-05 1984-04-12 Toyo Soda Mfg Co Ltd アルゴンガスの精製法
US5417957A (en) * 1992-10-05 1995-05-23 Air Products And Chemicals, Inc. Divalent cation exchanged lithium X-zeolite for nitrogen adsorption
GB2302824A (en) * 1995-06-29 1997-02-05 Inst Francais Du Petrole Adsorbents for use in processes for purifying gases
JPH1095612A (ja) * 1996-09-20 1998-04-14 Nippon Sanso Kk 銀含有ゼオライトとそれを用いたガス分離方法及びゼオライトの製造方法
JPH10128106A (ja) * 1996-10-31 1998-05-19 Tosoh Corp 酸素ｐｓａ用吸着剤、その製造法並びにそれを用いた酸素製造方法
EP0893157A1 (fr) * 1997-07-22 1999-01-27 Ceca S.A. Adsorbant zéolitique aggloméré amélioré, son procédé d'obtention et son utilisation pour la séparation non-cryogénique de gaz indusriels

Patent Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB937750A (en) * 1958-09-24 1963-09-25 Union Carbide Corp Metal loading of molecular sieves
US3626020A (en) * 1969-03-12 1971-12-07 Universal Oil Prod Co Separation of paraxylene from mixture of c aromatic utilizing crystalline aluminosilicate adsorbent
JPS5964510A (ja) * 1982-10-05 1984-04-12 Toyo Soda Mfg Co Ltd アルゴンガスの精製法
US5417957A (en) * 1992-10-05 1995-05-23 Air Products And Chemicals, Inc. Divalent cation exchanged lithium X-zeolite for nitrogen adsorption
GB2302824A (en) * 1995-06-29 1997-02-05 Inst Francais Du Petrole Adsorbents for use in processes for purifying gases
JPH1095612A (ja) * 1996-09-20 1998-04-14 Nippon Sanso Kk 銀含有ゼオライトとそれを用いたガス分離方法及びゼオライトの製造方法
JPH10128106A (ja) * 1996-10-31 1998-05-19 Tosoh Corp 酸素ｐｓａ用吸着剤、その製造法並びにそれを用いた酸素製造方法
EP0893157A1 (fr) * 1997-07-22 1999-01-27 Ceca S.A. Adsorbant zéolitique aggloméré amélioré, son procédé d'obtention et son utilisation pour la séparation non-cryogénique de gaz indusriels

Non-Patent Citations (3)

* Cited by examiner, † Cited by third party
Title
DATABASE WPI Section Ch Week 198421, Derwent World Patents Index; Class E36, AN 1984-130101, XP002133402 *
DATABASE WPI Section Ch Week 199825, Derwent World Patents Index; Class E36, AN 1998-280288, XP002133403 *
DATABASE WPI Section Ch Week 199830, Derwent World Patents Index; Class E36, AN 1998-341398, XP002133401 *

Cited By (19)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7300905B2 (en) 2001-01-05 2007-11-27 Questair Technologies Inc. Methods for their manufacture of adsorbent
EP2829318A1 (fr) 2001-01-05 2015-01-28 Air Products And Chemicals, Inc. Compositions de revêtement adsorbantes, stratifiés et éléments d'adsorption comprenant ces compositions et leurs procédés de fabrication et d'utilisation
EP2826554A1 (fr) 2001-01-05 2015-01-21 Air Products And Chemicals, Inc. Appareil pour fabriquer compositions de revêtement adsorbantes, stratifiés et éléments d'adsorption
EP2826552A1 (fr) 2001-01-05 2015-01-21 Air Products And Chemicals, Inc. Suspension utilisée pour la fabrication de revêtements adsorbants stratifiés pour PSA et leur procédé de préparation
EP2826553A1 (fr) 2001-01-05 2015-01-21 Air Products And Chemicals, Inc. Procédé de fabrication de compositions de revêtement adsorbantes, stratifiées et éléments d'adsorption comprenant ces compositions
US7902114B2 (en) 2001-01-05 2011-03-08 Xebec Adsorption Inc. Adsorbent coating compositions, laminates and adsorber elements
EP1243328A3 (fr) * 2001-02-13 2003-05-21 Air Products And Chemicals, Inc. Zéolite-X sélective pour argon/oxygène
US6432170B1 (en) 2001-02-13 2002-08-13 Air Products And Chemicals, Inc. Argon/oxygen selective X-zeolite
US6544318B2 (en) 2001-02-13 2003-04-08 Air Products And Chemicals, Inc. High purity oxygen production by pressure swing adsorption
AU755345B2 (en) * 2001-02-13 2002-12-12 Air Products And Chemicals Inc. Argon/oxygen selective X-zeolite
EP1243328A2 (fr) * 2001-02-13 2002-09-25 Air Products And Chemicals, Inc. Zéolite-X sélective pour argon/oxygène
US6658894B2 (en) 2001-11-19 2003-12-09 Air Products And Chemicals, Inc. Process and adsorbent for the recovery of krypton and xenon from a gas or liquid stream
CN1329101C (zh) * 2001-11-19 2007-08-01 气体产品与化学公司 从气流或液流中回收氪和氙的方法和吸附剂
EP1316357A1 (fr) * 2001-11-19 2003-06-04 Air Products And Chemicals, Inc. Procédé de récuperation du krypton et du xénon contenus dans un courant gazeux ou liquide
WO2003080236A1 (fr) * 2002-03-25 2003-10-02 Council Of Scientific And Industrial Research Procede pour l'elaboration d'un adsorbant de tamis moleculaire pour l'adsorption selective d'azote et d'argon
FR2853257A1 (fr) * 2003-04-02 2004-10-08 Air Liquide Systeme embarque de production de flux gazeux enrichi en oxygene et procede pour alimenter les voies aeriennes d'occupants d'un aeronef
WO2004089510A2 (fr) * 2003-04-02 2004-10-21 L'air Liquide,Societe Anonyme A Directoire Et Conseil De Surveillance Pour L'etude Et L'exploitation Des Procedes Georges Claude Systeme embarque de production de flux gazeux enrichi en oxygene et procede pour alimenter les voies aerennes d'occupants d'un aeronef
WO2004089510A3 (fr) * 2003-04-02 2004-11-18 Air Liquide Systeme embarque de production de flux gazeux enrichi en oxygene et procede pour alimenter les voies aerennes d'occupants d'un aeronef
US7582138B2 (en) 2003-04-02 2009-09-01 L'air Liquide, Societe Anonyme A Directoire Et Conseil De Surveillance Pour L'etude Et L'exploitation Des Procedes Georges Claude Carrier-board system for the production of oxygen-enriched gas streams and method for supplying the airways of the occupants of an aircraft

Also Published As

Publication number Publication date
AU2053200A (en) 2000-07-24

Similar Documents

Publication Publication Date Title
Hutson et al. Mixed cation zeolites: LixAgy‐X as a superior adsorbent for air separation
KR960016518B1 (ko) 2가 양이온 교환된 리튬 x-제올라이트를 사용한 질소 흡착 방법
CA2021175C (fr) Chabasite pour la separation des gaz
CA2085180C (fr) Adsorption d'azote sur une zeolithe x au lithium a cations ca et (ou) sr echanges
JP3776813B2 (ja) アルゴン／酸素選択性ｘゼオライト
AU701448B2 (en) Adsorptive separation of nitrogen from other gases
KR0165917B1 (ko) 기체분리 강화방법 및 여기에 사용되는 제올라이트 조성물
US6780806B1 (en) Lithium-based zeolites containing silver and copper and use thereof for selective absorption
US6537348B1 (en) Method of adsorptive separation of carbon dioxide
KR102128167B1 (ko) 개선된 rho 흡착제 조성물의 사용 방법
Hutson et al. Structural effects on adsorption of atmospheric gases in mixed Li, Ag–X‐zeolite
KR20150127131A (ko) 아르곤의 정제를 위한 흡착제 조성물
CA2013530A1 (fr) Procede de purification des gaz en vrac a l'aide d'absorbants a base de chabazite
KR20190037140A (ko) 개선된 rho 흡착제 조성물, 이의 제조 및 사용 방법
US5584912A (en) High micropore volume low silica EMT-containing metallosilicates
EP2035137B1 (fr) Matériaux a base de silicate, procédé de fabrication de ceux-ci et procédé d'utilisation desdits matériaux a base de silicate en vue d'une séparation de liquides par adsorption
WO2000040332A1 (fr) Zeolites a base de lithium contenant de l'argent et du cuivre, et leur utilisation pour l'adsorption selective
US6878657B2 (en) Process for the preparation of a molecular sieve adsorbent for the size/shape selective separation of air
CN100586554C (zh) 制备用于选择性吸附氮和氩的分子筛吸附剂的方法
EP1078685A2 (fr) Adsorbants nouveaux pour l'adsorption préférentielle de l'azote à partir d'autres gaz
CA2148773C (fr) Separation de gaz a l'aide de metallosilicates zsm-2 renfermant du lithium
EP1486254A1 (fr) Tamis moléculaire à performance améliorée dans la séparation d'air
JPH0685870B2 (ja) 吸着分離剤
JPH1095611A (ja) ガス吸着用ゼオライトおよびその製法ならびにこれを用いたガス吸着分離方法
Yang Air separation by pressure swing adsorption using superior adsorbents

Legal Events

Date Code Title Description
AK Designated states

Kind code of ref document: A1

Designated state(s): AL AM AT AU AZ BA BB BG BR BY CA CH CN CU CZ DE DK EE ES FI GB GD GE GH GM HR HU ID IL IN IS JP KE KG KP LS LT LU LV MD MG MK MN MW MX NO NZ PL PT RO RU SD SE SG SI SK SL TJ TM TR TT UA UG US UZ VN YU ZW

AL Designated countries for regional patents

Kind code of ref document: A1

Designated state(s): GH GM KE LS MW SD SL SZ TZ UG ZW AM AZ BY KG KZ MD RU TJ TM AT BE CH CY DE DK ES FI FR GB GR IE IT LU MC NL PT SE BF BJ CF CG CI CM GA GN GW ML MR NE SN TD TG

121 Ep: the epo has been informed by wipo that ep was designated in this application
DFPE Request for preliminary examination filed prior to expiration of 19th month from priority date (pct application filed before 20040101)
WWE Wipo information: entry into national phase

Ref document number: 09869623

Country of ref document: US

REG Reference to national code

Ref country code: DE

Ref legal event code: 8642

122 Ep: pct application non-entry in european phase