Abstract:
An oxygen-rich gas product of at least about 95% O 2  purity is obtained from ambient air in a single-stage pressure swing adsorption operation carried out in four steps, comprised of: (a) selective adsorption of nitrogen from the air feed at superatmospheric pressure, (b) followed, without intermediate purge or rinse, by pressure let down through countercurrent gas withdrawal to about atmospheric level, (c) desorption by evacuation of the adsorbent bed to subatmospheric pressure level of 1/20 (5%) or less than 1/20 th  of the pressure prevailing during the adsorption step; and (d) repressuring the evacuated bed with part of the oxygen-rich effluent obtained during the adsorption step. Optionally, the operating cycle may include pressure equalization between two columns performed intermediate steps (a) and (b), whereby O 2  recovery is increased. Under the prescribed conditions the oxygen production capacity at high purity level (˜95%) is substantially increased.

Description:
TECHNICAL FIELD 
     The present invention relates to separation of oxygen from air by pressure swing adsorption (PSA) and is particularly concerned with a process cycle for direct production of essentially nitrogen-free oxygen from ambient air. 
     BACKGROUND OF THE INVENTION 
     Numerous prior art patents describe various pressure swing adsorption processes for selective adsorption of nitrogen from ambient air whereby an oxygen-enriched product stream is recovered. While such oxygen enriched air has beneficial use in many diverse applications, essentially nitrogen free oxygen product is needed for some chemical processes, medical, welding and other uses. However, most known PSA processes can efficiently generate only up to about 80-93% O 2  product from ambient air because the efficiency of production decreases inordinately when higher purity oxygen product is demanded. 
     Where oxygen purity of about 95% or higher is desired resort must be had to purchase of liquid oxygen (LOX) or use of cryogenic means for recovery of oxygen from air. These alternatives require in the first instance transportation of the LOX to the site of use, and the latter instance cryogenic generation of the high purity oxygen at the site is often unattractive from an economical standpoint unless the plant size is sufficiently large. 
     PSA cycles have also been deviced for production of high purity oxygen by two-stage operating techniques whereby an oxygen-rich stream is first produced from ambient air followed by further removal therefrom of remaining nitrogen and argon impurities. Such two-stage operations are cumbersome, however, requiring many adsorbers and complex piping, in addition to which the overall efficiency of separation is quite low. 
     Among the various prior art patents directed to or disclosing PSA processes for production of oxygen-enriched product gas, only U.S. Pat. No. 3,717,974 contains an example directed to production of a product of 95% O 2  purity. Zeolitic adsorbents used for production of oxygen and nitrogen from air selectively adsorb nitrogen from the air feed but do not exhibit any selectivity between oxygen and argon. Consequently, the maximum possible O 2  purity that can be achieved by a PSA process is 95.7% O 2  (complete nitrogen removal). Thus, while certain of the prior art PSA schemes may be pushed to achieve or approach production from air of a gas product having about 95% O 2 , such result can be achieved only at the cost of poorer performance. The performance of the PSA process deteriorates as the purity of the oxygen product is increased; both the specific production capacity (in terms of lbs. of product oxygen/lb. of zeolite/cycle), as well as the oxygen recovery, decrease with increase in O 2  purity. As a result most PSA processes are designed to product about 85-92% O 2  as an optimum product. 
     Typical PSA multi-column systems for fractionation of ambient air and recovery of an effluent enriched in oxygen and/or in nitrogen are disclosed in U.S. Pat. Nos. 3,923,477; 4,013,429; 4,065,272 and 4,329,158. While certain of these and other patents disclose adsorption of nitrogen from atmospheric air at superatmospheric pressure and vacuum desorption of the nitrogen-laden adsorbent bed, none of these (except the previously noted U.S. Pat. No. 3,717,974) discloses the production of a product having an oxygen content of 95% or above. 
     Among the several operating modes disclosed in the cited U.S. Pat. No. 3,717,974 the production of a product of ˜95% O 2  purity is described only in Example 1 of the patent. To obtain the designated high purity O 2  product, a nine step process is employed: adsorption, four depressurization steps, followed by a purge at near ambient pressure, then three repressurization steps. The ˜95% O 2  product is obtained according to the example with a recovery of about 40% and at a production capacity of about 0.035 millipound moles/pound of the adsorbent. At the 70 psia adsorption pressure employed in the patent example the adsorption/desorption pressure ratio is about 4.8. 
     In accordance with the present invention, as hereinafter described, high purity oxygen product is obtained from atmospheric air at considerably higher yield than heretobefore by a simplified and cost efficient four step PSA operation, including adsorption at a moderate superatmospheric pressure and desorption at a moderate subatmospheric pressure within a prescribed adsorption/desorption pressure ratio. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a novel pressure swing adsorption (PSA) cycle for single stage production of essentially nitrogen-free (˜95% O 2  +5.0% Ar+N 2 ) oxygen product directly from ambient air. The key features of the invention are (1) the use of a superatmospheric adsorption step of up to about 8 atmospheres and a subatmospheric desorption step such that the absolute pressure during the adsorption step is at least about 20 times that employed during desorption; and (2) none of the valuable product is consumed for purge cleaning of the adsorbent columns. The high pressure ratio between the adsorption and desorption allows production of oxygen of about 95% purity with good efficiency and the absence of any purge step using product oxygen increases oxygen recovery. 
     Practice of the invention entails four simple steps: (1) passing the air at superatmospheric pressure through a bed of adsorbent selective in sorbing nitrogen, (2) depressuring the bed to about atmospheric pressure level, (3) evacuation to remove sorbed gas and (4) repressuring with product oxygen to adsorption pressure level. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic flow diagram illustrating a system adapted for practice of the invention. 
     FIG. 2 comprises plots of the midpoint pressures in the adsorption columns and effluent gas composition during (A) depressurization and (B) evacuation, respectively. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The process cycle according to the invention may be carried out in a multicolumn pressure swing adsorption system operated in parallel, wherein each of the columns, in turn, is on (a) the adsorption step for being charged with ambient air at superatmospheric pressure, followed in sequence by (b) pressure let down through gas withdrawal, (c) evacuation and (d) repressuring with part of the pure product oxygen withdrawn during the adsorption step. Each of the adsorbent columns is packed with a layer of a desiccant 25 at the feed end followed by a layer of molecular sieve zeolite adsorbent preferentially sorbing nitrogen, such as mordenite or type X, Y or A sieves or various cation exchanged forms of these adsorbents using a single or binary combination of cations selected from Groups I and II metals of the Periodic Table. The preferred desiccant is alumina, and the preferred zeolite layer is calcium exchanged molecular sieve A, X or Y. 
     To maintain continuity of operation three or four parallel adsorbent columns are needed. A four column system is illustrated in FIG. 1 of the accompanying drawings. The untreated air is compressed to a pressure level of at least about one and a half atmospheres and is introduced, at a temperature in the range of 50° to 120° F., into the then on-stream one of columns A, B, C or D. Assuming that column A is to be charged with the feed, the otherwise untreated ambient air is brought to the desired superatmospheric pressure level by compressor 10 discharging into feed manifold 12, passing into line 15 through open valve 20. The compressed air is cooled to near ambient temperature and any condensate removed in cooler/condenser 11 prior to its discharge into manifold 12. The feed gas entering the adsorbent column passes first through the layer of desiccant 25 and then into and through the succeeding layer of molecular sieve adsorbent in which nitrogen is selectively sorbed and a portion of the less strongly sorbed oxygen (plus argon) is discharged into manifold 30 by line 32 and open valve 41. Line 30 discharges the high purity oxygen into surge receiver 5, from which it may be withdrawn for desired use by line 31. The surge tank may not be needed in certain applications. 
     At the expiration of a pre-set on-stream time period the adsorption stroke in column A is terminated and the air feed is switched to another of the columns then starting the adsorption step, by opening one of the valves 21, 22, or 23 associated with that column. Each of the columns A through D goes through the same cycle sequence in turn; introduction of feed into columns B, C, D being effected via lines 16, 17, 18 respectively and discharge of unsorbed oxygen effluent into manifold 30 through its connecting line 33, 34 or 35 respectively. 
     At the termination of the adsorption stroke therein column A is subjected to depressuring by countercurrent withdrawal of gas therefrom into conduit 50 through then opened valve 51. Gas withdrawel is continued until the column on the desorption step is brought to about atmospheric pressure level. When column A is brought to about atmospheric pressure level, valve 51 is closed and evacuation of the column is initiated. 
     Evacuation of the column is accomplished through vacuum line 60 and vacuum pump 61. Thus, evacuation of column A is effected following the closing of valve 51, by opening valve 62. 
     During evacuation of column A, and of the companion columns in their turns, the column is brought to a subatmospheric pressure level in the range of 60 to 300 torr, which is at or less than 5% of the absolute pressure level maintained during the adsorption step. By the evacuation most of the sorbed nitrogen and oxygen (and other contaminants that may be present) are removed from the molecular sieve adsorbent layer, as well as the water from the desiccant layer. 
     The evacuation of column A is terminated by closing valve 62, following which column A is brought back to desired superatmospheric pressure by opening valve 72 for admission of pure oxygen product into the column from storage vessel 5 via supply line 71 or by directly transferring a portion of the O 2  -enhanced product gas being produced by a companion column then undergoing the high pressure adsorption stroke. When Column A is at the desired superatmospheric pressure level valve 72 is closed and valves 20 and 41 are reopened, for repetition of the described sequence of operational steps beginning with adsorption. 
     While various time periods can be empirically established for optimum efficiency, it is preferred to employ a system having four adsorption columns, wherein each of the four steps of the recited sequence is carried out for an equal period of time. Conveniently, a four minute cycle may be employed wherein a one minute time period is assigned to each of the steps of (a) adsorption, (b) depressuring, (c) evacuation and (d) repressuring. The operation in each of the four columns in a four minute cycle and the valve positions are tabulated in Table 1. 
     The initial duration of the adsorption stroke is preset so that the nitrogen in the feed gas continues to be adsorbed, and is terminated short of detrimental breakthrough of nitrogen in the O 2  product effluent. The time periods for the recited steps employed in regenerating the absorbent are designed to afford ample time for effluent completion of each of their assigned functions. 
     
                                           TABLE 1__________________________________________________________________________TIME    COLUMN  VALVE POSITION(min)    A B C D 20         21           22             23               41                 42                   43                     44                       51                         52                           53                             54                               62                                 63                                   64                                     65                                       72                                         73                                           74                                             75__________________________________________________________________________0-1 Ad Ds   Ev     Rp       o x x x o x x x x o x x x x o x x x x o1-2 Ds Ev   Rp     Ad       x x x o x x x o o x x x x o       x                                         x                                         x                                         x o x2-3 Ev Rp   Ad     Ds       x x o x x x o x x x x o o x       x                                         x                                         x                                         o x x3-4 Rp Ad   Ds     Ev       x o x x x o x x x x o x x x       x                                         o                                         o                                         x x x__________________________________________________________________________ Ad = Adsorption Ds = Desorption Ev = Evacuation Rp = Repressuring o = open x = closed 
    
     As described above, the nitrogen laden column following adsorption is next depressurized by gas withdrawal at the feed inlet end of the column. The gas thus withdrawn is comprised of unadsorbed oxygen (in the voids of the bed) and some co-adsorbed oxygen which, as illustrated in the drawings, is discharged into conduit 50. Instead of wasting the gas so withdrawn it can optionally be employed in the process cycle in partially repressuring a companion column, thus further increasing O 2  recovery. Assuming that the adsorption step in column A has been terminated and column D has been evacuated and is ready for repressurization, the gas withdrawn from column A during the countercurrent depressuring of that column, may be introduced into the evacuated column (D) counter to feed direction until the pressure in columns A and D is substantially equalized. Following such pressure equalization, column A is brought to about atmospheric pressure level by continued gas withdrawal, while column D is brought to designed superatmospheric adsorption pressure level with the high purity oxygen from surge vessel 5, or by direct transfer of part of the O 2  product from another column as earlier described. Suitable valving and piping needs to be provided in known manner to practice the pressure equalization step. 
     While the operation illustrated and described is directed to a preferred PSA system having four adsorbent columns operated in parallel, it will be understood that other arrangements may be employed. For further example, the four-step sequence employed in practice of the invention may be carried out in a three column arrangement utilizing a time schedule as shown in Table 2. It is also possible to carry out the cycle sequence of the present invention using only one or two adsorbent columns in conjunction with gas storage tanks. 
     
                       TABLE 2______________________________________TIME      COLUMN(min.)    A             B      C______________________________________  0-0.5   AD            EV     DS0.5-1.0   AD            RP     EV1.0-1.5   DS            AD     EV1.5-2.0   EV            AD     RP2.0-2.5   EV            DS     AD2.5-3.0   RP            EV     AD______________________________________ AD = Adsorption DS = Desorption EV = Evacuation RP = Repressuring 
    
     The operation according to the invention was tested by simulated runs using a column packed with a layer of alumina desiccant in the feed air end followed by a layer of calcium X molecular sieve adsorbent. The feed air was continuously passed through the desiccant and adsorbent beds at a pressure of 3 atmospheres, while discharging therefrom a produced primary effluent of enriched air of about 95% O 2  at essentially the feed pressure. The beds were depressurized to near ambient pressure level by countercurrent gas withdrawal, and then evacuated to about 75 torr. Repressuring of the evacuated column to 3 atmospheres was carried out employing a synthetic gas mixture composed of 95% O 2  and 5% Ar. Steady cyclic state was achieved after about 8-10 cycles. The material balances are tabulated in Table 3. FIG. 2 shows midpoint column pressure and effluent gas composition as functions of desorbed gas quantities, during (A) depressurization and (B) evacuation, respectively. The average O 2  concentrations of the depressurized and the evacuated gas from the column are about 18 and 10% (mole), respectively. The average O 2  concentration of these two streams is about 14%. 
     
                       TABLE 3______________________________________IN               OUT      To-                         To-% O.sub.2  tal    O.sub.2        % O.sub.2                                  tal  O.sub.2______________________________________Feed Air  21      1.31   0.275                      Effluent                              95.5  0.924                                         0.880Pressur-  95      0.83   0.795                      Depressure                              18.5  0.602                                         0.113ing Gas                    Evac.   10.0  0.616                                         0.062          2.14   1.070              2.142                                         1.055______________________________________ Feed Air Rate = 10.0 lb moles/hr/ft.sup.2 Feed Air Pres. = 3 atmospheres Feed Air Temp. = 21° C. O.sub.2 Product 0.094 (95% O.sub.2) O.sub.2 Recovery = 34% {All quantities are expressed in millipound moles/lb of adsorbent 
    
     As seen from the reported results, a large oxygen production capacity (0.094 millipound moles per pound of adsorbent) was obtained at 95% oxygen purity. The comparative numbers at 90% oxygen production level for typical PSA systems are in the order of up to about 0.08 millipound moles/lb. of adsorbent at 50% O 2  recovery (U.S. Pat. No. 4,013,429) and about 0.04 millipound moles per pound of adsorbent at 40% O 2  recovery (U.S. Pat. No. 4,329,158). The oxygen productivity of the complex PSA process disclosed in U.S. Pat. No. 3,717,974 is only 0.035 millipound moles per pound of adsorbent at 95% O 2  purity (40% recovery). Thus, the present invention provides substantially improved O 2  production capacity at desired high purity.