Process for the adsorption of nitrogen from gas mixtures by means of pressure swing adsorption with zeolites

In the pressure swing adsorption of nitrogen from gas mixtures with less polar gas components at temperatures of between 20.degree. and 50.degree. C., wherein the gas mixture is passed through an adsorber which is filled with packings of zeolite pellets and has an inlet zone and an outlet zone, the improvement which comprises providing at least two packings in the adsorber, a packing of Li-zeolite X in the inlet zone of the adsorber and a packing of at least one of Ca-zeolite A and Ca-zeolite X in the outlet zone of the adsorber.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an improved pressure swing adsorption 
process for the adsorption of nitrogen from gas mixtures with zeolite 
pellets. 
2. Description of Prior Art 
The production of oxygen from air at ambient temperatures is already 
performed on a large scale industrially with molecular sieve zeolites 
(c.f., for example, Gas Review Nippon, page 13, no. 5, 1985). Such methods 
exploit the preferential adsorption of nitrogen in comparison with oxygen, 
i.e. when the air is passed through a zeolite packing oxygen and argon are 
collected as the product on leaving the packing. The adsorbed nitrogen may 
be desorbed, for example, by evacuating the packing. In this case, the 
process is known as vacuum swing adsorption (VSA), in contrast with the 
pressure swing adsorption (PSA) process, which is also known. A continuous 
VSA process is characterised by the following processing stages: 
a) passage of air through zeolite packing (at, for example, ambient 
pressure of e.g. about 1 bar) and discharge of O.sub.2 -rich gas from the 
outlet side; 
b) evacuation of the packing with a vacuum pump (for example to a vacuum of 
approximately 100 to 300 mbar countercurrently relative to air flow); 
c) filling the packing with O.sub.2 -rich gas (for example to ambient 
pressure of e.g. about 1 bar countercurrently relative to air flow (see, 
for example, FIG. 1 hereinbelow)). 
In the PSA process, stage b) is performed at approximately ambient pressure 
of e.g. about 1 bar with purging with a portion of the O.sub.2 -rich gas. 
In the so-called PVSA process (a combination of VSA and PSA), separation 
is performed at 1.1 to 2 bar and desorption at approximately 200 to 700 
mbar (minimum pressure). The object of these processes is to achieve an 
elevated production rate (relative to the quantity of zeolite used) and to 
achieve an elevated O.sub.2 yield (ratio of the quantity of O.sub.2 in the 
product to the quantity of O.sub.2 in the introduced air). An elevated 
O.sub.2 yield results in low energy demand by the vacuum pump or air 
compressor. 
As a consequence of the three above-stated stages, there are generally 
three zeolite packings, i.e. three adsorbers, which are operated 
cyclically. In the case of the VSA process, adsorption may also be 
performed with 2 adsorbers (GB-A 1 559 325). 
The economic viability of such adsorption plants is influenced by capital 
costs, such as for example quantity of adsorbent, size of vacuum pump, and 
in particular by operating costs, such as the electricity consumption of 
the vacuum pumps. Zeolites have thus been developed with which it is 
possible to achieve elevated levels of nitrogen adsorption, such that the 
quantity of zeolite used may be kept low or even reduced. Ca zeolites A, 
as described in EP-A-128 545, are used for this purpose. 
Further developments in this area are directed towards increasing 
selectivity for nitrogen over oxygen. 
Elevated selectivity is achieved by using lithium zeolite X (EP-A 297 542). 
In comparison with Na zeolite X, a higher separation factor and higher 
N.sub.2 loading are achieved. 
Better energy consumption is also achieved with Li zeolite X in comparison 
with Na zeolite X (EP-A 461 478, Example 2). 
In order further to optimize adsorption processes for air separation, it 
has been proposed to use adsorbent packings which consist of zones having 
different types of zeolites. 
JP 87/148 304 discloses an oxygen enrichment process in which an absorber 
with particular arrangements of various types of zeolites is used instead 
of an absorber with a single zeolite packing. At the air inlet side, the 
adsorber contains zeolites of the Na--X, Na--Y or Ca--X type and, on the 
air outlet side, of the Ca--A type. 
In EP-A-374 631, a Ca zeolite A with low N.sub.2 adsorption is used in the 
air inlet zone, and a Ca zeolite A with elevated N.sub.2 adsorption is 
used in the outlet zone, wherein the CaO/Al.sub.2 O.sub.3 ratio of both 
zeolites is approximately equal. The different N.sub.2 loading capacities 
are a result of different levels of activation. 
EP-A 0 546 542 describes a packing arrangement in which Li zeolite X is 
used in the air inlet zone and Na zeolite X in the air outlet zone. 
Object 
The object of the invention is to provide a more energy efficient pressure 
swing adsorption process for the adsorption of nitrogen from gas mixtures 
with less polar gas components, with which process it is also possible to 
achieve improved O.sub.2 yields in comparison with the prior art. 
SUMMARY OF THE INVENTION 
It surprisingly proved possible to achieve this object with combinations of 
specific types of zeolites in the pressure swing adsorption process. 
The present invention provides a process for the adsorption of nitrogen 
from gas mixtures with less polar gas components, in particular from air, 
at temperatures of between 20.degree. and 50.degree. C. by means of 
pressure swing adsorption, in which process the gas mixture is passed 
through an adsorber which is filled with packings of zeolite pellets and 
has an inlet zone and an outlet zone, the improvement which comprises 
providing at least two packings in the adsorber, a packing of Li zeolite X 
in the inlet zone of the adsorber and a packing of at least one of zeolite 
A, which has been exchanged with cations of the alkaline earth metal group 
consisting of magnesium, calcium and strontium, and of zeolite X, which 
has been exchanged with cations of the alkaline earth metal group 
consisting of magnesium, calcium and strontium, in the outlet zone of the 
adsorber. 
DETAILED DESCRIPTION OF THE INVENTION 
In pressure swing adsorption processes, a distinction is in particular 
drawn between VSA processes (this process variant is preferably operated 
at evacuation pressures of between 100 and 400 mbar and adsorption 
pressures of between 1 bar and 1.1 bar), PSA processes (in this case, the 
process is preferably operated at a desorption pressure of 1 to 1.1 bar 
and an adsorption pressure of 2 to 6 bar) and PVSA (in this case, the 
process is operated at an evacuation pressure of between 200 and 700 mbar 
and an adsorption pressure of between 1.1 and 2 bar). 
According to the present invention, by using the combination of specific 
types of zeolites it proves possible not only to increase O.sub.2 yield 
but also, surprisingly, to reduce energy consumption. 
The zeolite X, which has been exchanged with cations, preferably has a 
molar SiO.sub.2 /Al.sub.2 O.sub.3 ratio of about 2.0 to 3.0 and a molar 
alkaline earth metal oxide/Al.sub.2 O.sub.3 ratio of about 0.45 to 1.0. 
At least two packings, a packing of Li zeolite X in the inlet zone of the 
adsorber and a packing of at least one of Ca zeolite A and Ca zeolite X in 
the outlet zone of the adsorber are preferred. 
In the Ca zeolite A and Ca zeolite X pellet packings in the outlet zone of 
the adsorber, the two types of zeolite may be present either as two 
separate packings or as a single packing consisting of a mixture of the 
two types of zeolites. 
Preferably, two packings are present in the adsorber. 
The Li zeolite X used is preferably a zeolite having a molar SiO.sub.2 
/Al.sub.2 O.sub.3 ratio of about 2.0 to 2.5 and of which about 80 to about 
100% of the AlO.sub.2 tetrahedron units are associated with lithium 
cations. The remaining cations are preferably sodium, magnesium, calcium 
or strontium ions or protons or mixtures thereof. 
The Ca zeolite X used preferably has a molar SiO.sub.2 /Al.sub.2 O.sub.3 
ratio of about 2.0 to 3.0 and a molar CaO/Al.sub.2 O.sub.3 ratio of about 
0.45 to 1.0. 
The Ca zeolite A used preferably has a degree of Ca ion exchange of about 
0.45 to 1.0. 
Other preferred combinations of packings are: 
a packing of Li zeolite X and a packing of at least one of Sr zeolite A and 
Sr zeolite X, 
a packing of Li zeolite X and a packing of at least one of Mg zeolite A and 
Mg zeolite X, 
a packing of Li zeolite X and a packing of at least one of Ca zeolite A and 
Ca zeolite X, 
a packing of Li zeolite X and a packing of at least one of a zeolite A, 
which has been exchanged with calcium and magnesium ions and has a molar 
CaO/Al.sub.2 O.sub.3 ratio of 0.05 to 0.95 and a molar MgO/Al.sub.2 
O.sub.3 ratio of 0.05 to 0.95, and of a zeolite X, which has been 
exchanged with calcium and magnesium ions and has a molar CaO/Al.sub.2 
O.sub.3 ratio of 0.05 to 0.95 and a molar MgO/Al.sub.2 O.sub.3 ratio of 
0.05 to 0.95, 
a packing of Li zeolite X and a packing of at least one of zeolite A, which 
has been exchanged with calcium and strontium ions and has a molar 
CaO/Al.sub.2 O.sub.3 ratio of 0.05 to 0.95 and a molar SrO/Al.sub.2 
O.sub.3 ratio of 0.05 to 0.95, and of a zeolite X, which has been 
exchanged with calcium and strontium ions and has a molar CaO/Al.sub.2 
O.sub.3 ratio of 0.05 to 0.95 and a molar SrO/Al.sub.2 O.sub.3 ratio of 
0.05 to 0.95, 
a packing of Li zeolite X and a packing of at least one of a zeolite A, 
which has been exchanged with strontium and magnesium ions and has a molar 
SrO/Al.sub.2 O.sub.3 ratio of 0.05 to 0.95 and a molar MgO/Al.sub.2 
O.sub.3 ratio of 0.05 to 0.95, and of a zeolite X, which has been 
exchanged with strontium and magnesium ions and has a molar SrO/Al.sub.2 
O.sub.3 ratio of 0.05 to 0.95 and a molar MgO/Al.sub.2 O.sub.3 ratio of 
0.05 to 0.95. 
The zeolite X which is present in the outlet zone preferably has a molar 
SiO.sub.2 /Al.sub.2 O.sub.3 ratio of 2.0 to 3.0 and a molar MeO/Al.sub.2 
O.sub.3 ratio (with Me.dbd.Ca, Sr) of 0.45 to 1.0 and a molar MeO/Al.sub.2 
O.sub.3 ratio (with Me.dbd.Mg) of 0.3 to 1.0. 
The zeolite A which is present in the outlet zone of the adsorber 
preferably has a molar MeO/Al.sub.2 O.sub.3 ratio (with Me.dbd.Ca, Sr) of 
0.45 to 1.0 and a molar MeO/Al.sub.2 O.sub.3 ratio (with Me.dbd.Mg) of 
0.30 to 1.0. 
The proportion of Li zeolite X in the total quantity of the packings in the 
adsorber is about 20 to 90%, preferably about 25 to 75%. The proportion is 
dependent upon the air inlet temperature and the pressure ratio between 
the maximum adsorption pressure and the minimum desorption pressure. 
For example, at an adsorption pressure of about 1 to 2 bar, the minimum 
evacuation pressure should preferably be between about 100 and 700 mbar, 
the adsorption cycle per adsorber should be about 20 to 80 seconds and the 
number of adsorbers should be between 1 and 3. 
Industrial performance of the process according to the invention is 
exhaustively described, for example, in Gas Separation and Purification 
1991, volume 5, June, pages 89 and 90. 
In addition to the above-stated Ca-exchanged zeolites A and X, it is also 
possible to use zeolites A and X which have been exchanged with other 
divalent cations, in particular magnesium, barium, strontium or mixtures 
thereof. The calcium in the zeolites A and X may be partially or 
completely replaced by the stated divalent cations (see U.S. Pat. No. 
3,313,091). 
The gas stream may preferably be dried before being passed through the 
zeolite packing, for example by being passed through a drying layer of 
silica gel.

DETAILED DESCRIPTION OF THE DRAWING 
Referring now more particularly to FIG. 2, there are shown three adsorbers 
A, B and C, supplied with starting gas mixtures through valved air blower 
C10, the gas passing through cooler/heater H10 and then into the adsorbers 
A, B and C. Each adsorber has a zeolite pellet packing of a composition 
described in the various examples hereinbelow. 
Valves 11A, 11B and 11C control the ingress of gas into the respective 
adsorbers, and valves 12A, 12B and 12C control egress of gas therefrom, 
controlled by vacuum pump V10. 
Valve sets 13A, 14A, 15A and 13A, 14B, 15B and 13A, 14C, 15C control the 
flow of gases between their respective adsorbers, other adsorbers and/or 
product blower G10. Valves 16ABC and 18ABC also serve to open or close 
their respective lines as needed or desired. 
The composition of the feed and product gases, the sequences of valve 
openings and closings and the composition of the zeolite pellets in the 
adsorbers are set forth in the illustrative examples which follow, wherein 
all parts are by weight unless otherwise expressed. 
EXAMPLES 
The zeolite X types used were produced by ion exchange of the corresponding 
Na zeolite X pellets (sample A). 
Sample A (Na zeolite X) 
Na zeolite X pellets were produced according to German patent 2 016 838, 
Example 2, wherein the pellets contained approximately 18% zeolite A and 
82% zeolite X. The molar SiO.sub.2 /Al.sub.2 O.sub.3 ratio was 2.3, the 
grain size 1 to 2 mm and the bulk density approximately 650 g/l. 
Activation was performed at 600.degree. C. with dry nitrogen. 
Sample B (Ca zeolite A) 
Ca zeolite A pellets were produced in accordance with EP-A 0 170 026, 
Example 2. Calcination was performed in a stream of nitrogen at 
500.degree. to 600.degree. C. The molar CaO/Al.sub.2 O.sub.3 ratio was 
0.72. 
Sample C (Ca zeolite X) 
The above-stated Na zeolite X pellets were subjected to Ca exchange prior 
to activation, wherein treatment was performed according to EP-A 0 170 
026, Example 15. Activation was then performed under N.sub.2 at 
600.degree. C. The molar CaO/Al.sub.2 O.sub.3 ratio was 0.75. 
Sample D (Li zeolite X) 
An Na zeolite X was subjected to lithium exchange (according to EP-A 297 
542) prior to activation. 12 liters of binder-free Na zeolite X pellets, 
produced according to DE-A 1 203 238, were placed in a column with a 
heatable jacket. 690 liters of 1 molar lithium chloride solution were then 
pumped through the pellet packing within 15 hours. The temperature was 
85.degree. C. Once ion exchange was complete, the pellets were washed with 
water, which had been adjusted to a pH of 9 with LiOH. Activation was then 
performed under nitrogen at 600.degree. C. The molar Li.sub.2 O/Al.sub.2 
O.sub.3 ratio was 0.96. 
The nitrogen adsorption performance of the samples may be found in Table 1 
and in FIG. 1. 
TABLE 1 
______________________________________ 
Adsorption characteristics of the samples: 
Sample A B C D 
______________________________________ 
N.sub.2 adsorption at 1 bar and 25.degree. C. in Nl/kg! 
9.25 13.5 14.25 
22 
N.sub.2 /O.sub.2 adsorption ratio at 1 bar and 25.degree. C. 
2.65 2.95 3.15 4.55 
______________________________________ 
Performance of testing 
The following parameters were held constant in the test plant and during 
performance of the testing: 
______________________________________ 
Packing diameter 500 mm 
Packing depth of the Al.sub.2 O.sub.3 layer at air inlet 
10% of MS depth 
Air inlet temperature 40.degree. C. 
Air outlet temperature 40.degree. C. 
Air pressure at inlet 1150 mbar (max.) 
Depth of zeolite layer 1600 mm 
Minimum evacuation pressure, inlet 
250 mbar 
Pressure at beginning of evacuation 
900 mbar 
Evacuation time/adsorption time 
30 seconds 
Transfer stage (BFP time) 
6 seconds 
______________________________________ 
The adsorbers were provided with insulation in order to prevent heat 
exchange with the surroundings. The wall thickness of the containers was 
approximately 1 mm. 
Test sequence for one adsorber cycle according to FIG. 2: 
______________________________________ 
C 10 - air blower 
H 10 - cooler/heater 
G 10 - product blower 
V 10 - vacuum pump 
A, B, C - adsorbers 
______________________________________ 
Time 0 sec.: 
Adsorber A has completed adsorption. 
Time 0-6 sec.=BFP time: 
Only valve 15 A is open on adsorber A. Only valves 12 C and 13 C are open 
on adsorber C. O.sub.2 -rich gas thus flows from adsorber A via valve 15 A 
and via control valve 17 ABC and valve 13 C into adsorber C. Adsorber C so 
completes its evacuation stage, wherein the pressure rises from the 
minimum level (for example 250 mbar) to a higher pressure. The pressure in 
adsorber A falls from its maximum level (for example 1150 mbar) to the 
initial evacuation pressure (for example 900 mbar). 
Adsorber B begins air separation, i.e. air passes through valve 11 B into 
adsorber B and O.sub.2 -rich product gas leaves valve 14 B and is passed 
to compressor G 10. 
Time 6-30 sec.: 
Only valve 12 A is open on adsorber A; adsorber A is evacuated with the 
vacuum pump V 10 from, for example 900 mbar, to, for example, 250 mbar. 
Adsorber B is at the adsorption stage as in "time 0-6 sec" and, 
simultaneously, O.sub.2 -rich gas is introduced into adsorber C via valve 
13 C, valve 18 ABC and 13 C. Only valve 13 C is open on adsorber C. The 
introduced quantity is calculated such that, at the end of this period, 
the pressure in adsorber C is, for example, 1080 to 1090 mbar. 
In the following cycle, adsorber C separates the air, then adsorber A, i.e. 
the "0-6 sec." and "6-30 sec." stages are repeated. 
The following parameters were also measured during performance of the 
testing: 
the quantity of O.sub.2 -rich product, 
the pressure profile at the adsorber inlet during the evacuation time, 
the evacuated quantity of gas. 
The evacuated quantity of gas and the quantity of O.sub.2 product are used 
to calculate the introduced quantity of air and thus the O.sub.2 yield 
(=quantity of O.sub.2 in product to quantity of O.sub.2 in air). 
All values relate to an O.sub.2 concentration in the product of 93 vol. %; 
the energy value from the vacuum pump and air blower was also converted 
for an O.sub.2 volume of 1000 m.sup.3 /h. 
The energy demand for the vacuum pump was calculated from the pressure 
profile during evacuation of the packing by referring to the 
characteristic curve (=energy demand as a function of evacuation pressure) 
of a known Roots blower with an evacuation capacity of 20000 m.sup.3 /h 
(at 1.03 bar). The energy demand of the air blower was calculated in 
accordance with the following formula: 
##EQU1## 
Example 1 (Comparison; Na Zeolite X) 
Sample A was used in the adsorber. The residual H.sub.2 O loading of the 
activated zeolite was below 0.5 wt. % (to DIN 8948; P.sub.2 O.sub.5 
method). The quantity of zeolite per adsorber was 190 kg. Oxygen 
enrichment was performed in accordance with the above explanations. The 
following data were obtained: 
______________________________________ 
Air temperature at inlet .degree.C.! 
40 
Quantity of product Nm.sup.3 /h! 
15.9 
O.sub.2 yield %! 45.5 
Calculated total energy demand KWh/Nm.sup.3 O.sub.2 ! 
0.46 
______________________________________ 
Example 2 (Comparison; Ca Zeolite A) 
Sample B was used in the adsorber (190 kg/adsorber). The residual H.sub.2 O 
loading of the activated zeolite was below 0.5 wt. %. The following data 
were obtained: 
______________________________________ 
Air temperature at inlet .degree.C.! 
40 
Quantity of product Nm.sup.3 /h! 
21.4 
O.sub.2 yield %! 52.5 
Calculated total energy demand KWh/Nm.sup.3 O.sub.2 ! 
0.395 
______________________________________ 
Example 3 (Comparison; Ca Zeolite X) 
Sample C was used in the adsorber (190 kg/adsorber). The residual H.sub.2 O 
loading of the activated zeolite was below 0.5 wt. %. The following data 
were obtained: 
______________________________________ 
Air temperature at inlet .degree.C.! 
40 
Quantity of product Nm.sup.3 /h! 
22 
O.sub.2 yield %! 52.5 
Calculated total energy demand KWh/Nm.sup.3 O.sub.2 ! 
0.40 
______________________________________ 
Example 4 (Comparison; Li Zeolite X) 
Sample D was used in the adsorber (190 kg/adsorber). The residual H.sub.2 O 
loading of the activated zeolite was below 0.5 wt. %. The following data 
were obtained: 
______________________________________ 
Air temperature at inlet .degree.C.! 
40 
Quantity of product Nm.sup.3 /h! 
23 
O.sub.2 yield %! 54 
Calculated total energy demand KWh/Nm.sup.3 O.sub.2 ! 
0.375 
______________________________________ 
Example 5 (Comparison; Li Zeolite X in Inlet Zone and Na Zeolite X in 
Outlet Zone) 
Above the zone with the desiccant, 95 kg of sample D were introduced into 
the adsorber, and, thereon, 95 kg of sample A. The following data were 
obtained: 
______________________________________ 
Air temperature at inlet .degree.C.! 
40 
Quantity of product Nm.sup.3 /h! 
18 
O.sub.2 yield %! 44.5 
Calculated total energy demand KWh/Nm.sup.3 O.sub.2 ! 
0.46 
______________________________________ 
Example 6 (Comparison; Ca Zeolite A in Inlet Zone and Li zeolite X in 
Outlet Zone) 
Above the zone with the desiccant, 95 kg of sample B were introduced into 
the adsorber, and, thereon, 95 kg of sample D. The following data were 
obtained: 
______________________________________ 
Air temperature at inlet .degree.C.! 
40 
Quantity of product Nm.sup.3 /h! 
22 
O.sub.2 yield %! 51 
Calculated total energy demand KWh/Nm.sup.3 O.sub.2 ! 
0.398 
______________________________________ 
Example 7 (According to the Invention; Li Zeolite X in Inlet Zone and Ca 
Zeolite X in Outlet Zone) 
Above the desiccant zone, 95 kg of sample D were introduced into the 
adsorber and, thereon, 95 kg of sample C. 
______________________________________ 
Air temperature at inlet .degree.C.! 
40 
Quantity of product Nm.sup.3 /h! 
26 
O.sub.2 yield %! 58 
Calculated total energy demand KWh/Nm.sup.3 O.sub.2 ! 
0.350 
______________________________________ 
Example 8 (According to the Invention; Li Zeolite X in Inlet Zone and Ca 
Zeolite A in Outlet Zone) 
Above the desiccant zone, 95 kg of sample D were introduced into the 
adsorber and, thereon, 95 kg of sample B. 
______________________________________ 
Air temperature at inlet .degree.C.! 
40 
Quantity of product Nm.sup.3 /h! 
25.5 
O.sub.2 yield %! 57.5 
Calculated total energy demand KWh/Nm.sup.3 O.sub.2 ! 
0.355 
______________________________________ 
The adsorber packing according to Example 7 exhibits a better O.sub.2 yield 
and lower energy demand than the Li zeolite X packing (Example 4; see FIG. 
3 and FIG. 4). O.sub.2 production costs are thus lower than in Example 4. 
The quantity of Ca zeolite X and the Ca content in Ca zeolite X are 
correlated to the inlet temperature. The Ca content should be increased at 
higher temperatures of the incoming air, and reduced at lower 
temperatures. 
The packing according to Example 8 produces the best results. The energy 
value and the O.sub.2 production rate are the best in comparison with the 
packings according to Examples 2 or 4. 
Example 5 achieves very poor energy values for the oxygen produced. 
It will be understood that the specification and examples are illustrative 
but not limitative of the present invention and that other embodiments 
within the spirit and scope of the invention will suggest themselves to 
those skilled in the art.