Process for adsorption

In a process for the separation of oxygen from a gas stream by adsorption, the improvement comprising using, as the adsorbent, a compound have the formula M.sub.x [M'(CN).sub.6 ].sub.y PA1 wherein M is an element having an atomic number of 21, 25 to 30, 39, 50, or 57 to 59; PA1 M' is an element having an atomic number from 24 to 27; and PA1 x and y are positive whole numbers such that the sum of the valence of M times x plus the valence of [M'(CN.sub.6)] times y is equal to zero.

TECHNICAL FIELD 
This invention relates to a process for the adsorption of oxygen from gas 
streams using a transition metal hexacyano compound as the adsorbent. 
BACKGROUND ART 
Gas separations utilizing solid adsorbent materials are well known, 
particularly those in which naturally occurring and synthetic zeolites are 
used. The synthetic zeolites are especially adapted to gas separations 
based on the size of the gas molecule. 
A number of different synthetic zeolites have been formulated and are 
commercially available for gas separations. For example, such molecular 
sieve adsorbent materials are commonly used to remove high boiling 
impurities such as water vapor and carbon dioxide upstream of further 
processing. Such usage is common for pretreatment of natural gas 
feedstocks and precleanup of air prior to cryogenic separation. Other 
applications include the upgrading of refinery process streams such as 
recycle hydrogen streams. Additionally, other zeolites are commonly used 
in adsorption processes to separate air. Such zeolites are utilized in 
either pressure swing or temperature swing adsorption processes, although 
the pressure swing processes are generally preferred. These zeolites are 
typically nitrogen selective, that is, the nitrogen component of the air 
stream is adsorbed preferentially to the oxygen component. As a result, 
the nitrogen component is loaded onto the adsorbent bed whereas the oxygen 
component tends to remain in the gas phase. Although the zeolite molecular 
sieve adsorbent materials are effective materials for separating air, they 
have one significant drawback. That drawback is related to the fact that 
by nature of their nitrogen selectivity, it is the major component of air 
that is adsorbed rather than the minor oxygen component. Since air 
composition is nominally 78 percent nitrogen, nitrogen selectivity for the 
adsorbent results in large adsorbent material requirements for such a 
separation process. It would be advantageous, therefore, for a separation 
process to adsorb oxygen rather than nitrogen and thereby reduce the 
adsorbent material requirements. 
The potential advantage of oxygen selective processes has been recognized 
and, for this purpose, oxygen selective carbon-type molecular sieve 
adsorbents have been made available. This type of adsorbent is rate 
selective, however. Consequently, these materials are necessarily used in 
nonequilibrium process cycles that maximize sorption rates of oxygen with 
respect to those of nitrogen. This, in turn, requires the use of rapid 
cycles, for example, cycles of about one minute in duration, which 
restricts cycle design to pressure swing adsorption processes and have 
relatively high power requirements. 
Thus, there is a need for adsorbent processes which are both oxygen 
selective and non-rate selective. 
DISCLOSURE OF THE INVENTION 
An object of this invention, then, is to provide a versatile adsorption 
process advantageously adaptable to conventional pressure or temperature 
swing separations. 
Other objects and advantages will become apparent hereinafter. 
According to the present invention, an improvement has been discovered in a 
process for the separation of oxygen from a gas stream by adsorption. The 
improvement comprises using, as the adsorbent, a compound having the 
formula: 
EQU M.sub.x [M'(CN).sub.6 ].sub.Y 
wherein M is an element having an atomic number of 21, 25 to 30, 39, 50, or 
57 to 59; 
M' is an element having an atomic number from 24 to 27; and 
x and y are positive whole numbers such that the sum of the valence of M 
times x plus the valence of [M'(CN).sub.6 ] times y is equal to zero. 
DETAILED DESCRIPTION 
The transition metal hexacyano compounds defined above are, among other 
thins, known pigments and catalysts as shown, for example, in U.S. Pat. 
Nos. 3,094,379 and 3,278,457, respectively. The characteristic of being an 
oxygen selective adsorbent is not recognized, however. As will be apparent 
from the atomic numbers, M can be any of the elements including scandium, 
manganese, iron, cobalt, nickel, copper, zinc, yttrium, tin, lanthanum, 
cerium, or praseodymium and M' can be any of the elements, chromium, 
manganese, iron, or cobalt. The formula subscripts x and y are chosen to 
form neutral molecules. 
Preferred adsorbent compounds are Zn.sub.2 [Fe(CN).sub.6 ], Zn.sub.3 
[Fe(CN).sub.6 ].sub.2, and Ce[Fe(CN).sub.6 ]. The Zn.sub.2 [Fe(CN).sub.6 ] 
is characterized by good loading and a high separation factor although its 
gas adsorption rate is relatively low. The Zn.sub.3 [Fe(CN).sub.6 ].sub.2 
compound is characterized by good loading and fast gas adsorption rates 
although its separation factor is moderate. The Ce[Fe(CN).sub.6 ] has a 
separation factor between those of the two zinc compounds, and its oxygen 
loading is much higher than those of either zinc compound. The rates of 
Ce[Fe(CN).sub.6 ] are similar to those of Zn.sub.2 [Fe(CN).sub.6 ]. The 
choice of adsorbent material will depend not only on the loading, ratio 
and separation factor, but also on the application. 
The separation method can utilize a pressure swing adsorption process 
whereby the oxygen-containing gas feed stream is contacted with the 
adsorbent bed material, which adsorbs at least some of the oxygen 
component and discharges an oxygen-depleted gas stream from the adsorbent 
bed. The adsorbent bed can be regenerated by reducing the pressure on the 
bed and thereby removing the oxygen-rich adsorbate. In a similar fashion, 
adsorbent material can be utilized in temperature swing adsorption 
processes whereby the feed gas stream containing the oxygen is passed 
through the adsorbent bed at a low temperature allowing at least some of 
the oxygen component to be adsorbed on the bed and discharging an 
oxygen-depleted gas stream from the adsorbent bed. The adsorbent bed can 
then be regenerated by raising the temperature of the bed to drive off the 
oxygen-rich adsorbate. Both the pressure swing and temperature swing 
adsorption processes are, aside from the adsorbent taught in this 
specification, conventional insofar as process steps are concerned. 
Conventional process conditions can also be applied here although 
preferred conditions may vary somewhat with the feed gas and from those 
utilized with other adsorbents. Thus, with air as the feed gas in the 
pressure swing mode, pressures can be in the range of about 0.01 
atmosphere to about 100 atmospheres, but are preferably in the range of 
about 1 to about 30 atmospheres for adsorption with adsorbent regeneration 
being carried out at about atmospheric pressure. The temperature range for 
the pressure swing cycles is about 150.degree. K. to about 350.degree. K. 
and is preferably in the range of about 290.degree. K. to about 
340.degree. K. In the temperature swing mode, temperatures can be in the 
range of about 195.degree. K. to about 373.degree. K. and are preferably 
in the range of about 273.degree. K. to about 373.degree. K. Optimum 
temperatures are about ambient for adsorption and in the range of about 
333.degree. K. to about 366.degree. K. for adsorbent regeneration. In 
general, all of the available adsorption processes can be used together 
with the transition metal hexacyano compounds defined above. Examples of 
this adsorption technology may be found in U.S. Pat. Nos. 2,944,627; 
3,024,867; and 3,636,679.