Process for separating nitrogen from less polar compounds

Details Process for separating nitrogen from less polar compounds Process for separating nitrogen from less polar compounds

1996-09-27

2002-01-08

Spitzer, Robert H. (Department: 1724)

Gas separation: processes

Solid sorption

Including reduction of pressure

C095S130000

Reexamination Certificate

active

06336956

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a new process for separating nitrogen from a gas mixture containing less polar gases.
The separation of the gases in air into nitrogen and oxygen, plus argon, is an established technique which makes use of the differential adsorption properties of gases on zeolite molecular sieves. The process universally recognized is the PSA or “Pressure Swing Adsorption”, which employs pressure differences in order to:
1- adsorb nitrogen at high pressure in order to enrich the gas phase in oxygen,
2- desorb nitrogen at low pressure in order to regenerate the adsorption properties of the zeolite.
3 types of PSA process are distinguished:
those where the high pressure is higher than atmospheric pressure and the low pressure is higher than or equal to atmospheric pressure; for example, a high pressure of approximately 3 b (bars) absolute and a low pressure of approximately 1 b (so-called superatmospheric or PSA scheme);
those where the high pressure is higher than atmospheric pressure, for example of the order of 1.5 b, and the low pressure is lower than atmospheric pressure, for example the order of 0.5 b (so-called transatmospheric scheme or Vacuum Pressure Swing Adsorption);
finally, those where the high pressure is lower than 1.2 b and the low pressure is lower than atmospheric pressure, for example a high pressure of approximately 1.1 b and a low pressure of approximately 0.25 b (subatmospheric scheme or Vacuum Swing Adsorption).
In the present specification reference to a PSA process will be used to denote these three types of process, without discrimination.
PRIOR ART
The adsorbents described in the prior art for making use of the PSA processes aiming at the separation of nitrogen from a gas mixture containing nitrogen are essentially zeolites of type A or X, exchanged with alkali or alkaline-earth metal cations or some other divalent cations. Thus,
patent McKee U.S. Pat. No. 3,140,932 describes the use of zeolites CaX (zeolite X exchanged with Ca cations), SrX, BaX and NiX,
patents McKee U.S. Pat. No. 3,140,933, Chao U.S. Pat. No. 4,859,217 and Kirner U.S. Pat. No. 5,268,023 recommend the use of zeolites LiX,
patents Coe U.S. Pat. No. 5,152,813 and Chao U.S. Pat. No. 5,174,979 describe the use of a zeolite X exchanged with Li and Ca,
patent Coe U.S. Pat. No. 5,258,058 recommends the use of a zeolite X exchanged with Li plus a cation from the list Ba, Co, Cu, Cr, Fe, Mg, Mn, Ni, Zn.
The literature abounds with studies concerning the adsorption of the gases from air on zeolites 5A (see, for example, the general works: D. M. Ruthven “Principles of adsorption and adsorption processes” 1987, John Wiley & Sons 1984; R. T. Yang “Gas separation by adsorption processes”, Butterworths 1987; R. M. Barrer “Zeolites and clay minerals as sorbents and molecular sieves”, Academic Press 1978; M. Suzuki “Adsorption Engineering”, 1990).
The choice of the adsorbent is based on the ability of the zeolite to adsorb much nitrogen at high pressure and to desorb much thereof when the pressure is lowered; this differential adsorptivity (or “breathing”) defines the gas quantity treated at each cycle, and therefore the productivity.
Zeolites other than A and X have hitherto been considered to be generally unsuitable for the separation of nitrogen by PSA because of the excessively high curvature of the isotherm, which prevents desorption at low pressure. Thus, patent Coe U.S. Pat. No. 4,925,460 clearly explains that Ca-chabazite is unsuitable for use in PSA, the form of the isotherm being unsuitable. Patent Leavitt U.S. Pat. No. 5,074,892 also describes very well the need to minimize the nitrogen adsorptivity at the low pressure of the cycle.
Thus, the state of the art recommends choosing the zeolites on the basis of a nitrogen adsorptivity which is as high as possible at the high pressure of the cycle and as low as possible at the low pressure of the cycle.
Besides the nitrogen adsorptivity, another determining factor for qualifying a high-performance zeolite in a PSA process is its adsorptivity differentiating between nitrogen and oxygen, that is its selectivity. This selectivity is expressed by the relationship:
S=q
N2
/q
O2
*P
O2
/P
N2
where q
N2
denotes the quantity of nitrogen adsorbed at the nitrogen partial pressure P
N2
,
and q
O2
denotes the quantity of oxygen adsorbed at the oxygen partial pressure P
O2
.
In the light of the criteria of nitrogen adsorptivity and of selectivity it has hitherto been considered that only zeolites A, X and chabazites exchanged with lithium were capable of being used in a PSA process on industrial scale.
In the present state of the art the selectivity is a quantity which is very difficult to calculate.
A known means for improving the adsorptivity of a zeolite consists in lowering the adsorption temperature.
For example, Patent EP 122,874 describes the production of oxygen from air by a PSA process employing a zeolite NaX at a temperature below the ambient. When compared with the usual zeolites 5A employed at ambient temperature, this process makes it possible to obtain an increase in the performance of the PSA process by lowering the temperature to −30° C.
Other publications show the advantage of low temperatures, for example Izami et al. “High efficiency oxygen separation with low temperature and low pressure PSA”, AIChE, San Francisco, November 1989, where measurements of performances of 5 zeolites in PSA are described as a function of temperature, with determination of an optimum between 0° C. and −15° C., according to the zeolite.
U.S. Pat. No. 3,973,931 makes it possible to ascertain that a low temperature can be reached spontaneously in a column in PSA operation.
U.S. Pat. No. 5,169,413 describes another PSA process operating below the ambient temperature with zeolites.
It thus appears that zeolites can be profitably employed at temperatures below the ambient (20° C.). In the works referred to-above, by Ruthven (pages 342, 343 and 362), Barrer (pages 103-158), Suzuki (page 36) and Yang (pages 26-44), it is even indicated that the adsorptivity of zeolites decreases when the temperature increases.
Furthermore, from the prior art it also emerges that, in general, there is a tendency to systematically set aside the idea of employing a zeolite exhibiting a nitrogen adsorption isotherm with a curvature that would be relatively high.
In the present specification it is chosen to represent said curvature by the parameter:
C
=
P
1
⁢
q
⁡
(
P
2
)
P
2
⁢
q
⁡
(
P
1
)
where q(P
1
) denotes the quantity of nitrogen adsorbed at pressure P
1
and
q(P
2
) that adsorbed at pressure P
2
; and
the pressures P
1
and P
2
are defined respectively from the high and low pressures of the PSA cycle in question.
Thus, P
1
is defined from the partial pressure of the nitrogen contained in a gas mixture, such as air, at the high pressure of the cycle. For example, for a cycle high pressure of 1.1 b, P
1
=1.1×0.78=0.858 (the nitrogen-containing gas mixture in this case being air; the molar concentration of nitrogen in air being 78%).
P
2
is defined from the partial pressure of the nitrogen contained in a given gas mixture, at the low pressure of the cycle. This gas mixture usually consists of the residual gas leaving the bed of adsorbent after desorption. For example, for a cycle low pressure of 0.35 b and a residual gas in which the molar concentration of nitrogen is 50%, P
2
=0.5×0.35=0.175.
Each adsorbent works at high and low pressures which depend on the cycle used and on the actual nature of the adsorbent. A person skilled in the art is capable, relying on his/her knowledge alone, of optimizing said pressures, this being for each adsorbent and cycle used.
In practice the high and low pressure are imposed by criteria such as:
utilization pressure of the oxygen produced, in the case of the high pressure;
technical limitations related to the plant intended to make use of the process, in particular blowers, compressors and vacuum pumps;
minimizing of the energy consump

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