Process for the production of a gas having a substantial oxygen content

The selective adsorption of nitrogen from air is used in a system including a number of adsorbers, to give oxygen enriched gas at elevated pressure, the adsorber is regenerated by depressurization under atmospheric pressure with a pump and by repressurization with oxygen enriched gas. The pumping duration is at most equal to the duration of the production step.

BACKGROUND OF THE INVENTION 
(a) Field of the Invention 
The present invention concerns the production of an oxygen enriched gas 
from air and by adsorption, according to a process of the "PSA" ("pressure 
swing adsorption") type carried out between an elevated pressure P.sub.M 
and a low pressure P.sub.m which is lower than the atmospheric pressure 
obtained by pumping. 
(b) Description of Prior Art 
The apparatuses of this type intended for the industrial production of 
oxygen by fractionning air over zeolites, for example of the 5A or 13X 
type, give oxygen enriched air up to 95% oxygen contents (the residual 5% 
essentially consisting of argon). 
In a large number of applications, a production quality of 90/93% oxygen 
content is however sufficient. In this same range of contents, the 
quantities of oxygen required by the application may be from a few 
tons/day to a few hundreds of tons/day. 
These known industrial apparatuses have been developed within the range of 
10 to 50 T/d of oxygen, and they have appeared to be very cost competitive 
as compared to the oxygen obtained by cryogenic means and supplied in 
liquid form. 
The different types of cycles proposed in these apparatuses generally 
comprise from 2 to 4 adsorbers, one of which is in production, while the 
other (or the others) are either being regenerated or are in intermediate 
phase (flushing, repressurizing . . . ). 
Since these cycles have a duration which generally varies between 90 
seconds and a few minutes, the volume of the adsorbers for a given cycle 
and a predetermined duration increases in proportion to the flow to be 
produced. The compliance with the rules of speed of flow of the gas in 
certain phases, to prevent the attrition of the absorbent, imposes a 
minimum cross section to the flow of gas, which, for large sizes, directly 
or indirectly becomes the limiting factor. In the case of adsorbers of 
vertical cylindrical shape and with vertical circulation of gas, the 
diameter of the adsorber becomes too large beyond a certain size of the 
apparatus (limitation of the diameter of the bottoms and sleeves, problems 
of transportation . . . ). 
In the case of adsorbers of horizontal cylindrical shape and with vertical 
circulation of gas, which enable larger flows than int he preceding case 
to pass therethrough, with the same diameter, the use of large flows 
present problems of gas distribution in the internal collectors on both 
sides of the adsorbent, as well as of an increase of dead volume in these 
collectors. On the other hand, in same phases, kinetic phenomenons require 
minimum durations to prevent a degradation of the performances, which 
determines, for an adsorbent of given granulometry, an optimum duration of 
cycle. 
Bearing in mind the requirements of attrition, kinetic and technological 
limitations, the limitation of such an apparatus for the production of 
oxygen is believed to be about 60 T/d. 
In the case where an application requires more important quantities of 
over-oxygenated air, for example 300 T/d, the actual solution is either to 
install a plurality of units in parallel (for example 3 units of 50 T/d 
each, for the production of 150 T/d), or to go to the solution of 
utilizing cryogenic means. 
SUMMARY OF INVENTION 
The problem that the invention aims at solving is to extend the actual 
tonnage limits of the apparatuses for the industrial production of oxygen, 
with a lower production cost than the one which would result from the use 
in parallel of a plurality of independent units. More specifically, it is 
an object of the invention to produce with a single unit, a quantity of 
oxygen that could be much higher than 60 T/d, which, in production cost, 
decreases the portion of fixed fees (civil engineering, general 
engineering, mounting, starting up), and also reduces the investment 
portion (material, adsorbent . . . ) as compared to the cost resulting 
from the juxtaposition of a plurality of units. 
These objects of the invention are reached, in a process of the type 
mentioned above wherein, on a plurality of n adsorbers, there is 
successively provided, cyclically according to a period T, on each of said 
adsorbers, a dephasing of duration T/n from one adsorber to the following: 
a) a step of co-current production of oxygen, of duration x, at a pressure 
at least equal to atmospheric pressure, with introduction of air at least 
during a substantial portion of said step of production; 
b) a step of depressurization, comprising at least in the last part 
thereof, a counter-current depressurization until reaching a 
sub-atmospheric pressure obtained by pumping; said pumping being followed, 
possibly, by a flushing-elution through a counter-current flow of oxygen 
enriched gas; the minimum cycle pressure reached during said pumping step 
being P.sub.m ; 
c) a step of repressurization comprising at least a phase of 
counter-current repressurization with oxygen enriched gas characterized by 
the combination of the following steps: 
d) the number of adsorbers is at least 3; 
e) the duration of the production step is longer than dephasing T/n; 
f) the sum of the durations of counter-current depressurization and the 
optional flushing-elution is at most equal to the duration of the 
production step; 
g) the step of pumping according to b) is carried out by means of "k" 
systems of pumping (k.gtoreq.1), each being adapted to a level of partial 
depressurization and exerting its action successively on a same adsorber 
during a pumping of duration y, so that 
##EQU1## 
with "k" lower by at least one unit than the maximum number of adsorbers 
which are in simultaneous production during at least a fraction of the 
time interval T/n. 
The expression system of pumping, means either a pump and its associated 
motor, or a pump stage or pump body and in this case, a plurality of 
pumping systems may be connected to a single motor, or a plurality of 
pumps mounted in parallel and pumping, at a given moment, the gas from a 
single and same adsorber. 
The depressurization gas, which is introduced counter-currently according 
to step c) is an initial depressurization gas from another adsorber and/or 
a production gas from still at least another adsorber. Possibly, a portion 
of the repressurization gas according to c) consists of air introduced 
co-currently. 
The maximum cycle pressure is generally between atmospheric pressure and 
1.6.times.10.sup.5 Pascal, while the minimum cycle pressure is between 
0.2.times.10.sup.5 and 0.5.times.10.sup.5 Pascal.

DESCRIPTION OF PREFERRED EMBODIMENTS 
As usual, it is pointed out that the expressions "co-current" and 
"counter-current" are used when the gas circulates int eh adsorber, 
respectively from the inlet orifice of the mixture to be treated towards 
the outlet orifice of the production gas and respectively vice-versa. On 
the pressure diagrams, the co-current direction extends towards the top of 
the sheet parallel to the axis of the ordinates "(pressure)", while the 
counter-current direction is opposite thereto. When the arrow which is 
indicative of the direction of gaseous flow extends through the diagram, 
this means that the gas passes through the adsorber from one orifice 
(inlet or outlet) to the other (outlet or inlet respectively). 
When the arrow originates or terminates on the diagram, this means that one 
of the orifices is closed, i.e. it is being emptied or filled 
respectively. 
On the drawings and in the description which follows the following 
abbreviations and designations are found: 
T duration--or period--of a cycle; 
n number of adsorbers in a group of adsorbers; 
T/n dephasing between two consecutive adsorbers; 
N number of adsorbers in simultaneous production 
t1 start of the step of co-current production; 
t2 end of the step of co-current production or start of the optional step 
of initial co-current depressurization; 
t3 end of the optional step of initial co-current depressurization or start 
of the step of counter-current depressurization; 
t4 end of the step of counter-current depressurization, before the optional 
flushing-elution; 
t5 end of the optional step of flushing-elution; 
t6 end of the optional step of partial repressurization up to an 
intermediate pressure; 
t7 end of the step of final repressurization; 
d1 duration of the step of co-current production; 
d2 duration of the step of optional co-current depressurization; 
d3 duration of counter-current depressurization before optional 
flushing-elution; 
d4 duration of flushing-elution; 
d5 duration of counter-current repressurization; 
d6 duration of co-current repressurization; 
d7 duration of counter-current pumping with flushing-elution, if any; 
k number of pumping system. 
Reference is now made to the different FIGS. 1 to 8, on which times 
corresponding to the cycle duration and the different steps are given by 
way of example, on the basis of a time of adsorption, including air 
admission in an adsorber, fixed at 60 s; this time being generally 
comprised between 30 and 120 s in the case of "PSA" industrial oxygen 
loaded with adsorbent either in the form of balls of diameter between 1 
and 3 mm, or in other forms of a granulometry with equivalent diameter. 
______________________________________ 
FIG. 1: 
______________________________________ 
T sec 120 sec 
n 4 
T/n 30 
N 2 
d1 60 sec 
d2 10 sec 
d3 20 sec 
d4 10 sec 
d5 20 sec 
d7 30 sec 
k 1 
______________________________________ 
This cycle has four adsorbers including two adsorbers in simultaneous 
production, each for a duration of 60 s and each adsorber is regenerated 
by counter-current pumping followed by elution for a duration of 30 s, by 
means of a single system of pumping, while ensuring a continuous operation 
of said pumping system. 
As compared to this cycle, with a cycle of the same type (same succession 
of steps, same time of adsorption of 60 s, continuous operation of the 
system of pumping, same volume for each adsorber) but including a single 
adsorber in production at a given moment of its operation, there is 
obtained a device with three adsorbers and a production reduced by half. 
Without substantial loss of yield, nor substantial increase of the energy 
consumption by cubic meter of oxygen produced, the apparatus with four 
adsorbers of FIG. 1 has the same production as two separate apparatus each 
including three adsorbers, representing a gain in productivity of 50%, and 
moreover the limit of production per unit has been multiplied by a factor 
of 2. 
______________________________________ 
FIG. 2: 
______________________________________ 
T 100 sec 
n 5 
T/n 20 
N 3 
d1 60 sec 
d2 10 sec 
d3 10 sec 
d4 10 sec 
d5 10 sec 
d7 20 sec 
k 1. 
______________________________________ 
The cycle of FIG. 2 is distinct from the preceding one by an additional 
adsorber among the group of adsorbers, and also through an additional 
adsorber which is in continuous production. The duration of cycle T is 
shortened to 100 sec, while the duration of production d1 is maintained at 
60 sec and the duration of pumping d7 is reduced to 20 sec. 
______________________________________ 
FIG. 3: 
______________________________________ 
T 120 sec 
n 6 
T/n 20 sec 
N 3 
d1 60 sec 
d2 10 sec 
d3 30 sec 
d4 10 sec 
d5 10 sec 
d7 40 sec 
k 2 
______________________________________ 
In this embodiment, the duration of pumping d7 of 40 sec, is twice that of 
dephasing T/n. Two systems of pumping (pump No. 1 and pump No. 2) are 
therefore used which operate continuously, each being adapted to its own 
suction and driving back pressure level. 
______________________________________ 
FIG. 4: 
______________________________________ 
T 90 sec 
n 3 
T/n 30 sec 
N successively 1 and 2 
d1 40 sec 
d2 10 sec 
d3 10 sec 
d4 10 sec 
d5 10 sec 
d6 20 sec 
d7 20 sec 
k 1 
______________________________________ 
With only three adsorbers, the single pump operates here discontinuously 
for a duration d7 which is 2/3 that of dephasing. On the other hand, two 
adsorbers are simultaneously in production, at least at a given moment 
corresponding to a fraction of the duration T/n of dephasing, a single 
adsorber being in production during the remaining fraction. 
It is noted that the repressurization is here carried out, during 10 
seconds, with introduction of gas at the two ends of the adsorbers, namely 
with co-current air, and an initial depressurization gas being withdrawn 
co-currently from another adsorber during an initial depressurization and 
being reintroduced counter-currently in the adsorber during 
repressurization. During the final repressurization stage, only air is 
sent co-currently. 
______________________________________ 
FIG. 5: 
______________________________________ 
T 144 sec 
n 6 
T/n 24 sec 
N successively 2 and 3 
d1 60 sec 
d2 12 sec 
d3 36 sec 
d4 12 sec 
d5 24 sec 
d7 48 sec 
k 2 
______________________________________ 
In this embodiment, the duration of pumping (d7=48 sec) requires two pumps 
(pump 1 and pump 2), one operating at a relatively high pressure level, 
the other at a less elevated level. Their operation on an adsorber lasts 
24 sec in each case, so that they both operate continuously. 
______________________________________ 
FIG. 6: 
______________________________________ 
T 120 sec 
n 6 
T/n 20 sec 
N 3 
d1 60 sec 
d2 0 sec 
d3 40 sec 
d5 20 sec 
d7 40 sec 
k 2 
______________________________________ 
In this embodiment, where the maximum cycle pressure P.sub.M is equal or 
only slightly higher than atmospheric pressure Pa, depressurization is 
entirely carried out by pumping with two pumps (pump 1 and pump 2), each 
operating during a dephasing T/n. There is no elution and the entire 
repressurization is carried out by withdrawing gas along the production 
flow. 
______________________________________ 
FIG. 7: 
______________________________________ 
T 120 sec 
n 6 
T/n 20 sec 
N 3 
d1 60 sec 
d2 10 sec 
d3 30 sec 
d5 20 sec 
d7 30 sec 
k 2 
______________________________________ 
Here the duration of pumping d7 (30 seconds) exceeds the 20 second 
dephasing. Even if the pressure reduction would be sufficiently moderate, 
it is suitable to use two pumps, wherein the first one (pump No. 1) 
operates only during 10 seconds, while the second one (pump No. 2), which 
is under more reduced pressure, operates during a duration of dephasing. 
This second pump No. 2 therefore operates continuously, while pump No. 1 
operates half the time. It should be noted that the repressurization is 
completely carried out in counter-current, with gas from the initial 
depressurization of another adsorber (duration d'5=10 seconds), then with 
a production gas (duration d"5=10 seconds). 
______________________________________ 
FIG. 8: 
______________________________________ 
T 120 sec 
n 4 
T/n 30 sec 
N 2 
d1 60 sec 
d2 0 sec 
d3 30 sec 
d4 0 sec 
d5 30 sec 
d7 25 sec 
k 1 
______________________________________ 
Here the duration of pumping d7 is shorter than the duration of the 
counter-current depressurization since it is preceded by a step of 
exposure to counter-current air of the adsorber. 
This step enables to initial the step of pumping starting at atmospheric 
pressure. 
______________________________________ 
SUMMARIZING TABLE 
FIG. FIG. FIG. FIG. FIG. FIG. 
1 2 3 FIG. 4 FIG. 5 
6 7 8 
______________________________________ 
T .sub.(sec) 
120 100 120 90 144 120 120 120 
n 4 5 6 3 6 6 6 4 
T/n 30 20 20 30 24 20 20 20 
N 2 3 3 1 or 2 2 or 3 
3 3 2 
d1.sub.(sec) 
60 60 60 40 60 60 60 60 
d2.sub.(sec) 
10 10 10 10 12 0 10 0 
d3.sub.(sec) 
20 10 30 10 36 40 30 30 
d4.sub.(sec) 
10 10 10 10 12 0 0 0 
d5.sub.(sec) 
20 10 10 10 24 20 20 30 
d6.sub.(sec) 
0 0 0 10 + 10 
0 0 0 0 
d7.sub.(sec) 
30 20 40 20 48 40 30 25 
k 1 1 2 1 2 2 2 1 
______________________________________ 
From the above table, it is noted that the pumping duration d7 is always 
shorter than or equal to the duration of the production step d1. As 
compared to the total duration of a cycle T, this pumping duration is 
between 0,20 (cycle of FIG. 2) and 0,33 (cycle of FIG. 5) of the duration 
of cycle T.