Method for plant and separating air by cryogenic distillation

Details Method for plant and separating air by cryogenic distillation Method for plant and separating air by cryogenic distillation

2001-09-19

2003-03-25

Doerrler, William C. (Department: 3744)

Refrigeration

Cryogenic treatment of gas or gas mixture

Separation of gas mixture

C062S646000

Reexamination Certificate

active

06536232

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a process and a plant for separating air by cryogenic distillation. In particular it relates to a process using three separation columns operating at a high pressure, a low pressure and a pressure which is intermediate between the high and low pressures.
BACKGROUND OF THE INVENTION
It is known from EP-A-0538118 to use a process of this type in order to separate air, the intermediate-pressure column having a bottom reboiler heated by nitrogen from the high-pressure column, thus reducing the heating of the bottom reboiler from the low-pressure column.
One aim of the invention is to reduce the energy consumption of the separation process with respect to the processes of the prior art.
Another aim of the invention is to produce oxygen with a purity of at least 95 mol %, or even at least 98 mol %, with an improved yield.
FIG. 1
shows a conventional process with a low-pressure column
103
operating at 1.3 bara enabling oxygen to be produced at 99.5 mol % with a yield of 92%.
A stream of 1 000 Nm
3
/h of air
1
at about 5 bara is divided into two in order to form a first stream
17
and a second stream 3 which is supercharged in a super-charger
5
at a higher pressure of about 75 bara.
The two streams
3
,
17
are cooled on passing through a heat exchanger
100
. The stream
17
is sent to the bottom of the high-pressure column
101
and the liquefied stream
3
in the heat exchanger
100
is expanded in a turbine
6
producing an at least partially liquid stream at its outlet, the fluid or mixture of fluids leaving the turbine
6
being sent at least in part to the high-pressure column
101
.
A rich liquid stream
10
from the high-pressure column
101
is cooled in the subcooler
83
before being expanded and sent to an intermediate level of the low-pressure column
103
.
A liquid airstream
12
is withdrawn from the high-pressure column
101
, cooled in the subcooler
83
, expanded and sent to the low-pressure column
103
.
A waste nitrogen stream
72
is withdrawn from the top of the low-pressure column
103
, sent to the subcooler
83
and then to the heat exchanger
100
where it is warmed.
A stream
31
of 193 Nm
3
/h of oxygen at 99.5 mol % is withdrawn in liquid form from the low-pressure column
103
, pumped in the pump
19
to 40 bara and vaporized in the heat exchanger
100
in order to form a pressurized gas stream.
A stream of 200 Nm
3
/h of gaseous nitrogen
33
is withdrawn from the top of the high-pressure column
101
and is partially heated in the heat exchanger
100
. At an intermediate temperature, part of the gas is expanded in a turbine
35
before being mixed with the waste gas
72
.
In another conventional diagram illustrated in
FIG. 2
, the low-pressure column operates at 4.8 bara and the high-pressure column
101
operates at 14.3 bara. This process produces oxygen at 99.5 mol % with a yield of 78%.
A flow of 1 000 Nm
3
/h of air
1
at about 14.3 bara is divided into two in order to form a first stream
17
and a second stream
3
which is supercharged in a super-charger
5
to a higher pressure of about 75 bara.
The two streams
3
,
17
are cooled on passing through a heat exchanger
100
. The stream
17
is sent to the bottom of the high-pressure column
101
and the liquid stream
3
is expanded in a turbine
6
producing an at least partially liquid stream at its outlet, the fluid or mixture of fluids leaving the turbine
6
being sent at least in part to the high-pressure column
101
.
A rich liquid stream
10
from the high-pressure column
101
is cooled in the subcooler
83
before being expanded and sent to an intermediate level of the low-pressure column
103
.
A liquid airstream
12
is withdrawn from the high-pressure column
101
, cooled in the subcooler
83
, expanded and sent to the low-pressure column
103
.
A waste nitrogen stream
72
is withdrawn from the top of the low-pressure column
103
, sent to the subcooler
83
and then to the heat exchanger
100
where it is warmed.
A stream
31
of 164 Nm
3
/h of oxygen at 99.5 mol % is withdrawn in liquid form from the low-pressure column, pumped in the pump
19
to 40 bara and vaporized in the heat exchanger
100
in order to form a pressurized gas stream.
No gaseous nitrogen stream is withdrawn from the top of the high-pressure column
101
(of course a high-pressure gaseous nitrogen stream is condensed conventionally in a reboiler-condenser associated with the low-pressure column).
It is known from EP-A-833118 and U.S. Pat. No. 5,657,644 to heat an intermediate-pressure column of a triple-column system with an argon-enriched gas which also serves to feed an argon-production column.
SUMMARY OF THE INVENTION
The inventors of the present application have discovered that, even without using an argon-separation column, purification of the oxygen at the bottom of the low-pressure column remains satisfactory for the production of high-purity oxygen.


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patent: 5682764 (1997-11-01), Agrawal et al.
patent: 5881570 (1999-03-01), Drnevich et al.
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patent: 6318120 (2001-11-01), Ha
patent: 6347534 (2002-02-01), Ha
patent: 0 687 876 (1995-12-01), None
patent: 0 924 486 (1999-06-01), None

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