Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2001-11-13
2004-02-17
Doerrler, William C. (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S298000, C062S643000
Reexamination Certificate
active
06691532
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to air separation units, and more particularly, to a method of designing or building air separation units by using libraries containing different module designs, and to air separation units constructed according to such a method.
BACKGROUND OF THE INVENTION
FIG. 1
is a schematic illustration of a portion of an air separation plant or unit
10
for the production of oxygen, nitrogen and/or argon. A main air compressor (MAC)
11
is used to produce a compressed air stream, e.g., at a pressure of 5-6 atmospheres, which is fed to pre-purification units (PPU)
12
a-b
in which carbon dioxide, water, trace hydrocarbons and other condensable substances are removed.
The air leaving the PPU
12
a-b
is then split into two streams. A main air stream
13
a
, comprising between about 40% to about 80% of the air volume leaving the PPU, is fed to main heat exchanger (MHE)
14
. Second stream
13
b
is passed to booster compressor
15
to produce a boosted air stream
13
c
having a pressure of about 10-70 atmospheres. The split ratio between the main air stream and the boosted air stream is a factor of a number of variables not the least of which is the desired product mix of the air separation plant. The boosted air is also fed to the MHE.
The MHE internals are of standard design, and the MHE is operated in standard fashion. The main air exits the MHE as saturated vaporous main air stream
13
d
at a temperature of about −187 C. (about −280 F.). The boosted air exits the MHE as liquid boosted air stream
13
e
at a temperature of less than about −187 C.
In certain air separation plant designs, main air side stream
13
f
is withdrawn from main air
13
a
, and passed through the MHE. The temperature of the main air side stream
13
f
is lowered within the MHE to about −140 C. (−220 F.), and it is withdrawn as vaporous main air side stream
13
g
. This side stream is then passed through expander
16
to lower its temperature to less than about −180 C. (−292 F.), and then fed as expanded main air side stream
13
h
directly into low pressure column
22
.
Both the vaporous main air and liquid boosted air streams
13
d
and
13
e
are then fed to high pressure column (HPC)
17
at about −187 C. for separation into an oxygen-enriched liquid stream and a nitrogen-enriched product. The gaseous nitrogen product stream
32
may be withdrawn from the upper section of the HPC at about −200 C. (about −300 F.).
The HPC internals are of any standard construction, e.g., structured packing, distillation trays, etc., and the HPC is operated in a conventional fashion. An oxygen-rich liquid (RL) stream
18
is withdrawn from the lower section of the HPC
17
, and comprises about 30-45% by volume oxygen, with the remainder being nitrogen, argon and residual air components such as xenon, krypton, and so on. Poor liquid (PL) stream
29
comprising essentially nitrogen, with various residual air components such as neon, etc., is withdrawn from the top section of the HPC
17
. The split of these two liquid streams
18
and
29
is typically 55% by volume (or mole %) rich liquid and 45% by volume (or mole %) poor liquid. Both the rich and the poor liquid streams
18
and
29
are fed separately to subcooler
20
, which is a refrigeration recovery heat exchange unit.
The resulting subcooled streams
21
a-b
are fed to low pressure column (LPC)
22
, which is typically located above and is thermally coupled with the HPC
17
. These subcooled streams enter the LPC
22
at a temperature of about −207 C. (about −316 F.) and at a pressure of between about 1 and 2 atmospheres. At this temperature and pressure, liquid oxygen
23
a
collects at the bottom of the LPC
22
from where liquid oxygen product stream
23
b
is withdrawn. The purity of the liquid oxygen product stream can vary from about 95% or less oxygen (low purity oxygen) up to and in excess of 99.9% oxygen (high purity oxygen). The actual purity or composition of the liquid oxygen stream depends in large part upon the manner in which other parts of the air separation plant are operated. If desired, another liquid stream
30
(also known as “intermediate liquid” stream) may be withdrawn from the HPC
17
and fed to the LPC
22
as an additional reflux stream.
Liquid oxygen generated in the lower section of the LPC
22
passes to the reboiler-condenser (R-C)
26
, a portion of which is submerged within liquid oxygen in sump
23
a
. Gaseous nitrogen
27
a
from the HPC
17
is condensed in R-C
26
by indirect heat exchange with liquid oxygen in sump
23
a
, resulting in partially reboiling of the liquid oxygen. The condensed nitrogen stream
27
b
from R-C
26
enters the HPC
17
. A poor liquid stream
29
is withdrawn from the HPC
17
and is passed to subcooler
20
. This poor liquid stream can be withdrawn either from the same point as nitrogen stream
27
b
or it may be withdrawn several stages below the nitrogen stream
27
b
feedpoint.
The liquid oxygen product stream
23
b
withdrawn from the sump
23
a
at a pressure of about 1 atmosphere can be transferred either directly, or optionally via subcooler
20
, to a liquid oxygen (LOX) storage tank
32
, and optionally, via subcooler
20
. If desired, oxygen can be withdrawn from the LOX storage tank
32
, and pressurized to about 5-70 atmospheres by a liquid oxygen pump P
24
. The pressurized LOX product stream then exchanges heat with the main and boosted air streams
13
a
and
13
c
in the MHE
14
, resulting in the formation of gaseous oxygen product stream
23
c
, which is recovered at a pressure of about 5-70 atmospheres.
Gaseous nitrogen waste stream
25
a
from the upper section of the LPC
22
is returned to the subcooler
20
for heat exchange with the rich and poor liquid streams from the HPC
17
, and then discharged from the air separation unit after additional heat exchange with the main and boosted air streams in the MHE
14
. Optionally, a product nitrogen stream (not shown) may also be withdrawn from the LPC
22
and recovered as a nitrogen product after undergoing heat exchange in subcooler
20
and MHE
14
.
Aside from the configuration shown in
FIG. 1
, many other variations are also possible. Depending on the capacity requirement, different designs for the MHE is available with a different number of heat exchanger cores, e.g., 2-10 or more cores. If argon product is desired, then another distillation column may be coupled to the LPC
22
for additional processing of a fluid stream withdrawn from LPC
22
. Moreover, the columns, compressors, expanders and other equipment can be arranged differently from that shown in
FIG. 1
, depending upon the refrigeration requirements of the plant. Labels A-N represent interface connections that will be addressed in conjunction with the discussion of FIG.
4
.
Applications requiring smaller quantities, e.g., less than 200 metric tons per day (MTPD), of oxygen or nitrogen will usually require only one product and the specifications for that product (e.g., purity, pressure, flows, etc.) are well known. The commercial solution for these applications is typically building a small pre-fabricated plant, e.g., a nitrogen generator, and installing it on the customer's site (known as an “on-site application”). The economic drivers for plants of this nature are low cost, repeatability, small “footprint” and the like. These plants are highly standardized, and all major air separation unit suppliers have plants of this nature in one form or another.
Applications requiring larger quantities of gas, e.g., 200-2000 or more MTPD, sometimes require more than one product and the specifications for these products vary significantly. The commercial solution for these applications typically is a scheme in which large quantities of the desired gas are piped “over the fence” from a production facility located next to the customer's plant. In this way, the delivered gas is metered in much the same way as any other utilities (e.g., ele
Andrecovich Mark J.
Edmiston Alexander
Krishnamurthy Ramachandran
Naumovitz Joseph Paul
Patriarca Joseph
Doerrler William C.
Neida Philip H. Von
Pace Salvatore P.
The BOC Group Inc.
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