U.S. patent application number 12/774249 was filed with the patent office on 2011-06-16 for separation method and apparatus.
Invention is credited to Henry Edward Howard, Semant Jain.
Application Number | 20110138856 12/774249 |
Document ID | / |
Family ID | 44141402 |
Filed Date | 2011-06-16 |
United States Patent
Application |
20110138856 |
Kind Code |
A1 |
Howard; Henry Edward ; et
al. |
June 16, 2011 |
SEPARATION METHOD AND APPARATUS
Abstract
A method and apparatus for producing an oxygen product in which
air is separated in an installation including one or more air
separation units having higher and lower pressure columns. An
exhaust stream produced from a turboexpander and optionally an
impure oxygen stream such as that derivable from higher pressure
column bottoms is rectified within an auxiliary column to produce
an oxygen containing stream that is introduced into the lower
pressure column of each of the air separation units to increase the
capacity of such columns. The pressure within the auxiliary column
is set by the pressure of the exhaust stream such that a
nitrogen-rich vapor stream extracted from the top of the auxiliary
column can be used in regenerating adsorbent within a
pre-purification unit utilized in connection with the
installation.
Inventors: |
Howard; Henry Edward; (Grand
Island, NY) ; Jain; Semant; (Buffalo, NY) |
Family ID: |
44141402 |
Appl. No.: |
12/774249 |
Filed: |
May 5, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12634810 |
Dec 10, 2009 |
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12774249 |
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Current U.S.
Class: |
62/645 |
Current CPC
Class: |
F25J 3/04084 20130101;
F25J 2205/30 20130101; F25J 2245/42 20130101; F25J 2205/64
20130101; F25J 2230/42 20130101; F25J 2235/50 20130101; F25J
3/04969 20130101; F25J 3/04872 20130101; F25J 3/0409 20130101; F25J
3/04824 20130101; F25J 3/04963 20130101; F25J 3/04448 20130101;
F25J 3/04951 20130101; F25J 3/04206 20130101; F25J 3/04181
20130101; F25J 3/04236 20130101; F25J 3/0429 20130101 |
Class at
Publication: |
62/645 |
International
Class: |
F25J 3/02 20060101
F25J003/02 |
Claims
1. A method of separating air, said method comprising: separating a
compressed and purified air stream in a cryogenic rectification
process employing a pre-purification unit to purify the air of
higher boiling impurities and at least one air separation unit
having a higher pressure column and a lower pressure column
configured to produce oxygen and nitrogen-rich fractions;
generating refrigeration within the cryogenic rectification process
by further compressing and partially cooling part of the compressed
and purified air stream and work expanding the part of the
compressed and purified air stream, after having been further
compressed, within a turboexpander to produce an exhaust stream;
introducing the exhaust stream into a bottom region of an auxiliary
column and rectifying the exhaust stream within the auxiliary
column to form an oxygen containing liquid as a column bottoms and
an auxiliary column nitrogen-rich vapor column overhead;
withdrawing at least one oxygen containing stream from the
auxiliary column having a lower nitrogen content than that of the
exhaust stream and introducing the at least one oxygen containing
stream into the at least one air separation unit for rectification
within the lower pressure column; withdrawing and warming an
auxiliary column nitrogen-rich stream, composed of the auxiliary
column nitrogen-rich vapor column overhead and introducing at least
a portion of the auxiliary column nitrogen-rich vapor stream into
the pre-purification unit so as to regenerate adsorbent within the
pre-purification unit with the auxiliary column nitrogen-rich vapor
stream; and the work expanding of the part of the compressed and
purified air stream within the turboexpander being conducted such
that the exhaust stream pressure of the exhaust stream sets the
pressure within the auxiliary column at a level that the auxiliary
column nitrogen-rich vapor stream is able to be introduced into the
pre-purification unit without further compression.
2. The method of claim 1, wherein the auxiliary column
nitrogen-rich vapor stream is at a higher pressure than that of at
least one lower pressure nitrogen-rich vapor stream composed of
lower pressure column nitrogen-rich vapor column overhead produced
in the lower pressure column of the at least one air separation
unit.
3. The method of claim 1, wherein: the cryogenic rectification
process generates at least one impure oxygen stream containing
oxygen and nitrogen and having an oxygen content no less than that
of the air; the at least one impure oxygen stream, along with the
exhaust stream, is introduced into a bottom region of an auxiliary
column and rectified along with the exhaust stream within the
auxiliary column to form the oxygen containing liquid as a column
bottoms and the auxiliary column nitrogen-rich vapor column
overhead; and the at least one oxygen containing stream withdrawn
from the auxiliary column has a lower nitrogen content of that of
the at least one impure oxygen stream and the exhaust stream and is
introduced into the lower pressure column of the at least one air
separation unit.
4. The method of claim 3, wherein the at least one impure oxygen
stream is composed of a crude liquid oxygen column bottoms produced
in the higher pressure column of the at least one air separation
unit.
5. The method of claim 3 or claim 4, wherein: at least part of the
at least one oxygen-rich liquid stream of high purity and composed
of an oxygen-rich liquid column bottoms produced in the lower
pressure column of the at least one air separation unit is pumped
to form a pumped liquid oxygen stream; another part of the
compressed and purified air stream is further compressed to form a
compressed air stream; the compressed air stream indirectly
exchanges heat with at least part of the pumped liquid oxygen
stream, thereby forming a liquid air stream from the compressed air
stream and an oxygen product from the at least part of the pumped
liquid oxygen stream; and intermediate reflux streams composed of
the liquid stream are introduced into the lower pressure column of
the at least one air separation unit above locations at which the
at least one oxygen containing stream is introduced into the low
pressure column and also, into the auxiliary column above the
bottom region thereof.
6. The method of claim 5, wherein: a higher pressure nitrogen-rich
column overhead produced in the higher pressure column of the at
least one air separation unit is condensed into a nitrogen-rich
liquid against vaporizing part of the oxygen-rich liquid column
bottoms; reflux liquid streams composed of the nitrogen-rich liquid
are introduced as reflux into the higher pressure column and the
lower pressure column of the at least one air separation unit and
into the auxiliary column; the nitrogen-rich liquid that is used in
forming the reflux liquid streams that are fed as the reflux to the
lower pressure column of the at least one air separation unit and
the auxiliary column is subcooled through indirect heat exchange
with the at least one lower pressure nitrogen-rich vapor stream and
the nitrogen-rich auxiliary column vapor stream; and at least one
lower pressure nitrogen-rich vapor stream composed of nitrogen-rich
vapor column overhead produced in the lower pressure column and the
auxiliary column nitrogen-rich vapor stream are fully warmed in the
main heat exchanger used in cooling the air to a temperature
suitable for its rectification within the at least one air
separation unit.
7. The method of claim 6, wherein the intermediate reflux streams
are also introduced into the higher pressure column of the at least
one air separation unit.
8. An apparatus for separating air comprising: a cryogenic
rectification installation configured to separate the air and
including a main compressor and a pre-purification unit in flow
communication with the main compressor to produce a compressed and
purified air stream, a main heat exchanger configured to cool the
compressed and purified air stream to a temperature suitable for
its rectification, at least one air separation unit connected to
the main heat exchanger and having a higher pressure column and a
lower pressure column configured to produce oxygen and
nitrogen-rich fractions, a refrigeration generation system and an
auxiliary column; the refrigeration generation system comprising a
booster compressor in flow communication with the main compressor
to further compress part of the compressed and purified air stream,
the booster compressor connected to the main heat exchanger and the
main heat exchanger configured to partially cool the part of the
compressed and purified air stream after having been further
compressed in the booster compressor and a turboexpander connected
to the main heat exchanger to work expand the part of the
compressed and purified air stream, after having been further
compressed and partially cooled and thereby produce an exhaust
stream; the auxiliary column connected to the turboexpander so as
to receive the exhaust stream in a bottom region thereof and
configured to rectify the exhaust stream, thereby form an oxygen
containing liquid as a column bottoms and an auxiliary column
nitrogen-rich vapor column overhead; the at least one air
separation unit connected to the auxiliary column so that at least
one oxygen containing stream is withdrawn from the auxiliary column
having a lower nitrogen content of that of the exhaust stream and
is introduced into the at least one air separation unit; the
pre-purification unit and the auxiliary column are connected to the
main heat exchanger such that an auxiliary column nitrogen-rich
vapor stream, composed of the auxiliary column nitrogen-rich vapor
column overhead, after having been warmed in the main heat
exchanger is introduced into the pre-purification unit so as to
regenerate adsorbent within the pre-purification unit with the
auxiliary column nitrogen-rich stream; and the refrigeration system
configured such that exhaust stream pressure of the exhaust stream
sets pressure within the auxiliary column at a level that the
auxiliary column nitrogen-rich stream is able to be introduced into
the pre-purification unit without further compression.
9. The apparatus of claim 8, wherein the auxiliary column
nitrogen-rich vapor stream is at a higher pressure than that of at
least one lower pressure nitrogen-rich vapor stream composed of
lower pressure column nitrogen-rich vapor column overhead produced
in the lower pressure column of the at least one air separation
unit.
10. The apparatus of claim 8, wherein: the auxiliary column is
connected to the at least one air separation unit so as to receive
at least one impure oxygen stream, together with the exhaust stream
in the bottom region thereof, the at least one impure oxygen stream
containing oxygen and nitrogen and having an oxygen content that is
no less than that of the air; the auxiliary column configured to
rectify the at least one impure oxygen stream along with the
exhaust stream, thereby forming the oxygen containing liquid as a
column bottoms and the auxiliary column nitrogen-rich vapor column
overhead; and the lower pressure column of the at least one air
separation unit is connected to the auxiliary column so that the at
least one oxygen containing stream is withdrawn from the auxiliary
column having a lower nitrogen content of that of the impure oxygen
stream and the exhaust stream and is introduced into the lower
pressure column for rectification within the lower pressure column
of the at least one air separation unit.
11. The apparatus of claim 10, wherein the auxiliary column is
connected to the higher pressure column of the at least one air
separation unit such that the at least one impure oxygen stream is
composed of a crude liquid oxygen column bottoms produced in the
higher pressure column of the at least one air separation unit.
12. The apparatus of claim 10 or claim 11, wherein: a pump is
connected to the lower pressure column of the at least one air
separation unit so that at least part of at least one oxygen-rich
stream of high purity and composed of an oxygen-rich liquid column
bottoms produced in the lower pressure column of the at least one
air separation unit, is pumped to form a pumped liquid stream; the
main heat exchanger is connected to the pump so that the at least
part of the pumped liquid stream is introduced into the main heat
exchanger and warmed to form an oxygen product; a further booster
compressor compresses another part of the compressed and purified
air stream, thereby to form a compressed air stream, the further
booster compressor is connected to the main heat exchanger such
that the pressurized liquid stream warms within the main heat
exchanger through indirect heat exchange with the compressed air
stream and the compressed air stream is thereby liquefied to form a
liquid air stream; and the lower pressure column of the at least
one air separation unit and the auxiliary column are connected to
the main heat exchanger such that intermediate reflux streams
composed of the liquid air stream are introduced into the lower
pressure column of the at least one air separation unit and the
auxiliary column above locations at which the at least one oxygen
containing stream is introduced into the lower pressure column and
above the bottom region of the auxiliary column.
13. The apparatus of claim 12, wherein: a heat exchanger is
connected to the higher pressure column and the lower pressure
column of the at least one air separation unit so that a higher
pressure nitrogen-rich column overhead produced in the higher
pressure column is condensed into a nitrogen-rich liquid against
vaporizing part of the oxygen-rich liquid column bottoms; the
higher pressure column and the lower pressure column of the at
least one air separation unit and the auxiliary column are
connected to the heat exchanger so that reflux liquid streams
composed of the nitrogen-rich liquid are introduced as reflux into
the higher pressure column and the lower pressure column of the at
least one air separation unit and into the auxiliary column; and a
subcooling unit is positioned between the lower pressure column of
the at least one air separation unit and the main heat exchanger so
that the nitrogen-rich liquid that is used in forming the reflux
liquid streams, that are fed as the reflux to the lower pressure
column of the at least one air separation unit and the auxiliary
column, is subcooled through indirect heat exchange with a lower
pressure nitrogen-rich vapor stream, composed of a nitrogen-rich
vapor column overhead produced in the lower pressure column and the
auxiliary column nitrogen-rich vapor stream.
14. The apparatus of claim 13, wherein the higher pressure column
of the at least one air separation unit is connected to the main
heat exchanger so that the intermediate reflux streams are also
introduced into the higher pressure column of the at least one air
separation unit.
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 12/634,810, filed Dec. 10, 2009.
FIELD OF THE INVENTION
[0002] The present invention relates to a method and apparatus for
separating air into oxygen and nitrogen-rich fractions. More
particularly, the present invention relates to such a method and
apparatus in which one or more air separation units having higher
and lower pressure columns are connected to an auxiliary column
that produces one or more oxygen containing streams that are lean
in nitrogen and that are introduced into lower pressure columns to
allow the air separation units to operate at a higher capacity and
a nitrogen-rich vapor stream that can be used in regenerating
adsorbent beds of a pre-purification unit used in purifying the air
without further compression.
BACKGROUND OF THE INVENTION
[0003] Large quantities of high purity oxygen are required for
purposes of coal gasification where the resulting synthesis gas
will be used in the production of chemicals. High purity oxygen is
typically defined by an oxygen content of 99.5 percent or better.
In certain large coal gasification installations, upwards of
between 10,000 and 15,000 metric tons per day of oxygen can be
consumed.
[0004] The cryogenic rectification of air is the preferred method
for large scale oxygen production. In cryogenic rectification, air
is compressed and purified of higher boiling contaminants such as
carbon dioxide, water vapor and hydrocarbons in a pre-purification
unit. Purification is necessary in that carbon dioxide and water
vapor will freeze within the main heat exchanger and hydrocarbons
will tend to collect in the oxygen to present an operational
hazard. Typically, the pre-purification step is conducted in a
pre-purification unit that has an adsorbent, for example, alumina
or a zeolite or silica gel or a combination of such materials to
adsorb the water vapor, the carbon dioxide and the hydrocarbons.
The adsorbent is employed in adsorbent beds that are operated in
accordance with an out-of-phase cycle in which one bed is placed
on-line and is actively adsorbing the impurities while another bed
is being regenerated, that is, being subjected to desorption to
desorb the impurities followed by repressurization to bring the
regenerated bed back on-line to allow the current on-line bed to be
regenerated. For such purposes, there can be two or more adsorbent
beds employed within a pre-purification unit.
[0005] The pre-purification unit is operated in an out-of-phase
cycle that is commonly either a temperature swing adsorption cycle
or a pressure swing adsorption cycle (or combinations thereof). In
a temperature swing adsorption cycle, compressed air that has been
cooled to ambient is introduced to an on-line bed to produce a
compressed and purified air stream. The off-line bed that is being
regenerated is subjected to a depressurization step and then, a
countercurrent heating step in which nitrogen waste gas is heated
in a heater and then introduced into the bed countercurrently to
the air flow. Since adsorbents have a lower adsorptive capacity at
higher temperature, the impurities will tend to desorb from the
adsorbent. The nitrogen waste gas is then introduced into the
adsorbent bed without being heated. This countercurrent purge with
the waste nitrogen desorbs and removes the contaminants previously
adsorbed by the adsorbent. A repressurization step is then
conducted with part of the product being produced by the on-line
bed to ready the off-line bed, now regenerated, to be brought back
to an on-line status. In a pressure swing adsorption cycle, the
off-line bed is regenerated by allowing the adsorbent bed to vent
to the atmosphere from its feed end. The adsorbent bed is then
countercurrently purged with waste nitrogen. Thereafter, the
adsorbent bed is then repressurized with product from the on-line
bed before being brought back to an on-line status. In both types
of cycles waste nitrogen from the air separation plant is used in
that it has a very low concentration of the impurities.
Furthermore, it is at a low pressure given the fact that the
impurities will tend to be adsorbed more readily at a higher
pressure. In a pre-purification unit that operates by pressure
swing adsorption, a greater flow rate of the waste nitrogen is
required than in a temperature swing adsorption cycle.
[0006] The compressed and purified air, which in certain plants can
be further compressed, is cooled to a temperature suitable for its
rectification and then rectified in distillation columns to
separate the components of the air. The distillation columns that
are employed in cryogenic rectification processes include a higher
pressure column and a lower pressure column. In the higher pressure
column, the air is rectified to produce a nitrogen-rich vapor
column overhead and a crude liquid oxygen column bottoms also known
in the art as kettle liquid. A stream of the crude liquid oxygen
column bottoms is further refined in the lower pressure column to
produce the oxygen product.
[0007] Distillation column diameters increase in proportion to the
square root of plant capacity or in other words the flow through
the columns. Shipping limitations result in a maximum vessel
diameter in the range of 6.0 to 6.5 m. As a consequence, the
design, construction and installation of an air separation plant
having an oxygen production capacity in excess of about 5000 metric
tons per day has not been found to be practical. In order to
overcome this limitation, typically multiple, parallel air
separation plant trains are constructed to operate in parallel
within an enclave. Unfortunately simple plant replication forfeits
many "economies of scale" in that the construction of additional
column shells carries with it considerable expense. Thus, even when
multiple air separation units having higher and lower pressure
columns are employed within an enclave of such units, it is
desirable that each such unit be constructed with the largest
capacity possible to limit the number of units employed within a
particular installation of air separation plants.
[0008] A critical limitation associated with a distillation column
involves the hydraulic flood point of any given column section.
Column diameters are typically defined by an approach to flood that
can be anywhere from 70 to 90 percent. Given equivalent pressure,
nitrogen has a lower mass density than oxygen. As the lighter (more
volatile) component of air, nitrogen flows to the top of the
associated (nitrogen/oxygen) rectification sections. As the column
vapor ascends it is progressively enriched in nitrogen. Conversely,
the descending liquid becomes richer in oxygen. As a consequence of
these thermodynamic aspects, the upper sections of the major low
pressure air distillation columns, known as the nitrogen
rectification sections, exhibit the highest volumetric loadings.
Given a fixed maximum diameter and packing selection, such sections
will limit capacity of each plant.
[0009] Another consideration of any air separation plant is that
the pressure of the nitrogen leaving the cold box sets the pressure
at the top of the lower pressure column. The pressure at the bottom
of the lower pressure column is set by adding to the pressure at
the top of the lower pressure column, the pressure drop through the
lower pressure column stages. The pressure at the top of the higher
pressure column is set by the temperature approach across the
condenser that is employed in condensing the nitrogen-rich vapor
column overhead of the higher pressure column against vaporizing
part of the oxygen-rich liquid column bottoms of the lower pressure
column. The pressure at the bottom of the higher pressure column is
a sum of the pressure at the top of the higher pressure column and
the pressure drop through the higher pressure column stages. After
accounting for the pressure drop through the main heat exchanger,
this pressure determines the main air compressor discharge
pressure. An increase of 1 psi in the pressure at the top of the
lower pressure column translates into an approximate 2.3 psi
increase in pressure at the top of the higher pressure column.
Thus, if pressure at the top of the lower pressure column can be
reduced, then substantial energy savings can be realized in
compressing the air since the required pressure will be lower.
[0010] In order to solve the foregoing problems, U.S. Pat. No.
6,227,005 discloses an air separation process in which compressed
and purified air is introduced into a distillation column unit
having a higher pressure column and a lower pressure column. A
pressurized oxygen product is produced by pumping an oxygen-rich
stream composed of an oxygen-rich liquid column bottoms of the
lower pressure column and then heating the resulting pumped liquid
in the main heat exchanger through indirect heat exchanger with
part of the compressed and purified air that has been further
compressed in a booster compressor. The crude liquid oxygen column
bottoms of the higher pressure column is rectified in an auxiliary
column to produce an oxygen-rich liquid that is introduced into the
lower pressure column for further refinement. Liquid air produced
as a result of the heat exchange between the compressed and
purified air and the pumped liquid stream is used as intermediate
reflux to the lower pressure column and the auxiliary column. Since
the oxygen-rich liquid produced in the auxiliary column is leaner
in nitrogen than the crude liquid oxygen column bottoms, there is
less nitrogen in the lower pressure column and therefore, the
capacity of the lower pressure column is increased. Furthermore,
the auxiliary column can be operated at a higher pressure than the
lower pressure column to produce a nitrogen-rich vapor at a higher
pressure than that produced in the lower pressure column. The lower
pressure column can be operated at a lower pressure to realize an
energy savings in initially compressing the air in the main
compressor.
[0011] There are, however, inherent operational limitations in the
air separation process illustrated in the above patent that stem
from the use of part of the nitrogen-rich vapor produced in the
higher pressure column to reboil the auxiliary column. As a result,
there is less boilup available in the lower pressure column to
strip argon from the liquid and consequently, the air separation
process is incapable of efficiently producing a high purity oxygen
product. Moreover, another result of using such nitrogen-rich vapor
is that the nitrogen containing overhead of the auxiliary column
will invariably be at a pressure of near 40 psig. The problem with
such a pressure is that it is not compatible with use as
regeneration gas for the adsorbent beds contained in the
pre-purification unit because, as mentioned above, the adsorbent
will have a higher capacity for the impurity at a higher pressure.
Typically, the nitrogen utilized for such purposes must be
generated at a pressure of between 5 and 10 psig so that after
accounting for heat exchanger and piping pressure drops, the
nitrogen can be delivered to regenerate the adsorbent beds at a
pressure of about 3 psig. While, the 40 psig nitrogen generated by
the air separation process described in this patent could be
reduced in pressure for such purposes, the pressure reduction would
be in effect an irreversible loss of the energy involved in
compressing the air in the first instance.
[0012] As will be discussed, the present invention provides a
method and apparatus for separating air that among other advantages
allows high purity oxygen to be produced while lower pressure
column capacity is increased and nitrogen-rich vapor can be
directly used in regenerating adsorbent contained in a
pre-purification unit.
SUMMARY OF THE INVENTION
[0013] In one aspect, the present invention provides a method of
separating air in which a compressed and purified air stream is
separated in a cryogenic rectification process. The process employs
a pre-purification unit to purify the air of higher boiling
impurities and at least one air separation unit having a higher
pressure column and a lower pressure column configured to produce
oxygen and nitrogen-rich fractions. Refrigeration is generated
within the cryogenic rectification process by further compressing
and partially cooling part of the compressed and purified air
stream and work expanding the part of the part of the compressed
and purified air stream, after having been further compressed,
within a turboexpander to produce an exhaust stream. The exhaust
stream is introduced into a bottom region of an auxiliary column
and rectified within the auxiliary column to form an oxygen
containing liquid as a column bottoms and an auxiliary column
nitrogen-rich vapor column overhead.
[0014] At least one oxygen containing stream is withdrawn from the
auxiliary column having a lower nitrogen content than that of the
exhaust stream and such stream is introduced into the at least one
air separation unit for rectification within the lower pressure
column. An auxiliary column nitrogen-rich vapor stream, composed of
the auxiliary column nitrogen-rich vapor column overhead is
withdrawn from the auxiliary column, warmed and thereafter, at
least a portion thereof is introduced into the pre-purification
unit so as to regenerate adsorbent within the pre-purification
unit. The work expanding of the part of the compressed and purified
air stream within the turboexpander is conducted such that exhaust
stream pressure of the exhaust stream sets the pressure within the
auxiliary column at a level that the auxiliary column nitrogen-rich
vapor stream is able to be introduced into the pre-purification
unit without further compression. The auxiliary column
nitrogen-rich vapor stream can be at a higher pressure than that of
at least one lower pressure nitrogen-rich stream composed of lower
pressure column nitrogen-rich vapor column overhead produced in the
lower pressure column of the at least one air separation unit.
[0015] The oxygen containing stream produced in the auxiliary
column will have a lower nitrogen content than the exhaust stream
that will have the effect of reducing the amount of nitrogen vapor
within the lower pressure column to in turn increase the capacity
of the lower pressure column and therefore, the entire cryogenic
rectification process. Additionally, since the exhaust stream is
being introduced into a bottom region of the auxiliary column, the
auxiliary column is not being reboiled with nitrogen-rich vapor
produced in the higher pressure column. This allows the lower
pressure column to produce the oxygen at a higher purity than in
the prior art discussed above if such high purity product is
desired. Furthermore, the auxiliary column, as in the prior art,
allows the lower pressure column to be operated at a lower pressure
to save energy costs in the main compression of the incoming air to
be separated. A major advantage of the method of the present
invention is that additionally, since the exhaust stream pressure
will set the pressure of the auxiliary column, the pressure of the
auxiliary column nitrogen-rich vapor stream can also be set to a
level that will allow such stream to be introduced into the
pre-purification unit without further compression. This saves on
the electrical power that would otherwise be consumed in such
compression and therefore, the ongoing operational costs of running
the process and plant incorporating the process as well as a
compressor that is normally used for such purposes. In this regard,
the ability to set the pressure of the lower pressure column also
allows the pressure of the auxiliary column nitrogen-rich vapor
stream to be set at a level that when taken in connection with
losses of piping and heat exchangers and possibly control valves
leading to the pre-purification unit will be optimal and not so
high as to require expansion valves or the like for pressure let
down purposes.
[0016] The cryogenic rectification process generates at least one
impure oxygen stream containing oxygen and nitrogen and having an
oxygen content no less than that of the air. This at least one
impure oxygen stream, along with the exhaust stream, can be
introduced into a bottom region of an auxiliary column and
rectified along with the exhaust stream within the auxiliary column
to form the oxygen containing liquid as a column bottoms and the
auxiliary column nitrogen-rich vapor column overhead. The at least
one oxygen containing stream that is withdrawn from the auxiliary
column has a lower nitrogen content of that of the at least one
impure oxygen stream and the exhaust stream and is introduced into
the lower pressure column of the at least one air separation unit.
The at least one impure oxygen stream can be composed of a crude
liquid oxygen column bottoms produced in the higher pressure
column. The use of the impure oxygen stream will act to reduce
nitrogen within the lower pressure column to a level below that of
the exhaust stream alone and will also allow for more auxiliary
column nitrogen-rich vapor to be generated. If the exhaust stream
were used alone, then the pre-purification unit would operate on
the basis of a temperature swing adsorption cycle.
[0017] At least part of at least one oxygen-rich liquid stream of
high purity and composed of an oxygen-rich liquid column bottoms
produced in the lower pressure column of the at least one air
separation unit can be pumped to form a pumped liquid oxygen
stream. In this regard, the term, "high purity" as used herein and
in the claims means a purity of at and above 99.5 percent oxygen by
volume. Another part of the compressed and purified air stream can
be further compressed to form a compressed air stream and the
compressed air stream indirectly exchanges heat with at least part
of the pumped liquid oxygen stream, thereby forming a liquid air
stream from the compressed air stream and an oxygen product from
the at least part of the pumped liquid oxygen stream. Intermediate
reflux streams composed of the liquid air stream are introduced
into the lower pressure column of the at least one air separation
unit above the location at which the at least one oxygen containing
stream is introduced into the lower pressure column and into the
auxiliary column above the bottom region thereof.
[0018] A higher pressure nitrogen-rich column overhead produced in
the higher pressure column of the at least one air separation unit
can be condensed into a nitrogen-rich liquid against vaporizing
part of the oxygen-rich liquid column bottoms. Reflux liquid
streams composed of the nitrogen-rich liquid are introduced as
reflux into the higher pressure column and the lower pressure
column of the at least one air separation unit and into the
auxiliary column. The nitrogen-rich liquid that is used in forming
the reflux liquid streams that are fed as the reflux to the lower
pressure column and the auxiliary column is subcooled through
indirect heat exchange with a lower pressure nitrogen-rich vapor
stream, composed of a nitrogen-rich vapor column overhead produced
in the lower pressure column of the at least one air separation
unit and the nitrogen-rich auxiliary column vapor stream. The at
least one lower pressure nitrogen-rich vapor stream and the
auxiliary column nitrogen-rich vapor stream are fully warmed in a
main heat exchanger used in cooling the air to a temperature
suitable for its rectification within the air separation unit. The
intermediate reflux streams can also be introduced into the higher
pressure column.
[0019] In another aspect, the present invention provides an
apparatus for separating air. In accordance with this aspect of the
present invention, a cryogenic rectification installation is
provided that is configured to separate the air. The installation
includes a main compressor and a pre-purification unit in flow
communication with the main compressor to produce a compressed and
purified air stream. A main heat exchanger is configured to cool
the compressed and purified air stream to a temperature suitable
for its rectification and at least one air separation unit
connected to the main heat exchanger and having a higher pressure
column and a lower pressure column configured to produce oxygen and
nitrogen-rich fractions.
[0020] A refrigeration generation system and an auxiliary column
are also provided. The refrigeration generation system comprises a
booster compressor in flow communication with the main compressor
to further compress part of the compressed and purified air stream.
The booster compressor is connected to the main heat exchanger and
the main heat exchanger is configured to partially cool the part of
the compressed and purified air stream after having been further
compressed in the booster compressor. A turboexpander is connected
to the main heat exchanger to work expand the part of the
compressed and purified air stream, after having been further
compressed and partially cooled and thereby produce an exhaust
stream. The auxiliary column is connected to the turboexpander so
as to receive the exhaust stream in a bottom region thereof and is
configured to rectify the exhaust stream, thereby to form an oxygen
containing liquid as a column bottoms and an auxiliary column
nitrogen-rich vapor column overhead. The at least one air
separation unit is connected to the auxiliary column so that at
least one oxygen containing stream is withdrawn from the auxiliary
column having a lower nitrogen content of that of the exhaust
stream and is introduced into the at least one air separation
unit.
[0021] The pre-purification unit and an auxiliary column are
connected to the main heat exchanger such that an auxiliary column
nitrogen-rich vapor stream, composed of the auxiliary column
nitrogen-rich vapor column overhead, after having been warmed in
the main heat exchanger, is introduced into the pre-purification
unit so as to regenerate adsorbent within the pre-purification unit
with the auxiliary column nitrogen-rich stream. The refrigeration
system is configured such that exhaust stream pressure of the
exhaust stream sets pressure within the auxiliary column at a level
that the auxiliary column nitrogen-rich vapor stream is able to be
introduced into the pre-purification unit without further
compression. The auxiliary column nitrogen-rich vapor stream can be
at a higher pressure than that of a lower pressure nitrogen-rich
stream composed of lower pressure column nitrogen-rich vapor column
overhead produced in the lower pressure column of the at least one
air separation unit.
[0022] The auxiliary column can be connected to the at least one
air separation unit so as to receive at least one impure oxygen
stream, together with the exhaust stream, in the bottom region
thereof. The at least one impure oxygen stream contains oxygen and
nitrogen and has an oxygen content that is no less than that of the
air. The auxiliary column is configured to rectify the at least one
impure oxygen stream along with the exhaust stream, thereby forming
the oxygen containing liquid as a column bottoms and the auxiliary
column nitrogen-rich vapor column overhead. The lower pressure
column of the at least one air separation unit is connected to the
auxiliary column so that the at least one oxygen containing stream
is withdrawn from the auxiliary column has a lower nitrogen content
of that of the at least one impure oxygen stream and the exhaust
stream and is introduced into the lower pressure column of the at
least one air separation unit for rectification within the lower
pressure column. The auxiliary column can be connected to the
higher pressure column of the at least one air separation unit such
that the at least one impure oxygen stream is composed of a crude
liquid oxygen column bottoms produced in the higher pressure column
of the at least one air separation unit.
[0023] A pump can be connected to the lower pressure column so that
at least part of at least one oxygen-rich stream of high purity and
composed of an oxygen-rich liquid column bottoms produced in the
lower pressure column of the at least one air separation unit, is
pumped to form a pumped liquid stream. The main heat exchanger is
connected to the pump so that the at least part of the pumped
liquid stream is introduced into the main heat exchanger and warmed
to form an oxygen product. A further booster compressor compresses
another part of the compressed and purified air stream, thereby to
form a compressed air stream. The further booster compressor is
connected to the main heat exchanger such that the pressurized
liquid stream warms within the main heat exchanger through indirect
heat exchange with the compressed air stream and the compressed air
stream is thereby liquefied to form a liquid air stream. The lower
pressure column of the at least one air separation unit and the
auxiliary column are connected to the main heat exchanger such that
intermediate reflux streams composed of the liquid air stream air
introduced into the lower pressure column of the at least one air
separation unit and the auxiliary column above locations at which
the at least one oxygen containing stream is introduced into the
lower pressure column of the air at least one air separation unit
and above the bottom region of the auxiliary column,
respectively.
[0024] A heat exchanger can be connected to the higher pressure
column and the lower pressure column of the at least one air
separation unit so that a higher pressure nitrogen-rich column
overhead produced in the higher pressure column is condensed into a
nitrogen-rich liquid against vaporizing part of the oxygen-rich
liquid column bottoms. The higher pressure column and the lower
pressure column of the at least one air separation unit and the
auxiliary column are connected to the heat exchanger so that reflux
liquid streams composed of the nitrogen-rich liquid are introduced
as reflux into the higher pressure column and the lower pressure
column of the at least one air separation unit and the auxiliary
column. A subcooling unit is positioned between the lower pressure
column of the at least one air separation unit and the main heat
exchanger so that the nitrogen-rich liquid that is used in forming
the reflux liquid streams, that are fed as the reflux to the lower
pressure column of the at least one air separation unit and the
auxiliary column, is subcooled through indirect heat exchange with
the lower pressure nitrogen-rich vapor stream and the auxiliary
column nitrogen-rich vapor stream.
[0025] The higher pressure column of the at least one air
separation unit can be connected to the main heat exchanger so that
of the intermediate reflux streams is also introduced into the
higher pressure column of the at least one air separation unit.
BRIEF DESCRIPTION OF THE DRAWING
[0026] While the specification concludes with claims distinctly
pointing out the subject matter that Applicants regard as their
invention, it is believed that the invention will be understood
when taken in connection with the accompanying sole FIGURE that
illustrates an apparatus for carrying out a method in accordance
with the present invention.
DETAILED DESCRIPTION
[0027] With reference to the FIGURE, a cryogenic rectification
installation 1 is illustrated that is designed to separate air and
thereby to produce an oxygen product. Cryogenic rectification
installation 1 is provided with a main heat exchanger 2 to cool the
air to a temperature suitable for its rectification within air
separation units 3 and 4 and thereby produce an oxygen product that
is discharged from the main heat exchanger 2 as an oxygen product
stream 96 and a nitrogen product stream 150, to be discussed in
more detail hereinafter. It is understood that air separation unit
4 as well as other possible air separation units that would be
operatively associated with an auxiliary column 100, also to be
discussed in more detail hereinafter, are optional in that the
present invention could be practiced with a single air separation
unit 3.
[0028] The air to be separated is introduced into apparatus 1 as an
air stream 10 that is compressed in a main compressor 12 to produce
a main compressed air stream 14 having a pressure in a range of
from between about 5 and about 15 bar(a). Main compressor 12 can be
a multi-stage intercooled integral gear compressor with condensate
removal. Main compressed air stream 14 is subsequently purified in
a pre-purification unit 16 to remove higher boiling impurities such
as water vapor, carbon dioxide and hydrocarbons from the air and
thereby produce a compressed and purified air stream 18. As well
known in the art and as discussed above, such unit 16 can
incorporate adsorbent beds operating in an out of phase cycle that
is a combination of temperature and pressure swing adsorption or
that is purely a temperature swing adsorption cycle or a pressure
swing adsorption cycle also described in detail above.
[0029] A part 20 of the compressed and purified air stream 18 is
cooled to a temperature suitable for its rectification within main
heat exchanger 2 as a main air feed and then is divided into
subsidiary main air feed streams 21 and 22 that are fed into the
bottom regions of higher pressure columns 44 and 46 of air
separation units 3 and 4 for rectification. Another part 23 of the
compressed and purified air stream is subsequently compressed in a
booster compressor 24, again preferably a multi-stage unit, to form
a compressed air stream 25 that can have a pressure in a range of
between about 25 and about 70 bar. Compressed air stream 25 can
constitute roughly between about 25 percent and about 35 percent of
the incoming air. As will be discussed, compressed air stream 25 is
liquefied within a main heat exchanger 2 against vaporizing a
second part 94 of a pumped liquid oxygen stream 88 to produce the
oxygen product stream 96 and a liquid air stream 26 in a subcooled
state. Another part 28 of the compressed and purified air stream 18
is compressed in a turbine loaded booster compressor 30 to a
pressure that can be in a range of between about 7 bar(a) and 10
bar(a) and then compressed in a compressor 32 to produce another
compressed air stream 34 that can have a pressure of between about
10 bar(a) and 15 bar (a). The compressed air stream 34 is partially
cooled within the main heat exchanger 2 to a temperature that is in
a range of between about 160 K and about 220 K and then expanded
within a turboexpander 36 to produce an exhaust stream 38 to supply
refrigeration to the air separation installation 1. Exhaust stream
38 is fed into a bottom region of an auxiliary column 100 that will
be discussed in greater detail hereinafter. Exhaust stream 38 would
comprise between about 5 to 10 percent by volume of the incoming
air. It is to be noted that the term, "partially cooled" as used
herein and in the claims means cooled to a temperature between the
warm and cold end temperatures of streams leaving and entering the
main heat exchanger 2.
[0030] It is to be noted that although main heat exchanger 2 is
illustrated as a single unit, in practice, main heat exchanger 2
could be a series of parallel units incorporating known aluminum
plate-fin construction. Moreover, the high pressure portion of main
heat exchanger 2 could be "banked", that is, fabricated so that the
portion used in exchanging heat between the first compressed stream
25 and the second part 94 of the pumped liquid oxygen stream 88
were in a separate high pressure heat exchanger. Thus, the term
"main heat exchanger" as used herein and in the claims can be taken
to mean a single unit or multiple units as described above.
Moreover, although booster compressor 30 is illustrated as being
mechanically connected to turboexpander 36 and compressor 32 is
provided to further compress the compressed and purified air,
single, separately driven booster compressors could be used in
place of the illustrated units. Furthermore the subcooling unit 124
to be discussed hereinafter could be incorporated into the main
heat exchanger in a manner known in the art.
[0031] Each of the higher pressure columns 44 and 46 are provided
with mass transfer contacting elements 48 and 50 such as structure
packing, dumped packing or sieve trays or a combination of such
elements as well known in the art. The introduction of primary feed
air streams 40 and 42 initiates formation of an ascending vapor
phase that becomes ever richer in nitrogen as it ascends higher
pressure columns 44 and 46, respectively. The ascending vapor is in
countercurrent contact with a descending liquid phase that becomes
ever richer in oxygen as it descends columns 44 and 46. As a
result, a crude liquid oxygen column bottoms 52 is formed in each
of the higher pressure columns 44 and 46, within bottom regions
thereof, and a higher pressure nitrogen-rich vapor at the top of
the higher pressure columns 44 and 46.
[0032] Lower pressure columns 54 and 56 of air separation units 3
and 4, respectively, operating at a lower pressure than higher
pressure columns 44 and 46, are each provided with heat exchangers
in the form of condenser reboilers 58 in the base of each of the
lower pressure columns 54 and 56. Streams 60 and 62 composed of the
higher pressure nitrogen-rich vapor column overhead of the higher
pressure columns 44 and 46, respectively, are condensed within
condenser reboilers 58 to produce nitrogen-rich liquid streams 64
and 66 and to partly vaporize an oxygen-rich liquid column bottoms
68 produced in each of the lower pressure columns 54 and 56. Such
vaporization initiates the formation of an ascending vapor phase
within lower pressure columns 54 and 56. The descending liquid
phase within lower pressure columns 54 and 56 is initiated through
introduction of reflux streams 70 and 72 that are composed of the
nitrogen-rich liquid streams 64 and 66. Mass transfer contacting
elements 74, 76 and 78 are located within each of the lower
pressure columns 54 and 56 to contact the descending liquid with
the ascending vapor and thereby to produce the oxygen-rich liquid
68 and a low pressure nitrogen-rich vapor column overhead in top
regions of the lower pressure columns 54 and 56.
[0033] Oxygen-rich streams 80 and 82 that are composed of the
oxygen-rich liquid column bottoms 68 and can be of high purity are
removed from lower pressure columns 54 and 56 and combined to form
a combined stream 84 that is pumped by a pump 86 to produce a
pumped liquid oxygen stream 88 that can have a pressure from
between about 10 bar(a) and about 50 bar(a). A first part of the
pumped liquid oxygen stream 88 can optionally be directly taken as
liquid product stream 92 and a second part 94 of the pumped liquid
oxygen stream 88 can, as described above, be warmed within the main
heat exchanger to produce the oxygen product as a product stream
96.
[0034] Within each of the lower pressure columns 54 and 56 as the
liquid phase descends, it becomes ever richer in oxygen, the
nitrogen being stripped out by the ascending vapor phase. The
section of the column where such action predominantly occurs is
within mass transfer contacting element 74. The sections of the
lower pressure columns occupied by mass transfer contacting
elements 76 and 78 are nitrogen rectification sections which serve
to enrich the ascending vapor in nitrogen content. In many
instances it is the uppermost sections that serve to constrain
plant capacity. In order to overcome this limitation, a
nitrogen-oxygen mixture which has been enriched in oxygen is
introduced into each lower pressure column 54 and 56 that is
generated in an auxiliary column 100 in lieu of crude liquid oxygen
or kettle liquid generated in the bottom region of each of the
higher pressure columns 44 and 46.
[0035] In cryogenic rectification installation 1, impure oxygen
streams, that in the illustrated embodiment constitute crude liquid
oxygen streams 102 and 104, are removed from higher pressure
columns 44 and 46, respectively. These streams are composed of the
crude liquid oxygen 52. The crude liquid oxygen streams 102 and 104
are then valve expanded to a pressure substantially at the
operating pressure of the auxiliary column 100 by expansion valves
106 and 108 and then introduced into a bottom region 101 of the
auxiliary column 100 with the exhaust stream 38 for rectification
to produce an oxygen containing liquid column bottoms 110 and an
auxiliary column nitrogen-rich vapor column overhead at the top of
auxiliary column 100. Pumps could be used to pump crude liquid
oxygen streams 102 and 104 if their pressure were not sufficient to
overcome gravitational head to the auxiliary column 100. Auxiliary
column 100 is refluxed by a reflux stream 112 that is made up of
the nitrogen-rich liquid streams 64 and 66 discussed above. In this
regard, nitrogen-rich liquid stream 64 and 66 are divided into
subsidiary streams 114, 116 and 118, 120, respectively. Subsidiary
streams 114 and 118 reflux the higher pressure columns 44 and 46,
respectively. Subsidiary streams 118 and 120 are combined to form a
combined stream 122 that is subcooled in a subcooling unit 124 and
then divided into reflux streams 70, 72 and 112. Reflux streams 70,
72 and 112 are valve expanded to an operational pressure of the
lower pressure columns 54 and 56 and the auxiliary column 100 by
expansion valves, 126, 128 and 130, respectively.
[0036] Auxiliary column 100 is provided with mass transfer
contacting elements 132 and 134 to contact ascending vapor and
descending liquid phases and thereby produce the oxygen containing
liquid column bottoms 110 and the auxiliary column nitrogen-rich
vapor column overhead. The exhaust stream 38 and flash-off vapor
produced by the introduction of crude liquid oxygen streams 102 and
104 into auxiliary column 100 as well as introduction of
intermediate reflux stream 158 (to be discussed) form the ascending
phase to be rectified. The descending liquid phase is produced by
reflux stream 112 and the intermediate reflux stream 158. As a
result of the distillation, the oxygen containing liquid column
bottoms 110 is leaner in nitrogen than the crude liquid oxygen
column bottoms 52 produced in the higher pressure columns 44 and
46. Oxygen containing streams 136 and 138 that are composed of the
oxygen containing liquid column bottoms 110 are removed from the
auxiliary column 100 and then introduced into the base of the
nitrogen rectification sections of the lower pressure columns 54
and 56 to reduce the nitrogen content within such sections of the
columns and to allow for a higher production rate without such
columns flooding. In this regard, such oxygen containing streams
136 and 138 might have a vapor content upon their introduction into
lower pressure columns 54 and 56. It is to be noted here that the
amount of total air fed to the auxiliary column can be adjusted in
accordance with plant refrigeration demands. For instance if there
is a need for increased liquid production (stream 92) a greater
flow of air may be directed through turbine 36 and hence into the
auxiliary column 100.
[0037] Nitrogen-rich vapor streams 140, 142 and 144, composed of
the nitrogen-rich vapor column overhead produced in such columns,
are removed from the lower pressure columns 54 and 56 and the
auxiliary column 100, respectively. Nitrogen-rich vapor streams 140
and 142 are combined to form a combined nitrogen-rich vapor stream
146. Combined nitrogen-rich vapor stream 146 is then partly warmed
within subcooling unit 148 to subcool combined nitrogen liquid
stream 122 and then is fully warmed within main heat exchanger 2 to
form a nitrogen product stream 150. Similarly, nitrogen-rich vapor
stream 144 is partly warmed within subcooling unit 124 to assist in
subcooling the combined nitrogen liquid stream 122 and then is
fully warmed within main heat exchanger 2 to form a warm waste
nitrogen stream 151.
[0038] It is to be noted that if nitrogen product stream 150 were
desired at high purity, an addition rectification section could be
placed at the top of the lower pressure columns 54 and 56. Another
possibility is to partition lower pressure columns 54 and 56 or
provide an annular divider to divide the lower pressure columns 54
and 56 to allow for the production of both high and low purity
nitrogen. Since, the warm waste nitrogen stream 151 is being used
to regenerate the adsorbent bed and high purity nitrogen is not
desired, then it is possible to reflux the auxiliary column 100
with a lower purity liquid nitrogen stream extracted from the
higher pressure columns 44 and 46.
[0039] The introduction of the oxygen containing streams 136 and
138 effectively unload the nitrogen rectification section of the
lower pressure columns 54 and 56. The upper rectification sections
of the low pressure columns still require sufficient reflux to
maintain high oxygen recovery. In order to achieve this condition,
the liquid air stream 26 is expanded to an operational pressure of
the higher pressure columns 44 and 46 by means of an expansion
valve 152 and then divided and subdivided into intermediate reflux
streams 154, 156 and 158 and optionally, intermediate reflux
streams 160 and 162. Intermediate reflux streams 154, 156 and 158
are valve expanded to lower the pressure of such streams by
expansion valves 164, 168 and 170 and then introduced as
intermediate reflux into lower pressure columns 54 and 56 above
locations at which the oxygen containing streams 136 and 138 are
introduced and auxiliary column 100, above the bottom region
thereof at which the impure oxygen streams are introduced. Optional
intermediate reflux streams 160 and 162 are introduced into the
higher pressure columns 44 and 46.
[0040] Warm waste nitrogen stream 151 is introduced into a process
heater 180 which may employ any number of means for heating
including and not limited to the use of electrical-resistive, gas
fired, steam or other heat exchanger fluid. The heating may take
place intermittently and is selectively activated. As described
above, to heat warm waste nitrogen stream 151 in connection with
the regeneration of adsorbent within adsorbent beds of
pre-purification unit 16 if such unit is operated in a temperature
swing adsorption cycle or a combination of a temperature swing
adsorption cycle and a pressure swing adsorption cycle. A portion
182 of stream 151 can be directed elsewhere as product (which may
be further compressed prior to sendout) the remaining fraction of
stream 151 being directed to pre-purification unit 16.
[0041] If pre-purification unit 16 is operated in accordance with
purely a pressure swing cycle, electrical heater 180 would not be
used. In such a situation a supplemental compressor 184 may be used
to compress a portion 186 of nitrogen gas 150 prior to combination
as a compressed nitrogen stream 188 with 151 and entry into
pre-purification unit 16.
[0042] As also mentioned above, the pressure of warm waste nitrogen
stream 151 is important in order to be effective in its role in
purging the adsorbent beds. The pressure of nitrogen-rich vapor
stream 144 is set so that no further compression thereof is
required and after piping pressure losses and losses within
subcooling unit 124 and main heat exchanger 2, warm waste nitrogen
stream 151 will typically be about 3 psig upon entry into
pre-purification unit 16. The pressure of nitrogen-rich vapor
stream 144 is in turn set by setting the pressure level within
auxiliary column 100 by in turn setting the pressure of exhaust
stream 38. There exists some latitude in the setting of the
pressure of exhaust stream 38 and this is accomplished by designing
the system resistance (heat exchanger pressure drop) to yield the
desired turbine exhaust pressure. From a process control standpoint
the pressure typically will be set by a back-pressure control valve
located at some point along the process path of stream 144. Once
this pressure is set, the pressure of crude liquid oxygen streams
102 and 104 will be set after expansion in valves 106 and 108 to a
pressure compatible with the pressure of exhaust stream 38. The
pressure of intermediate reflux stream 158 after expansion within
valve 170 will be set to be a pressure compatible with the level of
auxiliary column within which intermediate reflux stream 158 is
introduced. It is to be further noted, that lower pressure columns
54 and 56 preferably operate at a lower pressure than the auxiliary
column 100 such that nitrogen-rich vapor streams 140 and 142 have a
pressure of for example 3 to 5 psig. This will save on the amount
of energy being expanded in compressing the air in the main air
compressor 12.
[0043] As mentioned above, the present invention can be practiced
with a single air separation unit such as air separation unit 3. In
such case, there would be no other air separation units being fed
with subsidiary main feed air streams such as subsidiary main feed
air stream 22 and intermediate reflux streams such as intermediate
reflux streams 156 and 158. Furthermore, auxiliary column 100 would
not be fed with crude liquid oxygen stream 104. Nitrogen-rich
product stream 150 would be formed by nitrogen-rich vapor stream
140 and oxygen product stream 96 would be originated from the crude
liquid oxygen column bottoms 68 solely within lower pressure column
54. Having said this, the present invention has a greater
applicability than pumped liquid oxygen plants and could be used in
connection with any air separation unit designed to produce oxygen
and nitrogen-rich fractions. As such, all of the streams associated
with the production of liquid oxygen might not be present in a
possible embodiment of the present invention along with the
associated intermediate reflux streams. In this regard, it is
possible that the auxiliary column 100 in any embodiment be solely
fed with the exhaust stream 38. In such case, the resulting oxygen
containing stream 136 and 138 could be fed to the bottom regions of
the higher pressure columns 44 and 46 and then fed along with the
crude oxygen streams 102 and 104 directly into intermediate
locations of the lower pressure columns 54 and 56 with the effect
of reducing nitrogen within such columns. This, however, would
require a pump. A greater effect in increasing column capacity
would be to also feed the crude liquid oxygen streams 102 and 104
into the auxiliary column. In fact, this would be necessary to make
sufficient nitrogen-rich vapor for nitrogen-rich vapor stream 144
if it were eventually to be used in connection with a
pre-purification unit 16 operating in a pressure swing adsorption
mode of operation. The use of a pressure swing adsorption mode
would, however, also require the use of compressor 184 as has been
described above. However, the use of the exhaust stream 38 as the
sole feed to the auxiliary column 100 in a single air separation
unit embodiment is possible. However, in any embodiment of the
present invention considering that the turbine air flow only
constitutes about 5 to 10 percent of the air, the pre-purification
unit 16 would be constrained to be designed to operate in a
temperature swing adsorption cycle.
[0044] At the other extreme, although the auxiliary column 100 is
illustrated in connection with two air separation units 3 and 4, in
practice, an auxiliary column such as auxiliary column 100 should
be able to debottleneck 3 or 4 main air separation units, although
it is possible more air separation units would be used.
Additionally, although air separation units 3 and 4 are identical,
air separation units of different design and capability could be
used. For example, one air separation unit, as illustrated, could
be a conventional double column and the second unit may incorporate
argon recovery. The air separation units could also be of different
types. In this regard, the qualifying aspect of an air separation
unit is the utilization of a low pressure nitrogen rectification
section and most known oxygen production processes will have such a
section. As an example, the present invention is applicable to low
purity oxygen plants that employ air condensation within the base
of the lower pressure column. Such air condensation processes may
involve a total or a partial air condensation.
[0045] Although not illustrated, the present invention contemplates
that the auxiliary column 100 operates in a manner that is
independent of the associated air separation units. In particular,
not all of the air separation units need be in operation at any
time. If for instance, air separation unit 3 is out of service, the
auxiliary column could still function in connection with air
separation unit 4. Although the FIGURE depicts a common main heat
exchanger 2 and a subcooling unit 124 associated with the operation
of the air separation units 3 and 4, along with associated main air
compressor 12, turboexpander 36 and etc., it is possible to design
the cryogenic distillation installation in which each air
separation unit has dedicated components such as main heat
exchangers and subcooling units or partially dedicated and partial
common units. For example multiple pumps or a single pump 86 could
be used in the embodiment of the present invention shown in the
FIGURE. It is to be noted here that although the liquid air stream
26 is illustrated as being condensed against a second part 94 of
pumped liquid oxygen stream 88, it is possible to employ the
present invention in connection with pumped liquid nitrogen.
[0046] A combination of feed sources may be employed for an
auxiliary column system in accordance with the present invention.
In addition to impure oxygen liquid streams withdrawn from the
higher pressure columns 44 and 46, for example, crude liquid oxygen
streams 102 and 104, interstage fluids may be extracted from either
the higher or lower pressure columns associated with the air
separation units 3 and 4. All that is required for the impure
oxygen streams is that they contain an oxygen content that is no
less than that of air. For example, the impure oxygen streams could
be formed from part of the liquid air stream that is produced in
vaporizing a second part 94 of the pumped liquid oxygen stream 88.
In either case, by diverting such stream to the auxiliary column,
nitrogen would also be diverted to lower the nitrogen content in
the lower pressure columns 54 and 56. Also, such interstage fluids
could constitute a liquid air-like substance withdrawn from the
columns at the point of introduction of intermediate reflux
streams, for example, 160 and 162. Such liquid, known in the art as
synthetic liquid air, could likewise be used to divert nitrogen
from the lower pressure columns 54 and 56. As far as the
derivation, the same holds true for the intermediate reflux streams
that in the illustrated embodiment are designated by reference
numbers 154, 156, 160 and 162. These streams could be composed of
air or other air-like substance such as synthetic liquid air that
would have an argon content no less than that of air given that
such synthetic air, if withdrawn at the point of introduction of
streams 160 and 162, would in fact have an argon content greater
than air.
[0047] In the case where argon is produced from at least one of the
column systems, it is possible to route a portion of the vaporized
impure oxygen into the auxiliary column.
[0048] It should be noted that the feed source to the auxiliary
column 100 may be derived from only a single air separation unit,
for example air separation unit 3 or air separation unit 4 and then
be divided amongst the associated air separation units.
[0049] Although the present invention has been described with
reference to a preferred embodiment, as will occur to those skilled
in the art, numerous changes, additions and omissions can be made
to such embodiment without departing from the spirit and scope of
the present invention as set forth in the appended claims.
* * * * *