Note: Descriptions are shown in the official language in which they were submitted.
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Apparatus and method for separating air by cryogenic distillation
The present invention relates to an apparatus and to a process for the
separation
of air by cryogenic distillation.
In particular, it relates to an air separation apparatus comprising a double
column
with a first column operating at a first pressure and a second column
operating at a
second pressure, lower than the first pressure. The top of the first column
produces a
gas which condenses in a reboiler of the second column.
It is generally an objective of air separation apparatuses to look for the
lowest
possible energy consumption.
The purification of air is generally carried out at a pressure equal to or
greater
than that of the first pressure. This makes it possible to reduce the volume
of the
purification unit.
It is nevertheless known, from US 4 964 901, to purify a part of the air at
the first
pressure and the remainder of the air at the second pressure, using two
purification
units in parallel. The air purified at the second pressure is sent directly to
the second
column, while the air purified at the first pressure is separated in two, one
part being
sent directly to the first column and the remainder being boosted, cooled in a
heat
exchanger, expanded in a turbine coupled to the booster and sent to the second
column. Thus the turbine used is a blower turbine and the low pressure column
receives air which has been purified at two different pressures.
The process of US 5 934 105 purifies the air at a pressure above the second
pressure but below the first pressure; subsequently, the air intended for the
first column
is compressed and the air intended for the second column is expanded.
JPH11063810 and EP 1 050 730 are similar to US 5 934 105.
If all the flow which goes the second column is expanded in the turbine, as in
the
prior art, in order to maximize the energy gain, the air flow going to the
first column is
approximately 66% of the total purified flow, for example in order to produce
96%
oxygen. This means that it is necessary to pass 34% of the air flow at a
relatively low
pressure through the turbine.
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According to the present invention, between 6% and 8% of the air is expanded
in
an air turbine; thus the turbine according to the prior art is at least 4 to 5
times larger
due to the volume flow rate.
As the refrigerating capacity of the process according to the prior art is
fixed and
remains low since the process does not produce a liquid final product, this
means that
the expansion ratio of the turbine is very low which gives a turbine which is
inefficient
and in any case not at all standardized, indeed even nonexistent, among
suppliers of
cryogenic turbines.
In the case where it is desired to impose the air flow sent to the first
column in
order to maximize the energy gain, according to the prior art, in operation,
the
regulation of the refrigerating capacity cannot be done by a reduction in the
turbine
flow and thus will be done by adjusting the pressure upstream of the turbine,
that is to
say the purification pressure and ultimately the pressure of the blower. This
enormously complicates the regulation and makes it necessary to proportion the
purification to the lowest pressure which might be had with a refrigerating
capacity
lower than nominally expected or in a transient phase. According to the
invention,
provision is made for the purification pressure to be very close to the second
pressure.
The invention provides a process which consumes 1% less energy (2% less if a
turbine efficiency reduced by 5% pt is considered) compared to the prior art
(for
example, according to EP 1 050 730); according to the process of EP 1 050 730,
the
purification is carried out at a pressure between the first and the second
pressure.
The expansion ratio of the process of EP 1 050 730 is low, between 1.2:1 and
3.8:1, preferably between 1.4:1 and 2.5:1, while conventional cryogenic
turbines are in
an expansion ratio range of between 4:1 and 10:1. The invention uses an
expansion
ratio which remains at the low limit of this range, thus avoiding having a
significantly
degraded turbine efficiency.
In EP 1 050 730, the inlet pressure of the purification unit is typically 2.5
bara
(instead of approximately 1.3 bara according to the invention). This process
uses a
first compressor having several, typically two, stages with cooling between
two stages.
According to the invention, the compressor which compresses all the air has a
single
stage and thus no cooling between two stages.
The apparatus produces a gas flow enriched in oxygen with a particularly low
energy.
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US 5 666 824 describes a process according to the preamble of claim 1 but in
which the first flow is at least partially condensed in an intermediate
condenser of the
second column. While a gas is formed, it is itself condensed in another
intermediate
condenser of the second column and the liquid thus formed is sent to the top
of the
second column. Thus the first flow is not sent directly to the distillation.
W02013/014252 describes, in Figure 6, a process in which a first part of the
air
is cooled to its dew point in a heat exchanger where a flow of air expanded in
a turbine
is also cooled to its dew point. This is impossible since the waste nitrogen
which cools
the air flows has already been reheated in a subcooler. In this case, the
nitrogen is too
hot to cool the air flows to their dew point and the air flows will be cooled
at the very
most to a temperature approximately 10 C above the dew point.
Moreover, on calculating the refrigeration balance of Figure 6, it is found
that, by
using a compressor upstream of the turbine and by cooling to ambient
temperature
before expansion, a compression pressure of greater than 80 bar is required.
In this
case, the expansion ratio of the turbine is much higher than the values used
in industry.
Thus, it is not possible for a person skilled in the art to implement the
method of Figure
6 as described.
According to a subject matter of the invention, an air separation apparatus is
provided comprising a double column with a first column operating at a first
pressure
and a second column operating at a second pressure, lower than the first
pressure,
the second column having a bottom reboiler, means for sending nitrogen-
enriched gas
from the top of the first column to the bottom reboiler and means for sending
at least a
part of the condensed nitrogen-enriched gas from the bottom reboiler to the
top of the
first column, a heat exchanger, a purification unit, means for sending air to
the
purification unit at a third pressure greater than atmospheric pressure by at
most 1 bar,
a pipe for sending a first flow of air purified in the purification unit to
the heat exchanger
at a fourth pressure greater than the second pressure by at most 1 bar, a pipe
for
introducing the first flow of purified air cooled in the heat exchanger into
the second
column in order to be separated therein, a booster, a pipe for sending a
second flow
of air purified in the purification unit to the booster, a pipe for sending at
least a part of
the second flow, compressed by the booster up to a fifth pressure between the
first
pressure and 1 bar above the first pressure, to the heat exchanger, means for
producing refrigeration, a pipe for withdrawing at least one fluid enriched in
oxygen or
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nitrogen from a column of the double column connected to the heat exchanger
and a
pipe for exiting at least one fluid enriched in oxygen or nitrogen from the
heat
exchanger as product, the apparatus not comprising any means of expansion of
the
first flow and comprising only a single purification unit, characterized in
that the second
column does not comprise an intermediate condenser, the pipe for introducing
the first
flow of purified air being connected to the inside of the second column in
order to make
it possible for the first flow to participate in the distillation.
According to other optional aspects:
= the means for the production of refrigeration comprise at least one
turbine for
expansion of a part of the second flow and/or one turbine for expansion of a
nitrogen-
rich gas originating from the first column and/or means for sending a
cryogenic liquid
from an external source to the double column.
= the turbine for expansion of the part of the second flow is connected to
the
second column in order to send the expanded air there.
= the means for sending air to the purification unit at the third pressure
does
not comprise any compression means other than a single-stage compressor.
= the apparatus does not comprise any means for compression of the first
flow.
According to another aspect of the invention, there is provided a process for
the
separation of air by cryogenic distillation using a double column with a first
column
operating at a first pressure and a second column operating at a second
pressure,
lower than the first pressure, the second column having a bottom reboiler, in
which:
i) air containing water and carbon dioxide is sent to a single purification
unit at
a third pressure greater than atmospheric pressure by at most 1 bar,
ii) the purified air is separated into two,
ii) a first flow of air purified in the purification unit is sent to a heat
exchanger at
a fourth pressure greater than the second pressure by at most 1 bar,
iv) the first flow of purified air cooled in the heat exchanger is sent to the
second
column, without having expanded it,
v) a second flow of purified air is boosted to a fifth pressure between the
first
pressure and 1 bar above the first pressure, at least a part of the second
flow is sent
at the fifth pressure to the heat exchanger and the at least a part of the
second flow is
sent to the first column in gaseous form,
vi) refrigeration is provided in order to keep the process cold,
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vii) a nitrogen-rich gas from the first column is at least partially condensed
in the
reboiler and at least a part of the condensed nitrogen is returned to the
first column,
viii) a nitrogen-enriched liquid and an oxygen-enriched liquid are sent from
the
first column to the second column,
ix) an oxygen-enriched gas or a nitrogen-enriched gas is withdrawn from the
double column and it is reheated in the heat exchanger in order to form a
product of
the process, characterized in that the first air flow is sent directly into
the second
column in order to be separated therein without having been condensed in a
condenser.
According to other optional aspects:
= the entire first flow is sent to the second column.
= the first flow is sent to the second column at a level lower than or
equal to the
level of arrival of the oxygen-enriched liquid.
= the process does not produce any liquid product as final product and/or
no
liquid flow is withdrawn from the double column to serve as final product.
= the process is kept cold by expansion of a part of the second flow in a
turbine
from the fifth pressure to the second pressure.
= the part of the air expanded in the turbine represents between 6 vol% and
15
vol%, preferably between 6% and 8%, of the purified air.
= all the air is purified at a pressure which does not exceed 1.5 bara,
indeed
even does not exceed 1.3 bara.
= all the second flow is cooled in the heat exchanger down to an
intermediate
temperature of the heat exchanger, the inlet of the turbine is at the
intermediate
temperature of the heat exchanger and the part of the second flow sent to the
first
column is cooled in the heat exchanger down to the cold end of the latter.
= the first pressure does not exceed 6 bara.
= the second pressure does not exceed 1.5 bara.
= the oxygen-enriched gas contains at least 80 mol% oxygen.
= the oxygen-enriched gas contains at least 90 mol% oxygen.
= the oxygen-enriched gas contains less than 98 mol% oxygen.
= the first flow represents between 20 vol% and 30 vol% of the purified air
flow.
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= the second flow represents between 70 vol% and 80 vol% of the purified
air
flow.
= an oxygen-enriched gas and/or a nitrogen-enriched gas is (are) withdrawn
from the double column and it is (they are) reheated in the heat exchanger in
order to
form a product of the process by introducing it or by introducing them at the
cold end
of the heat exchanger.
= the first air flow and/or the part of the second flow intended for the
first column
is (are) cooled in the heat exchanger down to a temperature at least 5 C above
its
(their) dew point.
= an oxygen-enriched liquid is withdrawn and reheated in the heat exchanger
in order to form a product of the process.
= the oxygen-enriched liquid is pressurized before vaporizing it either in
a
dedicated vaporizer or in the heat exchanger.
= the oxygen-enriched liquid is vaporized by heat exchange with a part of
the
second flow or with a third flow of air pressurized to a pressure greater than
the fifth
pressure.
= the first air flow is subcooled between the heat exchanger and the second
column.
= the part of the air expanded in the turbine is subcooled between the
outlet of
the turbine and the second column.
The invention will be described in a more detailed manner with reference to
the
figure.
Figure 1 represents a process for the separation of air by cryogenic
distillation
according to the invention.
An apparatus for the separation of air by cryogenic distillation comprises a
double
column with a first column K3 operating at a first pressure and a second
column K4
operating at a second pressure, lower than the first pressure, the second
column
having a bottom reboiler M. The second column K4 does not contain an
intermediate
condenser.
In this example, the first pressure is 4.5 bara and the second pressure is
1.13
bara.
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A nitrogen-enriched gas is sent from the top of the first column to the bottom
reboiler M and at least a part of the condensed nitrogen-enriched gas from the
bottom
reboiler is sent to the top of the first column.
Air at atmospheric pressure is filtered in a filter A, compressed by a blower
B
having a single stage at a pressure at most 1 bar, preferably at most 0.5 bar,
above
atmospheric pressure, cooled by a cooling means C and purified of water and
carbon
dioxide in a single purification unit D in which the air 4 enters at a third
pressure greater
than atmospheric pressure by at most 1 bar, preferably by at most 0.5 bar. The
purification unit comprises two adsorbent beds used alternately to purify the
air, one
bed purifying the air while the other is regenerated.
The air purified in the unit D is divided into two in order to form two flows
6,8. The
air 8 is neither compressed nor expanded and is at a pressure which differs
from the
second pressure by a pressure equal to the pressure drops in the pipes and the
heat
exchanger G.
Preferably, the first flow 8 represents between 20 vol% and 30 vol% of the
flow 4
and the second flow 6 represents between 70 vol% and 80 vol% of the flow 4.
Thus, the air 8 is sent directly from the purification unit to the second
column K2
to be separated therein, entering the column in entirely gaseous form. The air
8 is
cooled in the heat exchanger G down to a temperature at least 5 C above its
dew
point.
The flow 6 is boosted in a booster E, cooled in a cooler F and sent to the
heat
exchanger G. The booster E boosts the air 6 up to a fifth pressure between the
first
pressure and 1 bar above the first pressure. The air 6 is divided into two
parts 30,32
at an intermediate level of the exchanger. The air 30 leaves the exchanger at
an
intermediate temperature of the latter, for example -125 C, is expanded in a
turbine 28
down to the second pressure and enters in gaseous form, mixed with the flow 8,
to be
separated in the second column K4.
The flow 30 can represent between 6 vol% and 15 vol%, preferably between 6%
and 8%, of the air 4.
The air 32 is cooled down to the cold end of the exchanger G and is sent to
the
bottom of the first column K3 in essentially gaseous form in order to be
separated
therein. The air 8 is cooled in the heat exchanger G down to a temperature at
least
C above its dew point.
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An oxygen-enriched liquid flow 34 is withdrawn at the bottom of the first
column
and sent to a level of the second column which is above the air inlet.
Alternatively, the
air can enter the second column at the same level as that of the arrival of
the liquid 34.
The expanded liquid 34 can be separated in a phase separator: the liquid
resulting from the phase separator is sent to the column K4 and the vapor
phase can
be mixed at the inlet of air 8,30 into the column K4.
A flow of liquid nitrogen 35 is withdrawn from the top of the first column and
sent
to the top of the second column.
Gaseous nitrogen 36 is withdrawn at the top of the second column K4 and is
heated in the subcooler S and subsequently in the exchanger G. A part 14 of
this gas
is used to regenerate the purification unit D.
Gaseous oxygen 29 is withdrawn at the bottom of the second column K4. The
flow 29 preferably contains at least 80 mol% oxygen, indeed even at least 90
mol%
oxygen, but preferably less than 98 mol% oxygen.
It will be noticed that the process does not produce any liquid flow as final
product.
The process does not produce any liquid flow to be vaporized in order to form
a final
gaseous product, optionally under pressure. It is, however, possible to
produce a small
amount of final gaseous product in this way, which can optionally be mixed
with the
main gaseous product.
Furthermore, a small flow of liquid might be produced.
In an alternative form, the air 8 and/or the air 30 can be subcooled in the
subcooler S and then be introduced into the second column K4. Otherwise, the
mixture
of the flows 8 and 30 can be subcooled in the subcooler S and then be
introduced into
the second column K4.
In the example described, the flow 29 is a flow of gaseous oxygen which is
heated
in the heat exchanger G from the cold end of the exchanger G. Alternatively,
the flow
29 can be a flow of oxygen-rich liquid pressurized to a pressure above that of
the
second column K4. The liquid 29 is vaporized either in a dedicated vaporizer
(not
illustrated) or in the heat exchanger G. The liquid 29 can be vaporized by
heat
exchange with all the air 32 in order to partially condense the air 32, which
will
subsequently be sent to the bottom of the first column K3. Otherwise the
liquid 29 can
be vaporized by heat exchange with a part of the air 32 in order to completely
condense
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this part of the air 32. The condensed air will subsequently be sent to the
bottom of the
first column K3 or to an intermediate point of the first and/or of the second
column.
Otherwise, a part of the purified air can be boosted in a booster to a
pressure
greater than that of the first column K3 in order to vaporize the liquid 29.
Date Recue/Date Received 2021-06-10
