Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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COMPRESSION METHOD AND AIR SEPARATION
Related Applications
[0001] This application is a continuation-in-part of prior U.S. Application
Serial No. 13/087,734, filed on April 15, 2011, which is incorporated herein
by
reference.
Field of the Invention
[0002] The present invention relates to a method and system for compressing a
gas and an air separation method and plant incorporating such method and
system
in which a gas or air is compressed in a series of compression stages to a
higher
pressure that is maintained both at normal operating conditions and during
turndown operational conditions when a lower flow rate of the gas or air is
required. More particularly, the present invention relates to such a method
and
system and air separation method and plant in which the compression stages
have
variable speed motors driving compressors and the speed of each of the
compressors is adjusted during turndown such that an initial of the
compression
stages operates at a point along a peak efficiency operating line.
Background of the Invention
[0003] Many industrial processes require the compression of a gas. For
example,
in air separation, air is compressed, cooled to a temperature at or near the
dew
point and then introduced into a distillation column system to separate the
air by
cryogenic distillation into its component parts, for example, nitrogen, oxygen
and
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argon. Many applications involve the liquefaction of a gas in which the gas is
compressed and then sufficiently cooled to produce a liquid.
[0004] Although in any compression application, it is possible to compress the
gas in a single stage, it is more common to compress the gas in sequential
compression stages. The reason for this is as the gas is compressed, its
temperature rises and the elevated gas temperature requires an increase in
power
to compress the gas. Where the gas is compressed in stages, the gas may be
cooled between stages to lower overall power requirements, as the process gets
closer to isothermal compression than if no interstage cooling is done. In a
typical
compressor installation utilizing individual stages, each stage uses a
centrifugal
compressor in which gases entering an inlet to the compressor are distributed
to a
vaned compressor wheel that rotates to accelerate the gas and thereby impart
the
energy of rotation to the gas. This increase in energy is accompanied by an
increase in velocity and a pressure rise. The pressure is recovered in a
static vaned
or vaneless diffuser that surrounds the compressor wheel and functions to
decrease the velocity of the gas and thereby increase the gas pressure of the
compressed gas.
[0005] The individual compressors of the compression stages can be driven by a
common driver, such as an electric motor, through a transmission that consists
of
a single bull gear driving driven gears connected to the shafts of the
compressors.
The problem with such an arrangement is that an air separation plant does not
always have the same demand placed upon it to deliver products. For example,
in
an air separation plant designed to produce oxygen, the demand can vary with
the
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time of day and the day of the week. Since, the major cost in operating an air
separation plant is electrical power, it is desirable to be able to turn down
the plant
and operate it to produce less oxygen than the plant is normally capable of
producing during normal operating conditions. This can be done by reducing the
flow of air into the multistage compression system used in compressing the air
for
the plant. The air flow can be reduced by reducing the speed of the
compressors.
Thus, in a geared arrangement, if the speed of the motor driving the
compressors
is decreased, the rotational speeds of all of the compressors will be reduced
by the
same amount. The problem with this is that the output pressure of the
multistage
compression system will also be reduced. Now, in case the oxygen or other gas
is
required to be produced at a particular pressure for a customer, such a
reduction in
pressure will also reduce the pressure of the product. As such, it is
sometimes not
possible to operate an air separation plant at turn down operating conditions.
These same types of problems would occur in any type of operation involving
the
compression of a gas in stages.
[0006] Another problem with the common geared arrangement is that all of the
compressors must be situated around the gearing. Further, there are
inefficiencies
in such a geared arrangement of compressors that arise from thermal losses
from
the gearing or gearbox. In order to avoid problems with having to mount
compressors about a gear box or thermodynamic inefficiencies, the gearing or
gearbox can be eliminated and the compressors can be individual driven and
controlled with speed controllers. For example, US Patent Appin. No.
2007/0189905, discloses a multistage compression system that includes
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compressors connected in series with interstage cooling between the
compressors.
Each of the compressors is individually controlled by a speed controller.
However, the speed of the compressors is controlled in this patent such that
when
the speed is increased or decreased, the ratio of the speed of any two motors
remains the same. Consequently, there exist the same difficulties in such a
system
as would exist in a geared system used in an air separation plant. Although
such a
system can be turned down and operated in a turned down operating condition,
the
delivery pressure will be reduced.
[0007] As will be discussed, the present invention provides methods and
apparatus that are particularly applicable to air separation in which a
multistage
compression system can be run under turn down operating conditions while
continuing to deliver compressed gas at the pressure obtained during normal
operational conditions.
Summary of the Invention
[0008] The present invention provides a method of compressing a gas. In
accordance with such method, the gas is compressed in a series of compression
stages, from a lower pressure to a higher pressure. The compression stages are
operating at a normal operating condition during which the gas is supplied at
the
higher pressure and at a higher flow rate and at a turndown condition during
which the gas is supplied at the higher pressure and at a lower flow rate. In
this
regard, the term "flow rate" as used herein and in the claims means mass flow
rate.
The compression stages have compressors driven by variable speed motors
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capable of driving the compressors at speeds that are able to be independently
adjusted for each of the compressors, thereby to adjust flow rate through the
compressors and pressure ratios across the compressors. In this regard, the
term
"variable speed motors" as used herein and in the claims means any type of
device
that can impart rotational motion to a compressor and in which the speed can
be
varied. Examples of variable speed motors include variable speed electric
motors,
variable speed hydraulic motors, variable speed internal combustion engines
and
variable speed stream turbines. During the turndown operating condition, the
speeds of the variable speed motors are adjusted and therefore, the speeds of
the
compressors and the pressure ratios across the compressors such that an
initial of
the compressors associated with an initial of the compression stages operates
at a
point along a peak efficiency operating line thereof, where an initial
pressure ratio,
across the initial of the compressors, is directly proportional to the flow
rate and
below that of the normal operating condition thereby obtaining the lower flow
rate.
Successive compressors, located in successive compression stages, downstream
of
the initial of the compression stages, operate at the lower flow rate and at
the
pressure ratios that will enable the gas to be delivered at the higher
pressure.
[0009] When a multistage compression system is operated in accordance with the
present invention, as set forth above, the flow rate is reduced during the
turn down
operating conditions while allowing the gas to be delivered at the pressure
that is
obtained during the normal operating conditions. Thus, the method of the
present
invention allows the multistage compression system to be used in applications
where a stable pressure is required under all operational regimes. In addition
to
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the foregoing, the method of the present invention is particularly energy
efficient
when the multistage compression system is operated during turn down
conditions.
The reason for this is that the power consumed by a multistage compression
system have multiple stages is greatest at the initial compression stage
because the
volumetric flow rate is greatest in such stage due to the fact that the gas
has the
lowest density while being compressed in the initial stage. As the gas is
successively compressed, the density increases and consequently less power is
consumed in subsequent compression stage. However, since the initial
compression stage is operated and turned down to a point along its peak
efficiency
operating line, the power consumed by such stage at turndown is less than
would
otherwise have been consumed had it not be operating along a point of its peak
efficiency operating line. The successive stages will recover the reduction of
the
pressure in the first stage of compression due to turn down and as such, will
very
likely not operate along their peak efficiency operating line. However, since
the
successive stages will consume less power than the initial compression stage,
the
loss in efficiency will be more than compensated for through operation of the
initial stage at its peak efficiency. Consequently, the operation of a
compression
system in accordance with the present invention enables both a reduced power
consumption and the higher pressure level of the normal operating condition to
be
obtained that would not be possible had the compression stages been turned
down
at a constant speed ratio.
[0010] Although there are applications of multiple stage multistage
compression
systems in which the compressors operate adiabatically, in most applications
the
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gas will be cooled between the compression stages and after having been
compressed in the series of compression stages.
[0011] In applications of the present invention involving air separation,
interstage
and after cooling will invariably be used. In this regard, the present
invention
provides a method of separating air in which the air is compressed in a series
of
compression stages, with interstage cooling between the compression stages and
after cooling to cool the air after having been compressed in the series of
compression states. The air is compressed within the compression stages from a
lower pressure to a higher pressure. The compression stages are operated at a
normal operating condition and a turndown operating condition. During normal
operating conditions, the air is supplied to a main heat exchanger at the
higher
pressure and at a higher flow rate. During the turndown condition, the air is
supplied air to the main heat exchanger at the higher pressure and at a lower
flow
rate. The air is cooled, after having been compressed, within the main heat
exchanger and is then introduced into a distillation column system to produce
return and product streams. The return and product streams are warmed within
the main heat exchanger to cool the air. The compression stages have
compressors driven by variable speed motors capable of driving the compressors
at speeds that are able to be independently adjusted for each of the
compressors,
thereby to adjust flow rate through the compressors and pressure ratios across
the
compressors. During the turndown operating conditions, the speeds of the
variable speed motors and therefore, the speeds of the compressors and the
pressure ratios across the compressors are adjusted such that an initial of
the
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compressors associated with an initial of the compression stages operates at a
point along a peak efficiency operating line thereof where an initial pressure
ratio
across the initial of the compressors is directly proportional to the flow
rate and
below that of the normal operating condition thereby obtaining the lower flow
rate.
Successive compressors, located in successive compression stages, downstream
of
the initial of the compression stages, operate at the lower flow rate and at
the
pressure ratios that will enable the gas to be delivered at the higher
pressure.
[0012] In a method of the present invention, during both normal operating
conditions and turndown operating conditions, the gas can be supplied from a
final compressor of a final of the compression stages. In an alternative
embodiment of the present invention, during normal operating conditions, the
gas
can be supplied from a final compressor at the pressure and at the higher flow
rate
and with an auxiliary compressor by-passed. During the turndown operating
conditions, when the intermediate and final compressor stages must operate
beyond their efficient pressure ratio conditions or beyond their mechanically
acceptable speeds, the auxiliary compressor can be set in flow communication
with the final compressor and the gas is then supplied from the auxiliary
compressor at the pressure and at the lower flow rate. In case of an
application of
a method of the present invention to an air separation plant, the gas would of
course be air. Additionally, in any method of the present invention, the
variable
speed motors can be direct drive motors. Speed controllers are connected to
the
direct drive motors to control the speed of each of the direct drive motors.
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[0013] In another aspect, the present invention provides a multistage
compression
system for compressing a gas. In accordance with such aspect, a series of
compression stages are provided to compress the gas from a lower pressure to a
higher pressure in a final stage of the compression stages. The multistage
compression system is configured to operate in a normal operating condition
and
in a turndown operating condition. During the normal operating condition the
gas
is supplied at the higher pressure from the compression stages and at a higher
flow
rate. During the turndown operating condition and the gas is supplied at the
higher pressure from the compression stages and at a lower flow rate. The
compression stages have compressors driven by variable speed motors capable of
driving the compressors at speeds that are able to be independently adjusted
for
each of the compressors, thereby to adjust flow rate through the compressors
and
pressure ratios across the compressors and variable speed controllers
connected to
the compressors and configured to independently adjust the speed of the
compressors. A master controller is connected to the variable speed
controllers
and is configured such that during the turndown operating condition, the
speeds of
the variable speed motors and therefore, the speeds of the compressors and the
pressure ratios across the compressors are adjusted such that an initial of
the
compressors associated with the initial of the compression stages operates at
a
point along a peak efficiency operating line thereof where an initial pressure
ratio
across the initial of the compressors is directly proportional to the flow
rate and
below that of the normal operating condition thereby obtaining the lower flow
rate.
Successive compressors, located in successive compression stages, situated
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downstream of the initial of the compression stages, operate at the lower flow
rate
and at the pressure ratios that will enable the compression stages to deliver
the gas
at the higher pressure.
[0014] As indicated above, intercoolers can be positioned between the
compression stages. An aftercooler can be connected to the series of
compression
stages such that the gas is cooled after having been compressed in the series
of
compression stages.
[0015] The present invention also provides an air separation plant employing a
series of compression stages to compress the air from a lower pressure to a
higher
pressure in a final stage of the compression stages. Intercoolers are
positioned
between the series of the compression stages and an aftercooler connected to
the
final stage of the compression stages. The multistage compression system is
configured to operate in a normal operating condition and in a turndown
operating
condition. During the normal operating condition the air is supplied at the
higher
pressure from the compression stages and at a higher flow rate. During the
turndown operating condition the air is supplied at the higher pressure from
the
compression stages and at a lower flow rate. The compression stages have
compressors driven by variable speed motors capable of driving the compressors
at speeds that are able to be independently adjusted for each of the
compressors,
thereby to adjust flow rate through the compressors and pressure ratios across
the
compressors and variable speed controllers connected to the compressors and
configured to independently adjust the speed of the compressors. A main heat
exchanger is connected to the multistage compression system and is configured
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cool the air, after having been compressed. A distillation column system,
configured to produce return and product streams, is connected to the main
heat
exchanger such that the air, after having been cooled in the main heat
exchanger,
is introduced into the distillation column system and the return and product
streams warm within the main heat exchanger to cool the air. A master
controller
is connected to the variable speed controllers and configured such that during
the
turndown operating conditions, the speeds of the variable speed motors and
therefore, the speeds of the compressors are adjusted such that an initial of
the
compressors associated with the initial of the compression stages operates at
a
point along a peak efficiency operating line thereof where an initial pressure
ratio
of the initial of the compressors is directly proportional to the flow rate
and below
that of the normal operating condition thereby obtaining the lower flow rate..
Successive compressors, located in successive compression stages, situated
downstream of the initial of the compression stages, operate at the lower flow
rate
and at the pressure ratios that will enable the compression stages to deliver
the
gas at the higher pressure.
[0016] In a multistage compression system in accordance with the present
invention or an air separation plant employing such a multistage compression
system, the compression stages can be configured such that during both normal
operating conditions and turndown operating conditions the gas is supplied
from a
final compressor of a final of the compression stages. Alternatively, the
compression stages can be provided with a final compressor in a final
compression stage of the compression stages and an auxiliary compressor in an
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auxiliary compression stage of the compression stages. A flow control network
is
provided having a by-pass line, a first valve positioned between the by-pass
line
and an aftercooler connected to the final compressor. A second valve is
positioned between the aftercooler and the auxiliary compressor. Each of the
first
valve and the second valves are operable to be set in a closed position and an
open
position such that during normal operating conditions, the first valve is set
in the
open position and the second valve is set in the closed position and the gas
is
supplied from a final compressor at the pressure and at the higher flow rate
through the by-pass line and with the auxiliary compressor by-passed. During
turndown operating conditions, the first valve is set in the closed position
and the
second valve is set in the open position such that the auxiliary compressor is
connected to the aftercooler and the gas is supplied from the auxiliary
compressor
at the pressure and at the lower flow rate. In case of an application of a
multistage
compression system of the present invention to an air separation plant, the
gas
would of course be air. In any embodiment of the present invention, the
variable
speed motors can be direct drive motors.
Brief Description of the Drawings
[0017] While the present invention concludes with claims distinctly pointing
out
the subject matter in accordance with the present invention, it is believed
that the
invention will be better understood when taken in connection with the
accompanying drawings in which:
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[0018] Fig. 1 is a schematic diagram of a multistage compression system for
carrying out a method in accordance with the present invention;
[0019] Fig. 2 is a graphical representation of a map of compressor performance
for the multistage compression system shown in Fig. 1 during normal
operational
conditions.
[0020] Fig. 3 is a graphical representation of a map of compressor performance
for the multistage compression system shown in Fig. 1 during turndown
operational conditions.
[0021] Fig. 4 is an alternative embodiment of Fig. 1; and
[0022] Fig. 5 is a schematic diagram of an air separation plant incorporating
a
multistage compression system shown in Fig. 1 or Fig. 4. for carrying out a
method in accordance with the present invention.
Detailed Description
[0023] With reference to Figure 1, a multistage compression system 1 in
accordance with the present invention is illustrated. Multistage compression
system 1 is a multistage compression unit having four compression stages 10,
12,
14 and 16 that is designed to compress a gas contained in a feed stream 18
from a
lower pressure to a higher pressure and thereby produce a compressed gas
stream
20 containing the gas at the higher pressure. It is understood that a
multistage
compression system in accordance with the present invention could include a
greater or lesser number of stages.
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[0024] Compression stage 10 is provided with a compressor 22 that is driven by
a
variable speed motor 24. It is understood that the compressor 22 can be
centrifugal compressor of the type described above that is driven by a
variable
speed electric permanent magnet motor. In compression stage 10, the variable
speed motor 24 is controlled by a speed controller 26 that could be a variable
frequency controller where the drive. 24 is a variable speed electric
permanent
magnet motor. It is to be pointed out that the variable speed motor 24 or any
other
motor employed in connection with the present invention could be another type
of
device, for example, a throttle controlled steam turbine. The successive
compression stages 12, 14 and 16 are similarly provided with compressors 28,
30
and 32, variable speed motors 34, 36 and 38 and speed controllers 40, 42 and
44,
respectively.
[0025] Multistage compression system 1 also incorporates interstage cooling
and
after cooling. As known in the art, as the gas is compressed in each stage of
compression, the temperature of the gas rises. As a result, the density of the
gas
decreases and more power has to be expended in a stage to compress the gas. By
cooling the gas between stages, the density of the gas is greater than without
such
cooling resulting in a power savings. This being said, it is possible to have
a
multistage compressor installation without intercooling or after cooling where
it is
desired to produce a heated gas that can be used in a downstream process. In
the
illustrated embodiment, however, such intercooling is illustrated and for such
purposes, after the gas is compressed in compressor 22, it is cooled in
interstage
cooler 46 before being fed to the inlet of the downstream compressor 28.
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Similarly, after the gas has been further compressed in compressor 28, it is
cooled
in interstage cooler 48 before being fed into the inlet of compressor 30 and
after
the gas has been compressed in compressor 30, the gas is cooled in interstage
cooler 50. After the gas has been compressed in compressor 32, it is cooled in
aftercooler 52 to remove the heat of compression. Interstage coolers 46, 48
and
50 are known in the art and can consist of liquid cooled, fin and tube heat
exchangers. Aftercooler 52 is preferably also incorporates liquid cooled, fin
and
tube heat exchanger construction or liquid cooled, direct contact heat
exchange.
[0026] Multistage compressor system 1 is designed to produce compressed gas
stream 20 at a specific pressure and flow rate. When multistage compressor
system 1 is operating in this manner, it is operating under a normal operating
condition. When it is desired to reduce the flow of compressed gas stream 20,
multistage compressor system 1 will still deliver the compressed gas stream at
the
same specific pressure that is required under the normal operating condition,
but
at the reduced flow. Under such circumstances, multistage compressor system 1
is said to be operating under a turndown operating condition. The control is
exercised by a programmable logic controller 54 that sends appropriate control
signals through a control network 56 that can be a set of data transmission
lines to
variable speed controllers 26, 40, 42 and 44 that act to control the speed of
motors
24, 34, 36 and 38 and therefore, the speed of the compressors 22, 28, 30 and
32.
[0027] With reference to Figure 2, a normalized map of compressor performance
is illustrated in which under the design operating conditions all of the
compressors
22, 28, 30 and 32 are operating along their peak efficiency operating line
where
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the pressure ratio is proportional to the flow through the compressors and
therefore, at peak efficiency and at 100% of their speed at the normal
operating
condition. In accordance with the present invention, where it is desired to
reduce
the flow under turn down operating conditions, the speed of the compressor 22
is
downwardly adjusted to a point along the peak efficiency operating line. The
adjustment of speed can be gradual along this operating line or simply
controlled
to reduce the speed directly from the normal operating condition to a point
corresponding to the turn down condition. In this regard, it is possible that
compromises would have to be made in the design of any compression train and
consequently, at the normal operating condition, compressor 22 or successive
downstream compressors might not be operating at exactly their stage-wise
aerodynamic peak efficiency. Therefore, during the normal operating condition,
it
is possible that compressors in a compression train might not be operating
exactly
on the peak efficiency operating line. Furthermore, if the compressors were of
different designs, then their operating lines would not converge as shown in
Figure 2. In any case, reducing the speed of the compressor 22 will reduce the
flow which in turn will result in a reduction of the pressure ratio across the
compressor 22. This reduction will have to be made up in the successive
compressors 28, 30 and 32 of the successive compression stages. One way to do
this is to compute the compression ratio between the turned down initial
compression stage 10 and divide the computed pressure ratio equally among the
successive compression stages 12, 14 and 16 (there may be more optimal methods
than to divide the computed ratio evenly over the remaining stages). As can be
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appreciated, there are potentially other ways to recover the pressure. In the
illustrated embodiment, the computation is performed in the programmable logic
controller 54 which also, generates the required control signals that are
applied to
the variable speed controllers through the control network 56. While one or
more
of the subsequent compressors 28, 30 and 32 may no longer be operating along
the peak efficiency operating line, the compressor 22 of the initial
compression
stage 10 will remain operating along the peak efficiency operating line and
therefore, be operated efficiently. Why this is important is that the
compressor 22
will invariably have the greatest electrical power requirements. Thus, during
turndown operating conditions, the multistage compressor system will
invariably
be more efficient than if the compressor speeds were stepped down together as
provided for in the prior art.
[0028] With additional reference to Figure 3 and the Table below, an example
of
the operation of multistage compression system 1 is illustrated that is
particularly
applicable to a cryogenic air separation plant of the type that is illustrated
in
Figure 4 to be discussed below.
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TABLE
Inlet Pressure, pica 15
Train Discharge Pressure, psia 90
Stage Count 4.0
Train Pressure Ratio 6.00
Design Stagewise Pressure Ratios 1.565
Turndown Rate 80%
1st Stage Peak Efficiency Pressure Ratio at Turndown 70%
Pin Pout Volumetric Pressure Normalized
psia Flow Ratio PR Flow
Stage 1 15.0 23.5 1.00 1.57 1.000 1.000
Full _________________________________________________________________
Stage 2 23.5 36.7 0.64 1.57 1.000 1.000
Flow _________________________________________________________________
Stage 3 36.7 57.5 0.41 1.57 1.000 1.000
Case _________________________________________________________________
Stage 4 57.5 90.0 0.26 1.57 1.000 1.000
Stage 1 15.0 16.4 0.80 1.10 0.700 0.800
80% __________________________________________________________________
Stage 2 16.4 29.0 0.73 1.76 1.126 1.143
Flow _________________________________________________________________
Stage 3 29.0 51.1 0.41 1.76 1.126 1.015
Case _________________________________________________________________
Stage 4 51.1 90.0 0.23 1.76 1.126 0.901
As shown in the Table, the inlet pressure of the feed stream 18 is about 15
psia
and the required discharge pressure is 90 psia of the compressed gas stream 20
for
a total pressure ratio of 6.00 across the multistage compressor system 1. Each
of
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the compressors 22, 28, 30 and 32 operate at a design pressure ratio of 1.57.
As
illustrated the flow through successive compressors decreases because the
volumetric flow is decreasing as the density of the gas decreases due to the
compression of the gas. During the turndown operating condition of 80 percent
of
the flow, the normalized pressure ratio across compressor 22 will be adjusted
to a
point along the peak efficiency operating line to be .7. If the required
pressure
ratio is equally adjusted among the successive stages 12, 14 and 16, then each
compressor 28, 30 and 32 will be operated at a normalized pressure ratio of
1.126
in accordance with the power law. In other words, 0.700 x 1.126 x 1.126 x
1.126
is equal to 1.00 or taking the actual pressure ratios, 1.10 x 1.76 x 1.76 x
1.76 is
equal to 6.00. Therefore, although multistage compression system 1 is being
turned down to 80 percent of flow, it will still deliver the compressed gas
stream
20 at 90 psia or in other words, at the design pressure that would be
delivered
during the normal operating condition. To achieve these conditions, the speed
of
the compressors 22, 28, 30 and 32 will be adjusted by their respective
variable
speed controllers 26, 40, 42 and 44 to speeds indicated by figure 3:
compressor 22
will be operated at approximately 83 percent of its design rotational speed;
compressor 28 will be operated at approximately 107 percent of its design
rotational speed; compressor 30 will be operated at approximately 104 percent
of
its design rotational speed; and compressor 32 will be operated at
approximately
103 percent of its design rotational speed. Here again, it is appropriate to
point
out that that although in the particular pressure recovery method discussed
above,
the successive compressor are operating at higher speeds, this may only be the
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case in the final compression stages in that upstream stages might be
operating at
a lower speed to save power. Programmable logic controller 54 could be
programmed to simply have a set of pre-set turn down conditions or
computations
could be made as set forth above to turn the multistage compression system 1
down to a set turn down flow rate that would be an input to programmable logic
controller 54.
[0029] Additionally, as illustrated, there exists pressure transducers 59, 60,
62 and
64 to measure the pressure ratios across the compressors 28, 30 and 32.
Pressure
transducer 58 is provided to measure the discharge pressure of compressor 22
and
fine tune the speed thereof to make certain it is at a specific location on
the
operating chart shown in Figure 2, for instance, when operating at the
turndown
operating condition. Signals referable to the pressures serve as a further
input by
means of data transmission lines 65, 66, 68, 70 and 72, respectively, to the
programmable logic controller 54. Also, a flow meter 74 able to send a signal
via
a data transmission line 76 to the programmable logic controller 54 along with
a
pressure transducer 78 and an associated data transmission line 80 could also
be
present.
[0030] This instrumentation and the programmable logic controller 54 may be
used to implement the control operation of the multistage compression system 1
described above. To such end, the programmable logic controller 54 can be
programmed to receive an input data referable to the desired flow rate and
then to
compute a reduced speed for variable speed motor 24 in accordance with an
algebraic representation of Figure 3 or a database containing the data within
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Figure 3 and generate a control signal referable to such computed reduced
speed.
The control signal is then input into the variable speed controller 26 through
data
transmission line 56 that in turn controls the speed of variable speed motor
24 to
obtain the reduced speed. Flow meter 74 generates a signal referable to the
mass
flow rate of compressed gas stream 20 that is transmitted as an input into
programmable logic controller 54 by means of data transmission line 80.
Programmable logic controller is programmed to respond to such signal and send
a revised signal to variable speed controller 26 should the flow rate not be
within
2 percent of the desired flow rate input into programmable logic controller
54.
When the flow rate has been set, the further computations are performed by
programmable logic controller 54 to compute the pressure ratios in a manner
set
forth in the above example, generate control signals to be transmitted by data
transmission line 56 to variable speed controllers 40, 42 and 44 to in turn
adjust
the speeds of variable speed motors 34, 36 and 38 and therefore, compressors
28,
30 and 32 to achieve the desired pressure ratios computed by programmable
logic
controller 54. Pressure signals referable to inlet and outlet pressures are
generated
by pressure transducers 59, 60, 62 and 64 and fed as another input into
programmable logic computer 54 that computes the actual pressure ratios and
then
as necessary generates and individually updates the control signals being sent
to
variable speed controllers 40, 42 and 44 until measured pressure ratios are
within
2 percent of the computed pressure ratios. Increased thereafter, data
generated on
the basis of the signal referable to mass flow as that is produced by flow
meter 74
is used to confirm that the desired train flow is achieved. If the desired
flow rate
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is not achieved, the process outlined above is repeated until the flow rate is
within
about two percent of the desired flow rate.
[0031] For purpose of a simplification, Figures 2 and 3 and the above example
assume aerodynamically identical compressor stages; that is, while physically
different and operating at differing pressure and volumetric flows, each stage
conforms to identical design rules and operates such that their fluid dynamic
flows
are effectively scaled variants of one another.
[0032] However, operation of multistage compression system 1 is not limited to
using aerodynamically identical compressor stages. Non-aerodynamically
identical stages may be used as well. This requires programmable logic
controller
54 to use for example, algebraic representations of each stages unique flow,
pressure and speed behavior.
[0033] With reference to Figure 4, a multistage compression system l' is
illustrated. For the sake of simplicity of explanation, where an element
illustrated
in this Figure 4 has been described in Figure 1, the same reference numbers
will
be used. In the multistage compression system l', a final compression stage
14'
is illustrated having a final compressor 30' and an auxiliary compressor 32"
in an
auxiliary compressions stage 16'. Also included is a flow control network 90.
The flow control network 90 is provided with a by-pass line 92, a first valve
94
positioned within the by-pass line 92 and a second valve 96 positioned between
the aftercooler 50 and the auxiliary compressor 32'. Each of the first valve
94 and
the second valve 96 are operable to be set in a closed position and an open
position and to be activated by the programmable logic controller 54' through
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electrical connections 98 and 100, respectively. The programmable logic
controller is programmed such that during the normal operating condition, the
first
valve 94 is set in the open position and the second valve 96 is set in the
closed
position and the compressed gas stream 20' is thereby supplied from the final
compressor 30' at the design pressure and at the higher flow rate through the
by-
pass line 92. Under such circumstances, the auxiliary compressor 32' is by-
passed. During a turndown operating condition, the first valve 94 is set in
the
closed position and the second valve 96 is set in the open position such that
the
auxiliary compressor 30' supplied compressed gas through the aftercooler 50
and
to the auxiliary compressor 32'. The auxiliary compressor 32' thereby supplies
the compressed gas stream 20' from its associated aftercooler 52 at the same
design pressure, but at the lower flow rate to be obtained during turndown
operation conditions. In this regard, the multistage compression system l' is
turned down in a similar fashion to the multistage compression system 1 in
that
the speed of the initial compressor 22 of the compression stage 10 is
downwardly
adjusted to a point along the peak efficiency operating line and the required
pressure ratio for the subsequent compressors 28, 30' and 32' are divided to
deliver the compressed gas stream 20' at the required design delivery pressure
in a
manner set forth above. The compressor system l' permits the delivery of
reduced
flow at design pressure while turning down compressor 30' and its preceding
stages each along its peak efficiency operating line. It might be necessary to
have
a series of compression stages in place of the singe compressor 32' in order
to
achieve the design pressure at reduced flow conditions.
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[0034] With reference to Figure 5, an air separation plant 2 is illustrated
that
incorporates a multistage compression system 110 that can either be in the
form
illustrated in connection with Figures 1 or 4 and controlled in a manner that
has
been described above. The compression system 110 compresses a feed air stream
112 to produce a compressed air stream designated by reference numeral 114
that
could therefore, be the compressed stream 20 or 20' produced by multistage
compression systems 1 and l', respectively. It is also to be pointed out that
air
separation plant 2 does not include a pre-purification unit to remove higher
boiling impurities from the feed air stream 112 in that the air separation
plant 2 is
designed to be used in an enclave of air separation plants as such, the feed
air
could be centrally processed. By the same token, if air separation plant 2
were to
be used as a stand-alone plant, a pre-purification unit could be located
downstream of the multistage compression system 110. Compressed air stream
114 is divided into a main compressed air stream 116 and two subsidiary
compressed air streams 118 and 120. Main compressed air stream 116 is cooled
within a main heat exchanger 122 to a temperature suitable for its
distillation in an
air separation unit 124.
[0035] Subsidiary compressed air stream 118 is compressed in a booster
compressor 126 to produce a boosted pressure air stream 128 that is cooled in
an
after-cooler 130 and then partially cooled within main heat exchanger 122 to
an
intermediate temperature between the cold and warm end temperature of main
heat exchanger 122. The boosted pressure air stream 128, after having been
partially cooled, is introduced into a turbo expander 138 coupled to the
booster
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compressor 126 to produce an exhaust stream 140 that is reintroduced into main
heat exchanger 122 and then fully cooled and introduced into the air
separation
unit 124. The purpose of the foregoing is to impart refrigeration into the air
separation plant 2 for purposes of overcoming warm end heat exchanger losses,
heat leakage into the cold box housing the air separation unit 124 and to
create
liquids. However, it is understood that there are air separation plants in
which
refrigeration is externally applied and also, air separation plants in
enclaves in
which the refrigeration is centrally applied. Also, there are yet other
methods of
using turbo expanders to supply refrigeration to air separation plants as are
well
known in the art. Consequently, the foregoing means used to impart
refrigeration
in air separation plant 2 is shown for purposes of illustration only and is
not
intended to limit the scope of the present invention.
[0036] Subsidiary compressed air stream 120 is in turn introduced into booster
compressor 142 to produce a boosted pressure air stream 144 that after cooling
in
aftercooler 144 is fully cooled in main heat exchanger 122 to produce a liquid
air
stream 146 that is also introduced into air separation unit 124. The
production of
boosted pressure air stream 144 is necessary to warm a pumped return stream
152
that could be liquid oxygen or liquid nitrogen and is shown for purposes of
illustration only.
[0037] Air separation unit 124, as known in the art, could be a double column
unit
having a high pressure column and a low pressure column operatively associated
within one another in a heat transfer relationship by a condenser reboiler.
Both of
such columns have mass transfer contacting elements such as trays, structured
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packing, random packing or a combination of such elements. These element
contact liquid and vapor phases of the air in a manner known in the art such
that
the liquid phases becomes ever more rich in oxygen as it descends and the
vapor
phase becomes ever more rich in nitrogen as its ascends in either the high or
low
pressure columns. As known in the art, the high pressure column produces a
crude liquid oxygen column bottoms that is further refined in the low pressure
column and a nitrogen-rich vapor column overhead that is condensed in the
condenser reboiler to produce reflux for both the high and low pressure
columns.
The low pressure columns produces an oxygen-rich liquid that can be removed as
return stream 148, pumped in a pump 152 to produce the pumped return stream
150 which is fully warmed within main heat exchanger 122 to produce a product
at pressure. The low pressure column also produces a nitrogen-rich vapor
column
overhead that could be removed as a return stream 154 and then, fully warmed
in
main heat exchanger 122 to produce a nitrogen product.
[0038] Although not illustrated, air separation unit 124 would incorporate
other
known heat exchangers such as a subcooling unit to subcool the reflux to the
low
pressure column and the crude liquid oxygen to be further refined in the low
pressure column and expansion valves to expand such streams to a suitable
pressure for introduction into the low pressure column. In the illustrated
embodiment, liquid air stream 146 could be expanded by expansion valves and
introduced into the low pressure column and also the high pressure column. The
exhaust stream 140 could be introduced into the low pressure column or
possibly
the high pressure column. The main heat exchanger is typically a brazed
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aluminum unit or units arranged in parallel. Further the main heat exchanger
could also incorporate a high pressure unit designed to cool the boosted
pressure
air stream 142 and to warm the pumped return stream 152. Another known
possibility is that the air separation unit 124 could incorporate an argon
column or
columns to produce an argon product. Also, air separation unit 124 could be a
single column designed to produce a nitrogen product.
[0039] As indicated above, multistage compression unit 110 is designed to
function to produce the compressed air stream 114 at a constant pressure both
during normal operational conditions of air separation plant 2 where air
separation
plant 2 is making products, such as pumped return stream 152, at a design
production rate and during turn down operating conditions where products are
produced at a lower flow rate. During turndown operation, the multistage
compression system 110 is turned down to reduce the flow rate of compressed
air
stream 114. Compressed air stream 114 is divided into a main compressed air
stream 116 and two subsidiary compressed air streams 118 and 120. The reduced
flow in compressed air stream 114 may be distributed among: subsidiary
compressed air stream 120 thereby reducing the production of liquid air stream
146; subsidiary compressed air stream 118 thus reducing the refrigeration
available to air separation plant 2; and/or reducing the flow rate of
compressed air
stream 116 so that production is decreased. Distribution of the reduced flow
from
compressed air steam 114 may be apportioned in any way among compressed air
streams 116, 118, 120 such that the sum of flows in compressed air steams 116,
118 and 120 totals the flow in compressed of stream 114. As an example in a
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plant having high and low pressure columns that is designed to produce a
pressurized oxygen product, the pumped return stream 152 would be made up of
oxygen-enriched liquid removed from the low pressure column. If it were
desired
during turn down operating conditions to reduce the production of the
pressurized
oxygen product, the flow of streams 116 and 120 would be reduced and the flow
rate of stream 118 could remain the same as during design operation. The speed
of the booster compressor 142 would have to be increased to allow the
discharge
pressure to remain at design.
[0040] Although the present invention has been described with respect to
preferred embodiments, as will occur to those skilled in the art, numerous
changes,
additions and omission can be made without departing from the spirit and scope
of the present invention as set forth in the appended claims.
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