U.S. patent application number 11/669324 was filed with the patent office on 2007-06-14 for cyrogenic air separation process and apparatus.
Invention is credited to Jean-Renaud Brugerolle, Bao Ha.
Application Number | 20070130992 11/669324 |
Document ID | / |
Family ID | 34681681 |
Filed Date | 2007-06-14 |
United States Patent
Application |
20070130992 |
Kind Code |
A1 |
Brugerolle; Jean-Renaud ; et
al. |
June 14, 2007 |
Cyrogenic Air Separation Process and Apparatus
Abstract
A low temperature air separation process and apparatus for
producing pressurized gaseous product in an air separation unit
using a system of distillation columns which include cooling a
compressed air stream in a heat exchange line to form a compressed
cooled air stream, sending at least part of the compressed, cooled
air stream to a column of the system, liquefying a process stream
to form a first liquid product, storing at least part of the first
liquid product in a storage tank, sending at least part of the
above first liquid product from the storage tank to the air
separation unit as one of the feeds, extracting at least one second
liquid product stream from a column of the column system and
pressurizing the at least one second liquid product stream,
vaporizing the above pressurized second liquid product stream to
form pressurized gaseous product in the heat exchange line and
extracting a cold gas without warming it completely in the heat
exchange line.
Inventors: |
Brugerolle; Jean-Renaud;
(Paris, FR) ; Ha; Bao; (San Ramon, CA) |
Correspondence
Address: |
AIR LIQUIDE
2700 POST OAK BOULEVARD, SUITE 1800
HOUSTON
TX
77056
US
|
Family ID: |
34681681 |
Appl. No.: |
11/669324 |
Filed: |
January 31, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10899688 |
Jul 27, 2004 |
|
|
|
11669324 |
Jan 31, 2007 |
|
|
|
60532219 |
Dec 23, 2003 |
|
|
|
Current U.S.
Class: |
62/615 ;
62/647 |
Current CPC
Class: |
F25J 3/04593 20130101;
F25J 3/04303 20130101; F25J 3/04581 20130101; F25J 1/0234 20130101;
F25J 3/04266 20130101; F25J 3/04606 20130101; F25J 2245/42
20130101; F25J 3/04575 20130101; F25J 3/04678 20130101; F25J
3/04836 20130101; F25J 3/04084 20130101; F25J 3/04351 20130101;
F25J 2210/50 20130101; F25J 2215/02 20130101; F25J 2210/42
20130101; F25J 1/0012 20130101; F25J 3/0406 20130101; F25J 3/04412
20130101; F25J 3/0429 20130101; F25J 3/04545 20130101; F25J 3/04496
20130101; F25J 2245/40 20130101; F25J 1/0221 20130101; F25J 1/0251
20130101; F25J 2210/62 20130101; F25J 3/04345 20130101; F25J
2210/40 20130101; F25J 3/04054 20130101; F25J 3/0409 20130101; F25J
3/04254 20130101; F25J 3/04224 20130101; F25J 2215/40 20130101;
F25J 3/04339 20130101 |
Class at
Publication: |
062/615 ;
062/647 |
International
Class: |
F25J 1/00 20060101
F25J001/00; F25J 3/00 20060101 F25J003/00 |
Claims
1. A low temperature air separation process for producing
pressurized gaseous product in an air separation unit using a
system of distillation columns which comprises the following steps:
a) cooling a compressed air stream in a heat exchange line to form
a compressed cooled air stream; b) sending at least part of the
compressed, cooled air stream to a column of the system; c) in a
first period of time, liquefying a process stream to form a first
liquid product and storing at least part of this first liquid
product; d) in a second period of time, sending the above stored
first liquid product to the air separation unit as one of the
feeds; e) pressurizing at least one second liquid product stream;
f) vaporizing the above pressurized second liquid product stream in
the heat exchange line to form pressurized gaseous product; and g)
during the above second period of time, extracting a cold gas from
the air separation unit at a temperature between about -195.degree.
C. and about -20.degree. C.
2. The process of claim 1 wherein the pressurized gaseous product
is oxygen product.
3. The process of claim 1 wherein the pressurized gaseous product
is nitrogen product.
4. The process of claim 1 wherein the air separation unit is within
a cold box and the cold gas is extracted from the air separation
unit cold box at a temperature between about -195.degree. C. and
about -20.degree. C.
5. The process of claim 1 wherein the process stream of step c)
contains any proportion of oxygen, nitrogen and argon.
6. The process of claim 1 wherein the process stream of step c) is
at least one of pure nitrogen, air, oxygen containing at least 37
mol. % oxygen, oxygen containing at least 65 mol. % oxygen, oxygen
containing at least 85 mol. % oxygen and oxygen containing at least
99.5 mol. %.
7. The process of claim 1 wherein the cold gas of step g) is chosen
from the group comprising a nitrogen rich gas, pure nitrogen gas,
air, a gas having a composition similar to air, an oxygen rich gas
and pure oxygen product.
8. The process of claim 1 wherein the second liquid product of step
e) is the same as the stored first liquid product of step c).
9. The process of claim 1 wherein step c) is performed if the
electricity rate is below a predetermined threshold.
10. The process of claim 7 wherein step c) is performed only if the
electricity rate is below a predetermined threshold.
11. The process of claim 1 wherein step d) is performed if the
electricity rate is above a predetermined threshold.
12. The process of claim 9 wherein step d) is performed only if the
electricity rate is above a predetermined threshold.
13. The process of claim 1 wherein step g) is performed if the
electricity rate is above a predetermined threshold.
14. The process of claim 11 wherein step g) is performed only if
the electricity rate is above a predetermined threshold.
15. The process of claim 1 wherein at least a portion of the cold
gas of step g) is heated and expanded in a hot expander to recover
energy.
16. The process of claim 1 wherein at least a portion of the cold
gas of step g) is injected into a gas turbine for energy
recovery.
17. The process of claim 1 wherein at least a portion of the cold
gas of step g) is recycled back to the air separation unit.
18. The process of claim 1 wherein the air separation unit supplies
pressurized gaseous oxygen product to an IGCC facility.
19. The process of claim 18 wherein the IGCC facility comprises a
gas turbine further comprising the following steps: a) extracting
air from the gas turbine if the rate of electricity is below a
predetermined threshold; and b) feeding above extracted air to the
air separation unit
20. The process of claim 18 comprising the step of injecting
pressurized cold gas to the gas turbine if the rate of electricity
is higher than a predetermined threshold.
21. The process of claim 1 wherein the refrigeration of vaporizing
LNG is recovered to reduce the liquefaction cost of the first
liquid product.
22. The process of claim 1 comprising reducing the flow of
compressed air in the heat exchanger if the rate of electricity is
above a predetermined threshold as compared to the amount of air
cooled in the heat exchanger if the rate of electricity is below a
predetermined threshold.
23. The process of claim 1 wherein the cold gas is removed from the
air separation unit without warming it in the heat exchange
line.
24. The process of claim 1 wherein the cold gas is removed from the
air separation unit after being warmed partially in the heat
exchange line.
25. A low temperature air separation process for producing
pressurized gaseous product in an air separation unit using a
system of distillation columns which comprises the following steps:
a) cooling a compressed air stream in a heat exchange line to form
a compressed cooled air stream b) sending at least part of the
compressed, cooled air stream to a column of the system c) in a
first period of time, liquefying a process stream to form a first
liquid product and storing at least part of the first liquid
product d) in a second period of time, sending the above stored
first liquid product to the air separation unit as one of the feeds
e) pressurizing at least one second liquid product stream f)
vaporizing the above pressurized second liquid product stream to
form pressurized gaseous product in the heat exchange line g)
during the above second period of time, extracting a cold gas from
the air separation unit and compressing the cold gas in a
compressor having an inlet temperature between about -180.degree.
C. and -50.degree. C. and an outlet temperature of at most
-20.degree. C. to form a pressurized gas.
26. The process of claim 26 wherein the pressurized gaseous product
is oxygen product.
27. The process of claim 26 wherein pressurized gaseous product is
nitrogen product.
28. The process of claim 26 wherein the process stream of step c)
contains any proportion of oxygen, nitrogen and argon.
29. The process of claim 26 wherein the process stream of step c)
is at least one of pure nitrogen, air, oxygen containing at least
37 mol. % oxygen, oxygen containing at least 65 mol. % oxygen,
oxygen containing at least 85 mol. % oxygen and oxygen containing
at least 99.5 mol. %.
30. The process of claim 26 wherein the cold gas of step g) is
chosen from the group comprising a nitrogen rich gas, pure
nitrogen, air, a gas having a composition similar to air, an oxygen
rich gas and pure oxygen product.
31. The process of claim 26 wherein step c) is performed if the
electricity rate is below a predetermined threshold.
32. The process of claim 32 wherein step c) is performed only if
the electricity rate is below a predetermined threshold.
33. The process of claim 26 wherein step d) is performed if the
electricity rate is above a predetermined threshold.
34. The process of claim 34 wherein step d) is performed only if
the electricity rate is above a predetermined threshold.
35. The process of claim 26 wherein step g) is performed if the
electricity rate is above a predetermined threshold.
36. The process of claim 36 wherein step g) is performed only if
the of electricity rate is above a predetermined threshold.
37. The process of claim 26 wherein the cold gas is compressed to a
pressure between 35 and 80 bars abs in the compressor.
38. The process of claim 26 wherein at least a portion of the
pressurized gas is heated and expanded in a hot expander to recover
energy.
39. The process of claim 26 wherein at least a portion of the
pressurized gas is injected into a gas turbine for energy
recovery.
40. The process of claim 26 wherein at least a portion of the
pressurized gas is recycled back to the column system of the air
separation unit.
41. The process of claim 26 wherein the air separation unit
supplies pressurized gaseous oxygen product to an IGCC
facility.
42. The process of claim 41 wherein the IGCC facility comprises a
gas turbine further comprising the following steps: a) extracting
air from the gas turbine if the rate of electricity is below a
predetermined threshold b) feeding above extracted air to the air
separation unit
43. The process of claim 26 comprising the step of injecting the
pressurized cold gas to the gas turbine if the rate of electricity
is higher than a predetermined threshold.
44. The process of claim 26 further comprising the following steps:
a) warming the pressurized gas in the heat exchange line; b)
cooling additional gas in the heat exchange line to form cold
additional gas; and c) cryogenically compressing cold additional
gas to higher pressure
45. The process of claim 44 wherein both gases are compressed to
between about 10 and about 20 bars abs.
46. The process of claim 26 wherein the refrigeration of vaporizing
LNG is recovered to reduce the liquefaction cost of the first
liquid product.
47. The process of claim 26 comprising reducing the flow of
compressed air in the heat exchange line if the rate of electricity
is above a predetermined threshold as compared to the amount of air
cooled in the heat exchange line if the rate of electricity is
below a predetermined threshold.
48. The process of claim 26 wherein the cold gas is removed from
the air separation unit cold box without warming it in the heat
exchange line.
49. The process of claim 26 wherein the cold gas is removed from
the air separation unit cold box after being warmed partially in
the heat exchange line.
50. The process of claim 26 comprising the step of warming the
pressurized gas in the heat exchange line.
51. The process of claim 1 wherein the air separation unit is
contained within a cold box and the extracting a cold gas from the
cold box at a temperature between about -195.degree. C. and about
-20.degree. C.
52. An air separation apparatus comprising: a) a system of
distillation columns; b) a heat exchange line; c) a cold box
containing at least the system of distillation columns and the heat
exchange line; d) a conduit for sending feed air to the heat
exchange line; e) a conduit for sending cooled feed air from the
heat exchange line to the column system; f) means for sending a
first liquid product to the column system; g) a conduit for
removing a liquid from a column of the column system; h) a conduit
for sending the liquid to the heat exchange line; i) a conduit for
removing vaporized liquid from the heat exchange line; and j) a
conduit for extracting a gas from a column of the system and for
removing the gas from the air separation apparatus without warming
the gas by traversing the heat exchange line in its entirety
53. The apparatus of claim 54 comprising means for storing the
first liquid product outside any column of the column system.
54. The apparatus of claim 54 comprising a gas compressor connected
to the conduit for extracting gas.
55. The apparatus of claim 54 comprising a conduit for removing the
gas from the air separation apparatus without warming the gas in
the heat exchange line.
56. The apparatus of claim 54 comprising means for liquefying a gas
to form the first liquid product.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application 60/532,219, filed Dec. 23, 2003, U.S. Non-Provisional
application Ser. No. 10/798,068, filed Mar. 11, 2004 and copending
U.S. Continuation-in-Part application Ser. No. 10/899,688 filed on
Jul. 27, 2004.
FIELD OF INVENTION
[0002] This invention relates to an air separation process and
associated equipment.
BACKGROUND OF THE INVENTION
[0003] Air separation is a very power intensive technology,
consuming thousands of kilowatts or several megawatts of electric
power to produce large quantities of industrial gases for tonnage
applications such as chemicals, refineries, steel mills, etc.
[0004] A typical liquid pumped process is illustrated in FIG. 1. In
this type of process, atmospheric air is compressed by a Main Air
Compressor (MAC) 1 to a pressure of about 6 bar absolute, it is
then purified in an adsorber system 2 to remove impurities such as
moisture and carbon dioxide that can freeze at cryogenic
temperature to yield a purified feed air. A portion 3 of this
purified feed air is then cooled to near its dew point in heat
exchanger 30 and is introduced into a high pressure column 10 of a
double column system in gaseous form for distillation. Nitrogen
rich liquid 4 is extracted at the top of this high pressure column
and a portion is sent to the top of the low pressure column 11 as a
reflux stream. The oxygen-enriched liquid stream 5 at the bottom of
the high pressure column is also sent to the low pressure column as
feed. These liquids 4 and 5 are subcooled before expansion against
cold gases in subcoolers not shown in the figure for the sake of
simplicity. An oxygen liquid 6 is extracted from the bottom of the
low pressure column 11, pressurized by pump to a required pressure
then vaporized in the exchanger 30 to form the gaseous oxygen
product 7. Another portion 8 of the purified feed air is further
compressed in a Booster Air Compressor (BAC) 20 to high pressure
for condensation in the exchanger 30 against the vaporizing oxygen
enriched stream. Depending upon the pressure of the oxygen rich
product, the boosted air pressure can be around 65 bar or sometimes
over 80 bar. The condensed boosted air 9 is also sent to the column
system as feed for the distillation, for example to the high
pressure column. Part of the liquid air may be removed from the
high pressure column and sent to the low pressure column following
subcooling and expansion. It is also possible to extract nitrogen
rich liquid from the top of the high pressure column then pump it
to high pressure (stream 13) and vaporize it in the exchanger in
the same way as with oxygen liquid. A small portion of the feed air
(stream 14) is further compressed and expanded into the column 11
to provide the refrigeration of the unit. Optionally alternative or
additional means of providing refrigeration may be used, such as
Claude expanders or nitrogen expanders.
[0005] Waste nitrogen is removed from the top of the low pressure
column and warms in exchanger 30. Argon is produced using a
standard argon column whose top condenser is cooled with oxygen
enriched liquid 5.
[0006] A typical 3,000 ton/day oxygen plant producing gaseous
oxygen under pressure for industrial uses can consume typically
about 50 MW. A network of oxygen plants for pipeline operation
would require a power supply capable of providing several hundreds
megawatts of electric power. In fact, the electric power is the
main operating cost of an air separation plant since its raw
material or feedstock is atmospheric air and is essentially free.
Electric power is used to drive compressors for air or products
compression. Therefore, power consumption or process efficiency is
one of the most important factors in the design and operation of an
air separation unit (ASU). Power rate, usually expressed in $/kWh,
is not constant during the day but varies widely depending upon the
peaks or off-peaks. It is well known that during the day the power
rate is the highest when there is strong demand--or peak
period--and the lowest during the low demand--or off-peak period.
Utility companies tend to offer significant cost reduction if an
industrial power user can cut back its power consumption during
peaks. Therefore, the companies operating air separation units
always have strong incentives to adjust the operating conditions of
the plants to track the power demand so that to lower the utility
cost. It is clear that a solution is needed to provide an
economical answer to this variable power rate issue.
[0007] It is useful to note that the periods when the power peaks
take place may be totally different from the product demand peaks,
for example, a warm weather would generate a high power demand due
to air conditioning equipment meanwhile the demand for products
remains at normal level. In several locations, the peaks occur
during the day time when the industrial output of manufacturing
plants, the main users of industrial gases, is usually at the
highest level and when combined with the high power usage of other
activities would cause very high demand on the electric grid. This
high power usage creates potential shortage and utility companies
must allocate other sources of power supply causing temporary high
power rate. Also, usually at night, the power demand is lower and
the power is available abundantly such that the utility companies
could lower the power rate to encourage usage and to keep the power
generating plants operate efficiently at reduced load. The power
rate at peaks can be twice or several times higher than the power
rate for off-peaks. In this application, the term "peak" describes
the period when power rate is high and the term "off-peak" means
the period when power rate is low.
[0008] For industrial power users, power rates are usually
negotiated and defined in advance in power contracts. In addition
to the daily variation of power rates, sometimes there are
provisions or allowances for interruptible power supply: during
periods of high power demand on the power grid, the utility
companies can reduce the supply to those users with a relatively
short advance notice, in return, the overall power rate offered can
be significantly below the normal power rate. This kind of
arrangement provides additional incentives for users to adapt their
consumption in line with the network management of the power
suppliers. Therefore, significant cost reduction can be achieved
only if the plant equipment can perform such flexibility. Based on
the power cost structure as set forth by the power contracts, the
users can define predetermined threshold or thresholds of power
rate to trigger the mechanism of power reduction: [0009] when power
rate is above the predetermined threshold, the power usage is
reduced to lower the cost. [0010] when power rate is below the
predetermined threshold, the power usage is increased to normal
level or even higher if desired.
[0011] A simple approach to address the problem of variable power
rate is to lower the plant's power consumption during peaks while
maintaining the product output in order to satisfy the customer's
need. However, the cryogenic process of air separation plants is
not very flexible since it involves distillation columns and the
product specifications require fairly high purities. Attempts to
lower the plant output in a very short time or to increase the
plant production quickly to meet product demand can have
detrimental effects over plant stability and product integrity.
Various patents have been written to suggest how to solve the
difficulties associated with the variable product demand of a
cryogenic plant.
[0012] U.S. Pat. No. 3,056,268 teaches the technique of storing
oxygen and air under liquid form and vaporizing the liquids to
produce gaseous products to satisfy the variable demand of the
customer, such as at metallurgical plants. The liquid oxygen is
vaporized when its demand is high. This vaporization is balanced by
a condensation of liquid nitrogen via the main condenser of the
double column air separation unit.
[0013] U.S. Pat. No. 4,529,425 teaches a similar technique to that
of U.S. Pat. No. 3,056,268 to solve the problem of variable demand,
but liquid nitrogen is used instead of liquid air.
[0014] U.S. Pat. No. 5,082,482 offers an alternative version of
U.S. Pat. No. 3,056,268 by sending a constant flow of liquid oxygen
into a container and withdrawing from it a variable flow of liquid
oxygen to meet the requirement of variable demand of oxygen.
Withdrawn liquid oxygen is vaporized in an exchanger by
condensation of a corresponding flow of incoming air.
[0015] U.S. Pat. No. 5,084,081 teaches yet another method of U.S.
Pat. No. 4,529,425 wherein another intermediate liquid, the oxygen
enriched liquid, is used in addition to the traditional liquid
oxygen and liquid nitrogen as the buffered products to address the
variable demand. The use of enriched oxygen liquid allows
stabilizing the argon column during the variable demand
periods.
[0016] In still another approach to address the variable product
demand, U.S. Pat. No. 5,666,823 teaches a technique to efficiently
integrate the air separation unit with a high pressure combustion
turbine. Air extracted from the combustion turbine during the
periods of low product demand is fed to the air separation unit and
a portion is expanded to produce liquid. When product demand is
high, less air is extracted from the combustion turbine and the
liquid produced earlier is fed back to the system to satisfy the
higher demand. The refrigeration supplied by the liquid is
compensated by not running the expander for lack of extracted air
from the combustion turbine during the high product demand.
[0017] The above publications addressed the technical issues of the
variable demand, especially the techniques used to maintain
stability of the distillation columns during the time when the
demand of the product varies widely. However, none of the above
directly address the aspect of potential savings and economy when
adapting the air separation plants to the power rate structure of
peak and off-peak periods to obtain cost reduction. Industry
practice also does not resolve the technical problems associated
with the adjustment of the air separation units during periods of
high power cost and with relatively unchanged product demand. In
fact, these two aspects of the operation of air separation units
are quite different by nature: one is governed by the customer's
variable demand and the other is governed by variable power cost
with relatively constant demand.
[0018] Therefore, there exists a need to come up with a
configuration for air separation plants permitting a reduction of
the power consumption during peaks, while maintaining a supply of
products to satisfy customer's demand. To make up for this
reduction of power, additional power consumption can be arranged to
take place during off-peak periods, at a much lower power rate.
Significant savings on power rate can therefore be achieved, since
a portion of the products is being produced at a low power rate and
supplied to the customers during periods of high power rate.
SUMMARY OF THE INVENTION
[0019] This invention offers a technique to resolve the problems
associated with the reduction of power consumption during peak
periods, while still being capable of maintaining the same product
output, so that power cost savings can be achieved. Key aspects
include: [0020] a) liquefying a process stream in off-peak periods
to produce a first liquid product; [0021] b) feeding the air
separation unit with the produced first liquid product in peak
periods; [0022] c) reducing air feed supplied by the air compressor
to maintain the total amount of oxygen contained in the feed
streams essentially the same; [0023] d) withdrawing at least one
product from the column system and raising its pressure by pumping,
and then vaporizing, in a heat exchanger to form gaseous product;
[0024] e) withdrawing a cold gas from the system at cryogenic
temperature; and [0025] f) cryogenically compressing the produced
cold gas to higher pressure with a cold gas compressor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] For a further understanding of the nature and objects for
the present invention, reference should be made to the following
detailed description, taken in conjunction with the accompanying
drawings, in which like elements are given the same or analogous
reference numbers and wherein:
[0027] FIG. 1 illustrates the prior art.
[0028] FIG. 2 illustrates the invention where the rate of
electricity is below a predetermined threshold level.
[0029] FIG. 2A illustrates the invention where the rate of
electricity is above a predetermined threshold level.
[0030] FIG. 3 illustrates one embodiment of the invention and the
equipment used in the liquefaction of air in the off-peak
periods.
[0031] FIG. 4 illustrates another embodiment with an independent
liquefier attached to the air separation unit used in the
liquefaction of air in the off-peak periods.
[0032] FIG. 5 illustrates the equipment used to produce liquid air
within the air separation unit.
[0033] FIG. 6 illustrates the liquid feed mode during peak
periods.
[0034] FIG. 7 illustrates that the cold compression of the cold gas
can be performed in a single step.
[0035] FIG. 8 illustrates an air separation unit based on that of
FIG. 2A in which cold low pressure nitrogen is compressed to
between 10 and 20 bar abs.
[0036] FIG. 9 illustrates the pressurized cold gas after a cold
compression in cold compressor can be heated and sent to a hot
expander for power recovery or power production.
[0037] FIG. 10 illustrates an application of the invention where
the compressed cold gas is sent to a gas turbine for power
recovery.
[0038] FIG. 11 illustrates an IGCC application.
[0039] FIG. 12 illustrates a general method for extracting cold gas
from the process when a liquid is fed to the system during peak
periods.
[0040] FIG. 13 illustrates an operating mode of the air separation
unit when the power peaks occur.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0041] According to the invention, there is provided a low
temperature air separation process for producing pressurized
gaseous product in an air separation unit using a system of
distillation columns which comprises the following steps: [0042] i)
cooling a compressed air stream in a heat exchange line to form a
compressed cooled air stream; [0043] ii) sending at least part of
the compressed, cooled air stream to a column of the system; [0044]
iii) in a first period of time, liquefying a process stream to form
a first liquid product and storing at least part of this first
liquid product; [0045] iv) in a second period of time, sending the
above stored first liquid product to the air separation unit as one
of the feed; [0046] v) pressurizing at least one second liquid
product stream; [0047] vi) vaporizing the above pressurized second
liquid product stream in the heat exchange line to form pressurized
gaseous product; and [0048] vii) during the above second period of
time, extracting a cold gas from the air separation unit at a
temperature between.
[0049] According to optional aspects of the invention: [0050] the
pressurized gaseous product is oxygen product [0051] the
pressurized gaseous product is nitrogen product [0052] the cold gas
is extracted from the air separation unit cold box at a temperature
between -195.degree. C. and -20.degree. C., preferably between
-180.degree. C. and -50.degree. C. [0053] the process stream of
step c) contains any proportion of oxygen, nitrogen and argon
[0054] the process stream of step c) is at least one of pure
nitrogen, air, oxygen containing at least 37 mol. % oxygen, oxygen
containing at least 65 mol. % oxygen, oxygen containing at least 85
mol. % oxygen and oxygen containing at least 99.5 mol. % [0055] the
cold gas of step g) is chosen from the group comprising a nitrogen
rich gas, pure nitrogen gas, air, a gas having a composition
similar to air, an oxygen rich gas and pure oxygen product [0056]
the second liquid product of step e) is the same as the stored
first liquid product of step c) [0057] step c) is performed if the
electricity rate is below a predetermined threshold [0058] step c)
is performed only if the electricity rate is below a predetermined
threshold [0059] step d) is performed if the electricity rate is
above a predetermined threshold [0060] step d) is performed only if
the electricity rate is above a predetermined threshold [0061] step
g) is performed if the electricity rate is above a predetermined
threshold [0062] step g) is performed only if the electricity rate
is above a predetermined threshold [0063] at least a portion of the
cold gas of step g) is heated and expanded in a hot expander to
recover energy [0064] at least a portion of the cold gas of step g)
is injected into a gas turbine for energy recovery [0065] at least
a portion of the cold gas of step g) is recycled back to the air
separation unit [0066] the air separation unit supplies pressurized
gaseous oxygen product to an IGCC facility [0067] the IGCC facility
comprises a gas turbine further comprising the following steps:
[0068] a) extracting air from the gas turbine if the rate of
electricity is below a predetermined threshold; and [0069] b)
feeding above extracted air to the air separation unit [0070]
injecting pressurized cold gas to the gas turbine if the rate of
electricity is higher than a predetermined threshold [0071] the
refrigeration of vaporizing LNG is recovered to reduce the
liquefaction cost of the first liquid product [0072] reducing the
flow of compressed air in the heat exchanger if the rate of
electricity is above a predetermined threshold as compared to the
amount of air cooled in the heat exchanger if the rate of
electricity is below a predetermined threshold [0073] the cold gas
is removed from the air separation unit without warming it in the
heat exchange line [0074] the cold gas is removed from the air
separation unit after being warmed partially in the heat exchange
line [0075] the cold gas is removed from the air separation unit
after being cooled by traversing the warm end of the heat exchange
line only
[0076] According to the invention, there is also provided a low
temperature air separation process for producing pressurized
gaseous product in an air separation unit using a system of
distillation columns which comprises the following steps: [0077] i)
cooling a compressed air stream in a heat exchange line to form a
compressed cooled air stream; [0078] ii) sending at least part of
the compressed, cooled air stream to a column of the system; [0079]
iii) in a first period of time, liquefying a process stream to form
a first liquid product and storing at least part of the first
liquid product; [0080] iv) in a second period of time, sending the
above stored first liquid product to the air separation unit as one
of the feeds; [0081] v) pressurizing at least one second liquid
product stream; [0082] vi) vaporizing the above pressurized second
liquid product stream to form pressurized gaseous product in the
heat exchange line; and [0083] vii) during the above second period
of time, extracting a cold gas from the air separation unit and
compressing the cold gas in a compressor having an inlet
temperature between -180.degree. C. and -50.degree. C. and an
outlet temperature of at most -20.degree. C. to form a pressurized
gas.
[0084] According to further optional aspects of the invention:
[0085] the pressurized gaseous product is oxygen product [0086] the
pressurized gaseous product is nitrogen product [0087] the process
stream of step c) contains any proportion of oxygen, nitrogen and
argon [0088] the process stream of step c) is at least one of pure
nitrogen, air, oxygen containing at least 37 mol. % oxygen, oxygen
containing at least 65 mol. % oxygen, oxygen containing at least 85
mol.% oxygen and oxygen containing at least 99.5 mol. % [0089] the
cold gas of step g) is chosen from the group comprising a nitrogen
rich gas, pure nitrogen, air, a gas having a composition similar to
air, an oxygen rich gas and pure oxygen product [0090] step c) is
performed if the electricity rate is below a predetermined
threshold [0091] step c) is performed only if the electricity rate
is below a predetermined threshold [0092] step d) is performed if
the electricity rate is above a predetermined threshold [0093] step
d) is performed only if the electricity rate is above a
predetermined threshold [0094] step g) is performed if the
electricity rate is above a predetermined threshold [0095] step g)
is performed only if the of electricity rate is above a
predetermined threshold [0096] the cold gas is compressed to a
pressure between 35 and 80 bars abs in the compressor [0097] at
least a portion of the pressurized gas is heated and expanded in a
hot expander to recover energy [0098] at least a portion of the
pressurized gas is injected into a gas turbine for energy recovery
[0099] at least a portion of the pressurized gas is recycled back
to the column system of the air separation unit [0100] the air
separation unit supplies pressurized gaseous oxygen product to an
IGCC facility [0101] the IGCC facility comprises a gas turbine
further comprising the following steps: [0102] a) extracting air
from the gas turbine if the rate of electricity is below a
predetermined threshold; and [0103] b) feeding above extracted air
to the air separation unit [0104] the process comprises the step of
injecting the pressurized cold gas to the gas turbine if the rate
of electricity is higher than a predetermined threshold [0105] the
process comprises the steps of: [0106] a) warming the pressurized
gas in the heat exchange line; [0107] b) cooling additional gas in
the heat exchange line to form cold additional gas; and [0108] c)
cryogenically compressing cold additional gas to higher pressure
[0109] both gases are compressed to between 10 and 20 bars abs
[0110] the refrigeration of vaporizing LNG is recovered to reduce
the liquefaction cost of the first liquid product [0111] the
process comprises reducing the flow of compressed air in the heat
exchange line if the rate of electricity is above a predetermined
threshold as compared to the amount of air cooled in the heat
exchange line if the rate of electricity is below a predetermined
threshold [0112] the cold gas is removed from the air separation
unit cold box without warming it in the heat exchange line [0113]
the cold gas is removed from the air separation unit cold box after
being warmed partially in the heat exchange line [0114] the cold
gas is removed from the air separation unit cold box after being
cooled by traversing the warm end of the heat exchange line only
[0115] the process includes the step of warming the pressurized gas
in the heat exchange line [0116] the air separation unit is
contained within a cold box and the extracting a cold gas from the
cold box at a temperature between -195.degree. C. and -20.degree.
C.
[0117] According to a further aspect of the invention, there is
provided an air separation apparatus comprising: [0118] a) a system
of distillation columns; [0119] b) a heat exchange line; [0120] c)
a cold box containing at least the system of distillation columns
and the heat exchange line; [0121] d) a conduit for sending feed
air to the heat exchange line; [0122] e) a conduit for sending
cooled feed air from the heat exchange line to the column system;
[0123] f) means for sending a first liquid product to the column
system; [0124] g) a conduit for removing a liquid from a column of
the column system; [0125] h) a conduit for sending the liquid to
the heat exchange line; [0126] i) a conduit for removing vaporized
liquid from the heat exchange line; and [0127] j) a conduit for
extracting a gas from a column of the system and for removing the
gas from the air separation apparatus without warming the gas by
traversing the heat exchange line in its entirety.
[0128] Preferably, the conduit for extracting a gas is not
connected to a reboiler-condenser of the apparatus.
[0129] According to further optional aspects, the apparatus
comprises: [0130] means for storing the first liquid product
outside any column of the column system [0131] a gas compressor
connected to the conduit for extracting gas [0132] an air
compressor having an inlet and an outlet, the inlet of the air
compressor being connected to a compressed air conduit at an
intermediate point of the heat exchanger [0133] a gas turbine
having an expander and a conduit for sending gas compressed in the
cold gas compressor to a point upstream the expander [0134] a
conduit for removing the gas from the air separation apparatus
without warming the gas in the heat exchange line [0135] means for
liquefying a gas to form the first liquid product
[0136] The invention will now be described in greater detail with
reference to the figures. FIGS. 2 to 13 show air separation
processes according to the invention.
[0137] The invention is in particular suitable for the liquid
pumped air separation process.
[0138] The process has at least two modes of operation, one
corresponding to the periods when the rate of electricity is below
a predetermined threshold (FIG. 2) and one corresponding to periods
when the rate of electricity is above a predetermined threshold
(FIG. 2A).
[0139] When the rate of electricity is below a predetermined
threshold, the apparatus operates according to FIG. 2 as follows.
Atmospheric air is compressed by a Main Air Compressor (MAC) 1 to a
pressure of about 6 bar absolute, it is then purified in an
adsorber system 2 to remove impurities such as moisture and carbon
dioxide that can freeze at cryogenic temperature to yield a
purified feed air. A portion 3 of this purified feed air is then
cooled to near its dew point in heat exchanger 30 and is introduced
into a high pressure column 10 of a double column system in gaseous
form for distillation. Nitrogen rich liquid 4 is extracted at the
top of this high pressure column and a portion is sent to the top
of the low pressure column 11 as a reflux stream. The
oxygen-enriched liquid stream 5 at the bottom of the high pressure
column is also sent to the low pressure column as feed. The two
liquids 4 and 5 are subcooled before being expanded. An oxygen
liquid 6 is extracted from the bottom of the low pressure column
11, pressurized by pump to a required pressure then vaporized in
the exchanger 30 to form the gaseous oxygen product 7. Another
portion 8 of the purified feed air is further compressed in a
Booster Air Compressor (BAC) 20 to high pressure for condensation
in the exchanger 30 against the vaporizing oxygen enriched stream.
Depending upon the pressure of the oxygen rich product, the boosted
air pressure is typically about 65 to 80 bar for oxygen pressures
of about 40-50 bar or sometimes over 80 bar. As an indication, the
flow of stream 8 represents about 30-45% of the total flow of
compressor 1. The condensed boosted air 9 is also sent to the
column system as feed for the distillation, for example to the high
pressure column. Part of the liquid air (stream 62) may be removed
from the high pressure column and sent to the low pressure column.
It is also possible to extract nitrogen rich liquid from the top of
the high pressure column then pump it to high pressure (stream 13)
and vaporize it in the exchanger in the same way as with oxygen
liquid. A small portion of the feed air (stream 14) is further
compressed and expanded into the column 11 to provide the
refrigeration of the unit. Optionally alternative or additional
means of providing refrigeration may be used, such as Claude
expanders or nitrogen expanders.
[0140] Waste nitrogen or low pressure nitrogen is removed from the
top of the low pressure column and all of the stream warms in
exchanger 30.
[0141] Argon is optionally produced using a standard argon column
whose top condenser is cooled with oxygen enriched liquid 5.
[0142] Nitrogen gas can be compressed to high pressure as needed by
compressors 45, 46 to yield a nitrogen product stream 48.
[0143] During this period when the rate of electricity is below a
predetermined threshold, air is liquefied by any means described in
FIGS. 3 to 5. For example, in FIG. 2, gaseous compressed air free
of moisture and CO2 (stream 47) is taken after the adsorber 2 and
sent to an external liquefier 60 to produce a liquid air stream 49.
This liquid air is stored in tank 50. Preferably no liquid air is
sent from the storage tank 50 to the column during this period.
[0144] When the rate of electricity is above the predetermined
threshold, the apparatus operates according to FIG. 2A as
follows:
[0145] Liquid air flows from the storage tank 50 to the high
pressure column 10 via conduit 60 connected to conduit 9 and to the
low pressure column 11 via conduit 61. Preferably liquefaction of
air in the liquefier does not take place during these periods.
[0146] When sending liquid air from the tank 50 to the column
system, the flow of the Main Air compressor 1 can be reduced by an
amount essentially equal to the amount of liquid air so that the
overall balance in oxygen of the feeds of the unit can be
preserved. As indicated above, the flow 14 of the expander 44 is
rather small and can be optionally eliminated and flow of
compressor 1 will be adjusted accordingly. The lost refrigeration
work resulted from the omission of the expander can be easily
compensated by the amount of the above liquid air. Therefore by
replacing the flow of stream 8 with a liquid air flow via 60, the
compressor 20 can be stopped and the flow of compressor 1 can be
reduced by 20-55%. These reductions result in a sharp drop in the
power consumption of the unit. Since the flow of various streams
feeding the column system remains similar, the distillation
operation will be undisturbed by those changes and the product
purities will not suffer. However, by feeding an important amount
of liquid air and by eliminating the boosted air portion 9 and
reducing the flow of compressor 1, the main exchanger 30 becomes
unbalanced in terms of ingoing and outgoing flows and
refrigeration. In order to restore the flow and refrigeration
balances, an outgoing cold gas flow at cryogenic temperature must
be extracted from the system. FIG. 2A illustrates a possible
arrangement of such operation in which part 40 of the waste
nitrogen from the low pressure column is removed from the system
without being warmed in the exchanger 30 or any other exchanger.
The stream 40 is optionally compressed in a compressor 70 whose
inlet is at a cryogenic temperature. The cold gas stream can be any
cold gas with suitable flow and temperature including gaseous
oxygen product at the bottom of the low pressure column 11. The
cold gas temperature leaving the cold box is from about
-195.degree. C. to about -20.degree. C., preferably between
-180.degree. C. and -50.degree. C. The main exchanger 30, and other
cryogenic heat exchangers such as subcoolers, constitute a heat
exchange system or sometimes called heat exchange line of an air
separation unit. This heat exchange line promotes heat transfer
between the incoming feed gases and the outgoing gaseous products
to cool the feed gases to near their dew points before feeding the
columns, and to warm the gaseous products to ambient
temperature.
[0147] The power needed to liquefy air is generally very high and
normally one cannot justify economically the use of liquid air to
replace the boosted air stream as described above. However, since
there exists a large difference in power rate between peak and
off-peak periods as explained earlier, it is conceivable to perform
the energy-intensive step of air liquefaction during the periods
when power rate is low, for example at night, such that the cost
incurred by this liquefaction step is not excessive. Therefore it
becomes clear that, during the peak periods, one can use this
liquid produced earlier inexpensively to feed the system and reduce
the flows or power consumed by the unit. Such maneuver sharply
reduces the power consumption of the unit. Consequently, the
expense of paying the high price of power during peak periods can
be minimized. In essence, this new invention allows producing the
molecules of gases needed for the distillation during low power
rate periods and then efficiently use those molecules during the
high power rate periods to achieve the overall cost savings.
[0148] The cold gas extracted from the system during peak time can
be compressed economically at low temperature to higher pressure.
The power consumed by this cold compression is low compared to a
warm compression performed at ambient temperature. Indeed, the
power consumed by a compressor wheel is directly proportional to
its inlet absolute temperature. A compressor wheel admitting at
100K would consume about 1/3 the power of a compressor wheel
admitting at ambient temperature of 300K. Therefore, by utilizing
cold compression, one can further improve the energy value of a gas
by raising its pressure at the expense of relatively low power
requirement. It is clear that the cold gas extracted from the
process, instead of subjecting it to a cold compression process,
can be used for other purposes, for example to chill another
process, to chill another gas, etc. Depending upon the
applications, instead of cold compressing the cold gas directly, it
is possible to warm the cold gas slightly by some other external
recovery heat exchangers to another temperature, still cryogenic
(less than -50.degree. C.) then compress it by cold compressor.
[0149] It is useful to note that traditional air separation units
also constantly discharge into the atmosphere small cold streams
such as non-condensible purge of condensers or liquid purge of
vessels or columns. These purge streams are usually very small in
flow, usually less than 0.2% of the total air feed. Unless there is
a rare gas recovery unit (Neon, Krypton, Xenon, etc.) that can
utilize those purge streams as feeds, they are rejected without any
cold recovery since their flow range is too small. Meanwhile, the
recovered cold gas of this invention is much larger in flow: its
minimum flow rate is at about 4% of the minimum gaseous air feed to
the system and can be as much as 70% of total air feed rate.
[0150] The liquefaction of air in the off-peak periods can be
conducted in another cryogenic plant, using different equipment as
illustrated in FIG. 3. Here air is compressed in compressor 100
sent to a liquefier 200 and then to storage tank 50. The liquid air
is sent from the storage tank 50 to an ASU as described in FIG. 2A
during peak periods, the storage tank being in this case outside
the cold box.
[0151] The liquefaction can also be performed by using an
independent liquefier attached to the air separation unit as
illustrated in FIG. 4 where air from main air compressor 1 is
divided, one part being sent to the liquefier 200 and the rest to
the ASU. Air from the liquefier is then sent to the storage tank 50
and thence back to the ASU during peak periods.
[0152] Alternatively the liquid air can be produced within the ASU,
using the same equipment as in the cases of integrated liquefier as
described in FIG. 5. FIG. 6 illustrates the liquid feed mode during
peak periods.
[0153] The liquid storage tank can be a vessel located externally
to the cold box or a vessel located inside the cold box. It is also
possible to use an oversized bottom of a distillation column as
liquid storage tank, in this case, the stored liquid has similar
composition as the liquid being produced at the bottom of the
vessel. The liquid level is allowed to rise at the bottom of the
column or vessel during the filling.
[0154] Some additional operating conditions of various process
parameters related to the invention will now be described: [0155]
The quantity of liquid air to be produced in off-peak time depends
upon the relative length of the off-peak duration over the length
of the peak duration. The shorter the off-peak time, the higher is
the required liquefaction rate and vice-versa. In the peak mode,
the liquid air feed rate can be about 20-30% of the total air feed
under normal conditions. [0156] FIG. 12 can be used to provide a
general guideline for extracting cold gas from the process when a
liquid 30 is fed to the system during peak periods: as shown, the
column system 71 is connected to the exchanger line 65, liquid
products 15, 16 are delivered by pumps 20, 21 to exchanger 65 for
vaporization. The total of all pressurized liquid product
vaporizing in the exchanger 65 is called the Total Vaporized
Liquid. Pressurized gases 31, 32 are cooled and condensed in
exchanger 65 against vaporizing products 15, 16 to yield liquid
feeds 25, 26 which are then expanded into the column system 71. The
total flow of all condensed pressurized streams is called the Total
Incoming Liquid. Cold gas 11 can be extracted from the system
according to the following guideline: its flow is about 1.6 to 2.6
times the Total Vaporized Liquid minus the Total Incoming Liquid:
Flow of cold gas=k [Total Vaporized Liquid-Total Incoming liquid],
with k=1.6 to 2.6 [0157] It is also possible to extract liquid
product (oxygen, nitrogen or argon) or a combination of those
liquid products along with the cold gas described above by
increasing the amount of liquid air feed, therefore supplying the
needed refrigeration for the production of liquid product or
products.
Additional Embodiments
[0157] [0158] 1. The cold compression of the cold gas can be
performed in a single step as illustrated above in FIG. 2A. When
the final pressure of the compressed cold gas is relatively low,
i.e. the compressed gas temperature remains at a low level then it
is possible to increase the compressed gas flow, as illustrated in
FIG. 7, by cooling additional air 85 from the Main air compressor 1
(or nitrogen gas) with the compressed cold gas from the cold
compressor 70 in exchange line 30 and then compressing the
additional gas to higher pressure in cold compressor 75. The two
cold compressed streams are then mixed upstream of the heat
exchange line 30 to form stream 95. This exchanger can be combined
with the main exchanger 30 of FIG. 2A. FIG. 8 also describes this
embodiment. [0159] FIG. 8 shows an ASU based on that of FIG. 2A in
which cold low pressure nitrogen 40 is compressed to between 10 and
20 bar abs., preferably 15 bar abs. The gas compressed in cold
compressor 70 is warmed at the warm end only of the heat exchanger
30. Part of the feed air compressed in main air compressor 1 is
purified, cooled in the exchanger 30 to an intermediate temperature
and then compressed in cold compressor 75 to the same pressure as
that at the outlet of cold compressor 70. The two streams
compressed in the cold compressors 70, 75 are then mixed and sent
for example to the combustion chamber of a gas turbine where the
mixed stream is heated then expanded in a turbine for power
recovery. [0160] 2. Another embodiment is described in FIG. 9, the
pressurized cold gas after a cold compression in cold compressor 70
can be heated and sent to a hot expander 110 for power recovery or
power production. This power being produced during peak time can be
very valuable and can be export to generate additional revenue. The
nitrogen from cold compressor 70 is warmed in exchanger 80 and
further warmed by heater 90 before being expanded in expander 110.
The exhaust gas from expander 110 is sent to exchanger 80 and used
to warm the cold compressed nitrogen. [0161] 3. FIG. 10 illustrates
the application where the compressed cold gas is sent to a gas
turbine for power recovery. Here the nitrogen from cold compressor
70 is sent to the combustion chamber 150 of the gas turbine, after
being mixed with air from gas turbine compressor 120. Fuel 140 is
also sent to the combustion chamber and the exhaust gas is expanded
by expander 130 to form gas 160. A compression arrangement similar
to the one illustrated in FIG. 8 or 9 using two compressors and
mixing cold compressed air with cold compressed nitrogen could also
be used in this application. [0162] 4. This invention may be used
to improve the economics of IGCC application. Indeed, the IGCC
(integrated gasification combined cycle) process is based upon the
concept of gasifying coal, petroleum coke, etc., using oxygen gas
to produce synthetic gas (syngas) which is then burned in a gas
turbine to generate power. A steam generation sub-system is added
to form a combined cycle for additional power generation. Since the
power demand from the IGCC usually fluctuates widely between day
and night, and the gasifier is not very flexible in terms of
throughput variations so that it is problematic to have a stable
operating mode. Furthermore the equipment is poorly utilized during
off-peak time. The problem is further compounded by the fact that
at night, with lower ambient temperature, the compressor of the gas
turbine can generate more flow to the turbine system. However, the
latter because of lower demand cannot utilize this additional
capacity. In a similar fashion, in the daytime, when the ambient
temperature is higher, the compressor of the gas turbine sees its
flow reduced and this, during the time where additional power
generation is desirable. By incorporating the features of this new
invention to an IGCC plant we can improve significantly the
performance of the unit thanks to the synergy of the air separation
plant and the IGCC: [0163] At night, as shown in FIG. 11, when the
power demand is low and higher compressor flow is available, air
from the compressor 120 of the gas turbine can be diverted to the
air separation plant to provide at least part of the flow and power
for the liquefaction of air. An elevated pressure ASU could also be
used advantageously since it can use the elevated pressure air from
the gas turbine directly. By taking more flow and consuming more
power, hence more syngas for the gas turbine, to liquefy the air
during off-peak time, the IGCC portion can be kept relatively
constant during the night time. In FIG. 11, block 170 represents
the gasifier and block 180 represents the synthetic gas/fuel
treatment, filtration, compression, etc. [0164] In the daytime, the
capacity of the air compressor 120 of the gas turbine is reduced
due to warmer ambient temperature. The air extraction of the night
mode can be stopped. The liquid air produced at night and sent to
storage 50 can then be used in the Air separation plant and its
power consumption is reduced, so that more power can therefore be
diverted to supply the high demand of the daytime. Furthermore, the
cold gas extracted from the ASU can be compressed economically in
cold compressor 70 to higher pressure for injection into the gas
turbine and to balance out the flow deficiency, thereby generating
even more power. [0165] For applications involving injecting
compressed gas into combustion turbine or gas turbine, the cold
compression arrangements of FIGS. 7 and 8 are well adapted: the
pressure requirement for the injected gas is about 15-20 bar which
is exactly the range of pressure called for by the process of those
figures, and by mixing the cold compressed air stream with the cold
compressed nitrogen rich gas as shown, one can assure a good supply
of oxygen required for the combustion process. [0166] 5. This
invention may be used advantageously as a distillation and
efficiency enhancement of an air separation unit. An embodiment of
this feature is illustrated in FIG. 13, which describes an
operating mode of the air separation unit when the power peaks
occur. Liquid air 30 produced during off-peak periods is fed to the
column system. Cold gas extracted from the top of the distillation
column is cold compressed to higher pressure as stream 13. A
portion of this higher pressure gas (stream 14) is recycled back to
the main exchanger 65 wherein it is liquefied to form a liquid
stream 15 and fed to the column system. This recycle and
liquefaction improves the vaporization of compressed liquid stream
23 in the main exchanger 65 and some flow reduction of liquid feed
30 can be achieved. Also, the presence of this liquid stream 15 at
the cold end of exchanger 65 would balance the cold end portion of
the plant, and prevent the liquefaction of stream 2 which could be
detrimental to the heat transfer in exchanger 65 and could cause
distillation problems in the column 30. If needed, a portion of the
compressed gas (stream 12) can also be cooled and recycled to the
top of the high pressure column to enhance the distillation of the
column system following cooling in heat exchange line 30 to form
stream 16. During off-peak periods, the air separation plant
operates according to the process described in FIG. 2 (for the
clarity of the drawing, the expanders and compressors of the
off-peak mode are not shown). The process of FIG. 2 is a typical
one for pumped liquid air separation plants, it is obvious to a
person skilled in the art that other liquid pumped processes such
as cold booster process or single Claude expander liquid pumped
process, etc., can also be utilized for the off-peak mode as well.
The liquid air needed for the peak periods could be produced by an
external liquefier as shown in FIG. 2. Of course, as mentioned
previously, an integrated liquefier can be implemented as well.
[0167] 6. An additional emoodiment may be used in cold recovery
from LNG vaporization. Cryogenic plants have been used to recover
the cold released from the vaporization of LNG in peak-shaving or
vaporization terminal LNG plants. This refrigeration is used to
lower the cost of producing liquid products in Air Separation
plants. With this invention, the refrigeration of vaporized LNG can
be used to lower the liquefaction cost of liquid air in off-peak
periods; which therefore, results in more cost savings when the
liquid is fed back to the ASU in peak periods as described in this
concept.
[0168] The above embodiments describe the use of liquid air as the
intermediate liquid to transfer the refrigeration and gas molecules
between the peak and off-peak periods. It is obvious to someone
skilled in the art that any liquid with various compositions of air
components can be used to apply this technique. For example, the
liquid can be an oxygen rich liquid extracted at the bottom of the
high pressure column containing about 35 to 42 mol. % oxygen or a
liquid extracted near the bottom of the low pressure column with
70-97 mol. % oxygen content, or even pure oxygen product. The
liquid can also be a nitrogen rich stream with little oxygen
content. It is useful to note when this nitrogen rich liquid stream
containing almost no oxygen is fed back to the air separation unit
during peak periods, the air feed flow will not be reduced but must
be maintained constant to satisfy the supply of oxygen molecules.
In this situation the power saving can be achieved for example by
shutting down the nitrogen product compressors (compressors 45, 46
of FIG. 2) and supplying the nitrogen product by cold compressors
that consume significantly less power. In another word, the concept
is applicable to an intermediate liquid of any composition of air
components.
[0169] The invention is developed for constant product demand under
variable power rate structure. It is clear that the invention can
be extended to a system with variable product demand as well. For
example, during periods with low demand in oxygen, one can apply
the concept by feeding liquid air to the system and reducing the
feed air flow. The unused oxygen can be stored as a liquid oxygen
product such that the distillation columns can be kept unchanged.
This liquid oxygen can be fed back to the system when the demand of
oxygen is high. By adjusting the flow of liquid air feed, oxygen
liquid, cold gas extraction and gaseous air feed, or another liquid
like liquid nitrogen, one can provide an optimum process satisfying
both variable product demand and variable power rate
constraints.
[0170] Although the invention has been described with reference to
certain preferred embodiments, those skilled in the art will
recognize that there are other embodiments of the invention within
the spirit and scope of the claims. Thus, the present invention is
not intended to be limited to the specific embodiments in the
examples given above.
* * * * *