Title: Process and apparatus for cryogenic separation of gases
Abstract: A back-up quantity of a "first" gas is supplied temporarily to maintain the level of production of the first gas from a cryogenic separation of a gaseous mixture comprising the first gas and at least one other gas in the event of reduction in the level of production of said first gas from the separation. In the event of reduction in the level of production of said first gas from the separation, liquefied first gas inventory is withdrawn from the or at least one of said cryogenic distillation systems and vaporised to produce said back-up quantity of first gas. The invention has particular application to the production of gaseous oxygen ("GOX") from the separation of air.
Patent Number: 6,889,524 Issued on 05/10/2005 to O'Connor, et al.
| Inventors:
|
O'Connor; Declan P. (Chessington, GB);
Andrew; Rebecca J. (Thames Ditton, GB);
Suggitt; Christopher (Woking, GB);
Higginbotham; Paul (Guildford, GB)
|
| Assignee:
|
Air Products and Chemicals, Inc. (Allentown, PA)
|
| Appl. No.:
|
630609 |
| Filed:
|
July 30, 2003 |
| Current U.S. Class: |
62/656; 62/50.2; 62/654 |
| Intern'l Class: |
F25J 003/00 |
| Field of Search: |
62/656,654,648,643,657,502
|
References Cited [Referenced By]
U.S. Patent Documents
| 3059439 | Oct., 1962 | Rice et al.
| |
| 5082482 | Jan., 1992 | Darredeau.
| |
| 5505052 | Apr., 1996 | Ekins et al.
| |
| 5526647 | Jun., 1996 | Grenier.
| |
| 5566556 | Oct., 1996 | Ekins et al.
| |
| 5941098 | Aug., 1999 | Guillard et al.
| |
| 6038885 | Mar., 2000 | Corduan et al.
| |
| 6062044 | May., 2000 | Bernard et al.
| |
| 6134916 | Oct., 2000 | Jahnke.
| |
| 6272884 | Aug., 2001 | Billingham et al.
| |
| 6357259 | Mar., 2002 | Higginbotham et al.
| |
| Foreign Patent Documents |
| 0 556 861 | Aug., 1993 | EP.
| |
| WO 9940304 | Aug., 1999 | EP.
| |
| 9940304 | Dec., 1999 | WO.
| |
Primary Examiner: Doerrler; William C.
Attorney, Agent or Firm: Jones, II; Willard
Claims
1. A process for the temporary supply of a back-up quantity of a "first" gas,
during the time taken for a vaporizer in a main back-up system to come fully on-line,
maintain the level of production of the first gas from a cryogenic separation of
a gaseous mixture comprising the first gas and at least one other gas in the event
of reduction in the level of production of said first gas from the separation,
said separation comprising:
separating the mixture, or a mixture derived therefrom, in at least one cryogenic
distillation system to produce liquefied first gas, the or each system retaining
a portion of said liquefied first gas as inventory; and
vaporizing further portion of said liquefied first gas by indirect heat exchange
against a process stream in at least one heat exchanger to produce said first gas;
said process comprising, in the event of reduction in the level of production
of said first gas from the separation, withdrawing liquefied first gas inventory
from the or at least one of said cryogenic distillation systems and vaporizing
the withdrawn liquefied first gas inventory to produce said back-up quantity of
first gas,
wherein at least a portion of the vaporization duty required to vaporize said
withdrawn liquefied first gas inventory is provided by heat inventory from the
or at least one of said heat exchangers.
2. The process according to claim 1 wherein the process operates when the or
at least one of the cryogenic distillation systems ceases to produce liquefied
first gas.
3. The process according to claim 1 wherein there is one cryogenic distillation
system and said system ceases to produce liquefied first gas, said process comprising
withdrawing liquefied first gas inventory from said cryogenic distillation system
and vaporizing the withdrawn liquefied first gas inventory to produce said back-up
quantity of first gas.
4. The process according to claim 1 wherein there is more than one cryogenic
distillation system and one of said cryogenic distillation systems ceases to produce
liquefied first gas, said process comprising withdrawing liquefied first gas inventory
from the cryogenic distillation system in which liquefied first gas production
has ceased and vaporizing the withdrawn liquefied first gas inventory to produce
said back-up quantity of first gas.
5. The process according to claim 1 wherein there is more than one cryogenic
distillation system and one of said cryogenic distillation systems ceases to produce
liquefied first gas, said process comprising withdrawing liquefied first gas inventory
from the or each cryogenic distillation system in which liquefied first gas production
has not ceased and vaporizing the withdrawn liquefied first gas inventory to produce
said back-up quantity of first gas.
6. The process according to claim 5 wherein, for each cryogenic distillation
system, said separation further comprises:
compressing said mixture to produce compressed mixture;
dividing said compressed mixture or a mixture derived therefrom into at least
two portions;
cooling a first portion by indirect heat exchange in a heat exchanger and feeding
the resultant cooled first portion to the cryogenic distillation system for separation;
further compressing a second portion in a booster compressor to produce further
compressed mixture; and
cooling and condensing said further compressed mixture by indirect heat exchange
in the or a further heat exchanger and feeding the resultant cooled and condensed
further compressed mixture to the cryogenic distillation system for separation,
said process further comprising, in the event of one of said cryogenic distillation
systems ceasing to produce liquefied first gas, increasing the flow of the second
portion through the booster compressor of the or each remaining cryogenic distillation
system such that the resultant increased flow of further compressed mixture through
said the or further heat exchanger of the or each remaining cryogenic distillation
system provides a portion of the vaporization duty required to vaporize said withdrawn
liquefied first gas inventory to provide said back-up quantity of first gas.
7. The process according to claim 1 wherein the process is initiated automatically
when the or at least one cryogenic distillation system ceases to produce liquefied
first gas.
8. The process according to claim 1 wherein liquefied first gas is stored for
vaporization in at least one vaporizer to produce back-up first gas in the event
of reduction in the level of production of said first gas from the separation,
said process operating only during the period of time required for the or each
vaporizer to come on-line.
9. The process according to claim 1 wherein the first gas is produced in more
than one cryogenic distillation system and is supplied to more than one downstream
processing unit, said process being operated only during the period of time required
to turndown or shutdown one of the downstream processing units in the event that
one of the distillation systems ceases to produce liquefied first gas.
10. The process according to claim 1 wherein the gaseous mixture is air and the
first gas is one of oxygen, nitrogen or argon.
11. The process according to claim 10 wherein the gaseous mixture is air and
the first gas is oxygen.
12. A process for the temporary supply of a back-up quantity of a "first" gas,
during the time taken for a vaporizer in a main back-up system to come fully on-line,
to maintain the level of production of the first gas from a cryogenic separation
of a gaseous mixture comprising the first gas and at least one other gas in the
event of reduction in the level of production of said first gas from the separation,
said separation comprising:
separating the mixture, or a mixture derived therefrom, in one cryogenic distillation
system to produce liquefied first gas, the cryogenic distillation system retaining
a portion of said liquefied first gas as inventory; and
vaporizing further portion of said liquefied first gas by indirect heat exchange
against a process stream in at least one heat exchanger to produce said first gas;
said process comprising, in the event of reduction in the level of production
of said first gas from the separation due to said cryogenic distillation system
ceasing to produce liquefied first gas, withdrawing liquefied first gas inventory
from the cryogenic distillation system and vaporizing the withdrawn liquefied first
gas, inventory to produce said back-up quantity of forst gas,
wherein at least a portion of the vaporization duty required to vaporize said
withdrawn liquefied first gas inventory is provided by heat inventory from the
or at least one of said heat exchangers.
13. A process for the temporary supply of a back-up quantity of a "first" gas,
during the time taken for a vaporizer in a main back-up system to come fully on-line,
to maintain the level of production of the first gas from a cryogenic separation
of a gaseous mixture comprising the first gas and at least one other gas in the
event of reduction in the level of production of said first gas from the separation,
said separation comprising:
separating the mixture, or a mixture derived therefrom, in more than one cryogenic
distillation system to produce liquefied first gas, each system retaining a portion
of said liquefied first gas as inventory; and
vaporizing further portion of said liquefied first gas by indirect heat exchange
against a process stream in at least one heat exchanger to produce said first gas;
said process comprising, in the event of reduction in the level of production
of said first gas from the separation due to one of said cryogenic distillation
systems ceasing to produce liquefied first gas, withdrawing liquefied first gas
inventory from the cryogenic distillation system in which liquefied first gas production
has ceased and vaporizing the withdrawn liquefied first gas inventory to produce
said back-up quantity of first gas,
wherein at least a portion of the vaporization duty required to vaporize said
withdrawn liquefied first gas inventory is provided by heat inventory from the
or at least one of said heat exchangers.
14. A process for the temporary supply of a back-up quantity of a "first" gas,
during the time taken for a vaporizer in a main back-up system to come fully on-line,
to maintain the level of production of the first gas from a cryogenic separation
of a gaseous mixture comprising the first gas and at least one other gas in the
event of reduction in the level of production of said first gas from the separation,
said separation comprising:
separating the mixture, or a mixture derived therefrom, in more than one cryogenic
distillation system to produce liquefied first gas, each system retaining a portion
of said liquefied first gas as inventory; and
vaporizing further portion of said liquefied first gas by indirect heat exchange
against a process stream in at least one heat exchanger to produce said first gas;
said process comprising, in the event of reduction in the level of production
of said first gas from the separation due to one of said cryogenic distillation
systems ceasing to produce liquefied first gas, withdrawing liquefied first gas
inventory from the or each one of said cryogenic distillation systems in which
liquefied first gas production has not ceased and vaporizing the withdrawn liquefied
first gas inventory to produce said back-up quantity of first gas,
wherein at least a portion of the vaporization duty required to vaporize said
withdrawn liquefied first gas inventory is provided by heat inventory from the
or at least one of said heat exchangers.
15. The process according to claim 14 wherein the gaseous mixture is air and
the first gas is oxygen.
Description
BACKGROUND OF THE INVENTION
The present invention relates to cryogenic separation of gases and, in particular,
to a process and apparatus for the temporary supply of a back-up quantity of a
"first" gas to maintain the level of production of the first gas from a cryogenic
separation of a gaseous mixture comprising the first gas and at least one other
gas in the event of reduction in the level of production of said first gas from
the separation. The invention has particular application to the production of gaseous
oxygen ("GOX") from the cryogenic separation of air.
GOX may be produced in a cryogenic air separation unit ("ASU"). Such an ASU may
be integrated with a downstream process that utilises the GOX in some way. For
example, the GOX may be used in the production of synthesis gas ("syngas") which
is a mixture of hydrogen and carbon monoxide and which may be used in the preparation
of higher molecular weight hydrocarbon compounds and/or oxygenates. A suitable
example of a process to produce hydrocarbons would be the Fischer-Tropsch process.
More than one ASU may be linked in parallel to produce GOX for the downstream process.
Some downstream processes, e.g. syngas production, gasification processes and
ethylene oxide production, require a substantially constant level of production
of GOX, that is the pressure or flow of the GOX must be maintained to within a
narrow range. These processes are often referred to as "oxygen-critical processes".
Thus, back-up systems must be in place to ensure the constant supply of GOX in
the event of a reduction in the pressure or flow of the GOX product for whatever
reason. In this connection, the pressure or flow of the GOX product may decrease
because a component of the ASU fails suddenly. For example, the main air compressor,
a booster air compressor (if present), an air pre-purifier, a liquid oxygen ("LOX")
pump or a valve may fail.
It is well known to provide back-up GOX from a storage reservoir of high pressure
("HP") LOX. In the event the pressure or flow of the GOX product drops below a
certain level, LOX may be taken from the reservoir and vaporised in a vaporiser
to produce back-up GOX at the required customer pressure. It is also well known
to provide back-up GOX from a storage reservoir of low pressure ("LP") LOX. In
the event the pressure or flow of the GOX product drops below a certain level,
LOX may be taken from the LP reservoir pumped to the desired pressure by one or
more back-up LOX pumps and vaporised in a vaporiser to produce back-up GOX.
The back-up system is brought on line on receipt of a trigger signal, such as
low product supply pressure. In the case of such a HP liquid back-up system, the
trigger signal causes a vaporiser oxygen control valve to open. For the LP liquid
back-up system, the trigger signal would also bring the, or each, back-up LOX pump
to its design operating point. However, vaporisers cannot instantly attain their
design vaporisation capacities when called upon to operate. The time taken to achieve
that capacity depends on the type of vaporiser installed. Generally, ambient vaporisers
have better response times than steam sparged water bath vaporisers due to relative
inventories and unit masses. For example, a steam sparged water bath vaporiser
must be kept warm so that it is ready for instantaneous use. Unfortunately, it
is simply not possible initially to force LOX through the warm vaporiser at the
design rate as the oxygen pressure drop through the vaporiser would be too high
at warm standby conditions. The vaporiser needs time to cool down to a point where
LOX may be vaporised at the necessary rate. This period of time may be up to 30
seconds within which time the oxygen-critical process may have been affected by
the reduction in pressure or flow of GOX thereto.
It is well known to have a GOX buffer vessel in communication with the GOX output
from the ASU(s) so that the GOX inventory of the line may be maintained high enough
so that no unacceptable drop in line pressure occurs during the time taken for
the vaporiser in the back-up system to come fully on-line. Such a buffer vessel
may be at line pressure or may be pressurised, in which case a valve would have
to used to reduce the pressure of the pressurised GOX before it would be released
into the GOX product line. One drawback of using the buffer vessel is the capital
cost involved.
WO-A-99/40304 (published on 12 Aug. 1999) comprises a combined cryogenic
air separation unit/integrated gasifier combined cycle power generation system
and describes a method for operating the ASU to vary its power consumption to maximise
net power production during peak demand periods while maintaining peak efficiency
when the power generation system operates at varying power production. The oxygen
production rate is maintained at a stable optimum level throughout the day and
is not subject to significant fluctuations during changes in power plant operating
conditions. Referring to FIG. 1 of WO-A-99/40304, in periods of off-peak power
demand, excess liquid oxygen generated by the ASU may be stored in the bottom of
the low pressure distillation column
6 or transferred through line
13
to vessel
21 where it is stored until such time as it is needed during periods
of high power demand in the integrated gasifier combined cycle system.
U.S. Pat. No. 6,062,044 (published on 16 May 2000) discloses the use of a liquid
oxygen storage tank to store excess liquid oxygen which can be used to satisfy
increases in oxygen demand.
It is an objective of the present invention to provide an alternative system
for
providing a back-up quantity of a first gas without having to use one or more expensive
buffer vessels or at least to allow the capacity of such buffer volume to be substantially
reduced. There is always an "inventory" (or store) of liquefied first gas in the
cryogenic separation system, usually in the sump of a distillation column. The
size of the inventory will depend on the size of the cryogenic distillation system
but there is usually more than enough liquefied first gas stored in the distillation
system itself to satisfy demand for the first gas during the time taken for the
vaporiser in the main back-up system to fully come on-line. The inventors have
devised a way of using this source of liquefied first gas to produce a back-up
quantity of first gas and maintain the level of production of the first gas.
BRIEF SUMMARY OF THE INVENTION
According to the present invention, there is provided a process for the
temporary supply of a back-up quantity of a "first" gas to maintain the level of
production of the first gas from a cryogenic separation of a gaseous mixture comprising
the first gas and at least one other gas in the event of reduction in the level
of production of said first gas from the separation, said separation comprising:
separating the mixture, or a mixture derived therefrom, in at least one
cryogenic distillation system to produce liquefied first gas, the or each system
retaining a portion of said liquefied first gas as inventory; and
vaporising a further portion of said liquefied first gas by indirect heat
exchange against a process stream in at least one heat exchanger to produce said
first gas; said process comprising, in the event of reduction in the level of production
of said first gas from the separation, withdrawing liquefied first gas inventory
from the or at least one of said cryogenic distillation systems and vaporising
the withdrawn liquefied first gas inventory to produce said back-up quantity of
first gas.
The inventory is initially withdrawn at a high enough rate to meet an acceptable
level of demand for the first gas; preferably at substantially the same rate at
which liquefied first gas is withdrawn when the distillation system is operational.
However, over the period of backup, the rate usually will continuously decrease.
One advantage of the invention is that expensive buffer vessels are either no
longer required or can be substantially reduced in volume, thereby enabling a significant
saving to be made to the overall capital expenditure for such processes.
The process operates usually when the or at least one of the cryogenic distillation
systems ceases to produce liquefied first gas (or "trips") but the process may
be applied in other circumstances, for example if a leak develops in one of the
process lines.
At least a portion of the vaporisation duty required to vaporise the withdrawn
liquefied first gas inventory is preferably provided by heat inventory, i.e. stored
heat, from the or at least one of the heat exchangers. There is a temperature gradient
between the "warm" end and the "cold" end of the or each heat exchanger. Heat stored
in the metal of a heat exchanger may be used to vaporise liquefied first gas inventory.
It is clearly not desirable for the heat exchanger to cool down to such an extent
that excessively cold first gas leaves the heat exchanger. However, the Inventors
have calculated that there is more than enough heat in the metal of the heat exchanger
to vaporise the withdrawn liquefied first gas inventory for the period of time
necessary for the vaporiser to come fully on-line.
In an embodiment of the process involving one cryogenic distillation system which
ceases to produce liquefied first gas, the process comprises withdrawing liquefied
first gas inventory from the cryogenic distillation system and vaporising the withdrawn
liquefied first gas inventory to produce said back-up quantity of first gas.
In another embodiment of the process involving more than one cryogenic distillation
system and one of the cryogenic distillation systems ceases to produce liquefied
first gas, the process comprises withdrawing liquefied first gas inventory from
the cryogenic distillation system in which liquefied first gas production has ceased
and vaporising the withdrawn liquefied first gas inventory to produce the back-up
quantity of first gas.
In an alternative, and presently preferred, arrangement of the embodiment involving
more than one cryogenic distillation system and one of the cryogenic distillation
systems ceases to produce liquefied first gas, the process comprises withdrawing
liquefied first gas inventory from the or each cryogenic distillation system in
which liquefied first gas production has not ceased and vaporising the withdrawn
liquefied first gas inventory to produce said back-up quantity of first gas. The
rate at which the liquefied first gas is withdrawn from the remaining (operational)
distillation systems is increased to accommodate the lack of contribution to the
first gas product stream from the failed distillation system. For example, in an
embodiment having two cryogenic distillation systems in parallel, one of which
fails, the remaining operational distillation system would produce first gas at
up to 100% over the normal operational rate, usually only for the short period
of time until the vaporiser of the back-up system comes fully on-line. In an embodiment
having three cryogenic distillation systems in parallel, one of which fails, the
remaining operational distillation systems would usually each produce first gas
at up to 50% over the normal operational rate for one distillation system. Again,
the increase in rate would usually only be for the short period of time until the
vaporiser of the back-up system comes fully on-line.
In this alternative arrangement, for each cryogenic distillation system, the
separation
may further comprise:
compressing said mixture to produce compressed mixture;
dividing said compressed mixture or a mixture derived therefrom into at
least two portions;
cooling a first portion of said compressed mixture by indirect heat exchange
in a heat exchanger and feeding the resultant cooled first portion to the cryogenic
distillation system for separation;
further compressing a second portion of said compressed mixture in a booster
compressor to produce further compressed mixture; and
cooling and condensing said further compressed mixture by indirect heat exchange
in the, or a further, heat exchanger and feeding the resultant cooled and condensed
further compressed mixture to the cryogenic distillation system for separation.
In such an embodiment, the booster compressor may well operate at below its maximum
operational rate. In such circumstances, the process may further comprise, in the
event of one of the cryogenic distillation systems ceasing to produce liquefied
first gas, increasing the flow of the second portion through the booster compressor
of the, or each, remaining cryogenic distillation system such that the resultant
increased flow of further compressed mixture through said the, or further, heat
exchanger of the, or each, remaining cryogenic distillation system provides a portion
of the vaporisation duty required to vaporise the withdrawn liquefied first gas
inventory to provide said back-up quantity of first gas.
Preferably, the process is initiated automatically when the or at least
one cryogenic distillation system ceases to produce liquefied first gas. In this
way, the time taken for the process to be up and running is likely to be significantly
less that if the process were to be initiated manually although it is to be understood
that such manual initiation is also within the scope of the present invention.
In preferred embodiments, there is a back-up quantity of liquefied first gas
stored
ready for vaporisation in at least one vaporiser to produce first gas in the event
of reduction in the level of production of said first gas from the separation.
In such embodiments, the process operates only during the period of time required
for the or each vaporiser to come on-line, i.e. to cool down sufficiently for liquefied
first gas to be vaporised at the rate necessary to maintain the required output
pressure or flow of first gas product.
The entire back-up system (liquid storage, pumps (if present), vaporizer, etc.)
could be eliminated, or greatly reduced in size, by use of another embodiment of
the invention. In general, if there are multiple ASUs then there will often be
multiple downstream processing units. If one of the ASUs were to trip then one
of the downstream processing units could be shutdown. It would not be necessary
to go to the substantial capital cost of liquid storage and vaporization facilities,
to keep the unit supplied with gas from the ASU. However, typically it would take
a significant time, e.g. 10 to 30 minutes, for one of the downstream processing
units to correctly and safely reduce capacity and shutdown. During this period,
the unit must continue to be supplied with gas from the ASU, albeit at a reducing capacity.
In this period, the pressurised LOX flow in the untripped ASUs could be increased
to substantially higher than the maximum steady state flow by static head increase
or pumping. The extra pressurised LOX flow would temporarily reduce liquid inventory
levels in the ASUs. The additional flow would be vaporized in the ASU main exchangers
by utilizing the thermal inventory of the main exchanger metal along with any spare
capacity in the untripped ASUs. Although such a situation could only continue for
a relatively short period before the oxygen product left the ASU at an excessively
cold temperature, the situation only is required to continue for the short period
it takes to unload and shutdown one of the downstream processing units. Thus, it
is proposed that at least a portion of the back-up quantity of gas could be supplied
from the untripped ASUs for the duration of the shutdown period.
Alternatively, the capacity of one of more of the downstream units
could be reduced. However, it may take as much as 10 to 30 minutes to achieve the
turndown and during that period the total oxygen demand may be larger than the
maximum continuous capacity of the online ASUs.
The process has particular application to cryogenic separations of air in which
the gaseous mixture is air and the first gas is argon, nitrogen or, especially,
oxygen. However, the invention has application in other cryogenic separations of
gaseous mixtures in which a liquid product is separated within a coldbox and then
vaporised within the coldbox to exit as a product gas. Examples of such separations
include the separation of a mixture of carbon monoxide (CO) and methane; the separation
of nitrogen from methane in a nitrogen rejection unit, in which a bottoms methane
rich stream is vaporised in a main exchanger against a condensing (unboosted) feed
stream; and the separation of nitrogen from CO in a hydrogen/carbon monoxide ("HYCO")
plant in which there is a separation column to separate nitrogen from CO resulting
in the CO being produced as a liquid, which is vaporised in the main exchanger.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
FIG. 1 is a general schematic representation of an embodiment of the present
invention as applied to the production of GOX from two ASUs arranged in parallel
for use in the production of syngas.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, GOX is produced in two ASUs
2,
4. The
first ASU
2 produces a stream
6 of GOX, which is combined with a
stream
8 of GOX from the second ASU
4. The combined stream
10
is divided into two portions
12,
14, the first portion
12 being
fed to a first syngas generation unit
16 and the second portion
14
being fed to a second syngas generation unit
18.
A back-up system is provided to produce back-up GOX in the event of a reduction
in the pressure or flow of GOX in stream
10. Back-up GOX is produced by
the vaporisation of LOX stored in a LOX storage vessel
20. When required,
LOX is withdrawn from the storage vessel as stream
22 and pumped in a pump
24 to produce a pumped LOX stream
26. The pumped LOX stream
26
is fed to a steam sparged water bath vaporiser
28, which is fed by a stream
30 of steam. A newly vaporised GOX stream
32 is fed via pressure
control valve
34 as stream
36 to GOX stream
10. However, pump
24 would not be required if the LOX storage vessel
20 operates at
an appropriate high pressure.
The back-up system is brought on-line by a control system. In normal operation
flow controllers
46,
48 monitor the oxygen produced from the ASUs
2,
4 and send control signals
42,
44 to adjust the
airflow to ASUs
2,
4 to match the oxygen demand of the customer.
In the event that the customer oxygen demand drops below the minimum capacity
of the ASUs
2,
4, flow controllers
60,
62 send control
signals
62,
64 to open GOX vent valves
66,
68 and vent
the excess GOX production to atmosphere via vent silencers
70,
72.
Pressure sensors
50,
52 monitor the pressure of GOX in streams
6,
8 respectively. If the pressure of GOX through one of the GOX
product streams
6,
8 drops, a control signal
54,
56
is sent to ASUs
2,
4 to increase the pressure of the LOX withdrawn
from the distillation system. If this pressure increase is achieved by use of LOX
pumps within units
2,
4, control signal
54,
56 adjusts
the output of the pump. If the pressure increase is achieved by static head increase
of the LOX within ASUs
2,
4, control signal
54,
56
adjusts a control valve in the LOX line exiting the distillation system.
Pressure controller
74 monitors the pressure of GOX in stream
10.
If the pressure of GOX in stream
10 drops, a control signal
76,
78
is sent to control valves
80,
82 so that the flow of GOX to stream
10 can be adjusted. Pressure controller
84 also monitors the pressure
of GOX in stream
10. The pressure setpoint of controller
84 is lower
than that of controller
74. If the pressure drops below the setpoint of
controller
84, a control signal
86 is sent to valve
34, which
opens to permit GOX from the vaporisation
28 of stored LOX to enter stream
10 and maintain the pressure of GOX in stream
10.
Flow controllers
88,
90 monitor flow of GOX in streams
12,
14
respectively. If the flow of GOX differs from the setpoint of controllers
88,
90,
a control signal
92,
94 is sent to flow control valves
96,
98 which would adjust the GOX flow accordingly. The setpoint of flow controllers
88,
90 is determined by the control system of syngas generation unit
16,
18. In the event of failure of one of the syngas generation units,
a trip signal
100,
102 would be sent to the ASUs
2,
4
to initiate a shutdown of one of the ASUs.
In the event that one of the ASUs
2,
4 trips and ceases to produce
LOX, a trip signal
38,
40 is sent to the back-up system. The trip
signal would immediately bring backup pump
24 to its design operating point
and would open backup control valve
34 to a preset position before surrendering
control of the valve to pressure controller
84.
In the event that one of the ASUs
2,
4 should trip and cease to
produce GOX, in one embodiment, a trip signal (not shown) would be sent to a secondary
LOX pump (not shown) of the ASU still operating which is normally kept at a cryogenic
temperature. The secondary pump would then begin to pump LOX inventory from the
distillation system (not shown) which would increase the flow of LOX through the
heat exchanger (not shown) thereby increasing the amount of GOX produced by the
ASU at least until the vaporiser
28 of the back-up system is fully on-line.
In another embodiment, a trip signal (not shown) would be sent to an oversized
LOX pump in the ASU still operating instructing the pump to pump more LOX inventory
from the distillation system through the heat exchanger to produce more GOX, again
at least until the vaporiser
28 of the back-up system is fully on-line.
Whilst the present process has been discussed with particular reference to
the production of oxygen from an air separation process, it is to be understood
that the process can be applied to the production of any gas using cryogenic separation
processes, such as those previously identified.
It will be appreciated that the invention is not restricted to the details described
above with reference to the preferred embodiments but that numerous modifications
and variations can be made without departing from the spirit and scope of the invention
as defined in the following claims.
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