CA2068171A1 - Production of the molecular sieve regeneration stream from the high pressure column to reduce the feed air pressure in air separation plants - Google Patents
Production of the molecular sieve regeneration stream from the high pressure column to reduce the feed air pressure in air separation plantsInfo
- Publication number
- CA2068171A1 CA2068171A1 CA002068171A CA2068171A CA2068171A1 CA 2068171 A1 CA2068171 A1 CA 2068171A1 CA 002068171 A CA002068171 A CA 002068171A CA 2068171 A CA2068171 A CA 2068171A CA 2068171 A1 CA2068171 A1 CA 2068171A1
- Authority
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- Canada
- Prior art keywords
- column
- stream
- pressure column
- high pressure
- nitrogen
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
- 230000008929 regeneration Effects 0.000 title claims abstract description 28
- 238000011069 regeneration method Methods 0.000 title claims abstract description 28
- 238000000926 separation method Methods 0.000 title claims abstract description 15
- 239000002808 molecular sieve Substances 0.000 title claims abstract description 11
- URGAHOPLAPQHLN-UHFFFAOYSA-N sodium aluminosilicate Chemical compound [Na+].[Al+3].[O-][Si]([O-])=O.[O-][Si]([O-])=O URGAHOPLAPQHLN-UHFFFAOYSA-N 0.000 title claims abstract description 11
- 238000004519 manufacturing process Methods 0.000 title abstract description 9
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims abstract description 84
- 229910052757 nitrogen Inorganic materials 0.000 claims abstract description 42
- XKRFYHLGVUSROY-UHFFFAOYSA-N Argon Chemical compound [Ar] XKRFYHLGVUSROY-UHFFFAOYSA-N 0.000 claims description 54
- 238000000034 method Methods 0.000 claims description 41
- 230000008569 process Effects 0.000 claims description 38
- 229910052786 argon Inorganic materials 0.000 claims description 28
- 239000007788 liquid Substances 0.000 claims description 20
- MYMOFIZGZYHOMD-UHFFFAOYSA-N Dioxygen Chemical compound O=O MYMOFIZGZYHOMD-UHFFFAOYSA-N 0.000 claims description 12
- 238000004821 distillation Methods 0.000 claims description 11
- 238000005057 refrigeration Methods 0.000 claims description 10
- 238000010992 reflux Methods 0.000 claims description 8
- 238000001179 sorption measurement Methods 0.000 claims description 8
- 230000006872 improvement Effects 0.000 claims description 6
- 238000010792 warming Methods 0.000 claims description 4
- 239000012535 impurity Substances 0.000 claims description 2
- 239000002699 waste material Substances 0.000 abstract description 7
- 239000003463 adsorbent Substances 0.000 abstract description 5
- 238000005265 energy consumption Methods 0.000 abstract 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 20
- 239000001301 oxygen Substances 0.000 description 20
- 229910052760 oxygen Inorganic materials 0.000 description 20
- 239000000047 product Substances 0.000 description 12
- 230000006835 compression Effects 0.000 description 9
- 238000007906 compression Methods 0.000 description 9
- 238000011084 recovery Methods 0.000 description 7
- 238000001816 cooling Methods 0.000 description 4
- 239000007789 gas Substances 0.000 description 3
- 230000009467 reduction Effects 0.000 description 3
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 2
- 150000001485 argon Chemical class 0.000 description 2
- 230000008901 benefit Effects 0.000 description 2
- 238000009835 boiling Methods 0.000 description 2
- 239000000356 contaminant Substances 0.000 description 2
- 238000004088 simulation Methods 0.000 description 2
- 239000007787 solid Substances 0.000 description 2
- 239000002918 waste heat Substances 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Chemical compound O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- VVTSZOCINPYFDP-UHFFFAOYSA-N [O].[Ar] Chemical compound [O].[Ar] VVTSZOCINPYFDP-UHFFFAOYSA-N 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- 229910002092 carbon dioxide Inorganic materials 0.000 description 1
- 239000001569 carbon dioxide Substances 0.000 description 1
- 238000009833 condensation Methods 0.000 description 1
- 230000005494 condensation Effects 0.000 description 1
- 229910001873 dinitrogen Inorganic materials 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 229930195733 hydrocarbon Natural products 0.000 description 1
- 150000002430 hydrocarbons Chemical class 0.000 description 1
- 239000012263 liquid product Substances 0.000 description 1
- 150000002829 nitrogen Chemical class 0.000 description 1
- 238000012856 packing Methods 0.000 description 1
- 230000007420 reactivation Effects 0.000 description 1
- 230000003068 static effect Effects 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 230000008016 vaporization Effects 0.000 description 1
Landscapes
- Separation By Low-Temperature Treatments (AREA)
Abstract
ABSTRACT
The current invention relates to the production of a molecular sieve (or other adsorbents) regeneration waste nitrogen stream (M/S
regeneration stream) from the high pressure column of a double column cryogenic air separation unit, which lowers the operating pressure of the low pressure column, which in turn lowers the discharge pressure of the main air compressor. As compared to conventional air separation plants, wherein typically a nitrogen product stream is produced from the low pressure column so that the produced nitrogen product stream can be used as the M/S regeneration stream, the present invention constitutes a significant saving in energy consumption.
The current invention relates to the production of a molecular sieve (or other adsorbents) regeneration waste nitrogen stream (M/S
regeneration stream) from the high pressure column of a double column cryogenic air separation unit, which lowers the operating pressure of the low pressure column, which in turn lowers the discharge pressure of the main air compressor. As compared to conventional air separation plants, wherein typically a nitrogen product stream is produced from the low pressure column so that the produced nitrogen product stream can be used as the M/S regeneration stream, the present invention constitutes a significant saving in energy consumption.
Description
~ ia ~
PRODUCTION OF THE MOLECULAR SIEVE REGENERATION
ST~EAM FROM THE HIGH PRESSURE COLUMN TO REDUCE
THE FEED AIR PRESSURE IN AIR SEPARATION PLANTS
TECHNICAL FIELD
The present invention is related to a cryogenic process for the distillation of air ;nto ;ts const;tuent components. In part;cular, the present invention relates to an improvement to a cryogenic air separation process for the production of a nitrogen waste stream suitable for the regeneration of the molecular sieve beds (M/S
regeneration stream) used to preclean the feed air to the air separation unit.
BACKGROUND OF THE INVENTION
Numerous process steps are known in the art of the production of waste streams, particularly, a nitrogen waste stream for use in the regeneration of a molecular sieve bed used to preclean the feed air to an air separation unit. Typically, this nitrogen waste stream is produced at pressure from the low pressure column of a double column air separation unit, thus setting the operating pressure of the low pressure column; unfortunately, this method does not allow for a reduction in the feed air pressure.
A way known in the art to lower the discharg~ pressure of main air compressor is to set the pressure of the low column pressure at a lower 2S operating pressure and then compressing the M/S regeneration stream to the required pressure. This compression requires a sizeable blower because the compression is low pressure compression9 i.e., close to atmospheric pressure, and the energy efficiency of a low pressure, low compression ratio blower is rather low. Furthermore, a heat exchanger (aftercooler) is necessary to extract the heat which is introduced to the M/S regeneration stream by the blower since the M/S regeneration stream is also used to cool the regenerated adsorption bed to its operation temperature after regeneration is complete. Therefore, this alternative method is a costly solution to reduction of discharge pressure of the main air compressor, and is not to be considered a 2 ~ 7 ~
superior cycle to a process in which the presCure of the low pressure column is set by the M/S regeneration stream. Obviously, if a solution can be found to set the low pressure column without introducing extra investment cost or energy requ;rement the result will be much more benef;cial.
SUMMARY OF THE INVENTION
The present invention relates to an improvement to a process for the separation of air by cryogenic distillation in a distillation column system having at least two distil1ation columns which operate at different pressures, a high pressure column and a low pressure column, and which are in thermal communicat;on with each other. In the process, a compressed feed a;r stream is precleaned in a molecular sieve bed by adsorption of impurities which wil~ freeze at cryogenic temperatures.
This precleaned, compressed feed air stream is then cooled to near its dew point and fed to the bottom of the high pressure column for rectification into a high pressure nitrogen overhead and a crude liquid oxygen bottoms. The crude liquid oxygen bottoms is reduced in pressure and fed to the low pressure column for distillation into a low pressure nitrogen overhead and an essentially pure liquid oxygen bottoms.
The improvement of the present invention is the production of a process stream for regeneration of the molecular sieve beds. This improvement comprises three steps: (a) removing a nitrogen-rich stream from the high pressure column; (b) isentropically expanding the removed nitrogen-rich stream in an expander so that the resultant pressure of the expanded, removed nitrogen-rich stream is in excess of the sum of the ambient pressure and the pressure drop associated with the transport of the expanded, removed n;trogen-rich stream from the expander through any interim process equipment to a vent to the atmosphere; and (c) warming the expanded, removed nitrogen-rich stream to recover refrigeration inherent to said stream.
The improvement of the present invention can also comprise warming the removed nitrogen-rich stream to an effective temperature prior to expansion of step (b~ so that, upon expansion of the warmed, removed nitrogen-rich stream, the temperature sf the expanded, removed nitrogen-rich stream is about the same as the dew point of the precleaned, -compressed feed air stream being fed to the bottom of the high pressure column.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a schematic of an embodiment of the process of the present invention.
Figure 2 is a schematic of an air compander air separation process for comparison with the embodiment of the present invention.
DETAILED DESCRTPTTON OF THE INVENTION
The air taken from the atmosphere contains ;mpurit;es such as water vapor, carbon dioxide, hydrocarbons and acid gases. Some of these substances would cause troubles of various kinds in the air separation unit were they not removed to an acceptable level. The removal of these contaminants is usually carried out in an adsorption bed which contains a solid adsorbent such as a zeol;t;c molecular sieve. These solid adsorbents, however, have to be regenerated after some time on stream.
Adsorption systems carrying out trace contaminant removal may be regenerated by thermal-swing (heating followed by cooling) or pressure-swing (pressurizing followed by depressurizing). This invention is applicable to either system in terms of the power-reduction achieved in the main air compressor. For regeneration of such adsorbents, a dry gas stream is need. This dry gas stream is conventionally taken from the product streams, usually the low purity nitrogen from the low pressure column if the air separation unit is mainly used for oxygen production. Since there is pressure drop associated with the transport of the regeneration stream through the adsorption bed during regeneration, often higher than 1 psi, the source pressure of this M/S regeneration stream and accordingly the pressure of low pressure column at the M/S regeneration stream offtake position has to be set accordingly. In a typical oxygen plant, the bulk of the nitrogen produced from the low pressure column is not used, except for chilling water in a waste heat recovery tower. Since the pressure drop in this waste heat recovery tower is usually low, the pressure of this bulk stream is not as high as the pressure needed for the M/S
regeneration stream. Also, it is not usually economical to use a turbine to recover the energy inherent in this bulk nitrogen. The 2 ~
PRODUCTION OF THE MOLECULAR SIEVE REGENERATION
ST~EAM FROM THE HIGH PRESSURE COLUMN TO REDUCE
THE FEED AIR PRESSURE IN AIR SEPARATION PLANTS
TECHNICAL FIELD
The present invention is related to a cryogenic process for the distillation of air ;nto ;ts const;tuent components. In part;cular, the present invention relates to an improvement to a cryogenic air separation process for the production of a nitrogen waste stream suitable for the regeneration of the molecular sieve beds (M/S
regeneration stream) used to preclean the feed air to the air separation unit.
BACKGROUND OF THE INVENTION
Numerous process steps are known in the art of the production of waste streams, particularly, a nitrogen waste stream for use in the regeneration of a molecular sieve bed used to preclean the feed air to an air separation unit. Typically, this nitrogen waste stream is produced at pressure from the low pressure column of a double column air separation unit, thus setting the operating pressure of the low pressure column; unfortunately, this method does not allow for a reduction in the feed air pressure.
A way known in the art to lower the discharg~ pressure of main air compressor is to set the pressure of the low column pressure at a lower 2S operating pressure and then compressing the M/S regeneration stream to the required pressure. This compression requires a sizeable blower because the compression is low pressure compression9 i.e., close to atmospheric pressure, and the energy efficiency of a low pressure, low compression ratio blower is rather low. Furthermore, a heat exchanger (aftercooler) is necessary to extract the heat which is introduced to the M/S regeneration stream by the blower since the M/S regeneration stream is also used to cool the regenerated adsorption bed to its operation temperature after regeneration is complete. Therefore, this alternative method is a costly solution to reduction of discharge pressure of the main air compressor, and is not to be considered a 2 ~ 7 ~
superior cycle to a process in which the presCure of the low pressure column is set by the M/S regeneration stream. Obviously, if a solution can be found to set the low pressure column without introducing extra investment cost or energy requ;rement the result will be much more benef;cial.
SUMMARY OF THE INVENTION
The present invention relates to an improvement to a process for the separation of air by cryogenic distillation in a distillation column system having at least two distil1ation columns which operate at different pressures, a high pressure column and a low pressure column, and which are in thermal communicat;on with each other. In the process, a compressed feed a;r stream is precleaned in a molecular sieve bed by adsorption of impurities which wil~ freeze at cryogenic temperatures.
This precleaned, compressed feed air stream is then cooled to near its dew point and fed to the bottom of the high pressure column for rectification into a high pressure nitrogen overhead and a crude liquid oxygen bottoms. The crude liquid oxygen bottoms is reduced in pressure and fed to the low pressure column for distillation into a low pressure nitrogen overhead and an essentially pure liquid oxygen bottoms.
The improvement of the present invention is the production of a process stream for regeneration of the molecular sieve beds. This improvement comprises three steps: (a) removing a nitrogen-rich stream from the high pressure column; (b) isentropically expanding the removed nitrogen-rich stream in an expander so that the resultant pressure of the expanded, removed nitrogen-rich stream is in excess of the sum of the ambient pressure and the pressure drop associated with the transport of the expanded, removed n;trogen-rich stream from the expander through any interim process equipment to a vent to the atmosphere; and (c) warming the expanded, removed nitrogen-rich stream to recover refrigeration inherent to said stream.
The improvement of the present invention can also comprise warming the removed nitrogen-rich stream to an effective temperature prior to expansion of step (b~ so that, upon expansion of the warmed, removed nitrogen-rich stream, the temperature sf the expanded, removed nitrogen-rich stream is about the same as the dew point of the precleaned, -compressed feed air stream being fed to the bottom of the high pressure column.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a schematic of an embodiment of the process of the present invention.
Figure 2 is a schematic of an air compander air separation process for comparison with the embodiment of the present invention.
DETAILED DESCRTPTTON OF THE INVENTION
The air taken from the atmosphere contains ;mpurit;es such as water vapor, carbon dioxide, hydrocarbons and acid gases. Some of these substances would cause troubles of various kinds in the air separation unit were they not removed to an acceptable level. The removal of these contaminants is usually carried out in an adsorption bed which contains a solid adsorbent such as a zeol;t;c molecular sieve. These solid adsorbents, however, have to be regenerated after some time on stream.
Adsorption systems carrying out trace contaminant removal may be regenerated by thermal-swing (heating followed by cooling) or pressure-swing (pressurizing followed by depressurizing). This invention is applicable to either system in terms of the power-reduction achieved in the main air compressor. For regeneration of such adsorbents, a dry gas stream is need. This dry gas stream is conventionally taken from the product streams, usually the low purity nitrogen from the low pressure column if the air separation unit is mainly used for oxygen production. Since there is pressure drop associated with the transport of the regeneration stream through the adsorption bed during regeneration, often higher than 1 psi, the source pressure of this M/S regeneration stream and accordingly the pressure of low pressure column at the M/S regeneration stream offtake position has to be set accordingly. In a typical oxygen plant, the bulk of the nitrogen produced from the low pressure column is not used, except for chilling water in a waste heat recovery tower. Since the pressure drop in this waste heat recovery tower is usually low, the pressure of this bulk stream is not as high as the pressure needed for the M/S
regeneration stream. Also, it is not usually economical to use a turbine to recover the energy inherent in this bulk nitrogen. The 2 ~
pressure of this stream is reduced through valves or pipes and, thus, the exergy of this bulk nitrogen is lost.
According to the present invention, the M/S regeneration stream is produced from the high pressure column by expanding it. That way, the operating pressure of the low pressure column is set by the lower pressure requirement stream, i.e., the bulk nitrogen sent to the waste tower. This lower operat;ng pressure in the low pressure column results in a lower discharge pressure for the main air compressor. One simplified embodiment of the process of the present in~ention is shown in Figure #1.
With reference to Figure #l, the compressed air from molecular sieve (and/or other adsorbent) bed, line l, is divided into two streams.
The first of these two streams, line 2, is fed directly to main heat exchangers 1~O and 104, wherein it is cooled to a temperature near its dew point and then fed, via line 8, into the bottom of high pressure column 106 for rectification into a high pressure nitrogen overhead and a crude liquid oxygen bottoms. The second of the two streams, line 3, is compressed in compressor 102 and then fed to heat exchangers 100 and 104, wherein this stream of higher pressure air is cooled and liquefied.
This liquefied air stream, line 6, is throttled and fed, via line 7, to high pressure column 106, thereby providing impure liquid reflux for the lower section of high pressure column 106. As an option9 a part of the liquefied stream can be fed, via line 22, to low pressure column 108 as additional liquid reflux.
A portion of the high pressure nitrogen overhead is removed from high pressure column 106, via l;ne 59, and condensed in reboiler-condenser 112 located in the bottom of low pressure column 108. This condensed portion is then fed, via line 57, to the top of high pressurP
column 106 to provide liquid reflux. The remaining portion o~ the high pressure nitrogen overhead is removed from high pressure column 106, via line 55, warmed in ma~n heat exchangers 104 and 100 to recover refrigeration and then recovered as high pressure nitrogen product, via line 50.
The crude liquid oxygen bottoms is removed from high pressure column 106, via line 190, subcooled in subcoolers 120 and 122, reduced in pressure and ~ed, via line 192, to low pressure column 108 for - s -distillation into a low pressure nitrogen overhead and a liquid oxygen bottoms.
A portion of the liquid oxygen bottoms is vaporized by heat exchange against the condensing high pressure nitrogen overhead portion 5 in reboiler-condenser 112. The remaining liquid oxygen is removed from the low pressure column, via line 42, boosted in pressure by its static head, and warmed and vaporized in subcooler 120 and main heat exchangers 104 and 100 to recover refrigeration and produce a gaseous oxygen product, via line 40. The pressure of this gaseous oxygen product can be then increased without distorting the temperature profiles in the heat exchanger.
The low pressure nitrogen overhead is removed from low pressure column 108, via line 210, warmed in subcoolers 122 and 120 and main heat exchangers 104 and 100 to recover refrigeration and then vented as waste nitrogen, via line 30.
The process of Figure 1 is shown incorporating argon side arm column 110. This argon side arm column is optional. ~ith reference to this argon side arm column, an argon-containing vapor side stream is removed, via line 230, from an intermediate and appropriate location of low pressure column 108 and fed to the bottom of argon column 110 for rectification into an argon overhead containing less than 5000 vppm oxygen and an argon-containing bottoms liquid. The argon-containing bottoms liquid is removed from argon column 110, via line 231, and returned to low pressure column 108. The argon overhead is removed from argon column 110, via line 65, and fed to and condensed in reboiler-condenser 114. The condensed argon overhead is removed from reboiler-condenser 114, via line 66, and split into two portions. The first portion, line 67, is returned to the top of argon column 110 as liquid reflux. The second portion, line 68, is revaporized in heat exchanger 200, purified ;n adsorber 202 and reliquefied in heat exchanger 204 thereby producing a pure liquid argon product. This pure liquid argon product is then removed from the process are pure liquid argon product, via line 70. Reboiler-condenser 114 is located atop argon column 110 is refrigerated with a liquid side stream from low pressure column 108 which is withdrawn between side stream draw, line 230, and oxygen-rich liquid feed, line 192. The precise location is chosen so as to provide sufficient refrigeration for the required condensation. In reboiler-7~
According to the present invention, the M/S regeneration stream is produced from the high pressure column by expanding it. That way, the operating pressure of the low pressure column is set by the lower pressure requirement stream, i.e., the bulk nitrogen sent to the waste tower. This lower operat;ng pressure in the low pressure column results in a lower discharge pressure for the main air compressor. One simplified embodiment of the process of the present in~ention is shown in Figure #1.
With reference to Figure #l, the compressed air from molecular sieve (and/or other adsorbent) bed, line l, is divided into two streams.
The first of these two streams, line 2, is fed directly to main heat exchangers 1~O and 104, wherein it is cooled to a temperature near its dew point and then fed, via line 8, into the bottom of high pressure column 106 for rectification into a high pressure nitrogen overhead and a crude liquid oxygen bottoms. The second of the two streams, line 3, is compressed in compressor 102 and then fed to heat exchangers 100 and 104, wherein this stream of higher pressure air is cooled and liquefied.
This liquefied air stream, line 6, is throttled and fed, via line 7, to high pressure column 106, thereby providing impure liquid reflux for the lower section of high pressure column 106. As an option9 a part of the liquefied stream can be fed, via line 22, to low pressure column 108 as additional liquid reflux.
A portion of the high pressure nitrogen overhead is removed from high pressure column 106, via l;ne 59, and condensed in reboiler-condenser 112 located in the bottom of low pressure column 108. This condensed portion is then fed, via line 57, to the top of high pressurP
column 106 to provide liquid reflux. The remaining portion o~ the high pressure nitrogen overhead is removed from high pressure column 106, via line 55, warmed in ma~n heat exchangers 104 and 100 to recover refrigeration and then recovered as high pressure nitrogen product, via line 50.
The crude liquid oxygen bottoms is removed from high pressure column 106, via line 190, subcooled in subcoolers 120 and 122, reduced in pressure and ~ed, via line 192, to low pressure column 108 for - s -distillation into a low pressure nitrogen overhead and a liquid oxygen bottoms.
A portion of the liquid oxygen bottoms is vaporized by heat exchange against the condensing high pressure nitrogen overhead portion 5 in reboiler-condenser 112. The remaining liquid oxygen is removed from the low pressure column, via line 42, boosted in pressure by its static head, and warmed and vaporized in subcooler 120 and main heat exchangers 104 and 100 to recover refrigeration and produce a gaseous oxygen product, via line 40. The pressure of this gaseous oxygen product can be then increased without distorting the temperature profiles in the heat exchanger.
The low pressure nitrogen overhead is removed from low pressure column 108, via line 210, warmed in subcoolers 122 and 120 and main heat exchangers 104 and 100 to recover refrigeration and then vented as waste nitrogen, via line 30.
The process of Figure 1 is shown incorporating argon side arm column 110. This argon side arm column is optional. ~ith reference to this argon side arm column, an argon-containing vapor side stream is removed, via line 230, from an intermediate and appropriate location of low pressure column 108 and fed to the bottom of argon column 110 for rectification into an argon overhead containing less than 5000 vppm oxygen and an argon-containing bottoms liquid. The argon-containing bottoms liquid is removed from argon column 110, via line 231, and returned to low pressure column 108. The argon overhead is removed from argon column 110, via line 65, and fed to and condensed in reboiler-condenser 114. The condensed argon overhead is removed from reboiler-condenser 114, via line 66, and split into two portions. The first portion, line 67, is returned to the top of argon column 110 as liquid reflux. The second portion, line 68, is revaporized in heat exchanger 200, purified ;n adsorber 202 and reliquefied in heat exchanger 204 thereby producing a pure liquid argon product. This pure liquid argon product is then removed from the process are pure liquid argon product, via line 70. Reboiler-condenser 114 is located atop argon column 110 is refrigerated with a liquid side stream from low pressure column 108 which is withdrawn between side stream draw, line 230, and oxygen-rich liquid feed, line 192. The precise location is chosen so as to provide sufficient refrigeration for the required condensation. In reboiler-7~
condenser 114, this refrigeration is provided by partially boiling the removed liquid stream, line 220. The partially vaporized side stream, line 221 is returned to low pressure column 108 at the same location as the liquid side stream, line 220 and provides additional boil-up for the upper sections of low pressure column 108.
The M/S regeneration stream is removed, via line 11, from high pressure column 106 at the appropriate location to provide the required purity. This location is preferably between the middle and the top of high pressure column 106. The withdrawn stream, line 11, is then warmed in cold section 104 of main heat exchangers 100 and 104. The warmed stream is removed, via line 12, and nearly isentropically expanded in expander 124 to a suitable pressure. This pressure is dictated by the pressure drop of the M/S regeneration stream in the adsorption bed and the main heat exchanger, as well as the pipe line between the expander and the adsorption bed. The temperature will be close to that of the low pressure nitrogen gas, line 31, at the cold end of main heat exchangers 104 and 100. These two parameters will determine the temperature to which the withdrawn stream, line 12, is warmed prior to expansion. The power generated from expander 124 can be used to drive compressor 102.
The refrigeration created by expansion of the M/S regeneration stream, line 12, is recovered by warming the expanded stream, line 13, and thus supplies the needed refrigeration to compensate for the losses due to heat leak, warm end temperature differences in main heat 2~ exchanger 100 and, ;f any, the refrigeration needed to produce the small amounts of liquid products such as liquid argon. When the production of liquid is small (e.g. only argon is produced as liquid) or zero, and the warm end temperature difference of the main heat exchanger is reasonable, the expander flow for such a cycle will be between 11% - 15%
of the column air, which is typically the flow rate Gf the M/S
regeneration stream.
By expanding the M/S regeneration stream from the high pressure column to the suitable pressure, the pressure of the low pressure column can be set by the required nitrogen product pressure, which is generally at a lower pressure, thus the pressure oP the low pressure column is lower. This in turn reduces the required discharge pressure of the main air compressor. In the cycle shown in Figure #1, no new equipment is added, and no extra energy is consumed. Specifically, in cycles where liquid oxygen is vaporized in the main exchanger a secondary benefit is derived, the pressure of the oxygen product will be higher due to the higher boiling temperature range of the high pressure air used to warm the oxygen. This higher pressure corresponds to a reduction in compression energy and compressor cost for the oxygen compression.
The advantages and the efficacy of the present invention are shown by the following example.
EXAMPLE
As a comparison, the embodiment of the process of the present invention as depicted in the Figure 1 and a conventional air compander cycle as depicted in Figure 2 of were computer simulated as air separation plants producing oxygen and small amounts of pure nitrogen and argon. The product specifications for oxygen, nitrogen and crude argon products for the simulation were as shown below.
Component IPurity ¦ Production ~ _ _ _ _ - 71 Oxygen >95% Oxygen _ possible 20High Pressure 2 vppm Oxygen0.006 of feed air to ¦¦
Argon 20~0 vppm oxygen Recover as much as <10 vppm nitrogen possible _ . .. _ __ , - - ------ ---~--.-c-.-, The pressure drops throughout the process, i.e. for the heat exchangers, subcoolers, pipe lines and columns, were the same for both cases. The total number of theoretical trays in each column were the same; the molar feed rate to the side arm argon column for both cases were the same (2S% of column air). The mean temperature difference in the main heat exchangers and subcoolers were kept the same, so were the efficiency of the expanders. The efficiency of the compressor (of the compander) was 77% for the process of the present invention cycle and 83% for the air compander cycle to account for the imbalance in flow rates of the compressor and expander of the compander of the process of the present invention. This was a conservative estimate for the process of the present invention. The results of simulation are shown in Table 1.
. . ,_ ~ . . . . _ . ....... . _ __ _ I .
MAC Oxygen Discharge Discharge l Process Pressure: Pressure: I Oxygen Argon Cycle psia psia Recovery Recovery Invention 76.2 27.1 _ 0.2094 O 835 Compander I
Cycle 81.3 24.7 0.2093 0.804 I
Ratio~
X~/X~ 0.93~ 1.094 1.0005 1.039 _ _ _ .
I X1 and X2 are the values for present invention and air compander l cycles, respectively.
Some important process parameters are listed in Table 2.
i Maximum Vapor _ - h . _ ._____ ' Flow: ~ -mollhr __psia Column _ Reflux Top of Bottom of Flow HP Column LP Column LP Column HP Column #-mol/hr l ~ , Present l Invention I
lcjrle 24,710 19,562 17.1 70.0 8,126 comlander 21,697 23,659 18.74 75.0 9,700 ~ ~ ~_ ~ /Y ~ 1.14 0 827 0.914 0.933 0.838 1 ; ~ . --- _ , ~ _ _ X1 and X2 are the values for present invention and air compander _ cycles, respectively.
From the data in Table 1, it is evident that, in using the process of the present invention, the discharge pressure of the main air compressor can be reduced by S psi, which alone means a saving of compression energy of the main air compressor by about 4%. The ~3~ ~
discharge pressure of oxygen product ~or the process of the present invention is increasPd by more than 2 psi, which is significant since the ratio of the two pressures is 1.094. The oxygen recoveries are not much different although the process of the present invention is somewhat better. Finally, argon recovery for the process of the present invention is significantly higher. In the cause of fairness, the argon recovery of the air compander cycle can be increased by some other arrangement, such as vaporizing oxygen by completely condensing a fraction of air and use this liquid air as reflux. However, this action will always incur some energy penalty such as higher oxygen compression energy.
Since whatever heat introduced by the compressors has to be extracted by cooling, lower air compression energy and oxygen compression energy in turn reduces the cooling duties of the cooling systems associated with the both compressors. Furthermore, since the M/S regeneration stream is expanded from high pressure column, no air is expanded into the low pressure column, the maximum vapor flow rate in the low pressure column of the process of present invention is 17%
smaller than in the air compander cycle. Volumetrically, it is about 11.5% smaller than that of the air compander cycle. This means that using the process of the present invention can reduce the size of the low pressure column diameter significantly. This is especially meaningful if a structured packing is used instead of conventional trays to effectuate the separation. The Yapor flow in the high pressure column of the process of the present invPntion is about 14% greater than that of the air compander cycle. This will increase the volume of the high pressure column by about 22%. Since the high pressure column is a lot shorter and smaller in diameter, this impact is smaller.
The ~ollowing additional changes are reali~ed in the nitrogen expander cycle (process of the present invention~ compared to the air compander cycle: (a) the diameter of the argon column the pipes and main heat exchanger in the process of the present invention have to be increased to accommodate the pressure decrease if the pressure drops are to be kept at the level used for the air compander cycle; (b~ the subcooler will be about 20% smaller in the process of the present invention, if the mean heat transfer ~T is kept the same as in the air compander cycle; (c) the reboiler/condenser is also smaller in the 7 ~
process of the present invention, since the liquid feed rate to the low pressure column is smaller, so are the vapor flows coming out from the low pressure column; and (d) the front end clean-up bed is somewhat bigger, so is the reactivation energy cost.
The embodiment described above has not exhausted the possible combinations of the concepts taught by the present invention.
Therefore, this embodiment should not be viewed as a limitation on the scope of this invention. The scope of the present invention should be ascertained by the ~ollowing claims.
The M/S regeneration stream is removed, via line 11, from high pressure column 106 at the appropriate location to provide the required purity. This location is preferably between the middle and the top of high pressure column 106. The withdrawn stream, line 11, is then warmed in cold section 104 of main heat exchangers 100 and 104. The warmed stream is removed, via line 12, and nearly isentropically expanded in expander 124 to a suitable pressure. This pressure is dictated by the pressure drop of the M/S regeneration stream in the adsorption bed and the main heat exchanger, as well as the pipe line between the expander and the adsorption bed. The temperature will be close to that of the low pressure nitrogen gas, line 31, at the cold end of main heat exchangers 104 and 100. These two parameters will determine the temperature to which the withdrawn stream, line 12, is warmed prior to expansion. The power generated from expander 124 can be used to drive compressor 102.
The refrigeration created by expansion of the M/S regeneration stream, line 12, is recovered by warming the expanded stream, line 13, and thus supplies the needed refrigeration to compensate for the losses due to heat leak, warm end temperature differences in main heat 2~ exchanger 100 and, ;f any, the refrigeration needed to produce the small amounts of liquid products such as liquid argon. When the production of liquid is small (e.g. only argon is produced as liquid) or zero, and the warm end temperature difference of the main heat exchanger is reasonable, the expander flow for such a cycle will be between 11% - 15%
of the column air, which is typically the flow rate Gf the M/S
regeneration stream.
By expanding the M/S regeneration stream from the high pressure column to the suitable pressure, the pressure of the low pressure column can be set by the required nitrogen product pressure, which is generally at a lower pressure, thus the pressure oP the low pressure column is lower. This in turn reduces the required discharge pressure of the main air compressor. In the cycle shown in Figure #1, no new equipment is added, and no extra energy is consumed. Specifically, in cycles where liquid oxygen is vaporized in the main exchanger a secondary benefit is derived, the pressure of the oxygen product will be higher due to the higher boiling temperature range of the high pressure air used to warm the oxygen. This higher pressure corresponds to a reduction in compression energy and compressor cost for the oxygen compression.
The advantages and the efficacy of the present invention are shown by the following example.
EXAMPLE
As a comparison, the embodiment of the process of the present invention as depicted in the Figure 1 and a conventional air compander cycle as depicted in Figure 2 of were computer simulated as air separation plants producing oxygen and small amounts of pure nitrogen and argon. The product specifications for oxygen, nitrogen and crude argon products for the simulation were as shown below.
Component IPurity ¦ Production ~ _ _ _ _ - 71 Oxygen >95% Oxygen _ possible 20High Pressure 2 vppm Oxygen0.006 of feed air to ¦¦
Argon 20~0 vppm oxygen Recover as much as <10 vppm nitrogen possible _ . .. _ __ , - - ------ ---~--.-c-.-, The pressure drops throughout the process, i.e. for the heat exchangers, subcoolers, pipe lines and columns, were the same for both cases. The total number of theoretical trays in each column were the same; the molar feed rate to the side arm argon column for both cases were the same (2S% of column air). The mean temperature difference in the main heat exchangers and subcoolers were kept the same, so were the efficiency of the expanders. The efficiency of the compressor (of the compander) was 77% for the process of the present invention cycle and 83% for the air compander cycle to account for the imbalance in flow rates of the compressor and expander of the compander of the process of the present invention. This was a conservative estimate for the process of the present invention. The results of simulation are shown in Table 1.
. . ,_ ~ . . . . _ . ....... . _ __ _ I .
MAC Oxygen Discharge Discharge l Process Pressure: Pressure: I Oxygen Argon Cycle psia psia Recovery Recovery Invention 76.2 27.1 _ 0.2094 O 835 Compander I
Cycle 81.3 24.7 0.2093 0.804 I
Ratio~
X~/X~ 0.93~ 1.094 1.0005 1.039 _ _ _ .
I X1 and X2 are the values for present invention and air compander l cycles, respectively.
Some important process parameters are listed in Table 2.
i Maximum Vapor _ - h . _ ._____ ' Flow: ~ -mollhr __psia Column _ Reflux Top of Bottom of Flow HP Column LP Column LP Column HP Column #-mol/hr l ~ , Present l Invention I
lcjrle 24,710 19,562 17.1 70.0 8,126 comlander 21,697 23,659 18.74 75.0 9,700 ~ ~ ~_ ~ /Y ~ 1.14 0 827 0.914 0.933 0.838 1 ; ~ . --- _ , ~ _ _ X1 and X2 are the values for present invention and air compander _ cycles, respectively.
From the data in Table 1, it is evident that, in using the process of the present invention, the discharge pressure of the main air compressor can be reduced by S psi, which alone means a saving of compression energy of the main air compressor by about 4%. The ~3~ ~
discharge pressure of oxygen product ~or the process of the present invention is increasPd by more than 2 psi, which is significant since the ratio of the two pressures is 1.094. The oxygen recoveries are not much different although the process of the present invention is somewhat better. Finally, argon recovery for the process of the present invention is significantly higher. In the cause of fairness, the argon recovery of the air compander cycle can be increased by some other arrangement, such as vaporizing oxygen by completely condensing a fraction of air and use this liquid air as reflux. However, this action will always incur some energy penalty such as higher oxygen compression energy.
Since whatever heat introduced by the compressors has to be extracted by cooling, lower air compression energy and oxygen compression energy in turn reduces the cooling duties of the cooling systems associated with the both compressors. Furthermore, since the M/S regeneration stream is expanded from high pressure column, no air is expanded into the low pressure column, the maximum vapor flow rate in the low pressure column of the process of present invention is 17%
smaller than in the air compander cycle. Volumetrically, it is about 11.5% smaller than that of the air compander cycle. This means that using the process of the present invention can reduce the size of the low pressure column diameter significantly. This is especially meaningful if a structured packing is used instead of conventional trays to effectuate the separation. The Yapor flow in the high pressure column of the process of the present invPntion is about 14% greater than that of the air compander cycle. This will increase the volume of the high pressure column by about 22%. Since the high pressure column is a lot shorter and smaller in diameter, this impact is smaller.
The ~ollowing additional changes are reali~ed in the nitrogen expander cycle (process of the present invention~ compared to the air compander cycle: (a) the diameter of the argon column the pipes and main heat exchanger in the process of the present invention have to be increased to accommodate the pressure decrease if the pressure drops are to be kept at the level used for the air compander cycle; (b~ the subcooler will be about 20% smaller in the process of the present invention, if the mean heat transfer ~T is kept the same as in the air compander cycle; (c) the reboiler/condenser is also smaller in the 7 ~
process of the present invention, since the liquid feed rate to the low pressure column is smaller, so are the vapor flows coming out from the low pressure column; and (d) the front end clean-up bed is somewhat bigger, so is the reactivation energy cost.
The embodiment described above has not exhausted the possible combinations of the concepts taught by the present invention.
Therefore, this embodiment should not be viewed as a limitation on the scope of this invention. The scope of the present invention should be ascertained by the ~ollowing claims.
Claims (8)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In a process for the separation of air by cryogenic distillation in a distillation column system having at least two distillation columns operating at different pressures, a high pressure column and a low pressure column, which are in thermal communication with each other, wherein a compressed feed air stream is precleaned in a molecular sieve bed by adsorption of impurities which will freeze at cryogenic temperatures, wherein this precleaned, compressed feed air stream is cooled to near its dew point and fed to the bottom of the high pressure column for rectification into a high pressure nitrogen overhead and a crude liquid oxygen bottoms, wherein the crude liquid oxygen bottoms is reduced in pressure and fed to the low pressure column for distillation into a low pressure nitrogen overhead and an essentially pure liquid oxygen bottoms, the improvement for producing a process stream for regeneration of the molecular sieve beds comprising the steps of:
(a) removing a nitrogen-rich stream from the high pressure column;
(b) isentropically expanding the removed nitrogen-rich stream in an expander so that the resultant pressure of the expanded, removed nitrogen-rich stream is in excess of the sum of the ambient pressure and the pressure drop associated with the transport of the expanded, removed nitrogen-rich stream from the expander through any interim process equipment to a vent to the atmosphere;
(c) warming the expanded, removed nitrogen-rich stream to recover refrigeration inherent to said stream.
(a) removing a nitrogen-rich stream from the high pressure column;
(b) isentropically expanding the removed nitrogen-rich stream in an expander so that the resultant pressure of the expanded, removed nitrogen-rich stream is in excess of the sum of the ambient pressure and the pressure drop associated with the transport of the expanded, removed nitrogen-rich stream from the expander through any interim process equipment to a vent to the atmosphere;
(c) warming the expanded, removed nitrogen-rich stream to recover refrigeration inherent to said stream.
2. The process of Claim 1 wherein the removed nitrogen-rich stream is warmed to a suitable temperature prior to expansion of step (b) so that the temperature of the expanded, removed nitrogen-rich stream is about the same as the dew point of the precleaned, compressed feed air stream being fed to the bottom of the high pressure column.
3. The process of Claim 1, wherein the precleaned, compressed feed air stream is split into two portions, wherein the first portion is cooled to near its dew point and fed to the high pressure column and wherein the second portion is further compressed in a booster compressor, liquefied and fed to an intermediate location of the high pressure column as impure liquid reflux.
4. The process of Claim 3, wherein at least a fraction of the liquefied second portion is fed to an intermediate location of the low pressure column as impure liquid reflux.
5. The process of Claim 3, wherein any work produced by the expander of step (b) is used to drive the booster compressor.
6. The process of Claim 41 wherein any work produced by the expander of step (b) is used to drive the booster compressor.
7. The process of Claim 1, wherein the distillation column system comprises three columns which includes an argon side arm column.
8. The process of Claim 2, wherein the distillation column system comprises three columns which includes an argon side arm column.
E:\JONES\APPLN\211P04400.WPF
11 May 1991
E:\JONES\APPLN\211P04400.WPF
11 May 1991
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US69986091A | 1991-05-14 | 1991-05-14 | |
| US07/699860 | 1991-05-14 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2068171A1 true CA2068171A1 (en) | 1992-11-15 |
Family
ID=24811229
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002068171A Abandoned CA2068171A1 (en) | 1991-05-14 | 1992-05-07 | Production of the molecular sieve regeneration stream from the high pressure column to reduce the feed air pressure in air separation plants |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2068171A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN105727685A (en) * | 2016-04-22 | 2016-07-06 | 莱芜钢铁集团电子有限公司 | Automatic control method and device for molecular sieve air regeneration valve |
-
1992
- 1992-05-07 CA CA002068171A patent/CA2068171A1/en not_active Abandoned
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN105727685A (en) * | 2016-04-22 | 2016-07-06 | 莱芜钢铁集团电子有限公司 | Automatic control method and device for molecular sieve air regeneration valve |
| CN105727685B (en) * | 2016-04-22 | 2018-06-29 | 莱芜钢铁集团电子有限公司 | A kind of autocontrol method and device of molecular sieve air regenesis valve |
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|---|---|---|---|
| EEER | Examination request | ||
| FZDE | Discontinued | ||
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Effective date: 19941108 |