Cryogenic rectification system for producing low purity oxygen using shelf vapor turboexpansion

Abstract
A cryogenic rectification system for producing low purity oxygen from an auxiliary column to a double column system and which can also effectively produce nitrogen gas product and/or one or more liquid products wherein the lower pressure column of the double column system is reboiled in part by turboexpanded shelf vapor which is condensed in an intermediate reboiler and preferably subcooled prior to passage into the lower pressure column.
Description




TECHNICAL FIELD




This invention relates generally to the cryogenic rectification of feed air and, more particularly, to the cryogenic rectification of feed air to produce low purity oxygen.




BACKGROUND ART




The demand for low purity oxygen is increasing in applications such as glassmaking, steelmaking and energy production. Low purity oxygen is generally produced in large quantities by the cryogenic rectification of feed air. However, conventional cryogenic rectification systems for producing low purity oxygen are relatively inefficient. Moreover, such conventional systems are not effective when gaseous nitrogen or one or more liquid products are desired.




Accordingly it is an object of this invention to provide a cryogenic rectification system which can more efficiently produce low purity oxygen.




It is another object of this invention to provide a cryogenic rectification system which can efficiently produce low purity oxygen and can also effectively produce gaseous nitrogen product and/or one or more liquid products.




SUMMARY OF THE INVENTION




The above and other objects, which will become apparent to one skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:




A method for producing low purity oxygen comprising:




(A) passing feed air into a higher pressure column and separating the feed air within the higher pressure column by cryogenic rectification into nitrogen-enriched vapor and oxygen-enriched liquid;




(B) passing oxygen-enriched liquid into a lower pressure column, condensing a first portion of the nitrogen-enriched vapor, and passing at least some of the resulting condensed first portion nitrogen-enriched liquid into the higher pressure column;




(C) turboexpanding a second portion of the nitrogen-enriched vapor, condensing the turboexpanded second portion of the nitrogen-enriched vapor, and passing the condensed second portion nitrogen-enriched liquid into the lower pressure column;




(D) producing by cryogenic rectification within the lower pressure column nitrogen-richer vapor and oxygen-richer liquid, and passing oxygen-richer liquid from the lower pressure column into an auxiliary column; and




(E) producing by cryogenic rectification low purity oxygen within the auxiliary column, and recovering low purity oxygen product from the lower portion of the auxiliary column.




Another Aspect of the Invention Is:




Apparatus for producing low purity oxygen comprising:




(A) a higher pressure column, a lower pressure column having a bottom reboiler and an intermediate reboiler, and means for passing feed air into the higher pressure column;




(B) means for passing fluid from the lower portion of the higher pressure column into the lower pressure column, means for passing fluid from the upper portion of the higher pressure column to the lower pressure column bottom reboiler, and means for passing fluid from the lower pressure column bottom reboiler to the higher pressure column;




(C) a turboexpander, means for passing fluid from the upper portion of the higher pressure column to the turboexpander, means for passing fluid from the turboexpander to the lower pressure column intermediate reboiler, and means for passing fluid from the lower pressure column intermediate reboiler into the lower pressure column;




(D) an auxiliary column and means for passing fluid for the lower portion of the lower pressure column to the upper portion of the auxiliary column; and




(E) means for recovering low purity oxygen product from the lower portion of the auxiliary column.




As used herein, the term “feed air” means a mixture comprising primarily oxygen and nitrogen, such as ambient air.




As used herein, the term “column” means a distillation or fractionation column or zone, i.e. a contacting column or zone, wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. For a further discussion of distillation columns, see the Chemical Engineer's Handbook, fifth edition, edited by R. H. Perry and C. H. Chilton, McGrawHill Book Company, New York, Section 13


, The Continuous Distillation Process.






The term “double column” is used to mean a higher pressure column having its upper portion in heat exchange relation with the lower portion of a lower pressure column. A further discussion of double columns appears in Ruheman “The Separation of Gases”, Oxford University Press, 1949, Chapter VII, Commercial Air Separation.




Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases is generally adiabatic and can include integral (stagewise) or differential (continuous) contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out at least in part at temperatures at or below 150 degrees Kelvin. (K).




As used herein, the term “indirect heat exchange” means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.




As used herein, the term “subcooling” means cooling a liquid to be at a temperature lower than the saturation temperature of that liquid for the existing pressure.




As used herein, the term “top” when referring to a column means that section of the column above the column mass transfer internals, i.e. trays or packing.




As used herein, the term “bottom” when referring to a-column means that section of the column below the column mass transfer internals, i.e. trays or packing.




As used herein, the term “reboiler” means a heat exchange device that generates column upflow vapor from column liquid. A reboiler may be located within or outside of the column. A bottom reboiler generates column upflow vapor from liquid from the bottom of a column. An intermediate reboiler generates column upflow vapor from liquid from above the bottom of a column.




As used herein, the terms “turboexpansion” and “turboexpander” mean respectively method and apparatus for the flow of high pressure gas through a turbine to reduce the pressure and the temperature of the gas thereby generating refrigeration.




As used herein, the terms “upper portion” and “lower portion” mean those sections of a column respectively above and below the midpoint of the column.




As used herein, the term “tray” means a contacting stage, which is not necessarily an equilibrium stage, and may mean other contacting apparatus such as packing having a separation capability equivalent to one tray.




As used herein, the term “equilibrium stage” means a vapor-liquid contacting stage whereby the vapor and liquid leaving the stage are in mass transfer equilibrium, e.g. a tray having 100 percent efficiency or a packing element height equivalent to one theoretical plate (HETP).




As used herein, the term “low purity oxygen” means a fluid having an oxygen concentration within the range of from 70 to 98 mole percent.











BRIEF DESCRIPTION OF THE DRAWINGS





FIG. 1

is a schematic representation of one preferred embodiment of the low purity oxygen cryogenic rectification system of this invention.





FIG. 2

is a schematic representation of another preferred embodiment of the low purity oxygen cryogenic rectification system of this invention wherein the fluid which drives the intermediate reboiler is compressed prior to being turboexpanded.











DETAILED DESCRIPTION




The invention will be described in detail with reference to the Drawings. Referring now to

FIG. 1

, feed air


40


is compressed in feed air base load compressor


1


to a pressure within the range of from 45 to 75 pounds per square inch absolute (psia). Compressed feed air


41


is cooled of the heat of compression in aftercooler


3


and passed in stream


42


to purifier


5


wherein it is cleaned of high boiling impurities such as water vapor, carbon dioxide and hydrocarbons. Resulting cleaned feed air stream


44


is divided into three portions designated


45


,


46


and


50


. About


3


to


20


percent of feed air


40


is passed in stream


50


to compressor


10


wherein it is compressed to a pressure generally within the range of from 55 to 110 psia. Resulting compressed feed air portion


51


is cooled of the heat of compression by passage through cooler


12


and resulting stream


52


is further cooled by partial traverse of main heat exchanger


18


by indirect heat exchange with return streams. Resulting feed air stream


60


is then turboexpanded by passage through turboexpander


14


to generate refrigeration and resulting turboexpanded feed air stream


61


is passed into lower pressure column


26


. The operation of turboexpander


14


serves to drive compressor


10


through shaft


134


.




About 24 to 35 percent of feed air


40


is passed in stream


46


to compressor


7


wherein it is compressed to a pressure sufficient to vaporize pumped liquid oxygen in stream


86


as will be more fully described below. This pressure may be within the range of from 75 to 1400 psia. Resulting compressed feed air portion


47


is cooled of the heat of compression by passage through cooler


8


and resulting stream


48


is cooled by passage through main heat exchanger


18


by indirect heat exchange with return streams. Preferably stream


48


is partially condensed, most preferably totally condensed, by passage through main heat exchanger


18


. Resulting feed air stream


56


is divided into streams


57


and


58


. Stream


57


is passed through valve


190


and as stream


59


into higher pressure column


24


. Stream


58


is passed through valve


194


and as stream


63


into lower pressure column


26


. Preferably, as shown in

FIG. 1

, stream


58


is subcooled, such as by passage through heat exchanger or subcooler


28


, and passed in stream


62


to valve


194


, prior to being passed into lower pressure column


26


in stream


63


. The remaining portion of feed air


40


is passed as stream


45


through main heat exchanger


18


wherein it is cooled by indirect heat exchange with return streams. Resulting cooled feed air stream


54


is passed to bottom reboiler


20


of auxiliary column


25


wherein it is partially condensed by indirect heat exchange with auxiliary column bottom liquid as will be more fully described below. The resulting partially condensed feed air is passed in stream


55


into higher pressure column


24


which forms a double column system with lower pressure column


26


.




Higher pressure column


24


is operating at a pressure generally within the range of from 40 to 70 psia. Within higher pressure column


24


the feed air is separated by cryogenic rectification into nitrogen-enriched vapor and oxygen-enriched liquid. Oxygen-enriched liquid is withdrawn from the lower portion of column


24


in stream


65


, subcooled by passage through heat exchanger


28


, passed in stream


66


through valve


191


and, as stream


67


, passed into lower pressure column


26


. Nitrogen-enriched vapor is withdrawn from the upper portion of column


24


in shelf vapor stream


75


. A first portion


76


, comprising from about 30 to 50 percent of stream


75


, is passed to lower pressure column bottom reboiler


22


wherein it is condensed by indirect heat exchange with lower pressure column bottom liquid. Resulting nitrogen-enriched liquid is withdrawn from bottom reboiler


22


in stream


77


and passed into higher pressure column


24


. If desired, a portion, shown by dotted line


78


, of stream


77


may be subcooled in subcooler


28


and passed as stream


79


through valve


193


and into lower pressure column


26


in stream


81


.




The remaining portion


34


of nitrogen-enriched vapor stream


75


is preferably warmed by partial traverse of subcooler


28


by indirect heat exchange with subcooling oxygen-enriched liquid


65


. The resulting warmed nitrogen-enriched vapor stream


83


is divided into streams


101


and


35


. Nitrogen-enriched vapor stream


101


is warmed by passage through main heat exchanger


18


and is removed from the system in stream


102


at least a portion of which is preferably recovered as product nitrogen. Nitrogen-enriched vapor stream


35


is passed to turboexpander


15


wherein it is turboexpanded to generate refrigeration. Resulting refrigeration bearing turboexpanded nitrogen-enriched vapor in stream


36


is passed to lower pressure column intermediate reboiler


23


wherein it is condensed by indirect heat exchange with lower pressure column descending liquid thus generating additional upflow vapor for the operation of lower pressure column


26


. In the embodiment of the invention illustrated in

FIG. 1

intermediate reboiler


23


is shown as being physically within column


26


although it is understood that this reboiler could also be located outside of column


26


. Intermediate reboiler


23


vaporizes column liquid taken from above the bottom of column


26


, generally from within the range of from 3 to 12 equilibrium stages above the bottom of column


26


. Resulting condensed nitrogen-enriched liquid from intermediate reboiler


23


is passed in stream


37


to heat exchanger


28


wherein it is subcooled and from there it is passed into the upper portion of lower pressure column


26


as additional reflux. In the embodiment of the invention illustrated in

FIG. 1

, the subcooled nitrogen-enriched liquid is withdrawn from heat exchanger


28


in stream


38


, passed through valve


177


and then in stream


39


combined with stream


81


for passage into column


26


. If stream


81


is not employed, stream


39


is passed directly into lower pressure column


26


. If desired, as shown in

FIG. 1

, a portion


82


of stream


38


may be recovered as product liquid nitrogen typically having a nitrogen concentration within the range of from 98 to 100 mole percent.




Lower pressure column


26


is operating at a pressure less than that of higher pressure column


10


and generally within the range of from 17 to 25 psia. Within lower pressure column


26


the various feeds are separated by cryogenic rectification into nitrogen-richer fluid and oxygen-richer fluid. Nitrogen-richer fluid is withdrawn from the upper portion of column


26


as vapor stream


90


, warmed by passage through heat exchangers


28


and


18


, and removed from the system in stream


93


which may, if desired, be recovered in whole or in part as product nitrogen.




oxygen-richer fluid, having an oxygen concentration generally within the range of from 60 to 90 mole percent, is passed from the lower portion of lower pressure column


26


into the upper portion of an auxiliary column. In the embodiment of the invention illustrated in

FIG. 1

, oxygen-richer liquid is withdrawn from the bottom of column


26


in stream


95


and passed into the top of auxiliary column


25


which has a bottom reboiler


20


.




The oxygen-richer liquid flows down auxiliary column


25


against upflowing vapor generated by the condensing feed air portion


54


, and in the process more volatile components (primarily nitrogen) are stripped out from the downflowing liquid into the upflowing vapor. By this cryogenic rectification stripping process the downflowing liquid forms low purity oxygen liquid at the bottom of auxiliary column


25


which is operating at a pressure generally within the range of from 17 to 25 psia. Vapor from the top of auxiliary column


25


is passed back to lower pressure column


26


in stream


96


.




Low purity oxygen fluid is withdrawn from the lower portion of auxiliary column


25


and recovered. The low purity oxygen fluid may be withdrawn from auxiliary column


25


as either vapor or liquid. The embodiment of the invention illustrated in

FIG. 1

is a preferred embodiment wherein low purity oxygen fluid is withdrawn as liquid from the lower portion of auxiliary column


25


in stream


84


and increased in pressure to form pumped liquid low purity oxygen stream


85


by passage through liquid pump


16


. If desired, a portion of stream


85


may be recovered as liquid low purity oxygen in stream


88


. The remaining portion of stream


85


, which could be all of stream


85


if no liquid product is recovered, is passed in stream


86


to main heat exchanger


18


wherein it is vaporized by indirect heat exchange with incoming feed air. Resulting vaporized low purity oxygen is recovered as product low purity oxygen gas in stream


87


.




The turboexpansion of shelf vapor followed by condensation in the intermediate reboiler provides a benefit when producing elevated pressure nitrogen gas product and/or liquid products in addition to the low purity oxygen product. The shelf turbine/intermediate reboiler arrangement of this invention enables the provision of additional refrigeration with essentially no penalty in mass transfer driving forces because it only reduces mass transfer driving forces in the lower section of the lower pressure column which has an excess mass transfer driving force. Thus the invention enables a reduction in the upper column turbine flow, thereby enabling greater elevated pressure nitrogen gas and/or liquid product recovery. This reduction in upper column turbine flow is better illustrated in the embodiment of the invention illustrated in

FIG. 2

wherein the use of feed air stream


50


which is ultimately turboexpanded and passed into the upper column is eliminated. The numerals in

FIG. 2

are the same as those of

FIG. 1

for the common elements, and these common elements will not be described again in detail.




In the embodiment of the invention illustrated in

FIG. 2

, the feed air that would have formed stream


50


remains in stream


45


. The increased vapor air flow that follows in column


24


improves the recovery potential of this embodiment, further increasing the amount of elevated pressure nitrogen gas and/or liquids that can be recovered as product.




Referring now to

FIG. 2

, stream


101


is passed in its entirety to heat exchanger


18


wherein it is warmed to near ambient temperature to form stream


102


. Preferably stream


102


is passed through one stage of compression in product compressor


120


which produces compressed nitrogen-enriched gas stream


103


for recovery as product nitrogen gas having a nitrogen concentration within the range of from 98 to 100 mole percent. A portion


150


of stream


102


passed to compressor


120


is withdrawn at an interstage level of compressor


120


, preferably after one stage of compression and intercooling in compressor


120


. Alternatively side stream


150


could be split off from stream


102


prior to passage to compressor


120


and could be compressed in a single stage compressor or in a single stage of a compressor performing a different function, such as compressor


1


or compressor


7


.




After withdrawal from the first stage intercooler, near ambient temperature stream


150


is preferably boosted further in pressure in booster compressor


110


, which is powered with energy withdrawn from turboexpander


15


through shaft


135


. The heat of compression from resulting stream


151


is removed in aftercooler


112


and resulting stream


152


is cooled by partial traverse of main heat exchanger


18


. Resulting stream


160


is turboexpanded by passage through turboexpander


15


to form stream


36


which is passed to intermediate reboiler


23


and further processed in a manner similar to that described with reference to FIG.


1


.




Although the invention has been described in detail with reference to certain preferred embodiments, those skilled in the art will recognize that there are other embodiments of the invention within the sprit and the scope of the claims.



Claims
  • 1. A method for producing low purity oxygen comprising:(A) passing feed air into a higher pressure column and separating the feed air within the higher pressure column by cryogenic rectification into nitrogen-enriched vapor and oxygen-enriched liquid; (B) passing oxygen-enriched liquid into a lower pressure column, condensing a first portion of the nitrogen-enriched vapor, and passing at least some of the resulting condensed first portion nitrogen-enriched liquid into the higher pressure column; (C) turboexpanding a second portion of the nitrogen-enriched vapor, condensing the turboexpanded second portion of the nitrogen-enriched vapor, and passing the condensed second portion nitrogen-enriched liquid into the lower pressure column; (D) producing by cryogenic rectification within the lower pressure column nitrogen-richer vapor and oxygen-richer liquid, and passing oxygen-richer liquid from the lower pressure column into an auxiliary column; and (E) producing by cryogenic rectification low purity oxygen within the auxiliary column, and recovering low purity oxygen product from the lower portion of the auxiliary column.
  • 2. The method of claim 1 wherein the oxygen-enriched liquid is subcooled prior to being passed into the lower pressure column.
  • 3. The method of claim 2 wherein the second portion of the nitrogen-enriched vapor is warmed by indirect heat exchange with the subcooling oxygen-enriched liquid prior to being turboexpanded.
  • 4. The method of claim 1 wherein all of the condensed first portion nitrogen-enriched liquid is passed into the higher pressure column.
  • 5. The method of claim 1 wherein at least some of the low purity oxygen product is recovered as liquid.
  • 6. The method of claim 1 comprising withdrawing low purity oxygen liquid from the lower portion of the auxiliary column, pumping the withdrawn low purity oxygen liquid to a higher pressure, vaporizing at least some of the pumped liquid low purity oxygen to produce low purity oxygen gas, and recovering the low purity oxygen gas as low purity oxygen product.
  • 7. The method of claim 1 wherein the condensed second portion of the nitrogen-enriched liquid is subcooled prior to being passed into the lower pressure column.
  • 8. The method of claim 1 wherein the second portion of the nitrogen-enriched vapor is compressed to form elevated pressure nitrogen-enriched vapor prior to being turboexpanded.
  • 9. The method of claim 8 wherein a portion of the elevated pressure nitrogen-enriched vapor is recovered as nitrogen gas product.
  • 10. The method of claim 1 further comprising recovering a portion-of the condensed second portion nitrogen-enriched liquid as product liquid nitrogen.
  • 11. Apparatus for producing low purity oxygen comprising:(A) a higher pressure column, a lower pressure column having a bottom reboiler and an intermediate reboiler, and means for passing feed air into the higher pressure column; (B) means for passing fluid from the lower portion of the higher pressure column into the lower pressure column, means for passing fluid from the upper portion of the higher pressure column to the lower pressure column bottom reboiler, and means for passing fluid from the lower pressure column bottom reboiler to the higher pressure column; (C) a turboexpander, means for passing fluid from the upper portion of the higher pressure column to the turboexpander, means for passing fluid from the turboexpander to the lower pressure column intermediate reboiler, and means for passing fluid from the lower pressure column intermediate reboiler into the lower pressure column; (D) an auxiliary column and means for passing fluid for the lower portion of the lower pressure column to the upper portion of the auxiliary column; and (E) means for recovering low purity oxygen product from the lower portion of the auxiliary column.
  • 12. The apparatus of claim 11 further comprising a subcooler wherein the means for passing fluid from the lower portion of the higher pressure column into the lower pressure column includes the subcooler, and the means for passing fluid from the upper portion of the higher pressure column to the turboexpander includes the subcooler.
  • 13. The apparatus of claim 11 wherein the auxiliary column has a bottom reboiler and further comprising means for passing feed air to the auxiliary column bottom reboiler and from the auxiliary column bottom-reboiler to the higher pressure column.
  • 14. The apparatus of claim 11 further comprising a liquid pump wherein the means for recovering low purity oxygen product from the lower portion of the auxiliary column includes the liquid pump.
  • 15. The apparatus of claim 11 wherein the lower pressure column intermediate reboiler is located within the lower pressure column.
  • 16. The apparatus of claim 11 wherein the lower pressure column intermediate reboiler is located from 3 to 12 equilibrium stages above the bottom of the lower pressure column.
  • 17. The apparatus of claim 11 wherein the means for passing fluid from the upper portion of the higher pressure column to the turboexpander includes a compressor.
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