Cryogenic air separation system with split kettle recycle

A cryogenic air separation system, which may be used to produce ultra high purity nitrogen or ultra high purity oxygen, wherein kettle liquid is vaporized in two steps using a split kettle top condenser and vapor from the first step compressed and then recycled to the cryogenic rectification column.

TECHNICAL FIELD
 This invention relates generally to cryogenic air separation and, more
 particularly, to cryogenic air separation wherein ultra high purity
 product may be produced.
 BACKGROUND ART
 Oxygen and nitrogen are produced commercially in large quantities and high
 purities by the cryogenic rectification of air. It is sometimes desired to
 employ oxygen or nitrogen at an ultra high purity, for example, for use in
 the electronics industry. While cryogenic air separation systems for
 producing oxygen or nitrogen at an ultra high purity are known, such
 system generally produce such product with a significantly reduced
 recovery.
 Accordingly, it is an object of this invention to provide an improved
 cryogenic air separation system for the production of oxygen or nitrogen
 at an ultra high purity.
 It is another object of this invention to provide an improved cryogenic air
 separation system which can produce oxygen or nitrogen at an ultra high
 purity and with high recovery.
 SUMMARY OF THE INVENTION
 The above and other objects, which will become apparent to those skilled in
 the art upon a reading of this disclosure, are attained by the present
 invention, one aspect of which is:
 A method for carrying out cryogenic air separation comprising:
 (A) passing feed air into a cryogenic rectification column and separating
 the feed air within the column by cryogenic rectification into
 nitrogen-enriched top fluid and oxygen-enriched kettle liquid;
 (B) partially vaporizing the oxygen-enriched kettle liquid by indirect heat
 exchange with nitrogen-enriched top fluid to produce oxygen-enriched
 kettle vapor and remaining oxygen-enriched kettle liquid;
 (C) compressing the oxygen-enriched kettle vapor and passing the resulting
 compressed oxygen-enriched kettle vapor into the cryogenic rectification
 column;
 (D) vaporizing remaining oxygen-enriched kettle liquid by indirect heat
 exchange with nitrogen-enriched top fluid; and
 (E) recovering some of the nitrogen-enriched top fluid as product nitrogen.
 Another aspect of the invention is:
 Apparatus for carrying out cryogenic air separation comprising:
 (A) a cryogenic rectification column and means for passing feed air into
 the cryogenic rectification column;
 (B) a split kettle top condenser, a phase separator, means for passing
 fluid from the lower portion of the cryogenic rectification column to the
 split kettle top condenser, and means for passing fluid from the split
 kettle top condenser to the phase separator;
 (C) means for passing fluid from the phase separator to the split kettle
 top condenser, and means for passing fluid from the upper portion of the
 cryogenic rectification column to the split kettle top condenser;
 (D) a compressor, means for passing vapor from the phase separator to the
 compressor, and means for passing fluid from the compressor to the
 cryogenic rectification column; and
 (E) means for recovering fluid from the upper portion of the cryogenic
 rectification column as product nitrogen.
 As used herein the term "feed air" means a mixture comprising primarily
 oxygen and nitrogen, such as ambient air.
 As used herein the term "ultra high purity oxygen" means a fluid having an
 oxygen concentration of at least 99.99 mole percent with a methane
 impurity of less than 10.sup.-8 mole percent.
 As used herein the term "ultra high purity nitrogen" means a fluid having a
 nitrogen concentration of at least 99.95 mole percent with an oxygen
 impurity of less than 10.sup.-8 mole percent.
 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 counter currently 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,
 McGraw-Hill Book Company, New York, Section 13, The Continuous
 Distillation Process.
 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. Distillation is
 the separation process whereby heating of a liquid mixture can be used to
 concentrate the more volatile component(s) in the vapor phase and thereby
 the less volatile component(s) 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
 counter current treatment of the vapor and liquid phases. The counter
 current contacting of the vapor and liquid phases can be adiabatic or
 nonadiabatic 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 terms "turbo expansion" and "turbo expander" 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 "subcooling" and "subcooler" mean respectively
 method and apparatus for cooling a liquid to be at a temperature lower
 than the saturation temperature of that liquid for the existing pressure.
 As used herein the terms "upper portion" and "lower portion" mean those
 sections of a column respectively above and below the mid point of the
 column.
 As used herein the term "phase separator" means a vessel wherein incoming
 two phase feed is separated into individual vapor and liquid fractions.
 Typically, the vessel has sufficient cross-sectional area so that the
 vapor and liquid are separated by gravity.
 As used herein the term "stripping column" means a column operated with
 sufficient vapor upflow relative to liquid downflow to achieve separation
 of a volatile component from the liquid into the vapor in which the
 volatile component becomes progressively richer upwardly.
 As used herein the term "split kettle top condenser" means a condenser
 wherein two different kettle liquid streams provide refrigeration to
 condense nitrogen-enriched vapor without rectification.

DETAILED DESCRIPTION
 The invention will be described in detail with reference to the Drawings.
 Referring now to FIG. 1, feed air 60 is compressed by passage through base
 load air compressor 30 to a pressure generally within the range of from 30
 to 300 pounds per square inch absolute (psia). Resulting compressed feed
 air 61 is cooled of the heat of compression by passage through cooler 31
 and then passed as stream 62 to purifier 32 wherein it is cleaned of high
 boiling impurities such as water vapor, carbon dioxide and hydrocarbons.
 Cleaned feed air stream 63 is cooled by indirect heat exchange with return
 streams in heat exchangers 15 and 16 and resulting cooled, cleaned,
 compressed feed air stream 64 is passed into cryogenic rectification
 column 10.
 Cryogenic rectification column 10 is operating at a pressure generally
 within the range of from 30 to 300 psia. Within cryogenic rectification
 column 10 the feed air is separated by cryogenic rectification into
 nitrogen-enriched top fluid and oxygen-enriched bottom fluid.
 Oxygen-enriched bottom fluid is withdrawn from the lower portion of column
 10 as liquid stream 65 and is subcooled by passage through subcooler 3.
 Resulting subcooled oxygen-enriched liquid stream 66 is passed through
 valve 67 and then as stream 68 is passed into split kettle top condenser 2
 wherein it is partially vaporized by indirect heat exchange with
 condensing nitrogen-enriched top fluid, as will be more fully discussed
 below, to form two phase stream 24. Generally from about 30 to 70 percent
 of stream 68 is vaporized by passage through split kettle top condenser 2.
 Two phase stream 24 is passed from split kettle top condenser 24 into phase
 separator 13 wherein it is separated into oxygen-enriched kettle vapor and
 remaining oxygen-enriched kettle liquid. Oxygen-enriched kettle vapor is
 withdrawn from separator 13 in stream 136, warmed by passage through
 primary heat exchangers 16 and 15 and then passed in stream 138 to
 compressor 36, driven by motor 39 wherein it is compressed to a pressure
 generally within the range of from 30 to 300 psia. Resulting compressed
 oxygen-enriched kettle vapor 139 is cooled of the heat of compression by
 passage through cooler 38 and resulting oxygen-enriched kettle vapor 140
 is cooled by passage through primary heat exchangers 15 and 16 and then
 recycled as stream 142 into cryogenic rectification column 10.
 Remaining oxygen-enriched liquid is passed from phase separator 13 in
 stream 143 through valve 144 and as stream 145 back into split kettle top
 condenser 2 wherein it is vaporized by indirect heat exchange with
 condensing nitrogen-enriched top fluid. The resulting vaporized remaining
 oxygen-enriched fluid 102 is warmed by passage through heat exchangers 3
 and 16 to form stream 104 which is turbo expanded by passage through
 turboexpander 37 to generate refrigeration. Resulting refrigeration
 bearing stream 105 is warmed by passage through primary heat exchangers 16
 and 15 thereby cooling incoming streams for passing refrigeration into the
 column to drive the separation. Resulting warmed stream 107 is then
 removed from the system.
 Nitrogen-enriched top fluid is withdrawn from the upper portion of
 cryogenic rectification column 10 as vapor stream 69. If desired, a
 portion 18 of the nitrogen-enriched top vapor may be warmed by passage
 through heat exchangers 3, 16 and 15 and then recovered as product
 nitrogen vapor in stream 19. Preferably the product nitrogen vapor in
 stream 19 is ultra high purity nitrogen. At least a portion 70 of
 nitrogen-enriched top vapor 69 is passed into split kettle top condenser 2
 wherein it is condensed by indirect heat exchange with oxygen-enriched
 liquid as was previously described. Resulting nitrogen-enriched liquid 71
 is passed as reflux 72 into column 10. Stream 71 has the same nitrogen
 concentration as does stream 70. If desired a portion 73 of stream 71 is
 passed through valve 74 and recovered as product nitrogen liquid in stream
 75. Preferably the product nitrogen liquid in stream 75 is ultra high
 purity nitrogen.
 FIG. 2 illustrates another preferred embodiment of the invention wherein
 ultra high purity oxygen may be produced. The numerals of FIG. 2 are the
 same as those of FIG. 1 for the common elements and these common elements
 will not be discussed again in detail.
 Referring now to FIG. 2, oxygen-containing liquid, generally having an
 oxygen concentration within the range of from 30 to 95 mole percent, is
 passed in stream 80 from cryogenic rectification column 10 through valve
 151 and as stream 81 into the upper portion of stripping column 120 which
 is operating at a pressure generally within the range of from 14 to 50
 psia. The oxygen-containing liquid passes down through stripping column
 120 against upflowing vapor and in the process more volatile impurities,
 e.g. argon, within the oxygen-containing liquid are passed or stripped out
 of the downflowing liquid into the upflowing vapor. The impurity
 containing vapor is removed from the upper portion of stripping column 120
 in stream 85 which is combined with stream 105 and then passed out of the
 system. As can be seen, in the embodiment illustrated in FIG. 2,
 turboexpander 37 is mechanically connected to compressor 36 thus serving
 to assist in driving compressor 36.
 The downflowing liquid collects in the bottom portion of stripping column
 120 as high purity oxygen liquid. A portion of the high purity oxygen
 liquid is withdrawn from the lower portion of column 120 in stream 82,
 passed through valve 121 and recovered as liquid oxygen product in stream
 83. Preferably the liquid oxygen product is ultra high purity oxygen.
 In the embodiment of the invention illustrated in FIG. 2, only a portion 50
 of compressed oxygen-enriched vapor 142 is passed directly in cryogenic
 rectification column 10. Another portion 55 of stream 142 is passed into
 bottom reboiler 125 in the sump of stripping column 120 wherein it is
 condensed by indirect heat exchange with high purity oxygen liquid thereby
 vaporizing some of the high purity oxygen liquid which serves as the
 aforesaid upflowing vapor. The resulting condensed oxygen-enriched fluid
 is passed out of reboiler 125 in stream 56 and then passed into cryogenic
 rectification column 10. If desired, all of stream 142 may be condensed in
 reboiler 125 before being recycled into column 10.
 Although the invention has been described in detail with reference to two
 preferred embodiments, those skilled in the art will recognize that there
 are other embodiments of the invention within the spirit and the scope of
 the claims. For example, a separate compressor for the oxygen-enriched
 vapor recycled into the cryogenic rectification column need not be
 employed, and this stream could be passed to the base load air compressor
 for compression and then passed into the cryogenic rectification column
 with the feed air. In another example, the oxygen-enriched recycle could
 be compressed at cryogenic conditions and passed to the cryogenic
 rectification column. The heat of compression may be removed by cooling
 the cryogenically compressed stream through the cold leg of the main heat
 exchanger to remove heat of compression prior to entering into the
 cryogenic rectification column. Moreover, some or all of the refrigeration
 needed to carry out the separations could be generated using a
 multicomponent refrigerant fluid circuit thereby reducing or eliminating
 entirely the need to use turboexpansion to generate the refrigeration.