Technical Field

(0001) The present inventions relates to enhancements to a dual column, nitrogen producing cryogenic air separation unit, and more particularly to improvements in the performance of such dual column, nitrogen producing air separation units in terms of increasing nitrogen recovery as well as reducing unit power consumption. The performance improvements are generally attributable to an enhanced air separation cycle that uses three condenser-reboilers and recycles a portion of the vapor from one or more of the condenser-reboilers to the incoming feed stream and or the compressed, purified air streams.

Background

(0002) Industrial gas customers often seek nitrogen product slates at volumes and pressures that typically require very large cryogenic air separation units. Such large scale or high volume nitrogen producing air separation units often use a dual distillation column arrangement, including a higher pressure column and a lower pressure column in which gaseous nitrogen products are withdrawn from the distillation columns at relatively high pressures or at two different pressures. In the conventional dual column nitrogen producing air separation unit, the higher pressure column and lower pressure column are thermally linked in a heat transfer relationship by a main condenser, which liquefies a portion of the nitrogen-enriched vapor from the overhead of the higher pressure column to be used as reflux to the higher pressure column. An example of a large volume nitrogen producing air separation unit is disclosed in U.S. Patent No. 4,453,957. Over the course of the past several decades numerous improvements to such large volume nitrogen producing cryogenic air separation units have been developed to address shortcomings in the performance of such large-scale nitrogen producing air separation cycles.

(0003) For example, U.S. Patent No. 5,098,457 discloses a double distillation column arrangement for large volume nitrogen production where the main condenser is not driven by reboiling a portion of the lower pressure bottoms liquid, but rather the main condenser is driven by a portion of the kettle liquid from the higher pressure column. More specifically, U.S. Patent No. 5,098,457 discloses a split kettle arrangement wherein a portion of the kettle liquid from the higher pressure column is re-boiled in the main condenser and another portion of the kettle liquid from the higher pressure column is directed to an intermediate location on the lower pressure column.

(0004) U.S. Patent No. 6,330,812 discloses another double distillation column arrangement for large volume nitrogen production that employs three condenser- reboilers including a double main condenser configuration where both main condensers are driven by reboiling kettle liquid from the higher pressure column while the third condenser-reboiler associated with the lower pressure column is driven by the oxygen- enriched liquid taken from the bottom of the lower pressure column.

(0005) Finally, U.S. Patent No. 6,257,019 discloses a triple distillation column arrangement for large volume nitrogen production. In addition to the conventional lower pressure distillation column and higher pressure distillation column each with a separate condenser-reboiler, the triple distillation column arrangement also utilizes an intermediate pressure distillation column and a third condenser-reboiler operatively associated with the intermediate pressure distillation column. The triple distillation column arrangement is believed to demonstrate very high nitrogen recoveries at comparatively lower power consumption levels. However, a key disadvantage to the triple distillation column arrangement is the higher capital costs associated with the additional column, the third condenser/reboiler, and additional compressors needed for the intermediate pressure column feed.

(0006) What is needed are further enhancements to such large-scale nitrogen producing cryogenic air separation units to improve nitrogen recovery and/or reduce the associated operating costs (i.e. power costs and reduced operating pressures) over the above-identified prior art systems and previously disclosed improvements thereto.

Summary of the Invention

(0007) The present invention may be characterized as an air separation unit comprising: (a) a main air compression system configured for receiving a stream of incoming feed air and producing a compressed air stream; (b) an adsorption based pre- purifier unit configured for removing impurities from the compressed air stream and producing a compressed, purified air stream; (c) a main heat exchange system configured to cool the compressed and purified air stream to temperatures suitable for fractional distillation; and (d) a distillation column system comprises a higher pressure column, a lower pressure column linked in a heat transfer relationship via a first condenser-reboiler, a second condenser-reboiler operatively associated with the higher pressure column, and a third condenser-reboiler operatively associated with the lower pressure column. The distillation column system produces a high pressure nitrogen product stream, a waste stream and a recycle stream that is a portion of the vapor from one or more of the condenser-reboilers that is recycled back to the incoming feed air stream and or the compressed, purified air stream.

(0008) The higher pressure column is configured to receive the cooled, compressed, purified air stream and produce a nitrogen enriched overhead and an oxygen-enriched kettle stream while the lower pressure column is configured and produce an overhead stream and an oxygen-enriched bottoms. The first main condenser-reboiler is configured to condense a first portion of the nitrogen enriched overhead from the higher pressure column against the oxygen-enriched bottoms from the lower pressure column to produce a nitrogen reflux stream for the higher pressure column and an ascending vapor stream in the lower pressure column from the boil-off of the oxygen-enriched bottoms.

(0009) The second condenser-reboiler is configured to condense a second portion of the nitrogen enriched overhead from the higher pressure column against a first split portion of the oxygen-enriched kettle stream from the higher pressure column to produce a liquid nitrogen stream, a recycle stream comprising a first portion of the boil- off of the oxygen-enriched kettle stream, and a diverted stream comprising a second portion of the boil-off of the oxygen-enriched kettle stream. The diverted stream is directed via a flow control valve to the lower pressure column and wherein the air separation unit is controlled, in part, by adjusting the relative flows of the recycle stream and the diverted stream;

(00010) The third condenser-reboiler is configured to condense the nitrogen overhead from the lower pressure column against the oxygen bottoms from the lower pressure column to produce a nitrogen reflux stream for the lower pressure column, a reverse reflux stream for the higher pressure column and a waste stream. In addition, a second split portion of the oxygen-enriched kettle stream is introduced into the lower pressure column at an intermediate location while a third portion of the nitrogen enriched overhead from the higher pressure column is taken as a high pressure nitrogen product stream.

(00011) Alternatively, the present invention may be characterized as an air separation unit comprising: (a) a main air compression system configured for receiving a stream of incoming feed air and producing a compressed air stream; (b) an adsorption based pre purifier unit configured for removing impurities from the compressed air stream and producing a compressed, purified air stream; (c) a main heat exchange system configured to cool the compressed and purified air stream to temperatures suitable for fractional distillation; and (d) a distillation column system comprises a higher pressure column, a lower pressure column linked in a heat transfer relationship via a first condenser-reboiler, a second condenser-reboiler operatively associated with the higher pressure column, and a third condenser-reboiler operatively associated with the lower pressure column. The distillation column system produces a high pressure nitrogen product stream, a first waste stream, a second waste stream and a recycle stream that is recycled back to the incoming feed air stream and or the compressed, purified air stream.

(00012) The higher pressure column is configured to receive the cooled, compressed, purified air stream and produce a nitrogen enriched overhead and an oxygen-enriched kettle stream while the lower pressure column is configured and produce an overhead stream, a first waste stream taken from an intermediate location of the lower pressure column, and an oxygen-enriched bottoms. The first main condenser-reboiler is configured to condense a first portion of the nitrogen enriched overhead from the higher pressure column against the oxygen-enriched bottoms from the lower pressure column to produce a nitrogen reflux stream for the higher pressure column and an ascending vapor stream in the lower pressure column from the boil-off of the oxygen-enriched bottoms. (00013) The second condenser-reboiler is configured to condense a second portion of the nitrogen enriched overhead from the higher pressure column against a first split portion of the oxygen-enriched kettle stream from the higher pressure column to produce a liquid nitrogen stream, a recycle stream comprising all or a portion of the boil-off of the oxygen-enriched kettle stream. The third condenser-reboiler is configured to condense the nitrogen overhead from lower pressure column against the oxygen bottoms from the lower pressure column to produce a nitrogen reflux stream for the lower pressure column, a reverse reflux stream for the higher pressure column and a second waste stream. In addition, a second split portion of the oxygen-enriched kettle stream is introduced into the lower pressure column at an intermediate location while a third portion of the nitrogen enriched overhead from higher pressure column is taken as a high pressure nitrogen product stream.

(00014) In some embodiments, the recycle stream may be recycled back to the main air compression system, preferably to an inter-stage location of the main air compressor while in other embodiments the recycle stream is compressed in a recycle compressor and recycled back to and combined with the compressed, pre-purified air stream. In all embodiments, refrigeration is preferably supplied to the air separation unit by use of a waste expansion refrigeration circuit.

(00015) In some further embodiments, a first waste expansion turbine is configured to expand the first waste stream to produce a first waste exhaust stream that is directed to the main heat exchange system to provide supplemental refrigeration for the air separation unit while the second waste stream is combined with the first waste exhaust stream. Optionally, the second waste stream may also be expanded in another waste expansion turbine with the resulting second waste exhaust stream combined with the first waste exhaust stream. In still other embodiments, the waste expansion turbine is configured to expand the first waste stream or a mixture of the first and second waste streams. Such embodiments may include one or more flow control valves operatively configured to direct the second waste stream to mix with the waste exhaust stream or to mix with the first waste stream. Brief Description of the Drawings

(00016) While the present invention concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it may be better understood when taken in connection with the accompanying drawings in which:

(00017) Fig. 1 is a schematic process flow diagram of an enhanced dual column, nitrogen producing cryogenic air separation unit;

(00018) Fig. 2 is a schematic process flow diagram of a dual column, nitrogen producing cryogenic air separation unit with further enhancements;

(00019) Fig. 3 is a schematic process flow diagram of yet another embodiment of the present dual column, nitrogen producing cryogenic air separation unit;

(00020) Fig. 4 is a schematic process flow diagram of still another embodiment of the present dual column, nitrogen producing cryogenic air separation unit; and (00021) Fig. 5 is a schematic process flow diagram of yet another embodiment of the present dual column, nitrogen producing cryogenic air separation unit.

Detailed Description

(00022) As discussed in more detail below, the disclosed cryogenic air separation systems and methods provide certain performance enhancements to large-scale, dual column, nitrogen producing cryogenic air separation units targeted to increase nitrogen recovery and reduce power consumption compared to prior art large-scale, dual column, nitrogen producing cryogenic air separation units.

(00023) Turning to Fig. 1, there is shown a schematic illustration of a large volume, nitrogen producing cryogenic air separation unit 10. In a broad sense, the depicted air separation unit 10 includes a main feed air compression train or system, a waste expansion refrigeration circuit, a main heat exchange system, and a distillation column system.

(00024) In the main feed compression train shown in Fig. 1, the incoming feed air 22 is typically drawn through an air suction filter house and is compressed in a multi-stage, intercooled main air compressor arrangement 24 to a pressure that can be between about 6.5 bar(a) and about 11 bar(a). This main air compressor arrangement 24 may include integrally geared compressor stages or a direct drive compressor stages, arranged in series or in parallel. The compressed air stream 26 exiting the main air compressor arrangement 24 is fed to a pre-purification unit 28 to remove impurities including high boiling contaminants. The pre-purification unit 28, as is well known in the art, typically contains two beds of alumina and/or molecular sieve operating in accordance with a temperature swing adsorption cycle in which moisture and other impurities, such as carbon dioxide, water vapor and hydrocarbons, are adsorbed. One or more additional layers of catalysts and adsorbents may be included in the pre-purification unit 28 to remove other impurities such as carbon monoxide, carbon dioxide and hydrogen to produce the compressed, purified air stream 29. Particulates may be removed from the feed air in a dust filter disposed upstream or downstream of the pre-purification unit 28. (00025) As shown in Fig. 1, the compressed, purified air stream 29 is fully cooled in heat exchangers 52A and 52B and exits the cold end of heat exchanger 52B as a fully cooled air stream 47. The fully cooled air stream 47 is introduced into higher pressure column 72 at a location proximate the bottom of the higher pressure column 72.

Cooling the compressed, purified feed air stream 29 in the heat exchangers 52A and 52B is preferably accomplished by way of indirect heat exchange with the warming streams which include the waste stream 93, a high pressure nitrogen product stream 105, the lower pressure expanded waste stream 196, and a recycle stream 100 from the distillation column system to produce cooled air streams suitable for rectification in the distillation column system.

(00026) The heat exchangers 52A and 52B are preferably brazed aluminum plate-fin type heat exchangers. Such heat exchangers are advantageous due to their compact design, high heat transfer rates and their ability to process multiple streams. They are manufactured as fully brazed and welded pressure vessels. For larger air separation units handling higher flows, the heat exchanger may be constructed from several cores which must be generally connected in series as illustrated in the drawings.

(00027) The illustrated distillation column system includes a higher pressure column 72, a lower pressure column 74, a first main condenser-reboiler 75, a second condenser- reboiler 85 and a third condenser-reboiler 95. The higher pressure column 72 typically operates in the range from between about 7 bar(a) to about 12 bar(a) whereas lower pressure column 74 operates at pressures between about 4.5 bar(a) to about 7 bar(a). Cooled feed air stream 47 is preferably a vapor air stream slightly above its dew point, although it may be at or slightly below its dew point, that is fed into the higher pressure column 72 for rectification resulting from mass transfer between an ascending vapor phase and a descending liquid phase that is initiated by a nitrogen based reflux stream. This separation process in higher pressure column 72 produces a nitrogen-rich column overhead 89 and crude oxygen-enriched bottoms liquid also known as kettle liquid 80 which is taken as kettle stream 88.

(00028) The higher pressure column 72 and the lower pressure column 74 are preferably linked in a heat transfer relationship via the first main condenser-reboiler 75 wherein a first portion 73 of the nitrogen-rich vapor column overhead extracted from the higher pressure column 72 is condensed within the first main condenser-reboiler 75 shown as a once-through heat exchanger being located in the base of lower pressure column 74 against the oxygen-rich liquid column bottoms 77 residing in the bottom of the lower pressure column 74. The boiling of oxygen-rich liquid column bottoms 77 initiates the formation of an ascending vapor phase within lower pressure column 74. The condensation produces a liquid nitrogen stream 81 that along with other liquid stream 82 and the reverse reflux stream 180 are used to reflux the lower pressure column 74 to initiate the formation of descending liquid phase therein. If desired, a portion of the streams 81, 82, or 180 may be withdrawn as liquid product.

(00029) A second portion 83 of the nitrogen-rich vapor column overhead extracted from the higher pressure column 72 is condensed within the second condenser-reboiler 85 shown as a once-through heat exchanger disposed in a separate condenser vessel 84. The second condenser-reboiler 85 is operatively associated with the higher pressure column 72 and configured to condense the second portion 83 of the nitrogen enriched overhead from the higher pressure column 72 against a subcooled first split portion 86 of the oxygen-enriched kettle stream 88 from the higher pressure column 72 to produce a liquid nitrogen stream 82 and a recycle stream 100 from the boil-off of the oxygen- enriched kettle stream. Liquid nitrogen stream 82 could be added to the liquid nitrogen reflux stream 81 and reverse reflux stream 180 that are used to reflux the lower pressure column 74. (00030) The remaining portion of the oxygen-enriched kettle stream, referred to as the second split portion 87, is subcooled and then flashed via valve 187 and introduced into an intermediate location of the lower pressure column 74, a number of stages above the first main condenser-reboiler 75. In addition, a third portion of the nitrogen-rich vapor column overhead extracted from the higher pressure column 72 which is not liquefied in either of the first main condenser-reboiler or the second condenser-reboiler but is withdrawn as a high pressure nitrogen product stream 105.

(00031) In the lower pressure column 74, the ascending vapor phase includes the boil- off from the first main condenser-reboiler 75. The descending liquid is initiated by a first portion 97 of nitrogen reflux stream 96 from the third condenser reboiler 95 which is released into lower pressure column 74. A second portion 188 of nitrogen reflux stream 96 from the third condenser reboiler 95 is pumped via pump 185 to form the reverse reflux stream 180 that is used along with streams 81 and 82 to reflux higher pressure column 72.

(00032) Lower pressure column 74 is also provided with a plurality of mass transfer contacting elements, that can be trays or structured packing or other known elements in the art of cryogenic air separation. The separation occurring within lower pressure column 74 produces a nitrogen overhead 92 and an oxygen-rich liquid column bottoms 77.

(00033) As indicated above, the third condenser-reboiler 95 is associated with the lower pressure column 74 and disposed in a vessel 94. The third condenser-reboiler 95 is configured to condense the nitrogen overhead 92 from the lower pressure column 74 against the portion of the oxygen bottoms liquid 77 that is not reboiled. That portion of the oxygen bottoms liquid 77 from the lower pressure column 74 is subcooled and the subcooled stream 176 is flashed via valve 177 into the boiling side of the third condenser-reboiler 95. The condensed liquid produced by the third condenser-reboiler 95 is nitrogen reflux stream 96 a portion of which is used to reflux the lower pressure column 74 while the vapor generated is withdrawn as waste stream 93 which is warmed in heat exchanger 52B and the warmed waste stream 193 is directed to the waste expansion circuit. (00034) The waste expansion refrigeration circuit shown in Figs. 1-3, includes a waste expansion turbine 195 operatively coupled to a generator 199. The waste expansion refrigeration circuit is configured to expand the warmed waste stream 193 in the waste expansion turbine 195 with the resulting waste expansion exhaust stream 196 being directed to a separate heat exchange passage within heat exchangers 52B and 52A to provide supplemental refrigeration necessary to cool the compressed, purified air stream 29 and exits the heat exchangers as a lower pressure gaseous nitrogen stream 120, which could suffice as a lower pressure nitrogen product stream. In this manner, the supplemental refrigeration created by the expansion of the waste stream is thus imparted indirectly to the main heat exchange system.

(00035) The recycle stream 100 is the boil off vapor stream exiting the second condenser-reboiler 85 and is preferably recycled back to the main air compression system and combined with the incoming feed air stream 22. As shown in Fig. 1, the heat exchangers 52A and 52B are configured to extract refrigeration from the recycle stream 100 and cool the compressed, purified air in part via indirect heat exchange with the recycle stream 100. The warmed recycle stream 102 may be optionally compressed in recycle compressor (not shown) and introduced to the main air compressor 24, preferably at an inter-stage location.

(00036) In a simulated operation of the air separation plant of the type depicted in Fig. 1, It was also found that the higher recycle flows may increase the oxygen content in the recycle steam which, in turn, increases the oxygen content of the feed air streams sent to the distillation column system. Thus, by controlling the recycle flow rate, the air separation unit operator has the ability to control the oxygen content in the recycle flow so as to optimize performance of the nitrogen producing, dual column air separation unit.

(00037) The embodiment depicted in Fig. 2 shows a further arrangement of the large- scale, dual column, nitrogen producing air separation unit. Many of the features, components and streams associated with the nitrogen producing air separation unit shown in Fig. 2 are similar or identical to those described above with reference to the embodiment of Fig. 1 and will not be repeated here. The key differences between the nitrogen producing air separation unit illustrated in Fig. 2 compared to the corresponding arrangement shown in Fig. 1 is the use of a direct feed conduit 130 and a flow control valve 135 configured to divert a portion of the boil-off vapor from second condenser-reboiler 85 directly to the bottom or a lower section of the lower pressure column 74.

(00038) There are two main techniques used to control the recycle flow in the dual column nitrogen producing air separation unit depicted in Fig. 1, including (1) controlling the backpressure of the 1st stg boiling stream, which would affect the deltaT (dT) in the condenser reboiler 85 and change the boil off vapor flow from the condenser-reboiler 85; and (2) controlling the liquid level of the condensing side of the second condenser-reboiler 85 which would affect the usable surface area for heat exchange in the second condenser-reboiler 85 and thus change the duty of the second condenser-reboiler 85.

(00039) In contrast to the embodiment of Fig. 1 where control of the boil off vapor flow in second condenser-reboiler 85 and/or of the liquid level in second condenser- reboiler 85 are used as control levers for effectively controlling the operation and performance of the distillation column system, in the embodiment shown in Fig. 2 control of the operation and performance of the distillation column system is affected, in part by diverting a portion of the boil off vapor flow from second condenser-reboiler 85 via valve 135 to the lower pressure column. Controlling the operation in this manner allows the air separation unit operator to make full use of the heat exchange surface area in the second condenser-reboiler and keep the plurality of condenser-reboilers operating very close to optimized design conditions while keeping the recycle flow at or near optimal conditions. This control feature is particularly useful in plant operating conditions where the distillation column pressures need to be reduced for whatever reason.

(00040) Turning now to Figs. 3-5, there is shown schematic diagrams of embodiments of the present system and method with further enhancements so as to control the make up of the waste stream and minimize the back pressure of the lower pressure column. Again, as many of the features, components and streams shown in Figs 3-5 are similar or identical to those described above with reference to Fig. 1 the descriptions will not be repeated here. (00041) In the embodiment of Fig. 3, the waste stream 93 originates or is taken from the boil-off vapor from the first main condenser-reboiler 75 in the lower pressure column 74 while the boil-off vapor 98 from the third condenser-reboiler 95 is merged with the exhaust stream 196 exiting the waste turbine 195 at a location upstream of the heat exchangers 52A, 52B such that refrigeration from the boil-off vapor 98 from the third condenser-reboiler 95 is captured in the main heat exchange system. In the arrangement depicted in Fig. 3, the waste stream 93 contains more nitrogen than that the boil-off vapor stream 98 from third condenser-reboiler 95, the lower pressure column is able to operate at much lower pressures than the embodiment in Fig. 1 reducing unit power costs by about 2% while sacrificing nitrogen recovery which is roughly 1% lower compared to the embodiment in Fig. 1.

(00042) The embodiment depicted in Fig. 4 shows a further arrangement of the large- scale, dual column, nitrogen producing air separation unit that is similar in many respects to the embodiment described above with reference to Fig. 3 but further includes a second turbine 205 configured to expand the boil-off vapor 98 from the third condenser-reboiler 95 and the expanded second exhaust stream 206 is merged with the exhaust stream 196 exiting the waste turbine 195 at a location upstream of the heat exchangers 52A, 52B such that refrigeration from the second exhaust stream is captured in the main heat exchange system. Although not shown, the second turbine 205 may be configured to drive a compressor within the air separation plant or may be coupled to a generator so as to reduce the net unit power required by the air separation unit.

(00043) The embodiment depicted in Fig. 5 shows a further arrangement of the large- scale, dual column, nitrogen producing air separation unit configured such that the warmed waste stream 193 can originate or be taken from either the boil-off vapor 93 A from the first main condenser-reboiler 75 in the lower pressure column 74 or a combination of the boil-off vapor 93 A from the first main condenser-reboiler 75 and the boil-off vapor 93B from the third condenser-reboiler 95. The arrangement includes flow control valves 99A and 99B which are controlled by the air separation unit operator to adjust the make-up of the waste stream, and in turn, impacts the operation and performance of the air separation unit. (00044) In a first mode of operation similar to the operation of the air separation unit of Fig. 3, the waste stream comprises the boil-off vapor 93A from the first main condenser-reboiler 75 in the lower pressure column 74 while the boil-off vapor 98 from the third condenser-reboiler 95 is merged with the exhaust stream 196 exiting the waste turbine 195 at a location upstream of the heat exchangers 52A, 52B. In a second mode of operation, the waste stream comprises the boil-off vapor 93A from the first main condenser-reboiler 95 in the lower pressure column 74 and the boil-off vapor 98 from third condenser-reboiler 95. In this manner the make-up of the warmed waste stream 193 directed to waste turbine 195 can be varied based on the selection of the waste stream from either the boil-off vapor stream 93A from first main condenser-reboiler or a combination of the boil-off vapor streams 93A and 93B from first main condenser- reboiler and third condenser-reboiler.

Examples

(00045) When compared against the various existing prior art air separation cycles for large scale dual column, nitrogen producing air separation units, the use of split kettle arrangement with three condenser-reboilers and a portion of the vapor from the second condenser-reboiler being recycled as contemplated by the present systems and methods generally reduces power consumption of the air separation unit by about 3.4% or more while concurrently increasing nitrogen recovery. Computer model simulations of a prior art dual column, nitrogen producing air separation unit against the embodiments shown in Figs. 1-3 and described herein are shown in Table 1.

(00046) Admittedly, each of the proposed embodiments of the present dual column, nitrogen producing air separation unit may require an increase in capital costs compared to the prior art dual column nitrogen producing air separation units as a result of using three condenser-reboilers, the recycle compressor, second turbine, as well as the increased size of the pre-purifier unit. Like many cryogenic air separation unit designs, design trade-offs to be considered is whether net nitrogen recovery and reduced power costs of the presently disclosed embodiments outweigh the additional capital costs for the air separation unit.

Table 1

(00047) While the present enhancements to a large-scale, dual column nitrogen producing air separation unit has been described with reference to several preferred embodiments, it is understood that numerous additions, changes and omissions can be made without departing from the spirit and scope of the present inventions as set forth in the appended claims.