Process to produce nitrogen using a double column plus an auxiliary low pressure separation zone

Abstract: An air feed [10] is cryogenically distilled to produce nitrogen by using an auxiliary low pressure separation zone [D2] in addition to the conventional high pressure column [D1] and low pressure column [D3]. The auxiliary low pressure separation zone [D2], which is operated at the same pressure as the low pressure column [D3] and which is heat integrated with the top of the high pressure column by means of a bottom reboiler/condenser [R/C1], pretreats the crude liquid oxygen [30] from the bottom of the high pressure column [D1]. The process can produce high pressure nitrogen [22] of various purity and nitrogen [22,62] at two different pressures and two different purities.

Description: The present invention relates to a process and an apparatus for the cryogenic distillation of an air feed. As used herein, the term "air feed" generally means atmospheric air but also includes any gas mixture containing at least oxygen and nitrogen. The target market of the present invention is high pressure nitrogen of various purity, varying from low purity (up to 98% nitrogen) to ultra-high purity (less than 1 part per billion of oxygen) such as the nitrogen which is used in various branches of the chemical and electronic industries. Some applications may require delivery of nitrogen at two different pressures and two different purities. In some other processes, all the nitrogen product may be required at high purity and a high pressure. It is an objective of the present invention to design an efficient cryogenic cycle that can be easily adapted to meet all of these needs. There are several processes known in the art of the production of nitrogen. The processes can be classified according to the number of distillation columns as single column cycles, single column with pre-fractionators or post-fractionators, double column cycles and cycles containing more than two distillation columns. A classic single column nitrogen cycle is taught in US-A-4,222,756. Vapor air is fed to the bottom of a rectifier, where it is separated into overhead vapor nitrogen and a bottom liquid, which is let down in pressure and boiled at the top of the column providing necessary reflux by indirect heat exchange with overhead vapor. The oxygen-enriched vapor from the top reboiler/condenser is discarded as a waste stream. An advantage of a single column nitrogen generator is its simplicity and low capital cost. A big disadvantage of this cycle is limited recovery of nitrogen. Various other types of single column nitrogen generators were proposed to increase nitrogen recovery. In US-A-4,594,085, an auxiliary reboiler was employed at the bottom of the column to vaporize a portion of the bottom liquid against air, forming additional liquid air feed to the column. A similar cycle enriched only with an air compander is taught in US-A-5,037,462. A single column cycle with two reboilers is taught in US-A-4,662,916. Yet another single column cycle, where a portion of the oxygen-enriched waste stream is compressed and recycled back to the column to further increase nitrogen recovery, is described in US-A4,966,002. Similarly, in US-A-5,385,024 a portion of the oxygen-enriched waste stream is cold companded and recycled back to the column with feed air. Nitrogen recovery in a single column system is considerably improved by addition of a second distillation unit. This unit can be a full distillation column or a small pre/post-fractionator built as a flash device or a small column containing just a few stages. A cycle consisting of a single column with a pre-fractionator, where a portion of a feed air is separated to form new feeds the main column is taught in US-A4,604,117. In US-A-4,927,441 a nitrogen generation cycle is taught with a post-fractionator mounted on the top of the rectifier, ' where oxygen-enriched bottom liquid is separated into even more oxygen-enriched fluid and a vapor stream with a composition similar to air. This synthetic air stream is recycled to the rectifier, resulting in.highly improved product recovery and cycle efficiency. Also, the use of two reboilers to vaporize oxygen-enriched fluid twice at different pressures improves the cycle efficiency even further. Classic double column cycles for nitrogen production are taught in US-A-4,222,756. The novel distillation configuration taught in this patent consists of the double column with an additional reboiler/condenser at the top, to provide reflux to the lower pressure column by vaporizing the oxygen-enriched waste fluid. Refrigeration is created by expanding nitrogen gas from the high pressure column. A similar distillation configuration (with different fluids expanded for refrigeration) is taught in GB-A-1,215,377 and US-A4,453,957. In US-A4,617,036, a side reboiler/condenser is employed instead of the heat exchanger at the top on the low pressure column. A dual column cycle with intermediate reboiler in the low pressure column is taught in US-A-5,006,139. A cycle for production of moderate pressure nitrogen and coproduction of oxygen and argon was described in US-A-5,129,932. The dual column high pressure nitrogen process taught in US-A4,439,220 can be viewed as two standard single column nitrogen generators in series (this configuration is also known as a splitcolumn cycle). US-A-4,448,595 differs from a split column cycle in that the lower pressure column is additionally equipped with a reboiler. In US-A-5,098,457, yet another variation of the split column cycle is shown where the nitrogen liquid product from the top of low pressure column is pumped back to the high pressure column, to increase recovery of the high pressure product. A triple column cycle for nitrogen production is described in US-A-5,069,699 where an extra high pressure distillation column is used for added nitrogen production in addition to a double column system with a dual reboiler. Another triple column system for producing large quantities of elevated pressure nitrogen is taught in US-A-5,402,647, the disclosure of which forms the basis of the preamble to the independent claims of the present application. In this invention, the additional column operates at a pressure intermediate to that of higher and lower pressure columns. US-A-5,231,837 by Ha teaches an air separation cycle wherein the top of the high pressure column is heat integrated with both the bottom of the low pressure column and the bottom of an intermediate pressure column. The intermediate column processes the crude liquid oxygen from the bottom of the high pressure column into a condensed top liquid fraction and a bottom liquid fraction which are subsequently fed to the low pressure column. All the prior art nitrogen cycles have the following disadvantage: recovery of high pressure nitrogen from the column system is limited and cannot be increased. US-A-4,775,399 discloses the so-called KELDIST technique in which at least part of the kettle liquid from a high presence cryogenic distillation column is depressurized and evaporated in conjunction with a counter-current vapor-liquid contact device, whereby two vapor streams of differing oxygen content are fed to different heights in a low pressure column operating at the same pressure as the contact device. The vapor stream with the higher oxygen content can be fed to the bottom of the low pressure column to provide boiled to that column. The present invention is a process for the cryogenic distillation of an airfeed to produce nitrogen, particularly high pressure nitrogen of various purity, varying from low purity (up to 98% nitrogen) to ultra-high purity (less than 1 part per billion of oxygen). The nitrogen may be produced at two different pressures and two different purities. The process uses an auxiliary low pressure separation zone In addition to the conventional high pressure column and low pressure column. The auxiliary low pressure separation zone, which is operated at the same pressure as the low pressure column and which is heat integrated with the top of the high pressure column by means of its bottom reboiler/condenser, pre-treats the crude liquid oxygen from the bottom of the high pressure column. According to a first aspect, the present invention provides a process for the cryogenic distillation of an air feed to produce nitrogen using a distillation column system comprising a high pressure column, a low pressure column and an auxiliary separation zone, said process being as defined in Claim 1. At least a remaining portion of said nitrogen rich overhead can be condensed in the second reboiler/condenser and fed as reflux to the low pressure column. In option (A), except for the portion removed as said high pressure nitrogen product, the entire amount of said nitrogen-enriched overhead usually will be condensed by indirect heat exchange against vaporizing oxygen-enriched liquid in the auxiliary low pressure separation zone. In option (A), the oxygen rich liquid stream suitably is reduced in pressure and vaporized in the second reboiled condenser to condense at least a portion of said nitrogen rich overhead. In option (A) a first oxygen-enriched vapor stream is removed from a location in the auxiliary low pressure separation zone between the distillation section and the first reboiler/ condenser; a second oxygen-enriched liquid stream is removed from the bottom of the auxiliary low pressure separation zone; and said first and second oxygen-enriched streams are fed to the bottom of the low pressure column. In option (A), a third portion of the nitrogen-enriched overhead is condensed in a first auxiliary reboiler/condenser and.at least a first part of the condensed third portion fed as reflux to the high pressure column; a first oxygen-enriched stream is removed from a location in the auxiliary low pressure separation zone between the distillation section and the first reboiler/condenser and fed to the bottom of the low pressure column; and a second oxygen-enriched liquid stream is removed from the bottom of the auxiliary low pressure separation zone, reduced in pressure and vaporized in said first auxiliary reboiler/condenser. in option (B(ii), a third portion of the nitrogen-enriched overhead is condensed in a second auxiliary reboiler/condenser, at least a part of the condensed third portion is fed as reflux to the high pressure column and/or at least a part of the condensed third portion reduced in pressure and fed as reflux to the low pressure column; an oxygen-enriched stream is removed from a location in the auxiliary low pressure separation zone between the distillation section and the first reboiler/condenser and fed to the bottom of the low pressure column; and the oxygen rich liquid stream reduced in pressure and vaporized in the second auxiliary reboiler/condenser. In an embodiment of option (A), a portion of the nitrogen-enriched vapor ascending the high pressure column is removed from an intermediate location as additional high pressure nitrogen product; a portion of the condensed nitrogen-enriched overhead from the high pressure column is collected as additional high pressure nitrogen product; and a portion of the oxygen-enriched liquid descending the low pressure column is removed from an intermediate location and fed to the top of the auxiliary low pressure separation zone. A portion of the condensed nitrogen rich overhead from the low pressure column can be pumped to an elevated pressure and fed to an intermediate location in the high pressure column or a portion of the nitrogen-enriched liquid descending the high pressure column removed from the high pressure column, reduced in pressure and fed to the top of the low pressure column. If desired, an additional air feed stream can be fed to an intermediate location in the low pressure column. The following is a description by way of example only and with reference to the accompanying drawings of presently preferred embodiments of the invention. In the drawings:- Figure 1 is a schematic drawing of one general embodiment of the present invention; Figure 2 is a schematic drawing of a second general embodiment of the present invention; Figure 3 is a schematic drawing of a third general embodiment of the present invention; Figure 4 is a schematic drawing of one embodiment of Figure 1 which illustrates one example of a further integration between the columns and/or separation zone of the present invention; Figure 5 is a schematic drawing of a second embodiment of Figure 1 which illustrates a second example of a further integration between the columns and/or separation zone of the present invention; Figure 6 is a schematic drawing of a third embodiment of Figure 1 which illustrates one example of how the present invention can be integrated with a liquid oxygen producing column; Figure 7 is a schematic drawing of a fourth embodiment of Figure 1 which illustrates a second example of how the present invention can be integrated with a liquid oxygen producing column; Figure 8 is a schematic drawing of a fifth embodiment of Figure 1 which illustrates a third example of how the present invention can be integrated with a liquid oxygen producing column; and Figure 9 is a schematic drawing of a first embodiment of Figure 4 which illustrates one example of how the various embodiments of the present invention can be integrated with a main heat exchanger, subcooling heat exchangers and a refrigeration generating expander. With reference to any or all of Figures 1-9, the process of the present invention comprises: (a) feeding at least a portion of the air feed [10] to the bottom of the high pressure column [D1]; (b) removing a nitrogen-enriched overhead [20] from the top of the high pressure column, collecting a first portion [22] as a high pressure nitrogen product, condensing a second portion in a first reboiler/condenser[R/C1] located in the auxiliary low pressure separation zone [D2] and feeding at least a first part [24] of the condensed second portion as reflux to an upper location in the high pressure column; (c) removing a crude liquid oxygen stream [30] from the bottom of the high pressure column, reducing the pressure of at least a first portion of it [across valve V1] and feeding said first portion to the top of the auxiliary low pressure separation zone; (d) removing a crude nitrogen overhead [40] from the top of the auxiliary low pressure separation zone and feeding it directly as a vapor to the low pressure column [D3] wherein the auxiliary low pressure separation zone is operated at the same pressure as the low pressure column, plus the expected pressure drop between the auxiliary low pressure separation zone and the low pressure column; (e) removing one or more oxygen-enriched streams [50a, 50b] from a lower location in the auxiliary low pressure separation zone in the vapor and/or liquid state, feeding at least a portion thereof directly to the low pressure column, and, optionally discarding any vapor portion thereof as a waste stream and/or at least partially vaporizing any liquid portion thereof at reduced pressure by indirect heat exchange against a third portion-of the nitrogen-enriched overhead from the top of the high pressure column; (f) removing a nitrogen rich overhead [60] from the top of the low pressure column, collecting at least an initial portion as a low pressure nitrogen product either directly as a vapor [62; 60 in Figure 3] and/or as a liquid [66 except in Figure 3] after condensing it in a second reboiler/condenser [R/C2 except in Figure 3]; and (g) removing an oxygen rich liquid stream [70] from the bottom of the low pressure column. An important feature of the present invention is the auxiliary low pressure separation zone which can consist of a distillation column with a reboiler/condenser in its bottom. Alternatively, the separation zone can consist of multiple reboiler/condensers and multiple distillation columns. The separation zone is heat integrated with the top of the high pressure column by means of its bottom reboiler/ condenser. The separation zone allows better control of the process and more layout flexibility in terms of giving one the option to physically decouple the main low pressure column from the high pressurspolumn. The separation zone is operated at the same pressure as the low pressure column, plus the expected pressure drop between the auxiliary low pressure separation zone and the low pressure column. It was unexpectedly found that, within the range of possible operating pressures between the pressure of the high pressure column and the pressure of the low pressure column, this is the optimum operating pressure for the separation zone: In addition, this leads to simpler flowsheets with easy flow communication between the separation zone and the low pressure column. In most embodiments of the present invention, and with reference to all but Figure 3: (i) step (f) further comprises condensing at least the remaining portion of the nitrogen rich overhead from the low pressure column in the second reboiler/condenser [R/C2] located at the top of the low pressure column and feeding at least a first part [64] as reflux to an upper location in the low pressure column; (ii) step (g) further comprises reducing the pressure of the oxygen rich liquid stream [70] [across valve V2], vaporizing it in the second reboiler/condenser [R/C2] located at the top of the low pressure column and discarding the vaporized stream [80] as a waste stream; and (iii) the entire amount of the nitrogen-enriched overhead [20] which is removed from the top of the high pressure column is condensed by indirect heat exchange against vaporizing oxygen-enriched liquid from the bottom of the auxiliary low pressure separation zone except for the portion [22] which is removed as the high pressure nitrogen product: (This is unlike US-A-5,231,837 by Ha discussed earlier where a portion of the overhead from the top of the high pressure column is also condensed against vaporizing oxygen-enriched liquid from the bottom of the low pressure column. In Ha, the top of the high pressure column is heat integrated with both the bottom of Ha's intermediate pressure column and the bottom of Ha's low pressure column. As a consequence, the feed air pressure must be higher in Ha which leads to an increased energy requirement.) Also in most embodiments of the present invention, and with reference to all Figures: (i) at least one of the one or more oxygen-enriched streams which is removed from the auxiliary low pressure separation zone in step (e) is removed in a state which is at least partially vapor; and (ii) in step (d), the crude nitrogen overhead [40] from the auxiliary low pressure separation zone is more specifically fed to an intermediate location in the low pressure column. In one general embodiment of the present invention, and with specific reference to Figure 1: (i) the auxiliary low pressure separation zone further comprjses a distillation section [S1] located above the first reboiler/condenser [R/C1]; and (ii) step (e) more specifically comprises removing a first oxygen-enriched vapor stream [50a] from a location in the auxiliary low pressure separation zone between the distillation section and the first reboiler/condenser, removing a second oxygen-enriched liquid stream [50b] from the bottom of the auxiliary low pressure separation zone and feeding the first and second oxygen-enriched streams to the bottom of the low pressure column. In Figure 1, it is generally sufficient for the separation zone's distillation section [S1] to have ten or less stages (or a packing height equivalent to ten or less stages). Also in Figure 1, the purity of the low pressure nitrogen product [62] can be equal to, lowerthan or even higherthan the purity of the high pressure nitrogen product [22], depending on one's needs. To achieve the desired purity level of this stream, an appropriate number of stages or packing height for the low pressure column must be provided. In a second general embodiment of the present invention, and with specific reference to Figure 2. (i) the auxiliary low pressure separation zone further comprises a distillation section [S1] located above the first reboiler/condenser [R/C1] in addition to further comprising a first auxiliary reboiler/condenser [R/C1a]; (ii) step (b) further comprises condensing a third portion [23] of the nitrogen-enriched overhead from the top of the high pressure col umn in the first auxiliary reboiler/condenser [R/C1a] and feeding at least a first part of the condensed third portion as reflux to an upper location in the high pressure column; and (iii) step (e) more specifically comprises removing a first oxygen-enriched stream [50a] from a location in the auxiliary low pressure separation zone between the distillation section and the first reboiler/condenser [R/C1] and feeding it to the bottom of the low pressure column, removing a second oxygen-enriched liquid stream [50b] from the bottom of the auxiliary low pressure separation zone, reducing its pressure [across valve V3], vaporizing it in the first auxiliary reboiler/condenser and discarding the vaporized stream [52] as a waste stream. and fed as reflux to an intermediate location In a third general embodiment of the present invention, and with specific reference to Figure 3: (i) the auxiliary low pressure separation zone further comprises a distillation section [81] located above the first reboiler/condenser [R/C1]; (ii) step (b) further comprises condensing a third portion [23] of the nitrogen-enriched overhead from the top of the high pressure column in a second auxiliary reboiler/condenser [R/C2a], feeding a first part [23a] of the condensed third portion as reflux to an upper location in the high pressure column, reducing the pressure of a second part [23b] [across valve V2] and feeding the second part as reflux to an upper location in the low pressure column; (iii) step (e) more specifically comprises removing a first oxygen-enriched stream [50a] from a location in the auxiliary low pressure separation zone between the distillation section and the first reboiler/condenser and feeding it to the bottom of the low pressure column; and (iv) step (g) further comprises reducing the pressure of the oxygen rich liquid stream [70] [across valve V3], vaporizing it in the second auxiliary reboiler/ condenser [R/C2a] and discarding the vaporized stream [80] as a waste stream. In Figure 3, it is also possible to feed the entire third portion [23] of the nitrogen-enriched overhead from the top of the high pressure column as discussed in (ii) above as reflux to either the high pressure column or the low pressure column. It should be noted that there are many opportunities for further integration in the above general embodiments between the columns and/or separation zone of the present invention. Figures 4 and 5 are two examples as applied to Figure 1 (common streams and equipment use the same identification as in Figure 1). With reference to Figure 4: (i) a portion of the nitrogen-enriched vapor [32] ascending the high pressure column is removed from an intermediate location in the high pressure column as additional high pressure nitrogen product; (ii) a second part [26] of the condensed second portion of the nitrogen-enriched overhead from the high pressure column is collected as additional high pressure nitrogen product; (iii) a portion of the oxygen-enriched liquid [42] descending the low pressure cofumn is removed from an intermediate location in the low pressure column and fed to the top of the auxiliary low pressure separation zone; and (iv) in step (f), a second part [68] of the condensed nitrogen rich overhead from the low pressure column Is pumped to an elevated pressure [in pump P1] and fed to an intermediate location in the high pressure column. In Figure 4, the liquid nitrogen recycle [68] to the high pressure column in (iv) above increases the recovery of the high pressure nitrogen products [22, 26, 32] from the high pressure column. Also in Figure 6, the oxygen-enriched liquid [42] recycle to the separation zone in (iii) above further increases recovery of the liquid high press,ure nitrogen product [26] from the high pressure column. Figure 5 is identical to Figure 4 except that the step described in (iv) above is replaced by the following: (iv) a portion of the nitrogen-enriched liquid [34] descending the high pressure column is removed from an intermediate location in the high pressure column, reduced in pressure [across valve V3] and fed to the top of the low pressure column. In Figure 5, stream [34] should be withdrawn from an appropriate level below the top of the high pressure column, especially if the purity of the low pressure nitrogen product [62, 66] is lower than the purity of the high pressure nitrogen product [22, 26, 32]. If these purities are equal, stream [34] can be withdrawn from the top of the high pressure column. It should further be noted that the present invention can be integrated with a liquid oxygen producing column to produce an ultra high purity liquid oxygen product. Figures 6, 7 and 8 are three examples as applied to Figure 1 (common streams and equipment use the same identification as in Figur 6). With reference to Figure 6: (i) the distillation column system further comprises a liquid oxygen producing column [D4] containing a third reboiler/condenser [R/C3] in its bottom; (ii) a hydrocarbon-depleted stream [36] is removed from an intermediate location in the high pressure column, reduced in pressure [across valve V4] and fed to the top of the liquid oxygen producing column; (iii) prior to reducing the pressure of the first portion of the crude liquid oxygen stream [30] from the bottom of the high pressure column and feeding it to the top of the auxiliary low pressure separation zone, said first portion is subcooled in the third reboiler/condenser [R/C3]; (iv) an overhead stream [92] is removed from the top of the liquid oxygen producing column and combined with the waste stream [80]; and (v) a liquid oxygen product [90] is removed from the bottom of the liquid oxygen producing column. . In Figure 6, the liquid oxygen producing column operates at a pressure close to atmospheric pressure (100 kPa), preferably at 16-30 psia (110-210 kPa). The withdrawal location of stream [36] in Figure 6 is selected high enough in the high pressure column such that all components less volatile than oxygen (especially hydrocarbons) are no longer present in the liquid phase or their concentration is below the acceptable limit. With reference to Figure 7: (i) the distillation column system further comprises a liquid oxygen producing column [D4] containing a third reboiler/condenser [R/C3] in its bottom; (ii) a hydrocarbon-depleted stream [36] is removed from an intermediate location in the high pressure column, reduced in pressure [across valve V4] and fed to the top of the liquid oxygen producing column; (iii) a second portion [12] of the air feed is further compressed [in compressor C2], at least partially condensed in the third reboiler/condenser [R/C3], combined with the first portion of the crude liquid oxygen stream [30] from the bottom of the high pressure column and fed to the top of the auxiliary low pressure separation zone; (iv) an overhead stream [92] is removed from the top of the liquid oxygen producing column, combined with the crude nitrogen overhead [40] from the top of the auxiliary low pressure separation zone and fed to an intermediate location in the low pressure column; and (v) a liquid oxygen product [90] is removed from the bottom of the liquid oxygen producing column. In Figure 7, the liquid oxygen producing column operates at an increased pressure vs Figure 6 (preferably 30-70 psia; 210-480 kPa) which is high enough so that the overhead stream [92] can be fed directly to the low pressure column, or as shown, combined with the crude nitrogen overhead [40] from the top of the separation zone and fed to an intermediate location in the low pressure column, This Increases the overall nitrogen recovery as compared to Figure 6. Also in Figure 7, the at least partially condensed air exiting the third reboiler/condenser [R/C3] may alternatively be fed directly to a suitable location in the high pressure column and/or the low pressure column. With reference to Figure 8: (i) the distillation column system further comprises a liquid oxygen producing column [D4] containing a third reboiler/condenser [R/C3] in its bottom; (ii) a hydrocarbon-depleted stream [36] is removed from an intermediate location In the high pressure column, reduced in pressure [across valve V4] and fed to the top of the liquid oxygen producing column; (iii) a second portion [12] of the air feed is further compressed [in compressor C2], at least partially condensed in the third reboiler/condenser [R/C3], combined with the first portion of the crude liquid oxygen stream [30] from the bottom of the high pressure column and fed to the top of the auxiliary low pressure separation zone; (iv) a hydrocarbon-depleted stream [44] is removed from an upper intermediate location in the low pressure column and combined with the hydrocarbon-depleted stream [36] which is removed from the high pressure column; (v) an overhead stream [92] is removed from the top of the liquid oxygen producing column and fed to an upper intermediate location in the auxiliary low pressure separation zone; and (vi) a liquid oxygen product [90] is removed from the bottom of the liquid oxygen producing column. In Figure 8, stream [44] can be a standalone feed to the liquid oxygen producing column, or as shown, an additional feed along with stream [36]. Also in Figure 8, the overhead stream [92] is preferably returned to the low pressure column at the same location where stream [44] is withdrawn. Alternatively, if the pressure of the liquid oxygen producing column [D4] is lower than the pressure of the low pressure column, then the overhead stream [92] can be combined with the waste stream [80]. It should further be noted that, for simplicity, the main heat exchanger and the refrigeration generating expander scheme have been omitted from Figures 1-8. The main heat exchanger and the various expander schemes can easily be incorporated by one skilled in the art. The candidates of likely streams to be expanded include: (i) at least a portion of the air feed, which after expansion, would generally be fed to an appropriate location in the distillation column system (as an example, this scheme is shown in Figure 9 discussed below); and/or (ii) at least a portion of one or more of the waste streams that are produced in the various embodiments, which after expansion, would generally be warmed in the main heat exchanger against the incoming air feed; and/or (iii) at least a portion of the low pressure nitrogen product from the top of the low pressure column (especially where this product stream must first be compressed to a fihal product specification), which after expansion, would generally be warmed in the main heat exchanger against the incoming air feed; and/or (iv) at least a portion of the high pressure nitrogen product (especially where high production of the high pressure nitrogen product is not needed), which after expansion, would generally be warmed in the main heat exchanger against the incoming air feed. It should further be noted that, for simplicity, other ordinary features of an air separation process have been omitted from Figures 1-8, including the main air compressor, the front end clean-up system, the subcooling heat exchangers and, if required, product compressors. These features can also easily be incorporated by one skilled in the art. Figure 9, as applied to Figure 4 (common streams and equipment use the same identification as in Figure 4) is one example of how these ordinary features (including the main heat exchanger and an expander scheme) can be incorporated. With reference to Figure 9: (i) prior to feeding the air feed to the bottom of the high pressure column in step (a), the air feed is compressed [in compressor C1], cleaned [in a clean-up system CS1] of impurities which will freeze out at 5 cryogenic temperatures (i.e. water and carbon dioxide) and/or other undesirable impurities (such as carbon monoxide and hydrogen) and cooled in a main heat exchanger [HX1] to a temperature near its dew point; (ii) prior to cooling the air feed stream in the main heat exchanger, an air expansion stream [12] is removed, further compressed [in compander compressor C2], partially cooled in the main heat exchanger and turbo-expanded [in expander E1] and fed to an intermediate location in the low pressure column; (iii) the high pressure nitrogen product [22, 32], low pressure nitrogen product [62] and waste stream [80] are warmed in the main heat exchanger; (iv) priorto warming the low pressure nitrogen product [62] and waste stream [80] in the main heat exchanger, said streams are warmed in a first subcooling heat exchanger [HX2] against the crude liquid oxygen stream [30] from the bottom of the high pressure column; (v) prior to warming the low pressure nitrogen product [62] and waste stream [80] in the first subcooling heat exchanger [HX2], said streams, along with the second part [68] of the condensed nitrogen rich overhead from the low pressure column, are warmed in a second subcooling heat exchanger [HX3] against the oxygen rich liquid stream [70] from the bottom of the low press ure column; and (vi) after being warmed in the main heat exchanger, the low pressure nitrogen product [62] is compressed to an elevated pressure [in compressor C3]. Computer simulations have demonstrated that, vis-à-vis the two cycles taught respectively in US-A-4,439,220 and GB-A-1,215,337 as discussed earlier, the present invention has the lowest specific power where specific power was calculated as the total power of the cycle divided by total nitrogen production. All three cycles were simulated to give the highest possible amount of gaseous high pressure nitrogen product at 132 psia (910 kPa). Refrigeration in all three cycles was provided by expanding a portion of the air feed directly to the low pressure column as shown in Figure 9.

Claims: A process for the cryogenic distillation of an air feed to produce nitrogen using a distillation column system comprising a high pressure column [D1], a low pressure column [D3] and an auxiliary separation zone [D2] comprising a distillation section and operated at the same pressure as the low pressure column, plus the expected pressure drop between the auxiliary separation zone and the low pressure column, said process comprising: (a) feeding at least a portion of the air feed [10] to the bottom of the high pressure column [D1]; (b) removing a nitrogen-enriched overhead [20] from the top of the high pressure column [D1], collecting a first portion [22] thereof as a high pressure nitrogen product, condensing a second portion thereof in a first reboiler/condenser [R/C1] located in the auxiliary separation zone [D2] below said distillation section [S1] thereof and feeding at least a first part [24] of the condensed second portion as reflux to the high pressure column [D1]; (c) removing a crude liquid oxygen stream [30] from the bottom of the high pressure column [D1], reducing [VI] the pressure of at least a first portion thereof and feeding said first portion to the top of the auxiliary separation zone [D2]; (d) removing a crude nitrogen overhead [40] from the top of the auxiliary separation zone [D2] and feeding it directly as a vapor to an intermediate location in the low pressure column [D3]; (e) removing an oxygen-enriched vapor stream [50a] from a location in the auxiliary separation zone [D2] between the said distillation section [S1] thereof and the first reboiler/condenser [R/C1] and feeding at least a portion thereof directly to the bottom of the low pressure column [D3]; (f) removing a nitrogen rich overhead [60] from the top of the low pressure column [D3], collecting at least an initial portion thereof as a low pressure nitrogen product either directly as a vapor [62] and/or as a liquid [66] after condensing it in a second reboiler/condenser [R/C2]; and (g) removing an oxygen rich liquid stream [70] from the bottom of the low pressure column [D3], wherein either (A) an oxygen-enriched liquid stream [50b] is removed from the bottom of the auxiliary separation zone [D2], as a second oxygen-enriched stream, and is fed to the bottom of the low pressure column [D3] or (B) a third portion [23] of the nitrogen-enriched overhead [20] is condensed in an auxiliary reboiler/condenser [Fig 2, R/C1a; Fig 3, R/C2a] against either (i) an oxygen-enriched liquid stream [50b] removed from the bottom of the auxiliary separation zone [D2], as a second oxygen-enriched stream, reduced in pressure [V3] and vaporized in said auxiliary reboiler/condenser [R/C1 a] or (ii) said oxygen rich liquid stream [70] from the low pressure column reduced in pressure [Fig 3, V3] and vaporized in said auxiliary reboiler/condenser [R/C2a]' and at least a first part of the resultant condensed third portion of the nitrogen-enriched overhead [23] of B(i) or B(ii) is fed as reflux to the high pressure column [D1]. A process of Claim 1, wherein at least a remaining portion of said nitrogen rich overhead [60] is condensed in a second reboiler/condenser [R/C2] and fed as reflux to the low pressure column [D3]. A process of either of the preceding claims, having option (A). A process of Claim 3, wherein, except for the portion [22] removed as said high pressure nitrogen product, the entire amount of said nitrogen-enriched overhead [20] is condensed by indirect heat exchange against vaporizing oxygen-enriched liquid in the auxiliary low pressure separation zone [D2]. A process of either Claim 3 or Claim 4, wherein said oxygen rich liquid stream [70] is reduced in pressure [V2] and vaporized in the second reboiler/ condenser [R/C2] to condense at least a portion of said nitrogen rich overhead [60]. A process of Claim 3, wherein: a portion [32] of the nitrogen-enriched vapor ascending the high pressure column [D1] is removed from an intermediate location as additional high pressure nitrogen product; a portion [26] of the condensed nitrogen-enriched overhead from the high pressure column [D1] is collected as additional high pressure nitrogen product; and a portion [42] of the oxygen-enriched liquid descending the low pressure column [D3] is removed from an intermediate location and fed to the top of the auxiliary low pressure separation zone [D2]. A process of Claim 6, wherein a portion [68] of the condensed nitrogen rich overhead from the low pressure column [D3] is pumped [P1] to an elevated pressure and fed to an intermediate location in the high pressure column [D1]. A process of Claim 6, wherein a portion [34] of the nitrogen-enriched liquid descending the high pressure column [D1] is removed from the high pressure column [D1], reduced in pressure [Fig 5 V3] and fed to the top of the low pressure column [D3]. A process of Claim 1 or Claim 2, having option B (i). A process of Claim 1 or Claim 2, having option B (ii). A process of any one of the preceding claims, wherein an additional air feed stream is fed to an intermediate location in the low pressure column [D3].

Inventors: AGRAWAL RAKESH, FIDKOWSKI ZBIGNIEW TADEUSZ, AGRAWAL, RAKESH, FIDKOWSKI, ZBIGNIEW TADEUSZ
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1997
Main classification: F25J3
IPC codes: F25J3