Process and device for the cryogenic distillation of air

Abstract

 Pressurised gaseous nitrogen ("GAN") is produced in a process in which liquid nitrogen ("LIN") is produced in a cryogenic air separation unit ("ASU") (124,150). The pressure of at least a portion of the LIN is increased (164) to produce pressurised LIN. A fluid (127) having an oxygen concentration at least equal to that of air is separated in an auxiliary cryogenic distillation column (196) to produce nitrogen-rich overhead vapour (159) and oxygen-enriched bottoms liquid (167). Heat and optionally mass is transferred (161) between at least a portion of the nitrogen-rich overhead vapour and at least a portion of the pressurised LIN to produce nitrogen-rich liquid and pressurised GAN (166). At least a portion (169) of said nitrogen-rich liquid is used as reflux to the ASU after suitable pressure adjustment (173). Such a process allows the production of high purity GAN without the loss of oxygen recovery typically observed in conventional pumped LIN cycles.

Description

[0001] The present invention relates to a process for the cryogenic distillation of air and, in particular, to the production of high purity, pressurised gaseous nitrogen ("GAN").

[0002] High purity GAN is typically prepared in a cryogenic air distillation process operating a pumped liquid nitrogen ("LIN") cycle. In such a cycle, LIN taken from the distillation system is pumped to the required product pressure and vaporised in the main heat exchanger by indirect heat exchange against condensing high pressure air. The resulting liquid air ("LAIR") is fed to the column system. However, the use of such a pumped LIN cycle usually leads to an unfavourable loss of oxygen recovery when compared to a high pressure ("HP") GAN cycle in which the nitrogen is taken as vapour from the HP column of a dual column distillation system, warmed in the main heat exchanger and compressed to the desired pressure. This is because, in the pumped LIN cycle, that part of the feed air that is condensed against boiling nitrogen is not pre-separated in the HP column into LIN and crude liquid oxygen as effectively as the vapour air feed in the HPGAN cycle. As a result, the low pressure ("LP") column of the dual column system has to do more of the separation, with the result that the overall oxygen recovery falls.

[0003] US-A-5419137 (Sweeney et al; published 30^th May 1995) discloses an air separation process producing high purity nitrogen using a dual distillation column system. Overhead vapour in the LP column is partially condensed by indirect heat exchange using a condenser against high purity LIN taken from the HP column. The resultant high purity GAN is removed as product but at low pressure and thus a separate nitrogen compressor is required to compress the GAN product to the required pressure. The condenser is located within the top section of the LP column. This reference also discloses an arrangement where the condenser is replaced with a stripping column (often referred to as a "top hat column") in direct heat and mass transfer relationship with the LP column.

[0004] US-A-4433989 (Erickson; published 28^th February 1984) discloses an air separation process producing GAN using a dual column system coupled with an auxiliary distillation column. In the auxiliary column, crude liquid oxygen from the sump of the HP column is separated into nitrogen-rich overhead vapour and oxygen-enriched bottoms liquid. LIN from the HP column is reduced in pressure and used as reflux to the auxiliary column. The overhead nitrogen-rich vapour is taken as product and no LIN product is withdrawn from this column.

[0005] US-A-4433989 also discloses a further auxiliary column in which air is separated in the further auxiliary column into oxygen-rich bottoms liquid and nitrogen-rich overhead vapour. The further auxiliary column is refluxed by condensing nitrogen overhead vapour by indirect heat exchange against pressurised LOX which is provided by pumping LOX taken from the LP column. Condensed overhead vapour is removed from the further auxiliary column and fed as reflux to the HP column.

[0006] US-A-6276171 (Brugerolle; published 21^st August 2001) discloses a gas turbine integrated with an air separation unit ("ASU"). LIN is produced in the ASU, pumped and fed to a nitrogen wash column. GAN is removed from the nitrogen wash column at a pressure between 8 and 25 bar (0.8 to 2.5 MPa), warmed, compressed and mixed with combustion exhaust gas from the gas turbine prior to work expansion. Bottoms liquid is removed from the wash column, expanded and fed to the ASU. The wash column is primarily intended to produce low purity nitrogen.

[0007] US-A-5596886 (Howard; published 28^th January 1997) discloses an air separation process for the production of gaseous oxygen ("GOX") and high purity nitrogen using a dual column system coupled with an auxiliary, nitrogen-enrichment column. Nitrogen-rich overhead vapour from the HP column is warmed, compressed, cooled and then fed to the auxiliary column which is refluxed with LIN. Nitrogen enriched overhead vapour is taken from the auxiliary column, condensed by indirect heat exchange against LOX from the LP column to form LIN. The LIN is pumped and a portion of the pumped LIN is fed to the top of the auxiliary column.

[0008] US-A-4790866 (Rathbone; published 13^th December 1988) discloses an air separation process using a dual column system coupled with an argon column. LIN is removed from the HP column and fed, after pressure reduction, to the condenser of the argon column where it assists condensing the overhead vapour of the argon column.

[0009] There is a need for a process capable of producing high purity GAN at a different pressure to the operating pressure(s) of the ASU without a reduction in the level of oxygen recovery.

[0010] According to a first aspect of the present invention, there is provided a process for the production of pressurised gaseous nitrogen ("GAN") comprising;
producing liquid nitrogen ("LIN") in a cryogenic air separation unit ("ASU");
increasing the pressure of at least a portion of said LIN to produce pressurised LIN;
separating a fluid having an oxygen concentration at least equal to that of air in an auxiliary cryogenic distillation column to produce nitrogen-rich overhead vapour and oxygen-enriched bottoms liquid;
transferring heat and optionally mass between at least a portion of said nitrogen-rich overhead vapour and at least a portion of said pressurised LIN to produce nitrogen-rich liquid and pressurised GAN; and
feeding at least a portion of said nitrogen-rich liquid as reflux to the ASU after suitable pressure adjustment.

[0011] The expression "nitrogen-rich" means that the nitrogen content is greater than that in air. The expression "nitrogen-enriched" means that the nitrogen content is greater after a particular process step has taken place. The expressions "oxygen-rich" and "oxygen-enriched" have analogous meanings to those given above for "nitrogen-rich" and "nitrogen-enriched". The purity of the nitrogen-rich liquid is usually less than the purity of the LIN and preferably has a purity of between about 85 mol % nitrogen to about 99 mol % nitrogen, e.g. about 95 mol % nitrogen.

[0012] Preferred processes of the present invention allow the production of high purity GAN at a different pressure from the column in which the high purity LIN is separated usually without the loss of oxygen recovery typically observed in conventional pumped LIN cycles in which nitrogen is boiled against condensing air.

[0013] Preferred processes also require fewer separation stages. For example, in embodiments where nitrogen-rich overhead vapour is condensed by indirect heat exchange against vaporising LIN, fewer stages are needed because the auxiliary column does not have to make pure LIN. In embodiments where heat and mass are transferred between LIN and nitrogen-rich overhead vapour, the top section of the auxiliary column operates at a higher reflux ratio than it would without the return stream to the ASU and thus fewer stages are needed to reach an equivalent purity level.

[0014] Preferred processes are also less susceptible to operating upsets than cycles in which high purity nitrogen is boiled directly in a wash column which has no return of lower purity nitrogen to the ASU. For example, in embodiments where nitrogen-rich overhead vapour is condensed by indirect heat exchange against vaporising LIN, the auxiliary column does not have to make pure LIN so the purity only has to be held in the column of the ASU producing the LIN. In embodiments where heat and mass is transferred between LIN and nitrogen-rich overhead vapour, the top section operates at a higher reflux ratio than it would without the return stream to the ASU, so the purity is less sensitive to operating fluctuations in the reflux ratio.

[0015] In preferred embodiments, the operating range of nitrogen production (i.e. the range of nitrogen production rates that can be supplied efficiently, for example, without having to vent excess nitrogen or feed excess air to the auxiliary column) may also be increased and nitrogen purity can more easily be maintained in the event that the performance of the column deteriorates.

[0016] Nitrogen-rich liquid is used as reflux in the ASU after suitable pressure adjustment. Where the ASU comprises a dual distillation column system, nitrogen-rich liquid is usually used as reflux in the LP column. Preferably, at least a portion of the oxygen-enriched liquid is also fed, possibly as reflux, to the ASU after suitable pressure adjustment or reduction. Where the ASU comprises a dual distillation column system, at least a portion of the oxygen-rich liquid is usually fed directly to the LP column (although it may be fed to the LP column via the HP column to recover any vapour that flashes off at the HP column pressure).

[0017] The oxygen concentration of the fluid is at least equal to that of air. The fluid may be air or oxygen-rich fluid from the ASU. The fluid may be gaseous or liquid. In process embodiments in which the fluid is liquid, then at least a portion of the liquid is usually vaporised by indirect heat exchange against a suitable process stream using a reboiler/condenser located in the sump of the auxiliary column. Suitable process streams include, for example, a side stream from an LP or HP column or, if present, from an argon column, or high pressure air supplied from a side stream of a booster air compressor, a recycle nitrogen stream or a stream of air or nitrogen from the main column system that has been compressed at cryogenic temperature.

[0018] The pressure of LIN may be increased by static head. However, it is preferred that LIN is pumped to increase the pressure thereof.

[0019] Nitrogen-rich overhead vapour may be condensed by indirect or direct heat exchange against pressurised LIN.

[0020] Heat may be exchanged indirectly between at least a portion of said nitrogen-rich overhead vapour and at least a portion of said pressurised LIN thereby condensing said nitrogen-rich overhead vapour to produce nitrogen-rich liquid and vaporising said pressurised LIN to produce pressurised GAN. In such process embodiments, heat may be exchanged indirectly using a reboiler/condenser. If the reboiler/condenser is located above the main distillation zone or section within the auxiliary column, then pressurised LIN may be passed through the reboiler/condenser thereby condensing nitrogen-rich overhead vapour surrounding the reboiler/condenser. If the reboiler/condenser is located outside the auxiliary column, then nitrogen-rich overhead vapour may be passed through the reboiler/condenser vaporising pressurised LIN surrounding the reboiler/condenser.

[0021] The auxiliary column usually comprises at least a main distillation zone. Heat and mass may be transferred directly between at least a portion of said nitrogen-rich overhead vapour and at least a portion of said pressurised LIN to produce nitrogen-rich liquid and pressurised GAN. In such embodiments, the auxiliary column preferably has at least a main distillation zone and vapour/liquid contact promoting means provided above the main distillation zone. The process comprises contacting directly nitrogen-rich overhead vapour with pressurised LIN in the contact promoting means to produce the nitrogen-rich liquid and pressurised GAN. The contact promoting means is usually a further distillation zone. In the further distillation zone, the liquid to vapour ratio is substantially above the minimum required to produce high purity nitrogen from the impure nitrogen vapour at the top of the main distillation zone. As a result, relatively few distillation stages can be used in comparison to a process that has no return of impure nitrogen liquid.

[0022] The auxiliary column may be refluxed using any suitable liquid such as LAIR. However, it is preferred that a portion of nitrogen-rich liquid is used to reflux the auxiliary column.

[0023] The ASU may comprise any suitable distillation column arrangement including a single distillation column that produces LIN. However, in preferred process embodiments, the ASU is a dual column system comprising an HP distillation column and an LP distillation column, the HP column being thermally integrated with the LP column via an ASU reboiler/condenser. In such embodiments, the process further comprising:

separating air in the HP column into HP nitrogen-rich overhead vapour and oxygen-rich bottoms liquid;

separating at least a portion of the oxygen-rich bottoms liquid in the LP column into LP nitrogen-rich overhead vapour and LOX;

cooling and at least partially condensing at least a portion of the HP nitrogen-rich overhead vapour in said ASU reboiler/condenser by indirect heat exchange against LOX to produce said LIN; and

refluxing the HP column with a portion of said LIN.

[0024] A portion of the oxygen-rich bottoms liquid produced in the HP column may be used as at least a portion of the fluid after suitable pressure adjustment. The oxygen-rich bottoms liquid may be pressurised by static head but is preferably pressurised using a pump.

[0025] In such process embodiments, the operating pressure of the auxiliary column is usually higher than the operating pressure of the LP column and preferably higher than the operating pressure of the HP column. In this connection, the typical operating pressure of the LP column is from about 1.2 bara (0.12 MPa) to about 4 bara (0.4 MPa) and of the HP column is from about 4 bara (0.4 MPa) to about 12 bara (1.2 MPa). Preferred operating pressures are about 4.8 bara (0.48 MPa) for the HP column and about 1.3 bara (0.13 MPa) for the LP column. The operating pressure of the auxiliary column may be from about 1.0 bara (0.10 MPa) to about 30 bara (3.0 MPa) and is usually from about 1.5 bara (0.15 MPa) to about 25 bara (2.5 MPa). Preferably, the operating pressure of the auxiliary column is about 12 bara (1.2 MPa) .

[0026] The pressurised GAN is usually produced at the operating pressure of the auxiliary column and is usually produced at a pressure from about 1.0 bara (0.1 MPa) to about 25 bara (2.5 MPa) and is usually of high purity, e.g. from about 99.9 mol % nitrogen to about 99.9999 mol % nitrogen, typically about 99.99 mol % nitrogen.

[0027] According to a second aspect of the present invention, there is provided apparatus for the production of pressurised GAN comprising:

a cryogenic ASU comprising at least one distillation column, for producing LIN;

pressurising means for pressurising LIN;

conduit means for feeding LIN from the top of the column of the ASU producing said LIN to the pressurising means;

an auxiliary cryogenic distillation column comprising a main distillation zone for separating a fluid having an oxygen concentration at least equal to that of air into nitrogen-rich overhead vapour and oxygen-enriched bottoms liquid;

transfer enabling means for enabling heat and optionally mass transfer between at least a portion of said nitrogen-rich overhead vapour and at least a portion of said pressurised LIN to produce nitrogen-rich liquid and pressurised GAN;

conduit means for feeding pressurised LIN from the pressurising means to the transfer enabling means;

pressure reducing means for reducing the pressure of nitrogen-rich liquid to produce reduced pressure nitrogen-rich liquid;

conduit means for feeding nitrogen-rich liquid from the auxiliary column to the pressure reducing means; and

conduit means for feeding reduced pressure nitrogen rich liquid from said pressure reducing means to the ASU as reflux.

[0028] The transfer enabling means may be condensing means for condensing at least a portion of nitrogen-rich overhead vapour by indirect heat exchange against pressurised LIN to produce nitrogen-rich liquid and pressurised GAN. In other embodiments, the transfer enabling means may be vapour/liquid contact promoting means, e.g. a further distillation zone, for promoting direct contact between nitrogen-rich overhead vapour and pressurised LIN, the contact promoting means being located within the auxiliary distillation column above the main distillation zone.

[0029] As mentioned above, the ASU may comprise a single distillation column. However, in preferred embodiments, the ASU comprises a dual distillation column system. In such preferred embodiments, the reduced pressure nitrogen-rich liquid may be fed either to the HP column or to the LP column. It is also preferably subcooled before being reduced in pressure to minimise the formation of flash vapour.

[0030] The apparatus preferably further comprises pressure reducing means for reducing the pressure of oxygen-enriched liquid to produce reduced pressure oxygen-enriched liquid, conduit means for feeding oxygen-enriched liquid from the auxiliary column to the pressure reducing means and conduit means for feeding reduced pressure oxygen-enriched liquid from the pressure reducing means to the ASU, possibly as reflux. If the ASU comprises a dual column system, the reduced pressure oxygen-enriched liquid is usually fed to the LP column, normally via the bottom of the HP column.

[0031] Each pressure reducing means may be any suitable means for reducing the pressure of a cryogenic liquid or gas. Preferably, however, the pressure reducing means is an expansion valve such as a Joule-Thompson valve.

[0032] The apparatus may be adapted and/or constructed to enable operation of any of the above-mentioned preferred process embodiments.

[0033] Particularly preferred embodiments of the process or apparatus are substantially as hereinbefore described with reference to the accompanying figures.

[0034] Preferred embodiments of the present invention will now be described with reference to the drawing in which:

FIGURE 1 is a diagrammatic representation of an auxiliary column suitable for use in respect of the present invention;

FIGURE 2 is a diagrammatic representation of a further auxiliary column suitable for use in respect of the present invention;

FIGURE 3 is a flowsheet of an embodiment of the process according to the present invention;

FIGURE 4 is a flowsheet of another embodiment of the process according to the present invention;

FIGURE 5 is a flowsheet of a further embodiment of the process according to the present invention; and

FIGURE 6 is a flowsheet of the process simulated by computer in the Example.

[0035] Referring to Figure 1, a stream 12 of air or an oxygen-rich fluid (such as oxygen-rich bottoms liquid from an HP column of a dual column ASU (not shown) following pressurisation using a pump (not shown) or a compressor (not shown) if the feed is vapour) is fed to the bottom of an auxiliary distillation column 14 where it is separated into nitrogen-rich overhead vapour and oxygen-enriched bottoms liquid. Where the fluid is a liquid, at least a portion of the liquid is vaporised by indirect heat exchange against a process stream using optional reboiler/condenser 16.

[0036] A stream 18 of pure LIN is taken from an ASU (not shown) and pumped in pump 20. A stream 22 of pressurised LIN is fed to a reboiler/condenser 24 located outside the auxiliary column 14 where it is vaporised by indirect heat exchange against nitrogen-rich overhead vapour from the auxiliary column 14 to produce a stream 26 of pressurised GAN and nitrogen-rich liquid which is used as reflux for the auxiliary column 14. A stream 28 of nitrogen-rich liquid is taken from the auxiliary column 14 and, after suitable pressure adjustment, fed as reflux to the ASU (not shown). A stream 30 of oxygen-enriched bottoms liquid is taken from the auxiliary column 14 and also, after suitable pressure adjustment, fed as reflux to the ASU (not shown).

[0037] The process depicted in Figure 2 is similar to that depicted in Figure 1 and the same reference numerals have been used to refer to the features that are equivalent in each figure. The following discussion relates only to the features depicted Figure 2 that are different from the features depicted in Figure 1.

[0038] In Figure 2, the reboiler/condenser 24 has been replaced with a further distillation zone or section 32 in the auxiliary column 14. Nitrogen-rich overhead vapour comes into direct contact with pressurised LIN from stream 22 in the further distillation column section 32. Both heat and mass is transferred to produce nitrogen-rich liquid (removed as stream 28) and pressurised GAN (removed as stream 26).

[0039] Referring now to Figure 3, a stream 100 of feed air is compressed in compressor 102 and water and carbon dioxide are removed from the resultant stream of compressed feed air using purifying unit 104. The purified compressed feed air is split into three streams 106, 108 and 122.

[0040] Stream 108 is cooled in the main heat exchanger 112 by indirect heat exchange against warming product streams to produce a stream 110 of cooled, compressed feed air which is then fed to the bottom of an HP column 124 of a dual distillation column system.

[0041] Stream 122 is further compressed in compressor 123 to produce a stream 126 of further compressed feed air which is then cooled in the main heat exchanger 112 to produce a stream 128 of cooled, further compressed feed air which is then fed, after suitable pressure adjustment, to an intermediate location on the HP column 124. A second stream 325 of further compressed feed air is removed from an intermediate stage of the compressor 123 and cooled in the main heat exchanger 112 and then fed as stream 127 to the bottom of an auxiliary column 196.

[0042] Stream 106 of compressed feed air is further compressed in compressor 115 and the further compressed feed air is cooled in the main heat exchanger 112 to an intermediate temperature between the warm and cold ends thereof whereupon it is removed as stream 116 and expanded in expander 118 to provide refrigeration for the process. The expanded stream 120 of feed air is fed to an intermediate location of the LP column 150 of the dual distillation column system.

[0043] The feed air fed as streams 110 and 128 to the HP column 124 is separated into HP column oxygen-rich bottoms liquid and HP column nitrogen-rich overhead vapour. The feed air fed as stream 127 to the auxiliary column 196 is separated into auxiliary column oxygen-rich bottoms liquid and auxiliary column nitrogen-rich overhead vapour. A stream 167 of auxiliary column oxygen-rich bottoms liquid is removed from the auxiliary column 196, reduced in pressure via valve 168 and combined with a stream of HP column oxygen-rich bottoms liquid from the HP column to form a stream 152 of oxygen-rich bottoms liquid which is fed, via valve 153, to an intermediate location of the LP column 150. A stream 130 of liquid is removed from an intermediate location of the HP column 124 and fed, after pressure reduction via valve 131, to the LP column 150.

[0044] A stream 158 of HP column nitrogen-rich overhead vapour is removed from the top of the HP column 124 and condensed in condenser 160 located in the sump of the LP column 150 by indirect heat exchange against LOX to produce a stream 162 of LIN. A portion of the LIN from stream 162 is fed as reflux to the top of the HP column 124. A second portion is fed as stream 170, following pressure reduction across valve 171, as reflux to the top of the LP column 150. A third portion is fed as stream 163 to a pump 164 where it is pumped to produce a stream 165 of pressurised LIN.

[0045] Stream 165 of pressurised LIN is boiled in reboiler/condenser 161 by indirect heat exchange against a stream 159 of auxiliary column nitrogen-rich overhead vapour to produce a stream 166 of pressurised GAN and a stream of nitrogen-rich liquid. The pressurised GAN stream 166 is warmed in the main heat exchanger 112 against cooling feed air to produce a product stream 168 of pressurised GAN. A portion of the nitrogen-rich liquid is fed as reflux to the top of the auxiliary column 196 and the remaining portion is fed as stream 169, after pressure reduction via valve 173, as reflux to the top of the LP column 150.

[0046] The feed streams to the LP column 150 are separated into LP nitrogen overhead vapour and LOX. A stream 180 of LOX is removed from the LP column 150 and pressurised in pump 182 to produce a stream 184 of pressurised LOX which is warmed in the main heat exchanger 112 to produce a stream 186 of GOX. A stream 172 of gaseous nitrogen is removed from the top of the LP column 150 and warmed in the main heat exchanger 112 to produce a stream 176 of LPGAN.

[0047] The processes depicted in Figures 4 to 6 are similar to that depicted in Figure 3 and the same reference numerals have been used to refer to the features that are equivalent in each figure. The following discussion relates only to the features depicted Figures 4 to 6 that are different from the features depicted in Figure 3.

[0048] In Figure 4, the auxiliary column 196 does not have a reboiler/condenser 161. Instead, the column has a further distillation zone 503 located above the main distillation zone 501. Stream 165 of pressurised LIN is fed to the top of the auxiliary column 196 and stream 166 of pressurised GAN is removed from the top of the auxiliary column 196. Stream 169 of nitrogen-rich liquid is removed from an intermediate location in the auxiliary column 196 and fed, after pressure reduction across valve 173, to the top of the LP column 150.

[0049] The process depicted in Figure 5 is similar to that depicted in Figure 4 with the auxiliary column 196 having a further distillation section 503 located above the main distillation section 501. An intermediate reflux stream to the HP column 124 is provided by a stream 169 of nitrogen-rich liquid which is reduced in pressure across valve 173. Reflux for the LP column 150 is provided by stream 170 which is withdrawn from an intermediate location in the HP column, reduced in pressure across valve 171 and then fed to the top of the LP column 150.

[0050] The process depicted in Figure 6 is similar to that depicted in Figure 3 with the auxiliary column 196 having a reboiler/condenser 161. The main difference is in the air feed of the process. In Figure 6, the stream 100 of air is compressed in compressor 102 and the compressed stream purified in purifier 104 to remove the water and carbon dioxide. A stream 105 of purified, compressed air is divided into three separate streams 107, 108 and 122.

[0051] Stream 108 of purified, compressed air is cooled in the main heat exchanger 112 and the cooled stream 110 is expanded in expander 118 to provide a portion of the refrigeration duty for the process. The resultant expanded air stream is fed to the bottom of the HP column 124.

[0052] Stream 122 is further compressed in compressor 123 to produce a stream 126 of further compressed air which is cooled in the main heat exchanger 112. The cooled, further compressed air is then fed, after pressure adjustment, as stream 128 to an intermediate location of the HP column 124.

[0053] Stream 107 is divided into two sub-streams. The first sub-stream is cooled in the main heat exchanger 112 and then combined, after pressure adjustment, with the cooled, further compressed air from stream 126 and fed as stream 128 to the HP column 124. The second sub-stream 325 is cooled in the main heat exchanger to form a stream 127 of cooled feed air which is fed to the bottom of the auxiliary column 196.

[0054] Stream 169 of nitrogen-rich liquid from the condenser 161 to the LP column 150, stream 165 of pumped LIN from the HP column 124 to the condenser 161, stream 170 of LIN from the HP column 124 to the LP column 150 and stream 130 of fluid from an intermediate location of the HP column 124 to the LP column 150 all pass through at least a portion of the main heat exchanger 112 in order that the temperature of each stream be adjusted to better match the temperatures of locations in the columns to which they are being fed. For simplicity, the streams are not shown as passing through the main heat exchanger 112 but instead the exchanger 112 has been shown as split up.

[0055] The process in Figure 6 also differs from that in Figure 3 in that a stream 190 of LIN and a stream 185 of pumped LOX are removed as product streams.

EXAMPLE

[0056] A computer simulation of the process depicted in Figure 6 has been carried out to exemplify the invention. In the simulation, the auxiliary column 196 had only 10 theoretical stages whereas it would need about 50 stages if it were to produce pure nitrogen directly with no return of impure nitrogen to the rest of the ASU. The results of the simulation are indicated in the Table 1.

TABLE 1

Feed
Streams		105	107	108	122	325
F	kmol/h	10000	2917	4667	2416	2519
P	bar (MPa)	13.4 (1.34)	13.4 (1.34)	13.4 (1.34)	13.4 (1.34)	13.4 (1.34)
T	deg C	30	30	30	30	30
%O2		20.96	20.96	20.96	20.96	20.96
%N2		78.11	78.11	78.11	78.11	78.11
%Ar		0.93	0.93	0.93	0.93	0.93
Phase		vapour	vapour	vapour	vapour	vapour

Product
Streams		168	176	185	186	190
F	kmol/h	2526	5693	42	1688	51
P	bar (MPa)	12.5 (1.25)	1.2 (0.12)	34 (3.4)	33.4 (3.34)	4.9 (0.49)
T	deg C	27	27	-177.7	27	-189.3
%O2		1 ppm	6.50	99.85	99.85	1 ppm
%N2		99.89	92.00	0.00	0.00	99.89
%Ar		0.11	1.50	0.15	0.15	0.11
Phase		vapour	vapour	liquid	vapour	liquid

Intermediate
Streams		127	165	166	167	169
F	kmol/h	2519	2526	2526	1189	1330
P	bar (MPa)	12.8 (1.28)	12.7 (1.27)	12.7 (1.27)	12.8 (1.28)	12.7 (1.27)
T	deg C	-161	-170.3	-165.4	-161	-164.3
%O2		20.96	1 ppm	1 ppm	33.90	9.40
%N2		78.11	99.89	99.89	64.90	90.00
%Ar		0.93	0.11	0.11	1.20	0.60
Phase		vapour	liquid	vapour	liquid	liquid

[0057] Throughout the specification, the term "means" in the context of means for carrying out a function, is intended to refer to at least one device adapted and/or constructed to carry out that function.

[0058] It will be appreciated that the invention is not restricted to the details described above with reference to the preferred embodiments but that numerous modifications and variations can be made without departing from the spirit or scope of the invention as defined by the following claims.

Claims

1. A process for the production of pressurised gaseous nitrogen ("GAN") comprising;
producing liquid nitrogen ("LIN") in a cryogenic air separation unit ("ASU");
increasing the pressure 124, 150 of at least a portion 163 of said LIN to produce pressurised LIN 165;
separating a fluid having an oxygen concentration at least equal to that of air in an auxiliary cryogenic distillation column 196 to produce nitrogen-rich overhead vapour 159 and oxygen-enriched bottoms liquid 167;
transferring heat and optionally mass 166 between at least a portion of said nitrogen-rich overhead vapour 159 and at least a portion of said pressurised LIN 165 to produce nitrogen-rich liquid and pressurised GAN 166; and
feeding 169 at least a portion of said nitrogen-rich liquid as reflux to the ASU after suitable pressure adjustment 173.

2. A process as claimed in any of the preceding claims wherein at least a portion of said oxygen-enriched liquid 167 is fed to the ASU after suitable pressure adjustment 168.

3. A process as claimed in any of the preceding claims wherein said fluid is air 127.

4. A process as claimed in Claim 1 or Claim 2 wherein said fluid is oxygen-rich fluid from the ASU.

5. A process as claimed in any of the preceding claims wherein a portion of said nitrogen-rich liquid is used to reflux said auxiliary column 196.

6. A process as claimed in any of the preceding claims wherein said portion of LIN 163 is pumped 164 to increase the pressure thereof.

7. A process as claimed in any of the preceding claims wherein heat is exchanged indirectly 161 between at least a portion of said nitrogen-rich overhead vapour 159 and at least a portion of said pressurised LIN 165 thereby condensing said nitrogen-rich overhead vapour to produce nitrogen-rich liquid 169 and vaporising said pressurised LIN to produce pressurised GAN 166.

8. A process as claimed in Claim 7 wherein said heat is exchanged indirectly using a reboiler/condenser 161.

9. A process as claimed in any of Claims 1 to 6 wherein heat and mass are transferred directly between at least a portion of said nitrogen-rich overhead vapour 159 and at least a portion of said pressurised LIN 165 to produce nitrogen-rich liquid 169 and pressurised GAN 166.

10. A process as claimed in Claim 9 wherein the auxiliary column 196 has at least a main distillation zone 501 and vapour/liquid contact promoting means 503 provided above the main distillation zone 501, said process comprising contacting directly nitrogen-rich overhead vapour with pressurised LIN 165 in said contact promoting means 503 to produce said nitrogen-rich liquid 169 and pressurised GAN 166.

11. A process as claimed in Claim 10 wherein said contact promoting means is a further distillation zone 503.

12. A process as claimed in any of the preceding claims wherein said fluid is gaseous.

13. A process as claimed in any of Claims 1 to 11 wherein said fluid is liquid.

14. A process as claimed in Claim 13 wherein at least a portion of said liquid is vaporised by indirect heat exchange against a process stream using a reboiler/condenser provided in the sump of said auxiliary column.

15. A process as claimed in any of the preceding claims wherein the ASU comprises a higher pressure ("HP") distillation column 124 and a lower pressure ("LP") distillation column 150, said HP column 124 being thermally integrated with said LP column 150 via an ASU reboiler/condenser 160, said process further comprising:

separating air 110, 128 in said HP column 124 into HP nitrogen-rich overhead vapour 158 and oxygen-rich bottoms liquid;

separating at least a portion of said oxygen-rich bottoms liquid in said LP column 150 into LP nitrogen-rich overhead vapour 172 and liquid oxygen ("LOX") 180;

cooling and at least partially condensing at least a portion of said HP nitrogen-rich overhead vapour 158 in said ASU reboiler/condenser 160 by indirect heat exchange against LOX to produce said LIN; and

refluxing said HP column with a portion of said LIN.

16. A process as claimed in Claim 15 further comprising using a portion of said oxygen-rich bottoms liquid as at least a portion of said fluid after suitable pressure adjustment.

17. A process as claimed in Claim 15 or Claim 16 wherein the operating pressure of said auxiliary column 196 is higher than the operating pressure of said LP column 150.

18. A process as claimed in any of Claim 15 to 17 wherein the operating pressure of said auxiliary column 196 is higher than the operating pressure of said HP column 124.

19. A process as claimed in any of the preceding claims wherein the operating pressure of the auxiliary column 196 is from about 0.10 MPa (1.0 bara) to about 3.0 MPa (30 bara).

20. A process as claimed in any of the preceding claims wherein the pressurised GAN is produced at a pressure from about 0.15 MPa (1.5 bara) to about 2.5 MPa (25 bara).

21. A process as claimed in any of the preceding claims wherein the purity of said pressurised GAN is from about 99.9 mol % nitrogen to about 99.9999 mol % nitrogen.

22. Apparatus for the production of pressurised GAN comprising:

a cryogenic ASU comprising at least one distillation column 124, 150, for producing LIN;

pressurising means 164 for pressurising LIN;

conduit means 163 for feeding LIN from the top of the column of the ASU producing said LIN to the pressurising means 164;

an auxiliary cryogenic distillation column 196 comprising a main distillation zone 501 for separating a fluid having an oxygen concentration at least equal to that of air into nitrogen-rich overhead vapour and oxygen-enriched bottoms liquid;

transfer enabling means 161, 503 for enabling heat and optionally mass transfer between at least a portion of said nitrogen-rich overhead vapour and at least a portion of said pressurised LIN to produce nitrogen-rich liquid and pressurised GAN;

conduit means 165 for feeding pressurised LIN from the pressurising means to the transfer enabling means 161, 503;

pressure reducing means 173 for reducing the pressure of nitrogen-rich liquid to produce reduced pressure nitrogen-rich liquid;

conduit means 169 for feeding nitrogen-rich liquid from the auxiliary column 196 to the pressure reducing means 173; and

conduit means for feeding reduced pressure nitrogen rich liquid from said pressure reducing means 173 to the ASU 150 as reflux.

23. Apparatus as claimed in Claim 22 wherein the transfer enabling means is condensing means 161 for condensing at least a portion of nitrogen-rich overhead vapour by indirect heat exchange against pressurised LIN to produce nitrogen-rich liquid and pressurised GAN.

24. Apparatus as claimed in Claim 22 wherein said transfer enabling means is vapour/liquid contact promoting means 503 for promoting direct contact between nitrogen-rich overhead vapour and pressurised LIN , said contact promoting means being located within the auxiliary distillation column 196 above the main distillation zone 501.

25. Apparatus as claimed in Claim 24 wherein the contact promoting means is a further distillation zone 503 in the auxiliary column 196.

26. Apparatus as claimed in any of Claims 22 to 25 further comprising:

pressure reducing means 168 for reducing the pressure of oxygen-enriched liquid to produce reduced pressure oxygen-enriched liquid;

conduit means for feeding oxygen-enriched liquid from the auxiliary column 196 to the pressure reducing means 168; and

conduit means 152 for feeding reduced pressure oxygen-enriched liquid from the pressure reducing means to the ASU.

Drawing