EP1202013B3

EP1202013B3 - Process and apparatus for the production of low pressure gaseous oxygen

Info

Publication number: EP1202013B3
Application number: EP20010308922
Authority: EP
Inventors: Rodney J. Allam; Alan Lindsay Prentice
Original assignee: Air Products and Chemicals Inc
Current assignee: Air Products and Chemicals Inc
Priority date: 2000-10-23
Filing date: 2001-10-19
Publication date: 2009-04-01
Anticipated expiration: 2021-10-19
Also published as: EP1202013B1; EP1202013A1

Description

The present invention relates generally to the production of oxygen gas ("GOX") and, in particular, to the production of low pressure GOX by the cryogenic distillation of air.
There is a considerable market, particularly in the glass and metallurgical industries, for low purity, e. g. from 90 to 98 wt %, low pressure, e.g. from 1.5 to 3.0 bar absolute (0.15 to 0.3 MPa), GOX. The GOX is used in processes requiring oxygen-enriched combustion in which the required pressure of the oxygen at the point of use is near atmospheric.
An O₂ vacuum swing absorption ("VSA") process is commonly used for applications requiring 90 to 93 % O₂. However, up to 98 % O₂ GOX is often required and, thus, cryogenic plant processes are also used. There are many prior public disclosures of processes using cryogenic distillation of airto produce a GOX product. A number of the disclosed processes use a liquid cryogen from an external source as a refrigerant. For example, in US-A-4853015 (Yoshino ) and US-A-4732595 (Yoshino ), liquid oxygen ("LOX") is injected into the low pressure column of a double column distillation system to provide refrigeration. In US-A-4732595 , an expander is used to provide some of the refrigeration requirement of the process. The use of such an expander increases the overall capital and running costs of the process and, as such, is undesirable.
In US-A-5408831 (Guillard et al ), air is distilled cryogenically in a double distillation column system without the use of an expander to provide a portion of the refrigeration duty. GOX at from 2 to 5 bar absolute (0.2 to 0.5 MPa) is taken from the low pressure ("LP") column of the distillation column system as product. It is an essential feature of the Guillard process that some refrigeration is provided by expansion of at least one gaseous product from an LP column of the distillation column system. Part of the refrigeration duty required to condense the feed air fed to the column system can be provided by LOX refrigerant from an external source. The refrigerant may be introduced into the LP column or into the GOX product at an intermediate location of the main heat exchanger. The actual temperature at which the LOX is introduced is chosen to minimise the risk of explosion of any hydrocarbon impurities.
In order to achieve the required pressure of the GOX product, the column system in US-A-5408831 is back-pressurised. As a result of the back-pressurisation of the column system, the air pressure is necessarily higher, at a pressure from 8 to 16 bar absolute (0.8 to 1.6 MPa), than that in processes without column back-pressurisation giving a significant power penalty of about 12% for a given air flow. Such a penalty represents an undesirable increase in operating cost especially when it is considered that power is the main operating cost of an air separation plant.
Processes producing low pressure GOX in which air compressor power is minimised without adversely effecting both the overall capital and running costs are desirable. In this connection, it is known to provide at least part of the refrigeration duty required to cool and at least partially condense by heat exchange feed air prior to cryogenic distillation through the use of a LOX refrigerant from an external source.
US-A-5505052 (Ekins et al ) discloses a process for the cryogenic distillation of air using a double column system having a high pressure ("HP") column and a LP column to produce GOX at a pressure of about 25 bar (2.5 MPa) for use in installations comprising, for example, electric arc furnaces adapted to produce stainless steel. Oxygen is withdrawn in liquid form from the base of the LP column, brought to the utilisation pressure by a pump and vaporised and reheated to about ambient temperature in the heat exchange line against the feed air. The gaseous oxygen is then fed to the installation.
A portion of the LOX withdrawn from the base of the column may be sent to storage, for example, during periods of low demand for GOX in the installation where it is kept until such time as the demand for GOX at the installation becomes high whereupon it is pumped to the utilisation pressure and vaporised and reheated to about ambient temperature in the heat exchange line against the feed air. The LOX from storage may travel through the same vaporisation passages through the heat exchange line as the LOX from the column system or it may travel through separate vaporisation passages. The gaseous oxygen is then fed to the installation.
Additional LOX may be added to the LOX in storage from tank trucks, for example, during prolonged periods of high demand for GOX. Alternatively, the storage facility may not be connected to the double column system and may be supplied only by tank trucks. In the exemplified embodiments of the process disclosed in Ekins et al, LOX, whether from the double column system or from storage, enters the heat exchange line at the cold end, i.e. the end at which cooled feed air exits the line.
In Ekins et al, both the LOX product from the distillation column and the additional LOX from storage are pumped to a pressure (about 25 bar (2.5 MPa)) that is substantially higher that the pressure (about 5 to 6 bar (0.5 to 0.6 MPa)) of the LP column. In addition, a portion of the total refrigeration duty requirement of the process is provided by an expander and a further portion is provided by the warming and evaporation of a stream of liquid argon.
There is a need for a process and apparatus for the production of low pressure GOX with lower capital and operating costs compared with existing processes. There is also a need for a process for the production of lower pressure GOX in which the risk of explosion resulting from deposition in the heat exchange line of impurities, for example hydrocarbons, CO₂ and N₂O, from LOX is reduced. This risk is discussed below in more detail in the general description of the invention.
According to a first aspect of the present invention, there is provided a process for the production of GOX, said process comprising:

cooling and at least partially condensing feed air by heat exchange using heat exchange means having a warm end and a cold end to produce cooled and at least partially condensed feed air;
distilling said cooled and at least partially condensed feed air in a distillation column system to produce LOX product;
removing a stream of said LOX product from the distillation column system, pressurising said stream (8) by hydrostatic pressure only and vaporising said pressurised LOX product stream by heat exchange against the feed air to produce GOX at a pressure from 0.15 MPa (1.5 bar absolute) to 0.3 MPa (3.0 bar absolute); and

(a) at a pressure greater than that of the LOX product entering the heat exchange means; or
(b) at a pressure substantially equal to that of the LOX product entering the heat exchange means and at an intermediate point between the warm and cold ends where the temperature of the heat exchange means is above the boiling temperature of the LOX refrigerant.

The heat exchange means comprises a "warm end" (or "hot end") and a "cold end". The warm end (or hot end) is the end at which the feed air enters the heat exchange means and the cold end is the end at which the cooled and at least partially condensed feed air leaves the heat exchange means. The terms "warm end" (or "hot end") and "cold end" are commonly used in the art to distinguish the two ends of heat exchange means by their relative temperatures.
GOX may be produced at a slightly elevated pressure by a known technique in which LOX product is withdrawn from the LP column of a double column system. LOX is withdrawn from the distillation column system and is vaporised and warmed by heat exchange against the feed air. A fraction of the feed air is condensed by heat exchange against the withdrawn LOX and, thus, there is less air vapour entering the distillation column system than there would otherwise be if the feed air were to be subjected to indirect heat exchange with GOX withdrawn from the column system. This has the effect of reducing the efficiency of the distillation when compared to a process in which GOX is withdrawn from the distillation column system. However, as only low purity GOX is required, there is no performance penalty as about 99.7 % of the O₂ becomes product. Therefore, the process is surprisingly efficient.
One reason for using LOX as the refrigerant is that the vaporised LOX refrigerant may be combined with the GOX produced by the vaporisation of the LOX product to produce GOX product. In this way, there is no wastage of vaporised refrigerant.
Preferably, the distillation column system comprises a multiple column system having a higher pressure ("HP") column and a lower pressure ("LP") column thermally integrated by the condensation of nitrogen overhead from the HP column against liquid bottoms in the LP column. A portion of the condensed HP column nitrogen overhead may be subcooled by heat exchange to produce a subcooled nitrogen stream which can be fed to the LP column.
In preferred processes, substantially all of the refrigeration duty required to keep the plant in energy balance is provided by the LOX refrigerant. Preferably, no refrigeration duty is provided by expansion of a process stream. Any heat leak into the process via the insulation and the fact that the product streams leave the heat exchanger at a temperature that is slightly lower then the entry temperature of the feed air is taken into account in calculating the amount of LOX refrigerant required.
Surprisingly, vaporising and warming LOX refrigerant separately from the product LOX has little effect on the temperature profiles of the main heat exchanger and causes only a very small increase in the amount of the refrigerant. This is particularly apparent for a process of the invention in which the LOX refrigerant is vaporised and warmed at substantially the same pressure as the LOX product but is introduced to the heat exchanger at an intermediate point between the warm and cold ends of the heat exchanger.
The LOX refrigerant is vaporised separately from the LOX product to reduce the risk of any problems resulting from the build up of hydrocarbon impurities such as ethylene due to deposition of CO₂ and N₂O on the interior wall surfaces of the boiling passages through the heat exchanger.
Commercial sources of LOX refrigerant, e.g. produced by an air separation plant, will contain hydrocarbons, CO₂ and N₂O impurities. The concentration of these impurities in the LOX refrigerant will vary depending on the plant producing it, the mode of operation of the plant and the ratio of LOX produced to feed air entering the plant. Concentrations of about 1500 ppb (vol.) CO₂ and about 3000 ppb (vol.) N₂O are typical.
As the LOX product boils at about 2.2 bar absolute (0.22 MPa), the vapour phase solubility of CO₂ and N₂O impurities is about 50 ppb (vol.) and about 500 ppb (vol.) respectively. If the LOX refrigerant were to be introduced directly into the LOX product stream, the impurity concentration of the combined LOX stream would be sufficiently increased to warrant concern about the unwanted and dangerous build up of impurity deposits in the heat exchanger. Even a very small amount of "slippage" of CO₂ and N₂O from the air purification will cause the concentration of CO₂ and/or N₂O to exceed the vapour phase solubility limit and result in at least partial blockage of the heat exchanger by deposited CO₂ and N₂O.
In one embodiment, vaporising LOX refrigerant without causing blockage of the heat exchanger by CO₂ and N₂O deposits is achieved by vaporising the refrigerant at a greater pressure than the LOX product such that, at the boiling temperature of the LOX refrigerant, the CO₂ and N₂O impurity concentrations are below the vapour phase solubility limits.
In an alternative embodiment, blockage of the heat exchanger by unwanted impurity deposits is avoided by injecting the LOX refrigerant into the heat exchange means at a pressure that is substantially equal to the pressure of the LOX product as it enters the heat exchange means, provided that the point of injection is between the warm and cold ends of the heat exchange means. Preferably, the temperature of the heat exchange means at the intermediate point of injection is from -165°C to -80°C, i.e. substantially above the O₂ boiling temperature.
In this alternative embodiment, as the preferred temperature of the point of injection in the heat exchanger is relatively warm, CO₂ and N₂O solubilities are relatively high and deposition on the surfaces of the heat exchanger will not occur. For the O₂ vaporisation pressure, i.e. about 1.5 bar absolute (0.15 MPa) to 3.0 bar absolute (0.3 MPa), vaporisation actually occurs at about -179°C to about -171°C respectively and any solid CO₂ and N₂O that form initially does not deposit on the metal but is carried onwards towards a warmer part of the heat exchanger and after a short distance the whole stream has reached or exceeded about -165°C to -80°C by which time CO₂ and N₂O will have sublimed into vapour and cannot precipitate.
Before vaporising the LOX product stream by heat exchange to provide GOX, it is pressurised hydrostatically by, for example piping the stream from the distillation column system to a lower elevation.
The pressure of the LOX product leaving the distillation column system is usually about 1.4 bar absolute (0.14 MPa). The pressure of the LOX refrigerant is preferably from 4 bar absolute (0.4 MPa) to 10 bar absolute (1.0 MPa). The pressure of the GOX product is from 1.5 bar absolute (0.15 MPa) to 3.0 bar absolute (0.3 MPa), preferably from 1.8 bar absolute (0.18 MPa) to 2.5 bar absolute (0.25 MPa).
The process may further comprise combining LOX refrigerant with the cooled and at least partially condensed feed air to further cool the feed air, preferably during plant cooldown. The process may also comprise introducing LOX refrigerant to the distillation column system under level control.
Preferably, the LOX refrigerant is provided by an air separation plant.
The process may further comprise withdrawing at least one nitrogen gas product stream from the distillation column system.
The feed air is preferably purified before heat exchange to reduce the CO₂ and N₂O impurity concentrations to a level which ensures that these impurity concentrations in the LOX product are below their vapour phase solubilities at the vaporising pressure and temperature heat exchange conditions. The feed air may be purified using, for example, either a temperature swing absorber system using alumina and CaX or a pressure swing adsorber system using alumina and 13X.
In a second aspect of the present invention, there is provided apparatus for carrying out the process of the first aspect of the present invention for producing gaseous oxygen, said apparatus comprising:

heat exchange means for cooling and at least partially condensing feed air to produce cooled and at least partially condensed feed air, said heat exchange means having a warm end and a cold end;
a distillation column system for distilling cooled and at least partially condensed feed air to produce LOX product;
conduit means to carry cooled feed air from the heat exchange means to the distillation column system; and
conduit means to carry LOX product from the distillation column system to the heat exchange means not having means for pumping the LOX product stream, whereby said stream is pressurized by hydrostatic pressure during passage from the distillation column system to the heat exchange means; and said apparatus further comprising either:
1. (a) conduit means to carry LOX refrigerant at a greater pressure than the pressure of the LOX product entering the heat exchange means from an external supply to the heat exchange means but does not comprise an expander for expanding a process stream to provide refrigeration duty; or
2. (b) conduit means to carry LOX refrigerant at a pressure that is substantially equal to the pressure of the LOX product entering the heat exchange means from an external supply to an intermediate point between the warm and cold ends of the heat exchange means where the temperature of the heat exchange means is above the boiling temperature of the LOX refrigerant and the heat exchange means has separate circuits for LOX refrigerant and LOX product.

The apparatus is preferably adapted or constructed to carry out any combination of the preferred features of the process discussed above.
It is preferable to minimise the amount of LOX refrigerant consumed by the process as it is expensive. The amount required is highly dependent on the number of transfer units (NTU) of the heat exchange means. NTU is defined as follows: $NTU = (T_{arihot} - T_{aircold}) / mean DT$

Where

T_airhot =: air temperature of hot end of heat exchanger;
T_aircold =: air temperature of cold end of heat exchanger
mean DT =: effective mean differential temperature between hot and cold stream in heat exchanger between air entry and exit.

The heat exchange means of the apparatus may have at least 55 NTU, preferably from 70 to 90 NTU and more preferably about 80 NTU.
The amount of LOX refrigerant consumed in the process is also very dependent on the heat gain through the insulation. Preferably, the cryogenic portion of the apparatus, i.e. the distillation column system and the heat exchange means, is vacuum insulated to reduce heat loss.
In particularly preferred embodiments, the LOX refrigerant passes through the heat exchange means via a separate circuit to the LOX product. The LOX refrigerant preferably passes through the heat exchange means via a single passage. Further, the LOX refrigerant is preferably introduced into the heat exchange means at an intermediate point between the cold and warm ends of the heat exchange means where the temperature of the metal of the heat exchange means is above the boiling temperature of the refrigerant.
The following is a description by way of example only and with reference to the accompanying drawing, which is a flowsheet of a presently preferred embodiment of the invention.
A purified and compressed feed air stream 1, having a concentration of CO₂ and N₂O low enough to prevent deposition in the main LOX circuit of the main heat exchanger E1, enters the main heat exchanger E1, preferably a plate-fin type, wherein it is cooled to a cryogenic temperature and at least partially condensed. An at least partially condensed feed airstream 2 is removed from the main heat exchanger E1 and fed to the high pressurecolumn C1 in an double distillation column system C1, C2 having a reboiler condenser E2.
The feed air stream 2 is distilled in the high pressure column C1 and a nitrogen-rich vapour stream 3 is condensed in the condenser E2 to produce a condensed nitrogen-rich stream 13. A portion 5 of the condensed nitrogen-rich stream 13 is returned to the high pressure column C1 as reflux to purify gas rising and the remaining portion 4 is sent to the top of the low pressure column C2 via the main heat exchanger E1 where it is subcooled. An oxygen-rich stream 6 is removed from the high pressure column C1 and fed to the low pressure column C2 at an intermediate location optionally via a heat exchanger to subcool the stream.
The two liquid streams 4, 6 entering the low pressure column C2 are distilled due to vapour rising from the reboiler E2. A low pressure waste nitrogen vapour stream 7 is withdrawn from the top of the low pressure column and warmed to ambient temperature in the main heat exchanger E1. A LOX product stream 8 is withdrawn from the bottom of the low pressure column C2 and piped to a lower elevation to gain static pressure before being vaporised and then warmed to ambient temperature in the main heat exchanger E1 to form GOX stream 9. In this way, GOX at a pressure typically from 1.8 to 2.5 bar absolute (0.18 to 0.25MPa) may be obtained directly from the plant.
Even though the cryogenic part of the plant is vacuum insulated to minimise heat loss, some refrigeration must be supplied to maintain a refrigeration balance. A LOX refrigerant stream 10 is introduced to a separate circuit of the main heat exchanger E1 at an intermediate point between the warm and cold ends and at a pressure equal to or higher than that for the LOX product stream 8 in a manner to avoid the deposition of CO₂ and N₂O. The LOX refrigerant stream 10 is vaporised and warming to ambient temperature to produce a stream 11 of vaporised LOX refrigerant which is combined with the GOX stream 9 to form a GOX product stream 12.
In a specific example, an airflow of 10000 Nm³/ h (167 Nm³/s) is compressed to about 6 bar absolute (0.6 MPa), purified,-cooled in the main heat exchanger E1 and fed to the H P column C1 at5.5 bar absolute (0.55 MPa). LOX refrigerant from an external source at a purity of about 99.8 % O₂ is injected into the main heat exchanger E1 at a flow rate of about 50 Nm³/h (0.8 Nm³/ s) where it is vaporised and warmed to ambient temperature. LOX product at about 95 % O₂ purity and at a contained O₂ flow of 2090 Nm³/h (35 Nm³/s) leaves the low pressure column C2 at about 1.4 bar absolute (0.14 MPa). The pressure of the LOX product stream is increased by about 0.8 bar absolute (0.08 MPa) due to static head and after vaporisation and warming leaves the main heat exchanger at 2.0 bar absolute (0.2 MPa). The two warmed GOX streams are combined giving a contained O₂ flow of 2140 Nm³/h (36 Nm³/s) of GOX.
The economics of the present invention compare favourably with those of O₂ VSA plants at product flows above 870 Nm³/h (15 Nm³/s). The present invention has the same or lower gas cost without the much higher capital cost or reliability issues of the O₂ VSA plants. In addition, the economics of the present invention also compare favourably with those of a cryogenic plant with an expander at a capacity of about 3480 Nm³/ h (58 Nm³/s). Again, the present invention is economic having the same gas cost and lower capital cost.
It is not obvious to introduce the LOX refrigerant into the heat exchanger at an intermediate point between the warm and cold ends to provide refrigeration for several reasons. First, it is less efficient thermodynamically to provide refrigeration by indirect heat exchange using a stream of LOX refrigerant injected into a heat exchanger separately from the LOX product rather then to provide equivalent refrigeration by injection of the LOX refrigerant directly into the LOX product stream before vaporisation in the heat exchanger. In addition, the design of a heat exchanger that is suitable for carrying out the invention is more complicated and therefore more expensive than a conventional heat exchanger. Further, it is simply inefficient thermodynamically to inject a LOX refrigerant stream into a warm part of a heat exchanger. One reason for injecting the LOX refrigerant in this way is to reduce the likelihood of an explosive energy release following the build up of hydrocarbons as a result of the deposition of dissolved impurities in the LOX.
Some of the advantages of the exemplified embodiment of the present invention are as follows:

the distillation column system is not back-pressurised and hence the air pressure is minimised;
GOX is produced with minimal air flow as substantially all of the O₂ in the feed air and all of the LOX refrigerant becomes GOX product;
as a result of the lack of back-pressurisation and minimal air flow, the air compressor power is minimised;
GOX is produced at about 2.0 bar absolute (0.2 MPa) directly from the cryogenic section; and
as a result of using vacuum insulation and a main heat exchanger with at least 55 NTU, the amount of LOX refrigerant is minimised.

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 scope of the invention as defined in the following claims.

Claims

A process for the production of gaseous oxygen ("GOX"), said process comprising:
cooling and at least partially condensing feed air (1) by heat exchange using heat exchange means (E1) having a warm end and a cold end to produce cooled and at least partially condensed feed air (2);

distilling said cooled and at least partially condensed feed air (2) in a distillation column system (C1, C2) to produce liquid oxygen ("LOX") product;

removing a stream (8) of said LOX product from the distillation column system (C1, C2), pressurising said stream (8) by hydrostatic pressure only and vaporising said pressurised LOX product stream (8) by heat exchange (E1) against the feed air (1) to produce GOX at a pressure from 0.15 MPa (1.5 bar absolute) to 0.3 MPa (3.0 bar absolute); and
separately from the LOX product, vaporising LOX refrigerant (10) from an external source by heat exchange (E1) against the feed air (1) to produce vaporised refrigerant (11) thereby providing a portion of the refrigeration duty required to cool and at least partially condense the feed air;
said process comprising the LOX refrigerant being injected into the heat exchange means (E1) either:
(a) at a pressure greater than that of the LOX product entering the heat exchange means; or

(b) at a pressure substantially equal to that of the LOX product entering the heat exchange means and at an intermediate point between the warm and cold ends where the temperature of the heat exchange means (E1) is above the boiling temperature of the LOX refrigerant.
A process as claimed in Claim 1, wherein the temperature of the heat exchange means (E1) at the intermediate point at which the LOX refrigerant is injected is from -165°C to -80°C.
A process as claimed in Claim 1 or Claim 2 further comprising combining the vaporised LOX refrigerant (10) with the GOX (9) produced by the vaporisation of the LOX product to produce GOX product (12).
A process as claimed in any one of Claims 1 to 3 wherein the distillation column system comprises a multiple column system having a higher pressure ("HP") column (C1) and a lower pressure ("LP") column (C2) thermally integrated by the condensation of nitrogen overhead from the HP column against liquid bottoms in the LP column.
A process as claimed in any one of Claims 1 to 4, wherein no refrigeration duty is provided by expansion of a process stream.
A process as claimed in any one of Claims 1 to 5, wherein the LOX refrigerant (10) provides all of the external refrigeration duty required to keep the process in energy balance.
A process as claimed in any one of Claims 1 to 6 wherein the pressure of the LOX refrigerant (10) is from 0.4 MPa (4 bar absolute) to 1.0 MPa (10 bar absolute).
A process as claimed in any one of Claims 1 to 7 wherein the pressure of the GOX product (12) is from 0.18 MPa (1.8 bar absolute) to 0.25 MPa (2.5 bar absolute).
A process as claimed in any one of Claims 1 to 8 further comprising combining LOX refrigerant with the cooled and at least partially condensed feed air to further cool the feed air.
A process as claimed in any one of Claims 1 to 9 further comprising introducing LOX refrigerant to the distillation column system under level control.
A process as claimed in any one of Claims 1 to 10 wherein the LOX refrigerant is provided by an air separation plant.
Apparatus for carrying out the process of Claim 1 to produce gaseous oxygen, said apparatus comprising:
heat exchange means (E1) for cooling and at least partially condensing feed air (1) to produce cooled and at least partially condensed feed air (2), said heat exchange means (E1) having a warm end and a cold end;

a distillation column system (C1, C2) for distilling cooled and at least partially condensed feed air (2) to produce LOX product (8);

conduit means to carry the cooled and at least partially condensed feed air (2) from the heat exchange means (E1) to the distillation column system (C1, C2); and

conduit means to carry LOX product (8) from the distillation column system (C1, C2) to the heat exchange means (E1);
said apparatus not having means for pumping the LOX product stream, whereby said stream is pressurized by hydrostatic pressure during passage from the distillation column system to the heat exchange means (E1) and said apparatus further comprising either:
(a) conduit means to carry LOX refrigerant (10) at a higher pressure than the pressure of the LOX product entering the heat exchange means from an external supply to the heat exchange means (E1) but does not comprise an expander for expanding a process stream to provide refrigeration duty; or

(b) conduit means to carry LOX refrigerant (10) at a pressure that is substantially equal to the pressure of the LOX product entering the heat exchange means from an external supply to an intermediate point between the warm and cold ends of the heat exchange means where the temperature of the heat exchange means is above the boiling temperature of the LOX refrigerant and the heat exchange means has separate circuits for LOX refrigerant and LOX product.
Apparatus as claimed in Claim 12 adapted or constructed to carry out the process of option (b) as defined in any one of Claims 2 to 11.
Apparatus as claimed in Claim 12 or Claim 13 wherein the cryogenic portion of the apparatus is vacuum insulated to reduce heat loss.
Apparatus as claimed in any one of Claims 12 to 14 wherein the heat exchange means (E1) has at least 55 NTU.
Apparatus as claimed in any one of Claims 12 to 15 wherein the heat exchange means (E1) has from 70 to 90 NTU.
Apparatus as claimed in any one of Claims 12 to 16 wherein the heat exchange means (E1) has about 80 NTU.
Apparatus as claimed in any of Claims 12 to 17 wherein, in option (a), the heat exchange means has separate circuits for LOX refrigerant and LOX product.
Apparatus as claimed in any of Claims 12 to 18 wherein, in option (b), the apparatus does not comprise an expander for expanding a process stream to provide refrigeration duty.
Apparatus as claimed in any one of Claims 12 to 19 wherein the LOX refrigerant (10) passes through the heat exchange means (E1) via a single passage.