Technical Field

This invention relates to processes and apparatus for separa- ting air by cryogenic fractional distillation to produce gaseous oxygen of 90 to 99.8% purity, plus optional co-product argon or nitrogen. The invention permits higher 0 ₂ production pressure with¬ out offsetting decreases in product purity or recovery. Oxygen is useful in the production of iron, steel, electricity (coal gasification combined cycle), and in many other applications.

Background Art

Cryogenic air separation processes usually incorporate a dual pressure or double column arrangement of distillation columns, wherein compressed, cleaned, and cooled feed air is supplied to a high pressure rectifier; the liquid bottom product (kettle liquid) is fed to a low pressure distillation column; a latent heat exchanger provides overhead reflux to the HP rectifier and reboil (either bottom or intermediate) to the LP column; and the LP column is refluxed by direct injection of HP rectifier overhead liquid nitrogen (LRJ product. Historically, the gaseous 0 ₂ product has usually been generated by latent heat exchange with HP rectifier overhead nitrogen, as shown for example in "Distillation of Air" by R.E. Latimer, Chemical Engineering Progress Volume 63 No. 2 February 1967, published by AIChE, New York. Given HP rectifier nitrogen at a typical pressure of 88 psia (6 atmospheres absolute) and a latent heat exchanger temperature differential of 3°F (1.67K), this establishes the oxygen boiling or evaporating conditions at 23.2 psia and -289°F.

There has been a continuing search for reliable, economical, and efficient means for increasing the oxygen evaporation pressure beyond that achievable from heat exchange with condensing HP rec¬ tifier nitrogen. These efforts have fallen mainly into three cate¬ gories, as follows:

a. Pumped LOX

In this category, the liquid oxygen (LOX) is pumped to the desired delivery pressure, and a fraction of the supply air (25 to 35%) is additionally compressed with an externally powered compressor sufficient to exchange latent heat with the evapora¬ ting LOX. This approach and the attendant problems are described in U.S. Patent 4604116. Prior art U.S. patents include 3110155 and 4372764.

b _^ TC LOXBOIL This variation of Pumped LOX does not require an additional externally-powered compressor. The 25 to 30% of the feed air which • totally condenses to evaporate the liquid oxygen is at the same general pressure as the feed air. Since the bubble point of air at a given pressure is about 4°F warmer than the saturation temperatur of nitrogen at the same pressure, the evaporating oxygen is also * 4°F warmer. This would increase the above cited 23 psia to 28 psia. Prior art examples of this technique include U.S. Patents 3277655, 4133662, and Russian Patent 756150. c. PC LOXBOIL The partial condensation liquid oxygen boil (PC LOXBOIL) approach varies from TC LOXBOIL in that all or a major fraction of the feed air (at feed pressure) is passed through the LOXBOIL heat exchanger, and hence only a minor fraction of the air condenses. Since the exiting vapor and liquid are in equilibrium, the liquid composition will be about 35% 0 ₂ , and the vapor about 17.5% 0 ₂ , as contrasted to the 21% 0 ₂ liquid (liquid air or "LAIR") obtained with TC LOXBOIL. The bubble point of 35% 0 ₂ in N ₂ liquid is about 7°F hotter than Tsat of N ₂ at the same pressure. A 7°F increase in evaporating temperature would increase the pre- viously cited 23 psia oxygen to 32.3 psia. Prior art examples incorporating PC LOXBOIL include U.S. Patents 3327489, 3251190, 3371496, and 4560398. The latter patent states that TC LOXBOIL is undesirable (p. 5 line 65) ^' .

SUBSTITUTESHEET

The use of companders (close coupled compressor and expander) is known generally in cryogenic plants and specifically in air separation plants. The vapor expander used to generate refrig¬ eration in low pressure gaseous oxygen plants generates shaft work approximating 2 to 3% of the main air compressor power. There is little cost difference between driving a small, warm end air compressor or an electrical generator with that shaft output. Prior art examples of cryogenic air separation companders using the refrigeration expander as driving end and a warm air compressor as driven end include U.S. Patents 3261128, 4375367, 4133662, and Russian Patent 756150. The latter two illustrate companded TC LOXBOIL, i.e. the warm air which is further com- pressed in the compander is then used for TC LOXBOIL. The additional pressure of the companded air raises its bubble point to 7 to 9°F higher than that of N ₂ at HP rectifier pressure, and hence 0 ₂ evaporation pressures equal to or greater than those of PC LOXBOIL are obtained by companded TC LOXBOIL.

International (PCT) Publication Number WO 86/06462 published 6 November 1986 discloses a means of increasing the recovery of crude argon from a dual pressure column by increasing the reboil rate up the lower portion of the argon sidearm column and correspondingly decreasing the reboil rate up a section of the nitrogen stripping stages of the low pressure column. This effect is accomplished by exchanging latent heat from an intermediate height of the argon sidearm to an intermediate height of the nitrogen stripping sec¬ tion of the low pressure column. U.S. Patent 4605427 describes a related technique of increasing argon recovery from triple pressure columns. pressure columns.

U.S. Patent 3729943 discloses a high purity oxygen plus argon configuration in which the argon sidearm is refluxed both at the top and at the bottom by latent heat exchange. The evaporating fluid may be N ₂ , which is subsequently expanded. Application 728264 discloses providing intermediate reflux to an argon sidearm by N ₂ evaporation, plus subsequent work expansion.

The problems with the prior art methods of increasing 0„ pressure without providing additional compression energy input are as follows. All of the LOXBOIL variations, with the exception of the above cited application 06/583817, share a commoD problem: limited availability of ₂ for column reflux. Less vapor fed to the HP rectifier inescapably means less L ₂ overhead product. For some air separation processes this does not pose a problem, because the requirement for LN ₂ reflux is inherently low. For example, conventional medium purity 0 ₂ plants (90 to 99% purity) with no argon coproduct and no signifi¬ cant amount of pressurized ₂ byproduct have adequate LN ₂ reflux for 95+% recovery of 0 even when LOXBOIL is used. Such plants are thus the ones disclosed in U.S. Patents 4133662, 4560398, 3251190, and Russian Patent 756150. Other processes, however, require more LN ₂ reflux than that available from the prior art UDXBOIL disclosures, and, as a result, if LOXBOIL were applied to those processes, the N ₂ reflux deficiency would cause product recovery to decline to such an extent that the increase in 0 ₂ delivery pressure has less value than the loss. Examples of processes which would suffer from the application of known LOXBOIL techniques are as follows: i) High purity oxygen plus argon.

Argon is recovered in a sidearm column which.is refluxed by evaporating part of the kettle liquid. It can readily be demonstrated on a McCabe-Thiele diagram that the more evaporated the feed to the LP column (the kettle liquid), the larger the minimum reflux requirement for that column. See for example p. 221 and p. 235 of "Separation Processes", second edition, by C. 3. King, McGraw Hill, New York, 1980. Thus high purity oxygen plus argon plants inherently require more LN ₂ reflux than do medium purity plants in order to achieve full (95+%) 0 ₂ recovery, and the LN ₂ reflux available from prior art TC LOXBOIL disclosures is not adequate for that purpose. This may be why no prior art disclosure of TC LOXBOIL cites production of high purity oxygen plus argon. ii) Pressurized N ₂ coproduct.

Any gaseous N ₂ withdrawn from the HP rectifier as product

also decreases the amount of LN ₂ reflux available. Thus even the prior art LOXBOIL flowsheets will begin to lose recovery if more than a nominal amount, say about 5%, of pressurized N ₂ is withdrawn, iii) Very low pressure medium purity plants. In some plants the HP rectifier overhead reboils an intermediate height of the LP column, not the bottom. The bottom is reboiled by condensing air. This further reduces the amount of vapor supplied to the HP rectifier (beyond the reduction due to LOXBOIL), and hence further reduces the amount of LN ₂ reflux available. Examples of this kind of plant incorporating either TC or PC LOXBOIL are in U.S. Patents 3277644, 3327489, and 3371496. It can be inferred that the limited use of these plants to date is at least partly due to the low 0 ₂ recoveries achieved due to the reduced availability of LN ₂ caused by TC or PC LOXBOIL. What is needed, and the primary objective of this invention, is a method and apparatus for obtaining the high 0 ₂ delivery pressure advantage of compressed or companded TC LOXBOIL dual pressure air separation while avoiding the offsetting disadvantage of reduced product recovery and/or purity encountered in all prior art disclosures.

Disclosure of Invention

The above and other useful advantages are obtained from a unique combination of steps (process) or apparatus as follows: In a dual or triple pressure air distillation plant comprised of HP rectifier and LP column with N ₂ rectifying section, liquid oxygen is evaporated by latent heat exchange with a minor fraction of the feed air which is at a pressure at least as high as the HP rectifier pressure, and substantially all of the minor fraction of air is as a result condensed. The resulting liquid air is split into at least two streams, one of which is supplied as intermediate reflux to the HP rectifier, and the other is supplied as intermediate reflux to the N ₂ rectifying section of the LP column, preferably after subcooling. At least 15% of the LAIR is supplied to each intermediate reflux location, and the preferred distribution is about one-third to the LP column and two-thirds to

the HP rectifier. The LAIR split can be effected by the coordinated action of two control valves controlling the respective liquid streams. The split proportions of the LAIR intermediate reflux are chosen so as to minimize the combined need for LN ₂ reflux to the two columns. Thus the otherwise harmful effects of insufficient LN ₂ reflux are unexpectedly avoided by the disclosed novel combina¬ tion of steps.

The minor fraction of air which is substantially totally condensed is preferably at a pressure greater than HP rectifier pressure, in order to further increase 0 ₂ production pressure. The added air pressure is preferably provided by a compander with a warm end air compressor which is driven by the refrigeration expander. Either air or N ₂ may be expanded. By using the compander, no additional input of external power is required to obtain the pressure increase.

The above improved combination of steps has general utility in any dual or triple pressure air distillation arrangement. However, the advantages are especially significant in flowsheets for producing high purity {99.5+%) oxygen plus coproduct argon; flowsheets for producing medium purity (90 to 99%) 0 ₂ plus substantial amounts of coproduct N _? ; and flowsheets for producing medium purity 0 ₂ using very low supply pressure (less than 4.5 ATA).

When producing high purity oxygen plus coproduct argon, it is further preferred to incorporate a means to increase argon recovery comprised of means for exchanging latent heat between argon rectifying section (sidearm) vapor from above a zone of couπtεr- current vapor-liquid contact, and liquid from an intermediate height of the nitrogen stripping section of the LP column (heat flow from rectifying to stripping section). Additionally or alternatively the argon recovery can also be increased by partially depressurizing part of the liquid nitrogen overhead product from the HP rectifier, evaporating it at an intermediate pressure by exchanging latent heat with vapor from above a zone of counter- current vapor-liquid contact in the argon sidearm; and work expanding the evaporated nitrogen with said work preferably powering said compressor for said minor fraction of air.

SUBSTITUTESHEET

Brief Description of the Drawings

Figure 1 is a simplified schematic flowsheet of an embodi¬ ment of the invention which is adapted for producing high purity oxygen plus coproduct argon. Figure 2 depicts an embodiment incorporating a nested double column so as to achieve a very low supply pressure, i.e., the HP rectifier reboils an intermediate height of the LP column, and which incorporates companded TC LOXBOIL plus two intermediate refluxes by split liquid air in order to improve recovery. Figure 3 is a high purity oxygen plus coproduct argon flowsheet analogous to Figure 1, except that argon recovery is increased by refluxing the argon sidearm by latent heat exchange with liquid nitrogen, and then work expanding the gaseous N _? .

Best Mode for Carrying Out the Invention Referring to Figure 1, low pressure column 1 and high pressure rectifier 2 jointly comprise a dual pressure column. Latent heat exchanger 3 condenses HP rectifier overhead vapor and reboils the LP column bottom. Compressed, dried and cleaned air is split, with a major fraction being cooled to near the dewpoint in main exchanger 4. The remaining minor fraction (about 25 to 30% of the total) is further compressed in compressor 5 while still near ambient temperature and then is cooled— first optionally by ambient cooling, and then by heat exchange with product exiting the cold box, e.g., as in main exchanger 4. After cooling to near its dewpoint, the minor fraction is sub¬ stantially totally condensed in LOX evaporator 6. The liquid air is split into two streams, one being injected to an inter¬ mediate height of the HP rectifier by control valve 7, and the other to an intermediate reflux height of the LP column 1 by control valve 8, preferably after subcooling in sensible heat exchanger 9.

A small part of the major air stream may be separated when only partially cooled and routed to expander 10 where it is expanded to LP column pressure and also produces work. The work is used to power compressor 5, preferably closely coupled in

a compander configuration. The remaining major fraction is rectified in the HP rectifier to LN ₂ overhead and kettle liquid bottoms. The kettle liquid is subcooled in 9, and then preferably split by coordinated action of control valves 11 and 12, with part being evaporated by latent heat exchanger 13 before being fed to the LP column, and the remainder fed directly, at a higher tray height. The exhaust air from expander 10 is also fed to the LP column at the same approximate height as the kettle liquid. The LP column has zones of counter-current vapor-liquid contact both above and below the feed location(s). Contact zones 1a and 1b are nitrogen rectifying zones; the liquid air through control valve 8 is intermediate reflux because it is introduced between zones 1a and 1b, i.e., because there are zones of nitrogen rectification via counter-current vapor-liquid contact both above and below it. Below the feed zone 1c are the nitrogen stripping zones 1d and 1e. Substantially nitrogen free vapor comprised of oxygen and argon from below zone 1e communicates with argon rectifier ("sidearm") 14, and bottom liquid from 14 returns to column 1. Below zone 1e is zone of counter-current vapor-liquid contact 1f, wherein argon stripping occurs. The argon sidearm 14 is refluxed by reflux condenser 13, and crude argon overhead product is withdrawn in.either vapor or liquid phase for further processing. Part of the liquid nitrogen from the HP rectifier overhead condenser 3 is subcooled, depressurized by valve 15, optionally phase separated in 16, and then directly injected as reflux into the LP column overhead. Gaseous nitrogen from there is warmed in exchangers 9 and 4 and exhausted. Liquid oxygen of product purity from the LP column bottom is further pressurized and routed to LOX evaporator 6 by means for one way flow 17, e.g., a pump or a check valve. Since the 0 ₂ pressure in 6 will.preferably be about 8 to 13 psi higher than at the bottom of LP column 1, the hydrostatic head associated with a barometric leg of liquid oxygen between 20 and 26 feet high will be sufficient to generate the added pressure. Thus using a barometric leg can eliminate the need for

other means of LOX pressurization such as a LOX pump. Finally product gaseous oxygen is withdrawn from 6.

In order to achieve increased argon recovery, Figure 1 also incorporates latent heat exchanger 18. This exchanger provides intermediate reboil to the nitrogen stripping section of column 1 owing to its functional location between contact zones 1d and 1e, i.e., at an intermediate height of the nitrogen stripping section. Of course, the actual physical location need not be inside column 1 as depicted. The condensing vapor which provides the intermediate reboil at 18 is taken from sidearm 14 above a zone of counter-current vapor-liquid contact (14a). The location may be either intermediate or at the top of sidearm 14, depending on whether contact zone 14b is present or not. More argon recovery is achieved when zone 14b is deleted, but heat exchanger 18 temperature differential is correspondingly decreased.

The split proportions of the liquid air are of critical importance to the successful achievement of the disclosed advantages from this invention, both for Figure 1 and for all other embodiments. This is because whereas a little bit of intermediate reflux is always helpful, too much can be as bad as or even worse than none at all. LOXBOIL necessarily causes a substantial quantity of liquid air to be available, on the order of 0.28 moles per mole of compressed air (m/mca). Without a split, i.e., if all were fed to either the LP column or HP rectifier as disclosed in the prior art, the recovery would be comparable to or even worse than that from PC LOXBOIL. It can be inferred that this is the reason U.S. Patent 4560398 states that TC LOXBOIL is undesirable. Surprisinglyj however, if at least about 15% of the liquid air (about 4% of the total air su plied) is fed as intermediate reflux to each N ₂ rectification, the recovery exceeds that from PC LOXBOIL. That is to say, the two liqui air stream flowrates should be within a factor of about six of each other. If the split is further optimized, then substantial additiona product recovery becomes possible with TC LOXBOIL: high purity 0 ₂ , c product argon, or other product increases. The exact proportions

of the optimal liquid air split are not at all critical, but generally fall between 1 to 1 and 2.5 to 1, and will vary depending on the flowsheet. For example at least approximately 2% more high purity oxygen can be recovered than with PC LOXBOIL. If in addition companding is used with the TC LOXBOIL, the 0 ₂ delivery pressure will be equal to or greater than that from PC LOXBOIL. The means for accomplishing counter-current vapor-liquid contact may be any known in the art: sieve trays, bubble caps, packing (random or structured), wire mesh, and the like. Referring to Figure 2, HP rectifier 1 reboils LP column 2 at an intermediate height via latent heat exchanger 19. The LP column bottom reboil is via partial condensation of a major fraction of the supply air in latent heat exchanger 20. Phase separator 21 (optional) removes the condensate from that stream. Since the liquid is of the approximate composition of kettle liquid, it is normally combined with the kettle liquid. Components 4, 5, 6, 8, 9, 12, 15, and 17 are described similarly as in Figure 1. Figure 2 depicts refrigeration via ₂ expansion vice air as in ^" 'Figure 1. Either method is approximately equiva- lent in overall results. A small stream (about 0.1 to 0.15 m/mca) of gaseous nitrogen is withdrawn from the HP rectifier overhead, partially warmed, and then work expanded in N„ expander 22, which powers compressor 5. If desired, the minor air fraction (TC LOXBOIL fraction) can be further compressed by externally powered compressor 23, including optional ambient cooling, to further increase the 0 ₂ delivery pressure. This option is particularly advantageous when the need for a separate Q ₂ compressor can be eliminated.

Column 2 of Figure 2 has 2 refluxes and 2 reboils, and rectifier 1 has 2 refluxes. By adjusting the relative heat duties of exchangers 19 and 20, and optimizing the liquid air split via valves 7 and 8, the operating lines of each column are caused to closely approach their equilibrium lines, i.e., very efficient column operation is achieved. This results in high recovery (95+%) of medium purity oxygen using a supply air pressure of only about 4.5 ATA, plus a delivery pressure of about 20 psia ( or higher

SUBSTITUTE SHEET

if an externally powered boost air compressor is incorporated). These results are unique, advantageous, and unexpected in view of the prior art.

Referring to Figure 3, another embodiment of the inventive entity of companded TC LOXBOIL plus LAIRSPLIT into two separate intermediate refluxes is disclosed. Figure 3 is for production of high purity oxygen plus coproduct crude argon as Figure 1. Components 1 to 9 and 11 to 16 have the same description as in Figure 1. Figure 3 incorporates an alternative means of increasing argon recovery to that disclosed in Figure 1 (latent heat exchanger 18). In Figure 3, part of the liquid nitrogen is partially depressurized by valve 24 and then evaporated in latent heat exchanger 25, which provides reflux to at least part of argon sidearm 14. The evaporated nitrogen, at a pressure between that of the HP rectifier and the LP column, is then work expanded in expander 26. The latter expander is preferably the only one present, and preferably used to drive compressor 5.

The high purity 0 ₂ flowsheets can utilize air expansion refrigeration as per Figure 1, nitrogen expansion refrigeration as per Figure 2, or partially depressurized nitrogen expansion as per Figure 3. The latter has the advantages of increased reboil through the argon stripper and the argon rectifier. The disad¬ vantages are larger flow through the expander (almost- double, e.g., 0.2 /mca) and reduced N ₂ reflux available. By careful optimization of the LAIRSPLIT, full 0 ₂ recovery can still be maintained, in addition to the increased reboil advantages. It will be recognized that the other method of increasing argon recovery—latent heat exchanger 18~could additionally be incor¬ porated in Figure 3 for even more argon recovery.

Notwithstanding that three figures all depict conventional dual pressure column configurations, it will be recognized by the artisan that the disclosed improvement is generic to any air separation flow¬ sheet incorporating both HP rectifier and LP column with N2 rectifying section (N ₂ ) removal column). This includes triple pressure flowsheets disclosed in U.S. Patents 3688513, 4507134, and 4578095, wherein the third pressure column is an argon removal column operating at a pressure slightly below the N2 removal column pressure.