Nitrogen Production By Low Energy Distillation

Technical Field

Process and apparatus are disclosed for distilling air to produce high yields of high purity nitrogen at lower .epergy consumption than has been possible heretofore. The disclosure also applies to other suba bient distilla¬ tions.

Background Art Recent large increases in demand for nitrogen have been experienced. One primary cause has been enhanced oil recovery by injection of pressurized nitrogen into the well. Production is reguired on a very large scale and at very high purity (typically less than 5 ppm O2). Under these conditions, the energy reguirement of the producing plant is a major component of the cost of the nitrogen. Accordingly much recent attention has been devoted to lowering the energy reguired for producing nitrogen. Prior art patents which, disclose reduced energy approaches to dual pressure distillative production of nitrogen include U.S. Patents 4453957, 4448595, 4439220, 4222756, and British Patent 1215377. These all involve supplying feed air to a high pressure rectifier, then routing the rectifier bottom product either directly or indirectly to a low pressure distillation column, and several also involve supplying reboil to the low pressure column by latent heat exchange with vapor from the HP rectifier. They also all incorporate a means' of increasing the reflux at the top of the LP column, whereby N2 purity and yield are increased, by exchanging latent heat between LP column overhead vapor and boiling depressurized LP column bottom product.

The '377 patent was one of the earliest disclosures of the basic configuration described above. It included the option of withdrawing some product 2 from the HP rectifier overhead, in addition to that withdrawn from the LP column overhead. The '957. patent discloses the same basic configuration, with the modifications of a different method of producing refrigeration and elimina¬ tion of any transport of liquid 2 from the HP rectifier - overhead to the LP column overhead. The '756 patent also involves the same basic configuration, also eliminates flow of LN2 from HP rectifier overhead to LP column over¬ head, and discloses yet another variation for producing refrigeration.

The '220 and *595 patents do- not involve- reboiling the LP column by latent heat exchange between HP rectifier vapor and LP column liguid. Rather, both of those patents disclose refluxiπg the HP rectifier by- exchanging latent heat with boiling deprεssurized kettle liguid (HP rectifier bottom product). The at least partially evaporated kettle liguid is then fed into the LP column for further separa¬ tion. This same technigue has been disclosed in processes for producing low purity oxygen, e.g. U.S. Patent's 4410343 and 4254629. The latter patent explains by means of a McCabe-Thiele diagram the advantage of this technigue-- that feeding 40% 0 ₂ vapor to the LP column is more efficient than feeding 40% O2 liguid to the same column.

The differences between the '220 patent and the '595 patent are that in the '220 patent the LP column is solely a rectifier with no source of reboil other than the vapor feed to it, whereas in the '595 patent the LP column has a stripping section and a reboiler supplied by total condensation of a minor fraction of the feed air. The latter means of reboiling the LP column is also disclosed in the U.S. Patent 4410343 for low purity..oxygen producing processes.

^■ Reboiling the medium pressure column of a three col¬ umn triple pressure configuration for producing high purity oxygen by latent heat exchange with partially condensing supply air is disclosed in U.S.. Patent 3688513. Providing intermediate reboil to a low pressure column by latent heat exchange between HP rectifier overhead vapor and partially evaporating LP column intermediate height liguid is disclosed in U.S. Patent 4372765.

The '220 patent has the disadvantage that the N2 recovery is low. Since the LP column is only a rectifier, the N2 content of the vapor feed (about 60%) sets a lower limit on the N2 content of the LP bottom liguid (about 40%), and hence recoveries only on the order of 80% are possible. The '595 patent has the disadvantage of reguiring significantly higher feed pressures than are actually necessary, while achieving lower recoveries than are possi¬ ble, due to inefficiencies involved in reboiling the LP column by total condensation and in feeding evaporated kettle liguid to the LP column.

Disclosure of Invention

The disadvantages of the prior art are overcome by providing a dual pressure air distillation process or apparatus in which: cooled and cleaned supply air at a single pressure is routed initially a partial condenser which rεboils the bottom of the LP column, and then at least a major fraction of the remaining uncondensed air is introduced into the HP rectifier, where it is rectified to kettle liguid bottom product and high purity overhead nitrogen. At least 15% and as much as 100% of the nitrogen overhead product is obtained as liguid and is routed to. the LP column overhead where it is directly injected as part of the reflux therefor. The remaining LP column overhead reflux is obtained by latent heat exchange with boiling depressurized LP column bottom liguid. The HP rec¬ tifier is refluxed by latent heat exchange with at least

one of boiling depressurized kettle liquid (Fig.2) and boil¬ ing LP column intermediate height liquid (Fig. 1).

The unexpected energy advantages made possible by partial condensation reboiling of the LP column by the supply air are only realized when the temperature differ¬ ence between the top and bottom of the HP rectifier is approximately the same as the temperature difference be¬ tween the bottom of the LP column and the LP column intermediate height where its vapor rate is substantially increased, either by intermediate reboil. or by introduc¬ tion of vapor feed (or both). Since the HP rectifier ΔT is usually 6 to 7°F, the corresponding LP bottom to LP in¬ termediate height ΔT should be 5 to 8°F. When the HP rectifier overhead is refluxed by- latent heat exchange with LP column intermediate height liquid, this is easily accomplished by choosing the appropriate tray height for the intermediate height, and selecting an LP column bottom reboil rate to just reach that tray height without pinching out. The LP column bottom section L/V necessary for that will be about 2.0 to 2.5, and usually about 2.2. This is adjusted by the amount of reboiler heat exchange surface provided. This will apply for a fairly wide range of N2 content in the LP column bottom liquid, e.g. 2% to 35%. On the other hand, if HP rectifier reflux is via latent heat exchange with kettle liguid, only a much more limited range of LP column bottom liguid concentrations can be tolerated--rσughly 17% to 25% N2 in the liguid. This is because the evaporated kettle liguid has a fixed composition of- about 66% N2, and therefore a fixed (equilibrium) entry point into- the LP column, and hence only a narrow range of bottom compositions will be within 5 to 8°F of that entry point temperature. If reflux is by partial evaporation of kettle liquid vice total evaporation, then higher N2 con¬ tent vapor is introduced into the LP column, which allows somewhat higher bottom liquid N2 contents (above 25%) while still retaining the low energy advantage.

The refrigeration necessary for the process can be developed in two preferred ways, or in other ways known in the prior art. The preferred ways are to either partially warm part of the HP rectifier 2 overhead product, expand it to slightly below LP column pressure, and add it to the product gas withdrawn from the LP column; or to partially warm an air stream taken from just before or preferably just after the partial condensation reboiler, expand it to LP column pressure, and introduce it into the LP column at an intermediate height which is above that associated with the HP rectifier reflux.

The former approach is slightly preferred, since the expanded N2 needn't be cooled back to LP column temperature, and the LP column diameter is somewhat smaller. With either refrigeration option above, and also with either HP rectifier reflux option, it is also possible to withdraw part of the N2 product from the HP rectifier over¬ head, although the major fraction of pro-duct will be with¬ drawn from the LP column overhead. It is also possible to coproduce low purity oxygen of from 70 to 95% purity, by adjusting the N2 content of the LP column bottom liquid.

Brief Description of Drawings

Figure 1 is a schematic representation of the pre¬ ferred embodiment wherein HP rectifier reflux is via latent heat exchange with LP column intermediate height liquid, and refrigeration is developed by expanding part of the HP rectifier overhead product and then adding it to the LP- nitrogen product. Figure 2 illustrates an alternative embodiment wherein HP rectifier reflux is via latent heat exchange with boiling depressurized kettle liquid, and refrigeration is via expanding part of the uncondensed air out of the partial condenser and then introducing it into the LP column.

• Best Mode For Carrying Out the Invention

Referring to Figure 1, block 101 represents the appar¬ atus for cleaning and cooling the supply air and rewarming the vapor streams exiting the cold box, and may be a re- versing exchanger, regenerator, conventional exchanger with mole sieve cleanup, or other configurations known in the art. 102 is the low pressure distillation column, having partial condensation bottoms reboiler 103 which receives the cooled and cleaned supply air. The partially condensed air, having at most about 30% liquid phase, is routed to optional phase separator 104, from which the uncondensed fraction of the supply air enters high pressure rectifier 105. Intermediate reboiler 106 supplies inter¬ mediate reboil to LP column 102 and overhead reflux to HP rectifier 105, and also supplies overhead product liquid nitrogen which is routed via subcooler 108 and expansion valve 109 to direct injection into LP column 102 overhead. Additional overhead product from HP rectifier 105 is with¬ drawn in vapor phase; and is expanded in refrigeration expander 110 after partial warming in heat exchange apparatus 101, plus optionally a minor fraction may be withdrawn as high pressure product via valve 111. The bottom liquid from HP rectifier 105 (kettle liquid), which may be combined with condensate from partial condeπ- sation reboiler 103, is routed via subcooler 108 and expan¬ sion valve 112 into LP column 102 as feed therefor, at a height above intermediate reboiler 106 height. The LP column bottom product liquid is also cooled in subcooler 108 and is expanded by valve 113 into reflux condenser 114, where it is boiled by latent heat exchange with condensing LP column overhead nitrogen. Product nitrogen at LP column pressure is withdrawn from the LP column overhead.

The following example operating conditions for the embodiment of Figure 1 are based on a computer simulation of that flowsheet. 100 moles/second (m) of air is com¬ pressed to 117 psia, and after cooling and cleaning enters

reboiler 103 at about 112.4 psia. 22m. of the air conden¬ ses in 103, and the partially condensed mixture exits at -273. °F. 78m of uncondeπsed air enters the HP rectifier at 112.2 psia, with an overhead pressure of 110 psia and about 40 theoretical stages. The overhead and bottom temperatures are -280.3°F ^' and -273.8°F respectively, for a column ΔT of 6.5°F. The overhead product, at less than 5 ppm O2 purity, consists of 14m of liquid N2 which is routed to the LP column overhead, plus 18.8m of gas¬ eous 2 which is used for refrigeration producing expansion plus, depending on the refrigeration needs, direct with¬ drawal at pressure. 45.2m of kettle liquid is combined with 22m condensate to yield 67.4m of liquid containing 67.5% N2, which is expanded into the LP column. 27.5m of LP column bottom product containing 20% N2 is expanded to 17.6 psia and totally evaporated to a vapor at -297.6°F by heat exchange with LP column overhead N2 at 59.3 psia ^• and -295°F. The LP column has .about 46 theoretical trays, and- intermediate reboiler 106 is located about 6 trays from the bottom, where the pressure is 62 psia, the tem¬ perature is -283°F, and the vapor and liquid phases con¬ tain 66% N2 and 41% N2 respectively. The LP column bottom temperature is -276.3°F, and hence the LP column ΔT between reboilεrs 103 and 106 is 6.7°F, or very close to the 6.5°F ΔT of the HP rectifier. The bottom section of the LP column has an L/V of about 2.2, whereas the V/L of the HP rectifier and LP rectifying section are about 1.65 and 1.8 respectively.

The expander- exhaust N2 is added to that from the LP column overhead, yielding 72.5m of high purity N2 (below 5 ppm O2) at a pressurε of 57 psia (exit the heat exchan¬ ger) plus 27.5m of atmospheric pressure waste gas con¬ taining 76% O2. Thus the N2 recovery is about 93% of that supplied the apparatus.

The above example of approximate conditions which can be expected in an operating plant reveals the unexpected .energy re.ductioπ advantage. obtained from partial conden¬ sation reboiling of the LP column (in conjunction with the other disclosed measures necessary to realize this advantage). The 100m of air supplied the reboilεr at 112.4 psia has a dewpoiπt of about -272.3°F. By the time that 22m of the air -is condensed, its temperature is -273.6°F. Thus the averagε effective temperature of latent heat release is about -272. °F. This provides a satisfactory reboiler heat exchange ΔT of 3.4°F with the -276.3°F LP column bottom liquid. If however, only 22m of air at 112.4 psia were supplied to reboiler 103 for total condensation reboil, the dewpoint would be the same, but the exiting bubble ^' pt. te peraturε would bε

-276.5°F. Howεver this is impossible, as it is actually colder than the LP column bottoms. In order to achieve the same average heat delivery temperature of 272.9°F by total condensation without temperature crossing at the cold end, it is necessary to raise the pressure to 117.4 psia. The lower supply pressure possible with the partial condensation approach equates to a lower εnεrgy require- mεnt providεd similar rεcoveries and product pressures are achievεd. The disclosed process actually achieves higher recoveries than most prior art low energy processes (93% in the example), which even further increases the realized energy savings. The high recovery is contingent upon the εssential transfer of liquid N2 from the HP rectifier to the LP column as reflux, which is coπtraindi- cated in the closest prior art disclosures. In the above example, in which the HP. rectifier overhead product was 14 + 18.8 = 32.8m, 14m or 42.7% of that product was supplied as LP column reflux. In general at least 15% and preferably more than 30% must be so supplied to achievε the disclosed low enεrgy plus high rεcovεry of high purity nitrogεn.

Onε additional prεcaution is important in order to achieve advantageous results with the Figure 1 flowsheet. The latent heat exchange from HP rectifier overhead vapor to LP column intermεdiatε liquid should preferably be by partial evaporation of the LP column intermediate liquid, as opposed to total evaporation. The reason here is similar to that described abovε: if only suf¬ ficient liquid is provided the intermεdiate reboiler such that total evaporation is required rather than partial evaporation, then the exiting vapor composition is the same as the entering liquid composition. The proper feed point for such a vapor, i.e., the tray having a vapor composition most closely approaching that vapor, would be several trays higher and colder than the tray where the liquid came from. Thus the vapor is introduced into the LP column several trays higher than necessary, re¬ quiring more rεboil in the lower section of the LP column to avoid pinching out, and hεπcε resulting in slightly less efficient operation. The way -to avoid the disadvantageous total evaporation intermεdiatε reboiling is to. supply more liquid to the reboilεr than is actually εvaporatεd, with the excess rεturnεd to the column as reflux. This is very easily done when the intermediate reboiler is physically located inside the LP column, as indicated schematically on

Figure 1. Obviously, however it could also be done for other reboiler locations.

Referring to Figure 2, two options to the Figure 1 flowsheet are illustratεd: using air vice N2 for refrig- eration expansion, and refluxing the HP rectifier by evaporating kettlε liquid vice LP column intermediate liquid. Either of these options may be applied indivi¬ dually to thε Figurε 1 flowshεet also, and at least in some conditions will achieve equally advantageous results. The 200-series components correspond to the 100-sεries counterparts of Figure 1, i.e., 201 corresponds to 101,

and only the new components will be further described.

Instead of all the uncondensed fraction of air from reboiler 203 and phase separator 204 being routed to HP rectifier 205, only a major fraction is routed, and a minor fraction, (depending on refrigeration requirements about 6 to 20% of the air supply) is routed to partial warming and then εxpansion in work-producing expander 215, and subsequently is fed into LP column 202 above the liquid feed introduction height (from valve 212). A major frac- tion (from 50 to 100%) of thε HP rεctifiεr overhead product is obtained in liquid phase and routed via subcooler 208 and expansion valve (i.e. pressure reducing valve) 209 for injection into- thε LP column overhεad as rεflux therefor. Any remaining HP rectifier overhead product may be with- drawn at pressure via valve 211. The HP rectifier reflux and the liguid N2 ovεrhεad product arε obtained from rεflux condenser 216, which is supplied depressurized liquid via valve 217 ^' from HP rectifier 2 ^' 05 and phase separator 204, and which in turn supplies vapor feed to LP column 202 at a height below the liquid feed hεight. The remaining liquid from rectifier 205 and separator 204 is routed via subcooler 208 and pressure reduction valve 212 and fed to the LP column.

It will be rεalized with respect to both of the above flows.heets plus obvious variants that different physical configurations may be eπcouπterεd without departing from the basic disclosed function, e.g. various other sensible heat exchange configurations, providing multiple units for some functions, and the like. ^' Also different operat- ing conditions may be employed, for instance differεnt heat exchanger ΔT's, component pressure drops, ambient prεssurε and tεmperature, and the like. It is known to remove products from differεπt locations (tray heights) to achieve more than one purity.

LP column pressures of 50 to 80 psia and HP column pres¬ sures of 100 to 190 psia, coupled with N ₂ recoveries of 80 to 99% of that in the supply air, are typical operating conditions under this disclosure. As cited above, various refrigeration expander variations are possible within the scope of the disclosed inventive entity. As another example of a preferred embodiment, expander 110 of Figure 1 can be replaced by an expander in the waste oxygen gas line. In that case for 100 mαles of air at 158 psia supplied to exchanger 101 18 moles is condensed at -265°F in reboiler 103 and the remaining vapor enters HP column 105, which operates between 149 and 152 pisa. 12.2 moles of liquid is supplied to reflux LP column 102 via valve 109. The LP column operates between 77 and 81 psia.

68.3 moles of oxygen enriched liquid air is supplied to the LP column at valve 112, and 28.5 moles of 73% 0 ₂ liquid is supplied to reflux condenser 114 via valve 113. Thε evaporated waste 0 at 24.4 psia is warmed to -120°F in exchangers 108 and 101, the expanded to

16.4 psia and -143°F, and then exhausted through the remainder of exchanger 101. All told, 51.25 moles of high purity N„ at 72 psia and 20 moles at 146.6 psia are recovered from 100 moles of air at 158 psia, for a recovery of 91.2% of the N _? available in the supply air.