Technical Field

This invention relates to processes and appara¬ tus for the separation of mixtures of non-condensable gases by fractional distillation at sub-ambient tem¬ peratures. The invention entails new combinations of steps or equipments which result in more efficient cryogenic distillation. Improved efficiency is more important in cryogenic distillation than in conven¬ tional distillation because of the high cost of supplying distillation column reflux at cryogenic temperatures.

Background Art

The disclosure of Streich et al in U.S. Patent 3688513 is cited as a prior art reference pertinent to this aDDlication. "Recycle distillation" describes a fractional distillation configuration or practice wherein there are two associated fractional distillation columns provided to accomplish a specified separation. The columns operate at different pressures, and the higher pressure column yields the specified overhead product (more volatile fraction) plus a non-specifi¬ cation bottom liquid which is transported to the lower pressure column for further separation. The lower pressure column yields the specified bottom product (less volatile fraction) plus an impure overhead product requiring further separation. That overhead product, which can be either gas or liquid phase (or both), is recycled back to the higher pres¬ sure column by pumping and/or compressing as appro- priate. Thus each column only partially accomplishes

the required distillation, and relies on the other column to accomplish the remaining part.

Fulton (U.S. Patent 2519451) discloses a liquid recycle distillation configuration which is narrowly applicable to distilling aqueous ammonia at above- ambient temperatures. The feed mixture is supplied initially to the lower pressure column, and the liquid overhead from that column is pumped to the higher pressure column. Steam is used to reboil both columns, and cooling water provides reflux to both columns. This configuration is not applicable to sub-ambient distillations. It also has the dis¬ advantage that all of the more volatile fraction (NH3) removed by the higher pressure column must first undergo the relatively inefficient total condensation in impure state at the lower pressure column overhead.

Both Haseldeπ (U.S. Patent 4025398) and Pagani et al., (U.S. Patent 4318782) disclose a vapor recycle distillation configuration in which vapor is recycled from the lower pressure column overhead to the higher pressure column. Pagani et al., disclose a configuration narrowly applicable to the distilla¬ tion of aqueous ammonia at above-ambient temperatures, wherein the need for a recycle vapor compressor is avoided by absorbing the lower pressure column overhead vapors into the liquid feed, and then pumping the resulting liquid mixture to the pressure of the higher pressure column. Pagani et al., also disclose a lower pressure column that operates only as a stripper, having no reflux other than the liquid from the higher pressure column. That configuration has the disadvantage that a large amount of impure vapor must be recycled to the higher pressure column. That configuration is also disadvantageous for sub-am- bient distillation because the absorption of vapor

into feed liquid requires substantial sub-cooling plus removal of the heat of absorption, and that cooling is difficult and expensive to supply in sub- ambient processes. Haselden discloses a configuration wherein the higher pressure column is solely a rectifier and the lower pressure column is solely a stripper. Feed mixture is supplied to the top of the lower pressure column together with recycle liquid from the higher pressure column, and they form the only reflux for the stripping column. A small amount of reboil is supplied at the bottom of the stripper by indirect application of external heat. Additional reboil is supplied at several heights of the stripper by indirect exchange. of latent heat with an appropriate height/location of the rectifier (higher pressure column). Since most of the reboil supplied to the stripper is by latent heat exchange and correspond¬ ingly supplies reflux to the rectifier, the external reflux requirement is very small, and is only supplied at the top of the rectifier. This configuration is also described in Figure 1 of the AIChE Journal arti¬ cle "Distillation with Secondary Reflux and Vapori¬ zation: A Comparative Evaluation" by R.S.H. Mah, J.J. Nicholas, and R.B. Wodπik, September, 1977, p. 652, Vol. 23 No. 5.

This configuration has the disadvantage that the compressor is located at the point of maximum the vapor vapor flow, and /must be compressed through a rela- tively large pressure ratio. The pressure ratio between the two columns is approximately the square root of the relative volatility, plus an increment for the heat exchanger ΔT. Finally, all heat exchanges are shown with latent heat flow from the rectifier to the stripper.

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In addition to Haselden, there are two other prior art disclosures of multiple latent heat ex¬ change between two associated distillation columns, operating at sub-ambient conditions. These are dis- closed by Geist (U.S. Patent 3277655) and Tamura et al., (U.S. Patent 4372765). Neither disclosure in¬ volves recycle distillation; in both the feed is transported from the high pressure column to the low pressure column after partial distillation, but no material is recycled back to the high pressure column. Both show two separate exchanges of latent- heat be¬ tween the two columns: one exchange between the HP column overhead and an l_P column midpoint, and one exchange between a HP column midpoint and the LP column bottom. The difference between the two involves handling of HP column midpoint reflux that is formed by latent heat exchange. Geist discloses transporting all of it to the LP column, whereas Tamura et al., disclose transporting only part to the LP column and returning the remaining part as reflux to the HP column.

The disadvantages with these configurations are that normally only a very limited amount of heat can be extracted from the HP column midpoint before adversely affecting the purity and amount of

HP column overhead liquid obtained. In some processes, such as air separation, all available liquid nitrogen is required to adequately reflux the LP column. Additionally, it is frequently not desirable to with- draw any liquid at all from a HP column idlength location.

The apparatus in which the exchange of latent heat between two distillation columns is normally accomplished is generally referred to as a "reboiler/ reflux condenser". Usually higher pressure gas con-

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denses on one side of the heat exchange surface while lower pressure liquid boils or evaporates on the other side. It may physically be located internal or external to either column it services. Reboilers and/or reflux condensers which connect to a mid- length location or tray height of a distillation column rather than the respective top or bottom are sometimes referred to as "side" or "intermediate" reboilers or refluxers. The mixture being separated by a fractional distillation column may be either binary or multi- component. Neither fraction will normally be a pure single component even in the case of a binary feed, but will contain at least traces of the other component. Thus when referring to the separation of a mixture of oxygen and nitrogen, calling the higher pressure column overhead product "nitrogen" merely signifies that its oxygen content has been reduced to below a desired maximum concentration. That concentration could vary from a few percent to a few parts per million depending on the end use. Terms such as "enriched bottom product" and "enriched nitrogen" sig¬ nify mixtures which have not yet attained the speci¬ fied purity, but are closer to that purity than was the column feed mixture. They are the mixtures which are recycled in the recycle configuration.

"Distillation column" signifies an apparatus with at least one feed entry point, at least one zone of countercurreπt vapor-liquid contact both above and below the feed point, plus an overhead and bottom.

"Rectifier" and "stripper" refer to columns in which the countercurreπt vapor-liquid contact only occurs respectively above and below the feed entry point. The contacting section can be of any known type-- bubble caps, sieve trays, packing, etc. A mldlength

location signifies a location betwe.en the feed point and either the overhead or bottom where there is coun- tercurrent vapor-liquid contact both above and below the location. 5. Cryogenic temperatures are those temperatures at which non-condensable gases, i.e., gases which are incapable of being liquefied at ambient temperature, can be liquefied.

Disclosure of Invention 0 The disadvantages of the prior art practice of sub-ambient distillation are overcome by providing a process (or apparatus) which comprises: a) supplying the mixture to be separated in fluid state to a first distillation column; b) distilling the mixture to overhead product and enriched bottom product liquid; c) transporting the enriched bottom product liquid to a second distillation column d) distilling the enriched bottom product 0 liquid to bottom product fluid and enriched over head fluid; e) transporting the enriched overhead fluid to the first distillation column; f) reboiling the first and second distillation columns by latent heat exchange with a condensing gas; g) refluxing the second distillation column by latent heat exchange with a boiling liquid; h) with drawing said bottom product fluid and said overhead product.

Further advantages are obtained by feeding a gas mixture initially to a rectification column, which reboils the two recycle distillation columns, and supplies both reflux and liquid feed to them. In 5 addition to the rectifier overhead providing reboil

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to the distillation columns, a midlength location can advantageously supply additional reboil to either.- Finally, either distillation column can exchange latent heat with the other. All of the above improvements taken together yield a particularly advantageous proc¬ ess for air separation wherein oxygen of greater than 97% purity can be obtained from supply air pressures lower than heretofore required. The above improvements, either in the same or in other combinations, will also be advantageous for othernoncoπdεnsable gas separation such as N2 -CH4, H2 - HD - D2, C2H4 - C2H5, various combinations of halocarbons, and others.

Sub-ambient distillation is frequently accom¬ plished by supplying the mixture as a compressed gas to a high pressure rectification column which reboils a lower pressure distillation column. The rectifica¬ tion column also supplies enriched liquid feed to the midlength of the distillation column and liquid over¬ head product as direct injection reflux to. the over- head of the distillation column. Thus the pressure ratio between the rectifier and the distillation column is indicative of the energy consumed by the distillation process, i.e., the amount the supply mixture must be compressed. Substituting the liquid recycle distillation columns for the single distillation .column. is one- way of reducing the required pressure ratio between the rectifier and the two columns. Incorporating multiple latent heat exchanges between the rectifier and distillation column is a second way of reducing the required pressure ratio. The disclosure herein reports the discovery that by properly incorporating both of the above im¬ provements in a single process, a greater reduction, in pressure ratio can be achieved than is possible with either improvement taken singly.

SUBSTITUTE SHEET

Whereas conventional low pressure air separation by a dual pressure column uses a pressure ratio of between 4.2 and 5.2 between the rectification column overhead and the distillation column overhead, either liquid recycle distillation or multiple latent heat exchange can reduce it to 3.8, and the two together can reduce it to 3.6 or less.

For example, consider a column discharging gaseous nitrogen with an overhead pressure of 18 psia. In conventional dual pressure air separation processes the rectification column overhead pressure would range from 75 psia to 92 psia. Either improvement described above would allow operation down to 68 psia, but the disclosed combination would allow operation with a rectification column overhead pressure between 50 and 67 psia.

Two essential aspects of the disclosed improve¬ ment of recycle distillation at sub-ambient tempera¬ tures are that the feed mixture be supplied initially to the higher pressure column of the two distillation columns, and that the lower pressure column

^" ...... . have a zone of counter- current vapor-liquid contact plus a supply of reflux located above the point at which recycle liquid from the higher pressure column is introduced. It is im-

.is portaπt to minimize the amount of fluid which/recycled from the lower pressure column to the higher pressure

•column, as that step is inherently inefficient. Any of the more volatile fraction that is introduced into the lower pressure column must be removed overhead. Thus by introducing the feed initially to the higher pressure column, at least some more volatile fraction is removed there, reducing the amount introduced to the lower pressure column. By incorporating a recti- fying section plus separate reflux in the lower pres¬ sure section, the overhead becomes more enriched

in the more volatile fraction, thus reducing the total mass flow rate required in order to remove the more volatile fraction. In summary, the two aspects de¬ scribed above combine to greatly reduce the overhead product flow rate which is recycled from the lower pressure column to the higher pressure column compared to prior art processes.

The lower pressure column overhead product which is recycled can be in any fluid state, i.e., gas and/or liquid. For the separation of close boiling components, it will normally be ^" liquid, to avoid the need for compression. For higher relative volatili¬ ties, the gas becomes progressively more enriched in the more volatile component than the liquid, and hence withdrawing the product as gas reduces the required flow rate. In some cases this advantage will more than offset the disadvantage of supplying a small compressor and adding a small amount of work inside the cold box.

Brief Description of the Drawings

The drawings are simplified schematic flowsheets showing only the columns plus their interconnections and latent heat exchanges. Other details which would actually be present but which are not essential to the disclosure are omitted, such as sensible heat exchangers or superheaters, control and instrumentation mechanisms, vapor or liquid supply or withdrawal points cleanup devices such as molecular sieve adsorbers, and the like. Figure 1 illustrates the basic configuration of sub-ambient liquid recycle distillation with feed supplied to the higher pressure distillation column and reflux supplied to the lower pressure distillation column. It also shows a high pressure

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rectifier which reboils both recycle columns and generates the feed for the higher pressure recycle column and reflux for both recycle columns.

Figure 2 illustrates a recycle distillation configuration wherein in addition to the latent heat exchange between the rectifier overhead and the two recycle columns, there is also a latent heat exchange between a rectifier midlength location and the bot¬ tom of one of the recycle columns, in this case the higher pressure one.

Figure 3 illustrates a recycle distillation configuration wherein the two recycle columns exchange latent heat. In this case the heat flow is from the lower pressure column to the higher pressure column.

Figure 4 illustrates a recycle distillation configuration which incorporates both of the improve¬ ments illustrated in Figures 2 and 3, and which is particularly advantageous in the application of air separation.

For ease of explanation, all of the figures show air as the mixture being separated, nitrogen as the more volatile fraction, and oxygen as the less volatile fraction. It will be understood that those labels are nonlimiting, and that the configurations illustrated, plus variations or modifications described in the text, apply equally to any other sub- ambient distillative separation.

Best Mode for Carrying Out the Invention

Referring to Figure 1, the higher pressure dis¬ tillation column 1 and lower pressure distillation column 2 are interconnected by means for transporting overhead fluid 4. Means for transport 3 may be a pump, valve, check valve, or simply a conduit or the like; means for transport 4 may be a pump, a com- - prεssor, or a barometric leg with one way valve or

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the like. The barometric leg will suffice when liquid is being transported and the absolute pressure dif¬ ference between the columns is suitably low, such as for low pressure air separation. Feed mixture in fluid state (gas and/or liquid) is introduced into col¬ umn 1 via means for feed introduction 5, e.g., a valve. The more volatile fraction (N2) is withdrawn from column 1 overhead, and the less volatile frac¬ tion (O2) is withdrawn from column 2 bottom as gas and/or liquid. Column 2 is refluxed by latent heat exchange with a boiling liquid in reflux condenser 6. Both columns are rεboiled by latent heat exchange with a condensing gas in reboiler/reflux condensers 7 and 8. Part of the condensate from reboilers 7 and 8 may be supplied to refluxer 6 via means for pressure reduction 9, where it is vaporized. A second part may be used to generate reflux for column 1, either by direct injec¬ tion through means for pressure reduction 10 as shown, or by provision of a second reflux condenser. The basic recycle distillation configuration as described above can be caused to operate by compres¬ sing the gas obtained from the boiling liquid which refluxes both columns back to the pressure required in reboilers 7 and 8, in a heat pump configuration. Alternatively, and preferably in the case of many separations such as air separation, a high pressure rectification column 11 may be employed in what can be termed a "feed heat pumped" configuration. The mix¬ ture to be separated, in a cleaned, cooled, pressurized gaseous state, is supplied to the bottom of column 11; the overhead gas condenses in reboiler/reflux conden¬ sers 7 and 8, and the condensate is divided in three parts: part refluxes column 11, part refluxes column 1, and part is used to generate reflux for column 2. Bottom liquid from column 11 is transported to column 1 via means for transport and pressure reduction 5,

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as previously described.

Various other liquid or gaseous supply or with¬ drawal points may be located at any point on the several columns and interconnections described. For 5 example, in air separatio processes frequently part of the gaseous 2 in column 11 overhead is withdrawn and routed to an expander which generates refrigera¬ tion required for the process.

The point at which overhead from column 2

10. is reintroduced into column 1 is not necessarily below the column 1 feed point (means for feeding 5) as shown. It may be at the same feed point, or above the feed point. The only stipulation is that it will be at some midlength location of the entire

15 column. This applies to all the configurations illustrated.

In addition to the same components 1 through 11, Figure 2 additionally illustrates an exchange of latent heat between a midlength location of column 0 11 and the bottom of column 1. Column 1 bottom liquid flows to reboiler/reflux condenser 12, and the resulting vapor liquid mixture is separated by separator 14 so as to return the vapor as reboil to column 1. An additional zone of countercurrent 5 vapor-liquid contact 13 is incorporated in column 1 to take advantage of the additional reboil.. Alterna¬ tively the additional latent heat exchange could be between columns 11 and 2 vice 11 and 1.

In addition to the same components through 11, 0 Figure 3 additionally illustrates an exchange of latent heat between column 1 and column 2. Gas from column 2 communicates with reboiler/reflux condenser 15, and liquid reflux returns to column 2. At the same time part of the downflowing liquid in 5 column 1 is vaporized in reboiler/reflux condenser 15

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and flows back up the column. Thus in the illustrated case the heat flow is from column 2 to column 1. The latent heat interchange illustrated is from a midlength location to another midlength location, but this is not generally required, as in some cases it will be from or to a bottom or overhead location, and/or in the other direction.

Figure 4 combines the improvements of both Figures 2 and 3, and has the same numbered components 1 through 15. This configuration is particularly advantageous for separating air to an oxygen product of about 97% purity or more. Both the liquid recycle configuration as in Figures 1 or 2, and the prior art multiply latent heat exchanged dual pressure columns suffer from the fact that only a limited amount of reboil is available at the bottom of the oxygen producing distillation column. This makes it impossible to separate out very much of the argon in the feed air from the oxygen, and an oxygen purity of 95 tα 96% results, with argon as the major impurity. With the Figure 3 and especially the Figure 4 configuration, substantially greater reboil is provided at the bottom of column.2, re¬ sulting in much greater oxygen purity. At the same time column 1 is made to operate more efficiently, as the reboil rate increases in several increments through the stripping section. Reboiler/reflux condenser 12 also causes the rectifier 11 to operate more efficiently, provided most or prefer- ably all of the reflux it generates is returned to column 11.

In the air separation application of Figure 4, cooled and cleaned air is introduced to rectifier 11 at 58 psia, and a liquid overhead at 56 psia and 91.1K and containing less than 1 % O2 is obtained

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overhead. Part of the gaseous nitrogen is supplied to an expander, e.g., 16 moles per hundred moles of compressed air (m/m) . About 35 m/m of the liquid nitrogen is withdrawn from column 11, the remainder refluxing it. Forty-nine m/m of enriched oxygen liquid

(about 42% O2) is withdrawn from the bottom of column 11 and supplied to column 1 via pressure reducer 5. Column 1 overhead is at about 17 psia and 79K enabling its gaseous nitrogen product to flow out of the cold box to atmosphere. Column 1 bottom product is about 93% oxygen liquid. Column 1 bottom is about 13.3 psia and 9iκ , thus a vacuum compressor would be required outside the hot box to assist the withdrawal of product oxygen. The overhead of column 1 is at about 12.4 psia and about 81.8K but the nitrogen gas actually exhausting from refluxer 6 is at 19.9 p.sia and 80K. Liquid nitrogen- is appor¬ tioned roughly 8 m/m through valve 9 and 27 m/m through valve 10. Approximately 2.0 m/m of approxi- mately 53% O2 liquid is returned to column 1 through means for transport 4, i.e., a liquid oxygen pump or a one-way valve. The reboil to column 1 can be comprised of 9 m/m vapor from separator 14, 16 m/m vapor to reboiler 8, and 22 m/m vapor to reboiler 15, or other suitable combination. The overhead pressure ratio between columns 1 and 2 is ^' 17/12.4 = 1.37 and the overhead pressure ratio between column ^' s 11 and 1 is 56/17 = 3.29. In general the pressure ratio between the two recycle distillation columns in this disclosure will be approximately the fourth root of the relative volatility of the two fractions separated For air, the relative volatility between oxygen and nitrogen at 90K is about 3.5, and the fourth root

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is ^' 1.37. The pressure ratio will always be less than the square root of the relative volatility with the disclosed configuration.

The operating values cited above are merely suggestive and not to be taken as limiting. The process is operative with air at various other column 1 pressures, with the other pressures adjusted proportionately. Separation of other fluids will en¬ tail completely different pressure and/or tempera¬ ture ranges. Additional features may be present, such as auxiliary distillation columns which with¬ draw a side vapor stream and return a side liquid stream to one of the disclosed columns.