WO1988002649A1

WO1988002649A1 - Interexchanged cascade ethanol distillation

Info

Publication number: WO1988002649A1
Application number: PCT/US1987/002659
Authority: WO
Inventors: Donald Erickson
Original assignee: Erickson Donald C
Priority date: 1986-10-10
Filing date: 1987-10-09
Publication date: 1988-04-21
Also published as: AU8177487A

Abstract

Reduced energy fractional distillation is achieved on a dilute feed mixture of azeotrope-forming substances such as beer by providing an interexchanged cascade, wherein two distillation columns are in both heat exchange and mass exchange relationship at intermediate heights. The high pressure column comprised of still (41) and rectifier (47) delivers intermediate height heat via exchanger (45) and intermediate height mass via valves (49) and (55) to lower pressure column comprised of stills (44) and (48) and rectifier (51). A second transfer of intermediate height heat is at exchanger (52). Beer feed is split between stills (41) and (44). Stillage-free water is withdrawn from stripper (48). Near-azeotropic ethanol is obtained at very high energy efficiency.

Description

Description INTEREXCHANGED CASCADE ETHANOL DISTILLATION

Technical Field

This invention relates to process and apparatus whereby highly unequal feed mixtures of azeαtrope-forming substances can be frac¬ tionally distilled at substantially reduced energy cost. In a second embodiment, the invention relates to fractional distillation of mixtures containing a nonvolatile dissolved substance requiring subsequent separation. In the most preferred embodiment the invention relates to distillation of mixtures exhibiting both of the above characteristics, for example the distillation of near azeotropic (about 85 mole percent) ethanol from a fermentation beer mixture comprised of about 2 to 5 mole percent ethanol; about 3 to 15 weight percent dissolved nonvolatile material, plus water.

Background Art

There is a long recognized need to reduce the energy consumption of fractional distillation of bulk commodity chemicals and petro¬ chemicals. The need is particularly acute for the separation of dilute mixtures and/or azeotropic mixtures, since ordinary distillation is notoriously inefficient in those circumstances.

One known method of reducing the energy demand for distillation is use of a cascade, wherein heat rejected from a higher pressure and temperature column is used to reboil a lower pressure and temperature column. The feed mixture is usually split between the two columns, although in some special circumstances it may be routed sequentially to the two columns in either order.

A distillation column may be comprised of a rectifier (rectifying section), a stripper (stripping section), or both, plus at least one feed point. "Rectifier" signifies that there is a zone of counter-current vapor-liquid contact above the feed point, and "stripper" signifies there is a zone of counter-current vapor- liquid contact below the feed point. "Intermediate height" signifies a height at which there is a zone or section of counter-current vapor- liquid contact both above and below that height. Any means of achieving counter-current vapor-liquid contact is contemplated: sieve trays, bubble cap trays, dumped packing, structured packing, and the like.

In cascades, the heat transfer from higher temperature column to lower temperature column is normally via latent heat exchange between condensing overhead vapor from the higher pressure column and evaporating bottoms liquid from the lower pressure column. Thus a single heat exchanger serves as both reflux condenser for the higher pressure column and reboiler for the lower pressure column. The problems with ccnventicnal cascades is that the temperature differ¬ ence between the two columns is the sum of the higher pressure column temperature differential plus the reboiler/reflux condenser temperature differential. This sum can be undesirably large, requiring large pressure- differences between the columns and either higher heat supply temperature at the hot end or excessively deep vacuum at the cold end. One way of reducing the required temperature and pressure differential between the columns of a cascade is disclosed in copending U.S. Patent application 872558 filed by Donald C. Erickson on June 10, 1986. Applying a cascade to an azeotropic distillation presents special problems. Whereas conventionaLdistillation requires a high vapor flow rate (reboil rate) at only the feed height, and the reboil can be reduced at other heights, with azeotropic distillation a very high reboil rate is required at the azeotropic height also (normally the top of the rectifying section). When the azeotropic mixture being distilled is a fermentation product comprised of water, ethanol, and nonvolatile organic matter ("beer"), still further problems arise. Reboiler temperatures above about 130°C cause severe fouling due to the heat sensitivity of the nonvolatiles. Also the dilute nature of the feedstock makes high reboil necessary in the "beer still" (the stripping section of the beer distillation column). Finally, the "stillage", which is the bottom liquid from the beer still, is fre¬ quently dried for use as livestock feed, and hence any process steps increasing its water content are harmful, whereas any process steps decreasing its water content are beneficial.

There are numerous prior art disclosures pertinent to applying cascades to azeotropic distillations such as ethanol-water. They fall into three groups. First are those wherein the primary recovery (beer distillation) column is the low temperature column in cascade with some other column(s). U.S. Patent 3445345 discloses the beer column cascaded with both light and heavy impurities removal columns. U.S. Patents 4256541, 4372822, and 4422903 disclose the beer still being cascaded with azeotrope-breaking ("dehydration" or "anhydrous") columns. In the second group, various columns are cascaded with the beer distillation column as the higher temperature column, i.e., the beer rectifier overhead vapor drives the lower temperature device. In this category, U.S. Patents 1860554 and 4217178 disclose both the anhydrous column and the azeotrope agent stripping column at the cold end; 4306942 discloses another beer column at the cold end (conven¬ tional cascade); 4309254 discloses a multiple effect evaporator at the cold end; 4582570 plus 4559109 disclose anhydrous column only at the cold end; and 4541897 plus 1940699 disclose both anhydrous column plus "open cycle" azeotrope stripping column at the cold end. The third group of cascades is characterized by the overhead vapor from the beer still driving the colder component (as opposed to overhead vapor from the beer rectifier). Both U.S. Patents 4422903 and 4582570 disclose that overhead vapor being totally condensed to drive a separate beer column; the patents differ in that one locates a dehydration column at the hot end of the cascade, and the other locates it at the cold end of the cascade. Finally, U.S. Patent 4428799 discloses driving a multiple effect evaporator via partial condensation of beer still overhead vapor, and the multiple effect evaporator in turn provides direct injection steam for reboil of a condensate stripper which receives liquid bottom product from a rectifier operating at the same pressure as the condensate stripper, and also receives the partial condensation liquid.

All of the patent disclosures in the first two groups except 4306942 have the problem that there is no reduction in the amount of energy necessary to drive the beer distillation column. For a typical feed composition of 4 mole percent ethanol, and a product composition of at least 190 proof and at least 99.9% recovery, The minimum steam demand is about 12.2 pounds steam per gallon (6.6 pounds) of pure ethanol. With U.S. Patent 4306942 a steam savings is achieved but the hot end temperature is too high and/or the cold end pressure is too low. With the patent disclosures in the third group, there is insufficient reboil through the near-azεotropic section (upper section) of the reboiler, and the ethanol purity accordingly is reduced. Even though the energy required to drive the beer stills is reduced, the lower alcohol purity, typically well under 180 proof, requires substantially greater energy in order to dry the alcohol to final dryness. This largely offsets the energy savings otherwise realized in the stripping section.

What is needed, and one objective of this invention is a process and corresponding apparatus which will permit a substantial reduction in the energy supplied to a beer distillation column below about 12 pounds per gallon of contained anhydrous ethanol, while still retain- ing a product purity of better than 180 proof and preferably better than 190 proof, and also without requiring excessively high temperatures for stillage reboil or substantial vacuum. For example, a steam demand of less than 8 pounds per gallon is preferred in combina¬ tion with a high pressure column pressure below.30 psia a low pressure column pressure above 6 psia. More broadly, the objective is to use one or more interexchangers to cascade two columns for distillation of an azeotrope-forming mixture whereby high reboil is obtained through both the contact zones near the azeotropic concentration, while reduced reboil rate is achieved through the central zones of the columns, and whereby the temperature differential between the two columns is less than the temperature differential across a single column.

A second need and also objective of this invention is a means for separately removing the water content from the beer rectifier bottom liquid without adding it to the stillage from the beer still, and also without requiring additional heat input to the distillation process.

The greatest need and hence the most preferred objective is to combine solutions to both of the above problems in a single apparatus and/or process. Example mixtures amenable to and benefitting from separation by the disclosed interexchanged cascade include ethanol- water (both synthetic and fermentation derived), isopropyl alcohol- water, methyl ethyl ketone-water, isobutyl alcohol-water, tert-butyl alcohol-water, acetone- ethanol, methyl cellosolve-water, and acetic acid water.

Disclosure of Invention

The above and other objectives are attained by providing proces and apparatus for distillation of a feed mixture comprised of: a) a distillation column comprised of stripper and rectifier; b) an auxiliary distillation column comprised of auxiliary stripper and auxiliary rectifier; c) reboiler for the auxiliary stripper which receives latent heat from partial condensation of the stripper overhead vapor; and d) conduits for routing the condensed fraction of the stripper overhead vapor to the auxiliary stripper and the uncondensed fraction to the rectifier.

Within the above genus, several species are disclosed as follows:

. rectifier bottom liquid is routed to the auxiliary stripper; * . rectifier overhead vapor provides intermediate reboil to the auxiliary rectifier; . part of the feed mixture is supplied to the auxiliary still; . part of the feed mixture is supplied to a separate auxiliary still; and . intermediate height liquid is withdrawn from the rectifier and fed to an intermediate height of the auxiliary rectifier at or above the height it is intermediate reboiled. In a second embodiment, process and apparatus are provided or distilling a feed mixture comprised of: a) a distillation column comprised of a stripping section, a rectifying section, and a feed point for at least part of said feed mixture; b) an auxiliary stripper which is fed bottom liquid from the rectifying section; c) a means for depressurization of the bottoms liquid from the stripping section, whereby a flash vapor is generated; and d) a means for directly injecting said flash vapor into the auxiliary stripper as reboil therefor. The most preferred embodiment combines both of the above inventions in application to beer distillation.

Brief Description of the Drawings

Figure 1 is a schematic flowsheet of a conventional beer distillation column. Figure 2 depicts two distillation columns in interexchanged cascade configuration, where the feed may optionally be split between the two columns or all fed to the higher pressure column. Figure 3 depicts a beer distillation column in combination with a condensate stripper which is reboiled by flash evaporation of the beer still stillage, thus providing "free" dewatering. Figure 4 depicts a flowsheet combining the inventive entities of both Figures 2 and 3.

Best Mode for Carrying Out the Invention

Referring to Figure 1, a conventional beer distillation column comprised of beer still 1 (stripping section) and beer rectifier 2

(rectifying section) is depicted. The column can be divided into two sections as suggested schematically, or combined into a single column, or divided into multiple units piped in parallel, as is known in the art. Reboiler 3 may be a simple thermosyphon type, or may incorporate forced circulation via pump 4 for improved heat transfer, as for example in a long tube vertical falling film exchanger. Reflux condenser 5 provides liquid reflux to the rectifier and optionally also condenses the product vapor. Impurities collectively known as fusel oil are withdrawn from an appropriate height of the rectifier as known in the art. In order to establish a representative reference by which to measure the energy improvement of the disclosed invention, the following Figure 1 operating conditions may be assumed to reflect a well-designed and operated plant. For 100 pound moles per hour of beer feed containing 4 mole percent ethanol and balance water, at 92°C, and for 20 theoretical trays in both the stripper and rectifier, a heat supply of 345,000 BTU/hour to reboiler 3 provides an L/V of 5.8 in the still and 0.74 in the rectifier, producing 4.7 pound moles per hour of ethanol product at .85 mole fraction ethanol (93.5 weight percent, or 192 proof). The beer still bottom is 22.4 psia and 112.3°C, and the rectifier overhead is 20.7 psia and 87.3 C. 12.5 pound moles per hour of rectifier bottom liquid at .04 mole fraction ethanol is returned to the beer still. The specific energy requirement is 12.2 pound/steam per gallon ethanol.

Referring to Figure 2, beer still M is reboiled^"by reboiler 12 plus optional pump 13. Beer still overhead vapor is partially con- densed in reboiler 14, which rebdils auxiliary stripper 15. The uncon¬ densed vapors are routed to rectifier 16. The condensate plus the bottom liquid from 16 are routed to auxiliary stripper 15 via means for pressure reduction 17, i.e., a valve, pump, or the like. Although still 15 is at a lower pressure than rectifier 16, a pump may be 0 required due to the hydrostatic liquid head of the feed point. Bottom liquid is withdrawn from still 15 and/or recirculated through reboiler 14 via optional pump 18. Still 15 has associated auxiliary rectifier 19 which receives vapor from and returns bottom liquid to 15. Recti¬ fier 16 receives overhead reflux from exchanger 20, which also provides 5 intermediate reboil to auxiliary rectifier 19. If additional reflux is needed it is obtained from optional trim reflux condenser 21. Auxiliary rectifier 19 is refluxed by exchanger 22. Intermediate height liquid is withdrawn from rectifier 16 via means for pressure reduction 23 (pump or valve), and routed to an intermediate height of 0 auxiliary rectifier 19 either directly or via exchanger 20 to be at least partially evaporated.

In addition to feeding reboiler 14 condensate plus rectifiers 16 and 19 bottom liquids to auxiliary still 15, it is also possible and in some cases preferable to feed part of the beer to it as well. In that event it is preferable to feed the beer to a lower height, typically 1 to 5 stages lower than the condensate feed height.

Referring to Figure 3 stripper 30 is reboiled by reboiler 31 and optional pump 32 as before. Condensate stripper 33 is reboiled by either or both of two sources of reboil. Liquid bottom product from stripper 30 can be flashed down to the approximate pressure of stripper 33 via depressurization valve 34, and the flash vapor separated from liquid product in phase separator 35, and the flash vapor directly injected into 33. Alternatively or additionally there may be a reboiler 36 which indirectly transfers latent heat to stripper 33 bottom liquid from partially condensing stripper 30 overhead vapor. All uncondensed vapor from stripper 30 overhead is. routed to rectifier 37, which is

UBSTITUTE C..EZT refluxed by reflux condenser 38. The bottom liquid from rectifier 37, plus any condensate from exchanger 36 if present, is fed to stripper 33. Overhead vapor from stripper 33 is condensed in condenser 39 and raised to rectifier 38 pressure by pump 40 and then fed to an intermediate height of rectifier 37. Referring to Figure 4, which is the preferred embodiment depict¬ ing the combined inventions and is also the figure most representative of the invention, a major fraction (typically 60 to 75%) of the feed mixture (beer) is fed to beer still 41 which is reboiled by reboiler 42 (and pump 43). The remainder is fed to auxiliary beer still 44, which is reboiled by reboiler 45 (and pump 46) via indirect latent heat exchange with partially condensing overhead vapor from 41. The uncon¬ densed vapor is routed to rectifier 47. Exchanger 45 condensate plus rectifier 47 bottom liquid are routed to condensate stripper 48 via letdown valve (or pump) 49. Overhead vapor from 44 is also routed to an intermediate height of condensate stripper 48 so as to add to the reboil through the top few trays. Stripper 48 is reboiled by directly injected steam from depressurization of beer still 41 stillage by valve

50 and optionally also by some of the steam generated in reboiler 45. Overhead vapor from 48 is routed to auxiliary rectifier 51, and bottom liquid from 51 is returned to 48. Auxiliary rectifier 51 also receives intermediate reboil from exchanger 52, thereby increasing the vapor flow through the near-azeotropic section of that column. Rectifier 51 receives overhead reflux from condenser 53. Condenser 54 is a trim condenser for column 47. Finally, and importantly, in order to enable rectifier 47 to attain near-azeotropic purity (above 180 proof and preferably above 190 proof) in spite of the fact that exchanger 45 causes the feed vapor ethanol concentration to be much higher than in the base case, it is necessary to withdraw a partial stream of inter¬ mediate height liquid from column 47, adjust it to the approximate pressure of column 5T via means for pressurization 55, and then feed it to column 51 at the approximate height of intermediate reboil from 52. Preferably it may be fed directly to 52 en route to 51. Columns 41 and 47 are at approximately the same pressure, and columns 44,--48 and

51 are all at appoximately the same lower pressure, about 1/3 to 1/2 the 41/47 pressure. The results of a computer simulation derived heat and mass balance of Figure 4 under similar operating conditions as Figure 1 are as follows. For the same 100M (lb-mαle/hr) feed to column 41 at the same temperature, pressure, and the same L/V ratio (5.8), the heat demand is reduced to 305,000 BTU/hour due to diverting the rectifier bottom liquid to a separate stripper. The reboil generated in 45 and by flash at 50 is sufficient not only to strip the condensate in 48, but to also strip an additional 40 M of feed in stripper 44, which operates at a bottom temperature and pressure of 96 C and 12.8 psia. Partially condensed stripper 41 vapor exits exchanger 45 at 98°C, having been 50% condensed (mole basis). The bottom stripping section of all 3 columns 41, 44, and 48 operate at approximately equal L/V ratios of about 5.8, and the top rectifying sections of both columns 47 and 51 arc at L/V of about 0.73. 6.6 M of 192 proof ethanol is obtained as product. 14.5 M of pure H 3 is obtained from the bottom of stripper 48, and hence the stillage evaporation duty is reduced by a corresponding amount (about 11%). The overall distillation energy requirement to reach nearly the azeotrope has been reduced to 305 KBTU per 5.6 lb-molc contained ethanol_, or about 7.8 lb steam per gallon. 2.7 of .41 mole fraction ethanol is transferred from 47 to 52/51. As mentioned above, it will be recognized that the disclosed inventive entities will find useful application in combinations other than the explicit configurations depicted in Figures 2, 3, and 4, by incorporation and/or deletion of other features known to the prior art. As non-limiting examples:

1) product ethanol can be withdrawn from both columns as shown, or from cither column alone, where the overhead product from the other rectifier is fed to the overhead of the withdrawal column; 2) feed can be preheated via heat exchange with stillage;

3) a dehydration column may be cascaded at the hot end of the beer still;

4) evaporation stages may be cascaded at the cold end; and

5) product can be withdrawn as liquid or vapor; ol sieve dehydration may be incorporated.

The intended scope of the claimed invention should be ascertained from the claims.

_ ^*"-¹¹- The selection of disclosed features will depend on specific conditions such as feed composition. Figures 2 and 4 are appropriate for typical fermentation feeds containing .04 mole fraction ethanol; Figure 3 finds application with more dilute feeds, or special circum¬ stances.

Claims

1. Apparatus for distillation of a feed mixture comprised of: a) a distillation column comprised of stripping section and rectifying section; b) an auxiliary distillation column comprised of auxiliary stripping section and auxiliary rectifying section: by c) means for reboiling the auxiliary stripping section/indirect exchange of latent heat with partially condensing vapor which is withdrawn from the stripping section; d) means for routing at least the uncondensed remainder of the said partially condensing vapor to the rectifying section; e) means for feeding at least the condensed portion of the said partially condensing vapor to the auxiliary distillation column; and f) means for feeding at least part of said feed mixture to said distillation column and means for withdrawing light product from at least one of rectifying section and the auxiliary rectifying section.

2. Apparatus according to claim 1 further comprised of means for exchanging latent heat between condensing overhead vapor from the rectifying section and intermediate height liquid from the auxiliary rectifying section, and means for returning the evaporated inter¬ mediate height liquid to the auxiliary rectifying section as intermediate reboil therefor.

3. Apparatus according to claim 2 further comprised of means for feeding a minor fraction of the feed mixture to the auxiliary distillation column, and wherein said feed mixture is fermen¬ tation beer and said light product is concentrated alcohol of at least 180 proof.

4. Apparatus according to claim 3 further comprised of means for with¬ drawing intermediate height liquid from the rectifying section and feeding it to an intermediate height of the auxiliary rectifying section and wherein the beer feed height of the auxiliary column is located below the partial condensate feed height.

5. Apparatus according to claim 2 wherein the feed mixture is fermented beer and further comprised of a second auxiliary stripping section; a means for feeding a minor fraction of the beer to said second auxiliary stripping section; a means for routing overhead vapor from said second auxiliary stripping section to said auxiliary rectifying section; means for routing bottom liquid from said auxiliary rectifying section to said auxiliary stripping section; and means for reboiling said second auxiliary stripping section by indirect exchange of latent heat between bottom liquid from the second auxiliary stripping section and partially condensing vapor from the rectifying section.

6. Apparatus according to claim 2 further comprising means for depressurizing the bottom liquid from the stripping section to the approximate pressure of the auxiliary stripping section; and means for routing the flash vapor generated thereby to the auxiliary stripping section as^" reboil therefor.

7. Apparatus according to claim 1 further comprised of means for with_¬ drawing- bottom liquid from said rectifying section and means for feeding it to said auxiliary stripping section.

a Apparatus according to claim 2 further comprised of means for routing rectifying section bottom liquid to said auxiliary strip¬ ping section, and means for withdrawing intermediate height liquid from said rectifying section and feeding it to at least one of an intermediate height of the auxiliary rectifying section and the said intermediate reboiler for the auxiliary rectifying section.

9. Apparatus for fractional distillation of a feed mixture comprised of: a) a distillation column comprised of a stripping section, a rectifying section,- and a feed point for at least part of said feed mixture; b) an auxiliary stripper which is fed bottom liquid from the rectifying section; c) a means for depressurization of the bottoms liquid from the stripping section, whereby a flash vapor is generated; and d) a means for directly injecting said flas vapor into the auxiliary stripper as reboil therefor.

10. Apparatus according to claim 9 further comprised of a condenser to condense the overhead vapors from the auxiliary stripper and a pump to feed the said condensed overhead vapors to an intermediate height of the rectifying section.

11. Apparatus according to claim 9 further comprised of an auxiliary rectifier which receives overhead vapor from and delivers bottom liquid to the auxiliary stripper.

12. Apparatus according to claim 11 further comprised of an intermediate reboiler for the auxiliary rectifier which is also an overhead reflux condenser for the rectifying section.

13. Apparatus according to claim 12 wherein the feed mixture is a product of fermentation comprised of water and ethanol; and further comprised of: a) .means for dividing the feed mixture into two fractions and feeding the lesser fraction to the auxiliary still; and b) means for reboiliπg the auxiliary still by indirect latent heat exchange between partially condensing vapor from the stripping section and bottom liquid from the auxiliary stripper.

14. Apparatus according to claim^' 12 wherein the feed mixture is a product of fermentation comprised of water and ethanol; and further comprised of: a) a second auxiliary stripper; b) a means for dividing the feed mixture into two streams and for feeding one stream to the second auxiliary stripper; and c) a means for reboiling the second auxiliary stripper by indirect latent heat exchange between stripping section overhead vapor and second auxiliary stripper bottom liquid.

15. Apparatus according to claim 14 further comprised of means for withdrawing intermediate height liquid from the rectifying section and transferring it to the auxiliary rectifier.

16. Apparatus according to claim 15 further comprised of means for evaporating at least part of said intermediate height liquid before feeding it to the auxiliary rectifier; and means for transferring overhead vapor from the second auxiliary still to an intermediate height of the auxiliary still.

17. Apparatus for fractional distillation of a mixture comprised of water and ethanol comprised of: a) a distillation column comprised of rectifier and beer still which is fed a major fraction of the mixture; b) an auxiliary beer still which is fed theremainder of the mixture and which is reboiled at least in part by indirect exchange of latent heat between partially condensing over¬ head vapor from the beer still and bottom liquid from the auxiliary beer still; c) an auxiliary stripper and an auxiliary rectifier which is fed vapor from the auxiliary stripper overhead, wherein the auxiliary stripper feed is bottom liquid from the rectifier and the auxiliary rectifier; and d) a combination overhead reflux condenser/intermediate reboiler which provides overhead reflux to the rectifier and inter¬ mediate reboil to the auxiliary rectifier.

18. Apparatus^"according to claim 17 further comprised of: a) means for transporting the uncondensed fraction of the beer still overhead vapor to the rectifier; b) means for feeding the condensed fraction of the beer still overhead vapor to the auxiliary stripper; c) means for reboiling the auxiliary stripper by direct injection of steam; d) means for providing at least part of the reboil steam of part c) by flask depressurization of the stillage from the bottom of the beer still; and e) means for transporting auxiliary beer still overhead vapor to an intermediate height of the auxiliary stripper.

19. A process for distilling an ethanol-water mixture to a specified concentration of at least about 80 mole percent ethanol comprising: a) stripping ethanol-containing vapor from at least part of the feed mixture in a beer still; b) rectifying at least part of the ethanol-containing vapor to specified concentration ethanol in a rectifier; c) separately stripping ethanol-containing vapor from the bottoms liquid resulting from said rectifying step in a separate stripper; and d) providing reboil steam to said separate stripper by at least one of: i) flash evaporating the bottoms liquid resulting from said stripping step and directly injecting the resulting flash steam; and ii) exchanging latent heat between partially condensing vapor from said stripping step and bottoms liquid from said separate stripping step.

20. Process according to claim 19 further comprised of: a) separately rectifying the overhead vapor from said separate stripping step to specified concentration ethanol in a separate rectifier; b) feeding at least part of said feed mixture to at least one of: i) said separate stripper; and ii) a second separate stripping step which includes reboiling by exchanging latent heat between bottom liquid and partially condensing vapor from said stripping step; c) exchanging latent heat between overhead vapor from said rectifier and intermediate height liquid from said separate rectifier; d) withdrawing intermediate height liquid from said rectifier, and feeding it to an intermediate height of said separate rectifier; and e) maintaining the pressure of the stripper and rectifier below about 27 psia and maintaining the pressure of the separate stripper and separate rectifier above about 6 psia.