NITROGEN PARTIAL EXPANSION REFRIGERATION FOR CRYOGENIC AIR SEPARATION

Abstract: A means of producing at least one of high purity nitrogen and low to medium purity oxygen (up to 97 % purity) at high recovery (above 96 % for oxygen). The LP column efficiency is improved to reduce the energy requirement, without offsetting reduction in LN2 reflux availability. Referring to Figure 1, this is done by providing intermediate height reboil to LP column (3) by a latent heat exchanger (10) in which HP rectifier (5) overhead N2 vapor which has been partially expanded in expander (9) is condensed and kettle liquid is evaporated. The condensed N2 is then used to reflux column (3) after depressurization by valve (13).

Description: Description Nitrogen Partial Expansion Refrigeration for Cryogenic Air Separation Technical Field This invention relates to processes and apparatus for separating air into at least one of nitrogen and low to medium high purity oxygen via cryogenic distillation. The invention makes possible a substantial reduction in the energy hitherto required for these products, by increasing the efficiency of the distillation step. Background Art Conventional cryogenic air separation processes normally involve at least two distillation ' columns: a "low pressure" column, from which is withdrawn fluid oxygen bottom product of specified purity plus gase ' ous nitrogen overhead product, plus a "high pressure rectifier" which receives the feed air, provides reboil to the LP column and LN ? reflux for both columns by indirect exchange of latent heat between the two columns, and provides oxygen enriched liquid air bottom product (kettle liquid) to the LP column. The conventional flowsheets provide the bulk of the refrigeration necessary for the process in either of two conven¬ tional manners: by work expanding either part of the HP rectifier overhead N 2 to nitrogen exhaust pressure (slightly below LP column overhead pressure), or expanding part of the feed air to LP column intermediate height pressure. U.S. Patent 3327488 illustrates the above two approaches in the same flowsheet, although for economic reasons usually only one or the other is used. The kettle liquid is low in 0 2 content, for example about 35% 0 2 . When kettle liquid is the primary feed to the LP column and there is only a bottoms reboiler, the bottom section of the column is very inefficient, i.e., has much more reboil than necessary. At least two methods have been disclosed in the prior art for reducing this inefficiency. U.S. Patent 4254629 discloses a configuration wherein one or two additional columns are incor¬ porated in order to evaporate at least part of the kettle liquid before being fed to the LP column. U.S. Patents 2753698, 3270514, and 4208199, disclose simpler approaches to the same objective. Secondly, it is possible to provide intermediate reboil to the LP column in addition to bottoms reboil. Numerous prior art references describe this- technique, including U.S. Patents 3210951, 3277655, 3251190, 3371496, 3688513, 4578095, and 4582518. Both of the techniques described above, as disclosed in the references, cause a serious reduction in the amount of LN~ reflux available to the LP column. 'This severely limits recovery of 0„. The majority of the overhead product of the HP rectifier, fairly pure nitrogen, is normally withdrawn as liquid for further cooling and subsequent direct injection into the LP column as overhead reflux therefor. Frequently a minor amount of gaseous N„ is also withdrawn: for expansion to produce refrigeration; for further compression and then recycle in an external heat pump (liquefaction cycles); or as a minor product directly. When withdrawn as either minor product or for refrigeration expansion, it causes a one-for-one reduction in the LN ? available for LP column reflux. It is known to use the power developed by the refrigera- tion expander to drive a warm-end compressor, for example in a compander configuration. See, for example, U.S. Patent Application 853461 filed 04/18/86 by Donald C. Erickson. It is also known to incorporate a cold expander driving a cold compressor, whereby no net refrigeration is obtained. U.S. Patent 4072023 illustrates this, showing cold compression of either the oxygen product or the supply to the HP rectifier. What is needed, and a primary objective of this invention, is process and apparatus whereby the efficiency of the lower section of the LP distillation column is improved by at least one of intermediate reboiling and kettle liquid evaporation, but without the accompanying substantial decrease in LN ? reflux avail¬ ability which has heretofore accompanied such improvement. More particularly it is desired to exchange the energy (excess reboil) presently wastefully consumed in the bottom section of the LP column for useful refrigeration work, thus minimizing or eliminating the need to expand N 2 vapor all the way to exhaust pressure to provide necessary refrigeration. Disclosure of Invention The above and other useful objects are provided by process and apparatus wherein a majority of the HP rectifier overhead product nitrogen is withdrawn as vapor, slightly super¬ heated as appropriate for a compensating stream, work expanded to an intermediate pressure above the LP column pressure and then supplied to a latent heat exchanger supplied with either of two evaporating liquids: LP column intermediate height liquid, or kettle liquid which has been depressurized to the approximate LP column pressure. The evaporated fluid adds to the intermediate reboil flow rate of the LP column, and the liquid N. condensate is routed preferably to the LP column overhead as direct injection reflux, and in some cases part may be pressurized and returned to the HP rectifier overhead as reflux therefor. In the above manner the N 2 vapor flowing through the expander is in lieu of reboil vapor which would otherwise flow through the LP column between the bottoms reboiler and the point of introduction of evaporated fluid. Since the reboil would otherwise be wasted in the column (i.e., not necessary for the desired separation), the work obtained at the N 2 expander is "free", i.e., at no additional input energy cost. The practical advantage is th ' at the conventional expander flow is no longer necessary. Since that conventional flow bypasses either the HP rectifier (air expansion) or the LP column (N 2 expansion), and hence represents a loss ofseparating power (i.e., LN 2 reflux), the new process avoids that loss. Thus various low energy or high efficiency flowsheets become possible which without the disclosed improvement would suffer offsetting low recovery of 0 2 due to lack of availability of sufficient LN ? reflux. With the disclosed process, subjecting N 2 to partial expansion vice a gas with substantial 0 2 content provides two important advantages. First, a given reboil rate up the HP rectifier yields more N 2 than a lower pressure 0 2 -contaiπing gas. Second, since the N 2 pressure is higher, the piping and heat exchange pressure drops are less severe, and heat transfer coefficients are improved. Also, the low pressure ratio expansion allows a very efficient expander. In general, any flowsheet incorporating nitrogen partial expansion refrigeration (NIPER) can utilize either varia¬ tion described above. When N 2 is condensed directly against LP column intermediate height liquid, the latent heat exchanger may be located either internal to or external to the column. Internal location is preferred In order to balance liquid flow rates without a pump. When kettle liquid is evaporated, external location is indicated and no pump is necessary. Hence overall LP column height may be reduced. NIPER is particularly useful for producing low purity oxygen (up to about 96% purity) and/or high purity nitrogen. It is usually not a preferred method of producing argon coproduct. It is particularly advantageous when incorporated in conjunction with other energy-saving or recovery-enhancing measures, since it tends to minimize disadvantageous side effects which would otherwise be present. Examples of other measures are presented in the figures. Brief Description of the Drawings Figures 1 through 7 are simplified schematic flowsheets illustrating preferred embodiments or configurations incorporating the disclosed improvement. Figure 5 is for high purity - as major product, and all the others are for low purity 0 ? . Figures 1, 6, and 7 illustrate depressurized kettle liquid being evaporated by the condensing N 2 , and the remaining figures Illustrate LP column intermediate height liquid being evpaorated (internal to the LP column). Figures 4 and 5 illustrate LP column bottom reboil by latent heat exchange with HP rectifier overhead vapor; in Figures 1, 2, 3, and 7 it is by partial condensation of supply air; and in Figure 6 it is by total condensa¬ tion of a co panded minor fraction of the supply air. Other distinc- ' "> tions between the flowsheets include how product 0 2 is evaporated, presence of kettle liquid split, and presence of liquid air split. These are elaborated upon below. Best Mode for Carrying Out the Invention Referring to Figure 1, pressurized supply air is cooled to 0 near its dewpoint in main exchanger 1, which may be any conventional type: reversing, regenerators, brazed plate fin ' , etc. Also cleanup of moisture, C0 2 , and hydrocarbons may be via any known technique, e.g., molecular sieve, reversing exchangers, and the like. The air is- routed to the bottoms reboiler 2 of LP column 3, where it partially 5 condenses. Optional phase separator 4 directs the uncondensed frac¬ tion to HP rectifier 5. Overhead vapor is divided; part is supplied to reboiler 6, from which LN 2 is returned to the HP rectifier as reflux, with optionally part also being supplied to reflux the LP column via subcooler 7 and depressurization valve 8. The remaining 0 N 2 vapor is superheated sufficiently to avoid condensation during work expansion, and also to compensate for heat exchange inefficiency of exchanger 1. It is then work-expanded in expander 9 and supplied to 2 condenser 10. If the expander exhaust temperature is close to the dewpoint, the partially depressurized 2 is directly supplied to 5 10; otherwise it may be sensibly cooled first. At condenser 10, kettle liquid which has been cooled in 11 and depressurized by valve 12 is at least partially evaporated, and then fed to LP column 3. The resulting liquid N 2 is cooled in 7, depressurized by valve 13, phase separated at 14, and directly injected into LP column 3 as 0 overhead reflux. Gaseous N ? is withdrawn from 3 and vented or put to other use, and product low purity 0 2 (e.g., about 90 to 97% purity) is evaporated by 2 and withdrawn. In Figure 2, also for production of low purity 0 2 , the changes from Figure 1 are as follows. First, rather than intermediate reboiler 6 producing more LN„ than required for HP rectifier reflux, it produces less, and hence some of the intermediate pressure LN 2 must be supplied by pump \6. Secondly, the partially depressurized N ? condenses by heat exchange with LP column intermediate height liquid in 15, vice with kettle liquid in 10. Thirdly, product 0 2 evap¬ oration in 17 is conducted separately from LP column bottoms reboil in 2, rather than together. Thus means for supplying liquid oxygen to evaporator 17 is required, i.e., a check valve, or pump 18, or barometric leg, or the like. Phase separator 19 then routes the con- - densats from 17 to the LP column via valve 20. In Figure 3, the basic embodiment or arrangement according to Figure 2 is combined with the technique disclosed in U.S. Patent . 4604116 for producing high pressure oxygen product at high energy efficiency and yield and without a separate oxygen compressor. A minor fraction of the supply air is supplied at elevated pressure to extra high pressure (EHP) rectifier 21, which is refluxed by latent heat exchange with boiling pumped LOX in 22. The pumped LOX exchanges sensible heat with both overhead 2 and kettle liquid from rectifier 21, in exchanger 23. The latter streams are then depres- ' surized by valves 24 and 25 respectively and undergo further separa¬ tion in rectifier 5 and/or column 3. A minor stream of liquid oxygen may optionally bypass evaporator 22 via valve 26. Figure 4 illustrates the use of NIPER in a more conventional dual pressure column configuration wherein the objective is to increase the yield of coproducts rather than decrease the air supply pressure as in Figures 1-3. As with all the flowsheets, numbered items which repeat earlier numbers have descriptions substantially the same as already described. The differences of Figure 4 from Figure 2 are that the LP column 3 is rεboiled at the bottom by reboiler 27 which exchanges latent heat directly with HP rectifier overhead 2 i.e., before partial depressurization. This makes it possible to addi¬ tionally withdraw substantial amounts of one or more co-products, .as indicated: liquid oxygen, liquid nitrogen, and/or high pressure gaseous nitrogen. Figure 5 is a further adaptation of Figure 4 so as to produce high purity nitrogen as the major product instead of low purity oxygen, while using NIPER. The essential additional features are that impure liquid oxygen from the bottom of column 3 is depressurized by valve 28 and then supplied to latent heat exchanger 29, thereby providing the large amount of LN~ reflux necessary for-high purity N ? plants. Product 2 is withdrawn both from the HP rectifier and LP column overheads. Typical operating conditions for Figure 5 are HP rectifier pressure about 135 to 150 psia, LP column pressure about 55 to 60 psia, N recovery of about 0.7 to 0.72 moles per mole of compressed air, and N intermediate pressure of about 95 to 120 psia. Figure 6 illustrates an extremely low energy, high effi¬ ciency arrangement made possible by NIPER for producing low purity oxygen. It differs most importantly from Figures 1 and 2 in that LP column 3 is bottom reboiled by total condensation of a minor frac¬ tion of the supply air in 30, and that fraction is compressed to above supply pressure by warm compressor 30 which is powered by expander 9. The compression heat may be removed by cooler 31. The extra pressure lets reboiler 30 operate at about the same temperature as reboiler 2 even though the HP rectifier 5 is 1 to 2°F cooler, and hence the required air supply pressure decreases from about 65 psia to about 59 psia (the rectifier pressure is about 6 psia lower than the supply pressure when using molecular sieves). Other beneficial efficiency and recovery-enhancing features illustrated by Figure 6 include a split of the liquid air by coordinated action of valves 34 and 35 so as to provide intermediate reflux to both recti¬ fier 5 and column 3; a split of kettle liquid by coordinated action of valves 12 and 33 so as to provide just sufficient kettle liquid to 10 for total evaporation, and the remainder directly to column 3 as liquid; and finally a supplemental expander 36 performing conven¬ tional expansion of air (or alternatively of N_) for those flowsheets wherein expander 9 cannot provide all required refrigeration. Figure 7 incorporates features of Figure 6 and of Figures 1 and 2. Once again it reflects an extremely efficient process, but the objective here is to increase byproduct yield (e.g., HP N 2 ) rather than decrease supply air pressure as in Figure 6. The differences froπr Figure 6 are that column 3 reboil is by partial condensation of feed air In 2, ; and a minor supply air fraction is further corn- Dressed in 37 and totally condensed in 38 to evaporate LOX. Also LN„ can be transferred in either direction, either from condenser 6 to LP overhead via valve 8 or from condenser 10 to HP rectifier overhead via valve 39 and pump 16, thus providing maximum flexi¬ bility, e.g., allowing different rates of coproduct withdrawal. It will be recognized that either or both NIPER variations can be incorporated in any of the above figures. All of the figures except Figures 4 and 5 reflect very high efficiency LP columns in which in addition to bottoms reboil, the reboil rate is further increased at two different vertically spaced heights. For Figure 4 it will be apparent that a second NIPER can be added to column 3 at a different height than condenser 15, using, for example, a kettle liqjid boiling condenser 10 and a second expander discharging at a different intermediate pressure. The second expander can increase the refriger¬ ation output, thus allowing withdrawal of more liquid coproduct, or alternatively could power a cold compressor so as to further increase 0 2 delivery pressure. Alternatively other known intermediate reboiler configurations could be added to Figure 4 besides NIPER. The various other energy-reducing and recovery-enhancing techniques illustrated in Figures 1-7 can similarly be applied in¬ dependently or in other combinations. Many more possible advantag¬ eous combinations and variations within the scope of the disclosed invention will occur to the artisan beyond those presented, and the intended scope is to be only limited by the claims. The N 2 intermediate pressure from expander 9 will normally be at least 1.5 times the LP column pressure, and more typically 2 times.

Claims: Claims 1. A process for obtaining at least one of oxygen and nitrogen from pressurized, cooled, and cleaned supply air by cryogenic distillation in an apparatus comprised of at least a high pressure rectifier and a low pressure distillation column, comprising: a) introducing at least part of a vapor obtained from said supply air into the HP rectifier; b) withdrawing pressurized gaseous nitrogen from the HP rectifier; c) partially .expanding the nitrogen to an intermediate pressure; d) condensing said partially expanded nitrogen by latent heat exchange with at least one of LP dis- tillation column intermediate height liquid and at least part of the depressurized kettle liquid; e) refluxing at least one of the HP rectifier and the distillation column by direct injection of the condensed nitrogen. 2. Process according to claim 1 further comprising partially warming said gaseous nitrogen prior to expansion; and wherein said inter¬ mediate pressure is at least 1.5 times the pressure of said LP column. 3. Process according to claim 2 further comprising partially condensing at least part of said supply air by exchanging latent heat with LP column bottom liquid, and providing the remaining uncondensed fraction from said partial condensing step as said vapor introduced to the HP rectifier. 4. Process according to claim 2 further comprising splitting rectifier liquid bottom product kettle liquid into two fractions, supplying the major fraction to said condensing heat exchange and supplying the minor fraction to the LP column as liquid. 5. Process according to claim 2 further comprising supplying at least a major fraction of said supply air directly to said HP rectifier as said introduced vapor. 6. Process according to claim 2 further comprising reboiling LP column bottom liquid by exchanging latent heat with a totally condensing minor fraction of said supply air which has been additionally compressed to above supply pressure. 7. Process according to claim 6 further comprising: powering said additional compression by said work expansion; and splitting the liquid air condensate and providing intermediate reflux to both the LP column and HP rectifier therefrom. 8. A dual pressure cryogenic distillation apparatus designed and dimensioned for air separation comprised of: a) high pressure rectifier; b) low pressure distillation column; c) conduit and heat exchange means for withdrawing gaseous N„ from said HP rectifier and controllably superheating it; d) expander for partially depressurizing said superheated N ? while producing refrigeration and power; e) latent heat exchanger for condensing said expanded N 2 and providing additional reboil to an intermediate height of said LP column; and f) means for introducing the condensed N 2 into the overhead of at least one of the HP rectifier and the LP column as reflux therefor. 9. Apparatus according to claim 8 further comprised of LP column bottoms reboiler in which latent heat is exchanged with partially condensing supply air. -11- 10. Apparatus according to claim 8 further comprised of a warm compressor powered by said expander which additionally compresses a minor fraction of supply air, means for totally condensing said supply air, and means for supplying the liquid air to interrned- iate reflux heights of both the HP rectifier and LP column. 11. In a subambient distillation apparatus designed, dimensioned, and adapted for separation of at least one of nitrogen and low purity oxygen from cleaned and cooled air, and comprised of high pressure rectifier and low pressure column, the improvement comprising means for providing refrigeration by work expansion of nitrogen vapor comprising: a) means for withdrawing HP rectifier overhead nitrogen as vapor and superheating it a controlled amount; b) means for expanding said superheated nitrogen to a pressure at least 1.5 times said LP column pressure so as to produce shaft work and refrigeration; c) means for condensing said partially expanded nitrogen by exchange of latent heat with at least one of: i) LP column intermediate height liquid, and; ϋ) at least part of the HP rectifier kettle liquid bottom product; and d) means for deprεssurizing at least part of said condensed N 2 to the approximate LP column overhead pressure and injecting it thereto asreflux therefor. 12. Apparatus according to claim 11 further comprised of means for splitting HP rectifier bottom liquid into two fractions, means for supplying one fraction to said means for condensing N 2 , and means for feeding the remaining fraction to the LPcolumn as liquid.

Inventors: ERICKSON DONALD, ERICKSON, DONALD
Applicants: ERICKSON DONALD, ERICKSON, DONALD
Application year: 1987
Main classification: F25J3
IPC codes: F25J3