MXPA97002046A - System for cryogenic rectification with power condensation in eta - Google Patents

Abstract

The present invention relates to a method for producing oxygen of lower purity by cryogenic rectification of air of feed in a double clutch having an upper pressure column and a lower pressure column, comprising: a) passing a first portion of the air feeding in the upper pressure column and separating the first portion of feed air into the upper pressure column by cryogenic rectification in oxygen enriched fluids enriched with nitrogen b) passing fluids enriched with oxygen and enriched with nitrogen from the upper pressure column towards the lower pressure column, c) partially condesar a second portion of the supply air by indirect heat exchange with fluid inside the liquid air pressure column and a first portion of air in steam; at least partially the first portion of air in steam by exchange indirect heat with fluid inside the lower pressure column, at a point above the point where the step is carried out, (c), to produce a second portion of liquid air, e) pass the first portion of liquid air and the second portion of liquid air in the lower pressure column, each at a point above the point where step (c) is carried out above the point where step (c) is carried out with the second portion of liquid air passed to the lower pressure column above where the first portion of liquid air is passed to the lower pressure column, f) separating the fluids passed to the lower pressure column by means of a rich fluid in the fluid in nitrogen and oxygen-rich fluid, and g) recover oxygen-rich fluid as an oxygen product of lower purity.

Description

SYSTEM FOR CRYOGENIC RECTIFICATION WITH CONDENSATION OF FEEDING AIR IN STAGES Field of the Invention This invention relates generally to cryogenic rectification and more particularly, to the production of oxygen of lower purity. Prior Art The cryogenic rectification of air to produce oxygen and nitrogen is a well-established industrial process. Normally, the feed air is separated in a double column system, in which nitrogen vapor is used from the shelf of seats or from the top of a column at higher pressures, to boil again the liquid of oxygen sediments in a column of lower pressure. The demand for oxygen of lower purity is increased in applications, such as glassmaking, steelmaking and energy production. Less boiling of steam is required in the lower pressure column separation sections, and less reflux of liquids in the lower pressure column enrichment sections, for the production of lower purity oxygen, which has a purity of oxygen of 97 mole percent or less, which is usually generated by the operation of a double column. Accordingly, oxygen of lower purity is generally produced in large quantities by a cryogenic rectification system, in which the feed air at the highest column pressure is used to re-boil the sediments of the pressure column lower, and then it is passed to the upper pressure column. The use of air instead of nitrogen to vaporize the sediments in the lower pressure column, reduces the air supply pressure requirements, and allows the generation of only the necessary boiling, in the separation sections of the lower pressure column. , either by feeding the appropriate portion of the air to the reboiler of the lower pressure column, or by partially condensing a larger portion of the total air fed. Although conventional air-boiling cryogenic rectification systems have been effectively used for the production of lower purity oxygen, their ability to generate reflux is limited in order to supply it to the upper part of the lower pressure column. This is the result of the fact that the condensation of a part of the feed air reduces the available steam, to generate reflux of nitrogen in the upper pressure column. More energy is consumed because the oxygen recovery is reduced, as a result of the reduced capacity to generate reflux. Accordingly, it is an object of this invention to provide a cryogenic rectification system to produce oxygen of lower purity, which employs a two-column arrangement and operates with reduced energy requirements, above conventional systems. SUMMARY OF THE INVENTION The foregoing and other objects which, upon reading the description, will be apparent to one skilled in the art, are obtained by the present invention, one aspect of which is: A method for producing oxygen of lower purity by rectification Cryogenic feed air in a double column, having an upper pressure column and a lower pressure column, comprising: (A) passing a first portion of the feed air into the upper pressure column and separating the first portion of air of feeding into the upper pressure column by cryogenic rectification of fluids enriched with oxygen and enriched with nitrogen; (B) passing fluids enriched with oxygen and enriched with nitrogen from the upper pressure column to the lower pressure column; (C) partially condensing a second portion of the feed air, by indirect heat exchange with fluid within the lower pressure column, to produce a first portion of liquid air and a first portion of air in steam; (D) condensing, at least partially, the first portion of air in steam by indirect heat exchange with fluid within the lower pressure column, at a point above the point where step (C) is carried out, to produce a second portion of liquid air; (E) passing the first portion of liquid air and the second portion of liquid air to the lower pressure column, each at a point above the point where step (C) is carried out; (F) separating the fluids passed to the lower pressure column by cryogenic rectification of the nitrogen-rich fluid and the oxygen-rich fluid; and (G) recovering oxygen rich fluid as an oxygen product of lower purity. Another aspect of the invention is: Apparatus for producing lower puy oxygen by: (A) a double column, having a first column and a second column; (B) means for passing a first portion of feed air to the first col umn; (C) means for passing fluid from the first column to the second column; (D) a first heat exchanger within the second column and means for passing a second portion of air to the first heat exchanger; (E) a second heat exchanger within the second column, at a point above the first heat exchanger, and means for passing steam from the first heat exchanger in the second heat exchanger; (F) means for passing liquid from the first heat and liquid exchanger from the second heat exchanger in the second column, each to a point above the first heat exchanger; and (G) means for recovering the oxygen product of lower purity, from the second column. As used herein, the term "oxygen of lower purity" means a fluid having an oxygen concentration of 97 mole percent or less. As used herein, the term "feed air" means a mixture comprising, primarily, nitrogen and oxygen, such as ambient air. As used herein, the terms "turbo-expansion" and "turbo-expander" mean, respectively, method and apparatus for the flow of high pressure gas through a turbine, to reduce the pressure and temperature of the gas , thus generating refrigeration. As used herein, the term "column" means a distillation column or fractionation zone, that is, a column or contact zone in which the liquid and vapor phases are contacted in countercurrent fashion to effect the separation of a mixture of fluids, such as by contacting the vapor and liquid phases on a series of vertically separated trays or plates, mounted within the column and / or packing elements, which may be structured packing elements and / or random packing. For an additional discussion of distillation columns, see "Chemical Engineer's Handbook," edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, E. U.A. , Section 13, The Continuous Distillation Process. The vapor and liquid separation procedures that are in contact depend on the difference in vapor pressures for the components. The high pressure (or more volatile or low boiling point) vapor component will tend to concentrate in the vapor phase, while the low pressure (or less volatile or high boiling point) vapor component will tend to Concentrate on the liquid phase. Partial condensation is the separation process by which the cooling of a vapor mixture can be used to concentrate the volatile component (s) in the vapor phase and, therefore, the ) less volatile component (s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations., as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contact of the vapor and liquid phase is adiabatic and can include integral or differential contact between the phases. The separation process arrangements that use the principles of rectification to separate mixtures are often interchangeably named, rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification procedure carried out, at least in part, at temperatures of or below -87.6 degrees Celsius. As used herein, the term "indirect heat exchange" means putting two fluid streams in heat exchange relationship, without any physical or intermixed contact of fluids between them. As used herein, the term "tray" means a contacting stage, which is not necessarily an equilibrium stage, and may mean another contact apparatus, such as the package having a separation capacity equivalent to a tray. As used herein, the term "equilibrium step" means a vapor-liquid contacting step, wherein the vapor and the liquid leaving said stage, are in mass transfer equilibrium, for example, a tray that has 100 percent efficiency or a packing element of height equivalent to a theoretical plate (AEPT). As used herein, the term "within a column" when referring to heat exchange, means functionally within the column, i.e., physically within that column or adjacent to that column, with the liquid in that column being passed to the heat exchange device. The liquid can be totally or partially vaporized and the resulting gas or gas-liquid mixture is returned to the column. Preferably the liquid is partially vaporized and the resulting gas-liquid mixture is returned to the column, at the same level as the liquid is extracted from the column. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic flow chart of a preferred embodiment of the cryogenic rectification system of the invention. Fig. 2 is a schematic flow diagram of another preferred embodiment of the cryogenic rectification system of the invention. Figure 3 is a representation of a preferred heat exchange arrangement in the practice of the invention, wherein the defined heat exchange within a column takes place outside the shell of the column. DETAILED DESCRIPTION OF THE INVENTION The invention serves to more closely eliminate the irreversibilities in the cryogenic distillation system of the lower pressure column of a double column system. This reduces the energy requirements of the system to a greater degree than is possible with conventional practice. Partially condensing a lower pressure supply air stream, in an intermediate heat exchanger in the lower pressure column against the liquid in the column, partially in reboiling, the operating line of this section of the column is approached to the balance line, thus reducing the system's energy requirements. The phase separation of the lower pressure, partially condensed feed air provides the opportunity for the incorporation of a second intermediate heat exchanger, at a higher level, in the lower pressure column. In this second intermediate heat exchanger, the steam separated from the first intermediate heat exchanger is preferably completely condensed against the liquid in the column, partially reboiled. The liquid that comes out of the heat exchanger does not mix with the liquid that enters the vaporization side. The liquids produced in each stage of intermediate heat exchanger, they are transferred to appropriate levels in the column, thus complementing the normally available reflux. The use of the second intermediate stage of heat exchange also reduces the irreversibilities in the column, and therefore reduces the energy requirements for the system. The cooling requirements for the system are covered by the turbo-expansion of a portion of supply air to the plant, to which the pressure has been increased above that used for partial condensation in the intermediate heat exchangers. A further reduction in energy requirements can be obtained by adding a second pair of intermediate heat exchangers, located at a higher level in the column that operates in almost the same way as the first pair. The second pair of intermediate heat exchangers is fed with the near-saturated lower pressure air from the main heat exchanger. The first pair of intermediate heat exchangers is heated with almost saturated air at a pressure somewhat higher than that of the second pair. The refrigeration for the cycle is balanced by turbo-expansion of a portion of the air to the plant, which has been increased above that of the first pair of intermediate heat exchangers. The invention will be described in greater detail with reference to the Drawings. Referring now to Fig. 1, the feed air 100 is compressed at a pressure generally within the range of 1.4 to 3.5 kg / cm2 absolute by passing through the base load compressor 31, and the current 60 of The resulting feed air is cleaned of high-boiling impurities, such as water vapor and carbon dioxide, by the passage through the purifier 50. A portion 63 of the feed air 61 compressed, cleaned, generally comprising of about 20 to 50 percent of the feed air 100, is extracted from the feed air to be used with the intermediate heat exchangers, as will be described later, more completely. The remaining feed air stream 62 is compressed by the passage through the compressor 32, at a pressure within the range of 2.8 to 7.03 kg / cm2 absolute, and the resulting feed air stream 79, is cooled by Indirect heat exchange with return flows. A portion 80 of the stream 79 of the feed air, generally comprising about 5 to 15 percent feed air 100, is removed after the partial travel of the main, turbo-expanded heat exchanger 1 through the pass through. of turbo-expander 30 to generate cooling, and passed as current 81 in column 11 of lower pressure. The remaining feed air stream 64, which preferably comprises the largest portion of the feed air and generally comprises about 35 to 75 percent of the feed air 100, is passed from the main heat exchanger 1 to the heater 23 of the feed air. product, where it is, at least, partially condensed by indirect heat exchange with oxygen as a boiling product. The resulting feed air stream 65 is passed as the first supply air portion in the first column 10 or higher pressure.

The first column 10 is the upper pressure column of a double column system, which also includes the second column 1 1 or lower pressure column. The upper pressure column 10 operates at a pressure within the range of 2.8 to 7.03 kg / cm2 absolute. Within a column 10 of higher pressure, the first portion of feed air is separated by cryogenic rectification in steam enriched with nitrogen and liquid enriched with oxygen. The steam enriched with nitrogen is extracted from the column 10 as the stream 82 and passed to the main condenser 20 where it is condensed by indirect heat exchange with boiling of the sediment liquid in the lower pressure column. The liquid 83 enriched with resulting nitrogen is divided into stream 84, which is returned to column 10 of higher pressure as reflux, and in stream 85 which is cooled by passage through heat exchanger 101 and passed to through valve 87 in column 1 1 of lower pressure, as reflux. The oxygen enriched liquid is extracted from the upper pressure column 10, as the current 71, is cooled by the passage through the heat exchanger 102 and is passed through the valve 73 in the lower pressure column 1 1. In the embodiment illustrated in Fig. 1, the current 71 is combined with the stream 68 from the first intermediate exchanger and this combined stream 75 is passed to the lower pressure column. The second column 1 or lower pressure, operates at a lower pressure than the upper pressure column and within the scale from 1.05 to 2.1 kg / cm2 absolute. The feed air stream 63 is cooled by the passage through the main heat exchanger 1, by indirect heat exchange with return currents. The resultant cooled, lower pressure feed air stream 66 is passed as a second supply air portion in the first intermediate heat exchanger 21, which is located within the lower pressure column 1 1, generally around the 2 to 15 equilibrium stages above the heat exchange of the sediment reboiler 20. Within the first intermediate heat exchanger 21, the second supply air portion 66 is partially condensed by indirect heat exchange with vaporization, preferably by partially vaporizing the liquid flowing down from column 1 1, thus generating flow vapor ascending for the column 1 1 and producing a first portion of liquid air and a first portion of air in vapor in the two-phase stream 67, which is passed from the first intermediate heat exchanger 21 in the phase separator 40. The first steam portion 99, which has a nitrogen concentration, which exceeds that of stream 66, is passed out from phase separator 40 to second intermediate heat exchanger 22, which is located within the column of lower pressure 1 1 above, in general, from about 1 to 10 stages of equilibrium above, of the first intermediate heat exchanger 21. Within the second intermediate heat exchanger 22, the first steam air portion 99 is, at least partially, and preferably is fully condensed by indirect heat exchange with vaporization, preferably partial vaporization of the liquid flowing down the column 1 1, thus generating additional upflow steam for column 1 1 and producing a second portion of liquid air.

The first portion 68 of liquid air, having an oxygen concentration exceeding that of stream 66, is passed out of phase separator 40, through valve 69 and into the lower pressure column 1 1 at a point at or above, generally up to 10 stages of equilibrium above, of the second intermediate heat exchanger 22. As previously mentioned, Fig. 1 illustrates an embodiment in which stream 68 is combined with stream 71 to form stream 75 which is then passed to column 1 1. Second portion 76 of liquid air, having a concentration of nitrogen that exceeds that of the stream 66, is passed out of the second intermediate heat exchanger 22, through the valve 77 and in the lower pressure column 1 1 at a point above, generally from 5 to 20 stages of equilibrium above, of the second intermediate heat exchanger 22. The first and second liquid air portions serve to provide additional reflux liquid in the lower pressure column 1 1, to improve the cryogenic separation within that column. Within the second column 1 1 or lower pressure, the different fluids that pass to that column are separated by cryogenic rectification in nitrogen-rich fluid and oxygen-rich fluid. The nitrogen-rich fluid is withdrawn from column 1 1 as steam stream 89, is heated by the passage through heat exchangers 101, 102 and 1 and is passed out of the system as stream of nitrogen 1, which can be recovered in whole or in part, as a product of nitrogen. The oxygen-rich fluid is extracted from column 1 1 and recovered, in whole or in part, as a low purity oxygen product. In the embodiment illustrated in Fig. 1, the oxygen rich fluid is extracted from the column 1 1, as liquid stream 92, which is passed in the product heater 23 where it is vaporized by indirect heat exchange with the first 64 air supply portion, condensation. The resulting oxygen-rich vapor stream 93 is heated by the passage through the main heat exchanger 1 and is recovered as a stream 94 of low purity oxygen product. If desired, a portion of the stream 92 can be recovered directly as a low purity liquid oxygen product. Fig. 2 illustrates another embodiment of the invention in which a second pair of intermediate heat exchangers is used, within the lower pressure column. The numbers in Fig. 2 correspond to those in Fig. 1 for the common elements and these common elements will not be described in detail again.

Referring now to Fig. 2, a third portion 103 of the feed air stream 61, generally comprising about 5 to 20 percent of the feed air 100, is taken from the stream 61 to be processed in the second pair of air. Intermediate heat exchangers. The stream 61 is then compressed at a higher pressure by the passage through the compressor 33, before being processed as described according to the embodiment illustrated in Fig. 1. The feed air stream 103 is heated by the passage through the main heat exchanger 1 and the resulting stream 104 is partially condensed in the third intermediate heat exchanger 24, which is located within the lower pressure column 1 1 , generally about 1 to 10 stages in equilibrium above the second intermediate heat exchanger 22. Within the third intermediate heat exchanger 24, the feed air stream 104 is partially condensed by the indirect heat exchange with the vaporization, preferably , partial vaporization, of the liquid flowing down column 1 1, thus generating upflow vapor for column 1 1 and producing a third portion of liquid air and an additional portion of vapor air in the two phase stream 105 , which is passed from the third intermediate heat exchanger 24 in the phase separator 41. The additional portion Air in vapor 106, which has a nitrogen concentration exceeding that of stream 103, is passed out of phase separator 41 in the fourth intermediate heat exchanger 25, which is located within the pressure column lower 1 1, above, generally about 1 to 10 stages of equilibrium above, of the third intermediate heat exchanger 24. Within the fourth intermediate heat exchanger 25, the additional vapor air portion 106 is, at least, partially, and preferably, it is fully condensed by indirect heat exchange with the vaporizing liquid flowing down from column 1 1, thus generating the additional upward flow vapor for column 1 1 and producing a fourth portion of liquid air. The third portion of liquid air 107, which has an oxygen concentration that exceeds that of stream 103, is passed through valve 108 and combined with stream 68 to form stream 109, which is then combined with the stream 71 to form stream 75, which is processed as described above. The fourth portion of liquid air 1 10, which has a concentration of nitrogen exceeding that of stream 103, is passed out of the fourth intermediate heat exchanger 25, through valve 1 1 1, and combined with the current 77 which is processed as written before. While Figures 1 and 2 illustrate the heat exchange associated with the heat exchangers 21, 22, 24 and 25, as physically occurs within the shell of the column, this is done to simplify the illustration of the method of the invention. In many cases, it is expected that one or more of said heat exchangers are physically located outside the shell of the column, i.e., functionally within the column. Figure 3 illustrates a generalized arrangement of said heat exchanger, functionally inside the column. Referring now to Figure 3, the liquid descending into the column 200 is collected and removed from the column as the stream 204. The means for collecting and extracting the liquid are well known to those familiar with the design of the distillation equipment. The liquid stream 204 is introduced to the heat exchanger 201, which may be a welded aluminum heat exchanger. As the liquid 204 travels the heat exchanger 201, at least it is partially vaporized by indirect heat exchange with a fluid 202 that, at least it is partially condensed. The fluid 202 represents the flow of steam in the heat exchanger, e.g. , current 66 or current 99 of Figure 1. The streams 202 and 204 flow in a countercurrent fashion within the heat exchanger 201. The partially vaporized liquid 205 leaves the heat exchanger 201 and is brought back to the column 200. Preferably, the partially vaporized liquid is returned to the column in such a way that the vapor portion 206 is able to mix with the vapor 209 , which arises within the column from below the point where the liquid 204 was originally extracted. The means to achieve this are commonly employed in the design of distillation columns when a two-phase stream is introduced to an intermediate location within the column. The liquid portion 207 of the stream 205 is decoupled from the vapor portion and is preferably distributed to those mass transfer elements, such as the packing or trays immediately below the level from which the liquid 204 was originally extracted. Means for decoupling the liquid from the vapor and for distributing the liquid, as described, are commonly employed in the design of distillation columns. Although from a functional point of view it is preferred to use all the liquid flowing down the column for stream 204, for this purpose, some design circumstances may dictate the use of only a portion of downflow liquid. As mentioned, stream 202 is, at least, partially condensed by the heat exchange within heat exchanger 201. The fluid in stream 203 is passed to the column. Stream 203 corresponds, for example, to stream 67 or stream 76 of Figure 1. Although the invention has been described in detail with reference to certain preferred embodiments, those skilled in the art will recognize that other embodiments of the invention exist within the spirit and scope of the claims.

Claims (8)

1. CLAIMS 1. A method for producing lower purity oxygen by cryogenic feed air rectification, in a double column having an upper pressure column and a lower pressure column, comprising: (A) passing a first portion of the feed air in the upper pressure column and separating the first portion of feed air into the upper pressure column by cryogenic rectification in oxygen enriched and nitrogen enriched fluids; (B) passing fluids enriched with oxygen and enriched with nitrogen, from the upper pressure column in the lower pressure column; (C) partially condensing a second portion of the feed air by indirect heat exchange with fluid within the lower pressure column, to produce a first portion of liquid air and a first portion of air in vapor; (D) at least partially condensing the first portion of air in steam by indirect heat exchange with fluid within the lower pressure column, at a point above the point where step (C) is carried out , to produce a second portion of liquid air; (E) passing the first portion of liquid air and the second portion of liquid air into the lower pressure column, each at a point above the point where step (C) is carried out; (F) separating the fluids passed to the lower pressure column, by cryogenic rectification in the nitrogen-rich fluid and the oxygen-rich fluid; and (G) recovering oxygen rich fluid, as an oxygen product of lower purity. The method of claim 1, further comprising: (H) partially condensing a third portion of the feed air, by indirect heat exchange with fluid within the lower pressure column, to produce a third portion of liquid air and a additional portion of air in steam; (I) at least partially condensing the additional portion of vapor air by indirect heat exchange with fluid within the lower pressure column at a point above the point where step (H) is carried out, to produce an additional portion of liquid air; and (J) passing the third portion of liquid air and the fourth portion of liquid air into the lower pressure column, each at a point above the point where step (H) is carried out. The method of claim 1, wherein the oxygen rich fluid is extracted from the lower pressure column as liquid and vaporized by indirect heat exchange with feed air, before recovery. 4. The method of claim 1, further comprising recovering nitrogen-rich fluid, such as nitrogen from the product. 5. Apparatus for producing oxygen of lower purity comprising: (A) a double column having a first column and a second column; (B) means for passing a first portion of feed air to the first column; (C) means for passing fluid from the first column to the second column; (D) a first heat exchanger within the second column and means for passing a second portion of supply air to the first heat exchanger; (E) a second heat exchanger within the second column, at a point above the first heat exchanger and means for passing steam from the first heat exchanger, in the second heat exchanger; (F) means for passing liquid from the first heat and liquid exchanger from the second heat exchanger in the second column, each at a point above the first heat exchanger; and (G) means for recovering oxygen product of lower purity from the second column. 6. The apparatus of claim 5, further comprising a third heat exchanger within the second column, means for passing a third portion of feed air into the third heat exchanger, a fourth heat exchanger inside the second column, at a point above the third heat exchanger, means for passing steam from the third heat exchanger in the fourth heat exchanger and means for passing liquid from the third heat exchanger and from the fourth heat exchanger in the second column, each at a point above the third heat exchanger. The apparatus of claim 5, further comprising a product heater, wherein the means for passing the first portion of feed air into the first column and the means for recovering oxygen of lower purity from the product of a second column, include both the product heater. The apparatus of claim 5, further comprising means for recovering nitrogen from the product from the second column.