High recovery cryogenic rectification system

Abstract: A cryogenic rectification system wherein oxygen-enriched liquid (107) from a higher pressure column (212) is vaporized against condensing nitrogen-containing fluid (208) which is employed as additional reflux (160) in the lower pressure column (210) thus increasing the column L/V ratio and the product recovery.

Description: Technical Field This invention relates generally to the cryogenic rectification of feed air, and is particularly advantageous for use in the production of elevated pressure product. Background Art Elevated pressure product, such as oxygen and nitrogen, produced by the cryogenic rectification of feed air is increasing in demand due to such applications as coal gasification combined-cycle power plants where all of the products from the cryogenic rectification plant may be used at the elevated pressure. One way of producing elevated pressure product from a cryogenic rectification plant is to compress the products produced by the plant to the requisite pressure. However, this approach is costly both because of the initial capital costs and because of the high operating and maintenance costs for the compressors. Another way of producing elevated pressure product from a cryogenic rectification plant is to operate the plant columns at a higher pressure. However, this puts a separation burden and thus a recovery burden on the system because cryogenic rectification depends on the relative volatilities of the components and these relative volatilities are reduced with increasing pressure. This is particularly the case where liquid oxygen and/or liquid nitrogen products are desired from the cryogenic rectification plant as this reduces the availability of high quality reflux which may be used to improve the separation and thus increase the product recovery at higher rectification pressures. A method of separating air known from EP-A-0 384 688 comprises: introducing a cooled feed air stream into a higher pressure rectification column, providing liquid nitrogen reflux for the higher pressure rectification column, and separating the air therein into oxygen-enriched and nitrogen-enriched fractions; withdrawing a liquid stream of oxygen-enriched fraction from the higher pressure column and passing it into a lower pressure rectification column in which it is separated into oxygen and nitrogen; withdrawing a nitrogen stream and a product oxygen stream from the lower pressure rectification column; withdrawing a liquid stream of nitrogen-enriched fraction from the higher pressure column and employing it as reflux in the lower pressure column; reboiling liquid oxygen in or from the lower pressure column; taking at least part of the nitrogen stream from the lower pressure rectification column, compressing it, cooling it, at least partially condensing it, and employing the resulting liquid nitrogen as additional reflux in the lower pressure column; and withdrawing a gaseous product stream of the nitrogen-enriched fraction from the higher pressure column. In this prior method a portion of the oxygen-enriched fraction withdrawn from the higher pressure column is introduced into a condenser-reboiler for cooling and at least partially condensing the nitrogen from the lower pressure rectification column employed as reflux in the lower pressure column, and then is passed from the condenser-reboiler into the lower pressure column, whereas a further portion of the oxygen-enriched fraction withdrawn from the higher pressure column is passed through a Joule-Thomson valve and then directly into the lower pressure column. It is an object of this invention to provide a cryogenic rectification system which can produce product at elevated pressure with improved recovery over that attainable with conventional systems. Summary Of The Invention The above and other objects which will become apparent to one skilled in the art upon a reading of this disclosure are attained by the present invention one aspect of which is: A cryogenic rectification method for producing product with high recovery as defined in claim 1. Another aspect of the present invention is a cryogenic rectification plant as defined in claim 3. As used herein, the term "column" means a distillation or fractionation column or zone, i.e., a contacting column or zone wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on vapor-liquid contacting elements such as on a series of vertically spaced trays or plates mounted within the column and/or on packing elements which may be structured and/or random packing elements. For a further discussion of distillation columns, see the Chemical Engineers' Handbook, Fifth Edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, "Distillation", B. D. Smith, et al., page 13-3, The Continuous Distillation Process . Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase while the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Distillation is the separation process whereby heating of a liquid mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby 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 contacting of the vapor and liquid phases is adiabatic and can include integral or differential contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out, at least in part, at low temperatures, such as at temperatures at or below 150°K. As used herein, the term "indirect heat exchange" means the bringing of two fluid streams into heat exchange relation without any physical contact or intermixing of the fluids with each other. As used herein, the term "feed air" means a mixture comprising primarily nitrogen and oxygen such as air. As used herein, the term "expander" means a device used for extracting work out of a compressed gas by decreasing its pressure. As used herein, the terms "upper portion" and "lower portion" mean those sections of a column respectively above and below the midpoint of a column. As used herein, the term "reflux" means the downflowing liquid phase in a column produced from condensing vapor. As used herein, the term "L/V ratio" means the ratio of the quantity of liquid flowing down a column to the quantity of vapor rising in the column. Brief Description Of The Drawing The single Figure is a schematic representation of a preferred embodiment of the invention wherein the condensing nitrogen-containing fluid is taken from the higher pressure column. Detailed Description In general, the invention is a system which improves product recovery, especially product oxygen recovery, by employing refrigeration from the lower portion of the high pressure column to condense nitrogen thus increasing the L/V ratio in the upper portion of the lower pressure column. The invention will be described in detail with reference to the Drawing. Referring now to the Figure, compressed feed air 101 which has been cleaned of high boiling impurities such as water vapor, carbon dioxide, and hydrocarbons and which preferably is at a pressure of about 10 bar (150 psia) is cooled by passage through heat exchanger 200 by indirect heat exchange with return streams. The resulting cooled feed air 102 is further cooled by passage through heat exchanger 202 by indirect heat exchange with return streams and resulting further cooled stream 153 is passed into first or higher pressure column 212. First or higher pressure column 212 is the higher pressure column of a double column cryogenic rectification apparatus and is operated at a pressure within the range of from 4.1 to 20.7 bar (60 to 300 pounds per square inch absolute (psia)). Within column 212 feed air is separated by cryogenic rectification into nitrogen-enriched fluid and oxygen-enriched fluid. Nitrogen-enriched fluid is withdrawn from column 212 as vapor stream 150 which is condensed by passage through main condenser 214 in indirect heat exchange with boiling column 210 bottoms. Resulting condensed nitrogen-enriched fluid 151 is passed out of main condenser 214 and a portion 152 is passed back into column 212 as reflux. Another portion 112 of nitrogen-enriched fluid 151 is subcooled by passage through heat exchangers 205 and 206, resulting stream 113 is expanded through valve 224 and resulting stream 114 is passed into column 210 as reflux. In the embodiment illustrated in the Figure stream 114 is combined with condensed nitrogen-containing fluid as will be discussed in greater detail below and this combined stream 164 is passed into column 210. Oxygen-enriched fluid is withdrawn from column 212 as liquid stream 107. The withdrawn oxygen-enriched liquid is subcooled by passage through heat exchanger 204 and resulting subcooled oxygen-enriched liquid 108 is reduced in pressure by passage through pressure reduction valve 222 to produce reduced pressure stream 109 which is essentially at the operating pressure of lower pressure column 210. A portion 110 of stream 109 is passed directly into column 210. Another portion 140 of stream 109 is passed into reflux heat exchanger 208 wherein it is vaporized by indirect heat exchange with condensing nitrogen-containing fluid which has been taken from the double column cryogenic rectification apparatus as will be discussed in greater detail below. Resulting vaporized oxygen-enriched fluid 111 is then passed out from reflux heat exchanger 208 and into column 210. Second or lower pressure column 210 is the lower pressure column of double column cryogenic rectification apparatus and is operated at a pressure lower than that of column 212 and within the range of from 1.0 to 13.8 bar (15 to 200 psia). Within column 210 nitrogen-enriched and oxygen-enriched fluids are separated by cryogenic rectification into nitrogen-rich fluid and oxygen-rich fluid. Oxygen-rich fluid is withdrawn from column 210 as stream 130 which is warmed by passage through heat exchangers 202 and 200 and recovered as oxygen product 132 having a purity within the range of from 50 to 100 percent. Nitrogen-rich fluid is withdrawn from lower pressure column 210 as vapor stream 116 which is warmed by passage through heat exchangers 206 and 205 by indirect heat exchange with subcooling nitrogen-enriched liquid. Resulting warmed nitrogen-rich vapor 117 is further warmed by passage through heat exchanger 204 by indirect heat exchange with subcooling oxygen-enriched liquid. Resulting further warmed nitrogen-rich vapor 118 is still further warmed by passage through heat exchangers 202 and 200 to produce nitrogen-rich vapor stream 120, which is removed from the process and may be recovered as nitrogen product having a nitrogen purity of at least 97 percent. It is understood that in the practice of this invention oxygen-rich fluid and nitrogen-rich fluid produced for recovery as product need not be recovered, in whole or in part, as product and may be simply removed from the system. A portion 300 of nitrogen-enriched vapor stream 150 is warmed by passage through heat exchanger 202 and resulting warmed nitrogen-enriched vapor 154 is expanded through expander 155 to generate refrigeration. Expanded nitrogen-enriched vapor 156 is passed as the nitrogen-containing fluid to reflux heat exchanger 208 wherein it is condensed by indirect heat exchange with vaporizing oxygen-enriched fluid. Resulting condensed nitrogen-enriched liquid 161 is subcooled by passage through heat exchanger 206. Resulting subcooled nitrogen-enriched liquid 162 is reduced in pressure through valve 226 and resulting reduced pressure stream 163 is passed into column 210 as additional reflux at a point above the point or points where oxygen-enriched fluid is passed into lower pressure column 210. In this illustrated embodiment stream 163 is first combined with stream 114 and the resulting combined stream 164 is passed into column 210. As indicated, the condensation of the nitrogen-containing fluid in the reflux heat exchanger against oxygen-enriched fluid and the subsequent introduction of the condensed nitrogen-containing fluid into the lower pressure column at a point higher than the introduction point of the oxygen-enriched fluid provides additional reflux for the lower pressure column thus improving the L/V ratio in the upper portion of the lower pressure column. The L/V ratio is efficiently increased because the nitrogen-containing fluid can be condensed against boiling oxygen-enriched fluid at a relatively low pressure, significantly lower than if it were condensed against oxygen-rich fluid such as by passage through main condenser 214. Furthermore, the lower pressure reduces flashoff losses incurred when the fluid is passed into the lower pressure column. The increased L/V ratio in the lower pressure column increases the recovery by reducing the concentration of the less volatile component on each tray in the upper portion of the column thus reducing the fraction of the less volatile component leaving each tray and leaving the column. Now, by the use of this invention feed air may be separated into both nitrogen and oxygen products under elevated pressure while still obtaining high product recovery. The invention can produce oxygen product with a recovery of at least 95 percent up to about 99.0 percent.

Claims: A cryogenic rectification method for the rectification of feed air and for producing product with high recovery, comprising: (A) providing all of the feed air (153) into a higher pressure column (212) and separating the feed air therein by cryogenic rectification into nitrogen-enriched fluid and oxygen-enriched fluid; (B) condensing nitrogen-enriched fluid (150) withdrawn from the higher pressure column (212) by indirect heat exchange with boiling bottoms from a lower pressure column (210) operating at a pressure less than that of the higher pressure column and passing resulting condensed nitrogen-enriched fluid (152) back into the the higher pressure column; (C) passing nitrogen-enriched fluid (114) into the lower pressure column (210); (D) withdrawing oxygen-enriched liquid (107) from the higher pressure column, subcooling the withdrawn oxygen-enriched liquid (107) by indirect heat exchange with nitrogen-rich fluid (117) taken from the lower pressure column (210), reducing the pressure of the entire withdrawn subcooled oxygen-enriched liquid (108) to about the operating pressure of the lower pressure column (210), vaporizing a portion (140) of the resulting reduced pressure oxygen-enriched liquid (109) by indirect heat exchange with condensing nitrogen-containing fluid (300, 156) taken from the higher pressure column (212), and passing another portion (110) of the resulting reduced pressure oxygen-enriched liquid (109) directly into the lower pressure column (210) at an intermediate level thereof; (E) passing vaporized oxygen-enriched fluid (111) into an intermediate level of the lower pressure column (210) and passing nitrogen-containing fluid (163) taken from the heat exchange with the oxygen-enriched fluid (140) into the lower pressure column at a point above the point where vaporized oxygen-enriched fluid (111) is passed into the lower pressure column; and (F) separating oxygen-enriched fluid and nitrogen-enriched fluid in the lower pressure column (210) by cryogenic rectification into nitrogen-rich fluid (116) and oxygen-rich fluid (130) for recovery as product (120, 132). The method of claim 1 wherein the nitrogen-enriched vapor (300) withdrawn from the higher pressure column (212) is expanded (155) before it is employed as the nitrogen-containing fluid (156) condensing by indirect heat exchange with oxygen-enriched fluid (140). A cryogenic rectification plant for the rectification of feed air, comprising: (A) a cryogenic rectification apparatus comprising a higher pressure column (212) and a lower pressure column (210), (B) means for passing all of the feed air (153) into the higher pressure column (212); (C) means (214) for condensing fluid (150) taken from the upper portion of the higher pressure column (212) by indirect heat exchange with bottoms of the lower pressure column (210), and means for passing resulting condensed fluid (152) back into the the upper portion of the higher pressure column (212); (D) a subcooler heat exchanger (204), pressure reducing means (222), a reflux heat exchanger (208); means for passing fluid (107, 108, 109, 110, 111) from the lower portion of the higher pressure column (212) to the subcooler heat exchanger (204), from the subcooler heat exchanger (204) to the pressure reducing means (222), and from the pressure reducing means (222) (1) directly into the lower pressure column (210) at an intermediate level thereof, without passing through another pressure reducing means, and (2) to the reflux heat exchanger (208) and from the reflux heat exchanger into an intermediate level of the lower pressure column (210); and means for passing fluid from the upper portion of the lower pressure column (210) to the subcooler heat exchanger (204); (E) means for passing fluid (300, 156) from the upper portion of the higher pressure column (212) to the reflux heat exchanger (208) and from the reflux heat exchanger into the lower pressure column (210) at a point above the point where fluid from the lower portion of the higher pressure column (212) is passed from the reflux heat exchanger into the lower pressure column; and (F) means for recovering product (120, 132) from the lower pressure column (210). The cryogenic rectification plant of claim 3 wherein the means for passing fluid from the upper portion of the first column (212) to the reflux heat exchanger comprises an expander (155).

Inventors: DRNEVICH RAYMOND FRANCIS, PAOLINO GERALD ANTHONY, DRNEVICH, RAYMOND FRANCIS, PAOLINO, GERALD ANTHONY
Applicants: PRAXAIR TECHNOLOGY INC, PRAXAIR TECHNOLOGY, INC.
Application year: 1993
Main classification: F25J3
IPC codes: F25J3