Cryogenic rectification regenerator system

Abstract: A cryogenic rectification system for producing nitrogen, especially at low production flowrates, wherein incoming feed air is cooled by a regenerator without need for cold end imbalance and wherein exogenous cryogenic liquid is added to the rectification column.

Description: Technical Field This invention relates generally to cryogenic rectification and, more particularly, to a method and an apparatus rectification for the production of nitrogen. Background Art A small user of nitrogen typically has liquid nitrogen delivered to a storage tank at the use site, and vaporizes the nitrogen from the tank to produce nitrogen gas as usage requirements dictate. This supply arrangement is costly because the nitrogen must be liquefied at the production plant, transported to the use site, and kept in the liquid state until required for use. It is preferable that nitrogen be produced at the use site as this eliminates the liquefaction, transport and storage costs discussed above, and, indeed, large users of nitrogen typically have a production plant on site for this purpose. However refrigeration to drive such a production plant is generally produced by turboexpansion of feed air or waste gas, and for smaller plants such use of turboexpanders is generally cost prohibitive. In addition, prepurification of the air stream to remove water and carbon dioxide is typically employed in conventional plants but this is cost prohibitive on smaller plants. Finally, the use of conventional heat exchangers, such as brazed aluminum heat exchangers, to cool the incoming air and warm the product and waste streams leaving the rectification column, are also cost prohibitive on a small scale. GB-A-1 463 075, which can be considered as the closest prior art, discloses a method for producing nitrogen by the cryogenic rectification of feed air, said method comprising: (A) cooling feed air by passing the feed air through a reversing heat exchanger and introducing the cooled feed air into a column; (B) passing exogenous cryogenic liquid into the column and separating the feed air by cryogenic rectification within the column into nitrogen vapor and oxygen-enriched liquid; (C) condensing a first portion of the nitrogen vapor by indirect heat exchange with oxygen-enriched liquid to produce oxygen-enriched vapor; (D) warming a second portion of the nitrogen vapor by indirect heat exchange with said cooling feed air; (E) recovering the warmed second portion of the nitrogen vapor as product nitrogen; and (F) passing oxygen-enriched vapor through the reversing heat exchanger. In this document the oxygen-enriched vapor is not warmed before passing it through the heat exchanger, which leads to unbalance problems in the case that a regenerator having a shell and a coil side were used instead of the simple reversing heat exchanger. In GB-A-978 833 there is disclosed a method for operating a regenerator, which regenerator comprises a casing which is filled with pebbles having great heat capacity as well as coiled conduits for product oxygen and product nitrogen which are embedded within the pebbles. A regenerator might be used to recapture most of the refrigeration which would otherwise pass out of the plant with the product and waste streams, and at the same time remove water and carbon dioxide, thus enabling commercially viable operation of a much smaller plant than currently possible while avoiding the need for prepurification. In addition, the regenerator is a low cost heat exchange device compared to other heat exchangers capable of the same heat transfer duty, such as brazed aluminum heat exchangers. However, a regenerator requires very small temperature differences between feed air and waste streams for extended operation, and, because the outgoing cold streams have less thermal capacity and are at a lower temperature than the feed air, an unbalance stream must be supplied to the cold end of the regenerator in order to ensure against debilitating frost buildup by maintaining small temperature differences between the feed air and the outgoing gases. The unbalance stream could be a portion of the feed air, a portion of the product or a portion of the waste stream. Whichever way the unbalance scheme is constructed, it is complicated and reduces any advantage the use of a regenerator might bring to the operation of a small nitrogen production plant. Accordingly, it is an object of this invention to provide a cryogenic rectification system for producing nitrogen which reduces the need for or does not require turboexpansion of a process stream to generate refrigeration and which employs regenerators having cold end unbalance requirements which are reduced over that required by conventional practice, or which are eliminated entirely. Summary of the Invention The above object is attained by the present invention, one aspect of which is a method for producing nitrogen by the cryogenic rectification of feed air as defined in claim 1. Another aspect of the invention is an apparatus for producing nitrogen by the cryogenic rectification of feed air as defined in claim 4. As used herein the term "feed air" means a mixture comprising primarily nitrogen and oxygen, such as ambient air or offgas from other processes. 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 a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. For a further discussion of distillation columns, see the Chemical Engineer's Handbook, fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, 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 whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate 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 generally adiabatic and can include integral (stagewise) or differential (continuous) 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 temperatures at or below 150 degrees Kelvin (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 "top condenser" means a heat exchange device that generates column downflow liquid from column vapor. As used herein the terms "upper portion" and "lower portion" mean those sections of a column respectively above and below the midpoint of the column. As used herein the term "regenerator" means a heat exchange device having a shell and one or more hollow coils passing therethrough. The coil side of the regenerator is the volume within the coil(s). The shell side of the regenerator is the volume within the shell but outside the coil(s). As used herein the term "cooling period" means a period of time during which feed air is passing through the shell side of the regenerator prior to being passed into a column, and as used herein the term "non-cooling period" means a period of time during which such feed air is not passing through the shell side of the regenerator. As used herein the term "exogenous cryogenic liquid" means a liquid which is not ultimately derived from the feed and is at a temperature of 150K or less. The exogenous cryogenic liquid is comparable in purity to the product nitrogen. Brief Description of the Drawings Figure 1 is a schematic representation of one preferred embodiment of the cryogenic rectification system of the invention. Figure 2 is a graph showing the temperature difference between feed air and waste flow under several conditions and the requirements for proper regenerator cleaning. Figure 3 is a graph showing the temperature difference across the top condenser in a typical embodiment of the invention. Detailed Description In the practice of this invention the use of exogenous cryogenic liquid addition reduces or removes entirely the need for turboexpansion to generate refrigeration and also increases the mass flow and therefore the total thermal capacity of the outgoing streams, causing the cold end temperature difference to decrease and reducing or eliminating the need for unbalance in the regenerator. The invention will be described in detail with reference to the Drawings. Referring now to Figure 1, feed air is compressed to typically between 206.8 to 1379 kPa (30 and 200 pounds per square inch absolute (psia)), after which it is typically cooled and free.water is removed. The compressed feed air stream 1 is then diverted through a switching valve 2 to the shell side 30 of one of a pair of regenerators 3, which generally contain a packing material, such as stones, within the shell. During such cooling period the feed air is cooled close to its dewpoint by passage through shell side 30 and all remaining water and most of the carbon dioxide is removed from the feed air by condensation. The cooled feed air is withdrawn from shell side 30 in stream 31 and is passed through check valve 4 to an adsorbent bed 5 for removal of hydrocarbons and any remaining carbon dioxide that exit with the feed air from the cold end of the regenerator. The adsorbent is typically a silica gel. The clean cold air is then passed into the lower portion of rectifying column 6 which contains mass transfer devices 7 such as distillation trays or packing and is operating at a pressure within the range of from 206.8 to 1379 kPa (30 to 200 psia). Within column 6 the feed air is separated by cryogenic rectification into nitrogen vapor and oxygen-enriched liquid. Nitrogen vapor, having a nitrogen concentration of at least 95 mole percent, is withdrawn from the upper portion of column 6 as stream 8 and divided into a first portion or reflux stream 10 and a second portion or product stream 9. Reflux stream 10 passes to top condenser 11 wherein it is condensed and returned to column 6 as liquid reflux. Product stream 9 is passed into the coil side of regenerators 3 and through coils 12 which are imbedded inside the regenerator packing material. Warm product leaving the regenerators (typically 5-15K colder than the incoming feed air) is then withdrawn from the coil side of the regenerators and recovered as product nitrogen 32 at a flowrate generally within the range of from 30 to 60 mole percent of the incoming feed air flowrate and having a nitrogen concentration of at least 95 mole percent. Oxygen-enriched liquid is withdrawn from the lower portion of column 6 as kettle liquid 13, and is pressure transferred to top condenser 11. This kettle liquid typically contains more than 30 mole percent oxygen. Kettle liquid in stream 13 is subcooled by passage through heat exchanger 17 prior to being passed into top condenser 11. The boiling pressure inside top condenser 11 is significantly lower than the pressure at which column 6 is operating thus allowing the transfer of the kettle liquid. The rate of flow of the kettle liquid is governed by a flow restricting device such as a control valve 14. Additional adsorbent may be located in the kettle liquid transfer line or in the condenser for final scavenging of residual hydrocarbons and carbon dioxide. The oxygen-enriched liquid in the top condenser is boiled against the condensing nitrogen reflux stream. Top condenser 11 operates at a much reduced pressure over that of the column 6. The pressure of the top condenser will be at least 68.9 kPa (10 psi) less than that at which column 6 is operating. This reduces the boiling temperature of the oxygen stream to below the temperature at which the nitrogen vapor, at column pressure, condenses. The resulting oxygen-enriched vapor 15, which will be termed the waste, passes out of top condenser 11 through a control valve 16 that regulates the boiling side pressure and hence the column pressure. The waste then passes in countercurrent heat exchange relation with the rising kettle liquid in a heat exchanger or superheater 17. Waste then passes through check valves 4 and into the cold end of the shell side of the regenerator 3 which does not have feed air passing through it, i.e. during a non-cooling period. The regenerators will switch via switching valves 2 between feed air and waste in a periodic fashion so that each regenerator experiences both cooling and non-cooling periods. The waste is withdrawn from the system in stream 33. Typically the nitrogen vapor will pass through a regenerator during both the cooling and non-cooling periods. Exogenous cryogenic liquid, which in the embodiment illustrated in Figure 1 is liquid nitrogen having a nitrogen concentration of at least 95 mole percent, is added from an external source to the column through line 18 to provide refrigeration to the system. The flow of the exogenous cryogenic liquid is regulated to maintain the liquid level inside the condenser 11 and is within the range of from 2 to 15 percent of the flowrate of nitrogen product stream 32 on a molar basis. Alternatively, some of the required exogenous cryogenic liquid may be added to the top condenser. One of the difficulties of regenerators is that for extended operation it is necessary to have very small temperature differences between the feed air and waste streams. As the feed air passes through the regenerator, water and carbon dioxide freeze out onto the packing material and the outer surface of the coils inside the regenerator. This frost must be removed by the returning cold waste stream or it will accumulate and eventually plug the regenerator. The waste stream has less mass flow than does the feed air coming in. Also it is at a lower temperature. Both of these facts tend to reduce the ability of the waste stream to hold moisture and carbon dioxide. Self cleaning depends on a delicate balance between the waste/air temperature difference (ΔT) and the waste/air flow and pressure ratios. Increasing the waste to air flow ratio reduces the amount of product recovered. Increasing the pressure ratio increases the column pressure which reduces separation efficiency and also consumes more power for compression. Thus the most effective means of assuring self cleaning is to ensure that the temperature differences are small. The variation of vapor pressure with temperature is such that the self cleaning requirements in terms of allowable ΔT are more severe for carbon dioxide than water. As a result, since water is removed at the warm end of the regenerator while carbon dioxide is removed at the cold end, large warm end temperature differences are more tolerable than large cold end temperature differences. Unfortunately the heat capacity of the high pressure air entering the plant exceeds that of the cold streams derived from the air coming out at lower pressure. This unbalances the regenerator such that tight temperature differences are obtainable at the warm end but not at the cold end. In order to make regenerators self cleaning, unbalance passages are conventionally used which increase the flow ratio of cold streams (referring to both the waste stream and product stream) to feed air in the cold end of the regenerator and cause the cold end temperature difference to tighten. While this may be accomplished in several ways, each arrangement increases the ratio of cold stream mass flow to air mass flow in the cold end of the regenerator and each requires additional piping, perhaps additional control and either additional coils within the regenerators or the addition of an additional adsorbent bed to remove carbon dioxide from air removed at an intermediate level in the regenerator. With the practice of this invention, wherein exogenous cryogenic liquid is added to the column at a flowrate within the range of from 2 to 15 percent of the flowrate of the nitrogen product stream on a molar basis, the requirement for cold end unbalance on the regenerator is reduced or even eliminated. The following example is provided to illustrate the invention and to provide comparative data. The example is not intended to be limiting. The example is presented considering a process arrangement similar to that illustrated in Figure 1. A steady state regenerator has a UA of 8.14 kW/°C (50,000 BTU/hr/F). A 45.4 kgmol/h (100 lbmols/hr) air stream enters the warm end of the regenerator at 48.9°C (120°F) and 689.5 kPa (100 psia). Waste and product streams enter the cold end of the heat exchanger at -167.8°C (-270°F). The waste stream flow is 27.2 kgmol/h (60 lbmols/hr) and pressure is 110.3 kPa (16 psia). The product stream flow is 18.1 kgmol/h (40 lbmols/hr) and pressure is 675.7 kPa (98 psia). The product stream is assumed to be pure nitrogen. The waste composition is set by mass balance (∼63 mole percent nitrogen). For the purposes of this analysis, it is assumed that the waste and product also exit the warm end of the heat exchanger at the same temperature. Figure 2 shows as Curve A the temperature difference between the air and a composite stream representing the sum of the returning cold streams as a function of air temperature when no exogenous cryogenic liquid is added to the column. This relationship is also shown at exogenous cryogenic liquid addition rates of 5 and 10 percent of the product flowrate on a molar basis as curves B and C respectively. It can be seen that increasing the exogenous cryogenic liquid addition rate reduces the cold end ΔT and increases the warm end ΔT. Also shown is the air/waste temperature difference required to remove carbon dioxide and water, curves D and E respectively, assuming that the waste and air streams are saturated throughout. This temperature difference is approximated using equation (1). where Pi(T) is the vapor pressure (kPa (psia)) exerted by component i at temperature T(°C(F)), P is the pressure (kPa (psia)), Q is the flow ((kgmol/h) lbmol/hr) and T is temperature at any point (°C (F)). Subscripts a and w refer to air and waste respectively. Equation (1) is an approximate relationship that serves to illustrate the form of the self cleaning curves. It represents the condition where at any point in the regenerator the waste stream at saturation can carry the same amount of water and carbon dioxide as the air stream. It can be seen from Figure 2 that in the absence of the addition of exogenous cryogenic liquid to the column, the air/waste temperature difference exceeds that required for carbon dioxide removal, that the system removes carbon dioxide more easily when exogenous cryogenic liquid is added to the column, and that at some minimum exogenous cryogenic liquid addition rate, the need for unbalance streams in the cold end of the regenerator is eliminated. Since the use of a turboexpander to generate refrigeration is not required, it is not necessary to maintain an elevated waste stream pressure. Thus, the pressure on the boiling side of top condenser need only be sufficient to drive the waste flow through the regenerator and piping to vent. The lower the pressure on the boiling side of the top condenser, the lower the temperature of the boiling mixture. For a fixed condensing pressure, this results in a large temperature difference in the top condenser. The heat duty in the condenser can be expressed as follows; Q = U c A c ΔT where Q is the heat transferred (W (BTU/hr)), U c is the overall heat transfer coefficient for the condenser (W/m 2 °C) (BTU/hrft 2 F)), A c is the area between the condensing and boiling regions (m 2 (ft 2 )) and ΔT is the temperature difference (°C (F)) between the boiling and condensing fluids. From equation (2) it is clear that increasing ΔT decreases the U c A c required for a given heat duty. As demonstrated, liquid addition allows the waste to operate at a pressure substantially lower than the column pressure. Since in most applications the nitrogen is required at pressure, the pressure difference between the condensing and boiling streams is generally at least 68.9 kPa (10 psi) and may exceed 344.7 kPa (50 psi). Figure 3 shows the temperature difference across the condenser for the case of pure nitrogen condensing at 689.5 kPa (100 psia)and a boiling waste stream with a vapor composition of 63 mole percent nitrogen. An additional advantage of operating the top condenser at high temperature differences is that while the condensing side heat transfer coefficient is not a strong function of temperature, the boiling side coefficient increases rapidly with temperature difference. Thus operating with a large pressure difference between the column and the top condenser results in larger overall heat transfer coefficients as well as larger ΔT. As a result, the area of the condenser is much reduced. A particularly advantageous embodiment of the invention employs a coil in shell top condenser. The waste liquid boils inside a shell with coiled tubes immersed in the liquid. Nitrogen from the upper portion of the column condenses on the inside of the tubes. Now by the use of this invention one can produce nitrogen by cryogenic rectification using regenerators, especially at lower production rates such as 566.3 m 3 -NTP (20,000 cfh-NTP) or less, without need for unbalancing the cold end of the regenerator.

Claims: A method for producing nitrogen by the cryogenic rectification of feed air using a regenerator having a shell side and a coil side, said method comprising: (A) cooling feed air (1) by passing the feed air through the shell side of a regenerator (3) during a cooling period, passing the cooled feed air into an adsorbent bed (5) for removal of hydrocarbons arid carbon dioxide, and introducing the cooled feed air into a column (6) having a top condenser (11) which is operated at a pressure that is at least 68.9 kPa (10 psi) less than the pressure at which said column is operated; (B) passing exogenous cryogenic liquid having a nitrogen concentration of at least 95 mole percent and a purity that is comparable to that of the product nitrogen to be produced into the column (6) and separating the feed air by cryogenic rectification within the column into nitrogen vapor (8) and oxygen-enriched liquid (13); (C) regulating the flow of the exogenous cryogenic liquid at a flowrate within the range of from 2 to 15 percent of the flowrate at which product nitrogen is recovered on a molar basis so as to maintain the liquid level inside the top condenser (11); (D) subcooling (17) oxygen-enriched liquid (13) withdrawn from the column (6) by indirect heat exchange with oxygen-enriched vapor (15) produced at the top condenser, expanding (14) the subcooled oxygen-enriched liquid and condensing a first portion (10) of the nitrogen vapor within the top condenser by indirect heat exchange with said subcooled, expanded oxygen-enriched liquid to produce said oxygen-enriched vapor; (E) warming a second portion (9) of the nitrogen vapor by indirect heat exchange with said cooling feed air (1) by passing said second portion of the nitrogen vapor through the coil side of the regenerator (3); (F) recovering the warmed second portion (9) of the nitrogen vapor as product nitrogen (32); and (G) passing oxygen-enriched vapor (15) through the shell side (30) of the regenerator (3) during a non-cooling period. The method of claim 1 wherein the exogenous cryogenic liquid is passed into the column (6) in the upper portion of the column. The method of claim 1 wherein the column (6) is operating at a pressure within the range of from 206.8 to 1379 kPa (30 to 200 psia). Apparatus for producing nitrogen by the cryogenic rectification of feed air comprising: (A) a regenerator (3) having a shell side (30) and a coil side; (B) an adsorbent bed (5); (C) a column (6) having a top condenser (11) adapted to be operated at a pressure that is at least 68.9 kPa (10 psi) less than the pressure at which said column is operated; (D) means for passing feed air into the shell side (30) of the regenerator (3), means for passing feed air from the shell side of the regenerator into the adsorbent bed (5) and from the adsorbent bed into the column (6), means (18) for passing exogenous cryogenic liquid having a nitrogen concentration of at least 95 mole percent into the column (6), and means for regulating the flow of the exogenous cryogenic liquid at a flowrate within the range of from 2 to 15 percent of the flowrate at which product nitrogen is recovered on a molar basis; (E) means for passing a first nitrogen vapor portion (10) from the column (6) into the top condenser (11); (F) a heat exchanger (17) and means for passing oxygen-enriched liquid (13) from the column into the heat exchanger and from the heat exchanger into the top condenser; (F) means for passing a second nitrogen vapor portion (9) from the upper portion of the column (6) into tile coil side of the regenerator (3) and means for recovering nitrogen vapor from the coil side of the regenerator as product nitrogen (32); and (G) means for passing oxygen-enriched vapor (15) from the top condenser (11) into the heat exchanger and from the heat exchanger into the shell side (30) of the regenerator (3). The apparatus of claim 4 wherein the means (18) for passing exogenous cryogenic liquid communicates with the column (6) in the upper portion of the column.

Inventors: BERGMAN THOMAS JOHN JR, BILLINGHAM JOHN FREDRIC, BERGMAN, THOMAS JOHN, JR., BILLINGHAM, JOHN FREDRIC
Applicants: PRAXAIR TECHNOLOGY INC, PRAXAIR TECHNOLOGY, INC.
Application year: 1998
Main classification: F25J5
IPC codes: F25J3, F25J5