MXPA98003572A - Separation of cryogenic air with recirculation of turbine calie - Google Patents

Abstract

The present invention relates to a method for carrying out the cryogenic air separation, comprising: (A) compressing feed air in a primary air compressor having a plurality of first through n compression stages to produce air of compressed feed; (B) cooling a first part of the compressed feed air, turbo expanding the first cold part and passing the first turboexpanded part to a cryogenic air separation plant; (C) comprising a part of compressed feed air , cooling the second compressed part, turboexpand at least a portion of the cooled second part, and recirculating at least some of the second turboexpanded part to the supply air enters the first and the n compression stage; (D) producing ld oxygen inside the cryogenic air separation plant, remove the ld oxygen from the cryogenic air separation plant, and vap orng the ld oxygen withdrawn through an indirect heat exchange with both the first cooling part of the supply air and the second cooling part of the supply air to produce gaseous oxygen, and (E) recovering the gaseous oxygen as a product.

Description

ÍF SEPARATION OF CRYOGENIC AIR WITH REC1RCULATION OF HOT TURBINE TECHNICAL FIELD This invention relates generally to the separation of cryogenic air and, more particularly, to cryogenic air separation systems, wherein the liquid of the air separation plant # Cryogenic is vaporized before recovery. BACKGROUND OF THE INVENTION Oxygen is produced commercially in large quantities through the cryogenic rectification of feed air in a Cryogenic air separation plant. Sometimes it may be desirable to produce oxygen at a higher pressure. Since gaseous oxygen can be removed from the cryogenic air separation plant and compressed to the desired pressure, it is generally preferred, for capital cost purposes, to remove the oxygen as liquid from the cryogenic air separation plant, increase its pressure, and then vaporize the pressurized liquid oxygen to produce the desired high pressure product oxygen gas. The removal of oxygen as a liquid from the cryogenic air separation plant removes a significant amount of refrigeration of the plant in need of a significant reintroduction of refrigeration in the plant. This is more the case when, in addition to the high pressure oxygen gas, it is desired to recover liquid product, for example, liquid oxygen and / or liquid nitrogen, from the plant. A very effective way to provide cooling in a cryogenic air separation plant is to turboexpand a stream of compressed gas and pass that stream, or at least the cooling generated by it, to the plant. In situations where significant amounts of liquid are removed from the plant, more than one of these turboexpanders is usually used. However, the use of multiple turboexpanders is complicated due to the small differences in turbine flows and pressures with respect to the cryogenic air separation plant and that the primary air compressor will cause an acute reduction in system efficiency causing The system is not economic. Accordingly, it is an object of this invention to provide an improved system for the cryogenic rectification of feed air using more than one turboexpander.

COMPENDIUM OF THE INVENTION The above objects and other objects, which will be apparent to those skilled in the art upon reading this description, are obtained through the present invention, an aspect of which is: A method for carrying out the separation of cryogenic air comprising: (A) compressing feed air in a primary air compressor having a plurality of first through n compression stages to produce compressed feed air; (B) cooling a first part of the compressed feed air, turboexpand the first cold part and passing the first turboexpanded part to a cryogenic air separation plant; (C) further comprises a second compressed-feed air part, cooling the second additional compressed part, turboexpanding at least a portion of the cooled second part, and recirculating at least some of the second turboexpanded part to the feed air between the first and the n compression stage; (D) producing liquid oxygen within the cryogenic air separation plant, removing the liquid oxygen from the cryogenic air separation plant, and vaporizing the liquid oxygen withdrawn through an indirect thermal exchange with both the first part of cooling the feed air as with the second cooling part of the feed air to produce gaseous oxygen; and (E) recovering gaseous oxygen as a product. Another aspect of the invention is: an apparatus for carrying out the cryogenic air separation comprising: (A) a primary air compressor having a plurality of first through n compression stages, a main thermal exchanger, a turboexpansor primary, and a cryogenic air separation plant; (B) means for passing the supply air to the first stage of the primary air compressor and means for removing the supply air from stage n of the primary air compressor; (C) means for passing the feed air from stage n of the primary air compressor to the main heat exchanger, from the main heat exchanger to the primary turbo expander, and from the primary turbo expander to the cryogenic air separation plant; (D) a lift compressor, a secondary turbo expander, means for passing the supply air from stage n of the primary air compressor to the riser compressor, from the riser compressor to the main heat exchanger, from the main heat exchanger to the secondary turbo expander , and of the secondary turboexpander to the primary air compressor between the first and the n compression stages; and (E) means for passing liquid from the cryogenic air separation plant to the main heat exchanger and means for recovering the steam from the main heat exchanger. As used herein, the term "liquid oxygen" means a liquid having an oxygen concentration greater than 50 mol%. As used herein, the term "column" means a column or zone of distillation or fractionation, i.e., a column or contact zone, wherein the liquid and vapor phases are brought into countercurrent contact to effect separation. of a fluid mixture, such as, for example, by contacting the vapor and liquid phases in a series of vertically separated trays or plates mounted within the column and / or in packing elements such as structured or random packing. For an additional discussion of the distillation columns see 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. The term "double column" is used to represent a higher pressure column having its upper end in heat exchanger ratio with the lower end of the lower pressure column. An additional discussion of the double columns appears in Ruheman's "The Separation of Gases," Oxford University Press, 1949, Chapter VII, Commercial Air Separation. The liquid vapor contact separation procedures depend on the difference in vapor pressures for the components. The component of high vapor pressure (or more volatile or less boiling) will tend to concentrate in the vapor phase, while the component of low pressure (or less volatile or high boiling) will tend to concentrate in the liquid phase. Partial condensation is the separation process by which the cooling of a vapor mixture can be used to concentrate the volatile components in the vapor phase and from this way less volatile components in the liquid phase. The rectification, or continuous distillation, is the separation process that combines vaporizations and successive partial condensations as obtained through an anti-current treatment of the vapor and liquid phases. The contact against The current of the vapor and liquid phases is generally adiabatic and may include integral (in-stage) or differential (continuous) contact between the phases. The provisions of the separation procedure that use the principles of rectification to separate mixtures are usually interchangeably called rectification columns, J I distillation columns, or fractionation columns. Cryogenic rectification is a rectification process performed at least in part at temperatures at or below 150 degrees Kelvin (K). As used herein, the term "indirect heat exchanger" means bringing the two fluid streams into a heat exchanger relationship without any physical contact or intermixing of the fluids with one another. As used herein, the term "air of "Feed" means a mixture comprising mainly oxygen and nitrogen, such as ambient air.As used herein, the terms "upper portion" and "lower portion" of a column represent those sections of the column respectively above and for Below the midpoint of the column As used herein, the terms "turboexpansion" and "turboexpander" respectively mean method and apparatus for the flow of high pressure gas through a turbine to reduce the pressure and temperature of the turbine. gas, thereby generating refrigeration As used herein, the term "compressor" means a machine that increases the pressure of a gas through the application of work. cryogenic air "means an installation to fractionally distill feed air, comprising one or more columns and the pipeline, valve equipment ula and thermal exchanger that serves them. As used herein, the term "primary air compressor" means a compressor that provides the largest portion of the air compression needed to operate a cryogenic air separation plant. As used herein, the term "lift compressor" means a compressor, which provides additional compression for purposes of obtaining higher air pressures required for the vaporization of liquid oxygen and / or turboexpans for processing together with a power plant. separation of cryogenic air. As used herein, the term "compression step" means an individual element, eg, a compression test, of a compressor through which the gas is increased under pressure. A compressor must be composed of at least one compression stage.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic representation of a preferred embodiment of the invention. Figure 2 is a schematic representation of another preferred embodiment of the invention. The numbers in the figures are the same for the common elements. - ^ W- DETAILED DESCRIPTION In the practice of this invention, a portion of the feed air bypasses the primary turboexpander, which turboexpands the feed air to the cryogenic air separation plant, and, rather, is turboexpanded in a secondary turboexpander and recirculated to the primary air compressor to a position between stages. This reduces the energy consumption required by the primary air compressor and thus increases the overall efficiency of the cryogenic air separation system. The invention will be described in greater detail with reference to the drawings. Referring now to Figure 1, the feed air 50 at approximately atmospheric pressure, is cleaned of particles by passing it through a filter housing 1. The resulting feed air 51 is then passed to the primary air compressor 13 which, in the embodiment of the invention illustrated in Figure 1, comprises 5 stages of compression, the fifth or last stage being stage n. In the practice of this invention, the primary air compressor will generally have at least 3 compression stages, and typically will have from 4 to 6 compression stages. The feed air 51 is passed to the first compression stage 2 of the primary air compound 13, where it is compressed and results in the supply air 52 being cooled through the passage in the intermediate heat exchanger 3. The air of feed 52 is then further compressed by passing through the second compression stage 4 of the primary air compressor 13 and the resulting feed air 53 is cooled by passing through the intermediate heat exchanger 5. The feed air 53 it is then further compressed through the passage of the third compression stage 6 of the primary air compressor 1 3 and the resulting feed air 54 is cooled through the passage of the intermediate heat exchanger 7. The feed air 54 then passes through the prepurifier 8, where it is cleaned of high boiling impurities such as carbon dioxide, water vapor and hydrocarbons. The clean feed air 55 then passes to a fourth compression stage 9 of the primary air compressor 13. Preferably, as in the embodiment of the invention illustrated in Figure 1, the feed air stream 55 is combined with a recirculation hot turbine, such as at junction 56, and the resultant combined feed air stream 57 # goes to the fourth compression stage 9, where it is compressed at a higher pressure. The resulting feed air stream 58 is cooled by passing through the intermediate heat exchanger 10 and then passed to the fifth compression stage 1 1 of the primary air compressor 13, where it is compressed at a higher pressure and from which it is compressed. withdrawal as a stream of compressed feed air 59 having a pressure within ^^ > the scale of 14.06 to 52.72 kilograms per absolute square centimeter (kg / cm2a). The primary air compressor 13 is driven by an external motor (not shown) with a rotary drive gearing gear 60. The compressed air supply 59 is cooled by passing it through the aftercooler 12 and divided into a first part 61 and a second part 62. The first part 61 comprises from about 50 to 55% of compressed feed air 59. The first part 61 is passed to the thermal exchanger principal 17, where it cools through indirect thermal exchange with return currents. After partial crossing of the main heat exchanger 17, the first cold part 63 is passed to the primary turboexpander 19, where it is turboexpanded at a pressure within the range of 4.5695 to 5 5.9755 kg / cm2a. The resulting first turbo-expanded part 64 is passed to a cryogenic air separation plant. In the embodiment illustrated in Figure 1, the cryogenic air separation plant 65 is a double column plant comprising first or upper pressure column 20 and a second column of pressure or lower 22, the first turbo-expanded part 64 is passed to the lower portion of the higher pressure column of 20. The second part 62 comprises from 45 to 50% of the compressed feed air 59. The second part 62 is does pass to the elevator compressor 15, where it is JBpfc compressed additionally at a pressure within the range of 35.15 to 98.42 kg / cm2a. The second compressed part 66 is cooled through the passage of the cooler 16 and then passed to the main heat exchanger 17, where it is cooled to through indirect thermal exchange with return currents. At least a portion of the cooled second part, shown in Figure 1 as stream 67, is removed after partial crossing of the main heat exchanger 17 and passes to the secondary turbo expander 18, where it is turboexpanded to a pressure within the scale from 5.2725 to 10.545 kg / cm2a. The resulting second turboexpanded portion 68 is heated through the partial crossing of the main heat exchanger 17 and then recirculated to the primary air compressor between the first and last stages, that is, in a position of between stages. In the embodiment illustrated in Figure 1, the hot turbine recirculation 69 is passed through the pressure control device 14 before being recirculated to the supply air 55 at a junction point 56 to be recirculated to the air compressor. primary air between the third and fourth compression stages of the primary air compressor 13. The pressure control device 14 can be, for example, a valve, a compressor or a blower. If desired, a portion of the second part 66 can completely pass through the main heat exchanger 17, where it is liquefied. This portion, shown at 70 in the embodiment illustrated in Figure 1, is passed through the valve 23 and toward the (Pt highest pressure column 20. Instead of the passage through the valve 23, the portion 70 can be passed through a dense phase, which is fluid or supercritical fluid, turbo machine to recover the pressure energy. , the arrow work recovered will activate an electric generator. The highest pressure column 20 operates at a pressure generally within the range of 4.5695 to 5.9755 kg / cm2a. Within the highest pressure column 20, the feed air fed to the column 20 is separated through rectification cryogenic towards steam enriched with nitrogen and liquid enriched with oxygen. The oxygen enriched liquid is withdrawn from the lower portion of the higher pressure column 20 as stream 71, subcooled through the passage to the subcooler 25, and passed through the valve 28 and into the lower pressure column 22. The steam enriched with nitrogen is withdrawn from the highest pressure column 20 as stream 72, and is passed to the main condenser 21, where it is condensed through indirect heat exchange with bottom liquid of the lowest boiling pressure column 22. The resulting nitrogen enriched liquid 73 is removed from the main condenser 21, a first portion 74 is returned to the higher pressure column 20 as reflux, and a second portion 75 is subcooled by passing through the subcooler 26. , and passes through valve 27, towards the lower pressure column 22. If desired, a portion of the liquid enriched with nitrogen can be recovered as nitrogen or product liquid having a nitrogen concentration of at least 99.99% molar. In the embodiment of the invention illustrated in Figure 1, a portion 76 of liquid enriched with nitrogen 75 is passed through valve 30 and recovered as a liquid nitrogen product 77. The lowest pressure column 22 it is operating at a lower pressure than that of the highest pressure column 20, and generally within the range of 0.0545 to 1.7575 kg / cm2a. Within the lower pressure column 22, the various feeds are separated through cryogenic rectification to f * steam enriched with nitrogen and liquid enriched with oxygen. The steam enriched with nitrogen is removed from the upper portion of the lower pressure column 22 as stream 78, heated by passage through the heat exchangers 26, 25 and 17, and removed from the system as stream 79, which it can be recovered as a product of nitrogen gas having a nitrogen concentration of at least 99.99 mole%. For purposes of purity control of the product, a stream containing nitrogen 80 is removed from the pressure column further low 22 below the level from which current 78 is removed. Current 80 is heated by passing through heat exchangers 26, 25 and 17 and removed from the system as stream 81. The liquid enriched with oxygen, that is, liquid oxygen, is removed from the lower portion of the lower pressure column 22 as liquid oxygen stream 82. If it is desired that a portion of the liquid enriched with oxygen can be recovered as a liquid oxygen product, such as the embodiment illustrated in FIG. Figure 1, where the current 83 is branched to the current 82, is passed through the valve 29 and recovered as a liquid oxygen stream 84. The oxygen enriched liquid is increased in pressure before vaporization. In the embodiment illustrated in Figure 1, the main portion 85 of the stream 82 is passed to a liquid pump 24, where it is pumped at a pressure within the range of 10,545 to 98.42 kg / cm a. The resulting pressurized liquid oxygen stream 86 is passed through the main heat exchanger 17, where it is vaporized through indirect heat exchange with the first cooling feed air part 61 and the second cooling feed part. 66. The resulting gaseous oxygen is removed from the main heat exchanger 17 as stream 87 and recovered as gaseous product oxygen having an oxygen concentration of at least 50 mol%. Liquid oxygen is advantageously vaporized by passing through the main heat exchanger 17 instead of a separate product boiler since this allows a portion of the cooling work of the stream 61 to be imparted to the stream 86 thereby reducing the pressure requirement of the air stream of high power 66. In addition, the need of a second heat exchanger apparatus for vaporization of the? > stream 86 is removed. Figure 2 illustrates another embodiment of the invention. The elements of the modality illustrated in Figure 2, which are common with those of the modality illustrated in Figure 1, do not will be discussed again in detail. Referring now to Figure 2, a further compressed second part 66, after passing through the cooler 16, is divided into the stream 88 and stream 89. The stream 89 is further compressed by passing through the compressor 31, is cooled from the compression heat by passing through the * cooler 32, and passes through the main heat exchanger 17, where it is liquefied. The resulting liquid feed air 90 is passed through the valve 23 and into the higher pressure column 20. Instead of passing through the valve 23, the air of. 5 power 20 can pass through a dense phase turbo machine to recover the pressure energy and typically the recovered arrow work will activate an electric generator. A stream 88 of the second part 66 is cooled through the passage of the main heat exchanger 17 and turboexpanded by passing through the secondary turbo expander 18. The resulting turboexpanded stream 91 is bifurcated into stream 92, which passes through the pressure control device 14 and is recirculated to the primary air compressor, and to stream 93, which is cooled in the main heat exchanger 17, passes through the valve 33, and is combined with the discharge stream of the primary turboexpander 64 to form the stream 94, which passes to the highest pressure column 20 of the cryogenic air separation plant 65. The embodiment of the invention illustrated in FIG. Figure 2 is particularly advantageous when the discharge of the The lift compressor 15 is insufficient to heat the vaporization oxygen stream 86. The bifurcation of the hot turbo expansion stream 91 to the streams 92 and 93 is advantageously employed in situations where the flow of the recirculation stream 92 is in excess of aq uel required for supply the desired flows of the finished product. By increasing the flow of current 93, called the recirculation bypass current, the energy consumption of the process can be reduced, allowing a more efficient production of liquid product. Now, with the practice of this invention, wherein at least one discharge portion of the hot turbine is recirculated towards the primary air compressor in an interstage position, the cryogenic air separation can be efficiently performed with the use of turboexpans. multiple 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 of the scope of the claims. For example, the cryogenic air separation plant may comprise a single column, or may comprise three or more columns, such as when the cryogenic separation plant comprises a double column with an argon lateral arm column. The elevating compressors 15 and 31 can be activated through an external motor, or through the expansion arrow work derived from the turboexpansors 18 and 1 9.

Claims (10)

# CLAIMS

1. - A method for carrying out cryogenic air separation, comprising: (A) compressing feed air in a primary air compressor having a plurality of first through n compression stages to produce compressed feed air; (B) cool a first part of the compressed feed air, turbo expand the first cold part and pass the 10 first part turboexpanded to a cryogenic air separation plant; (C) further comprises a second part of compressed feed air, cooling the second additional compressed part, turboexpand at least a portion of the second 15 cooled part, and recirculate at least some of the second turboexpanded part towards the supply air between the first and the n compression stage; (D) produce liquid oxygen within the cryogenic air separation plant, remove liquid oxygen from the plant 20 separating cryogenic air, and vaporizing the liquid oxygen withdrawn through indirect heat exchange with both the first cooling part of the supply air and the second cooling part of the supply air to produce gaseous oxygen; and 25 (E) recover gaseous oxygen as a product.

2. - The method according to claim 1, wherein a portion of the second turboexpanded part is combined with the first turboexpanded part and passes to the cryogenic air separation plant.

3. The method according to claim 1, further comprising recovering the liquid oxygen from the cryogenic air separation plant.

4. The method according to claim 1, further comprising producing liquid nitrogen within the cryogenic air separation plant and recovering the liquid nitrogen from the cryogenic air separation plant. 5. An apparatus for carrying out the cryogenic air separation comprising: (A) a primary air compressor having a plurality of first 15 through n compression stages, a main heat exchanger, a primary turboexpander, and a cryogenic air separation plant; (B) means for passing the feed air to the first stage of the primary air compressor and means for removing the 20 air supply of stage n of the primary air compressor; (C) means for passing the supply air from stage n of the primary air compressor to the main heat exchanger, from the main heat exchanger to the primary turbo expander, and from the primary turbo expander to the 25 Cryogenic air separation plant; (D) a lift compressor, a secondary turbo expander, # means for passing the supply air from stage n of the primary air compressor to the elevating compressor, from the elevating compressor to the main thermal exchanger,

5 main heat exchanger to the secondary turboexpander, and from the secondary turboexpander to the primary air compressor between the first and the n compression stages; and (E) means for passing liquid from the cryogenic air separation plant to the main thermal exchanger and 10 means for recovering steam from the main heat exchanger.

6. The apparatus according to claim 5, wherein the primary air compressor has at least three stages of compression.

7. The apparatus according to claim 5, wherein the means for passing the liquid from the cryogenic air separation plant to the main heat exchanger comprises a liquid pump.

8. The apparatus according to claim 5, wherein the cryogenic air separation plant comprises a double column comprising a higher pressure column and a lower pressure column.

9. The apparatus according to claim 8, wherein the means for passing the feed air from the primary turboexpander to the cryogenic air separation plant communicates with the higher pressure column.

10. - The apparatus according to claim 5, further comprising means for passing the feed air of the secondary turboexpander to the cryogenic air separation plant. SUMMARY A cryogenic air separation system, where the supply air is compressed in a multi-stage primary air compressor, a first part is turboexpanded and fed to a cryogenic air separation plant, a second part is turboexpanded and so less a portion of the second turboexpanded part is recirculated to the primary air compressor in an interstage position.