Patent Application
20110138856
The present invention relates to a method and apparatus for separating air into oxygen and nitrogen-rich fractions. More particularly, the present invention relates to such a method and apparatus in which one or more air separation units having higher and lower pressure columns are connected to an auxiliary column that produces one or more oxygen containing streams that are lean in nitrogen and that are introduced into lower pressure columns to allow the air separation units to operate at a higher capacity and a nitrogen-rich vapor stream that can be used in regenerating adsorbent beds of a pre-purification unit used in purifying the air without further compression.
Large quantities of high purity oxygen are required for purposes of coal gasification where the resulting synthesis gas will be used in the production of chemicals. High purity oxygen is typically defined by an oxygen content of 99.5 percent or better. In certain large coal gasification installations, upwards of between 10,000 and 15,000 metric tons per day of oxygen can be consumed.
The cryogenic rectification of air is the preferred method for large scale oxygen production. In cryogenic rectification, air is compressed and purified of higher boiling contaminants such as carbon dioxide, water vapor and hydrocarbons in a pre-purification unit. Purification is necessary in that carbon dioxide and water vapor will freeze within the main heat exchanger and hydrocarbons will tend to collect in the oxygen to present an operational hazard. Typically, the pre-purification step is conducted in a pre-purification unit that has an adsorbent, for example, alumina or a zeolite or silica gel or a combination of such materials to adsorb the water vapor, the carbon dioxide and the hydrocarbons. The adsorbent is employed in adsorbent beds that are operated in accordance with an out-of-phase cycle in which one bed is placed on-line and is actively adsorbing the impurities while another bed is being regenerated, that is, being subjected to desorption to desorb the impurities followed by repressurization to bring the regenerated bed back on-line to allow the current on-line bed to be regenerated. For such purposes, there can be two or more adsorbent beds employed within a pre-purification unit.
The pre-purification unit is operated in an out-of-phase cycle that is commonly either a temperature swing adsorption cycle or a pressure swing adsorption cycle (or combinations thereof). In a temperature swing adsorption cycle, compressed air that has been cooled to ambient is introduced to an on-line bed to produce a compressed and purified air stream. The off-line bed that is being regenerated is subjected to a depressurization step and then, a countercurrent heating step in which nitrogen waste gas is heated in a heater and then introduced into the bed countercurrently to the air flow. Since adsorbents have a lower adsorptive capacity at higher temperature, the impurities will tend to desorb from the adsorbent. The nitrogen waste gas is then introduced into the adsorbent bed without being heated. This countercurrent purge with the waste nitrogen desorbs and removes the contaminants previously adsorbed by the adsorbent. A repressurization step is then conducted with part of the product being produced by the on-line bed to ready the off-line bed, now regenerated, to be brought back to an on-line status. In a pressure swing adsorption cycle, the off-line bed is regenerated by allowing the adsorbent bed to vent to the atmosphere from its feed end. The adsorbent bed is then countercurrently purged with waste nitrogen. Thereafter, the adsorbent bed is then repressurized with product from the on-line bed before being brought back to an on-line status. In both types of cycles waste nitrogen from the air separation plant is used in that it has a very low concentration of the impurities. Furthermore, it is at a low pressure given the fact that the impurities will tend to be adsorbed more readily at a higher pressure. In a pre-purification unit that operates by pressure swing adsorption, a greater flow rate of the waste nitrogen is required than in a temperature swing adsorption cycle.
The compressed and purified air, which in certain plants can be further compressed, is cooled to a temperature suitable for its rectification and then rectified in distillation columns to separate the components of the air. The distillation columns that are employed in cryogenic rectification processes include a higher pressure column and a lower pressure column. In the higher pressure column, the air is rectified to produce a nitrogen-rich vapor column overhead and a crude liquid oxygen column bottoms also known in the art as kettle liquid. A stream of the crude liquid oxygen column bottoms is further refined in the lower pressure column to produce the oxygen product.
Distillation column diameters increase in proportion to the square root of plant capacity or in other words the flow through the columns. Shipping limitations result in a maximum vessel diameter in the range of 6.0 to 6.5 m. As a consequence, the design, construction and installation of an air separation plant having an oxygen production capacity in excess of about 5000 metric tons per day has not been found to be practical. In order to overcome this limitation, typically multiple, parallel air separation plant trains are constructed to operate in parallel within an enclave. Unfortunately simple plant replication forfeits many “economies of scale” in that the construction of additional column shells carries with it considerable expense. Thus, even when multiple air separation units having higher and lower pressure columns are employed within an enclave of such units, it is desirable that each such unit be constructed with the largest capacity possible to limit the number of units employed within a particular installation of air separation plants.
A critical limitation associated with a distillation column involves the hydraulic flood point of any given column section. Column diameters are typically defined by an approach to flood that can be anywhere from 70 to 90 percent. Given equivalent pressure, nitrogen has a lower mass density than oxygen. As the lighter (more volatile) component of air, nitrogen flows to the top of the associated (nitrogen/oxygen) rectification sections. As the column vapor ascends it is progressively enriched in nitrogen. Conversely, the descending liquid becomes richer in oxygen. As a consequence of these thermodynamic aspects, the upper sections of the major low pressure air distillation columns, known as the nitrogen rectification sections, exhibit the highest volumetric loadings. Given a fixed maximum diameter and packing selection, such sections will limit capacity of each plant.
Another consideration of any air separation plant is that the pressure of the nitrogen leaving the cold box sets the pressure at the top of the lower pressure column. The pressure at the bottom of the lower pressure column is set by adding to the pressure at the top of the lower pressure column, the pressure drop through the lower pressure column stages. The pressure at the top of the higher pressure column is set by the temperature approach across the condenser that is employed in condensing the nitrogen-rich vapor column overhead of the higher pressure column against vaporizing part of the oxygen-rich liquid column bottoms of the lower pressure column. The pressure at the bottom of the higher pressure column is a sum of the pressure at the top of the higher pressure column and the pressure drop through the higher pressure column stages. After accounting for the pressure drop through the main heat exchanger, this pressure determines the main air compressor discharge pressure. An increase of 1 psi in the pressure at the top of the lower pressure column translates into an approximate 2.3 psi increase in pressure at the top of the higher pressure column. Thus, if pressure at the top of the lower pressure column can be reduced, then substantial energy savings can be realized in compressing the air since the required pressure will be lower.
In order to solve the foregoing problems, U.S. Pat. No. 6,227,005 discloses an air separation process in which compressed and purified air is introduced into a distillation column unit having a higher pressure column and a lower pressure column. A pressurized oxygen product is produced by pumping an oxygen-rich stream composed of an oxygen-rich liquid column bottoms of the lower pressure column and then heating the resulting pumped liquid in the main heat exchanger through indirect heat exchanger with part of the compressed and purified air that has been further compressed in a booster compressor. The crude liquid oxygen column bottoms of the higher pressure column is rectified in an auxiliary column to produce an oxygen-rich liquid that is introduced into the lower pressure column for further refinement. Liquid air produced as a result of the heat exchange between the compressed and purified air and the pumped liquid stream is used as intermediate reflux to the lower pressure column and the auxiliary column. Since the oxygen-rich liquid produced in the auxiliary column is leaner in nitrogen than the crude liquid oxygen column bottoms, there is less nitrogen in the lower pressure column and therefore, the capacity of the lower pressure column is increased. Furthermore, the auxiliary column can be operated at a higher pressure than the lower pressure column to produce a nitrogen-rich vapor at a higher pressure than that produced in the lower pressure column. The lower pressure column can be operated at a lower pressure to realize an energy savings in initially compressing the air in the main compressor.
There are, however, inherent operational limitations in the air separation process illustrated in the above patent that stem from the use of part of the nitrogen-rich vapor produced in the higher pressure column to reboil the auxiliary column. As a result, there is less boilup available in the lower pressure column to strip argon from the liquid and consequently, the air separation process is incapable of efficiently producing a high purity oxygen product. Moreover, another result of using such nitrogen-rich vapor is that the nitrogen containing overhead of the auxiliary column will invariably be at a pressure of near 40 psig. The problem with such a pressure is that it is not compatible with use as regeneration gas for the adsorbent beds contained in the pre-purification unit because, as mentioned above, the adsorbent will have a higher capacity for the impurity at a higher pressure. Typically, the nitrogen utilized for such purposes must be generated at a pressure of between 5 and 10 psig so that after accounting for heat exchanger and piping pressure drops, the nitrogen can be delivered to regenerate the adsorbent beds at a pressure of about 3 psig. While, the 40 psig nitrogen generated by the air separation process described in this patent could be reduced in pressure for such purposes, the pressure reduction would be in effect an irreversible loss of the energy involved in compressing the air in the first instance.
As will be discussed, the present invention provides a method and apparatus for separating air that among other advantages allows high purity oxygen to be produced while lower pressure column capacity is increased and nitrogen-rich vapor can be directly used in regenerating adsorbent contained in a pre-purification unit.
In one aspect, the present invention provides a method of separating air in which a compressed and purified air stream is separated in a cryogenic rectification process. The process employs a pre-purification unit to purify the air of higher boiling impurities and at least one air separation unit having a higher pressure column and a lower pressure column configured to produce oxygen and nitrogen-rich fractions. Refrigeration is generated within the cryogenic rectification process by further compressing and partially cooling part of the compressed and purified air stream and work expanding the part of the part of the compressed and purified air stream, after having been further compressed, within a turboexpander to produce an exhaust stream. The exhaust stream is introduced into a bottom region of an auxiliary column and rectified within the auxiliary column to form an oxygen containing liquid as a column bottoms and an auxiliary column nitrogen-rich vapor column overhead.
At least one oxygen containing stream is withdrawn from the auxiliary column having a lower nitrogen content than that of the exhaust stream and such stream is introduced into the at least one air separation unit for rectification within the lower pressure column. An auxiliary column nitrogen-rich vapor stream, composed of the auxiliary column nitrogen-rich vapor column overhead is withdrawn from the auxiliary column, warmed and thereafter, at least a portion thereof is introduced into the pre-purification unit so as to regenerate adsorbent within the pre-purification unit. The work expanding of the part of the compressed and purified air stream within the turboexpander is conducted such that exhaust stream pressure of the exhaust stream sets the pressure within the auxiliary column at a level that the auxiliary column nitrogen-rich vapor stream is able to be introduced into the pre-purification unit without further compression. The auxiliary column nitrogen-rich vapor stream can be at a higher pressure than that of at least one lower pressure nitrogen-rich stream composed of lower pressure column nitrogen-rich vapor column overhead produced in the lower pressure column of the at least one air separation unit.
The oxygen containing stream produced in the auxiliary column will have a lower nitrogen content than the exhaust stream that will have the effect of reducing the amount of nitrogen vapor within the lower pressure column to in turn increase the capacity of the lower pressure column and therefore, the entire cryogenic rectification process. Additionally, since the exhaust stream is being introduced into a bottom region of the auxiliary column, the auxiliary column is not being reboiled with nitrogen-rich vapor produced in the higher pressure column. This allows the lower pressure column to produce the oxygen at a higher purity than in the prior art discussed above if such high purity product is desired. Furthermore, the auxiliary column, as in the prior art, allows the lower pressure column to be operated at a lower pressure to save energy costs in the main compression of the incoming air to be separated. A major advantage of the method of the present invention is that additionally, since the exhaust stream pressure will set the pressure of the auxiliary column, the pressure of the auxiliary column nitrogen-rich vapor stream can also be set to a level that will allow such stream to be introduced into the pre-purification unit without further compression. This saves on the electrical power that would otherwise be consumed in such compression and therefore, the ongoing operational costs of running the process and plant incorporating the process as well as a compressor that is normally used for such purposes. In this regard, the ability to set the pressure of the lower pressure column also allows the pressure of the auxiliary column nitrogen-rich vapor stream to be set at a level that when taken in connection with losses of piping and heat exchangers and possibly control valves leading to the pre-purification unit will be optimal and not so high as to require expansion valves or the like for pressure let down purposes.
The cryogenic rectification process generates at least one impure oxygen stream containing oxygen and nitrogen and having an oxygen content no less than that of the air. This at least one impure oxygen stream, along with the exhaust stream, can be introduced into a bottom region of an auxiliary column and rectified along with the exhaust stream within the auxiliary column to form the oxygen containing liquid as a column bottoms and the auxiliary column nitrogen-rich vapor column overhead. The at least one oxygen containing stream that is withdrawn from the auxiliary column has a lower nitrogen content of that of the at least one impure oxygen stream and the exhaust stream and is introduced into the lower pressure column of the at least one air separation unit. The at least one impure oxygen stream can be composed of a crude liquid oxygen column bottoms produced in the higher pressure column. The use of the impure oxygen stream will act to reduce nitrogen within the lower pressure column to a level below that of the exhaust stream alone and will also allow for more auxiliary column nitrogen-rich vapor to be generated. If the exhaust stream were used alone, then the pre-purification unit would operate on the basis of a temperature swing adsorption cycle.
At least part of at least one oxygen-rich liquid stream of high purity and composed of an oxygen-rich liquid column bottoms produced in the lower pressure column of the at least one air separation unit can be pumped to form a pumped liquid oxygen stream. In this regard, the term, “high purity” as used herein and in the claims means a purity of at and above 99.5 percent oxygen by volume. Another part of the compressed and purified air stream can be further compressed to form a compressed air stream and the compressed air stream indirectly exchanges heat with at least part of the pumped liquid oxygen stream, thereby forming a liquid air stream from the compressed air stream and an oxygen product from the at least part of the pumped liquid oxygen stream. Intermediate reflux streams composed of the liquid air stream are introduced into the lower pressure column of the at least one air separation unit above the location at which the at least one oxygen containing stream is introduced into the lower pressure column and into the auxiliary column above the bottom region thereof.
A higher pressure nitrogen-rich column overhead produced in the higher pressure column of the at least one air separation unit can be condensed into a nitrogen-rich liquid against vaporizing part of the oxygen-rich liquid column bottoms. Reflux liquid streams composed of the nitrogen-rich liquid are introduced as reflux into the higher pressure column and the lower pressure column of the at least one air separation unit and into the auxiliary column. The nitrogen-rich liquid that is used in forming the reflux liquid streams that are fed as the reflux to the lower pressure column and the auxiliary column is subcooled through indirect heat exchange with a lower pressure nitrogen-rich vapor stream, composed of a nitrogen-rich vapor column overhead produced in the lower pressure column of the at least one air separation unit and the nitrogen-rich auxiliary column vapor stream. The at least one lower pressure nitrogen-rich vapor stream and the auxiliary column nitrogen-rich vapor stream are fully warmed in a main heat exchanger used in cooling the air to a temperature suitable for its rectification within the air separation unit. The intermediate reflux streams can also be introduced into the higher pressure column.
In another aspect, the present invention provides an apparatus for separating air. In accordance with this aspect of the present invention, a cryogenic rectification installation is provided that is configured to separate the air. The installation includes a main compressor and a pre-purification unit in flow communication with the main compressor to produce a compressed and purified air stream. A main heat exchanger is configured to cool the compressed and purified air stream to a temperature suitable for its rectification and at least one air separation unit connected to the main heat exchanger and having a higher pressure column and a lower pressure column configured to produce oxygen and nitrogen-rich fractions.
A refrigeration generation system and an auxiliary column are also provided. The refrigeration generation system comprises a booster compressor in flow communication with the main compressor to further compress part of the compressed and purified air stream. The booster compressor is connected to the main heat exchanger and the main heat exchanger is configured to partially cool the part of the compressed and purified air stream after having been further compressed in the booster compressor. A turboexpander is connected to the main heat exchanger to work expand the part of the compressed and purified air stream, after having been further compressed and partially cooled and thereby produce an exhaust stream. The auxiliary column is connected to the turboexpander so as to receive the exhaust stream in a bottom region thereof and is configured to rectify the exhaust stream, thereby to form an oxygen containing liquid as a column bottoms and an auxiliary column nitrogen-rich vapor column overhead. The at least one air separation unit is connected to the auxiliary column so that at least one oxygen containing stream is withdrawn from the auxiliary column having a lower nitrogen content of that of the exhaust stream and is introduced into the at least one air separation unit.
The pre-purification unit and an auxiliary column are connected to the main heat exchanger such that an auxiliary column nitrogen-rich vapor stream, composed of the auxiliary column nitrogen-rich vapor column overhead, after having been warmed in the main heat exchanger, is introduced into the pre-purification unit so as to regenerate adsorbent within the pre-purification unit with the auxiliary column nitrogen-rich stream. The refrigeration system is configured such that exhaust stream pressure of the exhaust stream sets pressure within the auxiliary column at a level that the auxiliary column nitrogen-rich vapor stream is able to be introduced into the pre-purification unit without further compression. The auxiliary column nitrogen-rich vapor stream can be at a higher pressure than that of a lower pressure nitrogen-rich stream composed of lower pressure column nitrogen-rich vapor column overhead produced in the lower pressure column of the at least one air separation unit.
The auxiliary column can be connected to the at least one air separation unit so as to receive at least one impure oxygen stream, together with the exhaust stream, in the bottom region thereof. The at least one impure oxygen stream contains oxygen and nitrogen and has an oxygen content that is no less than that of the air. The auxiliary column is configured to rectify the at least one impure oxygen stream along with the exhaust stream, thereby forming the oxygen containing liquid as a column bottoms and the auxiliary column nitrogen-rich vapor column overhead. The lower pressure column of the at least one air separation unit is connected to the auxiliary column so that the at least one oxygen containing stream is withdrawn from the auxiliary column has a lower nitrogen content of that of the at least one impure oxygen stream and the exhaust stream and is introduced into the lower pressure column of the at least one air separation unit for rectification within the lower pressure column. The auxiliary column can be connected to the higher pressure column of the at least one air separation unit such that the at least one impure oxygen stream is composed of a crude liquid oxygen column bottoms produced in the higher pressure column of the at least one air separation unit.
A pump can be connected to the lower pressure column so that at least part of at least one oxygen-rich stream of high purity and composed of an oxygen-rich liquid column bottoms produced in the lower pressure column of the at least one air separation unit, is pumped to form a pumped liquid stream. The main heat exchanger is connected to the pump so that the at least part of the pumped liquid stream is introduced into the main heat exchanger and warmed to form an oxygen product. A further booster compressor compresses another part of the compressed and purified air stream, thereby to form a compressed air stream. The further booster compressor is connected to the main heat exchanger such that the pressurized liquid stream warms within the main heat exchanger through indirect heat exchange with the compressed air stream and the compressed air stream is thereby liquefied to form a liquid air stream. The lower pressure column of the at least one air separation unit and the auxiliary column are connected to the main heat exchanger such that intermediate reflux streams composed of the liquid air stream air introduced into the lower pressure column of the at least one air separation unit and the auxiliary column above locations at which the at least one oxygen containing stream is introduced into the lower pressure column of the air at least one air separation unit and above the bottom region of the auxiliary column, respectively.
A heat exchanger can be connected to the higher pressure column and the lower pressure column of the at least one air separation unit so that a higher pressure nitrogen-rich column overhead produced in the higher pressure column is condensed into a nitrogen-rich liquid against vaporizing part of the oxygen-rich liquid column bottoms. The higher pressure column and the lower pressure column of the at least one air separation unit and the auxiliary column are connected to the heat exchanger so that reflux liquid streams composed of the nitrogen-rich liquid are introduced as reflux into the higher pressure column and the lower pressure column of the at least one air separation unit and the auxiliary column. A subcooling unit is positioned between the lower pressure column of the at least one air separation unit and the main heat exchanger so that the nitrogen-rich liquid that is used in forming the reflux liquid streams, that are fed as the reflux to the lower pressure column of the at least one air separation unit and the auxiliary column, is subcooled through indirect heat exchange with the lower pressure nitrogen-rich vapor stream and the auxiliary column nitrogen-rich vapor stream.
The higher pressure column of the at least one air separation unit can be connected to the main heat exchanger so that of the intermediate reflux streams is also introduced into the higher pressure column of the at least one air separation unit.
While the specification concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention will be understood when taken in connection with the accompanying sole FIGURE that illustrates an apparatus for carrying out a method in accordance with the present invention.
With reference to the FIGURE, a cryogenic rectification installation 1 is illustrated that is designed to separate air and thereby to produce an oxygen product. Cryogenic rectification installation 1 is provided with a main heat exchanger 2 to cool the air to a temperature suitable for its rectification within air separation units 3 and 4 and thereby produce an oxygen product that is discharged from the main heat exchanger 2 as an oxygen product stream 96 and a nitrogen product stream 150, to be discussed in more detail hereinafter. It is understood that air separation unit 4 as well as other possible air separation units that would be operatively associated with an auxiliary column 100, also to be discussed in more detail hereinafter, are optional in that the present invention could be practiced with a single air separation unit 3.
The air to be separated is introduced into apparatus 1 as an air stream 10 that is compressed in a main compressor 12 to produce a main compressed air stream 14 having a pressure in a range of from between about 5 and about 15 bar(a). Main compressor 12 can be a multi-stage intercooled integral gear compressor with condensate removal. Main compressed air stream 14 is subsequently purified in a pre-purification unit 16 to remove higher boiling impurities such as water vapor, carbon dioxide and hydrocarbons from the air and thereby produce a compressed and purified air stream 18. As well known in the art and as discussed above, such unit 16 can incorporate adsorbent beds operating in an out of phase cycle that is a combination of temperature and pressure swing adsorption or that is purely a temperature swing adsorption cycle or a pressure swing adsorption cycle also described in detail above.
A part 20 of the compressed and purified air stream 18 is cooled to a temperature suitable for its rectification within main heat exchanger 2 as a main air feed and then is divided into subsidiary main air feed streams 21 and 22 that are fed into the bottom regions of higher pressure columns 44 and 46 of air separation units 3 and 4 for rectification. Another part 23 of the compressed and purified air stream is subsequently compressed in a booster compressor 24, again preferably a multi-stage unit, to form a compressed air stream 25 that can have a pressure in a range of between about 25 and about 70 bar. Compressed air stream 25 can constitute roughly between about 25 percent and about 35 percent of the incoming air. As will be discussed, compressed air stream 25 is liquefied within a main heat exchanger 2 against vaporizing a second part 94 of a pumped liquid oxygen stream 88 to produce the oxygen product stream 96 and a liquid air stream 26 in a subcooled state. Another part 28 of the compressed and purified air stream 18 is compressed in a turbine loaded booster compressor 30 to a pressure that can be in a range of between about 7 bar(a) and 10 bar(a) and then compressed in a compressor 32 to produce another compressed air stream 34 that can have a pressure of between about 10 bar(a) and 15 bar (a). The compressed air stream 34 is partially cooled within the main heat exchanger 2 to a temperature that is in a range of between about 160 K and about 220 K and then expanded within a turboexpander 36 to produce an exhaust stream 38 to supply refrigeration to the air separation installation 1. Exhaust stream 38 is fed into a bottom region of an auxiliary column 100 that will be discussed in greater detail hereinafter. Exhaust stream 38 would comprise between about 5 to 10 percent by volume of the incoming air. It is to be noted that the term, “partially cooled” as used herein and in the claims means cooled to a temperature between the warm and cold end temperatures of streams leaving and entering the main heat exchanger 2.
It is to be noted that although main heat exchanger 2 is illustrated as a single unit, in practice, main heat exchanger 2 could be a series of parallel units incorporating known aluminum plate-fin construction. Moreover, the high pressure portion of main heat exchanger 2 could be “banked”, that is, fabricated so that the portion used in exchanging heat between the first compressed stream 25 and the second part 94 of the pumped liquid oxygen stream 88 were in a separate high pressure heat exchanger. Thus, the term “main heat exchanger” as used herein and in the claims can be taken to mean a single unit or multiple units as described above. Moreover, although booster compressor 30 is illustrated as being mechanically connected to turboexpander 36 and compressor 32 is provided to further compress the compressed and purified air, single, separately driven booster compressors could be used in place of the illustrated units. Furthermore the subcooling unit 124 to be discussed hereinafter could be incorporated into the main heat exchanger in a manner known in the art.
Each of the higher pressure columns 44 and 46 are provided with mass transfer contacting elements 48 and 50 such as structure packing, dumped packing or sieve trays or a combination of such elements as well known in the art. The introduction of primary feed air streams 40 and 42 initiates formation of an ascending vapor phase that becomes ever richer in nitrogen as it ascends higher pressure columns 44 and 46, respectively. The ascending vapor is in countercurrent contact with a descending liquid phase that becomes ever richer in oxygen as it descends columns 44 and 46. As a result, a crude liquid oxygen column bottoms 52 is formed in each of the higher pressure columns 44 and 46, within bottom regions thereof, and a higher pressure nitrogen-rich vapor at the top of the higher pressure columns 44 and 46.
Lower pressure columns 54 and 56 of air separation units 3 and 4, respectively, operating at a lower pressure than higher pressure columns 44 and 46, are each provided with heat exchangers in the form of condenser reboilers 58 in the base of each of the lower pressure columns 54 and 56. Streams 60 and 62 composed of the higher pressure nitrogen-rich vapor column overhead of the higher pressure columns 44 and 46, respectively, are condensed within condenser reboilers 58 to produce nitrogen-rich liquid streams 64 and 66 and to partly vaporize an oxygen-rich liquid column bottoms 68 produced in each of the lower pressure columns 54 and 56. Such vaporization initiates the formation of an ascending vapor phase within lower pressure columns 54 and 56. The descending liquid phase within lower pressure columns 54 and 56 is initiated through introduction of reflux streams 70 and 72 that are composed of the nitrogen-rich liquid streams 64 and 66. Mass transfer contacting elements 74, 76 and 78 are located within each of the lower pressure columns 54 and 56 to contact the descending liquid with the ascending vapor and thereby to produce the oxygen-rich liquid 68 and a low pressure nitrogen-rich vapor column overhead in top regions of the lower pressure columns 54 and 56.
Oxygen-rich streams 80 and 82 that are composed of the oxygen-rich liquid column bottoms 68 and can be of high purity are removed from lower pressure columns 54 and 56 and combined to form a combined stream 84 that is pumped by a pump 86 to produce a pumped liquid oxygen stream 88 that can have a pressure from between about 10 bar(a) and about 50 bar(a). A first part of the pumped liquid oxygen stream 88 can optionally be directly taken as liquid product stream 92 and a second part 94 of the pumped liquid oxygen stream 88 can, as described above, be warmed within the main heat exchanger to produce the oxygen product as a product stream 96.
Within each of the lower pressure columns 54 and 56 as the liquid phase descends, it becomes ever richer in oxygen, the nitrogen being stripped out by the ascending vapor phase. The section of the column where such action predominantly occurs is within mass transfer contacting element 74. The sections of the lower pressure columns occupied by mass transfer contacting elements 76 and 78 are nitrogen rectification sections which serve to enrich the ascending vapor in nitrogen content. In many instances it is the uppermost sections that serve to constrain plant capacity. In order to overcome this limitation, a nitrogen-oxygen mixture which has been enriched in oxygen is introduced into each lower pressure column 54 and 56 that is generated in an auxiliary column 100 in lieu of crude liquid oxygen or kettle liquid generated in the bottom region of each of the higher pressure columns 44 and 46.
In cryogenic rectification installation 1, impure oxygen streams, that in the illustrated embodiment constitute crude liquid oxygen streams 102 and 104, are removed from higher pressure columns 44 and 46, respectively. These streams are composed of the crude liquid oxygen 52. The crude liquid oxygen streams 102 and 104 are then valve expanded to a pressure substantially at the operating pressure of the auxiliary column 100 by expansion valves 106 and 108 and then introduced into a bottom region 101 of the auxiliary column 100 with the exhaust stream 38 for rectification to produce an oxygen containing liquid column bottoms 110 and an auxiliary column nitrogen-rich vapor column overhead at the top of auxiliary column 100. Pumps could be used to pump crude liquid oxygen streams 102 and 104 if their pressure were not sufficient to overcome gravitational head to the auxiliary column 100. Auxiliary column 100 is refluxed by a reflux stream 112 that is made up of the nitrogen-rich liquid streams 64 and 66 discussed above. In this regard, nitrogen-rich liquid stream 64 and 66 are divided into subsidiary streams 114, 116 and 118, 120, respectively. Subsidiary streams 114 and 118 reflux the higher pressure columns 44 and 46, respectively. Subsidiary streams 118 and 120 are combined to form a combined stream 122 that is subcooled in a subcooling unit 124 and then divided into reflux streams 70, 72 and 112. Reflux streams 70, 72 and 112 are valve expanded to an operational pressure of the lower pressure columns 54 and 56 and the auxiliary column 100 by expansion valves, 126, 128 and 130, respectively.
Auxiliary column 100 is provided with mass transfer contacting elements 132 and 134 to contact ascending vapor and descending liquid phases and thereby produce the oxygen containing liquid column bottoms 110 and the auxiliary column nitrogen-rich vapor column overhead. The exhaust stream 38 and flash-off vapor produced by the introduction of crude liquid oxygen streams 102 and 104 into auxiliary column 100 as well as introduction of intermediate reflux stream 158 (to be discussed) form the ascending phase to be rectified. The descending liquid phase is produced by reflux stream 112 and the intermediate reflux stream 158. As a result of the distillation, the oxygen containing liquid column bottoms 110 is leaner in nitrogen than the crude liquid oxygen column bottoms 52 produced in the higher pressure columns 44 and 46. Oxygen containing streams 136 and 138 that are composed of the oxygen containing liquid column bottoms 110 are removed from the auxiliary column 100 and then introduced into the base of the nitrogen rectification sections of the lower pressure columns 54 and 56 to reduce the nitrogen content within such sections of the columns and to allow for a higher production rate without such columns flooding. In this regard, such oxygen containing streams 136 and 138 might have a vapor content upon their introduction into lower pressure columns 54 and 56. It is to be noted here that the amount of total air fed to the auxiliary column can be adjusted in accordance with plant refrigeration demands. For instance if there is a need for increased liquid production (stream 92) a greater flow of air may be directed through turbine 36 and hence into the auxiliary column 100.
Nitrogen-rich vapor streams 140, 142 and 144, composed of the nitrogen-rich vapor column overhead produced in such columns, are removed from the lower pressure columns 54 and 56 and the auxiliary column 100, respectively. Nitrogen-rich vapor streams 140 and 142 are combined to form a combined nitrogen-rich vapor stream 146. Combined nitrogen-rich vapor stream 146 is then partly warmed within subcooling unit 148 to subcool combined nitrogen liquid stream 122 and then is fully warmed within main heat exchanger 2 to form a nitrogen product stream 150. Similarly, nitrogen-rich vapor stream 144 is partly warmed within subcooling unit 124 to assist in subcooling the combined nitrogen liquid stream 122 and then is fully warmed within main heat exchanger 2 to form a warm waste nitrogen stream 151.
It is to be noted that if nitrogen product stream 150 were desired at high purity, an addition rectification section could be placed at the top of the lower pressure columns 54 and 56. Another possibility is to partition lower pressure columns 54 and 56 or provide an annular divider to divide the lower pressure columns 54 and 56 to allow for the production of both high and low purity nitrogen. Since, the warm waste nitrogen stream 151 is being used to regenerate the adsorbent bed and high purity nitrogen is not desired, then it is possible to reflux the auxiliary column 100 with a lower purity liquid nitrogen stream extracted from the higher pressure columns 44 and 46.
The introduction of the oxygen containing streams 136 and 138 effectively unload the nitrogen rectification section of the lower pressure columns 54 and 56. The upper rectification sections of the low pressure columns still require sufficient reflux to maintain high oxygen recovery. In order to achieve this condition, the liquid air stream 26 is expanded to an operational pressure of the higher pressure columns 44 and 46 by means of an expansion valve 152 and then divided and subdivided into intermediate reflux streams 154, 156 and 158 and optionally, intermediate reflux streams 160 and 162. Intermediate reflux streams 154, 156 and 158 are valve expanded to lower the pressure of such streams by expansion valves 164, 168 and 170 and then introduced as intermediate reflux into lower pressure columns 54 and 56 above locations at which the oxygen containing streams 136 and 138 are introduced and auxiliary column 100, above the bottom region thereof at which the impure oxygen streams are introduced. Optional intermediate reflux streams 160 and 162 are introduced into the higher pressure columns 44 and 46.
Warm waste nitrogen stream 151 is introduced into a process heater 180 which may employ any number of means for heating including and not limited to the use of electrical-resistive, gas fired, steam or other heat exchanger fluid. The heating may take place intermittently and is selectively activated. As described above, to heat warm waste nitrogen stream 151 in connection with the regeneration of adsorbent within adsorbent beds of pre-purification unit 16 if such unit is operated in a temperature swing adsorption cycle or a combination of a temperature swing adsorption cycle and a pressure swing adsorption cycle. A portion 182 of stream 151 can be directed elsewhere as product (which may be further compressed prior to sendout) the remaining fraction of stream 151 being directed to pre-purification unit 16.
If pre-purification unit 16 is operated in accordance with purely a pressure swing cycle, electrical heater 180 would not be used. In such a situation a supplemental compressor 184 may be used to compress a portion 186 of nitrogen gas 150 prior to combination as a compressed nitrogen stream 188 with 151 and entry into pre-purification unit 16.
As also mentioned above, the pressure of warm waste nitrogen stream 151 is important in order to be effective in its role in purging the adsorbent beds. The pressure of nitrogen-rich vapor stream 144 is set so that no further compression thereof is required and after piping pressure losses and losses within subcooling unit 124 and main heat exchanger 2, warm waste nitrogen stream 151 will typically be about 3 psig upon entry into pre-purification unit 16. The pressure of nitrogen-rich vapor stream 144 is in turn set by setting the pressure level within auxiliary column 100 by in turn setting the pressure of exhaust stream 38. There exists some latitude in the setting of the pressure of exhaust stream 38 and this is accomplished by designing the system resistance (heat exchanger pressure drop) to yield the desired turbine exhaust pressure. From a process control standpoint the pressure typically will be set by a back-pressure control valve located at some point along the process path of stream 144. Once this pressure is set, the pressure of crude liquid oxygen streams 102 and 104 will be set after expansion in valves 106 and 108 to a pressure compatible with the pressure of exhaust stream 38. The pressure of intermediate reflux stream 158 after expansion within valve 170 will be set to be a pressure compatible with the level of auxiliary column within which intermediate reflux stream 158 is introduced. It is to be further noted, that lower pressure columns 54 and 56 preferably operate at a lower pressure than the auxiliary column 100 such that nitrogen-rich vapor streams 140 and 142 have a pressure of for example 3 to 5 psig. This will save on the amount of energy being expanded in compressing the air in the main air compressor 12.
As mentioned above, the present invention can be practiced with a single air separation unit such as air separation unit 3. In such case, there would be no other air separation units being fed with subsidiary main feed air streams such as subsidiary main feed air stream 22 and intermediate reflux streams such as intermediate reflux streams 156 and 158. Furthermore, auxiliary column 100 would not be fed with crude liquid oxygen stream 104. Nitrogen-rich product stream 150 would be formed by nitrogen-rich vapor stream 140 and oxygen product stream 96 would be originated from the crude liquid oxygen column bottoms 68 solely within lower pressure column 54. Having said this, the present invention has a greater applicability than pumped liquid oxygen plants and could be used in connection with any air separation unit designed to produce oxygen and nitrogen-rich fractions. As such, all of the streams associated with the production of liquid oxygen might not be present in a possible embodiment of the present invention along with the associated intermediate reflux streams. In this regard, it is possible that the auxiliary column 100 in any embodiment be solely fed with the exhaust stream 38. In such case, the resulting oxygen containing stream 136 and 138 could be fed to the bottom regions of the higher pressure columns 44 and 46 and then fed along with the crude oxygen streams 102 and 104 directly into intermediate locations of the lower pressure columns 54 and 56 with the effect of reducing nitrogen within such columns. This, however, would require a pump. A greater effect in increasing column capacity would be to also feed the crude liquid oxygen streams 102 and 104 into the auxiliary column. In fact, this would be necessary to make sufficient nitrogen-rich vapor for nitrogen-rich vapor stream 144 if it were eventually to be used in connection with a pre-purification unit 16 operating in a pressure swing adsorption mode of operation. The use of a pressure swing adsorption mode would, however, also require the use of compressor 184 as has been described above. However, the use of the exhaust stream 38 as the sole feed to the auxiliary column 100 in a single air separation unit embodiment is possible. However, in any embodiment of the present invention considering that the turbine air flow only constitutes about 5 to 10 percent of the air, the pre-purification unit 16 would be constrained to be designed to operate in a temperature swing adsorption cycle.
At the other extreme, although the auxiliary column 100 is illustrated in connection with two air separation units 3 and 4, in practice, an auxiliary column such as auxiliary column 100 should be able to debottleneck 3 or 4 main air separation units, although it is possible more air separation units would be used. Additionally, although air separation units 3 and 4 are identical, air separation units of different design and capability could be used. For example, one air separation unit, as illustrated, could be a conventional double column and the second unit may incorporate argon recovery. The air separation units could also be of different types. In this regard, the qualifying aspect of an air separation unit is the utilization of a low pressure nitrogen rectification section and most known oxygen production processes will have such a section. As an example, the present invention is applicable to low purity oxygen plants that employ air condensation within the base of the lower pressure column. Such air condensation processes may involve a total or a partial air condensation.
Although not illustrated, the present invention contemplates that the auxiliary column 100 operates in a manner that is independent of the associated air separation units. In particular, not all of the air separation units need be in operation at any time. If for instance, air separation unit 3 is out of service, the auxiliary column could still function in connection with air separation unit 4. Although the FIGURE depicts a common main heat exchanger 2 and a subcooling unit 124 associated with the operation of the air separation units 3 and 4, along with associated main air compressor 12, turboexpander 36 and etc., it is possible to design the cryogenic distillation installation in which each air separation unit has dedicated components such as main heat exchangers and subcooling units or partially dedicated and partial common units. For example multiple pumps or a single pump 86 could be used in the embodiment of the present invention shown in the FIGURE. It is to be noted here that although the liquid air stream 26 is illustrated as being condensed against a second part 94 of pumped liquid oxygen stream 88, it is possible to employ the present invention in connection with pumped liquid nitrogen.
A combination of feed sources may be employed for an auxiliary column system in accordance with the present invention. In addition to impure oxygen liquid streams withdrawn from the higher pressure columns 44 and 46, for example, crude liquid oxygen streams 102 and 104, interstage fluids may be extracted from either the higher or lower pressure columns associated with the air separation units 3 and 4. All that is required for the impure oxygen streams is that they contain an oxygen content that is no less than that of air. For example, the impure oxygen streams could be formed from part of the liquid air stream that is produced in vaporizing a second part 94 of the pumped liquid oxygen stream 88. In either case, by diverting such stream to the auxiliary column, nitrogen would also be diverted to lower the nitrogen content in the lower pressure columns 54 and 56. Also, such interstage fluids could constitute a liquid air-like substance withdrawn from the columns at the point of introduction of intermediate reflux streams, for example, 160 and 162. Such liquid, known in the art as synthetic liquid air, could likewise be used to divert nitrogen from the lower pressure columns 54 and 56. As far as the derivation, the same holds true for the intermediate reflux streams that in the illustrated embodiment are designated by reference numbers 154, 156, 160 and 162. These streams could be composed of air or other air-like substance such as synthetic liquid air that would have an argon content no less than that of air given that such synthetic air, if withdrawn at the point of introduction of streams 160 and 162, would in fact have an argon content greater than air.
In the case where argon is produced from at least one of the column systems, it is possible to route a portion of the vaporized impure oxygen into the auxiliary column.
It should be noted that the feed source to the auxiliary column 100 may be derived from only a single air separation unit, for example air separation unit 3 or air separation unit 4 and then be divided amongst the associated air separation units.
Although the present invention has been described with reference to a preferred embodiment, as will occur to those skilled in the art, numerous changes, additions and omissions can be made to such embodiment without departing from the spirit and scope of the present invention as set forth in the appended claims.
This application is a continuation-in-part of U.S. patent application Ser. No. 12/634,810, filed Dec. 10, 2009.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12634810 | Dec 2009 | US |
| Child | 12774249 | US |
