Integrated air separation method

Abstract: An air separation method in which a pre-purification unit 16 is provided with three or more beds which are subjected to a pressure swing adsorption process. Each of the beds is subjected to feed, pressurisation, purge and repressurization stages. The purge stages are conducted with a waste stream from the air separation unit and the duration of the purge stage is equal to the total time of the cycle divided by the number of beds, and thus the waste stream is used continuously to purge the beds.

Description: The present invention relates to an integrated air separation method in which a pre-purification unit is integrated with an air separation unit to remove impurities from the air prior to its separation. More particularly, the present invention relates to such a process in which the pre-purification unit contains adsorbent beds which operate in accordance with a pressure swing adsorption cycle. It is known to separate air into its components by a variety of low temperature rectification processes. In accordance with such processes, air is compressed, cooled to a temperature suitable for its rectification, and then is introduced into a air separation unit which contains one or more columns in which the air is distilled into its component parts. The air separation unit can contain one or more distillation columns having trays or packing to provide intimate contact between vapour and liquid phases of the air to be separated. In addition to oxygen, nitrogen and argon, the air also contains higher boiling components such as carbon dioxide, hydrocarbons and water. If these higher boiling components are not removed from the air prior to the cooling of the air, such components will form ice in the main heat exchanger. Additionally, distillation columns tend to concentrate such higher boiling components as hydrocarbons. In case of hydrocarbons, this can present an operational hazard. In order to prevent such problems, pre-purification units are provided that are integrated with the air separation plant in order to remove the higher boiling components such as moisture, carbon dioxide and hydrocarbons. Such pre-purification is effected by beds of adsorbent operating in accordance with pressure swing adsorption cycles. A well known pressure swing adsorption cycle has feed, depressurisation, purge and repressurisation stages. During the feed stage, compressed air is fed to a bed to produce a purified air stream which is then cooled in the main heat exchanger. After the feed stage is complete, the bed is depressurised to atmospheric pressure. The depressurisation initiates desorption of the adsorbed impurities from the adsorbent. Depressurisation is completed by purging the bed with a waste stream, commonly waste nitrogen when available from a lower pressure column of a double column air separation unit. Thereafter, part of purified air from another bed undergoing a feed stage is routed to the bed that has just been subjected to a purge stage to repressurise the bed. It is possible to use two beds which are operated out of phase such that while one bed is being fed, the other bed is regenerated and is therefore being subjected to depressurisation, purge and repressurization stages. Even when more than two beds are used, the purge gas is used only intermittently. The problems that can be solved by the invention, particularly in terms of improving operational efficiency are identified below. In its broadest aspect the invention provides an integral air purification-air separation process in which the air is purified by pressure swing adsorption and a waste stream formed by rectification of the air is used continuously to purge the beds. According to the invention, there is provided an integrated air purification-air separation method comprising: separating air by compressing a feed air stream, cooling the feed air stream to a temperature suitable for its rectification, rectifying the air and thereby producing a waste stream; and prepurifying the feed air stream prior to said cooling thereof by pressure swing adsorption to produce a purified feed air stream; the pressure swing adsorption comprising subjecting each of at least three adsorbent beds to a repeating cycle including a feed stage to adsorb impurities contained within the feed air stream and thereby to produce said purified feed air stream, a depressurisation stage to initiate desorption of said impurities, a purge stage, comprising purging the adsorbent bed with the waste stream, to complete desorption of said impurities, and a repressurization stage to repressurise the beds; wherein the repeating cycle to which each bed is subjected is performed out of phase with the repeating cycles to which the other beds are subjected, and the duration of each individual purge stage is equal to the total duration of the individual cycle of which said individual purge stage forms a part divided by the total number of said adsorbent beds, and the waste stream is used continuously for purging the adsorbent beds. In a two bed PSA system, since one bed is producing while another bed is being regenerated, there exists a time period in which, although available, the purge stream is not being used to purge a bed. Hence, the present invention requires three or more beds so that the purge stream is continuously being used. The effect of the present invention is to increase the amount of time that each bed is exposed to the purge stream so that at the conclusion of the purge stream, more impurities have been desorbed from the bed than by prior art methods. As a result, more impurities can be adsorbed in the first instance. The application of the present invention is therefore to reduce the amount of adsorbent required for a pre-purification unit of an air separation plant. The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic flow diagram of an air separation plant for carrying out a method in accordance with the present invention; Figure 2 is a schematic diagram of a pre-purification unit for use in the plant shown in Figure 1; and Figure 3 is a schematic cycle diagram illustrating operation of the unit shown in Figure 2. With reference to Figure 1, in operation of an air separation plant 1 incoming air is filtered in a filter 10 to remove dust particles and the like and is then compressed in a compressor 12. The heat of compression produced through the compression of the air is removed by an aftercooler 14. Thereafter, the air is purified in a pre-purification unit 16 and is then cooled in a main heat exchanger 18 to a temperature suitable for its rectification within air separation unit 20. Air separation unit 20 can consist of a single distillation column that is used to separate the air into an oxygen-rich liquid column bottoms and a nitrogen-rich or even a high-purity tower overhead. The present invention is not limited by the actual distillation process used or the number of distillation columns used in carrying out the process. For instance, air separation unit 20 may be a double column in which higher and lower pressure columns are operatively associated with one another by a condenser-reboiler. The compressed and cooled air would be introduced into the bottom of the higher pressure column to produce a nitrogen rich tower overhead and an oxygen-rich column bottoms, known in the art as crude liquid oxygen. The crude liquid oxygen would be further refined in the lower pressure column into a top gaseous nitrogen fraction and a bottom liquid oxygen fraction. Air separation unit 20 may also be a single column oxygen or nitrogen generator. For purposes of explanation, it will be assumed that air separation unit 20 produces a waste nitrogen stream 22 which is typically impure. However, if an oxygen product is not desired, waste stream 22 may be formed from gaseous oxygen directly above the sump of a double column air separation unit. Additionally, air separation unit 20 produces a product stream 24 which may be liquid oxygen to be vaporised within main heat exchanger 18 or gaseous oxygen or nitrogen to be warmed within main heat exchanger 18 to ambient conditions. With reference to Figure 2 pre-purification unit 16 is provided with first, second and third adsorbent beds 26, 28 and 30. Each of the beds contains an adsorbent to adsorb the higher boiling impurities, for example, activated alumina (AA) 7 x 12 mesh beads manufactured by La Roche Chemicals, P.O. Box 1031, Airline Highway, Baton Rouge, La. 70821-1031. Compressed air from aftercooler 14 enters a header 32 and is sequentially routed to each of first, second and third adsorbent beds 26-30 to produce a product stream which is discharged from a product header 36 to main heat exchanger 18. Each of the first, second and third beds 26-30 is subjected to feed. depressurisation, purge and then repressurization stages by operation of on/off valves 38-66, which although not illustrated could be remotely activated by a programmable or analog controller. With additional reference to Fig. 3. a valve sequence chart follows to describe the operation of valves 38-66 to effect pressure swing adsorption process illustrated Fig. 3. Open valves are designated by the symbol "O". VALVE SEQUENCE CHART Valve Number 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 Time-minutes 0-0.5 0 0 0 0 0 0.5-12 0 0 0 0 0 0 12-12.5 0 0 0 0 0 12.5-24 0 0 0 0 0 0 24-24.5 0 0 0 0 0 24.5-36 0 0 0 0 0 0 In the first half minute, valves 38, 46, 48, 52 and 62 open. As a result, the feed air stream, formed from compressed air cooled in aftercooler 14, feeds to first adsorbent bed 26 to subject first adsorbent bed 26 to a feed stage. Since valve 62 is also open, the product is discharged to product header 36 from first adsorbent bed 26. Valves 46 and 52 are open to subject the second adsorbent bed 28 to the purge stage. Open valve 48 allows third adsorbent 30 to be subjected to the depressurisation stage. At the succeeding time interval, between a half and 12 minutes, first adsorbent bed 26 continues to be subjected to the feed stage. Second adsorbent bed 28 is now, however, being subjected to a repressurization stage. To this end, valves 56 and 58 are open to subject second adsorbent bed 28 to part of the product stream at pressure. Valves 54 and 48 have opened to subject third adsorbent bed 30 to the purge stage. During the time interval from 12 to 12.5 minutes first adsorbent bed 26 is now being subjected to depressurisation. To this end, valve bed 44 is open. Second adsorbent bed 28 is now subjected to the feed stage by setting valves 40 and 64 in the open position. Third adsorbent bed 30 continues to be subjected to purge. In the next time interval from 12.5 to 24 minutes first adsorbent bed 26 is subjected to the purge stage by opening valves 50 and 44. Third adsorbent bed 30 is repressurized by setting valves 58 and 60 in the open position. The next succeeding time interval, from 24 to 24.5 minutes, has second adsorbent bed 28 being subjected to a depressurisation stage by setting valve 46 in the open position. At the same time, third adsorbent bed 30, which previously was repressurized, is now subjected to the feed stage by setting valves 42 and 66 in the open position. The last time interval, from 24.5 to 36 minutes, has first adsorbent bed 26 being repressurized (prior to being brought on-line) with part of the product stream being produced from third adsorbent bed 30. Valves 56 and 60 are set in the open position for this purpose. Second adsorbent bed 28 is now subjected to the purge stage by opening valves 52 and 46. The above-reference sequence continuously repeats with beds acting in an adsorbing capacity followed by regeneration. As can be appreciated, in the illustrated embodiment, the time that each bed is exposed to the feed stage is equal to the purge stage and the total time of the cycle is equal to about three times the time of the purge stage or for that matter the time of the feed stage. The following is a calculated example to show the adsorbent savings when conducting a pressure swing adsorption cycle for a pre-purification unit in accordance with the present invention as compared with prior art two bed methods. It can be seen from this example that compared to a two bed method, about 4,808 Kg. of adsorbent are saved with the three bed method of the present invention. PRODUCT: CO 2 = 1 ppm peak ADSORBENT: Activated Alumina (AA) 7x12 mesh beads NO OF BEDS 2 3 Weight of Adsorbent (Kg.) 28,852 24,044 Weight of Adsorbent/Bed (Kg.) 14,426 8,015 Inner Diameter of Bed, Meters 2.7 2.25 Length of Bed, Meters 3 2.4 % Purge/Feed 41.5 50 % Vent Loss 1 0.5 % Max. Repressurization/Feed Flow 9 1.3 Feed Time, min. 12 12 Time Depressurisation, min. 0.5 0.5 Time Purge,min. 10 12 Time Repressurization, min. 1.5 11.5 Total Cycle Time,min. 24 36 For purposes of this example, the feed pressure and temperature is about 3 atmospheres at about 35° C. The feed rate is about 140 standard cubic meters per minute. The purge stream has a pressure of about 1 bara and a temperature of about 35° C. The flow rate for the purge stream is about 71 standard cubic meters minute. In addition to the aforementioned advantage of lower adsorbent requirement, maximum fluctuation in product flow rate from pre-purification unit 16 is reduced from about 9 to 1.3%. This improves the operation of the downstream air separation unit 20, one reason being that the fluctuation in the rate of air flow into the columns of the unit is reduced

Claims: An integrated air purification-air separation method comprising: separating air by compressing a feed air stream, cooling the feed air stream to a temperature suitable for its rectification, rectifying the air and thereby producing a waste stream; and prepurifying the feed air stream prior to said cooling thereof by pressure swing adsorption to produce a purified feed air stream; the pressure swing adsorption comprising subjecting each of at least three adsorbent beds to a repeating cycle including a feed stage to adsorb impurities contained within the feed air stream and thereby to produce said purified feed air stream, a depressurisation stage to initiate desorption of said impurities, a purge stage, comprising purging the adsorbent bed with the waste stream, to complete desorption of said impurities, and a repressurization stage to repressurise the beds; wherein the repeating cycle to which each bed is subjected is performed out of phase with the repeating cycles to which the other beds are subjected, and the duration of each individual purge stage is equal to the total duration of the individual cycle of which said individual purge stage forms a part divided by the total number of said adsorbent beds, and the waste stream is used continuously for purging the adsorbent beds. A method according to claim 1, wherein said waste stream comprises a waste nitrogen stream. A method according to claim 1 or claim 2, wherein said depressurisation stage is conducted by depressurizing the said adsorbent bed to atmospheric pressure. A method according to any one of the preceding claims, wherein said repressurization stage is conducted by introducing a part of said purified feed air stream into the said adsorbent bed. A method according to any one of the preceding claims, wherein said total number of beds is equal to three. A method according to claim 5, wherein the duration of each individual cycle is three times longer than the feed stage forming part of that cycle. A method according to claim 6, wherein the duration of the purge stage of the individual cycle is equal to the duration of the feed stage of that cycle.

Inventors: KUMAR RAVI, KUMAR, RAVI
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1996
Main classification: B01D53
IPC codes: B01D53, F25J3