Process for producing oxygen enriched product stream

Abstract: A two bed pressure swing adsorption process is disclosed having high yield and high production rate. The process utilizes fine or normal size zeolite sieve material and relatively short cycle times. A power saving is obtained by sequencing the various steps of the process to provide continuous use of a vacuum pump 16.

Description: This invention relates to a process for obtaining an oxygen enriched gas from a gas mixture containing principally oxygen and nitrogen as gas components, such as air, by means of pressure swing adsorption (PSA). Pressure swing adsorption systems and process have been widely used to produce oxygen enriched streams from mixed gases, including air, and a multitude of such systems and processes have been utilized. It is advantageous, in such systems, to utilize a relatively short time cycle for carrying out the process inasmuch as shorter times obtain good utilization of the sieve material used to adsorb one of the components. The short cycle times generally employ a finer particle size of sieve material to reduce diffusive resistance. Typical examples of short cycle times are shown and described in U.S. Patents 4,194,891 and 4,194,892. Oxygen production increased with the process in the aforementioned patents however the yield was fairly low, e.g., 10-20% yields. Conventional vacuum PSA processes do produce a higher oxygen yield (50-60%) however the production rate is somewhat low. The normal production rate of conventional three bed PSA processes is not particularly high; as an example, typically, the bed size factor is about 2000-2600 kg of zeolite per metric ton of oxygen produced per day. Optimally, one would obviously like to obtain the high production rate typified by faster cycle times and finer sieve particles along with the high yield typified by conventional three bed systems yet have a process that is inexpensive and relatively simple in operation. EP 334 495 discloses a two bed PSA apparatus for recovering an oxygen enriched gas from a mixed gas mainly comprising nitrogen and oxygen. The two adsorbers A and B are used in turn to adsorb nitrogen gas whilst product oxygen is passed to a gas reservoir C. Whilst such an apparatus is able to produce product gas of a comparatively high purity and yield, the cycle times are somewhat slow and the overall system is less efficient than would be desired. It is an aim of the present invention to provide a PSA process which achieves an enriched oxygen product that has a high oxygen yield as well as high production rate by solving the disadvantages of the prior art while minimising cost and maintaining simplicity of operation. According to one aspect of the present invention a process for producing an oxygen enriched product stream from a feed gas containing at least oxygen and nitrogen utilising two adsorption columns containing an adsorbent capable of selective adsorption of nitrogen and a product reservoir is characterised in that it consists of: (i) introducing gas into the outlet of a first adsorption column at a low pressure from the outlet of a second adsorption column at a higher pressure to substantially equalise the pressures in the columns while withdrawing gas from the inlet of the second column; (ii) after equalisation of step (i), introducing product gas contained within the product reservoir at a higher pressure into the outlet of the first column to backfill the first column while continuing to withdraw gas from the inlet of the second column; (iii) introducing feed gas into the inlet of the first column at a minimum predetermined pressure and recovering the product of enriched oxygen gas from the outlet of the first column to introduce said enriched oxygen stream to the product reservoir, while introducing a portion of the product stream to the outlet of the second column to purge the second column and simultaneously withdrawing gas from the inlet of the second column to desorb and evacuate nitrogen rich gas from the second column; (iv) introducing gas into the outlet of the second column from the outlet of the first column at an initially high pressure to equalise the pressures in the columns while withdrawing gas from the inlet of the first column; (v) after equalisation of step (iv), introducing oxygen enriched product gas contained within the product reservoir at a higher pressure into the outlet of the second column while continuing to withdraw gas from the inlet of the first column; (vi) introducing feed gas into the inlet of the second column at a predetermined minimum pressure and recovering the product of enriched oxygen gas from the outlet of the second column to introduce said enriched oxygen stream to the product reservoir while introducing a portion of the product stream to the outlet of the first column to purge the first column while simultaneously withdrawing gas from the inlet of the first column to desorb and evacuate nitrogen rich gas from the first column; and (vii) cyclically repeating steps (i) to (vi) while withdrawing oxygen enriched product gas from the product reservoir. According to a further aspect of the present invention an apparatus for carrying out the continuous production of an oxygen enriched stream of gas from a feed stream of air comprises two adsorption columns to cycle respectively said two adsorption columns between receiving said feed stream of air and producing said enriched oxygen stream, characterised by a vacuum pump, said vacuum pump being connected to both of said adsorption columns and adapted to withdraw gas continuously from one or the other of said adsorption columns during said repetitive cycle. The system uses short cycle times to gain good usage of the sieve material but requires only two beds, thus greatly simplifying the prior three-bed processes that heretofore were needed for high yields. As will be shown, one of the further features is the power saving by continuous operation of a vacuum pump with a two column system by simultaneously carrying out a equalization step supplying an oxygen enriched stream from the product to the outlet end of one column and a desorption to evacuate nitrogen rich gas from the inlet end of the same column. By this means, the vacuum pump is run continuously throughout this step as well as the remaining steps. Since the vacuum pump is not running idle or unused during any step of the overall process, its use, and thus its power consumption, is optimized. Thus, the two bed PSA process having high yield and production rate is achieved using relatively, fine particles of zeolite sieve material, such as 20-35 mesh size and even with larger particles such as 8-12 mesh size at short cycle times less than 30 seconds, preferably about 15-25 seconds. The range of pressure swing uses vacuum for desorption less than 300 mm Hg, preferably to 200 mm Hg and maximum product pressure less than 5 psig and preferably less than 3 psig. An embodiment of the present invention will now be described, by way of example, reference being made to the Figures of the accompanying diagrammatic drawings in which:- Figure 1 is a schematic flow diagram of the present invention; and Figure 2 is a column cycle of the present invention. Referring to Figure 1 and Figure 2, there is shown an apparatus for producing an enriched oxygen gas stream continuously generated from a gas containing principally oxygen and nitrogen, such as air. Each of the two adsorption columns A and B contains an adsorbent capable of selectively adsorbing nitrogen. The process is performed with relatively fine particles of zeolite of about 8-35 mesh, preferably about 12-20 mesh. Typical zeolite sieve material is available from various zeolite manufacturers in the form of beads or pellets. Control of each of the steps can be regulated by conventional means, e.g., a timer to control solenoid operated valves of standard commercial design. In step , valves 1A and 2A are closed, thereby closing off the lower or inlet end of the first column A. At the top, or outlet end of column A, valves 4A and 5A are closed and valve 3A is open. With respect to the second column B, valves 9B and 10B are closed, thus gas from the outlet of column B is being introduced into the outlet of column A through valve 3A and the flow is controlled by valve 8B. At the same time in Step 1, at the inlet end of column B valve 6B is closed but valve 7B is open, thus gas is being withdrawn from the inlet end of column B by means of vacuum pump 16. During this step, the pressure in column B which is initially higher than the pressure in column A is used to equalize the pressures in both columns, that is, column B may, at the beginning of step 1, be at a positive pressure of about 1010 mm Hg (4.84 psig) and is reduced to about 500 mm Hg (-5.03 psig) by the end of step 1 while column A commences step 1 at a pressure of about 260 mm Hg (-9.67 psig) and the pressure is raised to 470 mm Hg (-5.61 psig). At the end of step 1, therefore, the column pressures are essentially equalized. Step 1 may take place extremely rapidly, and preferably from about 2 seconds to 6 seconds and more preferably in about 4 seconds. In Step 2, valve 3A is closed and valve 5A and valve 24 opened and oxygen enriched product gas from the reservoir 18 is introduced into the outlet of column A and is controlled by metering valve 26 to backfill column A and to raise the pressure further in column A. Typically, since the pressure in product reservoir is about 800 mm Hg, the pressure within column A continues to increase from 470 mm Hg (-5.61 psig) to about 660 mm Hg (-l.93 psig). At the same time, gas is still being withdrawn from the inlet of column B through open valve 7B by vacuum pump 16 and the pressure within column B continues to decrease. That pressure may decrease, in step 2, from 500 mm Hg (-5.03 psig) to about 450 mm Hg (-6.00 psig). Again, the timing of step 2 is extremely rapid, preferably being from about 1 to 5 seconds, and more preferably about 3 seconds. In step 3, valve 1A is opened and air, or other feed gas containing principally oxygen and nitrogen under a predetermined feed pressure, is introduced to the inlet of column A. Inlet pressures may vary but the inlet pressure should have a minimum predetermined pressure of between 3 and 7 psig, and preferably about 5 psig. At the outlet of column A, valve 5A is closed and valve 4A is opened, thus the feed air passes through column A where nitrogen is adsorbed and an oxygen enriched product stream passes from the outlet end of column A, through valve 4A, check valve 11 and through line 20 to the product reservoir 18. During this step, the pressure in column A may increase from 660 mm Hg (-l.93 psig) to about 1010 mm Hg (4.84 psig) in producing oxygen enriched product. At the same time, in step 3, gas continues to be withdrawn from the inlet end of column B to desorb or evacuate nitrogen enriched gas from column B by means of vacuum pump 16. Simultaneously, with the desorption of column B via its inlet, valve 10B is open and valve 24 open and oxygen enriched product stream is introduced into the outlet of column B to purge column B. The flow of the oxygen enriched product stream is controlled by metering valve 26. Thus column B is both purged by an oxygen enriched stream of gas introduced into its outlet and desorbed by withdrawing nitrogen rich gas from its inlet. The pressure in column B thus continues to decrease, typically from about 450 mm Hg (-6.10 psig) to about 260 mm Hg (-9.67 psig) as the withdrawal by means of vacuum pump 16 continues uninterrupted. Step 3 is also carried out quite rapidly, in a range of cycle time preferably of about 10 seconds to 25 seconds and more preferably about 18 seconds. Continuing on to step 4, equalization again takes place, this time by introducing the now higher pressure gas of column A into column B. That is carried out by closing valve 4A at the outlet of column A and opening valve 3A. At the outlet of column B, valve 10B is closed and thus, gas passes from column A to column B for equalization of pressures controlled by metering valve 8B. At the same time, of course, the feed stream is cut off by closing valve 1A at the inlet to column A and valve 2A is opened so that gas can be withdrawn from the inlet end of column A through vacuum pump 16. The inlet of column B is closed completely by closing valve 7B. In step 4, therefore, the pressures within column A and B are approximately equalized, the pressure in column A is reduced from about 1010 mm Hg (4.84 psig) to about 500 mm Hg (-5.03 psig) while the pressure in column B is increased from about 260 mm Hg (-9.67 psig) to about 470 mm Hg (-5.61). Step 4 is carried out preferably in a time of from about 2 to about 6 seconds, and more preferably in about 4 seconds. In step 5, valve 3A is closed, thus closing entirely the outlet end of column A while gas continues to be withdrawn from the inlet end of column A drawing the pressure down, typically, from 500 mm Hg (-5.03 psig) to a further 450 mm Hg (-6.00 psig). Valve 10B and 28 are opened and oxygen enriched product from product reservoir 18 enters column B to backfill that column controlled by metering valve 30 such that the pressure in column B is increased from, typically, about 470 mm Hg (-5.61 psig) to about 660 mm Hg (-l.93 psig). Again, as in step 2, the backfilling step takes place in from about 1 to 5 seconds, and preferably in about 3 seconds. Finally, in step 6, valve 6B is opened, thus introducing the pressurized feed stream into the inlet of column B. Valve 9B is opened so that the oxygen enriched product stream from column B passes through check valve 12 and continues via line 20 to product reservoir 18. During step 6, valves 24 and 5A are opened to allow oxygen enriched gas to enter the outlet of column A to purge column A, controlled by metering valve 26. Simultaneously, with the purging of column A, gas continues to be withdrawn from the inlet of column A by vacuum pump 16 to desorb or evacuate nitrogen rich gas. Typically, again, the pressure within column A decreases from about 450 mm Hg (-6.00 psig) to about 260 mm Hg (-9.67 psig) while the pressure in column B increases from about 660 mm Hg (-l.93 psig) to about 1010 mm Hg (4.84 psig). The timing of step 6 can be from about 10 to about 25 seconds and preferably in about 18 seconds. At the completion of step 6, the entire sequence is repeated on a continual cyclic basis so that product is continuously taken from product reservoir 18 through valve 32 during each of the steps. As can be seen, the vacuum pump 16 is also continuously utilized to withdraw gas alternatively, from one or the other of the two columns, thus it is efficiently utilized to minimize power use throughout the process. EXAMPLES Using the apparatus described, with the sequence of steps as outlined, an operation of this method was conducted to obtain an oxygen enriched product stream. Two adsorption columns, A and B, each 2 inches in diameter and 15 inches in height were packed with Calcium X zeolite molecular sieve material in the form of beads commercially available from Laporte Co. for three runs and zeolite material in the form of pellets from Tosoh Company for one run. The production rate, and yields were obtained in accordance with the following table: Molecular sieves material (Zeolite) Laporte Tosoh Zeolum SA Size of MS 0.4-0.8 mm (beads) 1.5mm (pellets) Range of pressure swing 3.5 psig to-200 mm Hg Cycle times (second) 25 20 17 25 Oxygen purity (%) 93 93 93 93 Bulk density (kg/cm3) 681 620 Oxygen yield (%) 59 58 56 55 specific product (standard liter/hr of produced oxygen/liter of bed) 40 51 59 40 Bed size factor (kg zeolite per metric tons of oxygen per day) 535 420 375 487 From the above test, it can be seen that a high yield, high production rate at a high purity can be achieved by a two column system where vacuum is continuously applied to one or the other of the columns during each of the steps, thus the vacuum pump is efficiently used and power conserved. The cycles are extremely rapid to achieve good utilization of the sieve material, yet the construction and operation of the two bed system is obviously more advantageous than the more complex, more expensive three bed systems.

Claims: A process for producing an oxygen enriched product stream from a feed gas containing at least oxygen and nitrogen utilising two adsorption columns (A,B) containing an adsorbent capable of selective adsorption of nitrogen and a product reservoir (18) characterised in that it consists only of the steps of: (i) introducing gas into the outlet of a first adsorption column (A) at a low pressure from the outlet of a second adsorption column (B) at a higher pressure to substantially equalise the pressures in the columns while withdrawing gas from the inlet of the second column (B); (ii) after equalisation of step (i), introducing product gas contained within the product reservoir (18) at a higher pressure into the outlet of the first column (A) to backfill the first column (A) while continuing to withdraw gas from the inlet of the second column (B); (iii) introducing feed gas into the inlet of the first column (A) at a minimum predetermined pressure and recovering the product of enriched oxygen gas from the outlet of the first column (A) to introduce said enriched oxygen stream to the product reservoir (18), while introducing a portion of the product stream to the outlet of the second column (B) to purge the second column and simultaneously withdrawing gas from the inlet of the second column (B) to desorb and evacuate nitrogen rich gas from the second column (B); (iv) introducing gas into the outlet of the second column (B) from the outlet of the first column (A) at an initially high pressure to equalise the pressures in the columns (A, B) while withdrawing gas from the inlet of the first column (A); (v) after equalisation of step (iv), introducing oxygen enriched product gas contained within the product reservoir (18) at a higher pressure into the outlet of the second column (B) while continuing to withdraw gas from the inlet of the first column (A); (vi) introducing feed gas into the inlet of the second column (B) at a predetermined minimum pressure and recovering the product of enriched oxygen gas from the outlet of the second column (B) to introduce said enriched oxygen stream to the product reservoir (18) while introducing a portion of the product stream to the outlet of the first column (A) to purge the first column (A) while simultaneously withdrawing gas from the inlet of the first column (A) to desorb and evacuate nitrogen rich gas from the first column (A); and (vii) cyclically repeating steps (i) to (vi) while withdrawing oxygen enriched product gas from the product reservoir 18. A processes as claimed in Claim 1 characterised in that said feed gas introduced in steps (iii) and (vi) is air at a pressure of from about 0 mm Hg to 370 mm Hg (0 to about 7 psig) and said adsorbent is a fine particle zeolite of about 8-35 mesh. A process as claimed in Claim 2 characterised in that said fine particle zeolite is in the form of beads or pellets having a size of between about 12 and about 20 mesh. A process as claimed in Claim 1, 2 or 3 characterised in that said steps (i) and (iv) take place in about 2 to 6 seconds; said steps (ii) and (v) take place in about 1 to 5 seconds; and said steps (iii) and (vi) take place in about 10 to 25 seconds. A process as claimed in Claim 4 characterised in that said steps (i) and (iv) take place in about 4 seconds; said steps (ii) and (v) take place in about 3 seconds; and said steps (iii) and (vi) take place in about 18 seconds. A process as claimed in any one of Claims 1 to 5 characterised in that said step (i) and (iv) include equalizing the pressures in the columns (A, B) to a pressure of from about 470 to about 500 mm Hg; said product gas in steps (ii) and (v) is introduced at a pressure of about 800 mm Hg. A process as claimed in Claim 6 characterised in that said feed gas is introduced in steps (iii) and (vi) at a pressure of less than about 5 psig. A process as claimed in any one of Claims 1 to 7, characterised in that said steps (i) through (vi) are carried out in less than about 30 seconds.

Inventors: HIROOKA EIJI, HIROOKA, EIJI
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1991
Main classification: B01D53
IPC codes: C01B13, B01D53