Process for separating nitrogen gas by pressure swing adsorption system

Abstract: A process for separating nitrogen gas higher in purity than conventional products from a gas mixture predominantly composed of nitrogen gas and oxygen gas by the pressure swing adsorption system using carbon molecular sieve as adsorbent is characterized by the manner of communicating an adsorber after adsorption step with an adsorber after regeneration step for gas transfer from the former adsorber to the latter, the operation of discontinuing the gas transfer before establishment of pressure equalization, and the re­ generation promotion by causing back flow of part of product gas. The process can separate 99.99% pure nitrogen gas, which is never obtainable in the prior pressure swing adsorption processes, from air as raw material in an economic manner.

Description: This invention relates to a process for separating nitrogen gas by the pressure swing adsorption system (hereinafter referred to as "PSA system"). More particularly, it relates to a process for separating nitrogen gas of extremely high purity from a raw material gas mixture predominantly composed of nitrogen and oxygen, for example air, by means of an adsorber (adsorption bed) packed with a carbon (carbonaceous) molecular sieve (hereinafter referred to as "CMS") as an adsorbent. The process for separating nitrogen gas from such a mixed gas by the PSA system by utilizing the difference in rate of adsorption on CMS between oxygen gas and nitrogen gas is already in practical use. The equilibrium adsorption of oxygen gas and that of nitrogen gas on CMS are almost equal to each other under the same temperature and pressure conditions. Just after initiation of adsorption, however, these gases show a remarkable difference in rate of adsorption. Thus, oxygen gas and water vapor are each adsorbed on CMS almost to the equilibrium adsorption in about 60 seconds, while nitrogen gas and argon are adsorbed little. The separation of nitrogen gas by the PSA system utilizes this characteristic. The purity of nitrogen gas obtainable in practical processes using the PSA system is generally said to be at most 99.0% in the case of atmospheric pressure regeneration and at most 99.9% in the case of reduced pressure regeneration. It is regarded as difficult to separate nitrogen gas with a purity of 99.9% or higher in an economic manner by said system. It is a matter of course that any apparatus in which nitrogen gas having a high purity of more than 99.99% can be obtained is not known as yet. Therefore, in cases where nitrogen having a higher purity is required, it is a common practice to combinedly use a purification apparatus for converting the remaining 0.1 to 1.0% by volume of oxygen to water by reaction with hydrogen gas, ammonia or the like in the presence of a catalyst and removing the resulting water, or for removing such oxygen by contacting with a reducing metal. In operating such purification apparatus, however, a highly dangerous gas is used for reaction or regeneration, as mentioned above. In addition, there remain many problems, such as contamination with unreacted gases, and marked increases in cost of equipment and cost of operation. On the other hand, the prior art processes for producing high-purity nitrogen, typically the low temperature separation process, generally require a large-scale equipment, hence a large amount of investment. Accordingly, the site for plant construction is limited, and the cost of transportation to sites of consumption increases. For these reasons, the cost of nitrogen gas becomes very high. As mentioned above, the separation of nitrogen gas by the PSA system is advantageous in that nitrogen gas can be readily obtained at low cost and by relatively simple mechanisms but the product nitrogen gas has a limited range of applications because of its low purity. The purity may be increased by addition of a purification apparatus, but with many demerits, as mentioned above. DE 33 38 494 A1 discloses an adsorption process using a PSA system wherein subsequent to an adsorption phase the pressure in the adsorber is equalized via three pressure equalizing lines to an adsorber in which the regeneration is complete in such way that the respective inlet, outlet and middle portions of the adsorbers used are connected to each other. EP 0 121 042 A3 discloses a PSA process for the separation of nitrogen gas from pressurized air utilizing two adsorbers packed with CMS, and operable under the pressure swing adsorption system, which comprises the steps of: a) carrying out of the transfer of the gas under pressure remaining in an adsorber A in which the adsorption step is complete to an adsorber B in which the regeneration step under reduced pressure is complete through a piping connecting the raw material gas inlet of adsorber A to that of adsorber B and simultaneously through a piping connecting the product nitrogen outlet of adsorber A to that of adsorber B under reduced pressure and discontinuing the transfer before both adsorbers are equalized in pressure, b) pressurizing adsorber B and starting the adsorption step by introducing the raw material gas mixture into adsorber B, and c) carrying out the regeneration step in adsorber A, following the discontinuation of adsorber-to-adsorber gas transfer in step a), by releasing the gas remaining in said adsorber into the atmosphere to the neighborhood of atmospheric pressure followed by evacuation the adsorption and regeneration steps being alternately repeated in adsorbers A and B Accordingly, a gas supply source which can produce high-purity nitrogen gas while retaining the advantageous features of the PSA system has been highly demanded in the fields of chemical industry, food industry, semiconductor industry and metal heat treatment industry, among others. It is an object of the invention to provide a process for producing nitrogen gas, which is higher in purity than the prior art products obtained by the PSA system, from a gas mixture predominantly composed of nitrogen gas and oxygen gas by the PSA system using a carbon molecular sieve (CMS) as the adsorbent. This object is achieved by a process as defined in the patent claim 1. In this way, more than 99.99% pure nitrogen gas, which has never been obtained by the PSA system, can be separated in an economic manner. Fig. 1 is a schematic representation of an apparatus for nitrogen gas separation by the PSA system in which adsorber regeneration is carried out by means of a vacuum pump. A PSA system to which the present invention is applicable comprises the following constituent members: a plurality of adsorbers each packed with a carbon molecular sieve, a product stock tank for storing nitrogen gas separated, a vacuum pump, and pipelines and valves for connecting these members. A pressurized gas mixture predominantly composed of nitrogen gas and oxygen gas, for example air, enters one adsorber, where oxygen gas, water vapor and the like are adsorbed, and the unadsorbed components, such as nitrogen gas and argon, pass through the adsorber and enter the product stock tank and are further used for various purposes. The raw material gas supply is switched from one adsorber to another at certain time intervals, so that nitrogen gas can be obtained almost continuously. The gas in the adsorber in which the adsorption is complete is depressurized by discharge into the atmosphere or the adsorber is further evacuated by means of a vacuum pump, whereby the gases adsorbed are desorbed and the carbon molecular sieve is regenerated. Generally, prior to gas release into the atmosphere, the gas remaining in the adsorber and having a high nitrogen gas concentration is transferred to another adsorber in which the regeneration is complete. For this purpose, said adsorbers are brought into simultaneous communication of the product nitrogen outlet of the one adsorber with that of the other (hereinafter referred to as "upper communication") and of the raw material gas inlet of the one adsorber with that of the other (hereinafter referred to as "lower communication"). Since the gas transfer is generally carried out until the pressures in both adsorbers become equal to each other, this procedure is referred to as equalization. The prior art process which gives nitrogen gas having a maximum purity involves pressure equalization by connecting the adsorber inlets to each other and the adsorber outlets to each other. In that case, a decrease in space velocity results in a decrease in oxygen concentration in nitrogen gas but there is a certain limit to the decrease in said oxygen concentration; the purity of nitrogen gas cannot reach more than 99.99%. The present inventors made intensive investigations into the relationship between the nitrogen gas purity and the combination of various conditions in a process comprising discontinuing the gas transfer before the inside pressures in both adsorbers become equalized while varying the ratio between the gas transfer flow rates on the inlet and outlet sides and, as a result, found that nitrogen gas products having remarkably high purity can be obtained within a certain specific range of combination of such conditions. The present invention has been completed on the basis of this finding. By the way, such operation as varying the ratio between the gas transfer flow rates on the inlet and outlet sides has never been known at all in the prior art processes for separating nitrogen gas by PSA. The present invention provides a process for separating at least 99.99% pure nitrogen gas from a pressurized raw material gas mixture containing more than 78% of nitrogen gas by volume, oxygen and small amounts of other gases by the PSA system as mentioned hereinabove, which comprises the steps of: a) bringing the adsorber A in which the adsorption step is complete and the adsorber B in which the regeneration step under reduced pressure is complete into upper communication and lower communication simultaneously to thereby cause gas transfer from adsorber A under pressure to adsorber B under reduced pressure and discontinuing the gas transfer before both adsorbers are equalized in pressure, wherein the flow rate of gas transfer on the raw material gas inlet communication side is within the range of from about 3 to about 30% of the flow rate of gas transfer on the product nitrogen outlet communication side b) then bringing the product nitrogen outlet of adsorber B into communication with the inlet of a product tank to thereby cause back flow of product nitrogen into adsorber B and, when the gage pressure in adsorber B has arrived at about 50% or more of the maximum gage pressure during the adsorption step, starting the adsorption step by introducing the raw material gas mixture into adsorber B, and c) carrying out the regeneration step in adsorber A, following the discontinuation of adsorber-to-adsorber gas transfer in step a), by releasing the gas remaining in said adsorber into the atmosphere to the neighborhood of atmospheric pressure followed by evacuation to about 19998 Pa (150 torr) or below by means of a vacuum pump, the adsorption and regeneration steps being alternately repeated in adsorbers A and B, the time required for said regeneration step and for said adsorption step inclusive of the product nitrogen back flow process being each within the range of about 90 seconds to about 180 seconds. Referring to Fig. 1, the invention is now described in further detail. The pressurized raw material gas mixture enters adsorber 1 (adsorber A) via valve 5. Oxygen and other components to be removed are adsorbed on a CMS packed in the adsorber, and the thus-separated product nitrogen enters product tank 3 via valve 9 and is stored temporarily in said tank and then consumed. The pressure in said adsorber influences the purity of product nitrogen. The results of experiments made by the present inventors revealed that, for the process to be practical, said pressure should reach at least about 5.88 bar (5 kg/cm² (G)) and that the effect of the pressure is little in the range of 10.78 bar 10 kg/cm² (G)) and higher. Therefore, the range of 6.86-10.78 bar (6-10 kg/cm²(G)) is preferable. Furthermore, fluctuations of the pressure of the feed mixture gas should be avoided as far as possible. In particular in the separation of high purity (at least 99.99% pure) nitrogen, desorption, in small quantities, of oxygen and other components adsorbed on CMS, insufficient adsorptive separation due to the pressure difference within the adsorber and contamination with the low purity nitrogen remaining in the adsorber are crucial inhibitory factors. For reducing the quantity of raw material gas as required per unit quantity of product nitrogen, the adsorber 1 (adsorber A) in which the adsorption step is complete and the adsorber 2 (adsorber B) in which the reduced pressure regeneration step is complete are brought into upper communication and at the same time into lower communication by opening valves 11 and 12 following closure of valves 5 and 9 and the gas remaining in adsorber 1 (adsorber A) under elevated pressure and having a high nitrogen content is thus caused to migrate into adsorber 2 (adsorber B) under reduced pressure. The present inventors investigated the influences of various means of pressure equalization, such as upper equalization, lower equalization, upper and lower equalization, and serial equalization, on the purity of product nitrogen and, as a result, it was found that the nitrogen purity is lowest in the case of serial equalization, next lowest in the case of lower equalization, and increases in the order of upper equalization and upper and lower equalization. The term "serial equalization" as used herein means a gas transfer procedure which comprises bringing the product gas outlet of the adsorber in which the adsorption step is complete into communication with the raw material gas inlet of the adsorber in which the regeneration step is complete. Furthermore, it was found that, in upper and lower equalization, there is a region in which highest nitrogen purities can be obtained by adjusting the equalization conditions such that the gas transfer is discontinued before adsorbers are equalized in pressure and the rate of flow of the gas migrating through the upper communication portion is higher than that of the gas migrating through the lower communication portion. In this procedure, it is necessary that the gas flow rate in the lower communication portion is within the range of about 3 to about 30% of the gas flow rate in the upper communication portion and the orifices 15 and 16 serve to adjust these flow rates. This gas transfer must be discontinued before attainment of pressure equalization of both adsorbers. The reasons are not yet certain but it is presumable that the desorption of components adsorbed on CMS increases rapidly after pressure equalization. Valves 11 and 12 are closed and the gas transfer is thus discontinued when the absolute pressure ratio between the adsorber 2 (B), which is the gas receiving side, and the adsorber 1 (A), which is the gas supplying side, has arrived at a value within the range of 0.10 to 0.70, preferably 0.15 to 0.40. The diameters of orifices 11 and 12 are adjusted so that the time required for said gas transfer can be within 2 seconds. Then, the valve 10 is opened and product nitrogen is thus allowed to flow back from product tank 3 to adsorber 2 (B). Such flow back procedure has been used in the art for the purpose of rinsing low purity nitrogen retained in the piping in the upper communication portion after pressure equalization of both adsorbers. The quantity of back flow is not so large, however, since large quantities cause pressure fluctuations in the product tank. In the art, the flow back procedure is carried out simultaneously with raw material gas mixture feeding. In accordance with the invention, the flow back procedure is performed for the purpose of inhibiting formation of insufficiently separated nitrogen gas due to a great difference between the adsorber inside pressure and the pressure of raw material gas mixture in the initial phase of adsorption procedure rather than for the purpose of rinsing. Therefore, it is when, as a result of the back flow procedure, the gage pressure within adsorber 2 (B) has arrived at a level of about 50% or more, preferably 70% or more, of the maximum attainable gage pressure in the adsorption step that the adsorption procedure is started by opening the valve 7 and feeding the raw material gas mixture to adsorber 2 (B). The capacity of product tank 3 should preferably be selected such that the gage pressure in product tank 3 can be maintained at a level not lower than about 70%, more preferably not lower than 80%, of the maximum gage pressure attainable in the adsorber in the adsorption step even during the period of outflow of product nitrogen gas for consumption at a constant rate and simultaneous back flow to adsorber 2 (B). On the other hand, the regeneration of adsorber 1 (A) is carried out until the adsorption step in adsorber 2 (B) is completed, by opening the valves 6 and 13 following discontinuation of the adsorber-to-adsorber gas transfer, thus releasing the gas adsorbed and the gas remaining in said adsorber into the atmosphere until a pressure close to atmospheric pressure, then closing the valve 13, opening the valve 14 and evacuating said adsorber to a level within the range of about 0.2 bar (150 torr) and below, preferably 0.07-0.14 bar (50-100 torr), by means of vacuum pump 4. Thereafter, the adsorption and regeneration steps are repeated alternately in adsorber 1 (A) and adsorber 2 (B) in 180° phase shifts. The optimum period for said regeneration procedure or for the adsorption procedure inclusive of the product nitrogen gas flow back process, namely the optimum half-cycle period, is within the range of about 90 to about 180 seconds as compared with 60-90 seconds in the case of prior art processes for producing 99.0-99.9% pure nitrogen using the same CMS. The space velocity is an important dynamic index indicative of the efficiency of a nitrogen gas separation system based on PSA. The space velocity is defined as the flow rate of product nitrogen (measured at standard conditions) per minute per unit volume of CMS under constant adsorption procedure, namely of CMS packed in one adsorber in the case of the present invention. The performance characteristics of CMS itself has an influence on this characteristic. When the space velocity has a small value, the capacity required of the adsorber is great and, accordingly, the quantity of raw material gas mixture per unit quantity of product nitrogen increases because of an increase in quantity of gas released into the atmosphere, among others. The effect is not confined to the adsorber but extends to the compressor for raw material gas mixture, the vacuum pump and so forth, making the whole equipment large-scaled and causing the cost of operation, in particular the electric fee, to increase. Thus the above situation becomes a fatal inhibitory factor to practical use of the process. In the process according to the invention, it is premised that the space velocity should be not less than about 0.6 min⁻¹, preferably not less than 1.2 min⁻¹. Thus it is possible to install an equipment having a capacity of 1 to 1,000 Nm³/hr on any site demanding nitrogen and to supply high purity (at least 99.99% pure) nitrogen at low cost. The following examples merely illustrate the present invention. EXAMPLE 1 A PSA-based nitrogen gas separation apparatus as shown was used. Each adsorber was packed with a CMS showing an oxygen adsorption of 7.7 cm³/g in equilibrium at 25°C and one atmosphere and an oxygen/nitrogen adsorption ratio of 8.44 as measured after 60 seconds of adsorption. Air pressurized to 9.31 bar (8.5 kg/cm² (G)) was used as the raw material and the half-cycle period for adsorption or regeneration was 120 seconds. The minimum pressure attained in the regeneration step was 10665.6 Pa (80 torr). The gas transfer from the adsorber in which the adsorption step was complete to the adsorber in which the reduced pressure regeneration step was complete was carried out while the flow rate through the lower communication was adjusted to 6% of the flow rate through the upper communication. The gas transfer was discontinued when the ratio of the absolute pressure in the gas receiving side adsorber to that in the gas supplying side adsorber became 0.25. The time required for this gas transfer was 1.2 seconds. Then, product nitrogen was caused to flow back from the product tank to said receiving side adsorber for 0.2 second and, when the pressure in said adsorber arrived at 85% of the maximum gage pressure attained in the adsorption step, the feeding of raw material pressurized air was started. Even in the flow back step during which product nitrogen was supplied to an external equipment continued at a rate of 10 Nm³/h, the pressure in the product tank could be maintained at levels not lower than 85% of the maximum gage pressure attained in the adsorber under adsorption procedure. The capacity of the product tank used was 4 times that of the adsorbers. As for the quality of the product nitrogen obtained in this example, a residual oxygen concentration of 73 ppm and a dew point at atmospheric pressure of -70°C or below were obtained constantly at a space velocity of 1.5 min⁻¹. When the rate of supply from product tank to external equipment was reduced to 5 Nm³/h, the residual oxygen concentration in product nitrogen decreased to 5.7 ppm. COMPARATIVE EXAMPLE 1 A PSA-based nitrogen gas separation apparatus comprising two adsorbers packed with CMS as shown in Fig. 1 was used and nitrogen was separated from air pressurized to 7.84 bar (7 kg/cm² (G)) by performing alternately the adsorption procedure and the regeneration procedure under a reduced pressure of 10665.6 Pa (80 torr)(minimum achievable value). The half-cycle period was 100 seconds. The flow rate ratio in the gas transfer between the adsorber in which the adsorption step was complete and the adsorber in which the regeneration step was complete was equal. The adsorber-to-adsorber gas transfer was discontinued when the absolute pressure ratio of the gas in receiving adsorber to supplying side adsorber was 0.42. The product nitrogen qualities (in terms of residual oxygen concentration in ppm obtained in this procedure was 870 ppm. The raw material air was fed at a rate such that the sum total of the quantity of gas transferred in the above manner and the quantity of product nitrogen caused to flow back from the product tank into the gas-receiving side adsorber corresponded to 80% of the whole gas quantity in said adsorber corresponding to the maximum pressure in the adsorber. The product tank capacity was such that the gage pressure in the tank could be always maintained at levels not lower than 85% of the maximum pressure in each adsorber. Other basic conditions were the same as in Example 1.

Claims: A process for separating at least 99,99% pure nitrogen gas from a pressurized gas mixture containing more than 78% of nitrogen gas by volume, oxygen and small amounts of other gases in a nitrogen gas separation apparatus comprising two adsorbers packed with a carbon molecular sieve and operable under the pressure swing adsorption system, which comprises the steps of: a) carrying out of the transfer of the gas under pressure remaining in an adsorber A in which the adsorption step is complete to an adsorber B in which the regeneration step under reduced pressure is complete through a piping connecting the raw material gas inlet of adsorber A to that of adsorber B and simultaneously through a piping connecting the product nitrogen outlet of adsorber A to that of adsorber B under reduced pressure and discontinuing the transfer before both adsorbers are equalized in pressure, wherein the flow rate of gas transfer on the raw material gas inlet communication side is within the range of from about 3 to about 30% of the flow rate of gas transfer on the product nitrogen outlet communication side b) then bringing the product nitrogen outlet of adsorber B into communication with the inlet of a product tank to thereby cause back flow of product nitrogen into adsorber B and, when the gage pressure in adsorber B has arrived at about 50% or more of the maximum gage pressure during the adsorption step, starting the adsorption step by introducing the raw material gas mixture into adsorber B, and c) carrying out the regeneration step in adsorber A, following the discontinuation of adsorber-to-adsorber gas transfer in step a), by releasing the gas remaining in said adsorber into the atmosphere to the neighborhood of atmospheric pressure followed by evacuation to about 19998 Pa (150 torr) or below by means of a vacuum pump, the adsorption and regeneration steps being alternately repeated in adsorbers A and B, the time required for said regeneration step and for said adsorption step inclusive of the product nitrogen back flow process being each within the range of about 90 seconds to about 180 seconds. A process for separating nitrogen gas as claimed in Claim 1, wherein the gas transfer from adsorber A to adsorber B in step a) is discontinued when the ratio of the absolute pressure in the gas receiving side adsorber B to the absolute pressure in the supplying side adsorber A, has arrived at the range of about 0.10 to about 0.70. A process for separating nitrogen gas as claimed in Claim 1 or 2, wherein the period for the gas transfer from adsorber A to adsorber B is not longer than 2 seconds. A process for separating nitrogen gas as claimed in Claim 1, 2 or 3, wherein the maximum pressure attained in the step of adsorption and separation of the raw material gas mixture is at least about 5.88 bar (5 kg/cm²(G)). A process for separating nitrogen gas as claimed in Claim 1, 2, 3 or 4, wherein the space velocity is at least 0.6 min⁻¹.

Inventors: GEMBA TSUNEO, MATSUURA TETSUHIKO, TAMARU TAKESHI, GEMBA, TSUNEO, MATSUURA, TETSUHIKO, TAMARU, TAKESHI
Applicants: KURARAY CHEMICAL KK, KURARAY CHEMICAL CO., LTD.
Application year: 1989
Main classification: B01D53
IPC codes: B01D53, C01B21