PROCESS AND PLANT FOR THE SEPARATION OF NITROGEN AND OXYGEN FROM GASES RICH OF NITROGEN AND/OR OXYGEN

Abstract: Process for the separation of nitrogen or oxygen starting from gases rich of nitrogen and oxygen, preferably air, including the following steps: a) separation of nitrogen/oxygen by means of molecular sieves preferably, filtered and pressurised air flowing from the bottom of a first vessel containing molecular sieves counter current to a flow of pure separated gas; removing of the gas separated in the first vessel and delivering to a buffer tank; simultaneous regeneration of the second bed by means of pressure reduction, purging with a portion of pure separated gas and venting from the bottom of the secondary gas adsorbed; b) pressure equalisation in the two vessels; c) regeneration of the molecular sieves of the first vessel analogously to what described for the second bed. Furthermore, the invention concerns a plant for executing the process.

Description: PROCESS AND PLANT FOR THE SEPARATION OF NITROGEN AND OXYGEN FROM GASES RICH OF NITROGEN AND/OR OXYGEN Field of the invention The present invention concerns the field of the autonomous production of nitrogen and oxygen, with particular reference to the plants and the processes for the production of nitrogen or oxygen for applications in various fields of activity (nitrogen: wine manufacturing; laser cutting of metals; chemical industry, petroleum industry, pharmaceutical industry, food packaging and preservation, heat treatment of metals, aeronautics, aluminium die-casting, blanketing in general, etc.; and oxygen: welding, metal cutting, manufacturing of glass, burning plants, medical applications etc.) Known art At present the mostly used process for the "on-site" production of nitrogen and oxygen is the so-called PSA (Pressure Swing Absorption) method. This method is based on the capacity of some materials, called molecular sieves, to retain the oxygen (or the nitrogen) contained in a mixture of N2 and 02, generally air, and to selectively separate the one from the other so that the separated gas can be recovered and the other one (secondary gas) can be removed. In this process, the separation of nitrogen/oxygen is performed by variable timed cycles in function of the type of the molecular sieve used (Carbon Molecular Sieve (CMS) for the production of nitrogen and zeolite sieves for the production of oxygen). During these cycles the air alternatively flows through two molecular sieves, or beds. In the process, the filtered and compressed air flows through the first of the two vessels containing the sieves, and the nitrogen/oxygen obtained is delivered into a buffer tank, except for a portion of gas which is drawn and introduced from the top, with opposite flow, into the second vessel, in this step we have the regeneration of the bed in the second vessel and the discharge from the bottom of a gas mixture consisting of the oxygen/nitrogen adsorbed and of the air present in the bed interstices. At this point, the regenerated bed vessel, closed to the outside, is put into communication with the other vessel until pressures are equalized. In the subsequent step the pressurized air is sent through the lower group of valves to the second vessel, and the cycle continues analogously to what already described. A problem of fundamental importance inherent the above described PSA technology is that increasing the purity grade of the separated gas, both the productivity P (expressed as Nm3 / hr of gas produced per m3 of molecular sieve) and the efficiency E (expressed as % ratio between gas produced/gas introduced with the air) of the plants decrease considerably. In particular, in the nitrogen PSA plants and process, the target of obtaining an extremely pure product, with a 02 residual content lower than 0.1% ( .OOOppm), led to several improvements and modifications. A first modification concerns the timing of the pressure equalizing step and has been introduced by the patent EP 0121042. In this process the vessel of the bed which has already been regenerated and purged with pure nitrogen is put into communication with the exhausted bed vessel which contains, under pressure, a gaseous mixture with 02 contents superior to those of pure nitrogen. The invention aims to limit the equalizing time between 0.3 and 0.7 sec, so to reduce as much as possible the transfer of oxygen to the top of the regenerated bed and to produce nitrogen with oxygen residuals between 10 and 1000 ppm. The cited patent in the examples and in the graphics shows the following results: Another modification has been suggested by Ravi Jain and Shain Doong of the BOC Group. In this process in submitting the already regenerated and purged bed is submitted to a further purge with higher purity nitrogen, i.e. with a 02 content of 0.0%, coming from another source such as: cryogenic nitrogen or 02 free nitrogen coming from a catalytic purifying process (DEOXO). The results are as follows (p=8,46 bar, temperature 20°C). With the patent no. EP0433324 a process modification has been introduced in order to increase, with the same purity of the nitrogen product, Productivity P (Nm3/hr of nitrogen per m£ of CMS) and/or the Efficiency E (nitrogen produced / nitrogen fed). The modification consists of the anticipated shutting up of the venting valve of the bed vessel which is under regeneration. In this way the bed is submitted to a "rest time" equal to 20%-80% (preferably 35%-65%) of the desorption total time The best results obtained, also compared to the performances of other patented processes, are the following: The optimisation of Productivity P and Efficiency E in PSA plants is of pre-eminent importance, because the plant dimensions and then the volume of the molecular sieves (and therefore the capital expenditure needed for their installation) increases as P decreases and the demand of energy for air compression increases as E decreases: therefore, the higher P and E, the lower the nitrogen production total cost. Scope of the invention The aim of this invention is to overcome the limits of the already known PSA plants and processes for the separation from a nitrogen/oxygen enriched mixture, for example air, of nitrogen or oxygen. In particular an aim is to improve the performances of the PSA plants in terms of purity of the nitrogen/oxygen obtained (up to 10 ppm residual 02 content for the nitrogen process) without incurring in the penalization of efficiency E (Nm3 separated gas / Nm3 fed gas) and productivity P (Nm3 gas/hr x m.3 of molecular sieve) of the plant. Summary of the invention Said scopes have been achieved, according to the present invention, actively controlling the velocity of the wave front of the inlet pressurized air. This solution overcomes the problem of the longitudinal diffusion of the gas to be removed (secondary gas) through the molecular sieves and the contamination of the same with the secondary gas originated by the equalization step. According to the process, a part of the separated gas is fed back from a buffer tank containing the already produced gas to the top part of the adsorbing bed of the vessel in which the separation is taking place, in order to form a "wave front" opposite to the shock wave generated in the filtering bed by the inlet pressurized air. Furthermore, according to the invention, also the velocity of the inlet air front is modulated below a predetermined value. The process includes the following steps: A) separation of oxygen and nitrogen, by means of molecular sieves, of the already filtered and compressed air, flowing through a first vessel containing the molecular sieves; removing of the gas separated and delivery of the same to a buffer tank; at the same time the molecular sieves of a second vessel are regenerated by means of pressure reduction, purging with a portion of the separated gas product and discharge to atmosphere of the secondary gas adsorbed; B) equalizing of the pressures in the two vessels; C) regeneration of the bed of the first vessel analogously to what described for the second bed. The subsequent steps cyclically repeat steps A-B-C alternatively in the two vessels. According of the invention, in a nitrogen production process the pressure in the adsorption step reaches values preferably greater than 10.5 bar (a). The action of a wave front of pure separated gas which comes from the top of the bed which is starting the pressurization-production step, and is directed counter- current to the air flow introduced from the bottom involves several advantages: - elimination/reduction to the minimum of the secondary gas residual content in the top part of the bed, both in the gaseous and in the solid phase; - reduction/modulation of the wave front velocity of the air flowing from the bottom and consequent reduction of preferential ways of the air through the bed; - opposition to the diffusion towards the top, along the longitudinal axis of the bed, of the secondary gas introduced with the air from the vessel bottom; - total recovery of the pure separated gas introduced in the vessel for the complete regeneration of the top layers of the bed; - reduction of the mechanical stresses on the adsorbing beds. In other words, the invention enables to have adsorbing conditions very similar to the ideal ones, in which the air front gradually flows through molecular sieves layers in the full capacity of adsorbing the secondary gas that comes whit it, thus realizing a "mass transfer zone" of predetermined entity (depending by adsorbing kinetics) and the opportunity of optimizing the adsorbing bed size and shape in function of the amount and the quality of the gas to be separated. Finally, according to possible changes of some relevant parameters of the state of the adsorbent material, (or even when a change of the required performances occurs), a correction of the cycle working conditions is carried out, in order to maintain (or reach) the required quality/quantity of the separated gas to be produced. Brief description of the drawings These and other advantages will be more comprehensive thanks to the following description and attached drawings, given as a non limitative example, in which: - fig.1 schematically shows a PSA nitrogen/oxygen production plant according to the invention. Detailed description of the invention With reference to the attached drawing, a PSA process for the production of nitrogen starting from gases rich of nitrogen, preferably air, includes the following steps: A) introduction in the tower B1 of filtered feed air pressurized up to 12 bar with a initial velocity modulated by means of VM1 In function of the following factors: amount and quality of the gas produced, size of the towers and volume of CMS used for each single tower. In this step, simultaneously to an automatic and adjustable reflux of pure nitrogen from the buffer tank B3 to the top of tower B1 , through group R1 , the air is flowed through the bed. The bed is made of one or more layers of CMS, having different characteristics according to the quality and the amount of product required, in order to realize the separation of nitrogen, which remains concentrated in the gaseous state, and that flows through the CMS bed up to the upper outlet of the vessel, while the oxygen is adsorbed by the sieve. The vessel is equipped with proper devices in order to uniformly diffuse the gas along the cross sections of the bed. The back flow of pure nitrogen allows to obtain a very pure product. The nitrogen separated in B1 is sent into the buffer tank B3. B) Equalizing step of the pressure in vessels B1 and B2. The maximum pressure in the tower (B1 ) is reached with time gradients which are variable according to the performances required and the plant size, the type and the quality of CMS used, the specific conditions of the air feed and of the product which has to be delivered to the users. In particular, according to the invention the time of the pressure equalizing step is optimized by sizing the volumes of gas treated and the cross sections of the piping in order to avoid, on one hand, unwanted transfer of oxygen in the bed during the production step and, on the other hand, in order to limit the mechanical stresses acting on the beds. The first effect permits to define an upper limit of this step timing, while the second effect determines a lower limit thereof. According to a preferred process condition, the time of the equalizing step ranges between 1 and 2 seconds. The advantages obtained from the application of step B can be summahzed as follows: - energy saving; less stress on the CMS beds and therefore greater life time of the beds; - a first settlement of the air in the tower which is in the meantime regenerated (B2), allows the air to enter "gently" and to pass through the CMS bed without creating harmful turbolences or, even worst, "chimneys" which in the long run would bring to an inefficient adsorption of the CMS with consequent loss of flow rate and/or quality of the nitrogen produced. It is reminded that the maximum inlet pressure of the air into the towers is a technical choice, substantially depending on the type of CMS used from which, in turn, the plant productivity, expressed as Nm3 / hr of nitrogen/m3 of CMS, as well as the energy demand for the air compression, depend. C) Regeneration of the bed in the tower B1 by reducing the pressure to 1 bar(a), streaming through RS1 of a portion of the nitrogen product from B2 and discharging into the atmosphere the oxygen adsorbed by the bed. The tower B2 repeats the steps A-B-C- now described in exchange to B1 , in order to obtain continuity on the nitrogen production. Fig.1 shows the flow-chart of a plant according to the invention to perform the described process, in which: I is the pressurized air inlet; FC, FS, FP are air filters; RP1 , RP2 are pressure reducers respectively for the inlet air and for the nitrogen directed to the user N; B1 , B2 are the pressure vessels containing the adsorbing material; B3 is a buffer tank of the produced nitrogen; K1 ,K2 are 2 way electrovalves; M1-M4 are pressure gauges; CV1 is a check valve arranged in parallel with the valve K11 ; Rs1 is a flow regulator valve by which the streaming of nitrogen between B1 and B2 in the CMS regenerating step is performed; EX1 is a muffler for venting, from the bottom of B1 and B2, the air enriched with oxygen; RF2 is a flow regulator valve; During operations, the opening of the valves during each cycle occurs as follows: I) Tower B1 Step Open valves A) K1K4K5K6K8 B) K3K4K6K7 C) K2K3K5K7K8 II) Tower B2 Step Open valves A') K2K3K5K7K8 B') K3K4K6K7 C) K1 K4K5K6K8 As shown in fig.1 , immediately upstream tank B3, the plant includes a group R1 intended to control the reflux of nitrogen from tank B3 to vessel B1/B2 during step A/A'. The group R1 consist of a needle valve K1 1 , arranged in parallel with a check valve CV1 , by means of which it is possible to adjust and automatically perform a counterpressure in the vessel into which the pressurized air is entering. This makes the air wave front more regular and uniform, for the benefit of the quality of the nitrogen product (steps A,A'). In particular, in order to improve the quality of the nitrogen product the passage of nitrogen through valve K1 1 can be reduced and the air feed flow rate is at the same time reduced by means of the adjustable valve VM1. So operating, before the pressures upstream and downstream valve CV1 are equalized, thus interrupting the passage of nitrogen, the reflux of the pure nitrogen coming from B3 can reach the lowest layers of the bed during step A/A' and improve the quality of the nitrogen product. To perform the above stated steps, the plant solutions shown in the figure 1 have been chosen. However, the same functions can be performed according to the invention by alternative but equivalent ways. In particular, alternative solutions can concern the lower group G1 for the air inlet to vessels B1 and B2, the higher group G2 for the outlet of the nitrogen from the vessels, and the regulating group R1 placed upstream to the vessel B3. The plant can also comprise a control system F, represented in figure 1 with dashed lines, which provides for the continuous quality control of the produced nitrogen and for the calibration of the analyzer referencing to air. Hereinafter we state some examples of possible performances of the process according to the invention, all concerning the use of CMS of vegetal origin and feed air pressure of 10.5 bar(a), temperature 20°C and desorption at atmospheric pressure. In the cycles, the volume of refluxed gas from B3 to B1 and B2, adjusted by group R1 , at the varying of the air feed pressure from 8.5 to 10.5 bar(a) and the residual content in the nitrogen produced from 9%V to 50 ppm, ranges between 15 and 260% of the volume of produced nitrogen. The above described steps and components operate in an equivalent way in a plant and a process for the production of oxygen according to the teaching of the invention. Obviously, differences can be due to the different nature of the molecular sieves (zeolite instead of CMS) and of the separated gas (oxygen instead of nitrogen). The following is a preferred and non limitative example of a PSA process according to the invention for the production of oxygen starting from air. Each 33 litre tower B1 , B2 contains a molecular sieve consisting of 46 kg of zeolite material, the air inlet pressure of the air into the vessels is set to 4,5 bar. A first cycle of 60 seconds has the following step times: discharging of the tower to be regenerated, 40 seconds; charging of the tower to be put on production 17 seconds; equalization step time 3 seconds. A volume of 2000 Nlt/h of 95% pure oxygen are obtained for a volume of 24000 It/h of inlet air. The Efficiency is equal to 41 % for a delivering pressure of 3,5-3,8 bar, adjusted by the pressure reducer RP2. In order to increase productivity the same plant can perform a 30 second cycle (discharging step 20 seconds, charging step 9 seconds, equalization step 1 second) thus obtaining an Efficiency equal to 47% (4000 Nlt/h of oxygen over 42000 It/h of air). The maximum pressure of 4,5 bar is reached inside the towers 10-15 seconds before the cycle ends. During the equalization step the internal pressure of B1 , B2 is less then 2 bar, while the internal pressure of B3 decreases to 3,8-4 bar and then rises up to 4,5 bar in a time period which depends on the volume of B3 (predetermined according to volume and pressure of B1.B2), the cycle time and on the oxygen production. The back flow of the oxygen from B3 to B1/B2 is adjusted by K1 1 and is respectively equal to 35 It/min and 69 It/min in the two described cycles. The back flow of oxygen is calculated as a function of the volume and the pressure of the towers B1 ,B2 and of the desired hour production of oxygen. In the regeneration step, the streaming through RS1 is equal to 135 It/min for the already said volume of the towers B1 ,B3. When the conditions of the zeolite material is getting worst, the quality of the oxygen delivered can be preserved lowering the efficiency of the process. In this case, the inlet of the air is slowered (the zeolite material is subjected to lower mechanical stresses) and the charging and equalizing steps time is reduced. Furthermore, in the equalization step the delivering of oxygen can be reduced through RF2 and the back flow of oxygen from B3 to B1 , B2 is increased through valve K11.

Claims: CLAIMS 1. Process for the separation of nitrogen and oxygen from gases rich of nitrogen and/or oxygen, preferably air, including the following steps: A) separation of nitrogen/oxygen, by means of molecular sieves, from filtered and pressurized air passing through a first vessel containing said molecular sieves; removing of the nitrogen/oxygen separated in the first vessel and delivering of the same to a buffer tank; simultaneous regeneration of the second vessel by pressure reduction, purging with a portion of the nitrogen/oxygen product coming from the first vessel and discharge to atmosphere from the bottom of the oxygen/nitrogen adsorbed in the second vessel; B) equalizing of pressures in the two vessels; C) regeneration of the bed in the first vessel analogously to what described for the second vessel; subsequent steps A'-C cyclically repeating steps A-B-C alternatively for the two vessels; characterized by the fact that in said steps A/A' an adjustable portion of the produced nitrogen/oxygen back flows from the buffer tank to the vessel where the separation is taking place. 2. Process according to claim 1 , characterized by the fact that said gas to be separated is nitrogen and said molecular sieve consists of carbon molecular sieves (CMS). 3. Process according to claim 1 , characterized by the fact that said gas to be separated is oxygen and said molecular sieve consists of zeolite material. 4. Process according to claim 1-3, characterized by the fact that the time of said equalizing step B/B' ranges within a maximum lapse of time determined to avoid the unwanted transfer of the secondary gas in the tower starting the production step and a minimum lapse of time determined to limit the stress on the adsorbing beds due to pressure gradient. 5. Process according to claim 2, characterized by the fact that said step B/B' is performed in a time ranging between 1 and 2 seconds. 6. Process according to claim 3, characterized by the fact that said step B/B' is performed in a time ranging between 1 and 3 seconds. 7. Process according to claim 1 , characterized by the fact that said portion of separated gas coming back from the buffer tank to the vessel starting the production step is related to the volume of molecular sieve, the production rate of separated gas, its grade of purity and the air feed pressure. 8. Process according to claim 2 and 7, characterized by the fact that in the cycles the ratio between the volume of the portion of refluxed pure nitrogen and the volume of the net production of nitrogen ranges between 15 and 260%, in function of the nitrogen purity grade and the air feed pressure. 9. Process according to claim 2, characterized by the fact that said step A/A' provides for: air pressurization preferably at 10.5 bar(a) and more; reduction of the initial linear velocity of the air entering the adsorbing bed below 0.6 m/sec. 10. Process according to claim 1 -9, characterized by the fact that the molecular sieves are provided with upstream and downstream diffusers in order to protect the bed from shock waves and establish wave fronts as uniform as possible both of the back flow of the separated gas and of the pressurized inlet air. 11. Plant for the production of nitrogen or oxygen according to the process of claims 1-10, characterized by the fact of including: a pressurized air inlet (I); air filters (FC,FS,FP); optional pressure reducers (RPI.RP2) respectively for the inlet air and the separated gas directed to the user (N); pressure vessels (B1 , B2) containing molecular sieves; buffer tank (B3) for the separated gas product; air inlet group (G1 ) in the vessel (B1 , B2); feed air flow-rate controlling group (VM1 ); outlet group (G2) from (B1 , B2) of the separated gas product; a group (R1 ) immediately downstream of (G2) for the regulation of a return flow of separated gas from the tank (B3) to the vessel (B1/B2) during steps A/A'; pressure gauges (M1 M4); flow regulator valve (RS1 ) for the purge of (B1 ) and (B2) in the regenerating step of the molecular sieves; muffler (EX1 ) for the venting, from the bottom of B1 and B2, the air enriched with the secondary gas; flow meter (RF2) of the separated gas product. 12. Plant according to claim 11 , characterized in that said separated gas is nitrogen and said secondary gas is oxygen, the molecular sieves consisting of Carbon Molecular Sieves. 1 13. Plant according to claim 1 1 , characterized in that said separated gas is oxygen 2 and said secondary gas is nitrogen, the molecular sieves consisting of zeolite 3 material. 1 14. Plant according to claim 11-13, characterized by the fact that said group (R1 ) 2 is composed by a needle valve (K11 ), in parallel to a check valve for the 3 separated gas back flow (CV1 ). 1 15. Plant according to claim 1 1-13, characterized by the fact that said group (VM1 ) 2 is composed by a flow-rate control valve. i 16. Plant according to claim 1 1-13, characterized by the fact that said group (G2) ? is composed by 2/2 way valves (K6-K8). 1 17. Plant according to claim 11-16, characterized by the fact that said 2/2 way . electrovalves (k1-k8) work as follows: 3 Step Open valves 4 A) K1 K4K5K6K8 5 B) K3K4K6K7 6 C) K2K3K5K7K8 7 A') K2K3K5K7K8 8 B') K3K4K6K7 9 C) K1 K4K5K6K8 i 18. Plant according to claim 11-17, characterized by the fact that it comprises a 2 control device (F) which performs the continuous monitoring of the quality of the 3 separated gas product and the calibration of the analyzer instruments which 4 refer to air.

Inventors: MILLI OTTAVIO, MILLI, OTTAVIO
Applicants: ITALFILO DI MILLI OTTAVIO & C, MILLI OTTAVIO, ITALFILO DI MILLI OTTAVIO & C. S.A.S., MILLI, OTTAVIO
Application year: 1998
Main classification: B01D53
IPC codes: B01D53, C01B13, C01B21