WO1996001679A1

WO1996001679A1 - Method and apparatus for producing nitrogen by combined gas permeation and adsorption

Info

Publication number: WO1996001679A1
Application number: PCT/FR1995/000840
Authority: WO
Inventors: Christian Barbe
Original assignee: L'air Liquide, Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude
Priority date: 1994-07-08
Filing date: 1995-06-23
Publication date: 1996-01-25
Also published as: FR2722114A1; KR960704616A; FR2722114B1; JPH09502658A; CN1130361A; EP0717656A1

Abstract

A method for producing high-purity nitrogen by separating a nitrogen- and oxygen-containing gas mixture in a permeation separation apparatus (9) and feeding the resulting nitrogen-enriched gas into an adsorption apparatus (A, B). Part of the residual gas in the adsorption apparatus may be recycled by mixing it with the mixture to be separated or using it as a scavenging gas in the permeation separation apparatus (9).

Description

Process and installation for the production of nitrogen by combined gas permeation and adsorption

The present invention relates to a process and a plant for producing nitrogen and more particularly to a process and plant for producing high purity nitrogen by combined gas permeation and adsorption.

Nitrogen production by a generator installed nearby

^the immediate location of use is required for an increasing number of applications. The technologies currently used, called "non cryogenic technologies", are of two types: gas permeation on a polymer membrane and adsorption, generally on activated carbon with kinetic effect.

These two technologies have taken a significant place on the market for all applications where the maximum tolerated oxygen content is between 5 and 0.5%. Technology using gas permeation is generally preferred for lower flow rates, higher oxygen contents and mobile applications, while adsorption technology will be preferred for fixed applications with higher flow rates and higher purity. Means have been proposed which make it possible to produce nitrogen which is poorer in oxygen by these same technologies. We can cite for example the separation by cascade of membranes with two or even three stages

(US 4,894,068; US 4,119,417; US 5,240,471; US 5,102,432), and separation using PSAs (Pressure Swing Adsorption) with modified long cycles described in EP-A-466,093.

It is possible to add to the first separation step carried out either by membrane or by adsorption, a step of catalysis by hydrogen oxidation (deoxo), as described for example in EP-A-0.335.418A.

EP-A-0.537.614 describes a process for the production of nitrogen from a residual PSA-oxygen, using a permeation step.

All the processes described above have had only mediocre success on the market because they are generally not very economical.

At the ICOM conference in August 1993 in Heidelberg, a diagram entitled "Membrane PSA hybrid for N2 production" was presented. This diagram shows a 99.9% nitrogen production system from air compressed in which the air is purified first by a membrane, the non-permeate of the membrane being sent to a PSA which produces pure nitrogen. When a bottle of the PSA in the diagram becomes saturated with oxygen, it is depressurized by sending the residual gas retained in the bottle to a buffer tank (surge tank) and then the gas from the tank is recycled by mixing it with the air of power supply for the compressor.

The instantaneous flow of waste gas emitted by the oxygen saturated cylinder of a nitrogen PSA is very variable over time.

Those skilled in the art know that for a nitrogen PSA on carbon to function properly, the transition from the equilibrium pressure to atmospheric pressure must take place in less than five seconds and, preferably, in less than three seconds. In the case of the proposed scheme, this means that the residual gas buffer tank should be large in order to avoid braking the depressurization of the cylinder. Its size must also be such that the pressure at the end of regeneration is close to atmospheric pressure to ensure a good level of regeneration of the adsorbent.

In practice, these conditions are impossible to achieve economically. In the field of pure hydrogen production,

EP-A-0.241.313 discloses a process comprising a first step of separation by permeation, followed by a second step of separation by adsorption, the permeate from the first step being sent to a PSA whose residue is recycled and mixed with the gas to be treated. However, this process would not be used for the production of ultra-pure nitrogen, from a mixture of nitrogen and oxygen, because nitrogen "pinnate" less than oxygen.

The object of the invention is to produce high purity nitrogen more economically than non-cryogenic processes.

By examining the respective characteristics of the separation processes by permeation and by adsorption, we can make some general observations:

- gas permeation is particularly effective for low deoxygenation rates, that is to say between 2 and 10; it is favored by high walking pressures and it accommodates without problems with substantial moisture content in the supply air while producing a very dry gas;

- adsorption, especially of the PSA type on activated carbon, is effective for average deoxygenation rates, that is to say between 10 and 200, the performance is optimum for a moderate supply pressure (about 8 bar ); the increase in the operating pressure does not bring any appreciable gain, a substantial moisture content in the supply air has a negative effect on the performance and may even require pre-treatment of the air in the cases extremes. The proposed principle solution consists of a treatment of a gaseous feed mixture in two stages.

To this end, the process according to the invention is a process for producing nitrogen from a gaseous mixture to be treated with oxygen and nitrogen comprising a first step of separation of the mixture by gas permeation to produce an enriched gas. of nitrogen and an oxygen-enriched gas followed by a second step of separation of the nitrogen-enriched gas by oxygen adsorption, characterized in that when the adsorption apparatus becomes saturated, a first part of the retained residual gas is sent in the adsorption apparatus to the atmosphere and a second part of this waste gas is recycled in the first separation step.

Preferably, the first part of the gas is sent to the atmosphere before recycling the second part of the gas.

It is preferable that the first part of the waste gas be richer in oxygen than the second part of this gas. If possible, the first part is richer in oxygen than the gas mixture to be treated and the second part is as rich, or less rich in oxygen than this gas mixture.

In order to recycle this second part of the waste gas, it can be mixed with the gaseous mixture to be treated, preferably before a preliminary stage of compression of this mixture.

This second part of the waste gas can also be recycled by using it to sweep at least one unit of membranes used for the first separation step. In a more complex process, a first fraction of the second part of the waste gas is recycled as sweep gas and a second fraction of the second part is recycled by mixing it with the gas to be treated. In both cases of recycling, the residue from the adsorption stage has an oxygen content which varies periodically over time, therefore the feed rate of the membrane, which is formed by a mixture of air and waste will have an oxygen content which will vary periodically over time. Likewise, the flow rate of the non-permeate produced by the membrane will have a variable oxygen content periodically over time, if this flow rate is kept constant. It is advantageously possible to provide a step of analyzing the purity of the non-permeate, and to vary the withdrawal rate in order to keep this purity constant. More advantageously still, provision may be made for a step of delaying the introduction of the non-permeate towards the PSA by seeking to make the instant coincide with the moment when the oxygen content of this gas is maximum and the moment when an adsorption bottle freshly regenerated is put into service.

According to other characteristics, the rate of deoxygenation of the nitrogen-enriched gas is between 2 and 20 and preferably between 4 and 10. A step for cooling the nitrogen-enriched gas can be provided between the gas permeation and adsorption, in order to optimize the treatment temperatures for the two separation stages.

The subject of the invention is also an installation for producing nitrogen from a gaseous mixture of oxygen and nitrogen to be treated, comprising:

- a first permeation separation device;

- a second adsorption separation device;

- Means for sending the mixture to be treated to the first device;

- Means for sending the non-permeate from the first device to the second device;

- means for discharging a waste gas from the second device, characterized in that it comprises means for sending a first part of the waste gas to the atmosphere, and means for recycling a second part of the waste gas to the first device. The installation may include means for mixing the second part with the mixture to be treated, means for sending the second part in counter-current to the permeation side of at least one membrane unit of the first device to serve as sweep gas. , or both. In the case where the second part of the waste gas serves as purge gas and is also mixed with the mixture to be treated, it is preferable to provide means for sending a first fraction of the second part to serve as purge gas and means to send a second fraction of this second part to be mixed with the gas to be treated, the first fraction being richer in oxygen than the second.

Preferably, the installation comprises a means for cooling the nitrogen-enriched gas between the first and the second device. The installation is advantageously constructed so that the travel time of a molecule of the second part of the waste gas between the outlet of the adsorption apparatus and the inlet of said apparatus is on average equal to half a time -cycle of said device, reduced by a balancing time and a time of venting the first part of the waste gas.

Several examples of implementation of the invention will now be described with reference to the accompanying drawings, in which:

- Figures 1 to 7 schematically show high purity nitrogen production facilities according to the invention.

The nitrogen production installation shown in FIG. 1 makes it possible to produce 50 Nm ^ per hour of nitrogen containing less than 0.05% of oxygen. The installation essentially comprises an air compressor 1, coalescing filters 3, an activated carbon filter 5, a heating means 7, a membrane unit 9 and a PSA A, B, equipped with two adsorbers lined with carbon. active with kinetic effect.

After a pre-filtering step (not shown), the air is compressed by a screw type compressor lubricated at a pressure of 8 to 13 bar. The air is then deoiled in a conventional chain comprising coalescing filters 3 and an activated carbon filter; it is reheated to an optimum temperature for gas permeation (from 30 to 70 ° C. depending on the case) by a heating means 7. In this example, the air is electrically reheated. It could also be heated by direct or indirect recovery of compression heat or by steam.

The air is then partially deoxygenated in a membrane unit 9, with a deoxygenation rate between 4 and 7. This membrane unit 9 comprises a hollow fiber module, for example. The permeate from the membrane unit is sent to the atmosphere, the non-permeate (the gas enriched in nitrogen), being sent to a valve 19 controlled by an analyzer 23 so as to fix the oxygen content of the gas enriched in nitrogen at a certain level (10% oxygen, for example). This gas is simultaneously dried by the membrane unit, and is therefore available at a moisture content of less than 10 ppm. Its pressure is between 7 and 12 bar and its temperature between 30 and 60 ° C. The air is brought back to a temperature close to ambient or lower before the second stage.

In a second stage of separation by adsorption, the air is brought back to a temperature of approximately 20 ° C. then deoxygenated by adsorption in a PSA equipped with two adsorbers (A, B) filled with activated carbon, as described in EP-A -554.805, for example.

The PSA typically operates with a cycle time of 60 seconds, close to the cycle times for PSAs supplied by air. Nitrogen is produced in variable purity and is therefore stored in a buffer tank 17 before being sent to the customer.

During the cycle, the adsorber that receives the nitrogen-enriched gas becomes more and more oxygen-laden to the point where the gas produced by the PSA begins to enrich with oxygen. At this time, the PSA outlet valves are closed and the balancing phase begins, which has the effect of reducing the pressure of the adsorber from 8 to 4.5 bar, the oxygen-enriched waste gas then comes out PSA and, during a first depressurization period of approximately 3 seconds, this waste gas passes through the open valve 13 in a silencer 15 to then be sent to the atmosphere. We continue to depressurize by sending the waste gas to the atmosphere until its oxygen purity becomes lower than that of the ambient air and then the valve 13 controlled by a timer is closed and the valve 11 is opened to recycle the waste at the suction of the compressor 1 so that it mixes with the air to be separated. It is advantageous to interrupt the recycling of waste before the end of the depressurization phase by closing the outlet valve of the adsorber so as to keep the nitrogen relatively pure in the adsorber.

The Applicant has noted during the analysis that the composition of the waste gas vented during the depressurization phase was initially rich in oxygen, of the order of 30% for example, and that this content of oxygen drops to values below 1% at the end of depressurization. The Applicant has therefore decided to divide this waste gas into two parts using a timer, the first with an oxygen purity higher than that of air, and therefore to be rejected, and the second, which follows it, at a lower purity to that of air, this second part therefore being recycled in the separation installation.

It is also noted that the instantaneous hourly flow rate of depressurization waste gas is very high during the first seconds and appreciably lower during the rest of the phase.

This recycling of the second part of the waste gas obviously has an effect of varying the oxygen content of the feed gas or of the purge gas of the membrane unit, since the oxygen content of the second part of the waste is cyclically variable.

On the other hand, nitrogen PSA is more apt to treat an impure feed gas at the start of the production phase when the bottles have just been regenerated than at the end of the production phase (when the oxygen front has tendency to go out). The Applicant therefore provides a means of ensuring that the most impure nitrogen-enriched gas arrives at the PSA at the start of the production phase of the PSA cycle. Thus, the travel time of a molecule of the second part of the waste gas between the exit and the entry of the adsorption apparatus is chosen to be on average equal to the half-cycle time of the apparatus minus the equilibration time and the time for venting the first part of the waste gas. Obviously, the speed of the molecules varies over time, those coming out of the device last making it more slowly. It can be seen in FIG. 2 that the purity of this nitrogen-enriched gas can also be controlled indirectly by a flow limiter 19A which fixes the withdrawn flow.

However, as can be seen in FIG. 3, it is not imperative to control the purity of the gas enriched in nitrogen produced by the membrane unit 9. The flow rate withdrawn from the membrane in this diagram is fixed on average by a orifice plate 19B, so that the flow rate varies cyclically between two extreme values fixed by the pressure variations at the inlet of the PSA (A, B). FIGS. 4 to 6 repeat FIGS. 1 to 3 with the addition of an intermediate capacity 21 intended to introduce the delay time necessary for synchronization between the oxygen membrane output content of the gas enriched in nitrogen at the output of l membrane unit 9 and the PSA cycle (A, B). This capacity 21 which ensures the delay time calculated for the gas enriched with nitrogen supplying the PSA does not ensure homogenization of this gas. The capacity is achieved by a length of piping, which at the same time ensures cooling of the gas by contact with the atmosphere. However, this capacity 21 could also be a container provided with internal baffles. In the variant of FIG. 7, it can be seen that instead of recycling the second part of the waste gas at the suction of the compressor 1, this second part has the function of sweeping the permeate side of the membrane unit 9. This wiping has the effect of reducing the partial pressure of oxygen on the permeate side of the membrane unit and therefore of facilitating the separation, since the second part of the waste gas contains a low oxygen content (typically less than 10% ).

The contents of this second part will nevertheless vary cyclically, hence a slight variation in the content of the nitrogen-enriched gas produced by the membrane unit 9 during the end of the depressurization phase.

One could conceive of situations where the membrane unit 9 is replaced by a plurality of membrane units. In this case, the waste gas could be used to sweep one or some of these units. It is clear that the second part of the waste gas could be divided into two parts in order to mix a fraction with the gas to be treated, the remaining fraction being used as sweep gas. This arrangement is shown in Figure 7 in dotted lines. In this case, we would choose to send a first fraction richer in oxygen and at a higher flow rate through line 17A to be mixed with the gas to be treated and a second fraction less rich in oxygen and at a lower flow rate through line 17B, to serve as sweep gas. As the oxygen content and the flow rate of the second part of the waste gas decrease with time, preferably, the first fraction is that which leaves first from the adsorber, followed by the second fraction, the division being regulated by a time.

While we plan here to divide the first and second parts of the waste gas using a timer to reject the part that leaves first from the PSA and to recycle the one that leaves then, we could divide the parts of another way. In the variant of FIG. 7, it can be seen that the pipe 17B is partially closed by a calibrated orifice 12. In this way, the major part of the waste gas (the first part) passes to the silencer 15 which has low pressure drops. At the same time, a minor part (the second part) crosses the calibrated orifice 12 to be recycled.

Although the adsorption device described here is a PSA type device, one could consider the use of a TSA type device (Temperature Swing Adsorption).

In order to maximize the amount of nitrogen sent to the adsorption separation step, at least two units of membranes could be used to perform the separation by permeation of the gas mixture, the permeate from the first unit being sent to the second unit and the non-permeate thereof being mixed with the non-permeate of the first unit before being introduced into the adsorption system.

Claims

1. A method of producing nitrogen from a gas mixture to be treated with oxygen and nitrogen comprising a first step of separation of the mixture by gas permeation, in at least one membrane unit (9) to produce a gas enriched in nitrogen and a gas enriched in oxygen, followed by a second step of separation of the gas enriched in nitrogen by adsorption of oxygen in an adsorption apparatus (A, B), characterized in that, in order to depressurize the saturated adsorption device, a first part of the waste gas retained in the adsorption device is sent to the atmosphere and a second part of this waste gas is recycled in the first separation step.

2. The method of claim 1, wherein said first part of the waste gas is sent to the atmosphere before recycling the second part in the first separation step.

3. Method according to claim 1 wherein said first part of the waste gas is sent to the atmosphere at the same time as the second part of the waste gas is recycled.

4. The method of claim 1, 2 or 3, wherein the first part of the waste gas is richer in oxygen than the second part of the waste gas.

5. Method according to claim 4, wherein the first part of the waste gas has an oxygen content higher than that of the mixture to be treated and the second part of the waste gas has an oxygen content lower than that of the mixture to be treated.

6. Method according to one of the preceding claims, in which at least a fraction of the second part of the waste gas is recycled by mixing it with the mixture to be treated.

7. Method according to one of the preceding claims, in which at least a fraction of the second part is recycled by using it to scan at least one membrane unit (9).

8. The method of claims 6 and 7, wherein a first fraction of the second part of the waste gas is recycled by mixing it with the mixture to be treated and a second fraction of the second part of the waste gas by using it to sweep the minus one unit of membranes.

9. The method of claim 8, wherein the first fraction is richer in oxygen than the second fraction.

10. Method according to one of the preceding claims, dependent on claim 5, in which the travel time of a molecule of the second part of the waste gas between the outlet of the adsorption apparatus (A, B) and the input of said device is on average equal to a half-cycle time of said device, minus a balancing time and a time for venting the first part of the waste gas.

11. Method according to one of the preceding claims comprising a cooling step (19) of the nitrogen-enriched gas between the gas permeation and adsorption steps.

12. Method according to one of the preceding claims, in which the rate of deoxygenation of the nitrogen-enriched gas is between 2 and 20.

13. The method of claim 12, wherein the rate of deoxygenation of the nitrogen-enriched gas is between 4 and 10.

14. Method according to one of the preceding claims, in which the recycling of the second part of the waste gas is interrupted before the adsorber (A, B) is completely depressurized.

15. Installation for the production of nitrogen from a gaseous mixture of oxygen and nitrogen to be treated, comprising

- a first permeation separation device (9);

- a second adsorption separation device (A, B);

- Means for sending the mixture to be treated to the first device; - Means for sending a gas enriched in nitrogen from the first device to the second device;

- means (11, 13, 15) for discharging a waste gas from the second device, characterized in that it comprises means (13, 15) for sending a first part of the waste gas to the atmosphere, and means ( 11, 17) to recycle a second part of the waste gas to the first device (9).

16. Installation according to claim 15, comprising means (11, 17) for mixing the second part with the mixture to be treated.

17. Installation according to one of claims 15 and 16 comprising means (17) for sending the second part of the waste gas to the permeation side against the current of at least one membrane unit (9) of the first device for serving of purging gas.

18. Installation according to one of claims 15 to 17 comprising a cooling means (21) for cooling the nitrogen-enriched gas passing from the first to the second device.

19. Installation according to one of claims 15 to 18 wherein the travel time of a molecule of the second part of the waste gas between the outlet of the adsorption apparatus and the inlet of said apparatus is on average equal to a half-cycle time of said device, reduced by a balancing time and a time for venting the first part of the waste gas.