Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/22—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by diffusion
  • B01D53/229—Integrated processes (Diffusion and at least one other process, e.g. adsorption, absorption)
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/22—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by diffusion
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B21/00—Nitrogen; Compounds thereof
  • C01B21/04—Purification or separation of nitrogen
  • C01B21/0405—Purification or separation processes
  • C01B21/0433—Physical processing only
  • C01B21/0438—Physical processing only by making use of membranes
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B21/00—Nitrogen; Compounds thereof
  • C01B21/04—Purification or separation of nitrogen
  • C01B21/0405—Purification or separation processes
  • C01B21/0433—Physical processing only
  • C01B21/045—Physical processing only by adsorption in solids

Definitions

• 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.
• non cryogenic technologies are of two types: gas permeation on a polymer membrane and adsorption, generally on activated carbon with kinetic effect.
• EP-A-0.537.614 describes a process for the production of nitrogen from a residual PSA-oxygen, using a permeation step.
• 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.
• 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.
• - 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;
• the proposed principle solution consists of a treatment of a gaseous feed mixture in two stages.
• 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.
• the first part of the gas is sent to the atmosphere before recycling the second part of the gas.
• 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.
• this second part of the waste gas 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.
• 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.
• 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.
• 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.
• 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:
• 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.
• 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.
• 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.
• 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.
• 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.
• the air is electrically reheated. It could also be heated by direct or indirect recovery of compression heat or by steam.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• this capacity 21 could also be a container provided with internal baffles.
• 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 waste gas could be used to sweep one or some of these units.
• 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.
• the first fraction is that which leaves first from the adsorber, followed by the second fraction, the division being regulated by a time.
• adsorption device described here is a PSA type device, one could consider the use of a TSA type device (Temperature Swing Adsorption).
• 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.

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• Chemical & Material Sciences (AREA)
• Analytical Chemistry (AREA)
• Organic Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Engineering & Computer Science (AREA)
• General Chemical & Material Sciences (AREA)
• Oil, Petroleum & Natural Gas (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Separation Using Semi-Permeable Membranes (AREA)
• Separation Of Gases By Adsorption (AREA)

Applications Claiming Priority (3)

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• 1994
  • 1994-07-08 FR FR9408450A patent/FR2722114B1/fr not_active Expired - Fee Related
• 1995
  • 1995-06-23 WO PCT/FR1995/000840 patent/WO1996001679A1/fr not_active Ceased
  • 1995-06-23 EP EP95924359A patent/EP0717656A1/de not_active Withdrawn
  • 1995-06-23 JP JP8504146A patent/JPH09502658A/ja active Pending
  • 1995-06-23 KR KR1019960701122A patent/KR960704616A/ko not_active Withdrawn
  • 1995-06-23 CN CN95190613A patent/CN1130361A/zh active Pending

Non-Patent Citations (1)

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