FR2793310A1 - Cryogenic separation of air containing impurities in the form of aerosols - Google Patents

Abstract

The air flow passes through the following stages: (a) compression to a pressure greater than 1 bar; (b) cooling the air to a cryogenic temperature less than -120 deg. C; (c) cryogenic distillation of the air. The process includes at least one stage of eliminating at least part of the aerosols in the air flow. Installation for carrying out the process The aerosols are hydrocarbons, oils or oxidizable components and their size is of the order of 0.1 micro m to 0.2 micro m. At least one stage of eliminating the aerosols is carried out before stage (b) and/or at least part of the aerosols are eliminated by filtering the air flow by passing it through a cooling loop and across a granular bed by centrifuging the air or passing it through a cyclone. At least one stage of eliminating the aerosols is carried out after stage (b), inside the distillation column by filtering and/or purging a cryogenic liquid obtained from air. The filtration is carried out on a cryogenic liquid contained in the vaporizer/condenser bath in a medium pressure column. Preferably, the cryogenic liquid is a mixture of nitrogen and oxygen. In stage (a), the compression is to a pressure greater than 2 bar, preferably between 3 and 40 bar. The air is filtered prior to the compression stage. Nitrogen, oxygen or argon are recovered from the process.

Description

The aim of the present invention is to propose a separation process

  of atmospheric air by cryogenic distillation, in which an aerosol type impurity which is likely to be present in the air flow, in particular hydrocarbons and

oxidizable compounds.

  It is known that atmospheric air contains compounds or impurities, some of which must be removed before any cryogenic separation of the air, that is to say before the introduction of air into

  the heat exchangers of the cold box of an air separation unit.

  In particular, there may be mentioned compounds of the carbon dioxide (CO2) and / or water vapor (H2O) type, but also other compounds such as hydrogen (H2), carbon monoxide (CO), nitrogen oxides (NOx, N20 ...), sulfur oxides (SOx ...), hydrocarbons, such as butane, and the like. In addition, some aerosols may be generated during the

  compression of air, for example oils or the like.

  However, in the absence of such air pretreatment, there will inevitably occur condensation and solidification in ice of certain impurities, in particular CO2 and / or water vapor, when the air is cooled to temperature. cryogenic, which can cause problems with clogging of the cryogenic separation equipment or unit, in particular heat exchangers, distillation columns ... and, by that,

deterioration of it.

  To avoid these problems, it is customary to pre-treat the air to be

  separated by cryogenic prior to this cryogenic separation.

  This air pretreatment is usually called purification or purification "header" because performed upstream of the cryogenic separation unit. Currently, the air pre-treatment is carried out, as the case may be, by TSA process (Temperature Swing Adsorption) or by PSA process

(Pressure Swing Adsorption).

  o10 Such TSA or PSA air purification processes are in particular

  described in the following documents: US-A-3,738,084, US-A-5,531,808, US-

A-5,587,003 and US-A-4,233,038.

  However, until now, no document describes the elimination of aerosol-type impurities contained in the air, prior to the

  separation of said air by cryogenic distillation.

  However, the air, before and / or after compression, contains various pollutants of aerosol type which are composed of solid and / or liquid particles in suspension in the air, which are not stopped by the processes of

aforementioned pretreatment.

  Thus, aerosols formed from liquid particles may contain heavy hydrocarbons and industrial tests have shown that all or part of these liquid aerosols could reach the cryogenic air separation unit, despite the use of 'air pre-treatment steps. The problem which arises is therefore to avoid any deterioration of the cryogenic air separation unit due to a possible accumulation of these aerosols, in particular aerosols containing hydrocarbons and

other oxidizable components.

  In order to avoid such problems, the present invention proposes a partial or total elimination of the aerosols contained in an air flow to be separated by cryogenic way so as to avoid degradation of

  installation by aerosol accumulation.

  The present invention therefore relates to a method of cryogenic separation of an air flow containing at least liquid and / or solid aerosols as impurities, in which the air flow is subjected to the steps of: (a) compression at a pressure greater than 1 bar, (b) cooling the air flow to a cryogenic temperature below -120 C, (c) cryogenic distillation of the air, characterized in that it comprises at least one step elimination of

  minus part of the aerosols present in the air flow.

  Aerosols to be eliminated may already be present in the air before

  compression and / or be generated during air compression in step (a).

  Preferably, according to the method of the invention, the aerosols are allowed to pass in the various conventional stages of air prepurification and, then, a total or partial liquefaction of the air flow is carried out with

  elimination of aerosols by purging or by mechanical filtration.

  Depending on the case, the process according to the invention may include one or more of the following characteristics: - the aerosols are chosen from hydrocarbons, oils and

oxidizable compounds.

  - said aerosols have a size of the order of 0.10 pm to 0.20 pm.

  - at least one step of eliminating at least part of the aerosols

  is carried out prior to step (b).

  - at least part of said aerosols is eliminated by filtration of the air flow, by passage of the air flow in a cooling tower, by passage of the air flow through a granular bed, by centrifugation of the flow

  air or by passing the air flow through a cyclone-type device.

  - at least one step of eliminating at least part of the aerosols

  is performed subsequent to the cooling step (b).

  - at least one step of removing at least part of the aerosols is carried out, inside a portion of at least one distillation column, by filtration and / or purging on cryogenic liquid obtained from 'air. the filtration is carried out on cryogenic liquid contained in the liquid bath of the vaporizer / condenser or of the cryogenic liquid contained in a medium pressure column, preferably the cryogenic liquid

  is a mixture containing oxygen and nitrogen.

  - the purge is carried out on cryogenic liquid contained in the liquid bath of the vaporizer / condenser or of the cryogenic liquid contained in a medium pressure column, preferably the cryogenic liquid

  is a mixture containing oxygen and nitrogen.

  - in step (a), compression is carried out at a pressure greater than 2 bar, preferably from 3 to 40 bar, preferably at least 5 bar,

  preferably at a pressure of 6 bar to 25 bar.

  - it includes an air filtration step prior to step (a)

compression.

  - after cryogenic distillation, at least one compound is recovered

  chosen from nitrogen, oxygen or argon.

  - cryogenic air distillation is carried out at a pressure of 1 to

  bar, preferably of the order of 1 to 20 bar.

  In addition, the invention also relates to a cryogenic air separation installation capable of implementing the above method, comprising: - means for compressing air at a pressure greater than 1 bar, - means for cooling air flow to a cryogenic temperature below -120 C, - means for cryogenic air distillation comprising one or more cryogenic distillation columns and / or one or more evaporator / condenser, - means for elimination of aerosols to eliminate at least part of the liquid and / or solid aerosols which may be present

in the air flow.

  Preferably, the means for eliminating aerosols are chosen from filtration means, cooling towers, granular beds,

  centrifuge means and cyclone type devices.

  More generally, the problem of the presence of aerosols in the air is not due solely to atmospheric pollution because, tests 1o carried out in the context of the present invention, have shown that air contains between about 0.5 and 1 pg / Nm3 of organic or inorganic matter,

  for example dust or the like.

  In addition, another important characteristic to take into account concerning aerosols contained in atmospheric air is their

granulometry.

  Thus, measurements on industrial site have shown that the majority of aerosols in the atmosphere have a particle size of

  from 0.10 pm to 0.20 pm (particle size peak).

  However, as we will see below, aerosols with a diameter between approximately 0.05 to 1 pm, in particular those of the order of 0.10 pm to 0.20 pm, are the most difficult to trap and eliminate, and are therefore the more troublesome because they manage to pass through conventional prepurification stages which generally stop only the impurities in gaseous form. It follows then that these aerosols are then found until

  in the cold box of the air separation unit, as well as downstream of it.

  Among these harmful aerosols, there are in particular oily compounds, that is to say oils or lubricants, which can come directly from atmospheric air, from the air compressor used for compressing the air and / or machines of the cryogenic separation unit, for example

turbines, "boosters" ....

  According to the invention, the aerosols are stopped upstream of the cold box of the air separation unit, that is to say before the step of cooling the air to cryogenic temperature below -120 C. , and / or after this step

  cooling, that is to say downstream of the cold box.

  A first method to stop the aerosols contained in the

  air flow is the filtration of the air flow.

  Indeed, tests have shown that filters make it possible to stop

  all aerosol diameters with more or less efficiency.

  To do this, the filters can be arranged so as to filter and purify the air flow at the suction of the compressor and / or at any other place in the process, for example downstream of a zone of conventional top treatment. In order to ensure efficient removal of aerosols, it may be necessary to carry out several stages of filtration in different

installation locations.

  In addition, part of the aerosols can also be eliminated using the centrifugal force of the compressor, so as to eliminate the particles

  most sensitive to centrifugal force.

  Another method of aerosol removal is the use of a cooling tower, for example an air / water tower, in which bringing the air flow into contact with a cooling fluid allows

  to stop some of the aerosols.

  For example, some Venturi-type "scrubbers" have shown a high effectiveness in stopping aerosols: for example, particles of 0.05 μm are

  stopped with an efficiency of 50%.

  Another method is to use one or more

  granular beds which partially stop aerosols.

  However, to obtain an effective arrest of aerosols by a granular bed, it is advisable to choose correctly, according to the diameter (s) of the aerosols to be eliminated, the height of the granular bed, the diameter of the particles making up the bed, as well as their nature and their porosity, and

  also the speed of propagation of the air flow through the bed.

  Another possible method is to use a

  cyclone type which stops aerosols larger than 1 µm.

  According to the present invention, all or part of the aerosols present in the air, in particular hydrocarbons, are preferentially eliminated inside the so-called 'cold' part of the air separation unit (or ASU, for Air Separation Unit) located downstream of the expansion and / or cooling device for the air flow at cryogenic temperature, i.e.

  o10 below -120 C; this device is conventionally called "cold box".

  In other words, the air stream containing the aerosols is liquefied in the cold box, then the solid aerosol particles contained in the cryogenic liquid resulting from this liquefaction are removed by

  filtration and / or purging of ASU fluids.

  To do this, we proceed, for example, according to one of the channels given below, the channel being to be chosen in particular according to the type of installation, for example ASU with film or bath vaporizer / condenser, ASU production liquid oxygen ..., depending on the nature and quantity of aerosol pollutants in the air ....: - channel I: filtration of the cryogenic liquid rising from the column

  medium pressure to the ASU low pressure column.

  - channel 2 filtration of the ASU vaporizer / condenser liquid.

  - channel 3 draining the ASU vaporizer / condenser liquid and / or

  draining the liquid from the ASU medium pressure column bath.

  - track 4: purging the liquid from the medium pressure column bath

  the ASU with production of liquid from the vaporizer / condenser.

  - track 5: Liquid filter and liquid purge in vaporizer / condenser bath.

  In order to choose the most suitable route, atmospheric pollution, that is to say the quantity of harmful aerosols, can be measured either directly at the intake of compressor 10 with a particle counter or any other means. measuring aerosols, either upstream of the cold box 8, or directly in the liquid bath 3 of the

vaporizer / condenser 7.

  The different purification routes given above have been illustrated in FIG. 1 which represents a block diagram of an air separation installation comprising an ASU 9, which is provided with

  means of aerosol removal.

  This air separation installation comprises, in series, air compression means, such as a compressor 10; an air / water tower 11 or, alternatively, an exchanger without direct contact between the fluids, intended to eliminate part of the aerosols as explained above; a conventional air pre-treatment zone 12 comprising two containers 12a, 12b containing one or more adsorbents and / or catalysts; a cold box 8 in which the compressed air is cooled, a part which can be liquefied; and an ASU 9 composed of a medium pressure column 5, a

  evaporator / condenser 7 and a low pressure column 6.

  More specifically, the ASU 9 comprises, moreover, filters 1, 2 and purge means 4 and 5 'intended to eliminate all or part of the aerosols

  contained in an air flow to be separated by cryogenic distillation.

  Filtration of the liquid going up towards the low pressure column When one chooses the way 1 above, that is to say that one wishes to carry out an effective filtration 1 of the cryogenic liquid going up from the medium pressure column 5 towards the column low pressure 6, this filtration 1 must be absolute, that is to say an almost total stopping of the particles must be carried out (elimination efficiency> 99%), and operated on all the flow rates of the liquids because if some of the flux is in gaseous form, the

  pressure losses become too great.

  Filtration can be carried out by means of one or more filters 1, for example filters made of metal, metal alloy or sintered ceramic

  or not, membrane filters resistant to cryogenic temperatures.

  Preferably, these filters 1 are regenerable or exchangeable periodically. In addition, since it is necessary, in certain cases, to introduce air in gaseous form into column 6 at low pressure, an absolute filtration of this air will then also be carried out before its introduction, for example at HEPA (High Efficiency Particulate Air) or ULPA (Ultra Low Penetration Air) type filter to avoid

  o10 contamination of fluids by impure air.

  It should be noted that this solution has the advantage of not allowing solid aerosol particles to pass through the low pressure column 6 and also of having, in the vaporizer / condenser 7, a cryogenic liquid.

  free of all solid aerosol particles.

  However, a drawback of this solution is the possible presence of solid aerosol particles in the 5 'bath of liquid rich in

  oxygen (40 to 60% oxygen) from the medium pressure column 5.

  Purification of the vaporizer / condenser liquid According to this solution (channel 2), the solid particles are eliminated

  contained in the vaporizer / condenser liquid 7.

  To do this, we proceed by filtration 2 and / or purge 4 of the liquid from the

bath 3 of the vaporizer / condenser.

  Although this solution does not make it possible to remove all the aerosol particles from the liquid in the vaporizer / condenser bath, it nevertheless makes it possible to effectively control the concentration of solid aerosols in this bath and therefore to minimize the risk of degradation of the

  vaporizer / condenser linked to these solid particles.

  In this case, filtration is done on a defined flow of liquid, which can

279331.0

  be all of the liquid pumped for a film vaporizer / condenser 7, by

example.

  If the vaporizer / condenser 7 installed requires a liquid that is substantially free of solid aerosol particles, the vaporizer / condenser must operate in a closed body and all of its liquid flow must then be filtered with an absolute filter, such as a metallic or ceramic sinter, a filtered

  membrane, or granular bed for example.

  This solution is less relevant than the filtration of liquids rising in column 6 low pressure because the liquids in column o10 low pressure 6 may, in this case, still contain particles

  harmful solids, such as heavy hydrocarbons.

  In case of heavy pollution, the filtered flow can be increased to

  decrease or minimize the content of aerosol particles in the liquid.

  In order to verify the effectiveness of this aerosol particle elimination route, measurements were carried out on an installation similar to that

of figure 1.

  These measurements have shown that the air, at the inlet of the cold box 8, contained approximately 2.6 × 104 particles per liter of air for a flow rate of 180,000 m 3 / h, said particles measured having a diameter of between approximately 2 o 0.20 and 0.30 pm and this, despite the presence of the

  pretreatment 12 upstream of the cold box 8.

  After liquefying the air in the cold box 8 and filtering the liquid from the vaporizer / condenser 7 according to the invention, almost all of the aerosol particles have been eliminated, that is to say the efficiency of the shutdown aerosols from

  0.20 to 0.30 prl is almost 100%.

  Purging the liquid from the vaporizer / condenser bath The simplest solution is to remove the aerosol solid particles contained in the vaporizer / condenser bath 7 by means of a purge

  4 of the vaporizer / condenser liquid 7 (channel 3).

  The purge rate is to be chosen according to the content of solid particles not to be exceeded and it is therefore a function of atmospheric pollution. Liquid filter and purging of the vaporizer / condenser bath liquid It is also possible to combine two of the previous solutions, for example, coupling a purge to a filtration of the vaporizer / condenser bath liquid, so as to further improve the elimination of o aerosols, especially when the aerosol particle content is high. When the production of the air separation unit (ASU) is gaseous oxygen under pressure by vaporization of liquid oxygen, the liquid will preferably be filtered before vaporization in the main exchanger. However, this filtration is only necessary if there is no

  treatment before liquid production (see Fig. 1).

  The present invention is particularly well suited to implementation on an industrial site with high atmospheric pollution, in particular on an industrial site close to a site or a hydrocarbon refining area or on an industrial site subjected to combustion which generate fumes containing significant quantities of aerosols based on hydrocarbons.

Claims (13)

  1. Method for the cryogenic separation of an air flow containing at least liquid or solid aerosols as impurities, in which the air flow is subjected to the steps of: (a) compression at a pressure greater than 1 bar, (b) cooling the air flow to a cryogenic temperature below -120 C, (c) cryogenic distillation of the air, characterized in that it comprises at least one step of removing at least   minus part of the aerosols present in the air flow.   2. Method according to claim 1, characterized in that the aerosols are chosen from hydrocarbons, oils and compounds oxidizable.   3. Method according to one of claims 1 or 2, characterized in   that said aerosols have a size of the order of 0.10 pm to 0.20 pm.   4. Method according to one of claims 1 to 3, characterized in that   that at least one step of eliminating at least part of the aerosols is carried out prior to step (b) and / or that at least part of said aerosols is removed by filtration of the air flow, by passing the air flow through a cooling tower, by passing the air flow through a granular bed, by centrifuging the air flow or by passing the air flow in a cyclone type device.   5. Method according to one of claims 1 to 4, characterized in that   that at least one step of eliminating at least part of the aerosols is   performed subsequent to the cooling step (b).   6. Method according to claim 5, characterized in that at least one step of removing at least part of the aerosols is carried out, inside a portion of at least one distillation column, by filtration   and / or purging on cryogenic liquid obtained from air.   7. Method according to one of claims 5 or 6, characterized in that   that the filtration is carried out on cryogenic liquid contained in the liquid bath of the evaporator / condenser or cryogenic liquid contained in a medium pressure column, preferably the cryogenic liquid is a   mixture containing oxygen and nitrogen.   8. Method according to one of claims 5 or 6, characterized in that   that the purge is carried out on cryogenic liquid contained in the liquid bath of the vaporizer / condenser or of the cryogenic liquid contained in lo a medium pressure column, preferably the cryogenic liquid is a   mixture containing oxygen and nitrogen.   9. Method according to one of claims 1 to 8, characterized in that   in step (a), compression takes place at a pressure greater than 2   bar, preferably from 3 to 40 bar.   10. Method according to one of claims 1 to 9, characterized in that   that it includes an air filtration step prior to the compression step (a).   11. Method according to one of claims 1 to 10, characterized in that   that we recover nitrogen, oxygen or argon.   12. Cryogenic air separation installation capable of   in ceuvre a method according to one of claims 1 to 11, comprising:   - means for compressing air at a pressure higher than I bar, - means for cooling the air flow to a cryogenic temperature below -120 C, - means for cryogenic air distillation comprising a or more cryogenic distillation columns and / or one or more vaporizer / condenser, - aerosol elimination means to eliminate at least part of the liquid and / or solid aerosols likely to be present 3 o in the air flow.   13. Installation according to claim 12, characterized in that the aerosol elimination means are chosen from filtration means, cooling towers, granular beds, means of   centrifuge and cyclone type devices.