Patent Publication Number: US-2002005117-A1

Title: Removal of chemical and biological agents from air

Description:
BACKGROUND TO THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The invention relates to the removal of chemical and biological agents from air and in particular to such removal from air for breathing by humans.  
       [0003] 2. Brief Review of the Prior Art  
       [0004] The removal of chemical and biological agents from air for breathing by humans has long been a problem. GB-A-2238490 contains a discussion of previously known methods and systems for such removal. GB-A-2238490 proposes a system for the removal of such agents including compressing the air, purifying the compressed air in a pressure swing adsorber and filtering the compressed air through a filter intercepting particulates.  
       [0005] Such systems have been successful in removing a wide variety of chemical and biological agents. The invention seeks to provide an improved system of this general type.  
       SUMMARY OF THE INVENTION  
       [0006] According to a first aspect of the invention, there is provided a method of removing chemical and biological agents from air for breathing by humans comprising removing higher boiling point agents from the air and then passing the air through an adsorbent in a pressure swing adsorber to remove lower boiling point agents and water vapour.  
       [0007] According to a second aspect of the invention, there is provided a system for removing chemical and biological agents from air for breathing by humans comprising means for removing higher boiling point agents from the air and a pressure swing adsorber for receiving air from the removing means and including an adsorbent for removing lower boiling point agents and water vapour from the air.  
       [0008] The following is a more detailed description of an embodiment of the invention, by way of example, reference being made to the accompanying drawings in which: BRIEF 
     
    
    
     DESCRIPTION OF THE DRAWINGS  
     [0009]FIG. 1 is a schematic representation of a system for removing chemical and biological agents from air for breathing by humans,  
     [0010]FIG. 2 is a plan view from above of a combined coalescer and filter incorporated in the filter of FIG. 1 and,  
     [0011]FIG. 3 is a section on the line A-A of FIG. 2. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0012] Referring to the drawing, the system comprises a compressed air inlet  10  that leads from a source of compressed air  10   a.  This source  10   a  may, for example, be a dedicated turbine or may be an air bleed in the case where, for example, the apparatus is used in an aircraft or other system having a power source producing compressed air. In this latter case, the pressure of the inlet air may need to be controlled to make it compatible with the requirements of the system.  
     [0013] The air inlet  10  leads to a combined coalescer and filter  11 . The combined coalescer and filter  11  is best seen in FIGS. 2 and 3. Referring to those Figures, the combined coalesces and filter  11  comprises a bowl  30  closed at its upper end by a combined inlet/outlet  31 . A liquid drain  12  is provided at the lower end of the bowl  30 . These parts may, for example, be made of metal. The combined inlet/outlet  31  includes a central vertical outlet passage  32  surrounded by a vertical wall  33  having an initial generally straight portion  34  leading from an inlet  35  that is tangential to the succeeding semi-cylindrical portion of the wall  36 . The wall then terminates in a further generally straight portion  37 .  
     [0014] The bowl  30  encases a coalescer  38  and an HEPA filter  39 . The coalescer  38  is an annular pleated element with an incorporated lint-like layer. The HEPA filter  39  is located within the core of the coalescer  38  co-axially therewith and may, for example, be formed by a pleated glass fibre material having a rating of, for example, 0.2 microns. The outlet passage  32  extends from the centre of the HEPA filter  39 .  
     [0015] It will be appreciated that the coalescer  38  and the HEPA filter  39  are of conventional design and will not, therefore, be described in further detail.  
     [0016] The outlet passage  32  of the filter  11  connects with first and second inlet branches  14   a,    14   b,  each controlled by respective first and second inlet valves  15   a,    15   b.  The first and second inlet branches  14   a,    14   b  lead to respective first and second pressure swing adsorbers  16   a,    16   b.  The first and second pressure swing adsorbers  16   a,    16   b  are preferably identical so only the first pressure swing adsorber  16   a  will be described; it being understood that the second pressure swing adsorber  16   b  is preferably similarly constructed.  
     [0017] The first pressure swing adsorber  16   a  comprises a housing  17   a  having an inlet  18   a  connected to the associated branch  14   a  and an outlet  19   a.  Adjacent the outlet  19   a  is a layer of mesoporous carbon  20   a.  Preferably, this is the mesoporous carbon  20   a  sold under the trade name BAX 1100, although other mesoporous carbons may be used.  
     [0018] In this specification, the term “mesoporous carbon” refers to porous carbon material in which the pores have an average diameter of between 20 Å and 500 Å. It will be appreciated that this does not exclude some of the carbon being microporous or macroporous but requires simply that, considered overall, the carbon is broadly mesoporous.  
     [0019] After the layer of mesoporous carbon  20   a,  there is a layer  21   a  of 13X Zeolite, although other adsorbents may be used. The adsorbent material is in the form of beads or pellets which may be bonded to one another by a polymeric binding agent, such as polyethylene. The construction and operation of such adsorbents is described in more detail in, for example, GB-B-2238490 and so will not be described here in detail.  
     [0020] The first outlet  19   a  from the first pressure swing adsorber  16   a  and the second outlet  19   b  from the second pressure swing adsorber  16   b  are controlled by respective first and second outlet valves  29   a,    29   b  and combine in a single feedline  22  which is connected to a zone  22   a  to be provided with air for breathing by humans. There is also a purge line  23  leading from the zone  22   a  and dividing into first and second purge inlet branches  24   a,    24   b  connected to the outlet sides of respective first and second pressure swing adsorbers  16   a,    16   b.  The first and second purge inlet branches  24   a,    24   b  are controlled by respective first and second purge inlet valves  25   a,    25   b.  In addition, the first and second adsorbers  16   a,    16   b  have respective first and second purge outlet lines  26   a,    26   b  leading to a single purge outlet  27 . The first and second purge outlet lines  26   a,    26   b  are controlled by respective purge outlet line valves  28   a,    28   b.    
     [0021] The apparatus operates as follows.  
     [0022] The externally sourced compressed air is fed to the filter  11 . The apparatus is used in situations where the air may include contamination by nuclear, biological or chemical agents. The purpose of the apparatus is to remove such agents, hereinafter referred to as NBC agents, from the air. It will also be appreciated that the air is very likely to include water vapour and water and other aerosols and, particularly in humid climates, the level of such aerosols can be high.  
     [0023] The air is received by the combined coalescer and filter  11  entering through the tangential inlet  35 . The air then passes around the semi cylindrical wall  38  and water and other liquid particles will tend to coalesce on the wall  37  and then drop through the bowl  30  to the drain  12 . The air, along with mists and other small particles such as bacterial particles, then enters the bowl  30  and passes through the coalescer  38 . This coalesces the mists which then drop through the coalescer  38  to the liquid drain  12 . Finally, the air, with any remaining particles, passes to the HEPA filter  39  where small contaminated particles and biological particles are removed. The air then leaves via the outlet passage  32 . Accordingly, the air leaving the filter  11  through the outlet  13  is depleted of liquid aerosols, contaminated particles and biological particles but may still contain molecular contaminants.  
     [0024] The air then passes to one of the first and second pressure swing adsorbers  16   a,    16   b.  As will be described below, the first and second pressure swing adsorbers  16   a,    16   b  are cycled to regenerate them so that, at any one time, one pressure swing adsorber  16  is operative while the other pressure swing adsorber  16  is being regenerated. For the purposes of the present description, it will be assumed that the first pressure swing adsorber  16   a  is adsorbing and the second pressure swing adsorber  16   b  is being regenerated. In this case, the first inlet valve  15   a,  the first outlet valve  29   a,  the second purge inlet valve  25   b  and the second purge outlet valve  28   b  are open and the second inlet valve  15   b,  the second outlet valve  29   b,  the first purge inlet valve  25   a  and the first purge outlet valve  28   a  are closed. As a result, all the air from the filter  11  passes to the first pressure swing adsorber  16   a.    
     [0025] In the first pressure swing adsorber  16   a,  the air contacts the mesoporous carbon  20   a.  The usefulness of the mesoporous carbon  20   a  is based on the property of NBC agents which is that they are all capable of being physically adsorbed. Thus, whilst the chemistry and molecular size and shape of NBC agents may vary significantly, this basic adsorption phenomenon is common to all NBC agents. However, as the boiling point of the NBC agent decreases, so does the efficiency of physical adsorption by carbon filters.  
     [0026] In this regard, it will be appreciated that NBC agents that have very low boiling points will in general evaporate before they reach the apparatus. Those that have very high boiling points will be in the form of liquid or solid particles and will be removed by the filter  11 . Accordingly, the mesoporous carbon  20   a  and the 13X Zeolite  21   a  are required to remove NBC agents having boiling points between such very low and very high boiling points. Accordingly, the terms “higher boiling point” and “lower boiling point” when used in relation to NBC agents are relative terms describing agents between these extremes.  
     [0027] Accordingly, the mesoporous carbon  21   a  adsorbs all of the relatively higher boiling point NBC agents. Mesoporous carbon is preferred because microporous carbon will adsorb but not desorb and macroporous carbon will not adsorb. The air leaving the mesoporous carbon  20   a  is thus depleted of high boiling point NBC agents. It thus contains lower boiling point NBC agents and water vapour, which is also not adsorbed by the mesoporous carbon  20   a.    
     [0028] The 13X Zeolite particles  21   a  adsorb lower boiling point NBC agents and water vapour very efficiently. It has been found that the efficiency of the 13X Zeolite  21   a  is increased significantly by the removal of the higher boiling point NBC agents by the mesoporous carbon  20   a,  since 13X Zeolite has a tendency to react chemically with adsorbed higher boiling point NBC agents which can thus not subsequently be desorbed. This eventually reduces the capability of the 13X Zeolite  21   a  to adsorb lower boiling point contaminants so that NBC agents may pass through the adsorber to give contaminated air at the outlet.  
     [0029] After the air has passed through the 13X Zeolite  21   a,  it leaves via the outlet  19   a  and passes to the zone in which the air is to be breathed by humans. The air is depleted of all NBC agents and also of water vapour. It will be appreciated that the air may be conditioned by a suitable conventional air conditioner before being passed to the zone to adjust the temperature and humidity of the air.  
     [0030] As discussed above, the first and second pressure swing adsorbers  16   a,    16   b  are cycled. The cycle time may, for example, be comparatively short, for example less than 10 seconds and preferably 8 seconds. As one pressure swing adsorber  16   a,    16   b  is treating air from the filter  11 , the other pressure swing adsorber  16   b,    16   a  is being purged and regenerated. The purge regeneration of the second pressure swing adsorber  16   b  will now be described.  
     [0031] Low pressure air from the zone is passed through the purge line  23  and then through whichever branch  24   a  or  24   b  has the associated purge inlet valve  25   a  or  25   b  open. For the purposes of this description, it will be assumed that the second purge inlet valve  25   b  is open (with the first purge inlet valve  25   a  closed) so that the air passes through the second branch  24   b  to the second pressure swing adsorber  16   b.  In the second pressure swing adsorber  16   b,  the low pressure air passes across and through the 13X Zeolite layer  21   a  releasing trapped molecules from the Zeolite molecular sieve and passing them with the air to the mesoporous carbon  20   b  layer performing a similar purging operation on molecules adsorbed by the mesoporous carbon  20   b.  The air, together with the desorbed molecules (which will include water vapour and NBC agents) then passes through the second purge outlet line  26   b  to the purge outlet  27  from which they are released into the ambient atmosphere outside the zone.  
     [0032] The operation of the valves  15   a,    15   b,    25   a,    25   b,    28   a,    28   b,    29   a,    29   b  is controlled by a control system  31 . This cycles the adsorption/purging of the first and second pressure swing adsorbers  16   a,    16   b  in accordance with a predetermined cycle. The volumes of the mesoporous carbon  20  and of the 13X Zeolite  21  are, however, calculated to provide sufficient adsorption to deal with all expected volumes and concentrations of NBC agents.  
     [0033] It is expected that an apparatus as described above with reference to the drawing will not suffer from any build-up of retained NBC agents and water vapour, that is to say that the purge cycle will always leave generally the same low level of retained matter over repeated cycling. However, in order to ensure that there has been no build-up of such retained matter, it is possible to subject the apparatus to a thermal decontamination clean-up cycle. This cycle is performed when there is no threat of NBC agents during decontamination.  
     [0034] In the clean-up cycle, heated air (at, for example, 180° C.-190° C.) is used in the purge cycle (rather than unheated air) and the heated air sweeps through the first and second pressure swing adsorbers  16   a,    16   b  and this acts to remove any residual matter from the first and second pressure swing adsorbers  16   a,    16   b  and pass it to the purge outlet  27 . It may take a number of purge cycles with heated air to heat-up the 13X Zeolite layers  21   a  and  21   b  to desorb them.  
     [0035] An apparatus described above with reference to the drawing has been tested by Chemical and biological defence Agency at Porton Down. The test used a single pressure swing adsorber  16  of the kind described above with reference to the drawings operating under the same conditions of flow velocity and residence times as would be used in a working apparatus. The performance of the pressure swing adsorber  16  was tested using the nerve agent simulant DMMP (dimethyl methylphosphonate), the mustard agent simulant 2CEEE (2 chloro ethylethylether) GD (Soman), HD (mustard agent), PS (Chloropicrin), AC (hydrogen cyanide), CK (Cyanogen Chloride), isobutene, PFIB (perfluoroisobutene), freon  134   a,  the low boiling point simulant freon  23  and the general contaminants Avtur (aviation fuel), octane and ethanol. In each case, assessments were carried out under A 1  (hot dry) and B 3  (hot humid) conditions of meteorology (with the exception of assessments using GD and HD which were carried out under A 1  conditions only (to minimize the risks associated with handing these more hazardous agents)). The agent and simulant doses used generally ranged between 300-120,000 mg min m −3  .  
     [0036] The air leaving the outlet  19  was analysed using sensitive flame ionization detectors, a gas chromatograph and a mass spectrometer for any traces of the agents and simulants. In addition, for certain of the substances set out above, samples of the air were taken and analysed using sensitive adsorbent sampling tubes and the Chemical Agent Monitor (CAM). These additional techniques provided greater levels of detection sensitivity to try and demonstrate whether there would be any risk of personnel being exposed to low level dosages of agents such as DMMP since low level exposure to nerve agents can give rise to miosis which can affect the ability of operators to use weapon systems or to manoeuvre. In addition, during and after the challenge of the apparatus with agents and simulant, the inlet and purge airstreams were also monitored to determine the concentrations in the inlet  18  and the purge outlet  27 . In addition, the pressure and temperature of the air in the inlet  18  was also monitored and recorded. The dewpoint of the air at the inlet  18  and the outlet  19  was also monitored.  
     [0037] The dosages of the agents and simulants detailed above in general exceeded those which might be expected to be encountered in real situations. Thus, if the apparatus described above with reference to the drawings is able to deal satisfactorily with such dosages, it will certainly deal with the lower dosages which are more likely to be encountered in the field.  
     [0038] The results of the tests were as follows.  
     [0039] In all cases, the pressure swing adsorber was found to provide full protection against the agents and simulants under both A 1  and, where used, B 3  meteorology conditions, meaning that no agent or simulant was detectable in the air leaving the outlet  19 . The performance of the pressure swing adsorber  16  was shown to be unaffected by sustained challenge with high concentrations of water vapour. During assessments under B 3  conditions, frequent weighing of the pressure swing adsorber  16  established that the water content of the pressure swing adsorber  16  stabilized (i.e. the weight of the pressure swing adsorber  16  did not continue to rise). This is plainly an essential prerequisite if the pressure swing adsorber  16  is to provide sustained protection against the NBC agent since, if the water content of the pressure swing adsorber  16  continues to increase, it will become saturated with water vapour and will lose its capacity for adsorbing agents and simulants which will then begin to pass into the outlet gas. The results obtained by weighing correlated with the observation that the dewpoint of the outlet air remained low (&lt;−50° C.) throughout all the tests.  
     [0040] During the tests, the pressure swing adsorber  16  was periodically purged using unpressurized air as described above. The test showed that the pressure swing adsorber  16  desorbed the bulk of the retained NBC agents during normal operation. The results demonstrated that the adsorber could be operated in the pressure swing adsorption mode for extended periods of time without compromising the working capacity of the adsorber  16 . Thus, the ability of the pressure swing adsorber  16  to defeat challenge with agents and simulants over sustained periods of operation did not depend on the use of the thermal clean-up cycle.  
     [0041] The test also included a clean-up cycle using heated air at a temperature of about 180°. The test showed that this clean-up cycle removed contaminants that were not desorbed during the cycling of the pressure swing adsorber  16 . On the basis of the sustained challenges to which the pressure swing adsorber  16  was exposed, and an analysis of the results of the clean-up cycle, the tests assessed the regeneration of the pressure swing adsorber  16  as resulting in the restoration of the adsorption capacity to at least 98% of its initial value. This small reduction in capacity, which was difficult to quantify accurately, was thought to be due to the presence of a small percentage of microporosity in the mesoporous carbon  20 , which traps higher boiling point components. This small reduction has no impact on the performance of the pressure swing adsorber  16  so that the working capacity of the adsorber  16  is unaffected.  
     [0042] These tests demonstrate that the system described above with reference to the drawings offers the capability to provide broad band protection against a range of NBC hazards under worldwide conditions of meteorology. It also has the ability to protect against the threat from toxic industrial chemicals and battlefield contaminants.  
     [0043] As described above, the apparatus includes two pressure swing adsorbers  16   a,    16   b,  each having two layers, a layer of mesoporous carbon  20   a,    20   b  and a layer of 13X Zeolite molecular sieve  21   a,    21   b.  The mesoporous carbon layers  20   a,    20   b  adsorb higher boiling point NBC agents while the 13X Zeolite  21   a,    21   b  adsorbs the lower boiling point agents. It will be appreciated, however, that the mesoporous carbon  20   a,    20   b  could be separate from the 13X Zeolite  2 l a,    21   b  and be subjected to a separate purge. In addition, the higher boiling point NBC agents could be removed by a material other than mesoporous carbon and the lower boiling point agents could be removed by a material other than 13X Zeolite, although tests so far have shown mesoporous carbon and 13X Zeolite to be the most efficient materials for these purposes. It will be appreciated that, while the system described above with reference to the drawings has two pressure swing adsorbers  21   a,    21   b,  there may be more than two such adsorbers. For example, there may be three such adsorbers or, for larger applications any multiple of two or three adsorbers.