Abstract:
A multi-bed PSA pressure vessel includes a hollow tubular body having one or more end plates releasably secured thereto. Secured to the end plate(s) are pipes which extend along the interior of the hollow tubular body. Each of the pipes terminates at a location along the length of the tubular body different from the remaining pipes. In a preferred embodiment, a plurality of perforated spacing flanges are secured to the pipe and some adjacent flanges defining between them a space for receiving adsorbing sieve material.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to pressure swing adsorption (PSA) techniques for the separation of a preselected gas or gases from a gas mixture including said gas or gases. 
     PSA techniques are used in a wide variety of applications where it is desired to separate a particular gas, for example, oxygen or nitrogen from a gas mixture containing oxygen or nitrogen, for example, air. 
     PSA techniques are also used when it is desired to control or modify an atmosphere in a container. For example, many refrigerated containers utilise a modified atmosphere for the transportation of produce such as foodstuffs. Fruit and vegetables when transported in closed containers frequently give off carbon dioxide and ethylene, the levels of which have to be controlled if the produce is to be maintained in a fresh, edible state. Known PSA systems for controlling or modifying the atmosphere in a container are frequently bulky and difficult to service and repair. Bulk is a problem particularly when the container whose atmosphere is to be controlled forms part of a land vehicle such as a lorry where space is at a premium. Carbon dioxide control in modified atmosphere containers is known to be effected by the use of a chemical such as sodalime which adsorbs excess carbon dioxide. The arrangement is such that the level or percentage by volume of carbon dioxide in the container is monitored and when a preselected level is reached, a fan circulates the gas mixture in the container over the sodalime which adsorbs the carbon dioxide. This continues until a preselected level of the carbon dioxide in the gas mixture is reached after which the fan is shut down. 
     This arrangement is simple but has a number of limitations in particular the disposal of the spent sodalime and its replacement. 
     It is an aim of the present invention to provide a multi-bed PSA pressure vessel which is relatively compact and easy to assemble and disassemble. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a multi-bed PSA pressure vessel assembly comprises a hollow tubular body, an end plate releasably secured to an end of the hollow tubular body, the end plate having secured thereto at least two pipes extending into the hollow tubular body and each pipe terminating at a location along the length of the hollow tubular body different from the or the remaining pipes. 
     In a preferred embodiment a plurality of spacing flanges are secured to at least one of the pipes such that at least some adjacent spacing flanges define between them spaces for receiving adsorbing sieve material. The spacing flanges may be perforated to allow the passage therethrough of a gas. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Embodiments of the invention will now be described, by way of example, reference being made to the Figures of the accompanying diagrammatic drawings in which: 
     FIG. 1 is a schematic diagram of a PSA system for use in controlling or modifying the atmosphere in a closed container; 
     FIG. 2 is a schematic cross-sectional view through a first embodiment of a multi-bed PSA pressure vessel assembly according to the present invention; 
     FIG. 3 is a schematic cross-sectional view through a second embodiment of a multi-bed PSA pressure vessel assembly; and 
     FIG. 4 is a perspective detail of the embodiment shown in FIG.  3 . 
    
    
     DETAILED DESCRIPTION 
     As shown in FIG. 1, a PSA system for controlling the atmosphere within a closed container  10  containing, for example fruit comprises a compressor  3  from which extends a pipe  3 ′ to a first vessel  1  containing a water adsorbing material such as alumina or a water selective zeolite. Flow of gas through the pipe  3 ′ is controlled by a valve  7 . A pipe  10 ′ extends from the pipe  3 ′ and communicates with the interior of the container  10 . The flow of gas through the pipe  10 ′ is controlled by a valve  8 . A pipe  1 ′ extends between the first vessel  1  and a second vessel  2  containing a carbon dioxide adsorbing material. A pipe  5 ′ extends from the pipe  1 ′ and communicates with atmosphere. The flow of gas through the pipe  5 ′ is controlled by a valve  5 . A pipe  4 ′ extends from the second vessel  2  and communicates with a pressure regulating valve  4 . A further pipe  6 ′ extends as shown from the pipe  3 ′ at a location between the compressor  3  and the valve  7  to the pipe  4 ′ at a location between the second vessel  2  and the pressure regulating valve  4 . 
     In use, when a predetermined level of carbon dioxide has been reached in the container  10 , the compressor  3  will be activated causing the gas mixture atmosphere in the container  10  to pass along the pipe  10 ′ through open valve  8  and into the first vessel  1  where the gas mixture is dried. The dried gas mixture then passes through the pipe  1 ′ and enters the second vessel  2  where the carbon dioxide is adsorbed. At this stage, valves  5  and  6  are closed. The adsorption pressure in the second vessel  2  is maintained by the pressure regulating valve  4 . 
     After several minutes of operation of the compressor  3  the sieve of carbon dioxide adsorbing material in vessel  2  will be saturated with carbon dioxide and will require regeneration. This is achieved by opening the valves  5  and  6 , and closing valve  7 . Compressed gas now flows via pipe  6 ′ to the second vessel  2  and passes the desorbed gas to atmosphere via pipe  5 ′. When the sieve in the vessel  2  has been regenerated, valve  5  is closed thereby diverting gas through pipe  1 ′ to the first vessel  1 . Valve  8  is then opened so that the gas passes from first vessel  1  through valve  8  and pipe  10 ′ to return to the interior of the container  10  with water vapour from the vessel  1 . 
     This cycle is repeated until the carbon dioxide level in the interior of the container  10  reaches a predetermined lower level. 
     This system although effective to control the level of carbon dioxide within the container  10  is bulky, requiring two vessels  1  and  2  with all the associated valves and pipe work. 
     The present invention is concerned with a PSA system which is similar in operation to that described with reference to FIG. 1 but which is less bulky and relatively easy to assemble and dismantle. 
     As shown in FIG. 2, a multi-bed PSA pressure vessel assembly includes a hollow tubular body  12  in the form of a generally right circular cylinder closed at its lower (as shown) end and in use closed at its opposite upper (as shown) end by an end plate  20 . The end plate  20  is releasably connected by bolts or studs (not shown) to a flange  22  surrounding the open upper end of the body  12 . Rigidly connected to the end plate  20  are pipes  14 ,  16 ,  18  and  24  which depend therefrom and into the body  12 . Each pipe  14 ,  16 ,  18  and  24  terminates at a different location within the body  12 . 
     Mounted on the central (as shown) pipe  14  are five separating flanges  26 ,  28 ,  30 ,  32  and  34 . The separating flanges are perforated to permit the passage therethrough of gas and may be enveloped in a gauze skin. The separating flanges are made from a resilient material and adjacent flanges  26 ,  28  and  30 ,  32  are held apart by resilient spacers  38 . The separating flange  34  is spaced from the closed lower end of the body  12  again by resilient spacers  38 . The space between flanges  32  and  34  is filled with a water adsorbing sieve; and the space between flanges  28  and  30  is filled with a carbon dioxide adsorbing sieve. 
     The mode of operation is similar to that described with reference to the PSA system of FIG. 1 in that atmospheric feed air from the container is pumped by the compressor down the pipe  14  into the space between the separating flange  34  and the closed end of the body  12 . The gas then rises through the perforations in flange  34  and any water/water vapour is adsorbed by the water adsorbing sieve located between the separating flanges  32  and  34 . The dried gas then passes up through the perforations in flanges  32  and  30  to enter the carbon dioxide adsorbing sieve material where carbon dioxide is removed from the gas. Ultimately, dry gas relatively free from carbon dioxide is passed back to the container via the pipe  16 . Pipe  24  is fitted with a pressure regulating valve (not shown) to maintain the absorption pressure in the vessel  12 . 
     After several minutes of operation of the compressor, the sieve of carbon dioxide adsorbing material in the space between the separating flanges  28 ,  30  will be saturated with carbon dioxide and will require regeneration. This is achieved by passing a purge gas from the compressor down the pipe  18  some of which gas will pass through the separating flanges  32  and  34  where it will entrain moisture and then pass up through the pipe  14  to be returned to the container. The remaining gas will pass up through the spacing flange  30  through the carbon dioxide adsorbing sieve through the separating flange  28  and into pipe  16  where the gas stream will be diverted to atmosphere. 
     Any propensity for the sieve material to be fluidised is dampened by introducing a gas under pressure through the pipe  24  and into the header space above the separating flange  26  and the lower surface of the endplate  20 . The gas pressure above the separating flange  26  will cause it to flex which in turn will cause the spacers  38  to flex and the remaining separating flanges thereby compacting the sieve material between them and effectively locking the beds of sieve material in place. 
     When replacing the sieve material during routine maintenance or repairing/unblocking the perforated flanges  26  to  34  all that is necessary is access to one (the top as shown) end of the pressure vessel assembly. In order to dismantle the assembly, the bolts/studs holding the flanges  20 ,  22  together are undone after which the flange  20  together with the connected pipes  14 ,  16 ,  18  and  24  and separating flanges  26 ,  28 ,  30 ,  32  and  34  can be withdrawn from the body  12  for access to the interior thereof. 
     Referring now to FIGS. 3 and 4, the multi-bed pressure vessel assembly includes a hollow tubular body  42  in the form of a flexible tube closed at its lower (as shown) end and located in an outer vessel  44 . The outer vessel  44  at its upper (as shown) end is formed with a flange  46  to which is releasably attached in a gas tight manner an end plate  48 . As shown, the end plate  48  closes off the upper end of the body  42 . Rigidly fixed to the end plate  48  are three pipes  50 ,  52  and  54  which depend from the end plate  48  and into the interior of the pressure vessel  42 . Each pipe  50 ,  52  and  54  terminates at a difference location within the body  42 . 
     Attached to the central pipe  52  are four spacing flanges  60 ,  62 ,  64  and  66 . The spacing flanges are perforated to allow the passage therethrough of gas and each may be enveloped in a gauze material. A water adsorbing sieve material is positioned between the spacing flanges  64  and  66  and a carbon dioxide adsorbing material is positioned between the spacing flanges  60  and  62 . A conduit  70  extends through the outer vessel  44  and communicates with the space defined by the inner surface of the outer vessel  44  and the outer surface of the body  42 . 
     In use, when the level of carbon dioxide reaches a predetermined level in the container (not shown), a compressor is activated, as with the previously described embodiments, which passes the gases in the container down the central pipe  52 . The gases will then rise through the perforations in the spacing flange  66  and through the bed/sieve of water/water vapour adsorbing material where it is dried. The dried gas then passes up through the perforations in the spacing flanges  64  and  62  and passes through the bed/sieve of carbon dioxide adsorbing material where substantially all the carbon dioxide is removed from the gas. Ultimately, the gas passes through the perforations in the spacing flange  60  to exit the body  42  via the pipe  54  en route back to the interior of the container. 
     After several minutes the carbon dioxide sieve material will require regeneration and this is achieved by stopping the flow of gas down the pipe  52  and passing a purge gas down pipe  50 . Some of the purge gas will pass downwardly through the perforations in the spacing flange  64 , through the water adsorbing sieve material and hence through the perforations in the spacing flange  66  to return moist gas up the pipe  52  to the interior of the container. 
     The remaining purge gas will pass upwardly through the perforations in the spacing flange  62  and hence through the carbon dioxide adsorbing sieve. The purge gas together with desorbed carbon dioxide then passes through the perforations in the spacing flange  60  to exit the body  42  via pipe  54  where it is vented to atmosphere. 
     In order to prevent fluidisation of the sieve material in the flexible body  42  gas under pressure is passed through the conduit  70  to exert a force on the outer surface of said flexible body for locking the beds of sieve material in place. 
     As described with reference to FIG. 2, routine maintenance and the repair of the pressure vessel assembly is rendered relatively easy in that by releasing the end plate  48  from the flange  46  access is readily available to the separating flanges and the interior of the flexible body  42 . 
     Although in the above described embodiments reference has been made, by way of example, to using a carbon dioxide adsorbing sieve for removing carbon dioxide from a gas mixture, clearly other sieve materials can be used to remove other gases. For example, an ethylene adsorbing sieve could be used in place of or together with the carbon dioxide adsorbing sieve to remove ethylene. 
     Furthermore, although reference has been made to spacing flanges these are not essential since if the particle sizes of the respective sieve materials are similar no mechanical interface is necessary. 
     Alternatively, the sieve materials could be separated, for example, by ceramic particles or balls which are non-reactive to the gases passing through the hollow tubular body of the pressure vessel assembly.