Patent Publication Number: US-2021170329-A1

Title: Plant and method for the separation of a gas mixture containing a plurality of components, in particular for obtaining biomethane

Description:
The present disclosure relates in general to a plant and a method for the separation of a gas mixture containing a plurality of gaseous components, and more in particular for the separation of a gas mixture into at least a first final flow of gas enriched in a first component and into a second final flow of gas enriched in a second component of said plurality of components. 
     In particular, the plant and the method according to the disclosure are especially suitable to be used with an initial mixture of biogas in order to extract a first flow enriched in methane or bio-methane (CH 4 ), and a second flow enriched in carbon dioxide (CO 2 ), and they will be described hereinafter with reference to this specific application without intending however to limit in any way their application to other types of gas mixtures for the extraction of flows enriched in gases other than the ones mentioned above. 
     As is known, in the energy sector, over the course of the last decades, the attention to research, development and sustainable exploitation of energy resources alternative to traditional ones has grown significantly, at the same time paying great attention to all aspects linked to environmental impact and eco-sustainability. 
     To this end, various technological trends have been explored and, among these, diverse technologies have been developed aimed at exploiting gas masses of natural origin, the so-called biogas, resulting for example from fermentation of animal sewage, from waste or material of vegetal origin, from landfills, from wastewater treatment, etc. 
     These technologies are mainly based on the use of special membranes which make it possible to selectively separate the gas mixtures into various components and they are used in plants having several separation stages, according to multiple operational configurations which vary depending on the type of initial mixture to be treated and, above all, on the final result to be obtained. 
     In fact, the plant configuration for a given gas mixture and successive separation into the component or components desired, depends on numerous factors, such as: the type of final product to be extracted; the acceptable percentage of purity of the desired product and the commercial value of said product; the number of stages and machinery necessary to realize the plant as a whole. 
     In particular, one of the parameters which has the most impact on the choice of design is given by the cost of the membranes and by that of the compression means associated with the various separation stages and which are necessary to guarantee an adequate efficiency of the membranes; in fact, the result of the separation which can be obtained by means of the membranes in a separation stage depends not only on the characteristics of the membranes but also on the pressure ratio between the high pressure and low pressure sides of the membrane itself. The higher the pressure ratio, the better is the result of the maximum separation achievable. 
     Therefore, one of the drawbacks of the solutions of the known art is that, in order to obtain an adequate purity of the desired gas, it is necessary to use a plurality of stages and a plurality of separate compressors driven by corresponding engines which increase the complexity and the costs for the realization, operation and maintenance of the plants. Alternatively, it is necessary to increase the overall working surface of the membranes thus, in practice, increasing the number of membranes to be used, whose cost per unit is notoriously high. 
     In any case, in plants with multiple stages, for example with three or more membrane-based gas separation stages, after having gone through the initial separation stages, typically two, the gas mixture may still contain a significant percentage of the component which has to be extracted and, therefore, is then treated by means of one or more further membrane separation stages. 
     In this case, even though the quantity of residual gas to be extracted is still considerable, the cost of carrying out and operating the additional one or more stages is negatively disproportionate to the additional quantity of gas which can be extracted. 
     It follows that, where the plants use these additional stages, the overall cost-benefit ratio is reduced; on the contrary, not using said additional stages reduces the quantity and the quality of the final desired result, with emission into the atmosphere of some of the gas having environmental impact. 
     Therefore, despite the fact that the solutions available on the market today make it possible to obtain fairly good results, it is clear from what has been pointed out above that it is still necessary to find new solutions which allow a further improvement compared to the present state of the art. 
     To this end, there is provided a plant for the separation of a gas mixture containing a plurality of gaseous components, comprising:
         a first membranes-based gas separation stage, adapted to receive in input a flow of said gas mixture and to separate it in a first flow of retentate gas, initially enriched in a first component of said plurality of components, and in a first flow of permeate gas, initially enriched in a second component of said plurality of components;   a second membranes-based gas separation stage adapted to receive in input said first flow of retentate gas and to separate it into a final flow of retentate gas further enriched in said first component and in a second flow of permeate gas suitable to be recirculated in the plant upstream of said first membranes-based gas separation stage;   a third gas separation stage with adsorption with oscillating pressure, said third gas separation stage being adapted to receive said first flow of permeate gas in input and to separate it, by means of adsorption, in a recirculable gas flow suitable to be recirculated in the plant upstream of said first membranes-based gas separation stage and in a final gas flow further enriched in said second component.       

     The present disclosure also provides a method for the separation of a gas mixture containing a plurality of gaseous components, comprising:
         separating, by means of a first membranes-based gas separation stage, a flow of the gas mixture entering the plant, in a first flow of retentate gas, initially enriched in a first component of said plurality of components, and in a first flow of permeate gas initially enriched in a second component of said plurality of components;   separating, by means of a second membranes-based gas separation stage, said first flow of retentate gas into a final flow of retentate gas further enriched in said first component and in a second flow of permeate gas suitable to be recirculated in the plant upstream of the first membranes-based gas separation stage;   separating, by means of a third gas separation stage with adsorption with oscillating pressure, said first flow of permeate gas initially enriched in a second component of said plurality of components, in a recirculable gas flow suitable to be recirculated in the plant upstream of said first membranes-based gas separation stage and in a final gas flow further enriched in said second component.       

    
    
     
       Further characteristics and advantages of the present disclosure will become more apparent from the following detailed description of an exemplary but non-limiting embodiment thereof, as illustrated in the accompanying drawings, in which: 
         FIG. 1  is a block diagram schematically showing an embodiment of a plant for the separation of a gas mixture according to the present disclosure; 
         FIG. 2  is a flow diagram schematically showing a method for the separation of a gas mixture according to the present disclosure; 
         FIG. 3  schematically shows an operating sequence relative to a first embodiment of a gas separation stage with adsorption with oscillating pressure to be used in the plant of  FIG. 1  and to carry out the method of  FIG. 2 ; 
         FIG. 4  schematically shows a second embodiment of a gas separation stage with adsorption with oscillating pressure to be used in the plant of  FIG. 1  and to carry out the method of  FIG. 2 ; 
         FIG. 5  schematically shows an example of a membrane-based gas separation module to be used in the plant of  FIG. 1  and to carry out the method of  FIG. 2 ; 
         FIG. 6  schematically shows an operating sequence relative to the second embodiment of a gas separation stage with adsorption shown in  FIG. 4 . 
     
    
    
     It should be noted that in the following detailed description, identical or similar components, from both structural and/or functional points of view, may be indicated with the same reference numbers, independently from the fact that they are shown in different embodiments or in different components of the present description; furthermore, it should be noted that, in order to illustrate this description clearly and concisely, the drawings are not necessarily to scale and certain characteristics of the description may be shown in a rather schematic form. 
     Furthermore, when the term “adapted” or “configured” or “shaped”, or similar, is used in this context with reference to any component whatsoever as a whole or to any part whatsoever of a component, it must be understood as comprising correspondingly the structure and/or configuration and/or the shape and/or the positioning of the component or part to which it refers. In particular, when said terms refer to hardware or software electronic means, they are to be understood as including circuits or parts of electric circuits, as well as software/firmware, for example algorithms, routines and programs in general, under execution in and/or resident in any given storage medium whatsoever. 
       FIG. 1  schematically shows an embodiment of a plant  100  for the separation of an initial gas mixture  101  containing a plurality of gaseous components according to the present disclosure. 
     In particular, the plant  100  is configured to separate the gas mixture  101  into at least a first final flow of gas enriched in a first gaseous component (A) and into a second final flow of gas enriched in a second gaseous component (B), forming part of the plurality of components of the mixture  101 . 
     According to a preferred but non-limiting embodiment, the initial gas mixture  101  is made up of a mixture of biogas resulting for example from fermentation of animal sewage and may comprise mainly and in varying quantities methane (CH 4 ), carbon dioxide (CO 2 ), ammonia (NH 3 ), hydrogen sulfide (H 2 S), water (H 2 O), nitrogen (N 2 ), oxygen (O 2 ); other substances may be present in smaller and variable quantities. These gas mixtures, before entering the supply line, for example by means of a blower  41 , can be subjected to pre-treatment, for example in scrubbers, according to embodiments well known to persons skilled in the art and, for this reason, not described in detail herein. 
     In this case, the plant  100  is advantageously configured in such a way that the final gas flow further enriched in the first component (A) is a flow of gas enriched in bio-methane (CH 4 ), and the final gas flow further enriched in the second component (B), is a flow of gas enriched in carbon dioxide (CO 2 ). 
     As shown in  FIG. 1 , the plant  100  comprises at least:
         a first membrane-based gas separation stage  10 ;   a second membrane-based gas separation stage  20 ; and advantageously   a third gas separation stage  30  with adsorption with variable or oscillating pressure, indicated with the international terminology “Pressure Swing Adsorption”.       

     According to a particularly preferred embodiment, and according to the modalities described in greater detail in the following description, the third gas separation stage ( 30 ) is of the type with adsorption with regeneration at sub-atmospheric pressure, indicated with the international terminology Vacuum Swing Adsorption (VSA), or Vacuum Pressure Swing Adsorption (VPSA). 
     In a possible embodiment, the first membrane-based gas separation stage  10 , and the second membrane-based gas separation stage  20 , each comprise at least a module  110 , preferably a plurality of modules  110  connected in parallel to each other and an exemplary embodiment of which is schematically shown in  FIG. 5 . 
     In particular, each module  110  comprises a casing  111  which contains one or, more preferably, multiple hollow polymeric fibers  112 ; the casing  111  is provided with a gas inlet port  113  for the intake of the gas flow entering into the corresponding separation stage, with a first gas outlet port  114  and with a second gas outlet port  115 , to allow the outflow of flows of retentate or permeate gases, obtained as a result of the interaction between the gas introduced and the membrane or membranes of the stage itself. 
     For example, the membranes can be of the type CO-810FSC or CO-810FC or CC-1610NFH or CC-1610SEH, marketed by the company UBE Europe GmbH. 
     As illustrated in  FIG. 1 , upstream of the first membrane-based gas separation stage  10 , besides the blower  41 , a compressor  42  is provided to bring the pressure of the flow of the gas mixture  101  to be treated to an adequate pressure when entering the first separation stage  10 , for example between 9 and 10 barg. 
     Therefore, the first membrane-based gas separation stage  10  receives the compressed flow of gas mixture coming out of the compressor  42  and is configured to separate it into a first flow of retentate or not permeate gas  11 , initially enriched in the first component (A), and into a first permeate gas flow  12  initially enriched in the second component (B). 
     As illustrated in  FIG. 1 , the second membrane-based gas separation stage  20  is positioned downstream of the first membrane-based gas separation stage  10 , for example near the first gas outlet port  114 , and is suitable to receive the first incoming retentate gas flow  11 , initially enriched in said first component (A), and to separate it again into a final retentate gas flow  21  further enriched in said first component (A) and into a second permeate gas flow  22 . 
     The final flow of retentate gas  21 , further enriched in said first component (A), comprises for example between 95% and 98% of methane gas (CH 4 ). 
     In this case, therefore, bio-methane is obtained having adequate purity which can be used as an energy source, introducing it, for example, into a methane gas distribution network, so avoiding its emission into the environment and preventing at the same time its negative greenhouse effect. 
     The second flow of permeate gas  22 , which may contain percentages of the gaseous component (A) more or less significant, can be further recycled, for example by pre-circulating it towards the area upstream of the first separation stage  10 . 
     For example, the second flow of permeate gas  22  is re-circulated upstream of the compressor  42  where it can be added to a new flow of gas mixture  101  coming out of the blower  41  and be treated again together with said new flow, exactly as described above. Advantageously, in the plant  100  according to the present disclosure, the first flow of permeate gas  12 , initially enriched in said second component (B), is sent directly, without subjecting it to a further compression, to the third gas separation stage  30 , positioned downstream of the first separation stage  10  for example near the second gas outlet port  115 . 
     In its turn, the third gas separation stage  30  with adsorption with oscillating pressure, is configured to receive in input the first flow of permeate gas  12  from the first separation stage  10  and to separate it, by means of adsorption, into a flow of recirculable gas  31  suitable to be re-circulated in the plant upstream of the first membrane-based gas separation stage  10 , and into a final gas flow  32  further enriched in the second component (B). 
     In particular, the further flow of recirculable gas  31 , which may still contain more or less significant percentages of the gaseous component (A) and/or of the gaseous component (B), is recirculated directly upstream or downstream of the blower  41 , depending on the pressure made available by the separation stage  30 , where a new flow of gas mixture  101  can be added and treated again together with it, exactly as described above. 
     In the plant  100  according to the disclosure, the third gas separation stage  30  comprises a plurality of separation tanks, i.e. two or more tanks, suitable to be connected to a supply line of the first flow of permeate gas  12  and each containing adsorbing means  1  for the separation, by means of adsorption, of the first flow of permeate gas  12  into said flow of re-circulable gas  31  and into said final gas flow  32  further enriched in said second component (B). 
     As will be seen in the following detailed description, according to the embodiment shown in  FIG. 3  there are foreseen two separation tanks  50  and  51 , while according to the embodiment shown in  FIGS. 4 and 6  there are foreseen three separation tanks  50 ,  51 ,  52 . 
     The adsorbing means  1 , depending on the gas mixture to be treated and above all on the final components (A) and/or (B) to be obtained, can be composed of one or more adsorbing filters, for example filters made of beds of molecular sieves, such as zeolites and aluminophosphates, silica gel, alumina, resins and/or polymers. 
     In particular, the tanks of the plurality of separation tanks  50 , 51 , 52  are operatively connected to each other and to the various supply lines of the plant  100 , by means of one or more valves schematically shown in the various figures by the reference number  40 , so that during operation of the plant  100 , under steady state operational conditions, they are switched in rotation among them with:
         at least a first tank  50  of the plurality of separation tanks isolated from the supply line of the first flow of permeate gas  12 , i.e. where the flow inside the first tank  50  is temporarily interrupted, and is subjected to a regeneration phase of the adsorbing means  1  contained therein by applying inside the first tank  50  a first pressure, preferably lower than the atmospheric pressure by operating a vacuum pump  61 ; and, at the same time:   at least one of the remaining separation tanks  51 ,  52  is connected to the supply line and continues to be fed, at a second pressure higher than said first pressure, for example between 0.3 and 1 barg, with the first flow of permeate gas  12 , continuing the separation process in order to generate the flow of re-circulable gas  31  and the final gas flow  32  further enriched in the second component (B).       

     In this way, in rotation, while the adsorbing means  1  of at least one tank saturated during the previous adsorbing phases are regenerated, operation of the plant  100  and production of gases enriched in components (A) and (B) continue thanks to one or more of the remaining tanks which remain operative on-line. 
     According to a first possible embodiment of the plant  100  of the disclosure shown in  FIG. 3 , the third gas separation stage  30  comprises a first separation tank  50  and a second separation tank  51  containing respective adsorbing means  1  dedicated to the separation of the first flow of permeate gas  12 ; furthermore, a storage tank  55  is provided connected to said first and second separation tanks  50  and  51  for the storage of at least a part of the final gas flow  32  further enriched in said second component (B), for example carbon dioxide (CO 2 ). 
     In particular, according to this first embodiment, a part of the final gas flow  32  produced by the separation tanks  50  and  51  is stored in the storage tank  55  and is destined to be subsequently re-introduced inside the first and the second separation tanks for washing the adsorbing means  1  contained therein during the respective regeneration phase. 
     In a second possible embodiment, as schematically shown in  FIG. 4 , the third gas separation stage  30  comprises a first separation tank  50 , a second separation tank  51  and a third separation tank  52 , operatively connected to each other and to various plant supply lines by means of one or more valves  40 , so that during operation of the plant  100 , in particular under steady state operational conditions, they are switched in rotation among them with:
         at least a first tank of the plurality of separation tanks, for example initially the first tank  50 , isolated from the supply line of the first flow of permeate gas  12 , i.e. where said flow inside the first tank  50  is temporarily interrupted, and is subjected to a regeneration phase of the adsorbing means  1  contained therein by applying inside the first tank  50  a first pressure, preferably lower than the atmospheric pressure by operating a vacuum pump  61 ; and at the same time   a second tank of the plurality of separation tanks, for example the tank  51 , which remains connected to the supply line and continues to be fed, at a second pressure higher than said first pressure, for example between 0.3 and 1 barg, with the first flow of permeate gas  12 , continuing the separation process in order to generate the flow of recirculable gas  31  and the final flow of gas  32  further enriched in the second component (B);   and the third separation tank, for example the third tank  52 , having finished the regeneration phase of its adsorbing means  1  and still isolated from the supply line  102  of the first flow of permeate gas  12  and waiting to be reconnected thereto.       

     In particular, in this second embodiment, reconnection of the third separation tank  52  to the supply line  102  of the first flow of permeate gas  12  takes place by connecting simultaneously the flow of permeate gas  12  to the third separation tank  52  and to the second separation tank  51 , so that the flows of gas which pass through the second tank  51  are at least partially deviated also inside the third tank  52 . 
     When the adsorbing means contained in the second tank  51  have reached the saturation point, the third tank  52  replaces the second tank  51  for production of the flow  31  while the second tank  51  is isolated from the supply line  102 . Afterwards, the first tank  50 , which meanwhile has been regenerated, is connected to the second separation tank  51  in order to temporarily equalize the pressure inside the first tank  50  with that inside the second tank  51  at a pressure level intermediate between the first pressure and the second pressure. 
     Once this pressure equalization has taken place, the first tank  50  remains isolated and waits to be reconnected to the supply line  102  in order to replace the third tank  52  in the production of the flow  31  when the latter reaches its saturation point, while the second tank  51  remains isolated from said supply line  102  and is subjected to the regeneration phase, which originates the production of the flow  32 . 
     In this way, in rotation, and similarly to what previously indicated, while the adsorbing means  1  of at least one tank, saturated during the previous adsorbing operations, are being regenerated, operation of the plant  100  and production of gases enriched in components (A) and (B) continue thanks to a second tank, with the further advantage that a third tank, in turn, participates in the production of gas flows  31  and  32  or also acts as a storage tank which in the first embodiment is constituted by the tank  55  dedicated exclusively to storage. 
       FIG. 2  schematically illustrates a method  200  for the separation of an initial gas mixture  101  containing a plurality of gaseous components, to be carried out in a plant  100  as previously described, comprising:
           210 : separating, by means of a first membrane-based separation stage  10 , a flow of the gas mixture entering the plant  100 , into a first flow of retentate gas  11 , initially enriched in a first component (A) of said plurality of components, and into a first flow of permeate gas  12  initially enriched in a second component (B) of said plurality of components;     215 : separating, by means of a second membrane-based gas separation stage  20 , said first flow of retentate gas  11  into a final flow of retentate gas  21  further enriched in said first component (A) and into a second flow of permeate gas  22  suitable to be re-circulated in the plant  100  upstream of the first membrane-based gas separation stage  10 , for example upstream of the compressor  42 ;     220 : separating, by means of a third separation stage  30  with adsorption with oscillating pressure (PSA), said first flow of permeate gas  12  initially enriched in a second component (B) of said plurality of components, into a recirculable gas flow  31  suitable to be re-circulated in the plant upstream of said first membrane-based gas separation stage  10 , for example upstream, or alternatively, if the pressure allows it, downstream of the blower  41 , and into a final gas flow  32  further enriched in said second component (B).       

     In particular, as previously described with reference to the plant  100 , the third gas separation stage  30  with adsorption with oscillating pressure (PSA), and in particular of the type at sub-atmospheric pressure (VSA or VPSA), comprises a plurality of separation tanks  50 ,  51 ,  52  and the step  220  of separating by means of the third stage comprises the phase  221  of switching in rotation among them the plurality of separation tanks when the plant is at steady state operations, so that:
         at least a first tank  50  of said plurality of separation tanks is isolated from the supply line  102  of the first flow of permeate gas  12  and subjected to a regeneration phase of the adsorbing means  1  contained therein, by applying inside the first tank  50  a first pressure, preferably lower than the atmospheric pressure;   and at least one of the remaining separation tanks  51 ,  52  is connected to the supply line  102  and fed, at a second pressure higher than said first pressure, for example between 0.3 and 1 barg, with the flow of permeate gas  12  to be separated into said re-circulable gas flow  31  and into said final gas flow  32  further enriched in said second component (B).       

     In the case of the first embodiment of the third separation stage  30  shown in  FIG. 3 , i.e. where the third stage  30  comprises a first separation tank  50 , a second separation tank  51 , and a further storage tank  55  connected to the first and second separation tanks  50  and  51 , the phase  221  of switching comprises the following steps:
           222 : storing in said storage tank  55  at least a part of the final gas flow  32  further enriched in said second component (B) outgoing from one or more of said first and second separation tanks  50  and  51 ;     224 : re-introducing at least a part of the final gas flow  32 , stored in the storage tank  55 , into each of said first and second separation tanks  50 ,  51  for washing the adsorbing means  1  contained therein during the respective regeneration phase.       

     In practice, in this embodiment and as shown in the sequence in  FIG. 3 , once the plant is operating at steady state, for example while the second tank  51  is subjected to a regeneration phase and is on standby not connected to the supply line  102 , the first flow of permeate gas  12  passes through the first tank  50  (Step  1 ) at a second pressure higher than the first pressure, with the adsorbing means  1  contained therein which adsorb in particular the component (B), for example carbon dioxide (CO 2 ), while the flows of re-circulable gas  31  are emitted and sent upstream of the blower  41  and here added to the flows of the gas mixture  101  which flow continually into the plant in order to be treated. 
     When the first tank  50  is being saturated, there is a brief transitory phase (Step  2 ) when both tanks  50  and  51  are connected to the supply line  102 , then they move on to an operational situation (Step  3 ) when only the second tank  51  is connected to the supply line  102  and where the first flow of permeate gas  12  passes through with the adsorbing means  1  contained therein that adsorb in particular component (B), for example carbon dioxide (CO 2 ), while the flows of recirculable gas  31 , still containing a significant percentage of component (A), for example methane, are emitted and sent upstream or downstream of the blower  41  and there added to the flows of gas mixture  101  which continually flow into the plant to be treated once again. Meanwhile, the first tank  50  begins the regeneration phase during which it is first connected to the storage tank  55 , and the gas rich in component (B) contained therein, for example carbon dioxide (CO 2 ), is re-introduced into the first tank  50  for washing the adsorbing means  1  contained therein before carrying out the true regeneration process. The flow obtained in this phase contributes to the formation of the re-circulable flow  31  and is therefore recirculated for example upstream of the blower  41 . 
     After washing, inside the first tank  50  a first pressure lower than the atmospheric pressure is applied, for example by means of connection to a vacuum pump  61  (Step  4 ); in this way, what entrapped in the adsorbing means is removed, and in particular the second component (B), for example carbon dioxide (CO 2 ), thus obtaining the final flow  32  further enriched in said second component. This flow  32  may contain a percentage of carbon dioxide (CO 2 ) even higher than 99% and which can be released for example into the atmosphere (off-gas), or reused for other purposes, for example alimentary purposes, and in part stored in the storage device  55 . 
     Finally, in said situation (Step  5 ), the newly regenerated separation tank  50  is put on standby while waiting to be reconnected with the supply line  102 , similarly to the situation described in Step  1  for the second tank  51 . 
     When the latter reaches the saturation point, everything described above is repeated with the tanks reversed. 
     In the second embodiment of the third separation stage  30  shown in  FIG. 4 , i.e. where the third stage  30  comprises a first separation tank  50 , a second separation tank  51 , and a third separation tank  52 , all totally similar and interchangeable among each other, the separation phase  220  comprises the sub-phase  221  of switching in rotation said first, second and third separation tanks, during operation of the plant, so that:
         at least a first tank, for example tank  50 , is isolated from the supply line  102  of the first flow of permeate gas  12  and subjected to a regeneration phase of the adsorbing means  1  contained therein, saturated with what previously adsorbed, by applying inside said first tank a first pressure, in particular lower than the atmospheric pressure;   a second tank, for example the second tank  51 , is connected to the supply line  102  and fed at a second pressure higher than said first pressure with the first flow of permeate gas  12  to be separated;   and a third separation tank, for example the third tank  52 , which has finished its regeneration phase, is at a pressure intermediate between that of the other two tanks, and is isolated from the supply line  102  of the first flow of permeate gas  12  while waiting to be reconnected to it.       

     In particular, in this embodiment, the switching sub-phase  221  comprises the following steps:
           228 : at a first instant, of so-called pre-saturation, in other words where the adsorbing means  1  contained in the separation tank  51  are close to saturation, connecting simultaneously the flow of permeate gas  12  to the third tank  52  and to the second tank  51  so that the gas flows which pass through the second tank are at least partially deviated also inside the third tank  52 ; and subsequently:     230 : at a second instant, of so-called saturation, where the adsorbing means  1  contained in the second separation tank  51  reach saturation, disconnecting the second tank  51  from the supply line  102  and connecting it to the first tank  50 , which in the meantime has finished the regeneration phase, in order to temporarily equalize between them the pressures inside the first tank  50  and the second tank  51  at a pressure level intermediate between the first pressure and the second pressure; and subsequently:     232 : placing said regenerated first tank  50  and following the equalization of the pressures, in a position isolated from the supply line  102  of the first flow of permeate gas  12  and waiting to be reconnected thereto and, at the same time, subjecting said second tank  51  to a regeneration phase of the adsorbing means  1  contained therein saturated with what previously adsorbed, by applying inside said second tank  51  a first pressure, in particular lower than the atmospheric pressure.       

     In, practice, in this embodiment, when the plant is operating at steady state, a first tank  50  (STEP  10 ) is fed, by the supply line  102 , with the first flow of permeate gas  12  at a higher pressure (second pressure or greater pressure) with its own adsorbing means  1  which adsorb in particular the component (B), for example carbon dioxide (CO 2 ), while the flows of re-circulable gas  31 , still containing a considerable percentage of the component (A), for example methane, are emitted and sent upstream of the blower  41  and here added to the flows of gas mixture  101  which flow continually inside the plant in order to be treated. At the same time, the second tank  51 , just regenerated and then brought to a pressure intermediate between that of regeneration and that of the supply line  102 , is isolated from the supply line  102  and is on standby waiting to be reconnected thereto, while the third tank  52  is in the regeneration phase by applying inside a first pressure, i.e. a pressure lower than the atmospheric pressure, for example by means of connection to a vacuum pump  61 . 
     In this way, all that is entrapped in the adsorbing means is removed and, in particular, the second component (B), for example carbon dioxide (CO 2 ), thus obtaining the final flow  32  further enriched in said second component, for example having a quantity of carbon dioxide (CO 2 ) even higher than 99% which, for example, can be released into the atmosphere (off-gas) or reused for other purposes, for example alimentary purposes. 
     Also in this case, when the first tank  50  is undergoing saturation, there is a brief transitory phase (STEP  11 ) where both the first and second tanks  50  and  51  are connected to the supply line  102 , while the third regenerated tank  52  is on standby waiting for its turn to be reconnected to the line  102 ; then (STEP  12 ) an operational situation occurs when only the second tank  51  is connected to the supply line  102 , where the first flow of permeate gas  12  flows through at a second pressure higher than the first pressure with the adsorbing means  1  therein contained which adsorb in particular the component (B), while the flows of recirculable gas  31  are emitted at the outlet. 
     During this time, the first tank  50  is connected to the third tank  52  on standby to equalize the pressures therein contained to an intermediate level between the first pressure of the third tank following regeneration and the high pressure inside the first tank  50  following the previous passing flow of permeate gas  12 . 
     In this way, quantities of gas rich in the component (A), for example methane, are discharged into a tank which will subsequently go on-line, and which is partially pressurized while, advantageously, the tank to be regenerated is depressurized thus reducing consumption of the vacuum pump in the successive regeneration phase. 
     Once the pressures have been equalized, (STEP  13 ), the third tank  52  is isolated and on standby waiting for saturation of the second tank  51 , while the first tank  50  is subjected to regeneration by applying inside it a first pressure lower than the atmospheric pressure, for example by means of connection to a vacuum pump  61 ; in this way, what entrapped in the adsorbing means  1  is removed, and in particular the second component (B), for example carbon dioxide (CO 2 ), thus obtaining again a final flow  32  further enriched in said second component. 
     In the successive stage (STEP  14 ), the first regenerated tank  50  is on standby waiting for its turn to be reconnected to said line  102 , while there is a brief transitory phase when both the second tank  51  and the third tank  52  are connected to the supply line  102 . 
     Subsequently (STEP  15 ), only the third tank  52  is connected to the supply line  102  and where the first flow of permeate gas  12  passes through at a second pressure higher than the first pressure with the adsorbing means  1  contained therein adsorbing in particular the component (B), while the flows of recirculable gas  31  are emitted at the outlet. 
     During this time, the second tank  51  is connected to the first tank  50  on standby, so as to equalize the pressures contained therein to a level intermediate between the first pressure inside the first tank following regeneration and the high pressure inside the second tank  51  following the previous passage of permeate gas  12 . 
     Once the pressures have been equalized, (STEP  16 ), the first tank  50  remains isolated on standby waiting for saturation of the third tank  52 , while the second tank  51  is subjected to regeneration by applying inside it a first pressure lower than the atmospheric pressure, for example by means of connection to a vacuum pump  61 ; in this way what is entrapped in the adsorbing means is removed and, in particular, the second component (B), for example carbon dioxide (CO 2 ), thus obtaining again a final flow  32  further enriched in said second component. 
     The procedure then continues with cyclic rotational switching of the tanks in the various operational situations, similarly to that previously described. 
     As can easily be understood by a person skilled in the art, the rotational switching of the tanks in the various operational situations previously described, may take place in moments or instants preferably predefined and pre-set, plant by plant, and/or said instants may also be calculated in real time and/or however redefined with respect to pre-set timing, during operation and on the basis of signals provided by suitable sensors associated for example to the tanks. 
     Similarly, in particular the moment or first instant when a tank is near saturation and the moment or second instant when saturation is considered reached, are preferably predefined and preset, application by application, in the control systems of a specific plant; and/or said first and second instants can be calculated in real time and/or recalculated and reset during operation on the basis of signals provided by suitable sensors associated for example to the tanks. 
     In practice, it has been evidenced how the plant  100  and the method  200  according to the disclosure make it possible to accomplish the scope as well as the objects prefixed. In particular, the use of a third separation stage with adsorption with variable or oscillating pressure, preferably of the vacuum pressure type, makes it possible to obtain adequate results in terms of desired purity of the gases selectively obtained from the initial biogas mixture, while utilizing a simplified constructive solution compared to solutions known in the art, which makes it possible to reduce the compression stages and, more generally, to reduce consumption and operational costs of the entire plant. 
     Naturally, without prejudice to the scope of the disclosure, many variations may be applied to the above-described exemplary and non-limiting embodiments and implementation details thereof, without departing from the spirit of the invention. For example, it is possible to modify the number of modules within the various separation stages and to use membranes of a different type; at least some stages of the method  200  may be carried out in a different sequence with respect to that described above for merely illustrative purposes, or all simultaneously; depending on the necessity and specific applications, it is possible to obtain final enriched gas flows with the desired percentage and degree of purity. For this purpose, the plant  100  can be suitably equipped with one or more regulation valves  60 , appropriately controlled, for example by a plant control unit not illustrated in the drawings, so as to regulate the flow of gas entering and/or leaving the respective separation stages, for example by modifying the rate of flow, etc.