Apparatus and method for absorbing a first gas species from a gas mixture

Abstract

Separating a gas from a mixture comprising at least two gas species may involve: 310 - feeding the gas mixture into an adsorption bed via a first opening, with a second opening of the bed closed, 320

Description

The present invention relates to apparatuses and methods for adsorbing a first gas species from a gas mixture, and more specifically to swing (i.e., temperature and / or pressure swing) adsorption. BACKGROUND Adsorption is a phenomenon whereby gases tend to be trapped onto solid surfaces under high pressure. Desorption refers to the subsequent lowering of pressure to remove / release the gases trapped to said surfaces. Different gas species will adsorb / desorb differently to different solids or as they are typically referred, “adsorbents”. Adsorption / desorption can be encouraged using temperature and / or pressure. Pressure swing adsorption (PSA) is a technique that exploits this effect by using pressure to separate a gas species from a mixture of gases. An example relates to the recovery of Krypton (Kr) gas from a Kr and Nitrogen (N2) mixture. The technique exploits the affinity of a gas species and the preferential adsorption behaviours for a particular adsorbent material. The adsorbent material (i.e., zeolites, activated carbon) are essentially a ‘trapping material’ that preferentially adsorbs the gas species from the mixture of gases at a particular pressure. To subsequently desorb the gas species from the adsorbent material, the pressure in the vicinity of the adsorbent material is lowered and the gas species removed. Vacuum pressure swing adsorption (VPSA) is a variant of PSA whereby the desorption process is performed at vacuum. The ratio of an adsorbent’s adsorption capacities for two gas species is typically referred to as ‘selectivity’ or more generally as the ‘adsorption ratio’. A higher adsorption ratio is favourable for more efficient and effective recovery of a particular gas species. SUMMARY OF THE INVENTION A conventional PSA / VPSA process consists of four main steps referred to as feed or feeding, adsorption, rinse and desorption. The feed step refers to the provision of a gas mixture to a first opening or inlet of an adsorption bed containing an adsorbent whilst the second opening or outlet to the bed is closed. In the feed step the bed becomes pressurised. The adsorption step refers to opening the second opening of the bed whilst maintaining the flow of the gas mixture into the bed to encourage the adsorption of a first gas species from the gas mixture. The rinse step refers to the flowing of a gas with high purity of the first gas species through the bed to increase the purity of the adsorbed first gas species . The desorption step refers to reducing the pressure in the bed to depressurise the bed and encourage desorption of the adsorbed first gas species from the bed. In many applications, the performance of PSAA / PSA techniques is evaluated in respect of ‘product purity’ and ‘gas recovery rate’. However, there tends to be a trade-off between product purity and gas recovery rate, with PSAA / PSA techniques being tuned to preferentially increase purity to the detriment of recovery rate, or vice versa. Hence there is a requirement for an adsorption apparatus and method that increases gas recovery rate with minimal compromise on product purity. In a first aspect, there is provided an apparatus for adsorbing a first gas species from a gas mixture comprising the first gas species and at least a second gas species, the apparatus comprising: at least one adsorption bed containing an adsorbent for adsorbing the first gas species from the gas mixture; means for supplying the gas mixture to the at least one adsorption bed for adsorption of the first gas species from the gas mixture; means for supplying a first rinse gas to the at least one adsorption bed for rinsing the at least one adsorption bed, wherein the first rinse gas comprises the first gas species at a first purity, wherein the means for supplying the first rinse gas comprises means for storing the first rinse gas; means for supplying a second rinse gas to the at least one adsorption bed for rinsing the at least one adsorption bed, wherein the second rinse gas comprises the first gas species at a second purity, the second purity being greater than the first purity; and means for desorbing the first gas species from the at least one adsorption bed; wherein the apparatus is configured to provide a gas flow path for the second rinse gas from the means for supplying the second rinse gas to the means for storing the first rinse gas via the at least one adsorption bed. It will be appreciated that the apparatus of the first aspect may utilise pressure swing and / or temperature swing for the adsorption / desorption cycles. The apparatus may comprise valve means for providing the gas flow path. The valve means may be configured to be operable between a plurality of configurations, wherein the plurality of configurations comprises a first configuration for feeding the at least one adsorption bed with the gas mixture, wherein in the first configuration the valve means fluidly connects the means for supplying the gas mixture to a first opening of the at least one adsorption bed and closes a second opening of the at least one adsorption bed; wherein the plurality of configurations further comprises a second configuration for adsorption of the first gas species from the gas mixture, wherein in the second configuration the valve means fluidly connects the means for supplying the gas mixture to the first opening of the at least one adsorption bed, and further fluidly connects the second opening of the at least one adsorption bed to a waste; wherein the plurality of configurations further comprises a third configuration for coarse rinsing the at least one adsorption bed with the first rinse gas, wherein in the third configuration the valve means fluidly connects the first opening of the at least one adsorption bed to the means for supplying the first rinse gas, and further fluidly connects the second opening of the at least one adsorption bed to the waste; wherein the plurality of configurations further comprises a fourth configuration for fine rinsing the at least one adsorption bed with the second rinse gas, wherein in the fourth configuration the valve means fluidly connects the first opening of the at least one adsorption bed to the means for supplying the second rinse gas, and further fluidly connects the second opening of the at least one adsorption bed to the means for storing the first rinse gas, thereby providing the gas flow path between the means for supplying the second rinse gas and the means for storing the first rinse gas, via the at least one adsorption bed; and wherein the plurality of configurations further comprises a fifth configuration for desorbing the first gas species from the at least one adsorption bed, wherein in the fifth configuration the valve means fluidly connects the at least one adsorption bed to the means for desorbing the first gas species. The apparatus may be a PSA or VPSA apparatus. The apparatus may be a temperature swing adsorption apparatus. The means for storing the first rinse gas may be a tank also referred to herein as a buffer tank. In the fifth configuration the valve means may fluidly connect the means for desorbing to the first opening. In such a configuration the second opening may be closed. The apparatus disclosed herein tends to improve gas recovery rate with minimal comprise on recovered product purity. Product purity can be considered as the purity of the desorption product extracted from the process i.e., the purity of the first gas species in the desorption product. The gas recovery rate is the net amount of the first gas species recovered divided by the amount of said first gas species in the original gas mixture. The amount of the first gas species recovered can be determined as the gross amount of the first gas species recovered (in the desorption product) minus the amount of the first gas species used for the fine rinsing. The apparatus disclosed herein tends to use the first rinse gas to perform a coarse rinse of the at least one adsorption bed to raise the in-bed concentration of the first gas species prior to rinsing with the higher purity second rinse gas. The apparatus disclosed herein tends to therefore mitigate prolonged rinsing using the second higher purity rinse gas. Furthermore the apparatus disclosed herein mitigates excessive wastage of higher purity second rinse gas. This tends to be because when the valve means is operated in the fourth configuration, the used gases are not directed to waste but are captured in the means for storing the first rinse gas. Hence the used higher purity rinse gas (second rinse gas) can be captured and stored for later use as a first rinse gas. Such gases would otherwise be lost to waste in prior art systems. For the apparatus disclosed herein, in-bed concentration of adsorbed first gas species tends to be maintained at a high purity (for instance up to 5N or 99.999% purity) because of the exposure to the gas mixture and the coarse and fine rinse gases. However, the amount of the first gas species used for the fine rinsing tends to be reduced (because of the coarse and fine rinsing) thereby improving gas recovery rate. Accordingly, the apparatus disclosed herein tends to enable high purity gas i.e., gas at the second purity (i.e., 5N gas or 99.999% purity) to be recovered with an improved gas recovery rate. Particularly in semiconductor fabrication processes, such as modern etching processes, gas species such as Krypton are used in large volumes. Such gases are costly and have high carbon footprint in their production. The apparatus herein thus tends to reduce overall cost and carbon footprint of such semiconductor fabrication processes by recovering more efficiently and with higher purity, such gas species from gas mixtures output from semiconductor manufacturing. Additionally, less high purity second rinse gas is lost to waste owing to the recirculation of said gases to the means for storing. Accordingly the recovered gases tend to be reusable (as both recovered desorption product and as a subsequent first (coarse) rinse gas stored in the means for storing). The valve means may comprise: a first valve connecting the means for supplying the gas mixture to the first opening of the at least one adsorption bed; a second valve connecting the second opening of the at least one adsorption bed to the waste; a third valve connecting the means for supplying the first rinse gas to the to the first opening of the at least one adsorption bed; a fourth valve connecting the means for supplying the second rinse gas to the first opening of the at least one adsorption bed; a fifth valve connecting the second opening of the at least one adsorption bed to the means for storing the first rinse gas; and a sixth valve connecting the first opening of the at least one adsorption bed to the means for desorbing the first gas species from the adsorbent. In the first configuration the second, third, fourth, fifth and sixth valves may be closed and the first valve may be open; in the second configuration the third, fourth, fifth and sixth valves may be closed and the first and second valves may be open; in the third configuration the first, fourth, fifth and sixth valves may be closed and the second and third valves may be open; in the fourth configuration the first, second, third and sixth valves may be closed and the fourth and fifth valves may be open; and in the fifth configuration the first, second, third, fourth and fifth valves may be closed and the sixth valve may be open. The plurality of configurations may further comprise a sixth configuration for supplementary fine rinsing the at least one adsorption bed using the second rinse gas, wherein in the sixth configuration the valve means fluidly connects the first opening of the at least one adsorption bed to the means for supplying the second rinse gas, and further connects the second opening of the at least one adsorption bed to the waste. The supplementary fine rinsing may be used to raise the concentration of the first gas species adsorbed into the adsorbent prior to the valve means being configured in the third configuration. This tends to enable the means for storing to see / receive a higher concentration of the first gas species when the apparatus is operated in the third configuration. In the sixth configuration the first, third, fifth and sixth valves may be closed and the second and fourth valves may be open. The valve means may be configured to operate between the plurality of configurations in the following order: the first configuration, the second configuration, the third configuration, the fourth configuration, then the fifth configuration. The valve means may be configured to operate between the plurality of configurations in the following order: the first configuration, the second configuration, the third configuration, the sixth configuration, the fourth configuration, then the fifth configuration. The valve means may operate in the various configurations for respective pre-determined periods of time and at pre-determined flow rates. Both may be tailored or tuned to achieve product purities of the recovered desorption product and recovery rates, as per customer need. The apparatus may comprise a control means for controlling the valve means to operate between the plurality of configurations. The control means may comprise at least one memory and at least one processor coupled with the at least one memory. The at least one memory may comprise instructions which when executed by the at least one processor causes the valve means to operate between the plurality of configurations as disclosed herein. The first gas species may be Krypton and the second gas species may be Nitrogen. The gas mixture may be 80:20 mix of Krypton to Nitrogen, for instance. The adsorbent may comprise a zeolite. The adsorption ratio of the first gas species to the second gas species for the adsorbent may be greater than or equal to 3 for a pressure of 1 bar. As noted herein, Krypton gas is used in large quantities for semiconductor manufacturing processes such as etching. Owing to the lack of abundance of Krypton and the costs and carbon footprint involved in sourcing and extracting Krypton from air or other sources, the recovery of Krypton and its re-use is particularly pertinent for the semiconductor manufacturing industry. Alternatively, the first gas species may be Argon, another gas used in abundance in the semiconductor industry, or any species that is worthwhile recovering for a given application. The at least one second gas may be any gas species flowing together with the first gas, for instance. The means for supplying the gas mixture may be configured to supply the gas mixture at a first pressure; the means for storing the first rinse gas may be configured to store the first rinse gas at second pressure, the second pressure being greater than the first pressure; the means for supplying the second rinse gas may be configured to supply the second rinse gas at the second pressure; and the means for desorbing the first gas species from the adsorbent may be configured to reduce the pressure in the at least one adsorption bed to a third pressure, wherein the third pressure is less than the first pressure. The at least one adsorption bed may initially be configured at a minimum pressure, for instance, 50mbar. The at least one adsorption bed may contain minimal gas, initially. When the means for suppling the gas mixture is connected to the first opening of the at least one adsorption bed in the first configuration, the pressure in the at least one adsorption bed tends to increase to the first pressure. The first pressure may be 1 bar, for instance. When the apparatus is operated in the second configuration, the flow rate to the at least one adsorption bed tends not to change. An optional back pressure regulator may be used, for instance. In the second configuration the adsorbent preferentially adsorbs the first gas species according to an adsorption ratio. The remainder of the gas mixture tends to be lost to the waste with minimal loss of the first gas species. When operated in the third configuration, the first rinse gas with the first purity flows from the means for storing to the at least one adsorption bed. This achieves a coarse rinse of the at least one adsorption bed. Resultantly, the inbed concentration of the first gas species tends to increase, up to, the first purity. The second pressure of the first rinse gas in the means for storing may be 2 bar, for instance and may reduce towards the first pressure when operated in the third configuration. When operated in the fourth configuration the second rinse gas rinses the at least one adsorption bed. The in-bed pressure is maintained and the concentration of the first gas species increases in-bed (towards the second purity). Concurrently, the pressure in the means for storing the first rinse gas tends to be pressurised back to the second pressure with higher purity gases (i.e., 90-99% first gas species purity may remain in gases exiting the second opening after propagating through the at least one adsorption bed) that flow out of the second opening of the at least one adsorption bed. When operated in the fifth configuration, the pressure in the at least one adsorption bed is swung to a lower pressure - the third pressure. This tends to encourage desorption of the first gas species from the adsorbent. The means for desorbing the first gas species may be configured to evacuate the at least one adsorption bed. The setting of the first, second and third pressures may be dependent upon the adsorbent, the first gas species, the gas mixture, the purity requirement and the recovery rate required for the first gas species. The disclosure herein provides specific examples in this respect, but these are not intended to be limiting. The first purity of the first rinse gas may be 90-99.9%. The second purity of the second rinse gas may be 99.999% purity. The at least one adsorption bed may comprise a plurality of adsorption beds for instance with respective first openings and second openings connected in parallel. The adsorption beds may be deliberately asynchronized in their operation to support continuous first opening flow and / or second opening flow. The adsorption beds in the apparatus disclosed herein may have no direct bed-to-bed interaction, tending to provide a simpler design. According to a second aspect, there is provided a method for adsorbing a first gas species from a gas mixture comprising the first gas species and at least a second gas species, the method comprising: feeding a first opening of at least one adsorption bed with the gas mixture with a second opening of the at least one adsorption bed closed, wherein the adsorption bed contains an adsorbent for adsorbing the first gas species from the gas mixture; feeding the first opening of the at least one adsorption bed with the gas mixture with the second opening of the at least one adsorption bed open, such that the gas mixture can flow through the at least one adsorption bed from the first opening to the second opening and to a waste; coarse rinsing the at least one adsorption bed with a first rinse gas comprising the first gas species at a first purity, wherein the coarse rinsing comprises flowing the first rinse gas from a means for storing the first rinse gas through the at least one adsorption bed from the first opening to the second opening and to a waste; fine rinsing the at least one adsorption bed with a second rinse gas comprising the first gas species at a second purity, wherein the second purity is greater than the first purity, wherein the fine rinsing comprises flowing the second rinse gas through the at least one adsorption bed from the first opening to the second opening and to the means for storing the first rinse gas; and desorbing the first gas species from the at least one adsorption bed. The method may optionally comprise supplementary fine rinsing the at least one adsorption bed using the second rinse gas after performing the coarse rinsing but prior to the fine rinsing, wherein the supplementary fine rinsing comprises flowing the second rinse gas through the at least one adsorption bed from the first opening to the second opening and to the waste. The methods described herein may be performed using the apparatus of the first aspect. Aspects of the disclosure herein may be provided or embodied as a program or plurality of programs arranged such that when executed by a computer system or one or more processors it / they cause the computer system or the one or more processors to operate in accordance with the methods disclosed herein. For instance, a control means for a valve means may be configured to operate in accordance with aspects of the disclosure herein to enable the methods disclosed herein. Such a program or plurality of programs may be stored on a machine readable storage medium. It will be appreciated that the technical benefits and advantages of the first aspect apply equivalently to the further aspects. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1A is a schematic illustration (not to scale) of an example apparatus according to the first aspect; Figure 1B is a schematic illustration (not to scale) of the apparatus of Figure 1A showing the gas flow path in the first configuration; Figure 1C is a schematic illustration (not to scale) of the apparatus of Figure 1A showing the gas flow path in the second configuration; Figure 1D is a schematic illustration (not to scale) of the apparatus of Figure 1A showing the gas flow path in the third configuration; Figure 1E is a schematic illustration (not to scale) of the apparatus of Figure 1A showing the gas flow path in the fourth configuration; Figure 1F is a schematic illustration (not to scale) of the apparatus of Figure 1A showing the gas flow path in the fifth configuration; Figure 1G is a schematic illustration (not to scale) of the apparatus of Figure 1A showing the gas flow path in the sixth configuration; Figure 2A is a graph showing the performance results of a conventional PSA apparatus and an example of an apparatus according to the first aspect; Figure 2B is a graph showing the performance results of Figure 2A and the performance results of a further example apparatus according to the first aspect; and Figure 3 shows an illustration of a method according to the second aspect. DETAILED DESCRIPTION Figure 1A is a schematic illustration (not to scale) of an example apparatus 100 according to the first aspect. The apparatus 100 is for adsorbing a first gas species from a gas mixture comprising the first gas species and at least a second gas species. In the present example, a Krypton / Nitrogen separation problem is discussed to describe the apparatus 100. More specifically, the first gas species is Krypton with the gas mixture containing a second gas species that is Nitrogen. Even more specifically, the gas mixture comprises Krypton and Nitrogen gas in a ratio 80:20. The apparatus 100 comprises at least one adsorption bed 110 containing an adsorbent (not visible) for adsorbing the first gas species from the gas mixture, wherein the at least one adsorption bed 110 comprises a first opening 112 and a second opening 114. The adsorbent comprises zeolite having a Krypton:Nitrogen adsorption ratio of 3 at a pressure of 1 bar. The at least one adsorption bed 110 is shown a sealed vessel with the first opening 112 and second opening 114. The apparatus 100 is shown as having at least one adsorption bed 100 denoted ‘adsorption bed T up to ‘adsorption bed X’. This is for illustrative purposes to indicate that the disclosure herein is not limited to examples comprising only a single adsorption bed. For the purposes of illustration, the apparatus 100 shall be described with respect to adsorption bed 1 i.e., the apparatus 100 shall be described as comprising one adsorption bed 110 on the understanding that the same description will be relevant for each bed 110 in the plurality of beds. The apparatus 100 comprises a means 120 for supplying the gas mixture to the at least one adsorption bed 110 for adsorption of the first gas species from the gas mixture. The means for supplying the gas mixture 120 may be fluidly connected to an output of a semiconductor fabrication equipment, or, may be fluidly connected to an output of other gas recovery systems, for instance, and may receive the gas mixture therefrom. The means 120 for supplying the gas mixture may comprise one or more pipes or pipework, for instance. The means 120 for supplying the gas mixture is configured to supply the gas mixture at a first pressure which may be 1 bar. The apparatus 100 also comprises means 130 for supplying a first rinse gas to the at least one adsorption bed 110 for rinsing the at least one adsorption bed 110. The first rinse gas comprises the first gas species at a first purity. The first purity is 90-99.9%. The means 130 for supplying the first rinse gas comprises means 132 for storing the first rinse gas. The means 132 for storing the first rinse gas is a buffer tank. The means 130 for supplying the first rinse gas comprises one or more pipes or pipework connecting the means 132 for storing the first rinse gas to the adsorption bed 110. The means 132 for storing the first rinse gas is configured to store the first rinse gas at second pressure, the second pressure being greater than the first pressure. The second pressure is denoted Ptank.max and may be 2 bar. In the present example it is required to recover 99.999% (5N purity) Krypton product from the gas mixture. A means 140 for supplying a second rinse gas to the at least one adsorption bed 110 for rinsing the at least one adsorption bed 110 is also provided. The second rinse gas comprises the first gas species at a second purity, the second purity being 99.999% i.e., greater than the first purity. The means 140 for supplying the second rinse gas comprises one or more pipes or pipework connecting the means 140 for supplying to the adsorption bed 110. The means 140 for supplying the second rinse gas is configured to supply the second rinse gas at the second pressure. The apparatus 100 further comprises means 150 for desorbing the first gas species from the at least one adsorption bed 110. One or more pipes or pipework connect the means 150 to the at least one adsorption bed 110. The means 150 for desorbing the first gas species from the at least one adsorption bed 110 is configured to reduce the pressure in the at least one adsorption bed 110 to a third pressure, wherein the third pressure is less than the first pressure. The third pressure may be vacuum. Prior to describing the connections of the various components of the apparatus 100, the term ‘gas recovery rate’ will be introduced. The gas recovery rate can be expressed according to Equation 1 in the context of Krypton recovery: Gas reowy rate speetotn f&e SafKtmixeurr Equation 1 Accordingly, to obtain a higher gas recovery rate, the recovered amount or quantity of a gas species needs to be increased, the quantity of gas species used in the first opening mixture needs to be reduced, or both. The net quantity of gas species recovered is the gross quantity of the gas species recovered minus the quantity of gas species consumed for the fine rinsing operation. The apparatus 100 further comprises valve means 161, 162, 163, 164, 165, 166. More specifically a first valve 161 connects the means 120 for supplying the gas mixture to the first opening 112 of the at least one adsorption bed 110. A second valve 162 connects the second opening 114 of the at least one adsorption bed 110 to a waste 170. A third valve 163 connects the means 130 for supplying the first rinse gas to the to the first opening 112 of the at least one adsorption bed 110. A fourth valve 164 connects the means 140 for supplying the second rinse gas to the first opening 112 of the at least one adsorption bed 100. A fifth valve 165 connects the second opening 114 of the at least one adsorption bed 100 to the means 132 for storing the first rinse gas. A sixth valve 166 connects the first opening 112 of the at least one adsorption bed 110 to the means 150 for desorbing the first gas species from the at least one adsorption bed 110. The valve means 161, 162, 163, 164, 165, 166 is configured to be operable between a plurality of configurations as will now be described. In a first configuration for feeding the at least one adsorption bed with the gas mixture, the valve means 161, 162, 163, 164, 165, 166 fluidly connects the means 120 for supplying the gas mixture to the first opening 112 of the at least one adsorption bed 110 and closes the second opening 114 of the at least one adsorption bed 110. In a second configuration for adsorption of the first gas species from the gas mixture, the valve means 161, 162, 163, 164, 165, 166 fluidly connects the means 120 for supplying the gas mixture to the first opening 112 of the at least one adsorption bed 100, and further fluidly connects the second opening 114 of the at least one adsorption bed 100 to the waste 170. In a third configuration for coarse rinsing the at least one adsorption bed 110 with the first rinse gas, the valve means 161, 162, 163, 164, 165, 166 fluidly connects the first opening 112 of the at least one adsorption bed 100 to the means 130 for supplying the first rinse gas, and further fluidly connects the second opening 114 of the at least one adsorption bed to the waste 170. In a fourth configuration for fine rinsing the at least one adsorption bed 110 with the second rinse gas, the valve means 161, 162, 163, 164, 165, 166 fluidly connects the first opening 112 of the at least one adsorption bed 100 to the means 140 for supplying the second rinse gas, and further fluidly connects the second opening 114 of the at least one adsorption bed 100 to the means 132 for storing the first rinse gas, such that gases flowing out of the second opening 114 of the at least one adsorption bed 100 can be stored in the means 132 for storing for subsequent use as the first rinse gas, thereby providing the gas flow path between the means 140 for supplying the second rinse gas and the means 132 for storing the first rinse gas, via the at least one adsorption bed. In a fifth configuration for desorbing the first gas species from the at least one adsorption bed 110, the valve means 161, 162, 163, 164, 165, 166 fluidly connects the at least one adsorption bed 110 to the means 150 for desorbing the first gas species. In the present example, a sixth configuration of the valve means 161, 162, 163, 164, 165, 166 is also operable. The sixth configuration is for supplementary fine rinsing the at least one adsorption bed 110 using the second rinse gas. In the sixth configuration the valve means 161, 162, 163, 164, 165, 166 fluidly connects the first opening 112 of the at least one adsorption bed 110 to the means 140 for supplying the second rinse gas, and further connects the second opening 114 of the at least one adsorption bed 110 to the waste 170. With the description of Figure 1 A, the gas flow paths in the first, second, third, fourth, fifth and sixth configurations will now be described with reference to Figures 1B-1G. Figure 1B is a schematic illustration (not to scale) of the apparatus 100 of Figure 1A showing the gas flow path in the first configuration. In the first configuration the second 162, third 163, fourth 164, fifth 165 and sixth 166 valves are closed and the first valve 161 is open. A gas flow path 181 is established from the means 120 for supplying the gas mixture to the first opening 112 of the at least one adsorption bed 110. Prior to opening the first valve 161 the adsorption bed 110 is at a minimum pressure, denoted Pbed.min, which may be 50mbar. The adsorption bed 110 may comprise near zero of any gas. The gas mixture flowing into the adsorption bed 110 increases the pressure in the adsorption bed 110 to the first pressure i.e., 1 bar. When the adsorption bed 110 pressure reaches the first pressure (a condition denoted Pbed.max), the valve means 161, 162, 163, 164, 165, 166 may be configured to the second configuration. Figure 10 is a schematic illustration (not to scale) of the apparatus 100 of Figure 1A showing the gas flow path in the second configuration. In the second configuration the third 163, fourth 164, fifth 165 and sixth 166 valves are closed and the first 161 and second 162 valves are open. A gas flow path 182 is established from the means 120 for supplying the gas mixture to the waste 170 via the at least one adsorption bed 100. With the adsorption bed 110 pressure reaching Pbed.max in the first configuration, the second valve 162 is opened while the gas mixture flow is maintained to the adsorption bed 110. In the second configuration the adsorbent in the adsorption bed 100 preferentially adsorbs more Krypton molecules that Nitrogen. In the second configuration, Nitrogen tends to be discharged to the waste 170 with negligible Krypton. Figure 1D is a schematic illustration (not to scale) of the apparatus 100 of Figure 1A showing the gas flow path in the third configuration. In the third configuration the first 161, fourth 164, fifth 165 and sixth 166 valves are closed and the second 162 and third 163 valves are open. A gas flow path 183 is established between the means 130 for supplying the first rinse gas, and the waste 170 via the at least one adsorption bed 110. The third configuration may be initiated after a pre-determined amount of time in the second configuration. In the third configuration the high purity first rinse gas of Krypton can flow from the buffer tank 132 to the at least one adsorption bed 110 to rinse the bed 110. The in-bed concentration (both adsorption and gas phase) of Krypton will be increased. The rinsing in the third configuration is referred to as a coarse rinse owing to the in-bed Krypton concentration only being able to be increased up to the purity level of the first rinse gas in the buffer tank 132. Prior to the third configuration, the buffer tank 132 stored the first rinse gas at a second pressure denoted Ptank.max which in the present example may be 2 bar. When the valve means 161, 162, 163, 164, 165, 166 is in the third configuration, the pressure in the buffer tank 132 decreases to between 1 bar and 2bar. A proportion of the Krypton in the first rinse gas will be lost to the waste 170 with the Nitrogen. However, this is a loss of the coarse rinse gas, not the higher purity fine rinse gas as will now be discussed. Figure 1E is a schematic illustration (not to scale) of the apparatus 100 of Figure 1A showing the gas flow path in the fourth configuration. In the fourth configuration the first 161, second 162, third 163 and sixth 166 valves are closed and the fourth 164 and fifth 165 valves are open. A gas flow path 184 is established between the means 140 for supplying the second rinse gas the means 132 for storing the first rinse gas via the at least one adsorption bed 110. The second rinse gas is provided at a pressure equal to the second pressure i.e., 2 bar. Constant pressure is maintained within the at least one adsorption bed 110. The flow of gas out of the at least one adsorption bed 110 is of high purity i.e., 90% Krypton. This flow of gas out of the at least one adsorption bed 110 pressurises the buffer tank 132 back to the second pressure or Ptank-max. The second rinse gas also increases the in-bed concentration or Krypton to 5N or 99.999%. Figure 1F is a schematic illustration (not to scale) of the apparatus 100 of Figure 1A showing the gas flow path in the fifth configuration. In the fifth configuration the first 161, second 162, third 163, fourth 164 and fifth 165 valves are closed and the sixth 166 valve is open. A gas flow path 185 is established between the at least one adsorption bed 110 and the means 150 for desorbing the first gas species. The fifth configuration may be operated after a predetermined amount of time in the fourth configuration. The means 150 for desorbing the first gas species evacuates the at least one adsorption bed 110. Desorption of the Krypton in the adsorbent occurs and high purity (i.e., 5N or 99.999%) Krypton flows out of the adsorption bed 110 via the first opening 112. This desorption product can subsequently be reused. In addition, following the operation of the valve means 161, 162, 163, 164, 165, 166 in the fifth configuration, the apparatus 100 can be considered as being returned to a state suitable for repeating the PSAA / PSA cycle. As described for the apparatus 100, the valve means 161, 162, 163, 164, 165, 166 may be operated between the plurality of configurations in the order of the first configuration, the second configuration, the third configuration, the fourth configuration, the fifth configuration. A further, sixth configuration may be integrated into the cycle as described with reference to Figure 1G. Figure 1G is a schematic illustration (not to scale) of the apparatus 100 of Figure 1A showing the gas flow path in the sixth configuration. In the sixth configuration the first 161, third 163, fifth 165 and sixth 166 valves are closed and the second 162 and fourth 164 valves are open. A gas flow path 186 is established between the means 140 for supplying the second rinse gas and the waste 170 via the at least one adsorption bed 110. The sixth configuration may be operated after the third configuration but prior to the fourth configuration. In the sixth configuration the second rinse gas of higher purity than the first rinse gas rinses the adsorption bed 110. Whilst this tends to result in additional loss of, in the present example, Krypton, performing such a fine rinse prior after the coarse rinse of the third configuration tends to mean that the in-bed concentration of Krypton is increased to a higher level prior to the fine rinse of the fourth configuration. Resultantly, in the fourth configuration, the buffer tank 132 receives gases of higher purity Krypton for storage as a first rinse gas for future PSA / VPSA cycles. For conciseness, the various configurations of the valve means 161, 162, 163, 164, 165, 166 are summarised in Table 1 where a ‘0’ indicates a closed valve position and a T indicates an open valve position. Configuration / Step First Valve Second Valve Third Valve Fourth Valve Fifth Valve Sixth Valve First / Feed 1 0 0 0 0 0 Second / Adsorption 1 1 0 0 0 0 Third / Coarse Rinse 0 1 1 0 0 0 Fourth / Fine Rinse 0 0 0 1 1 0 Fifth / Desorption 0 0 0 0 0 1 Sixth / Supplementary Rinse 0 1 0 1 0 0 Table 1 Figure 2A is a graph showing the performance results of a conventional PSA apparatus and an example of an apparatus according to the first aspect. The first set of performance results 210 relate to the conventional PSA apparatus. The second set of performance results 220 relate to the apparatus 100 as described herein when a supplementary rinse (i.e., operation of the valve means in the sixth configuration) is not performed. The performance results 210, 220 show the impurity level in parts per million on a logarithmic scale, against gas recovery rate. A brief discussion of the performance results 210, 220 follows. The performance results 220 for the apparatus 100 according to the first aspect show that a gas recovery rate of 0.4 can be achieved at an impurity level of 10ppm. In contrast for the conventional system results 210 it is shown that a gas recovery rate of 0.4 is achievable for an impurity level of approximately WOOppm. The performance results 220 for the apparatus 100 according to the first aspect show that a gas recovery rate of approximately 0.5 can be achieved at impurity levels between 10ppm and 100ppm. In contrast for the conventional system results 210 it is shown that a gas recovery rate of 0.5 is achievable for an impurity level between WOOppm and WOOOppm. The performance results 220 for the apparatus 100 according to the first aspect show that a gas recovery rate of approximately 0.55 and 0.6 can be achieved at impurity levels between 101 ppm and WOOppm. In contrast for the conventional system results 210 it is shown that a gas recovery rate of 0.55 and 0.6 is achievable for an impurity level between WOOppm and WOOOppm. Overall, the performance results 210, 220 show that the apparatus 100 according to the first aspect can achieve a given gas recovery rate at a lower level of impurity when compared to the convention system. Figure 2B is a graph showing the performance results of Figure 2A and the performance results of a further example apparatus according to the first aspect. Specifically, the performance results 210 of the conventional PSA system are shown, the performance results 220 of the apparatus 100 without supplementary fine rinse / sixth configuration are shown. In addition, the performance results 230 for the apparatus 100 performing the supplementary fine rinse / sixth configuration is also shown. The sixth configuration has been operated in the apparatus 100 after the coarse rinse (third configuration) but prior to the fourth configuration. A brief discussion of the performance results 230 relative to the performance results 220 follows to illustrate the benefit of the supplementary fine rinse (i.e., of operating the valve means in the sixth configuration). The performance results 230 show that by including a supplementary fine rinse operation in the apparatus 100 a gas recovery rate of 0.55 can be achieved for impurity levels below 100ppm. Figure 3 shows an illustration of a method 300 according to the second aspect. The method 300 is a method for adsorbing a first gas species from a gas mixture comprising the first gas species and at least a second gas species. The method may be performed using the apparatus 100. A first step 310 comprises feeding a first opening 112 of at least one adsorption bed 110 with the gas mixture with a second opening 114 of the at least one adsorption bed 110 closed, wherein the adsorption bed 110 contains an adsorbent for adsorbing the first gas species from the gas mixture. A further step 320 comprises feeding the first opening 112 of the at least one adsorption bed 110 with the gas mixture with the second opening 114 of the at least one adsorption bed 110 open, such that the gas mixture can flow through the at least one adsorption bed 110 from the first opening 112 to the second opening 114 and to a waste 170. A further step 330 comprises coarse rinsing the at least one adsorption bed 110 with a first rinse gas comprising the first gas species at a first purity, wherein the coarse rinsing comprises flowing the first rinse gas from a means 132 for storing the first rinse gas through the at least one adsorption bed 110 from the first opening 112 to the second opening 114 and to the waste 170. A further step 340 comprises fine rinsing the at least one adsorption bed 110 with a second rinse gas comprising the first gas species at a second purity, wherein the second purity is greater than the first purity, wherein the fine rinsing comprises flowing the second rinse gas through the at least one adsorption bed 110 from the first opening 112 to the second opening 114 and to the means 132 for storing the first rinse gas. A further step 350 comprises desorbing the first gas species from the at least one adsorption bed 110. It will be understood that the various steps 310, 320, 330, 340, 350 may define a cycle i.e., following the step 350 the method 300 may repeat in a further cycle starting with step 310 again. This cycle may be repeated a plurality of times. The method 300 may optionally comprise supplementary fine rinsing the at least one adsorption bed 110 using the second rinse gas prior to performing the coarse rinsing, wherein the supplementary fine rinsing comprises flowing the second rinse gas through the at least one adsorption bed 110 from the first opening 112 to the second opening 114 and to the waste 170. Apparatus, including the apparatus 100, for implementing the above arrangement, and performing the method 300 steps described above, may be provided by configuring or adapting any suitable apparatus, for example one or more computers or other processing apparatus or processors, and / or providing additional modules. The apparatus may comprise a computer, a network of computers, or one or more processors, for implementing instructions and using data, including instructions and data in the form of a computer program or plurality of computer programs stored in or on a machine readable storage medium such as computer memory, a computer disk, ROM, PROM etc., or any combination of these or other storage media. It should be noted that certain of the process steps depicted in the flowchart of Figure 3 and described above may be omitted or such process steps may be performed in differing order to that presented above and shown in Figure 3. Furthermore, although all the process steps have, for convenience and ease of understanding, been depicted as discrete temporally-sequential steps, nevertheless some of the process steps may in fact be performed simultaneously or at least overlapping to some extent temporally. Conventional PSA / VPSA cycles may contain a feed step, an adsorption step and a desorption step. Such conventional cycles may further comprise a fine rinse step. However, the fine rinse step tends to rinse an adsorption bed with the discharged gas going to waste (i.e., to a waste stream). The apparatus and methods disclosed herein tend to recirculate and capture, in a buffer tank, the discharged gases resulting from the fine rinse step that would otherwise go to waste. Hence a proportion of a first gas species i.e., Krypton in a fine rinse gas recirculates in the PSA / VPSA system. This tends to avoid direct loss of a first gas species i.e., Krypton. Furthermore, the captured and stored gases can be reused as a coarse rinse gas in a subsequent PSA / VPSA cycle, which tends to save on the amount of higher purity fine rinse gas that is required in the subsequent cycle. The disclosure herein provides an apparatus for adsorbing a first gas species from a gas mixture comprising the first gas species and at least a second gas species, the apparatus comprising a buffer tank. The apparatus may be configured to rinse an adsorption bed using a rinse gas. The apparatus may be configured to flow the rinse gas through the adsorption bed to the buffer tank such that the gases are not wasted after rinsing but are stored in the buffer tank for subsequent use as a coarse rinse gas. The gases stored in the buffer tank may be reused in subsequent PSA / VPSA cycles as a coarse rinse gas. The disclosure herein provides a method for adsorbing a first gas species from a gas mixture comprising the first gas species and at least a second gas species, the method comprising: feeding at least one adsorption bed with the gas mixture with a second opening of the at least one adsorption bed closed; feeding the first opening of the at least one adsorption bed with the gas mixture with the second opening of the at least one adsorption bed open; coarse rinsing the at least one adsorption bed with a first rinse gas comprising the first gas species at a first purity; fine rinsing the at least one adsorption bed with a second rinse gas comprising the first gas species at a second purity, wherein the second purity is greater than the first purity, wherein the fine rinsing comprises flowing the second rinse gas through the at least one adsorption bed from the first opening to the second opening and to a means for storing the first rinse gas; and desorbing the first gas species from the at least one adsorption bed. Whilst the disclosure and examples described herein may refer to a Krypton recovery application, this is not intended to be limiting. The disclosure herein is broadly applicable to gas purification applications, where the gas species to preserve is preferentially adsorbed relative to other gas species (i.e., impurities) in a gas mixture. Whilst the disclosure and examples described herein may refer to six configurations or steps in a PSAA / PSA cycle, it should be noted that the sixth configuration is optional. Whilst the disclosure and examples described herein may refer to one adsorption bed, plural adsorption beds may be used in parallel whilst being deliberately asynchronized. This tends to support continuous first opening flow and / or provide continuous second opening flow. This further tends to support a simpler design owing to a lack of bed-to-bed interaction. Whilst the disclosure and examples described herein may refer to one adsorption bed processing a gas mixture in continuum, it will be understood that a gas mixture may be processed in batches. Whilst the disclosure and examples described herein may refer to up to six valves, additional valves may be used, such as for reasons of safety. The six valves are described herein to illustrate the functionality of the valve means. Whilst the disclosure and examples described herein may detail adsorption and desorption cycles that are exemplified using pressure swing adsorption, the adsorption and desorption cycles may alternatively or in addition be controlled using temperature swing, REFERENCE NUMERALS 100 - apparatus 110 - adsorption bed 112 - first opening 5 114 - second opening 120 - means for supplying a gas mixture 130 - means for supplying a first rinse gas 132 - means for storing a first rinse gas 140 - means for supplying a second rinse gas 10 150 - means for desorbing a first gas species 161 - first valve 162 - second valve 163 - third valve 164 - fourth valve 15 165 - fifth valve 166 - sixth valve 170 - waste 181 - gas flow path 182 - gas flow path 20 183 - gas flow path 184 - gas flow path 185 - gas flow path 186 - gas flow path 210 - performance results 25 220 - performance results 230 - performance results 300 - method 310 - step of method 320 - step of method 330 - step of method 5 340 - step of method 350 - step of method

Claims

1. An apparatus for adsorbing a first gas species from a gas mixture comprising the first gas species and at least a second gas species, the apparatus comprising:at least one adsorption bed containing an adsorbent for adsorbing the first gas species from the gas mixture;means for supplying the gas mixture to the at least one adsorption bed for adsorption of the first gas species from the gas mixture;means for supplying a first rinse gas to the at least one adsorption bed for rinsing the at least one adsorption bed, wherein the first rinse gas comprises the first gas species at a first purity, wherein the means for supplying the first rinse gas comprises means for storing the first rinse gas;means for supplying a second rinse gas to the at least one adsorption bed for rinsing the at least one adsorption bed, wherein the second rinse gas comprises the first gas species at a second purity, the second purity being greater than the first purity; andmeans for desorbing the first gas species from the at least one adsorption bed;wherein the apparatus is configured to provide a gas flow path for the second rinse gas from the means for supplying the second rinse gas to the means for storing the first rinse gas via the at least one adsorption bed.

2. The apparatus of claim 1, wherein the apparatus comprises valve means configured to be operable between a plurality of configurations, wherein the plurality of configurations comprises:a first configuration for feeding the at least one adsorption bed with the gas mixture, wherein in the first configuration the valve means fluidly connects the means for supplying the gas mixture to a first opening of the at least one adsorption bed and closes a second opening of the at least one adsorption bed;a second configuration for adsorption of the first gas species from the gas mixture, wherein in the second configuration the valve means fluidly connects the means for supplying the gas mixture to the first opening of the at least one adsorption bed, and further fluidly connects the second opening of the at least one adsorption bed to a waste;a third configuration for coarse rinsing the at least one adsorption bed with the first rinse gas, wherein in the third configuration the valve means fluidly connects the first opening of the at least one adsorption bed to the means for supplying the first rinse gas, and further fluidly connects the second opening of the at least one adsorption bed to the waste;a fourth configuration for fine rinsing the at least one adsorption bed with the second rinse gas, wherein in the fourth configuration the valve means fluidly connects the first opening of the at least one adsorption bed to the means for supplying the second rinse gas, and further fluidly connects the second opening of the at least one adsorption bed to the means for storing the first rinse gas, thereby providing the gas flow path between the means for supplying the second rinse gas and the means for storing the first rinse gas, via the at least one adsorption bed; anda fifth configuration for desorbing the first gas species from the at least one adsorption bed, wherein in the fifth configuration the valve means fluidly connects the at least one adsorption bed to the means for desorbing the first gas species.

3. The apparatus of claim 2, wherein the valve means comprises:a first valve connecting the means for supplying the gas mixture to the first opening of the at least one adsorption bed;a second valve connecting the second opening of the at least one adsorption bed to the waste;a third valve connecting the means for supplying the first rinse gas to the to the first opening of the at least one adsorption bed;a fourth valve connecting the means for supplying the second rinse gas to the first opening of the at least one adsorption bed;a fifth valve connecting the second opening of the at least one adsorption bed to the means for storing the first rinse gas; anda sixth valve connecting the first opening of the at least one adsorption bed to the means for desorbing the first gas species from the at least one adsorption bed.

4. The apparatus of claim 3, wherein:in the first configuration the second, third, fourth, fifth and sixth valves are closed and the first valve is open;in the second configuration the third, fourth, fifth and sixth valves are closed and the first and second valves are open;in the third configuration the first, fourth, fifth and sixth valves are closed and the second and third valves are open;in the fourth configuration the first, second, third and sixth valves are closed and the fourth and fifth valves are open; andin the fifth configuration the first, second, third, fourth and fifth valves are closed and the sixth valve is open.

5. The apparatus of any one of claims 2-4, wherein the plurality of configurations further comprises a sixth configuration for supplementary fine rinsing the at least one adsorption bed using the second rinse gas, wherein in the sixth configuration the valve means fluidly connects the first opening of the at least one adsorption bed to the means for supplying the second rinse gas, and further connects the second opening of the at least one adsorption bed to the waste.

6. The apparatus of claim 5 when dependent upon claim 3 or 4, wherein:in the sixth configuration the first, third, fifth and sixth valves are closed and the second and fourth valves are open.

7. The apparatus of any one of claims 2-6, wherein the valve means is configured to operate between the plurality of configurations in the following order: the first configuration, the second configuration, the third configuration, the fourth configuration, then the fifth configuration.

8. The apparatus of any one of claims 5-6, wherein the valve means is configured to operate between the plurality of configurations in the following order: the first configuration, the second configuration, the third configuration, the sixth configuration, the fourth configuration, then the fifth configuration.

9. The apparatus of any one of claims 2-8, further comprising a control means for controlling the valve means to operate between the plurality of configurations.

10. The apparatus of any preceding claim, wherein the first gas species comprises Krypton and the second gas species comprises Nitrogen.

11. The apparatus of any preceding claim, wherein the adsorbent comprises a zeolite.

12. The apparatus of any preceding claim, wherein:the means for supplying the gas mixture is configured to supply the gas mixture at a first pressure;the means for storing the first rinse gas is configured to store the first rinse gas at second pressure, the second pressure being greater than the first pressure;the means for supplying the second rinse gas is configured to supply the second rinse gas at the second pressure; andthe means for desorbing the first gas species from the adsorbent is configured to reduce the pressure in the at least one adsorption bed to a third pressure, wherein the third pressure is less than the first pressure.

13. The apparatus of any preceding claim, wherein:the first purity is 90-99.9%; andthe second purity is a 99.999% purity.

14. The apparatus of any preceding claim, wherein the at least one adsorption bed comprises a plurality of adsorption beds connected in parallel.

15. A method for adsorbing a first gas species from a gas mixture comprising the first gas species and at least a second gas species, the method comprising:feeding a first opening of at least one adsorption bed with the gas mixture with a second opening of the at least one adsorption bed closed, wherein the adsorption bed contains an adsorbent for adsorbing the first gas species from the gas mixture;feeding the first opening of the at least one adsorption bed with the gas mixture with the second opening of the at least one adsorption bed open, such that the gas mixture can flow through the at least one adsorption bed from the first opening to the second opening and to a waste;coarse rinsing the at least one adsorption bed with a first rinse gas comprising the first gas species at a first purity, wherein the coarse rinsing comprises flowing the first rinse gas from a means for storing the first rinse gas through the at least one adsorption bed from the first opening to the second opening and to a waste;fine rinsing the at least one adsorption bed with a second rinse gas comprising the first gas species at a second purity, wherein the second purity is greater than the first purity, wherein the fine rinsing comprises flowing the second rinse gas through the at least one adsorption bed from the first opening5 to the second opening and to the means for storing the first rinse gas; anddesorbing the first gas species from the at least one adsorption bed;wherein the method optionally comprises supplementary fine rinsing the at least one adsorption bed using the second rinse gas prior to performing the coarse rinsing, wherein the supplementary fine rinsing comprises flowing the 10 second rinse gas through the at least one adsorption bed from the first opening to the second opening and to the waste.

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