Apparatus and method for purifying gases and method of regenerating the same

Abstract

A method and device for purifying a process gas mixture, such as a cryogen gas, in which impurity components of the mixture are removed by de-sublimation via cryo-condensation. The gas mixture is cooled to a temperature well below the condensation temperature of the impurities, by direct exchange of the gas mixture with a cooling source disposed in a first region of the device. The de-sublimated or frozen impurities collect about the cooling region surfaces, and ultimately transferred to a portion of the device defining an impurities storage region. The output-purified gas is transferred from the impurities storage region, is optionally passed through a first micrometer sized filter, through a counter-flow heat exchanger, and ultimately up to an output port at room temperature. A method of purging the collected impurities and regenerating the device is also disclosed.

Claims

1. A gas purifier for removing gaseous impurities from a cryogen gas comprising: a housing having an inlet for receiving a cryogen gas to be purified and a purified gas outlet, said housing defining a hollow interior which defines a first region in an uppermost interior portion thereof and a second region in a lower interior portion thereof; a coldhead disposed in the first region and operative to contact a flow of the cryogen gas sought to be purified received through the inlet, the coldhead being operative to cool the cryogen gas to a temperature sufficient to de-sublimate at least one gaseous impurity present in the cryogen gas; a heat exchanger disposed in the first region of the interior of the housing and connected to the purified gas outlet, the heat exchanger comprising an elongate, tubular segment which coils around the coldhead; a heater disposed within said first region of the interior of the housing, the heater being operative to cause sublimation of the at least one impurity de-sublimated in the first region, the heater being positioned between the coldhead and the heat exchanger; and a collection mechanism coupled to the purified gas outlet, the collection mechanism being disposed within the second region and selectively positioned therein such that the cryogen gas passes therethrough, flowing from the second region towards the first region, and through the outlet while retaining the at least one de-sublimated impurity within the interior of said housing, the collection mechanism being connected to the heat exchanger; the heater and the coldhead being adapted for cooperative use in a first mode wherein the heater is activated and the coldhead is deactivated to cause sublimation of the at least one impurity de-sublimated in the first region; the heater and the coldhead further being adapted for cooperative use in a second mode wherein the heater is deactivated and the coldhead is reactivated to maintain the temperature of the collection mechanism below a predetermined maximum temperature; wherein: the second region of the interior of the housing includes at least one surface to retain the at least one de-sublimated impurity formed in the first region; the housing comprises a vertically-oriented Dewar; and the collection mechanism includes a filter mechanism comprising a sheet of metallic wire mesh; the gas purifier further comprises at least one first sensor disposed within the interior of the Dewar in the first region and in operative communication with the coldhead and the heater, the coldhead and the heater being operative to selectively activate and deactivate to transition between the first and second modes in response to information received from the at least one first sensor; the collection mechanism further comprises a filter cartridge assembly and at least one second sensor disposed on said filter cartridge assembly and in operative communication with the coldhead and the heater, the at least one second sensor being operative to monitor the temperature of said filter cartridge assembly as the cryogen gas flows from the second region towards the first region; and the at least one first sensor and the at least one second sensor being configured to monitor changes in temperature within the purifier as the cryogen gas flows from the second region toward the first region through the collection mechanism.

2. The gas purifier of claim 1 wherein the metallic wire mesh includes a plurality of micropores formed therein, the micropores having a size ranging from 1 to 25 micrometers.

3. The gas purifier of claim 1 further comprising a second heater disposed within the second region of the interior of the Dewar, the second heater being operative to liquefy and facilitate evaporation of the at least one de-sublimated impurity disposed within the second region of the interior of the Dewar.

4. The gas purifier of claim 1 wherein the cryogen gas sought to be purified is helium and the at least one impurity comprises oxygen.

5. The gas purifier of claim 4 wherein the at least one impurity further includes nitrogen.

6. The gas purifier of claim 1 further comprising: a first pressure sensor in communication with the inlet to measure an inlet pressure of cryogen gas flowing through the inlet, and a second pressure sensor in communication with the outlet to measure an outlet pressure of the cryogen gas flowing through the outlet.

7. The gas purifier of claim 6 wherein the metallic mesh defines a plurality of apertures having an aperture size from 1 micrometer to 25 micrometers.

8. The gas purifier of claim 6 wherein the cryogen gas comprises helium and the gaseous impurities comprise oxygen and nitrogen.

9. The gas purifier of claim 6, wherein the first region defines a first zone formed within an uppermost portion of the interior chamber and a second zone formed adjacent to the first zone, and the second region defines a third zone disposed below the second zone within a lowermost portion of the interior chamber, the gas purifier further including a second heater disposed within the third zone of the interior chamber of the Dewar, the second heater being operative to liquefy and facilitate the evaporation of the de-sublimated impurities collected within the impurities storage region of the third zone.

10. The gas purifier of claim 1, wherein the filter cartridge assembly includes a funnel and an outlet conduit extending therefrom.

11. The gas purifier of claim 1, wherein the housing defines a longitudinal axis, the sheet of metallic wire mesh extending perpendicular to the longitudinal axis.

12. The gas purifier of claim 11, wherein the sheet of metallic wire mesh includes a mesh disc.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other features and advantages of the various embodiments disclosed herein will be better understood with respect to the following description and drawings, in which like numbers refer to like parts throughout, and in which:

(2) FIG. 1A is a pressure-temperature phase diagram, at constant volume, for helium (He), nitrogen (N.sub.2), oxygen (O.sub.2) and hydrogen;

(3) FIG. 1B is a pressure-temperature phase diagram similar to FIG. 1A but corresponding to a particular case of FIG. 1A for a working pressure of 2 bar absolute that includes water, Xe and Ne, and including a scale on the right side thereof specifying the volume concentration of a given impurity at each temperature;

(4) FIG. 2A is a cross-sectional view of a gas purifier apparatus constructed in accordance with a preferred embodiment of the present invention wherein the purifier apparatus is shown receiving an input of cryogen gas to be purified whereby the latter is shown cooling down from room temperature;

(5) FIG. 2B is the cross-sectional view of the purifier apparatus of FIG. 2A wherein the cryogen gas is shown undergoing purification after initial cool down, such purification being reflected by a frost of de-sublimated impurities forming within the upper-most portion of the interior of the apparatus;

(6) FIG. 3A is the cross-sectional view of FIGS. 2A and 2B wherein the purifier is shown undergoing a soft regeneration process;

(7) FIG. 3B is the cross-sectional view of FIGS. 2A-2B and FIG. 3A wherein the purifier is shown purifying a gas after a sublimation/impurity displacement process;

(8) FIG. 4A is a graph depicting fluctuations of several parameters (e.g., flow rate, incoming pressure, outgoing pressure, and temperatures as a function of time during an impurity de-sublimation process;

(9) FIG. 4B is a graph depicting exemplary fluctuations of several parameters (e.g., flow rate, incoming pressure, outgoing pressure, and temperatures) as a function of time during an impurity de-sublimation process occurring during a soft regeneration;

(10) FIG. 4C is a graph which is representative of a month of operation of a prototype of the present invention between two N.sub.2 regenerations (140K) during which the system automatically performed 11 soft regeneration processes;

(11) FIG. 5 is the cross-sectional view of FIGS. 2A-2B and 3A-3B wherein the purifier is shown undergoing a regeneration process as accomplished by the combined effort of first and second heaters operative to displace impurities from a de-sublimation area to an impurities storage area (heater 1) and ultimately liquefied and evaporated (heater 2) through a vent valve opened to the atmosphere; and

(12) FIG. 6 is a partially-exploded view of a filter mechanism for use with the gas purifiers of the present invention as constructed in accordance with a preferred embodiment.

DETAILED DESCRIPTION OF THE INVENTION

(13) The detailed description set forth below is intended as a description of the presently preferred embodiment of the invention, and is not intended to represent the only form in which the present invention may be implemented or performed. The description sets forth the functions and sequences of steps for practicing the invention. It is to be understood, however, that the same or equivalent functions and sequences may be accomplished by different embodiments and that they are also intended to be encompassed within the scope of the invention.

(14) Bearing the foregoing in mind, the present invention is directed to methods and devices for purifying a process gas mixture (i.e., cryogen gas) in which the gaseous impurity components of the mixture are removed by de-sublimation. In this regard, the working principle of this invention is cryo-condensation, which is a method well-known in the art to essentially freeze-out undesired components (i.e., impurities) from a given gas mixture by cooling down the mixture well below the condensation temperature of the impurities sought to be removed. FIG. 1 depicts a pressure-temperature phase diagram for a helium gas mixture having impurities of N.sub.2, .sub.2 and H.sub.2.

(15) Considering that the initial molar fraction, Y.sub.j, at Room Temperature (RT), of an impurity represented by the index j in the gas mixture, can be approximated by the ratio of its partial pressure, P.sub.j, to the total pressure of the mixture, P.sub.m (the approach is valid for ideal gases or small molar fractions),

(16) $Y_j=\frac{P_j}{P_m}$

(17) The partial pressure of a frozen impurity at any temperature below its condensation temperature, T.sub.cj, that is, for any T<T.sub.cj(P.sub.j), is given by the vapor pressure of the condensate at T; in other words, it can be represented by the solid line separating Vapor (V) and Solid (S) phases for the specific impurity. As illustrated in FIG. 1, the continuous lines correspond to the saturation V-S, V-L lines for each component, the total Pressure (P) of the mixture being typically 2 bar. The respective dashed lines with the arrows indicate the partial pressure of the respective components of the mixture during their cool down. When a given component reaches the de-sublimation V.fwdarw.S line, then it follows this continuous line, decreasing with T, and does not leave this line when heating up until all the frozen mass becomes vapor, or liquid first and then vapor, depending on total condensed amount of the impurity. As will be appreciated, Y.sub.j(T) dramatically decreases by orders of magnitude once the sublimation (V.fwdarw.S) line is reached and T is further decreased.

(18) Thus, for helium (He) at room temperature and 2 bar having small volume concentrations (<1% in total) of mainly N.sub.2 and O.sub.2 after cool down of the mixture below 30 K, the concentration of O.sub.2 and N.sub.2 in the gas phase will be reduced to below 0.5 ppm and to negligible values once the mixture is cooled below 20 K.

(19) In the example illustrated in FIG. 1, the dashed lines, with their corresponding arrows, indicate the Pj-T trajectory of the vapor phase for each component, (j=N.sub.2, O.sub.2, H.sub.2), during initial cool down. It is an isobaric process until the temperature reaches the condensation (de-sublimation) value of the given component. Then, when the sublimation S-V saturation line is reached, the impurities are immediately frozen and their corresponding partial pressures on the mixture are determined by the vapor pressure of the condensates. Further decreasing of the temperature dramatically reduces the vapor pressure of the frozen impurity.

(20) The same principles also apply with respect to purging or removing the collected de-sublimated impurities. In this context, and after a certain time frozen impurities are accumulated, the system is heated for regeneration (sublimation of the impurities), discussed more fully below, whereby each frozen component will follow first the S-V solid line, back up until all the condensate mass becomes vapor if the resulting partial pressure is smaller than the triple point pressure, or until the triple point through the S-V line first, and then, further up in partial pressure trough the L-V saturation line, until all the accumulated mass of the impurity becomes finally vapor.

(21) Referring now to FIGS. 2A-3B and 5, and initially to FIGS. 2A and 2B, there is shown an embodiment of a gas purifier or apparatus 10 for purifying gases as constructed in accordance with the present invention. As illustrated, the apparatus 10 is configured as a vertically-oriented housing, namely, a vertical vapor shielded helium Dewar 12 having an elongate, generally cylindrical configuration. With greater particularity, the Dewar 12 includes a gas inlet 14 for receiving a cryogen gas to be purified and a post-purification gas outlet 16. The gas inlet and outlets 14, 16 are disposed proximate the top end of the Dewar 12 as viewed from the perspective shown in FIGS. 2A-3B, with the gas inlet 14 fluidly communicating with an elongate, generally cylindrical interior chamber 17 of the Dewar 12. The interior chamber 17 is defined by an inner container 18 of the Dewar 12 which is concentrically nested within an outer container 20 thereof. A vacuum chamber 22 of the Dewar 12 is defined between the inner and outer containers 18, 20. Though not shown in the drawings, the Dewar 12 may also be outfitted with several radiation shields within prescribed interior regions thereof.

(22) That portion of the interior chamber 17 disposed proximate the gas inlet and outlets 14, 16, which is commonly referred to as the neck of the Dewar 12, receives and accommodates a cooling device or coldhead 24 of the apparatus 10. The coldhead 24 includes three separate sections, including a first section 24a, a second section 24b, and a third section or cold tip 24c. In this regard, as labeled in FIGS. 2A-3B, the first section 24a of the coldhead 24 defines a first stage thereof, with the second and third sections 24b, 24c collectively defining a second stage thereof The coldhead 24 is a known component in the art, an example being a Gifford-McMahon (GM) two-stage closed cycle refrigerator (refrigerator compressor not shown). The first section 24a (i.e., the first stage) of the coldhead 24, in combination with a corresponding portion of the inner container 18, defines a first part of a deep cooling region within the interior chamber 17, labeled as Zone 1 in FIGS. 2A-3B. The second and third sections 24b, 24c (i.e., collectively the second stage) of the coldhead 24, in combination with a corresponding portion of the inner container 18, define a second part of the deep cooling region within the interior chamber 17, labeled as Zone 2 in FIGS. 2A-3B. That remaining portion of the interior chamber 17 extending below Zone 2 as viewed from the perspective shown in FIGS. 2A-3B and labeled as Zone 3 defines an impurities storage zone or region whereby frozen impurities are collected following de-sublimation thereof in Zones 1 and 2. As will be described with greater particularity below, also disposed within Zone 3 are hardware components necessary to provide an optional filtering system operative to ensure that any impurities, typically in their solid, de-sublimated form, do not become reintroduced into the purified cryogen gas stream generated by the apparatus 10 and methods of the present invention.

(23) In a preferred implementation of the apparatus 10, the same is provided with a counter-flow heat exchanger 26. The heat exchanger 26 comprises an elongate, tubular segment of a material having prescribed thermal transmission characteristics which is coiled in the manner shown in FIGS. 2A-3B. In this regard, the heat exchanger 26 is formed in such that the outer diameter of the coils thereof is less than the inner diameter of the interior chamber 17 as allows the heat exchanger 26 to be advanced into the neck region of the Dewar 12, and in particular the interior chamber 17 thereof. At the same time, the inner diameter of the coils of the heat exchanger 26 is sized to circumvent the coldhead 24, thus allowing for the effective advancement of the coldhead 24 into the interior of the heat exchanger 26. As seen in FIGS. 2A-3B, in a preferred implementation, the heat exchanger 26 is sized relative to the coldhead 24 such that the outermost pair of coils is disposed generally proximate respective ones of the distal ends of the first and third sections 24a, 24c, the lowermost coil of the heat exchanger 26 thus being located at approximately the junction between Zones 2 and 3. However, those of ordinary skill in the art will recognize that this relative sizing between the coldhead 24 and heat exchanger 26 is exemplary only, and may be modified without departing from the spirit and scope of the present invention. In the apparatus 10, the upper end of the heat exchanger 26 terminating proximate the upper end of the first section 24a is fluidly coupled to the gas outlet 16.

(24) In the apparatus 10, the lower end of the heat exchanger 26 proximate the third section 24c is defined by a straight portion which extends generally along the axis of the interior chamber 17. Along these lines, in accordance with a preferred fabrication method, the heat exchanger 26 is formed from the aforementioned elongate segment of tubular material stock, with one section thereof being coiled, and one section being maintained in a generally straight configuration.

(25) The apparatus 10 further preferably comprises a first heater 30. The first heater 30 is electrically connected to a suitable power supply, and may be positioned between the coldhead 24 and the heat exchanger 26 proximate to the junction between the first and second stages, and hence Zones 1 and 2. In a preferred implementation, the first heater 30 may be wound onto portions of the coils of the heat exchanger 26 in the aforementioned location. The use of the first heater 30 will be described in more detail below. In addition, disposed on a prescribed location of the third section 24c or cold tip of the coldhead 24 is a sensor 32 (e.g., a thermal diode, thermometer). The sensor 32 electrically communicates with both the coldhead 24 and the first heater 30, and is operative to selectively toggle each between on and off states for reasons which will also be described in greater detail below.

(26) As further seen in FIGS. 2A-3B, in accordance with the present invention, the lower end of the heat exchanger 26 as defined by the distal end of the straight portion thereof is fluidly coupled to a collection mechanism that is operative to receive purified cryogen gas within Zone 3 and transfer the same to gas outlet 16 via the heat exchanger 26 with de-sublimated impurities being left behind within Zone 3. The collection mechanism is disposed in Zone 3 and may simply include a device such as a funnel, font or other like device. In a preferred embodiment, the collection mechanism comprises a filter cartridge assembly 34 which is shown with particularity in FIG. 6.

(27) The use of the filter cartridge assembly 34 as the collection mechanism, or as part of the collection mechanism, is optional within the apparatus 10. In FIGS. 2A-3B and 5, the apparatus 10 is depicted as including the filter cartridge assembly 34 as the collection mechanism. When viewed from the perspective shown in FIGS. 2A-3B, such filter cartridge assembly 34 is positioned within Zone 3 at a lower portion of the interior chamber 17 defined by Dewar 12. With greater specificity, the filter cartridge assembly 34 is positioned within the interior chamber 17 at an orientation sufficient to enable helium gas to be collected and passed therethrough, and thereafter through the heat exchanger and the gas outlet 16 in sequence, while leaving remaining de-sublimated and/or liquefied impurities within an impurities collection/storage region of Zone 3 as will be described in greater detail below.

(28) In the embodiment depicted in FIG. 6, the filter cartridge assembly 34 comprises a cylindrically configured, hollow collection member 36 into which the purified gas flows. After entering the collection member 36, the gas is passed through a filtering mechanism residing within the interior thereof. Exemplary filtering mechanisms which may be integrated into the filter cartridge assembly 34 include a bulk filter 38 or a thin layer filter 40, these filtering mechanisms being adapted to prevent impurities from being reintroduced within the cryogen gas sought to be purified through the use of the apparatus 10. The filter cartridge assembly 34 further comprises a funnel 42 which is attached to the collection member and effectively encloses the filtering mechanism therein. The funnel 42 is fluidly coupled to one end of an elongate, tubular outlet conduit 44 also included in the filter cartridge assembly 34. As seen in FIGS. 2A-3B, that end of the outlet conduit 44 opposite the end attached to the funnel 42 is fluidly connected to the heat exchanger 26, and more particularly to the distal end of the generally straight, non-coiled section thereof. The functionality of the filter cartridge assembly 34 (if included in the apparatus 10) based on preferred material selections for the particular filtering mechanism integrated therein will be described in more detail below.

(29) The apparatus 10 further preferably comprises a second heater 46. The second heater 46 is also electrically connected to a suitable power supply and, when viewed from the perspective shown in FIGS. 2A-3B, is preferably positioned between the lower or bottom end of the interior chamber 17 and the filter cartridge assembly 34. Within the apparatus 10, this particular region of the interior chamber 17 adjacent to its lower end is characterized as the aforementioned impurities storage region thereof The use of the second heater 46 will also be described in more detail below. In addition, disposed on a prescribed location of the filter cartridge assembly 34 (if included) is a sensor 48 (e.g., a thermal diode, thermometer) which electrically communicates with the coldhead 24 and the first heater 30. The sensor 48 is operative to monitor the temperature of the filter cartridge assembly 34 for reasons which will be described in more detail below as well.

(30) Having thus described the structural features of the apparatus 10, an exemplary method of using the same will now be described with reference to the FIGS. 2A-3B. FIGS. 2A and 2B depict the apparatus 10 receiving a cryogen gas to be purified at room temperature and during purification after initial cool down. The gas mixture enters Zone 1 through the gas inlet port 14 and is precooled by the first stage of the coldhead 24. The cooling of the gas mixture by the coldhead 24 is supplemented by the further cooling attributable to a direct heat exchange with the output gas flowing through the coils of the heat exchanger 26. As will be appreciated by those skilled in the art, the heat exchange facilitated by the heat exchanger 26 advantageously helps to minimize the cooling power extracted from the coldhead 24.

(31) In accordance with a preferred embodiment, the incoming gas will be cooled to a temperature of 30 K or less, and preferably 10 K. In operation of the apparatus 10, the speed of the gas molecules for a typical input flow rate of 30 L/min decreases rapidly from a few cm/s down to 1-2 cm/min due to density increases. Some impurities in the gas introduced into Zone 1 via the gas inlet 14 may immediately reach super-saturation at some point down in Zone 1 and will start coating at least portions of the surfaces within that portion of the neck of the interior chamber 17. In greater detail, these frozen impurities (labeled as 50a in FIGS. 2B and 3B) may start coating portions of the first section 24a (i.e., the first stage) of the coldhead 24, one or more coils of the heat exchanger 26 which reside in Zone 1, and/or a corresponding portion of the inner container 18 which defines Zone 1. Thereafter, the gas mixture reaches Zone 2 where it is deep cooled down to a temperature at which all the remaining impurity components are de-sublimated and coat several different surfaces in Zone 2. In greater detail, these remaining frozen impurities (labeled as 50b in FIGS. 2B and 3B) coat at least portions of the second and third sections 24b, 24c (i.e., the second stage) of the coldhead 24, one or more coils of the heat exchanger 26 which reside in Zone 2, and/or a corresponding portion of the inner container 18 which defines Zone 2.

(32) In order for the apparatus 10 to run in as continuous a manner as possible such that minimal time and effort are expended to dislodge or otherwise transfer the de-sublimated impurities 50a, 50b collected within Zones 1 and 2, the present invention further contemplates regeneration processes, and more particularly a soft regeneration process, operative to remove such impurities 50a, 50b from Zones 1 and 2 to the aforementioned impurities storage region of Zone 3. FIG. 3A illustrates the apparatus 10 as effectuating such soft regeneration (i.e., sublimation) process. As shown, the coldhead 24 is deactivated and first heater 30 concurrently activated until the third section 24c or cold tip of coldhead 24 reaches the sublimation and/or liquefaction temperature of the frozen impurities 50a, 50b in Zones 1 and 2. This causes the frozen impurities 50a, 50b to sublimate and/or liquefy, and fall down towards the impurities storage region of the interior chamber 17. As they fall, the impurities are again subjected to low de-sublimation temperatures. Since the impurities are again supersaturated in the gas mixture, they consequently are again frozen (such re-frozen impurities being labeled as 50c in FIGS. 3A and 3B), and may adhere to surfaces within Zone 3 and/or finally fall down into the impurity storage region. During the regeneration process, which can be repeated as often as needed, the temperature in the lower portion of Zone 3, including the temperature of the filter cartridge assembly 34 therein, does not change substantially as its temperature remains less than 20 K, while the temperature of the third section 24c of the coldhead 24 rises up to 90-100 K, ensuring complete sublimation/liquefaction of impurities within Zones 1 and 2.

(33) Along those lines, during the regeneration or sublimation process, the temperature of the filter cartridge assembly 34 is monitored via sensor 48. It is contemplated that the regeneration process will be interrupted (the first heater 30 deactivated and the coldhead 24 reactivated) if the temperature of the filter cartridge assembly 34 starts to approach 30 K, to thus guarantee that the impurities level at the gas output 16 remains negligible (less than 0.05 ppm). In this regard, it is desirable that the temperature in at least the lower portion of Zone 3 remains at or below the de-sublimation temperature of the impurities to insure that no sublimated impurities resulting from the regeneration process contaminate the gas flowing into the cartridge filter assembly 34 and thereafter to the gas outlet 16 via the heat exchanger 26. As a consequence of the very high efficiency of the heat exchanger 26, it is almost always free of frost and condensates, resulting in the temperature of the filter cartridge assembly 34 (which is fluidly coupled to the heat exchanger 26) typically remaining in the range of 5 K-20 K. Optionally, the exterior surface of the coldhead 24 and/or that of the heat exchanger 26 may be coated with an ice resistant material so that the solid impurities and frost are repelled by the resulting slippery coated surfaces and directly fall down into the impurities storage region, thus minimizing the frequency of the regeneration processes.

(34) This soft regeneration process, which was derived from finding that the impurities are frozen and collected in Zones 1 and 2, is nothing less than a cleaning process for the coldhead 24 during which the coldhead 24 is OFF and first heater 30 is ON. This process displaces the impurities 50a, 50b down into Zone 3, thus cleansing the heat exchanger 26 and the coldhead 24 that therefore recovers its cooling capacity. Several processes of this kind can be done at regular intervals of time, or when considered necessary, to increase the purifying time period between two regenerations.

(35) More particularly, as indicated above, it is contemplated that the initiation of the soft regeneration process can be facilitated in any one of several different ways. One way could be based on process initiation automatically at prescribed, timed intervals (e.g., once a day). Another could be based on the functionality of the sensor 32 attached to the third section 24c or cold tip of the second stage of the coldhead 24. As indicated above, the sensor 32 is preferably a thermal diode or thermometer which electrically communicates with both the coldhead 24 and the first heater 30. The efficacy of the apparatus 10 is premised, in large measure, on its thermal stability. Along these lines, when the temperature of the cartridge assembly 34 reaches a minimum threshold and starts to increase, this often means that the efficiency of the coldhead 24 and the heat exchanger 26 is being degraded, thus compelling the need for the initiation of the soft regeneration process. The sensors 32, 48, working in concert with each other, effectively monitor the thermal stability of the apparatus 10, with the sensor 32 being operative to selectively toggle the coldhead 24 and the first heater 30 between on and off states as may be needed to facilitate the initiation of the soft regeneration process. Along these lines, it is also contemplated that the sensor 32 may be operative to terminate any regeneration process by deactivating the first heater 30 and reactivating the coldhead 24 once it senses that the temperature in Zones 1 and 2 has reached the highest sublimation temperature of the specific impurities within the gas entering the interior chamber 17 via the gas inlet 14.

(36) In less common circumstances, an excessive amount of build-up of frozen impurities 50c in Zone 3 could create a partial blockage within the interior chamber 17 as gives rise to a pressure drop between the gas inlet 14 and the gas outlet 16. In this regard, it is contemplated that the apparatus 10 may also be outfitted with two pressure sensors, one which is operative to monitor inlet pressure within Zones 1 and 2, and the other which is operative to monitor outlet pressure at the gas outlet 16 fluidly communicating with the heat exchanger 26. In an exemplary embodiment, these two pressure sensors labeled as 19 and 21 in FIG. 2A, are positioned such that the pressure sensor 19 is located at and fluidly communicates with the gas inlet 14, with the pressure sensor 21 being located at and fluidly communicating with the gas outlet 16. In the event the aforementioned pressure drop is detected by these pressure sensors based on a comparison of the pressure in Zones 1 and 2, and the pressure in the heat exchanger 26 (which would be commensurate to the reduced pressure in Zone 3 attributable to the complete or partial blockage therein), the pressure sensors could be used to trigger the regeneration process. The pressure sensors would further be operative to thereafter discontinue such regeneration process upon sensing that the previously imbalanced pressure levels have equalized within the apparatus 10. An exemplary illustration of this functionality is graphically depicted in FIG. 4A.

(37) The soft regeneration process (cleansing of the coldhead 24) allows for an extension in the periods between high T (150 K) regenerations, therefore allowing the purifying periods to be much longer. The ability to use the soft regeneration is attributable, at least in part, to the high available volume in Zone 3 (especially when using a small filter cartridge assembly 34), and thus the higher available volume to collect frozen impurities displaced from Zones 1 and 2. Moreover, the fact that Zone 3 remains very cold as indicated above ensures that the purity at the gas output 16 is not affected by the sublimation process, so that the apparatus 10 continuously feeds the liquefiers or any device connected at its output. In this regard, FIG. 3B represents the situation in which, after a regeneration process, impurities are stored in Zone 3 and new impurities are being de-sublimated in Zones 1 and 2.

(38) When the amount of impurities collected in solid form in Zone 3 is estimated to be of the order of the belly volume (i.e., available volume in the impurities storage region), or when any blockages caused by frost are frequent and cannot be eliminated by the soft regeneration or sublimation processes, the apparatus 10 must necessarily be subject to a more robust regeneration process. To accomplish this objective, the second heater 20 in the impurities storage region may be activated, and used to sublimate, liquefy, and evaporate the stored impurities (labeled as 52 in FIG. 5). Heating the whole system to about 120-150 K guarantees that all the stored impurities 52 are evaporated, with the inner container 18 thereafter being evacuated with a pump and refilled again with a gas mixture to start a new purification cycle. In this regard, and for sake of clarification, the first and second heaters 30, 46 are necessary in the practice of the present invention; first heater 30 in the deep cooling region for performing the soft regeneration, and second heater 46 in the bottom of the Dewar 12 or impurities storage region for additional heating during the standard high T regenerations.

(39) The soft regeneration method, however, cannot be implemented with any embodiments designed for coalescing impurities, as some prior art systems such as those disclosed in U.S. patent application Ser. No. 13/937,186, entitled CRYOCOOLER-BASED GAS SCRUBBER, filed on Jul. 8, 2013. Notwithstanding, in a new embodiment using the small filter cartridge assembly 34, it is possible to implement such method. The method provides for a huge improvement in the art, since the coldhead 24 and heat exchanger 26 both maintain efficiency unaltered, and the down time for removing impurities can be dramatically reduced. In fact, by adequate design of the interior of the Dewar 12, it is possible to store impurities during very long periods, potentially as long as the maintenance period of the coldhead 24.

(40) As previously explained, in certain embodiments of the present invention, it is contemplated that the filter cartridge assembly 34 may be integrated into the collection mechanism of the apparatus 10 and operative to ensure that any of the impurities held within Zone 3 or the impurities storage region do not somehow become reintroduced into the purified cryogen gas stream that is ultimately collected from Zone 3 and passed upwardly through the Dewar 12 for reuse once output from the gas outlet 16. The filter cartridge assembly 34 integrated as part of the apparatus 10 and as described above is specifically designed to have a compact, thin profile that not only provides exceptional filtering capability, but eliminates the large, excessively bulky wool glass cartridge designs typically in use.

(41) In operation of the apparatus 10 as outfitted with the filter cartridge assembly 34, the purified gas (e.g., helium) is introduced into the collection member 36 of the filter cartridge assembly 34 and thereafter passed through its filtering mechanism, i.e., the bulk filter 38 or thin layer filter 40. After passing through either of these filtering mechanisms, the purified gas passes through funnel 42 and upwardly through outlet conduit 44, and ultimately passes to gas outlet 16 via heat exchanger 26. In the embodiment shown, the filter mechanisms represented by the bulk filter 38 and the thin layer filter 40 represent two alternative types of filtering means, with bulk filter 38 representing a prior art glass wool or fiberglass-based filtering mechanism that is operative to provide sufficient surface area to trap any impurities that might otherwise become reintroduced into the cryogen gas. In the alternative, the thin layer filter 40 represents a thin layer of material having a plurality of micrometer-sized holes through which the gas is filtered. Such the thin layer filter 40, discussed more fully below, may preferably be formed from a metallic mesh material or may be formed from nylon mesh, the latter being preferred.

(42) With greater particularity, a very small 2D nylon mesh filter used as the thin layer filter 40 plays the same role than a big wool glass cartridge and gives much more room available for storing impurities during the necessary and very important soft regeneration processes to maintain the efficiency of the heat exchange during long periods of time. In fact, it is presently believed that there is not necessarily a need for a wool glass cartridge typically constituting the bulk filter 38, as use of a filter cartridge assembly 34 outfitted with the thin layer filter 40 is functional in a manner wherein impurities at the level of 0.1 ppm never arrive to the gas outlet 16 when such filter cartridge assembly 34 is placed near the bottom of the Dewar 12. The filter cartridge assembly 34 can accommodate different micrometer size thin layer filters 40 that can be used to avoid dragging of impurities towards the gas outlet 16. In this regard, it is contemplated that a single or a combination of planar nylon and/or metallic mesh discs having a hole size ranging from 1-25 m and a diameter of approximately 25 mm can be utilized with the nylon mesh having hole sizes ranging from 1-25 m and the stainless steel mesh having a 25 m hole size. Other types of materials and hole sizes would be readily understood by those skilled in the art and readily integrated in the practice of the present invention.

(43) Those of ordinary skill in the art will recognize that the size and/or shape of the filter cartridge assembly 34 as shown in FIGS. 2A-3B and 5 may vary (e.g., may be smaller than that depicted) without departing from the spirit and scope of the present invention. In this regard, the overall size and shape will be dictated, to at least some degree, by the selection of the particular filtering mechanism that is to be integrated therein. Irrespective of the specific size or shape of the filter cartridge assembly 34, it is contemplated that the annual gap defined between the circumferential surface thereof of greatest diameter and the inner diameter of the inner container 18 will be sufficient to allow for the desired flow of sublimated impurities into the impurities storage region and the flow of purified gas into the underside of the collection member 36.

(44) Prototype Development and Test Results

(45) A prototype apparatus built with the purpose of verifying the invention ideas, was implemented using a two stage coldhead of 1.5 W cooling power at 4.2 K, placed in the neck of a Helium Dewar of 10 L capacity, similar to prior art systems. The apparatus had a heater wound on top of an output heat exchange tube, and a sensor attached in said tube, just below the cold tip of the coldhead second stage, to implement in a controlled manner the sublimation/displacement of solid impurities trapped on the deep cooling region, i.e., in the Dewar neck region. The sublimation/displacement process consisted of stopping the coldhead and activating the heater for about 10-60 minutes until the cold tip sensor indicated 100 K, a temperature at which the collected impurities in Dewar neck region are sublimated/liquefied, and transported to the impurities storage region, i.e., to the Dewar bottom.

(46) By performing periodic sublimation/displacement cycles of the solid impurities from the deep cooling region to the storage region, the efficiency of the heat exchanged between the input gas flow, the coldhead, and the output gas through the heat exchanger was maintained nearly optimal at any time. Thus, the prototype was operative to purify from 10.sup.6 to 10.sup.7 sL of Helium gas containing from 100 ppm to 1000 ppm total volume ratios of N.sub.2 and O.sub.2, without interruption for regeneration. Output flow rate peaks as large as 50 sL/min, and average flow rates in excess of 30 L/min, could be maintained with sufficiently long periods of time (>12 hours) between soft regenerations, without affecting the output purity of the processed gas. The whole apparatus and its components could be scaled in size and power for higher flow rates.

(47) Filter Assembly

(48) As revealed in the testing of the prototype, there is strong evidence that the role of a glass wool cartridge serving as the filtering mechanism is confined to avoiding possible dragging of solid impurities only when sudden high output flow rates develop (>30 L/min). The thermodynamics of gas mixtures also indicated that impurities are totally frozen until the level corresponding to the vapor pressure and temperature on the coldhead deep cooling region located on the upper part of the Dewar. This leads to the conclusion that the size of the filter cartridge assembly is not necessarily of importance in the purification process, with the smaller size the better. Thus, as indicated above, a simple small planar 2D filter in the micrometer range size serving as the filtering mechanism in the filter cartridge assembly could potentially perform the same role as any glass wool cartridge of any size serving as the filtering mechanism.

(49) To demonstrate it experimentally, there was built a very small canister in which a single or a combination of planar Nylon and/or metallic mesh discs, having different hole sizes in the micrometer scale range (1, 5, 10, 25 m) and a diameter of 25 mm were installed. Used were Nylon mesh discs with 1, 5, and 10 m hole sizes, and stainless steel mesh disc for the 25 m hole size. Also added were two 25 mm diameter stainless steel grids with 1 mm holes, one on each side of the 2D pancake filtering device, to provide mechanical strength against pressure differences. The design allowed for simple exchange of the meshes for easy testing of different combinations if necessary.

(50) Referring to FIG. 4C, after 30 days of operation, a total of 1,000,000 L, having an average impurity concentration of 300 ppmV, were purified. About 300 cc of solid impurities were collected (1,000,000 L*300 ppms of impurities/10.sup.6=300 L of gas impurities=>300 L(gas)/1000 (L(gas)/L(solid))=0.300 L(solid)=300 cc (solid)). During such period, starting and ending with standard air regenerations (140 K), eleven soft regenerations were automatically performed by the system. It is clear that soft regenerations for that level of impurities (300 ppmV) are only necessary when the incoming gas flow exceeds 20 L/min.

(51) During that period many automatic soft regenerations were performed by the system. Those processes were launched as soon as the lost of efficiency was detected by the increase of the canister temperature. FIG. 4B is a graph depicting exemplary fluctuations of several parameters (e.g., flow rate, incoming pressure, outgoing pressure, and temperatures) as a function of time during an impurity de-sublimation process occurring during a soft regeneration. The data is very clean, thus clearly establishing the correlation between coldhead space T and a small pressure drop (incoming pressure minus outgoing pressure) appearing during the cool down. This is of the order of 0.1 psi/L/min and becomes negligible as soon as coldhead space T is below 20 K, when the molar volume of the solid impurities reaches a minimum constant value. Since this is a limit situation equivalent to that having 2 ATLs 160 connected to the ATP in FAST mode (24 L/min flow rate), it was concluded there was no need to reduce the gas flow impedance of the prototype. Along these lines, the small observed pressure drop is not believed to be attributable to the filter assembly within the system, but occurs in the deep cooling region and is the result of the volume change of the solid impurities with temperature. In any event, it will be apparent for those of ordinary skill in the art that a gas flow impedance reduction could be easily implemented when necessary, e.g. by increasing the available space for solid impurities in the coldhead deep cooling space (zones 1 and 2) and/or above the canister (zone 3), since those are the zones where the pressure drop takes place and not on the output filter nor on the interior of the heat exchanger exhaust tube.

(52) Furthermore, this effect also limits the output flow and can be used, together with the corresponding T increase, as a double check for the system to decide when to perform a soft regeneration. Furthermore, if a pressure drop develops while the filter is at a temperature below 10 K, it will indicate that clogging is starting to be produced in the coldhead deep cooling space (zones 1 and 2) or on the impurities storage region (zone 3) and a standard regeneration should be performed.

(53) With the 2D filter there is also much more room available for the pure cold He phase in zone 3, than in prior art, thus allowing transients of high flow (>30 L/min) at the output during much longer time before the thermal stability is lost.

(54) Foreseeable Modifications

(55) At present, it is believed that a number of minor, foreseeable modifications with respect to previous art may be made to enhance the practice of the present invention as presently disclosed. For example, a bypass valve to maintain a minimum input flow of 5 L/min when there is no flow demand at the output may not be necessary. In fact partial clogging-unclogging on the deep cooling region may appear spontaneously, even with continuous input-output flows above 10 L/min, but only for high impurities concentration. A soft regeneration would be sufficient to periodically eliminate this problem and there would be no need for a heater on the 2D filter output device. In fact, there is contemplated future improvements wherein the filter may be thermally anchored to the Dewar bottom so that the filter sensor also senses the temperature (T) of the bottom for the low temperature regenerations to be performed, maintaining the heating until the liquid phase of the impurities is completely evaporated, as in the prior art (Quantum Designs ATP model), such as that described in U.S. patent application Ser. No. 13/937,186 entitled CRYOCOOLER-BASED GAS SCRUBBER filed Jul. 8, 2013.

(56) It is further contemplated that only this filter/Dewar bottom sensor may be all that is strictly necessary since, as demonstrated in the testing, the soft regenerations can be controlled only with the filter temperature that should never exceed 30 K. The size/power of the coldhead is of importance to guarantee larger maximum flow rates during longer periods of time before each soft regeneration.

(57) Accordingly, additional modifications and improvements of the present invention may also be apparent to those of ordinary skill in the art. Thus, the particular combination of parts and steps described and illustrated herein is intended to represent only certain embodiments of the present invention, and is not intended to serve as limitations of alternative devices and methods within the spirit and scope of the invention.