Patent Publication Number: US-2022228667-A1

Title: Back-pressure control valve

Description:
TECHNICAL FIELD 
     The present invention relates to a back-pressure control valve. 
     BACKGROUND ART 
     In a supercritical fluid chromatograph (SFC), a supercritical fluid is used as a mobile phase. Generally, carbon dioxide is used as a supercritical fluid. In the supercritical fluid chromatograph, the pressure and temperature of carbon dioxide are controlled in order to keep the carbon dioxide supplied to a separation column in a supercritical state. A back-pressure control valve is used to control the pressure of carbon dioxide. For example, the pressure of carbon dioxide is controlled to be not less than 10 MPa by the back-pressure control valve. In Patent Document 1, a pressure control valve which is the back-pressure control valve is described. 
     The pressure control valve (hereinafter referred to as the back-pressure control valve) described in Patent Document 1 has a pressure control block formed of a hard material such as stainless. An opening is provided in one outer surface of the pressure control block, and a planar pressure control surface is formed on the bottom portion of the opening. In the pressure control block, an inlet flow path and an outlet flow path are formed. One end of the inlet flow path is connected to a flow path of the supercritical fluid chromatograph, and the other end opens at the pressure control surface. One end of the outlet flow path opens at the pressure control surface, and the other end is opened to an atmospheric pressure. 
     A sheet-like valve element is arranged above the pressure control surface in the opening. A gap is formed between the pressure control surface and the valve element. The gap amount between the pressure control surface and the valve element is adjusted by upward and downward movement of the valve element by an actuator. Thus, the pressure in the inlet flow path is adjusted. 
     [Patent Document 1] WO 2017/130316 A1 
     SUMMARY OF INVENTION 
     Technical Problem 
     In a supercritical fluid chromatograph, a modifier made of an organic solvent is mixed with a mobile phase for adjustment of separation of sample into components. In the back-pressure control valve, the pressure in the inlet flow path is as high as not less than 10 MPa in order to keep carbon dioxide in a mobile phase in a supercritical state. Further, the pressure in the outlet flow path is an atmospheric pressure. Thus, the pressure in the gap between the pressure control surface and the valve element falls rapidly. 
     As a result, cavitation occurs in the mobile phase in the back-pressure control valve. The pressure control surface of the back-pressure control valve is eroded due to cavitation. Such erosion is likely to occur in a case where a modifier including an organic solvent in particular is used. 
     As such, Patent Document 1 describes that the pressure control surface of the back-pressure control valve is coated with DLC (Diamond-Like Carbon) having hardness higher than that of a hard material of the pressure control block. Thus, erosion of the pressure control surface is suppressed. 
     On the other hand, it is desired to further improve the durability and lifetime of the back-pressure control valve by further suppressing erosion of the pressure control surface of the back-surface control valve. 
     An object of the present invention is to provide a back-pressure control valve durability and lifetime of which are improved. 
     Solution to Problem 
     As results of various repeated experiments and studies, the inventor of the present invention has discovered that it was possible to suppress erosion caused by cavitation by forming the pressure control surface of the back-pressure control valve using a soft material conversely rather than forming the pressure control surface using a hard material, and created the following invention. 
     A back-pressure control valve according to one aspect of the present invention includes a main body having an inner space, a valve element that is arranged in the inner space of the main body and has an opposing surface opposite to one surface of the inner space, a driver that moves the valve element such that a distance between the opposing surface of the valve element and the one surface in the inner space changes, and a resin coating formed on one of the one surface in the inner space and the opposing surface of the valve element, wherein the main body has a first flow path that guides a fluid to a pressure control space formed between another surface out of the one surface and the opposing surface of the valve element, and the resin coating, and a second flow path that discharges a fluid from the pressure control space. 
     Advantageous Effects of Invention 
     With the present invention, a back-pressure control valve durability and lifetime of which are improved can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view showing the structure of a back-pressure control valve. 
         FIG. 2  is a schematic diagram showing one example of the configuration of a supercritical fluid chromatograph. 
         FIG. 3  shows images representing results of a first durability test in regard to the back-pressure control valve. 
         FIG. 4  shows images representing results of a second durability test in regard to the back-pressure control valve. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A back-pressure control valve and a supercritical fluid chromatograph including the back-pressure control valve according to embodiments will be described below in detail with reference to the drawings. 
     (1) Configuration and Operation of Back-Pressure Control Valve 
       FIG. 1  is a schematic cross sectional view showing the configuration of the back-pressure control valve  100 . The back-pressure control valve  100  of  FIG. 1  includes a pressure control block  10 , a resin coating  20 , a diaphragm  30  and a driver  80 . The pressure control block  10  is one example of a main body, and the diaphragm  30  is an example of a valve element. 
     The pressure control block  10  is formed of a hard material such as a metallic material. A metallic material is an example of a first material. In the present embodiment, the pressure control block  10  is formed of stainless. The material of the pressure control block  10  is not limited to this. A concave portion  11  is formed in an upper portion of the pressure control block  10 . The concave portion  11  has a flat bottom surface  12 . The upper end of the concave portion  11  is open. In the present embodiment, the concave portion  11  is columnar. The concave portion  11  is an example of an inner space. 
     An inlet flow path  14  extending obliquely upwardly from a lower portion in one side portion of the pressure control block to the concave portion  11  is formed. Further, an outlet flow path  15  is formed to extend obliquely upwardly from a lower portion in the other side portion of the pressure control block  10  to the concave portion  11 . The inlet flow path  14  is an example of a first flow path, and the outlet flow path  15  is an example of a second flow path. 
     One end of the inlet flow path  14  opens at the outer surface of the pressure control block  10 , and the other end of the inlet flow path  14  opens at the bottom surface  12 . One end of the outlet flow path  15  opens at the outer surface of the pressure control block  10 , and the other end of the outlet flow path  15  opens at the bottom surface  12 . 
     The resin coating  20  is formed on the bottom surface  12  of the concave portion  11 . The resin coating  20  is formed of resin having hardness lower than that of a metallic material. In the present embodiment, PEEK (polyetheretherketone) is used as a resin material. Due to the reasons described below, the thickness of the resin coating  20  is preferably not more than 50 μm. The thickness of the resin coating  20  is not less than 10 μm and not more than 50 μm, for example. Further, the thickness of the resin coating  20  is preferably not less than 10 μm and not more than 30 μm. Hereinafter, the upper surface of the resin coating  20  is referred to as a pressure control surface  21 . In the resin coating  20 , holes  21   a ,  21   b  that respectively communicates with the other end of the inlet flow path and the other end of the outlet flow path  15  are formed. 
     In the concave portion  11  of the pressure control block  10 , the flat-plate shaped diaphragm  30  is arranged to be opposite to the pressure control surface  21 . The diaphragm  30  is provided to be movable in an up-and-down direction in the concave portion  11 . While the diaphragm  30  is formed of PBT (polybutylene terephthalate) in the present embodiment, the material of the diaphragm  30  is not limited to this. The diaphragm  3  may be formed of another resin material. A resin material is an example of a second material. A pressure control space SP is formed between the lower surface of the diaphragm  30  (hereinafter referred to as an opposing surface  31 ) and the pressure control surface  21 . 
     In this manner, the pressure control space SP is formed of the opposing surface  31  of the diaphragm  30  formed of a resin material and the pressure control surface  21  of the resin coating  20 . With such a configuration, both of the pressure control surface  21  and the opposing surface  31  are formed of a resin material that is softer than a metallic material. It is preferable that hardness of one of the pressure control surface  21  and the opposing surface  31  is high for highly accurate pressure control. Therefore, the thickness of the resin coating  20  is preferably small. Therefore, as described above, the thickness of the resin coating  20  is preferably not more than 50 μm. 
     The diaphragm  30  is driven by the driver  80  in the up-and-down direction. The driver  80  is constituted by a stepping motor  40 , a mobile member  50 , a piezo element  60  and a valve stem  70 . The mobile member  50  is attached to a rotation shaft of the stepping motor  40 . The valve stem  70  is attached to the upper surface of the diaphragm  30  to extend in the up-and-down direction. The piezo element  60  is attached between the mobile member  50  and the valve stem  70 . 
     The rotation shaft of the stepping motor  40  is rotated, so that the mobile member  50  is moved in the up-and-down direction. Therefore, the position of the diaphragm  30  in the up-and-down direction can be roughly adjusted by rotation of the stepping motor  40 . Further, the thickness of the piezo element  60  changes in accordance with an applied voltage. Therefore, it is possible to finely adjust the position of the diaphragm  30  in the up-and-down direction by changing a voltage applied to the piezo element  60 . Thus, the gap amount between the pressure control surface  21  and the opposing surface  31  of the diaphragm  30  can be adjusted by an operation of the driver  80 . That is, the volume of the pressure control space SP can be adjusted. 
     When the back-pressure control valve  100  is operated, a mobile phase is supplied to the pressure control space SP through the inlet flow path  14  and the hole  21   a  as indicated by the arrow A 1 . As indicated by the arrow A 2 , a mobile phase in the pressure control space SP is discharged to outside of the pressure control block  10  through the hole  21   b  and the outlet flow path  15 . In this case, the driver  80  adjusts the gap amount between the pressure control surface  21  and the opposing surface  31  of the diaphragm  30 , whereby the pressure of the mobile phase supplied through the inlet flow path  14  can be controlled. A downstream portion of the outlet flow path  15  is open to an atmospheric pressure. 
     At this time, the pressure of the mobile phase in the upstream portion of the pressure control space SP is as high as the pressure control 10 MPa to 40 MPa. In contrast, the pressure of the mobile phase in the downstream portion of the pressure control space SP is close to an atmospheric pressure. Therefore, cavitation is likely to occur in the pressure control space SP. In the present embodiment, the pressure control surface  21  is formed of the upper surface of the resin coating  20 . Thus, erosion of the pressure control surface  21  caused by cavitation is suppressed as described below. 
     (2) Supercritical Fluid Chromatograph 
       FIG. 2  is a schematic diagram showing one example of the configuration of the supercritical fluid chromatograph using the back-pressure control valve  100  of  FIG. 1 . The supercritical fluid chromatograph  1  of  FIG. 2  includes a CO 2  pump  110 , a modifier pump  120 , a mixer  130 , an autosampler  140 , a separation column  150 , a detector  160 , a pressure sensor  170 , a controller  180  and the back-pressure control valve  100 . 
     The CO 2  pump  110  extracts carbon dioxide (CO 2 ) from a cylinder  111  while pressurizing carbon dioxide. The modifier pump  120  extracts a modifier from a modifier container  112 . In the present embodiment, methanol is used as a modifier. The mixer  130  mixes the carbon dioxide extracted by the CO 2  pump with the modifier extracted by the modifier pump  120 , and supplies a liquid mixture to the separation column  150  as a mobile phase through the autosampler  140 . 
     The autosampler  140  introduces a sample into the mobile phase supplied to the separation column  150  from the mixer  130 . A mobile phase and a sample are introduced into the separation column  150 . The separation column  150  separates an introduced sample into components. The mobile phase and sample that have been led out from the separation column  150  flow through a flow cell of the detector  160 . The detector  160  detects the components of sample in the mobile phase flowing through the flow cell. 
     The mobile phase and sample that are led out from the flow cell of the detector  160  flow into the inlet flow path  14  of the back-pressure control valve  100  of  FIG. 1  and flows out from the outlet flow path  15 . The pressure sensor  170  detects the pressure at a position farther upstream than the back-pressure control valve  100 . The controller  180  controls the driver  80  of the back-pressure control valve  100  based on the pressure detected by the pressure sensor  170 . Thus, the pressure at a position farther upstream than the back-pressure control valve  100  is adjusted to a set value. Carbon dioxide extracted from the CO 2  pump  110  is kept in a liquid in a supercritical state by the pressure control carried out by the back-pressure control valve  100  and the temperature control carried out by a cooling device (not shown). 
     (3) Inventive Example and Comparative Example 
     A durability test, described below, was carried out with use of the supercritical fluid chromatograph  1  of  FIG. 2  in order to evaluate durability of the back-pressure control valve  100  according to the present embodiment. In an inventive example, the back-pressure control valve  100  of  FIG. 1  was used. In a comparative example, a back-pressure control valve, having the same configuration as the back-pressure control valve  100  of  FIG. 1  except that a DLC coating was formed instead of the resin coating  20  on a bottom surface  12  of a concave portion  11  of a pressure control block  10 , was used. The durability test was also carried out in regard to a reference example. In the reference example, a back-pressure control valve, having the same configuration as the back-pressure control valve  100  of  FIG. 1  except that a bottom surface  12  of a concave portion  11  of a pressure control block  10  was exposed, was used. In the back-pressure control valve of the reference example, a pressure control surface is formed of stainless of a pressure control block. 
     First, a first durability test was carried out with use of the back-pressure control valves of the inventive example, the comparative example and the reference example. In the first durability test, a mobile phase was supplied to a back-pressure control valve at a relatively large flow rate. Further, a second durability test was carried out using the back-pressure control valves of the inventive example and the comparative example. In the second durability test, a mobile phase was supplied to a back-pressure control valve at a relative small flow rate. 
     In the first durability test, a mobile phase was supplied to the back-pressure control valve of each of the inventive example, the comparative example and the reference example from an inlet flow path at a flow rate of 80 mL/min, and the pressure in an upstream portion of the back-pressure control valve was set to 15 MPa. Methanol was mixed with a mobile phase as a modifier. The concentration of modifier of the mobile phase is 20%. 
       FIG. 3  shows images representing results of the first durability test in regard to the back-pressure control valves of the comparative example, the inventive example and the reference example. In  FIG. 3 , the images of the pressure control surfaces before and after the first durability test are shown. 
     In  FIG. 3 , the upper left image shows the pressure control surface before the test in the comparative example, and the lower left image shows the pressure control surface after the test in the comparative example. In the comparative example, after 406 liters of a mobile phase was supplied to the back-pressure control valve, the pressure control surface made of a DLC coating was already eroded. 
     In  FIG. 3 , the upper central image shows the pressure control surface before the test in the inventive example, and the lower central image shows the pressure control surface after the test in the inventive example. In the inventive example, even after a mobile phase of 488L was supplied to the back-pressure control valve, the pressure control surface made of the resin coating was hardly eroded. 
     In  FIG. 3 , the upper right image shows the pressure control surface before the test in the reference example, and the lower right image shows the pressure control surface after the test in the reference example. In the reference example, after 694 liters of a mobile phase was supplied to the back-pressure control valve, a large area of the pressure control surface formed of stainless was eroded. 
     Next, in the second durability test, a mobile phase was supplied from the inlet flow path  14  to the back-pressure control valve of each of the inventive example and the comparative example at a flow rate of 1.5 mL/min, and the pressure in the upstream portion of the back-pressure control valve was set to 10 MPa. Methanol to which 0.1% of trifluoroacetic acid was added was mixed with the mobile phase as a modifier. The concentration of modifier in the mobile phase is 40%. 
       FIG. 4  shows images representing the results of the second durability test in regard to the back-pressure control valves of the comparative example and the inventive example. In  FIG. 4 , images of the pressure control surfaces before and after the second durability test are shown. 
     In  FIG. 4 , the left upper image shows the pressure control surface before the test in the comparative example, and the lower left image shows the pressure control surface after 8 hours has elapsed from the start of test in the comparative example. In the comparative example, the pressure control surface made of a DLC coating was eroded after about 68 hours has elapsed from the start of supply of the mobile phase to the back-pressure control valve. Thus, the hole in the inlet flow path was connected to the hole in the outlet flow path in the pressure control surface. As a result, it was difficult to control the pressure in the upstream portion of the back-pressure control valve to a set value. 
     In  FIG. 4 , the upper right image shows the pressure control surface before the test in the inventive example, and the lower right image shows the pressure control surface after about 222 hours has elapsed since the start of test in the inventive example. In the inventive example, the pressure control surface made of the resin coating was not eroded even after about 222 hours has elapsed from the start of supply of the mobile phase to the back-pressure control valve. Thus, a pressure could be controlled accurately even after the test. 
     From the results of the first durability test, in a case where the flow rate was relatively large such as the time when a sample was separated into components, it was found that erosion was sufficiently suppressed in the pressure control surface formed of the resin coating as compared to the pressure control surface formed of the DLC coating. Further, from the results of the second durability test, in a case where the flow rate was relatively small such as the time when sample components were analyzed, it was found that erosion of the pressure control surface made of the resin coating was suppressed although the pressure control surface formed of the DLC coating was eroded. 
     (4) Effects of Embodiments 
     In the back-pressure control valve  100  according to the present embodiment, the resin coating  20  is formed on the bottom surface  12  of the concave portion  11  of the pressure control block  10 . In this case, the pressure control surface  21  is formed of the upper surface of the resin coating  20 . Thus, even in a case where a supercritical fluid including an organic solvent is supplied as a mobile phase to the space between the pressure control surface  21  and the opposing surface  31  of the diaphragm  30  for a long period of time, erosion of the pressure control surface  21  caused by cavitation is suppressed. As a result, the durability and lifetime of the back-pressure control valve  100  are improved. 
     (5) Other Embodiments 
     In the above-mentioned embodiment, the pressure control block  10  is formed of a metallic material, the diaphragm  30  is formed of a resin material, and the resin coating  20  is formed on the bottom surface  12  of the pressure control block  10 . However, the pressure control block  10  may be formed of a resin material, the diaphragm  30  may be formed of a metallic material, and the resin coating  20  may be formed on the opposing surface  31  of the diaphragm  30 . 
     While the resin coating  20  is formed of PEEK in the above-mentioned embodiment, the resin coating  20  may be formed of a ketone resin other than PEEK. Further, another resin having a mechanical property (compression stress, a tensile strength, etc.) similar to that of PEEK and having relatively high hardness may be used. For example, the resin coating  20  may be formed of Fluorine resin such as PTFE (Polytetrafluoroethylene). Further, the resin coating  20  may be formed of another resin such as PPS (Polyphenylene sulfide) or PBT (Polybutylene terephthalate). 
     While the back-pressure control valve  100  is used in the supercritical fluid chromatograph in the above-mentioned embodiment by way of example, the back-pressure control valve  100  may be used in a supercritical fluid extraction device (SPE). 
     (6) Aspects 
     It is understood by those skilled in the art that the plurality of above-mentioned illustrative embodiments are specific examples of the below-mentioned aspects. 
     (Item 1) A back-pressure control valve according to one aspect may include a main body having an inner space, a valve element that is arranged in the inner space of the main body and has an opposing surface opposite to one surface of the inner space, a driver that moves the valve element such that a distance between the opposing surface of the valve element and the one surface in the inner space changes, and a resin coating formed on one of the one surface in the inner space and the opposing surface of the valve element, wherein the main body may have a first flow path that guides a fluid to a pressure control space formed between another surface out of the one surface and the opposing surface of the valve element, and the resin coating, and a second flow path that discharges a fluid from the pressure control space. 
     With the back-pressure control valve according to item 1, even in a case where a supercritical fluid including an organic solvent is supplied as a mobile phase to the space between the one surface of the inner space of the main body and the opposing surface of the valve element for a long period of time, erosion of the resin coating caused by cavitation is suppressed. As a result, the durability and lifetime of the back-pressure control valve can be improved. 
     (Item 2) The back-pressure control valve according to item 1, wherein the main body may be formed of a first material, the valve element may be formed of a second material that is softer than the first material, and the resin coating may be formed on the one surface of the inner space. 
     With the back-pressure control valve according to item 2, the pressure control space is formed between the resin coating formed on the one surface of the main body having hardness higher than that of the valve element, and the opposing surface of the valve element. Even in a case where cavitation occurs in this pressure control space, erosion of the resin coating can be suppressed. 
     (Item 3) The back-pressure control valve according to item 1, wherein the first material may be a metallic material, the second material may be a resin material, and the resin coating may have hardness lower than hardness of the metallic material. 
     According to the item 3, erosion of the one surface of the main body formed of a metallic material can be suppressed. 
     (Item 4) The back-pressure control valve according to item 1, wherein the resin coating may be formed of a ketone resin. 
     According to item 4, erosion of the resin coating formed of a ketone resin can be suppressed. 
     (Item 5) The back-pressure control valve according to item 1, wherein the resin coating may be formed of polyetheretherketone. 
     According to item 5, erosion of the resin coating formed of Polyetheretherketone can be suppressed sufficiently. 
     (Item 6) The back-pressure control valve according to item 1, wherein the resin coating may have a thickness of not more than 50 μm. 
     According to item 6, a pressure can be controlled with high accuracy.