Patent Publication Number: US-2021164956-A1

Title: Measuring apparatus and method for determining the total organic carbon of a dissolved sample

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is related to and claims the priority benefit of German Patent Application No. 10 2019 132 869.1, filed on Dec. 3, 2019, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a measuring apparatus for determining the total organic carbon of a dissolved sample and to a method for determining with a measuring apparatus the total organic carbon of a sample in a liquid medium. 
     BACKGROUND 
     For process water monitoring in the pharmaceutical industry but also in other industries which produce pure water and ultrapure water, it is necessary to measure the total organic carbon (TOC) in liquids in the trace range. In most measuring apparatuses used for this purpose, the organic carbon is converted thermally or by means of UV radiation into CO 2  and detected. 
     In general, one of the following two approaches is used for this purpose:
         (i) Digestion of a defined sample volume and conversion of the resulting amount of CO 2  into a gas stream followed by detection of the CO 2  concentration in the gas stream in an NDIR (non-dispersive infrared sensor) measuring cell; and   (ii) Digestion of the liquid by means of UV radiation and measuring the concentration of CO 2  dissolved in the liquid by the increased electrical conductivity of the liquid.       

     In the latter method, the organic carbon is digested into CO 2  by UV radiation (in the wavelength range of &lt;200 nm). A vacuum Hg lamp is generally used as the UV source. The CO 2  being produced dissolves in the liquid and increases the electrical conductivity. The increase in electrical conductivity correlates with the CO 2  concentration in the liquid or with the original carbon concentration or with the proportion of organically bound carbon originally present in the liquid. 
     According to the state of the art, there are several design variants for implementing the latter method, i.e., digestion of the liquid by means of UV radiation and measuring the concentration of CO 2  dissolved in the liquid by the increased electrical conductivity of the liquid. The two most common variants are discontinuous differential conductivity measurement and continuous differential conductivity measurement. 
     In continuous differential conductivity measurement, measured values are not acquired cyclically as in the discontinuous differential conductivity measurement but continuously. To this end, electrical conductivity is first measured in a first flow conductivity measuring cell. The sample is then digested in a UV flow reactor. Conductivity is then measured again in a second flow conductivity measuring cell. The carbon concentration can be calculated from the increase in conductivity. 
     A continuous measurement signal is advantageously produced, as a result of which concentration peaks can be reliably detected. 
     Disadvantageous is that, although the residence time of the sample in the digestion reactor is constant due to the constant sample flow, different organic compounds have different digestion rates. Consequently, the sample may not be completely digested or the degree of digestion between the various substances may differ. Measurement errors can arise as a result. 
     It is also disadvantageous that digestible compounds, e.g., organic acids, or compounds (e.g., CHCl 3 ) in case of which, in addition to CO 2 , other conductive and soluble compounds are produced by the digestion contribute to a basic conductivity of the sample and can likewise lead to measurement errors. 
     Drift of the conductivity measuring cells also leads to measurement errors. 
     It should also be taken into account that the temperature increase of the liquid (due to the heat of the UV lamp, for example) influences the conductivity, which is why either heat exchangers or the like are necessary to keep both measuring cells at a uniform temperature or a computational temperature compensation is necessary. 
     Devices known from the prior art for the continuous measurement of organic carbon in liquids require a relatively large installation space on account of components, such as a first conductivity measuring cell, a heat exchanger with applicable heating cartridges or a cooling unit, a digestion reactor, and also a second conductivity measuring cell. 
     SUMMARY 
     On the basis of these preliminary considerations, the object of the present disclosure is to improve a measuring apparatus and a method, for example, for the continuous measurement of the total organic carbon (TOC) in liquids, to the effect that at least some of the disadvantages described above are reduced. 
     The present disclosure achieves this object by the subject matter of claim  1  and also by a method having the features of claim  16 . 
     A measuring apparatus according to the present disclosure for determining the total organic carbon of a sample, for example, one dissolved in a liquid medium, has a reactor block made of electrically conductive and corrosion-resistant material. The material is preferably metallic. Example materials are stainless steel, graphite, titanium, and/or platinum. 
     The formation of the reactor block from a metal promotes the thermal conductivity of the reactor block. 
     The reactor block takes the form of a housing with a corresponding housing wall and with an inlet into the reactor block and an outlet from the reactor block. 
     The reactor block may, for example, take the form of a cube, a cuboid, or a cylinder in which an inlet and an outlet are arranged at defined locations, through which the dissolved sample can be introduced into the measuring apparatus and discharged. 
     The reactor block can furthermore have a housing wall with a wall thickness of at least 0.5 mm. The wall thickness of the housing wall can also be designed to be at least 15 mm and, as a further example, 21-200 mm thick, as a result of which design-related advantages can arise, e.g., holes of larger diameter in the wall for accommodating measuring components of the corresponding diameter. 
     The UV lamp heats the reactor block. Optionally, the reactor block may be heated externally or internally or be actively cooled. The relatively large wall thickness of the reactor block allows preheating of the sample or of the liquid medium up to the temperature of the flow chamber through the reactor block itself. 
     Optionally and advantageously, the reactor block has a first connecting channel in the housing wall, which is arranged parallel to the longitudinal axis of the reactor block. Depending on the wall thickness of the housing wall and the corresponding hole, this first connecting channel can have a diameter of a different size. The arrangement of the first connecting channel in the wall of the reactor block saves additional installation space. 
     Furthermore, the housing wall encloses a flow chamber in which digestion of the sample for determining the organic carbon occurs. The flow chamber may be a digestion chamber. 
     The housing wall delimits the flow chamber from the environment. The term “environment” refers to the space outside the reactor block. 
     The flow chamber is designed to accommodate a light source, for example, a UV light source, and to pass through the sample or the liquid medium with the sample which is to be irradiated with UV light. The UV light source can advantageously be part of the measuring apparatus according to the present disclosure. It can be arranged in the flow chamber so as to be replaceable. The flow chamber can take the form of a self-contained cavity with openings for the passage of liquid media. A plurality of openings may be provided. The UV light source can preferably be a vacuum Hg lamp. UV radiation, preferably in the wavelength range &lt;200 nm, has proven to be favorable. 
     The measuring apparatus has at least one conductivity measurement device. The conductivity measurement device may consist of one or more components. A plurality of components can be arranged to form a single component composed of several components. 
     The reactor block is designed as an electrode, for example, as an external electrode of the conductivity measurement device. As a result, the reactor block takes the form of a housing and at the same time an external electrode. 
     Such a compact arrangement of a measuring apparatus integrating several components into one assembly with the result of increased measurement reliability in the detection of total organic carbon in liquids has previously not been known. 
     The formation of the reactor block from a metal additionally promotes the thermal conductivity of the reactor block. 
     The reactor block is designed to absorb heat, which is generated, for example, by the energy of the UV light source in the reactor chamber, and to dissipate it to the environment. As a result, the reactor block is also designed as a heat exchanger to compensate for temperature differences between the conductivity measuring cells integrated in the reactor block and to keep the temperature in the flow chamber of the reactor block constant at defined heat levels. 
     Advantageous is the arrangement of the first connecting channel in the housing wall of the reactor block because the sample supplied into the first connecting channel can already be preheated to a uniform reactor temperature as a result of the thermal conductivity of the reactor block. 
     Preheating advantageously reduces the response time of the analyzer, whereby faster response times are possible with limit value monitoring. 
     Overall, preheating of the sample to the reactor temperature and maintaining a constant temperature in the chamber of the reactor block have a beneficial effect because measurement reliability is increased as a result. 
     Overall, this also makes it possible to dispense with heat exchangers, heating cartridges, and/or cooling units as separate components. 
     As a result, the present disclosure advantageously also reduces the installation space required for a measuring apparatus and ensures increased measurement reliability. In addition, the integration of various components in a single assembly makes it possible to manufacture a measuring apparatus in a space-saving manner. 
     The use of fewer components also advantageously reduces the risk of failure as well as leakages and leaky locations. It is also advantageous that a plurality of functions is integrated into one component, thereby reducing production costs. 
     The present disclosure is suitable for the use of measurements for determining the concentration of organic compounds in liquids in the trace range. 
     Advantageous embodiments of the present disclosure are the subject-matter of the dependent claims. 
     In an advantageous embodiment of the present disclosure, a first conductivity measuring cell of the conductivity measurement device can be arranged in the reactor block with at least two corresponding electrodes, wherein the two electrodes are an internal electrode and an external electrode, and wherein a first internal electrode is arranged within the first conductivity measuring cell and corresponds with the reactor block as an external electrode. The first conductivity measuring cell can be arranged upstream of the flow chamber in which digestion of the sample takes place. 
     In the aforementioned embodiment, a second conductivity measuring cell of the conductivity measurement device is also arranged with at least two corresponding electrodes, wherein the two electrodes are an internal electrode and an external electrode, and wherein a second internal electrode is arranged within the second conductivity measuring cell and corresponds with the reactor block as an external electrode. The second conductivity measuring cell can be arranged downstream of the flow chamber in which digestion of the sample takes place. 
     The internal electrodes comprise an electrically conductive material. The temperature inside the two conductivity measuring cells is also uniform on account of their integration in the reactor block. 
     The first connecting channel may preferably be formed between the inlet and the first conductivity measuring cell. Furthermore, there must be a passage between the first conductivity measuring cell and the flow chamber and a passage between the flow chamber and the second conductivity measuring cell. Lastly, a second connecting channel may be arranged between the second conductivity measuring cell and the outlet. 
     The second connecting channel may be less than 20% of the length of the first connecting channel. 
     Advantageously, a supply and discharge device is connected upstream of the inlet of the reactor block. In this case, the supply and discharge device comprises a supply line of the measuring apparatus to a first valve and/or a pump, wherein a third connecting channel extends between the first valve and the pump. A fourth connecting channel is arranged between the pump and the inlet of the reactor block, and a fifth connecting channel is arranged between the outlet of the reactor block and the second valve. Advantageously, a sixth connecting channel extends between the second valve and the first valve. A discharge line is arranged to start from the second valve. The third, fourth, and fifth connecting channels can be formed, for example, by a tube or a hose which extend outside the reactor block. 
     The supply and discharge device is preferably part of the measuring apparatus. The upstream supply and discharge device expands the measuring apparatus in such a way that a sample can be conveyed through the measuring apparatus in an annular flow or circuit. 
     Measurement errors caused by different digestion rates can be compensated with an annular flow. 
     Drift of the conductivity measuring cells can also be corrected by pumping the sample liquid in the circuit until the conductivity in both measuring cells has reached a stable value. This makes it possible to ensure that the sample is completely oxidized. 
     Drift of the measuring apparatus can occur due to deposits and other effects. However, this can advantageously be corrected. To this end, the evaluation unit is advantageously designed such that an offset measurement, for example, an offset correction, can be carried out during the determination of a measured value. 
     Useful here is that the first and second valves can be switchable between multiple operating modes. A first operating mode ensures a constant supply into and discharge from the measuring apparatus, and a second operating mode ensures that the medium is returned in the circuit. Valve control based on the respective operating modes can be performed by a control and evaluation unit. 
     It is also expedient in this respect that the control and evaluation unit also controls the conductivity measurement by the conductivity measurement device, wherein the control and evaluation unit is designed to determine a content of organic carbon in liquid media taking into account the conductivity determined in the first and second conductivity measuring cells. 
     The flow chamber of the measuring apparatus can be cylindrical. The flow chamber is arranged centrally and parallel to the longitudinal axis of the reactor block and symmetrically to the UV light source and preferably encloses the latter. 
     The lateral distance between the outer inner wall, facing away from the UV light source, of the lateral surface of the flow chamber and the center of the longitudinal axis of the UV light source, which corresponds to the longitudinal axis of the reactor block, can be less than 8 mm. The lower distance between the outer inner wall of the lower circular section surface of the flow chamber and the lower center of the longitudinal axis of the UV light source can also be less than 8 mm. A sample stream that passes close to the UV light source makes possible an increase in radiation intensity and thus also a complete oxidation of the carbon-containing compounds in the sample. 
     The first and second conductivity measuring cells may be arranged at the same radial distance from the longitudinal axis of the reactor block. 
     This arrangement ensures a predictable digestion over the entire vertical lateral surface of the preferably cylindrical flow chamber. 
     The reactor block can be formed in several parts, thereby facilitating the provision of holes and milled recesses in the production process. As a result, components of the reactor block can also advantageously be replaced more easily in the event of failure. 
     The reactor block can preferably be made of stainless steel. However, the reactor block may also be made of other metallic materials or metal alloys, e.g., brass or the like, which are electrically conductive and corrosion-resistant. The use of stainless steel as a heat exchanger is known in a wide range of industries, e.g., in condensing boiler technology. Fields of application include the use of heat, heating of a medium, and cooling of a medium. These properties act synergistically in the stainless-steel reactor block in the solution of the inventive task. 
     Advantageously, the first connecting channel and the second connecting channel, the inlet and the first passage, and the second passage and also the outlet are of equal diameter. This diameter is preferably more than 0.5 mm, particularly preferably between 1.5 and 4 mm. 
     It is furthermore advantageous that an annular gap is formed in each case between the walls of the first and second conductivity measuring cells and the internal electrode arranged in the respective conductivity measuring cell, and that the first and the second annular gaps have identical volumes and geometric dimensions. The volume and geometric dimension may be equal to the diameter of the first and second connecting channels, of the inlet, of the first and second passages, and of the outlet. 
     A sealing device, for example, a sleeve that is transparent at least to UV radiation from the UV light source and into which the UV light source is inserted can be arranged in the flow chamber. This allows the UV lamp to be replaced in the event of a defect even if the reactor block is filled with liquid. In this case, the liquid medium with the sample is located between the sealing device and the inner wall of the flow chamber. 
     It is also advantageous if a third annular gap is arranged between the inner wall of the lateral surface of the flow chamber and the outer wall of the sealing device of the UV light source, said gap having a diameter which is equal to the diameter of the first and second connecting channels and preferably measures between 0.5 and 4 mm. Within this annular gap, the sample flows around the UV light source. The radiation from the UV light source converts the organic components in the sample to CO 2 . The CO 2  forms carbonic acid in the liquid medium, especially, in water, said carbonic acid being detectable with the aid of the conductivity measuring cells as a result of the change in conductivity. At the same time, ozone formation due to overlarge gap widths of the annular gap should be avoided or minimized. 
     The sample stream, which is ensured in the system and distributed widely over the surface of the UV light source, has an advantageous effect on measurement reliability and measurement accuracy. 
     A method according to the present disclosure for determining the total organic carbon of a liquid sample, for example, one dissolved in a liquid medium, with a measuring apparatus, for example, with the measuring apparatus according to the present disclosure, comprises the following steps:
         I. Providing the measuring apparatus;   II. Introducing the liquid sample through the inlet of the reactor block into the first connecting channel in which the sample is preheated to the reactor temperature;   III. Conveying the sample into the first conductivity measuring cell and first measurement of conductivity by the control and evaluation unit;   IV. Supplying the sample from the first conductivity measuring cell into the flow chamber, in which the sample flows around the UV light source in a third annular gap, wherein radiation from the UV light source digests the sample with the formation of CO 2 ;   V. Transferring the sample from the flow chamber into the second conductivity measuring cell and performance of the second measurement of conductivity by the control and evaluation unit, and   VI. Discharging the sample from the second conductivity measuring cell via the second connecting channel through the outlet of the reactor block.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages, features, and details of the present disclosure become apparent from the following description, in which exemplary embodiments of the present disclosure are explained in more detail with reference to the drawings. The person skilled in the art will also expediently consider individually the features disclosed in combination in the drawing, the description, and the claims and combine them into meaningful further combinations. The following are shown: 
         FIG. 1  shows a schematic view of a measuring apparatus in a constant flow mode; and 
         FIG. 2  shows a schematic view of a supply and discharge device as part of the measuring apparatus in an annular flow mode. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a measuring apparatus  100  for determining the total organic carbon of a sample dissolved in a liquid medium. The measuring apparatus has a reactor block  1 . 
     The reactor block  1  comprises a metallic material and is made of an electrically conductive, corrosion-resistant material, preferably of stainless steel, with a wall thickness of more than 0.5 mm. 
     The reactor block  1  is designed as an external electrode and is connected to an evaluation unit  18  by a first connection  20  by means of a supply cable  19 . The reactor block  1  is designed to absorb and dissipate heat to the outside and thereby acts as a heat exchanger. 
     The reactor block  1  has a liquid inlet  5  through which the sample is conveyed into a connecting channel  9 . The connecting channel  9  is arranged in the wall of the reactor block  1 . The heat generated by a UV light source  4  is absorbed by a housing wall  27  of the reactor block  1 , whereby the sample introduced into the first connecting channel  9  is preheated to the reactor temperature. 
     The sample flows from the first connecting channel  9  into a first annular gap  29  of a first conductivity measuring cell  7  and flows around a first internal electrode  2 . The first internal electrode  2 , which, with the reactor block  1  as an external electrode, corresponds with the control and evaluation unit  18  via the first connection  20  for the supply cable  19 , has a second connection  21  for the supply cable  19  to the control and evaluation unit  18 . The first internal electrode  2  is inserted in the wall of the reactor block  1 , a direct contact and thus a short circuit between the electrodes being prevented as a result of an electrical isolation of the mounting point, e.g., in the form of a polymer seal or a polymer casting. A first measurement of the conductivity of the sample is carried out by the evaluation unit by means of the first internal electrode  2  and the external electrode of the first conductivity measuring cell  7 . 
     From the first conductivity measuring cell  7 , the sample is supplied through a first passage  10  into a flow chamber  12  into which the UV light source  4  is introduced in a medium-tight manner. The sample flows around the luminous part of the UV light source  4  in a third annular gap  31  and is digested by the radiation of the UV light source  4 . 
     The UV light source can have a connection head  4   b  with a seal  4   a . The seal  4   a  can take the form of a quartz glass sleeve with a spherically capped enclosure of the UV light source  4 . The seal can also be formed from other materials. 
     The seal  4   a  prevents liquid contact with the UV light source  4  and advantageously simplifies replacement of the UV light source. 
     The sample is transferred through a second passage  11  from the flow chamber  12  into a second annular gap  30  of a second conductivity measuring cell  8  and flows around a second internal electrode  3 . The second internal electrode  3 , which, with the reactor block  1  as an external electrode, corresponds with the control and evaluation unit  18  via the first connection  20  for the supply cable  19 , has a third connection  22  for the supply cable  19  to the evaluation unit  18 . The second internal electrode  3  is inserted in the wall of the reactor block  1 , a direct contact and thus a short circuit between the electrodes being prevented as a result of an electrical isolation of the mounting point, e.g., in the form of a polymer seal or a polymer casting. By means of the first internal electrode  2  and the reactor block  1  serving as the external electrode of the second conductivity measuring cell  7 , a second measurement of the conductivity of the sample is carried out by the evaluation unit  18 . The terms “control and evaluation unit” and “evaluation unit” are used synonymously in the present application. 
     From the second conductivity measuring cell  8 , the sample is discharged through a second connecting channel  13  in an outlet  6  of the reactor block  1 . 
       FIG. 2  shows a supply and discharge device  200  as part of the measuring apparatus  100  for determining the total organic carbon of a sample dissolved in a liquid medium with a measuring apparatus  100  in an annular flow mode. The internal structure of the reactor block  1  is not shown in  FIG. 2  but is constructed analogously to  FIG. 1 . 
     In this variant, the sample is conveyed via a supply line  14  to a valve  15   a  and via a third connecting channel  23  to the pump  16  through the fourth connecting channel  24  into the inlet  5  of the reactor block  1 . 
     The sample passes through the flow chamber in a manner analogous to  FIG. 1 . After the sample exits the outlet  6 , the sample may be returned by means of a pump  16  and the fourth connecting channel  24  to the inlet  5  of the reactor block  1  via a fifth connecting channel  25  via a second valve  15   b  connected in an annular flow and via a downstream sixth connecting channel  26  via the first valve  15   a  connected in an annular flow. 
     As a result, the sample keeps flowing through the reactor chamber in an annular flow or circuit until the conductivity in both conductivity measuring cells  7  and  8  has reached a stable value.