Medium for quantitatively removing water from a gas stream, device for this purpose, and use of said medium

Abstract

The invention relates to a material for quantitatively removing water from a gas stream, wherein the material comprises a mixture of a molecular sieve and calcium chloride provided with an indicator. The invention also relates to the use of said material in devices for chemical analysis in which a quantitative removal of water is of particular importance.

Description

[0001] Means for the quantitative removal of water from a gas stream and device therefor and use thereof

[0002] The invention relates to a mixture for quantitatively removing water from a gas stream.

[0003] The purity of gases plays a major role in a variety of processes. A key aspect is water removal. Especially in gas analysis, it is necessary to quantitatively remove the water contained in the gas stream to be analyzed. The term "quantitative" in the context of the invention is understood to mean reducing the water content to a value of less than 100 ppm.

[0004] The need for such water removal affects all existing analytical systems and modes for measuring nitrogen, carbon, and oxygen, as well as, for example, TOC analysis or IRMS analysis. Typically, in such drying processes, the gas stream to be analyzed is passed through a tube partially filled with the drying agent, forcing the gas to pass through the drying agent, thereby removing water.

[0005] To date, phosphorus pentoxide desiccants on an inert carrier material, known primarily under the name Sicapent®, or magnesium perchlorate have been used in gas analysis, as these desiccants are capable of actually removing water quantitatively. However, these materials have in common that they are not only highly hygroscopic, but also, as strong oxidizing agents, tend to form acids and explosives when combined with other substances. The use of these materials therefore requires a high degree of occupational safety and appropriate training for the personnel working with them. A further disadvantage of these materials is their very complex and correspondingly energy-intensive production, which also results in high waste volumes.

[0006] Other common drying agents are either not suitable for truly quantitatively removing water or are not inert to all gas streams. For example, the well-known drying agent silica gel frequently undergoes side reactions, so the analysis result is distorted by pre-drying, as components in the drying agent are either bound or altered.

[0007] The invention is therefore based on the object of providing a simple and safe material for the quantitative removal of water, which is particularly suitable for drying a gas stream prior to downstream gas analysis. To achieve this, the drying agent must not react with the sample gases or interact excessively, as this would lead to a change in the measurement signal.

[0008] This object is achieved with a material according to claim 1.

[0009] Such a material according to the invention comprises a mixture of molecular sieve and calcium chloride containing an indicator. The molecular sieve is capable of quantitatively removing the water contained therein. It thus ensures the necessary dryness of the gas stream (dry efficiency), especially for elemental analysis.

[0010] The term "molecular sieve" or molecular sieve in the context of this invention is to be understood as a functional designation, particularly for natural and synthetic zeolites. However, the terminology also encompasses other materials that, like cellulites, have the inherent property of having a high adsorption capacity for gases, vapors, and dissolved substances with specific molecular sizes. Such molecular sieves, especially natural and synthetic zeolites, have a comparably large internal surface area, preferably in the range of 500 to 760 m². 2 / g. They also have a very large pore diameter, which is in the same order of magnitude as the diameter of the molecules to be adsorbed. The pore diameter, expressed in angstroms, is generally used to classify molecular sieves.

[0011] The advantage of using a molecular sieve is that it does not dissolve when absorbing water, thus retaining its solid shape and structure even when using a drying tube. When used in elemental analysis, this prevents clogging and the entrainment of liquid fractions with the gas stream, which would at the very least lead to measurement interruptions or incorrect measurements. Furthermore, persistent entrainment of material would require more frequent changes of the desiccant and, in the worst case, lead to damage to the instrument.

[0012] While the use of a molecular sieve alone would ensure suitable and sufficient drying, the nature of so-called molecular sieves makes it impossible to apply an indicator to detect when the molecular sieve's drying capacity has been exhausted due to their dark color and / or their ability to retain water in the sample. Therefore, the user remains uncertain as to whether the desiccant can still be used or whether the water is no longer completely removed, and the residual water contained in the gas stream has long since distorted the measurement result.

[0013] Calcium chloride is therefore added to the mixture according to the invention as a second drying agent. The use of pure calcium chloride as the sole drying agent would not be sufficient to ensure quantitative removal of water, which is particularly important for gas analysis. However, due to its colorless, opaque crystals, calcium chloride is well suited to visualizing a distinct color change when the calcium chloride contains an indicator. Due to the ability of calcium chloride to absorb large amounts of water and then dissolve itself in the water of crystallization, the color change upon exposure to water is particularly visible. The indicator thus allows the water absorption to be visually monitored.

[0014] The combination of molecular sieve and calcium chloride also has the advantage that even after completion of the aforementioned dissolution process of calcium chloride, a completely liquid phase cannot form over the entire diameter of a drying tube or other container containing the agent according to the invention, since the solid structure of the molecular sieve is retained.

[0015] Further advantages of using calcium chloride include the fact that, like the molecular sieve, it is inert to virtually all gas streams being measured. This applies not only to the typically used carrier gases helium and argon, but also to carbon dioxide. Furthermore, it has the advantage that, as already mentioned, calcium chloride acts as a desiccant, meaning that the addition of an indicator to a carrier material does not dilute the desiccant material itself. This, in turn, has the advantage that the total amount of mixtures can remain the same when compared to material containing an indicator, which plays a role in factors such as pressure drop, dead volume, and downtime.

[0016] It has been found that the material according to the invention has a significantly higher water absorption capacity per volume than previously known drying materials. In comparison, the water absorption of Sicapent® is approximately twice as low over a wide range of blends due to its significantly lower bulk density.

[0017] In summary, the advantage of the new invention is that it provides a way to use a new desiccant for C, N, and O determinations using simple chemicals available in large volumes, which simultaneously indicates the desiccant's creation. Furthermore, the hazard potential of the material has been significantly reduced, making processing, preparation, and use safer and easier. Due to the increased absorption capacity per volume, the operating time could be extended by a factor of 2 while maintaining the same container volume.

[0018] Molecular sieves between 3 and 4 angstroms, in particular various 3 Å molecular sieves, can be used for the mixture. This means that preferably at least 70 wt.%, preferably at least 90 wt.%, have a pore diameter above the pore diameter. 3 Å molecular sieves are particularly preferred because they absorb and release gas to a lesser extent during measurements compared to larger-pore molecular sieves, thus preventing falsification of measurement results.

[0019] Furthermore, it has proven advantageous if the proportion of molecular sieve in the total mass of the mixture is between 60 and 95 wt.%, preferably between 80 and 93 wt.%, and particularly preferably between 85 and 90 wt.%. The remaining difference to the total amount is then accounted for by the calcium chloride contained in some form of the indicator, so that its proportion is between 5 and 40 wt.% based on the total amount of the drying material, preferably 7 to 20 wt.% and particularly preferably between 10 and 15 wt.%. This ensures highly efficient drying and, at the same time, a clearly visible color change when the drying capacity is exhausted. The indicators used, which display a color change depending on the humidity in the surrounding medium, can be either inorganic or organic.

[0020] Inorganic indicators are often available at lower prices on the market. Among inorganic indicators, copper(II) sulfate (Cu(II)SO4) is particularly preferred. Copper(II) chloride can also be used effectively. Its advantage is its safety. When used in conjunction with calcium chloride, the color changes from colorless to blue as soon as the primary drying agent, the molecular sieve, is no longer suitable for quantitatively removing the water.

[0021] Organic substances often exhibit very clearly recognizable color differences. Phenolphthalein is a particularly suitable organic substance. Phenolphthalein has the particular advantage that when the molecular sieve's capacity is exhausted, as its pores completely fill with water, it exhibits a color change to white, which is clearly visible against the dark brown color of the molecular sieve. This "front" shifts further and further in the direction of flow toward the gas outlet over the course of the measurements. As soon as this color change reaches the gas outlet, there is no longer a sufficient amount of desiccant suitable for completely removing the water. However, if the mixture according to the invention is stored for an extended period in such a way that it is exposed to atmospheric humidity, so that the water is not carried through by a gas stream, this changes the reactions that lead to the color change of the indicator.The indicator (In) then does not have the structure FMn, which leads to a white color, but is present as In. 2- and thus turns pinkish-violet. This pink color clearly indicates that moisture has slowly diffused in. This also makes it easier to detect the cause of the exhaustion of a column's absorption capacity, particularly storage errors.

[0022] Furthermore, the calcium chloride (CaCl2) containing the indicator can be coated with the indicator, so that the indicator lies on the surface of the calcium chloride acting as the carrier material. Alternatively, it is also possible to mix the indicator with the calcium chloride and form it into granules or beads, for example, by pressing. The latter is less complex to manufacture than the aforementioned coating, but depending on the type of indicator used, the color change is sometimes more visible with a coating due to the higher local concentration on the surface.

[0023] In summary, the desiccant is characterized by a particularly long service life, easy handling, and an optical indicator that ensures visual traceability of water absorption. Furthermore, unlike previously used materials, the material is not classified as hazardous.

[0024] The invention further encompasses a device for drying gases in the sense of quantitatively removing the water, particularly in preparation of a gas stream for subsequent analysis. Such a device comprises a gas-tight housing and a gas inlet and a gas outlet. The volume defined by this housing is at least partially filled with the material according to the invention according to at least one of claims 1 to 6. The material according to the invention can be used directly in any gas analysis, particularly in any elemental analysis, without modifying the system. A filled tubular reactor represents a particularly simple embodiment of such a device.

[0025] The device can in principle be made from any material that is inert to the gas stream to be analyzed and is gas-tight. Furthermore, it must be at least partially transparent. Metals, glass, and plastic are particularly conceivable as base materials. When metals and opaque plastics are used, a viewing window made of glass or a correspondingly transparent plastic must be provided so that the indicator changes color and the exhaustion of the drying agent's service life is visible. Using glass, and sometimes plastic, as the base material, has the advantage that such viewing windows are not necessary. When a color change is shifted along the length of the container, especially the tube, the effect is clearly visible. Glass is particularly preferred as a housing material due to its inert properties.

[0026] In principle, it is also conceivable to fill the device according to the invention sequentially with a series of materials, for example to provide the flow direction first a carbon dioxide absorbing material and then the drying agent according to the invention from the mixture.

[0027] Finally, the invention also relates to the use of the material according to the invention according to any one of claims 1 to 6 and / or a device according to the invention according to claim 7 or 8 for the quantitative removal of water in a gas analysis system. In particular, this relates to gas analysis systems for the determination of nitrogen, carbon, hydrogen, and oxygen (CHNO). The material according to the invention is particularly suitable for completely removing water from the gas flow head without changing the flow behavior and, at the same time, reliably indicates the exhaustion of its drying capacity through the indicator it contains. The use of the material is not limited to specific devices but can be used in all applications that identify non-acidic, dry gas streams. Other conceivable areas of analysis include TOC analysis and IRMS analysis.Even with a thermal conductivity detector (TCD), a dry gas flow is the basic requirement for precise, linear and repeatable results across the entire measuring range.

[0028] Further embodiments of the invention will become apparent from the examples, as well as the figures and their associated descriptions. Each feature is intended to be disclosed individually or in any combination. Some of the illustrations are slightly simplified and schematic.

[0029] They show:

[0030] Fig. 1 : a schematic design of a device according to the invention and

[0031] Fig. 2: schematically shows the use of a device according to the invention in a gas analysis system

[0032] Figure 1 shows the device 10 according to the invention, which is designed as a preferably cylindrical housing 11 with a gas inlet 12 and a gas outlet 13. Inside, there is a filling 14 containing the material according to the invention, which is introduced in such a way that the gas stream flowing in through the gas inlet 12 must always pass through the material before it can escape via the gas outlet 13.

[0033] Figure 2 shows a highly simplified gas analysis system 20, preferably an elemental analysis system, in which the material according to the invention is used for the quantitative removal of water. Via lines 21, 22 and 23, the gas stream is first passed through a device for removing carbon dioxide 24 and then through a device 10 according to the invention before being analyzed in the detector 25. As an alternative to the graphic representation, the functions of the devices 24 and 10, i.e. carbon dioxide removal and drying, can also be arranged in one device such that this device has two segments into which a corresponding material is filled. In particular, this can be a tube which is filled with the mixture according to the invention in a first part and with a drying agent in a second part, downstream of the gas flow flowing through.

[0034] In addition, pre-drying (not shown) may be provided before the removal of carbon dioxide.

[0035] Example 1

[0036] The following example shows how many measurements can be carried out with which type of absorbent for the quantitative removal of water before the indicator contained in each case indicates a complete load with water.

[0037] The values ​​given are averages over three service life tests each.

[0038] The following example shows that due to the excellent drying properties of the molecular sieve, its drying capacity is increased by a factor of 2 for an identical filling volume, which in turn significantly increases the service life of a column. Example 2

[0039] To determine the drying effectiveness of the material according to the invention, an analyzer without a desiccant was used, followed by various desiccants. For better comparability, an automatic liquid sample injection was used. The drying efficiency (amount of water removed relative to the total amount of water present) can be determined from the ratio of the values ​​obtained using the respective desiccant to the analysis without the desiccant. The average of 10 analyses was used for the calculation.

[0040] List of reference symbols

[0041] 10 Device

[0042] 11 housings

[0043] 12 Gas inlet

[0044] 13 Gas outlet

[0045] 14 Filling with inventive material

[0046] 20 Gas analysis system

[0047] 21 - 23 Line

[0048] 24 Drying device

[0049] 25 detector

Claims

Protection claims 1. Material for the quantitative removal of water from a gas stream, characterized in that the material is a mixture of molecular sieve and calcium chloride provided with an indicator.

2. Material for the quantitative removal of water from a gas stream according to claim 1, characterized in that the molecular sieve has a pore size of 3 A or 4 A, preferably 3 A, to at least 70 wt.%.

3. Material for the quantitative removal of water from a gas stream according to one of the preceding claims, characterized in that the proportion of the molecular sieve in the total mass of the mixture is between 60 and 95 wt.%, preferably between 80 and 93 wt.% and particularly preferably between 85 and 90 wt.%.

4. Material for the quantitative removal of water from a gas stream according to one of the preceding claims, characterized in that the indicator is an inorganic compound, in particular copper(II) sulfate.

5. Material for the quantitative removal of water from a gas stream according to one of the preceding claims, characterized in that the indicator is an organic compound, in particular phenolphthalein.

6. Material for quantitatively removing water from a gas stream according to one of the preceding claims, characterized in that the calcium chloride is at least partially coated with the indicator or calcium chloride and indicator are present together in granules.

7. Device (10) for absorbing water with a gas-tight housing (11) which has a gas inlet (12) and a gas outlet (13) and whose volume is at least partially filled with a material according to one of claims 1 to 6.

8. Device (10) according to claim 7, characterized in that the housing is at least partially transparent.

9. Use of a material according to one of claims 1 to 6 and / or a device according to claim 7 or 8 for the quantitative removal of water in a gas analysis system (20) for the determination of nitrogen, carbon, hydrogen and / or oxygen.

10. Use of a material according to any one of claims 1 to 6 and / or a device according to claim 7 or 8 for the quantitative removal of water in a TOC analysis or an IRMS analysis.

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