ABSORPTION AGENT FOR THE QUANTITATIVE REMOVAL OF CARBON DIOXIDE FROM A GAS FLOW AND USE OF THE SAME

Description

[0001] The invention relates to a mixture for the quantitative removal of carbon dioxide from a gas stream.

[0002] In the analysis of gas streams, it is necessary to quantitatively remove carbon dioxide (CO₂), whereby the term "quantitative" within the meaning of the invention is to be understood as reducing the carbon dioxide content to a value of less than 1 ppm. This applies in particular to the analysis of gas streams using a thermal conductivity detector, IR spectroscopy, or a mass spectrometer. Therefore, the gas streams to be supplied for analysis are usually purified, typically by providing an absorbent in a tube through which the gas stream is passed.

[0003] Until now, the use of sodium hydroxide (NaOH) as an absorbent was generally known, which proceeds according to the following reaction equation: 2 NaOH + CO₂ → Na₂CO₃ + H₂O

[0004] Sodium hydroxide exhibits sufficient reactivity towards carbon dioxide to ensure quantitative removal. However, a disadvantage in analytical applications is the water released during this process, which must be removed for accurate measurements in subsequent analysis.

[0005] However, sodium hydroxide cannot be used in its pure form, as this would lead to the formation of sodium carbonate (NaCO3). This sodium carbonate would form solid agglomerates, thereby adversely altering the flow profile of the gas stream or, in the worst case, causing a complete blockage.

[0006] Another disadvantage of using this material in gas stream analysis is that sodium carbonate primarily forms on the surface of the bulk material, while sodium hydroxide remains inside. When rinsed with water, both the carbonate and the hydroxide dissolve, resulting in a strongly alkaline and therefore hazardous solution.

[0007] In the past, sodium hydroxide was therefore applied to a support material, preferably a silicate support. Quartz glass fragments, preferably in a 1:1 ratio to sodium hydroxide, were also usually applied to this support material to mechanically prevent the formation of larger sodium carbonate agglomerates. However, the production of such a material is complex. Furthermore, blockages with the disadvantages described above cannot be reliably and completely avoided in all applications.

[0008] The use of so-called soda lime, a mixture of sodium hydroxide and calcium hydroxide, is also known from the prior art. Besides carbon dioxide removal in elemental analysis, this mixture, also known as soda lime, is used in rebreathers to bind exhaled carbon dioxide.

[0009] The calcium hydroxide (Ca(OH)₂) contained in the mixture does not possess sufficient reactivity on its own to remove the required amount of carbon dioxide from the gas stream and is therefore unsuitable as an absorbent for the quantitative removal of carbon dioxide. In the mixture of sodium and calcium hydroxide, sodium hydroxide reacts with carbon dioxide on the surface in a known reaction. In a second reaction, the sodium carbonate formed from the reaction of sodium hydroxide with carbon dioxide reacts with the calcium hydroxide. This reaction produces calcium carbonate. Simultaneously, the sodium hydroxide is regenerated, thereby significantly increasing the absorbent's capacity. The overall reaction equations are as follows: 2 NaOH + CO₂ → Na₂CO₃ + H₂O (rapid reaction) Na₂CO₃ + Ca(OH)₂ → 2 CaCO₃ + 2 NaOH

[0010] However, these reactions are highly dependent on water, which can be partly explained by the fact that the chemical equilibrium reactions leading to the formation of carbonate and hydrogen carbonate ions, which are necessary intermediates in the two reactions, only occur in the presence of water.

[0011] However, gas streams to be analyzed are usually pre-purified to such an extent that they are not saturated with water and would therefore carry away this excess water when flowing through the material. This is disadvantageous for several reasons: If the water-containing stream actually enters the detector, it would distort its signal. Alternatively, the gas stream would have to be dried after CO₂ absorption, which would involve a high consumption of drying agent. Furthermore, the removal of water from the system would negatively affect the ongoing equilibrium reactions, so that the second reaction with calcium hydroxide would hardly be able to proceed, and within a very short time the situation would be comparable to using pure sodium hydroxide.

[0012] From CN 113 750 728 A, the use of a mixture of NaOH and Ca(OH)₂ for the removal of carbon dioxide after catalytic oxidation is known. In addition to the soda lime, water and an indicator are added. Optionally, further adsorbents such as molecular sieves (5A and 13X), or a CO₂ adsorbent (consisting of NaOH, pearlite, and indicator), as well as additional desiccants, can be added. The weight proportions of the total mixture are not disclosed.

[0013] It is therefore an object of the invention to provide a material that can be produced simply and safely for the quantitative removal of carbon dioxide from a gas stream for downstream gas analysis, in particular elemental analysis. This also includes ensuring that the composition of the gas stream, especially with regard to its water content, is not altered in such a way that the downstream desiccant would be rapidly consumed and / or the detector signal would be distorted.

[0014] This problem is solved with a material according to claim 1.

[0015] Such a material comprises a mixture of sodium hydroxide, calcium hydroxide, and at least one drying agent. The additional use of a drying agent ensures that the gas stream being analyzed does not carry away significant amounts of water from the absorption of the carbon dioxide. However, it should be noted that most drying agents are not pH-neutral. As an example of a classic drying agent, Sicapent® is a typical acidic agent, which would therefore react with sodium or calcium hydroxide and is thus unsuitable as a mixing component.

[0016] Furthermore, the desiccant must not remove water so permanently that it is no longer available for the reaction equilibrium. Surprisingly, it was found that the use of molecular sieves, also called molecular sieves, meets these requirements.

[0017] The molecular sieve does not react with sodium hydroxide or calcium hydroxide. At the same time, it reliably retains the water in the mixture according to the invention, thus avoiding any additional interference with elemental analysis or downstream steps such as drying. However, the molecular sieve does not remove the water from the mixture so completely that the regeneration of sodium hydroxide by calcium hydroxide would be prevented, as it remains within the pores and maintains an equilibrium. The simultaneous fulfillment of all three requirements by the desiccant makes it possible to use sodium hydroxide and calcium hydroxide for the quantitative removal of carbon dioxide.

[0018] The positive effect of the indirect provision of water in the molecular sieve is so great that the service life of the material according to the invention is higher than with the use of an identical amount of sodium and calcium hydroxide without the use of a drying agent, disregarding the negative effects of water carried out.

[0019] The term molecular sieve, as used in the invention, is to be understood as a functional designation for natural and synthetic zeolites and other substances that have a high adsorption capacity for gases, vapors, and dissolved substances of specific molecular sizes. The invention relates in particular to natural and synthetic zeolites. Such molecular sieves have a comparatively large internal surface area, preferably in the range of 500–760 m² / g, and have a very homogeneous pore diameter, which is on the order of the diameter of molecules. The pore diameter, usually expressed in angstroms, is generally used to classify molecular sieves.

[0020] For the mixture, molecular sieves with a pore diameter between 3 and 5 Å, in particular various 3 Å and 4 Å molecular sieves, can be used, meaning that preferably at least 70 wt.%, and preferably at least 90 wt.%, have this pore diameter. 3 Å molecular sieves are particularly preferred because they absorb and release less gas during the measurement, thus reliably preventing falsification of measurement results. At the same time, they reliably retain water, so that no additional load is placed on any subsequent drying process.

[0021] Due to the already described influence of water on the reactions taking place, it is preferred that the mixture of sodium hydroxide, calcium hydroxide and a drying agent already contains water from the beginning, so that the water content during operation is higher than that which results from the reaction of the sodium hydroxide with the carbon dioxide.

[0022] According to the invention, the mixture comprises 0.5 to 5 wt.% sodium hydroxide, 20 to 70 wt.% calcium hydroxide and 25 to 79.5 wt.% desiccant, preferably 1 to 3 wt.% sodium hydroxide, 39 to 59 wt.% calcium hydroxide and 38 to 62 wt.% desiccant, particularly preferably 1.5 to 3 wt.% sodium hydroxide, 39 to 59 wt.% calcium hydroxide and 38 to 59.5 wt.% desiccant, wherein presaturation with water is additionally carried out, so that it comprises 1 to 3 wt.% sodium hydroxide, 33 to 55 wt.% calcium hydroxide, 3 to 10 wt.% water and 32 to 63 wt.% desiccant, preferably 1.5 to 3.0 wt.% sodium hydroxide, 35 to 50 wt.% calcium hydroxide, 4 to 10 wt.% water and 37 to 59.5 wt.% desiccant. The relatively small amount of sodium hydroxide reliably prevents the local formation of sodium carbonate and the associated blockages of the gas flow.At the same time, the amount of calcium hydroxide ensures a long service life for the absorbent. The amount of desiccant and, if necessary, the presaturation with water are precisely calibrated to reliably prevent water breakthrough while simultaneously ensuring that the reaction of the calcium hydroxide is not limited by the amount of water present.

[0023] It has also proven advantageous if the mixture consists of at least 90 wt.%, preferably 95 wt.%, of sodium hydroxide, calcium hydroxide, at least one drying agent and water, so that other components do not influence the reaction.

[0024] In a preferred embodiment, however, the material contains an indicator for carbon dioxide and / or an indicator for water. This has the advantage that, during use of the material, for example when filling it with a bulk material, any storage defects that may have led to an exhaustion of the absorption capacity even before use are visible. Similarly, the material can also be used in a container with at least a partially transparent outer casing, so that the end of its service life can be visually detected.

[0025] In a particularly simple embodiment, it is a pH indicator which, when the available calcium hydroxide is exhausted and the second overall reaction therefore no longer takes place, indicates the resulting change in pH.

[0026] The desiccant can be used in any desired form, although a granular form, in particular a spherical or rod-shaped design, has proven to be particularly advantageous for a uniform flow profile of the gas stream.

[0027] The invention further comprises a device for absorbing carbon dioxide. Such a device has a gas-tight housing with a gas inlet and a gas outlet. The volume defined by this housing is at least partially filled with the material according to any one of claims 1 to 4 of the invention. A particularly simple embodiment of such a device is a filled tubular reactor.

[0028] The device can be made of any material that is inert to the gas stream being analyzed and is gas-tight. Metals, glass, and plastic are particularly suitable. Glass, and to some extent plastic, has the advantage of being transparent; plastic is also particularly suitable due to its impact resistance and, in some cases, low manufacturing costs.

[0029] In this context, it is also possible to fill this device segmentally with a series of materials, for example, first providing the material according to the invention in the direction of flow and then a desiccant.

[0030] Finally, the invention also relates to the use of the material according to any one of claims 1 to 5 and / or a device according to claim 5 for the quantitative removal of carbon dioxide in a gas analysis system, particularly in elemental analysis and / or IRMS. As described, the material according to the invention is particularly suitable here because, in addition to quantitative removal, it must also be ensured that the gas stream freed of carbon dioxide has not been altered by other components such as water or that its flow behavior has not been changed.

[0031] Further embodiments of the invention are shown in the example and the figures with their accompanying descriptions. Each feature, individually and in any combination, is considered disclosed. Some of the illustrations are slightly simplified and schematic.

[0032] They show: Fig. 1: a schematic embodiment of a device according to the invention and Fig. 2: a schematic representation of the use of a device according to the invention in a gas analysis system

[0033] Figure 1 Figure 10 shows the device 10 according to the invention, which is equipped as a housing 11, preferably cylindrical, with a gas inlet 12 and a gas outlet 13. Inside, there is a filling 14 with the material according to the invention, which is introduced in such a way that the gas flow entering through the gas inlet 12 must in any case pass through the material before it can escape via the gas outlet 13.

[0034] Figure 2Figure 1 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 carbon dioxide. The gas stream is first guided via lines 21, 22 and 23 over the device 10 according to the invention and then a drying device 24 before being analyzed in the detector 25.

[0035] As an alternative to the graphical representation, the functions of devices 10 and 24, namely carbon dioxide removal and drying, can also be arranged in a single 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 in a first part with the mixture according to the invention and in a second part, downstream with respect to the flowing gas stream, with a desiccant.

[0036] Furthermore, a pre-drying step, not shown, may be required before the removal of carbon dioxide. Example

[0037] The following example shows how many measurements with which type of absorbent can be performed for the quantitative removal of carbon dioxide before a signal originating from the carbon dioxide that is no longer completely absorbed and / or from water that is no longer completely removed by the downstream drying device is recorded in the detector. The values ​​given are averages over five service life tests each. Absorption material Number of measurements Sodium hydroxide on carrier material 300 Sodium hydroxide / calcium hydroxide (section S1) followed by drying via a flow-through section S2 (S1=S2) 30 Sodium hydroxide / calcium hydroxide (section S1) followed by drying via a flow-through section S2, where S2=2*S1 60 Inventive mixture 500

[0038] It is evident that the material according to the invention is superior to all other absorption materials with regard to service life. Furthermore, the subsequent drying section and the associated pressure losses can be comparatively short when using the mixture according to the invention. Reference symbol list

[0039] 10 Device 11 Housing 12 Gas inlet 13 Gas outlet 14 Filling with material according to the invention 20 Gas analysis system 21 - 23 Line 24 Drying device 25 Detector

Claims

1. Material for the quantitative removal of carbon dioxide from a gas stream, the material containing a mixture of sodium hydroxide, calcium hydroxide and at least one drying agent, and the drying agent being a molecular sieve, characterized in that the mixture comprises 1 to 3% by weight of sodium hydroxide, 33 to 55% by weight of calcium hydroxide, 3 to 10% by weight of water and 32 to 63% by weight of drying agent.

2. Material for the quantitative removal of carbon dioxide from a gas stream according to claim 1, characterized in that at least 70% by weight of the molecular sieve has a pore size of 3 to 5 Å.

3. Material for the quantitative removal of carbon dioxide from a gas stream according to one of the preceding claims, characterized in that the mixture consists of at least 90% by weight of sodium hydroxide, calcium hydroxide, at least one drying agent and water.

4. Material for the quantitative removal of carbon dioxide from a gas stream according to one of the preceding claims, characterized in that the material contains an indicator for carbon dioxide and / or an indicator for water.

5. Device (10) for absorbing carbon dioxide, having 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 4.

6. Use of a material according to any one of claims 1 to 4 and / or a device according to claim 5 for the quantitative removal of carbon dioxide in an analytical system which operates by means of gas analysis (20).