Adsorbent for quantitatively removing carbon dioxide in gas flow and application thereof

Abstract

The invention relates to a material for quantitatively removing carbon dioxide in airflow. The material contains a mixture of sodium hydroxide, calcium hydroxide and at least one drying agent, wherein the drying agent is a molecular sieve. The invention also relates to the use of said material and gas analysis system in a suitable device.

Description

[0001] The present invention relates to a mixture for quantitatively removing carbon dioxide from a gas stream.

[0002] When analyzing a gas stream, it is necessary to remove carbon dioxide (CO2) quantitatively, which in the present invention means reducing the carbon dioxide content to below 1 ppm. This refers in particular to analyzing the gas stream by means of a thermal conductivity detector, an infrared spectrometer or a mass spectrometer. Therefore, the gas stream to be analyzed is usually purified, i.e. an adsorbent is contained in a tube through which the gas stream passes.

[0003] So far, the use of sodium hydroxide (NaOH) as an adsorbent is well known, which proceeds according to the following reaction equation:

[0004] 2NaOH+CO2→Na2CO3+H2O

[0005] Sodium hydroxide is reactive enough with carbon dioxide to ensure quantitative removal. However, one drawback in the analysis is that water is liberated during the process and must be removed for proper measurement in downstream analysis.

[0006] However, sodium hydroxide cannot be used in pure form because this would form sodium carbonate (NaCO3). This sodium carbonate would form solid lumps that would adversely affect the flow conditions of the gas stream and in the worst case cause complete blockage.

[0007] Another disadvantage when used as an analytical gas stream is that sodium carbonate forms mainly on the surface of the block, while sodium hydroxide remains inside the material. When flushed with water, both carbonate and hydroxide dissolve, forming a highly alkaline and dangerous solution.

[0008] Therefore, in the past, sodium hydroxide was applied on a support material, preferably a silicate support. In most cases, quartz glass fragments were also added to this support material, preferably in a 1:1 ratio with the sodium hydroxide, in order to mechanically prevent the formation of larger sodium carbonate agglomerates. However, the production of this material is very complex. Moreover, the above-mentioned disadvantages cannot be completely reliably avoided in all possible applications.

[0009] The use of soda lime, a mixture of sodium hydroxide and calcium hydroxide, is also known. In addition to removing carbon dioxide in elemental analysis, this mixture (also called soda lime) is also used in respirators to bind exhaled carbon dioxide.

[0010] Calcium hydroxide (Ca(OH)2) itself is not sufficiently reactive to remove the required amount of carbon dioxide from a gas stream and is therefore not suitable as an adsorbent for quantitative removal of carbon dioxide. In a mixture of sodium hydroxide and calcium hydroxide, the sodium hydroxide reacts with the carbon dioxide on the surface in a known manner in a first reaction. In a second reaction, the sodium hydroxide reacts with the carbon dioxide to form sodium carbonate which reacts with the calcium hydroxide. This reaction forms calcium carbonate. At the same time, the sodium hydroxide is regenerated, which greatly increases the capacity of the adsorbent. The overall reaction equation is as follows:

[0011] 2NaOH+CO2→Na2CO3+H2O (fast)

[0012] Na2CO 3+ Ca(OH)2→2CaCO3+2NaOH

[0013] However, these reactions are highly dependent on water, in part because the chemical equilibrium reactions that promote the formation of carbonate and bicarbonate ions, which are necessary intermediate stages of these two reactions, can only occur in the presence of water.

[0014] However, the gas stream to be analyzed is usually already purified beforehand and is therefore not saturated with water, so that excess moisture is expelled when flowing through the material. However, this is disadvantageous for a number of reasons: If a water-containing gas stream actually reaches the detector, this would falsify the signal. In addition, the gas stream must be dried after absorbing the carbon dioxide, which requires the consumption of large amounts of desiccant. Furthermore, the expulsion of water from the system has a negative impact on the ongoing equilibrium reaction, so that the second reaction with calcium hydroxide is difficult to carry out and within a short time the reaction situation will be comparable to that with pure sodium hydroxide.

[0015] The object of the present invention is therefore to provide a simple and safe material for quantitatively removing carbon dioxide from a gas stream for downstream gas analysis, in particular elemental analysis. This also includes that the composition of the gas stream, in particular the water content, must not vary, so that the downstream desiccant is rapidly consumed and / or the signal of the detector is distorted.

[0016] The material as claimed in claim 1 solves this problem.

[0017] One such material includes a mixture of sodium hydroxide, calcium hydroxide, and at least one desiccant. The additional use of a desiccant ensures that the gas stream being analyzed does not absorb a significant amount of water by absorbing carbon dioxide. However, it should be noted that most desiccants are not pH neutral. For example, It is a typical acidic desiccant that reacts with sodium hydroxide or calcium hydroxide and is therefore not suitable for use as a mixture component.

[0018] Furthermore, desiccants cannot permanently remove moisture to the point where it is no longer available for reaction equilibrium. Surprisingly, it was discovered that the use of molecular sieves (also known as molecular sieves) can meet these requirements.

[0019] Molecular sieves do not react with sodium hydroxide or calcium hydroxide. At the same time, according to the present invention, molecular sieves can reliably retain the moisture in the mixture, so there is no additional impact on elemental analysis or downstream stages (such as drying). But at the same time, molecular sieves will not completely remove moisture from the mixture, preventing calcium hydroxide from regenerating sodium hydroxide, because sodium hydroxide is still present in the pores and has a balancing effect. The desiccant used can meet these three requirements at the same time, so sodium hydroxide and calcium hydroxide can be used to quantitatively remove carbon dioxide.

[0020] The positive effect of indirectly providing water in the molecular sieve is so significant that, without taking into account the negative effects of the discharged water, the service life of the material according to the invention is longer than when the same amount of sodium hydroxide and calcium hydroxide is used without a desiccant.

[0021] For the purposes of the present invention, the term molecular sieve is to be understood as a functional name for natural and synthetic zeolites and other substances which have a high adsorption capacity for gases, vapors and dissolved substances of a specific molecular size. The present invention relates in particular to natural and synthetic zeolites. Such molecular sieves have a relatively large internal surface area, preferably in the range of 500-760 m 2 / g, and has a very uniform pore size, which is on the order of the diameter of a molecule. The pore sizes given are often used to classify molecular sieves.

[0022] 3 to 4% can be used in the mixture Molecular sieves, especially various and Molecular sieves, this means that preferably at least 70% of the molecular sieves have such pore size, more preferably at least 90% of the molecular sieves have such pore size. Molecular sieves are particularly popular because they absorb and release relatively little gas during the measurement process, thus reliably preventing the measurement result from being falsified. At the same time, they also reliably retain moisture and therefore do not place an additional burden on any downstream drying processes.

[0023] Due to the effect of water on the above reaction, the mixture of sodium hydroxide, calcium hydroxide and desiccant preferably contains water from the beginning, so that the water content during the operation is higher than the water content produced by the reaction of sodium hydroxide and carbon dioxide.

[0024] According to the present invention, the mixture includes 0.5 to 5wt% of sodium hydroxide, 20 to 70wt% of calcium hydroxide and 25 to 79.5wt% of desiccant, preferably 1 to 3wt% of sodium hydroxide, 39 to 59wt% of calcium hydroxide and 38 to 62wt% of desiccant, more preferably 1.5 to 3wt% of sodium hydroxide, 39 to 59wt% of calcium hydroxide and 38 to 59.5wt% of desiccant. Desiccant, and pre-saturated with water, so that sodium hydroxide accounts for 1 to 3wt%, calcium hydroxide accounts for 33 to 55wt%, water accounts for 3 to 10wt%, desiccant accounts for 32 to 63wt%, preferably sodium hydroxide accounts for 1.5 to 3.0wt%, calcium hydroxide accounts for 35 to 50wt%, water accounts for 4 to 10wt%, and desiccant accounts for 37 to 59.5wt%. The amount of sodium hydroxide is relatively small, which can reliably prevent sodium carbonate from being generated locally, thereby avoiding gas flow obstruction. At the same time, the amount of calcium hydroxide can ensure the service life of the adsorbent. The amount of desiccant and, if necessary, the water presaturation are adjusted in such a way that water breakthrough can be reliably avoided, while the conversion of calcium hydroxide is not limited by the amount of water.

[0025] The inventors have also found that it is advantageous if the mixture contains at least 90 wt %, preferably 95 wt % of sodium hydroxide, calcium hydroxide, at least one desiccant and water, so that the other components do not affect the reaction.

[0026] However, in a preferred embodiment, the material contains a carbon dioxide indicator and / or a water indicator. This has the advantage that when using the material, for example in bulk filling, any storage faults leading to exhaustion of the absorption capacity can be seen before use. Likewise, the material can also be used in a container with an at least partially transparent outer shell, so that the end of the service life can also be detected visually.

[0027] In a particularly simple embodiment, this is a pH indicator which shows the resulting change in pH when the calcium hydroxide has been used up and the secondary overall reaction no longer occurs.

[0028] The desiccant can be used in any form, with granular forms, in particular spherical or rod-shaped designs, proving to be particularly advantageous for a uniform flow of the gas stream.

[0029] The invention also includes a device for absorbing carbon dioxide. The device has a gas-tight housing with a gas inlet and a gas outlet. As described in one of claims 1 to 4 of the invention, the volume defined by the housing is at least partially filled with the material of the invention. A particularly simple embodiment of the device is a filled tubular reactor.

[0030] The device can be made of any material that is inert to the gas flow to be analyzed and is gas-tight. Metal, glass and plastic are particularly suitable. Glass and plastic have the advantage of being transparent in some cases, while plastic is particularly suitable due to its resistance to breakage and, in some cases, its lower manufacturing cost.

[0031] In this case, it is also possible to fill the device with the various materials in stages, for example by first providing the material according to the invention in the flow direction and then providing a desiccant downstream.

[0032] Finally, the invention also relates to the use of a material according to 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, in particular in elemental analysis and / or IRMS. As already mentioned above, the material according to the invention is particularly suitable because, in addition to the quantitative removal, it must be ensured that the gas stream from which the carbon dioxide is removed is not altered by other components (such as water) or that its flow properties are not altered.

[0033] Further embodiments of the invention are shown in the examples and figures with the associated description. Each feature can be described individually or in any combination. Some of the figures are slightly simplified and schematic.

[0034] It shows:

[0035] Figure 1 : Schematic diagram of the device described in the present invention

[0036] and

[0037] Figure 2 : Schematic diagram of the use of the device described in the present invention in a gas analysis system.

[0038] Figure 1 The device 10 according to the invention is shown, 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 filled with the material according to the invention, which is inserted in such a way that the gas flow flowing in from the gas inlet 12 must always pass through the material before escaping through the gas outlet 13.

[0039] Figure 2 A very simple gas analysis system 20 is shown, preferably an elemental analysis system, in which the material according to the invention is used for quantitative removal of carbon dioxide. The gas stream first passes through the device 10 according to the invention via lines 21, 22 and 23, then through a drying device 24 and finally analyzed in a detector 25.

[0040] As an alternative to the diagrammatic method, the functions of the devices 10 and 24, i.e., removal of carbon dioxide and drying, can also be arranged in one device, so that the device has two parts filled with the corresponding materials. Specifically, it can be a pipe, the first part of which is filled with the mixture according to the invention and the second part is filled with a desiccant, the second part being downstream relative to the passing gas flow.

[0041] Furthermore, before removing the carbon dioxide, a pre-drying treatment (not shown) may be performed. Example

[0042] The following examples show how many measurements can be carried out with which adsorbent to remove CO2 quantitatively before the detector registers a signal from CO2 that is no longer completely absorbed and / or water that is no longer completely removed by the downstream drying device. The values ​​given are the average of 5 service life tests in each case:

[0043]

[0044] The material of the invention has been shown to outperform all other adsorption materials in terms of service life. In addition, when using the mixture of the invention, the downstream drying section and the associated pressure loss can be designed to be relatively short.

[0045] Reference List

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[0047]

Claims

1. A material for quantitatively removing carbon dioxide from a gas stream, the material comprising a mixture of sodium hydroxide, calcium hydroxide and at least one desiccant, wherein the desiccant is a molecular sieve, characterized in that: The mixture includes 1 to 3 wt % of sodium hydroxide, 33 to 55 wt % of calcium hydroxide, 3 to 10 wt % of water and 32 to 63 wt % of a desiccant.

2. The material for quantitatively removing carbon dioxide from a gas stream according to claim 1, characterized in that: At least 70% by weight of the molecular sieves have pore sizes of 3 to 3. A material for quantitatively removing carbon dioxide from a gas stream according to any one of the preceding claims, characterized in that The mixture consists of at least 90 wt % of sodium hydroxide, calcium hydroxide, at least one desiccant and water.

4. A material for quantitatively removing carbon dioxide from a gas stream according to any one of the preceding claims, characterized in that The material contains a carbon dioxide indicator and / or a water indicator.

5. A device (10) for absorbing carbon dioxide, comprising a gas-tight housing (11) with a gas inlet (12) and a gas outlet (13), the volume of the housing being 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 quantitative removal of carbon dioxide in an analysis system operated by gas analysis (20).

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