JPH0373326B2 - - Google Patents

Description

[Detailed description of the invention]

Purpose of invention

[Industrial application field]

The present invention relates to a method for selectively adsorbing and separating nitrogen dioxide in a mixed gas of strongly acidic nitrogen dioxide and weakly acidic carbon dioxide and/or non-acidic gas, such as nitrogen dioxide present in air. The present invention also allows nitrogen dioxide and carbon dioxide in a mixed gas of nitrogen dioxide, carbon dioxide and non-acidic gas to be
This invention relates to a method for selectively sequential adsorption separation.

[Conventional technology]

Conventionally, as a separation adsorbent for acidic gas,
Strongly basic adsorbents such as soda lime (chemical composition: CaO/NaOH), caustic soda, and bicarbonate of soda (NaHCO ₃ ), and porous adsorbents such as zeolite and activated carbon are used. However, strongly basic adsorbents adsorb not only strongly acidic nitrogen dioxide and sulfur dioxide, but also all acidic gases including weakly acidic carbon dioxide. Therefore, a strongly basic adsorbent cannot separate weakly acidic gas and strongly acidic gas. For this reason, even if the purpose is to remove and separate only nitrogen dioxide contained in the exhaust gas from a nitric acid plant or the nitration process of organic compounds, the adsorption of nitrogen dioxide quantity decreases. In addition, after adsorbing acid gas with soda lime or sodium bicarbonate,
Since it is difficult to recover the adsorbed gas by desorption and regenerate the adsorbent, the only option is to dispose of the adsorbent as it is or to use it for various purposes. In this way, when a strong basic adsorbent is used as an adsorption/separation agent for strongly acidic gases, its usage efficiency is low, and
There was a problem that there was little possibility of reusing the adsorbed gas by recovering it. On the other hand, porous adsorbents such as zeolites have a smaller adsorption capacity per unit weight of acidic gas than basic adsorbents. Furthermore, since these adsorbents adsorb gases by a non-specific physical adsorption mechanism, all gases that can be condensed into their pore structures are adsorbed. For this reason, porous adsorbents cannot selectively adsorb only acidic gases, and even when desorbed by heating, a mixture of multiple gases coexists, making it difficult to use them effectively. There was a problem.

[Problems to be solved]

One object of the present invention is to solve the above problems and to selectively separate nitrogen dioxide from a mixed gas of strongly acidic nitrogen dioxide and weakly acidic gas and/or non-acidic gas. I can do it, and also.
An object of the present invention is to provide an adsorption separation method that allows easy desorption recovery of adsorbed and separated gases and reuse of adsorbents. Another object of the present invention is to provide a method for selectively and sequentially adsorbing and separating nitrogen dioxide and carbon dioxide in a mixed gas of nitrogen dioxide, carbon dioxide, and non-acidic gas. Composition of the invention

[Means for solving the problem] Generally, base adsorbents adsorb acidic gases, but
The inventor discovered that among basic adsorbents, adsorbents with weak basic strength, particularly magnesium oxide, zinc oxide, and beryllium oxide, adsorb strongly acidic nitrogen dioxide but not weakly acidic carbon dioxide. Ta. The basic aspect of the present invention, which was completed based on the above findings, is to use a mixed gas of nitrogen dioxide, which is a strongly acidic gas, and carbon dioxide, which is a weakly acidic gas, and/or a non-acidic gas. , MgO,
It is characterized by selectively adsorbing nitrogen dioxide by contacting with an adsorbent mainly composed of a weakly basic metal oxide consisting of one or more of ZnO and BeO. Another aspect of the present invention is to use a mixed gas of nitrogen dioxide, which is a strongly acidic gas, and carbon dioxide, which is a weakly acidic gas, and a non-acidic gas, with a weak base consisting of one or more of MgO, ZnO, and BeO. Nitrogen dioxide in this mixed gas is selectively adsorbed and separated by introducing it into a first packed bed filled with an adsorbent mainly composed of oxidized metal oxides, and then the mixed gas flowing out from the first packed bed is treated with a strong base. It is characterized by introducing the gas into a second packed bed filled with an adsorbent mainly composed of metal oxides, selectively adsorbing and separating carbon dioxide gas in the mixed gas, and causing non-acidic gas to flow out from the second packed bed. .

[Effect]

In this invention, by using an adsorbent mainly composed of a weakly basic metal oxide, nitrogen dioxide can be selectively separated or removed from a mixed gas at room temperature. In addition, by heating the adsorption layer after the adsorption and collection of the divalent nitrogen, the adsorbed nitrogen dioxide can be released and recovered, and the adsorbent can be regenerated at the same time. Therefore, energy-saving and resource-saving gas separation is realized.
This advantage is simple compared to the conventional catalytic reduction method used as a method for removing nitrogen dioxide from the air, which requires the packed bed of catalyst for catalytic reduction to be kept at a high temperature of around 350°C at all times. will be understood. In addition, while it is difficult to remove 100% of nitrogen dioxide with the catalytic reduction method, in the present invention, the adsorbent packed bed completely removes nitrogen dioxide during the period until adsorption breakthrough occurs due to saturated adsorption of acidic gas. Nitrogen dioxide can be separated. Furthermore, since it can be designed as a highly mobile and portable device by utilizing room-temperature adsorption, it can be applied to small-scale, unsteady applications.

[Embodiment]

The weakly basic metal oxide used in the adsorbent in the present invention is selected from one or more of magnesium oxide, zinc oxide, and beryllium oxide. The table below shows the nitrogen dioxide (NO ₂ ) concentration of various metal oxides, metal ion exchangers and porous adsorbents.
The results of measuring the adsorption characteristics of Also,
FIG. 1 shows an example of the measuring device, in which nitrogen gas N ₂ is supplied from a nitrogen cylinder 1 to flow rate adjusting devices 2A and 2.
B through B at a constant flow rate of 10 ml/min, and after passing one side through nitrogen dioxide generator 3,
Mix both at (25℃) to obtain a NO ₂ concentration of 0.49%.
(v/v) and introduced into the adsorbent packed bed 5 in the tank, and the airflow flowing out from the packed bed 5 was introduced into a gas chromatograph 7 equipped with an autosampler 6, and the NO ₂ leakage concentration was measured at 6 minute intervals. It was designed to measure 100 mg of adsorbent each with an inner diameter of 4 mm.
It was used by filling it into a glass tube.

【table】

[Table] As is clear from the table, among various metal oxides, magnesium oxide (MgO) and zinc oxide (ZnO), which are weakly basic oxides, have a particularly large
Adsorption amount of NO ₂ is observed, and beryllium oxide (BeO)
Also shows the second largest amount of adsorption. Because these metal oxides have a small specific surface area and the number of NO ₂ adsorbed molecules is greater than the number of metal atoms in these solids, NO ₂ is not only adsorbed on the surface but also absorbed into the interior. considered to be a thing. In contrast, other transition metal oxides MnO ₂ ,
V ₂ O ₅ , CuO, Fe ₂ O ₃ , TiO ₂ and the like also exhibit NO ₂ adsorption, but the amount of adsorption is small. Also, porous adsorbents such as activated carbon, silica gel, and zeolite can _{reduce NO2}
The amount of adsorption is much lower than expected from its large specific surface area, and is
It is only about ~30%. Furthermore, in the case of metal-supported cation exchange resins, no remarkable effect is observed, as in the case of the above-mentioned transition metal oxides. The form of the basic metal oxide used in the present invention is:
Commercially available powdered reagents or industrial products can be used as they are, but if they absorb a large amount of acidic gas or water vapor, a change similar to deliquescence will occur, which may cause clogging when filled and used. Playback may become difficult. Therefore, instead of granulating the basic metal oxide alone, we use a strong carrier, a binder for granulation, an activator to impart porosity to the granulated metal oxide, etc. It is preferable to add it, granulate it into spheres, cylinders, pellets, etc., and ripen it by heat treatment such as heating. Examples of such carriers include commonly used carriers such as titanium oxide, silica gel, alumina, silicon carbide, and pumice. Next, the acid gas adsorption separation method according to the present invention will be specifically explained with reference to the drawings. Figure 2 shows MgO, ZnO and
A method using only an adsorbent (hereinafter referred to as "weakly basic adsorbent") mainly composed of a weakly basic metal oxide consisting of one or more types of BeO is shown. In other words, when air containing carbon dioxide (CO ₂ ) and nitrogen dioxide (NO ₂ ) is introduced from one end of a packed bed filled with zinc oxide pellets as a weakly basic adsorbent, nitrogen dioxide, a strongly acidic gas, is released. The packed bed adsorbs and collects carbon dioxide, which is a weakly acidic gas, and is released from the other end along with air without being adsorbed. Zinc oxide that has adsorbed nitrogen dioxide can be heated to about 450°C after the inflow of mixed air is stopped and clean air is passed through it to release nitrogen dioxide and regenerate it as an adsorbent. Figure 3 shows a method using two types of adsorbents with different base strengths. That is, similarly to the above, a second packed bed filled with soda lime as a strong basic adsorbent is connected in series to a first packed bed filled with zinc oxide as a weak basic adsorbent. When air containing carbon dioxide and nitrogen dioxide is introduced from the inlet side of the first packed bed, nitrogen dioxide is
In the second packed bed, carbon dioxide is selectively adsorbed and separated, and air containing no acid gas flows out from the second packed bed. FIG. 4 shows a method in which a dehydrating agent and a porous adsorbent are combined in the first and second packed beds shown in FIG. That is,
The inlet side of the first packed bed connected in series is filled with a dehydrating layer filled with a dehydrating agent such as granular sodium sulfate, and the outlet side of the second packed bed is filled with a non-selective porous adsorbent such as activated carbon. The layers are connected and used. In this case, if air containing vapors of non-acidic organic compounds such as hydrocarbons and halogenated hydrocarbons in addition to nitrogen dioxide, carbon dioxide, and water vapor is introduced from the dehydration layer side, water is first removed in the dehydration layer. After nitrogen dioxide and carbon dioxide are sequentially adsorbed and removed in the first and second packed beds, organic vapors are adsorbed and removed in the porous adsorption layer, and from the last layer, clean air containing no acid gas or organic vapors is produced. flows out. In this way, acidic gases and organic vapors are sequentially and selectively adsorbed and separated. In this case as well, after adsorbing a certain amount of nitrogen dioxide in the first packed bed, the inflow of mixed air is stopped, the first packed bed is heated, and clean air is introduced to desorb nitrogen dioxide. The adsorbent can be regenerated. Further, regarding the second packed bed and the porous adsorbent layer, there are cases where desorption and regeneration can be performed in the same manner as described above, and cases where the used adsorbent is taken out from the packed bed and replaced with a new adsorbent.

【Example】

Next, the present invention will be specifically explained using examples. In addition, in the following description, parts represent parts by weight. Example 1 Mix 20 parts of zinc oxide, 10 parts of aluminum hydroxide, and 1 part of glucose, then add dilute nitric acid (approximately 10 parts) until the whole becomes cake-like (1:10), and then
Mixed for 45 minutes. This is made into pellets using a pellet extruder, heated to 550℃ in air, and then heated to 550℃~600℃.
It was baked for 2 hours at ℃. This was naturally cooled as it was to prepare an adsorbent mainly composed of zinc oxide. This pellet-like adsorbent was filled in a glass tube with an inner diameter of 20 mm to a length of 40 mm, and nitrogen dioxide NO ₂ (0.49%) was filled.
The leaked nitrogen dioxide concentration was measured by introducing air containing nitrogen dioxide at different flow rates and by connecting a portion of the effluent gas to the measuring device shown in Figure 1.The following results were obtained. Mixed air flow rate NO ₂ leakage concentration (ml/min) (ppm) 250 Not detected 500 〃 1000 50 2000 350 As is clear from the above results, the flow rate is 250 to 2000
In the ml/min range, the leakage concentration of _NO2 is 0~
350ppm, and the adsorption separation rate is the original concentration (4900ppm)
However, it reached over 93%, which shows that it has extremely high practical value. Example 2 40 parts of basic zinc carbonate, 10 parts of aluminum hydroxide and 10 parts of powdered zeolite were mixed, and dilute nitric acid was added and further mixed until the whole became cake-like (1:10). This was formed into cylindrical particles using an extruder, and the temperature was slowly raised to 550°C in a rotary kiln. Next, the mixture was calcined at 550°C to 600°C for about 2 hours, and then naturally cooled to prepare an adsorbent mainly composed of zinc oxide. Similar to Example 1, this adsorbent made of cylindrical particles was filled to a depth of 40 mm in a glass tube with an inner diameter of 20 mm, and the concentration of nitrogen dioxide was varied at a constant flow rate (500 ml/min).
The leaked concentration of nitrogen dioxide in the effluent gas was measured, and the following results were obtained. Concentration of leakage concentration of nitrogen dioxide NO ₂ in mixed gas (%) (ppm) 0.01 Not detected 0.1 〃 0.5 〃 As is clear from this data, the initial leakage of NO ₂ over a wide concentration range of 0.01 to 0.5 The concentration is so low as to be undetectable, and its adsorption and separation properties are stable. Effects of the Invention According to the present invention, as explained above, nitrogen dioxide can be selectively adsorbed and separated at room temperature, and the adsorbed and separated nitrogen dioxide can be desorbed and recovered by heating, etc. Since it can also be recycled and reused, it has high practical value in terms of energy and resource conservation. In addition, since adsorption separation occurs at room temperature, it can be designed as a device with good mobility and portability, and is useful for small-scale applications such as analytical devices.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of an apparatus for measuring the adsorption characteristics of nitrogen dioxide by various solids. Figures 2 and 3
FIG. 4 is an explanatory diagram showing an embodiment of the method of the present invention, respectively. 1... Nitrogen cylinder, 2A, 2B... Flow rate adjustment device, 3... Nitrogen dioxide generator, 4... Constant temperature bath, 5
... Adsorbent packed bed, 6 ... Auto sampler, 7
……Gas chromatograph.

Claims (1)

[Claims] 1. A mixed gas of nitrogen dioxide, which is a strongly acidic gas, and carbon dioxide, which is a weakly acidic gas, and/or a non-acidic gas, is made of one or more of MgO, ZnO, and BeO. An adsorption separation method for acidic gases, which is characterized by selectively adsorbing nitrogen dioxide by bringing it into contact with an adsorbent mainly composed of weakly basic metal oxides. 2. Acidic gas according to claim 1 using an adsorbent in which a weakly basic metal oxide is supported on one or more carriers of alumina, silica gel, titanium oxide, silicon carbide, and pumice. adsorption separation method. 3 A mixed gas of nitrogen dioxide, which is a strongly acidic gas, and carbon dioxide gas, which is a weakly acidic gas, and a non-acidic gas is mixed with a weakly basic metal oxide mainly consisting of one or more of MgO, ZnO, and BeO. Nitrogen dioxide in this mixed gas is selectively adsorbed and separated by introducing it into a first packed bed filled with an adsorbent, and then the mixed gas flowing out from the first packed bed is made into a mixture containing mainly strong basic metal oxides. Selective sequential adsorption of acidic gas, which is characterized by introducing the adsorbent into a second packed bed, selectively adsorbing and separating carbon dioxide gas in the mixed gas, and causing non-acidic gas to flow out from the second packed bed. Separation method. 4. Selection of the acidic gas according to claim 3, which includes the step of passing the mixed gas through a dehydration layer filled with a dehydrating agent to remove moisture in the gas prior to introduction into the first packed bed. sequential adsorption separation method. 5. Claim 3 or 4 includes a step of introducing the mixed gas flowing out from the second packed bed into a bed filled with a porous adsorbent such as activated carbon or zeolite to adsorb and remove organic matter. The selective sequential adsorption separation method for acidic gases described.