JP4382770B2 - Carbon dioxide purification method - Google Patents

Description

  The present invention relates to a method for purifying carbon dioxide. Specifically, it recovers combustion exhaust gas such as petroleum and liquefied natural gas (LNG), and removes impurities contained in trace amounts in carbon dioxide, which is the main component. The present invention relates to a method for purifying carbon dioxide, and more particularly to a preliminary purification step in the step for purifying carbon dioxide.

As a method for obtaining carbon dioxide, a method is known in which combustion exhaust gas such as petroleum and natural gas is recovered, and various impurities are removed from the exhaust gas to purify carbon dioxide. Impurities contained in the recovered flue gas vary greatly depending on the type of fuel, combustion conditions, etc., but are mainly nitrogen, oxygen, carbon monoxide, nitrogen oxides (NO, NO ₂ etc.), sulfur compounds (SOx, H ₂ S etc.). In order to obtain product carbon dioxide, it is necessary to remove these impurities. In particular, for use as a food additive, the allowable impurity concentration is severely limited.

  In general, carbon dioxide is purified by preliminarily purifying it by a combination of steps such as adsorption, absorption, reduction reaction and decomposition by oxidation reaction, followed by liquefaction step and distillation step. There are various combinations of the steps in the pre-purification, and an optimal combination is selected according to conditions such as temperature and pressure of the combustion exhaust gas as a raw material and the composition of impurities contained therein.

Among the various impurities, a component having a dew point greatly different from that of carbon dioxide can be separated and removed relatively easily by a liquefaction process or a distillation process. However, nitrogen dioxide (NO ₂ ) has the property of being mixed into the liquid when carbon dioxide is liquefied, and needs to be completely removed before the distillation step. Specifically, NO ₂ is generally removed by adsorption or a reduction reaction by hydrogenation as shown in the following formula.

2NO ₂ + 4H ₂ → N ₂ + 4H ₂ O

  Although carbon monoxide can be separated in the liquefaction step, it becomes a catalyst poison in the above-described reduction reaction. Therefore, it is desirable to remove in advance by an oxidation reaction represented by the following formula, for example.

2CO + O ₂ → 2CO ₂

For example, as a method for purifying carbon dioxide using combustion gas from a converter gas boiler as a raw material, the carbon dioxide is concentrated by a pressure fluctuation adsorption method, the pressure is increased to a predetermined pressure, and a sulfur compound is removed by a desulfurization tower. It is introduced into a hydrocracking tower under conditions of -100 ° C. and a pressure of about 300-500 kPa (gauge pressure) to decompose oxygen and nitrogen oxides into nitrogen and water. A method is proposed in which trace amounts of SO ₂ , H ₂ S, NO _{2 and the} like remaining in a deodorizing tower are removed, and finally high-purity carbon dioxide is obtained with a liquefied distillation apparatus (for example, Patent Document 1). reference.).

  In addition, when a reduction reaction by hydrogenation is used for removing impurities in carbon dioxide, in order to avoid the influence of the catalyst poison due to the contained carbon monoxide, only carbon monoxide is removed in advance or an excessive amount of carbon monoxide is removed. There is a method of oxidizing and removing simultaneously with oxygen. For example, in order to avoid the influence of the catalyst poison due to carbon monoxide, a method of reducing and removing nitrogen oxide in the presence of hydrogen after first oxidizing and removing carbon monoxide in the presence of oxygen has been proposed (for example, (See Patent Document 2). At this time, the oxidation reaction conditions of carbon monoxide are platinum or palladium as a catalyst, the reaction temperature is 100 to 200 ° C., and the subsequent reduction reaction of nitrogen oxides in the presence of hydrogen is 40 to 100 ° C. .

In general, it is known that the reaction rate between oxygen and a reducing gas is higher than other reaction rates, and it is considered that nitrogen oxides are reduced after oxygen is reduced (for example, Patent Document 3). reference.). Therefore, in the method of removing nitrogen oxides and carbon monoxide simultaneously, conventionally, in addition to hydrogen for nitrogen oxide reduction, hydrogen for reducing the total amount of oxygen added excessively for carbon monoxide oxidation is used. There was a need to supply. For example, when the raw material carbon dioxide contains 200 ppm of oxygen and 100 ppm of nitric oxide (NO), 400 ppm and 100 ppm of hydrogen are consumed for each reduction when simultaneously reduced at a temperature of 100 to 120 ° C. ( For example, see Patent Document 4.) That is, depending on the composition of the raw material carbon dioxide, there may be a case where more hydrogen is required to reduce excess oxygen than hydrogen required to reduce the nitrogen oxides to be removed.
Japanese Patent Laid-Open No. 5-124808 JP-A-4-219309 JP-A 51-87470 JP-A-2-43923

  In the method of removing carbon monoxide in advance, it is possible to select the optimum conditions for each of the oxidation reaction and reduction reaction. become. Considering this, it is better that the number of components is small, and a method capable of simultaneously removing nitrogen oxides and carbon monoxide is desirable.

  However, in the method of removing at the same time, as described above, the hydrogen necessary for the reduction of excess oxygen may be larger than the hydrogen necessary for the reduction of nitrogen oxides, which increases the running cost. There was a problem.

  Here, since the reduction reaction of nitrogen oxides is performed at a high temperature, it is preferable to perform energy recovery by installing heat exchangers before and after the reaction tower in view of thermal efficiency. However, in the reduction reaction of nitrogen oxides and oxygen, water vapor is generated. As a result of excessive oxygen reduction, if a large amount of water vapor is generated, it may condense in the heat exchanger and become susceptible to corrosion. It is done. In particular, when an aluminum plate fin heat exchanger is used, problems such as a hole being formed in the flow path due to corrosion are expected.

  Therefore, the present invention suppresses the generation of water vapor due to the reduction of excess oxygen as much as possible even when the oxidation reaction of carbon monoxide and the reduction reaction of nitrogen oxide are simultaneously performed in the reaction tower, and efficiently preliminarily purifies carbon dioxide. It aims at providing the purification method of the carbon dioxide which can be performed.

  In order to achieve the above object, the carbon dioxide purification method of the present invention introduces a mixed gas containing carbon dioxide as a main component into a reaction tower, and the nitrogen oxide in the mixed gas is reduced by a reduction reaction with hydrogen. A method for purifying carbon dioxide in which carbon oxides are simultaneously removed by oxidation reaction with oxygen, wherein the reaction conditions in the reaction column are determined as unreacted oxygen in the reaction column outlet gas derived from the reaction column outlet. It is characterized in that it is set to a condition that remains.

  Furthermore, in the method for purifying carbon dioxide of the present invention, the reaction temperature in the reaction tower is set to 110 ° C. or lower, and the amount of hydrogen in the mixed gas at the inlet of the reaction tower reduces oxygen in the mixed gas. The amount of hydrogen in the mixed gas at the inlet of the reaction tower is obtained by exchanging heat between the mixed gas introduced into the reaction tower and the outlet gas of the reaction tower. The oxygen pressure is set to an amount such that the water vapor pressure in the reaction tower outlet gas after the temperature lowering derived from the heat exchanger for raising the temperature of the mixed gas and lowering the reaction tower outlet gas is less than the saturated water vapor pressure. Is added to the mixed gas from outside the system.

  In the carbon dioxide purification method of the present invention, it is necessary to remove nitrogen oxides contained in the raw carbon dioxide by a reduction reaction, and carbon monoxide is present in the target carbon dioxide, and its influence as a catalyst poison. Even when excessive oxygen is required to minimize the amount of hydrogen, the amount of hydrogen supply required for the purification of carbon dioxide can be reduced by discharging some oxygen from the reaction tower while remaining unreacted.

  This is because the reduction of oxygen is suppressed by supplying less than an equivalent amount of hydrogen to the reduction reaction of oxygen, and by setting the reaction temperature to 110 ° C. or lower, which is low as a general catalytic reaction. It is. In addition, carbon monoxide and nitrogen oxide can be removed simultaneously in one reaction tower, and the effect of the present invention is particularly remarkable when the concentration of carbon monoxide and oxygen contained in the raw carbon dioxide is relatively high. Become.

  Furthermore, since the amount of oxygen to be reduced is small, the amount of water vapor produced is small, and there is less concern about device corrosion in the subsequent process, and the adsorption capacity required for the moisture adsorber can be reduced.

  FIG. 1 is a system diagram showing an example of a purification apparatus to which the carbon dioxide purification method of the present invention is applied.

  First, a high-temperature combustion exhaust gas from a power generation facility or the like that uses petroleum or LNG as a raw material is cooled and then subjected to processing such as dust removal, and is pressurized to a predetermined pressure by the compressor 11 as a raw material gas. The main component of the raw material gas is carbon dioxide, which contains a small amount of carbon monoxide, nitrogen monoxide, nitrogen dioxide, oxygen, hydrogen, hydrogen sulfide, sulfur oxide, water vapor and the like.

  In the case where oxygen and hydrogen are not included in the source gas, oxygen and hydrogen may be appropriately added from outside the system through the pipes 50 and 51. When supplying oxygen, air may be supplied instead. Moreover, when adding hydrogen from the piping 50, the density | concentration is made less than the equivalent hydrogen concentration required in order to reduce | restore all the mixed oxygen.

  The pressurized source gas is introduced into the deodorizing tower 12 and the drying tower 13 to remove sulfur compounds, water vapor and the like contained in the source gas. Thereafter, the raw material gas is heated to a predetermined temperature by the heat exchanger 14 and the heater 15. The heated exhaust gas is introduced into the reaction tower 16.

  The reaction tower 16 includes a catalyst layer that simultaneously removes carbon monoxide and nitrogen oxides. As shown in FIG. 2A, the catalyst layer in the reaction tower 16 may be one in which the catalyst layers A and B are filled with different catalysts. As shown in FIG. 2B, the same catalyst layer A is used. May be filled in two stages. Further, as shown in FIG. 2 (C), the reaction tower can be divided into two towers 16a and 16b. Even when different catalysts are filled, carbon monoxide and nitrogen oxides in carbon dioxide can be removed simultaneously in one catalyst layer.

  In the reaction tower 16, the nitrogen oxides in the raw material gas are simultaneously removed by reduction with hydrogen and carbon monoxide by oxidation with oxygen. Is set so that unreacted oxygen remains in the reaction tower outlet gas derived from the above, and carbon monoxide and nitrogen monoxide are removed by an oxidation reaction and a reduction reaction, respectively, by a catalyst filled therein. At this time, the oxidation reaction and the reduction reaction are simultaneously performed in one catalyst layer, and the reaction temperature is 110 ° C. or less, preferably 90 to 100 ° C.

The oxygen reduction reaction (O ₂ + 2H ₂ → 2H ₂ O) is suppressed by reducing the hydrogen concentration equivalent to that necessary for reducing all the mixed oxygen and by setting the reaction temperature to 110 ° C. or lower. It is possible to reduce hydrogen consumed for oxygen reduction. Oxygen that has not reacted is led out from the reaction column 16 by suppressing the reduction reaction of oxygen.

  The source gas (reaction tower outlet gas) containing nitrogen, oxygen, hydrogen, water vapor and the like recovers thermal energy by the heat exchanger 14. At this time, the amount of hydrogen in the raw material gas at the inlet of the reaction tower 16 is set to an amount smaller than the equivalent amount necessary for reducing oxygen in the raw material gas, and the oxygen reduction reaction is suppressed. The water vapor content in the source gas derived from the reaction tower 16 is also reduced. Therefore, even if the temperature of the raw material gas decreases in the heat exchanger 14, the water vapor is hardly condensed.

  The raw material gas exiting the heat exchanger 14 is introduced into a dehumidifying tower 17 that is used for switching, and water vapor and the like are adsorbed and removed. A raw material gas derived from the dehumidifying tower 17 and containing nitrogen, oxygen, and hydrogen is introduced into the carbon dioxide liquefier 18 and liquefied and distilled to obtain product liquefied carbon dioxide LCO from the pipe 19. The dehumidifying tower 17 may be a normally used dehumidifying device. For example, an adsorber filled with zeolite or the like may be used.

  Trace amounts of nitrogen, oxygen, and hydrogen are discharged from the carbon dioxide liquefier 18 into the purge gas in the pipe 52. Depending on the conditions, it is possible to reuse oxygen and hydrogen in the purge gas by collecting the purge gas and introducing it into the raw material gas.

An experiment was conducted in which carbon monoxide and nitrogen monoxide were simultaneously removed from a raw carbon dioxide gas containing a small amount of carbon monoxide and nitrogen monoxide using one reaction tower filled with a platinum-based catalyst. The oxygen concentration in the raw carbon dioxide was about 150 to 300 ppm. The hydrogen concentration necessary for the reduction of oxygen is about 300 to 600 ppm. In order to suppress the reduction reaction of oxygen, the hydrogen concentration at the inlet of the reaction tower was set to 50 to 100 ppm. The experiment was conducted at a temperature of 90 to 110 ° C. and a pressure of 600 to 900 kPa (absolute pressure). The results are shown in Table 1.

  In the conventional method, as described above, a reaction temperature of 40 to 120 ° C. is used for the reaction aiming at reduction of nitrogen oxides, and a reaction temperature of 100 to 200 ° C. is used for simultaneous removal of carbon monoxide and nitrogen monoxide. Temperature is used.

In general, the higher the temperature, the more the reaction is promoted. However, when the reaction temperature exceeds 120 ° C., ammonia (NH ₃ ) may be generated. In any case, it was confirmed that carbon monoxide and nitric oxide can be removed simultaneously, and that ammonia is not generated at 110 ° C. or lower. About 10 to 20% of the oxygen contained in the raw carbon dioxide was considered to have been reduced by hydrogen, but the remaining about 80 to 90% could be confirmed at the outlet of the reaction tower without reacting. That is, by reducing the hydrogen concentration at the inlet of the reaction column to be equal to or less than the equivalent hydrogen concentration necessary for the reduction of oxygen, it was confirmed that the reduction of oxygen was suppressed, in other words, the reduction of the amount of hydrogen required to be added. Moreover, the fact that about 80 to 90% of oxygen is unreacted also suppresses the amount of water vapor generated.

It is a systematic diagram which shows an example of the refiner | purifier to which the carbon dioxide purification method of this invention is applied. It is explanatory drawing which shows the structural example of a reaction tower.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Compressor, 12 ... Deodorizing tower, 13 ... Drying tower, 14 ... Heat exchanger, 15 ... Heater, 16 ... Reaction tower, 17 ... Dehumidification tower, 18 ... Carbon dioxide liquefying apparatus

Claims (5)

A mixed gas containing carbon dioxide as a main component is introduced into a reaction tower, and nitrogen oxides in the mixed gas are simultaneously removed by reduction with hydrogen and carbon monoxide by oxidation with oxygen. A method for purifying carbon dioxide, characterized in that a reaction condition in the reaction tower is set to a condition in which unreacted oxygen remains in a reaction tower outlet gas derived from the reaction tower outlet. . The method for purifying carbon dioxide according to claim 1, wherein the reaction temperature in the reaction tower is set to 110 ° C or lower. The method for purifying carbon dioxide according to claim 1 or 2, wherein the amount of hydrogen in the mixed gas at the inlet of the reaction tower is set to an amount smaller than an equivalent amount necessary for reducing oxygen in the mixed gas. . The amount of hydrogen in the mixed gas at the reaction tower inlet is such that the mixed gas introduced into the reaction tower and the reaction tower outlet gas are heat-exchanged to raise the temperature of the mixed gas and lower the temperature of the reaction tower outlet gas. The method for purifying carbon dioxide according to any one of claims 1 to 3, wherein the water vapor pressure in the reaction tower outlet gas derived from the heat exchanger is lower than the saturated water vapor pressure. . The method for purifying carbon dioxide according to any one of claims 1 to 4, wherein the oxygen is added to the mixed gas from outside the system.

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