Adsorbents for removing low-concentration nitrogen oxides

Abstract: The invention provides first to fourth adsorbents for removing low-concentration nitrogen oxides. The first adsorbent comprises a carrier of gamma-alumina, and ruthenium supported thereon. The second adsorbent comprises a carrier of anatase-type titania, and ruthenium supported thereon. The third adsorbent comprises ceramic paper retaining a carrier of anatase-type titania thereon, and ruthenium supported on the ceramic paper. The fourth adsorbent comprises ceramic paper retaining a carrier of anatase-type titania thereon, and a ruthenium halide and a halide of addition metal which are co-supported on the ceramic paper. These adsorbents are free of the influence of moisture and therefore usable without necessitating energy-consuming dehumidification or only with dehumidification on a reduced scale.

Description: BACKGROUND OF THE INVENTION The present invention relates to adsorbents for efficiently removing nitrogen oxides (NOx) which are contained in low concentrations in gases discharged by ventilation from various highway tunnels, mountain tunnels, underwater tunnels, underground roads, sheltered roads and the like (hereinafter referred to collectively as "highway tunnels"). Highway tunnels, especially those having a large length and a great vehicular traffic volume, must be ventilated at a considerable rate in order to ensure the drivers' and passengers' health and increase the minimum distance of distinct vision. Air is withdrawn also from tunnels of relatively short length for ventilation to control air pollution due to carbon monoxide (CO), NOx and the like which are concentrically released from the inlet and outlet of the tunnel when the tunnel is located in an urban or suburban area. However, if the gas discharged by ventilation is released as it is in the neighborhood, the quality of the environment of the area concerned will not be improved. FR patent 2 211 286 discloses NOx removal from exhaust gas, but does not teach means for adsorbing Nox contained in a few ppm in ventilation gas of a highway tunnel. Exactly the same problem as above will be encountered in the case where roadway tunnels or sheltered tunnels are to be constructed as a pollution control measure for existing roads. On the contrary, the present invention relates to adsorbents for efficiently removing NOx which is present in low concentrations in gases removed from highway tunnels by ventilation. PRIOR ART The gas released from highway tunnels by ventilation is characterized in that it has a low NOx concentration of about 5 ppm and ordinary temperature, and varies greatly in quantity with the volume of traffic. Processes have heretofore been investigated for removing NOx produced from fixed sources in order to purify the combustion exhaust gas from boilers. These processes are divided generally into the following three types. (1) Catalytic Reduction Process In this process, NOx in the exhaust gas is selectively reduced to harmless nitrogen and water vapor using ammonia as a reducing agent. This process is most generally used for denitrating exhaust gases from boilers. However, the gas to be treated by the process must be heated to at least 200°C, so that the process is not economical for treating gases from highway tunnels since the gas, having ambient temperature and being large in quantity, needs to be heated with a large amount of energy. (2) Wet Absorption Process This process utilizes the fact that nitrogen dioxide (NO 2 ) and nitrogen trioxide (N 2 O 3 ) can be readily absorbed by a liquid absorbent such as water or an aqueous alkali solution. Nitrogen monoxide is oxidized using the liquid absorbent with an oxidation catalyst or ozone injected into the absorbent, and the resulting NO 2 and N 2 O 3 are caused to be absorbed by the absorbent, or the absorbent is used with an oxidizing agent added thereto. However, this process is complex because NOx is accumulated in the absorbent in the form of nitrates and nitrites, necessiting the maintanance and aftertreatment of the absorbent. The process further has an economical problem since the cost of the oxidizing agent per mole is higher than that of ammonia for use in the catalytic reduction process (3) Dry Adsorption Process This process removes NOx from the exhaust gas with the use of a suitable adsorbent. Several processes of this type had been investigated before the catalytic reduction process was placed into wide use for denitrating boiler exhaust gases. Nevertheless, since the boiler exhaust gas has (a) a high NOx concentration, (b) a high temperature, and (c) a high water content, the dry adsorption process is economically inferior to the catalytic reduction process and has not been introduced into use. However, when the dry adsorption process was evaluated for purifying the gas resulting from the ventilation of highway tunnels, the process was found to be simple but economical, and entirely different from the same process as applied to the treatment of boiler exhaust gases. We have already proposed adsorbents intended to efficiently adsorb and remove NOx present in a low concentration of 5 ppm, i.e. a low-concentration NOx adsorbent which comprises at least one copper salt supported on natural or synthetic zeolite and selected from among copper chloride, double salts of copper chloride and amine complex salt of copper chloride (see Unexamined Japanese Patent Publication N° 299642/1989), and an adsorbent wich comprises vanadium supported on a carrier of anatase-type titania (see the specification of Japanese Patent Application N° 340627/1990). Howewer, these adsorbents have the problem of exhibiting an impaired adsorbing property (deterioration) when used at an increased water (or moisture) content. Accordingly, to enable these adsorbents to exhibit a satisfactory adsorbing property, the moisture content of gases needs to be not higher than about -35°C in terms of dew point (up to about 200 ppm), so that the gas to be treated must be dehumidified by a dehumidifying step before the removal of NOx. An object of the present invention is to provide an adsorbent which is free of the influence of moisture for removing NOx. Another object of the invention is to provide such an adsorbent which is free of poisoning with SOx present in the gas to be treated. Still another object of the invention is to provide such an adsorbent which comprises a ruthenium halide and which retains high activity for a prolonged period of time without permitting the separation of the halogen. SUMMARY OF THE INVENTION The present invention provides an agent for removing low-concentration NOx by adsorption. (Such an agent will hereinafter be referred to merely as "adsorber"). Said adsorbent comprises a carrier comprising gamma-alumina, and ruthenium halide supported on the carrier. Said adsorbent is not affected by moisture, so that the dehumidifying step which must be executed conventionally before the removal of NOx with use of a large amount of energy can be dispensed with or carried out to a diminished extent. This achieves great savings in energy and savings in the space needed (reduction in the size of equipment) unlike the conventional process. The NOx adsorbed by the adsorbent can be readily desorbed by heating to facilitate regeneration of the adsorbent. Accordingly, the adsorbents of the invention are suited for use with NOx adsorption rotors for continually and repeatedly adsorbing NOx and desorbing NOx (for regeneration). BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing a relation between the time and the breakthrough ratio ; FIG. 2 is a graph showing a relation between the time and the breakthrough ratio ; FIG. 3 is a graph showing a relation between the time and the breakthrough ratio ; FIG. 4 is a graph showing a relation between the amount of Ru supported and the breakthrough time ; DESCRIPTION OF THE PREFERRED EMBODIMENTS The adsorbent of the present invention comprises a carrier comprising gamma-alumina, and ruthenium halide supported thereon. When desired, the carrier may contain, in addition to gamma-alumina, organic auxiliary forming agents, inorganic forming agents (serving as binders or diluents) such as silica sol and silica-alumina, and inorganic fibrous substances such as ceramic fibers. The carrier is prepared by kneading the alumina with such an auxiliary forming agent and fibrous substance which are used as required, then forming the mixture into a desired shape, and drying and baking the shaped mixture. The gamma-alumina carrier may be a commercial alumina carrier, or an alumina carrier prepared from alumina sol. Ruthenium halide is supported on the carrier preferably in an amount, calculated as ruthenium metal, of at least about 0.01 wt. %, more preferably 0.1 to 5 wt. %, based on the adsorbent. The amount of ruthenium halide to be supported is adjusted generally by varying the ruthenium concentration of the immersion bath to be used, temperature of the bath, immersion time or the like. The carrier is caused to support ruthenium halide thereon generally by dissolving or dispersing ruthenium chloride (RuCl 3 ) or like ruthenium compound in a suitable medium to obtain an immersion bath, and immersing the alumina carrier in the bath, although this method is not limitative. The immersed carrier is thereafter withdrawn from the bath, washed with water and dried in air at 100 to 120°C. The dried product is baked at 300 to 500°C when so required. When the adsorbent is to be used continuously by repeating adsorption and desorption for regeneration, it is sometimes desirable to treat the dried product at a temperature slightly higher thant the highest temperature at which the adsorbent is to be used. The shape of the adsorbent is not limited specifically. Preferably it is shaped to have a large contact area and permit smooth passage of gases therethrough, as is the case with solid cylinders, balls, Raschig rings or a honeycomb structure. In treating large quantities of gases such as ventilation gases from highway tunnels, it is required that the adsorbent be diminished in the resistance to the gas flow to ensure a minimized pressure loss. The adsorbent preferable for use in this case is an adsorbent which comprises a flat sheet-corrugated sheet multilayer structure having a honeycomb cross section, and ruthenium supported thereon, the multilayer structure being composed of alternately arranged flat sheets of ceramic paper retaining a carrier of gamma-alumina thereon and corrugated sheets of ceramic paper retaining the same carrier thereon. The ceramic paper is prepared from ceramic fibers by the papermaking process. Ceramic papers commercially available are also usable. The adsorbents embodying the present invention are used for removing NOx present in a low concentration in the ventilation gas of highway tunnels by contact with the gas. The adsorbent having NOx adsorbed thereto can be heated for the desorption of NOx, and the regenerated adsorbent is exposed to the ventilation gas for a continuous treatment. Example 1 Commercial gamma-alumina (brand name, Sanbido-AN, product of Shokubai Kagakukogyo Co., Ltd) as comminuted and sieved to 8- to 14-mesh sizes was immersed in an amount of 7 ml in 10 ml of an aqueous solution of ruthenium chloride (RuCl 3 ), 0.38 wt. % in Ru concentration, at room temperature for 20 hours. The alumina was then washed with water and thereafter dried at about 110° C for 2 hours to obtain a Ru-supporting alumina adsorbent (amount of RU supported : 0.6 wt. %) A 5-ml quantity (3.5 g) of the adsorbent was packed in a stainless steel reactor tube having an inside diameter of 22 mm, treated at about 300° C for 1 hour while passing dry air (moisture content : about 50 ppm) through the tube at a rate of 2.5 NL/min and then allowed to cool to room temperature. After cooling the passage of dry air was discontinued, and air adjusted to a moisture content of 500 ppm and containing 3.5 ppm of NOx was introduced into the adsorbent layer at a rate of 2.5 NL/min. Immediately after the start of introduction, the outlet gas of the reactor tube was checked for NOx concentration by a chemiluminescent detector. FIG. 1 shows variations in the NOx concentration of the outlet gas with time. Plotted as ordinate in FIG. 1 is a value obtained by dividing the NOx concentration of the outlet gas by the NOx concentration of the inlet gas. The value will hereinafter the reffered to as a "breakthrough ratio.") The drawing, showing the result of Example 1, indicates that the time taken for the NOx concentration of the outlet gas to reach 10 % of the concentration at the inlet (breakthrough ratio: 0.1), i.e., 0.35 ppm, is 30.0 minutes. (The time will be referred to as "10 % breakthrough time.") Comparative Examples 1 and 2 An adsorbent comprising vanadium (V) supported on titania was prepared by heating a titanic acid slurry (TIO 2 content : about 30 wt. %) in air at 400° C for 5 hours to obtain anatase-type titanium oxide (titania) for use as a carrier, and impregnating the carrier with ammonium metavanadate (NH 4 VO 3 ). The adsorbent was used under the same conditions as in Example 1 to measure the outlet NOx concentration similarly. FIG.1 also shows variations in this concentration with time as the result of Comparative Example 1. Further a Cu-supporting zeolite adsorbent was prepared by impregnating commercial Y-type zeolite serving as a carrier with cupric chloride (CuCl 2 ). This adsorbent was used under the same conditions as in Example 1 to measure the outlet NOx concentration similarly. FIG. 1 shows variations in the NOx concentration with time thus determined as the result of Comparative Example 2. The drawing reveals that at a moisture content of 500 ppm the Ru-supporting alumina adsorbent (Example 1) exhibits an exceedingly higher property than the V-supporting titania adsorbent (Comparative Example 1) and the Cu-supporting zeolite adsorbent (Comparative Example 2). Example 2 An adsorbent was prepared in the same manner as in Example 1 using a gamma-alumina carrier which was obtained by pelletizing "Colloidal Alumina 200," manufactured by Nissan Chemical Industries, Ltd, drying the pelletized alumina at about 110° C for 44 hours and thereafter baking the alumina in air at 400°C for 24 hours. This adsorbent was used under the same conditions as in Example 1 to measure the outlet NOx concentration similarly. FIG. 2 shows variations in the NOx concentration with time thus determined as the result of Example 2. The diagram reveals that the alumina carrier prepared from colloidal alumina (alumina sol, Example 2) and the commercial alumina carrier are both usable as such with no noticeable difference found therebetween in the NOx adsorbing property. Example 3 A 5-ml quantity (3.5 g) of an adsorbent prepared in the same manner as in Example 2 was packed in a stainless steel reactor tube having an inside diameter of 22 mm, treated at about 300°C for 1 hour while passing dry air (moisture content : about 50 ppm) through the tube at a rate of 2.5 NL/min and then allowed to cool to room temperature. After cooling, the passage of dry air was discontinued, and air (temperature : 24.5°C, relative humidity : 49 %, moisture content : about 15,000 ppm) containing 3.5 ppm of NOx was introduced into the adsorbent layer at a rate of 2.5 NL/min. Immediately after the start of introduction, the gas from the outlet of the reactor tube was checked for NOx concentration. FIG. 3 shows variations in the NOx concentration of the outlet gas with time as the result of Example 3 along with the result achieved in Example 2 (moisture content : 500 ppm). The diagram reveals that the adsorbent retains its NOx adsorbing property even at an increased water content, ensuring efficient removal of NOx at the moisture content of the atmosphere. Example 4 The same carrier as used in Example 2 was crushed and sieved to 8- to 14-mesh sizes, then immersed in an aqueous ruthenium chloride solution of specified concentration at room temperature for 20 hours, washed with water and thereafter dried. In this way, adsorbents were prepared with varying amounts of Ru supported on the carrier. A 5-ml quantity (3.5 g) of each of these adsorbents was packed in a stainless steel reactor tube having an inside diameter of 22 mm and used under the same conditions as in Example 1 to measure the outlet NOx concentration and determine 10 % breakthrough time. FIG 4 shows the relation between the amount of Ru supported and the 10 % breakthrough time thus established. The diagram reveals that as the amount of Ru supported increases, the 10 % breakthrough time increases to result in a higher NOx adsorbing property. It is seen, however, that when the amount of Ru exceeds about 3 wt. %, the 10 % breakthrough time becomes almost definite.

Claims: An adsorbent for removing low-concentration nitrogen oxides comprising a carrier comprising gamma-alumina, and ruthenium halide supported on the carrier. An adsorbent as defined in claim 1 wherein the carrier contains an organic or inorganic auxiliary forming agent and an inorganic fibrous substance in addition to gamma-alumina. An adsorbent as defined in claims 1 or 2 wherein the amount of ruthenium halide supported is 0.1 to 5 wt.% calculated as ruthenium metal and based on the adsorbent. An adsorbent according to anyone of claims 1 to 3, comprising a flat sheet-corrugated sheet multilayer structure having a honeycomb section, the multilayer structure being composed of alternately arranged flat sheets of ceramic paper retaining a carrier as defined in the preceding claims and corrugated sheets of ceramic paper retaining the same carrier thereon. A method of removing low-concentration nitrogen oxides from a ventilation gas of a highway tunnel, mountain tunnel, underwater tunnel, underground road, sheltered road or the like by bringing an adsorbent as defined in any one of claims 1 to 4 into contact with the gas. A method as defined in claim 5 wherein the adsorbent is heated after adsorbing nitrogen oxides to desorb the adsorbed nitrogen oxides and regenerate the adsorbent, and the regenerated adsorbent is brought into contact with the ventilation gas to conduct a continuous treatment.

Inventors: AKIYAMA MASAKI, FUKUJU ATSUSHI, ICHIKI MASAYOSHI, INAZUMI CHIKASHI, KOBAYASHI HIDETSUGU, ONITSUKA SHIGENORI, SAIRYO YUKI, WATANABE TAKANOBU, AKIYAMA, MASAKI, FUKUJU, ATSUSHI, ICHIKI, MASAYOSHI, INAZUMI, CHIKASHI, KOBAYASHI, HIDETSUGU, ONITSUKA, SHIGENORI, SAIRYO, YUKI, WATANABE, TAKANOBU
Applicants: HITACHI SHIPBUILDING ENG CO, POLLUTION RELATED HEALTH DAMAG, HITACHI ZOSEN CORPORATION, THE POLLUTION- RELATED HEALTH DAMAGE COMPENSATION AND PREVENTION ASSOCIATION
Application year: 1992
Main classification: B01J20
IPC codes: B01J20, B01D53