CATALYST STRUCTURE AND GAS PURIFICATION APPARATUS

Abstract: A catalyst unit is constructed by alternately stacking rectangular catalyst elements (1) formed by cutting catalyst support plates coated with a catalytic material and having ribs (2) inclined at 45° to one specified side edge (1a) of each thereof, and those turned upside down in a case, and the catalyst unit is disposed in a gas passage with the ribs (2) of the catalyst elements (1) inclined at an angle greater than 0° and smaller than 90° to the direction of the gas flow (6). When the catalyst unit comprising the catalyst elements (1) coated with a denitrating catalyst is placed in the gas passage, the ribs (2) of the catalyst elements (1) blocks the gas flow (6) and generate turbulent currents on the downstream side thereof to promote the contact of ammonia and NOx contained in the exhaust gas with the catalyst. When the gas flow (6) is disturbed, the thickness of laminar films covering the surface of the catalyst elements (1) is reduced, so that ammonia and NOx can be easily diffused and the catalytic activity can be enhanced. The catalyst elements exhibits their catalytic abilities effectively without entailing significant pressure loss in the gas flow.

Description: TECHNICAL FIELD The present invention relates to a catalyst unit for purifying a gas and more particularly, to a catalyst unit employing a plate-shaped catalyst elements for efficiently reducing nitrogen oxides (hereinafter referred to as "NOx") with ammonia (NH 3 ). BACKGROUND ART The NOx contained in the exhausted gas discharged by power plants, plants, automobiles and such is a substance causative of photochemical smog and acid rain. An exhaust gas denitrating method that employs NH 3 as a reducing agent for selective catalytic reduction has been prevalently used as an effective denitrating method principally in thermal power plants. A titanium oxide (TiO 2 ) catalyst containing vanadium (V), molybdenum (Mo) or tungsten (W) as an active ingredient is used. Particularly, a catalyst containing vanadium (V) is highly active, difficult to be deteriorated by impurities contained in exhaust gases and effective even at low temperatures. Therefore, such a catalyst is the most currently prevalent denitrating catalyst (JP-A No. 50-128681). Generally, catalyst elements are formed in the shape of a honeycomb or a plate. Various methods of manufacturing catalyst elements have been developed. A well-known flat catalytic plate is formed by coating and cladding a mesh base member formed by working a thin metal sheet in a metal lath and spraying aluminum over the metal lath, a textile fabric or a nonwoven fabric with a catalyst. This flat catalytic plate is worked to obtain a plate-shaped catalyst element 1 having ribs 2 of a wavelike cross section, and flat sections 3 in an alternate arrangement as shown in Fig. 2. A plurality of such catalyst elements 1 are stacked in layers in a case 4 with the ribs 2 extended in the same direction to construct a catalyst unit 8 (JP-A No. 54-79188 and JPO filing No. 63-324676) as shown in Fig. 20. Since this known catalyst unit 8 causes comparatively small pressure loss and cannot be easily clogged with soot and coal ashes, the catalyst unit 8 is employed prevalently in denitrating apparatuses for denitrating the exhaust gases of boilers for thermal power generation. The number of power generating installations equipped with gas-turbines or combinations of gas-turbines and waste heat recovery boilers has progressively increased in recent years to cope with peak power demand in summer. Most of such power generating installations are located in the suburbs of cities, and exhaust gas processing apparatus must be highly efficient and compact in view of site condition and pollution control. Under such circumstances, a method of efficiently reducing the NOx content of exhaust gases proposed in JP-A No. 55-152552 employs a catalyst unit 8 constructed by stacking catalyst elements 1 as shown in Fig. 2 so that the respective ribs 2 of the adjacent catalyst elements 1 extend perpendicular to each other, and disposes the catalyst unit 8 with the ribs 2 of the alternate catalyst elements 1 extending perpendicular to the direction of gas flow 6 and with the ribs 2 of the rest of the catalyst elements 1 extending in parallel to the direction of gas flow 6 as shown in Fig. 21. A catalyst unit 11 proposed in JP-U No. 52-6673 is formed by working metal laths or metal sheets to obtain corrugated sheets 9 having successive ridges 10 of a wavelike cross section and not having any flat section as shown in Fig. 23, constructing a support structure by stacking the corrugated sheets 9 so that the respective ridges 10 of the adjacent corrugated sheets 9 extend across each other as shown in Fig. 24, and a catalyst is supported on the support structure to complete the catalyst unit 11. The catalyst unit 8 of Fig. 20 needs the following improvements to construct a high-efficiency, compact exhaust gas processing apparatus. Fig. 25 shows some of gas passages defined by the catalyst elements 1 stacked with the ribs 2 extended in parallel to the direction of gas flow 6. Catalyst units 8 of this type causing very small pressure loss, the exhaust gas processing apparatus employing the catalyst unit 8 of this type requires small power for operation. However, since the flows of gas in the gas passages of the catalyst unit 8 is not very turbulent and the distance of movement of the components of the gas in the gas passages is small, the catalytic reaction rate (overall reaction rate) is small and the catalyst is unable to fully exhibit its activity. A catalyst unit as shown in Figs. 23 and 24 is similarly disclosed in EP-A-0 433 222. When the catalyst unit 8 is constructed by stacking the catalyst elements 1 so that the ribs 2 extend in parallel to the direction of gas flow 6 as shown in Fig. 20 the rigidity of the catalyst unit with respect to the direction in which the ribs 2 are extended (longitudinal direction) is very large, while the rigidity of the same with respect to the direction perpendicular to the longitudinal direction is small. Therefore there are slight differences in width between the gas passage in the longitudinal direction of the ribs 2 and that perpendicular thereto. In the catalyst unit 8 shown in Fig. 21, in which the respective ribs 2 of the adjacent catalyst elements 1 are perpendicular to each other, the ribs 2 extending perpendicular to the direction of gas flow 6 exert high gas disturbing effect to promote the component substances of the gas to be subjected to a catalytic reaction. However, those ribs 2 work as barriers against the flow of the gas causing a large pressure loss. A small degree of freedom of changing draft loss and performance is a problem in the catalyst unit 8 shown in Fig. 21. Since the catalyst unit 8 is constructed by alternately stacking catalyst elements 1 of the same shape, the opening ratio of the catalyst unit 8 does not change and hence draft loss does not decrease significantly even if the pitch of the ribs 2 (the distance between the adjacent ribs) is changed. Furthermore, since the length of the catalyst elements 1 must be equal to the size of the frontage of the catalyst unit 8, it is difficult to change the length of the catalyst elements 1 optionally. Naturally, two types of catalyst elements 1 of different shapes, e.g. those different in the pitch of the ribs 2, may be alternately stacked, but such two types of catalyst elements 1 requires complex manufacturing processes entailing increase in manufacturing costs. In the catalyst unit 8 shown in Fig. 21, the pitch of the ribs 2 that affect significantly the effect of the catalyst on reaction rate and pressure loss is an important factor. Although the ribs 2 are arranged at equal pitches, the distance between the inlet end of the catalyst unit 8 and the first rib 2 and that between the last rib 2 and the outlet end of the catalyst unit 8 with respect to the direction of gas flow 6 are not particularly specified. Since the catalyst unit 8 shown in Fig. 21 is constructed by stacking the catalyst elements 1 of a given length obtained by cutting a continuous catalytic sheet provided with the ribs 2 at given pitches at given intervals, in some cases, the distance between the end of the catalyst unit 8 and the first rib 2 increases when the amount of the catalyst necessary for catalytic reaction increases, i.e., when the length of the catalyst elements 1 is increased. Consequently, the flat section bends and it is difficult to form a uniform flow passages and it is possible that the end section of the catalyst element bends as shown in Fig. 22 to block the gas passage, lowering the performance of the catalyst unit 8 due to increase in the draft resistance and unbalanced gas flows. The corrugated catalyst elements 9 of the catalyst unit 11 shown in Fig. 24 do not have any sections corresponding to the flat sections 3 of the catalyst elements 1 shown in Fig. 2. Therefore, when the height of the ridges 10 is substantially equal to that of the ribs 2 of the catalyst elements 1 shown in Figs. 20 and 21 the ridges 10 of the adjacent corrugated catalyst elements 9 are in contact at a very large number of contact points. Therefore, when the gas flow 6 flows across the section of the cubic catalyst unit 11, the numerous contact points of the ridges 10 cause draft resistance against the gas flow 6, increasing pressure loss. Catalyst elements as disclosed in DE-U-85 27 885 have wave form ribs formed at a pitch. Most of the ridges of the ribs are in contact with flat sections extending between the ribs, and the gas flow will block at the contact area between ribs and flat sections. DE-A-43 16 132 discloses catalyst units which are composed of a stack of a plurality of plate-like catalyst elements having ribs formed as a pitch and flat sections extending between the ribs and catalyst elements having ribs formed without a flat section extending between the ribs. Accordingly, it is a first object of the present invention to solve problems in the prior art and to provide a catalyst unit capable of enhancing the turbulence of a gas to be processed in the gas passages thereof to suppress the formation of laminar films and of further enhancing catalytic actions. A second object of the present invention is to solve problems in the prior art and to provide a catalyst unit capable of causing a gas to be processed to diffuse satisfactorily over catalytic surfaces without increasing pressure loss and of enhancing the performance of the catalyst. A third object of the present invention is to solve problems in the prior art and to achieve exhaust gas purification by using a catalyst unit capable of enhancing the performance of the catalyst by further leveling the flow velocity distribution of a gas to be processed without causing pressure loss in the gas flow. DISCLOSURE OF THE INVENTION Generally, reaction between a gas flow flowing through a pipe and a catalyst supported on the wall of the pipe is expressed by: 1/K = 1/Kr + 1/Kf where K is overall catalytic reaction velocity constant, Kr is unit surface area reaction velocity constant, and Kf is film coefficient of mass transfer of a gas, which indicates the easiness of diffusion of a gas over catalytic surfaces. As is obvious from the expression, the catalytic performance of a catalyst can be improved by increasing film coefficient of mass transfer of the gas. The present invention is intended to solve a problem to enhance the performance of the catalyst by improving the diffusion of the gas over the surface of catalytic surfaces to the largest possible extent without increasing the pressure loss of the gas that flows through the catalyst unit. The present invention will more readily be understood by reference to the accompanying drawings, however, these drawings are intended to illustrate the invention and are not to be construed to limit the scope of the invention. First, a catalyst unit provided with catalyst elements of the present invention provided with ribs and arranged with the ribs at an angle equal to or greater than 30° and equal to or smaller than 60° to the direction of gas flow will be described. A mode of stacking the component catalyst elements provided with the aforesaid oblique ribs will be described. Figs. 6(a) to 6(d) show catalyst units constructed by stacking catalyst elements in accordance with the present invention with the ribs thereof inclined at an angle equal to or greater than 30° and equal to or smaller than 60°. In the catalyst unit shown in Fig. 6(a), rectangular catalyst elements 1 each provided with parallel ribs 2 formed at a given pitch between flat sections 3 so as to extend at an angle  meeting an inequality, 30° ≤  ≤ 60° to a specified side edge 1a of a flat rectangular plate catalyst element 1 are stacked with the alternate catalyst elements 1 facing up and the catalyst elements 1 between the former catalyst elements 1 facing down. In the catalyst unit shown in Fig. 6(b), rectangular catalyst elements 1 each provided with parallel ribs 2 formed at a given pitch between flat sections 3 so as to extend at an angle  meeting an inequality, 30° ≤  ≤ 60° to a specified side edge 1a of a flat rectangular plate, and rectangular catalyst elements 1' each provided with parallel ribs 2' formed at a given pitch between flat sections 3' so as to extend in parallel to the specified side edge 1a of the flat rectangular plate are stacked alternately, in which the catalyst elements 1' underlie the catalyst elements 1, which applies to the catalyst units shown in Figs. 6(c) and 6(d). In the catalyst unit shown in Fig. 6(c), rectangular catalyst elements 1 each provided with parallel ribs 2 formed at a given pitch between flat sections 3 so as to extend at an angle  meeting an inequality, 30° ≤  ≤ 60° to a specified side edge 1b of a flat rectangular plate, and rectangular catalyst elements 1' each provided with parallel ribs 2' formed at a given pitch between flat sections 3' so as to extend in parallel to the specified side edge 1b of the flat rectangular plate are stacked alternately. In the catalyst unit shown in Fig. 6(d), pairs of the rectangular catalyst elements 1 and 1' shown in Fig. 6(b) and pairs of catalyst elements 1 and 1' shown in Fig. 6(c) are stacked alternately. The gas flow 6 flows into each of the catalyst units in accordance with the present invention formed by stacking the catalyst elements 1 and 1' as shown in Figs. 6(a) to 6(d) in a direction perpendicular to one side edge 1c of a flat rectangular plate of the catalyst unit. In the catalyst units in accordance with the present invention shown in Figs. 6(a) to 6(d), the ridges of the ribs 2 of the two adjacent catalyst elements 1, and the ridges of the ribs 2 and 2' of the two adjacent catalyst elements 1 and 1' are in point contact with each other, and portions of the ribs 2 and 2' extending on the opposite sides of the points of contact are inclined at the given angle  to the specified side edge 1a or 1b of a flat rectangular plate (= the catalyst element 1). Since the ribs 2 and 2' are inclined to the direction of gas flow 6 and slit passages of a predetermined width are formed in gas passages defined by the flat sections 3 and 3' of the catalyst elements 1 and 1' and the ribs 2 and 2' of the corresponding adjacent catalyst elements 1 and 1', the degree of blocking the gas is small though, a certain pressure loss is caused. Consequently, the unbalanced flow velocity of the gas flowing through the slit passages and in the neighborhood of the ribs 2 and 2' is leveled off. Furthermore, as shown in Fig. 7, turbulent flows are generated on the downstream side of the ribs of the catalyst unit of the type shown in Fig. 6(a) to promote the contact between, for example, NOx and NH 3 contained in the exhaust gas, and the catalyst. The disturbance of the gas flow 6 reduces the thickness of the laminar films formed over the catalytic surfaces, which facilitates the diffusion of NOx and NH 3 and improves catalytic activity greatly. The gas flow 6 disturbed by the portions of the ribs 2 or the ribs 2 and 2' in point contact with each other flows through the slit passages for a predetermined distance. The degree of turbulence of the gas flow 6 is decreased as the gas flows through the slit passages. Therefore, the pressure loss is not extremely large, the thickness of the laminar films on the catalytic surfaces is reduced, and hence the gas diffuses satisfactorily to increase the catalytic performance greatly. Since the gas to be processed flows into the catalyst units constructed by stacking the catalyst elements 1 and 1' as shown in Figs. 6(a) to 6(d) in a direction oblique to the ribs 2 and 2', the flow passage area of the gas passages is not reduced sharply and reduced gradually and continuously or stepwise, as compared with the reduction of the flow passage area in the catalyst unit shown in Fig. 27 (plan view of the catalyst unit shown in Fig. 21) or Fig. 24, so that pressure loss is comparatively small. In the prior art catalyst unit (JP-A No. 55-152552) shown in Fig. 27, slit passages are formed between the catalyst element 1 having the ribs 2 parallel to the gas flow 6, and the catalyst element 1' having the ribs 2' perpendicular to the gas flow 6 (Fig. 28 showing a cross section taken on line A-A in Fig. 27). Since the ribs 2' perpendicular to the gas flow 6 are arranged at predetermined intervals, the gas flow 6 is blocked by the ribs 2' in the catalyst unit, so that the pressure loss is very large as compared <ith that which is caused by the catalyst units shown in Fig. 6. In the prior art catalyst unit 11 (JP-U No. 52-6673) shown in Fig. 24, the catalyst elements do not have any sections corresponding to the flat sections 3 and 3' shown in Fig. 6 or 27, and the ridges 10 are in point contact with each other at positions far many than positions of contact between the ribs 2 and 2' of the catalyst unit shown in Fig. 6 or 27. Therefore, when the height of the ridges 10, i.e., the depth of the furrows, is the same as that of the ribs shown in Fig. 6 or 27, the gas is disturbed by the portions of the ridges 10 in point contact with each other in the catalyst unit 11 and a pressure loss far greater than that is caused by the catalyst units shown in Fig. 6 results. In the catalyst unit in accordance with the present invention shown in Fig. 6, the ribs 2 of each catalyst element are inclined to the specified side edge 1a or 1b (side edges 1a and 1b are parallel to the direction of the gas flow 6) of the catalyst element 1 at the angle  equal to or greater than 30° and equal to or smaller than 60°. When the angle  is determined so that the opposite ends of the longest rib 2a of each catalyst element 1 are in contact with side walls 12a and 12b at positions near the inlet and the outlet end of the gas passage, respectively, as shown in Fig. 14, the gas flow 6 that flows through a flat passage (the slit passages) defined by the flat section 3a adjacent to the longest rib 2a must inevitably flow over the longest rib 2a toward the outlet end, whereby the degree of contact of the gas with the catalyst is increased accordingly. On the other hand, when the angle  is determined so that the opposite ends of the longest rib 2a of each catalyst element 1 are on the inlet and the outlet end of the gas passage, respectively, as shown in Fig. 15, the gas flow 6 that flows into the slit passage defined by the flat section 3a adjacent to the longest rib 2a need not flow over the longest rib 2a and is able to flow through the slit passage toward the outlet end. In this case, the degree of contact of the gas with the catalyst is smaller than that in the case illustrated in Fig. 14. In the prior art catalyst unit shown in Fig. 20, the ribs 2 of all the catalyst elements 1 are parallel to the direction of flow 6. Therefore, each catalyst element 1 has a large bending strength with respect to a direction parallel to the direction of the gas flow 6 and a small bending strength with respect to a direction perpendicular to the direction of gas flow 6. Therefore, the catalyst elements bend easily and the width of the spaces between the catalyst elements 1 is irregular as shown in Fig. 26. In the catalyst unit in accordance with the present invention, in which the ribs 2 of the alternate catalyst elements 1 are inclined at an angle equal to or greater than 30° and equal to or smaller than 60° to the direction of the gas flow 6, the rigidity with respect to a direction perpendicular to the direction of the gas flow 6 is increased and the catalyst elements 1 do not bend. Accordingly, the flow passage area rarely varies irregularly and gas passages having regularly varying flow passage area can be formed. The gas passages of regularly varying flow passage area, in combination with the gas mixing effect of the ribs 2 and 2', reduce greatly the probability of formation of regions in which the rate of catalytic reaction is low. Thus, the present invention has not only the effect of preventing the reduction of catalytic performance attributable to the irregular variation of the flow passage area of the gas passages but also the effect of reducing the probability of forming gas passages having irregularly varying sectional area. Although the ribs 2 and 2' disturb the gas flow 6 to promote the contact between components of catalytic reaction and the catalytic surfaces and enhance the activity, the pressure loss caused by the catalyst elements 1 and 1' is smaller than that caused by the catalyst elements stacked with the ribs 2 and 2' extending in a direction perpendicular to the direction of gas flow 6, because the catalyst elements 1 and 1' are arranged alternately with the ribs 2 and 2' extending at an angle equal to or greater than 30° and equal to or smaller than 60° to the direction of the gas flow 6. The ribs of the plate-shaped catalyst elements in accordance with the present invention may be of any shape, provided that the ribs and the flat sections are arranged alternately in parallel to each other. The ribs may be formed in any one of cross sections of an S-curved shape, a zigzag shape and a convex relief shape as shown in Figs. 3(a) to 3(e) by way of example. The height of the ribs of the catalyst elements to be used for denitration is in the range of 1.5 to 14 mm. Excessively little height increases pressure loss and excessively big height increases the volume of the catalyst necessary for securing the same performance. The width of the flat sections is dependent on the bending strength of the catalyst element; a greater width is more advantageous provided that the catalyst element does not bend, because a greater width is more effective in reducing pressure loss. A desirable width is in the range of 5 to twenty-five times the height of the ribs from the surface of the flat section. According to the invention, the width of the flat sections of catalyst element to be used for denitration is in the range of about 10 to about 150 mm. The foregoing catalyst elements are used in suitable combinations to construct catalyst units in accordance with the present invention. The catalyst unit of the present invention can be applied to various catalytic reaction apparatuses for processing gases, such as catalytic deodorizing apparatuses, catalytic combustors and fuel reformers. The use of the catalyst unit of the present invention in exhaust gas denitrating apparatuses for denitrating an exhaust gas by reducing the NOx contained in the exhaust gas under the existence of an ammonia is the most typical application of the present invention. For example, a denitrating apparatus (Fig. 12) provided with at least one catalyst unit of the present invention comprising catalyst elements coated with a denitration catalyst in the passage of a NOx-containing exhaust gas is able to denitrate the exhaust gas at a high NOx removal efficiency entailing a comparatively low pressure loss in the exhaust gas. The pressure loss can be limited to a value within a range allowed by a system, such as a plant, in which a denitrating apparatus is used by using a catalyst unit of the present invention comprising the aforesaid catalyst elements coated with a denitration catalyst in combination as shown in Fig. 13 with an ordinary low-pressure-loss denitrating apparatus having catalyst elements disposed with its ribs extended in parallel to the direction of the gas flow (a honeycomb denitrating apparatus having a structure having a honeycomb-shaped cross section or a plate type denitrating apparatus as shown in Fig. 20 having a structure constructed by stacking a plurality of flat plates at intervals). Since some system, such as a plant, places restrictions in pressure loss on the catalyst unit and a pressure loss that may be caused when only the catalyst unit of the present invention is used is excessively high in some cases, the pressure loss can be limited to a value in the allowable range by using the catalyst unit of the present invention in combination with an ordinary catalyst unit that causes a low pressure loss. The catalyst element of the present invention has a high effect in mixing the gas in the catalyst unit. Therefore, the irregularity of, for example, the ammonia concentration at the outlet of a catalytic denitrating apparatus is less irregular than that at the outlet of a catalytic denitrating apparatus that causes a less pressure loss even if the ammonia concentration at the inlet of the catalytic denitrating apparatus is locally irregular, so that a catalytic apparatus disposed on the downstream side of the catalytic denitrating apparatus is able to operate effectively. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a fragmentary perspective view of a catalyst unit in an embodiment according to the present invention; Fig. 2 is a perspective view of a catalyst element in an embodiment according to the present invention; Figs. 3(a), 3(b), 3(c), 3(d) and 3(e) are sectional view of catalyst elements having ribs and employed in the present invention; Fig. 4 is a sectional view of a catalyst element employed in an embodiment of the present invention; Fig. 5 is a perspective view of a catalyst element in an embodiment according to the present invention; Figs. 6(a), 6(b), 6(c) and 6(d) are schematic plan view for assistance in explaining modes of stacking catalyst elements employed in an embodiment according to the present invention; Fig. 7 is a typical view illustrating gas flow in a catalyst unit for assistance in explaining the effect of the present invention; Figs. 8(a) and 8(b) are fragmentary perspective views of a catalyst unit in an embodiment according to the present invention; Fig. 9 is a fragmentary perspective view of a catalyst unit in an embodiment according to the present invention; Fig. 10 is a diagram showing the denitrating ability of Example 1 of the present invention normalized by the denitrating ability of Comparative example 1; Fig. 11 is a diagram showing pressure loss caused by Example 1 of the present invention normalized by pressure loss caused by Comparative example 1; Fig. 12 is a block diagram of an exhaust gas purifying apparatus comprising two catalyst units in an embodiment according to the present invention successively arranged on an exhaust gas passage; Fig. 13 is a block diagram of an exhaust gas purifying apparatus comprising a catalyst unit in an embodiment according to the present invention, and a denitrating apparatus that causes a low pressure loss disposed on the downstream side of the catalyst unit, arranged on an exhaust gas passage; Fig. 14 is a schematic plan view of a catalyst unit in an embodiment according to the present invention disposed on a gas passage; Fig. 15 is a schematic plan view of a catalyst unit in an embodiment according to the present invention; Fig. 16 is a graph showing the relation between overall reaction rate and gas flow velocity for catalyst units in Examples 1 and 8 of the present invention and Comparative example 2 and a catalyst unit 11 shown in Fig. 24; Fig. 17 is a graph comparatively showing the relation between pressure loss and gas flow velocity for the catalyst units in Examples 1 and 8 of the present invention and Comparative example 2 and the catalyst unit 11 shown in Fig. 24; Fig. 18 is a diagram comparatively showing data representing pressure loss caused by the catalyst units in Examples 1 and 8 of the present invention and Comparative example 2 and the catalyst unit 11 shown in Fig. 24 on the basis of the same denitrating ability; Fig. 19 is a block diagram of an exhaust gas purifying apparatus comprising two low-pressure-loss catalytic denitrating apparatus in accordance with the present invention; Fig. 20 is a side perspective view of a prior art catalyst unit; Fig. 21 is a perspective view of a prior art catalyst unit; Fig. 22 is a view for assistance in explaining problems in the catalyst unit of Fig. 21; Fig. 23 is a plan view of a prior art catalyst element; Fig. 24 is a perspective view of a catalyst unit constructed by staking catalyst elements like that shown in Fig. 23; Fig. 25 is a fragmentary perspective view of a catalyst unit for assistance in explaining problems in the prior art; Fig. 26 is a fragmentary perspective view of a catalyst unit for assistance in explaining problems in the prior art; Fig. 27 is a view of assistance in explaining a mode of stacking prior art catalyst elements; and Fig. 28 is a typical view showing a gas flow in a catalyst unit constructed by stacking catalyst elements in the mode illustrated in Fig. 27. BEST MODE FOR CARRYING OUT THE INVENTION Preferred embodiments of the present invention will be described in detail hereinafter. First, catalyst units embodying the present invention comprising catalyst elements disposed with its ribs inclined at an angle equal to or greater than 30° and equal to or smaller than 60° to the direction of a gas flow will be described. Example 1 A paste of about 36% in moisture content was prepared by kneading a mixture of 67 kg of a metatitanic acid slurry (30% by wt. TiO 2 , 8% by wt. SO 4 ), 2.4 kg of ammonium paramolybdate ((NH 4 ) 6 ·Mo 7 O 24 ·4H 2 O), and 1.28 kg of ammonium metavanadate (NH 4 VO 3 ) and evaporating water contained therein by a heated kneader. The paste was extruded in 3 mm diameter round cords, pellets obtained by pelletizing the cords were dried by a fluidized bed drier, and then the dried pellets were baked at 250°C for 24 hr in the atmosphere to obtain granules. The granules were crushed by a hammer mill to reduce in size to obtain a powder of 5 µm in average grain size as a first component. The composition of the first component was V/Mo/T = 4/5/91 in atomic ratio. A mixture of 20 kg of the power thus obtained, 3 kg of inorganic Al 2 O 3 ·SiO 2 fibers and 10 kg of water was kneaded by a kneader for 1 hr to obtain a claylike catalytic paste. Flat catalytic plates of about 0.9 mm in thickness and 500 mm in length were fabricated by coating portions forming meshes of 500 mm wide, 0.2 mm thick metal laths of SUS 304 surface-treated for roughening by aluminum spraying with the catalytic paste. The flat catalytic plates were worked by pressing to obtain catalyst elements having ribs 2 of a wavelike cross section arranged at given pitches between flat sections 3 as shown in Fig. 2, and then the thus worked catalytic plates were baked at 550°C for 2 hr in the atmosphere after air drying to obtain catalyst elements 1. In the catalyst elements 1 as shown in Fig. 4, the height h of the ribs 2 from the surface of the flat section 3 is 2.5 mm and the width P of the flat sections 3 is 80 mm. The catalyst elements 1 thus fabricated were cut in a rectangular shape so that the ribs 2 are inclined at 45° to one side edge 1a of each catalyst element 1 to complete rectangular catalyst elements 1 shown in Fig. 5. The catalyst elements 1 and those turned upside down were stacked alternately as shown in Fig. 1 in a case having 2 mm thick walls, not shown, to construct a layer-built catalyst unit of 150 mm by 150 mm by 500 mm (length) shown in Fig. 6(a). This catalyst unit is disposed in a gas passage so that the ribs 2 of the catalyst elements 1 are inclined at 45° to the direction of the gas flow 6 as shown in Fig. 1. Since the catalyst elements 1 are of the same shape and the catalyst unit can be constructed simply by alternately stacking the catalyst elements 1 and those turned upside down, the catalyst unit can be mass-produced at reduced manufacturing costs. Example 2 A catalyst unit of a construction shown in Fig. 6(b) or 6(c) (Fig. 8(a) or 8(b)) was constructed by alternately stacking catalyst elements 1 (the catalyst elements of Example 1 cut in a rectangular shape so that the ribs 2 are inclined at 45° to the side edge 1a) and catalyst elements 1' (the catalyst elements cut in a rectangular shape so that all the ribs 2 are parallel to a specified side edge 1a). The catalyst unit is disposed in a gas passage with the ribs 2 of the catalyst elements 1 inclined at 45° to the direction of the gas flow 6. Example 3 Catalytic plate sets each formed by superposing the catalyst element 1 and the catalyst element 1' employed in Example 2 as shown in Fig. 6(b), and catalytic plate sets each formed by superposing the catalyst element 1 and 1' as shown in Fig. 6(c) were stacked alternately as shown in Fig. 6(d) to obtain a catalyst unit shown in Fig. 9. The catalyst unit is disposed in a gas passage with the ribs 2 of the catalyst elements 1 thereof inclined at 45° to the direction of the gas flow 6. Comparative Example 1 Catalyst elements 1 as shown in Fig. 2 having ribs 2 of 5 mm in height from the surface of flat sections 3 were fabricated by working the flat catalytic plates from which the catalyst elements 1 of Example 1 were formed, and then the catalyst elements 1 thus fabricated were cut in a rectangular shape so that the ribs 2 are parallel to one specified side edge 1a of each catalyst element 1 to complete rectangular catalyst elements 1. The catalyst elements 1 were stacked in a case 4 as shown in Fig. 20 to construct a catalyst unit 8 of 150 mm by 150 mm by 500 mm (depth). The catalyst unit 8 is disposed in an exhaust gas passage with the ribs 2 of the catalyst elements 1 extended in parallel to the direction of the gas flow 6. Each of the catalyst units in Examples 1 to 3 and Comparative example 1 was installed in a reactor forming the exhaust gas passage, and an LPG combustion exhaust gas was passed through the reactor to measure the denitrating abilities of the catalyst units in Examples 1 to 3 and Comparative example 1 under conditions tabulated in Table 1. The distribution of NOx concentration at the exhaust gas outlet ends of the catalyst units was measured and the uniformity of the exhaust gas flow was examined. Measured results are shown in Table 2. Reaction temp. 350°C Areal velocity 60 m/h Exhaust gas LPG combustion exhaust gas NO 90 ppm NH 3 108 ppm Catalytic units NOx conc. at outlet (ppm) Mean denitration eff. (%) Mean Standard dev. Ex. 1 10.5 2.3 89 Ex. 2 15.3 3.1 83 Ex. 3 14.4 3.0 84 Comp. Ex. 1 31.5 9.9 65 As is obvious from Table 2, the ranges of distribution of NOx concentration at the exhaust gas outlets of the catalyst units in Examples 1, 2 and 3 are very narrow and the flow of the exhaust gas in the catalyst units in Examples 1, 2 and 3 is uniform over the respective entire cross sections of the catalyst units in Examples 1, 2 and 3. The mean denitration efficiencies of the catalyst units of Examples 1, 2 and 3 are far higher than that of Comparative example 1. The blocking effect of the ribs 2 of the catalyst elements shown in Fig. 7 in addition to the uniform passage shape gives the catalyst units in Examples 1, 2 and 3 high denitrating abilities. Example 4 Catalyst elements 1 the same as those employed in Example 1 were cut in a rectangular shape so that the ribs 2 are inclined at 30° to a specified side edge 1a to obtain rectangular catalyst elements 1 shown in Fig. 5. The catalyst elements 1 and those turned upside down were stacked alternately as shown in Fig. 1 in a case having 2 mm thick walls, not shown, to construct a layer-built catalyst unit of 150 mm by 150 mm by 500 mm (length) shown in Fig. 6(a). As shown in Fig. 1, this catalyst unit is disposed in an exhaust gas passage with the ribs 2 of the catalyst elements 1 inclined at 30° to the direction of the gas flow 6. Example 5 Catalyst elements 1 of a shape shown in Fig. 4 fabricated by the same processes as those for fabricating the catalyst elements 1 employed in Example 1, the catalyst elements 1 were cut to obtain rectangular catalyst elements 1 having ribs 2 inclined at an angle  of 60° to one side edge 1a as shown in Fig. 5. The catalyst elements 1 and those turned upside down were stacked alternately as shown in Fig. 1 in a case having 2 mm thick walls, not shown, to construct a layer-built catalyst unit of 150 mm by 150 mm by 500 mm (length) shown in Fig. 6(a). As shown in Fig. 1, the catalyst unit is disposed in an exhaust gas passage with the ribs 2 of the catalyst elements 1 inclined at 60° to the direction of the gas flow 6. Comparative Example 2 The catalyst elements 1 (Fig. 2) from which the rectangular catalyst elements 1 employed in Example 1 were stacked so that the respective ribs of the adjacent catalyst elements 1 extend perpendicular to each other as shown in Fig. 21 to obtain a catalyst unit. The catalyst unit was disposed with the ribs 2 of the alternate catalyst elements 1 were parallel to the direction of the gas flow 6. Each of the catalyst units in Examples, 1, 4 and 5 and Comparative example 2 was installed in a reactor and an LPG combustion exhaust gas was passed through the reactor to measure the denitrating abilities of the catalyst units in Examples 1, 4 and 5 and Comparative example 2, and pressure losses caused by the same catalyst units under conditions tabulated in Table 1. Fig. 10 shows the denitrating abilities normalized by the denitrating ability of Comparative example 1, and Fig. 11 shows pressure losses normalized by the pressure loss caused by Comparative example 1. Denitrating ability testing conditions were temperature: 350°C, NH 3 /NO mole ratio: 1.2 and gas flow velocity: 8 m/sec. As is obvious from Figs. 10 and 11, although Comparative example 2 had a high denitrating ability, the same caused a large pressure loss. The pressure losses caused by Examples 1, 4 and 5 were scarcely different from the pressure loss caused by Comparative example 1 in which the ribs were extended in parallel to the direction of the gas flow. Although slightly lower than the denitrating ability of Comparative example 2, the denitrating abilities of Examples 1, 4 and 5 were satisfactorily high. As is obvious from Figs. 10 and 11, when the angle of inclination of the ribs of the catalyst elements to the direction of the gas flow is equal to or greater than 30° and equal to or smaller than 60°, the catalyst units are able to effectively exert their denitrating abilities without significantly causing pressure loss (=ΔP) in the gas flow. Example 6 Two catalyst units of 150 mm by 150 m by 250 mm (depth) constructed by stacking catalyst elements coated with a denitrating catalyst and the same as those employed in Example 1, except that the length (depth) was 250 mm, were arranged in series along the direction of the gas flow as shown in Fig. 12, and a gas was passed through the catalyst units under the conditions tabulated in Table 1 to measure pressure loss and denitrating efficiency. Example 7 A catalyst units of 150 mm by 150 m by 250 mm (depth) constructed by stacking catalyst elements coated with a denitrating catalytic paste and the same as those employed in Example 1, except that the length (depth) was 250 mm, and a denitrating apparatus (150 mm by 150 mm by 150 mm) that causes a small pressure loss, employing the plate type catalyst unit shown in Fig. 20 were arranged in series in an exhaust gas passage through which an exhaust gas containing nitrogen oxides flows along the direction of the gas flow as shown in Fig. 13. The catalyst elements 1 of the plate type catalyst unit were coated with the denitrating catalytic paste employed in Example 1. The catalyst unit was disposed on the upstream side of the denitrating apparatus. A gas was passed through the catalyst unit and the denitrating apparatus under the conditions tabulated in Table 1 to measure pressure loss and denitrating efficiency. Comparative Example 3 Two catalyst units that cause a comparatively small pressure loss like the catalyst unit employed in Example 7 were arranged in series along the direction of the gas flow as shown in Fig. 19. A gas was passed through the catalyst units under the conditions tabulated in Table 1 to measure pressure loss and denitrating efficiency. The denitrating abilities of the catalyst units in Examples 6 and 7 and Comparative example 3 and pressure losses caused by the same catalyst units were measured by passing an LPG combustion gas through the catalyst units under the conditions tabulated in Table 1. Measured results are shown in Table 3. Catalyst units Denitrating eff. (%) Pressure loss (mmH 2 O) Ex. 6 89 14 Ex. 7 81 11 Comp. Ex. 3 65 9 As is obvious from Table 3, although Comparative example 3, i.e., a catalytic apparatus employing the catalyst units that cause lesser pressure loss, causes a comparatively small pressure loss, the denitrating efficiency of Comparative example 3 is low, although the denitrating efficiency of Example 6, i.e., a catalytic apparatus employing the catalyst units of the present invention, is very high, Example 6 causes a pressure loss larger than that caused by Comparative example 3, and although the denitrating efficiency of Example 7, i.e. a catalytic apparatus employing the catalyst unit of the present invention and a catalyst unit that cause a comparatively low pressure loss disposed on the downstream side of the former catalyst unit, is lower than that of Example 6, Example 7 causes a pressure loss lower than that caused by Example 6. In Example 7, the catalyst unit of the present invention disposed on the upstream side of the catalyst unit that causes a comparatively small pressure loss has a gas disturbing effect. The disturbing effect reduces the irregularity of the distribution of ammonia concentration at the outlet of the catalyst unit of the present invention even if the distribution of ammonia concentration at the inlet of the catalyst unit and, consequently, the catalyst unit disposed on the downstream side works effectively. The following experiments were conducted to compare the catalytic performance of the catalyst unit of the present invention and that of the prior art catalyst unit. (1) A catalyst unit (I) similar to that of the catalyst unit in Example 1 of a stacking system shown in Fig. 6(a) was constructed by stacking catalyst elements 1 having flat sections 3 of 80 mm in width and ribs 2 of 2.5 mm in height from the surface of the flat sections 3 so that the ribs 2 of the catalyst elements 1 are inclined at 45° to the direction of the gas flow 6. (2) A catalyst unit (II) similar to that shown in Fig. 21 was constructed by stacking catalyst elements 1 having flat sections 3 of 80 mm in width, ribs 2 of 4 mm in height from the surface of the flat sections 3 and open meshes so that the inclination  of the ribs 2 of the catalyst elements 1 to the direction of the gas flow 6 is 90°. (3) A catalyst unit (III) similar to that of the catalyst unit in Comparative example 2 of a stacking system shown in Fig. 21 was constructed by stacking catalyst elements 1 having flat sections 3 of 80 mm in width, ribs 2 of 2.5 mm in height from the surface of the flat sections 3 and stopped up meshes so that the inclination  of the ribs 2 of the catalyst elements 1 to the direction of the gas flow 6 is 90°. (4) A catalyst unit (IV) was constructed by stacking the corrugated catalyst elements 10 shown in Fig. 24 having ridges 9 of 5 mm in height and coated with the same catalytic coating as that employed in (1) to (3) so that the respective ridges 9 of the adjacent catalyst elements 10 intersect each other perpendicularly and are inclined at 45° to the direction of the gas flow 6. An LPG combustion gas was passed through the catalyst units (I), (II), (III) and (IV) to examine the relation between overall reaction velocity constant and gas flow velocity and the relation between pressure loss and gas flow velocity. Results of the examination are shown in Figs. 16 and 17. Fig. 18 shows normalized pressure losses caused by the catalyst units (I), (II), (III) and (IV), respectively, for the same denitrating ability. As is obvious from Figs. 16, 17 and 18, the pressure losses caused by the catalyst unit in accordance with the present invention are very small as compared with that caused by the catalyst unit (IV) shown in Fig. 24 for the same denitrating ability. The difference in pressure loss (about 130 mmH 2 O) between the catalyst unit in Example 1 (catalyst unit I) and the catalyst unit shown in Fig. 24 (catalyst unit IV) for the same denitrating ability is equivalent to a power cost difference of $240,000 for annual power generation of 73,000 kW. CAPABILITY OF EXPLOITATION IN INDUSTRY The catalyst unit in accordance with the present invention exerts a low draft resistance, disturbs the gas flow to improve catalytic ability and has a compact construction. The catalyst unit in accordance with the present invention is applicable to various catalytic gas processing apparatuses, such as deodorizing apparatuses, catalytic combustors and fuel reformers. The use of the catalyst unit of the present invention in exhaust gas denitrating apparatuses for denitrating an exhaust gas by reducing the NOx contained in the exhaust gas under the existence of an ammonia reducer is the most typical application of the present invention.

Claims: A catalyst unit comprising a housing having an inlet and an outlet defining a direction for gas flow(6) therebetween as an axial direction and side walls extending therebetween; a plurality of catalyst plate elements (1, 1') mounted within said housing between said inlet and said outlet, each catalyst plate element supporting a catalytic material and having parallel ribs (2, 2') space across one dimension thereof with flat sections (3, 3') separating the ribs; wherein a plurality of catalyst plate elements (1, 1') are comprised with a plurality of first catalyst plate elements (1) supporting a catalytic material and having parallel ribs (2) spaced across one dimension thereof with flat sections (3) separating the first ribs (2), said first ribs (2) arranged at a first angle with said axial direction and a plurality of second catalyst plate elements (1') alternating with said first catalyst plate elements (1) to form a stack, each second catalyst plate element (1') supporting a catalytic material and having parallel ribs (2') spaced across one dimension thereof with flat sections (3') separating the second ribs (2'), said second ribs (2') arranged at a second angle, different from said first angle with said axial direction; wherein said flat sections (3, 3') define opposing surfaces for each catalyst plate elements (1, 1') and wherein each of said catalyst plate elements (1, 1') has ridges formed by said ribs (2, 2') and extending from both of its opposing surfaces, whereby contact between adjacent catalyst plate elements (1, 1') in said stack is point contact between their respective ridges; wherein at least one of said first and second angles is equal to or greater than 30° and equal to or smaller than 60°, the height of said ribs being in the range of 1.5 to 14 mm and the width of the flat sections being in the range of about 10 to about 150 mm. A catalyst unit of claim 1, wherein said first (1) and second (1') catalyst plate elements are identical but are arranged with the opposing sides of said second catalyst plate elements (1') reversed relative to the opposing sides of said first catalyst plate elements (1). A catalyst unit of claim 1 wherein said first angle is 0° so that said first ribs extend in parallel to said axial direction and to the direction of the gas flow (6), and said second angle is greater than 30° and smaller than 60°. A catalyst unit of claim 3 further comprising: a plurality of third catalyst plate elements having third ribs arranged parallel to the axial direction and to the direction of the gas flow, said third catalyst plate elements being interposed between adjacent first and second catalyst plate elements; and a plurality of fourth catalyst plate element which are identical to and reversed relative to said third catalyst plate elements and alternately arranged in said stack in contact with either the first or the second catalyst plate elements. A catalyst unit of claim 1, wherein each of said catalyst plate elements has two types of ribs (2) of different heights. A catalyst unit of claim 1, wherein said first catalyst plate elements (1) each have two types of ribs (2) of different heights, and said second catalyst plate elements (1') have ribs (2') of the same height. A catalyst unit of claim 1, wherein said ribs have a cross-section of an S-curved shape, a zigzag shape or a convex relief shape. A catalyst unit of claim 1, wherein said catalytic material supported on said first and second catalyst plate elements (1, 1') is a denitrating catalyst. A catalyst unit of claim 1, wherein each catalyst plate element has one rib longer than its other ribs, and wherein said one longer rib has opposing ends in contact with side walls near the inlet and the outlet of said housing respectively.

Inventors: ISHIKAWA TOMIHISA, KATO YASUYOSHI, KURODA HIROSHI, MICHIMOTO TAKASHI, MORII MASAHARU, MORITA ISATO, MUKAI MASATO, NAGAI YOSHINORI, YAMADA AKIHIRO, ISHIKAWA, TOMIHISA, KATO, YASUYOSHI, KURODA, HIROSHI, MICHIMOTO, TAKASHI, MORII, MASAHARU, MORITA, ISATO, MUKAI, MASATO, NAGAI, YOSHINORI, YAMADA, AKIHIRO, ISHIKAWA, Tomihisa Kure Works,, KATO, Yasuyoshi, Kure Research Laboratory,, KURODA, Hiroshi Babcock-Hitachi Kabushiki Kaisha, MORII, Masaharu Bab-Hitachi Engineering Co., Ltd., MORITA, Isato Kure Works,, NAGAI, Yoshinori Kure Works,
Applicants: BABCOCK HITACHI KK, BABCOCK-HITACHI KABUSHIKI KAISHA
Application year: 1995
Main classification: B01D53
IPC codes: B01D53, B01J35, F01N3