Honeycomb monolith structure

Abstract

Ho Honeycomb monolith structure, especially for use in mass transfer-limited processes or processes for the selective catalytic reduction (SCR) of nitrogen oxides, comprising: a plurality of cell walls defining a plurality of polygonal channels, the plurality of cell walls and channels extending in parallel along a common direction from an entrance end to an outlet end of the structure in the fluid flow direction. The transversal cross section of a polygonal channel has the shape of a convex elongated polygon, wherein at least 50% of the internal angles between two adjacent walls of the convex polygon are above 90 degrees and wherein the cell diameter ratio L.sub.L/L.sub.S is greater than 1.5.

Claims

1. A honeycomb monolith structure, comprising: a plurality of cell walls defining a plurality of polygonal channels, the plurality of cell walls and channels extending in parallel along a common direction from an entrance end to an outlet end of the structure in a fluid flow direction and where the channels are open in both ends, wherein a transversal cross section of a polygonal channel has a convex polygon shape, wherein at least 50% of the internal angles between two adjacent walls of the convex polygon are greater than 90 degrees and wherein a cell diameter ratio L.sub.L/L.sub.S (longest over shortest diameter) is greater than 1.5 and wherein a majority of the channels have the same flow resistance.

2. The honeycomb monolith structure according to claim 1, wherein more than 50% of the internal angles between two adjacent walls of the convex polygon shape are greater than 90 degrees.

3. The honeycomb monolith structure according to claim 1, wherein all of the internal angles between two adjacent walls of the convex polygon shape are greater than 90 degrees.

4. The honeycomb monolith structure according to claim 1, wherein the cell diameter ratio L.sub.L/L.sub.S is between 2 and 10.

5. The honeycomb monolith structure according to claim 1, wherein the cell walls have a thickness between 0.01 and 5 mm, including the boundary values.

6. The honeycomb monolith structure according to claim 1, wherein the L.sub.S is between 0.5 and 20 mm, including the boundary values.

7. The honeycomb monolith structure according to claim 1, wherein the L.sub.L is between 0.75 and 200 mm, including the boundary values.

8. The honeycomb monolith structure according to claim 1, wherein the polygonal shape is selected from the group consisting of a pentagonal shape, a hexagonal shape, and an octagonal shape.

9. The honeycomb monolith structure according to claim 1, wherein the convex polygons are arranged in a closest packing tessellation.

10. The honeycomb monolith structure according to claim 1, which is made by extrusion of a ceramic material.

11. The honeycomb monolith structure according to claim 1, wherein at least one inside corner is rounded.

12. The honeycomb monolith structure according to claim 10, wherein a radius of curvature L.sub.R is about equal to half the smallest diameter L.sub.S.

13. The honeycomb monolith structure according to claim 1, for use as a catalyst, or as a support for one or more catalysts, or as a solid for one or more non-catalytic processes, or as a support for one or more chemicals used in a non-catalytic process.

14. The honeycomb monolith structure according to claim 13, for use as a catalyst in selective catalytic reduction (SCR) of nitrogen oxides.

15. A catalyst structure, comprising the honeycomb monolith structure according to claim 1, wherein the polygonal channels of the honeycomb monolith structure have a pentagonal shape or a hexagonal shape.

16. A stacked structured reactor comprising at least two honeycomb monolith structures according to claim 1 stacked along a common flow direction.

17. The honeycomb monolith structure according to claim 4, wherein the cell diameter ratio L.sub.L/L.sub.S is between 2 and 6.

18. The honeycomb monolith structure according to claim 4, wherein the cell diameter ratio L.sub.L/L.sub.S is between 2.5 and 4.

19. The honeycomb monolith structure according to claim 5, wherein the cell walls have a thickness between 0.1 and 1.5 mm, including the boundary values.

20. The honeycomb monolith structure according to claim 5, wherein the cell walls have a thickness between 0.2 and 1.1 mm, including the boundary values.

21. The honeycomb monolith structure according to claim 6, wherein the L.sub.S is between 1 and 10 mm, including the boundary values.

22. The honeycomb monolith structure according to claim 6, wherein the L.sub.S is between 2 and 6 mm, including the boundary values.

23. The honeycomb monolith structure according to claim 7, wherein the L.sub.L is between 1.5 and 100 mm, including the boundary values.

24. The honeycomb monolith structure according to claim 7, wherein the L.sub.L is between 6 and 60 mm, including the boundary values.

25. The honeycomb monolith structure according to claim 10, wherein the ceramic material is a ceramic paste.

26. The honeycomb monolith structure according to claim 11, wherein all of the inside corners are rounded.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The honeycomb monolith structure according to the invention will be further described with reference to the accompanying drawings, wherein:

(2) FIG. 1 shows a prior art honeycomb monolith with square channel geometry (wall thickness=0.7 mm, L.sub.S=4.2 mm)

(3) FIG. 2 shows a honeycomb monolith with an elongated hexagon channel geometry according to the present invention (wall thickness=0.7 mm, L.sub.S=3.2 mm)

(4) FIG. 3a shows a prior art square channel geometry with a given L.sub.L and L.sub.S

(5) FIG. 3b shows a pentagon channel geometry according to the present invention

(6) FIG. 3c shows a channel geometry as an elongated hexagon according to the present invention

(7) FIG. 4 shows a channel geometry as an elongated hexagon with rounded corners according to the present invention

(8) FIG. 5a shows elongated hexagons according to the present invention in closest packing

(9) FIG. 5b shows elongated pentagons according to the present invention in closest packing

(10) FIG. 6a shows elongated convex polygons with 12 internal corners according to the present invention

(11) FIG. 6b shows elongated convex polygons wherein the corners have been rounded such that a smooth convex surface is obtained.

(12) FIG. 6c shows elongated convex polygons with rounded corners (allentoids) in staggered configuration.

(13) FIG. 7 shows the pressure drop as a function of the diameter ratio (L.sub.L/L.sub.S) and channel geometry, wherein the channel wall thickness is kept constant. Benchmark=Square channel geometry.

(14) FIG. 8 shows chemical conversion as a function of the diameter ratio (L.sub.L/L.sub.S) and channel geometry, wherein the channel wall thickness is kept constant. Benchmark=Square channel geometry.

(15) FIG. 9 shows one particular embodiment of a honeycomb monolith structure according to the invention with elongated hexagonal channels with an L.sub.L/L.sub.S ratio of (9.08/3.20)=2.84, a cell wall thickness of 0.7 mm, and a cell density of 1.86 cells/cm.sup.2.

(16) FIG. 10 shows another particular embodiment of a honeycomb monolith structure according to the invention with elongated hexagonal channels with an L.sub.L/L.sub.S ratio of (13.87/3.20)=4.33, a cell wall thickness of 0.7 mm, and a cell density of 2.87 cells/cm.sup.2.

(17) FIG. 11a shows a photo of the reference monolith after 48 hours exposure to fly-ash.

(18) FIG. 11b shows a photo of a hexagonal monolith after 48 hours exposure to fly-ash.

(19) FIG. 11c shows a graph of the open frontal area of the monoliths as a function of time.

(20) FIG. 12 shows the results from pressure drop experiments carried out with monoliths that have square or elongated hexagonal channels.

DETAILED DESCRIPTION OF THE INVENTION

(21) The invention is based on the finding that an improved performance can be gained for mass-transfer limited reactions from a multi-channel honeycomb monolith reactor with a modified channel transversal cross sectional shape compared to prior art reactors.

(22) The honeycomb channel structure according to the present invention has a transversal cross sectional shape which is the shape of a convex polygon, wherein at least 50% of the internal angles between two adjacent walls are above 90 degrees and wherein the cell diameter ratio L.sub.L/L.sub.S is greater than 1.5. This definition excludes the prior art rectangular shaped cell structures, but includes pentagonal, hexagonal, heptagonal, octagonal, nonagonal, decagonal and polygonal structures with a higher number of angles.

(23) Within the context of the present invention, a honeycomb monolith structure is defined as a structure comprising a plurality of channels, or cells, or passages, separated by thin walls, extending in parallel along a common direction from an entrance end to a outlet end of the structure in the direction of a fluid, such as a liquid or a gas, flowing through said plurality of channels, or cells, or passages (longitudinal axis/fluid flow direction). The channels are open in both ends.

(24) Within the context of the present invention, a convex polygon is defined as a polygon wherein all internal angles between two adjacent walls are in the range from 90 to 180, this in contrast to a concave polygon, wherein one or more of the internal angles between two adjacent walls are below 90.

(25) Within the context of the present invention, the cell diameter ratio L.sub.L/L.sub.S is defined as the ratio of the longest internal diameter (L.sub.L) over the smallest internal diameter (L.sub.S) of a cell, wherein both diameters are measured perpendicular to the common direction from an entrance end to an outlet end of the structure in the flow direction of a fluid, wherein both diameters are measured as the length of a straight line through the centre of gravity of the cell with a start- and endpoint at a wall delimiting the cell.

(26) It is preferred that all or the majority of the channels in the honeycomb monolith structure have the same flow resistance (hydraulic diameter). With the majority or bulk of the channels it is meant most of the channels except for the channels in the periphery of the structure. Some of these will consist of parts of a cell as can be seen in the figures. It is preferred that the bulk of the cells have the same transversal cross-sectional shape and dimensions and that all the walls have the same thickness, however, it is recognized that there may be structural benefits when some of the walls in the monolith are thicker (increasing mechanically strength), or that the dimensions of the transversal cross-sectional shape may change along the common direction from an entrance end to an outlet end of the structure in the fluid flow direction. However, all the channels must be open in both ends and all or the main part of the channels have equal to nearly the same shape and the same flow resistance (hydraulic diameter).

(27) Surprisingly, it has been found that by selecting an elongated shape for the channel cross-section according to the invention, it is possible to decrease the pressure drop significantly, compared to the prior art, while still maintaining or even improving the conversion rate. In addition, because of the increased hydraulic diameter of these channel designs, combined with the larger total internal angle (sum of all internal angles), it is found that the novel channel geometries also can improve, over prior art for monolith reactors, treating flue gasses which contain particular material (high dust, fly ash, soot) since the novel design has less dust depositions and higher erosion resistance.

(28) The limitations on the longest diameter (L.sub.L), substantially equal to the width of the cell, chosen for a given wall thickness, will in general be defined by the required material strength and structural properties of the monolith structure, i.e. dependent on the material chosen for the monolith structure walls. The limitation on the smallest diameter (L.sub.S), substantially equal to the height of the cell, will in general be linked to the dimensions of any particular material in the gas flow.

(29) According to the invention, a honeycomb monolith structure is provided, especially for use in mass transfer-limited processes, comprising a plurality of cell walls defining a plurality of polygonal channels, the plurality of cell walls and channels extending in parallel along a common direction from an entrance end to an outlet end of the structure in the fluid flow direction, wherein the transversal cross section of a polygonal channel has the shape of a convex polygon, wherein at least 50% of the internal angles between two adjacent walls of the convex polygon are above 90 degrees and wherein the cell diameter ratio L.sub.L/L.sub.S is greater than 1.5. The channels are open in both ends. All or the main part of the channels have equal to nearly the same shape and the same flow resistance (hydraulic diameter).

(30) According to one embodiment, more than 50% of the internal angles between two adjacent walls of the convex polygon are greater than 90 degrees.

(31) According to one embodiment, all the internal angles between two adjacent walls of the convex polygon are greater than 90 degrees.

(32) According to one embodiment, the cell diameter ratio L.sub.L/L.sub.S is 2 or more, more preferably 2.5 or more. According to one embodiment, the cell diameter ratio L.sub.L/L.sub.S ranges between 2 and 10, preferably 2 and 6, and most preferably between 2.5 and 4. In principle, the cell diameter ratio L.sub.L/L.sub.S can have any value of more than 1.5. However, the maximum width is determined by the obtainable stability and mechanical strength of the elongated channels and hence, by the choice of material the walls are made of, the production method, etc.

(33) According to one embodiment the wall thickness ranges between 0.01 and 5 mm, preferably between 0.1 and 1.5 mm, most preferably between 0.2 and 1.1 mm, including the boundary values.

(34) According to one embodiment the L.sub.S ranges between 0.5 and 20 mm, preferably between 1 and 10 mm, most preferably between 2 and 6 mm, including the boundary values.

(35) According to one embodiment the L.sub.L ranges between 0.75 and 200 mm, preferably between 1.5 and 100 mm, most preferably between 6 and 60 mm, including the boundary values.

(36) According to one embodiment, the transversal cross section of a polygonal channel has a pentagonal, hexagonal or octagonal shape. An example of a pentagonal shape is illustrated in FIG. 3b, an example of a hexagonal shape is illustrated in FIG. 3c.

(37) According to one embodiment, the convex polygons are arranged in a closest packing tessellation. FIG. 5a shows hexagons in closest packing and FIG. 5b shows pentagons in closest packing. Using a closest packing arrangement, the open frontal area (OFA) is maximized.

(38) According to one embodiment, the longitudinal slots may also have one or more rounded internal corners. When an internal corner in the cross-section polygon is rounded, the radius of curvature (L.sub.R) of the rounded corner is half or less than the smallest diameter (L.sub.S), i.e. L.sub.RL.sub.S, as has been shown in FIG. 4 for two radius of curvature, in the left pane where L.sub.R is about half of L.sub.S, and in the right pane where L.sub.R is much smaller than L.sub.S. Preferably, all internal corners are rounded. In FIG. 6a it is shown elongated convex polygons with 12 internal corners, while FIG. 6b shows elongated convex polygons wherein the corners have been rounded such that a smooth convex surface is obtained. FIG. 6c shows elongated convex polygons with rounded corners (allentoids) in a staggered configuration.

(39) According to one particular embodiment, shown in FIG. 9, the honeycomb monolith structure according to the invention has elongated hexagonal channels with an ratio of (9.08/3.20)=2.84, a cell wall thickness of 0.7 mm, and a cell density of 1.86 cells/cm.sup.2.

(40) According to another particular embodiment, shown in FIG. 10, the honeycomb monolith structure according to the invention has elongated hexagonal channels with an L.sub.L/L.sub.S ratio of (13.87/3.20)=4.33, a cell wall thickness of 0.7 mm, and a cell density of 2.87 cells/cm.sup.2.

(41) The novel honeycomb monolith structure according to the invention may be a catalyst, or a support for one or more catalysts, or a solid for one or more non-catalytic processes, or a support for one or more chemicals used in a non-catalytic process. Preferably, the novel honeycomb monolith structure according to the invention may be used as a catalyst in selective catalytic reduction (SCR) of nitrogen oxides.

(42) Multiple honeycomb monolith structures according to the invention may also be stacked along their common flow direction making a stacked structured reactor.

(43) The honeycomb structure according to the invention can be produced using any suitable means known to the skilled person. In particular, a method of producing the honeycomb structure according to the invention comprises at least an extrusion moulding step, a cutting step, a drying step, and a firing step. In particular, the extrusion moulding step uses an extrusion moulding die having a plurality of slit grooves which are arranged corresponding to the shape of the arrangement of the cell walls in the polygonal cell honeycomb structure according to the invention.

(44) In an extrusion moulding step, at first, a ceramic raw material powder is prepared having the desired composition. Next, water, ceramics binder and other additives of a predetermined amount are added to the ceramic material and subsequently mixed together in order to make the ceramic paste. This ceramic paste is extruded using the moulding die to produce the polygonal cell honeycomb structure according to the invention.

(45) In the cutting step, the shaped body having a honeycomb structure is cut into a plurality of parts of a predetermined length.

(46) In the firing step, the dried honeycomb structure parts are fired at a predetermined temperature, for example at 1400 C. for a given ceramic material. After completion of the firing step, the polygonal honeycomb structure of the invention is produced, as shown in FIG. 2.

(47) Alternatively, the structure can also be made by additive layer manufacturing, such as disclosed in WO 2012/032325 (Johnson Matthey PLC, 2012).

(48) The invention will now be further illustrated with the following examples, without being bound to it.

EXAMPLES

Example 1: Conversion and Pressure Drop Properties Improvements (CFD Simulations (Computational Fluid Dynamics))

(49) FIG. 7 and FIG. 8 show a lower pressure drop combined with an equal or increased NO-conversion for a range of cell diameter ratios (L.sub.L/L.sub.S) for a relevant example of an honeycomb monolith structure with elongated hexagons and pentagons as cells as displayed in FIGS. 5a, 5b and 6b, with the same wall thickness as the prior art benchmark square channel geometry (as shown in FIG. 1 and FIG. 3a), used for catalytically reducing nitrogen oxides (NO) to elementary nitrogen and water. The inner height is reduced from 4.2 mm in the benchmark square channel to 3.2 mm in the elongated hexagons and pentagon (i.e. by 25%). It can be seen that at a cell diameter ratio (L.sub.L/L.sub.S) of about 2.5, the pressure drop is much reduced compared to the pressure drop for the benchmark square channel geometry, and is decreasing further with increasing values of the cell diameter ratio (L.sub.L/L.sub.S) for all honeycomb monoliths according to the invention. Furthermore, the NO-conversion is at or near the same level (82%) as for the benchmark square channel geometry. It can also be observed that an even higher NO-conversion can be obtained with the honeycomb monolith structure than with the benchmark square channel geometry, at the expense of pressure drop, which might be of value for systems with a relative high overpressure.

Example 2: Increase in OFA/Reduction in Catalytic Volume

(50) For the case in Example 1, where CFD simulations showed a decrease in the pressure drop while maintaining similar NO-conversion rates for an elongated hexagonal shaped channel with a cell diameter ratio (L.sub.L/L.sub.S)=2.5, additional benefits are recognized.

(51) First, the open frontal area (OFA) is increased from 73.5% to 76.3% when comparing with the prior art benchmark square channel monolith (as shown in FIG. 1). The increase in OFA reduces the amount of frontal area hit by dust particle for high-dust flue gas, reducing the risk for dust depositions and fouling. Likewise, the hydraulic diameter is increased by 14% as well as an increase in all the internal angles from 90 to 120, which again reduces the risk for dust depositions and also erosion.

(52) Secondly, another benefit of the increased OFA, is that the total volume of catalyst in the honeycomb monolith is reduced by 7.5%, which reduces the material cost when extruding the honeycomb monolith structure according to the present invention.

Example 3: Fly-Ash Resistance

(53) Experiments were carried out to demonstrate the dust attrition properties related to different geometry of the monolith. Comparative studies were carried out on monoliths produced in the same material and with the same production method where one had a benchmark structure with square channel geometry (L.sub.S=4.2 mm) and a monolith with elongated hexagonal channel geometry (L.sub.L/L.sub.S=3.2). The experiments related to fouling and attrition were performed on equipment where air containing solid fly-ash particles at a relevant range of superficial velocities (usually around 5 m/s in front of the sample) are entered from the top through a monolith sample standing vertically. The solid concentration is relevant for high dust applications in coal-power plants, i.e. from 10 g/Nm.sup.3-30 g/Nm.sup.3. The solid particles used in the experiments are fly-ash captured by electrostatic precipitators in coal-power plants, fly-ash that is commonly added to Portland cement.

(54) The experimental set-up consists of a fan that blows dry air (dew point at 4 C.) through an electrical heater, a screw feeder of fly-ash to an eductor where the amount fed is monitored by a weight cell and a gas cyclone after the monolith sample holder. The holder containing the monolith sample has straight walls with the necessary length to achieve a fully developed flow as has been verified by CFD simulations. The pressure drop is measured over the sample holder and the exposed monolith is monitored by weight measurements and by regular taken photographs. Image analysis is used to monitor parameters like the open frontal area of the different monoliths or simply the number of channels that are plugged.

(55) Post-analysis of the monoliths measure the depth of plugs and the amount of fouling on the monolith surfaces. The results are shown in FIG. 11c, where the open frontal area is shown as a function of time. The plugging rate is quite linear over time. The open frontal area is higher with the new design and this gives slightly lower plugging. The pictures 11a and b show the result after 48 hours.

Example 4: Pressure Drop of Monolith Tested in Pressure Drop Rig

(56) For the measurement of pressure drop through monolith, a monolith sample is installed in a pressure drop test system. The system consists of three parts: fan, controller and measuring part. The fan is employed to gain a range of air flow rate which leads to a range of superficial velocity in front of a monolith sample. In a typical run, a monolith sample is tested for pressure drop in the range of 0 to 10 m/s in the duct which generally has the size of 15 cm by 15 cm but can be varied in terms of its monolith size to be tested. The monolith sample is placed in the measuring duct which is straight enough to achieve a fully developed flow. The controller is adjusted to achieve a predetermined gas superficial velocity. In order to confirm the gas superficial velocity, a velocity meter probe is inserted in the sampling hole located before the monolith sample. A static pressure probe is also used in the same sampling hole to measure its upstream static pressure. Downstream static pressure can be measured through another sampling hole located after the monolith sample. The differential pressure can be decided by these two static pressures. In general, the measurement is repeated with a variety of gas flow rates.

(57) FIG. 12 shows the results from pressure drop experiments carried out with monoliths that have square or elongated hexagonal channels. The same structures and dimensions as described in Example 1 were employed but two cell diameter ratio (L.sub.L/L.sub.S), 2.79 and 4.29, were chosen for the testing of hexagonal channels. The graph shows the pressure drop as a function of the gas superficial velocity explained above. The results confirm the structural benefit of elongated hexagon, found by CFD simulations as describe in Example 1, from pressure drop point of view, compared to conventional benchmark square channel shape.