STRUCTURED PACKING ELEMENT WITH REDUCED MATERIAL REQUIREMENT

Abstract

A structured packing element for a column for at least one of mass transfer and heat exchange between a heavy fluid phase and a light fluid phase. The structured packing element comprises at least two layers of a grid comprising openings that are surrounded and separated from each other by separating elements. At least two of the layers are arranged in a longitudinal direction parallel and in touching contact with each other such that an open space extending from one end to an opposite end of the layers is provided between the layers such that at least one of the heavy fluid phase and the light fluid phase may flow therethrough. An average width of at least 50% of the separating elements between adjacent openings is at least 15 times a layer material thickness and is between 70% and 125% of an average hydraulic diameter of the adjacent openings.

Claims

1. A structured packing element for a column for at least one of mass transfer and heat exchange between a heavy fluid phase and a light fluid phase, the structured packing element comprising: at least two layers of a grid comprising openings that are surrounded and separated from each other by separating elements, wherein at least two of the at least two layers are arranged in a longitudinal direction of the packing element parallel and in touching contact with each other such that an open space extending from one end to an opposite end of the at least two layers is provided between the at least two layers such that at least one of the heavy fluid phase and the light fluid phase may flow therethrough, wherein an average width of at least 50% of the separating elements between adjacent ones of the openings is at least 15 times a layer material thickness and is between 70% and 125% of an average hydraulic diameter of the adjacent ones of the openings, wherein a maximum distance between at least two of the at least two layers measured in plane perpendicular to the longitudinal direction is at least 4 times larger than the average width of the separating elements, and wherein the average width of one of the separating elements is determined by dividing up the one of the separating elements into individual sections each having a section length di, for each of the individual sections a shortest distance bi between adjacent edges within the individual sections is measured and a sum of products di.Math.bi is divided by a sum of di to yield the average width of the one of the separating elements, and the layer material thickness is a thickness of a material of one of the at least two layers and is measured at an outer edge of the at least two layers with a micrometer screw.

2. The structured packing element according to claim 1, wherein the average width of at least 75% of the separating elements between adjacent ones of the openings is between 70% and 125% of the average hydraulic diameter of the adjacent ones of the openings.

3. The structured packing element according to claim 1, wherein the average width of at least 50% of the separating elements between adjacent ones of the openings is 1.5 to 4 mm.

4. The structured packing element according to claim 1, wherein the maximum distance between at least 50% of the at least two layers measured in the plane perpendicular to the longitudinal direction is 4 to 15 times larger than the average width of the separating elements.

5. The structured packing element according to claim 1, wherein for at least 50% of the at least two layers, a ratio of a total area of the openings divided by a sheet area of the at least two layers is between 20% and 38%.

6. The structured packing element according to claim 5, wherein for each of the at least two layers, the ratio of the total area of the openings divided by the sheet area of the at least two layers is between 20% and 38%.

7. The structured packing element according to claim 1, wherein at least 50% of the openings of each of the at least two layers have a hydraulic diameter that is between 50 and 150% of the average hydraulic diameter of all of the openings.

8. The structured packing element according to claim 1, wherein a hydraulic diameter of at least 50% of the openings of each of the at least two layers is 1.25 to 5 mm.

9. The structured packing element according to claim 1, wherein at least 50% of the openings have a rectangular or quadrilateral cross-section, wherein shorter characteristic lengths of the rectangles or quadrangles are 1 to 4 mm, wherein longer characteristic lengths of the rectangles or quadrangles are 2 to 8 mm, and wherein the average width of the separating elements between adjacent rectangles is 1.5 to 4 mm.

10. The structured packing element according to claim 1, wherein at least 50% of the openings have a rectangular or quadrilateral cross-section, wherein a ratio of shorter characteristic lengths of the rectangles or quadrangles divided by longer characteristic lengths of the rectangles or quadrangles is 0.4 to 0.7.

11. The structured packing element according to claim 1, wherein the maximum distance between each of the at least two layers measured in the plane perpendicular to the longitudinal direction is at most 15 times larger than the average hydraulic diameter of the openings.

12. The structured packing element according to claim 1, wherein at least 50% of the at least two layers comprise periodic deformations, and wherein the open space between the at least two layers is defined by the periodic deformations.

13. The structured packing element according to claim 12, wherein the periodic deformations are corrugations comprising a plurality of alternately oriented peaks and valleys, wherein the peaks of one of the at least two layers contact the valleys of an adjacent one of the at least two layers and the valleys of one of the at least two layers contact the peaks of an adjacent one of the at least two layers, and wherein adjacent ones of the at least two layers are oriented such that the peaks and valleys of the adjacent ones of the at least two layers intersect in crisscross fashion with the peaks and valleys of the ones of the at least two layers extending obliquely relative to the longitudinal direction.

14. A mass transfer column comprising at least one structured packing element according to claim 1.

15. A method of performing at least one of mass transfer and heat exchange using a structured packing element according to claim 1.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0069] The invention will be explained in more detail hereinafter with reference to the drawings.

[0070] FIG. 1 is a schematic side view of a mass transfer column including several structured packing elements according to one embodiment of the present disclosure.

[0071] FIG. 2A is an explosion view of a part of the sheets of a structured packing element according to one embodiment of the present disclosure.

[0072] FIG. 2B is a schematic side view of the structured packing element shown in FIG. 2A.

[0073] FIG. 2C shows two layers of the structured packing element shown in FIG. 2A.

[0074] FIG. 3 is a fragmentary view of a structured packing element according to another embodiment of the present disclosure.

[0075] FIG. 4 is a schematic view of a part of a structured packing element according to one embodiment of the present disclosure.

[0076] FIG. 5 is a schematic view of a part of a structured packing element according to an embodiment of the present disclosure.

[0077] FIGS. 6A-6F are schematic views of different embodiments of the grid structure of the layers of the structured packing element of the present disclosure.

[0078] FIGS. 7A and 7B are schematic views illustrating the determination of the average width of a separating element and of the average hydraulic diameter of an opening.

[0079] FIG. 8 illustrates the determination of the least deformed portion of a structured packing element made of corrugated layers.

[0080] FIG. 9 is a true to scale plan view illustrating the determination of the parameters for a specific example in accordance with the present disclosure.

[0081] FIGS. 10A-10D show schematic views of the cross-section of a packing layer to illustrate the definitions of the areas A.sub.P (FIG. 10B), A.sub.S (FIG. 10C), A.sub.M, V.sub.M and a.sub.M (FIG. 10D).

[0082] FIG. 11 shows the efficiency curves obtained in example 1 and comparative example 1 for a head pressure of the distillation column of 960 mbar.

[0083] FIG. 12 shows the efficiency curves obtained in example 1 and comparative example 1 for a head pressure of the distillation column of 100 mbar.

[0084] FIG. 13 shows the pressure drops obtained in example 1 and comparative example 1 for a head pressure of the distillation column of 100 mbar.

[0085] FIG. 14 shows the efficiency curves obtained in example 2 and comparative examples 2 and 3 for a head pressure of the distillation column of 960 mbar.

[0086] FIG. 15 shows the efficiency curves obtained in example 2 and comparative examples 2 and 3 for a head pressure of the distillation column of 100 mbar.

[0087] FIG. 16 shows the pressure drops obtained in example 2 and comparative examples 2 and 3 for a head pressure of the distillation column of 100 mbar.

[0088] FIG. 1 shows is a schematic side view of a mass transfer column 10 and more specifically a distillation column 10 according to one embodiment of the present disclosure (the transparent inside of the figure is only for illustrative purposes). Also for illustrative purposes, the grid structure of the layers in not shown in FIG. 1, but only in FIG. 6. The distillation column 10 comprises a plurality of structured packing elements 12, which are arranged in form of two beds 14, 14. Above each of the two beds 14, 14 a distributor 16, 16 is arranged to evenly distribute the liquid over the cross-section of the bed, while leaving enough space for the vapor to ascend through it. Below each bed 14, 14 a grid-like holding device 18 and a collector 20 are arranged, wherein the grid-like holding device 18 keeps the bed 14 at its position and the collector 20 collects the liquid trickling down from the bed 14, while sufficient open space is left in the collector for the vapor to ascend.

[0089] During the operation of the distillation column 10, gas ascends as light phase from the bottom to top, whereas liquid as heavy phase descends in counter-current from the top to the bottom of the distillation column 10. More specifically, the liquid is essentially homogenously distributed by the distributor 16 over the cross-section of the bed 14 and trickles down along the surfaces of the layers of the structured packing elements 12. Between the different layers of the structured packing elements 12 open spaces are provided, which are filled with the gas and provide a path for the gas to ascend, while it is driven by a pressure gradient. By allowing the liquid to spread on the surface of the layers of the structured packing elements 12, a large interface is created between the two phases so that an efficient heat and mass transfer between the liquid and the gas is established at the interface. At the bottom of the bed 14, the liquid is collected in the collector 20 and guided via the pipe 22 down to the distributor 16 above the second bed 14.

[0090] FIGS. 2A to 2C show a structured packing element 12 of the so-called cross-channel corrugated sheet packing type. For illustrative purposes, the grid structure of the layers in not shown in FIGS. 2A-2C, but only in FIGS. 6A-6F. The structured packing element 12 is assembled from a plurality of corrugated sheets 24, 24, which are parallel and in touching contact with each other. Each of the corrugated sheets 24, 24 is a grid as described above and as described in further detail below according to FIGS. 6A-6F. At the bottom right of FIG. 2C the grid-structure of a part of the corrugated sheet 24 is schematically indicated. As it is understood from the above specification, indeed at least two and preferably all of the corrugated sheets 24, 24 consist of such grids, which is not shown in FIGS. 2A to 2C only for illustrative reasons. In the present embodiment, the corrugated sheets 24, 24 are made of expanded sheet material, i.e. they are prepared by cutting and stretching a thin metal plate and then deforming the expanded sheet metal to corrugated sheets 24, 24.

[0091] The corrugated metal sheets 24, 24 are fixed to each other by means of several rods (not shown) penetrating the corrugated sheets 24, 24 perpendicular to the longitudinal section of the corrugated sheets 24, 24, wherein the rods are fixed with the first and last corrugated sheet by means of a washer and a nut or by bending the rods or by any other means (not shown). Each corrugated sheet 24, 24 comprises a plurality of alternately oriented peaks 26 and valleys 28, wherein adjacent corrugated sheets 24, 24 are oriented such that the corrugations 26, 28 of the adjacent corrugated sheets 24, 24 intersect in crisscross fashion with the corrugations 26, 28 of the corrugated sheets 24, 24 extending obliquely relative to the longitudinal direction, thus forming inclined channels 30 which continuously cross one another. More specifically, angle ? between each of the peaks 26 and each of the valleys 28 with respect to the longitudinal direction V is 10? to 60?, preferably 20? to 50? and most preferably 25? to 47?, wherein the peaks 26 and valleys 28 of adjacent layers 32, 32 or 24, 24, respectively, are oriented in opposing directions. The channels 30 define a maximum distance D between adjacent corrugated sheets 24, 24, such as for example of 20 mm. These channels 30 positively influence the flows of the gas phase and of the liquid phase within the structured packing element 12 and facilitate the mass transfer between the phases. That is, the gas phase and liquid phase are brought into contact in the channels 30 of the structured packing element 12 and the mass transfer as well as the heat transfer between the phases is thus facilitated. More specifically, ascending gas comes into contact with liquid, which is present on the surface of the corrugated sheets 24, 24 defining the channels 30, when it flows downwardly through the mass transfer column. All in all, the light phase flows through the open space or channels 30, respectively, without a bypass flow through the openings 40 of the grid 38 of the layers 32, 32 of the structured packing element 12. This leads to a particular efficient mass and energy transfer between the light and heavy phases. Moreover, the crisscross fashion of the channels 30 leads to an optimal distribution of the phases from left to right.

[0092] FIG. 3 shows a fragmentary view of a structured packing element in accordance with an embodiment. The structured packing element of FIG. 3 is similar to that shown in FIGS. 2A to 2C except that corrugated sheets 24, 24 do not comprise linear extending peaks and valleys, but that the peaks 26, 26 and valleys of the corrugated sheets 24, 24 are bended in the terminal zones or portions 33, 33, respectively, so as to extend in the terminal portions 33, 33 of the corrugated sheets 24, 24 essentially in the vertical direction. The terminal zones are the uppermost and the lowermost zones 33, 33 of the corrugated sheets 24, 24 extending from the upper and lower edges of the corrugated sheets 24, 24 30%, preferably 25% and more preferably 20% or less along the length of the corrugated sheets 24, 24, which is the direction along the longitudinal direction of the corrugated sheets 24, 24. Each of the terminal zones 33, 33 may have peaks 26, 26 and valleys with a different height than those of the central zone, which is the zone of the layer between the two terminal zones 33, 33. Such features as the different height or the bend may be present in both terminal zones 33, 33 of the corrugated sheets 24, 24 only.

[0093] In FIG. 3, the solid lines depict the peaks 26 of the corrugations in the face of the corrugated sheet 24 presented to the viewer, while the broken lines 26 depict the peaks of the corrugations in the corresponding face of the corrugated sheet 24 immediately behind the one in view. By bending the terminal portions 33, 33 so as to extend in the terminal portions 33, 33 of the corrugated sheets 24, 24 essentially in the vertical direction, the flow resistance of the terminal portions 33, 33 of the corrugated sheets 24, 24 is reduced compared to the flow resistance of the portions being located between the terminal portions 33, 33 of the corrugated sheets 24, 24. This leads to a reduced pressure loss of the structured packing element.

[0094] FIG. 4 shows two layers of a structured packing element 12 in accordance with an embodiment. For illustrative purposes, the grid structure of the layers in not shown in FIG. 4, but only in FIGS. 6A-6F. The structured packing element 12 comprises several layers 32, 32, wherein each layer is deformed to provide several dimples 34, wherein each of the dimples 34 comprises a dimple valley wall 36. Adjacent layers 32, 32 are arranged parallel to each other in touching contact with each other. In other words, adjacent layers 32, 32 touch each other so that at least some of the dimple valley walls 36 of a layer 32 contact the non-dimpled surface part of an adjacent layer 32. Thus, the dimples 34 in fact act as spacer so as to define a channel 30 or open space, respectively, between the layers 32, 32 defining a maximum distance D between the single layers 32, 32 so as to allow the light phase or gas, respectively, to ascend though the channel 30.

[0095] FIG. 5 shows a structured packing element 12 in accordance with one embodiment. For illustrative purposes, the grid structure of the layers in not shown in FIG. 5, but only in FIGS. 6A-6F. The structured packing element 12 comprises several layers 32, 32, wherein each layer is deformed so that two adjacent layers 32, 32 touching each other define a channel 30 or open space, respectively, with a hexagonal cross-section between the layers 32, 32 defining a maximum distance D between the single layers 32, 32 so as to allow the light phase or gas, respectively, to ascend though the channel 30. The layers 32, 32 could be arranged in inclined orientation with alternating angle relative to the longitudinal direction such that the channels 30 of the layers 32, 32 cross each other. Despite a preferred gas flow following those channels 30 alternatively in the right or left direction, the average gas flow would follow the longitudinal direction V.

[0096] FIGS. 6A to 6F are schematic views of different embodiments of the grid 38 that forms the layers 32 of the structured packing element 12 according the present disclosure, which are for instance suitable to be used in a structured packing element as shown in any of FIGS. 2A to 2C, 3, 4 and 5. The grid 38 of the layer 32 of the structured packing element shown in FIG. 6A comprises openings 40 with a quadrilateral cross-section, wherein the openings 40 are surrounded and separated from each other by separating elements 42. The separating elements 42 are thin strips with an average width b of, for example, 2 mm, wherein the separating elements 42 completely surround the openings 40. The two side lengths a.sub.1, a.sub.2 of the openings 40 are selected so as to result in an opening 40 with a suitable hydraulic diameter d of, for instance, 3 mm. As known in the field, the hydraulic diameter d may be calculated in accordance with the formula 4 A/P, wherein A is the cross-sectional area of the opening 40 and P is the perimeter of the opening 40.

[0097] In accordance with the present disclosure, the average width b of the separating elements 42 between adjacent openings 40 is between 70% and 125% of the average hydraulic diameter d of the adjacent openings 40, 40. Moreover, the maximum distance D between at least two adjacent of the at least two layers 32,32 measured in the plane being perpendicular of the longitudinal direction V is at least 4 times larger than the average width b of the separating elements 42. By adjusting these relations, the wetting of the surface of the layers 32, 32 of the structured packing element 12 can be further improved andas a consequence thereofthe mass transfer efficiency based on a given capacity of the structured packing element 12 is increased despite the fact that less physical area and less material are present.

[0098] The grid 38 may be simply produced of expanded sheet material, i.e. by cutting and stretching a thin metal plate and then deforming the expanded sheet metal to the desired form, such as to a corrugated sheet.

[0099] Grids 38 with a different geometry of the openings 40 and a different geometry of the separating elements 42 are shown in FIGS. 6B to 6F. The openings 40 of the grids 38 of FIGS. 6B and 6C are quadrilateral, whereas the openings 40 of the grid 38 of FIG. 6D are irregular and the openings 40 of the grids 38 of FIGS. 6E and 6F are ellipsoid. They may also be of lenticular shape.

[0100] Subsequently, the determination of the width b of a separating element 42 and of the hydraulic diameter d of an opening 40 of the grid 38 of a structured packing element in accordance with the present disclosure is described under reference to FIGS. 7A and 7B. First of all, several plan views of one of the openings 40, 40 of the structured packing element 12 are made by taking three photographic pictures 44a, 44b, 44c of the opening 40 under different angles. The plan views of the opening 40 photographic pictures 44a, 44b, 44c are taken along the normal axis of the plane defined by the adjacent edges 48, 48 of the separating element 42. The photographic picture 44b that renders the opening 40 largest is then taken as the plan view of the opening 40. One reference length z is used to determine lengths and sizes on the plan view. This is achieved by identifying or marking a certain distance z on the real object in the neighborhood of the opening 40 and measuring its length. The ratio between the effective length z of this distance appearing on the plan view and the distance z measured on the real object is used for scaling all other distances that are measured on the plan view.

[0101] For determining the width b of a separating element 42, the separating element 42 of the plan view is divided up into individual sections 46 designated i=1, 2, 3 . . . n each having a section length d.sub.i. For each of the sections, the shortest distance b.sub.i between the adjacent edges 48, 48 within the sections 46 is measured. The sum of the products d.sub.ib.sub.i divided by the sum of d.sub.i multiplied with the coefficient z/z yields the average width b of the separating element 42.

[0102] The hydraulic diameter of the opening 40 is calculated with the formula 4 A/P, wherein A is the cross-sectional area of the opening 40 and P is the perimeter of the same opening 40. The cross-sectional area of the opening 40 is subdivided in a number j=1, 2, 3 . . . m of sections 50 each having a simple shape. The area of each section 50 is designated A.sub.j and is calculated using basic measures and basic geometric formula. The area A of the opening 40 is obtained by summing up all areas A.sub.j determined in the opening 40.

[0103] The perimeter P of the opening 40 is determined by subdividing the perimeter P of the opening 40 in a number of k=1, 2, 3 . . . K individual straight lines P.sub.k that approximate the opening 40 best and represent it by a closed polygon. By summing up the lengths of these straight lines P.sub.k, the perimeter P is obtained. Again, the lengths must be translated into real lengths using the ratio z/z as defined previously.

[0104] FIG. 8 illustrates the determination of the least deformed portion of a structured packing element made of corrugated sheets 24, 24 as layers 32, 32. As set out above, in preferred embodiments of the present disclosure, the layers 32, 32 of the structured packing element 12 are made of expanded sheet metal, i.e. by cutting and stretching a thin metal plate and then deforming the expanded sheet metal to, for instance, corrugated sheets 24, 24. After this processing, the openings and separating elements are likely to be distorted and/or stretched around the peaks 26 and the valleys 28 of the corrugations of the corrugated sheets 24, 24.

[0105] However, the inclined flanks, which are defined by the approximately straight portion of the grid connecting the peaks 26 and the valleys 28, include openings and separating elements of almost unmodified sizes, because deformation is less pronounced there. It is therefore preferred according to the present disclosure to measure the dimensions only in the portion of the layer that is less deformed, which is designated a least deformed portion of the corrugated sheets 24, 24. This least deformed portion is defined as follows. The corrugated sheet has an average layer width W. This average layer width is determined by the amplitude of the majority of the peaks 26 and valleys 28 of the layer 24. An upper and a lower plane represented by two dashed lines in the figure below are drawn to touch the majority of the peaks 26 and the valleys 28 of the layer. The distance between these two dashed lines is called average layer width W, and it is typically around half the maximum distance D. The value W is very often a constant value, but it may vary in the most general case as the two planes do not need to be parallel and a packing element may contain layers of different width. A third center plane 52 is defined, which is placed in such a way, that from each point on this center plane 52, the distance measured to the upper and the lower plane is identical. The least deformed portion of the corrugated layer 24, which shall be considered when determining the characteristic dimensions of the grid, is bordered by an upper and a lower limiting plane 54, 54, which is positioned at ?20%, more preferably ?30% and most preferably ?40% of W around the center plane 52. The openings and separating elements in this least deformed portion, i.e. the openings and separating elements found between these two limiting planes 54, 54, are analyzed when determining the parameters, such as average hydraulic diameter of the holes and average widths of the separating elements. According to one embodiment of the present patent application for at least 90% of the holes between the limiting planes the following is valid: Each opening with its surrounding system of separating elements shall have the same appearance and identical hydraulic diameter d. The surrounding separating elements shall on average have the same width b. A structured packing element layer that fulfills this statement is considered a layer made of a uniform grid.

[0106] The above observations are also valid for layers of any different shape. It is not limited to corrugated layers.

[0107] FIG. 9 is a true to scale plan view illustrating the determination of the parameters for a specific example in accordance with the present disclosure, namely for an opening 40 having the shape of a rhombus (i.e. an equilateral quadrilateral). The dimensions are chosen for illustrative purpose only and are not necessarily typical for the present disclosure. It should even be noted that there was no intention to make them fit into the advantageous ranges of dimensions in accordance with the present disclosure.

[0108] The reference length z has been measured to be 6.25 mm. By measuring the length z as it appears in the printed picture (plan view), one obtains z. All other lengths that can be measured in the figure are multiplied by the coefficient z/z obtained as described above. Accordingly, the longer characteristic length (or diagonal, respectively) e.sub.1 of the rhombus is 10 mm long and the shorter characteristic length (or diagonal, respectively) e.sub.2 is 8 mm. The side length a.sub.1 of the rhombus is 6.4 mm, which computes as a.sub.1=?.Math.?(e.sub.1.sup.2+e.sub.2.sup.2). The separating element 42 has a width of b=1 mm. The area of the opening 40 and the length of the perimeter P length are obtained by the following equations:

A=e.sub.1.Math.e.sub.2/2

P=4.Math.a.sub.1=2.Math.?(e.sub.1.sup.2+e.sub.2.sup.2)

[0109] Numerical values therefore are A=40 mm.sup.2 and P=25.6 mm. The resulting hydraulic diameter is d=4.Math.A/P=e.sub.1.Math.e.sub.2/?.Math.(e.sub.1.sup.2+e.sub.2.sup.2).

[0110] The hydraulic diameter of approximately d=6.25 mm is visualized in FIG. 9 by means of a circle of diameter d.

[0111] If the grid 38 of the structured packing element is very regularly composed of the rhombic openings 40 and the separating elements 42 as shown in FIG. 9, a unit cell U can be defined, which is a typical repetitive element of the grid containing all its features and dimensions. The unit cell U is a quadrilateral (rectangle) U covering an opening 40, wherein the quadrilateral U extends through midpoints 58 of the four junction points 56 of the separating elements 42. The unit cell U has a short unit cell height u.sub.2 and a long unit cell width u.sub.1. While the short unit cell height u.sub.2 is aligned with the short diagonal e.sub.2 of the opening 40, the long unit cell height u.sub.1 is aligned with the long diagonal e.sub.1 of the opening 40. It is possible to determine areas in the unit cell U, namely those of the entire unit cell U, those of the openings 40 and those of the separating elements 42. To determine u.sub.1 and u.sub.2, the lengths e.sub.3 and e.sub.4 of the small rhombus at the crossing of two separating elements 42 must be known. Based on geometrical considerations, the following expressions are obtained.

e.sub.3=b.Math.?(1+e.sub.1.sup.2/e.sub.2.sup.2)

e.sub.4=e.sub.3.Math.e.sub.2/e.sub.1

u.sub.1=e.sub.1+e.sub.3

u.sub.2=e.sub.2+e.sub.4

[0112] The resulting dimensions are e.sub.3=1.60 mm, e.sub.4=1.28 mm, u.sub.1=11.6 mm, u.sub.2=9.28 mm. The ratios e.sub.2/e.sub.1 and u.sub.2/u.sub.1 are equal and the value therefore is 0.8.

[0113] The dimensions of an expanded sheet are commonly specified by the unit cell dimensions u.sub.1, u.sub.2 and the width b of the separating element. Under those circumstances, the hydraulic d may be computed as follows:

e.sub.1=u.sub.1?b.Math.?(1+u.sub.1.sup.2/u.sub.2.sup.2)

e.sub.2=e.sub.1.Math.u.sub.2/u.sub.1

d=e.sub.1.Math.e.sub.2/?(e.sub.1.sup.2+e.sub.2.sup.2)

[0114] As a simplification, when dealing with expanded metal sheet with typical ratios of e.sub.2/e.sub.1 below 0.5, the following approximations may be sufficiently accurate to determine the characteristic lengths e.sub.1 and e.sub.2 based on u.sub.1 and u.sub.2:

e.sub.1=u.sub.1?b.Math.u.sub.1/u.sub.2

e.sub.2=u.sub.2?b

[0115] Furthermore, expanded metal sheet can be characterized by means of the stretching factor, which is defined as f.sub.s=u.sub.2/2b. In the current example, its value is f.sub.s=4.64.

[0116] The area of the unit cell U is A.sub.u=u.sub.1.Math.u.sub.2=107.7 mm.sup.2. The area of the openings in the unit cell U is A.sub.o=e.sub.1.Math.e.sub.2=2.Math.A=80 mm.sup.2. The area of the solid portion in the unit cell U is obtained by adding the area of the four separating elements and twice the area of the small rhombus at the crossing of two separating elements:

A.sub.b=2b?(e.sub.1.sup.2+e.sub.2.sup.2)+e.sub.3.Math.e.sub.4.

[0117] The resulting area is A.sub.b=27.66 mm.sup.2 and the sum A.sub.u=A.sub.b+A.sub.o is maintained. Within the unit cell U, the ratio of the total area of the openings 40 divided by the total surface area of the layer is A.sub.o/A.sub.u=74.3%, which we call the void fraction of the layer. A value close to the void fraction could have been obtained by using the stretching factor as follows

1?(1/f.sub.s)=78.5%

which is an indication of the raw material savings for a given sheet area if the sheet is stretched.

[0118] The physical area of the unit cell U is A.sub.P=A.sub.b. The sheet area of the unit cell U is A.sub.S=A.sub.u. The geometrical area A.sub.M of the unit cell U is twice the sheet area, which is 2 A.sub.u and 215.4 mm.sup.2. The specific area a.sub.M is undefined in this context as the shape of the layer is not specified.

[0119] FIG. 10 shows schematic views of the cross-section of a packing layer in order to explain how to distinguish the various expressions for the area of surfaces. FIG. 10A shows a cross-section of a typical layer of the structured packing element 12. The material forming the separating elements 42 is represented by the black lines, whereas the white portions represent openings 40, 40 in the layer 32, 32. Each black portion is a cross-section through a separating element 42. The thickness of the black lines represents the layer material thickness s. In FIGS. 10B to 10D the areas on this layer are represented just by thinner lines that follow the layer contour. The physical area A.sub.P of a structured packing layer 32, 32 is shown in FIG. 10B. It is the sum of the surface measured on one selected side of all its separating elements 42. The edges 48, 48 of the separating elements 42 do not contribute to this area. Rather, A.sub.P counts only surface that is physically present. Thus, holes do not contribute to the value. The physical area A.sub.P of the structured packing element 12 is the sum of the physical area A.sub.P of all layers 32, 32 comprised therein. FIG. 10C defines the sheet area A.sub.S of a packing layer. It is obtained by adding both the area of the openings in the layer and the physical area A.sub.P of the layer. The sheet area A.sub.S of the structured packing element 12 is obtained by summing the sheet area A.sub.S of all layers 32, 32 comprised therein. The geometrical area of the layer A.sub.M as defined in FIG. 10D adds up both sides of the layer as if there were no openings 40, 40 or holes. In other words, the geometrical area A.sub.M is approximately obtained by multiplying the sheet area A.sub.S of the packing layers by two, because both sides of the layers account for the geometrical area A.sub.M. The specific area a.sub.M is defined as the geometrical area A.sub.M of the structured packing element divided by the volume V.sub.M that the structured packing element occupies.

EXAMPLES AND COMPARATIVE EXAMPLES

[0120] Structured packing element 12 as shown in FIG. 2 were tested in a distillation column. The commonly known standard procedure determines the pressure drop over the packing bed and the mass transfer efficiency using a binary mixture under total reflux condition. EP Patent No. 0 995 958 B1 describes such a test with oxygen and argon at a pressure of 22 psia. U.S. Pat. No. 6,874,769 B2 describes to test structured packing elements by using the close-boiling binary mixture para- and ortho-xylene. A binary mixture of similar ideal characteristics as the latter was used in the present disclosure, namely monochlorobenzene (as low-boiler) and ethylbenzene (as high-boiler). Other standard close-boiling ideal binary mixtures to assess the performance of distillation equipment are specified in U. Onken, W. Arlt: Recommended Test Mixtures for Distillation Columns, 2nd Ed. 1990, The Institution of Chemical Engineers, Rugby, England. ISBN 0-85295-248-1.

[0121] The bottom of a distillation column was filled with a sufficient amount of the binary mixture to maintain a decent liquid level during operation of the column. The reboiler was started, a part of the liquid mixture was continuously vaporized, and the vapor rose towards the head of the column. The flow rate of the vapor can be expressed in terms of the factor F and is commonly determined indirectly via the energy balance at the reboiler or at the condenser at the column head. The condenser cooled the vapor such that it condensed back to liquid. Under the preferred total reflux conditions, the entire amount of liquid was sent back to the top of the packing bed, where it was distributed by means of a distributor. The distributor is typically a device comprising channels that receive the liquid and provide an evenly spaced set of orifices through which the liquid can trickle down onto the top packing of the structured packing bed. After trickling through the structured packing bed, the entire amount of liquid was collected at the bottom of the column or at the bottom of the bed by means of a collector from where it was sent back to the bottom of the column. At the bottom the liquid joined the liquid pool from which it was vaporized again. A constant head pressure p was established by controlling the cooling duty of the condenser in combination with a vacuum pump to remove surplus inert gases.

[0122] After a certain time of operation at constant reboiler duty a steady state condition was achieved. At this point, the pressure drop over the packing bed and temperatures at relevant points along the column were read, and top and bottom samples of the mixture were taken from the distributor at the top of the packing bed and from the collector at the lower end of the packing bed or from the sump. Several operating points were measured by varying the heat (and cooling) duty, which affects the factor F (vapor flow) and the related liquid flow through the packing bed while the head pressure was kept unchanged. The same experiment was repeated for several settings of the head pressure.

[0123] The compositions of the samples were analyzed by means of a calibrated gas chromatograph. The top and bottom samples varied by the amount of low-boiler they contain. More low-boiler, i.e. the compound with the lower boiling temperature, was found in the top sample than in the bottom sample. Once the binary compositions were known, the equation according to Fenske (M. R. Fenske, Ind. Engng. Chem. 24, p. 482, 1932) was applied to determine the number of Theoretical Stages per Meter (NTSM). Sometimes, the inverse value HETP was used, which is called Height Equivalent to a Theoretical Plate.

HETP=1/NTSM

[0124] A high NTSM (or a low HETP) means a good mass transfer efficiency. The factor F is defined by:

F=v.sub.G?.Math.?.sub.G

wherein v.sub.G is the average velocity of the rising vapor, which can be determined from the mass flow rate via an energy balance at the reboiler. The second variable ?.sub.G is the vapor density at the relevant vapor/liquid equilibrium. Due to the change in pressure and temperature along the column the vapor density and other physical properties of the fluids varied along the column, but the relevant information is available for the binary mixture. Such variations require to select an appropriate definition of the factor F. It may be determined by means of the properties valid under the conditions at the top or at the bottom of the packing bed. Alternatively, an average value may be computed taking into account the variation over the entire bed. For comparison purpose any of the possible approaches works, provided the same approach is used for all tests.

[0125] A high factor F means a high mass flow rate in the column. The value of F that is achievable is usually limited by the flooding which determines the capacity of a packing. Sometimes the capacity factor c is used instead of F, which is obtained by dividing F by the square root of the density difference of the liquid and the vapor.

[0126] The pressure drop over the packing bed was another relevant result of the experiment. It was obtained as the difference of the pressure readings at the top and at the bottom of the packing bed after dividing by the bed height H.sub.B:

?P/?z=(p.sub.top?p.sub.bottom)/H.sub.B

[0127] Five kinds structured packing elements were used in the examples and comparative examples, which were named P1-250, R-250, P2-500, P3-500 and R-500. While the structured packing elements P1-250 and P2-500 were cross-channel corrugated sheet packings made of layers according to the present disclosure, the structured packing elements P3-500, R-250 and R-500 were reference structured packing elements. More specifically, the structured packing elements R-250 and R-500 were known standard cross-channel corrugated sheet packings with punched holes (leading to approximately 10% void fraction of the layers) and surface texturing as described in GB 1,569,828 and in U.S. Pat. No. 4,981,621, which are commercially distributed under the names Mellapak 250.Y and Mellapak 500.X. All structured packing elements had a height of around 200 mm. The relevant parameters of the aforementioned structured packing elements are summarized in table 1.

TABLE-US-00001 TABLE 1 Parameter P1-250 P2-500 P3-500 R-250 R-500 Angle of corrugation ? 45? 30? 30? 45? 30? Specific area a.sub.M (m.sup.2/m.sup.3) 250 500 500 250 500 Hydraulic diameter d (mm) 2.46 2.46 1.54 Width of separating element b (mm) 2 2 0.7 b/d= 81% 81% 45% Max. distance D (mm) 22.5 13 13 22.5 13 D/b= 11.3 6.5 18.6 Side length a.sub.1 (mm) 3.1 3.1 1.9 Longer diagonal e.sub.1 (mm) 5.5 5.5 3.4 Diagonal ratio e.sub.2/e.sub.1 0.5 0.5 0.5 Grid thickness g (mm) 1.2 1.2 0.75 Void fraction of the layer 30% 30% 47% 10% 10% Layer material thickness s (mm) 0.1 0.1 0.1 0.1 0.1 b/s= 20 20 7 present disclosure reference reference

Example 1 and Comparative Example 1

[0128] The structured packing element according to the disclosure P1-250 and the reference structured packing element R-250 were tested in a distillation column with a 1 m inner diameter at total reflux using monochlorobenzene and ethylbenzene at head pressures of p=960 mbar (close to atmospheric) and p=100 mbar. The packing beds were 4.3 m high. The obtained efficiency curves are shown in FIG. 11 and FIG. 12. In both cases, the structured packing element P1-250 according to the disclosure showed in comparison to the reference structured packing element R-250 a higher mass transfer efficiency (higher NTSM) and even a slightly extended capacity, which is characterized by the factor F, where the efficiency suddenly plummets. It is remarkable and surprising that the structured packing element P1-250 with 30% less material (and 20% less physical area A.sub.P) than the reference structured packing element R-250 achieves the better mass transfer result.

[0129] The pressure drops of both structured packing elements is shown in FIG. 13 and were very similar. Accordingly, the structured packing element P1-250 according to the disclosure had a higher pressure drop at low F-factor, but the slope was lower, which provided the new packing with its capacity advantage and a lower pressure drop at high flow rates.

Example 2 and Comparative Examples 2 to 3

[0130] The structured packing element P2-500 according to the disclosure and the reference structured packing element R-500 were tested in a column with a 0.25 m inner diameter at total reflux using monochlorobenzene and ethylbenzene at head pressures of p=960 mbar and p=100 mbar. Furthermore, the structured packing element P3-500 was tested. Despite its similarity with P2-500, structured packing element P3-500 was quite different in as much as significant geometrical parameters are set to values outside the numeric value ranges as specified in the present disclosure. More specifically, for the structured packing element P3-500 the ratio of the average width of the separating elements to the average hydraulic diameter of the adjacent openings b/d was 45% and the ratio of the average width of the separating elements to the layer material thickness b/s was 7, i.e. both of these ratios were outside the numeric value ranges as specified therefore in the present disclosure. The packing beds with P2-500 and P3-500 were 2.4 m high and the packing bed with the reference R-500 had a height of 2.6 m.

[0131] The obtained efficiency curves for these structured packing elements are shown in FIG. 14 and FIG. 15 and the obtained pressure drops for these structured packing elements in FIG. 16.

[0132] The better efficiency of the structured packing element P2-500 according to the present disclosure in comparison to the structured packing elements P3-500 and R-500 can be easily derived in FIGS. 14 and 15 for both, a head pressure of 960 mbar as well as a head pressure of 100 mbar. The spread in mass transfer efficiency is especially striking at the low head pressure. Interestingly, P3-500 has a nice capacity, but the efficiency is significantly below that of R-500. Both structured packing elements P2-500 and P3-500 have initially a higher pressure drop than that of R-500, but as F increases, they gain advantage and the higher capacity of both can also be recognized in this graph.