Abstract:
An apparatus for a heat transfer or mass transfer process, comprising a column or divided column having at least one pair of converging walls or wall portions and, within at least a region of the column or divided column bounded by at least one pair of converging walls or wall portions, a structured packing having a corrugation angle of at least about 50°; a method of heat and/or mass transfer applicable to the apparatus; and a method of installation of structured packing into a relevant apparatus.

Description:
FIELD 
       [0001]    The present invention generally relates to structured packing. Structured packing has particular application in heat and/or mass exchange columns, especially in cryogenic air separation processes, although it may be used in other applications, such as heat exchangers, for example. 
       BACKGROUND 
       [0002]    The term “column” as used herein means a distillation or fractionation column or zone, i.e. a column or zone wherein liquid and vapour phases are countercurrently contacted to effect separation of a fluid mixture, such as by contacting of the vapour and liquid phases on packing elements or on a series of vertically-spaced trays or plates mounted within the column. 
         [0003]    A divided wall column is a system of thermally coupled distillation columns. In divided wall columns, at least one dividing wall is located in the interior space of the column. The dividing wall generally is vertical. Two different mass transfer separations may occur on either side of the dividing wall, for example. 
         [0004]    The term “packing” means solid or hollow bodies of predetermined size, shape and configuration used as column internals to provide surface area for the liquid to allow heat and/or mass transfer at the liquid-vapour interface during countercurrent flow of two phases. Two broad classes of packings are “random” and “structured”. 
         [0005]    “Random packing” means packing wherein individual members have no specific orientation relative to each other or to the column axis. Random packings are traditionally small, hollow structures with large surface area per unit volume that are loaded at random into a column. 
         [0006]    “Structured packing” means packing wherein individual members have specific orientation relative to each other and to the column axis. Structured packings usually are made of thin metal foils stacked in layers. 
         [0007]    In processes such as distillation, it is advantageous to use structured packing to promote heat and/or mass transfer between counterflowing liquid and vapour streams. Structured packing, when compared with random packing or trays, offers the benefits of higher efficiency for heat and/or mass transfer with lower pressure drop. It also has more predictable performance than random packing. 
         [0008]    The separation performance of structured packing is often given in terms of height equivalent to a theoretical plate (HETP), which is the height of packing over which a composition change is achieved that is equivalent to the composition change achieved by a theoretical plate. The term “theoretical plate” means a contact process between gaseous and liquid phases such that the existing gaseous and liquid streams are in equilibrium. The smaller the HETP of a particular packing for a particular separation, the more efficient the packing, because the height of the packing bed being used decreases with HETP. 
         [0009]    Cryogenic separation of air is carried out by passing liquid and vapour in countercurrent contact through a distillation column. A vapour phase of the mixture ascends with an ever increasing concentration of the more volatile components (e.g., nitrogen) while a liquid phase of the mixture descends with an ever increasing concentration of the less volatile components (e.g., oxygen). Various packings or trays may be used to bring the liquid and gaseous phases of the mixture into contact to accomplish mass transfer between the phases. 
         [0010]    There are many processes for the separation of air by cryogenic distillation into its components (i.e. nitrogen, oxygen, argon, etc.). A typical cryogenic air separation unit  10  is shown schematically in  FIG. 1 . High (or higher) pressure feed air  1 , typically at a pressure of from 2 to 10 bar (200 to 1000 kPa), is fed into the base of a high (or higher) pressure distillation column  2 . Within the high pressure column  2 , the air is separated into nitrogen-enriched overhead vapour and oxygen-enriched bottoms liquid. The oxygen-enriched bottoms liquid stream  3  is fed from the high pressure distillation column  2 , after suitable pressure reduction (not shown), typically to a pressure of from 1.1 to 2 bara (110 to 200 kPa absolute), into a low (or lower) pressure distillation column  4 . Nitrogen-enriched vapour stream  5  is passed into a condenser  6  where it is condensed to provide reboil to the low pressure column  4 . The nitrogen-enriched liquid stream  7  is partially returned via stream  8  as reflux to the top of high pressure column  2 , and is partially fed via stream  9  into the top of low pressure column  4  as liquid reflux. 
         [0011]    Low pressure column  4  consists of a lower section  11 , in which is placed structured packing  20 , and upper narrower section  12  in which is placed structured packing  21 . A separate low pressure column  13 , also known as an auxiliary or sidearm column, comprising structured packing  22  is provided for production of an argon-enriched stream  14 . 
         [0012]    In the low pressure column  4 , the streams  3  and  9  are separated by cryogenic distillation into oxygen-rich and nitrogen-rich components. Structured packings  21  and  20  may be used to bring into contact the liquid and gaseous phases of the oxygen and nitrogen to be separated. The nitrogen-rich overhead component is removed as a vapour stream  16 . The oxygen-rich bottoms component is removed as a liquid stream  17 . Alternatively the oxygen-rich component can be removed from a location in the sump surrounding reboiler/condenser  6  as a vapour. A waste stream  15  also is removed from the low pressure distillation column  4 . 
         [0013]    Feed stream  18  is removed from an intermediate point between lower section  11  and upper section  12  of low pressure column  4  and is passed to column  13 . A condenser  25  is provided in the upper portion of column  13  to generate a reflux from the feed stream  18 . Passage of this feed stream in countercurrent flow with reflux from condenser  25  through structured packing  22  creates an argon-enriched overhead vapour stream  14 , and oxygen-enriched bottoms liquid stream  19  which is returned to the low pressure column  4  above structured packing  20  and below structured packing  21  as reflux. 
         [0014]      FIG. 2  shows an alternative arrangement for cryogenic distillation of air to provide nitrogen, oxygen and argon, in which a divided low pressure column is used in place of the separate column  13  for argon production in  FIG. 1 . Such an arrangement is described in, for example, U.S. Pat. No. 6,240,744 (Agrawal et al.). Features in common with the arrangement in  FIG. 1  have the same reference numbers. In this arrangement, the vapour stream leaving the top of structured packing  20  at the lower end of low pressure column  4  is divided into two portions, of which the first rises to the structured packing  21  and the second rises to the structured packing  22  which is divided from the structured packing  21  by dividing wall  23  and from the upper part of low pressure column  4  by end wall  24 . In the Figure, the dividing wall is shown as a flat wall centrally mounted in the low pressure column  4 , such as to divide the column  4  along its diameter into two equally sized sections of semi-circular cross section; however, as explained in U.S. Pat. No. 6,240,744, many other arrangements for the separation of structured packing  22  from structured packing  21  are possible. The vapour portion entering structured packing  21  is separated into a nitrogen-rich overhead vapour stream and an oxygen-rich bottoms liquid stream as described for  FIG. 1 . The vapour portion entering structured packing  22  is separated into an argon-enriched overhead vapour stream  14  and an oxygen-enriched bottoms liquid, as described for column  13  in  FIG. 1 , but without requiring the additional expense and complication of providing the second column  13 . Further, with the arrangement of  FIG. 2  it is not necessary to adapt low pressure column  4  by the narrowing depicted in  FIG. 1  to compensate for the withdrawal of vapour from column  4  to pass to column  13  in order that the mass transfer performance of column  4  is maintained despite the reduced vapour flow in the upper section  12  compared with lower section  11 . 
         [0015]    US2010/0096249 (Kovak) describes a divided exchange column into which trays or structured packing are placed. The document discloses division of the column into two sections by a chord wall (both equal and unequal divisions are contemplated) and also division of the column into three sections by means of radial walls intersecting at the centre of the cylindrical column. 
         [0016]    Other prior art relating to the structure of columns used in cryogenic distillation of air includes: 
         [0017]    U.S. Pat. No. 5,339,648 (Lockett et al.), U.S. Pat. No. 5,946,942 (Wong et al.), EP1162423 (Messer AGS GmbH), US2006/0005574 (Glatthaar et al.), U.S. Pat. No. 7,357,378 (Zone et al.), U.S. Pat. No. 6,250,106 (Agrawal et al.), US2006/0260926 (Kovak), and U.S. Pat. No. 5,669,236 (Billingham et al.). 
         [0018]    None of the prior art known to the inventors considers in detail the nature of the structured packing required to obtain an optimal result in columns that are not circular in cross-section but have a cross-section having at least one pair of converging walls or wall portions, such as those including at least one corner or angle in their cross-section, such as divided columns. The prior art instead simply discusses the generic use of any structured or random packing and/or trays in partitioned or divided wall columns. 
         [0019]    Structured packing is defined in the present invention as a thin metal or plastic foil that has been perforated, fluted and corrugated to meet specific requirements for its intended application. A representation of a typical structured packing is shown in  FIG. 3 , in which is shown a foil  40  which is corrugated by folding along fold lines  45 , and which has a pattern of fluting, that is, depressions and/or elevated areas  50  in the form of horizontal striations, formed for example by embossing the foil  40 , and a pattern of perforations, or through holes,  55 . The perforations  55  and the texture formed by elevated/depressed areas  50  aid liquid/vapour spreading on the surface of foil  40 , thus improving the heat and mass transfer efficiency of the packing. Typically, the surface area of the foil occupied by the perforations is from about 5% to about 20%. Typically, the fluting may be in the form of horizontal striations, or a bidirectional surface texture in the form of fine grooves in crisscrossing relation. 
         [0020]    Within a layer of structured packing in a column, multiple foils are oriented vertically (that is to say, with the plane of the foil substantially parallel to the axis of the column), with adjacent foils having their corrugations oriented transversely (that is to say, if a first foil has its corrugations running from bottom left to top right, an adjacent foil will be oriented such that its corrugations run from bottom right to top left). Such an arrangement is depicted in FIG. 3 of U.S. Pat. No. 4,296,050 (Meier). It is conventional to rotate successive layers of structured packing, typically by an angle of about 90° about the column axis with respect to the underlying layer, in order to improve the flow characteristics. Such an arrangement is shown in FIG. 4 of U.S. Pat. No. 4,296,050 (Meier). However, each rotation increases the pressure drop through the column comprised of the packing. 
         [0021]    EP1036590 (Sunder et al.) describes optimum ranges of several packing parameters, e.g. a surface area density of from about 350 to about 800 m 2 /m 3 , a corrugation angle (i.e. the angle between the horizontal and the longitudinal axis of the corrugation when the packing element is vertical in the column) of from about 35 to about 65°, and open area of perforations of from about 5 to about 20%. There is no discussion in this document of the use of divided wall columns or non-cylindrical columns. 
         [0022]    U.S. Pat. No. 5,876,638 (Sunder et al.) and U.S. Pat. No. 5,901,575 (Sunder) also discuss developments in structured packing. 
       SUMMARY 
       [0023]    It is an aim of the present invention to provide a structured packing that is optimised for use in a column whose cross-section is not wholly rounded, that is, a column whose cross-section has at least one pair of converging walls or wall portions, such as a divided wall column in which the division creates at least one corner or angle within the column. In particular, it is an aim of the present invention to provide a structured packing that is optimised for use in such a column in a cryogenic distillation apparatus, in particular one used in the separation of components of air. 
         [0024]    Accordingly, in a first aspect, the present invention provides an apparatus for a heat transfer or mass transfer process, comprising a column or divided column having at least one pair of converging walls or wall portions and, within at least a region of the column or divided column bounded by, or lying between, at least one pair of converging walls or wall portions, a structured packing having a corrugation angle of at least about 50°. 
         [0025]    In a second aspect, the present invention provides a method of heat and/or mass transfer, comprising supplying one or more fluids to a column or column division having at least one pair of converging walls or wall portions and which, within at least a region of the column or divided column bounded by, or lying between, at least one pair of converging walls or wall portions, contains a structured packing having a corrugation angle of at least about 50° such that the one or more fluids contact the structured packing in order to effect heat transfer and/or mass transfer. 
         [0026]    In the context of the present invention, a column division is a part of a column physically separated from the remainder of the column by at least one dividing wall arranged substantially to co-extend with the longitudinal axis of the column. That is, where the column has its longitudinal axis positioned vertically, as is usual in use, the or each column division is created by the presence of a substantially vertical wall within the column that physically segregates a part of the column volume from the remainder, such as to prevent the mixing of fluid present in the column division with fluid present in the remainder of the column over the vertical distance over which the dividing wall extends. 
         [0027]    It is believed by the present inventors that the presence of at least one pair of converging walls or wall portions, and in particular the presence of an angle or corner, in the cross-section of a column or column division restricts the mixing of fluid within the column in an edge zone close to the column or column division wall, resulting in reduced efficiency of mass transfer and/or heat transfer within the column where a structured packing optimised for use in a cylindrical column is used. The present invention provides benefit in terms of cost savings and increased efficiency of mass transfer by use of a structured packing optimised for use in a column or column division having at least one pair of converging walls or wall portions, which optimisation has not previously been considered necessary. 
         [0028]    In a third aspect, the present invention provides a method of upgrading an apparatus for a heat transfer or mass transfer process, which apparatus comprises a column or column division whose cross-section comprises at least one pair of converging walls or wall portions and which contains a structured packing having a corrugation angle of less than about 50°, comprising the steps of: 
         [0000]    removing the structured packing having a corrugation angle of less than about 50° from at least a region of the column or column division bounded by, or lying between, the at least one pair of converging walls or wall portions, and
 
replacing the structured packing having a corrugation angle of less than about 50° with a structured packing having a corrugation angle of at least about 50°.
 
         [0029]    Preferably, the structured packing having a corrugation angle of about 50° or more has a corrugation angle of about 55° or more. 
         [0030]    In a fourth aspect, the present invention provides a method of installation of structured packing into an apparatus for a heat and/or mass transfer process, which apparatus comprises a column or column division having at least one pair of converging walls or wall portions, comprising the steps of: 
         [0000]    providing a structured packing having a corrugation angle of at least about 50°, and
 
installing the said structured packing into at least a region of the column or column division bounded by, or lying between, at least one pair of converging walls or wall portions.
 
         [0031]    The following preferred features apply to all aspects of the invention, where appropriate, and may be combined. 
         [0032]    The term “at least one pair of converging walls or wall portions” describes the situation wherein column walls, or parts of column walls, approach each other increasingly closely. The walls or parts of walls need not intersect or contact one another as a result of their convergence, but may intersect or contact one another to form an angle or corner in the cross-section of the column. 
         [0033]    Suitably, the structured packing is used across the whole of the cross-sectional area of the said column or column division, and not only in a region bounded by, or lying between, at least one pair of converging walls or wall portions. 
         [0034]    Preferably, the corrugation angle of the structured packing used in the present invention is between about 50° and about 70°, more preferably between about 55° and about 65°, and is most preferably about 60°. 
         [0035]    Preferably, the column or column division comprises at least one internal angle of less than or equal to about 120° in its cross-section, more preferably less than or equal to about 100°, such as less than or equal to about 90°. It is believed that the disruption to mixing within the edge zone increases with the acuteness of the angle or angles present in the cross-section of the column or column division, and so greater benefit is obtained for the present invention where the angle or angles are more acute. 
         [0036]    Suitably, the column or column division cross-section may be an irregular cross-section which includes a corner or angle, or may be an irregular or regular polygon, or may be a figure formed by the intersection of a chord with a circle or other rounded shape resulting in one or more angles or corners. For example, the column or column division may be of hexagonal, pentagonal, square, rectangular, triangular, semicircular, part-circular, or quarter-circular cross-section. Again, it is expected that the benefits of the invention will be greater the more acute the angle or angles present in the cross-section. 
         [0037]    The column or column division may comprise one, two, three, four, five, six or more angles or corners. It is expected that the benefit of the invention will increase with the number of angles present that are able to disrupt mixing. Preferably, at least one, and more preferably all, of the angles are about 120° or less, such as about 100° or less, more preferably about 90° or less, such as about 70° or less. 
         [0038]    Preferably, the invention is applied to a column division formed by providing at least one dividing wall within the column which is in contact with the outer wall of the column in at least one place. Preferably, the column which is to be divided has a circular cross-section, and at least one of the divisions of the column thus formed has a non-circular cross-section which comprises at least one angle or corner. Suitably, the column may be divided into more than two divisions, such as three, four, five, six, ten or twenty divisions, by an appropriate number of dividing walls, which may intersect each other and/or the column wall to form the required number of divisions. The dividing walls may be the same as one another or may take different forms, and may individually form a straight line or a curved line within the cross-section of the column to be divided. The dividing walls may be the same lengths or different lengths, and the divisions formed may be regular or irregular shapes or polygons, and may be of the same or different cross-section and/or cross-sectional area as one another. These parameters can be selected depending on the intended use of the divided column. 
         [0039]    Preferably, however, the column is divided into two divisions by a single dividing wall. Where the column which has been divided has a circular cross-section, preferably the dividing wall is a chord wall. Where the column to be divided is not circular in cross-section, the column is preferably divided by a dividing wall that extends across the cross-section of the column such that each end of the dividing wall intersects the column wall in different places. In either case, the cross-sectional areas of the column divisions may be selected according to the required flow of fluid through each column division. Suitably, where the flow through each division is to be equal, the column divisions are of equal cross-sectional area, and, in this case, where the column which has been divided has a circular cross-section, the column divisions are of semi-circular cross-section. 
         [0040]    Preferably, the structured packing has a surface density of from about 350 to about 800 m 2 /m 3 . Preferably, the fluting of the structured packing is in the form of horizontal striations. Preferably, the open area of the perforations is in the range of from about 5 to about 20%. 
         [0041]    Preferably, the column size is greater than about 0.5 m in diameter, such as greater than or equal to about 0.9 m in diameter, more preferably greater than or equal to about 1 m in diameter, where the column is of circular cross-section, or is of greater than the equivalent cross-sectional areas (that is, greater than about 0.196 m 2 , greater than or equal to about 0.64 m 2 , and greater than or equal to about 0.79 m 2  respectively) where the cross-section is of another shape. 
         [0042]    Preferably, the maximum column diameter is about 15 m, such as about 10 m, about 9 m, about 8 m, about 7 m, about 6 m, about 5 m, or about 4 m, for a column of circular cross-section. Again, where the column cross-section is of another shape, the maximum column size is of the corresponding maximum cross-sectional area, that is, about 177 m 2 , about 78.5 m 2 , about 63.6 m 2 , about 50.3 m 2 , about 38.5 m 2 , about 28.3 m 2 , about 19.6 m 2 , or about 12.6 m 2  respectively. 
         [0043]    Preferably, the invention is applied in a cryogenic separation process, such as a cryogenic air separation (cryogenic distillation) process, which includes, but is not limited to, the separation of air into nitrogen-enriched, oxygen-enriched and argon-enriched streams. Thus, it may be applied to the cryogenic separation of air into nitrogen- and oxygen-enriched streams. Suitably, the invention is applied to the cryogenic separation of air into oxygen- and argon-enriched streams. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]    The foregoing summary, as well as the following detailed description of exemplary embodiments, is better understood when read in conjunction with the appended drawings. For the purposes of illustrating embodiments of the invention, there is shown in the drawings exemplary embodiments of the invention; however, the invention is not limited to the specific methods and instruments disclosed. In the drawings: 
           [0045]      FIG. 1  shows a schematic representation of a known arrangement for the cryogenic distillation of air; 
           [0046]      FIG. 2  shows a schematic representation of a known arrangement for the cryogenic distillation of air, in which a divided wall (partitioned) column is used; 
           [0047]      FIG. 3  shows a schematic representation of structured packing; 
           [0048]      FIG. 4  shows a schematic representation of the longitudinal axes of corrugation in structured packing when placed in a column; 
           [0049]      FIG. 5  shows in plan view the appearance of structured packing in (a) a column having a circular cross-section, and (b) a column having a semi-circular cross-section; 
           [0050]      FIGS. 6 to 15  illustrate plan views of examples of column arrangements to which the present invention is applicable; 
           [0051]      FIG. 16  shows the results obtained for a prior art structured packing when used in a column having a circular cross-section and a column having a semi-circular cross-section in terms of HETP; 
           [0052]      FIG. 17  shows the results obtained for a prior art structured packing when used in a column having a circular cross-section and a column having a semi-circular cross-section in terms of pressure drop; 
           [0053]      FIG. 18  shows the results obtained for a structured packing according to the present invention when used in a column having a circular cross-section and a column having a semi-circular cross-section; and 
           [0054]      FIG. 19  shows the results obtained in terms of pressure drop for a structured packing according to the present invention when used in a column having a circular cross-section and a column having a semi-circular cross-section. 
       
    
    
     DETAILED DESCRIPTION 
       [0055]    The present invention is applicable to columns in which, in the plan view of the column, there are at least one pair of converging walls or wall portions, such as where one or more dividing walls create angles or corners, or such as where the cross-section is not wholly rounded but instead has at least one angle or corner. It is believed by the present inventors that the advantages of the present invention in terms of separation efficiency are obtained for all such columns. 
         [0056]    When designing an optimum structured packing for a particular column, the skilled person is aware that a number of “trade-offs” are used in determining the best overall parameters. For example, the mass transfer efficiency and pressure drop are found to be higher for a corrugation angle of 45° than for a corrugation angle of 60°, whereas the operating capacity is lower for a 45° than 60° corrugation angle; these effects must be balanced in order that the chosen packing exhibits acceptable mass transfer efficiency, pressure drop and operating capacity for a particular column. 
         [0057]    When passing through structured packing in a column, fluid flows mainly along the channels formed by the corrugations in the foil. Part of this fluid flowing in these channels mixes with the fluid flowing in the adjacent criss-crossing channels which are in a transverse diagonal direction as explained above. Also due to the presence of apertures in the foil, some of the fluid mixes with fluid flowing through adjacent channels. Fluid mixing in these ways is important to correct any composition imbalance that may develop within a cross-section of a distillation column, and is a significant factor in the separation efficiency of the column. It can be seen from  FIG. 4  that a 45° angle provides a longer lateral distance compared with a 60° angle within the column for the fluid to travel in each layer of structured packing, and thus this provides a larger area for mixing of fluid from separate channels to take place within each layer of structured packing. 
         [0058]    EP1036590 teaches an optimum corrugation angle of 35°-65° for cylindrical columns. Within this range it is a common industrial practice to use packings with a corrugation angle of about 45° to provide the trade off of parameters such as mass transfer efficiency, pressure drop and operating capacity as described above for a particular cylindrical column. 
         [0059]    The present inventors are aware of no prior art in which is discussed the optimisation of structured packing for either non-cylindrical columns or divided columns in which there is at least one pair of converging walls or wall portions, or in which it is disclosed or suggested that the optimal parameters for structured packing for use in either non-cylindrical columns or divided columns in which there is at least one pair of converging walls or wall portions are different from those for conventional cylindrical columns. However, the present inventors have surprisingly discovered that the optimum packing for a divided column or a column in which there is at least one pair of converging walls or wall portions is different from that for a column of circular cross-section. It is believed that this difference is due to the difference in mixing behaviour of fluids in the column at the “edge zone”, explained below, for the two types of column. 
         [0060]    It has surprisingly been found by the present inventors that the use of a corrugation angle of about 60° in a column or column division in which the cross-section has at least one pair of converging walls or wall portions provides the same separation efficiency as the use of the same packing in a cylindrical column. However, use of packing having a corrugation angle of about 45° in a column or column division in which the cross-section has at least one pair of converging walls or wall portions results in significant degradation of the separation efficiency compared with the same packing used in a cylindrical column. 
         [0061]    In a cylindrical column, fluid can flow freely within the annular edge zone close to the column wall. The edge zone is the lateral distance from the column wall in which a corrugation channel in the structured packing will end at the wall rather than at the structured packing in the layer above or the layer below. It is calculated as (layer height of the structured packing)/(tan [corrugation angle]). A typical layer height for such structured packing is about 200 mm, and with a corrugation angle of 45° an annular edge zone of about 200 mm would be present within which mixing may take place. However, in a column or division of a column where at least one pair of converging walls or wall portions is present, fluid instead tends to accumulate in the region in which the walls or wall portions converge, and so thorough mixing and composition balancing of the fluids in these regions does not take place. As a result, column performance in terms of separation efficiency is impaired compared with an equivalent cylindrical column. 
         [0062]    Without wishing to be bound by theory, one possible explanation by the present inventors is that the maintenance of the separation efficiency for the 60° corrugation angle packing is as a result of a smaller edge zone close to the wall of the column or column division formed when using structured packing having a corrugation angle of about 60° compared with that observed for a structured packing having a corrugation angle of about 45°. As a result, the expected increase in composition imbalance due to poor mixing of fluids in the region of the column in which the walls or wall portions converge is significantly reduced or avoided completely, and so separation efficiency is maintained compared with use of the same packing in a cylindrical column. 
         [0063]    Accordingly, the optimum structured packing corrugation angle for use in a column or column division in which the cross-section has at least one pair of converging walls or wall portions is different from the angle in the prior art for cylindrical columns. None of the prior art of which the inventors are aware discusses any possible difference in the performance of structured packing in columns of different cross-section, despite the widespread use of divided columns in the distillation industry for over 50 years. 
         [0064]    The finding that separation efficiency does not degrade at a corrugation angle of about 60° for a column or divided column having at least one pair of converging walls or wall portions relative to a more commonly used angle of about 45° permits advantage to be taken of the higher operating capacity of 60° corrugation angle packing—i.e. the “trade-off” for this column unexpectedly shifts in favour of a corrugation angle of about 60°. This is of particular benefit in a system such as that depicted in  FIG. 2 , in which a divided column is used, as the higher operating capacity and cost benefits of such a system can be obtained with the present invention without requiring a 30-50% addition to the height of the divided column to compensate for the poorer mass transfer characteristics of that column when used in conjunction with a conventional 45° corrugation angle structured packing. 
         [0065]    Examples of columns to which the present invention is applicable are shown in  FIGS. 6-15 . It will be appreciated by the skilled person that these are merely examples, and that other column cross-sections or divided wall column arrangements are possible to which the invention will equally apply. 
         [0066]      FIG. 6  shows a column in plan view that has been divided by a plurality of intersecting dividing walls into a number of column zones each with a square cross-section.  FIG. 7  shows a column in plan view that has been divided by a plurality of intersecting dividing walls into a number of column zones each with a hexagonal cross-section. It will be appreciated that other arrangements of tessellating polygonal column zone cross-sections may be used. Equally, it will be appreciated that the benefits of the present invention will be obtained with the use of a single column having a polygonal cross-section. It is expected that the benefits of the invention will be greater for the use of square or rectangular cross-section columns, such as those shown in  FIG. 6 , than for hexagonal columns such as those shown in  FIG. 7 , as the angles formed between the column walls are more acute for the square or rectangular columns, and so the effect of the angles in creating edge zones in which fluid mixing is reduced is expected to be greater. The closer to circular is the cross-section of the column or column division, the less the benefit of the present invention will be obtained. 
         [0067]      FIG. 8  shows a column in plan view in which two dividing walls each span the radius of the circular cross-section of the column in order to divide one-quarter of the area of the cross-section from the remaining three-quarters. An angle or corner of 90° is formed at the middle of the cross-section of the column, and at the intersections of the dividing walls with the circumference of the column the dividing walls are perpendicular to a tangent to the column wall at the point of intersection. It is anticipated that the benefit of the invention will be obtained in both of the column divisions thus formed. In the smaller division, the angles between the two dividing walls and between each dividing wall and the circular column wall are expected to restrict fluid mixing significantly. Similarly, the angles formed between each dividing wall and the circular column wall in the larger division are expected significantly to reduce column mixing; the reflex angle at the junction of the two dividing walls at the centre of the column may also affect fluid mixing, but this is expected to be to a much lesser extent than that observed at the acute angles at the circular column wall. 
         [0068]    Again, it will be appreciated that other arrangements of two dividing walls are possible in which a larger or smaller angle is formed between the two dividing walls. 
         [0069]      FIG. 9  shows a column in which a dividing wall which is a chord of the circular cross-section of the column splits the column cross-sectional area into two unequal parts. An angle or corner is formed at each end of the dividing wall where it intersects the circumference of the column. It is expected that the advantages of the present invention will be obtained in both of the divisions of the column, but that a greater degree of advantage will be obtained for the smaller division, as the more acute angles formed between the circular column wall and the dividing wall are expected to more significantly retard fluid mixing close to these corners. 
         [0070]      FIG. 10  shows a column having two dividing walls that are chords of the circular cross-section of the column, which in this case are placed parallel to one another to define three regions within the column: two sectors and an area between the sectors crossing the centre of the column which is close to rectangular in cross-section. Again, angles or corners are formed where the chords intersect the circumference of the column. The benefit of the present invention is expected to be greater for the two sector divisions, having more acute angles formed at the junctions between the dividing wall and the circular column wall, than the centre division, although all three areas are expected to derive some benefit from the invention. 
         [0071]      FIG. 11  shows a column having three dividing walls that are chords of the circular cross-section of the column, and which each intersect an adjacent dividing wall at the point where they intersect the circumference of the column such that they define a column region of triangular cross-section in the centre of the column. Angles or corners are formed at each point of intersection. It will be appreciated that alternative arrangements are possible in which the walls need not intersect one another when intersecting the circumference (that is, a number of sectors may be formed whose dividing walls do not intersect with one another), and that more than three dividing walls may be provided that are chords to the circumference of the column. It is expected that the benefit of the present invention would be obtained for all of the divisions of this column. 
         [0072]      FIG. 12  shows a column in which the dividing walls do not intersect the column wall but define a region of triangular cross-section within the column. Similarly,  FIG. 13  shows an arrangement where the dividing walls enclose a region of square cross-section within the column without any of the dividing walls intersecting with or contacting the column wall. It will be appreciated that more walls may be provided to enclose regions having different polygonal cross-sections. Also, one or more of the dividing walls may intersect with the circumference of the column, and the walls may be of different lengths, thus resulting in a central region of irregular polygonal cross-section. It is expected that the benefit of the present invention will be obtained in both areas formed by the column division, as fluid mixing will be restricted both by the converging walls in the outer division and the angles formed by the intersecting walls of the inner division, but that the benefit will be greater for the inner division than the outer division. The benefit obtained by the triangular central area in  FIG. 12  is expected to be greater than that obtained for the square inner area in  FIG. 13  due to the more acute angles of the column in the former case. 
         [0073]      FIG. 14  shows a column in which a dividing wall encloses a region having circular cross-section, which dividing wall contacts the circumference of the column. Thus, the dividing wall and the column circumference converge to form angles at the point of contact between the dividing wall and the column circumference. It will be appreciated that the dividing wall need not enclose a circular area but may form any generally rounded shape. It will also be appreciated that the dividing wall need not contact the column wall, but must be arranged such that at least a part of the dividing wall and at least a part of the column wall converge. It is expected in this case that the benefit of the present invention will be obtained only in the outer column division, as the inner column division is of circular cross section, whereas the acute angles formed by the contact between the column wall and the dividing wall will have a restrictive effect on fluid mixing in the outer column section. 
         [0074]      FIG. 15  shows a column in which three dividing walls extend radially from the centre of the column to the circular column wall, thus dividing the cylindrical column into three equal segments. The benefit of the present invention is expected to be obtained for all of the segments due to the restricted fluid mixing caused in the corners of each segment. 
       EXAMPLES 
       [0075]    A comparison of the performance of structured packing according to the prior art with structured packing according to the invention was conducted in a cryogenic distillation apparatus including either a column with a D-shaped cross-section or a column with a circular cross section for the separation of argon from oxygen. For the column with a circular cross section, approximately 20 layers of packing, where each layer of packing is approximately 210 mm in height and 900 mm in diameter, are stacked on top of each other at 90° orientations inside a cryogenic distillation column. For the column with a D-shaped cross-section, approximately 20 layers of packing, where each layer of packing is approximately 210 mm in height and has a 900×450 mm semi-circular area, are stacked on top of each other at 90° orientations inside a cryogenic distillation column. All the comparisons were conducted under total internal reflux at a column pressure of 0.4 barg (40 kPa gauge). The separation of binary mixtures of Argon/Oxygen were studied by measuring the composition of the liquid and vapour streams entering and leaving the column to ascertain the mass transfer efficiency and pressure drop. Both structured packings used conform to the general type shown in  FIG. 3 , with horizontal striations, open area of perforations of about 10%, and each has a surface area of approximately 500 m 2 /m 3 . The structured packings differ in their corrugation angle, which is 45° in the case of the prior art packing (Packing A) and 60° in the case of the packing according to the invention (Packing B). The performance is presented in terms of the HETP and the measured dynamic pressure drop through both packings, both of which are presented as functions of K v , the density corrected superficial gas velocity, which is defined as: 
         [0000]        K   v   =U [(ρ v /ρ L −ρ v ) 0.5 ],
 
         [0000]    wherein U=superficial velocity of the vapour phase in the column in m/s; ρ v =density of the vapour phase in the column in kg/m 3 ; and ρ L =density of the liquid phase in the column in kg/m 3 . The values of HETP, K v  and pressure drop have been normalized in order to compare the performance of the packings in the two different forms of columns used. 
         [0076]    The results obtained for the prior art structured packing A in a column of circular cross-section and a column of semi-circular cross-section are shown in  FIGS. 16 and 17 . Two datasets are shown for circular cross-section columns (circular and triangular datapoints) and one for semi-circular cross-section columns (rhomboid datapoints). It can be seen from  FIG. 16  that the HETP value is higher by around 30-50% (depending on K v ), and thus the separation efficiency is lower, for this packing in a semi-circular cross-section column compared with a column of circular cross-section. The curves begin to converge at a normalized K v  of around 1.35, at which point both curves begin to show a steep increase of HETP with increasing K v , which latter observation implies that the operating capacity is similar for both the columns used.  FIG. 17  shows that the pressure drop of packing A is similar in both column types used. In addition, the loading point of both columns, which in this case is defined as the K v  at which the pressure drop increase becomes more rapid with further incremental increases in K v , are similar at a K v  of 1.05. 
         [0077]    The results obtained for the structured packing B according to the present invention are shown in  FIGS. 18 and 19 . Again, two datasets are shown for circular cross-section columns (in this case, circular and rhomboid datapoints) and one for semi-circular cross-section columns (triangular datapoints). It can be seen from  FIG. 18  that a structured packing B with a corrugation angle of 60° (compared to 45° for the prior art packing A) unexpectedly does not show a significant increase in HETP for the semi-circular cross-section column compared with the circular cross-section column (compare with  FIG. 16 ). In fact, the datapoints for the semi-circular column in general lie between the two sets of datapoints for the circular cross-section column. As for  FIG. 16 , the HETP for all datasets starts to increase rapidly on increase of K v  at a similar value, so again it can be inferred that the operating capacity for the packing is similar for both column types used.  FIG. 19  shows, similarly to  FIG. 17 , that the pressure drop of the packing and the loading point of the packing is similar in both column types used. 
         [0078]    An overall comparison of the performance of the two packings in the two column types is presented in Table 1 in terms of relative HETP and relative operating capacity. The relative HETPs are representative of the HETPs along the flatter parts of the curves in  FIGS. 16 and 18 , prior to the rapid increase in HETP at higher K. Relative operating capacities are evaluated at the K v  at which the HETPs start to converge. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Relative HETP 
                 Relative operating 
               
               
                   
                 Corrugation 
                 (semicircular/ 
                 capacity (semicircular/ 
               
               
                   
                 Angle (°) 
                 circular) 
                 circular) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Packing A 
                 45 
                 1.3-1.5 
                 1.0 
               
               
                 Packing B 
                 60 
                 1.0 
                 1.0 
               
               
                   
               
             
          
         
       
     
         [0079]    Thus, it can be seen that the operating capacity of Packing A is similar in both the semi-circular and circular cross-section column. The operating capacity of Packing B is also similar in both the semi-circular and circular cross-section column, although higher than Packing A. The higher corrugation angle structured packing B also provides a similar separation efficiency in both the semi-circular and circular cross-section column, whereas Packing A loses separation efficiency in the semi-circular cross-section column. Thus, use of packing B in semi-circular columns permits use of a lower column height than would otherwise be expected if the relative HETP was the same as Packing A, and thus use of Packing B provides a more cost-effective trade-off of height versus operating capacity than use of packing A. 
         [0080]    Whilst the invention has been described with reference to a preferred embodiment, it will be appreciated that various modifications are possible within the scope of the invention. 
         [0081]    In this specification, unless expressly otherwise indicated, the word ‘or’ is used in the sense of an operator that returns a true value when either or both of the stated conditions is met, as opposed to the operator ‘exclusive or’ which requires that only one of the conditions is met. The word ‘comprising’ is used in the sense of ‘including’ rather than in to mean ‘consisting of’. All prior teachings acknowledged above are hereby incorporated by reference.