LIQUID-VAPOUR CONTACT COLUMN

Abstract: A packing for a liquid-vapour contact column is provided which comprises metallic sheets defining liquid-vapour passages and having at least one wettable surface comprised of a coating of porous metal. The sheets are perforated to permit vapour to pass between them. The porous metal coating is formed by spraying a particulate mixture of metal and a plastics material onto the sheet to form a coating of plastics particles embedded in metal and then heating to volatilize the plastic thereby forming porosity in the metal coating.

Description: This invention relates to liquid-vapour contact columns. In a liquid-vapour contact column, for example a distillation column an ascending vapour is intimately contacted with a descending liquid. Members providing surfaces on which such contact can take place are disposed in the column. The members may comprise perforate, spaced, horizontal trays across which liquid flows and through which the ascending vapour passes. The vapour and liquid thus come into contact with one another. Downcomers are used to conduct liquid from the exit end of one tray to the entrance end of the tray below. An alternative method of contacting liquid and vapour is to use a packing in the columns. Column packings typically have a regular structure with defined flow channels. They have a large active mass exchange surface and give high separation performance with low pressure drop. The packing elements, typically consist of thin corrugated sheets of material such as gauze stacked on top of one another to form open, intersecting channels at an angle to the column axis. As a result, the gas flowing through the packing is mixed in the direction of the parallel elements. By transposing successive elements, radial mixing is induced over the entire column cross-section. Although structured column packings are widely used in distillation columns operating at above ambient temperature for fractionating liquid mixtures, they are not widely used commercially in separating gas mixtures, eg air, at cryogenic temperatures except on a small scale. In practice, there is a difficulty in limiting the height of the column when for example, air is being distilled. The invention aims at providing a packing for a distillation or other liquid-vapour contact column which is able to ameliorate this difficulty. According to the present invention there is provided a packing for a liquid-vapour contact column, comprising at least one array of liquid-vapour contact members each defining liquid-vapour contact passages therethrough each having a wettable surface, each member being of metallic sheet and each said wettable surface including a multiplicity of open pores, wherein said surface is formed by spraying a particulate mixture of metal and plastics material onto a surface of the metallic sheet to form a coating comprising particles of plastics embedded in metal on the said surface, and heating the thus formed coating to volatise or otherwise remove the plastics material and thereby form the pores in the coating. The invention also provides a method of making a packing for a liquid-vapour contact column comprising, spraying a particulate mixture of metal and plastics material onto metallic sheets to form on each sheet at least one coating comprising particles of plastics embedded in metal on the said surface, heating the coating to volatalise or otherwise remove the plastics material and thereby form the pores in the coating, forming liquid vapour contact passages in the sheet, and forming the sheets into an array in the column. The percentage porosity of the coating depends on the mass ratio of plastics powder to metal powder. The average size of the pores depends largely on the average size of the plastics particles. Accordingly, the invention enables the porosity to be varied independently of the average pore size and thus enables there to be prepared a surface specially tailored to the properties of a gas mixture being separated by distillation at cryogenic temperatures. Typically, the plastics particles can have an average size in the range 15 to 150 micrometres. Typically, the mass ratio of metal particles to plastic particles in the mixture that is sprayed onto the surface of the substrate has a value in the range of 4:1 to 1:1 so that the coating has a porosity of from 20 to 50%, although if desired more porous coatings may be formed. The average size of the metal particles is not critical to the invention and may be lower or higher than the average size of the plastics particles. Similarly, the thickness of the coating is not critical to the invention. In our experiments, we have prepared coatings comprising a single porous layer 5, 254 and 381 micrometres (10 and 15 thousands of an inch) in depth, and coatings comprising two such porous layers, one layer having a difference average pore size from the other. The distribution of pore sizes is preferably designed to give optimum liquid hold up and wetting and to result in a minimum HETP (height equivalent to theoretical plate). The plastic and metal particles may each have a regular or irregular geometry but their flow properties must allow their use in a spray coating process. Coated sheets for use in making a packing according to the invention typically have surfaces that comprise a network of open, re-entrant pores or cavities having an average size in the range 15 to 150 micrometres (and more typically a size in the range 15 to 50 micrometres). The nature of the coating is such that it provides the finished sheets with excellent wetting properties. Such properties are conducive to good liquid-vapour contact. Moreover, the wetting properties enable the packing to have a relatively low HETP and hence a relatively low pressure drop. Such characteristics are particularly beneficial when separating a mixture of cryogenic fluids of similar volatilities in particular air. Each sheet is preferably formed with alternate crests and troughs so as to define the liquid-vapour contact passages. In addition, the sheets are preferably perforate to enable vapour to pass from each passage to an adjacent one. The sheets are preferably arranged such that in alternate sheets the passages slope in different directions. The mixture of plastics and metal is preferably sprayed onto the substrate by plasma spraying. Alternatively, flame spraying may be employed. The spraying process may be controlled so as to give the axes of the pores any desired orientation, although typically each pore has an axis perpendicular to the surface of the substrate. Generally, the mixture comprises separate particles of plastics and metals, although if desired the mixture may comprise composite particles of plastics and metal. The metal coating may have the same or a different composition from that of the sheets. Typically, the metal comprises aluminium or copper or an alloy based on aluminium or copper. It is possible to select the plastics particles from a wide range of different plastics materials, but in our experiments we have used polyester particles. The sheets are preferably of aluminium or copper. In the event that the plastics particles are of polyester, a temperature in the range 500 to 550°C is preferably employed to effect volatilisation of the deposited polyester. The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic perspective view of a structured packing according to the invention. Figure 2 is a schematic plan view of a fragment of the packing shown in Figure 1. Figure 3 is a schematic side view of a fragment of the packing shown in Figure 1, Figure 4 is a schematic section of a fragment of a sheet forming part of the packing shown in Figures 1 to 3. Figure 5 is a schematic section of a fragment of an alternative sheet that can form part of the packing shown in Figures 1 to 3. Figure 6 is an electronmicrograph of the surface of a sheet forming part of a packing according to the invention showing the surface at a magnification of 500 times actual size. Figure 7 is an electronmicrograph similar to Figure 6 but at a magnification of 5000 times actual size. Referring to Figures 1 to 3 of the drawings, a structured packing comprises a multiplicity of vertical sheets 2 of heat conductive metal, usually aluminium or copper. In Figures 2 and 3, just fragments of three adjacent sheets 4, 6 and 8 are shown. Viewed from one side each sheet is provided with alternate peaks and troughs. The surfaces between neighbouring peaks form liquid-vapour contact passages 10. Similarly, viewed from the other side, each sheet is also provided with alternate peaks and troughs and the surfaces bewteen neighbouring peaks also form liquid-vapour contact passages 10. The passages 10 defined by one set of alternate sheets slope from left at the top to right at the bottom, and on the other set of the alternate sheets from right at the top to left at the bottom. The sheets are preferably all identical to one another. The different directions of slope of the passages 10 is obtained by inverting one set of alternate sheets with respect to the other set. The sheets are each formed with perforations 12 which permit liquid and vapour to flow from one passage 10 to a adjacent passage 10. Typically from 5 to 15% of the surface area of each sheet is occupied by the perforations 12. Each sheet may also be provided with a multiplicity of small parallel ridges 14 extending laterally from one side of the sheet to the other (as shown in Figure 3). As shown in Figure 4, each sheet has a surface layer 16 of porous metal on both its sides. The porous metal has the same composition as the sheet. Each layer preferably has a thickness in the range of 0.1 mm to 1.0 mm. In an alternative embodiment illustrated in Figure 5, there are a plurality of layers of porous metal on each side of the sheet. Typically, each inner layer 18 has a smaller average pore size then each outer layer 20, although if desired, this difference in the average pore size may be reversed. One example of the method according to the invention, a sheet such as shown in Figures 1 to 3 and typically of aluminium is provided with a porous aluminium-silicon coating by the following procedure. The surface is first cleaned by shot-blasting. The surface is then plasma sprayed with a proprietory blend of silicon-aluminium alloy and polyester powder (Metco 601 NS). The plasma is formed by supplying to the spraying chamber argon at a pressure of 689.50KPa (100 psig) and hydrogen at a pressure of 344.74 Kpa (50 psig). Spray rates in the range 2.27 Kg(5 pounds) to 3.17 Kg(7 pounds) per hour of the mixture of plastics powder and metal powder are employed. Continuous matrices of aluminium with dispersed particles of polyester are formed on the surface of the aluminium samples. The polyester is then driven off by heating in a vacuum for two hours at a temperature in the order of 540°C. This left surfaces on each sample comprising a network of open re-entrant pores having axes generally disposed at right-angles to the surface of the substrate. If desired, the passages 10 and perforations 12 may be formed after coating. Figures 6 and 7 are electromicrographs of a suitable surface formed by plasma spraying a mixture of 60% by weight of aluminium and 40% by weight of polyester onto an aluminium substrate and then baking the resulting coated substrate for two hours at 500°C. The coating had a thickness of 0.38 mm. Figure 6 shows the coated surface at a magnification of 500 times actual size, and Figure 7 shows the surface at a magnification of 5000 times actual size.

Claims: 1. A packing for a liquid-vapour contact column, comprising at least one array of liquid-vapour contact members each defining liquid- vapour passages therethrough each having a wettable surface, each member being of metallic sheet and each said wettable surface including a multiplicity of open pores, wherein said surface is formed by spraying a particulate mixture of metal and plastics material onto a surface of the metallic sheet to form a coating comprising particles of plastics embedded in metal on the said surface, and heating the thus formed coating to volatalise or otherwise remove the plastics material and thereby form the pores in the coating. 2. A packing as claimed in Claim 1, in which the said coating has a porosity of from 20% to 50%. 3. A packing as claimed in Claim 1 or Claim 2, in which the average size of the pores is from 15 to 150 micrometres. 4. A packing as claimed in Claim 3, in which the average size of the pores is from 15 to 50 micrometres. 5. A packing as claimed in any one of the preceeding claims, in which each sheet is formed with alternate crests and troughs so as to define the liquid-vapour contact passages. 6. A packing as claimed in Claim 5, in which the sheets are perforated to enable, in use, vapour to pass from each passage to an adjacent one. 7. A packing as claimed in Claim 5 or Claim 6, in which the sheets are arranged such that in alternate sheets the passages slope in different directions. 8. A method of making a packing for a liquid-vapour contact column comprising, spraying a particulate mixture of metal and plastics material onto metallic sheets to form on each sheet at least one coating comprising particles of plastic embedded in metal on the said surface, heating the coating to volatalise or otherwise remove the plastics material and thereby form the pores in the coating, forming liquid-vapour contact passages in the sheets, and forming the sheets into an array in the column. 9. A method as claimed in Claim 8, in which the plastics particles have an average size in the range 15 to 150 micrometres. 10. A method as claimed in Claim 8 or Claim 9, in which the mass ratio of metal particles to plastics particles in the mixture that is sprayed onto the sheets has a value in the range of 4:1 to 1:1. 11. A method as claimed in any one of Claims 8 to 10, in which the mixture of plastics and metal is deposited by plasma spraying or flame spraying. 12. A method as claimed in any one of Claims 8 to 11, in which the plastics particles are of polyester.

Inventors: LAVIN, JOHN TERENCE
Applicants: THE BOC GROUP PLC
Application year: 1989
Main classification: B01D53
IPC codes: B01J19, C23C4