LIQUID-VAPOUR CONTACT APPARATUS

Abstract: A liquid-vapour contact apparatus for use in distilling air on board a ship has pairs l0 and l2 of vertical outer walls disposed in the manner of the vertical faces of a cuboid. The apparatus includes a plurality of spaced, parallel, vertical plates l4. Between each pair of adjacent plates l4 there is a plurality of spaced, generally horizontal, liquid-vapour contact members 20 each including a horizontal tray member 26 and having edges 22 and 24 in fluid-tight engagement with the respective plates l4. There are a plurality of downcomers 34 for conducting liquid from tray member 26 to tray member 26, there being separate flow paths for liquid between each pair of adjacent plates l4. The two outermost plates form the walls l2 of the apparatus. Each of the walls l0 comprises alternate edge portions l5 of plates l4 and spacer bars or members l6. The apparatus may accordingly be manufactured by vacuum brazing in the manner of a matrix heat exchanger.

Description: This invention relates to liquid-vapour contact apparatus suitable for use in fractionally distilling a liquid mixture comprising two or more components. Conventional distillation columns for use in separating air employ circular or part-circular liquid-vapour contact trays. It has been found that aboard ship such columns perform badly as a result of the motion of the ship causing maldistribution of the liquid on the trays. Our UK patent specification l 478 459 relates to a distillation column whose trays are each provided with spaced vertical baffles that limit displacement of the liquid across the tray in a direction normal to that of the liquid flow. The aforesaid UK patent specification describes an embodiment of such a column in which trays with generally rectangular liquid-bearing surfaces are employed. However, it has been found necessary to employ generally tubular column walls as difficulties arise in fabricating a vessel (for containing the trays) in the shape of a cuboid. Moreover, when a cylindrical vessel is employed it is found necessary to use separate reboilers and condensers. These requirements tend to add unnecessarily to the overall size of the apparatus thereby giving rise to further disadvantage since on board ship there is generally limited space available for an air separation column. The invention aims at providing a liquid-vapour contact apparatus that can for example be fabricated by vacuum brazing and that has vertical outer walls disposed in the manner of the vertical faces of a cuboid. According to the present invention there is provided a liquid-vapour contact apparatus having vertical outer walls disposed in the manner of the vertical faces of a cuboid, said apparatus including a plurality of spaced, parallel, vertical plates, there being between each pair of adjacent plates firstly a plurality of spaced, generally horizontal, liquid-vapour contact members each having one edge in engagement with one of said pair of plates and an opposite edge in engagement with the other of said pair of plates, and a plurality of downcomers for conducting liquid from tray to tray in downward vertical sequence, whereby there are separate flow paths for liquid between each pair of adjacent plates, and wherein the outermost plates provide one pair of opposed, vertical, outer walls of the apparatus, and each of the other pair of such walls comprises alternate plate edges portions and vertical spacer members. Preferably, each plate, save the said outermost ones, has a plurality of apertures formed therethrough for the passage of vapour, such apertures being disposed such that in operation of the column no liquid passes therethrough. The presence of such apertures helps to limit at any level in the apparatus differences in composition between the vapour between one pair of adjacent plates and the vapour between another pair of adjacent plates. If desired, the liquid-vapour contact apparatus according to the invention may form part of a unitary apparatus additionally including a reboiler and a condenser. The apparatus according to the invention is particularly suited for air separation use on board ship. Liquid-vapour contact apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure l is a schematic perspective view of a distillation plant including liquid-vapour contact apparatus according to the invention. Figure 2 is a section through the line II-II in Figure l and, Figure 3 is a section through the line III-III in Figure l. Referring to the drawings, a distillation apparatus or column 2 formed as a unitary article has at its bottom, a reboiler 4 and at its top a condenser 6, and extending therebetween a liquid-vapour contact apparatus 8 in accordance with the invention. The distillation apparatus 2 is generally cuboidal in shape. The apparatus 8 has a first pair of opposed, vertical, outer walls 10 and a second pair of opposed, vertical, outer walls l2. As shown in Figure 2, the apparatus 8 includes a plurality of vertical plates l4 that are spaced equally apart from one another and extend in parallel with one another. The two outermost ones of the plates l4 form the walls l2 of the apparatus 8. As shown in Figure 2, the walls l0 are each formed by arrangement of edge portions l5 of the plates l4 and longitudinal spacing bars or members l6 extending from top to bottom of the apparatus 8. The apparatus 8 additionally includes a plurality of liquid-vapour contact members 20, as shown in Figures 2 and 3. Between each pair of adjacent plates l4 there is a vertical array of such members 20. The members 20 each include a horizontal tray member 26 having a horizontal liquid bearing surface and perforations 36 formed therethrough. The peforations 36 are equally spaced from one another and have vertical axes. The members 20 have edges 22 and 24 in fluid-tight engagement with the plates l4 that they respectively abut. The liquid-vapour contact members 20 also include vertical partition members 28 which provide a weir 30 for each tray member 26 and a wall of a downcomer 34 for conducting liquid from said tray member to the tray member 26 immediately therebelow. Each downcomer 34 has a horizontal, liquid-collecting member 35 which is typically formed integral with the tray member 26 onto which said liquid is to be conducted by that downcomer in operation of the distillation apparatus 2. With reference to Figure 3, each plate l4 except the two that constitute the walls l2 has a multiplicity of orifices 38 formed therethrough. The orifices 38 may be arranged in rows 39, the distance between adjacent rows 39 being the same as the distance between adjacent tray members in each said array of such tray members. The position of each row 39 is such that in operation only vapour fully disengaged from liquid can pass therethrough. As shown in Figure 3, therefore, the rows of 39 of orifices 38 are typically positioned just below the level of respective tray members 26. The liquid-vapour contact members 20 (including the members 26, 28 and 35) may comprise an integral pressing or may be fabricated from two or more pieces of metal. Typically, the apparatus 8 is formed of aluminium, although stainless steel and copper are acceptable alternatives. Where the members 20 are formed from more than one piece of metal, they may be prefabricated or alternatively joined together with other parts of the apparatus 8. Typically, in order to fabricate the apparatus 8, an assembly of all the plates l4, spacer members l6, and liquid-vapour contact members 20 is made as shown in Figures l to 3. All of the abutting surfaces may then be vacuum brazed together to form a unitary apparatus. Typically, the reboiler 4 and the condenser 6 may be brazed to the apparatus 8 at the same time. In general, the method of manufacture may resemble that employed to make matrix heat exchangers. It is to be appreciated that the perforations 36 and orfices 38 are made prior to the commencement of the vacuum brazing. It is not essential that the various abutting surfaces be vacuum brazed together in order to make fluid-tight joints therebetween. Other joining methods such as welding may be employed. However, vacuum brazing is generally preferred. Each pair of adjacent plates l4 defines therebetween a discrete and separate flow path for the liquid from that defined by the other pairs of adjacent plates l4. In operation liquid from the condenser 6 flows onto each top tray member 26 at the top of a vertical array flows therealong and is conducted by a downcomer 34 to the next tray member 26 therebelow. In Figure 3, the liquid flow path is illustrated by the arrows 42. As it passes along each tray member 26, liquid comes into intimate mass exchange relationship with vapour ascending the column from the reboiler 4 and passing through the perforations 36 in the tray members 26. Thus, the liquid as it descends the apparatus 8 tends to become richer in the less volatile component or components of the mixture being distilled, and the vapour as it ascends the column becomes richer in the more volatile component or components of the mixture being distilled. In Figure 3, the upward passage of the vapour is indicated by the arrows 40. It is also to be appreciated that there will be some exchange of vapour across the plates l4 through the orifices 38. Such exchange of vapour helps to limit any disparity between the composition of vapour between one pair of adjacent pipes l4 and that of vapour at the same level between another pair of plates l4. The apparatus shown in Figure l to 3 of the accompanying drawings is particularly intended for the distillation of air on board ship in order to produce either a nitrogen or an oxygen product, or both. The plates l4 help to limit displacement of the liquid on the tray members 26 in a direction generally at right angles to the direction of flow. Typically, on board a ship, the apparatus is mounted such that one wall l2 faces the stern and the other wall l2 faces the bow.

Claims: A liquid-vapour contact apparatus (2) having vertical outer walls (10, 12) disposed in the manner of the vertical faces of a cuboid, said apparatus including a plurality of spaced, parallel, vertical plates (14), there being between each pair of adjacent plates (14) firstly a plurality of spaced, generally horizontal, liquid-vapour contact members (20) each having one edge in engagement with one of said pair of plates (14) and an opposite edge in engagement with the other of said pair of plates, and secondly a plurality of downcomers (34) for conducting liquid from member (20) to member (20) in downward vertical sequence, whereby there are separate flow paths for liquid between each pair of adjacent plates (14), and wherein the outermost plates (14) provide one pair (12) of opposed, vertical, outer walls of the apparatus, and each of the other pair (10) of such walls comprises alternate plate edges (15) and vertical spacer members (16). Apparatus as claimed in claim 1, wherein each plate (14), save the outer most ones has a plurality of apertures formed therethrough for the passage of vapour, such apertures being disposed such that in operation of the column no liquid passes therethrough. Apparatus as claimed in claim 1 or claim 2, forming part of a unitary apparatus additionally including a reboiler (4) and a condenser (6).

Inventors: AITKEN, WILLIAM HARROLD, PORTER, JOHN FREDERICK
Applicants: THE BOC GROUP PLC
Application year: 1986
Main classification: B01D3
IPC codes: B01D3