Distributor for packed liquid-vapor contact column

Abstract: A liquid distributor for a packed liquid-vapour contact column comprises a header tank (1); a liquid distribution plate (5) having vapour riser passages (8) and a multiplicity of discrete reservoir cells (4) each having an aperture for flow of liquid therefrom into the column; and conduit means (2,3,7) for feeding liquid from the header tank (1) into each cell (4). The liquid distribution plate preferably has a uniform criss-cross structure with alternating vapour riser passages (8) and reservoir cells (4) of identical shape and size. The conduit means may have two or more sections (3,7) each feeding a discrete group of reservoir cells (4) from a location of the header tank at an elevation spaced from that of the other section(s). The distributor compensates for column sway or tilt when mounted on, for example, a ship.

Description: The present invention relates to the distribution of liquid feed into a liquid-vapour contact column and provides a distributor for that purpose. It has particular, but not exclusive, application to columns, especially for cryogenic air separation, at a location, such as off-shore, subject to movement causing the column to sway or tilt from the vertical. It is well known that the use of structured packing in distillation columns has many advantages where low pressure drop is important. However, packed column performance is very dependent on creating and maintaining a balance between the downward flow of liquid and upward flow of vapour locally in the packing. The distribution of the liquid and vapour within the packing is influenced by the initial presentation of these fluids to the packing, and the particular characteristics of the packing. For a variety of reasons, the distribution of liquid within the packing is more sensitive than the distribution of vapour to initial presentation. Typically, initial presentation of liquid is made by a liquid distributor, consisting of a collection of interconnecting open troughs with irrigation holes in the base which feed liquid to the packing below. A uniform liquid level in the troughs of the liquid distributor is a minimum requirement to achieve uniform flow from the irrigation holes. In practice, difficulties arise in creating uniform initial liquid distribution. For example, a variety of factors affect uniformity of liquid level in a liquid distributor, including hydrodynamic resistance internal to the liquid distributor, misalignment during installation of the distributor, and column tilt. The design of liquid distributors for off-shore applications presents some particularly demanding problems. The action of both wind and waves causes significant movement of a shipboard distillation column; this movement is particularly of concern when it causes the column to tilt away from the vertical in a swaying motion, causing side-to-side movement of liquid within a liquid distributor. The uniformity of liquid level in a typical liquid distributor is significantly compromised during ship movement, causing flow non-uniformity from the irrigation holes, leading to maldistribution within the packed column. Numerous approaches to providing initial liquid distribution have been proposed including trough distributors, fractal distributors, overflowing distributors, and irrigation-hole-multiplying distributors (see, for example US-A-4,816,191 (Berven & Meyer), US-A-5,132,055 (Alleaume et al) & US-A-5,354460 (Kearney et al), DE-A-2,752,391 (Streuber), and a paper by M.Kearney & V.Kochergin entitled "A Liquid Distributor for Industrial Chromatography Columns: An Approach Based on Fractal Geometry" " presented at the Fifth World Congress of Chemical Engineering in San Diago, California, July 14 - 18, 1996). All of these rely to some extent on a uniform liquid level to achieve flow uniformity and are subject to liquid level non-uniformity created by the aforementioned mechanisms. For example, none of these approaches is suitable for a non-vertical column, nor for a periodically tilting column on, for example, a ship. Alternative approaches to those relying on a uniform liquid head in a trough have been reported. Pluss and Bomio (I. Chem. E. Symp. Ser. 104 (1987) A259) describe a pipe-distributor which was shown to be superior to a trough-type distributor in a tilting column. This pipe distributor is a type of pressurised distributor consisting of a closed manifold, such as a pipe, with irrigation holes directing liquid down into the packing. The number and size of the holes are such that there is a back pressure significantly greater than the difference in head caused by the distributor being tilted. In this way, the distribution becomes relatively even regardless of column tilt. Unfortunately, there are practical difficulties associated with the pressurised distributor approach. For example, if the distributor back pressure were generated by a pump, the pumped liquid would have to be subcooled to avoid flashing in the pressurised manifold because flashing would lead to maldistribution by formation of bubbles impeding liquid flow in the manifold. Subcooling can be problematical if the liquid is entering the coolest section of the distillation column and there are no cooler fluids to provide subcooling duty. The pumped option is also difficult if liquid is being fed from one section of a column to another, without leaving the column shell. Another approach, although fundamentally similar to the pressurised distributor, was used by Tanner, et al., (Trans. IChemE 74 (1996) A177) in which a high liquid head in a distributor was shown to yield superior mass transfer performance to a low head in a tilted column, because the variation in head due to tilt on a percentage basis was lower with the high average head than with the low average head. Unfortunately, the elevation required to provide the required back pressures for good distribution is excessive and would severely impact on both column height and cost. FR-A-2203659 discloses a liquid distributor plate for a packed liquid-vapour column having two sets of nozzles distributed over the plate between vapour riser passages. The nozzles produce solid cone spray patterns so that each set produces a spray pattern which is substantially uniform over the projected area of the plate therebeneath. One set of nozzles is fed directly from the upper surface of the plate. Each nozzle of the other set is surrounded by a respective tube extending upwardly from the plate so that the nozzle receives liquid only when the liquid on the plate reaches an overflow level corresponding to the upper end of the tube. US-A-5262094 discloses a frothing plate for a packed liquid-vapour column having laterally spaced rows of packed upwardly extending plate portions extending across the column. The bases of the rows are perforated to permit upward vapour and downward liquid flow through the plate. The opposed walls of the rows also are perforated to allow liquid received in the spaces between the row to pass into the adjacent rows but the base sections between the row are impermeable. It is an object of the present invention to provide a liquid distributor which permits of uniform distribution to a liquid-vapour contact column during sway or tilt such as that encountered by a shipboard cryogenic air separation distillation column. In one aspect, the present invention is a liquid distributor for a packed liquid-vapour column comprising a header tank; a liquid distribution plate having a uniform criss-cross structure with alternating vapour riser passages and a discrete reservoir cells, each cell having an aperture for flow of liquid therefrom into the column and providing sufficient liquid residence time to buffer out feed variations; and conduit means for feeding liquid from the header tank into each cell. In another aspect, the invention is a liquid distribution plate for a packed liquid-vapour column comprising a uniform criss-cross structure with alternating vapour riser passages and discrete reservoir cells, each cell having an aperture for flow of liquid therefrom into the column and providing sufficient liquid residence time to buffer out feed variations. The distributor of the invention is a two stage distributor consisting of a primary distributor (viz. the header tank & conduit means) and secondary distributor (viz. the liquid distribution plate). The primary distributor feeds liquid to the secondary distributor, which feeds liquid to the packing. The secondary distributor consists of a large number of individual reservoir cells which cannot communicate with each other and the primary distributor feeds liquid individually into each cell in the secondary distributor. The cross-sectional area and height of each cell reduces the effect of column tilting on the head of liquid above the liquid flow aperture(s) therein and hence on the liquid flow through the aperture(s) into the column packing. As described above, the prior art had two main approaches to solving the problem of non-uniform liquid levels in liquid distributors leading to initial maldistribution of the liquid (viz. pressurised distributors and tall head tanks). The problems associated with these have been primarily associated with pumps and excessive column height, respectively. The present invention circumvents these problems by combining the concept of a pressurised distributor with a modest-sized header tank to create a primary distributor. The primary distributor works in conjunction with a secondary distributor composed of a lattice of small vessels (reservoir cells) which act to buffer any short-comings of the primary distributor. No pumps are required, and thus the problems associated with subcooling, etc., are avoided. Moreover, the modest-sized header tank requires little or no extra column height, resulting in a significant capital cost savings over the prior art. The present invention is characterized by the combination of a header tank and (secondary) distribution plate, whose construction is such that any short-comings of distribution from the header tank are dampened. As mentioned above, the distributor of the present invention can be considered as having a primary distributor constituted by the header tank and conduit means and a secondary distributor constituted by the distribution plate. The secondary distributor has a large number of individual reservoir cells which cannot communicate with each other but which each have one or more apertures through which liquid is fed to the column packing. The primary distributor feeds liquid from the header tank through the conduit means into each reservoir cell in the secondary distributor. Typically, the conduit means consists of one or more main supply pipes connected to the header tank, branch pipes extending from the main pipe(s) over rows or series of reservoir cells, and secondary delivery tubes dedicated to feeding a single cell from an adjacent branch pipe. Since the primary distributor only requires relatively modest back pressure, only modest elevation of the header tank is required thereby limiting the additional height of the column required to accommodate the distributor of the present invention. The primary distributor is only able to smooth out the impact of column movement to a limited extent and hence the feed to the secondary distributor will still vary to an unacceptable level. This variation is overcome by the reservoir capacity of the cells, each cell being designed to provide sufficient liquid residence time to buffer out feed variations. This is accomplished by dimensioning the cross-sectional area, height and aperture size for each cell according to column dimensions and performance so that anticipated column tilting has minimal effect on the head of liquid above the cell aperture(s). Due to the fact that the liquid level in a cell will vary linearly with the difference between flow in and flow out, but the flow out is proportional to the square root of the liquid level, the variation in flow out of the cell is very effectively dampened compared to the inlet flow. The combination of primary and secondary distributors described above is designed to cope with horizontal column oscillations, and, in addition, the primary distributor is designed with sufficient back pressure to overcome the offsets in average position away from the vertical. Preferably, the volume of each reservoir cell is of uniform cross-section through the liquid distribution plate and suitably is of rectangular, especially square, cross-section. Usually, each cell is open at its top and has a bottom wall containing a single centrally located liquid flow aperture. Conveniently, all the cells in the body of the liquid distribution plate are of the same shape and size but those at the edges are shaped to fit the column wall. The liquid distribution plate has vapor riser passages alternating with the reservoir cells, which passages preferably are of the same shape and size as the reservoir cells but open at both ends to permit the free flow of vapor therethrough. In a presently preferred embodiment, the distribution plate is of a checkerboard (i.e. uniform criss-cross) structure with cells of identical square cross-section covering the whole cross-section of the column above the packing and with open (vapour riser) passages alternating with reservoir cells. The distributor of the present invention can be adapted to accommodate a column required to operate over a wide range of flows. Here the difficulty would be in maintaining liquid buffer capacity and level in the reservoir cells of the secondary distributor and also back pressure in the primary distributor at low flow rates. This difficulty is overcome by providing conduit means which comprises at least two sections each feeding a discrete group of reservoir cells from a location of the header tank at an elevation spaced from that of the other section(s). Thus two or more primary distributor circuits are provided which are fed by the same header tank but feed different groups of reservoir cells in the secondary distributor. The feeds to these primary distributors are at different elevations within the header tank such that at high rates all of the primary distributors are filled with liquid, but at turndown some are empty. Thus at turndown, only some of the reservoir cells in the secondary distributor will be active, the others will be empty. In this way, liquid levels are always maintained at adequate levels. The arrangement of cells being fed by two primary distributors is such that the liquid irrigation of the packing is still uniform at low rates, although the irrigation density will be half that at high rates. It is preferred that the distributor of the invention is used in a column packed with structured packing arranged to reduce the effect of column tilt on the liquid-vapour contact within the column. In particular, it is preferred that there are one or more layers of structured packing sheets arranged so that, viewed in the axial direction of the column, packing sheets in the body of each layer are arranged in one or more sections in which at least one band surrounds a central core, said band being formed of segments in which the packing sheets in each segment extend rectilinearly in a different direction to those in the adjacent segments. This arrangement is the subject of our co-pending European Patent Application No. 99300347.4 (EP-A-0930097) of the same filing date. The following is a description, by way of example only and with reference to the accompanying drawings, of presently preferred embodiments of the present invention. In the drawings: Figure 1 is a schematic representation of a liquid distributor in accordance with the present invention and Figure 2 is a plan view of the liquid distribution plate of the distributor of Figure 1. Referring first to Figure 1, a distributor in accordance with a presently preferred embodiment of the present invention comprises a header tank 1 from which a main distribution pipe 2 extends for the delivery of liquid from the header tank. A plurality of pipe branches 3 extend from the main pipe 2 over rows of reservoir cells 4 in a liquid distribution plate 5 (see Figure 2) located above structural packing 6 in a distillation column (not shown) of a cryogenic air separation unit mounted on a ship (not shown). Each pipe branch 3 has a series of spaced delivery tubes 7 individually aligned with a respective reservoir cell 4 of the respective row. As shown in Figure 2, the distribution plate 5 is a circular disc covering the whole cross-section of the column above the packing and having identical bores (4,8) of square cross-section formed therein in uniform criss-cross rows. Alternate bores 8 (vapour riser passages) are open at both the top and bottom to permit the free flow of vapour therethrough and the remaining bores 4 (reservoir cells) are blind being closed at their bases except for a central hole (not visible) permitting limited flow of liquid therethrough. Example A distributor as described above with reference to the figures is fitted to a shipboard cryogenic air separation column of 2.9 m diameter packed with structured packing sheets in accordance with the teaching of our co-pending European Patent Application No. 99300347.4 (EP-A-0930097). The column produces about 1000 tons/day (900 tonnes) of oxygen with a liquid flowrate of 0.042 m 3 /s and a vapour flowrate of 5.04 m 3 /s. The distributor is dimensioned for ±12° tilting of the column from the vertical with a 15 second period and an average list of 2° from the vertical. Each reservoir cell in the distribution plate is about 0.050 m x 0.050 m square and 0.38 m tall with a hole size of 0.005 m, yielding a 30 second inventory of liquid. The height of the liquid head in the header tank is 1.2 m and the total height of the distributor system is about 3 m. This system gives an overall flow variation of ±5% during operation. In contrast, if the same column were fitted with a conventional pipe distributor, a liquid head of 7.5 m would be required to achieve ±5% flow variation. The total height of the conventional pipe distributor would be about 9 m (i.e. about 3 times more than is required by the exemplified distributor of the present invention).

Claims: A liquid distribution plate for a packed liquid-vapour column comprising a uniform criss-cross structure with alternating vapour riser passages (8) and discrete reservoir cells (4), each cell having an aperture for flow of liquid therefrom into the column and providing sufficient liquid residence time to buffer out feed variations. A liquid distribution plate as claimed in Claim 1, wherein the volume of each reservoir cell (4) is of uniform cross-section through the plate (5). A liquid distribution plate as claimed in either of the preceding claims, wherein each reservoir cell (4) is open at its top and has a bottom wall containing said aperture. A liquid distribution plate as claimed in any one either of the preceding claims, wherein the reservoir cells (4) are of rectangular cross-section. A liquid distribution plate as claimed in Claim 4, wherein the reservoir cells (4) are of square cross-section. A liquid distribution plate as claimed in Claim 5, wherein the vapour riser passages (8) and reservoir cells (4) in the body of the liquid distribution plate are of square cross-section. A liquid distributor for a packed liquid-vapour contact column comprising a header tank (1); a liquid distribution plate (5) as defined in Claim 1; and conduit means (2,3,7) for feeding liquid from the header tank (1) into each cell (4). A liquid distributor as claimed in Claim 7, wherein the liquid distributor plate (5) is as defined in any one of Claims 2 to 6. A liquid distributor as claimed in Claim 7 or Claim 8, wherein the conduit means comprises at least two sections (3, 7) each feeding a discrete group of reservoir cells (4) from a location of the header tank (1) at an elevation spaced from that of the other section(s).

Inventors: ARMSTRONG PHILLIP ANDREW, KALBASSI MOHAMMAD ALI, MILLER DEREK, ARMSTRONG, PHILLIP ANDREW, KALBASSI, MOHAMMAD ALI, MILLER, DEREK
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1999
Main classification: B01D3
IPC codes: B01J4, B01D3