CA2225293A1 - Structured packing for an exchange column - Google Patents

Abstract

This invention relates to a packing section for a mass transfer or heat transfer column consisting of a plurality of fixed helical screw flutes which allow the liquid to flow down the flutes, and the vapor to flow up the spaces between the flutes. In addition, the core of the fluting consists of a tubular conduit through which a heating or cooling medium can be passed, thus allowing heat to be added or withdrawn from the flutes. The surface of the flutes may consist of corrugations, grooves, apertures, slits, or wire screen to increase the surface area of contact between the flowing streams.

Description

CA 0222~293 1997-12-19 TFM File No. 187.1 TITT F: STRUCTURED PACKING FOR AN EXC~ANGE COLUMN

s FTFT n OF TT-TF I~VF~TION
The present invention relates to heat exch~nge columns in general, and in particular to a packing element or packing section for a mass transfer column or heat exchange column that allows the addition or extraction of heat.

10 F~A(~KGROUNT) OF T~TF TNVFNTION
Mass transfer and heat transfer columns are commonly used for rectification, till~tion, dephlegmation, gas absorption, liquid stripping ,heat recovery, and other gas-liquid and liquid-liquid interactions. Situations exist in which the heat produced or consumed by the fluids in contact will cause the temperature within the column to increase 15 or decrease, thus inhibiting the desired effectiveness of the heat or mass transfer operation.
For example, if it is desired to saturate warm dry air with water vapor by counter-current contact of air and liquid water in a mass transfer column, the evaporation of water into the air stream will cause the air to be cooled, at which lowered temperature theequilibrium water content of the air is less than if the air was still at its original 20 temperature. Another example would be in the absorption of propane and butane from a natural gas stream using a lean oil absorber. In this process, the absorption of propane and butane into the liquid lean oil causes the absorber temperatures to increase, which reduces the amount propane that can be absorbed into the lean oil.

As is known, there are many types and configurations of packing for mass or heattransfer columns. Random dumped p~ckings are m~nllf~ct~lred in shapes designed to distribute the fluids over a large surface area while m~ g a minimllm flowing pressure drop. Early structured packings consisted of bricks or wooden slats also 5 arranged to promote a high degree of surface wetting and high flow area. US Patents 4,296,050 and 4,643,853 teach the art of designing structured packing elements with a plurality of corrugated metal sheets and describe various surface enhanc~ment~ such as grooves and apertures that increase the effectiveness of liquid distribution over the plate surfaces. None of the foregoing methods allow for the addition or extraction of heat other o than that provided by the contact of the mixing fluids.
Methods of extracting or adding heat to the mass or heat transfer column as usedin industrial applications may consist of heat exchangers located externally to the transfer column. Reboilers to add heat to the bottom section, and condensers to remove heat from the top section are usual configurations for distillation columns. Side heaters or side 15 reboilers are used on industrial distillation columns which allow the liquid flowing down the column to be collected from the column trays, passed through an external heat exchanger and then the heated liquid is reintroduced to the ~i~till~tion column at a location of lower elevation than that at which it was extracted. The present invention would, in many cases, allow heat extraction or addition within the column itself, and thus possibly 20 elimin~te or reduce the size of reboilers, side heaters or condensers.
US Patent 3,894,133 describes the use of a refrigerant stream to extract heat from an absorber column with cross-flow liquid trays in a manner to m~int~in iso-thermal fluid contact conditions. This design has the disadvantage of low heat transfer area per unit CA 0222~293 1997-12-19 volume of the co~ ,",~nt vessel, and low fluid contact area per unit volume of the conLail~lllent vessel.
Some columns using random dumped packings contain pipe coils through which heating or cooling fluid is passed to control the temperature of the gas-liquid cont~ctinp~
5 process. This design has the disadvantage of having low heat transfer area per volume of p~cking, and having potential cavities in the packing which allow channeling of the liquid into a large stream that is poorly contacted by the vapor.
US Patent S,472,044 describes the use of a convoluted array of heat e~h~n~e tubes through which heating or cooling medium flows and the exterior of which is used as lo a contact surface for gas-liquid interaction. Although this design has high heat transfer per unit volume of the containment vessel, the fluid contact area is restricted to the area provided by the tubes themselves, and the contact area is not suitable for the purposes of many structured packing applications.
US Patent 5,174,928 teaches an assembly for cont~cting liquid and gas streams 5 inside of a double plate panel with heat transfer to or from an outside fluid. US Patent 5,596,883 teaches the use of plate-fin heat exchangers in light component stripping applications. Both of these designs have a high heat exchange area per unit of containment vessel volume, but exhibit an undesireable characteristic of flooding or liquid carryover at low fluid mass rates per unit of volume due to the small hydraulic radius of 20 the flow passages in which the contact of the downward flowing liquid and upward flowing vapor occurs.
In order to better address the above noted problems, the present invention provides an improved packing element which consists of a plurality of fixed helical or spiral screw flutes which allow the liquid to flow down the flutes, and the vapor or light CA 0222~293 1997-12-19 fluid to flow up the spaces between the flutes. The flutes are spaced and arranged such that they become what is known as a packing section for liquid-vapor contact. The core of the fluting consists of a tubular conduit through which a heating or cooling medium can be passed, thus allowing heat to be added or withdrawn from the flutes, thereby affecting 5 the temperature of the packing in an advantageous manner, or enabling the recovery of heat or cooling.

SU~/IMA~Y OF THF INVENTION
It is an object of the invention to provide structured packing for a mass or heat 0 transfer column that allows addition or extraction of heat as described herein, and results in low pressure drops, large extended heat transfer area, and intim~te contact between the process fluids.
A plurality of central tubes are used to convey cooling or heating medium.
Around the periphery of each tube is one or more helically wound fins or flutes, preferably 15 but not restricted to a helix angle of 45 degrees. The surface of the flute contains surface enhancements or irregularities such as corrugations and apertures to enhance the intim~te cont~r,ting of the fluids, typically of liquid flowing down the packing and a vapor flowing upwards, although a heavy liquid flowing downward and a light liquid flowing upward would also be a favorable use of the invention. Multiple fluted tubes are arranged in an 20 array such that the flutes attached to one tube overlap the flutes of the adjacent tubes.
Liquids droplets which fall off the edge of the helix will then descend upon the flute of the a~jacrnt tube and continue down and around the adjacent helix or across the corrugated surface in a tortuous path. Gas flowing upward flows up the corkscrew shaped passage between upper and lower flutes until it meets the edge of the flute of an adjacent tube CA 0222~293 1997-12-19 which forces the stream to divide and continue its generally upward saw-tooth shaped path, continuously being forced against liquid droplets falling off the edges of ~cljacent flutes or flowing down the helix corrugations. The tortuous flow paths and dividing and re-mixing provide intim~te contact between the fluids that is required for efficient mass 5 transfer operation.
The number of flutes or fins per unit of tube length can be varied dependent on the extended surface area required to add or extract the heat from the process fluids. The distance that the fins overlap can also be varied such that they can overlap to the point where the flute is in contact with the wall of the adjacent tube, however less overlap of the 0 fins is readily accomplished and desirable in the multiple flute case, by notching the flutes with a slot having the required overlap prior to assembling the tubes into a multiple tube unit. In those cases where the vapor flow pressure drop is a concern, the notch where the fins overlap can be made wider to allow the vapor more area for flow.
The corrugations, perforations, or other surface enhancement of the helix impede5 the formation of liquid streams that would form on a smooth helix, scattering them into droplets or ret~ining the liquid in the corrugations to increase the surface area of the liquid. Other surface enhancements such as axial slits, perforations, screens, or surface texture that would enhance the capillary effects of the liquid are also advantageous configurations.
The ends of the tubes through which the heating or cooling me~illm passes can beinstalled into a tube-sheet as is common in shell and tube exchangers, or they may be connected together with return bends or U-tubes as is common in air-conditioningapplications, or they may be otherwise connected by pipe headers in parallel or series flow, or other coll,bina~ions advantageous to the skilled heat transfer designer, dependent CA 0222~293 1997-12-19 on the available or required heating or cooling medium flow rate or properties. The heating or cooling medium is transferred to the exterior of the vessel that houses the packing section, where heat is added, extracted, re-circulated or recovered as required by the process configuration.

P~RTFF nF!~Cl~TPTION OF THE FIGURES
Embodiments of the invention will now be described, by way of example only, withreference to the accompanying figures, wherein:
Figure 1 shows an assembly of one embodiment of a packing section cont~ining an 0 assembly of single flute helix tubes;
Figure la is a close-up view of a portion of a flute showing a corrugated surface;
Figure 2 is a side view of part of an assembly of double flute helix tubes according to another embodiment of the invention;
Figure 3 shows the packing section of fig. 1 with two possible methods of sealing 15 the packing section against the inner wall of the containment vessel in which the packing section is installed; and, Figures 4a to 4g show the helical flutes of the present invention with seven alternate versions of flute surface enhancements to improve contact between fluids.

20 nET~TT Fn DFSCRIPTION OF THE INVENTION
Figure 1 shows a packing assembly, generally indicated by reference numeral 10, according to one embodiment of the present invention. It consists of a plurality of tubes 11 typically arranged to stand vertically. The diameter and length of each tube 11 may be any practical size but is plerelelllially 0.25 inches to 2 inches (6.4 mm to 50.8 mm) in CA 0222~293 1997-12-19 outside diameter with a length of 1 foot to 40 feet (.3 m to 12.2 m). Around each tube 11 is affixed one or more fins or flutes 12 wrapped spirally, or helically, thereabout. The method of affixation may be by welding, peening, heat shrink, friction fit, tube expansion, adhesive, brazing or other suitable bonding method. The thickness of the fin or flute 12 is 5 plerelelllially but not limited to the range of 0.020 inches to 0.125 inches (.51 mm to 3.18 mm). The flute may be affixed to the tube at any helix angle from 15 degrees to 75 degrees but pl~rerelllially at 45 degrees. The flute may be affixed radially to the tube at right angles plus or minus 40 degrees to the axis of the tube but pr~rel~ ially at a right angle or slightly deviated dowllw~~dly from a right angle to better accommodate the flow o of fluid and the angle of corrugations on the flutes. By way of example in the fig.1 embodiment the flute height is equal to the outside diameter of the corresponding tube, the helix angle at the tube's outer wall is 45 degrees, and the overlap ratio of ~djacent flutes is 1.0 (i.e. the amount one flute extends over an adjacent flute, expressed as a ratio of the flute's radial width).
The surface of each flute 12 is pl~relenLially corrugated with angular or sinusoidal corrugations or steps 13 which generally extend radially from the flute/tube interface to the outer edge of the flute. Each corrugation has a corrugation height and spacing of the same order as the width of the liquid stream that is expected to be dispersed, or the droplet size of that stream, typically in the range of 1/64 inch to 3/16 inch (.4 mm to 4.8 20 mm) in height and 1/32 to 3/8 inch (.8 mm to 9.5 mm) in circull~erelllial spacing to a maximum step height of one fin spacing. To increase the surface area of the liquid, the surface of the flute may contain serrations, apertures, slits, grooves, or be surfaced with mesh, screen, or other surface texturing in lieu of, or in addition to, the corrugations, as discussed in greater detail below.

CA 0222~293 1997-12-19 Figure 2 shows another embodiment where each tube 11 carries a double flute helix configuration indicated by 14a and 14b. The radial dimension or width "f" of each flute from the tube 11 to the edge ofthe flute may be any practical size but is pl~rerenlially 0.25 tube diameters to 8 tube diameters. The axial spacing "s" of the flutes longi~l(lin~lly 5 along each tube may be from 0.1 inches to 6 inches (2.5 mm to 152.4 mm) but ispr~ ially of the same order as the height of the fin 13 or less. The helix angle of the flute is shown as the angle "delta" in FIG. 2. To enhance the draining of liquid from one flute to the flute of an ~dj~cçnt tube, the -flute ~tt~rhment angle to the tube "alpha" in FIG.

2 can be plus or minus 40 degrees from a right angle ~tt~chment, and preferably in most o applications has a slightly downward slope. By way of example, in the fig.2 embodiment the flute width "f" is equal to the outside diameter "d" of the tube. The helix angle "delta"
is 45 degrees, and the overlap ratio of the flutes is 0.5. The fins of FIG. 2 are shown with a small downward slope, angle "alpha", which would allow liquid condensed at the tube to drain radially to the outer edge of the flute.
Notches in the flutes for assembly and tube spacing purposes are shown in FIG. 2by reference numeral 15. The depth of the notches is such that the amount of overlap of the flutes can be fixed and the assembly of many fluted tubes into one unit is made easier by laying the notches of successive fluted tubes onto the notches of the previous row. The notches may be rect~n~ r, tri~n~ r, circular or other shape, and may be either cut out 20 or bent out of the flute material. For the sixty degree tri~ng~ r centerline spacing, where the centerline spacing is the distance between the tubes' longitutlin~l axes shown as the distance "p" in FIG. 2, the notches on an individual fluted tube would consist of six straight rows, 60 degrees apart, for the full axial length of the fluted tube. The notches may be full overlap depth on alternating fluted tubes instead of one half of the overlap CA 0222~293 1997-12-19 depth on each tube or combinations of full, half, or no flute overlap depth that f~.ilitate consistent assembly. The notches are not required to be circulllrelel~lially symmetric.
Other configurations such as a 90 degree square centerline spacing between the tubes would require a di~erenl notch configuration. In the fig.2 example the notches in the 5 flutes are cut to a radial depth of 0.25 times the flute's radial width "f" and 60 degrees apart.
Figure 3 shows two methods of sealing the outside periphery of the packing assembly or section 10 to prevent the upward or downward flowing fluids from bypassing the packing assembly in the gap between the packing and the interior wall of the10 containment vessel in which the packing section is installed. The sealing can be accomplished by several methods including open cell or closed cell foam material 30, either flexible or rigid, cut to the inside diameter of the vessel and resting against the flute outside diameter, or layers of material such as sheet metal 31 cut to the inside diameter of the vessel and fitting against the tube outside diameter which mimic the effect of the flutes 5 on the next row of adjacent tubes which do not exist due to the location of the vessel wall.
Preferably, the flutes can be merely bent vertically upwards or dowllwa~ds thus redllcin~
the outside tli~met~r of the packing section until a friction fit to the inside diameter of the con~ail~ ent vessel is achieved. This forces the upward and downw~d flowing fluids at the coll~ahllllent vessel wall into the packing section passages. One skilled in the art of 20 containment vessel design and/or heat exchanger design will recognize a number of acceptable means that could be used such as sealing strips or continuous edge gaskets by which the flow of process fluids along the outer perimeter of the pacl~ing section can be ~ prevented, of which FIG. 3 shows only two possible methods.

CA 0222~293 1997-12-19 Figures 4a to 4h show various flute surface enhancements to promote fluid interaction. Fig. 4a shows an angular flute corrugation where the corrugation surfaces 16 are generally flat and meet along distinct lines to form distinct troughs 18 and "knife edge"
ridges or peaks 17 for instance, and fig.4b shows a sinusoidal corrugation similar to that of 5 fig.2 where the surfaces are curved to form smooth, continuous interfaces. In the fig.4a and 4b embodiments the troughs and peaks may all have the same radial slope "alpha"
relative to the tube; or a reversing slope may be provided, namely the slope may alternate between s~lcces~ive troughs, for example, so that fluid in one trough is directed toward the tube and in an ~djac~nt trough away from the tube. A further variation is shown in fig.4c 0 where an intlPnt~tion or channel 19 in each flute surface 16 to direct liquid toward the tube and then toward the flute's outer edge one or more times. A portion 20 of the flute may be cut out adjacP.nt to a lower end of each channel 19 to allow liquid to drain from the channel onto another flute surface. Yet another variation in fig.4d is to provide the flute with a pattern of upwardly ext~n(ling dimples 21 so that successive dimples block or interfere with fluid flow so as to divide the flow path of the liquid thereon. If desired, one may combine the dimples 21 in fig.4d with the angular corrugations of fig. 4a, or the sinusoidal corrugations of fig.4b may also be employed. Figure 4e shows a flute with spaced radial grooves 22 and interspersed holes 23. Figure 4f shows both wide and narrow slits 24 and 25, respectively, on the flute surface wherein the narrow slits 25 serve 20 to increase the capillary action of the liquid and the wider slits 24 serve to increase the flow rate of the vapor. Finally, fig.4g & 4h show side and plan views, respectively, of another flute design with relatively large surface lln~ tions 26 and edge notches 27 that may be suitable for large liquid flow volumes at low gas rates, or which may be suitable for assembly of the tubes into a relatively large structured packing section.

The above description is int~n(~ed in an illustrative rather than a restrictive sense, and variations to the specific configurations described may be apparent to skilled persons in adapting the present invention to specific applications. Such variations are inten~led to form part of the present invention insofar as they are within the spirit and scope of the 5 claims below.

Claims (20)

We claim:

1. A packing section for a mass and heat exchange column having opposed first and second ends comprising:
a plurality of elongate core members disposed in a spaced, parallel relation;
a flute element extending helically about each of said core members, said flute elements of adjacent core members overlapping one another to form a plurality of tortuous paths for intermixing first and second countercurrently moving fluids, said first fluid flowing by gravity along said flute elements from said first end of the column, and said second fluid rising from said second end of the column through said paths and between said flutes.

2. The packing section of claim 1 wherein at least some of said core members are of a hollow tubular construction for passing a heat transfer medium therethrough to add or remove heat from said intermixing first and second fluids, thereby transferring heat into or out of said column.

3 The packing section of claim 1 wherein said flute element forms a first flute, and a second flute extends helically about each of said core members and in parallel relation to said first flute to define a double helix configuration.

4. The packing section of claim 2 wherein at least some of said flute elements have a plurality of surface irregularities to enhance said intermixing of the first and second fluids, said irregularities comprising spaced corrugations extending radially from said core members across said flute elements.

5. The packing section of claim 4 wherein said corrugations form troughs having a height and spacing of the same order as the width of the droplet size of said first liquid.

6. The packing section of claim 5 wherein said troughs are sloped in at least one of radially downwardly away from said core member to urge said first liquid away from said core member toward an outer edge of the flute element, and radially upwardly from said core member to said outer edge of the flute element to urge said first liquid toward said core member.

7. The packing section of claim 4 wherein said irregularities further include at least one of channels, dimples, apertures, slits and surface undulations spaced along the length of said flute elements.

8. In combination with a mass and heat exchange column having a longitudinal axis, an upper portion for introducing a first fluid which flows downwardly by gravitytherethrough, and a lower portion for introducing a second fluid which flows upwardly therethrough, at least one packing section for said column comprising:
a plurality of spaced, elongate tubular conduits disposed in a generally parallel relation to said axis for passing a heat transfer medium therethrough to add or remove heat into or out of said column;

a flute wrapped helically about each of said conduits and defining a plurality of tortuous paths with the flutes of adjacent conduits for intermixing said first and second fluids and effecting a distribution of said first and second fluids across said packing section.

9. The combination of claim 8 wherein said flutes have a plurality of notches to allow the flutes of adjacent conduits to overlap, wherein the extent of said overlap alters said tortuous paths.

10. The combination of claim 9 wherein said overlap is up to one radial width of said helical flute.

11. The combination of claim 9 wherein said notches of one flute form gaps with adjacent flutes to allow said first and second liquids to pass therethrough to another part of said tortuous paths, thereby alleviating pressure drop in said second fluid flow.

12. The combination of claim 8 wherein the helical angle of said flutes is between 15 and 75 degrees.

13. The combination of claim 8 wherein said flute extends radially from said conduit and is sloped downwardly at a first angle to said longitudinal axis to urge said first liquid away from said conduit.

14. The combination of claim 13 wherein said first angle is between 50 and 130degrees.

15. In a mass and heat exchange column having a first portion for introducing a first fluid which flows by gravity therethrough and. a second portion for introducing a second fluid which rises therethrough, the improvement consisting of at least one packing section within said column comprising:
a plurality of elongate tubular conduits extending through said column in a spaced, parallel relation, said conduits passing a heat exchange medium therethrough to transfer heat into or out of said column;
a plate-like flute element extending helically about each of said conduits and overlapping with like flute elements of adjacent conduits to form a plurality of tortuous paths for intermixing said first and second fluids and effecting a distribution of said first and second fluids across said at least one packing section.

16. The column of claim 15 wherein each of said flute elements has opposed radially inner and outer edges wherein said inner edge is connected to said conduit, and a plurality of surface enhancements for promoting said intermixing and distribution of the first and second fluids.

17. The column of claim 16 wherein said surface enhancements include cut out portions spaced along said outer edge.

18. The column of claim 16 wherein said surface enhancements include snaking channels upon said flute element to direct said first liquid toward said tubular conduit to promote heat exchange with said heat exchange medium and then to direct said first liquid toward said outer edge of the flute element.

19. The column of claim 17 wherein said surface enhancements include snaking channels upon said flute element to direct said first liquid toward said tubular conduit to promote heat exchange with said heat exchange medium and then to direct said first liquid toward said outer edge of the flute element.

20. The column of claim 17 wherein said surface enhancements include at least one of a plurality of upwardly extending dimples to disperse fluid flow along said flute element, a plurality of interspersed apertures to allow said first and second fluids to travel through portions of said flute element, and a combination of narrow slits for increasing the capillary action of said first fluid and wide slits for increasing the flow rate of said second fluid.