US2692128A - Fractionating tray - Google Patents

Description

Oct. 19, 1954 v Q BOWLES FRACTIONATING TRAY 3 Sheets-Sheet 1 Filed Sept. 26 1950 i WH o l a l] I] W" mm INVENTOR- irrmrz fl flaw/es BY W 24 flGE/VT 0A ATTORNEY Oct. 19, 1954 v. o. B OWLES FRACTIONATING TRAY 3 Sheets-Sheet 2 Filed Sept. 26, 1950 INVENTOR Verizon 0 jaw/es ,%Mm@4 /l6E/ 7' 0/? ATTORNEY Oct. 19, 1954 V. O. BOW LE 5 FRACTIONATING TRAY 3 Sheets-Sheet 3 Filed Sept. 26, 1950 INVENTOR Vzrnan 030M449 fli/vr 0R ATTORNEY Patented Oct. 19, 1954 FRACTIONATING TRAY Vernon 0. Bowles, Rye, N. Y., assignor to Socony- Vacuum Oil Company, Incorporated, a corporation of New York Application September 26, 1950, Serial No. 186,860

6 Claims. I

This invention relates to 'fractionating towers and, more particularly, to the trays used therein in which a gas passes through a liquid.

Fractionating towers of the type conventionally employed in the refining industries for fractionating hydrocarbon liquids and the like have heretofore generally been constructed by mounting a plurality of rigid horizontal. plates at various elevations within a cylindrical tank. These horizontal plates are ordinarily pierced by a plurality of orifices over which are placed bubble caps so that-vapors rising in the. tower pass'up through the orifices and bubble down into the liquid on the plate. The downwardly flowing liquid passes from plate to plate by means of downcomers to afford contact between vapor and liquid at a number of various levels as determined by the relative position of the horizontal plates or bubble trays;

The trays of the bubble cap type of tower are usually constructed of metal and supported by annular flanges spaced along the outer edge of the trays and secured to the inside wall of the tank. To afford the necessary rigidity and strength, the trays must be constructed of relatively thick metal plate and must frequently have supporting beams to avoid deflection of the plate. As heretofore recognized, the maintenance; and particularly the cleaning of heavy bubble cap type trays is cumbersome. It. is, for example, well known that during fractionation of: petroleum hydrocarbons, a gummy or carbonaceous deposition takes placeon the plates due to polymerization, coking and the. action of sulfur. The cleaning of heavy bubblev cap plates made up of numerous bubble caps, risers, and associated fastenings in such instances is a tedious operation, accomplished only at the expenditure of considerable time and labor.

Furthermore, while the trays are preferably constructed of a corrosion-resistant metal, such as stainless steel, the cost of fabricating atray of such material having sufiicient thickness, together with supporting beams, to provide the necessary rigidity and strength has heretofore been an expensive proposition. Common practice, accordingly, has been to construct the'trays of a less expensive material such as cast iron or steel. A tray fabricated from such metals a1- fords the necessary thickness in construction at a comparativelyreasonable cost, but such trays are heavy and cumbersome and are unsatisfactory when liquid and vapors coming in contact therewith are of a corrosive nature. Corrosion of the trays necessitates frequent replacement or repair thereof, which, as pointed out herein- :a-bove, is not only'an expensive-proposition but a laborious and time-consuming operation due to the complexity of design and to the weight of. the heavy metal parts ordinarily used in fabrication.

With the use of ordinary bubble cap trays, there'is, furthermore, the disadvantage of: rising vaporsfrequently passing onlythrougha portion 7 of the: liquid on a given plate This .inefficient operation is: caused by the difference in liquid level 011117118 plate, since the depth of liquid is greatest at the point at which liquidis admitted to the: plate and least: at the point where liquid leaves the plate;vv Risingvapor coming. in contact withzsuch' liquid layer tends to seek out the spot of leastresistance and fiowsthrough the liquid at the point of minimum depth. Accordingly, often awconsiderable portion of each. tray is rendered useless for its intended purpose, since no contact between. liquid and vapor takes place thereon.

It is an object of this invention to overcome the. aforesaid disadvantages in fractionating tray construction and to provide a fractionating tower wherein. a corrosion-resistant metal can be employed in .relatively thin sections for fabrication of'thetray.

A further object is the provision of a' fractionating tray wherein the functional members making up said tray serve as light weight structural beams extending transversely across the tank, thereby permitting construction of a comparatlvely' light tray which is easy to fabricate; assemble,.and. maintain in operation.

A still further object of this invention is to provide a fracti'onating tray of sections having inherent structural strength and simple design so that both: the cost of fabrication in the shop and installation in the field is minimized, and therefore to afford a' tray which can be installed at lesscost than other types of fractionatingtrays presently in use.

A very important object of this invention is the provision of av fractionating tray in which a highly e'fifl'cient contact between counter-flowing liquid andvapor is attained as the result of. the design which provides equal slot submergence under all conditions of flow.

A fractionating tray construction designed to fulfill the aforesaid objects is hereinafter described, together with other features. of the invention.

The invention will be more readily understood by reference to the accompanying drawings wherein:

Figure 1 is' an elevational view, partly in sec- 3 tion, of a portion of a fractionating tower having trays constructed in accordance with the invention.

Figure 2 is a plan view of a fractionating tray embodying one form of the invention.

Figure 3 is an isometric view illustrating in detail the nature of the end plates.

Figure 4 is an isometric View of a fractionating tray fabricated in accordance with the invention.

Figure 5 is an enlarged view in section of the members which make up the fractionating tray.

Figure 6 is a detailed side elevational view showing the arrangement of slots in the fractionating members.

Figure 7 is a plan view of a portion of a fractionating tower similar to Figure 2 but illustrating an alternate end arrangement of the trays.

Figure 8 is a plan view of a portion of a fractionating tower similar to Figure 2 but illustrating another alternate end arrangement of the trays.

Figure 9 is an isometric view of a preferred embodiment of the invention designed to reduce pressure drop due to vapor flow through the fractionating sections of the tower.

Referring more particularly to the drawings in which like reference characters denote like parts, i designates a cylindrical tank which forms the shell of the fractionating tower. Since the tank may be of any desired height and diameter, only a portion thereof is shown. A plurality of trays is mounted at various elevations throughout the tank, the number of said trays depending upon the requirements of the fractionation to be performed. While only two trays have been shown in Figure 1, it will be understood that additional trays may be positioned above and below the particular set of trays illustrated. A plurality of support rings 2 is welded or otherwise secured to the inside wall of tank I. Resting upon and affixed to each support ring are the members which make up the fractionating tray. Adjacent to the low point of each tray are downcomers 3 through which liquid passes. Each downcomer is sealed by the level of liquid on the tray below, thus preventing the upward passage of vapors therethrough.

The invention, as set forth hereinabove, is particularly directed to the construction and mounting of the fractionating trays. The trays, as will be seen, are composed of a series of troughs 4 extending transversely across the tank and positioned in a plurality of difierent planes as distinguished from a tray entirely in a single horizontal plane. The troughs are each fitted with end closure plates l5, as shown in detail by Figure 3 and Figure 5. Each of the troughs is secured to and variously elevated above support ring 2 in stepwise fashion by bolts 5 held in position by nuts 5 and by end closure plates [5. The bolts and end closure plates, as shown, are of varying from about 5 to about of the total tower I cross-section. A related advantage of the present tray construction is the high percentage of riser area attained; that is, the cross-sectional area between the troughs 6 through which the vapors pass upwardly. Existing bubble cap trays achieve riser areas ranging from 6 to 15% whereas the instantly described tray achieves a riser area of 25 to 30% or greater. Moreover, the tray construction of this invention has an equally high area corresponding to the annular area in existing bubble cap tray design. These large areas which the trays of this invention thus afiord result in low pressure drop of vapor flow through the tray, which is particularly advantageous in vacuum fractionation.

To each trough is attached an L-shaped section composed of a horizontal leg I attached to the downstream edge of the trough and a vertical leg 8, the lower edge of which is slotted or serrated. The aforementioned L-shaped section interfits in the next succeeding trough, thereby providing a passage 9 between each pair of adjacent troughs. The L-shaped section overhanging the upstream trough of highest elevation is attached to a Vertical plate Hi which in turn is attached to horizontal extension H contiguous with and affixed to support ring 2 by bolts l2. The downstream wall of the trough of lowest elevation l3 extends vertically below the horizontal level of said trough to a point sufficiently above plate I l of the next lower tray so that liquid can easily flow thereunder but sufficiently below the top of vertical plate it of said lower tray to provide an adequate vapor seal.

The downcomers are staggered on alternate trays in such manner as to cause a back and forth travel of liquid in a horizontal direction across each successive tray in addition to the vertical flow from tray to tray. The horizontal flow of liquid across each tray is caused by the hydraulic gradient between the point where liquid enters the tray and the point where it leaves the tray. The stepwise construction of the troughs is such that the elevation of each trough with respect to the one adjacent to it diilers by at least the maximum expected hydraulic gradient. With this horizontally. progressively descending arrangement of the troughs, a constant operating slot submergence is achieved and each slot or trough releases equal amounts of vapor regardless of liquid flow and vapor flow so long as the hydraulic gradient between one trough and the next does not exceed the difference in their elevation. As a practical matter, this elevation difference between adjacent troughs is subject to computation and for most conditions employing preferred aspects of this invention should bev of the order of 0.25 inches. This difference in trough elevation may, however, range from 0.05 to 1.5 inches depending on design conditions, it being understood, however, that in every instance the bottom of each succeeding trough is lower than that of the preceding one by a distance equivalent to at least the maximum expected hydraulic gradient between said troughs.

The vertical plate ll] of each tray is afllxed through horizontal extension I I to support ring 2 in such manner that it is perpendicular to the flow of liquid proceeding across each tray after the same has entered the tray through downcomer 3. Likewise, since the vertical walls of each of the troughs 4 and the overhanging vertical legs 8 of each of the L-shaped sections are parallel to vertical plate Hi, these members are also located perpendicular to the horizontal flow of liquid across the tray. The result of this perpendicular arrangement of troughs and interfitting L-shaped sections to the line of liquid flow is a rippling effect. as the liquid travels across each tray and passes downwardly through the tank. The combination of the rippling efiect of .5 the liquid and the bubbling: action of the rising vapors passing therethroughwthus affords an intimate degree of contact between vapor and liquid.

Referring to the plan viewof thefractionating tray shown. in Figure. 2, itwill be seen that the troughs. 4 and interfitting Lashaped sections made up of legs 1. and 8- :extend transversely across tank I" and are suitably stiffened. by one or more webs 14'. These plate-like metal strips extend. across the trough section of. the tray in aright-angle .direction to. the troughs and serve to stiifen the vertical) walls of the troughsection against deflection particularly 'invth'e central portion of the tray. The troughs and overhanging L-shaped sections. are fitted with. .end closure plates 15. These plates overlap: one another, as shown, and are welded or otherwise fastened to the trough and PL-shaped sections. providing a suitably tight end. closure therefor. The end plates restv on support ring 2. The small clearance between. the shell: of tank I and. the end closure plates may, if desired, be filled with packing, although .the use of packing is not considered essential.

The. arrangement: of end closure plates is further illustrated in detail. by the isometric view of Figure 3.. Each plate is provided with a slot l6 which 'interfitsuover the verticalitrough edgev located at the junctureof thenext lower end plate and, the upstream wall. .of troughv 4;. This arrangement is shown in Figure. 3. for one of the troughs,. said: slot :being designated by numeral I]. This interlocking. arrangement of end plates eliminates the. need. for: additional bolts or other fastening. means and affords. easy. means for removing and inserting the trays during cleaning or repair. The endarrangement. of the closure plates may be substantially circular as shown in Figure2 or may alternatelybe arranged in polygonaloutline as shown. by Figures 7 and 8.

An. isometric view of a fractionating tray of the type described herein .is illustratedby Figure 4. As. will be seen, the verticalleg 81 of the interfitting. L-shaped sectionis serrated with the serrations l8 pointing. downward and extending into the adjacent downstream: trough to provide an effectivebubbling meansfor passing rising vapor through liquid flowing. perpendicularly across the trough. Instead of. being serrated, vertical leg 8 'may alternately be pierced. by a plurality of triangular-shaped slots 19, as shown in Figure 6.

The passage 9 between each pair of adjacent troughs and the vflow of vapor therethrough is shown in detail :by-Figure 5. It will be seen that this passage. proceeds upwardly between the downstream wall 20: of one trough and the upstream wall 21 of an. adjacent trough, over the edge of the wall 2:1. and downwardly between'said wall and the overhanging vertical leg 8; Thus, during operation vapors from the tray below pass up through the passage 9 between two adjacent troughs, over the. upstream edge of the downstream trough and then through the slotted or serrated edge-of leg 8 where bubbles are released into the liquid on this trough. The path of the vapors is indicated by the dotted line in Figure 5.

A preferred embodiment of the trough arrangement of thisinvention is shown by the improvement illustrated in Figure 9. Referring to this drawing, it, willbe seen that the upstream edge of trough 4' is provided with a backwardly curved extension 22 whichmay be considered to be a continuation of the upstream wall of the trough suitably curved and spot welded orotherwise afiixed to the outer upstream wall of the trough at 23. The curved back edge of the trough so formed facilitates vapor flow through the fractionating sections of the tower by providing: a streamlined flow for the vapors travelling. upward through passages 9. The line of now of said vapors is indicated by arrows in FigureJQ. In addition to facilitating vapor flow and reducing pressure drop of vapors flowing through the tray, the aforementioned streamlined curved extension stiifens the edge of the trough making it mechanically more rigid and imparting greater structural strength thereto.

Construction and mounting of fractionating trays in accordance with this invention. has proved advantageous in that each of the members making up the tray serves as structural beams extending transversely across. the tank: so that it thereby becomes possible to constructthe trays of a light, corrosive-resistant.metal at lower cost than other constructions employing heavier metal. The described arrangement of a. series of troughs and interfitting L.-shape'd sections forming a bubbling device which in itself is.a structural member of the tray is further advantageous from an operating point of View. For example, it has been found. that with the fractionating tray construction of this invention, a tray can be designed for inch deflection. a 9-foot span using 16 gauge stainless steel. This advantage resulting in reduced tray weight contributes to reducing its initial cost to as much as 50 per cent of that of fractionating trays currently being employed. The fractionating tray herein described composed of a number of sections, which vary in dimension only lengthwise, can be made from one set of dies. Thus, a minimum of initial fabrication work is required. Moreover, as a result of the simplicity and compactness of the design, considerably less installation and maintenance labor is required.

While there has been described various preferred embodiments of the present invention, it will be understood that the invention is not limited in scope to the specific details of construction illustrated and described but that the same is subject to considerable variation without departing from the underlying features of. the invention as expressed in the following claims.

I claim:

1. In a fractionating tower comprisingan elongated tank, a plurality of support rings afiixed at various levels to the inner periphery of said tank and a plurality of elements for effecting fractionation supported on said rings, the improvement wherein said elements comprise a series of trough-like members spaced at uniform intervals transversely across said tank in a direction substantially perpendicular to the line of liquid flow and arranged in stepwise fashion in a plurality of different planes so that the bottom of. each succeeding member is lower than that of the preceding one by a distance equivalent to the maximum expected hydraulic gradient, each of said members comprising a trough having attached thereto an L-shaped section, a horizontal leg of said section being afiixed to the downstream edge of said trough and a vertical leg of said section being serrated, pointing downward and overhanging into the trough of an adjacent downstream member, each of the members-makingup said series having plates affixed to and enclosing either end thereof, which'plates are slotted sov as to interfit over the vertical trough edge of 7 the adjacent member, thereby providing interlocking assembly of said trough-like members and tight end closure therefor.

2. A lightweight tray for use in a fractionating tower comprising a plurality of trough-like members which serve as structural beams extending transversely across the tower in stepwise fashion in a plurality of different planes so that the bottom of each succeeding member is lower than that of the preceding one by a distance equivalent to the maximum expected hydraulic gradient, each of said members comprising a trough having attached thereto an L-shaped section, a hori zontal leg of said section being affixed to the downstream edge of said trough and a vertical leg of said section being serrated, pointing downward and overhanging into the trough of an adjacent downstream member; a plurality of slotted plates and a plurality of stiifening webs, the ends of each of said members being enclosed by said plates, which plates have slots therein designed to interfit over the vertical walls of adjoining members, said walls in turn being strengthened against deflection by said stiffening webs which extend across the troughs of said members in a right angle direction thereto to afford a resulting rigid tray composed of stiffened interlocking troughs.

3. A lightweight tray for use in a fractionating tower, comprising a plurality of troughs spaced at uniform intervals transversely across the tower and arranged in stepwise fashion in a plurality of difierent planes so that the bottom of each succeeding member is lower than that of the preceding one by a distance equivalent to the maximum expected hydraulic gradient, a plurality of L-shaped sections having the horizontal legs thereof attached to one edge of each of the aforementioned troughs, the vertical legs of said sections being serrated and pointing downward and overhanging into adjacent troughs, thereby affording passages for the upward flow of vapor therethrough, a plurality of end closure plates aihxed to either end of the troughs and overhanging sections, each of said plates being slotted and the slots therein engaging the vertical wall of the next adjoining trough, thereby affording interlocking assembly of said troughs and tight end closure therefor,

4. In combination with a tower, a fractionating tray comprising a plurality of troughs spaced at uniform intervals transversely across the tower and arranged in stepwise fashion in a plurality of different planes so that the bottom of each succeeding trough is lower than that of the preceding one by a distance equivalent to the maximum expected hydraulic gradient, a plurality of L-shaped sections having the horizontal legs thereof attached to the downstream edges of each of the aforementioned troughs, the vertical legs of said sections being serrated and pointing downward and overhanging into adjacent troughs, thereby affording passages for the upward flow of vapor therethrough, a plurality of end closure plates perpendicularly affixed to either end of the troughs and overhanging sections, each of said end closure plates overlapping one another and having a recessed slot extending vertically from the lower edge thereof at a point corresponding to the juncture of the upstream wall of the adjacent downstream trough and the end closure plate aflixed to said adjacent trough, the slotted edge in each of said plates engaging the upstream wall of the adjacent downstream trough to provide interlocking assembly of the troughs with attached overhanging sections and tight end closure therefor.

5. A fractionating tray comprising a support ring, a plurality of troughs afiixed to and variously elevated above said support ring in stepwise fashion in a plurality of different planes so that each trough during operation releases substantially equivalent amounts of vapor, a plurality of interfitting L-shaped sections, the horizontal legs of which are attached to the downstream edges of eachof said troughs and the vertical legs of which are pierced by a plurality of openings, said vertical legs pointing downward and overhanging into downstream adjacent troughs, a plurality of end closure plates aflixed to either end of said troughs and overhanging sections, each of said end closure plates being providedwith a slot designed to interfit over the vertical trough edge of the adjacent downstream trough at the juncture of the upstream wall of said trough and the endplate to which said trough is affixed, said troughs, together with the L-shaped sections attached thereto, being positioned on said support ring and held in place thereon by said end closure plates in a direction substantially perpendicular to the line of liquid flow.

6. A fractionating tower including a tank, a support ring afiixed to the interior of said tank, a plurality of troughs affixed to and variously elevated above said support ring in stepwise fashion in a plurality of different planes so that each trough during operation releases equivalent amounts of vapor, a plurality of L-shaped sections having the horizontal legs thereof attached to the downstream edges of each of said troughs and the vertical legs thereof pierced by a plurality of openings and pointing downwards overhanging into adjacent downstream troughs to provide passages for upward vapor flow, a plurality of end closure plates affixed to either end of said troughs and overhanging sections, each of said end closure plates being slotted at the juncture of the next lower end plate and the upstream wall of the trough to which said lower end plate is afiixed, thereby affording means for interlocking assembly of the stepwise arranged troughs and overhanging sections by engagement of the slot on the end plate affixed to the adjacent upstream trough and overhanging section with the upstream wall of the next lower trough, a plurality of curvediextensions attached to the upstream edges of each of the aforementioned troughs to facilitate said vapor fiow, means for conducting liquid downwardly and horizontally through said tank in a direction perpendicular to the stepwise arranged troughs and overhanging sections to afford a flow of liquid across each tray and to thereby effect intimate contact between upwardly flowing vapor and downwardly flowing liquid.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 829,700 Drees Aug. 28, 1906 1,605,263 Millard Nov. 2, 1926 1,822,323 Stover et al Sept. 8, 1931 2,510,590 Kraft June 6, 1950 FOREIGN PATENTS Number Country Date 839,695 France Jan. '7, 1939

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