US2772081A

US2772081A - Vapor-liquid contacting apparatus

Info

Publication number: US2772081A
Application number: US214076A
Authority: US
Inventors: Henry J Hibshman; James J Trexel
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1951-03-06
Filing date: 1951-03-06
Publication date: 1956-11-27
Anticipated expiration: 1973-11-27
Also published as: NL164663C

Description

yNOV- 27; 1956 H. J. HlBsHMAN ErAl. 2,772,081

A-VFORMLJQUID CONTACTING APPRATUS Filed March 6, 1951 3 Sheets-Sheet l l nu mi mnu my up) A i4 Z Gumbofr-mecsA NOV' 27, 1956 H. J. HIBSHMAN Erm. 2,772,081

VAPOR-LIQUID CONTACTING APPARATUS Filed MarchG, 1951v 3 Sheets-Sheet` 2 CLttonnevz Nov. 27, 1956 H. J. HlBsHMAN rAx. 2,772,081

VAPOR-LIQUID CONTACTING APPARATUS Y 23% CLbor'net United States Patent VAPOR-LIQUID CONTACTIN G APPARATUS Henry J. Hibshman, Plainfield, and James J. Trexel,

Roselle, N. J., assignors to Esso Research and Engineering Company, a corporation of Delaware Application March 6, 1951, Serial No. 214,076

1 Claim. (Cl. 251-114) The present invention is concerned with an improved apparatus for operating a countercurrent-vapor-liquid treating zone. The invention is more particularly concerned with an improved fractionation zone and is especially directed to apparatus for contacting upflowing vapor and downilowing liquid utilizing contacting trays or their equivalent. ln accordance with the present invention, the capacity of a tray and the entire treating zone is markedly increased by providing apparatus for efficiently and readily contacting the countercurrent owing phases. In accordance with the present invention, a directional vapor stream is produced on the respective trays which facilitates the passage of the downflowing liquid across the tray, thus reducing liquid holdup on the tray.

lt is well known in the art to carry out many chemical reactions and separations wherein vapor and liquid are contacted in a countercurrent manner, such as in a hydrocarbon fractionation zone. Normally the liquid passes from one zone to a lower zone by means of downcomers or their equivalent while the vapors pass upwardly from zone to zone through chimneys in the tray, around various types of bell caps into the liquid phase disposed on the top of the tray. The liquid phase passes from zone to zone over weirs on the respective trays into downcomers and passes onto the tray in the zone below. The height of the liquid phase on the tray is determined by the height of the Weir. The downcomer from the zone above must of necessity extend below the top of the liquid phase on the tray.

In liquid-gas contacting operations of this character, the capacity of the tray to a large extent is determined by the degree to which the downiiowing liquid can be moved across the tray.

Aside from limitations of auxiliary equipment such as furnaces, feed pumps, and condensers, the capacity of a fractionation tower may be limited by any of several factors. Basically, these are limitations to passage of liquid down and vapor up the tower in such a manner that eliicient contacting is achieved.

The requirement of eicient contacting means that the limitation may be one of too rapid or free passage of'one or more of the phases through the tower, as well as restrictions to ow of the phases. Tray dumping, liquid running down through bubble cap chimneys, is an example of too free ow of liquid. Downcomer filling with backup'of `liquid on the tray is the opposite type of limitation, resulting in poor eiciency because of excessive entrainrnent and ultimately in tower iiooding. A similar highentrainment effect is produced by excessive vapor rates. ln a typical bubble cap tower, each of these limitations predominates over a different vapor rate range..

Oneloperating disadvantage comprises excessive liquid holdup which, in the absence of a downcomer limitation and obstructions on the tray, is determined by the linear velocity at which liquid is able to pass across the tray. `Fora given liquid velocity across the tray the ,Y 2,772,081 Patented Nov. 27,-1956 liquid holdup is directly proportional to the volume of liquid flowing across the tray in a unit of time. Since the liquid on the tray is aerated by the vapor, kthe volume occupied by the liquid is proportional to the velocity of vapor in the tower and the amount of liquid holdup on the tray. At normal tray spacings a tower will ultimately be limited in capacity by the liquid ow approaching the tray above, resulting in excessive entrainment. Accordingly, higher capacities can be reached if the liquid holdup is reduced. Reduced holdup can be accomplished by the present invention which directs the vapor stream through the trays in such a manner as to push liquid across the tray at a faster rate.

In accordance with the present invention liquid holdup on the respective trays is prevented by providing conf current flow of vapor and liquid across the tray. `In

other words, a directional ilow of vapors is provided in' order to increase the velocity of the liquid flowing across the trays, thus preventing liquid holdup.

The present invention may be readily understood by reference to the drawings illustrating embodiments `of the same. Figure l illustrates a typical bubble cap tray fractionation zone. Figure 2 illustrates a typical bubble cap chimney and bubble cap, while Figure 3v is a top view of the same. Figure 4 illustrates a typical lean-to device, while Figure 5 is a top view of the same. Figure 6 illustrates a portion of a tray wherein conventional bubble caps are partially blanked off, while Figure 7 illustrates a Venetian blind type of directional device.

Referring specifically to Figure l, the numeral 10 designates a bubble cap tray column, contacting vessel. The vessel lil is conventionally provided with a series of vertically spaced, transverse, perforated plate elements 1l, forming a vertical series of superimposed, Contact chambers or zones. These chambers or zones are in communication one with another by way of the passageways l2 formed by the plate perforations, and downcomers 13 disposed at alternate sides of the vessel from plate to plate. The downcomers extend from the surface of one plate downwardly into vertically spaced relation to the surface of the plate next below. As shown in Figure l, the passageways 12 through the plate are each provided with bubble cap elements 14. In addition, each plate 11 is provided with a weir member 15, at the entrance to the downcomer 13, extending upwardly from the plate surface to a level above the lower end of the downcomer from the plate next above. The vessel is also provided with a feed line 16. An outlet from the vessel for gaseous materials is provided as by conduit 17. At the lower end of the vessel is an outlet 19 for heavier materials. In operation, it is to be underf stood that any number of trays may be utilized. In accordance with the present invention, for the purpose of description, it is assumed that a vaporous feed is introduced at an intermediate point of fractionating zone 19 by means of line 16. Temperatures and pressures are adjusted so that vapors ow upwardly in zone 10 through the chimneys l2 and around bubble caps 14 into a liquid maintained on the top of the plate, the height of which is determined by weir 15. The liquid phase on the plate comprises dissolved or condensed constituents of the vaporous feed introduced by means of line 16. Vapors ow through the liquid phase on the top of each plate from the zone below through the chimneys and around the bell caps into the liquid disposed on the top of the plate, the height of which is determined by weirs.

Referring specifically to Figure 2, illustrating a conventional bubble cap, the vapors flow upwardly through chimney 21 on plate 2l), around the upperlip 23 of the chimney and down around the skirt 24 of bell cap i bafile 32.

Y the open end facing Vthe downcomer.

Vduce therinlet head Vand prevent tower flooding dueto rectional baffles on standard bubble caps. Vused 'consist of a Vrectangular piece of sheet metal,

`zz" nm 'the liquid phase maintained on the rop Qf par@ Y2t): VFigure" 3 is a top" viewn of the' conventional bubble:

cap;Y a Y l Y Y Figure 4 illustrates abubble capnor equivalent meansV for securing the' directional owof vapors Vas they flow upwardly from one plate to another. These vapors ow upwardly through arhole 31 in plate 30Yinto a lean-to Y The baffle resembles a lean-to with vertical side, open front and. faces Ythe downcomer. Figure 5V is atop view of the lean-to baffle of the present invention.-

l Figure 6v shows ytwo conventional bubble capshalfof which are blanked off with the opening in the direction ofthe doyvncorner.iv Vapors iiow'upwardly throughchinln'eys lllniii` platenir'tl, around thehalf-open skirt of bubble caps (12 4and 43 into the VliquidY phase maintained on ftopzof plate 45. Thusjthe liquid andl uptiowing vapors H ow cncurrently, transversely', across plate 40 in the direction of the downcorner 44. Y i

Figura] illustrates a VenetianI blind type of tray. vapors' Vtliiv,Iupwardly through

holes

51, 52 and'SS in plate 50. These vapors then pass' directionally in con-A current ow with the liquid owing transversely across the plate in the direction of downcomer 54.

Another advantage of the present invention kis connected with reduced tray inlet liquid heads. Inra normal Vbubble cap tray there is a tendency for liquid to build. i up to a higher level atY the inlet side ofthe tray than It has been'ioundY thatl theV anywhere else on theV tray. use ofNlean-to type baiiies in place of bubble caps produces thejopposite effect. In kthis case, the inlet head is lower than anywhere else on the tray. The baiiiesV cronsist'of sheet metal lean-tos withvertical sides with premature downcorner lling, it is only necessary to have the first row or two of bubble capsV and chimneys replaced with these lean-to type baffles. The rest of the tray may have Venetian blind type baffles. Y

Studies Vin a three tray vertical half section of a ten foot diameter distillation tower have shown that tower Y' capacities may be increased as much as 41% by the use of lean-toY type baffles A in place of conventional bubble capsl and chimneys. The baffles employed consisted of sheet metal lean-tos with vertical sidesattached to the tray by conventional bubble cap tie-down bolts with the open end facing the downcor'ner. This design produced a directional vapor stream which facilitated they passage of the liquid across thertray, thus reducing liquid holdup on the tray. 4It has also been found that it may be benecial to completely blank off the holes left by removal of bubble caps immediately adjacent to the downcorner.

In this case, the action of the directional air stream produced, by the lean-to baflies inV conjunction with' the curved side ofthe tower acts as a centrifugal device to throw out and Ydeaerate liquidy from the vapor stream.

The design of the lean-to baies is not critical. Hemispherical-baiiies open on one side towards the downcomer',square or rectangular box-type baiiles open towards the downcomer; elbow-type baffles and similar Y devices may. be used. A simple and effective arrange- "ment may be fabricated by punching plate trays to pro-V vide thev` required openings, leaving iiaps of metal adjacn'treach opening inclined to direct vapors to the downcom ,ei` A; Y'

' The'bafes may be equipped with slots on their open ends Vfand they may have holes or clearance with the tray oor on the sides other than that facing the 'down- Y corner.4 However, they must produce a directional vapor stream leaving the tray in the direction lof Vthe 'downcomen Studies in a three-tray vertical half section of a ten foot diameter bubble cap tower have shown Vthat ca pacityrnay b e increased as much as 21% when using `di- The battles In order to re-V tray floor.

side ofall the bubble Vcap on arstandard iifty-six bubble Y i curved tofcover one half of the'v circumference of'aV standard' Bubble cap. 'rnerruateiis efisnct'a'siz'etat Y it covers the entire slot area and extends down to the With these baies installed on the upstreamV` caphalf-section gtray, the capacity init was increased 21% when using an enlarged downcom'e1.1.Althongh the bale's used in this work oiwlred onehaliof the bubble cap circumference, other lengths of batlies maybe used.,Y

The" baiiie sizes should be determined by that. giving. the' greatest directional effect to tlie vapor 'and liquidY on the"V V1. Y

tray while effecting tlieleast increase in tray pressure drop. The baiiie' beattaelie'd to the bubble-:cap i either by means of appropriate circumferential c'lamps tack-welding or appropriate clips which engage the bubble cap slots as Vthe baiiie is slipped over the bubble'cap.

Y It is a frequent occurrence that production is limited in refinery operations by distillation tower capacity. .The

limitation is often inthe distillation tower'its'elf as con-1 trasted to auxiliaryrequipment such as furnaces, heat ,Y exchangers and pumps. ilifhen' feed stock is available,` there is' a very attractivev incentive inincreased productionV vof high prot incremental product through increased tower capacity. Y Y Y Studies in a three-'tray vertical half section of a Vten Y foot diameter distillation tower have shown that increase!V in capacity results from removal of conventional bubble lcaps and chimneys and `substitution of Venetian-blind battles on the tray.Y The baiiiles consist of rectangular"` strips of sheet metal approximately Vl0 inches wide, These are positioned above the holes in the tray exposed by removal of the bubble caps and chimneys so that theV bailies run across the tower perpendicularly to the di-`V Y rection or" liquid iiow across the tray. The baiiies are Vil() arranged like an almost closed Venetian blind so that"V suicient velocity and pressure dropV are obtained so thatV Yessentially no liquid flows down through the batiies, andA tray dumping is avoided. The distance between thev ov'er-V lapping portion of the battles Yis from about one half toi one inch. The batiies may be curved or bent and may` contain slottedcorrugated, or vg'rooved edges and sur? faces.V What is claimed isi Y k ApparatusV particularly Yadapted for fractionation com-L prising Va vertical tower containing a plurality Voi horizontally disposed vertically spaced plates extending substantiallyV across the tower intersecting vertically positioned conduits extending through each of said plates terminating above andbelow each plate, inV which theVV said plates are provided with aV large number of perfora-V tions distributed over a substantialV portion of their area, and lean-to means'as'sociated with each of said perforations, said lean-to means being Vfixed to said plate and inclined upwardly towards the said conduit;V` associated with and extending downwardly from Yeach plate, said*Y lean-to means being further characterized by having side closure members joined toV said lean-to means and ex# tending downwardly to the said plate adjacent to the'v perforation provided in the plate, said lean-to Ymeans Y being still further characterized in that the leanato member covers a substantially greater area than the area of Y the corresponding perforation.

References Cited Yin the tile of Vthispatei'ttk UNITED srATns PATENTS