HIGH-CAPACITY VAPOR/LIQUID CONTACTING TRAY

Abstract: This concept concerns a vapor/liquid-contacting tray which is useful in distillation and related mass transfer or heat transfer applications where liquid flows down and gas or vapor flows up through a vessel. In this device, the perforated area (3) is relatively small, so the liquid is entrained upward. The liquid flow is then directed downward into a large disengaging area (5). The downward momentum of the liquid and the large disengaging allow for high liquid and vapor rates without overloading the vapor/liquid disengaging capacity of the device.

Description: HIGH-CAPACITY VAPOR/LIQUID CONTACTING TRAY This application is related to International Application PCT/98US/23670 This invention is directed to a vapor/liquid contacting tray and a separation process which employs this series of trays Contacting trays such as the one of this invention are useful in distillation and related mass transfer or heat transfer applications where liquid flows down and gas or vapor flows up through a vessel Liquid entrainment is a problem frequently encountered in distillation tray technology Gas-liquid contacting employing plate and tray columns and potential problems involved are discussed in Section 18 of The Chemical Engineers' Handbook, Fifth Edition, by Robert H Perry and Cecil H Chilton Tray designs with baffles and dispersers of various types are illustrated and discussed U S Patent No 5,837, 105 describes a device for de-entraining liquid using a number of de-entrainment devices (such as vane packs) on the trays to accomplish the vapor/liquid de-entrainment U S Patent No 4,356 132 discloses a mass-transfer apparatus which comprises a vertical cylindrical housing, contact trays accommodated inside the housing and made up of perforated and non-perforated sectors, the neighboring trays being arranged so that the perforates sectors of one tray overtie the non- perforated sectors of another tray and vice-versa The overflow passageways of the contract trays are positioned on the non-perforated sectors Overflow deflectors are arranged on the contact trays, and baffles are adapted to extend downwardly from each contact tray from the borderline between the perforated and non-perforated sectors and inclined toward the perforated sector The device does not use multiple baffles vertically located between the perforated area and the impermeable roof, in order to decrease entrainment of liquid Summary of the Invention The instant invention imparts co-current flow of both gas and liquid with substantially all the liquid being purposefully entrained upward into the vanes (or baffles) where the liquid is deflected downward, at an angle, into a disengaging area opposite the perforated area where the liquid gas efficiently separate In this respect it operates differently to the device shown in U S 4,356, 132 (see Figures 3 and 6, solid and dashed flow lines) The present invention has a large sloping open disengaging area extending well below the plane of and opposite the perforated area for enhanced separation of liquid from vapor, which is absent from the known device In the present invention, the phase separation can take place in an essentially empty disengagement area by the forces of gravity and liquid downward inertia, without the aid of special de-entrainment devices Although they are not necessary, mesh pads, vanes, plates, louvers or other means to assist phase separation or other means of assisting phase separation may be placed within the disengagement area An impermeable "roof directly above the bubbling area, so liquid cannot be thrown directly up against the perforated area of the tray above There is at least one outlet weir (or baffle, or alternatively, vane) or other surface projecting up at some angle from the level of the perforated area at or near the exit end of the perforated area, to help direct the froth upwards initially Complete entrainment of the liquid is not necessary in the functioning of this invention Some liquid can simply spill over the outlet weir It is preferred, however, to initially impart some upward momentum to the bulk of the liquid There is at least one baffle that then helps direct the liquid at least partially downward, such that the liquid is moving generally downward, rather than upward, as it enters the disengaging zone where the vapor flows upward to the next tray One or more curved turning vanes may also be used to direct the liquid momentum downward Many FCC units are limited by the downstream gas plant The trays of the instant invention can help debottleneck gas plants other high-pressure towers, and amine towers with high liquid loads The capacity benefits of this invention are probably highest at conditions of high liquid flux (greater than 814 I mm "1 m 2 of tower area) The Drawings Figure 1 illustrates a conventional sieve tray Figure 2 illustrates the sieve tray of the instant invention in a single pass mode Figure 3 illustrates a sieve tray of this invention in a multi-pass mode Figure 4 illustrates the effects of single versus multiple vanes on entrainment of liquid by plotting liquid entrainment (percent) against liquid rate Figure 5 illustrates the effect of disengaging area size on Cf flooding factor employing straight weirs by plotting Cf flooding factor v liquid rate Figure 6 illustrates the effect of disengaging area size on C f flooding factor employing straight weirs v sloped weirs, by plotting Cf flooding factor v liquid rate Figure 7 illustrates the single pass mode of this invention, with louvers opposite the perforated area Figure 8 illustrates tray performance with and without louvers by plotting Cf flooding factor against liquid load Figure 9 illustrates the effect of baffle (or vane) radius on C f flooding factor A conventional sieve tray is illustrated in Figure 1 Liquid that is entrained upwards from the tray can be thrown up against the perforated area of the tray above, and qet carried up to the next tray This backmixing of liquid up the column, contrary to the desired downward liquid flow, can greatly decrease the separation efficiency of the column and can cause flooding (hydraulic overloading) of the column Liquid enters the tray at 1 , becomes entrained with the rising vapor 2, and is thrown against the perforated area of the tray 3 above, to be carried to the next tray The tray design of this invention, and the process steps involving its use, are illustrated in Figure 2 There is an impermeable roof directly above the bubbling area, which prevents liquid from being thrown directly up against the perforated area of the tray above There are baffles that help direct the liquid at last partially downward These baffles could alternately be described as curved turning vanes, and direct liquid flow downward There is little or no direct line of sight from the perforated area of one tray to the perforated area of the next tray in sequence, i e the trays are vertically staggered Vapor entrained with liquid enters the tray through perforated area 1 The impermeable roof 2 above the perforated area prevents the entrained liquid from rising further in the column The outlet weir 3 (which may be tilted as illustrated in order to maximize the disengaging area), which is adjacent to the perforated area through which vapor travels upward from one tray to another, and baffles 4, deflect vapor and entrained liquid into the disengaging area 5 which are preferably not attached to the root or the perforated area The root 2 separates downcomer area 5(a) from the engaging area 5(b) De-entrained liquid then moves downward to the tray 6 below, while the vapor moves upward through the perforated area 7 into the tray above Similar trays are arranged above and below the tray shown in staggered configuration As Figure 2 illustrates, there is little or no direct line of sight from the perforated area of one tray to the perforated area of the next tray in sequence Less than 50% of the perforated area of each tray is directly in view from the perforated area of the next tray in sequence A multi pass mode may be provided by the use of trays arranged in parallel as shown in Figure 3 It is strongly preferred that the baffles which are located between the trays have a downwardly curved trailing edge, in order to increase the C f ioo d factor, discussed below Such baffles are referred to here as turning vanes The trailing edge of the turning vane possesses a shape which approximates a ninety-degree arc This arc suitably has a radius from 6 to 50mm and preferably from 12 to 25mm This shape may be obtained by straight segments or a combination of straight and curved segments This disengaging area comprises preferably more than 40%, and more preferably from 45 to 60% of the area of the given "pass" of the tower Devices such as perforated mesh pads, baffles, louvered plates, and vanes or similar devices may be placed in the disengaging area, especially against the far wall 8, as shown in Figure 2 When liquid sprays against the far wall, there may be some advantage to mounting these devices in order to keep the liquid from splashing or being blown upward, although satisfactory performance may be obtained without such devices The preferred ratio of perforated area at a given tray level to the total cross- sectional area of the tower is 1 10 to 1 2, and most preferably 1 5 to 2 5 This is smaller than the percentage of perforated area usually found in conventional trays Circular holes are preferred for the perforated areas of this tray, but any shape of hole or slot or any other device, including valves or bubble caps, that allows vapor passage would be acceptable here in general, the number and size of perforations are chosen to be small enough to avoid excessive weeping of liquid down through the perforations, and large enough to avoid excessive pressure drop Devices on or above the deck such as vanes, momentum breaker bars or deck orifice shrouds may be used to moderate the liquid momentum and influence the pattern of vapor flow up into the liquid Close tray spacing is recommended in for the invention If the tower is operating in a pinched regime (which is common in the target applications for this tray, such as debutanizer bottom sections), then the increased capacity associated with this tray can be used to increase the reflux ratio In this way the efficiency of the tray may be enhanced Figure 4 Is a plot illustrating the benefit of multiple baffles or vanes which are vertically located between the perforated area and the impermeable roof, as opposed to the single baffle system disclosed in the prior art, in reducing liquid entrainment An approximately six-fold decrease in entrainment (as percentage of liquid feed) is obtained The following Example is offered to demonstrate that multiple baffles vertically located between the perforated area and the impermeable roof decrease liquid entrainment dramatically A 46cm diameter column was used to determine the capacities of trays Liquid was fed to the top of the tray section and gas was fed to the bottom Liquid that was entrained upwards from the topmost tray was captured and measured It is desirable to minimize entrainment A tray with two baffles unattached to the impermeable roof, such as that shown in Figure 2 had only 1 25 liters/minute entrainment, with a liquid feed rate of 340 liters/mm and a gas rate of 14 16m 3 /mιnute Another device (not shown) has essentially the same geometry as shown in Figure 2, except for a single straight baffle protruding downward from the impermeable roof This device has higher entrainment (4 8 liters/mm ) at the same liquid and gas feed rates thereby demonstrating improved performance for multiple baffles The instant invention demonstrates the benefit of an increased disengaging area Disengaging area is defined as the area cross section of the tower defined by DA = 100 - (BA + DBA) where, DA = disengaging area (or downcomer top area) BA = bubbling area (perforated area) DBA = downcomer bottom area Disengaging area was increased in the instant invention by reducing bubbling area This can be accomplished by reducing perforated area or sloping the outlet weir in toward the perforated area Data supporting the improved performance at larger disengaging area is shown in the (volume of liquid per minute per unit cross-sectional area or tower) Figures 5 and 6 Flooding C f actor or C ioo d factor, is plotted versus liquid load for data at two different disengaging areas The higher disengaging area gives the higher C f iood values (high Cnoo value is better) meaning a higher gas rate is needed to flood the device when higher disengaging area is used Figure 5 shows the effect of disengaging area when straight outlet weirs are used and Figure 6 shows the effect of increased disengaging area brought about by sloping the outlet weir The preferred value for disengaging area for the instant invention is from 45-70% Figure 7 illustrates the presence of louvered grates 9 in a single pass application in the disengaging area 5 opposite the perforated area, in order to further enhance disengagement of liquid from vapor Figure 8 demonstrates the performance improvement (higher C F ioo d ) when the louvers are used compared to without the louvers Figure 9 shows the effect of baffle radius on the flooding factor with higher floodity factors corresponding to the smaller vane radius relative to the relative liquid rate

Claims: Claims 1 A vapor/liquid separation process which comprises passing streams of vapor and liquid through a series of trays, each tray comprising at least one perforated area through which vapor comprising entrained liquid travels upward from one tray to another, wherein the perforated area of each tray is covered by an impermeable roof in order to inhibit the rise of entrained liquid, each tray further comprising at least one outlet weir which is adjacent to the perforated area, and wherein at least one tray of the series further comprises at least two baffles, each of which is vertically located between the perforated area and the impermeable roof, one baffle being a topmost baffle and other baffle or baffles being located vertically below it, wherein all baffles aid in the deflection of vapor and entrained liquid to a disengaging area between the trays, where vapor is separated from liquid, the vapor moving upward and the liquid falling downward 2 The process of claim , wherein at least one baffle is located between each tray and the next one in sequence 3 The process of claim 2, wherein each baffle possesses a curved trailing edge having a shape approximating a ninety-degree arc 4 The process of claim 3, wherein said arc has a radius of from 6 to 25mm 5 The process of claim 1 , in which the preferred ratio of perforated are at a given tray level to the cross-sectional area of the tower is from 1 10 to 1 2, thereby optimizing Cnood value 6 The process of claim 1 , in which the preferred ratio of perforated area at a given tray level to the total cross-sectional area of the tower is from 1 5 to 2 5, thereby further optimizing Cnood value 7 The process of claim 1 , in which perforations of the perforated area are circular 8 The process of claim 1 , in which vanes or mesh devices are mounted within the disengaging zone in order to prevent liquid from splashing upward 9 The process of claim 1 , wherein louvered grates are present in the disengaging area opposite the perforated area, in order to further enhance disengagement of liquid from vapor 10 A vapor/liquid separation device which comprises a series of trays, each tray comprising at least one perforated area through which vapor comprising entrained liquid travels upward from one tray to another, wherein said perforated area of each tray is covered by an impermeable roof in order to inhibit the rise of entrained liquid, each tray further comprising at least one outlet weir which is adjacent to the perforated area, and wherein at least one tray of the series further comprises at least two baffles, each of which is vertically located between the perforated area and the impermeable roof but is not attached to either, one baffle being a topmost baffle and other baffle or baffles being located vertically below it, wherein all baffles aid in the deflection of vapor and entrained liquid to a disengaging area between the trays, where vapor is separated from liquid, the vapor moving upward and the liquid falling downward

Inventors: BUCHANAN JOHN SCOTT, STOBER BERNE KIM, BUCHANAN, JOHN, SCOTT, STOBER, BERNE, KIM
Applicants: MOBIL OIL CORP, MOBIL OIL CORPORATION
Application year: 2000
Main classification: B01D3
IPC codes: B01D3