WO1999026718A1

WO1999026718A1 - High-capacity vapor/liquid contacting device

Info

Publication number: WO1999026718A1
Application number: PCT/US1998/023670
Authority: WO
Inventors: John Scott Buchanan; Berne Kim Stober
Original assignee: Mobil Oil Corporation
Priority date: 1997-11-20
Filing date: 1998-11-06
Publication date: 1999-06-03
Also published as: AU1451799A; EP1100615A1; EP1100615A4; CA2311253A1; JP2001523567A

Abstract

A vapor/liquid contacting tray is disclosed which is useful in distillation, mass transfer and heat transfer processes where the liquid flows down and the vapor flows up through the vessel. In the device, the perforated area (1, 7) of the tray is relatively small, so the liquid is entrained upward. The liquid flow is directed downward into a large disengaging area (5). The downward momentum of the liquid and the large disengaging area (5) allow for high liquid and vapor flow rates without overloading the vapor/liquid disengaging capacity of the device. The device is also equipped with outlet weirs (3) and baffles (4) to deflect the flow of the vapor and liquid passing through the vessel.

Description

HIGH-CAPACITY VAPOR/LIQUID CONTACTING DEVICE

This invention is directed to a series of vapor/liquid contacting trays 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.

Co-pending application S.N. 08/833,490 (which is commonly owned with the instant invention) is directed to a means of de-entraining liquid. In S.N. 08/833,490, a plurality of de-entrainment devices (such as vane packs) on the trays are specified to accomplish the vapor/liquid de-entrainment.

A series of conventional sieve trays 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 get 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.

In the instant invention, the phase separation can take place in an essentially empty de-entrainment zone 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 de-entrainment zone.

Figure 2 illustrates the instant invention. There is an impermeable "roof directly above the bubbling area of one tray 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 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 at least partially 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, as shown in Figure 3, may also be used to direct the liquid momentum downward. Figures 2 and 3 illustrate that it is desirable that the perforated area of one tray not be in the direct line with the perforated area of another tray in sequence. 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 about 13.58 l/s/m² (20 gpm/ft²)) of tower area.

Figure 1 illustrates a conventional sieve tray. Liquid enters the tray at 1 , and at a high rate, 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.

Figure 2 illustrates a series of sieve trays of the instant invention. 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.

Figure 3 illustrates a series of sieve trays with an alternate form of baffles 1 (curved turning vanes) for directing liquid flow downward.

Figures 2 and 3 illustrate a series of sieve trays with little or no direct line of sight from the perforated area of one tray to the perforated area of the next tray in sequence. The tray design of this invention, and the process steps involving its use, are illustrated in Figure 2. Vapor entrained with liquid enters the tray through perforated area 1. The impermeable roof above the perforated area prevents the entrained liquid from rising further in the column. The outlet weirs 3 and 4, which are adjacent to the perforated area through which vapor travels upward from one tray to another, and baffle 4 deflect vapor and entrained liquid into the disengaging area 5. De-entrained liquid then moves downward to tray 6 below, while the vapor moves upward through the perforated area 7 into the tray above.

As figures 2 and 3 illustrate, 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.

It is strongly preferred that the baffles which are located between the trays have a downwardly curved trailing edge. 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 has a radius of from one-fourth inch to 2 inches, and preferably from one-half inch to one inch. The shape may be obtained by straight segments or a combination of straight and curved segments.

The disengaging area comprises preferably more than 40%, and more preferably from 45 to 60% of the area of a given "pass" of the tower. Devices such as perforated mesh pads, baffles, louvered plates or vanes, may be placed in the disengaging area, especially against the far wall 8. 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 can 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 for the instant invention. If the tower in which the tray series is located 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 tower may be enhanced.

Claims

CLAIMS:

1. 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 at least one baffle which is adjacent to the impermeable roof which 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 series of trays of claim 1 , wherein at least one baffle is located between each tray and the next one in sequence.

3. The series of trays of claim 1 , wherein the baffle possesses a curved trailing edge having a shape approximating a ninety degree arc, the arc having a radius of from o one-fourth inch to 2 inches.

4. The series of trays of claim 1 , wherein the arc has a radius of from one-half inch to 1 inch.

5. The series of trays 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 1:10 to 1:2.

6. The series of trays 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.

7. The series of trays of claim 1 , in which perforations of the perforated area are circular.

8. The series of trays 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 series of trays of claim 1 , wherein the perforated area of one tray is not in direct line with the perforated area of the next tray in sequence.

10. A process for separation of vapor from liquid, the process employing the series of trays of claim 1 , wherein vapor comprising entrained liquid travels upward through the perforated area of one tray and is deflected by the impermeable roof and the outlet weir to said disengaging area, where vapor is separated from liquid, the vapor moving upward and the liquid falling downward.