US2288958A - Fractionating column - Google Patents

Description

Search R0 July 7, 1942. R. B. SMITH FRACTIONATING COLUMN Filed Nov. '7, 1940 IIlII-IIII-IIII INVENTOR fieaazhyflarlawdlwzz ATTORNEYS APt'Ali/ll U8.

1 Tatented July 7, 1942 uulbll h FRACTIONATING COLUMN Reading Barlow Smith, Hammond, Ind., asslgnor to Sinclair Refining Company, New York, N. Y., a corporation of Maine Application November 7, 1940, Serial No. 364,683

(Cl. 261-1l2) 8 Claims.

This invention is concerned with fractional distillation and provides an improved fractionating column of great capacity in terms of material fractionated in unit space in unit time.

Heretofore customary commercial fractionating columns of the bubble tray type, such, for example, as those employed in the petroleum industry, have an HETP (height equivalent to one theoretical plate) of 2 to 4 feet. Attempts have been made to decrease this figure by various expedients, including the use of finer tower packing to provide more contact between liquid and vapor, but such packing generally is impractical in commercial operations, especially in the petroleum industry, because it is so easily plugged by gum or foreign material derived from the stock undergoing distillation.

I have developed an improved tower packing in the form of a tray which provides additional contact between liquid and vapor (with resultant increased tower capacity in terms of material fractionated in a given space in unit time, i. e.. a decreased HETP) and in which is incorporated a simple cleaning mechanism for periodically removing accumulations of gum and the like.

My invention contemplates a tray for a fractionating column comprising a contact grid formed of a plurality of strips spaced in substantially parallel alignment with slots between, a cleaning grid of similar construction having its strips aligned to move in the slots of the contact grid, and means for moving the cleaning grid relative to the contact grid so that the strips of the former move in the slots of the latter. In the preferred form of tray of my invention, the strips of both cleaning grid and contact grid are relatively thin and are set on edge with the slots in the contact grid just wide enough to accommodate the strips of the cleaning grid. During normal operation of the fractionating tower, the cleaning grid is positioned slightly above 01' below (preferably below) the contact grid so as not to interfere with passage of liquid and vapor in countercurrent through the tray. The strips of the cleaning grid preferably are serrated and in normal operation the teeth of these strips project partially into the slots of the cleaning grid. When it is necessary to clean the tray, the cleaning grid is forced into the contact grid and frees accumulations which have formed in the slots.

The trays preferably are disposed in a sloping position, a number of trays being superposed in a fractionating tower with alternate trays sloping in opposite directions. In this structure, each tray may be provided with independent means for moving its cleaning grid with respect to its contact grid so that the trays may be cleaned separately and without interfering with the operation of the tower. Weirs may be provided between trays for carrying excess flow of liquid down the tower and vapor risers may also be provided for passing vapor upwardly through the trays. These and other features of my invention will be more thoroughly understood in the light of the following detailed description taken in conjunction with the accompanying drawing in which:

Fig. l is a fragmentary plan of a fractionating tray of my invention;

Fig. 2 is a partial elevation of the tray of Fig. 1 provided with means for moving the cleaning grid with respect to the contact grid and shows the normal or operating positions of the grids with respect to each other;

Fig. 3 is a plan view through a fractionating column constructed in accordance with my invention; and

Fig. 4 is a partial elevation through the fractionating column of Fig. 3.

Referring now to Figs. 1 and 2, it will be observed that the tray is circular in plan in order to fit within a cylindrical tower or column having a wall It. The tray comprises a contact grid ll formed of a plurality of thin strips llA, NB, NC, ND, HE disposed edgewise and parallel to each other with slots in between wide enough to accommodate the strips l2A, I23, MC, MD and HE of a cleaning grid l2. The tray is set in a sloping position and in normal operation the cleaning grid is disposed slightly below the contact grid. The strips of the cleaning grid are provided with teeth l3, l4, IS on their upper edges. In normal operation these teeth project but slightly into the slots of the contact grid.

' The apparatus is provided with a grid moving mechanism l6. Conveniently, this moving mechanism may comprise a rack ll fastened at its upper end to the cleaning grid and adapted to be moved upwardly and downwardly by a pinion I! mounted on the end of a crank l9 that projects through a gastight gland 20 on the wall of the fractionating column. When the crank 19 is rotated the pinion is rotated and the rack moved either upward or downward depending upon the direction of rotation. Upward movement of the rack forces the cleaning grid upwardly through the contact grid so that the lower portion or blade 2| of each strip of the cleaning grid is pushed through the corresponding slot of the contact grid and thus cleans it.

In order to adjust the exposed surface of the tray or the resistance thereof to fiow of liquids and vapors therethrough, the position of the cleaning grid with respect to the contact grid may be varied. Thus, if the teeth of the strips on the cleaning grid project far into the slots of the contact grid, the resistance of the tray to the passage of liquid and vapor will be increased.

Referring now to Figs. 3 and 4, it will be observed that a fractionating tower 25 of cylindrical section is equipped with a plurality of superposed trays 26, 21, 28 similar in structure to the tray just described in conjunction with Figs. 1 and 2. These trays are sloped alternately in opposite directions. Each tray is provided with a series of vertical cylindrical pipes or vapor risers 29A, 29B, 29C which pass through the trays. Each tray is also provided with a downcomer 30 having a weir or hydraulic seal 3| at its lower end immediately above the next lower tray. Each tray is provided with a separate actuating mechanism 32 for moving its cleaning grid relative to its contact grid. The actuating mechanism may be of various forms, for example, that described in detail with reference to Figs. 1 and 2.

In the operation of the fractionating tower of Figs. 3 and 4, liquid runs down the tower in countercurrent with rising vapor. Some of the liquid and some of the vapor pass in countercurrent with each other through the slots in the respective trays. Excess liquid may flow over the tray and through the downcomer and weir to the next tray. Excess vapor passes upwardly through the trays in the vapor risers.

The strips of both cleaning grid and contact grid may be held together by spacer bolts (not shown) passing horizontally and transversely through the strips.

The thickness of the strips of cleaning grid and contact grid depends upon the physico chemical characteristics of the material to be fractionated. The surface tension of the material to be treated plays an important part in the determination of the thickness of strip. In the fractionation of light hydrocarbons, such as butane or pentane, optimum thickness of strip is in the neighborhood of to Q2 of an inch.

As indicated above, the fractionating tower may be provided with a single mechanism for moving all of the cleaning grids in unison. However, in order that the cleaning of each tray might be accomplished independently at any time during the operation without excessive interference with the flow on the tower, the separate actuating mechanisms for each cleaning grid are preferred.

The trays may, under some circumstances, be disposed in a substantially level position but it is strongly recommended that the trays be sloped substantially, thereby increasing hydraulic gradient and reducing to a minimum the weir length required for maintaining a depth of liquid on the tray. For optimum operation and to assure greatest contact between vapor and liquid, it is undesirable that a large pool of liquid be entrained by the grid. Thus, a liquid depth in the neighborhood of V inch is desirable. With the trays arranged as shown in Figs. 3 and 4, this relatively small liquid depth can be maintained on the grid without an excessively large overflow weir at the end of the downcomer. The provision of the smaller weir and downcomer makes possible a substantial saving in tower space.

Upflowing vapors are brought into contact with the liquid from underneath the grid. It is not necessary that large amounts of vapors pass through the grid since the principal contact between vapor and liquir occurs underneath the grid among falling films of liquid.

A fractionating tower constructed in accordance with my invention offers relatively small resistance to the passage of liquid and vapor. Consequently, higher vapor velocities and smaller tower cross sections obtain. The greater vapor velocity results in formation of thinner gas films in the apparatus and increased diffusion, thus improving the efliciency of each tray.

As mentioned above, the cleaning grid can be employed to vary the free area and the resistance of the grid to the passage of liquid and vapor. Thus, by varying the position of the cleaning grid with respect to the contact grid, the tower may be adjusted to afford optimum conditions for the fractionation of various types of material and for varying rates of throughput. For example, at low throughputs it may be desirable to have a different free area in the grid than for high throughputs and this change in free area may be obtained easily by adjusting the relative positions of cleaning grid and contact grid.

To summarize, my invention permits the maintenance of large film contacts between vapor and liquid in the fractionating column, thus obtaining high diffusion rates and a nearer approach to complete equilibrium between vapor and liquid in each tray, with consequent reduction in the number of trays required for a given job of fractionation. Secondly, the tray lS easily cleaned and is not subject to objection which have applied heretofore to tower trays and packing which afforded large surface area. Thirdly, the cleaning mechanism of the tray can be used to adjust free area of the contact grid and thus adapt the tray for different types of fractionation. Fourthly, the grid type tray permits reduction in the amount of liquid entrained in the tower together with higher permissible vapor velocities and closer tray spacing. Lastly, the sloping grid type trays permit maintenance of a low liquid level on the tray with a minimum weir length. In sum, the use of fractionating columns equipped with trays of my invention results in a substantial reduction in the HETP of the column and a substantial increase in capacity of the tower in terms of throughput.

I claim:

1. In a fractionating column, a tray for promoting contact of liquid and vapor which comprises a contact grid formed of a plurality of substantially parallel strips spaced from each other with slots between, a cleaning grid of similar construction having its strips interposable between those of the contact grid in the slots and means for moving the cleaning grid relative to the contact grid so that the strips of the cleaning grid move in the slots of the contact grid.

2. In a fractionating column, a tray for promoting contact of liquid and vapor which comprises a contact grid formed of a plurality of thin strips set edgewise and spaced substantially parallel to each other with slots between, a cleaning grid of similar construction having its strips interposable between those of the contact grid in the slots, and means for moving the cleaning grid relative to the contact grid so that the strips or the cleaning grid move in the slots of the contact grid.

3. In a fractionating column, a tray for promoting contact of liquid and vapor which comprises a contact grid formed of a plurality of substantially parallel strips spaced from each other with slots between, a cleaning grid of similar construction having its strips interposable between those of the contact grid in the slots, the strips of the contact grid being provided with teeth disposed in the slots of the contact grid, and means for moving the cleaning grid relative to the contact grid.

4. In a'fractionating column, a tray for promoting contact of liquid and vapor which comprises a contact grid having a substantially s1oping upper surface and formed of a plurality of substantially parallel strips spaced from each other with slots between them, a cleaning grid of similar construction having its strips interposable between those of the contact grid in the slots, and means for moving the cleaning grid relative to the contact grid 50 that the strips of the cleaning grid move in the slots of the contact grid.

5. In a fractionating column, a tray for promoting contact of liquid and vapor which comprises a contact grid formed of a plurality of thin strips set edgewise and parallel to each other with slots between a cleaning grid of similar construction having its strips interposable be tween those of the contact grid in the slots, and means for moving the cleaning grid vertically relative to the contact grid so that the strips of the cleaning grid move in the slots of the contact grid.

6. In a fractionating column, the combination search R001 which comprises a plurality of superposed trays the upper surfaces of which slope alternately in opposite directions, each tray comprising a contact grid formed of a plurality of substantially parallel strips spaced from each other with slots between and a cleaning grid of similar construction having its strips movable in the slots of the contact grid, and means for moving the cleaning grid of each tray relative to the contact grid.

7. In a fractionating column, the combination which comprises a plurality of superposed trays the upper surfaces of which slope alternately in opposite directions, each tray comprising a contact grid formed of a plurality of substantially parallel strips spaced from each other with slots between and a cleaning grid of similar construction having its strips movable in the slots of the contact grid, and independent means for moving each cleaning grid relative to its contact grid so that the strips of said cleaning grid move in the slots of said contact grid.

8. In a fractionating column, a tray for promoting contact of liquid and vapor which comprises a contact grid formed of a plurality of substantially parallel strips spaced from each other with slots between, a cleaning grid formed of a plurality of substantially parallel strips adapted to fit in the slots of the contact grid, the strips of the cleaning grid having teeth on their upper edges adapted to be disposed in the slots of the contact grid, and means for moving the cleaning grid upwardly relative to the contact grid.

READING BARLOW SMITH.

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