US3444697A - Distributed heat exchange fractionating column - Google Patents

Description

May 20, 1969 J. R. MUENGER DISTRIBUTED HEAT EXCHANGE FRACTIONATING COLUMN Filed June 30, 1967 Sheet of 2 United States Patent 3,444,697 DISTRIBUTED HEAT EXCHANGE FRACTIONATING COLUMN James R. Muenger, Beacon, N.Y., assignor to Texaco Inc., New York, N.Y., a corporation of Delaware Filed June 30, 1967, Ser. No. 650,472 Int. Cl. F25j 3/00; B01d 3/00 US. Cl. 62-42 7 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of the invention This invention relates to a fractionating column for cryogenic service having a distributed heat exchange means extending throughout a substantial portion of the column.

Description of the prior art Fractionating columns for low temperature rectification or gas-separation often include internal boilers and condensers. Such columns may consist of a single column, or may comprise both exhausting and enriching sections as in a compound fractionating column, or may be a double fractionating column. The term compound fractionating column, as used herein, shall be understood to mean a fractionating column having both an exhausting or stripping section and an enriching or rectification section. As used herein, a double column is one in which the condenser for one column serves as a boiler for the other. Such columns are well known in the art and are illustrated, for example, in FIGURES 12-34 and 12-35, respectively of Perrys Chemical Engineers Handbook, fourth edition, McGraw-Hill Book Company, Inc., New York, 1963.

In the compound column, when applied to air-separation for example, cooling in the condenser may be accomplished by evaporating a liquid refrigerant and heating in the boiler may be accomplished by condensing a vapor. The use of latent heats, rather than sensible heats, of heat exchange media permits an approach to reversible heat transfer which is limited only by the temperature drop due to the heat exchanger walls and the boundary films. When air is separated in a compound tower, all of the cooling required in the top part of the column takes place in a condenser at the top of the column, which is at the lowest temperature existing in the column, while all the heating required in the bottom part of the column takes place in a boiler at the bottom of the column which is at the highest temperature existing in the column. These facts contribute substantially to the thermodynamic losses of the compound tower and prevent even an idealized compound tower from being a reversible device in the thermodynamic sense.

Although the fractionating column of this invention will be described in terms of an air rectification column, it will be understood that it is equally applicable to use with any other fluid which may be separated in a fractionating column.

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SUMMARY OF THE INVENTION An object of the present invention is to provide a plate-type fractionating column for cryogenic service capable of providing inherent higher efliciencies than are presently obtainable in the conventional compound tower. This object is reached by applying the principle of distributed heat exchange in the tower sections patterned after the requirements of an idealized tower for thermodynamic reversibility.

Another object of the present invention is to provide a plate-type fractionating column for cryogenic service wherein column service heat exchange takes place principally through a vertical tube bundle.

This invention provides a distributed heat exchange column comprising bubble plates, weirs and downcomers and having a tube bundle with extended heat transfer surface with tubes extending vertically through the bubble plates over a relatively large section of the column. A preferred apparatus is in the form of a sub-assembly which is readily inserted or removed from a pressure tight cylindrical column body shell.

BRIEF DESCRIPTION OF THE DRAWING The invention will be further understood by reference to the accompanying drawing in which:

FIGURE 1 is an elevation of a preferred embodiment of a fractionating column constructed in accordance with the present invention;

FIGURE 2 is a vertical cross sectional view of the fractionating column of FIGURE 1, taken along the line 22 of FIGURE 3;

FIGURE 3 is a cross sectional view of the fractionating column of FIGURE 1, taken along the line 3-3 of FIGURE 2;

FIGURE 4 is a fragmentary view in larger scale of the encircled portion of FIGURE 3; and

FIGURE 5 is a partial vertical cross sectional taken along the line 5-5 of FIGURE 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT The following detailed description of a preferred embodiment of apparatus constructed in accordance with the present invention is directed to a specific example of a compound column for air separation using gaseous helium as both heating and cooling fluids for tower servicing, i.e. cooling helium to supply heat in the lower part of the column and warming helium to provide refrigeration in the upper part of the column.

FIGURES l and 2 display a plate type fractionating column 1, having support legs 2 which are attached to a lower bottom cup or base assembly 3 having a circular flange 4 substantially about its periphery adjacent the open end or top side. A hollow cylindrical body shell 5 has a lower flange 6 and an upper flange 7, respectively, on its lower and upper end portions. The lower flange 6 is shown joined to the flange 4 of the base assembly 3 forming joint 8. A hollow cap assembly 9 has a projecting circular flange 10 at its open end which forms joint 11 with the upper flange 7 of the cylindrical body shell 5.

Tower 1 is suitably insulated against heat leakage from the surroundings. This may be done by placing the tower in a cold box along with other cryogenic components of the total system as is customary in air separation systems, or it may be done by separately insulating the tower. In the latter type of installation, the tower may be surrounded by a pressure-tight shell with the intervening space filled with blanket or particulate insulation kept under vacuum. Reflective layers or reflective particles in the insulation eflectively curtail radiative heat transfer while the vacuum and insulation combine to control conview vective and conductive heat transfer. The latter type of insulation, known as super insulation, has proved to be very effective. Heat leakage through the tower structure from the warm end to the cold end of the tower is discouraged wherever possible by heat dams such as flange gaskets.

The base assembly has depending therefrom a heating fluid outlet or helium return connection 12 which is in communication with the lower tube bundle 2.1, as will be described below, and an oxygen or bottom product tap 13 to remove the heavier component. Bottom product tap 13 can be suitably devised, when air is the fluid being rectified, for a liquid oxygen (LOX)) take-off or for gaseous oxygen removal by suitable changes in the tap and in the duty of the bottom heat exchanger as will be obvious to those skilled in the art. A heating fluid inlet or helium feed connection 14 also depends from the lower bottom cup 3, and is in communication, through a lower feed manifold 22, with the lower portion of the bottom tube bundle 21 as will be described below.

The cap assembly 9 includes a refrigerant fluid outlet orhelium return connection 15 in communication with an upper tube bundle 30, a nitrogen or upper product tap 16 to remove the lighter component, and a refrigerant fluid inlet or helium feed connection 17 in communication with the upper tube bundle, as will be described below. Lifting eyes 18 are provided on the top assembly and on the cylindrical body shell for installing the units onto the base assembly during construction or for removing the units should dismantling of the column 1 become necessary.

A feed connection 19 extends outside the central portion of the outer periphery of the cylindrical body shell and joins a feed distributor 20 shown as a perforated pipe inside the column. The feed connection acts as an inlet into the column for introducing the fluid to be rectified. In this example, air is supplied to the fractionating column through the feed connection 19.

As shown in FIGURE 2, the helium feed connection 14 is connected to the lower tube bundle 21 by way of a manifold 22, base connection tubes 24, and tube headers 23. A flexible or expansion section 25 is provided in the helium feed connection suitably in the form of a metal bellows, between the lower bottom cap of the tower and the manifold. A manhole 26 is provided in the bottom cap 3 to facilitate assembly and subsequent inspections or maintenance. The lower tube bundle 21 is connected to the upper manifold 28, by the collecting upper headers 27, and upper connectors 61. A flexible joint 48 connects the upper manifold 28 to the return connection 12, and flexible joint 29.

Similarly, an upper tube bundle 30 is linked to inlet connection 17, through a flexible connection 31, distributing manifold 32, inlet connection tubes 63, and headers 33. Upper tube bundle 30 is joined at its lower or outlet end to collection headers 34, which are in turn connected to manifold by outlet connection tubes 64. Manifold 35 is in communication with return connection 15 through return riser conduit 50 (see also FIGURE 5). Flexible sections 36 and 65 join either end of riser 50 to return conduit 15 and manifold 35, respectively. A manhole 37 is provided in the cap assembly 9 to aid assembly and subsequent inspection or maintenance.

Structurally connected within the inside of the body shell is an upper liner which lies within and is con centric with the cylindrical body shell 5. The liner has a peripheral flange which when joined to the flange of the cap becomes the flange 10 of the cap assembly, thus supporting the liner and its afiixed parts relative to the cap. This liner acts as a supporting frame for sieve trays 38, weirs 49, downcomers 39, which are vertically spaced apart and for the upper tube bundle 30 which together form a unit assembly which may be inserted into and removed from the column as a unit. Plates 38 suitably of conventional bubble plate design having openings 70 therein, cover the-entire cross sectional area inside the liner with the exception of sector 75 (see FIGURE 3) at the side of the tray where they join downcomers 39 and weirs 49. Th eindividual tubes 46 of the tube bundle pierce the trays 38 and preferably make good thermal contact therewith. The tube bundle cross section in a preferred embodiment forms a uniform matrix within these boundaries. The active sieve area of the plate preferably encompasses the entire cross sectional area including the portion through which the tubes pass and can extend a distance beyond, short of the overhanging downcomer (see particularly FIGURES 3 and 4). Construction of the lower section of the tower is similar and is understood by reference to the drawing and the above description.

In operation, a refrigerant, for example helium, enters the top tube bundle at a temperature sufliciently colder than the nitrogen leaving the tower that heat is absorbed from the tower contents, condensing nitrogen on the outside tube surfaces of the upper part of the tube bundle. Condensation of gases from the tower contents on the outside surfaces of the tubes continues the length of the tube bundle at the same time, liquid on the plates of the fractionating column is also cooled by the refrigerant by heat transfer through the walls of the tubes which pass through the plates and, in turn, also efiect condensation of gas bubbling through liquid on the plates adding to the efficiency of the desired heat exchange. The temperature of the condensate increases as it moves downwardly from plate to plate in the column with the composition of the condensate becoming increasingly richer in oxygen content. Helium inside the tubes meanwhile is warmed by its acceptance of the heat released by condensation of air components outside the tubes. After completing its desired cooling function, the helium leaves the top tube bundle and the tower as previously indicated.

The heat exchange medium, e.g. helium entering the bottom of the tower is at a temperature such that the bottom product, e.g. oxygen, cools the heat exchange medium which thus furnishes heat to the tower contents and vaporizes some of the liquid at the bottom of the tower. As the helium flows upward through the lower tube bundle, it continues to give heat to the tower contents, causing additional vaporization of liquid of even poorer oxygen content. The creation of an upwardly flowing vapor stream in the lower part of the tower is, in effect, a staged stripping operation, necessary for the production of high purity oxygen product. As in the case of the upper bundle, conduction of heat from the metal plates to the tubes facilitates rapid and eflicient heat transfer.

The air to be rectified, previously chilled to approximately saturation temperature, entering the column through feed connection 19 and feed distributor 20, joins upwardly flowing vapor from the bottom section of the tower and passes upward through openings 70 in the plates 38 above the feed inlet. In so doing, it also passes the collecting headers 34, outlet connection tubes 64, and manifolds 35 of the upper tube bundle 30, resulting in some additional beneficial condensation and rectification. As the vapor passes up through the succssive plates, it comes into intimate contact with the liquid on the plates and is cooled and rectified. In its passage between plates, it is also further cooled by contact with tubes comprising the upper tube bundle. These actions result in partial condensation and staged rectification until the lighter product or vapor rising from the top tray is practically pure nitrogen.

The liquid so generated collects on the plates, overflowing the weirs 49 on each plate to flow downward through the open area 75 between the liner 4G and downcomer 39 in the column to the next lower plate. Downcomers 39 extend below the liquid level of the next lower plate to maintain a liquid seal preventing vapor from bypassing the plates. The downcomers 39 and weirs 49 are attached to the side of each tray. The side of the tray having the downcomers and weirs attached thereto is alternated for successive adjacent trays in the column.

The liquid generated in the upper section of the tower enters the lower section as feed. As it descends from plate to plate it is subjected to a staged stripping by the action of the distributed heating of the lower tube bundle 21 and by the action of the upflowing vapor so generated. The liquid becomes progressively richer in oxygen content as it passes downward in the tower, being practically pure oxygen by the time it reaches the bottom of the tower.

In this example, liquid oxygen is removed through tap 13 in the base and gaseous nitrogen is removed through tap 16 in the cap 9, as products of the separation.

The following is a specific example of a column in accordance with the present invention in which the column effects separation of air to produce 150 tons per day of liquid oxygen o 98 percent purity at 294 F. and 565 tons per day of gaseous nitrogen of 97 percent purity at -3l5 F. in which helium is the heat transfer fluid in both the upper and lower tube bundles. In this example, 715 tons per day of air at 308 F. and 1 percent wet is fed to the column operated at 20.7 p.s.i.a. The minimum temperature drop in the upper and lower tube bundles across the tube walls is 5 F. and the pressure drop measured from the top of the tube bundles to the bottom is 2.5 p.s.i. The upper tube bundle is supplied with 858 tons per day of helium at 357 F. and 182.5 p.s.i.a.; the lower tube bundle is supplied with the same flow rate of helium at 28l F. and 527.5 p.s.i.a., the pressures and temperatures being determined by characteristics of the heat pump system as well as by the tower requirements. The upper tube bundle is made up of 800 /s-inch internal diameter tubes and the lower tube bundle comprises 800 A -inch internal diameter tubes. The overall tower dimensions are approximately 5 feet in diameter by 39 feet in height. The upper section contains 20 plates spaced 1 foot apart while the lower section contains 13 plates spaced 1 foot apart.

The advantages of distributed heat exchange in fractionating towers are not limited to the separation of air into two component streams. Distributed heat exchange can be applied to other separations, including those having several feeds and several takeotfs. Obviously, the advantages accrue in towers of large, moderate, or small size. The invention, therefore, is not restricted to the particular separation in the above examples, namely, separation of air into liquid oxygen and gaseous nitrogen, nor to the specific choice of helium as the heat exchange medium. Ihe specific examples do, however, illus trate a practical construction of a distributed heat exchange column which can be applied to arious separation problems by those skilled in the art.

I claim:

1. A plate-type fractionating column for cryogenic service comprising:

(a) a closed vertical cylindrical body shell;

(b) a lower tube bundle vertically disposed within the lower portion of said body shell;

(c) means for supplying heat exchange fluid to the lower end of said lower tube bundle;

(d) means for withdrawing said fiuid from the upper end of said lower tube bundle;

(e) an upper tube bundle vertically disposed within the upper portion of said body shell;

(f) means for supplying heat exchange fluid to the upper end of said upper tube bundle;

(g) means for withdrawing said fluid from the lower end of said upper tube bundle;

(h) vertically spaced substantially horizontal bubble plates within said body shell pierced by said tube bundle tubes;

(i) means for introducing fractionation feed material to said body shell intermediate said tube bundles;

(j) means for withdrawing product from the upper portion of said fractionating column above said upper tube bundle; and

(k) means for withdrawing product from the lower portion of said fractionating column below said lower tube bundle.

2. A plate-type fractionating column according to claim 1 wherein said tube bundles are each surrounded by a liner attached to said bubble plates and to said body shell for structural support of said plates and tube bundles therein.

3. A plate-type fractionating column according to claim 1, wherein said enclosed hollow cylindrical body shell is a composite structure comprising:

(a) a hollow cap assembly having a circular flange at its open end;

(b) a hollow cylindrical body shell having flanges integral with its upper and lower open ends; and

(c) a base assembly in the form of a hollow cylindrical cup having a circular flange at its open end.

4. A plate-type fractionating column according to claim 3 comprising:

(a) an upper liner which supports plates, weirs, downcomers, tube bundle and manifolds associated therewith having an external flange about its outer periphery which is sandwiched between said flanges of said upper body shell and said cap assembly so as to structurally support said liner; and

(b) a bottom liner which supports plates, weirs, downcomers, tube bundle and manifolds associated therewith having an external flange about its outer periphery which is sandwiched between said flanges of said lower end of said body shell and said base assembly, thus structurally supporting said bottom liner.

5. A plate-type fractionating column as defined in claim 1 in which comprises:

(a) a lower feed manifold in communication with the bottom end of said lower tube bundle;

(b) a heating fluid feed conduit in communication with said lower manifold;

(c) a lower return manifold in communication with the upper end of said lower tube handle;

(d) a return conduit in communication with said lower return manifold;

(e) an upper feed manifold in communication with the upper end of said upper tube bundle;

(f) an upper refrigerant feed conduit extending through the upper part of said body shell in communication with said upper feed manifold;

g) an upper return manifold in communication with the lower end of said upper tube bundle; and

(h) an upper return conduit extending through the upper portion of said body shell in communication with said upper manifold return.

6. In a plate-type fractionating column for cryogenic service comprising a closed vertical cylindrical hollow body shell having spaced horizontal plates with weirs and downcomers attached to alternate sides of adjacent plates, a top product tap at the top of said body shell for obtaining the lighter separated product from said column, a base product tap at the base of said body shell for obtaining the heavier separated product from said column, a feed inlet in said body shell intermediate said top product tap and said base product tap, a lower heat exchange means in the lower portion of said body shell below said feed inlet, and an upper heat exchange means in the upper portion of said body shell, the improvement which comprises:

(a) an upper vertical tube bundle comprising said upper heat exchange means piercing said plates in the upper portion of said body shell;

(b) an upper liner within and structurally supported by said body shell supporting said tube bundle, plates, weirs and downcomers in said upper portion;

(c) a lower vertical tube bundle comprising said lower heat exchange means piercing said plates in the lower portion of said body shell; and

(d) a lower liner within and structurally supported by said body shell supporting said tube bundle, plates, weirs and downcomers in said lower portion.

7. In a plate-type fractionating column for cryogenic service comprising a closed vertical cylindrical hollow body shell having spaced horizontal plates with weirs and downcomers attached to alternate sides of adjacent plates, a top product tap at the top of said body shell for obtaining the lighter separated product from said column, a base product tap at the base of said body shell for obtaining the heavier separated product from said column, a feed inlet in said body shell intermediate said top product tap and said base product tap, a lower heat exchange means in the lower portion of said body shell below said feed inlet, and an upper heat exchange means in the upper portion of said body shell, the improvement which comprises the said lower and upper heat exchange means comprising:

(a) a vertical tube bundle located within said column and extending over a substantial portion thereof;

(b) means for introducing heat transfer fluid into one end of said tube bundle;

(c) means for withdrawing said heat transfer fluid from the opposite end of said tube bundle;

(d) an open ended housing within the cylindrical hollow body shell surrounding said tube bundle;

(e) the said horizontal plates being pierced by tubes of said tube bundle and supported by said housing to form a unit assembly; and (f) means for supporting said unit assembly vertically within said fractionating column.

NORMAN References Cited UNITED STATES PATENTS YUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner.

US. Cl. X.R.

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