US3048373A

US3048373A - Heat exchange apparatus and method

Info

Publication number: US3048373A
Application number: US681404A
Authority: US
Inventors: Robert D Bauer; Carnot E Bellinger
Original assignee: Phillips Petroleum Co
Current assignee: Phillips Petroleum Co
Priority date: 1957-08-30
Filing date: 1957-08-30
Publication date: 1962-08-07
Anticipated expiration: 1979-08-07

Description

Aug. 7, 1962 R. D. BAUER ETAL 3,043,373

HEAT EXCHANGE APPARATUS AND METHOD Filed Aug 50, 1957 FIG. 5

HEATING 22 HEATING FLUID OUTLET BOTTOM PRODUCT OUTLET INVENTORS R.D. BAUER c .E. BELLINGER A TTORNEVS 3,048,373 Patented Aug. 7, 1962 3,048,373 HEAT EXCHANGE APPARATUS AND METHOD Robert D. Bauer, Bartlesville, Okla, and Carnot E. Bellinger, Sweeny, Tex., assignors to Phillips Petroleum Company, a corporation of Delaware Filed Aug. 30, 1957, Ser. No. 681,404 5 Claims. (Cl. 257-239) This invention relates, in general, to apparatus and method for the heat exchange of liquids. More particularly, this invention relates to heat exchange apparatus of a tube-shell type wherein the liquid to be heated is partally vaporized in the shell of said heat exchange aparatus. In one aspect, this invention relates to an improved reboiler for use with a fractionation tower. In another aspect, this invention relates to an improved fractionation process.

In many heat transfer operations, it is desirable to subject a liquid to indirect heat exchange in order to vaporize a portion of the liquid for the purpose of either separating and recovering the vaporized portion or concentrating the liquid from which a portion has been vaporized. In either type operation, it is necessary, in order to provide an economical process, that the liquid feed not be subjected to any higher temperature than is necessary to effect vaporization of the desired portion of the feed; that is, the vapor evolved from the liquid feed should not be superheated because the heat carried away in such superheated vapor is lost from the heat exchange process. If the amount of heat lost in such superheated vapor is very large, the heat exchange process is very uneconomical.

In many conventional fractionating processes, a feed stream is fed into a vapor-liquid contacting device or tray located between the upper and lower ends of a vertical column. Although some heat may be supplied to the liquid feed before entering the column, it is necessary to supply heat in some manner to the lower section of the column if fractionating action is to occur on the trays below the point of introduction of the feed. It is standard practice to supply such heat by withdrawing a portion of the liquid from the bottom of the fractionation column and returning it to some elevated point in the column as the vapor, thereby supplying heat to the lower section of the column. It is desirable that the vapor notbe returned to the column in a superheated condition because the vapor-liquid equilibrium existing on the trays to which the vapor is returned would be upset. The vaporization of the liquid withdrawn from the bottom of the column is often effected, as standard practice in the art, in a horizonally disposed heat exchanger of the tube and shell type wherein the liquid withdrawn is vaporized in the shell of the heat exchanger by the transfer of heat from a heat exchange liquid circulated in the tubes. In such aparratus, it is difiicult to avoid superheating the vapor to be returned to the fractionation column.

An object of this invention is to provide shell-tube type heat exchange apparatus and method wherein liquid is vaporized in the shell of said apparatus without superheating. Another object of this invention is to provide a horizontally disposed heat exchange apparatus of the shell and tube type wherein vapor binding around the tubes is eliminated. Another object of this invention is the provision of a liquid outlet accumulator disposed in a horizontal heat exchange apparatus of the shell and tube type to eifect control of the level of the liquid being vaporized within the shell of said apparatus. Another object of this invention is the provision of a feed distributor disposed above the tubes in a horizontal heat exchange :apparatus of the shell and tube type to form a curtain of liquid feed for contacting the vapors evolved from the liquid being heated. Another object of this invention is to provide an improved fractionation process utilizing an external reboiler.

Other aspects, objects and advantages of the invention are apparent from a consideration of the accompanying disclosure, drawings and the appended claims.

In accordance with this invention, heat exchange apparatus of the shell and tube type is provided for vaporizing a portion of the liquid to be heated within the shell of said apparatus, said apparatus comprising, in combination, a horizontally disposed shell, a plurality of heat exchange tubes located in the lower portion of said shell and adapted for the circulation of the heat exchange fluid therethrough, a liquid feed inlet means located in the top of said shell, a liquid feed distributing means horizontally disposed above said heat exchange tubes in the vapor space between said tubes and said shell and adapted to receive said liquid feed and to provide a curtain of liquid feed for contacting the vapors evolved from the liquid being heated, and a vapor outlet means located in the top of said shell, and a liquid collecting zone disposed within the shell of said apparatus adjacent one end of said heat exchange tubes and having a liquid outlet means for withdrawing from said apparatus heated liquid and adapted for controlling the level of liquid maintained in contact with said heat exchange tubes.

In accordance with another concept of this invention, there is provided a vertically disposed baffle in a horizontally disposed heat exchange apparatus of the shelltube type wherein a liquid which is fed into said apparatus through the top thereof is heated and a portion vaporized in the shell of said apparatus by the circulation of a heat exchange fluid through heat exchange tubes submerged in a body of said liquid maintained in said apparatus and the vapors evolved are withdrawn from the top thereof, said baffle dividing said heat exchange apparatus into a heat exchange zone and a liquid collecting zone, said baffle having a first opening iri the lower portion thereof, permitting the how of heated liquid from said heat exchange zone into said liquid collecting zone and a second opening in the upper portion thereof, permitting the flow of vapor from said liquid collecting zone into said heat exchange zone.

In accordance with another concept of this invention, there is provided a liquid collecting zone within a horizontally disposed heat exchange apparatus of the shelltube type wherein a liquid which is fed into said apparatus through the top thereof is heated and a portion vaporized in the shell of said apparatus by the circulation of a heat exchange fluid through heat exchange tubes submerged in a body of said liquid maintained in said apparatus and the vapors evolved are withdrawn from the top thereof, said liquid collecting means having a liquid outlet means for the removal of liquid heated in said apparatus and collected in said liquid collecting zone and liquid level control outlets located in the top and bottom of said liquid collecting zone for controlling the level of liquid in the liquid collecting zone and thereby controlling the level of the liquid being heated to prevent superheating of the vapors evolved.

in accordance with another concept of this invention,

gen e /s there is provided a liquid feed distributor tray means in a horizontally disposed heat exchange apparatus of the shell-tube type for contacting the vapors evolved from the liquid heated in the shell of said heat exchange apparatus with a shower of entering liquid feed in order to prevent superheating the vapors evolved from said liquid, said liquid feed distributor tray means being horizontally disposed above the heat exchange tubes adjacent the top of the shell of said heat exchange apparatus. 'In a preferred embodiment, the liquid feed distributor tray means has serrated vertical walls and extends the substantial length of the heat exchange Zone.

In accordance with another concept of the invention, there is provided a method of preventing superheating of vapors evolved in a horizontally disposed heat exchange apparatus of the shell-tube type wherein a liquid is vaporized by circulation of a heat exchange liquid through heat exchange tubes submerged in said liquid, said method comprising controlling the level of liquid in said heat exchange zone by regulating the level of liquid in an adjacent liquid collecting means in open liquid communication with said heat exchange zone.

In accordance with another concept of this invention, there is provided a method of controlling the level of liquid being heated in a heat exchange zone in a horizontally disposed heat exchange apparatus of a shell-tube type, said method comprising regulating the level of liquid in an adjacent liquid collecting zone in open liquid communication with said heat exchange zone to thereby control the level of liquid in said heat exchange zone.

In accordance with another concept of this invention, there is provided a method for separating a stream in a. fractionation tower, said method comprising withdrawing a liquid stream from the bottom of said tower, vaporizing a portion of said liquid stream in an external reboiler without superheating the vapor evolved, and returning said vapor onto a tray at an elevated location in said fractionation tower.

Referring to the drawings forming a part of this application, FIGURE 1 is a longitudinal sectional view of the heat exchange apparatus, FIGURE 2 is a sectional view taken along the line IIII of FIGURE 1, FIGURE 3 is a sectional view taken along the line III-III of FIGURE 1, FIGURE 4 is a perspective view of a preferred liquid feed distributor tray, and FIGURE 5 is a plan view of another torm of bafile.

In FIGURE 1, there is shown a heat exchange apparatus, comprising a shell containing a number of heat exchange tubes 11 arranged in a bundle in the lower portion of shell 10. Shell 10 comprises a cylindrical body havingone end closed by a unitary end wall 12 and a removable head 13 at the opposite end. Tubes 11 are secured at one end to a tube sheet 14 which is clamped between a flange 15 of shell 10 and a flange 16 of removable head 13. At the opposite end, the tubes are secured to a floating tube sheet 17 to which a floating head 18 is secured.

Removable head 13 comprises an inlet header 19 having an inlet connection 20 and an outlet header 21 having an outlet connection 22. Floating head 18 provides a reversing chamber at the opposite ends of tubes 11 for directing the heating fluid in its passage from inlet header 19 through a number of the tubes 11 in one direction and then through the remaining tubes 11 in the opposite direction to the outlet header 21. The heating fluid may be any suitable fluid having an elevated temperature, such as steam or flue gases, and preferably steam is employed. It is to be understood that a number of tubes 11 and any desired arrangement of the tubes can be used. Although tubes 11 are shown in FIGURE 1 as straight horizontally disposed elongated tubes, tubes having other configurations, such as continuous helical tubes, can be used. It is to be noted in the embodiment of FIGURE 1 that the entire tube bundle can be removed as a unit from shell 10 by disengaging flanges l5 and 16.

Shell 1! is provided with a liquid inlet 23 and a vapor outlet 24 in the top thereof. A bottoms product outlet. 25 is provided in the bottom of shell 10 adjacent the enclosed end 12. Liquid inlet 23 extends a short distance into shell 13' so that the open end of liquid inlet 23 terminates immediately above or within horizontally disposed liquid feed distributor tray 26 which is located above tubes 11 in the top of shell iii. Liquid feed distributor tray 26 extends a substantial length of. heat exchange tubes 11 and is mounted substantially horizontally so that the liquid feed entering through liquid inlet 23 overflows the vertical walls of liquid feed distributor tray 26 and thereby forms a continuous curtain of liquid feed. The upper edges of liquid feed distributor tray 26 can be constructed in a smooth continuous manner or can be constructed with discontinuous edges, such as serrated form shown in FIGURE 4.

Vertical baflle 27, extending from the top of shell 10 to a point adjacent the bottom of shell 10 is located in the closed end thereof adjacent floating head 18 to thereby form a liquid collecting zone 28 in the closed end of shell 10. Bottoms product outlet 25 in the bottom of shell 10 is located within liquid collecting zone 28. The opening between the end of battle 27 in the bottom of shell 10 permits the flow of heated fluid from the heating zone surrounding heat exchange tubes 11 into liquid collecting zone 28 and then out of the heat exchange apparatus through bottoms product outlet 25. The flow of product through bottoms product outlet 25 is controlled by valve 29 in accordance with the liquid level 30 in liquid collecting zone 28 as determined by liquid level controller 31 operatively connected to valve 29. Vapors evolved in liquid collecting zone 28 are returned to the vapor space above exchange tubes 11 through opening 31 in bafiie 27 adjacent the top of shell 10.

In operation, the liquid to be heated and/or vaporized is supplied to the heat exchange apparatus through liquid inlet 23 and discharged into liquid feed distributor tray 26 which it overflows and forms a vertical curtain of fall'- ing liquid, Some level of liquid, such as level 32, is maintained in the heat exchange zone and the liquid is heated by the circulation of heating fluid through heat exchange tubes 11. The vapors evolved from the heated liquid collect in the vapor space in the top of shell 10 above heat exchange tubes 11 and are Withdrawn through vapor outlet 24. The heated liquid flows from around heat exchange tubes 11 underneath vertically disposed baffle 27 and into the liquid collecting zone 28 from which a portion is removed through bottoms product outlet 25. The level of liquid 32 surrounding heat exchange tubes 11 is controlled by the level of liquid 30 in liquid collecting zone 28. The level of liquid 30 in liquid collecting zone 28 is in turn determined by the position of valve 29 in bottoms product outlet 25 as determined by liquid level controller 31. Ordinarily, liquid level 32 will be maintained at an elevation suflicient to completely just sufficiently submerge all of heat exchange tubes 11; however, as desired, liquid level 32 can be maintained at an elevation above or below the uppermost heat exchange tubes 11. Preferably, all heat exchange tubes 11 are surrounded by liquid in order to avoid vapor binding around the tubes and also to avoid superheating the vapor evolved from the body of heated liquid. Depending upon the rate of flow of liquid into the heat exchange apparatus, liquid level 36 in liquid collecting zone 28 will ordinarily be maintained at a substantial lower elevation than the elevation of liquid level 32 and the lower edge of vertically disposed bafile 27 will at all times be submerged; however, liquid level 30 may, in some operations, be maintained at an elevation substantially equal to the elevation of liquid level 32.

In any operation wherein the vapors evolve from the body of heated liquid may have become superheated, as for example, when the elevation of liquid level 32 is maintained at some point which exposes a few of heat exchange tubes 11 to heat transfer contact with the vapor, the falling curtain of liquid overflowing liquid feed distributor tray 26 contacts such superheated vapor and prevents superheated vapor from being discharged from the heat exchange apparatus through vapor outlet 24.

In FIGURE is shown a modified bafile 27 which extends completely across the cross-section of shell and is provided with three circular openings 33 in the lower section thereof adjacent the bottom of shell 10. If desired, baffle 27 can be constructed with square openings, elongated slots, or an opening of any other configuration which will provide for the flow of fluid from the heat exchange zone into the liquid collecting zone. It is, therefore, understood that this invention is not limited to the use of an underflow baffle, as shown in FIGURE 3.

The heat exchange apparatus of this invention is useful, generally, for heating a liquid for the creation of vapor without superheating the vapor obtained. The heat exchange apparatus of this invention is very useful as an external reboiler for supplying heat to the lower section of a fractionation tower by heating a liquid fraction withdrawn from the bottom of said tower to generate the vapors fraction which is returned to the lower section of said tower at some point above the bottom of said tower. In such an operation in combination with a fractionating tower, the liquid fraction withdrawn from the bottom of the fractionating tower enters the heat exchange apparatus of this invention through liquid inlet 23 and is heated by the passage of steam through heat exchange tubes 11. The vapor generated, at vapor-liquid equilibrium condition, is withdrawn from the heat exchange apparatus through vapor outlet 24 and returned to the lower section of the fractionation tower. Bottoms product is obtained from the heat exchange apparatus of this invention through outlet 25 from liquid collecting zone 28. The apparatus of this invention can also be employed as an external reboiler in combination with separation columns of other types, such as a deethanizing absorber tower wherein the bottoms product is continuously contacted in the lower or stripping section of the tower with an absorption oil which is continuously recycled through the heat exchange apparatus of this invention in order to inject the absorption oil as a vapor into the bottom of said tower. The apparatus of this invention is not limited to use in combination with separation columns, as above described, but can be used in any unit operation wherein it is desired to generate a vapor stream by heating a liquid stream.

The heat exchange apparatus of this invention is particularly advantageous in the heating and vaporization of liquid streams in which a variation in composition is encountered, for example, a bottoms fraction obtained from a separation tower. Changes in composition of the liquid stream supplied to a horizontally disposed shelltu'be type heat exchange apparatus results in the constant changing of the elevation of the liquid level in the heat exchange apparatus so that, in some operations, some of the heat exchange tubes are exposed to the vapor evolved from the liquid. The failure to keep the heat exchange at all times surrounded by the liquid to be heated results in a loss of heat transfer capacity because the heat transfer coefiicients of a gas are much lower than those of a liquid. The exposure of the heat exchange tubes to the vapor also results in superheating of the vapor, and, therefore, a loss of heat. The apparatus of this invention overcomes these difiiculties by permitting the level of liquid in contact with the heat exchange tubes to be maintained substantially constant, even with a liquid feed of varying composition.

Specific Example In applying the invention to a debutanizing operation wherein butane and lighter are separated overhead, in a debutanizing tower, from a charge comprising butane and lighter plus heavier components including up to octanes,

the following conditions exist in the reboiler unit along with the following stream compositions.

By a simple bubble point, calculation, it can readily be shown that the vapor removed via 24 is substantially in equilibrium with liquid removed via 25, at p.s.i.a. and 278 R, which is desired and effected by the invention. That is, no superheating of the produced vapors occurs.

Reasonable variation and modification are possible within the scope of the foregoing disclosure, drawings and the appended claims to the invention, the essence of which is that there have been provided an apparatus and method for efliciently heating a liquid and vaporizing a portion thereof without superheating the vapor produced, the apparatus comprising, in combination, in a horizontally disposed heat exchange apparatus of the shell-tube type wherein a liquid which is fed into said apparatus through the top thereof is heated and a portion vaporized in the shell of said apparatus by the circulation of a heat exchange fluid through heat exchange tubes submerged in a body of said liquid maintained in said appartus and the vapors evolved are withdrawn from the top thereof, a vertically disposed baffle dividing the shell of said heat exchange apparatus into a heat exchange zone and a liquid collecting zone and adapted to provide for the flow of heated liquid from said heat exchange zone to said liquid collecting zone through an opening in the lower portion of said baffle, and a liquid feed distributor tray means horizontally disposed above said heat exchange tubes in said heat exchange zone for developing a shower of liquid which contacts the vapors evolved from the heated fluid and prevents the discharge of superheated vapors from said heat exchange apparatus; the method comprising (1) as a method of operating a separation column, passing a portion of the bottom fraction into a heat exchange zone, vaporizing a portion of said bottom fraction without superheating the vapor formed, and returning said vapor to the lower section of said separation column to thereby supply heat to said separation column, (2) as a method of controlling the level of liquid in the shell of a horizontally disposed heat exchange apparatus of the shell-tube type, regulating the level of a liquid in an adjacent liquid collecting zone in open liquid communication with said heat exchange zone.

We claim:

1. In a horizontally disposed heat exchange apparatus of the shell-tube type, vertically disposed bafiie means dividing said apparatus into a heat exchange zone and an unobstructed liquid collection zone, said baffle means having a first opening in the lower portion thereof to provide for the flow of liquid from said heat exchange zone into said liquid collection zone, said baffle means having a second opening in the upper portion thereof to provide for the free and unobstructed flow of vapor from said liquid collection zone into said heat exchange zone, a bundle of heat exchange tubes disposed in the lower portion of said heat exchange zone, liquid feed inlet means and vapor outlet means both located directly above said bundle, liquid valved outlet means located in the bottom of said liquid collection zone, and a liquid feed distributor tray means located in said heat exchange zone entirely above said bundle and directly 'below said liquid feed inlet means and said vapor outlet means, said tray distributor means adapted to provide a curtain of said liquid feed for contacting evolved vapor.

2. Apparatus according to claim 1, further comprising liquid level control means operatively connected to said liquid collection zone and said liquid valved outlet means and adapted to control the level of liquid in said zones.

3. Apparatus according to claim 2 wherein said tray distributor means is a horizontally disposed rectangular 10 tray having vertical walls the top edge of which is serrated.

4. A heat exchange method comprising continuously introducing liquid feed into the top of a horiozntally disposed heat exchange zone the bottom portion of which is provided with a bundle of heat exchange tubes substantially immersed in said liquid, distributing said introduced liquid feed in the form of a downwardly descending curtain above said bundle, evaporating only a portion of said introduced liquid feed, allowing evolved vapor to 5. The method according to claim 4, wherein said level of liquid in said zones is controlled by balancing the withdrawal of liquid from said liquid collecting zone against the flow of liquid from said heat exchange zone into said liquid collecting zone.

References Cited in the file of this patent UNITED STATES PATENTS 277,280 Hoeveler May 8, 1883 1,555,435 Rohrer Sept. 29, 1925 1,617,081 Price Feb. 8, 1927 1,760,907 Jones June 3, 1930 2,091,757 Hanny Aug. 31, 1937 2,302,993 Graham Nov. 24, 1942 2,413,102 Ebert et al. Dec. 24, 1946 2,428,768 Bertram Oct. 14, 1947 2,524,715 Rathbun Oct. 3, 1950 2,729,600 Beach Jan. 3, 1956 FOREIGN PATENTS 588,820 England June 4, 1947 OTHER REFERENCES Badger et a1.: Introduction to Chemical Engineering, McGraw-Hill Book Company, Inc., New York, 1955, page 182, Figs. 5-l0.

Kern: Process Heat Transfer, published by Mc- Graw-Hill Book Company, Inc., New York, 1950, pages 471, 472.