US3456719A - Transfer line heat exchanger - Google Patents

Description

July 22, 1969 E. H. PALCHIK 3,456,719

TRANSFER LINE HEAT EXCHANGER Filed Oct. 5, 1967 s Sheets-Sheet 1 IN VENTOR. Edward H. Polchik Fig. l.

772mm & Arronuevs y 1969 E. H. PALCHIK 3,456,719

TRANSFER LINE HEAT EXCHANGER Filed Oct. 5, 1967 5 Sheets-Sheet 2 H.P. Water 1' 70 LP. 62 Wqter 76 56 I g L; l Gas "m Fig. 4.

INVENTOR. Edward H. Pulchik WZW ATTORNEYS y 1969 E. H. PALCHIK TRANSFER LINE HEAT EXCHANGER 5 Sheets-Sheet 5 Filed Oct. 5, 1967 INVENTOR. Edward H. Polchik ATTORNEYS 3,456,719 TRANSFER LINE HEAT EXCHANGER Edward H. Palchik, Queens Village, N.Y., assignor to The Lummus Company, New York, N.Y., a corporation of Delaware Filed Oct. 3, 1967, Ser. No. 672,479 Int. Cl. F28d 7/12, 7/02 US. Cl. 165-142 Claims ABSTRACT OF THE DISCLOSURE Background of the invention This invention relates generally to heat exchangers and, more particularly, to heat exchangers used to quench hot gases issuing from cracking or pyrolysis heaters. Such heat exchangers are called transfer line exchangers or cracked gas coolers.

It is known that olefinic hydrocarbons may be prepared by thermal cracking of gaseous or completely evaporable liquid hydrocarbons in metal tubes, in a mixture with steam, at temperatures above 750 C. This forms cracked gas rich in olefins, such as ethylene and propylene, and also containing higher olefins, diolefins, and other cracking products. To prevent secondary reactions, the highly reactive gas mixture must be quickly cooled. This cooling is in practice accomplished by direct injection of coolants, as for instance liquid hydrocarbons, into the reaction mixture, or by indirect cooling, e.g. with the aid of water, in a cracked-gas cooler.

In general, to improve heat recuperation, indirect cooling is preferred. A disadvantage of indirect cooling, however, is that deposits of coke or other cracking products are often formed, especially in the connecting duct between the cracking furnace and the cracked gas cooler, or in the individual cooling tubes of the cooler. These deposits increase the pressure difference in the system and interfere with the uniform distribution of the hot cracked gas on the cooling surfaces. The plant must then be shut down at longer or shorter intervals and the coking products must be cleaned out.

The connecting duct (between the cracking furnace and the cracked-gas cooler) used to deliver the cracked gas to the cooling tubes, is, in general, so designed that it gradually widens in diameter from the diameter of the cracking tube or tube manifold to the diameter of the cooler shell. To avoid vortices, which prolong the residence time in this area and thereby cause secondary reactions, the angle of the cone to the direction of flow must be kept as small as possible. When the cooler shell is of large diameter, however, the connecting line forms a relatively large space, and it has been found that the long residence time of the cracked gas in this space results in coking and decreased olefin yield.

To prevent condensing heavy polymers on the gas side of the tube, the tube wall temperature must be controlled. When the cooling medium is evaporating water, the pressure must be very high (80 to 100 atmospheres) to give the required tube wall temperature. One type of exchanger construction well suited to such pressure is the double tube design, because it does not require an excessively thick shell or tube sheet. However, high severity 3,456,719 Patented July 22, 1969 pyrolysis requires extremely rapid cooling and very low pressure drop across the cooler. This requires many parallel tubes of small diameter. A disadvantage of the double tube exchanger is that tubes cannot be located on close centers. When many parallel tubes are required the volume of the inlet device becomes large, forming an adiabatic retactor, the same as a large or long transfer line. The adiabatic reaction reduces the temperature and causes coke formation, which increases the pressure drop and limits the length of operation between cleanings.

Heretofore, a variety of expedients have been employed to overcome these problems. Generally, these have involved some sort of modification of the inlet cone connecting the heater outlet with the tube side of the exchanger. Extension of the tubes beyond the tube sheet into the narrow end of the cone is one proposal. Building up of the cone to a general trumpet-bell configuration is another proposal. Use of gas-directing baflles which partially fill the cone is still another proposal. While all of these proposals generally reduce the residence time of the gas in the cone, reduce gas eddying, or both, their net effect is an amelioration of the problem rather than a solution to it.

Objects It is a general object of the present invention to provide an improved heat exchanger which overcomes the foregoing problems.

Another object of the present invention is to provide a transfer line exchanger directly attachable to a heater outlet.

Still another object of the present invention is to provide a transfer line exchanger having essentially no unoccupied cone volume between the heater outlet and the heat exchange surface.

Yet another object of this invention is to provide a transfer line exchanger without any high pressure tube sheets.

A still further object of the invention is to provide a transfer line exchanger which will quench hot gases quickly and which will minimize coking problems.

Still another object of the invention is to provide a transfer line exchanger which can be de-coked without taking the exchanger out of service.

Various other objects and advantages of the invention will become clear from the following description of several embodiments thereof, and the novel features will be particularly pointed out in connection with the appended claims.

Summary of invention In essence, the present invention comprises a heat exchanger having a generally conical inlet on the shell side for the hot gases, and heat exchange surfaces running the length thereof and extending into the conical portion. In one embodiment, the heat exchange surfaces comprise a group of concentric hollow cylinders surrounding a central pipe or bayonet, the latter extending essentially to the heater outlet. Means are provided for circulating a cooling medium through the cylinders and bayonet. The cylinders may be massive with internal pipes for carrying fresh coolant to the bottom, or they may be comprised of a ring of individual straight tubes attached to a toroidal ring at the lower end, half the pipes bringing fresh coolant down and the other half carrying coolant up for discharge. Gas-directing bafiles are preferably provided on the lower extremity of each hollow cylinder.

In another embodiment, tubes carrying the coolant run through the exchanger completely, a conventional tube sheet being used for support. In this embodiment, a central bayonet pipe is also employed directly over the gas inlet. Gas-directing baffles are also employed.

The drawings Understanding of the invention will be facilitated by referring to the following detailed description in conjunction with the accompanying drawings, in which:

FIGURE 1 is a perspective view of a simplified embodiment of the invention, partially cut away;

FIGURES 2 and 3 are elevation views, in section of two types of hollow cylinder construction which can be employed in the invention;

FIGURE 4 is a cross-sectional elevation of a different embodiment of the invention;

FIGURE 5 is a cross-sectional elevation of manifolding suitable for use in the embodiment of FIGURE 1; and

FIGURE 6 is a plan view of the manifolding of FIG- URE 5.

Description of embodiments With reference to FIGURE 1, a cracking heater 10 is provided with a vertical outlet 12, to which is attached the inlet 14 of the exchanger of the invention, indicated generally at 16.

Exchanger 16 comprises a steel shell 18 with suitable insulation 20 lining the interior wall and defining a conical inlet section 22 and a straight section 24. Suitable structural support (not shown) for exchanger 16 must of course be provided. Insulation 20 may be cast or rammed, and a thin sheet metal cover (not shown) can be provided.

Heat exchange surfaces comprise a central bayonet or tube 26 and surrounding, concentric, hollow cylinders 28, 30, 32. It will be understood that the number of such cylinders provided will depend on the size of the installation. Within bayonet 26 there is a central coolant supply pipe 34 which is connected to coolant supply manifold 36. Pipe 34 extends almost to the bottom of bayonet 26, so that coolant will flow down through pipe 34, up through bayonet 26, and out through outlet manifold 38. In similar fashion, each of the cylinders 28, 30, 32 is provided with a plurality of internal coolant supply pipes 40 all of which are connected to coolant manifold 36. It will be understood that while only one inlet and outlet manifold are shown, a plurality thereof will be used, preferably one of each for each cylinder. It should be further noted that the inlet and outlet manifolds are conveniently used as supports for holding the various cylinders in place, since they are themselves cooled by the coolant.

In the operation of the exchanger, it is important that the gas flow be directed against the entire available heat exchange surface and, to this end, it is preferred to include flow-directing bafiles or vanes on the lower edge of the several hollow cylinders. Such an arrangement is illustrated in FIGURE 2. Cylinder 30 is provided with coolant supply pipes 40 as in FIGURE 1, and. V-shaped ring 42 is welded to the lower edge thereof. The particular angle thereof on each hollow cylinder is selected to distribute gas flow between the various cylinders in proportion to the heat exchange surface available.

A different type of hollow cylinder is illustrated in FIGURE 3. In this embodiment the cylinder is formed of a plurality of straight tubes 44 which are attached to and communicate with a toroidal manifold 46. Half of the tubes 44 are connected at their upper end to the coolant supply manifold, and the other half are connected to the coolant outlet. A V-shaped gas deflector 42 is also provided. It will be noted that the heat exchange surface presented by tubes 44 is considerably larger than that of cylinder 30.

In FIGURE 4 there is shown an embodiment which more closely resembles a conventional shell and tube exchanger. This exchanger has a shell 48, insulation 50, a conical inlet section 52 and a straight, cylindrical section 54. Gas inlet 14 is the same as shown in FIGURE 1. An upper tube sheet 56 and a lower tube sheet 58 support a plurality of tubes 60 through which the coolant flows. Coolant is supplied at inlet 62 and is withdrawn at outlet 64. A bayonet tube 66 having a central coolant supply tube 68 is located directly above the gas inlet, in the same manner as illustrated in FIGURE 1. However, tube 68 requires a separate, higher pressure source of coolant 70 so that it will be forced back up bayonet tube 66 and into chamber 72 where it will mix with coolant passing into tubes 60. A plurality of gas-directing baffles 74 distribute the gas throughout the exchanger and the gas exits through one or more outlets '76.

The embodiment of FIGURE -1 is advantageous in that the conventional high pressure tube sheets are completely eliminated. On the other hand, structural support of the concentric cylinders presents certain engineering problems. The embodiment of FIGURE 4 does require tube sheets, but support of the tubes 60 is simplified, being done in the conventional manner.

In FIGURES 5 and 6 there is illustrated a manifold arrangement which will also serve to support the cylindrical cooling surfaces of the embodiment of FIGURE 1. To simplify illustration, these drawings are out of proportion. Two toroidal manifolds 80, 82 are welded together to for-m a rigid ring-shaped structure. Coolant supply tubes 40 are attached to manifold 82 so as to receive coolant therefrom. Cylindrical housing 30 is welded to both manifolds, but orifices 84 in manifold allow fluid to pass thereinto. Coolant is supplied to manifold 82 by four inlet conduits 88. Since this structure is at the cold end of the exchanger, high temperature alloys are not required. Support for the structure on the shell of the exchanger is provided by the eight equi-spaced conduits which are, of course, self-cooling.

It is to be noted that the transfer line exchanger of the present invention, and in particular the general type of exchanger illustrated in FIGURE 1, is readily adapted for on-stream de-coking by means of thermal shock. By successively shutting off the coolant supply, for a short period, to each of the concentric layers of heat exchange surface, the temperature of that layer approaches that of the surrounding gaseous medium, the layer expands thermally and will tend to crack and spall any coke deposited thereon. The spalled coke particles are carried away in the gas stream and removed by an appropriate conventional device downstream. Since there is no bottom tube sheet, the concentric layers are free to expand downwardly, and the expansion will be the greatest at the bottom, where coke deposition will be the greatest.

It is to be further noted that, in any of the embodiments of the invention, the amount of available heat exchange surface may be increased by the use of vertical fins attached directly to the exposed envelope or tubes. Incorporation of such fins depends on design requirements for heat transfer, allowable metal temperature and the like.

The proper size of the exchanger and mass velocity of gases passing therethrough must be determined from process considerations for a given installation. However, it is clear that the invention has several distinct advantages over conventional cracked gas coolers where the gas is on the tube side, and a large number of relatively small diameter tubes are employed. In such coolers, plugging and fouling can occur relatively easy, and high mass velocities are necessary. In the instant case, plugging is essentially impossible because the gas is not confined in a small space. Further, any coke that does form can be removed much more readily than from a conventional unit, as pointed out above. Also, by eliminating the conical inlet section and, in effect, putting it inside the exchanger, all of the problems associated with such inlets are avoided.

Various changes in the details, steps, materials and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as defined in the appended claims.

What is claimed is:

1. A transfer line exchanger for quench-cooling of cracking heater efiluent gases comprising:

an upright enclosure having a gas outlet at its upper end and a gas inlet at its lower end, said inlet including attachment means for direct connection to the outlet of a cracking heater;

a generally conical inlet section having said gas inlet at its narrow end in communication therewith;

a generally cylindrical section in communication with said conical section at its lower end and said gas outlet at its upper end;

a plurality of hollow, spaced heat exchange surfaces extending longitudinally through said conical and cylindrical sections, and having upper and lower ends; said heat exchange surfaces comprising a plurality of coaxial concentric cylinders of progressively smaller diameter extending progressively further into said conical section; and an axial bayonet tube located directly over said inlet and extending substantially thereto, said tube having means for circulating a cooling medium therein; and

means for supplying a heat exchange medium to, and Withdrawing said medium from, the hollow interior of said heat exchange surfaces.

2. The exchanger as claimed in claim 1, wherein each said cylinders comprise:

a plurality of straight tubes forming a cylindrical body;

and

a toroidal manifold at the lower end thereof and in fluid communication with each of said tubes, a first portion of said tubes acting to supply a cooling medium and the remainder of said tubes acting to discharge a cooling medium.

3. The exchanger as claimed in claim 1 and additionally comprising fiow-directing vanes on the edges of said heat exchanger surfaces nearest said gas inlet.

4. The exchanger as claimed in claim 1, wherein the means for supplying and withdrawing the heat exchange medium comprises:

first manifold means connected to a source of said heat exchange medium and communicating with the interior of said heat exchange surfaces at one end; and

second manifold means communicating with the interior of said heat exchange surfaces at the other end, said second manifold means being connected to a cooling medium outlet.

5. The exchanger as claimed in claim 4, wherein said rst and second manifold means are located at the upper end of said heat exchange surfaces and serve to support the same.

References Cited UNITED STATES PATENTS 1,419,337 6/1922 Werner -142 2,518,688 8/1950 Hasche 260-679 2,645,673 7/ 1953 Hasche 260-679 FOREIGN PATENTS 1,007,044 4/ 1957 Germany.

MEYER PERLIN, Primary Examiner ALBERT W. DAVIS, Assistant Examiner US. Cl. X.R.

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