US3477495A - Inlet cone device and method - Google Patents

Description

Nov. 11, 1969 w TUCKER 3,477,495

INLET CONE DEVICE AND METHOD Original Filed May 13, 1966 INVENTOR.

United States Patent M 3,477,495 INLET CONE DEVICE AND METHOD William Tucker, Great Neck, N.Y., assignorv to The Lummus Company, New York, N.Y., a corporation of Delaware Original application May 13, 1966, Ser. No. 549,852, now Patent No. 3,374,832, dated Mar. 26, 1968. Divided and this application Jan. 19, 1968, Ser. No. 725,969

Int. Cl. F28f 1/00, 11/00 U.S. Cl. 165-1 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the transfer of hydrocarbon fluids from heaters to shell and tube heat exchangers in conical connecting conduits. In essence, the process of the present invention comprises the steps of excluding the hydrocarbon fluids from regions between the tube sheet of the heat exchanger and the narrow end of the conduit so that the hydrocarbon fluid will flow therethrough and into said tubes; and providing inert fluids at pressures above that of the hydrocarbon fluid within said region.

This application is a division of application Ser. No. 549,852, filed May 13, 1966, now US. Patent 'No. 3,374,- 832 granted Mar. 26, 1968.

This invention relates to a novel inlet device for connecting the output of a cracking heater with the inlet of a transfer line heat exchanger, together with a method of operating the same.

In the production of olefins from normally gaseous hydrocarbons by thermal cracking or in the steam reforming of naphtha and the like, it is necessary that the reaction products be cooled very quickly from the cracking temperature to a temperature below the point where secondary reactions can proceed. Such secondary reactions decrease yield and cause coke formations which decrease the length of time a heater can be kept in service. In conventional practice, the heater eflluent is passed in a conduit to a heavily insulated inlet cone which is in direct communication with the tube side of a transfer line exchanger (also referred to as a cracked gas cooler), the cone being necessary because of the substantially larger diameter of the exchanger. The insulation, which may be internal or external, is provided in an attempt to keep the gas as hot as possible until it actually reaches the exchanger. It is also possible to cool the effluent directly by injection of a suitable cooling medium but while this effects the desired cooling very rapidly, it results in a significant loss of recovered high pressure steam. As a result, indirect cooling in a transfer line exchanger is generally preferred.

Operating experience has demonstrated that for certain feedstocks, particularly ethane and propane, and for high severity naphtha cracking, the inlet cone is the most critical zone for coke buildup in the entire heater-transfer line system. This is apparently due to the flow characteristics of the hot gases within the cone. Coke may build up on the walls of the cone and eventually break off, causing blockage of gas flow through a portion of the exchanger. Further, it is difficult to distribute the flow to all of the exchanger tubes equally; tubes with low gas flow tend to foul quickly. Ultimately, the resulting high pressure drop requires that the whole heater-exchanger system be shut down and the cone and tubes cleaned out, even though there may be essentially no coke buildup in the heater itself.

High severity pyrolysis requires extremely rapid cooling and very low pressure drop across the cooler. This is accomplished by providing many parallel tubes of small 3,477,495 Patented Nov. 11, 1969 diameter, but such an arrangement demands that the inlet cone have a large volume. This turns the inlet cone into an adiabatic reactor, the same as a large or long transfer line. The adiabatic reaction is contrary to the principles of short residence time cracking, and reduces the temperature and causes coke formation, increasing pressure drop and shortening run length.

The area of the heater outlet tubes is typically about one-half of the area of the exchanger tubes, and with a conventional inlet cone the volume thereof may be as much as one third of the volume of the heater coil. Since discharge from the heater is at maximum temperature, the adiabatic reaction is rapid, with a significant effect on yield and coke formation.

Prior workers have addressed themselves to this problem, but a truly satisfactory solution has been elusive. It has been suggested, for example, to weld extensions onto the exchanger tubes and bring them close together near the narrow end of the cone, the object being to get the gas into the exchanger tubes without any opportunity for turbulent flow in the cone area, and to reduce adiabatic reaction by reducing cone volume. With such a design it is possible for gas to enter between the tubes and deposit coke, causing tube deformation and, ultimately, failure. A second disadvantage of this design in that it is often not desirable to weld the inlet cone tubes to the exchanger tubes, because this can impose restrictions on the free movement of the tube sheet and impose thermal stress. This is particularly true with ethanepropane feedstocks, where water-cooled fixed tube exchangers are preferred, and the tube sheet must remain flexible.

It is thus a general object of the present invention to prevent coke buildup in the inlet cone between a heater .and a transfer line exchanger.

Another object of the invention is to provide an improved inlet cone for connecting a heater to a transfer line exchanger.

Still another object of the invention is to provide a novel method of transferring cracked gas from a heater to a transfer line exchanger.

Yet another object of the invention is to provide means for passing cracked gas from a heater to a transfer line exchanger which effectively prevents coke buildup and which does not involve welding all of the tube extensions onto the exchanger tubes.

A still further object of the invention is to pass cracked gas from a heater to a transfer line without coke buildup, without reducing yield and without loss of heat recovery potential.

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

In essence, the present invention is based on the combination of inlet cone tubes, the bulk of which are loosely slip-fitted into the exchanger tubes, together with a steam purge of the space between the exchanger tube sheet and the inlet cone tube sheet, the steam being supplied at a higher pressure than the heater efi luent. In an alternative embodiment the inlet cone tube sheet is eliminated by using hexagonal tubes welded to each other. The steam effectively purges the cone area of any hydrocarbons which may enter. Because of the slip-fit of the inlet cone tubes within the transfer line exchanger tubes, the purged steam can pass around the outside of the inlet cone tubes and into the exchanger tubes. Also because of this feature, the tube sheet of the exchanger is not restricted in movement. Lastly, a much smaller quantity of insulation is required by the arrangement of the invention. In some instances, it may be desirable to weld some of the tubes to simplify assembly and stabilize the position of the tubes in the cone.

Understanding of the invention will be facilitated by referring to the accompanying drawings, which are intended to be illustrations only and which are not to be interpreted in a limiting sense.

In the drawings:

FIGURE 1 is a cross-sectional elevation of a heater outlet, inlet cone and transfer line exchanger in accordance with the present invention; and

FIGURE 2 is an elevation of the inlet cone tube openings in an alternative embodiment of the invention.

With reference to FIG. 1, the heater outlet is indicated generally at 10, the inlet cone at 12, and the transfer line exchanger at 14. The heater outlet is provided with insulation 16, a protective sleeve 18 and a flange 20 for attachment to inlet cone 12. A suitable gasket 22 seals the joint.

Inlet cone 12 comprises a flanged steel shell 24 with some insulation 26 between heater outlet flange 20 and the inlet cone tube sheet 28. This tube sheet (28), which may be either dished, as shown, flat or dished in the reverse direction, supports a plurality of tubes 30, of which only a few are shown for ease of illustration. It will be understood that there are as many tubes 30 as there are tubes in exchanger 14. At at least one point on inlet cone 12 there is an opening 32 with a suitable fitting 34 for connection to a source 36 of steam. This need not be especially high pressure steam, as long as it is superheated and at a pressure higher than the effluent gas from the heater. Inlet cone 12 may be externally insulated.

A second gasket 40 effects a seal between inlet cone 12 and exchanger 14. As shown, exchanger 14 comprises a shell 42 with suitable flanges, a tube sheet 44 supported therein and tubes 46 attached to tube sheet 44, but it Will be understood that many different exchanger designs are employed. The ends of tubes 30 extend into tubes 46, there being a loose, fluid-permeable fit therebetween.

The most eflicient exchangers for this type of service have a large number of small diameter tubes. Doublewall tubes may sometimes be employed. The large number of tubes makes construction of tube sheet 28 diflicult, since almost all of the area is required for the tubes and there is very little area for supporting structure. And of course, the closer tube sheet 28 can be located to the narrow end of the cone, the more effective the device will be. This problem is somewhat lessened, however, by the fact that the tubes 30 are of somewhat smaller outside diameter than tubes 46.

This problem can be substantially reduced by employing tubes with hexagonal ends to fabricate tubes 30, as shown in FIG. 2. In this embodiment, the tubes are essentially self-supporting, being welded together to form a honeycomb type structure, shield 50 only being required to seal the tubes in the neck of inlet cone shell 24. Again for purposes of simplicity and ease of understanding, only 31 tubes are shown in FIG. 2 when in fact a much larger number would be employed. It will also be understood that shield 50 need not be fabricated of metal, but may be refractory or a suitable packing material.

As noted hereinabove, the significant operating feature is that the steam pressure in area 38 be maintained at a higher level than the pressure of the heater eflluent. While the steam temperature is not considered critical, the steam must be free of entrained water to avoid thermal shock.

Various changes in the details, steps, materials and arrangements 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. In particular, While the general shape of the device is referred to herein as generally conical, it will be understood that it may in fact be pyramidal, tetragonal or any configuration which presents a small opening to the heater outlet and a large opening to the exchanger inlet.

What is claimed is:

1. In the transfer of hot, hydrocarbon fluids from heaters to shell and tube heat exchangers in conical connecting conduits, the improvements comprising:

excluding said hydrocarbon fluid from a region extending between the tube sheet of said heat exchanger and the narrow end of said conduit, said fluid flowing therethrough and into said tubes in discrete tubes; and

providing an inert fluid at a pressure higher than said hydrocarbon fluid within said region.

2. The method as claimed in claim 1, wherein said inert fluid is steam.

3. The method as claimed in claim 2, and further comprising purging said steam from said region into the tubes of said heat exchanger.

References Cited UNITED STATES PATENTS 12/1929 Smith -70 9/1961 Laist 165-134 US. Cl. X.R. 165--7O

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