GB2096759A

GB2096759A - Heat exchanger for quenching hot gas

Info

Publication number: GB2096759A
Application number: GB8207772A
Authority: GB
Original assignee: Mitsui Petrochemical Industries Ltd; Mitsui Engineering and Shipbuilding Co Ltd
Current assignee: Mitsui Petrochemical Industries Ltd; Mitsui Engineering and Shipbuilding Co Ltd
Priority date: 1981-03-23
Filing date: 1982-03-17
Publication date: 1982-10-20
Also published as: KR860000534Y1; KR830009465A; FR2502316B1; IT8220350A0; GB2096759B; JPS57157991A; FR2502316A1; IT1153477B; JPS628714B2

Abstract

The heat exchanger has a steam drum (12), a plurality of vertical heat transfer tubes (15) depending from the steam drum and an envelope (25) connected to the steam drum and surrounding the heat transfer tubes, the lower ends of which are closed. A water chamber (13) is disposed in the steam drum and a plurality of vertical inner tubes (16) are connected to a bottom of the water chamber to extend into respective heat transfer tubes. One or more radial projections (32) are secured to outer surfaces of the heat transfer tubes to extend in the longitudinal direction thereof. Hot gas, particularly formed by cracking petroleum enters into a bottom end (30) of the envelope and is quenched while flowing through gas passages defined by the outer surfaces of the heat transfer tubes and the projections. A lower portion the heat exchanger constitutes a quenching section while an upper portion constitutes a heat recovery section. The efficiency of heat recovery is improved and pressure loss of the gas is decreased. <IMAGE>

Description

SPECIFICATION Heat exchanger for quenching hot gas This invention relates to a heat exchanger for quenching hot gas, and more particularly a heat exchanger for quenching cracked gas obtained by heat decomposing such petroleum raw material as naphtha, kerosene, L.P.G., etc., for the purpose of olefine production as ethylene, propylene or the like, and for recovering heat of the cracked gas.

Various types of heat exchangers have been developed for this purpose but heat exchangers have not yet been developed that can operate continuously over a long time and having a compact construction and can be used for various raw materials. More particularly, the most serious problem that prevents continuous operation is so-called coking phenomenon caused by the deposition of coke contained in the cracked gas onto the heat transfer surface. Such coke deposition decreases heat transfer and increases the pressure loss of the cracked gas, thus making it impossible to efficiently recover heat. Furthermore, when the coking phenomenon occurs it is necessary to remove the deposited coke at a predetermined interval.

A typical prior art quenching heat exchanger is disclosed in Japanese Laid Open Patent Specification No. 12321 of 1974 in which vertical tubes are provided in a coolant for passing cracked or decomposed gas. In the heat exchanger of this type, since the cracked gas flows through tubes of a relatively small diameter, when coking occurs, the effective diameter of the tubes is decreased with the result that the pressure loss in the tubes increases rapidly thus increasing the yield of undesirable components or obliged to interrupt the operation.

In this type of the heat exchanger wherein the cracked gas flows through the tubes, when a heavy material is cracket, the field of use of the heat exchanger would be narrowed. Where a quenching type heat exchanger designed for such light raw material as naphtha is used to crack heavy raw material, the pressure loss increases abruptly, so that such cracking is impossible. Conversely when a heat exchanger designed for cracking heavy raw material is used for light raw material, the temperature at the gas exit port becomes very high thus greatly decreasing the quantity of recovery of high pressure steam.

Moreover, as the strength of the heat transfer tubes against thermal strain is limited it is necessary to operate the heat exchanger while continuously supplying coolant (high pressure water) so that so-called on-line decoking is impossible in which the coke deposited on the surface of the heat transfer tubes is burned by hot steam and air mixed gas after completely evaporating off the coolant.

Instead of passing gas through tubes, a heat exchanger has also been proposed in which such coolant as water is passed through the tubes while the cracked gas is passed on the outside of the tubes as disciosed in U.S. Patent No. 3,433,298 dated March 18, 1969. This type of heat exchanger is more advantageous over the aforementioned Japanese Patent Laid Open Specification No. 1 2321 of 1 974 in the following points. In the former type the coking is formed in the tube and its growth is suppressed by the flow velocity of gas so that when the flow velocity is very low, the heat transfer tubes would be perfectly clogged, so that it necessary to design the heat exchanger for a certain degree of mass velocity.With the latter type in which the coke deposits on the outer periphery of the tubes even where the flow velocity is very low, clogging of the tubes does not occur and the coke does not grow beyond a definite thickness. Because the coke deposits on the outer periphery of the tubes so that it peels off when the thickness of the deposited coke increases beyond a predetermined value. In this type, the deposited coke can be removed by the on-line decoking method.

In this type, however, the opposite ends of a plurality of coolant tubes protrude to the outside of a housing surrounding the tubes and the coolant flows in the tubes from the lower side to the upper side.

Such construction of the heat exchanger with the opposite ends protruding to the outside is not only complicated but also makes it difficult to assemble or disassemble.

Moreover, with this type, since adjacent water tubes are interconnected by staggered fins and pipe walls, when subjected to gas of a temperature higher than 8000C, the water tubes tend to warp or cracks are formed due to thermal stress.

The invention makes it possible to provide a novel heat exchanger for quenching hot gas that can recover heat more efficiently than the prior art heat exchanger, can prevent increase in the pressure loss and can operate over a long time.

Another advantage of this invention is that it makes it possible to provide a compact heat exchanger for quenching cracked hot gas having a relatively simple construction.

Another advantage of this invention is that it makes it possible to provide a novel heat exchanger for quenching hot gas capable of being readily assembled and disassembled.

A further advantage of this invention is that it makes it possible to provide a novel heat exchanger for quenching hot gas that can be used for cracking not only light oils but also heavy oils.

A still further advantage of this invention is that it makes it possible to provide an improved heat exchanger for quenching cracked gas capable of efficiently recovering heat because coke deposited on heat transfer tubes peels off.

According to the invention, there is provided a heat exchanger comprising a plurality of heat transfer tubes with their lower ends closed, and a plurality of inner tubes respectively inserted into the heat transfer tubes, the lower ends of the innertubes being open. Coolant, usually water is supplied into the inner tubes via a coolant chamber disposed above the inner tubes and then caused to flow into a space between the inner tubes and the heat transfer tubes through the opened ends of the inner tubes thus creating a natural flow of the coolant without using any pump. At least one spacer extending in the longitudinal direction of each heat transfer tube is disposed on the outer surface thereof.Flow passages of the hot gas are defined by the outer surfaces of adjacent heat transfer tubes and the spacers to pass the hot gas flow in the longitudinal direction of the heat transfer tubes along the outer surfaces thereof from lower to the upper side.

According to this invention there is provided a heat exchanger for quenching hot gas comprising a steam drum; a cylindrical envelope which is gas tightly connected to a bottom of the steam drum; a plurality of heat transfer tubes depending from the bottom of the steam drum, the lower ends of the heat transfer tubes being closed; a coolant chamber disposed in the steam drum and adapted to divide down flow from up flow of the coolant; a plurality of inner tubes extending from the bottom of the coolant chamber through respective heat transfer tubes, the lower ends of the inner tubes being open; at least one spacer secured to the outer surface of each heat transfer tube and extending in the longitudinal direction thereof; a hot gas inlet pipe opening into a gas diffusion space at the lower end of the envelope; at least one gas exit port provided above the upper ends of the spacers, whereby the hot gas is quenched while it passes through the gas inlet pipe, the gas diffusion chamber and gas passages defined by the outer surfaces of adjacent heat transfer tubes and the spacers therebetween so as to discharge hot gas having a predetermined composition through the gas exit port.

In the accompanying drawings: Fig. 1 is a longitudinal sectional view showing a preferred embodiment of the heat exchanger for quenching hot gas; Fig. 2 is a cross-sectional view taken along a line 1I--II in Fig. 1; Fig. 3 is a view similar two Fig. 2 showing a modified heat exchanger; Figs. 4 and 5 are longitudinal sectional views showing modified embodiments of this invention.

A heat exchanger 10 shown in Fig. 1 comprises a steam drum 12 made of carbon steel and containing therein a water or coolant chamber 1 3 also made of carbon steel. A plurality of vertical heat transfer tubes 1 5 depend from the bottom of the steam drum 12, the heat transfer tubes 1 5 being constructed to be gas tight. These heat transfer tubes 1 5 have the same diameter and regularly arranged on the same plane at an equal spacing, the ends of the tubes 1 5 being closed. Vertical inner tubes 1 6 depending from the bottom of the coolant chamber 1 3 are inserted in respective heat transfer tubes 1 5 with proper spacings therebetween.Coolant such as water introduced into the chamber 1 3 in the drum 12 through an inlet port 1 8 flows downwardly through respective inner tubes 1 6 and then flows upwardly from the bottom openings 1 6a through spaces between the inner tubes 1 6 and the heat transfer tubes 1 5 to enter into the steam drum 12 and while passing between the heat transfer tubes 1 5 and inner tubes 16, the coolant is heated by high temperature cracked gas to be described below to form steam. Such flow naturally occurs during the operation of the heat exchanger so that it is not necessary to use any circulation pump. At the top of the steam drum 12 is provided a steam dischargevalve 1 9.

The heat transfer tubes 1 5 are surrounded by a polygonal envelope 25 comprising an inner shell 21, a heat insulator 22 and an outer shell 23, and the upper end of the envelope 25 is welded to the bottom of the steam drum 12 2 or connected to the bottom with a flange. The inner shell 21 is made of an alloy such as stainless steel and incoloy that can resist against a relatively high temperature while the heat insulator 22 is made of light castabie, and carbon steel is used for the outer shell 23. Gas outlet ports 28 and 29 are provided near the upper end of the envelope 25 and at a substantially mid point along the longitudinal length.The lower end of the envelope 25 extends slightly beyond the lower ends of the heat transfer tubes 1 5 and a gas inlet pipe 30 is provided at the lower end of the envelope 25 for admitting high temperature gas formed by cracking such petroleum raw material as naphtha, kerosene, light oil and LPG. Between the inlet pipe 30 and the lower ends of the heat transfer tubes 1 5 is defined a funnel shaped gas diffusion chamber to uniformly diffuse the admitted gas so as to cause it to uniformly contact with the heat transfer tubes 1 5 as far as possible.

According to the embodiment shown in Fig. 1 the lower portions of the heat transfer tubes act as a quenching section A, while the upper portions as a heat recovery section B.

The quenching section A constituted by the lower portions of the heat transfer tubes 1 5 includes spacers 32 acting as heat transfer fins and extending in the longitudinal direction of the heat transfer tubes, as shown in Figs. 2 and 3. In Fig. 2, seven heat transfer tubes are used, while in Fig. 3 six heat transfer tubes are provided. In Fig. 2, one or a plurality of spacers 32 is secured to the outer surface of each heat transfer tube to extend in the radial direction. For example, each one of the heat transfer tubes 1 5A is provided with a single radial spacer 32A, whereas each one of the heat transfer tubes 1 5B is provided with two radial spacers 32B, 1200 apart and each one of the heat transfer tubes 1 5C is provided with three radial spacers 32C, 1 200C apart. The radial length of all spacers is the same and their outer ends oppose the peripheries of adjacent heat transfer tubes with a small gap, 1 mm, for example. These gaps are provided for the purpose of preventing damage of the heat transfer tubes when they undergo thermal expansion. With the construction shown in Fig. 2, the spacers of adjacent heat transfer tubes define longitudinal gas passages 35 having substantially triangular cross-sectional configuration. Although not shown in Fig. 2, the heat transfer tubes are also provided with radial spacers with their outer ends spaced a little from the inner wall of the polygonal envelope 25. Thus, the spacers function are to maintain a definite spacing between adjacent heat transfer tubes and to define gas flow passages together with the outer surfaces of the heat transfer tubes.

In a modification shown in Fig. 3, five heat transfer tubes 1 5A each having a single spacer 32A and dne heat transfer tubes 1 5D provided with 1 80O spaced spacers 32D and 32E are combined.

Accordingly, each gas passage 35 defined by spacers has a substantially square cross-sectional configuration. Like Fig. 2, the outer ends of the spacers confront the outer surfaces of adjacent heat transfer tubes with small gaps.

When the quenching section A provided with the spacers characterizing the invention is used, the spacers rectify the flow of the gas flowing through the gas passages so that the gas flows uniformly along the surfaces of the heat transfer tubes. Furthermore, the spacers increase the heat transfer area for the gas flowing through the gas passages. Where the thickness of the spacers is selected suitably, it is possible to decrease the cross-sectional area of the gas passages in the envelope 25 to a desired value, thus increasing the flow velocity of the gas.

Since radial spacers are provided on the outer surfaces of the heat transfer tubes in the longitudinal direction thereof, use of the quenching section promotes the quenching action due to the increase in the heat transfer area and increase in the gas flow velocity thereby decreasing deposition of coke.

Since the gas quenching section A is constructed to cause the gas to flow through the gas passages defined by the outer surface of the heat transfer tubes and the spacers, coke deposits on the outer surfaces of the heat transfer tubes and the spacers. Since the deposited coke film readily peels off, there is no fear of clogging the gas flow passage. Accordingly, the quenching section decreases the speed of forming the coke layer than the conventional heat exchanger in which gas is passed through the inside of the heat transfer tubes.

Moreover, in the heat exchanger of this invention, as the inner tubes are disposed concentrically in the heat transfer tubes the coolant flows through the inner tubes and thence upwardly through the spaces between the inner tubes and the outer heat transfer tubes by natural circulation, so that the construction is much simpler than that of the prior art heat exchanger, which simplifies assembling and disassembling operation.

In a preferred embodiment of this invention, each heat transfer tube has an outer diameter of 50.8 mm, and each spacer has a width of 6 mm, a height of 1 5 mm and a length of 1 60 cm. Where the number of the heat transfer tubes 12 is 19, and the spacers are arranged as shown in Fig. 2, and a raw material having a specific gravity of 0.84 and a Bureau of Mines Correlation Index of 30 is used, even when the heat exchanger is used continuously for 60 or more days no decoking is necessary.

Usually, the quenching section A is designed such that it can cool the cracked gas from a cracking temperature to a temperature of 600--6500C at which secondary or tertiary reaction of the cracked gas does not occur. The cracked gas is then cooled further by the heat recovery section B to be described hereinbelow to recover heat.

The heat recovery section B is constituted by the upper portions of the heat transfer tubes 1 5 to efficiently recover the heat of the gas passed through the quenching section A at a high speed. For this reason, the cross-sectional area of the gas passage is made to be larger in the heat recovery section B than that in the quenching section A so as to decrease the gas velocity or to increase the heat transfer coefficient of the gas by creating a turbulence of the gas. For this reason, a plurality of projections 40 aligned in the longitudinal direction are provided for all or selected ones of the heat transfer tubes 1 5 at the upper one half thereof.The projections 40 project from the outer surfaces of the heat transfer tubes in the radial direction and have the same height as that of the spacers and the outer ends of the projections confront the outer surfaces of the adjacent heat transfer tubes with small gaps therebetween. Preferably each projection 40 has a width of 6 mm, a height of 15 mm, and an axial length of 50 mm.

The heat recovery section B functions are similar to, those of the quenching section A in a manner described above. Projections 40 creates turbulence in the gas flow to increase the flow velocity of the gas or to increase the heat transfer coefficient and to maintain the desired spacings between adjacent heat transfer tubes. In this embodiment, two gas exit ports 28 and 29 are provided. One exit port 28 is located at a portion sufficiently spaced from the gas quenching section A, that is near the bottom of the steam drum 12, that is on the down stream side of the heat recovery section B. The other exit port 29 is located adjacent the upper end of the quenching section A, that is near the downstream side of the gas passages 35 defined by the spacers 32 and the heat transfer tubes 1 5.

Where two gas exit ports 28 and 29 are provided, the exit port 29 is used for heavy oil cracked gas and the exit port 28 is used for the light oil cracked gas. Because, if the increase of pressure loss between the exit port 29 and the exit port 28 is unreasonable in comparison with the heat recovery between the exit port 29 and the exit port 28 in the case of using the exit port 28 for heavy oil cracked gas, the exit port 29 is used for heavy oil cracked gas.

Where either one of the exit ports 28 and 29 is used, the other exit port is closed.

The heat exchanger described above has excellent characteristics as can be noted from the following theorectical consideration. More particularly, the relationship between factors that determined the characteristics of the quenching heat exchanger or quencher and the excellent characteristic thereof is shown in the following Table. It is ideal to satisfy all of the items 1 through 5 in the Table.

Practically, however, it is impossible to satisfy all these items.

TABLE

Factor Mass velocity Gas exit temp. Diameter of (at start (at start heat transfer Effect of run) of run) tube . .

1 Increase of larger lower smaller quenching effect 2 lower gas exit larger lower smaller temp. at coking 3 less rising of larger higher smaller gas exit temp.

4 lower gas pressure smaller higher larger loss at coking 5 less increase of larger higher larger gas pressure loss More particularly, to lower in the gas exit temperature for the purpose of improving heat recovery and to decrease in the pressure loss for the purpose of preventing decrease in the yield of ethylene contradict with each other. The prior art heat exchanger in which the cracked gas is passed through tubes was designed on a compromise of these contradictory factors.In the case of a quencher in which the cracked gas is passed on the outside of the tubes it is possible to adopt an ideal design corresponding to the cracking conditions of respective raw materials because it is possible to determine as desired the gas flow velocity and the tube diameter by the spacers that support the heat transfer tubes in accordance with the gas temperature along the length of the heat transfer tubes.

To assure operation of the heat exchanger over a long time there have been used the following two designs which contradict with each other.

(1) to design the gas flow velocity to be small as far as possible. This design is adopted for the purpose of decreasing the gas pressure loss as far as possible at the time of starting the running to decrease the pressure loss at the time of depositing coke in order to assure long running.

(2) to design to increase as far as possible the gas velocity.to restrain growth of a coke layer.

These two designs have the following disadvantages respectively.

A. in the case of the former, it is impossible to accomplish the quenching effect of cracked gas.

B. in the case of the latter since the gas pressure loss at the time of starting of the running is made large, even when a small quantity of coke deposits, the absolute value of the gas pressure loss increases.

For this reason, according to this invention, it is necessary to provide spacers between the heat transfer tubes at portions into which the cracked gas is admitted for increasing the gas velocity.

Generally, the speed of growing coke is lower at the inlet port than at the exit port so that this design is advantageous for increasing the gas velocity. Further, at the inlet port the gas temperature is higher and the gas velocity is larger than at the exit port so that the deposition of coke is small. In the heat recovery section following the quenching section, since nothing is attached to the heat transfer tubes except projections that prevent deflection of the heat transfer tubes, gas velocity becomes low. However, different from the prior art type in which the cracked gas is passed through the tubes, although the gas flow velocity has decreased, the coke film does not grow beyond a definite thickness so that it does not stop the flow of the gas. Accordingly, according to this invention, it is possible to decrease the temperature at the gas exit port in a range in which the mass velocity is relatively low, whereby the heat exchanger of this invention is effective for versatility ranging from light oil to heavy oil.

Figs. 4 and 5 diagrammatically illustrate other embodiments of the heat exchanger for quenching cracked gas, in which heat transfer tubes 1 5 depending from the steam drum 12 and inner tubes 1 6 depending from the water chamber 13 are shown. Other portions are also shown diagrammatically.

In Fig. 4, the gas exit port 29 shown in Fig. 1 is omitted so that the gas introduced flows to the exit port 28 through the quenching section A and the heat recovery section B. This modification can be used for the cracked gas produced by light oil cracking or heavy oil cracking.

In Fig. 5, the heat recovery section B shown in Fig. 4 is omitted so that the heat exchanger comprises only the quenching section A in which spacers 32 are provided along substantially the entire length of the heat transfer tube 1 5. This construction is used for the cracked gas produced by a light oil cracking. When the quantity of deposited coke is very small and the gas pressure loss does not remarkably increase for light oil cracking, the exchanger of this invention is used for gas produced by light oil cracking. Accordingly, when the construction shown in Fig. 5 can be used, the increase in the pressure loss to be is taken into consideration. With the heat exchanger shown in Fig. 5, the construction can be made compact.

It should be understood that the invention is not limited to the specific embodiments described above and that many changes and modifications will be obvious to one skilled in the art.

Although in the foregoing description the heat exchanger of this invention has been described in connection with cracked gas, it will be clear that the invention can also be used to any hot gas required to be quenched and the heat thereof should be recovered.

Claims

1. A heat exchanger for quenching hot gas comprising: a steam drum; a cylindrical envelope which is gas tightly connected to a bottom of said steam drum; a plurality of heat transfer tubes depending from said cylindrical envelope, lower ends of said heat transfer tubes being closed; a coolant chamber disposed in said steam drum and adapted to divide down flow from up flow of a coolant; a plurality of inner tubes extending from a bottom of said coolant chamber through respective heat transfer tubes lower ends of said inner tubes being open; at least one spacer secured to an outer surface of each heat transfer tube and extending in the longitudinal direction thereof; a hot gas inlet pipe opening into a gas diffusion space at a lower end of said envelope; at least one gas exit port provided above an upper ends of said spacers;; whereby said hot gas is quenched while it passes through said gas inlet pipe, said gas diffusion chamber, and gas passages defined by outer surfaces of adjacent heat transfer tubes and the spacers therebetween so as to discharge said gas having a predetermined composition through said gas exit port.

2. The heat exchanger according to claim 1 wherein said spacer extends in the radial direction from the outer surface of each heat transfer tube and an outer end of said spacer confronts an adjacent heat transfer tube with a small gap therebetween.

3. The heat exchanger according to claim 1 wherein each gas passage defined by said heat transfer tubes and said spacers has a substantially regular triangular cross-sectional configuration.

4. The heat exchanger according to claim 1 wherein each gas passage defined by said heat transfer tubes and said spacers has a substantially square cross-sectional configuration.

5. The heat exchanger according to claim 1 wherein a heat recovery section is provided between said gas exit port and a downstream end of said spacers.

6. The heat exchanger according to claim 5 wherein said heat recovery section comprises a radial projection provided for at least one of said heat transfer tubes.

7. The heat exchanger according to claim 6 wherein all of said heat transfer tubes are provided with a plurality of projections which are spaced in the longitudinal directions of said heat transfer tubes.

8. The heat exchanger according to claim 5 wherein another hot gas exit port is provided near a downstream side of said spacer.

9. A heat exchanger substantially as hereinbefore described with reference to, and as illustrated in, Figures 1 and 2 or modified as shown in any one of Figures 3 to 5 of the accompanying drawings.