JP2006010309A - Heat exchange method, and heat exchanger - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat exchanger showing improved durability against metallic powdering and stress corrosion. <P>SOLUTION: In the heat exchange method consecutively cooling a first fluid by carrying out indirect heat exchange with a second fluid, the first fluid is consecutively led into at least two concentric U-shaped pipe bundles respectively dividing at least one first heating area and a second heating area, the second fluid is led into a shell side part of the U-shaped pipe bundles, the heating areas are partially separated from each other by a wall, the first heating area is a comparatively cold area, the second heating area is a comparatively hot area, the pipe bundle of the comparatively cold first heating area is made by low alloy steel, the pipe bundle of the comparatively hot second heating area is made by heat resistant and corrosion resistant alloy, and each step of drawing out the cooled second fluid and the heated second fluid is included. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a heat exchanger and a heat exchange method in which the heat exchanger can be used. In particular, the present invention relates to a heat exchanger that can be used as a steam superheater and has improved durability against metal dusting and stress corrosion.

  Steam reforming is very often an important step in producing carbon monoxide rich synthesis gas. In this reaction, methane and steam are converted into a gas composition comprising hydrogen, carbon dioxide, carbon monoxide, steam and methane under the supply of heat by the steam reforming. The temperature of the synthesis gas after reforming is very often between 750 and 1050 ° C. This hot synthesis gas is then cooled in the boiler or in the boiler and superheater.

  One of the severe conditions associated with coolers for reformed gases is corrosion known as metal dusting. Metal dusting is a corrosion degradation due to the erosion of carbon monoxide rich gas into iron and / or nickel based alloys. The basic reaction by metal powdering is the decomposition of carbon monoxide by a reduction reaction or a Baudal reaction. Metal dusting occurs only when the metal surface temperature is below the equilibrium temperature of these reactions. The equilibrium temperature is generally 750 to 850 ° C. However, if the temperature is low, typically the reaction does not occur at a significant rate below 450 ° C. This means that there is an intermediate metal surface temperature to avoid contact with the gas in the cooler for the modified gas. These temperature ranges are 450-800 ° C for nickel-based high alloys and 400-800 ° C for low alloy steels.

  The heat transfer surface of the waste heat boiler can be designed to cool by effective heat transfer to boiling water and therefore generally avoid metal dusting conditions. However, when used as a cooler for synthesis gas, the superheater should be considered susceptible to metal dusting erosion.

  Another severe condition to consider in superheater design is that it can be stress corroded by superheated wet steam. Nickel-based alloys are very sensitive to stress corrosion, but low alloy steels are not. Therefore, the nickel base alloy should be contacted only with the dry stream.

  Accordingly, it is an object of the present invention to provide a heat exchanger that exhibits improved durability against metal dusting and stress corrosion.

Solution

The present invention relates to a heat exchange method in which the first fluid is subsequently cooled by indirectly exchanging heat with the second fluid,
The first fluid is subsequently introduced into at least two concentric U-tube bundles each separating at least one first heating zone and a second heating zone;
Introducing a second fluid into the barrel side of the U-tube bundle, each heating zone being partially separated from one another by a wall, wherein the first heating zone is a relatively cold zone; and The second heating zone is a relatively hot zone, the first relatively cold heating zone tube bundle is made of low alloy steel, and the second relatively hot heating zone tube bundle is heat resistant. Made of corrosion-resistant alloy,
Withdrawing the second cooled fluid and the heated first fluid
It relates to a method as described above, characterized in that it comprises stages.

  The present invention also provides a heat exchanger for use in the above method, wherein the heat exchanger has a plurality of heat exchange surfaces for allowing heat transfer between the first and second fluids. A U-tube, the U-tube disposed in at least two successive concentric tube bundles, a tube bundle each divided into at least one first and second heating region, and each heating region partially separated from the other by a wall. The first heating region is a relatively cold region and the second heating region is a relatively hot region, and the first relatively cold heating region tube bundle is made of low alloy steel. The second heat exchanger is characterized in that the second relatively hot zone tube bundle is made of a heat-resistant and corrosion-resistant alloy.

  FIG. 1 illustrates a heat exchanger having two heating zones.

  FIG. 2 illustrates a horizontal cut surface of the heat exchanger.

  FIG. 3 illustrates a heat exchanger having three heating zones.

  The present invention is useful as a superheater and to avoid metal dusting and stress corrosion by properly selecting the combination of metal alloy and gas / water vapor flow through a predetermined pattern of heat exchange tube bundles. It relates to the designed heat exchanger. This heat exchanger is suitable for heat exchange between the first fluid and the second fluid. Examples of such fluids are water vapor (first fluid) and synthesis gas (second fluid). Hot syngas from the steam reforming reactor is cooled by steam in this heat exchanger.

  The heat exchanger consists of a thin tubular plate U-tube type. A plurality of U-tubes for transporting the first fluid are arranged in parallel and spaced from the central inlet and the barrel outlet for the second fluid. Trunk side heat exchange is facilitated by disk-like and donut-like baffles. A plurality of tubes are arranged in a tube bundle, each tube side corresponding to an individual heating zone.

  The first fluid, e.g. water vapor, flows in the tubes and the second fluid, e.g. reformed gas, flows around these tubes, i.e. the barrel sides of the tubes, thereby ensuring a heat exchange surface. .

  The essential principle of the present invention is that at least two tube bundles are present in the heat exchanger and they are concentrically connected to one tubular sheet. Each compartment for each tube bundle is separated by a metal wall having an opening in the middle or end, through which the second fluid passes and the second fluid passes from one compartment to the other. When flowing into the compartment, it is divided into several streams.

  The second fluid flows countercurrently and cocurrently with respect to the first fluid within each tube bundle compartment as indicated by the arrows in FIGS.

The heat exchanger of the present invention is described in further detail below:
1 and 3, the flow directions of the first fluid and the second fluid are indicated by curved arrows.

  FIG. 1 relates to one embodiment of the present invention having two heating zones separated by a wall. A first fluid, such as water vapor, enters the heat exchanger through inlet 1. The first fluid then enters the compartment containing the U-tube in the first tube bundle and defining the first heating zone 2. After passing through the U-tube of the first heating zone in indirect heat exchange with the second fluid, the first fluid contains the U-tube in the second tube bundle and the second heating zone 3 Enter the second compartment that defines

  The U-tube of the second tube bundle is disposed subsequent to the U-tube of the first tube bundle. In FIG. 1, the tube bundle defining the second heating zone 3 is located at the innermost position in the heat exchanger, whereas the tube bundle defining the first heating zone 2 is located at the innermost position and these two The two tube bundles are separated by a wall 12. Wall 12 may be made of metal and is provided with openings 15 and 16 that allow a second fluid stream to be split into several streams as it flows from one compartment to another. It has a structure. The first fluid passes through the U-tube in the second heating zone 3 in indirect heat exchange with the second fluid. After passing through the second heating zone 3, the first fluid is now heated and leaves the heat exchanger through the outlet 4.

  A second fluid, such as synthesis gas, or some other hot gas that requires cooling enters the heat exchanger through inlet 5. The inlet 5 is led to a central tube 13 located in the middle of the innermost tube bundle. The central tube 13 has an opening 14 which allows the second fluid to leave the central tube 13 and the second heating zone 3 on the barrel side of the tube bundle defining this heating zone. to go into. Advantageously, the opening 14 is not located at the end of the central tube 13 to ensure both co-current and counter-current.

  The second fluid enters the middle of the heating zone 3 through the opening 14 and the fluid is then divided to flow towards both ends of the tube bundle. The second fluid thus contacts the outer surface, i.e., the barrel side of the U-tube of the innermost tube bundle, and is cooled by indirect heat exchange with the first fluid. The second fluid then passes through the end openings 15 and 16 of the wall 12 and is divided into two tube bundles defining the first and second heating zones 2 and 3. Opening 15 is at the lower end of wall 12 and opening 16 is at the upper end of wall 12. The second fluid then traverses the barrel side of the tube bundle that defines the first heating zone 2. The first heating zone 2 surrounds the innermost tube bundle that defines the second heating zone 3. The gas then flows from the end openings 15 and 16 through the tube bundle towards the middle of the heating zone 2. Another cooled second fluid then leaves the heat exchanger through the first heating zone 2 and the outlet 6.

  FIG. 2 shows the arrangement of the associated tube bundles in the heat exchanger. The wall 12 divides the heating area into two compartments to form heating areas 2 and 3. The tube bundle is located in a heat exchanger having a tube bundle in the heating region 2 located at the outermost part and a tube bundle in the heating region 3 located at the innermost part.

  In one embodiment of the invention, the heat exchanger may have three heating zones as shown in FIG. In this case, there is a third bundle of U-shaped tubes around the second tube bundle. This third bundle defines a heating zone 11 that allows further heat exchange between the first fluid and the second fluid. The second fluid enters the middle of this heating zone through a central opening 17 in the wall 18 that separates the outer tube bundle from the two inner tube bundles. Thereby, the wall 18 separates the heating zone 11 from the heating zones 2 and 3. The fluid is then split into a stream that flows in the direction of both ends of the tube bundle.

  The wall separating the septa may therefore have openings at its ends (15 and 16) or at its middle (17). Therefore, if there are several heating zones, the next opening in each wall alternates by being present either at the end of the wall or in the middle. This ensures that the second fluid flow is both cocurrent and countercurrent to the first fluid flow in each heating zone. Effective heat exchange is thereby realized.

  The second fluid is thus cooled by subsequent flows through two or three tube bundles. If two heating zones are present as shown in FIG. 1, the first fluid is heated by the subsequent flow through the tube. That is, start with the outermost bundle having the lowest temperature and leave after flowing through the innermost bundle with the hottest and therefore highest temperature. Therefore, the outermost tube bundle defining the heating region 2 corresponds to a cold region (low temperature region), and the innermost tube bundle defining the heating region 3 corresponds to a hot region (high temperature region).

  When three heating regions exist as shown in FIG. 3, the intermediate heating region 2 between the heating regions 3 and 11 is between the hottest region (high temperature region) and the coldest region (low temperature region). Has an intermediate temperature.

  The baffle can be located in the heating zone to improve the heat distribution. Baffles that are particularly suitable for heat exchangers are disk-shaped or donut-shaped. These have the effect of allowing the second fluid to move in a zigzag motion through the heating area and additionally help in positioning the U-tube. The baffles 7, 8 and 9 shown in FIG. 1 are held in place by rods. The baffle 7 is hot, i.e. experiences a high temperature, and the baffle 8 is cold, i.e. experiences a low temperature. The baffle 10 in the center tube is a hot baffle. Each baffle may also be arranged in the embodiment shown in FIG.

  The hot (hot) tube bundle that defines the heating zone 3 must be made of a material that is resistant to metal dusting. This may be a high alloy such as, for example, an austenitic nickel / chromium / iron-alloy, for example Inconel®. The walls that define the channels in which the baffles, rods, and tube bundles are placed must be resistant to metal dusting. The cold (cold) tube bundle defining the heating zone 2 may be low alloy steel, and most often the baffles and rods may be made of a low alloy material. When a third tube bundle is present as shown in FIG. 3, the middle / center bundle tube is low alloy steel, but the rod, baffle and wall / channel are Inconel®. Also good. The low alloy steel may be, for example, ferritic iron, chromium, molybdenum, or carbon steel.

  A feature of the heat exchanger of the present invention is that the U-tube is a material that is durable against metal dusting even when the material surface is hot enough to pose a risk of metal dusting. The U-tube may be a relatively inexpensive low alloy steel when placed in a relatively low temperature region. Low alloy steels are not sensitive to wet stress corrosion. If the first fluid is water vapor, it enters the low alloy steel U-tube and the water vapor does not contact the high alloy U-tube before it is completely dried.

  Since the heat exchanger of the present invention has improved durability against metal powdering and stress corrosion, it improves its heat exchange performance.

  A typical method in which the heat exchanger is useful is the steam reforming method described below.

  Hot effluent, such as carbon monoxide-containing reformed gas, eg, synthesis gas from a reforming reactor, is passed to a waste heat boiler where the temperature of the effluent is, for example, 1050 using steam supplied from a steam drum. The temperature is lowered from 0 ° C to 475 ° C. The cooled effluent is then sent to the heat exchanger of the present invention where the temperature is further reduced to 360 ° C. by heat exchange with steam. This heat exchanger functions as a steam superheater. The steam used can be supplied from a steam drum, whereby it is heated, for example from a temperature of 320-400 ° C.

FIG. 1 illustrates a heat exchanger having two heating zones. FIG. 2 illustrates a horizontal section of the heat exchanger. FIG. 3 illustrates a heat exchanger having three heating zones.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... First fluid inlet 2 ... First heating region 3 ... Second heating region 4 ... First fluid outlet 5 ... Second fluid Inlet 6 ... Second fluid outlet 7, 8, 9, 10 ... Baffle 11 ... Heating region 12 ... Wall 13 ... Tubes 14, 15, 16, 17 ... Opening 18 ... wall

Claims (10)

In a heat exchange method in which the first fluid is subsequently cooled by indirectly exchanging heat with the second fluid,
The first fluid is subsequently introduced into at least two concentric U-tube bundles each separating at least one first heating zone and a second heating zone;
Introducing a second fluid into the barrel side of the U-tube bundle, each heating zone being partially separated from one another by a wall, wherein the first heating zone is a relatively cold zone; and The second heating zone is a relatively hot zone, the first relatively cold heating zone tube bundle is made of low alloy steel, and the second relatively hot heating zone tube bundle is heat resistant. Made of corrosion-resistant alloy,
Withdrawing the second cooled fluid and the heated first fluid
A method as described above, comprising each step. The heat exchange method of claim 1, wherein the first fluid is water vapor and the second fluid is a modified gas. The heat exchange method according to claim 1, wherein the heat-resistant and corrosion-resistant alloy is an austenitic nickel / chromium / iron-alloy. The heat exchange method according to claim 2, wherein the heated first fluid is superheated steam. 2. A heat exchanger for use in the method of claim 1 wherein a plurality of U-tubes having heat exchange surfaces to allow heat transfer between the first and second fluids, at least two sequential concentric surfaces. The U-tube arranged in the tube bundle, the tube bundle divided into at least one first and second heating region, respectively, each heating region partially separated from the other by the wall, the first heating region being compared Cold zone and the second heating zone is a relatively hot zone, the first relatively cold heating zone tube bundle is made of low alloy steel and the second relatively hot zone The above heat exchanger is characterized in that the tube bundle is made of a heat-resistant and corrosion-resistant alloy. The heat exchanger according to claim 5, wherein the heat exchanger has three tube bundles, and the third bundle is located between the first tube bundle and the second tube bundle. 6. A heat exchanger according to claim 5, wherein the heat resistant and corrosion resistant alloy is an austenitic nickel / chromium / iron alloy. 6. A heat exchanger according to claim 5, wherein the heat exchanger has disk-like and donut-like baffles. 7. The heat of claim 6, wherein the intermediate third bundle tube comprises low alloy steel and the baffle and rod holding the baffle in place and the intermediate bundle wall comprises a heat resistant and corrosion resistant alloy. Exchanger. 6. The wall separating the heating zones is made of metal, and the wall is positioned to divide the flow of the second fluid into several flows by passing through a plurality of openings in the wall. The heat exchanger as described in.

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