The invention concerns a heat exchanger and a process for heat exchange in which the heat exchanger is applicable. In particular the invention relates to a heat exchanger useful as a steam super heater and having improved resistance to metal dusting and stress corrosion.
Steam reforming is most often an essential step in the production of carbon monoxide rich synthesis gas. In this reaction methane and steam is hereby converted under supply of heat to a gas composition comprising hydrogen, carbon dioxide, carbon monoxide, steam and methane. The temperature of the synthesis gas after reforming is most often between 750° C. and 1050° C. The hot synthesis gas is subsequently cooled in a boiler or in a boiler and a super heater.
One of the severe conditions related to coolers for reformed gas is the corrosion known as metal dusting. Metal dusting is a deteriorating attack of the carbon monoxide rich gas on alloys based on iron and/or nickel. A basic reaction by metal dusting is the decomposition of carbon monoxide in a reduction reaction or the bouduard-reaction. Metal dusting only takes place when the metal surface temperature is below the equilibrium temperature of these reactions. That will typically be between 750° C. and 850° C. However, if the temperature is lower, typically below 450° C., the reaction will not take place at a significant rate. This means that there is a metal temperature surface intermediate, which should be avoided for contact with gas in reformed gas coolers. These temperature ranges are between 450-800° C. for nickel based high alloys and 400-800° C. for low alloy steels.
The heat transfer surfaces of waste heat boilers are cooled by the effective heat transfer to the boiling water and can therefore normally be designed to avoid conditions of metal dusting. However, super heaters when applied as coolers for synthesis gasses have to be considered as subject to metal dusting attack.
Another severe condition to consider in the design of super heaters is the possibility of stress corrosion from the wet steam which is to be superheated. Nickel based alloys are very sensitive to stress corrosion, whereas low alloy steels are not. Nickel based alloys should therefore only be in contact with dry steam.
It is therefore an objective of the invention to provide a heat exchanger which shows improved resistance to metal dusting and stress corrosion.
The invention provides a heat exchange process comprising sequentially cooling of a first fluid by indirect heat exchange with a second fluid and comprising the following steps:
The invention also provides a heat exchanger for use in the above process, the heat exchanger for use in the above heat exchange process, the heat exchanger comprising a plurality of U-tubes securing a heat exchange surface for allowing heat transfer between a first and a second fluid, the U-tubes arranged in at least two sequential concentric tube bundles, the tube bundles defining at least a first and second heating zone respectively, each heating zone partially separated from the other by a wall, the first heating zone being a colder zone and the second heating zone being a hotter zone, the tube bundle of the first colder heating zone being made of a low alloy steel and the tube bundle of the second hotter heating zone being made of a temperature and corrosion resistant alloy.
The invention concerns a heat exchanger which is useful as a super heater and is designed to avoid metal dusting and stress corrosion by a proper selection of a combination of metal alloys and gas/steam flow through a pre-defined pattern of heat exchange tube bundles. The heat exchanger is suitable for heat exchange between a first and a second fluid. An example of such fluids is steam (first fluid) and synthesis gas (second fluid). The hot synthesis gas from a steam reforming reactor is cooled by steam in the heat exchanger.
The heat exchanger is of the U-tube type with a thick tube sheet. A plurality of U-tubes for transfer of the first fluid are arranged parallel and spaced apart with a central inlet and a peripheral outlet for the second fluid. The shell side heat exchange is enhanced by disc and doughnut baffles. The plurality of tubes is arranged in tube bundles, each tube bundle corresponding to a particular heating zone.
The first fluid, for instance steam, flows in the tubes and the second fluid, for instance reformed gas, flows around these tubes, i.e. on their shell side, thereby securing a heat transfer surface.
The essential principle of the invention is that at least two tube bundles are present in the heat exchanger and they are connected to one tube sheet in concentric rings. The compartments for each tube bundle are separated by metallic walls with openings in their middle or at their ends through which the second fluid passes and is divided into several streams when flowing from one compartment to the other.
The second fluid flows both countercurrent- and concurrent to the first fluid within each tube bundle compartment, as shown by the arrows in
The heat exchanger of the invention will be described in more detail in the following:
In
The U-tubes of the second tube bundle are placed sequentially after the U-tubes of the first tube bundle. In
The second fluid, for instance synthesis gas, or any other hot gas that requires cooling, enters the heat exchanger through inlet 5. Inlet 5 leads to a central pipe 13 placed in the middle of the innermost tube bundle. This central pipe 13 has openings 14 allowing the second fluid to leave the central pipe 13 and enter the second heating zone 3 on the shell side of the tube bundles defining this heating zone. It is preferable that the openings 14 are not located at the ends of the central pipe 13, in order to ensure both concurrent and countercurrent flow.
The second fluid enters the middle of heating zone 3 through the openings 14 and the fluid is then divided to flow towards the two ends of the tube bundle. The second fluid thus contacts the external surfaces i.e. the shell side of the U-tubes of the innermost tube bundle and is cooled in indirect heat exchange with the first fluid. The second fluid thereafter passes through end openings 15 and 16 in the wall 12 separating the two tube bundles defining the first and second heating zones 2 and 3. The opening 15 is at the lower end of the wall 12 and the opening 16 is at the upper end of the wall 12. The second fluid then passes across the shell side of the tube bundles defining the first heating zone 2, which surrounds the innermost bundle defining the second heating zone 3. The gas then flows in the tube bundle from the end openings 15 and 16 towards the middle of the heating zone 2. The further cooled second fluid then leaves the first heating zone 2 and the heat exchanger through outlet 6.
In an embodiment of the invention, the heat exchanger can have three heating zones as shown in
The walls separating the compartments can therefore have openings at either their ends (15 and 16) or in their middle (17). When several heating zones are at present the openings in each subsequent wall therefore alternate by being either at the end of the wall or in its middle. This ensures that the flow of the second fluid is both concurrent and counter current to the flow of the first fluid in each heating zone. Effective heat exchange is thereby realised.
The second fluid is in this way cooled by subsequent flow (divided flow) through the two or three tube bundles. When two heating zones are present as shown in
When three heating zones are present as shown in
Baffles can be placed in the heating zones in order to improve the heat distribution. Baffles particularly suitable for the heat exchanger are of the disc and doughnut type. These have the effect of allowing the second fluid to travel through the heating zones in a zig-zag movement and additionally they assist in positioning the U-tubes. The baffles 7, 8 and 9 shown in
The hot (high temperature) tube bundle defining heating zone 3 must be made of a material resistant to metal dusting. This could for example be a high alloy such as austenitic nickel/chromium/iron alloy, for instance Inconel®. The baffles, rods and walls defining the channels in which the tube bundles are situated must also be resistant to metal dusting. The cold (low temperature) tube bundle defining heating zone 2 may be of low alloy steel and in most cases the baffles and rods may also be of low alloy material. If a third bundle of tubes are present as shown in
Characteristic for the heat exchanger of the invention is that the U-tubes are of materials resistant to metal dusting when the material surface is hot enough to give a risk of metal dusting. The U-tubes can be of cheaper low alloy steel when situated in colder zones. Low alloy steel is not sensitive to wet stress corrosion. When the first fluid is steam, it enters U-tubes of low alloy steel, and the steam will not come in contact with the U-tubes of high alloys before it is completely dry.
The heat exchanger of the invention shows an improvement in its heat exchange performance due to it enhanced resistance towards metal dusting and stress corrosion.
A typical process in which the heat exchanger is useful is in a steam reforming process as described in the following:
Hot effluent, for instance a carbon monoxide containing reformed gas such as synthesis gas from a reforming reactor, is passed to a waste heat boiler where the temperature of the effluent is reduced from, for instance 1050° C. to 475° C., using steam supplied from a steam drum. The cooled effluent is then sent to a heat exchanger of the invention where the temperature is further reduced to 360° C. by heat exchange with steam. The heat exchanger functions as a steam super heater. The steam used can be supplied from the steam drum and it is thereby heated from a temperature of for instance 320° C. to 400° C.
Number | Date | Country | Kind |
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PA 2004 00998 | Jun 2004 | DK | national |