I would like to ask about the corrosion issue of the heat exchange tubes in the reboiler.
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This post was last edited by Wang Genrong on 2018-8-1 at 15:06. The reboiler we have here is the reboiler for the ethyl acetate esterification tower; the fluid flowing through the tubes is a mixture of ethanol, acetic acid, and ethyl acetate； The shell side is 0.5 MPa steam. The temperature at the outlet at the top of the reboiler is around 110–113°C. The material of the heat exchange tube is 316L. It corroded to this state in just two or three months. Is there a problem with the material chosen for the equipment?
As can be seen from the diagram, it appears that the corrosion perforations are all located around the welds where the heat exchange tubes are joined. Is that really the case? The original poster needs to confirm. Also, is this heat exchanger a U-tube heat exchanger? Is the location of the corrosion perforation in the U-tube section? If it occurs in these two locations, it is likely a quality issue with the heat exchange tubes. The reasons are: 1. Chemical composition segregation caused by the welding joints at the seams of the heat exchange tubes; 2. Cold bending deformation altered the original condition of the material. It is suspected that no solution treatment was carried out after the tube bundles were joined and bent.
This reboiler has straight tubes, and the corrosion is present at the weld edges as well as in areas far from the welds along the middle of the tubes.
Well, then we need to see how the medium affects the corrosion of the material at this temperature. I’ll look up some information later to check if there are any issues with the material choice
Well, thanks for your thoughtful reply and help:lol
Under these operating conditions, 316 or 316L is not suitable; a graphite reboiler is recommended. -
I checked some more information; regarding acetic acid and ethyl acetate as media, 316L should have good corrosion resistance and should not corrode and perforate within just over 2 months. I still think it’s necessary to consider the following aspects: 1. Are there any other trace components in the medium, such as chloride ions (not only in the tube side but also considering the quality of the steam)? 2. Is the corroded area at the medium inlet? 3. Was there any noticeable vibration of the tube bundle during equipment operation? 4. Is the reboiler vertical? Is the corroded area near the liquid level (the interface between liquid and gas)?
Judging from the appearance of the pipe corrosion, it is corrosion from the inside out of the pipe. Therefore, it is possible that the working fluid contains components that corrode 316L. Such as halogen elements, sulfide ions. The process of producing ethyl acetate via gasification of syngas tends to carry a small amount of these ions. Also, check whether there are any components other than 316L in the tube sheet of the pipe or at other points of contact, such as the tube sheet and other supports. It may cause electrocorrosion. If the reboiler generates high noise during operation, it will exert lateral stress on the heat exchange tubes. So stress corrosion could also be the cause.
Based on the corrosion morphology, the corrosion appears to have started inside the tube and gradually intensified until perforation occurred. If the material selection is correct: (1) it is necessary to verify whether there are any substances in the fluid flowing through the pipe that could be corrosive to 316L, such as chloride ions. (2) Is the perforation regular, and are areas with alternating dry and wet conditions common? Trace amounts of corrosive substances can easily accumulate in this area, causing perforation. (3) Is this heat exchanger operating in reverse or in the same direction? Is the perforation location above or below the steam inlet? The temperature here is high, which can easily cause the material to vaporize instantly, resulting in flashing that washes against the tube walls.
There seems to be an issue with the material chosen for the pipes; the selected material is of low quality, the composition of the material is complex, and the operating temperature is quite high. It is recommended to conduct a qualitative analysis of the material before selecting an appropriate one
Silicon carbide heat exchangers can be used; it’s convenient to add QQ: 4630138 for further discussion