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We are having corrosion in our FCC gas recovery section. It is producing a fouling in the reboiler deetanizer. We are injecting water from the FCC SWS (Sour water system) that is recycled. We inject water before the interstage heat exchanger gas compressor and before the heat Exchanger main tower. The water from interestage gas compressor vessel and high pressure vessel are sent to before the main tower heat exchanger. All water from the main tower top vessel are sent to the SWS (sour water system). What is it the typical FCCU water system ? Is our wash water system good in order to reduce corrosion ? Do I need to inject any chemical for that ? Where is it the point to inject? Is it necessary to inject water before the high pressure vessel ? And what about the water quality.
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There are several factors that can affect the effectiveness of your anti-corrosion water wash. The system you describe, where water is routed first to high pressure service and then cascaded to lower pressure is an older design and not as effective as cascading water from low to high pressure. Cyanide solubility in water increases with pressure so it is likely that you are capturing cyanide in the high pressure water wash which is then released to travel through the system again when the water is used at a lower pressure. Modern unit design collects water from the main fractionator overhead receiver. Much of this water is recycled to the main column overhead receiver exchanger inlet. Additional fresh water (BFW or SWS water) is added to the recycle stream to control wash water pH and cyanide level. Excess receiver water is used to wash the interstage exchanger. Water from the interstage receiver is used to wash the high pressure exchanger. Water from the high pressure receiver is usually routed to the sour water stripper. It will likely be costly to reverse the direction of your water cascade. Adding more makeup water and/or using chemicals such as ammonium polysulfide will directionally help. Wash water pH and cyanide should be monitored to assure water quality.
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We found that injecting recycled water from the Main Fractionator OH Acc (low pressure) to the Main Fractionator OH condensers works very well. We would then inject recycled water with ammonia polysulphide into the compressor discharge. We would monitor for PH and cyanides weekly to insure water quality.
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