Alan R. English
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.