On the material of the propylene distillation column in ethylene plants and the methods for starting up and shutting down
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I’ve heard that in ethylene plants, the propylene distillation columns – I’m new to the petrochemical industry; I used to work in the chlor-alkali sector – are made of Q345R material. Therefore, the design temperature for such columns must be around -16°C. However, at atmospheric pressure, the temperatures of propane and propylene drop below -40°C (propane at -42°C, propylene at -48°C). I know that the operating temperature for these two columns is around 50°C, but doesn’t introducing or removing liquid phases during startup and shutdown cause temperatures to drop below this level? How is it avoided in the operating procedures? What if a leak occurs in the reboiler? How is that handled? Has the tower cracked due to ultra-low temperatures? (I once accidentally introduced liquid HCl at -35°C into the discharge main, and as a result the pipeline cracked immediately.) I appreciate your advice, thank you. Please focus on explaining how to avoid the problems
When draining the C3 tower heat exchanger during shutdown, pressure must be maintained while draining; the liquid phase must be completely drained before venting the gas phase to relieve pressure. It is strictly prohibited to relieve pressure with liquid present!
There is a very important rule: first apply pressure when filling with liquid, and release pressure after pouring the liquid in.
The key is to follow the operating procedures strictly. Pressurize first then add liquid; pour the liquid in first then release pressure. I haven’t heard that low-temperature materials are required for the propylene distillation tower in ethylene plants; ordinary carbon steel is used. The material after the LD line throttle valve changes to 304 stainless steel.