What is the most likely cause of a frozen chlorine cylinder?

The most likely cause of a frozen chlorine cylinder is related to the feed rate being too high. When the feed rate of a chlorine cylinder is set excessively high, it can lead to rapid gas flow. Chlorine is stored in cylinders under pressure as a liquid, and when it’s fed into the system too quickly, it can’t vaporize properly. This rapid change from liquid to gas can result in temperatures dropping significantly, leading to freeze-up in the area of the regulator or valve.

In a properly functioning system, the feed rate should be balanced to allow for adequate vaporization without causing extreme cold conditions that could lead to freezing. Therefore, managing the feed rate is crucial for maintaining the appropriate operating conditions and preventing freeze-ups, making it the correct answer.