Abnormal temperature detection for fixed bed reactors

A fixed bed processing system, comprising:

• a reactor vessel having a reactor inlet and a reactor outlet;
• at least one fixed bed of catalyst particles within the reactor vessel;
• one or more temperature-sensor structures within the reactor vessel;
• a detector in fluid communication with the reactor outlet, wherein the one or more temperature-sensor structures are composed of a material that undergoes a structural change at a threshold temperature, and wherein the one or more temperature-sensor structures have a first rate of introducing a detectable substance into the reactor at a temperature below the threshold temperature and a second rate of introducing the detectable substance at a temperature above the threshold temperature, the second introduction rate being at least about 5 times the first introduction rate.