A fixed bed reactor shows declining conversion over time. How would you diagnose the cause?

Diagnostic approach: Check operating conditions (T, P, feed composition vs design), verify instrumentation accuracy, sample catalyst for analysis (coke content, metal dispersion, poison accumulation, physical integrity). Pressure drop trend indicates fouling or bed movement. Temperature profile changes reveal deactivation patterns - uniform decline suggests sintering, inlet-concentrated suggests feed contamination. Compare with fresh vs used catalyst activity tests. Mass balance closure verifies analytical accuracy. Historical correlation with feed source changes may identify poisons.