Reasons for the darkening color of coal tar hydrogenated refined oil

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The color of the oil products in the coal tar hydrogenation and refining stage gradually darkens, shifting from light red to deep red, then to dark green and finally to very dark green. The sulfur and nitrogen contents are within acceptable limits, and the bromine value is also low. Improvements are seen when the reaction temperature is increased, but the changes are not significant. Let’s discuss the reasons for this. The process in my facility is as follows: Pretreatment is used to remove asphalt and light components with temperatures below 200°C; the remaining heavy tar then enters the reaction section. The inlet temperature for the first reactor is 280°C, while the outlet temperature for the third reactor is 370°C. We believe that the problem lies in the efficiency of separation in the pretreatment vacuum distillation column – the liquid level at the bottom of the column remains at 100%, which causes some of the heavy components to end up in the reaction section. I would appreciate it if experts could analyze whether this assumption is correct