Consultation on issues with phthalic anhydride heat transfer oil systems

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Dear colleagues, I have a question regarding the heat transfer oil used in the phthalic anhydride production process. Basic details: production capacity of 30,000 tons per year, phthalic anhydride produced via the naphthalene method. The switchable condenser uses heat transfer oil for cooling and heating. The heat transfer oil system consists of a cold oil system (operating temperature: 50 degrees) and a hot oil system (operating temperature: 180–220 degrees). In the cold oil system, a water cooler is used to cool the heat transfer oil, while a cold oil pump is employed to circulate the oil (to the switchable condenser); the flow rate of this pump is 500 m3/h. No high-level tank is set up in the system. Hot oil system: Steam available on site is used to heat the heat transfer oil in the heat storage oil heater, and a heat storage oil pump is employed to circulate the heat transfer oil within the heater and the heat storage tank. The flow rate of this pump is 250 m3/h, which ensures that the flow velocity of the heat transfer oil in the heat storage oil heater reaches 2.2 m/s. In addition, the hot oil system is equipped with a hot oil tank as well. After being temperature-regulated by heat exchange, the heat transfer oil in the hot oil tank and the heat storage oil tank is pumped to the switching condenser using a hot oil pump. No high-level tank is set up in the system. The current problem is that, two years after using a synthetic heat transfer oil with a maximum operating temperature of 300 degrees, the oil has suffered significant degradation (an increase in residual carbon and acid value), and a large amount of this oil needs to be replenished. Have any of the experts encountered this problem? Is it because a high-level tank has not been installed in the system? If a head tank is to be installed, should cold oil and hot oil share one head tank, or should two separate head tanks be used? Additionally, is there any difference between naphthalene phthalic anhydride and ortho-phthalic anhydride when selecting heat transfer oils? Is it necessary to use biphenyl-biphenyl ether-type gas-liquid two-phase heat transfer oil in the naphthalene process? Thank you very much for all your valuable suggestions and opinions!