Experimental study on thermal stability and degradation behavior of tripropylene glycol and tetrapropylene glycol under closed-system conditions

This study investigates the thermal behavior of mono-, di-, tri-, and tetrapropylene glycol (MPG, DPG, TPG, and TePG) under elevated-temperature conditions, motivated by the high reboiler temperatures required for their separation and the limited literature on their thermal stability. Thermal stability experiments were carried out using a laboratory-designed apparatus operating at high temperatures under closed-system conditions. Two types of experiments were performed: long-term tests with sampling after 24 h and short-term tests with sampling every 3 h. All collected samples were analyzed using gas chromatographic methods. Long-term experiments conducted at 438.15 K for MPG, DPG, and TPG indicate limited formation of degradation products under these conditions. For TePG, experiments performed at 438.15 K for 48 h and at 473.15 K for 24 h show a slight increase in degradation products with increasing thermal exposure, suggesting the onset of degradation. Short-term experiments for TPG and TePG at 523.15 K reveal more pronounced degradation behavior, with TePG exhibiting a more complex pattern characterized by the formation and growth of degradation products, indicating greater susceptibility to thermal cleavage and secondary transformations. Thermal degradation is strongly temperature-dependent, with TPG and especially TePG—relevant for high-temperature reboiler conditions—showing increased susceptibility, leading to the formation of lower glycols and degradation products that reduce product purity.