## 2 Experiments and Signal Acquisition

Table 2: Operating parameters of the columns. the distillation process and facilitated the direct observation of the gas-liquid two-phase interaction during the acquisition process. The acquisition method shown in the figure could accurately collect the passive acoustic signals of the target distillation column under different operating conditions and reduce the influence of environmental noise. The behavior of bubbles can be categorized into three types: bubble formation, bubble coalescence, and bubble bursting Bubble formation refers to the creation of bubbles when the ascending gas phase enters the liquid phase on the upper tray through the sieve holes. Bubble coalescence occurs when two or more bubbles merge within the liquid phase to form a new bubble. Bubble breakup is the process in which a single bubble breaks into multiple bubbles due to external forces within the liquid or on the liquid surface. Different bubble behaviors result in varying amounts of energy being transferred to the external environment by the gas-liquid interactions, causing differences in the information carried by the passive acoustic signals. When the reboiler heating voltage reached 150 V, it was observed that the liquid layer height on the sieve trays gradually increased to 30 mm and remained stable. From the top window, it was observed that the bubble diameter in the liquid phase ranged between 8 and 15 mm, indicating that the distillation column was in a normal mass transfer state. As shown in Figure 3a, the generation, coalescence, and breakup of bubbles occurred almost simultaneously. The passive acoustic signal collected at this time was classified as N1. When the reboiler heating voltage stabilized at 170 V, the liquid layer height on the sieve trays further increased. Due to the increased gas flow rate, the gas velocity through the perforations also increased, leading to a faster necking and breakup of gas bubbles. Consequently, the bubble diameter decreased, and the number increased, with bubble generation and breakup dominating and bubble coalescence in the liquid phase being almost unobservable. More liquid was carried to the upper trays, as shown in Figure 3c, indicating entrainment flooding. The passive acoustic signal collected at this time was classified as F1. However, as the heating voltage gradually increased from 150 V to 170 V, the distillation column began to deviate from the normal mass transfer state. Observations from the top window revealed that entrainment started to occur during this phase. However, based on the top column temperature and the reflux flow rate, the mass transfer process inside the column was not severely affected. Further observations from the top window indicated that although the bubble size did not significantly differ from normal mass transfer, the liquid layer height on the sieve trays increased to 40-50 mm, as shown in Figure 3b. Bubble generation increased, and bubble breakage on the liquid surface intensified, potentially  
Figure 2: Schematic diagram of the signal acquisition device. leading to different acoustic characteristics. The passive acoustic signal collected in this state was labeled as T1, indicating a potential flooding fault. When the reboiler heating voltage was increased to 190 V, observations from the top window showed that the liquid layer height on the sieve trays rapidly decreased until it disappeared, with the light component vapor accumulating at the top and upper trays drying out. As shown in Figure 3e, as the liquid phase decreased, bubble generation almost ceased, with bubble coalescence becoming the dominant behavior. The passive acoustic signal collected at this time was classified as F2. Similarly, as the voltage increased, the distillation column did not undergo a sudden state change, as shown in Figure 3d; the state before tray drying was classified as the potential dry tray fault state T2. To simulate a heating failure due to reasons such as circuit aging or short-circuiting, the reboiler heating voltage was significantly reduced.