## 2 Experiments and Signal Acquisition  
A laboratory-scale sieve tray column was employed in this study, and the parameters of the distillation column are provided in Table 1. The distillation apparatus used in this experiment is shown in Figure 1. The sound acquisition equipment used was a ICD-UX565, SONY, Guangzhou, China, with a sampling frequency range of 1-44,100 Hz, capable of covering the audible sound range. The equipment was placed on the outside of the distillation column, at a distance of 2 mm from the column body. The upper and lower sections of the distillation column were equipped with viewports, allowing for the observation of the relative motion state of the fluid inside the column during operation. The experiment was conducted at ambient temperature and pressure. The system within the column was chosen to be a 15% ethanol-water solution [30]. This selection aimed to simulate the purification process of the original fermentation broth of bioethanol, thereby better mimicking the actual production process and obtaining results with higher reliability and comparability. Initially, approximately 30.0 L of feed liquid was added to the feed tank and thoroughly stirred with a feed pump to ensure uniformity. The specific gravity of the feed was recorded to calculate the concentration, and the sample was set aside for later use. Then, a certain amount of feed liquid was added to the reboiler using the feed pump. After fully opening the cooling water valves of the top condenser, the heating power switch of the reboiler \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Distillation Column Parameters**} \\ \hline Column height & 2230 mm \\ Tray type & Sieve tray \\ Tray spacing & 100 mm \\ Number of trays & 9 \\ Column diameter & 50 mm \\ Downcomer diameter & 5 mm \\ Sieve pore diameter & 2 mm \\ Free area fraction of trays & 8.7\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of distillation columns. was turned on, and the voltage was set to 150 V. The two-phase contact state within the column was observed through the viewports. Once the reflux liquid began to appear at the top of the column, the temperature of the reflux liquid was maintained, and operational parameters such as the top temperature, reboiler temperature, reflux flow rate, and reflux temperature were recorded every minute while continuing to observe the two-phase contact situation within the column. After the operation of the entire column stabilized, the specific gravity of the top product was measured and recorded at room temperature. When the ethanol mass fraction of the top product reached over 95% and remained stable, the passive acoustic signals of the equipment were collected. Subsequently, the heating voltage of the reboiler was continuously increased to raise the gas velocity within the column and change the operating state of the distillation column. The operating parameters of the columns are shown in Table 2.  
The instrument for collecting passive acoustic signals is shown in Figure 2. The recording equipment was fixed on the outside of the distillation column, at a distance of 1-2 mm from the column. As the temperature of the column was high during operation, this distance not only ensured that the sensor would not exceed the normal operating temperature range and thus be damaged during the acquisition process but also effectively avoided the non-acoustic interference caused by the sensor being close to the equipment. As illustrated, the distillation column was equipped with upper and lower viewports that were made of glass, which could withstand the operating pressure and temperature of  
Figure 1: Distillation equipment used in this study. \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Operating Parameters**} \\ \hline Maximum reboiler duty & 3.19 kW \\ Heater voltage & 150–190 V \\ Operation state & Total reflux \\ F factor & 0.25–0.4 (m/s)\(\cdot\)(kg/m\({}^{3}\))\({}^{0.5}\) \\ Gas flow velocity & 3.0–4.6 m/s \\ \hline \hline \end{tabular}
\end{table}