Pressure pulsations in sieve-tray columns.

Destructive vibrations occur in industrial sieve-tray columns under certain flow conditions. The vibrations result from regular pulsations of the gas flow and pressure in the column. These pulsations have been studied with a 3 m. high perspex model sieve-tray column having a single active test-tray using air and water as process fluids. The effects on the amplitude and frequency of the pressure pulsations of gas velocity, tray liquid head, liquid crossflow and tray and column geometry were determined using pressure transducers with real time signal analysis. Pulsations were produced with fifteen tray geometries, including five hole diameters between 4.76 mm and 15.87 Mm. Most of the pulsations were produced at gas velocities below 12 mls and liquid heads below 200 N/m2. The pulsation fundamental frequency varied between 12 Hz. and 40 Hz., with an r.m.s. amplitude of up to 60 N/m2• The gas-liquid behaviour on the sieve-tray was studied using high speed cine-photography and electrical conductivity probes. This showed conclusively that, during the occurrence of pulsations, the gas-liquid interaction was highly regular and synchronised with the pressure pulsations. Detailed measurements were obtained of the degree of synchronisation and of the liquid motion on the tray. Several modes of gas-liquid interaction were identified. During pulsation production, the pulsating jet and the imperfect bubble were most common. Calculated fluctuations in the gas flow rate through the test-tray based on measured pressures agreed well with the results of the film analysis. A model of the synchronisation process is proposed based upon control by the pulsating jet. The model explains the measured limits of pulsation occurrence and permits some prediction of these limits for systems other than air-water. A simplified system flow analysis accounts for effects of gas density and column geometry on the pulsation frequency. The form of the empirical correlation of frequency with experimental variables is consistent with a physical description of the pulsating jet.