Air was used as the gas phase and demineralized water as the liquid phase in all experiments carried out. At the start of each experimental run, the clear liquid height was set at 0.16 m in the gas-liquid separator (corresponding to a clear liquid height of 1.74 m from the base of the monolith airlift column). Measurements were made of the gas holdup in the riser, the downcomer liquid velocity and the rate of gas-liquid mass transfer. For each experiment, the internal loop airlift reactor was operated in such a way that no gas was present in the downcomer. Each set of experiments was first carried out with the column operated without vibration. This was followed by a corresponding experimental set wherein the vibration system was utilised. In all vibration experiments carried out, the vibrator was programmed to generate low frequency sine wave oscillations. The amplitude of vibration was set at 0.5 mm while the frequency was set at 60 Hz.
The gas holdup in the riser section of the airlift reactor was measured by sealing the top of the riser with a pre-calibrated plastic stopper at the moment gas flow into the column was shut down. In this way, gas was trapped in the riser. The height of the trapped gas was read using a graduated rule affixed on the riser tube, from which the volume of gas and thus, the gas holdup were determined. For each gas flow rate, the gas holdup experiments were done thrice and an average holdup value taken. The figure below gives a representation of the procedure used to measure the volume of gas bubbles in the riser:
Representation of the procedure used for determining the gas holdup in the riser of the airlift reactor column
To measure the liquid velocity in the downcomer section of the airlift reactor, a tracer was injected 1.54 m from the base of the column (corresponding to a distance of 0.045 m beneath the top of the downcomer), and detected by a conductivity probe placed 1.371 m below. The tracer used was a saturated NaCl solution and between 0.2 and 0.5 mL was injected depending on the superficial gas velocity into the reactor column. The conductivity probe used consisted of two copper wires, which were connected to a Consort K920 conductivity meter and a PC. RTD data obtained were interpreted to obtain the mean residence time, from which the liquid velocity in the downcomer was determined for a given superficial gas velocity.
The volumetric mass transfer coefficient kLa was determined by means of a dynamic oxygen absorption technique. An oxygen electrode (YSI Incorporated Model 5331) inserted 0.15 m from the base of the reactor column was used to measure the change in dissolved oxygen concentration. Readings given by the electrode were fed to the PC via an ammeter and an analogue-to-digital converter card. The change in dissolved oxygen concentration was reflected as a change in electrical current displayed on the ammeter. Sensitivity of the sensor to the presence of dissolved oxygen was ensured by the application of a 0.13 g/mL KCl solution between its tip and an outer membrane, made of Teflon. Prior to conducting the mass transfer experiments, the time constant of the oxygen sensor was determined.
Dissolved oxygen was stripped from the liquid phase to a negligible concentration by the use of nitrogen sparged through the gas distributor capillaries. After the stripping operation, a step input of air was introduced into the column, with the uptake of oxygen into the liquid phase continuously monitored by the oxygen sensor. Sufficient time was given in each experimental run for the oxygen saturation concentration in the liquid, CL* to be reached. Data obtained were then interpreted to obtain volumetric mass transfer coefficient values, based on a model of the reactor developed.
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