Experimental Set-up

Experiments were carried out in a set-up consisting of an internal loop airlift reactor column and a vibration control system. The airlift reactor comprises of an outer column (the reactor column), a riser column and a gas-liquid separator unit all constructed of polyacrylate. The riser column, with an internal diameter of 0.069 m was placed concentrically in the reactor column, which has an internal diameter of 0.10 m. The annular space between the riser and reactor columns forms the downcomer section of the reactor. The lower end of the riser tube was supported by means of three metal pins placed at a distance of 0.11 m from the bottom of the reactor. The gas-liquid separator with an internal diameter of 0.29 m and a height of 0.36 m was mounted at a height of 1.58 m from the base of the reactor.

The bottom of the reactor column was sealed by means of a 0.4 mm-thick silicon rubber membrane tightly sandwiched between two metal discs each with a diameter of 0.096 m. Below the membrane was a pressurized air chamber. This chamber ensures that the membrane remains in a horizontal position after the reactor column is filled with liquid (a condition necessary for the membrane to be properly displaced by the vibrator) by allowing for pressure compensation. Air was fed into the reactor through 11 stainless steel capillary gas distributors each with an internal diameter of 0.9 mm. The outlet points on the gas distributors were at height of 0.12 m from bottom of the reactor. A twelfth stainless steel capillary, positioned at the centre of the reactor served as a tracer injection point for RTD experiments. The rate of air flow into the reactor was controlled using pre-calibrated rotameters.

An air-cooled vibration exciter (TIRAvib 5220, TIRA Maschinenbau GmbH, Germany) was attached to the bottom of the reactor column. This vibrator was coupled to a power amplifier. It can be operated within a frequency range of 2 - 5000 Hz. Depending on the frequency, the amplitude can be varied between 0.0001 -12 mm. The vibration system was fully automated and controlled from a PC using Signal Calc 550 Vibration Controller software (Data Physics Corporation, United States).

Schematic representations of the experimental set-up, the reactor dimensions and the gas distributor are given below:

Schematic representation of the experimental set-up