Answer to Question #302563 in Chemical Engineering for SAVARA RAVITEJA

Question

. A magnetic stirrer agitated vessel is used for absorption of CO2 in water at 298 K and 2

atm pressure. Water is fed at a rate of 1 L/min and the carbonated water leaves the vessel

continuously so that a constant volume is maintained in the contactor. The outlet water

contains 2.3 g CO2/L. The interfacial area of gas-liquid contact is 80 m2

/ m3

; the volume of

gas-liquid dispersion is 8 L. The solubility of CO2 in water can be found using Henry’s law.

At 298 K, the Henry’s law constant for CO2 is 1640 atm/mole fraction and it’s diffusivity in

water is 1.92 x 10-9 m/s. The density of liquid is 997 kg/ m3

. Calculate:

(i) The thickness of the liquid film, if the Film theory is applicable.

(ii) The contact time between the liquid element with the gas if the Penetration theory is

applicable.

(iii) The surface renewal rate if the Surface-renwal theory is applicable. (

Answer 1

Volume

Magnetic stirrers are most effective for smaller volume samples. Typical lab bench models are rated for maximum volumes of 20 L and lower. Overhead stirrers can range in their top volumes from 2‑80 L and higher, depending on the model.  Unless specified, both magnetic and overhead stirrers set their maximum volume based on water viscosity.

Viscosity

Viscosities close to water can be stirred by both magnetic and overhead stirrers, but higher viscosity mixing is where the overhead stirrer shines. This can be especially critical for applications increasing in viscosity as they progress, such as polymerizations or lotions as they cool. With no direct connection to its agitating motor, the magnetic stir bar can be stopped by the fluid’s resistance in higher viscosity batches. In contrast, the impeller of overhead stirrers is inserted into a chuck or collet connected to its drive motor.  Refer to the overhead stirrer’s viscosity and torque specs to ensure the motor is suitable for the viscosity of your sample, and check for the ability to maintain the set rpm under changing viscosities. Models with torque display can be helpful, signalling the completion of a batch when the torque indicates a certain viscosity is reached.