Mass transfer of gases in agitated tanks has a wide range of applications in the real world. For example, it was used to dissolve oxygen into a fermenter or fish tanks. In addition, one of the Senior Design project groups is using this technique to produce liquid phase methanol. In their process, natural gas and steam are reformed in a furnace to make hydrogen and carbon oxides. The hydrogen and carbon oxides react under pressure in a gas phase reactor in the presence of a catalyst to produce methanol. In all of the described systems above, it involved sparging a gas into a stirred tank. Various correlations had been published by professional scientists to related mass transfer coefficient (kLa) with impeller speed and gas superficial velocities. They will be used to compare to the experimental results.
This experiment was broken down into two parts, the absorption of oxygen into the water from the air, and the desorption of oxygen out of the water by using Nitrogen. In the absorption part of the lab, a stream of air was injected into the tank from the bottom of the tank. An impeller run at constant RPM will ensure the oxygen is homogenously spread out in the tank. In the desorption cycle, air will be switched off and a stream of Nitrogen will be introduced to the system.
The rate of mass transfer of oxygen into vessel is primarily determined the kLa, which is liquid phase mass transfer coefficient. The kLa is affected by the temperature, flow rate and impeller speed. The concentration of oxygen in the tank will initially be zero when no oxygen is injected; then, concentration will increase steadily to saturation. At saturation, the concentration will not increase even when more oxygen is injected into the system. The set up looked like this:This system consists of a vessel that holds approximately 250 liters of water, with an internal diameter, DT, of 2 feed. An 11-inch, 6-blade impeller along with four 2.5 by 24 inch baffles are being installed at the bottom of the vessel. The vessel is made of stainless steel, and the heating jacket is made of 9 rounds of coil. A stream of steam and cooling water are being used to control the heating and cooling of the system.
In this experiment, a stream of air is injected into the vessel tank full of water through a circular distributer. An impeller is used to ensure the mixture in the tank is homogenous. The speed of the impeller is controlled by a motor. Dissolve oxygen probes are installed at two locations within the tank to monitor the change in oxygen concentration over time. Air will first be flowed through the tank at a fixed flow rate, temperature, and RPM of the impeller. When the oxygen concentration in water is saturated, Nitrogen will be switched on to flush the system free of oxygen. Once the concentration of oxygen dropped down to zero, air will be turned on again at a new operating condition of choice.