Injecting gases through a diffuser into a liquid is an important aspect of many operations in the chemical process industries (CPI). Important applications include dissolving reactant gases into a liquid phase for further reaction (such as in hydrogenation, oxidation, ozonation), as well as carbonation of beverages, stimulation of fermentation processes (Figure 1), aeration of wastewater for treatment, stripping of air or oxygen from chemicals, stripping volatile organic compounds (VOCs) from liquid chemicals, removal of moisture from fuels and others. This one-page reference provides information on key aspects of gas diffusion in liquids, including mass-transfer rate, agitation effects and equipment selection.
The main purpose of a sparging system is to increase the gas-to-liquid mass-transfer efficiency, (a ratio of the amount of active gas component dissolved in liquid to the amount of gas injected). Low mass-transport efficiency leads to an elevated gas-injection rate. In this case, the increased gas volume raises the cost to achieve the desired results. The gas-to-liquid mass-transfer efficiency is primarily controlled by the mass-transfer resistance of the liquid phase.
Fast and efficient mass transfer is correlated with fine bubble propagation, which increases the gas surface area in contact with liquid.
Engineered porous metal or ceramic materials create fine bubbles according to the requirements of the application. Porous materials allow large volumes of gas to be passed with very high specific area. For example, with equal volumes of gas, 1-mm bubbles would have 6.35 times more gas-liquid contact surface area than 6.35 mm (1/4-in.) bubbles .
The gas-to-liquid mass-transfer rate per unit volume is calculated using: KLa(C* – C), where KL is the liquid-phase mass-transfer coefficient that is dependent on the diffusivity, liquid viscosity, temperature, and mixing; a is the interfacial area of gas bubbles in contact with liquid; C* is the saturated concentration of the gas in liquid; and C is the concentration in bulk liquid.
By sparging small gas bubbles with high surface-to-volume ratio into the liquid, the interfacial area a is increased and the gas mass-transfer rate is improved. The mass-transfer driving force (C* – C) also has a big impact on the gas dissolution rate, as the high-purity gas is used instead of the lower-purity gas. For example, the saturated concentration of oxygen in water from pure oxygen is five times higher than that from air, resulting in a large increase in the oxygen dissolution rate with pure oxygen.
Gas diffuser considerations
Spargers are chosen based on the design and operating conditions of the process The type and configuration of the sparger used depend on factors such as whether a process is a continuous process or batch, as well as the gas flowrate, tank size, mechanical agitation, operating pressure and temperature.
Materials of construction. Metal spargers are used in high-temperature, corrosive or oxidizing conditions, whereas ceramic spargers are sufficient for mild conditions.
Gas exit velocity. The gas exit velocity at the sparger surface is an important design criterion for sparger selection. The actual gas volumetric flowrate for exit velocity is calculated using the pressure (P) that is the sum of tank headspace pressure (PHeadspace), liquid head pressure at the sparger (PLiquid), and pressure drop across the sparger element (ΔP). The minimum sparger surface area is based on the gas exit-velocity limit for the process.
The exit velocity limit is lowest for the static sparging operation when there is no mechanical agitation of the liquid phase. For agitated tank sparging and dynamic sparging, where liquid has high forced velocity along the sparger surface, the gas exit-velocity limits are significantly higher, requiring smaller spargers for the same gas flow. The exit-velocity limit for agitated tank sparging and dynamic sparging depends on the impeller speed and liquid velocity, respectively.
Agitation effects. Apart from using a properly designed sparger, it is important to focus on the mixing of gas and liquid. In chemical process applications, the reactor vessel is often closed, so that the unreacted high-purity gases, such as hydrogen or oxygen, are not vented through the system. In these applications, specially designed mixing impellers are used, depending on the operating conditions of the reactor. Typically, one impeller turbine is located above the sparger to shear and disperse the gas bubbles. Agitation at the liquid surface may also be required to entrain the headspace gas into the liquid phase.
Diffuser sizing. The size of a sparger depends largely on the superficial gas exit velocity from the porous sparger surface. This value is calculated from the actual cubic feet per minute (ACFM) per square foot of sparger surface area (ACFM/ft2). The ACFM is calculated at the liquid pressure and temperature found at the sparger (the ACFM is not based on gas pressure)*.
Editor’s note: Portions of the text in this column were adapted from the following article: Air Products Inc., Gas Sparging, Chem. Eng., September 2012, p. 21.
*An additional reference is the following publication: Mott Corp., Gas-liquid Contacting Part Selector and Design Guide, www.mottcorp.com.