Gas liquid mass transfer in bioreactors is affected by many factors. Diffusion coefficient, liquid-phase density, and fluid viscosity are intrinsic parameters of the fluid. They leave little room for changes in studies on intensifying gas liquid mass transfer. So, turbulent kinetic energy dissipation rate, bubble diameter, local gas holdup, and surface tension are the main research targets for mass transfer intensification.
Intensification of Gas Liquid Mass Transfer via Turbulent Kinetic Energy Dissipation Rate
Turbulent kinetic energy and average fluid velocity are key factors for turbulent kinetic energy dissipation rate. So, researchers need to increase the turbulent kinetic energy and average fluid velocity in gas-liquid bioreactors. For mechanically agitated bioreactors, you can optimize the reactor’s basic parts (like the tank body, impellers, or baffles). This can greatly boost the turbulent kinetic energy and average fluid velocity of the bioreactor.
- Impeller Design for Intensifying GasLiquid Mass Transfer
Mechanically agitated bioreactors are widely used in many fields. Their core part is the impeller. It directly affects the gas-liquid two-phase flow in the reactor. So, designing the impeller structure is the first step to improve the mass transfer efficiency of bioreactors.
The standard Rushton turbine (Figure 1(a)) has drawbacks in use. Many researchers have put forward ways to improve its structure:
- (b) Self-similar impeller: It is based on the self-similar characteristics of nonlinear theory. It aims to intensify the gas-liquid dispersion process. Studies show this impeller breaks gas cavities. It reduces negative pressure zones. This increases the relative power demand (RPD). It also makes the gas holdup distribution more uniform.
- (c) Grid-disk impeller: It replaces the solid disk of the standard Rushton turbine with a grid disk. This enhances the reactor’s gas dispersion and axial pumping capacity. It also lowers energy needs.
- (d) Fan-shaped impeller: This impeller uses less power under the same operating conditions. It also has high-efficiency oxygen mass transfer performance. Its oxygen transfer efficiency is 35%–66% higher than that of the standard Rushton turbine.
- (e) Perforated straight-bent impeller: Compared with non-perforated impellers, it improves gas-liquid mixing. It reduces power loss. This then increases the relative power demand (RPD).
- Tank and Baffle Design for Intensifying GasLiquid Mass Transfer
You can use impeller design and combination methods to intensify gas liquid mass transfer. You can also design the reactor tank structure and baffles properly. This can also intensify the gas liquid mass transfer process. The tank structure and baffle arrangement directly affect the flow pattern inside the bioreactor. This then influences gas liquid mass transfer.
Lu and others studied a shaken conical-bottom bioreactor (Figure 2). They compared the mixing and mass transfer characteristics before and after installing baffles. They tested three baffle arrangements: vertical, inclined, and horizontal. All three arrangements broke the quasi-steady flow state without baffles. The mass transfer coefficient increased by 38.1%, 40.0%, and 33.6% respectively.
Photobioreactors are key equipment for microalgae cultivation. They are often used in biological carbon fixation processes.
Kumar and others installed butterfly-shaped baffles (Figure 3) in a dual-column photobioreactor (DC-PBR). They optimized the baffle size and the angle between the wings. The results show these butterfly-shaped baffles create vertical flow vortices inside the reactor. They enhance the light/dark cycle. This reduces the mixing time by 20%. It increases the mass transfer coefficient by 32%. It also improves the photochemical efficiency and electron transport rate by 20%. The biomass growth rate goes up by 33%.
Intensification of Gas Liquid Mass Transfer via Bubble Diameter and Gas Holdup
Research on bubble diameter in bioreactors has reached the nano-micron scale. When the bubble diameter is as small as the nano-micron scale, it shows different mass transfer characteristics from macro bubbles. Nano-micro bubbles are hundreds to thousands of times smaller than regular bubbles. This greatly increases the mass transfer area. Also, the mesoscopic effects across the nano-micro scale can make the mass transfer coefficient even larger. This significantly enhances interphase mass transfer.
For gas-liquid bioreactors, getting a nano-micro interface depends on making a micro-dispersed phase. Compared with regular dispersed phases, micro-dispersed phases (microbubbles or microdroplets) have advantages. They have a larger specific surface area. Their phase interfaces are more stable. They stay longer in the reactor. Usually, the diameter of the micro-dispersed phase is directly related to the flow channel characteristics. So, designing gas distributors properly can effectively increase the reactor’s gas holdup. It can reduce aeration pressure. It also improves interphase transfer and reaction processes.
Intensification of Gas Liquid Mass Transfer via Surface Tension
The physicochemical properties of the fluid (like density, viscosity, and surface tension) have a big impact on gas liquid mass transfer. Surface tension is easier to adjust than density and viscosity for mass transfer intensification. So, you can add a non-aqueous phase to reduce surface tension. This can overcome the mass transfer limitations of bioreactors.
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