Average shear rate for non-Newtonian fluids in a concentric-tube airlift bioreactor

Global performance parameters for different pneumatic bioreactors operating with water and glycerol solution: experimental data and CFD simulation

Global variables play a key role in evaluation of the performance of pneumatic bioreactors and provide criteria to assist in system selection and design. The purpose of this work was to use experimental data and computational fluid dynamics (CFD) simulations to determine the global performance parameters gas holdup ([Formula: see text]) and volumetric oxygen transfer coefficient (k L a), and conduct an analysis of liquid circulation velocity, for three different geometries of pneumatic bioreactors: bubble column, concentric-tube airlift, and split tube airlift. All the systems had 5 L working volumes and two Newtonian fluids of different viscosities were used in the experiments: distilled water and 10 cP glycerol solution. Considering the high oxygen demand in certain types of aerobic fermentations, the assays were carried out at high flow rates. In the present study, the performances of three pneumatic bioreactors with different geometries and operating with two different Newtonian fluids were compared. A new CFD modeling procedure was implemented, and the simulation results were compared with the experimental data. The findings indicated that the concentric-tube airlift design was the best choice in terms of both gas holdup and volumetric oxygen transfer coefficient. The CFD results for gas holdup were consistent with the experimental data, and indicated that k L a was strongly influenced by bubble diameter and shape.

Application of hydrodynamic cavitation in ballast water treatment

Ballast water is, together with hull fouling and aquaculture, considered the most important factor of the worldwide transfer of invasive non-indigenous organisms in aquatic ecosystems and the most important factor in European Union. With the aim of preventing and halting the spread of the transfer of invasive organisms in aquatic ecosystems and also in accordance with IMO's International Convention for the Control and Management of Ships Ballast Water and Sediments, the systems for ballast water treatment, whose work includes, e.g. chemical treatment, ozonation and filtration, are used. Although hydrodynamic cavitation (HC) is used in many different areas, such as science and engineering, implied acoustics, biomedicine, botany, chemistry and hydraulics, the application of HC in ballast water treatment area remains insufficiently researched. This paper presents the first literature review that studies lab- and large-scale setups for ballast water treatment together with the type-approved systems currently available on the market that use HC as a step in their operation. This paper deals with the possible advantages and disadvantages of such systems, as well as their influence on the crew and marine environment. It also analyses perspectives on the further development and application of HC in ballast water treatment.

Oxygen transfer and shear rate in surface aerator

Shear rate in a surface aeration system exerts a profound effect on its performance, affecting the mixing pattern, the power requirement and oxygen transfer process. Based on theoretical analysis, it is shown that the shear rate gamma in the fluid is a function of the rotational speed N of the impeller in surface aeration systems. Analysis shows that the shear rate varies linearly with N in laminar flow and its behaviour is non-linear in a turbulent flow regime. Experimental correlations of gamma with N have been developed in the present study for different sized surface aerators. Scale-up or scale-down criteria for oxygen transfer rate have been developed, which relate oxygen transfer rate to shear rate in surface aeration systems.

Determination of the average shear rate in a stirred and aerated tank bioreactor

A method for evaluating the average shear rate ((.)gamma(av)) in a stirred and aerated tank bioreactor has been proposed for non-Newtonian fluids. The volumetric oxygen transfer coefficient (k(L)a) was chosen as the appropriate characteristic parameter to evaluate the average shear rate ((.)gamma(av)). The correlations for the average shear rate as a function of N and rheological properties of the fluid (K and n) were obtained for two airflow rate conditions (phi(air)). The shear rate values estimated by the proposed methodology lay within the range of the values calculated by classical correlations. The proposed correlations were utilized to predict the (.)gamma(av) during the Streptomyces clavuligerus cultivations carried out at 0.5 vvm and four different rotational impeller speeds. The results show that the values of the average shear rate ((.)gamma(av)) varied from 437 to 2,693 s(-1) by increasing with N and flow index (n) and decreasing with the fluid consistency index (K).