Vol 50, No 6 (2016)

The use of human mesenchymal stem cells (hMSCs) in regenerative medicine is a potential major advance for the treatment of many medical conditions, especially with the use of allogeneic therapies where the cells from a single donor can be used to treat ailments in many patients. Such cells must be grown attached to surfaces and for large scale production, it is shown that stirred bioreactors containing ~200 μm particles (microcarriers) can provide such a surface. It is also shown that the just suspended condition, agitator speed N_JS, provides a satisfactory condition for cell growth by minimizing the specific energy dissipation rate, ε_T, in the bioreactor whilst still meeting the oxygen demand of the cells. For the cells to be used for therapeutic purposes, they must be detached from the microcarriers before being cryopreserved. A strategy based on a short period (~7 min) of very high ε_T, based on theories of secondary nucleation, is effective at removing >99% cells. Once removed, the cells are smaller than the Kolmogorov scale of turbulence and hence not damaged. This approach is shown to be successful for culture and detachment in 4 types of stirred bioreactors from 15 mL to 5 L.

Mechanisms of mass transfer intensification in the process of membrane cleaning using supercritical fluids were investigated. Transport properties, hydrodynamics of the solvent flow, mutual solubility of the solvent and the oil contaminant, and capillary effects occurring inside the porous membrane as factors affecting the overall process performance were studied. The analysis was performed using empirical correlations for the transport property coefficients and a model of the process implemented to CFD code developed using the OpenFOAM environment. Supercritical carbon dioxide exhibits favourable transport properties, which are highly tunable with process parameters and contribute to low mass transfer resistance. The investigated process is controlled by diffusive mass transfer inside the membrane pores, so increasing solvent flow rate has limited impact on the overall process rate. Mutual solubility of the oil and solvent phase leads to the effect of swelling of the oil phase, which promotes faster completion of cleaning. Capillary effects inside the pores may be another factor of process acceleration which requires further investigation.

An effect of the impeller eccentricity on the process characteristics in an agitated vessel was analyzed on the basis of our own experimental and numerical results obtained within the turbulent range of the Newtonian liquid flow. Mixing time, power consumption, local and mean values of the heat transfer coefficients and distributions of the transport coefficients (shear rate and friction coefficient) at the vicinity of the vessel wall were studied experimentally within the wide range of the operating and geometrical parameters of the agitated vessel equipped with eccentrically located high-speed impeller. Numerical simulations of hydrodynamics in an agitated vessel with eccentrically located axial flow impeller (up-pumping propeller or downpumping HE-3 impeller) were carried out using CFD method.

Similarities in the flow and heat transfer characteristics between simulation results of a round impinging jet and experimental data obtained on an agitated vessel with an axial flow impeller are presented in this paper. The electrodiffusion method was used in measuring the local heat transfer coefficients on a flat bottom of an agitated vessel. A small axial impeller has been built to provide a clearly defined flow imitating a submerged confined jet impinging the vessel bottom. Our simulation and experimental results show that the flow pattern in the impinging jet region below the axial flow impeller can be compared to the stagnation region of a round impinging jet with a corresponding tangential velocity component. CFD simulations of an impinging jet showed also the importance of different boundary conditions on small electrodes used with the electrodiffusion method and give an approximation of necessary correction factor.

The study reveals the hydrodynamics at the surface of a submerged tubular membrane module integrated in a stirred membrane bioreactor. The reactor is equipped with a conventional six flat-blade impeller imposing radial circulation across the membrane interface. Simulation and computer visualization of “real” flow using a Reynolds-averaged Navier-Stokes model and CFD methodology are employed. A variety of model solutions at various mixing intensity are obtained and the mixing conditions are assessed by delineation of the near-wall zones and identification of the zones' shear rate and shear stress values. Shear rate non-uniformity along the surface of the tubular module is visualized. Shear stress values as high as 160 Pa at the membrane module lower section and as low as 0.6 Pa at the module upper section has been determined. Referring to reported data for shear stress near flat plate stirred filtration cells and external narrow-channel cross-flow systems, the mixing conditions are expected to allow enhanced access of the retentate fluid to the membrane surface, as well as possible low membrane fouling potential related to microfiltration practice.

The process of convection and diffusion in a Taylor regime of gas–liquid flow through microchannels is modeled based on a three-layer mass transfer model in the axisymmetric formulation. The circulation circuit of Taylor vortices is described in the form of outer and inner layers surrounded by a thin film that does not participate in circulation. Due to the assumption about the predominance of convection over radial diffusion in the inner layer (the Peclet number order of magnitude is nearly 10⁵) and a uniform concentration distribution over the cross sections of each layer, the problem is confined to a quasi-one-dimensional problem with boundary conditions of special type. The obtained numerical solutions allow us to determine the kinetics of mass transfer from a liquid to a channel wall in detail, calculate the average mass-transfer coefficient, and reveal optima in the dependences of the mass-transfer coefficient on the two-phase flow velocity and the capillary diameter; furthermore, there is also an optimum for the length of a liquid slug. The obtained results allow us to understand the pattern of Taylor convection and diffusion fluxes and to reveal the reasons for a decrease in mass fluxes under nonoptimal conditions. This enables the correction of selected geometric and process parameters during the design of microreactor equipment.

Dynamic programming is applied to the optimization of a cascade of stirred tank reactors in which an isothermal first-order reaction takes place. Use of this method is illustrated by calculation of polymerization parameters for a four-reactor cascade from experimental data available from the literature.

Self-sustained oscillations of fluidized bed have been considered, as well as the concerted periodic change in their integral parameters, including the average porosity and height, the flow rate of a fluidizing agent, and the pressure drop across the apparatus. The main factors are revealed, the joint effect of which at a certain combination of the system parameters results in the development and occurrence of this phenomenon. Mathematical models have been formulated for four reactor schemes, i.e., open and closed devices with blowing and open and closed devices that blow open with the suction of a fluidizing gas. Solutions are obtained in the form of damped oscillations and undamped oscillations with finite amplitude. An analysis of the stability of stationary solutions has been carried out. Calculations have been performed for the conditions of experiments known from the literature. The results of a comparison of the calculated and measured values for the frequencies of fluidized bed oscillations have been presented.

This article deals with specific structural features of the vapor–liquid equilibrium diagram of the butyl propionate–propionic acid–butyl butyrate–butyric acid four-component system, which is of industrial importance and contains biazeotropic ester–acid constituents. The positions of some isomanifolds in the concentration tetrahedron and in its constituents are reported.