Hydrodynamic and mass transfer studies in an external-loop air-lift bioreactor for immobilized animal cell culture

Air-lift bioreactors containing suspended or immobilized animal cells have been used for the production of a variety of high-value biologicals. In the bioprocessing industry, there is a need to study and quantify the relationships between bioreactor-system properties such as mixing, flow, mass transfer, and cell processes. In the present study, the performance of a 1-L external-loop air-lift bioreactor was investigated by studying gas-liquid oxygen transfer, mixing time, liquid velocity and gas hold-up at various aeration rates. These studies were performed over a range (0-25%) of loadings of small (500-800 μm) calcium alginate beads to investigate the effect of using various concentrations of cell immobilization matrices on the physical properties of the system. At an aeration rate of 0.5 vvm, the mixing time was decreased by 50%, from 75 s at 0% bead loading to 38 s at 10% bead loading. A minimum liquid velocity of 10 cm/s was required to keep the alginate beads in suspension. As bead loading increased, flow within the reactor went from turbulent conditions to the transition zone. At all bead loadings tested, the gas hold-up increased by only 2% with an increase in aeration rate from 0.1 to 1.0 vvm, regardless of whether the total reactor volume (i.e., liquid and beads) or the liquid volume was used in calculating the hold-up. A mathematical correlation was developed for expressing the dependence of the volumetric mass-transfer coefficient, k₁a, on aeration rate (vvm) and microbead loading. With this equation it was possible to predict, within 20%, the k₁a knowing the gas-flow rate and the volume percentage of microbeads present in the bioreactor. A theoretical study was also performed to calculate the oxygen transfer from the bulk liquid to the center of microcapsules containing animal cells using experimental k₁a data. The results suggest that whereas there is no oxygen limitation at 10 to 15% microcapsule loading, there is a potential mass-transfer problem at 25% loading if the bioreactor is operated at an aeration rate of less than 1.06 vvm.     

Silicalite crystal growth investigated by atomic force microscopy

Atomic force microscopy has been used to image the various facets of two morphologically distinct samples of silicalite. The smaller (20 microm) sample A crystals show 1 nm high radial growth terraces. The larger (240 microm) sample B crystals show growth terraces 1 to 2 orders of magnitude higher than the terraces on sample A with growth edges parallel to the crystallographic axes. Moreover, the terraces on the (010) face are significantly higher than the terraces on the (100) face - inconsistent with the previously proposed 90 degrees intergrowth structure. Sample A highlights that under certain synthetic conditions, silicalite grows in a manner akin to zeolites Y and A, via the deposition of layers comprising, in the case of silicalite, pentasil chains. It is probable that the rate of terrace advance is identical on the (010) and (100) faces, and it is the rate of terrace nucleation that dictates the overall growth rate of each facet and hence the relative size expressed in the final crystal morphology. Analysis of the growth terraces of sample B and detailed consideration of the structures of both MFI, and a closely related material MEL, lead to the proposal of a generalized growth mechanism for silicalite including the incorporation of defects within the structure. These defects are thought to be responsible for both the relative and the absolute terrace heights observed and may also explain the hourglass phenomenon observed by optical microscopy. The implications of this growth mechanism, supported by results of infrared microscopy, generate a new dimension to the continuing debate on the existence of intergrowths within one of the most important structures relevant to zeolite catalysis.     

Excitation of kinetic Alfvén waves by streaming ions in a dusty magnetoplasma with generalized (r,q) distribution function

We investigate the parallel streaming effects on the dispersion characteristics of a kinetic Alfvén wave (KAW) in a low β dusty magnetoplasma. To analyse the influence of streaming ions obeying generalized (r, q) distribution function, hot and magnetized electrons, and mobile charged dust, a theoretical approach has been used for the instability analysis by employing two potential theories. A linear kinetic dispersion relation of Alfvén waves is derived, whose solutions are used to interpret the numerical and analytical results. The solutions of dispersion relation indicate that the characteristics of KAWs are transformed when generalized (r, q) distribution function is employed instead of its Maxwellian counterpart. We also found that the unstable modes have a strong dependence on spectral indices r and q , dust parameters, and plasma β . For the excitation of KAWs, the streaming velocity has been observed to be within the sub-Alfvén range, whereas when it extends to the super-Alfvén range, the growth rates are significantly suppressed. The observations further show that an ambient magnetic field and superthermal particles inhibit the growth of an electromagnetic wave to a significant degree and have a stabilizing effect on the wave mode, whereas an increasing concentration of low-energy particles contributes to enhancing growth rates.     

Synthesis and Crystal Structure of a Novel Self-assembled 2D Coordination Polymer of Chloridobis(imidazolidine-2-thione)thiocyanato dicopper(I)

Abstract  A copper(I) complex, poly(chloridobis(imidazolidine-2-thione)thiocyanato dicopper(I)), [Cu₂(Imt)₂(SCN)Cl] _n (1) (Imt = Imidazolidine-2-thione) has been prepared by the reaction of CuCl₂ with imidazolidine-2-thione and potassium thiocyanate in the ratio of 1:1:2. Compound 1 crystallizes in the monoclinic space group P2₁/a in the form of a coordination polymer, consisting of 2D layers. The solid-state structure is composed of dinuclear units having each copper(I) ion tetrahedrally coordinated. These units aggregate through bridging Imt and thiocyanate leading to a supramolecular 2D-network. Index Abstract  The title complex, poly(chloridobis(imidazolidine-2-thione)thiocyanato dicopper(I)), [Cu₂(Imt)₂(SCN)Cl] _n (Imt = Imidazolidine-2-thione) was prepared by the reaction of CuCl₂ with imidazolidine-2-thione and potassium thiocyanate in the ratio of 1:1:2. The crystal structure consists of dinuclear units having each copper(I) ion tetrahedrally coordinated. These units aggregate through bridging Imt and thiocyanate leading to a supramolecular 2D-network.