The effect of flow regimes in the distillation of systems containing two liquid phases

Single phase solutions containing three components have been observed to exhibit foaminess near a single to two liquid phase boundary. It was seen, in a sintered plate column under mass transfer conditions, that distillation systems where the liquid appeared as one phase in one part of a column and two phases in another part, exhibited foaminess when the liquid concentration was near the one phase to two phase boundary. Various ternary systems have been studied in a 50 plate. 30mm i.d. Oldershaw column and it was observed that severe foaming occurred in the middle section of the column near the one liquid phase to two liquid phase boundary and no foaming occurred at the end of the column where liquid was either one phase or two phase. This is known as Ross type foam. Mass transfer experiments with Ross type ternary systems have been carried out in a perspex simulator with small and large hole diameter trays. It was observed that by removal of the more volatile component, Ross type foam did not build up on the tray. Severe entrainment of liquid was observed in all cases leading to a 'dry' tray, even with a low free area small diameter hole tray which was expected to produce a bubbly mixture. Entrainment was more severe for high gas superficial velocities and large hole diameters. This behaviour is quite different from the build up of foam observed when one liquid phase/two liquid phase Ross systems were contacted with air above a small sintered disc or with vapour in an Oldershaw distillation column. This observation explains why distillation columns processing mixtures which change from one liquid phase to two liquid phases (or vice versa) must be severely derated to avoid flooding. Single liquid phase holdups at the spray to bubbly transition were measured using a perspex simulator similar to that of Porter & Wong (17). i.e. with no liquid cross flow. A light transmission technique was used to measure the transition from spray regime to bubbly regime. The effect of tray thickness and the ratio of hole diameter to tray thickness on the transition was evaluated using trays of the same hole diameter and free area but having thickness of 2.38 mm, 4 mm, and 6.35 mm. The liquid holdup at the transition was less with the thin metal trays. This result may be interpreted by the theory of Lockett (101), which predicts the transition liquid holdup in terms of the angle of the gas iet leaving the holes in the sieve plate. All the existing correlations have been compared and none were found to be satisfactory and these correlations have been modified in view of the experimental results obtained. A new correlation has been proposed which takes into account the effect of the hole diameter to tray thickness ratio on the transition and good agreement was obtained between the experimental results and the correlated values of the liquid holdup at the transition. Results have been obtained for two immiscible liquids [kerosene and water] on trays to determine whether foaming can be eliminated by operating in the spray regime. Kerosene was added to a fixed volume of water or water was added to a fixed volume of kerosene. In both cases, there was a transition from spray to bubbly. In the water fixed system. the liquid holdup at the transition was slightly less than the pure kerosene system. Whilst for the kerosene fixed system, the transition occurred at much lower liquid holdups. Trends In the results were similar to those for single liquid phase. New correlations have been proposed for the two cases. It has been found that Ross type foams, observed in a sintered plate column and in the Oldershaw column can be eliminated by either carrying out the separation in a packed column or by the addition of defoaming additives.

Bioactive glass sol-gel foam scaffolds:evolution of nanoporosity during processing and in situ monitoring of apatite layer formation using small- and wide-angle X-ray scattering

Recent work has highlighted the potential of sol-gel-derived calcium silicate glasses for the regeneration or replacement of damaged bone tissue. The work presented herein provides new insight into the processing of bioactive calcia-silica sol-gel foams, and the reaction mechanisms associated with them when immersed in vitro in a simulated body fluid (SBF). Small-angle X-ray scattering and wide-angle X-ray scattering (diffraction) have been used to study the stabilization of these foams via heat treatment, with analogous in situ time-resolved data being gathered for a foam immersed in SBF. During thermal processing, pore sizes have been identified in the range of 16.5-62.0 nm and are only present once foams have been heated to 400 degrees C and above. Calcium nitrate crystallites were present until foams were heated to 600 degrees C; the crystallite size varied from 75 to 145 nm and increased in size with heat treatment up to 300 degrees C, then decreased in size down to 95 rim at 400 degrees C. The in situ time-resolved data show that the average pore diameter decreases as a function of immersion time in SBF, as calcium phosphates grow on the glass surfaces. Over the same time, Bragg peaks indicative of tricalcium phosphate were evident after only 1-h immersion time, and later, hydroxycarbonate apatite was also seen. The hydroxycarbonate apatite appears to have preferred orientation in the (h,k,0) direction.

Multifunctional bioactive glass scaffolds coated with layers of poly(d,l-lactide-co-glycolide) and poly(n-isopropylacrylamide-co-acrylic acid) microgels loaded with vancomycin

A new family of multifunctional scaffolds, incorporating selected biopolymer coatings on basic Bioglass® derived foams has been developed. The polymer coatings were investigated as carrier of vancomycin which is a suitable drug to impart antibiotic function to the scaffolds. It has been proved that coating with PLGA (poly(lactic-co-glycolic acid)) with dispersed vancomycin-loaded microgels provides a rapid delivery of drug to give antibacterial effects at the wound site and a further sustained release to aid mid to long-term healing. Furthermore, the microgels also improved the bioactivity of the scaffolds by acting as nucleation sites for the formation of HA crystals in simulated body fluid. © 2013 Elsevier B.V. All rights reserved.

Novel bioactive scaffolds incorporating nanogels as potential drug eluting devices

Big advances are being achieved in the design of new implantable devices with enhanced properties. For example, synthetic porous three-dimensional structures can mimic the architecture of the tissues, and serve as templates for cell seeding. In addition, polymeric nanoparticles are able to provide a programmable and sustained local delivery of different types of biomolecules. In this study novel alternative scaffolds with controlled bioactive properties and architectures are presented. Two complementary approaches are described. Firstly, scaffolds with nanogels as active controlled release devices incorporated inside the three-dimensional structure are obtained using the thermally induced phase separation (TIPS) method. Secondly, a novel coating method using the spraying technique to load these nanometric crosslinked hydrogels on the surface of two-dimensional (2D) and three-dimensional (3D) biodegradable scaffolds is described. The scanning electron microscopy (SEM) images show the distribution of the nanogels on the surface of different substrates and also inside the porous structure of poly-a-hydroxy ester derivative foams. Both of them are compared in terms of manufacturability, dispersion and other processing variables.

Atmospheric plasma treatment of porous polymer constructs for tissue engineering applications

Porous 3D polymer scaffolds prepared by TIPS from PLGA (53:47) and PS are intrinsically hydrophobic which prohibits the wetting of such porous media by water. This limits the application of these materials for the fabrication of scaffolds as supports for cell adhesion/spreading. Here we demonstrate that the interior surfaces of polymer scaffolds can be effectively modified using atmospheric air plasma (AP). Polymer films (2D) were also modified as control. The surface properties of wet 2D and 3D scaffolds were characterised using zeta-potential and wettability measurements. These techniques were used as the primary screening methods to assess surface chemistry and the wettability of wet polymer constructs prior and after the surface treatment. The surfaces of the original polymers are rather hydrophobic as highlighted but contain acidic functional groups. Increased exposure to AP improved the water wetting of the treated surfaces because of the formation of a variety of oxygen and nitrogen containing functions. The morphology and pore structure was assessed using SEM and a liquid displacement test. The PLGA and PS foam samples have central regions which are open porous interconnected networks with maximum pore diameters of 49 μm for PLGA and 73 μm for PS foams. (Figure Presented) © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.