Elisabeth Rodier

Microwave Vacuum Drying of Porous Media : Experimental Study and Qualitative Considerations of Internal Transfers

This paper deals with experimental results obtained on a laboratory scale dryer. Two porous media were dried, a packing of initially water saturated glass beads and a packing of initially unsaturated pharmaceutical granules. For the first one, the main parameter is permeability of the bed varying the beads diameter and for the second one it is the porosity of the bed varying the amount of product. These experiments were carried out at fixed incident power and pressure level. In the first period of the drying, it was observed that permeability was a limiting parameter to internal mass transfers when free liquid water is expulsed, revealing high gas pressure gradients inside the product. They occur when we have a massive vaporisation with ebullition. As for the porosity of the granules bed, it has no effect on mass transfers. In the second period, we have no influence of both parameters on the drying kinetics. In the last period, so in the hygroscopic field or below the irreducible saturation, the internal mass transfers are governed by the electromagnetic energy absorption.

Comparison Between Static and Dynamic Methods for Sorption Isotherm Measurements

Abstract Sorption isotherms of Microcrystalline Cellulose (MCC), pharmaceutical granules, PolyEtherBlocAmide (PEBA) membrane and sewage sludges were measured at various temperatures using three different experimental gravimetric methods: the saturated salt method, the Dynamic Vapor System (DVS) apparatus and a simultaneous gravimetric and calorimetric measurement techniques using a Setaram TGA-DSC111. A comparison between the salts method and DVS is proposed at ambient temperature for the MCC, the pharmaceutical granules and the PEBA membrane. The comparison between the static method and the DVS shows that the results are consistent as long as the apparent moisture content in the material is high. When the apparent diffusion coefficient becomes low, the difficulty to reach the thermodynamic equilibrium appears in the saturated salts method. Then, DVS and TGA-DSC apparatus were used to investigate the desorption isotherms for the MCC between the ambient temperature and 90°C. Finally, an application on biological products is proposed: the desorption isotherms for two sewage sludges coming from the same wastewater treatment plant were determined with the DVS and the TGA-DSC equipments at 45°C. The accuracy of the TGA-DSC is good as long as the apparent diffusion coefficient is above 10−9 m2/s. When this threshold value is overstepped, the desorption isotherms are overestimated.

Drying of Silica Gels to Obtain Aerogels:Phenomenology and Basic Techniques

Silica gels form a large family of materials, obtained from more or less complex processes; their elaboration involves several steps that basically include a sol gel transition or a precipitation, followed by drying. This latter operation generally leads to dry materials with a structure significantly different from that in the wet state. If this structure is preserved after drying, a new variety of silica gels is obtained, known as aerogels. For the last 30 years, aerogels have been the subject of an increasing number of research works, because of their extraordinary

Physicochemical characterization of d-mannitol polymorphs: The challenging surface energy determination by inverse gas chromatography in the infinite dilution region

Nowadays, it is well known that surface interactions play a preponderant role in mechanical operations, which are fundamental in pharmaceutical processing and formulation. Nevertheless, it is difficult to correlate surface behaviour in processes to physical properties measurement. Indeed, most pharmaceutical solids have multiple surface energies because of varying forms, crystal faces and impurities contents or physical defects, among others. In this paper, D-mannitol polymorphs (α, β and δ) were studied through different characterization techniques highlighting bulk and surface behaviour differences. Due to the low adsorption behaviour of β and δ polymorphs, special emphasis has been paid to surface energy analysis by inverse gas chromatography, IGC. Surface energy behaviour has been studied in Henrys domain showing that, for some organic solids, the classical IGC infinite dilution zone is never reached. IGC studies highlighted, without precedent in literature, dispersive surface energy differences between α and β mannitol, with a most energetically active α form with a γ(s)(d) of 74.9 mJ·m⁻². Surface heterogeneity studies showed a highly heterogeneous α mannitol with a more homogeneous β (40.0 mJ·m⁻²) and δ mannitol (40.3 mJ·m⁻²). Moreover, these last two forms behaved similarly considering surface energy at different probe concentrations.

Cocoa Butter Saturated with Supercritical Carbon Dioxide: Measurements and Modelling of Solubility, Volumetric Expansion, Density and Viscosity

The use of supercritical carbon dioxide technology for lipid processing has recently developed at the expense of traditional processes. For designing new processes the knowledge of thermophysical properties is a prerequisite. This work is focused on the characterization of physical and thermodynamic properties of CO2-cocoa butter (CB) saturated mixture. Measurements of density, volumetric expansion, viscosity and CO2 solubility were carried out on CB-rich phase at 313 and 353 K and pressures up to 40 MPa. The experimental techniques have previously been compared and validated. Density measurements of CB and CB saturated with CO2, were performed using the vibrating tube technology at pressures ranging from 0.1 to 25 MPa. Experimental values correlated well with the modified Tait equation. CO2 solubility measurements were coupled to those of density in the same pressures ranges. At 25 MPa, the solubility of CO2 is 31.4 % and 28.7 % at 313 and 353 K. Phase behaviour was investigated using a view cell in order to follow the expansion of the CB-rich phase with the rise in pressure. Volumetric expansion up to 47 % was measured and correlated to the CO2 solubility. Phase inversion was observed at 313 K and 40 MPa. Lastly, we developed an innovative falling ball viscometer for high pressure measurements. Viscosity measurements were carried out up to 25 MPa showing a viscosity reduction up to 90 % upon CO2 dissolution. These results were correlated with two empirical models. A new model here presented, was favourably compared with the Grunberg and Nissan model. All the experimental results are consistent with the available literature data for the CB-CO2 mixture and other fat systems. This work is a new contribution to the characterization of physical and thermodynamic behaviour of fats in contact with CO2 which is necessary to design supercritical fluid processes for fats processing.

MICROWAVE VACUUM DRYING OF POROUS MEDIA: VERIFICATION OF A SEMI-EMPIRICAL FORMULATION OF THE TOTAL ABSORBED POWER

Vacuum drying experiments were performed on a laboratory scale dryer with two porous materials: a packing of initially saturated glass beads and of unsaturated pharmaceutical granules. Several incident powers and two vacuum pressure levels were tested in order to demonstrate different drying mechanisms. The drying kinetics, temperature of the product and the absorbed power are presented and the coupling between the drying rate and the absorbed power is shown. A semi-empirical formulation of the total absorbed power is proposed taking into account the water content, the temperature and the dielectric properties of each phase. This formulation is based on a quasi-static assumption which allows the local electric field inside the material to be expressed with an analytical equation.

Development of Characterization Techniques of Thermodynamic and Physical Properties Applied to the CO2-DMSO Mixture

This work is focused on the development of new characterization techniques of physical and thermodynamic properties. These techniques have been validated using the binary system DMSO-CO2 for which several studies of characterization have been well documented. We focused on the DMSO-rich phase and we carried out measurements of volumetric expansion, density, viscosity and CO2 solubility at 298.15, 308.15 and 313.15 K and pressures up to 9 MPa. The experimental procedures were compared and validated with the available literature data on SC-CO2-DMSO system. We made density and CO2 solubility measurements, using respectively the vibrating tube technology and two static analytical methods. Lastly, we developed an innovative falling body viscosimeter for high pressure measurements. All the measurements made are in good agreement with the already published data in spite of very different experimental techniques. This work is a contribution to the understanding of the DMSO-CO2 binary as it implements new viscosity data. Moreover, it opens new perspectives about the determination of the properties of other systems such as polymers-CO2 and fats-CO2, which are essential for supercritical process design such as extraction, crystallization, chromatography and synthesis reaction.

Modelling non-homogeneous flow and residence time distribution in a single-screw extruder by means of Markov chains

Non-homogeneous velocity distribution of the flow in the channel of a single-screw extruder is taken into account by a new model developed on the basis of the Markov chains. This model allows calculating the Residence Time Distribution (RTD) as well as the influence of the operating conditions on the process at any velocity distribution in the channel. It has been used to represent experimental results on mass flow rate and RTD previously obtained by extrusion of an acrylic polymer, Eudragit E100, at different temperatures and screw rotation speeds. The diffusion coefficient is the only adjusting parameter of the model. It was shown that it does not depend on the screw rotation speed and a correlation between this diffusion coefficient and the barrel temperature was found. The model provides global understanding of the transport kinetics of the flowing material through the extruder according to its behaviour and better describes the progress of the polymer flow all along the barrel from the hopper to the die.

Experimental Study on Microwave Vacuum Drying of Two-Porosity Level Media

Abstract Microwave vacuum drying experiments were performed on a laboratory scale dryer with a two-level porosity material: a packed bed of porous alumina beads. The incident microwave power and the vacuum pressure level were fixed, the main varying parameters being the beads diameter and porosity, and the mean pore diameter. The drying kinetics and the evolution of the product temperature are presented. The drying kinetics can be divided into two main periods. The first one corresponds to the drying from the bed voids according to evaporation mechanism that we describe with a stagnant film law. The second one corresponds to the drying from the particle pores and we divide it into two parts: we suggest that the former is dominated by capillarity driven moisture transport, and the latter is limited by the desorption kinetics of the few water layers left.

Microencapsulation by a Solvent-Free Supercritical Fluid Process: Use of Density, Calorimetric, and Size Analysis to Quantify and Qualify the Coating

Coating of microparticles or microencapsulation is a widely used operation in which a thin layer of a coating agent is deposited onto a solid particle. Currently, the technique faces two challenges: being solvent-free and being applicable for coating particles smaller than 80 microns. In this work, several techniques are used to test the feasibility of a new solvent-free coating process using supercritical fluid technology. Some model microparticles, glass beads, and an active compound (AC) are coated with Precirol®, which is a mixture of fats. The process involves two steps: first supercritical CO2 is dissolved in molten Precirol®, then the melt solution is sprayed onto the host particles by a rapid expansion. The particles coated in this way are examined by particle size analysis, environmental scanning electron microscopy (ESEM), and IR spectrophotometry. Also mass ratios of host particle to coating are evaluated using both helium pycnometry to determine sample density and calorimetry to measure the heat of fusion of the coating agent. The results show that these analytical techniques can be used for qualitative analysis as well as quantification of the coating. Thus they give relevant information on reproducibility and feasibility of the new process.