Influence of Particle Size on the Drying Kinetics of Single Droplets Containing Mixtures of Nanoparticles and Microparticles: Modeling and Pilot-Scale Validation (original) (raw)
Related papers
Fundamental Investigation of the Drying of Solid Suspensions
Industrial & Engineering Chemistry Research
In this work, a comprehensive series of experiments is conducted to investigate the drying behavior of micro-and nanosized particle dispersions. To this end, an acoustic levitator was used to study the drying kinetics of single droplets. The temporal evolution of the actual droplets was recorded using a complementary metal oxide semiconductor (CMOS) camera, and the solid grains produced at the end of drying were investigated by scanning electron microscopy (SEM) imaging. At the end of drying, the grains show different morphologies as a function of the particle size, concentration, and initial droplet volume. We combine these experimental data to show the drying behavior is dependent on all the parameters and that the data all collapse when plotted against the Pećlet number. This resulted in a novel characteristic diagram which allows one to predict the shape of the dried colloidal droplet based on Pe. Our results extend the fundamental understanding of the mechanisms controlling the drying of droplet suspensions.
Processes
The spray drying technique is suitable for different kinds of liquid dispersions and can be easily optimized to produce solid particles with tailored properties. The spray drying technique is a complex process. As an example, it is difficult to track drying kinetics, shape, and morphological changes on the scale of a single droplet. To better understand the effect of drying process variables on dried particle formation, it is useful to observe the drying of single droplets. Fundamental processes, such as mass and heat transfer, can then be easily monitored and compared with theoretical models. Acoustic levitation enables droplet/particle suspension in the air without any mechanical contact. Experiments in the acoustic levitator can be used to mimic the drying process in the spray dryer. The drying kinetics of single droplets of PEG6000 into solid particles was studied. Droplets with an initial polymer concentration (PEG6000 aqueous solution of 5%, 10%, and 15% (w/w)) were investigat...
Chemical Engineering Science, 2020
Spray drying allows tuning the physical properties of the resulting powders widely. However, targeted process design is complicated by the interplay between process characteristics and the non-ideal physical properties exhibited by multicomponent mixtures, such as pharmaceutical formulations. This work presents a mechanistic model describing the drying of single droplets. The model includes heat and mass balances, non-ideal vapour-liquid equilibria, and population balances describing the evolution of particulates within the drying droplet. The model is applied to (up to) ternary mixtures of solvents, polymers and solutes, and predicts properties of the drying droplet such as the time of shell formation and the size of the particle obtained after drying. A comparison with experimental data from single droplet drying experiments carried out at defined relative vapour saturation and temperature shows that the model can be used to predict shell formation (as well as other properties) for the systems studied here.
Aerosol Science and Technology
Experimental studies of particle formation from solution droplets were conducted using a newly developed monodisperse spray drying process. Solutes beclomethasone dipropionate and caffeine were dissolved in ethanol, pressurized hydrofluoroalkane propellant 134a, and mixtures thereof. Solutions were atomized into monodisperse microdroplets using a custom droplet generator installed in a laboratory scale spray dryer, enabling drying and collection of the resulting monodisperse microparticles. The effects of droplet diameter, solution concentration, solvent composition, and drying rate on the physical properties of the dried particles were evaluated. Particle morphology and size were assessed using ultramicroscopy and image analysis of micrographs. Extent of crystallinity and polymorphism were investigated using Raman spectroscopy. The drying temperature was found to have a large effect on the morphology of amorphous beclomethasone dipropionate particles. Particles dried near room temperature were spheroidal to ellipsoidal with prevalent surface concavities and evidence of shell buckling; increasing the drying temperature for fixed droplet size and composition resulted in a transition to more spherical, smooth-surfaced particle morphologies. Crystalline caffeine microparticles were made up of assemblies of multiple crystallites. The measured length and breadth of these crystallites was found to be correlated with the time available for crystal nucleation and growth as calculated using a particle formation model. The results highlight the abilities and limitations of currently available particle formation models in elucidating the relationships between the size, composition, and evaporation rate of drying solution droplets and the physical properties of the resulting particles. The work demonstrates the suitability of monodisperse spray drying as an experimental technique for investigating the fundamentals of particle formation from solution droplets.
Study of dynamics of drying processes in FE2O3 and SIO2 nanocolloid droplets
2014
This work is devoted to comparison of changes in geometry and in light transmittance of evaporating drop of nanocolloid deposited on flat surface. One of the focuses of paper is aimed at differentiation of drying process into separate stages, each being characterized by particular behavior of suspension nanoparticles inside the droplet, the last of which ends by forming of ring-shaped patterns (known as "coffee ring effect") that appear after all the fluid has evaporated. Dynamics of colloids` evaporation was studied according to two observing techniques. The first one was based on spectrometric measuring of light transmitted through droplets. The second method was based on registering of geometrical properties of drying puddles that were changing in time due to evaporation. Patterns left by droplets of nanocolloids were alike in generalcollars were formed along the perimeter of initial droplet. According to coffee ring effect a major part of suspended particles assembled in these collars though a thin film of suspended substance deposited within initial drop perimeter. The results of drying dynamics experiments carried out in two different techniques appeared to be also different. The second (geometrical) method showed a uniform decrease of drop height while the first (spectrometric) method showed non-uniform changing of parameters. The time of drying according to the second technique was less than the time of drying received in experiments undertaken with the first technique.
Anais da Academia Brasileira de Ciências
A model including simultaneous droplet heating and water evaporation is proposed to simulate temperature, shrinking and mass profiles of a spherical droplet subjected to convective drying, being valid for the first drying stage. Experimental data on drying skim milk and colloidal silica obtained in the literature were used for validation, but there is no restriction in the model that prevents it from being suitable for other materials. There were not significant differences observed concerning to the droplet components (dissolved or insoluble materials). The initial heating time of the particle upon reaching the constant temperature is relatively short (Δt ≈ 7s) for both simulated materials and water evaporation during the first drying stage occurs mostly at the wet bulb temperature of the air. Discrepancy between simulated and experimental values did not exceed 9% for skim milk and 7% for colloidal silica in this first stage, indicating good applicability of the model. Considering the applicability of the model in a more generic way, Whitaker correlation evaluated at the film temperature showed better results. Finally, the small discrepancy found is discussed and some improvements are proposed.
The Journal of …, 2008
The method of formation of nanoparticle aggregates such as high-coverage spherical shells of microspheres or 3-D micro crystals grown in the geometry unaffected by a substrate is described. In the reported experiment, the evaporation of single levitated water droplet containing 200 nm diameter polystyrene spheres was studied. Successive stages of the drying process were discussed by analyzing the intensity of light elastically scattered by the evaporating droplet. The numerically simulated self-assembly coincides nicely with the observed morphologies resulting from transformation of a droplet of suspension into a solid microcrystal via kinetically driven self-assembly of nanostructures.
Self-organization of colloidal particles during drying of a droplet: Modeling and experimental study
Advanced Powder Technology, 2018
Formation of structurized micro/nanoparticle aggregates in spray drying process is analyzed theoretically and experimentally. Colloids of mono-and bimodal particle size distribution are used as the precursors to demonstrate different patterns of particle self-organization inside the drying droplet. In case of monodisperse primary particles their self-organization in the final aggregate results in either a hollow or a full (packed) spherical structure. For primary particles with bimodal size distribution, either the layered structure of aggregates is formed (with smaller particles forming outer layer and the bigger particles captured inside) or the ordering of bigger particles on the aggregate surface is observed, depending on process parameters. Numerical investigations allow to predict and explain the conditions at which selfassembling of particles within powder aggregates takes place.
Morphological transformations during drying of surfactant-nanofluid droplets
Journal of Industrial and Engineering Chemistry
The effect of surfactants with different chain length on the drying dynamics of nanosized dispersion droplets and on the final morphology of the grains formed after water evaporation is investigated experimentally. An acoustic levitator was used to examine the drying dynamics of single droplets and SEM imaging was used to characterise the morphology of the final dried grains. Results show that the drying of drops with high molecular weight surfactants leads to more irregular grains and that the grain morphology is related to surface tension driven instability of the evaporating droplets which may lead to formation of hollow dried grains.
Chemical Engineering Science, 2011
Experimental data on the drying behavior of suspension droplets is limited, despite its importance in industrial applications for material processing, chemical or the food industry involving spray dryers. This fact is particularly significant for high load and temperature conditions, as found in such industrial applications. In this work, the drying behavior of acoustically levitated multiphase droplets has been experimentally investigated. The acoustic tube levitator has been modified in order to allow experiments to be performed at high temperature conditions. The flow rate, temperature and relative humidity of this air stream can be controlled by an air conditioning system. A CMOS camera and a backlight illumination system are used to measure the droplet cross-sectional area and vertical position of the droplet during the drying process. The experiments have been performed using water-glass particle suspensions. The glass particles have a mean particle size and relative density of 13 mm and 2.5, respectively. The effect of the air temperature (60 1C o To 120 1C), initial volume of the droplet (0.05 mloV 0 o 0.7 ml), initial solid mass load (0.01 o Y S o 0.5) and relative humidity of the air (0.05 o HR o 0.45) on the mean porosity of the grain, first drying period duration and liquid evaporation rate has been analyzed by means of a parametric screening matrix and also by means of a central composite design (CCD) experimental design. The most important parameters to be considered for the porosity and the drying behavior in the range of variables analyzed are the initial solid mass load and the initial droplet volume. The relative humidity of the air exerts a moderate influence on the drying behavior of the droplet and the temperature has only a very low impact on the mean porosity. In addition, particular attention should be given to the drying behavior of small droplets, which result in a very low mean porosity values for high solid mass loads. The CCD confirms that the initial droplet volume, the solid mass load and their interaction exert significant influence on the three responses.