Single Droplet Drying Research Articles

A multiscale investigation on protein addition toward steering agglomeration and yield in spray drying

The positive effect of protein drying aids that reduce the stickiness of sugar-rich products on the spray drying yield is known. However, as agglomeration also depends on stickiness, one can expect an effect of these drying aids on interparticle collisions. Therefore, the effect of protein addition to maltodextrin systems on agglomeration and yield in spray drying was investigated using single droplet drying and pilot-scale spray drying experiments. Single droplet drying was used to compare the sticking regimes of whey and pea proteins at different concentrations. Adding protein increased the chance of creating agglomerates upon forced collisions between a drying droplet and a glass bead. This was reflected in an increased fraction of agglomerated particles in pilot-scale experiments with injection of fines. Pea protein decreased the fraction of non-agglomerated primary particles from 38% to 22% when the protein content in the liquid feed was 50 g/kg dry basis. In pilot-scale spray drying, protein addition improved the yield. Industrially, these results can be used to steer formulations regarding agglomeration and yield.

Single droplet drying of dairy‐based systems at spray drying like temperature–time trajectories

Upon spray drying of dairy powders, the distribution of components within the droplets can become nonuniform. In this study, the influence of the composition on the drying behaviour, morphological development and surface composition were studied using single droplet drying. The addition of fat was found to result in an earlier locking point (1.1 instead of 1.8 s). The fat content did not influence the morphological development, but the protein and lactose ratios did influence whether the dried particle was smooth or wrinkled. Confocal Raman microscopy revealed that the initial fat, lactose and protein contents influenced the dry surface composition. For example, fat was found on the surface of emulsions that were either rich in fat or in whey protein.

Single droplet drying with stepwise changing temperature-time trajectories: Influence on heat sensitive constituents

Optimization procedures for industrial spray drying processes mainly rely on empirical understanding. Mechanistic understanding of the process is limited, but can be enhanced by studying the drying of single droplets. We here report on a new sessile single droplet drying platform, using two air streams to represent the inlet and outlet air of a spray dryer to simulate changing conditions in a spray dryer. Accurate temperature measurements confirmed the temperature profiles and their imposition onto a drying droplet. Single droplets of solutions containing β-galactosidase and maltodextrin were dried with different temperature-time trajectories, with the inactivation of the enzyme as indicator for the thermal load on the droplet. The locking point is found to be an important parameter: the air temperature before this point does not influence the enzyme inactivation much, but a high air temperature after the locking point results in significant inactivation. The β-galactosidase inactivation was also successfully predicted with a coupled drying and inactivation model.

Describing the Drying and Solidification Behavior of Single Suspension Droplets Using a Novel Unresolved CFD-DEM Simulation Approach

With increasing computational capacities and advances in numerical methods, a simulation-based process and product design approach for spray dried products has become the focus of many researchers. In this context, a novel unresolved CFD-DEM simulation approach for single suspension droplets is presented, which models the drying and solidification process in a hot gas environment. The solidification process is described by the formation of bonds between the primary particles in the suspension droplet, showing a plausible solidification behavior, which starts when a critical solid concentration is reached at the droplet surface. The drying conditions for the single-droplet simulations are determined from a large-scale spray dryer simulation for different droplet size classes. The resulting solid particles from the CFD-DEM simulation show a higher tendency to form hollow particles at high drying rates when the locking point is reached at earlier stages of the drying process. Using these results, suitable operating conditions for the spray dryer can be determined, which favor the formation of particles with a targeted morphology.

A porous-crust drying model for a single dairy droplet

The development of a novel numerical model for droplet drying is the topic of this paper. The three main stages of droplet drying are distinguished, viz. unhindered evaporation of a ’wet’ particle (the droplet), restricted drying at a falling rate due to the formation of a crust around a wet core, and inert heating of the dry porous particle. Each stage is mathematically detailed to replicate all phenomena occurring throughout the drying process. The focus, however, is on the falling rate drying regime which is described in terms of Stefan diffusion of water vapour through the pores of a thickening crust. To this end, the model needs the material properties. This permits the droplet characteristics to be determined by composition rather than through single-droplet drying experiments. Finally, the model is validated against five of such experiments from literature using skim milk. Good agreement is found at each comparative case for the particle mass and temperature throughout the various drying regimes providing that for good reasons in three cases a lower drying air temperature is applied than reported for the experiments. The model is capable of predicting the entire drying process at low computational cost and without requiring empirical input.

Hanging a droplet with minimized intervention

Single droplet drying (SDD) by glass filament apparatus is a powerful tool for tracking droplet drying kinetics. However, the level and influence of intrusion of glass suspender into droplet have not been fully elucidated. In this work, the hanging processes of droplets by suspenders are numerically simulated by the volume of fluid (VOF) method, aimed at improved design of suspenders that can offer reduced suspender-droplet contact and increased droplet sphericity. Furthermore, the hanging simulation has been extended to small-sized droplets similar to those in spray drying operation which are too small to be addressed by the existing SDD technique. A comparison diagram of droplet suspension states at two scales has been constructed, which provides different suspension states of droplets as determined by the size of the suspender and the contact angle. The quantitative results can guide the design of minimal-intervention suspenders for droplets with specific properties.

Application prospect of reaction engineering approach in studying drying behavior of single droplet during spray drying of traditional Chinese medicine

Spray drying technology is one of the most commonly used unit operations in the production of traditional Chinese medicine(TCM) preparations, offering advantages such as short drying time and uniform product quality. However, due to the properties of TCM extracts, such as high viscosity, strong hygroscopicity, and poor flowability, there is limited scope to solve the problems of wall adhesion and clumping in spray drying from the macroscopic perspective of pharmaceutical production. Therefore, it has become a trend to study and optimize the spray drying process from the microscopic point of view by investigating single droplet evaporation behavior. Based on the reaction engineering approach(REA), the single droplet drying system, as a novel method for studying droplets, collects parameter data on individual TCM droplets during the drying process and uses the REA to process the data and establish predictive models. This approach is crucial for understanding the mechanism of TCM spray drying. This paper summarized and analyzed the cha-racteristics of various single droplet systems, the application of REA in single droplet drying systems, and its significance in optimizing the process, predicting drying states, and shortening the development cycle in the field of TCM spray drying, and looked ahead to the prospects of this method, including the introduction of new parameters and imaging techniques, aiming to provide a reference for further research in the field of TCM spray drying.

Modeling and simulation of single droplet drying in an acoustic levitator

We present a mathematical model for the full drying process of a single protein formulation droplet taking into account the convective impact arising from its levitation by a standing ultrasound wave. Using the finite element method allows us to compute the evaporation rate directly from the fully resolved heat and mass transfer within and around the levitated droplet. We apply our model to simulate the drying kinetics of pure water and aqueous phosphoglycerate kinase (PGK) droplets under various levitation and drying conditions. Empirical data from the literature are used to validate and discuss our numerical results. The acoustic streaming turns out to accelerate not only the first but also the second drying stage. Moreover, the dehydration of the protein molecules is found to be primarily responsible for their enzymatic inactivation throughout the drying process.

Impact of Different Saccharides on the In-Process Stability of a Protein Drug During Evaporative Drying: From Sessile Droplet Drying to Lab-Scale Spray Drying

ObjectivesSolid biopharmaceutical products can circumvent lower temperature storage and transport and increase remote access with lower carbon emissions and energy consumption. Saccharides are known stabilizers in a solid protein produced via lyophilization and spray drying (SD). Thus, it is essential to understand the interactions between saccharides and proteins and the stabilization mechanism.MethodsA miniaturized single droplet drying (MD) method was developed to understand how different saccharides stabilize proteins during drying. We applied our MD to different aqueous saccharide-protein systems and transferred our findings to SD.ResultsThe poly- and oligosaccharides tend to destabilize the protein during drying. The oligosaccharide, Hydroxypropyl β-cyclodextrin (HPβCD) shows high aggregation at a high saccharide-to-protein molar ratio (S/P ratio) during MD, and the finding is supported by nanoDSF results. The polysaccharide, Dextran (DEX) leads to larger particles, whereas HPBCD leads to smaller particles. Furthermore, DEX is not able to stabilize the protein at higher S/P ratios either. In contrast, the disaccharide Trehalose Dihydrate (TD) does not increase or induce protein aggregation during the drying of the formulation. It can preserve the protein’s secondary structure during drying, already at low concentrations.ConclusionDuring the drying of S/P formulations containing the saccharides TD and DEX, the MD approach could anticipate the in-process (in) stability of protein X at laboratory-scale SD. In contrast, for the systems with HPβCD, the results obtained by SD were contradictory to MD. This underlines that depending on the drying operation, careful consideration needs to be applied to the selection of saccharides and their ratios.

Release and migration of Hg during desulfurization wastewater evaporation process: Whole process evaluation by experimental and theoretical study.

Hot flue gas evaporation technology is an effective strategy for zero liquid discharge of desulfurization wastewater. However, there is a potential risk that heavy metals such as Hg may be released from the wastewater during evaporation, disrupting the original balance of the power plant or even exceeding the Hg emission standard. Wastewater evaporation and Hg release behavior were obtained using a single droplet drying system. At an evaporation temperature of 300°C, approximately 18.5% of Hg was released in the constant wet-bulb temperature period, and the remaining was released in the following evaporation periods. Furthermore, a fixed-bed experiment, in combination with density functional theory calculations, was used to investigate the possible migration mechanisms of released Hg. The results revealed that high HCl concentration, introduced fly ash, and precipitated evaporation products play a crucial role in the fate of Hg, and 85.3% of Hg finally turned into less harmful particulate-bound Hg. This study provides a new and effective strategy for evaluating the migration process of pollutants in wastewater treatment. Moreover, it will serve as an essential reference for advanced wastewater treatment and heavy metals control technologies in the future.

Neural network modeling of the dynamic inactivation of probiotics during single droplet drying for improved cell viability

Dairy products containing probiotics are often dried to improve shelf life and facilitate transportation. A reliable dynamic inactivation model has long been pursued to optimize the production by maximizing probiotics' survival during drying. How to take care of the dynamic drying process experienced by the cells for precise prediction of their survival remains a challenging task. In this work, a multi-task convolutional self-attention network (CSAN) has been developed for dynamic modeling of probiotics inactivation during single droplet drying (SDD). The convolution self-attention approach together with a unique double-branch architecture allows the neural network (NN) to learn effectively from historical data and predict inactivation dynamics throughout the whole drying process. In terms of prediction accuracy, our model ( R 2 > 0.96) outperforms many other existing models ( R 2 < 0.6 in most circumstances). By resorting to this model, two optimal SDD conditions have been identified with the resultant terminal solid contents higher than 90 wt% and cell survival ratios higher than 0.65. • A dynamic inactivation model for probiotics survival prediction during drying • Effective leaning of historical data by a convolutional self-attention neural network • A double-branch architecture allows precise prediction of inactivation dynamics • Optimized conditions identified for improved cell viability and sufficient solvent removal

Spray Drying of PEG6000 Suspension: Reaction Engineering Approach (REA) Modeling of Single Droplet Drying Kinetics

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 investigated at different gas drying temperatures. The size of the droplet, moisture content, and the shape evolution of the droplet/particle during the drying process were studied. The experimental drying curves were compared with the Reaction Engineering Approach (REA). The REA models were shown to provide a very good agreement for drying behavior, with a relative error of about ±3% between the initial and predicted droplet mass. This model can be implemented into the large-scale modeling of spray drying using Computational Fluid Dynamics (CFD).

Study on drying behavior of additive droplets by reaction engineering approach

A prediction method of drying behaviors for five types of additives droplets based on Reaction Engineering Approach and single droplet drying was introduced in this study. The experimental measurement results showed that the density, viscosity and surface tension of these droplets were in the range of 1.16–1.56 g / m L , 12.86 × 10 − 2 –142.25 × 10 − 2 P a ⋅ s and 35.23–52.05 m N / m , respectively. The volume change rate and drying rate were calculated using machine vision and image processing. The results showed that the volume change rate and the drying rate were in the range of 0.05–16.83 and 1.83 × 10 −3 –7.21 × 10 −3 g / m i n for these five types of additives droplets. The theoretical results showed that the Reynolds number and apparent activation energy ( Δ E v ) were in the range of 0.42–3.54 and 1.88–4.27 k J / m o l for these droplets. The linear relation between heating temperature and Δ E v was established as Δ E v = 111.58 T h − 4709.84 (R 2 = 0.998), indicating that the heating temperature plays a leading role. The prediction method can be applied as a simplified and accurate model to predict drying behaviors based on material specific properties. • The phase transition phenomenon was quantitatively analyzed by using Reaction Engineering Approach. • Drying behavior of additives was studied by single droplet drying using machine vision and image processing. • Heating temperature plays a leading role in the single droplet drying experiment. • Significant linear relationship was found between temperature and apparent activation energy.

The dual effect of shellac on survival of spray-dried Lactobacillus rhamnosus GG microcapsules

Heat shock and hygroscopicity are two main factors that resulted in low viability of probiotics in spray-dried microcapsules during storage. Hydrophobic polyester shellac was combined with whey protein isolate (WPI) to solve this problem. The results suggested that although the survival rate after drying decreased from 20.63% to 0.01% with increased shellac to WPI ratio, the 1:1 shellac-WPI provided the best protection among all samples during storage. The consistence between moisture-adsorption-isotherm and bacterial inactivation constants confirmed the moisture barrier effect of shellac under moderate humidity. Single-droplet drying and differential scanning calorimeter revealed that shellac addition reduced the drying rate and glass transition temperature of microcapsules, which in turn decreased the membrane integrity and growth capability of the probiotics after drying. This study revealed the dual effect of hydrophobic material on instant and long-term survival of spray-dried probiotic microcapsules, which provided new sight to the design of composite wall materials.

Phase Diagram of Dairy Protein Mixes Obtained by Single Droplet Drying Experiments.

Dairy powders are mainly produced by droplet spray drying, an articulated process that enables the manufacture of high added-value goods with a long shelf life and well-preserved functional properties. Despite the recent advances, a full understanding of the mechanisms occurring at the droplet scale in drying towers and, consequently, of the impact of process parameters and processed fluid characteristics on the powder properties is far from being achieved. In the wake of previous studies based on a laboratory scale approach, in this work, we provided a global picture of the drying in droplets of dairy protein mixes, i.e., whey proteins and casein micelles, which represent crucial dairy powder ingredients. Using profile visualization and optical microscopy, we explored the shape evolution in droplets with a range of protein contents and compositions typical of commercial powder production. The observation favored the evaluation of the specific role of each protein on the evaporation dynamics, and led to the construction of a phase diagram predictive of the dry droplet shape starting from the characteristics of the initial protein dispersions. Our outcomes represent a further step shedding light on the paradigm linking the physics of drying at the microscale and the nutritional properties of complex dairy powders.

Measurement of Secondary Structure Changes in Poly-L-lysine and Lysozyme during Acoustically Levitated Single Droplet Drying Experiments by In Situ Raman Spectroscopy.

Drying processes such as spray drying, as commonly used in the pharmaceutical industry to convert protein-based drugs into their particulate form, can lead to an irreversible loss of protein activity caused by protein secondary structure changes. Due to the nature of these processes (high droplet number, short drying time), an in situ investigation of the structural changes occurring during a real drying process is hardly possible. Therefore, an approach for the in situ investigation of the expected secondary structural changes during single droplet protein drying in an acoustic levitator by time-resolved Raman spectroscopy was developed and is demonstrated in this paper. For that purpose, a self-developed NIR–Raman sensor generates and detects the Raman signal from the levitated solution droplet. A mathematical spectral reconstruction by multiple Voigt functions is used to quantify the relative secondary structure changes occurring during the drying process. With the developed setup, it was possible to detect and quantify the relative secondary structure changes occurring during single droplet drying experiments for the two chosen model substances: poly-L-lysine, a homopolypeptide widely used as a protein mimic, and lysozyme. Throughout drying, an increase in the β-sheet structure and a decrease in the other two structural elements, α-helix, and random coil, could be identified. In addition, it was observed that the degree of structural changes increased with increasing temperature.

Evaporation Characteristics of a Single Desulfurization Wastewater Droplet in High-temperature Gas

Understanding the high-temperature evaporation process of atomized wastewater droplets is a prerequisite for designing an efficient spray evaporation wastewater treatment facility. However, some dynamic characteristics during the evaporation of high-temperature wastewater droplets have not yet been fully understood. Here, the convective evaporation process of desulfurization wastewater droplet in high-temperature gas, which is commonly encountered in the spray drying technology to treat desulfurization wastewater in power plants, was investigated adopting the single droplet drying (SDD) device. In-situ droplet information including the morphology, radius, temperature, and mass was recorded. The influence of bulk temperature and the initial concentration of dissolved solids on evaporation kinetics was studied. To explore the development of resistances during the evaporation process as well as develop a reliable and flexible droplet evaporation model useful for CFD simulations, experimental results were correlated with a modified Reaction Engineering Approach (REA) framework, which revised the previous linear shrinkage model into a hybrid model to illustrate the droplet/particle deformation considering the possible inflation phenomenon at elevated temperatures. The results show that crust formation and internal bubble nucleation induced by wastewater components contributed to unique evaporation features including decreasing evaporation rate and inflation of semi-dried particles. Increasing bulk temperature enhanced the evaporation rate and the degree of inflation. Finally, the rapid and accurate prediction of the modified REA model elucidated its effectiveness to illustrate the whole evaporation and drying process.

Numerical study of single droplet drying in an acoustic levitator before the critical point of time

When using an acoustic levitator to perform single droplet drying studies, one has to be aware of its significant impact on the heat and mass transfer. In this paper, we propose a model for the first drying stage of an acoustically levitated water droplet containing suspended solids. Both the realistic geometry of the acoustic levitator and the deformation of the levitated droplet are taken into account by our numerical simulations. Empirical data from the literature are used to validate our single droplet levitation and drying model in case of pure water and binary water-maltodextrin droplets. We found that the acoustic streaming inside the levitated droplet has a retarding effect on its critical point of time when the solute forms a solid crust at the free phase boundary. Moreover, the temperature dependence of the sound wave length turned out to have a crucial impact on the performance of the acoustic levitator.

Spray drying of desulfurization wastewater: Drying characteristics, product analysis and potential risk assessment

The Wastewater Evaporation System (WES) was a reliable and economical way to treat desulfurization wastewater. However, the evaporation mechanism was not clear, and the evaporation product properties were lack of awareness. In this study, the evaporation and product properties were revealed by the single droplet drying (SDD) method and spray drying system. It was found that the evaporation process of wastewater involved the constant rate and falling rate period. The component of the evaporation product from the SDD and spray drying method were similar, mainly consisted of MgCl2, CaCl2, NaCl and CaSO4. Besides, the potential risks of WES were assessed. After wastewater evaporation, the Electrostatic Precipitator (ESP) removal efficiency was enhanced due to the aggregation of fine particles and the enhanced specific resistance. For the influence of WES on comprehensive fly ash utilization, the chloride mass fraction in the final product was lower than 0.06% and could meet strict regulations.

Chloride release characteristics of desulfurization wastewater droplet during evaporation process using the single droplet drying method

The desulfurization wastewater evaporation technology is an effective way to realize zero-liquid discharge (ZLD). However, the release of chloride during the evaporation process is still a great challenge, which may cause severe chloride enrichment in the wastewater. In this work, the single droplet drying (SDD) method was utilized to investigate the chloride release characteristics during the evaporation process. The three periods of chloride release were first proposed based on the chloride release ratio and wastewater droplet's features (mass, temperature, and diameter). With the removal of the free moisture, the first period (Free water evaporation) was initiated, and almost no released chloride was detected; In the unstable shell period, 19.8% chloride would release there, mainly due to the increased droplet's temperature; Approximately 10% chloride would release during the shell pyrolysis period, which was directly related to the hydrolysis of the chloride-based salt hydrates in the evaporation particle. Besides, the chloride release characteristics at different cations species and ambient temperature were investigated. The desulfurization wastewater with a higher Mg2+ concentration would be more likely to release chloride during the evaporation process. With the temperature increased from 150 to 300℃, the chloride release ratio would improve from 14.56 to 29.8% due to the disruption of the shell and the hydrolysis degree at a higher temperature. In practical engineering practices, a balance point between chloride release and evaporation efficiency should be found, and the Mg2+ in the wastewater should be pre-treated before entering into the evaporation system.