Unraveling the droplet drying characteristics of crystallization‐prone mannitol – experiments and modeling

Spray‐dried mannitol is a potential lactose replacement in pharmaceutical formulations, yet the drying behavior of individual mannitol droplets within the spray chamber has not been fully understood. This work explored the drying characteristics of mannitol by employing the reaction engineering approach (REA) in data analysis. A glass filament single droplet drying technique was used to monitor the changes in droplet temperature, mass, and diameter. The drying kinetics data obtained clearly demonstrated the droplet “wet‐bulb” period, the crust formation, and the crystallization phenomena. The master activation‐energy curves developed from REA modeling responded sensitively to varying drying temperatures, which could have led to different crystallization events. The deviation of these plots from the expected norms that do not encounter a phase change was used effectively to discern the physics involved. A REA kinetic model was proposed to assist in process optimization of large‐scale spray‐drying operations. © 2017 American Institute of Chemical Engineers AIChE J, 63: 1839–1852, 2017     

甘露醇喷雾干燥过程中液滴粒度分布变化的群体粒数衡算模拟和实验研究

利用群体粒数衡算（population balance，PB）计算机模拟和实验研究了甘露醇水溶液的喷雾干燥过程中液滴的粒度分布的变化规律。液滴干燥过程中的颗粒粒度的萎缩速率，在群体粒数衡算模型中描述为液滴的逆（或负）生长项，通过单个液滴反应动力学方法（reaction engineering approach，REA）获得。基于单个液滴干燥的反应工程方法模型REA和群体粒数衡算模型PB集成建立了PBREA模型。PBREA 模型的求解是通过高分辨率数值方法。本文模拟研究了不同工况下，不同粒径液滴的干燥时间、液滴平均含湿量以及液滴粒度分布随时间的变化。结果显示，液滴粒径越大，干燥时间越长，模型预测的颗粒平均粒径为实验值的1.0~1.5倍，粒度分布跨度是实验值的0.61~0.89倍。模拟误差主要来源于液滴及颗粒粒径分布统计精度、单个静止液滴与群体运动液滴干燥的差异、热导率及扩散系数是经验值3个方面。在使用Buchi 290 小型喷雾干燥仪进行的实验中，使用了图像采集和分析方法得到了液滴及颗粒的数密度分布，并和模拟结果做了对比。结果表明该模型可以有效地预测喷雾干燥过程中干燥颗粒的平均粒度及分布跨度。     

Influence of Particle Size on the Drying Kinetics of Single Droplets Containing Mixtures of Nanoparticles and Microparticles: Modeling and Pilot-Scale Validation

The particle size of the primary particles is an important parameter influencing the drying behavior of droplets. In this work, the influence of particle size on the drying kinetics and grain properties was analyzed for droplets containing silica nanoparticles, microparticles, and mixtures of the two. The presence of microparticles was found to increase the drying rate and shrinkage of the droplet. The drying curves were modeled using a reaction engineering approach (REA) model. Finally, different suspensions were dried in a pilot-scale spray dryer in order to prove the influence of the particle size obtained in the levitator tests.     

The Basics of a Reaction Engineering Approach to Modeling Air-Drying of Small Droplets or Thin-Layer Materials

In this article, the development of a reaction engineering approach (REA) to modeling air drying of thin-layer materials or solids droplet/particles, a 10-year-old approach, is critically reviewed. This approach for drying data reduction has been seen to be successful in several applications (droplet drying and layer drying). The historical aspects and its possible relations to the characteristic drying rate curve (CDRC) model, one of the most well-known drying kinetics models, have been articulated. Possible improvements and future prospects are discussed.     

热烟气环境下脱硫废水液滴蒸发特性研究进展

利用热烟气作为热源喷雾蒸发脱硫废水具有工艺简单、处理效率高等优点,但脱硫废水液滴蒸发机制不清是各种热烟气蒸发工艺存在的共性问题。本文综述了脱硫废水热烟气蒸发工艺中液滴（群）蒸发特性研究,首先介绍了脱硫废水液滴在高温烟气中的蒸发过程,指出其蒸发过程与纯水滴存在明显差异,原因在于盐溶液蒸汽压降效应及析盐成壳。进而分析了脱硫废水烟道蒸发工艺中液滴群蒸发实验研究现状,总结过程参数如烟温、烟速及废水水质对蒸发的影响。接着介绍了相关数值模拟研究及干燥动力学模型,用于获取工艺参数和不同水质脱硫废水蒸发特性的内在关联。同时为获得干燥动力学模型所需脱硫废水蒸发信息,简述了单液滴蒸发测试技术及其在含固含盐液滴蒸发中的相关研究。最后提出可将反应工程法模型耦合CFD精确描述脱硫废水液滴蒸发进程,将宏观尺度下液滴群蒸发信息和单液滴蒸发特性结合,揭示脱硫废水液滴蒸发的内在机制。     

A differential shrinkage approach for evaluating particle formation behavior during drying of sucrose,lactose, mannitol,skim milk,and other solid-containing droplets

Particle formation process during drying of solid-containing droplets exerts profound influence on the property and quality of dried particles. A differential shrinkage approach is proposed to describe the particle formation behavior of different materials, by evaluating how the droplet shrinkage kinetics of the given material(s) deviates from the ideal shrinkage. Sucrose was selected as the reference material to establish ideal shrinkage kinetics, as its shrinkage at 10, 30, and 50?wt.% initial concentrations well followed the ideal shrinkage at all temperatures tested (70, 90, and 110?°C). Comparing the differential shrinkage kinetics of sucrose, lactose, and mannitol showed that mannitol has a strong crust-forming tendency, which could not be explained by the difference in the solubility of the three materials. By establishing a differential shrinkage curve for each material, this approach offers a straightforward and powerful method to evaluate the particle formation property of the material at various initial concentrations.     

Mapping the Shrinkage Behavior of Skim Milk Droplets During Convective Drying

This article investigated the influence of drying rate on the shrinkage behavior of skim milk (SMP) droplets during convective drying, behavior which was then mapped using the Peclet number, Pe. The objective was to demonstrate the potential use of the Peclet number to predict the particle formation pathway during drying by defining a material-specific perfect shrinkage threshold using only the knowledge of initial process conditions and droplet characteristics. The shrinkage mapping was undertaken by drying relatively large and statically suspended droplets through adjustment of the drying air humidity. Its influence on droplet shrinkage behavior was observed through droplet diameter profile. Analysis of lactose, a non-skin-forming material, revealed perfect shrinkage behavior. Experiments with SMP, a skin-forming material, revealed that deviation from perfect shrinkage behavior occurred beyond the Peclet number threshold of 0.23–1.27 when droplets and the solid diffusion front were modelled as an equivalent sphere. The developed threshold was further used to predict the shrinkage behavior of atomized SMP droplets undergoing rapid drying in a spray tower.     

Sensitivity Analysis of the Reaction Engineering Approach to Modeling Spray Drying of Whey Proteins Concentrate

Whey proteins concentrate (WPC) powder is an important protein source for humans and is commonly produced from whey using a spray-drying technique. Predicting several drying parameters as well as the parameters that govern the quality of the product is essential before manufacturing WPC in industries. Drying kinetics is an essential tool for predicting the drying rate and various parameters that are rate dependent. However, there have been only a few studies published previously on both modeling WPC drying and dryer-wide simulations using different computational tools. In this article we review the application of a reaction engineering approach (REA) to model the droplet drying process. Results based on dryer-wide simulations using the REA are presented. More importantly, a sensitivity analysis of the REA using drying of WPC and skim milk droplets is reported. This analysis will be helpful to select an appropriate drying kinetics model and forms a benchmark for the future WPC drying modeling work.     

Comparative study of droplet drying models for CFD modelling

CFD simulation is used to study wall deposition and agglomeration phenomena commonly encountered in industrial spray dryers. This paper initially provides a comparison of two drying kinetics models: Characteristic Drying Curve (CDC) and Reaction Engineering Approach (REA). Comparisons are made with experimental data with application to carbohydrate droplet drying obtained from past workers. These models were then extrapolated to actual drying conditions to assess their performance. The REA model predicts the progressive reduction in drying rate better than the CDC model for the carbohydrate droplets. A modified CDC model incorporating a convex falling rate produced better agreement than the conventional linear falling rate model. Further analysis showed that the REA model can be extended to simulate the particle surface moisture which may affect the agglomeration process. The proposed concept was compared with reported simulation results from a diffusion model which showed reasonable fit with data.     

Modeling of Drying Curves of Silica Nanofluid Droplets Dried in an Acoustic Levitator Using the Reaction Engineering Approach (REA) Model

The use of nanoparticles has become of great interest in different industrial applications. The spray drying of nanofluids forms nanostructured grains, preserving the nanoparticle properties. In this work, individual droplets of silica nanofluids were dried in an acoustic levitator. Tests were carried out under different experimental conditions to study the influence of the variables on the drying process. The drying curves were experimentally obtained and an REA model was used to obtain the theoretical curves and the correlations for the activation energy. The critical moisture content theoretically obtained was used to predict the grain diameter.     

Enhanced methods for experimental investigation of single droplet drying kinetics and application to lactose/water

In this work, a suspension device is presented that can be used for the investigation of single droplet drying kinetics at temperatures from ambient to 200°C. Evaluation is hybrid, using both camera data and a water mass balance. In this way, drying kinetics can be captured for shrinking droplets and also after a constant diameter of the wet particle has been reached and during inflation/deflation periods. Results can be further enhanced by X-ray μ-CT investigation of the internal morphology of dried particles. It is proposed to read critical conditions for crust formation from a plot of evaporation flux over water mass fraction. Repeatability and accuracy of the experimental and evaluation methods were checked with pure water droplets. Moreover, the entire evaluation was conducted for aqueous solutions of lactose with various initial concentrations.     

Particle formation from droplet containing nanoparticles: Internal bubble growth

ABSTRACT

Modeling spray drying is based on the behavior of individual droplets involved in the process. The drying history of single droplets is generalized to the entire spray by incorporating drying kinetics equations into Euler–Lagrange models of spray drying process. The morphological evolution of single droplet is determined by parameters of the drying agent and properties of solid material in the droplet. In this study, the possible causes for bubble inception are discussed and different morphological evolutions and deformations are examined. A novel mathematical model of single droplet containing insoluble nanoscale particles and internal bubble has been developed and parametrically studied.     

A new approach to modelling of single droplet drying

A model was developed to predict the change in droplet mass and temperature when it is exposed to hot air, as in spray drying of droplets containing solids. The droplet was assumed first to undergo sensible rapid heating with no mass change. Then the droplet experiences some shrinkage, with no temperature change but rapid mass losses, followed by a period of crust formation with a significant change in droplet mass and temperature and finally a short period of sensible heating of the dried particle. The model, unlike previous models, accounts for shrinkage and for the temperature distribution in the droplet. It provided a good prediction for the change in droplet temperature and mass for some of the experimental measurements available in the literatures.     

CFD Evaluation of Droplet Drying Models in a Spray Dryer Fitted with a Rotary Atomizer

Computational fluid dynamics (CFD) modeling of spray dryers requires a simple but sufficiently realistic drying model. This work evaluates two such models that are currently in discussion; reaction engineering approach (REA) and characteristic drying curve (CDC). Two versions of the CDC, linear and convex, drop in drying rate were included. Simulation results were compared to the overall outlet conditions obtained from our pilot-scale experiments. The REA and CDC with a linear drop in drying rate predicted the outlet conditions reasonably well. This is contrary to the kinetics determined previously. Analysis shows that the models exhibit different responses to changes in the initial feed moisture content. Utilizing different models did not result in significantly different particle trajectories. This is due to the low relaxation time of the particles. Despite the slight differences in the drying curves, both models predicted similar particle rigidity depositing the wall. For the first time in a CFD simulation, the REA model was extended to calculate the particle surface moisture, which showed promising results for wet particles. Room for improvement was identified when applying this concept for relatively dry particles.     

A composite reaction engineering approach to drying of aqueous droplets containing sucrose,maltodextrin (DE6) and their mixtures

Recently, several studies have been published on the spray drying of sucrose and other low‐molecular‐weight sugars which are typically sticky materials. Sticky materials can not be processed under normal drying conditions and may require addition of high‐molecular‐weight carbohydrates such as maltodextrin. Predicting appropriate drying conditions are however difficult due to the unavailability of drying kinetics. In this article, we have formulated the drying kinetics model using the reaction engineering approach (REA) for the drying of aqueous sucrose and aqueous maltodextrin (DE6) droplets. The relative activation energy was empirically obtained based on experimental measurements. To model the drying of droplets containing both solutes (sucrose and maltodextrin), a new “composite” REA has been established and presented here for the first time. Results demonstrated that the composite REA forms a reliable framework to model the drying of aqueous solutions of pure carbohydrates and their mixtures. © 2008 American Institute of Chemical Engineers AIChE J, 2009     

蒸发结晶过程中悬浮溶液液滴形态的变化

利用单轴声悬浮装置观察了毫米大小的无机盐水溶液悬浮液滴蒸发和结晶的动态过程，为认识伴随均匀成核和结晶过程的溶液液滴形态变化规律提供了一个理想的环境．通过对ＮＨ４Ｃｌ溶液液滴广泛的实验观察发现，开始结晶后通常先在液滴表面的局部形成外壳，局部外壳发展到完全成壳经过３种可能的生长历程，即局部外壳向液滴内的生长，沿液滴表面的扩展，以及从局部外壳上个别点向外的枝状物或晶须生长．多数溶质液滴都经历与ＮＨ４Ｃｌ液滴相同的过程演化到完全成壳，但完全成壳后的形态变化却各不相同．初步考察了溶质性质与形态变化的关系，以及二元混合溶液液滴的形态变化规律，并定性地讨论分析了声场存在对于观察结果代表性可能的影响．     

Modeling of Convective Drying of Sawdust Using a Reaction Engineering Approach

Drying as a simultaneous heat and mass transfer process can be modeled via the reaction engineering approach (REA) where the apparent activation energy of the material is established and related to its moisture content during drying. This relationship is unique as the normalized activation energies can be collapsed into a single equation irrespective of the drying conditions and dryer types. Here, REA was applied to model the drying kinetics of sawdust using convective hot air in a laboratory setup. The normalized (relative) activation energy curve generated from one drying experiment was employed to predict the drying kinetics and temperature profiles. The REA can describe well the convective drying kinetics of sawdust, and major physics of the drying process was captured well with this technique.     

Single Droplet Drying Technique to Study Drying Kinetics Measurement and Particle Functionality: A Review

Advances in the study of the rate processes in spray drying have helped improve product quality. Single droplet drying (SDD) is an established method for monitoring the drying kinetics and morphological changes of an isolated droplet under a controlled drying environment, mimicking the droplet convective drying process in spray drying. To enhance particle quality requires understanding of both the particle formation process and knowledge of how different particle properties are affected by the drying conditions used. The latest development in the SDD technique enables evaluation of these aspects by incorporating a dissolution test in the drying experiment. The experiment is realized by attaching a solvent droplet to a dried/semi-dried single particle in situ and then video-recording the resultant morphological changes. Some of the particle (e.g., crystallinity) properties obtained under different drying conditions can be modelled using the measured droplet drying kinetics. This paper reviews the applications of SDD experiments in measuring the drying kinetics and monitoring the droplet morphological changes during drying. Some examples of extending the glass filament SDD technique to examine particle functionalities are discussed. SDD experiments are shown to be a powerful tool for particle engineering due to its ability to study both the external convective transport process of a single droplet and to understand the different particle functionalities of the resultant single dried particle.     

A Model for Drying of an Aqueous Lactose Droplet Using the Reaction Engineering Approach

Spray drying is the primary method for manufacturing of food powders from liquids. Optimal design and optimization of spray drying operations at the fundamental level require both modeling of the drying characteristics of a single droplet and dryer wide simulations using computational fluid dynamics (CFD). An accurate yet simple model for drying of a single droplet, which does not require solution of partial differential equation, is ideal input for CFD simulations. The reaction engineering approach (REA) is shown to be appropriate in this regard. It has been successfully used for prediction of skim and whole milk droplet drying behavior under various drying conditions. In this study, an aqueous lactose solution was dried in droplet form and the appropriate REA model parameters obtained. The change of diameter of the droplet during drying was measured experimentally and compared with the model results.