Kinetics of melt crystallization in falling films

Falling film melt crystallization (I): Model development, experimental validation of crystal layer growth and impurity distribution process

Abstract: This paper was concerned with the model development and experimental validation of the detailed crystal layer growth and multi-ions impurity distribution process in the falling film melt crystallization (FFMC) model. The phosphoric acid (PA) was separated and purified by FFMC to obtain a hyperpure phosphoric acid (HPA), which was a vital electronic chemical in IT industry. To establish a valid model, which offered an easy and convenient path of the simulation, dynamic heat and mass balance, approaches were adopted to describe the variation of crystal layer growth rate along the crystallizer. An impurity balance approach was adopted to describe the change of distribution coefficient for multi-ion impurity. A criterion was proposed to determine the formation of branched-porous (B–P) structure. The model was validated by experimental results with various equipments and operational conditions and a good agreement was obtained. The effective distribution coefficient K eff for multi-ion impurities were less than 0.2 (Na + ), 0.25 (Fe 3+ ) and 0.35 (Ca 2+ ) with proper operation conditions. The resulting model was directly exploited to understand crystal layer growth and impurity distribution behaviors in FFMC from laboratory to industrial scale. More significantly, the model proposed a method for the separation effect evaluation and the key operational conditions (feed rate and cooling rate) determination which could readily develop optimal crystal layer growth route during industrial crystallization. In addition, the model was a vital base to describe the subsequent purification step of FFMC: sweating process.

Research Progress and Model Development of Crystal Layer Growth and Impurity Distribution in Layer Melt Crystallization: A Review

Abstract: Layer melt crystallization has been widely utilized in numerous chemical industries because of its high selectivity for pure products, low energy consumption, and the convenience to industrialization. This review will lay out the research progress and process model development of the key processes (crystal layer growth and impurity distribution) involved in layer melt crystallization. First, the nucleation mechanism, the preparation approaches of the initial crystal layer, and classic experimental configurations are illustrated. Second, modeling approaches are outlined to release the progress of separation effect evaluation, parameter optimization, and sweating process simulation in layer melt crystallization. Novel theories (fractal, porous media, and so on) and technologies (gradient freezing, sonocrystallization, and so forth) with suitable interpretation are potential solutions for the shortcomings of the current process research. Consequently, application areas related to layer melt crystallization a...

Using complex layer melt crystallization models for the optimization of hybrid distillation/melt crystallization processes

Abstract: Layer melt crystallization is a highly selective method for the separation of narrow boiling mixtures which are difficult to separate with conventional separation techniques like distillation due to low driving forces. Contrawise, layer melt crystallization has the drawback of limited capacity due to the direct connection between crystal product and required cooled surface. Here, the combination of the high throughput distillation and highly selective layer melt crystallization into an integrated hybrid process can lead to enormous benefits. Since the separation efficiency of the crystallization is not predictable, it has to be described with empirical correlation. Here, studies from literature use strongly simplified correlations by, e.g. assuming complete separation. This bears the serious risk of overestimating the efficiency of the hybrid process. Further, the effective post purification step sweating was not implemented into hybrid processes in studies from literature. This study fills this gap in literature. A distilliation/melt crystallization hybrid process is optimized by realistically describing crystallization separation efficiency and by implementing sweating. The required crystallization models are presented and experimentally validated. The optimization of the hybrid process is done with different modelling depths and the results underline impressively the importance of the adequate description of the crystallization separation efficiency.

Heat transfer and its effect on growth behaviors of crystal layers during static layer melt crystallization

Abstract: The growth behaviors of crystal layers during static layer melt crystallization was studied from the perspectives of morphology structure, growth rates and temperature evolution. The temperature distributions of the melt and crystal layer were deduced. With these results, the heat transfer during crystallization was analyzed by considering the relative influences of natural convection, heat conduction in the melt and latent heat of crystallization. A model correlating growth rate with physical and experimental parameters was derived based on energy conservation. Effective thermal conductivity of crystal layers was evaluated. It was confirmed that the structure and density of the crystal layer can significantly affect the thermal conductivity. According to the temperature curves of melt, the static layer melt crystallization process in a tubular crystallizer can be divided into four stages as nucleation stage, fast growth stage, slow growth stage and steady state stage.

The growth of a deposited layer on a cold surface.

Abstract: An approximate analysis of the growth of, and the temperature within, a deposit which may occur in a cold surface in a gas stream is developed with the convective heat transfer to the surface of the deposit taken into account. Simple “short time” and “long time” solutions are obtained. A numerical example which may be of interest in connection with cryogenic surfaces in a hypersonic wind tunnel is presented.