Model-based analysis of stirred cooling crystallizer of high aspect ratio crystals with linear and nonlinear breakage

Influence of ammonium nitrate on the crystallisation of ammonium sulfate

This study aims to explore the influence mechanism of ammonium nitrate produced by ozone denitrification on the crystallisation of ammonium sulfate, a by-product of ammonia desulfurisation. The laser method was used to study the influence of ammonium nitrate on the solubility and metastable zone width of ammonium sulfate. An experiment on the influence of ammonium nitrate on the particle size of ammonium sulfate was designed, and the influence mechanism was explored through scanning electron microscopy and X-ray diffraction. The findings showed that the addition of ammonium nitrate increased the size and aspect ratio of ammonium sulfate crystals. The addition of ammonium nitrate inhibited the dissolution of ammonium sulfate and widened its metastable zone. The addition of ammonium nitrate covered the active sites of crystal nucleus growth, which inhibited the formation of crystal nuclei to a certain extent, and crystal growth dominated the crystallisation process. Moreover, the addition of ammonium nitrate induced the preferred orientation of the specific crystal plane of ammonium sulfate, and the addition of a small concentration of ammonium nitrate decreased the crystallinity of ammonium sulfate. The research results can provide a reference for crystallisation optimisation and quality improvement of ammonium sulfate in the ammonia desulfurisation process.

Comparison of the Dynamic and Thermal Behavior of Different Ideal Flow Crystallizers

In this simulation study, we compare the dynamics and thermal behavior of different ideal flow crystallizers. The first step in creating mathematical models for the crystallizers was the implementation of the population balance equation. The population balance equation was completed with mass balance equations for the solute and the solvent as well as in the case of non-isothermal crystallizers with an energy balance equation. The solution to the population balance equation, which is a partial differential equation, can only be performed numerically. Using the method of moments, which calculates the moments of the population density function, gives a mathematically simpler model for simulating and analyzing the crystallizers. All crystallizers studied are considered mixed suspension and mixed product crystallizers. In this simulation study, the investigated crystallizers are the batch mixed suspension and mixed product isothermal crystallizer, the batch mixed suspension and mixed product non-isothermal crystallizer, and the continuous mixed suspension and mixed product removal (CMSMPR) non-isothermal crystallizer equipped with a cooling jacket. We consider citric acid as the solid material to be crystallized, and a water–glycol system is used as a cooling medium. Considering the nucleation kinetics, we applied both primary and secondary nucleation. In the case of a crystal growth kinetic, we assumed a size-independent growth rate. The highest expected value and the variance of the crystal product occur in the isotherm batch case, which can be explained by the high crystallization rate caused by the high supersaturation. Contrary to this, in the non-isothermal batch case, the final mean particle size and variance are the lowest. In continuous mode, the variance and mean values are between the values obtained in the two other cases. In this case, the supersaturation is maintained at a constant level in the steady state, and the average residence time of the crystal particles also has an important influence on the crystal size distribution. In the case of non-isothermal crystallization, the simulation studies show that the application of the energy balance provides different dynamics for the crystallizers. The implementation of an energy balances into the mathematical model enables the calculation of the thermal behavior of the crystallizers, enabling the model to be used more widely.

On the Breakage of High Aspect Ratio Crystals in Filter Beds under Continuous Percolation

Purpose

This work details experimental observations on the effect of liquid flow percolating through packed beds of crystals to elucidate how the filtration pressure severely alters the size distribution and crystal shape. Pressure filtration is widely used in the pharmaceutical industry, and frequently results in undesired size distribution changes that hinder further processing.

Methods

The percolation methodology presented fixes fluid flow through a bed of crystals, resulting in a pressure over the bed. X-ray computed tomography (XCT) provided detailed observations of the bed structure. Detailed 2D particle size data was obtained using automated microscopy and was analysed using an in-house developed tool.

Results

Crystal breakage is observed when the applied pressure exceeds a critical pressure: 0.5–1 bar for ibuprofen, 1–2 bar for β-L glutamic acid (LGA) and 2–2.5 bar for para amino benzoic acid (PABA). X-ray computed tomography showed significant changes in bed density under the applied pressure. Size analysis and microscope observations showed two modes of breakage: (i) snapping of long crystals and (ii) shattering of crystals.

Conclusion

LGA and PABA have a similar breakage strength (50 MPa), ibuprofen is significantly weaker (9 MPa). Available breakage strength data may be correlated to the volumetric Gibbs free energy. Data from 12 and 35 mm bed diameters compares well to literature data in a 80 mm filter; the smaller, easy to operate percolation unit is a versatile tool to assess crystal breakage in filtration operations.

Electronic supplementary material

The online version of this article (10.1007/s11095-020-02958-x) contains supplementary material, which is available to authorized users.