An investigation of the acoustic emission generated during crystallization process of salicylic acid

Highlights

• Crystal size distribution could be estimated at the steady-state.

• Acoustic Emission (AE) is obviously influenced by the injection rates.

• AE signals are related to the basic crystallization phenomena.

Abstract

Due to its non-invasive and non-destructive nature, Acoustic Emission (AE) has been successfully used in a wide variety of solids elaboration process. But few papers reported that solution crystallization process give rise to acoustic emission signals that could be related to the basic crystallization phenomena. This study is intended to evaluate the potential for using acoustic emission to monitor the physicochemical crystallization process. The potential relationship between the generation of acoustic signals and the physicochemical crystallization process of salicylic acid (SA) is analyzed by intensity of AE signal and pH value of the solution. Due to its great impact on end-product quality, SA crystal particles are investigated under the influence of injection rates. The results demonstrate that the mode of the particle size distribution is inversely related to the injection rates. In addition, the intensity of AE is obviously influenced by the injection rates. Moreover, the changing tendencies of AE are related to the basic crystallization phenomena, and the intensity of AE can be an important technical parameter to characterize the physicochemical crystallization process. The valuable AE information could increase process understanding and provide a basis for innovative online monitoring and control application.

Introduction

It is reported that over 90% of all active pharmaceutical ingredients (API) would go through a crystallization step at some stage of their production cycles [1]. As other high value-added chemical unit operations in pharmaceutical industry, crystallization, as a main unit operation strongly affecting the end-product characteristics, requires to be monitored during time. Crystallization is a complex process and many parameters may affect the process of crystallization and the final quality of products such as, solution concentration, standing time, solvent–anti-solvent ratio, temperature, stirring speed and solution injection rate. Apart from the reacting conditions, the properties of particulates, especially particle size distribution is considered to be one of the most critical quality-affecting attributes.

The traditional method for particle size detection includes screening method, microscope method, sedimentation method, and inductive method. In recent years, advanced detection technology has been developed rapidly, and it has widened the detection range of particle size. There are Focused Beam Reflectance Measurement (FBRM) [2], [3], laser diffraction [4], [5], mass spectrometry [6] and pressure fluctuations [7]. Due to its simple, low cost, no special requirements, screening method is often used in industry for particle size measurement. The microscope method is suitable for the measurement of fine particles, and requires the clean test environment. However due to the unsteady-state dynamic features of the crystallization performed by using batch processing, these techniques are not only relatively complex to apply, but also relatively difficult to interpret. Moreover, a lot of chemical production processes are often carried out in hard process conditions, for example in high pressure and high temperature conditions, corrosive media and dust content. As a result, until today it is still difficult to do on-line detection and controlling particle size as all existing technologies exhibit major limitations depending on the basic feature of the slurry, properties of the particles.

As far as crystallization control is concerned, there is a need to develop reliable monitoring techniques that can be used for on-line detection. Although several methodologies are suggested to monitor the crystallization process, e.g. differential scanning calorimetry (DSC), pulsed nuclear magnetic resonance (pNMR), X-ray diffraction (XRD), rheology and polarized light microscopy (PLM) [8], only two of them can actually monitor microstructural characteristic. Furthermore, all these techniques are applied off-line in the assumption that the tested batch is the same as the entire production. Therefore, developing a simple, rapid and accurate on-line detection technology to monitor the crystallization process and particle size is of great significance for quality control.

Acoustic measurement technique, based on vibration energy produced during manufacturing, processing or material transport, has shown potential as a basis for the development of on-line monitoring and control systems. It has been applied in various areas of research and industrial process monitoring, such as tablets [9], slurries [10], powder blending [11], heterogeneous reactions [12], [13], various fluidized bed processes [14], [15], [16], [17], and end-point detection in high shear granulation [18], [19], [20], [21]. However, few studies deal with the AE monitoring of crystallization processes [22], [23], [24].

Acoustic monitoring techniques, in comparison to optical techniques such as spatial filtering technique, near infrared and image analysis [25], [26], [27], [28], [29], [30], do not require a window or port into the process vessel. So, there is no need for equipment modifications and it can avoid the inaccurate measurements or impossible of data collection caused by fouling optical probe head or window. Thanks to the large amount of collected data, one can explore a new path for gaining a new point of view on basic crystallization phenomena, to increase process understanding and provide a basis for innovative online monitoring and control application.

This work aims at evaluating the potential for using acoustic emission to monitor the physicochemical crystallization process. With this objective, the chemical reaction of sodium salicylate with sulfuric acid was selected as a model-system. The acoustic waves emitted by the process under consideration can be characterized by an important number of parameters which, roughly speaking, are related to the frequency of the acoustic bursts. Multivariate data were collected and try to find the relationship between the stages of production of salicylic acid particles and try to characterize the basic crystallization phenomena. The intensities (the counts) of AE signals and their relationships with the mode of the particles distribution were analyzed at five different injection rates.