Crystal-size distribution measurement

D. M. Ruthven and K. F. Loughlin (University of New Brunswick, Fredericton, N. B., Canada) We would like to re-emphasize the significance of the effects of crystal size distribution in the analysis of sorption curves. The variations in diffusivity which are reported in this paper show precisely the trend which is to be expected if the effect of crystal size distribution is considered. We feel sure that if crystal size distribution measurements are available, it should be possible to obtain the correct values of diffusivity using the type of analysis which we presented in our comment on the paper of Kondis and Dranoff. 

Other techniques also based on mass are sedimentation and light scattering/diffraction. The most popular technique offered by several companies is Fraunhofer diffraction. In each case, including sieves, there will be some difficulty in measuring small particles in the presence of larger particles. This will manifest itself in poor estimation of the small size population density, a shortcoming that will become more obvious when nucleation rates are determined from crystal size distribution measurements. 

Although magma density is a function of the kinetic parameters fP and G, it often can be measured iadependentiy. In such cases, it should be used as a constraint ia evaluating nucleation and growth rates from measured crystal size distributions (62), especially if the system of iaterest exhibits the characteristics of anomalous crystal growth. 

The population balance analysis of the idealized MSMPR crystallizer is a particularly elegant method for analysing crystal size distributions at steady state in order to determine crystal growth and nucleation kinetics. Unfortunately, the latter cannot currently be predicted a priori and must be measured, as considered in Chapter 5. Anomalies can occur in the data and their subsequent analysis, however, if the assumptions of the MSMPR crystallizer are not strictly met. 

Several authors have presented methods for the simultaneous estimation of crystal growth and nucleation kinetics from batch crystallizations. In an early study, Bransom and Dunning (1949) derived a crystal population balance to analyse batch CSD for growth and nucleation kinetics. Misra and White (1971), Ness and White (1976) and McNeil etal. (1978) applied the population balance to obtain both nucleation and crystal growth rates from the measurement of crystal size distributions during a batch experiment. In a refinement, Tavare and... 

Figure 8.10 Predicted and measured (averaged) crystal size distributions for barium sulphate (Re = 30,000, C o = 0.015 kmol Cbo = 1-500 kmol Rii = 1).

Garside, J. and Jancic, S.J., 1978. Prediction and measurement of crystal size distribution for size-dependent growth. Chemical Engineering Science, 4331. 

Crystallization from solution is a widely utilized separation and purification technique in chemical industry. It is characterized by the formation of a spectrum of differently sized crystals. This spectrum, called the Crystal Size Distribution or CSD, is highly important for the performance of the crystallizer, the crystal handling equipment like centrifuges and dryers, and the marketability of the produced crystals. However, in many industrial crystallizers, the observed CSD s show large transients due to disturbances or are unstable because of the internal feedback mechanisms of the crystallization process ). The main limitation for effective CSD control was the lack of a good on-line CSD measurement device, but recent developments show that this hurdle is taken (2). 

The observed transients of the crystal size distribution (CSD) of industrial crystallizers are either caused by process disturbances or by instabilities in the crystallization process itself (1 ). Due to the introduction of an on-line CSD measurement technique (2), the control of CSD s in crystallization processes comes into sight. Another requirement to reach this goal is a dynamic model for the CSD in Industrial crystallizers. The dynamic model for a continuous crystallization process consists of a nonlinear partial difference equation coupled to one or two ordinary differential equations (2..iU and is completed by a set of algebraic relations for the growth and nucleatlon kinetics. The kinetic relations are empirical and contain a number of parameters which have to be estimated from the experimental data. Simulation of the experimental data in combination with a nonlinear parameter estimation is a powerful 1 technique to determine the kinetic parameters from the experimental... 

Higgins M. D. (2000) Measurement of crystal size distributions. Am. Mineral. 85, 1105-1116. 

Crystal size distribution (CSD) is measured with a series of standard screens. The openings of the various mesh sizes according to the U.S. Standard are listed in Example 6.6, and according to the British Standard in Figure 16.4. Table 12.1 is a complete listing. The size of a crystal is taken to be the average of the screen openings of successive sizes that just pass and just retain the crystal. 

On-line particle sizing by ultrasonic (acoustic attenuation) spectroscopy was developed for use during batch crystallization processes.14 Crystallization of the alpha polymorph of (l) -glutamic acid from aqueous solution was monitored by continuously pumping the crystallizing solution through an on-line ultrasonic spectrometer. The method enabled measurement of the crystal size distribution and solid concentration throughout the... 

Peterson T (1996) A refined technique for measuring crystal size distributions in thin section. Contrib Mineral Petrol 124 395-405... 

Higgins M. D. (1996a) Crystal size distributions and other quantitative textural measurements in lavas and tuff from Egmont volcano (Mt Taranaki), New Zealand. Bull Volcanol. 58, 194-204. 

Crystal Size Distribution. The measured crystal size distribution followed a log-normal form, suggested as characteristic for most small particles by Herdan (6). Figure 1 shows results obtained with the 4A crystal powder and with the 2 types of pellets formed from it. Here and below, the Linde crystal powder as received, the pelletized powder, and the pellets formed from the elutriated powder will be referred to as CPR, CPS, and ECPS, respectively. Clearly, the pelletizing process did not alfect the size distribution of the original material. Furthermore, the elutriated particles do have a somewhat larger average size. 

When the value of s is known from measurements of crystal size distribution, this expression may be evaluated numerically to give W as a function of Dt/jx. The value of D//x may then be obtained by matching the experimental data to the theoretical curve. 

Crystal size distribution (CSD) is measured with a series of standard screens. The openings of the various mesh sizes according to the... 

Sensors for particle size characterization used for crystallization include ultrasound attenuation measurement/ " laser diffraction/ and laser backscatteiing/ commercially called focused beam reflectance measurement (FBRM). Ultrasonic attenuation spectroscopy has been used to monitor the crystallization process parameters such as the crystal size distribution, concentration, and the onset of nucleation during batch crystallization of L-glutamic acid/ Off-line laser diffraction has been used to measure the crystal size distribution in the development of the crystallization process for a pharmaceutical intermediate/ ... 

This disadvantage is compensated by the ability to measure a wide size range from below 10 pm to above 3 mm and the fact that PSDs can be measured at very high concentrations (0.5 to >50 percent of volume) without dilution. This eliminates the risk of affecting the dispersion state and makes this method ideal for in-line monitoring of e.g., crystallizers (A. Pankewitz and H. Geers, LABO, In-line Crystal Size Distribution Analysis in Industrial Crystallization Processes by Ultrasonic Extinction, May 2000). 

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