(391f) Effect Of Crystal Aging And Agglomeration On Particle Breakage In Saturated Solutions

Particle breakage in stirred vessels such as crystallizers occurs due to collision of crystals with each other and with the mechanical parts of the crystallizer such as the stirrer, baffles, vessel wall etc. Researchers investigating the particle breakage phenomenon in crystallizers usually use a non-solvent in order to isolate breakage from other phenomena such as crystal aging and agglomeration. One type of crystal aging is Ostwald ripening, where large particles grow larger at the expense of smaller particles dissolving in a saturated solution. Ostwald ripening poses an obvious concern to the experimentalist researching particle breakage. If a significant quantity of small particles is formed, it could dissolve and significantly change the shape and size of the larger particles. Another type of aging is due to small temperature oscillations. It has been shown in the past that some crystals (for example, terephthalic acid, barium-lead sulfate) undergo shape changes in their saturated solutions when they are subjected to small temperature fluctuations. This type of aging poses a bigger concern because even in a temperature-controlled crystallizer there is a chance that temperature maybe slightly higher or lower than the set point at various locations in the vessel. For example, the temperature could be slightly higher at the tip of the impeller compared to the wall of the vessel. This could cause a suspended crystal in the saturated solution to experience random periods of high and low temperatures due to stirring and thereby accelerate the aging process and consequently change the shape and size of the crystals. While using non-solvents prevents other phenomena from occurring simultaneously, it doesn't always adequately represent real crystallization conditions. This is because of the difference in properties of saturated solutions and non-solvents. Only a few systems have been studied in the past using saturated solutions. Consequently, very little experimental data is available on crystal breakage behavior in saturated solutions. Although, it is common knowledge that phenomena like agglomeration, Ostwald ripening and aging occur in saturated solutions, the severity of such phenomena and their impact on breakage behavior is not known. The limited number of systems tested cannot be considered as representative of all crystallization processes. This work studies different systems in saturated solutions and quantitatively investigates the effects of Ostwald ripening, aging and agglomeration on crystal size and shape. Since it is impossible to follow one single particle in a stirred vessel, the particle is studied using a microscope hot-cold stage to control the temperature, and a microscope to determine changes in particle size and shape. Computer software is employed to measure various dimensional characteristics of particles to quantitatively analyze changes in shape and size. To investigate the effect of temperature fluctuations, the temperature of the hot-cold stage is varied randomly between 1 to 3 °C. Different sizes of crystals in saturated solutions and non-solvents are studied to determine if the original crystal size has a significant effect. Testing for agglomeration and Ostwald ripening is performed by adding small crystals to a saturated solution with a large crystal, and observing the large crystal. Images at regular time intervals are taken and analyzed. A variety of systems with different solubilities are tested to determine the effect solubility has on the results. Results of these experiments and comparisons between systems are presented.