Spherical Crystallization of Benzoic Acid

University dissertation from Stockholm : KTH

Abstract: Spherical agglomerates of benzoic acid crystals have been successfully prepared by drowning-out crystallization in three solvent partial miscible mixtures. Benzoic acid is dissolved in ethanol, bridging liquid is added and this mixture is fed to the agitated crystallizer containing water. Fine crystals are produced by crystallization of the substance, and the crystals are agglomerated by introduction of an immiscible liquid called the bridging liquid. The concentration of solute, agitation rate, feeding rate, amount of bridging liquid and temperature are found to have a significant influence on the physico-mechanical properties of the product. The product particle characterization includes the particle size distribution, morphology and mechanical strength.Many of the solvents such as chloroform, toluene, pentane, heptane, cyclo hexane, diethyl ether and ethyl acetate were used as bridging liquids. Among the selected solvents ethyl acetate and di ethyl ether could not form the spherical agglomerates. Characteristics of the particles are changing with the bridging liquid used. Range of the operation for spherical agglomeration is very narrow and was shown that only at certain conditions the spherical agglomerates are produced. Increased amount of bridging liquid, decrease in feeding rate and temperature causes the particle size to increase. Particle morphology depends on the bridging liquid used, amount of bridging liquid and the temperature. Particles look completely spherical from the experiments where toluene is used as bridging liquid.The mechanical strength of single agglomerates has been determined by compression in a materials testing machine, using a 10N load cell. For single particle compression an approximate estimation of the true stress is presented. Compression characteristics for single agglomerates are compared with data on particle bed compression. Low elastic recovery and high compressibility of the single particles and of bed of particles reveals that the spherical agglomerates prepared in this work have a plastic behavior which is expected to be favorable for direct tabletting. Some of the stress–strain curves are J-shaped with no clear fracturing of the particles, and are well correlated by an exponential–polynomial equation.

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