Particle engineering of benzoic acid by spherical agglomeration

The influence of process conditions on the properties of benzoic acid spherical agglomerates, are investigated. Agglomerates are produced in a fed-batch agitated tank process. Benzoic acid is dissolved in ethanol and the solution is mixed with the bridging liquid, before being fed into an agitated aqueous solution. A broader investigation has been performed using heptane as the bridging liquid, and in further experiments different bridging liquids are compared. The results show that the bridging liquid has an influence on the product properties, with diethyl ether and ethyl acetate being at the extreme end with no agglomerates formed at all. Using any of the other five solvents (chloroform, toluene, heptane, pentane, or cyclohexane) spherical agglomerates are formed, as long as a sufficient amount of the bridging liquid is used. The results show that the particle size and strength increase with increasing amount of bridging liquid, and with decreasing temperature. At amount of bridging liquid producing optimum particle shape, the largest agglomerates are produced when using either cyclohexane operating at 5 °C, or using toluene in a process at 20 °C. The highest particle fracture stress is obtained using toluene as the bridging liquid regardless of temperature. The particle shape depends on the bridging liquid, and becomes completely spherical when toluene or pentane is used. For four of the solvents the particle morphology improves with decreasing temperature, but for cyclohexane the result is the opposite. By continued agitation beyond the completion of the feeding, particle size and strength gradually increases and also the shape gradually becomes more spherical. High compressibility and low elastic recovery suggest that the particles are favorable for direct tabletting. The results are analyzed and discussed against capillary theory and granulation mechanisms.