Acetaminophen was synthesized by reacting p-aminophenol with acetic anhydride. Overall materials balance was compiled. Three different crystallization paths (i.e., cases I, II, and III) involving various modes in agitation and addition of sodium hydroxide were taken to produce acetaminophen particles with different particle size distributions (PSDs) and polymorphism. The concentration profile of acetaminophen, temperature, and pH values throughout reaction and crystallization were monitored mainly by in-process control (IPC). The rates of nucleation, crystal growth, and agglomeration during crystallization were quantified by the mixed-suspension mixed-product removal (MSMPR) formalism. The average specific cake resistance for filtration, Krischer rate-moisture plot for drying, Carr's index for powder flowability, and dissolution rate for drug delivery of acetaminophen crystals generated in cases I, II, and III were thoroughly studied and compared.
■ INTRODUCTIONIsolation and purification of solid intermediates and final product from chemical reaction are often achieved by crystallization followed immediately afterwards. 1−3 Reaction crystallization can be thought of as a bottom-up fabrication 4,5 spanning over an enormous time and length scale 6 from the production of angstrom leveled molecules to the generation of nanometer sized nuclei and the growth of submicrometer-sized primary crystals, and all the way up to the generation of micrometer-sized agglomerates. Besides the reaction-related factors such as the completion of reaction, the impurity profiles and levels, and the optimization of yield, 7 other significant engineering impacts brought about by the solid state properties of the final crystallization product, such as polymorphism, 8 crystal habit, particle size, and distribution, 9 on the downstream behaviors 10 involving filter cake resistance, 9,11 drying kinetics, 12 flowability, 13,14 and dissolution rate 5 should be fully considered for ensuring rugged and reproducible scale-up operations. Many of the previous efforts have already been placed on those separate areas, for example: (a) chemical preparation, 15,16 (b) impurity effect on crystallization kinetics, 17 (c) crystallization route on polymorphism, 18,19 (d) solvent selection on crystal habits and agglomerates, 20−22 (e) agitation speed and antisolvent feeding rate on agglomeration, 23 (f) different types of crystallizers on crystal size distribution (CSD), 24 (g) optimization of particle size distribution (PSD), 25 (h) prediction on agglomerate type, 26 (i) rates of nucleation, growth, and aggregation, 27,28 (j) spherical agglomeration, 29 (k) drying of drug substances, 30,31 and (l) crystal growth kinetics and dissolution behavior. 32,33 However, making stable and reproducible pharmaceutical products required monitoring and controlling of the desired PSD from crystallization all the way through various agitated dryers. 34,35 The collective behaviors manifested by different PSDs in blending, wet granulation, and dissolution have also been inve...