congresos y reuniones científicas
Melt granulation: effects of operating variables on particles growth mechanisms
Workshop; 6th International Granulation Workshop; 2013
Granulation processes are usually classified according to the binder nature as wet, dry o melt. Nowadays, dry or melt granulation are considered attractive strategies to overcome operating problems caused by moisture. The amount of articles related with wet granulation processes using aqueous solutions as liquid binders is vast. Among others, Smith and Nienow [1], Pont et al. [2] and Hemati et al. [3], studied the influence of the process variables on the particles growth kinetics when aqueous solutions are atomized into beds of solids fluidized by hot air. These experimental studies revealed that many variables can affect the particles growth mechanisms: binder (composition, viscosity, surface tension, flowrate, droplet size), seeds properties (size, shape, porosity), atomization and fluidization air flowrates, nozzle location, bed temperature, etc. For wet granulation and low excess gas velocities, agglomeration was identified as the main particle growth mechanism. For higher gas velocities, the higher particles circulation rate improved the liquid distribution on the seeds surface, reducing bed quenching by lumps formation. Moreover, the higher gas velocities increased the frequency and energy of the inter-particle collisions and particle-wall impacts that are responsible for the breakage of the solidified binder bridges formed between primary seeds. The same behaviour was observed, through experimental work, for different fluidized granulation systems (i.e., with different operating variables and material parameters). Besides, the growth regions were found to be very sensitive to the product and type of granulation unit. Unfortunately, the theories and results obtained for wet granulation are not fully appropriate for describing fluidized-bed melt granulation [4,5]. As for wet granulation, many authors were focused on identifying the influence of some of the more important experimental variables on the product quality. Regarding the studies based on fluidized-bed granulation by atomizing molten binders, Abberger et al. [6], Seo et al. [7], Tan el al. [5] and Boerefijn and Hounslow [8] investigated the effects of binder spray rate and droplet size, seeds size, bed temperature, atomization air pressure and fluidization air velocity on the process performance, using polyethylene glycol as a model binder and glass ballotini or lactose as seeds. In addition, the binder flowrate and the seeds sizes were relatively low. In this work, the influence of the operating variables of a pilot-scale fluidized-bed granulator (binder flowrate, fluidization air velocity, bed temperature, atomization air flowrate and seeds sizes) on the urea particles growth mechanisms is studied. Particularly, relatively large urea seeds (from 1.5 to 3.7 mm) and high binder flowrates (between 0.24 and 1 kg/min of molten urea; for a bed mass of about 2 kg) were used. The results indicate that the melt flowrate and the fluidization air velocity strongly affect agglomerates formation. A regime map, derived from the performed experiments, is presented to predict the process variables ranges that guarantee the production of granules by coating. [1] P.G. Smith, A.W. Nienow, Chemical Engineering Science. 38 (1983) 1223–1231. [2] V. Pont, K. Saleh, D. Steinmetz, M. Hemati, Powder Technology. 120 (2001) 97–104. [3] M. Hemati, R. Cherif, K. Saleh, V. Pont, Powder Technology. 130 (2003) 18–34. [4] G. Walker, C. Holland, M.N. Ahmad, D. Craig, Chemical Engineering Science. 60 (2005) 3867–3877. [5] H. Tan, a Salman, M.J. Hounslow, Chemical Engineering Science. 61 (2006) 1585–1601. [6] T. Abberger, A. Seo, T. Schaefer, T. Schæfer, International Journal of Pharmaceutics. 249 (2002) 185–197. [7] A. Seo, P. Holm, T. Schæfer, T. Schaefer, European Journal of Pharmaceutical Sciences. 16 (2002) 95–105. [8] R. Boerefijn, M.J. Hounslow, Chemical Engineering Science. 60 (2005) 3879–3890